Studies in Surface Science and Catalysis 46
ZEOLITES AS CATALYSTS, SORBENTS AND DETERGENT BUILDERS Applications and Innovations
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Studies in Surface Science and Catalysis Advisory Editors:B. Delmon and J.T. Yates
Vol. 46
ZEOLITES AS CATALYSTS. SORBENTS AND DETERGENT BUILDERS Applications and Innovations Proceedings of an International Symposium, Wurzburg, F.R.G., September 4-8,1988
Editors
H.G. Karge Fritz-Haber-lnstitut der Max-Planck-Gesellscha ft, Faradayweg 4-6, D- 1000 Berlin 33 (West)
J. Weitkamp University of Stuttgart, Institute of Chemical Technology I, Pfaffenwaldring, D- 7000 Stuttgart 80, F. R. G.
ELSEVIER
Amsterdam - Oxford - New York -Tokyo
1989
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010, U S A . L l b r i r y of Congress C 8 t 8 I o g l n g - l n - P u b l l c a t I n n
Zeolites a s c a t a l y s t s . s o r b e n t s . and d e r e r g e n r b u l l d e r r
Data
applications
and i n n o v a t i o n s P r O c e e d l n g s o f an i n t e r n a t t o n a l s y n p o s i u n . WuPZbuPg. F . R . G . . Smptemb8r 4-8. 1988 / 0 d l T O r S . H.G. K a r g n . J. He I r k a m p . p. cm. ( S t u d l e i I n r u r f a c n s c l e n c o and c a t i l y s l s , 4 6 ) P r o c a e d l n p s o f t h e I n r 8 r n a r l o n a l Symposium on "Zeolltss a s C a r a l y s t s . S o r b e n t r . and D s t n r g 8 n t B u l l d a r s . " I n c l u d e s indexes. Blbliography p. ISBN 0-444-87383-x 1. ZeoIites--Congrcsrsr. 2 . Sorbents--Congresres. 3 . D8Tmrponts. Synthetlc--Conprssses. I . Kargm. H. 0 . t n e l l m u r 0 . ) 11. W8ltkamp. J. ( J e n s ) 111. I n r s r n a t l o n a l Symposium on "Zeolltss as C a t i l y s t s . S o r b a n t s . ana D e t e r g e n t B u l l d 8 r S ' (1988 H u r z b u r g . Germany) i V . Sa-les. ~ ~ 2 4 5 . ~ 5 198s z m 660 . 2 9 9 5 - - d c 2 0 89- 1464 CIP
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V
CONTENTS
.......................... XI11 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . XIV International Scientific Committee . . . . . . . . . . . . . . . . . . .X I V Executive Committee ........................... XV Preface
Financial Support
...........................
xv
I . CATALYSIS AND CATALYSIS-RELATED PROPERTIES
Skeletal Rearrangement Reactions of Olefins, Paraffins and Aromatics over Aluminophosphate based Molecular Sieve Catalysts J.A. Rabo, R.J. Pellet, P.K. Coughlin, E.S. Shamshoum
.........1
Catalytic and Physical Properties of Silicon-Substituted AlP04-5 Molecular Sieves K.J. Chao, L.J. Leu
...........................
On the Nature of the Catalytic Activity of SAPO-5 Ch. Minchev, V. Kanazirev, V. Mavrodinova. V. Penchev, H. Lechert
19
. . . . 29
Molecular Orbital Calculations on the Structural and Acidic Characteristics of Aluminophosphates (AlPO). Si 1 icoaluminophosphates (SAPO) and Metaluminophosphate (MeAPO) Based Molecular Sieves R. Carson, E.M. Cooke, J. Dwyer, A. Hinchliffe, P.J. O'Malley
. . . . . . 39
Relation between Paraffin Isomerisation Capability and Pore Architecture of Large-Pore Bifunctional Zeolites J.A. Martens, M. Tielen, P.A. Jacobs.
..................
49
Respective Influences of the Geometric and Chemical Factors in the Conversion o f Aromatics over Acidic Zeolites F. Fajula, M. Lambret, F. Figueras
. . . . . . . . . . . . . . . . . . . 61
Para-Selectivity of Pentasil Zeolites 3.-H. Kim. S. Namba, T. Yashima
.....................
71
v1
Hydrogen Spillover in the Conversion of n-Alkanes on Zeolites K.-H. Steinberg, U. Mroczek, F. Roeher
. . . . . . . . . . . . . . . . 81
Intermediates in the Formation of Aromatics from Propene and 2-Propanol on H-ZSM-5 Zeol i tes H. Lechert, C. Bezouhanova, C. Oimitrov, V. Nenova
. . . . . . . . . . . 91
Zeolite and Matrix Structures and their Role in Catalytic Cracking C.J. Groenenboom
...........................
99
Evaluation of Non-Commercial Modified Large Pore Zeolites in FCC E. Jacquinot, F. Raatz, A. Macedo. Ch. Marcilly
. . . . . . . . . . . . 115
Surface-Metals Interactions in Fluid Cracking Catalysts During the Upgrading of Vanadium Contaminated Gas Oils M.L. Occelli, J.M. Stencel
.......................
127
Highly Dispersed Pt and Pt-Cr Clusters in Pentasils and their Activity in Transformations of Lower Alkanes E.S. Shpiro, G.J. Tuleuova, V.I. Zaikovskii, O.P. Tkachenko, T.V. Vasina, O.V. Bragin, Kh.M. Minachev
. . . . . . . . . . . . . . . . 143
Conversion of Light Alkanes into Aromatic Hydrocarbons. 3. Arornatization of Propane and Propene on Mixtures of HZSM5 and of Ga2O3 N.S. Gnep, J.Y. Doyemet, M. Guisnet
. . . . . . . . . . . . . . . . . . 153
Shape-Selective Catalysis in Zeolites with Organic Substrates Containing Oxygen R.F. Parton, J.M. Jacobs, O.R. Huybrechts and P.A. Jacobs.
. . . . . . . 163
lhe Use of Zeolite Catalysts for the Synthesis of Nitrogen-Containing Organic Intermediates W.F. Hoelderich
............................
193
Comparison of the Alkylation of Anisole and Phenol wifh Methanol on Pentasil and Ultrastable Zeolites R.F. Parton. J.M. Jacobs. H.v. Ooteghem, P.A. Jacobs
. . . . . . . . . . 211
Acidity Effect o f ZSM 5 Zeolites on Phenol Methylation Reaction N.S. Chang, C.C. Chen, S.J. Chu, P.Y. Chen, T.K. Chuang
. . . . . . . . 223
VII
A new Catalyst for MIBK Synthesis - Palladium on ZSM-5 Zeolites P.Y. Chen, S.J. Chu, N.S. Chang, T.K. Chuang, L.Y. Chen.
. . . . . . . . 231
Acetylene Hydration on Zeolite Catalysts: An I.R. of the Surface Species Gy. Onyestyak, J. Papp Jr.. 0. Kallo
Spectroscopic Study
. . . . . . . . . . . . . . . . .241
Preparation and Characterization of Mo-HY Zeolites A. Meszaros-Kis, J. Valyon
251
Propylene Metathesis Reaction over Mo/Y-Zeolites M. Caniecki
259
......................
..............................
Molybdenum-Oxide-Modif ied Pentasil Zeolites I.M. Harris, J. Dwyer, A.A. Garforth, C.H. McAteer, W.J. Ball
. . . . . 271
Surface and Catalytic Properties of the New Zeolite Type LZ 132 Z. TvaruZkova, M. Tupa, P. Jiru. A. Nastro. 6. Giordano, F. Trifiro
. . 281
PtRh-Doped Zeolites as Three-Way-Catalysts: SIMS Analysis as a Tool for the Selection of Suitable Zeolite Types C. Plog, J. Haas, J. Steinwandel
. . . . . . . . . . . . . . . . . . . 295
Transformation o f Ethanethiol over Zeolites M. Ziol’ek, P. Decyk, M. Derewinski, J. Haber
. . . . . . . . . . . . . 305
Study o f the Structure and the Redox Reactivity of NaX Encapsulated Co( 1 I)-Phthalocyanine G. Schulz-Ekloff, 0. Wohrle, V. Iliev, E. Ignatzek, A. Andreev Selective Reduction o f Nitric Oxide over Zeolite-Supported Iridium Catalyst R. Myrdal, St. Kolboe
. . . . 315
........................
Metal-Doped Zeolites for Selective Catalytic Reduction of Nitrogen Oxides in Combustion Gases J.Haas, J.Steinwande1, C. Plog
327
. . . . . . . . . . . . . . . . . . . 337
Vlll
Preparation of NiHZSM-5 Catalyst for Isomerization o f c8 Aromatics. Solid-state Incorporation of Nickel B. Wichterlova. S. Beran, L. Kubelkova, J. Novakova, A. SmieSkova, R. Sebik
347
Formation of Carbocations from c6 Compounds in Zeolites I. Kiricsi, H. Forster, G. Tasi
355
............................... ....................
An Infrared and Catalytic Study o f Isomorphous Substitution in Pentasil Zeolites M.F.M. Post, T. Huizinga, C.A. Emeis, J.M. Nanne, W.H.J. Stork
. . . . 365
Calorimetric lnvestigation o f the Acidity of Dealuminated Y-Type Zeolites Using Various Basic Probes A. Auroux. Z.C. Shi, N. Echoufi, Y . Ben Taarit
. . . . . . . . . . . . 377
The Acidity of a Modified Faujasite Structure, Zeolite CSZ-1 S. Cartlidge. R.L. Cotterman, M.L. Howes
. . . . . . . . . . . . . . . 389
Control of Catalytic Properties of ZSM-5 made by Fast and Template-free Synthesis A. TiAler, P. Polanek, U. Girrbach, U. Muller, K.K. Unger Theoretical Studies of BrBnsted Acidity in Zeolites R. Vetrivel. C.R.A. Catlow. E.A. Colbourn. M. Leslie
. . . . . . . 399
. . . . . . . . . 409
N M R and I R Studies of Zeolites of the Erionite Type F. Roefiner, K.-H. Steinberg, 0. Freude, M. Hunger, H. Pfeifer
. . . . 421
Specific Platinum Particles Properties in Basic Zeolites A. de Mallmann, D. Barthomeuf
....................
11.
429
SORPTION
Fundamental Research and Modeling for a Technical Process of Selective Adsorption of Normal Paraffins ("Parex"-Process of DDR) by Zeolite A W. Schirmer, K. Fiedler, H. Stach, M.Suckow
. . . . . . . . . . . . . 439
Sorbex Technology for Industrial Scale Separation J.A. Johnson, A.R. Oroskar
......................
451
IX
Gas Oil Dearomatization by Adsorption A. Laktic, J. Muhl. I. Beck. M. Beer
. . . . . . . . . . . . . . . . . 469
Modeling Diffusion Pathways in MFI Materials by Time-Resolved Powder Diffraction Techniques B.F. Mentzen
.............................
477
Measurement of Intracrystalline Diffusivities of HZSM-5 Zeolite at Higher Temperatures and Predictions of Shape Selectivity K. Hashimoto, T. Masuda, M. Kawase
. . . . . . . . . . . . . . . . . . 485
Measurement of Hydrocarbon Diffusion Coefficient in a Non-Isobaric Chromatographic Column of Zeolite Crystal Powder E. Aust, W. Hilgert, G. Emig
.....................
495
Molecular Mobility of Benzene and p-Xylene in M F I Type Zeolites M. Bulow, J. Caro, B. Rohl-Kuhn, B. Zibrowlus
. . . . . . . . . . . . 505
Diffusion of n-Hexane and 3-Methylpentane in H-ZSM-5 Crystals of Various Sizes P. Voogd, H. van Bekkum
........................
519
Comparative Study by Deuteron Solid State NMR Spectroscopy of the Dynamics o f Benzene and Olefins in Faujasite- and Mordenite-Type Zeolites B. Boddenberg. R. Burmeister, G. Spaeth
k
. . . . . . . . . . . . . . . 533
Probing the Hydrogen Sorption States in Zeolites A by Infrared Spectroscopy and Low-Temperature Gas Chromatography Supplemented by Theoretical Calculations H. Forster, W. Frede, G. Peters
. . . . . . . . . . . . . . . . . . . 545
Fourier-Transform Infra-Red Photoacoustic Spectroscopy, A Useful Technique for the Study of Strongly Physisorbed Molecules J. Philippaerts, E.F. Vansant, Y.A. Yan
. . . . . . . . . . . . . . . 555
Adsorption Properties of Large Crystals of ZSM-5 Zeolite as a Function of the Degree of Dealumination J. Kornatowski, M. Rozwadowski, A. Gutsze, K.E. Wisniewski
. . . . . . 567
X
Self-Consistent-Charge Xu Calculations of Sorption Complexes of Nitrous Oxide Attached to Transition Metal Occupied Zeolite Clusters 0. Zakharieva-Pencheva, M. Grodzicki, H. Forster
. . . . . . . . . . . 575
IR Study o f the Adsorption of Benzene on HZSM5 A. Jentys, J.A. Lercher
......................
565
Adsorption Separation o f Methylnaphthalene Isomers on X and Y Zeolites V. Solinas. R. Monaci, E. Rombi, M.Morbidelli
. . . . . . . . . . . . 595
Oxygen Enrichment of Air with Molecular Sieve Zeolites Using the PSA/VSA Technique G. ReiR
..............................
607
Adsorption and Diffusion o f Different Hydrocarbons in MFI Zeolite of Varying Crystallite Size D.H. Lin, V. Ducarme, 6. Coudurier, J.C. Vedrine
. . . . . . . . . . . 615
High Resolution Sorption Studies of Argon and Nitrogen on Large Crystals of Aluminophosphate A1 Po4-5 and Zeolite ZSM-5 U. Muller, K.K. Unger, 0. Pan, A. Mersmann, Y. Grillet, F. Rouquerol. J. Rouquerol
......................
625
A new Potential Large-Scale Application o f Zeolites as Fire-Retardant Material H.K. Beyer, G. Borbely, P. Miasnikov, P. Rozsa
. . . . . . . . . . . . 635
111. ION EXCHANGE AND DETERGENT BUILDING
Industrial Production of Zeolites E. Roland
645
Calcium and Magnesium Exchange in Na-A, Ma-X and their Precursor Gels L.V.C. Rees
661
.............................
.............................
Fundamentals of Phosphate Substitution in Detergents by Zeolites Cobuilders and Optical Brighteners M.J. Schwuger, M. Liphard
-
.....................
673
XI
Zeolite A - A Builder for Liquid Detergents? W. Leonhardt, B.-M. Sax
691
Development and Performance of Zeol ite-A-Bui It Non-Phosphate Detergents H. Upadek, P. Krings
701
.......................
........................
Simultaneous Separation of Suspended Sol ids, Ammonium and Phosphate Ions from Waste Water by Modified Clinoptilolite J. Olah. J. Papp, A. Meszaros-Kis, Gy. Mucsi, 0. Kallo
. . . . . . . . 711
IV. MODIFICATION AND CHARACTERIZATION Framework and Non-Framework A1 Species in Dealuminated Zeolite Y P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs Characterization of Calcined FAPO-5 S. Schubert, H.M. Ziethen, A.X. Trautwein, F. Schmidt, H.-X. J.A. Martens, P.A. Jacobs
. . . 721
Li,
......................
Control of Pore-Opening Size of Zeolites Y.F. Chu, C.F. Keweshan, E.F. Vansant
735
. . . . . . . . . . . . . . . . . 749
Structural-Modification Technique for Zeolites: Chemisorption o f Si2H6 Y. Yan, J . Verbiest, J. Philippaerts, E.F. Vansant. P. De Hulsters I\ new
. . . 759
Dealumination of the Zeolites Offretite and Erionite Studied by Sol id-State 29Si- and 27A1-MAS NMR Spectroscopy K.P. Lillerud, M.Stocker
769
Gal liation and 180-exchange Reactivities of ZSM-5 and ZSM-11 A. Endoh, K. Nishimiya, K. Tsutsumi, T. Takaishi
779
Thermal Decomposition of Ironpentacarbonyl in Zeolites of Faujasite Type. A Study of the Influence of Argon, HE, H2/CO Gas Mixture and Various Si/Al Ratios Using Mossbauer, ESR and Mass Spectroscopy H.M. Ziethen. A.X. Trautwein
709
....................... ...........
....................
Investigation of Ultra Stable Y by Differential Thermal Analysis after Injection of Water Vapour A. Yoshida, K. Inoue
. . . . . . . . . . . . . . . . . . . . . . . . . 801
Properties of Hydrothermal Low-Damaged 5A and 1OX Zeolites R. Schollner, H. Siege1
. . . . . . . . . . . . . . . . . . . . . . . . 811
Phase Transformations and Changes in Lattice Parameters of ZSM-5 as a Function of A1 Content and Temperature G.T. Kokotailo, L. Riekert, A. TiRler
. . . . . . . . . . . . . . . . . 821
The Effect of Sorbates and Elevated Temperatures on the Structures of Some Zeolite Catalysts C.A. Fyfe, G.T. Kokotailo, H. Strobl, H. Gies, G.J. Kennedy, C.T. Pasztor, G.E. Barlow
. . . . . . . . . . . . . . . . . . . . . . 827
The Effect of Temperature and Sorption of p-Xylene and Benzene on the Structure of ZSM-5 G.T. Kokotailo, L. Riekert, A. TiSler.
. . . . . . . . . . . . . . . . . 843
The Characterization of Modified ZSM-5 Catalysts prepared via a Sol id-state Reaction for Propane Aromatization Y. Yang, X. Guo, M. Oeng, L. Wang, Z. Fu
. . . . . . . . . . . . . . . . 849
Author Index
......,.....,.. ............ ....
Subject Index
. . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . 863
Studies in Surface Science and Catalysis (other volumes in the series). .
859
. 869
XI11
PREFACE
This symposium was one of the smaller zeolite meetings held in between the big International Zeolite Conferences. It is the latest in the series of meetings which started in 1978 in Szeged, Hungary, and continued in 1980 in Villeurbanne (Lyon), France; 1982 in Bremen, FRG; 1984 in Prague, Czechoslovakia; 1985 in Si6fok, Hungary; and 1987 in Nieuwpoort, Belgium. These conferences are intended t o cover selected fields from the broad area of zeolite research and application. For the Wurzburg Symposium emphasis was placed on zeolite catalysis, sorption and detergent builders. With respect t o catalysis, particular attention was paid to the synthesis of fine chemicals with the help of zeolite catalysts. An overview of this important field was given in two invited lectures which were complemented by a number of related oral and poster contributions. Special efforts were made to bring together experts from industry and academia. This endeavour was successful, as was reflected both in the invited lectures, oral and poster presentations and in the composition of the audience. An expected trend was the ever increasing interest in the so-called new generation molecular sieves both for research and application. A remarkable number of the presented studies were devoted t o these materials. Innovations, however, were mostly visible in developing and refining methods and techniques of investigation either in experiment or theory. Obviously, successful application of highly sophisticated tools of solid state and surface science t o problems of zeolite research now becomes possible. This seems t o promise a growth in our knowledge of and deeper insight into the subtle details of zeolite properties and behaviour.
Hellmut G. Karge Fritz Haber institute Max Planck Society, Berlin
January, 1989
Jens Weitkamp institute of Chemical Technology I University of Stuttgart
XIV
ACKNOWLEDGMENTS The organizers of the international Symposium on "ZEOLITES AS CATALYSTS, SORBENTS AND DETERGENT BUILDERS", held at Wurzburg, Federal Republic of Germany, September 4-8, 1988 express their appreciation to the members of the International Scientific Committee who carried out the difficult and important task of paper selection. They also wish t o thank the members of the Executive Staff for their great efforts made during preparation and running of the symposium. Particular thanks are due to Mrs. R. Stottko for the coordination and execution of the finances, to Dr. St. Ernst for his active participation in the organizational work and to Mrs. M. Rahimi, Mrs. E. Stankewitz and Mrs. J. Reiffel for their most valuable assistance in preparing the proceedings. Furthermore, the organizers thank the authors for submitting their manuscripts for publication in these proceedingsand the referees for spending time and effort in order to ensure the high scientific standard of the contributions. Last but not least, the help and generous financial support of collaborating organizations and sponsors from the industry is gratefully acknowledged.
Hellmut G. Karge
Jens Weitkamp.
INTERNATIONAL SCIENTIFIC COMMlllEE H. BEYER, HungarianAcademy of Sciences, Budapest, Hungary. 5. CARTLIDGE, Grace GmbH, Worms, FRG. P. CHRISTOPHLIEMK, Henkel KGaA, Dusseldorf, FRG. J. DWYER, The University of Manchester, England.
F. FAJULA, Ecole Nationale Supkrieure de Chimie, Montpellier, France. F. FETTING, Technische Hochschule Darmstadt, FRG.
W. H6LDERICH. BASF AG, Ludwigshafen, FRG. W. KEIM, RWTH Aachen, FRG, and DMGK, Hamburg, FRG. A. KISS, Degussa AG, Hanau, FRG.
H. KRAL, Dechema, FrankfurVM., FRG.
xv E.4. LEUPOLD, Hoechst AG, FrankfuWM., FRG. R. MAUREL, lnstitut de Recherches sur la Catalyse, Villeurbanne, France. I.E. MAXWELL, Koninklijke/Shell, Amsterdam, The Netherlands. L. MOSCOU, Akzo Chemie BV, Amsterdam, The Netherlands. G. OHLMANN, Academy of Sciences of the GDR, Berlin-Adlershof, GDR. L. PUPPE, Bayer AG, Leverkusen, FRG.
M.S. SPENCER, ICI, Billingham, England. J.B. UYlTERHOEVEN, Katholieke Universiteit Leuven, Belgium.
EXECUTIVE COMMllTEE R. AMBERG, Berlin (West).
C.H.BERKE, Stuttgart, FRG. A. BREHM, Oldenburg, FRG. C.-Y. CHEN, Oldenburg, FRG.
H. DARMSTADT, Berlin (West). St. ERNST, Oldenburg, FRG.
TH. KROMMINGA, Stuttgart, FRG. D. LINDNER, Oldenburg, FRG. G.W. MULLER, Stuttgart, FRG. M. NEUBER, Karlsruhe, FRG. W. NIESSEN, Berlin (West). E. PERNKLAU, Stuttgart, FRG. E. STANKEWITZ, Berlin (West). R. STOTTKO, Berlin (West).
W. WACHSMANN, Berlin (West).
FINANCIAL SUPPORT Akzo Chemie, Ketjen Catalysts, Amsterdam, The Netherlands. BASF AG, Ludwigshafen, FRG. BP International Limited, Sunbury-on-Thames, England.
Bayer AG, Leverkusen, FRG.
XVI
Degussa AG, Frankfurt, FRG. Deutsche Forschungsgemeinschaft(DFG), Bonn, FRG. Deutscher Akademischer Austauschdienst (DAAD), Bonn, FRG. DGMK German Society for Petroleum Sciences and Coal Chemistry, Processing and Application Division, Hamburg, FRG. Exxon Chemical Holland B.V., Rotterdam, The Netherlands. Grace GmbH, Worms, FRG. Henkel KGaA, Dussseldorf, FRG. Hoechst AG, Frankfurt, FRG. lnstitut Frangais du Petrole, Rueil Malmaison, France. International Zeolite Association (IZA). Kali-Chemie AG, Hannover, FRG.
Max-Planck-Gesellschaft, Munchen, FRG. Perkin Elmer GmbH, Uberlingen, FRG. Rutgerswerke AG, Frankfurt, FRG. SKW Trostberg AG, Trostberg, FRG.
Sud-Chemie AC, Miinchen, FRG.
I. CATALYSIS AND CATALYSIS-RELATED PROPERTIES
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H.G. Karge, J . Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SKELETAL REARRANGEMENT REACTIONS OF OLEFINS,
PARAFFINS AND AROMATICS OVER
ALUMINOPHOSPHATE BASED MOLECULAR S I E V E CATALYSTS
J u l e A. Rabo, Regis J . P e l l e t , P e t e r K. C o u g h l i n and Edwar S. Shamshoum Union Carbide C o r p o r a t i o n , O l d Saw M i l l R i v e r Road, Tarrytown, NY 10591, U.S.A.
ABSTRACT Medium pore aluminophosphate based m o l e c u l a r s i e v e s w i t h t h e -11, -31 and -41 c r y s t a l s t r u c t u r e s a r e a c t i v e and s e l e c t i v e c a t a l y s t s f o r 1-hexene i s o m e r i z a t i o n , hexane d e h y d r o c y c l i z a t i o n and c 8 a r o m a t i c r e a c t i o n s . With o l e f i n feeds, t h e y promote i s o m e r i z a t i o n w i t h l i t t l e loss t o competing hydride transfer and c r a c k i n g r e a c t i o n s . With C8 aromatics, they e f f e c t i v e l y c a t a l y z e xylene i s o m e r i z a t i o n and ethylbenzene d i s p r o p o r t i o n a t i o n a t v e r y low xylene loss. As a c i d components i n b i f u n c t i o n a l c a t a l y s t s , t h e y a r e s e l e c t i v e f o r p a r a f f i n and c y c l o p a r a f f i n i s o m e r i z a t i o n w i t h low c r a c k i n g activity. I n these r e a c t i o n s t h e medium p o r e aluminophosphate based s i e v e s a r e g e n e r a l l y l e s s a c t i v e b u t s i g n i f i c a n t l y more s e l e c t i v e t h a n t h e medium pore z e o l i t e s . S i m i l a r i t y w i t h medium p o r e z e o l i t e s i s d i s p l a y e d b y an o u t s t a n d i n g r e s i s t a n c e t o coke induced d e a c t i v a t i o n and b y a v a r i e t y o f shape s e l e c t i v e actions i n catalysis. The e x c e l l e n t s e l e c t i v i t i e s observed w i t h medium pore aluminophosphate based s i e v e s i s a t t r i b u t e d t o a unique combinat i o n o f m i l d a c i d i t y and shape s e l e c t i v i t y . S e l e c t i v i t y i s a l s o enhanced by t h e presence o f t r a n s i t i o n metal framework c o n s t i t u e n t s such as c o b a l t and manganese which may e x e r t a chemical i n f l u e n c e on r e a c t i o n i n t e r m e d i a t e s . INTRODUCTION According t o recent reports,
t h e aluminophosphate based m o l e c u l a r s i e v e s
have a c i d i c c a t a l y t i c a c t i v i t y f o r a broad a r r a y o f p e t r o l e u m r e f i n i n g and petrochemical
reactions.
activities
were
molecular
sieves.
reported
In
early
for
a
SAPO
These
studies
number
of
molecular
Subsequently,
substitution
aluminophosphate
certain
2)
n-butane
cracking
(SAPO)
sieves
showed
weak
acidity
by
i t was found t h a t t r a n s i t i o n m e t a l
comparison w i t h z e o l i t e s . into
(ref.
silicoaluminophosphate
crystals
resulted
i n enhanced
a c i d i t y as i n d i c a t e d b y enhanced butane c r a c k i n g a c t i v i t y ( r e f .
3).
With
medium pore species t h e g e n e r a l l y m i l d a c i d i t y combined w i t h u n i q u e shape selectivity reactions.
has Thus,
resulted
in
a r e v i e w of
improved
phosphate based m o l e c u l a r s i e v e s catalytic
cracking
reforming
(ref.
conversion
(refs.
(ref.
7),
4),
aromatic
9,lO)
and
catalytic
selectivity
in
several
t h e p a t e n t l i t e r a t u r e r e v e a l s t h a t aluminohave
shown c a t a l y t i c
hydrocracking alkylations
(ref. (ref.
activity
in fluid
5), dewaxing ( r e f . 6), 8 ) , methanol t o o l e f i n
i n o l e f i n oligomerization
(refs.
8.11).
In
s e v e r a l cases o l e f i n s p l a y an i m p o r t a n t r o l e , e i t h e r as f e e d c o n s t i t u e n t s o r
as r e a c t i o n i n t e r m e d i a t e s .
The enhanced s e l e c t i v i t y o f c e r t a i n SAPO's f o r
o l e f i n r e a c t i o n s has a l r e a d y been noted. SAPO-34,
s m a l l p o r e SAPO's,
such as
were found v e r y e f f e c t i v e a t i n t e r c o n v e r t i n g l i g h t o l e f i n s such as
ethylene,
propylene
and
butylenes
12).
oligomeric products ( r e f , very
Thus,
selective
for
with
little
loss
to
paraffinic
or
The medium p o r e SAPO's were a l s o a c t i v e and
oligomerization
of
p r o p y l e n e and butenes t o o l e f i n i c
11) o r t o d i s t i l l a t e s w i t h o u t the production o f p a r a f f i n s o r
gasoline ( r e f . aromatics. The
present
aluminophosphate
paper
on
reports
molecular
the
sieves
in
catalytic model
properties
hydrocarbon
of
selected
reactions.
The
m o l e c u l a r s i e v e s were s e l e c t e d t o r e p r e s e n t l a r g e and medium p o r e s i z e s w i t h
a v a r i e t y o f framework elements i n c l u d i n g t r a n s i t i o n m e t a l s , i n a d d i t i o n t o aluminum and phosphorus.
Model
r e a c t i o n s were chosen t o e x p l o r e c a t a l y t i c
performance i n p a r a f f i n , o l e f i n and a r o m a t i c rearrangement r e a c t i o n s t o probe molecular
sieve
character,
shape
selectivity
and
catalytic
activity,
p a r t i c u l a r l y for reactions i n v o l v i n g o l e f i n s or o l e f i n r e a c t i o n intermediates. EXPERIMENTAL M o l e c u l a r Sieve C a t a l y s t P r e p a r a t i o n The aluminophosphate based m o l e c u l a r s i e v e s used i n t h e p r e s e n t s t u d y were prepared a c c o r d i n g t o procedures d e s c r i b e d i n US P a t e n t ( r e f s .
2, 13, 14).
The p r e p a r a t i o n o f medium p o r e r e f e r e n c e m o l e c u l a r sieves, LZ-105 z e o l i t e and s i l i c a l i t e , have a l s o been d e s c r i b e d elsewhere ( r e f s . 15, 16). For 1-hexene i s o m e r i z a t i o n and f o r a c i d c a t a l y z e d C8 a r o m a t i c r e a c t i o n s a l l m o l e c u l a r s i e v e s were e v a l u a t e d i n t h e i r c a l c i n e d ,
C8 aromatics,
the study o f
selected
SAPO m o l e c u l a r
exchanged o r steam t r e a t e d as n o t e d i n T a b l e I V . used
in
paraffin
interconversions, platinum-loaded
the
cyclizationlisomerization calcined
molecular
sieve
c h l o r i d e d gamma alumina powder.
powdered s t a t e .
For
s i e v e s were aluminum
For b i f u n c t i o n a l c a t a l y s t s and
ethylbenzene-xylene
powder
was
mixed
with
These m i x t u r e s were t h e n
bound u s i n g s i l i c a s o l and e x t r u d e d t o f o r m 1/16" e x t r u d a t e s which were d r i e d and c a l c i n e d a t 500°C.
The b i f u n c t i o n a l c a t a l y s t s were p r e p a r e d t o c o n t a i n
about 0.5% p l a t i n u m and about 40 t o 50% SAPO m o l e c u l a r s i e v e i n t h e f i n i s h e d catalysts. Cata 1ys t Eva1u a t ion The powdered m o l e c u l a r
sieves
were e v a l u a t e d f o l l o w i n g t h e t r e a t m e n t
d e s c r i b e d above, w i t h o u t f u r t h e r a c t i v a t i o n .
C8
aromatic
isomerization
tests
continuous f l o w microreactors.
were
The 1-hexene i s o m e r i z a t i o n and
conducted
in
tubular,
fixed
bed,
The c a t a l y s t bed c o n t a i n e d one gram m o l e c u l a r
3
s i e v e powder and one t o t h r e e grams o f s i m i l a r l y s i z e d q u a r t z c h i p s used as diluent.
The r e a c t o r was heated t o t h e chosen r e a c t i o n temperatures i n a
fluidized
sand
bath,
and
the
r e a c t i o n t e m p e r a t u r e was
monitored b y
a
Typical runs l a s t e d 3 t o 5 hours
thermocouple .located i n t h e c a t a l y s t bed.
d u r i n g which samples were c o l l e c t e d e v e r y 30 minutes. P a r a f f i n c y c l i z a t i o n and i s o m e r i z a t i o n s t u d i e s were a l s o conducted i n t h e m i c r o r e a c t o r system. The f e e d used i n t h i s s t u d y was t e c h n i c a l grade n-hexane c o n t a i n i n g 87% n-hexane, 9% methylcyclopentane and 4% isopentanes. T y p i c a l l y about 0.35 g o f ground e x t r u d a t e s (20/80 mesh) were mixed w i t h quartz
c h i p s and loaded t o t h e r e a c t o r .
The r e a c t o r was heated t o t h e
r e a c t i o n temperature i n f l o w i n g hydrogen, p r i o r t o hexane f e e d i n t r o d u c t i o n . Runs l a s t e d f o r 24 hours and samples were c o l l e c t e d o n l y a f t e r 20 h o u r s on feed t o p e r m i t c a t a l y s t l i n e - o u t .
L i q u i d and gaseous p r o d u c t s were c o l l e c t e d
and analyzed as d e s c r i b e d above. The
bifunctional
platinum
-
SAP0
catalysts
were
evaluated
for
C8
a r o m a t i c r e a c t i o n s i n t h e presence o f hydrogen i n bench s c a l e r e a c t o r s . T y p i c a l l y , 25 t o 50 cc c a t a l y s t e x t r u d a t e s were d i l u t e d w i t h about f o u r volumes of s i m i l a r l y s i z e d q u a r t z chips, and p l a c e d i n a 1 1/4" ID r e a c t o r w i t n q u a r t z c h i p s placed i n b o t h end zones.
Feed which c o n s i s t e d o f e i t h e r
17% ethylbenzene + 83% m-xylene o r 40% ethylbenzene + 60% m-xylene was t h e n pumped over
t h e c a t a l y s t bed.
p r o d u c t s were c o l l e c t e d d a i l y .
Bench s c a l e r u n s l a s t e d s e v e r a l days and Both gas and l i q u i d p r o d u c t s were q u a n t i f i e d
and analyzed and m a t e r i a l balances were determined.
T y p i c a l l y t h e s e balances
were w i t h i n 2% o f c l o s u r e . The r e a c t i o n c o n d i t i o n s f o r a l l c a t a l y s t t e s t s a r e g i v e n a l o n g w i t h t h e t e s t r e s u l t s i n Tables I t o V. RESULTS A N D DISCUSSION Reactions o f O l e f i n s Olefins
p l a y a key r o l e as
intermediates
r e f i n i n g and petrochemical r e a c t i o n s .
Therefore,
i n a number o f
petroleum
t o b e t t e r understand t h e
f u n c t i o n and u t i l i t y o f aluminophosphate-based c a t a l y s t s , i t i s d e s i r a b l e t o examine t h e i r c a t a l y t i c c h e m i s t r y w i t h model o l e f i n s .
As mentioned above,
t h e s i l icoaluminophosphate m o l e c u l a r s i e v e s have a l r e a d y been r e p o r t e d t o be a c t i v e f o r o l i g o m e r i z a t i o n o f l i g h t o l e f i n s t o g a s o l i n e range p r o d u c t s ( r e f . 11).
The r e s u l t s o f t h a t s t u d y a r e reproduced i n T a b l e I which p r e s e n t s
conversion and s e l e c t i v i t y d a t a f o r p r o p y l e n e o l i g o m e r i z a t i o n t o l i q u i d products. These d a t a were o b t a i n e d u s i n g l a r g e , medium and s m a l l - p o r e SAPO's and medium pore r e f e r e n c e LZ-105 z e o l i t e .
4
TABLE I Vapor Phase P r o p y l e n e 01 i g o m e r i z a t i o n M o l e c u l a r Sieve P o r e Size, A Run Temperature, "F Pressure, p s i g P r o p y l ene WHSV Time on Stream,hr. p r o p y l e n e Conversion, % C5+ S e l e c t i v i t y a a)
SAPO-5 8 700 25 0.98 4.3 0
SAPO- 11 6 700 25 0.94 4.2 86.3 77.0
SAPO-3 1 1 700 50 1.04 5.5 76.2 82.7
SAPO-34 4.3 700 25 0.53 2.33 41.6 19.5
LZ- 105 6 703 25 0.90 3.5 81.6 37.2
C5+ S e l e c t i v i t y = (C5+ y i e l d , wt%)/(C3= c o n v e r s i o n , wt%)x100. The
data
show
that
the
l a r g e pore
SAPO-5
with
a
undirectional
non
i n t e r s e c t i n g channel system ( r e f . 17) was i n a c t i v e f o r o l i g o m e r i z a t i o n b y t h e t i m e t h e f i r s t sample had been taken. a c t i v i t y f o r butane c r a c k i n g ( r e f .
2),
Since SAPO-5 e x h i b i t e d s i g n i f i c a n t t h e lack o f oligomerization a c t i v i t y
was a t t r i b u t e d t o r a p i d c a t a l y s t coking, r e s u l t i n g i n d e a c t i v a t i o n . The s m a l l p o r e SAPO-34 h a v i n g a c r y s t a l s t r u c t u r e analogous t o c h a b a z i t e was
also
ineffective for
c o n v e r s i o n was observed,
propylene oligomerization. most o f
While 40% p r o p y l e n e
the products consisted o f ethylene
butenes, w i t h o n l y 20% s e l e c t i v i t y t o l i q u i d p r o d u c t s .
and
L a r g e r o l i g o m e r s were
c o n c e i v a b l y formed i n t h e cages o f t h e -34 c r y s t a l s t r u c t u r e b u t c o u l d n o t escape through t h e m o l e c u l a r s i e v e ' s s m a l l p o r e openings. products
observed,
were
p r o p y l e n e oligomers.
presumably
formed
by
the
The l i g h t o l e f i n
cracking
of
trapped
T h i s s c r a m b l i n g o f l i g h t o l e f i n s b y SAPO-34 was a l s o
observed by K a i s e r ( r e f . 12). I n c o n t r a s t t o SAPO-5 and SAPO-34, exhibited products.
significant While
oligomerization
the
s t r u c t u r e o f SAPO-11
structure
of
t h e medium p o r e SAPO-11 and SAPO-31 activity
the
has been r e p o r t e d .
and s e l e c t i v i t y t o
SAPO-31
is
as
yet
liquid
unknown,
the
I t comprises u n i d i r e c t i o n a l ,
non
i n t e r s e c t i n g channels formed by 10 member oxygen r i n g s ( r e f . 17). Conversions as h i g h as 86%, with C5+ p r o d u c t s e l e c t i v i t i e s as h i g h as 83%, were r e p o r t e d under t h e s c r e e n i n g c o n d i t i o n s .
The performance o f t h e medium p o r e SAPO's
a l s o d i f f e r e d s i g n i f i c a n t l y from t h a t o f t h e medium p o r e z e o l i t e r e f e r e n c e , LZ-105,
which a l s o achieved h i g h p r o p y l e n e conversion,
b u t a t o n l y 37% C5+
selectivity. The LZ-105 i s s t r u c t u r a l l y r e l a t e d t o ZSM-5 b u t i s p r e p a r e d w i t h o u t an o r g a n i c template. The l i q u i d p r o d u c t s formed over LZ-105 were h i g h l y aromatic,
i n c o n t r a s t t o t h e SAPO p r o d u c t s which were p r e d o m i n a n t l y
o l e f i n i c . Furthermore, t h e major gas p r o d u c t formed over LZ-105 c o n s i s t e d o f l i g h t paraffins.
The LZ-105
preponderance
hydrogen
of
o l i g o m e r i z a t e underwent
p r o d u c t d i s t r i b u t i o n was
transfer
secondary
reactions reactions,
in
which
attributed t o olefin
presumably o v e r
feed
strong
the and acid
5
s i t e s , t o produce l i g h t p a r a f f i n s and aromatics.
It was concluded t h a t , t h e
medium p o r e SAPO'S l a c k e d t h e a c i d s t r e n g t h r e q u i r e d t o c a t a l y z e h y d r i d e s h i f t r e a c t i o n s , and p o s s i b l y l a c k e d s p a t i a l r e q u i r e m e n t s t o f o r m t h e b u l k y t r a n s i t i o n s t a t e s e n v i s i o n e d f o r t h e s e h y d r i d e s h i f t r e a c t i o n s ( r e f . 11).
This lack
of h y d r i d e s h i f t a c t i v i t y r e s u l t e d i n h i g h s e l e c t i v i t y t o o l e f i n i c g a s o l i n e .
In t h e p r e s e n t study, t h e r e a c t i o n s o f 1-hexene as c a t a l y z e d b y a number of t h e s e m o l e c u l a r s i e v e s a r e summarized i n T a b l e 11. presented for
large,
medium
and
small
pore
As b e f o r e , d a t a a r e
SAPO's.
The
influence
of
t r a n s i t i o n metal framework elements i s a l s o e x p l o r e d w i t h s e l e c t e d MeAPO and
In a d d i t i o n t o o l i g o m e r i z a t i o n and h y d r i d e t r a n s f e r
MeAPSO m o l e c u l a r s i e v e s . reactions
observed
with
propylene
feed,
double
bond
and
skeletal
i s o m e r i z a t i o n s , c y c l i z a t i o n and c r a c k i n g a r e a l s o p o s s i b l e u s i n g 1-hexene as model r e a c t a n t , TABLE I 1 Reactions o f I-Hexene over A1 uminophosphate-Based M o l e c u l a r Sieves Run C o n d i t i o n s : Run Temperature 650°F Pressure 40 p s i g WHSV 5.5 l / h r M o l e c u l a r Sieve Pore Size, A T o t a l Conversion,% Selectivities: Double Bond Isomerization, % Skel e t a 1 Isomerization, % Oligomerization, % Cracking, %
SAPO -5 8
SAPO -11 6
FAPO -11 6
MnAPO -11 6
SAPO -31 7
FAPO -31 7
85.1
84 .5
90.1
89.6
86.0
89.1
94.5
93.7
79.1
46.2
22.1
28.0
82.2
42.5
43.2
2.4
10.9 5.6 3.2
41.9 4.3 3.3
70.6 2.4 1.5
63.6 0.9 1.9
14.3 1.7 0.9
52.8 0.9 1.3
44.6 4.1 2.6
12.2 55.1 25.6
MnAPSO -31 7
LZ-105 6
As w i t h propylene, t h e medium p o r e SAPO's a r e again q u i t e a c t i v e and a l s o quite selective.
Thus,
SAPO-11 gave over 90% 1-hexene conversion w i t h over
90% s e l e c t i v i t y t o isomerized products, h a l f o f which were s k e l e t a l isomers. It i s
important t o note t h a t
medium p o r e SAPO-11
i s s e v e r a l o r d e r s of
magnitude more coke r e s i s t a n t than t h e l a r g e p o r e SAPO-5. minutes
on
hexene
feed,
the
SAPO-5
was
catalytically
A f t e r o n l y 30 i n a c t i v e whereas
i s o m e r i z a t i o n a c t i v i t y w i t h SAPO-11 remained unchanged t h r o u g h o u t t h e t h r e e hour l o n g m i c r o - s c r e e n i n g t e s t .
It i s envisioned t h a t
t h e enhanced coke
r e s i s t a n c e o f t h e medium pore SAPO-11 i s due t o a p r o d u c t shape s e l e c t i v i t y . Due t o s p a t i a l c o n s t r a i n t s w i t h i n t h e SUPO-11 s t r u c t u r e , b u l k y coke p r e c u r s o r s a r e formed a t a g r e a t l y reduced r a t e r e l a t i v e t o t h e i r r a t e o f f o r m a t i o n i n
6
U n t i l now t h i s r e s i s t a n c e t o coke formation has o n l y been observed
SAPO-5.
w i t h t h e medium pore ZSM type molecular sieves ( r e f . 18). Under t h e m i l d e r c o n d i t i o n s o f t h i s screening s t u d y w i t h SAPO-11, l i t t l e o l i g o m e r i z a t i o n activity
was
observed,
but
importantly,
almost
no
cracking
of
hexene
occurred. I n c o n t r a s t , under i d e n t i c a l t e s t c o n d i t i o n s , t h e z e o l i t e r e f e r e n c e LZ-105 had v e r y h i g h a c t i v i t y f o r both o l i g o m e r i z a t i o n and cracking, f o r m i n g over 30% l i g h t products.
Most o f t h e isomerized hexene o l e f i n s were consumed
i n these secondary r e a c t i o n s .
The h i g h i s o m e r i z a t i o n s e l e c t i v i t y observed
w i t h t h e medium pore SAPO's i s probably a t t r i b u t a b l e t o s i g n i f i c a n t l y m i l d e r a c i d s t r e n g t h than t h a t possessed b y the r e f e r e n c e z e o l i t e c a t a l y s t . The i n c o r p o r a t i o n o f t r a n s i t i o n elements i n t o the c r y s t a l s t r u c t u r e o f medium pore aluminophosphate molecular sieves enhances s k e l e t a l i s o m e r i z a t i o n selectivity
even
beyond
that
with
observed
the
SAPO's.
Thus,
the
s i l icoaluminophosphate SAPO-11 gives 42% conversion t o s k e l e t a l isomers w i t h 3% cracked product w h i l e t h e same aluminophosphate c r y s t a l phase c o n t a i n i n g manganese
as
a
framework
constituent,
MnAPO-11,
i s o m e r i z a t i o n w i t h o n l y 2% cracked product.
gives
64%
skeletal
The i r o n c o n t a i n i n g FAPO-11 was
even more a c t i v e and s e l e c t i v e w i t h 71% and 1.5% i s o m e r i z a t i o n and c r a c k i n g a c t i v i t i e s , respectively.
S i m i l a r trends can be seen f o r t h e manganese and
i r o n s u b s t i t u t e d aluminophosphates w i t h t h e -31 s t r u c t u r e . P a r a f f i n Reactions The dehydrocycl i z a t i o n o f p a r a f f i n s r e p r e s e n t s a v e r y i m p o r t a n t c l a s s o f reactions
occurring
dehydrocyclization aromatics.
low
in
the
gasoline
octane
paraffins
converted
to
Through
high
octane
isomers t y p i c a l l y have v e r y s i g n i f i c a n t l y enhanced octane
numbers r e l a t i v e t o normal p a r a f f i n s . very
are
process.
P a r a f f i n i s o m e r i z a t i o n a l s o boosts g a s o l i n e octane i n r e f o r m i n g
s i n c e branched are
reforming
often
accompanied
by
D e h y d r o c y c l i z a t i o n and i s o m e r i z a t i o n
undesirable
cracking
reactions
enhancing
product octane a t t h e expense o f g a s o l i n e y i e l d . Shape s e l e c t i v e c r a c k i n g can enhance octane by c r a c k i n g away t h e low octane p a r a f f i n s t o form gas. The intermediacy o f o l e f i n s i n these r e a c t i o n s has been p r e v i o u s l y demonstrated ( r e f . 19). A simp1 i f i e d mechanism f o r t h e dehydrocycl i z a t i o n , i s o m e r i z a t i o n and c r a c k i n g o f n hexane i s summarized below: Pt
nG
-
nG--
CYCLO ~e
-
LIGHT OLEFINS
BENZENE LIGHT PARAFFINS
7
Here,
f i r s t o l e f i n i c i n t e r m e d i a t e s a r e generated over p l a t i n u m .
These
i n t e r m e d i a t e s a r e e i t h e r cracked t o f o r m l i g h t e r o l e f i n s o r c y c l i z e d and isomerized over t h e a c i d i c c h l o r i d e d alumina.
The o l e f i n s and naphthenes
thus formed a r e f i n a l l y dehydrogenated over t h e p l a t i n u m t o f o r m p a r a f f i n s Z e o l i t i c a c i d s such as m o r d e n i t e ( r e f . 20) and ZSM-5
and aromatics.
( r e f . 21)
have been s u b s t i t u t e d f o r t h e c h l o r i d e d alumina as a c i d c a t a l y s t components. I n t h e p r e s e n t study, m i x t u r e s o f supported p l a t i n u m and aluminophosphate based m o l e c u l a r s i e v e s have been t e s t e d f o r n-hexane rearrangement r e a c t i o n s under
reforming
conditions
(900°F,
200
psig).
Results
are
summarized
g r a p h i c a l l y i n F i g u r e s 1 and 2, where s e l e c t i v i t i e s a r e p l o t t e d as f u n c t i o n s Of
n-hexane conversion.
Data a r e presented f o r t h e l a r g e , medium and s m a l l
pore SAPO m o l e c u l a r s i e v e s mixed w i t h
a Pt-alumina
catalyst.
Data f o r
p l a t i n u m mixed w i t h c h l o r i d e d alumina o r mixed w i t h s i l i c a l i t e a r e p r e s e n t e d f o r reference.
I n F i g u r e 1, t h e r a t i o o f iso-hexanes t o cracked p r o d u c t s i s
p l o t t e d a g a i n s t conversion. pore
SAPO-5
however,
is
less
The c a t a l y s t c o n s i s t i n g o f p l a t i n u m and l a r g e
active
i t appears s i m i l a r
distribution of pore SAPO-34
than
reference
i n selectivity,
platinum-chlorided
i n t h a t i t produces a s i m i l a r
isomerized and cracked p r o d u c t s .
i s quite active f o r
alumina;
Mixed p l a t i n u m and s m a l l
hexane conversions
but
selectivity for
isomerized hexanes i s s i g n i f i c a n t l y lower than observed w i t h t h e r e f e r e n c e , and
a
significant
observed.
increase
in
the
amount o f
These r e s u l t s a r e t o be expected.
l i g h t cracked
products
is
SAPO-34 w i t h t h e c h a b a z i t e - t y p e
s t r u c t u r e possesses cages i n which t h e l a r g e hexene isomers can form; however, due t o t h e small pore opening o f t h e -34 s t r u c t u r e ,
t h e isomers once formed
can n o t escape w i t h o u t c r a c k i n g t o l i g h t e r p r o d u c t s .
The n o b l e m e t a l - l o a d e d
medium pore
SAPO's
-11
and
-41
exhibit
excellent
hexane
isomerization
s e l e c t i v i t i e s when a p p l i e d t o g e t h e r w i t h p l a t i n u m , p r o d u c i n g 2 t o 4 t i m e s t h e amount o f isomers compared t o r e f e r e n c e p l a t i n u m - c h l o r i d e d alumina c a t a l y s t . I n contrast, mixture,
t h e medium pore s i l i c a l i t e
and
supported p l a t i n u m c a t a l y s t
w h i l e a c h i e v i n g s i g n i f i c a n t l y h i g h e r n-hexane
conversion,
i s far
l e s s s e l e c t i v e than t h e medium p o r e SAPO c a t a l y s t m i x t u r e and t h e c h l o r i d e d alumina-platinum
reference,
p r o d u c i n g a l a r g e amount o f cracked p r o d u c t s .
S i m i l a r s e l e c t i v i t y t r e n d s a r e observed f o r t h e dehydrocycl i z a t i o n r e a c t i o n s of n-hexane.
Thus i n F i g u r e 2 t h e benzene t o l i g h t p r o d u c t s r a t i o i s p l o t t e d
a g a i n s t conversion f o r t h e same s e t o f c a t a l y s t s ,
and again t h e medium p o r e
SAPO's e x h i b i t s u p e r i o r s e l e c t i v i t y t o b o t h r e f e r e n c e m a t e r i a l s and t o b o t h l a r g e and small pore SAPO c a t a l y s t systems.
These r e s u l t s a r e i n complete
agreement w i t h t h e hexene i s o m e r i z a t i o n d a t a d e s c r i b e d above,
wherein t h e
medium pore SAPO's show s i g n i f i c a n t l y reduced a c t i v i t y f o r c r a c k i n g r e a c t i o n s while maintaining h i g h skeletal isomerization a c t i v i t y .
8 12r
11
-
A A
10. 0
A
!i 5 *9 c 9-
7-
x
I
'i
2
+n +
1-
Q o A 1
.
1
1
1
,
Fig. 1 . I s o m e r i z a t i o n / C r a c k i n g S e l e c t i v i t y R a t i o p l o t t e d as a f u n c t i o n o f n-hexane conversion: ASAPO- 11 , X SAPO-41, +SAPO-5, OSAPO-34, A S i l i c a l i t e and O P t / c h l o r i d e d - a l u m i n a r e f e r e n c e .
+
I
.75-
A
x x
A
2
I I
s
A
.50-
z W
++
.25W
m
0
I
0
I
10
1
1
.
A
,
30 40 50 60 7 0 80 n - HEXANE CONVERSION, WT% 20
I
90
F i g . 2. D e h y d r o c y c l i z a t i o n / C r a c k i n g S e l e c t i v i t y R a t i o p l o t t e d as a f u n c t i o n of n-hexane conversion: ASAPO-11, XSAPO-41, +SAPO-5, OSAPO-34, A S i l i c a l i t e and O P t / c h l o r i d e d - a l u m i n a r e f e r e n c e .
9
Aromatic Reactions I n a d d i t i o n t o reforming,
t h e i s o m e r i z a t i o n o f C8
a r o m a t i c s t o produce
para-xylene i s another area where o l e f i n i c i n t e r m e d i a t e s may p l a y a s i g n i f i c a n t m e c h a n i s t i c r o l e . The p r o d u c t i o n o f p a r a - x y l e n e i s o f i n t e r e s t t o t h e petrochemical i n d u s t r y because o f i t s use as monomer i n p o l y e s t e r p r o d u c t i o n . I n a d d i t i o n t o C a r o m a t i c i s o m e r i z a t i o n , t h e r e a r e a number o f i m p o r t a n t 8 r o u t e s t o para-xylene i n c l u d i n g t h e a l k y l a t i o n o f t o l u e n e w i t h methanol and the disproportionation o f
toluene.
The c a t a l y t i c
p r o p e r t i e s o f t h e SAP0
m o l e c u l a r s i e v e s f o r t o l u e n e m e t h y l a t i o n r e a c t i o n s have been d e s c r i b e d ( r e f . 11).
While b o t h l a r g e and medium pore SAPO's were a c t i v e f o r t h e a l k y l a t i o n
reaction,
t h e medium pore m a t e r i a l s were d i s t i n g u i s h e d b y r e m a r k a b l y h i g h
s e l e c t i v i t y f o r methylation reactions, with disproportionation o f the toluene feed r e p r e s e n t i n g l e s s than 2% o f t h e t o t a l conversion. pore SAPO-5 had n e a r l y 60% d i s p r o p o r t i o n a t i o n
By comparison, l a r g e
s e l e c t i v i t y and t h e z e o l i t e
r e f e r e n c e LZ-105 had n e a r l y 80% d i s p r o p o r t i o n a t i o n s e l e c t i v i t y . d i s p r o p o r t i o n a t i o n a c t i v i t y o f t h e medium p o r e SAPO's, m i l d acid character,
The v e r y low
attributed t o their
r e s u l t e d i n reduced l o s s e s o f t o l u e n e t o benzene and
increased xylene y i e l d s r e l a t i v e t o LZ-105 and SAPO-5. In
the
present
study,
silicon
and
transition
metal
substituted
aluminophosphate m o l e c u l a r s i e v e s have a l s o been e v a l u a t e d f o r a c t i v i t y and selectivity for
p r o d u c t i o n v i a C8
para-xylene
commercial p r a c t i c e ,
C8
and
naphtha
from
fraction
pyrolysis
aromatic isomerization.
In
aromatic c u t s a r e o b t a i n e d f r o m r e f o r m a t e g a s o l i n e streams.
o f ethylbenzene which
Both
feeds
contain
a
significant
i s d i f f i c u l t t o s e p a r a t e f r o m xylenes
by
and must be c a t a l y t i c a l l y c o n v e r t e d t o o t h e r p r o d u c t s .
p h y s i c a l techniques,
T h i s c a t a l y t i c conversion can b e accomplished b y one o f two c o m n e r c i a l l y a v a i l a b l e approaches. as ZSM-5
(ref.
22)
I n one approach,
m o n o - f u n c t i o n a l a c i d c a t a l y s t s such
isomerize xylenes and a l s o c o n v e r t ethylbenzene t o non
aromatics by selective ethyl group disproportionation. '8 D i s p r o p o r t i o n a t i o n o f xylenes t o non C8 a r o m a t i c s i s a competing s i d e r e a c t i o n l o w e r i n g t h e u l t i m a t e para-xylene y i e l d . not
convert
catalysts
the
ethyl-benzene
to
xylenes.
To
A c i d c a t a l y s i s a l o n e can this
end,
c o n t a i n i n g b o t h a hydrogenation-dehydrogenation
f u n c t i o n such as
p l a t i n u m as w e l l as an a c i d i c f u n c t i o n such as mordenite typically
been
hydrogenated
over
employed. Pt
to
In
the
latter
ethylcyclohexene
approach,
which
is
bifunctional
(ref.
2 3 ) , have
ethylbenzene
then
isomerized
is to
dimethylcyclohexene o v e r t h e a c i d c a t a l y s t f u n c t i o n , and f i n a l l y c o n v e r t e d t o xylenes b y dehydrogenation over t h e platinum.
10
2Hzf
CRACKED PRODUCTS
Conceptually, yields,
this
b u t since
route
offers
naphthenic
the
possibility
intermediates
of
are present,
enhanced
xylene
s i g n i f i c a n t acid
c a t a l y z e d r i n g opening and y i e l d l o s s o f a r o m a t i c s a r e a l s o p o s s i b l e . review o f a v a i l a b l e patent l i t e r a t u r e data
(refs.
because o f these u n d e s i r a b l e s i d e r e a c t i o n s , isomerization
yields
about
the
same
22,
23b) suggests
A
that
t h e b i f u n c t i o n a l approach t o
amount
of
xylene
as
does
the
monofunctional route.
C, Aromatic Reactions Witnout Hydrogen v -
I n t h e p r e s e n t study,
t h e alumino-phosphate
m o l e c u l a r s i e v e s have been
used a l o n e and w i t h added p l a t i n u m and hydrogen t o i s o m e r i z e C feeds.
8
aromatic
In an i n i t i a l s c r e e n i n g s t u d y , a s e r i e s o f l a r g e t o medium p o r e s i z e
molecular
sieves
rearrangements a t conditions
all
isomerization xylene
were
evaluated
1000°F
without
molecular
sieves
o f m-xylene
isomers,
while
trimethylbenzenes
varies
for
catalytic
added metal
activity
for
and hydrogen.
evaluated
give
m-xylene
Under
essentially
these
complete
f e e d t o a thermodynamic e q u i l i b r i u m m i x t u r e of
the
disproportionation
significantly.
summarized i n T a b l e 111 and i n F i g u r e 3,
The
activity
results
to
of
toluene
this
study
and are
where x y l e n e d i s p r o p o r t i o n a t i o n
a c t i v i t y i s p l o t t e d as a f u n c t i o n o f m o l e c u l a r s i e v e p o r e s i z e .
I n general,
a rough t r e n d can be seen i n which t h e m o l e c u l a r s i e v e s with l a r g e r p o r e s i z e s a r e more a c t i v e f o r t h i s u n d e s i r a b l e s i d e r e a c t i o n . chosen
screening
conditions,
SAPO-5,
MAPO-5
and MAPO-36,
m o l e c u l a r s i e v e s w i t h a p p r o x i m a t e l y 8 angstrom p o r e s i z e s , to
22
%
disproportionation
while
SAPO-11,
Thus under t h e
MAPO-11
all
l a r g e pore
c a t a l y z e f r o m 12 and
SAPO-41
a p p r o x i m a t e l y 6 angstrom pores s u f f e r o n l y 2 t o 7 % x y l e n e l o s s e s .
with These
r e s u l t s a r e c o n s i s t e n t w i t h t h e concept o f t r a n s i t i o n s t a t e shape s e l e c t i v i t y where a b u l k y ,
bimolecular
i n t e r a c t i o n o f two xylenes t o f o r m t o l u e n e and
t r i m e t h y l b e n z e n e i s d i f f i c u l t t o a c h i e v e i n t h e a p p r o x i m a t e l y 6 angstrom s i z e channels.
11
TABLE 111 m-Xylene Reactions Catalyzed by A1 uminophosphate-Based Molecular Sieves Run Conditions: Run Temperature Pressure WHSV
1000°F 100 p s i g 5.6 l / h r . a
Molecular Sieve SAPO-5 MAPO-5 MAPO-36 SAPO-3 1 SAPO-4 1 SAPO- 11 MAPO- 1 1
I:
Pore Size 0.8 0.8 0.8 0.65 0.6 0.6 0.6
b Pore Volume 0.31 0.17 0.22 0.16 0.16
m-Xyl ene Dispropor t ionat ion, % Conversion 20.7 22.4 12.6 15.1 7.1 3.2 1.8
Determined by McBain Bakr g r a v i m e t r i c adsorption s t u d i e s ( r e f . 3). Determined by water adsorption a t s a t u r a t i o n ( r e f . 3). 3oL
z
0
2 -
0 24
1 0
E
g18-
* z
0 v)
212-
> z
8w z
2
6-
X
#
01
I
0.8
0.7
0.8
0.9
PORE SIZE, NM
Fig. 3. Xylene losses due t o d i s p r o p o r t i o n a t i o n are a f u n c t i o n o f molecular sieve c a t a l y s t ' s pore s i z e . I n t h e next phase o f t h e present study, sieves were evaluated f o r m-xylene feed.
the
several medium pore molecular
c a t a l y t i c performance with an ethylbenzene and
Again, t h e molecular sieves were t e s t e d with no added metal
12
o r hydrogen.
Several of
t h e s e aluminophosphate based m o l e c u l a r s i e v e s d i d The r e s u l t s o f t h i s s t u d y
c o n t a i n t r a n s i t i o n m e t a l s as framework elements. are summarized i n Table I V and F i g u r e 4.
From t h e Table i t can be seen t h a t
a l l m o l e c u l d r s i e v e s y i e l d p a r a - x y l e n e i n amounts e q u i v a l e n t t o thermodynamic equilibrium.
Surprisingly,
the
cobalt
and manganese-containing
molecular
s i e v e s w i t h t h e -31 t y p e s t r u c t u r e d i d n o t promote t h e e q u i l i b r a t i o n o f meta t o ortho-xylene,
so t h a t v e r y h i g h p a r a / o r t h o s e l e c t i v i t i e s were observed.
Since SAPO-31 w i t h i d e n t i c a l framework s t r u c t u r e does n o t e x h i b i t t h e h i g h para and low o r t h o - x y l e n e
s e l e c t i v i t y observed w i t h t h e MeAPSO-31 m o l e c u l a r
s i e v e s , t h e i r h i g h s e l e c t i v i t y m i g h t p o s s i b l y be i n t e r p r e t e d as p r o d u c t shape selectivity
due
to
restricted
pore
size.
Obviously
the
Mn+2 framework
c a t i o n s a r e l a r g e r than t h e s i l i c o n and aluminum c a t i o n s t h e y a r e r e p l a c i n g , and t h i s c o u l d c o n c e i v a b l y r e s u l t i n r e s t r i c t i o n s i n t h e m o l e c u l a r
sieve
channels.
"plug
gauge-sized"
However,
McBain-Bakr
g r a v i m e t r i c adsorption
studies with
molecules suggest t h a t MeAPSO-31 I s have p o r e s i z e s and volumes
s i m i l a r t o those o f t h e m e t a l - f r e e SAPO-31, which does n o t appear t o b e p a r a s e l e c t i v e ( r e f . 3 ) . Furthermore, SAPO-11 w i t h s l i g h t l y s m a l l e r p o r e s i z e t h a n CoAPSO-31 and MnAPSO-31 does n o t show n e a r l y t h e p a r a s e l e c t i v i t y observed w i t h the t r a n s i t i o n metal-containing structure.
m o l e c u l a r s i e v e s w i t h t h e -31
An a l t e r n a t e e x p l a n a t i o n i s needed.
crystal
I t may b e t h a t t h e framework
t r a n s i t i o n metal i o n s i n t h e s e m o l e c u l a r s i e v e s e x e r t a chemical i n f l u e n c e on the
intermediates
para-isomer.
in
Alternately,
isomerization,
favoring
the
formation
of
the
a c i d s i t e s may be u n i q u e l y l o c a t e d i n t h e MeAPSO
m o l e c u l a r s i e v e s such t h a t access o f r e a c t a n t s and r e a c t i o n i n t e r m e d i a t e s t o these acid s i t e s i s s p a t i a l l y constrained, f a v o r i n g p a r a - s e l e c t i v i t y . TABLE I V m-XylenelEthylbenzene Reactions w i t h Medium Pore S i z e Aluminophosphate Based M o l e c u l a r Sieves Run C o n d i t i o n s : Run Temperature Pressure WHSV
800°F 100 p s i g 5.6 l / h r .
M o l e c u l a r Sieve SAPO- 11 SAPO-11 (Al'3 exchanged) SAPO-11 (Steam t r e a t e d ) SAPO-31 SAPO-31 (Steam t r e a t e d ) MnAPSO- 11 COAPSO-11 MnAPSO-31 COAPSO-31 LZ-105
P ar a/Or t h o Xylene R a t i o
:1.52 ; 0.78 1.56 0.88 1.81 3.59 3.06 0.99
%Para-Xyl ene Equilibration 96 102 63 100 102 104 91 120 111 100
% D i s p r o p o r t io n a t ion Xylenes Ethylbenzene 9.9 23.2 6.6 20.1 0.2 6 .O 31.6 56.3 1.7 10.4 5 .O 23.3 0.0 7.7 0.0 17.7 1.7 23.0 23.6 58.5
13
Figure 4 plots ethylbenzene vs xylene disproportionation activity for the same series of catalysts. Data were obtained at a range of conversions by varying reaction temperature. Again a1 1 catalysts were tested without added metal or hydrogen. Data obtained with medium pore zeolite reference LZ-105 are also presented for comparison. The MeAPSO-31 molecular sieves are again distinguished by superior selectivity. Thus at comparable ethylbenzene conversions, the CoAPSO and MnAPSO-31 molecular sieves exhibit the lowest activity for the undesirable xylene disproportionation while SAPO-11 and SAPO-31 are considerably less selective. Data obtained with LZ-105 is intermediate in selectivity. Again the enhanced selectivity observed with the MeAPSO-31 molecular sieves may be due to a transition metal specific influence on the reaction intermediates. Shape selective effects by themselves cannot explain differences between these materials and the transition metal-free SAPO's.
./ rn
J
6
13
20
27
% XYLENE LOSS
Fig. 4. Selectivity for ethylbenzene conversion plotted against xylene losses for A MeAPSO's, 0 LZ-105 and SAPO's.
14
C Aromatic -
R e a c t i o n s w i t h Hydrogen
I n a final
phase o f t h e c u r r e n t s t u d y , c a t a l y s t s c o n s i s t i n g o f s e l e c t e d
SAPO's and s u p p o r t e d p l a t i n u m have been e v a l u a t e d i n t h e presence o f hydrogen for
C8 a r o m a t i c i s o m e r i z a t i o n .
bifunctional
Two c a t a l y s t s were prepared,
one c o n t a i n i n g 40% o f t h e i n t e r m e d i a t e p o r e SAPO-11 and t h e o t h e r c o n t a i n i n g 40% o f l a r g e pore SAPO-5.
Both c a t a l y s t s were prepared t o c o n t a i n about 0.6% The c a t a l y s t s were e v a l u a t e d a t 8OO0F,
p l a t i n u m supported on alumina.
185
p s i g and a t a space v e l o c i t y o f 1 and a hydrogen t o hydrocarbon r a t i o o f 14. The f e e d used f o r t h e s e t e s t s c o n t a i n e d 17% e t h y l b e n z e n e and 83% m-xylene, simulating
C8
a
aromatic
cut
obtained
from
reformate
gasoline.
Each
c a t a l y s t was e v a l u a t e d f o r s e v e r a l days on stream, and a t t h e h i g h hydrogen/ hydrocarbon r a t i o employed, addition,
the
l i t t l e or
SAPO-11-containing
n o d e a c t i v a t i o n was
catalyst
ethylbenzene
+
effectiveness
i n c o n v e r t i n g ethylbenzene
60% m-xylene
feed
s i m i l a r t o a p y r o l y s i s naphtha C8
in
was
order
to
Typical
with
better
t o xylenes,
cut.
observed.
evaluated
a
In
40%
evaluate
and t o model
its
a feed
performance d a t a f o r each
c a t a l y s t a f t e r s e v e r a l h o u r s on t h e 17% e t h y l b e n z e n e f e e d a r e summarized i n Table V . formed
Performance i n d i c a t o r s were c a l c u l a t e d assuming t h a t t h e naphthenes during
processing
would
in
commercial
practice
be
recycled,
and
t h e r e f o r e t h e i r f o r m a t i o n would c o n t r i b u t e t o n e i t h e r e t h y l b e n z e n e c o n v e r s i o n nor t o xylene losses. A c c o r d i n g t o t h e t e s t s b o t h c a t a l y s t s promote near-complete
xylene
equilibration,
and
the
catalyst
containing
achieves h i g h e r ethylbenzene c o n v e r s i o n t h a n t h e SAPO-11 c a t a l y s t , 44% r e s p e c t i v e l y . However,
SAPO-5 68% and
t h e l a r g e p o r e m o l e c u l a r s i e v e i n c u r s n e a r l y 22%
x y l e n e l o s s e s w h i l e t h e SAPO-11 based c a t a l y s t a c t u a l l y produces 2.1% more xylenes than were p r e s e n t i n i t i a l l y i n t h e feed.
With SAPO-5 these l o s s e s
a r e due t o d i s p r o p o r t i o n a t i o n r e a c t i o n s p r o d u c i n g benzene, aromatics,
and a l s o t o non c y c l i c p r o d u c t f o r m a t i o n ,
t o l u e n e and C9+
suggesting s i g n i f i c a n t
r i n g opening and c r a c k i n g .
With t h e SAPO-11 c a t a l y s t
t h e r e i s much l e s s
disproportionation
and almost n o ring-opened
p a r a f f i n i c products
activity,
a r e observed. While t h e p r e s e n t s t u d y has n o t examined t h e performance o f z e o l i t e based catalysts,
Table
V
summarizes
patent
m o r a e n i t e and p l a t i n u m / a l u m i n a m i x t u r e . obtained
under
similar
conditions
literature
data
(ref.
23b)
for
a
Data f o r t h e P t and SAPO-11 m i x t u r e are
presented
for
comparison.
m o r d e n i t e c a t a l y s t i s s i g n i f i c a n t l y l e s s s e l e c t i v e t h a n SAPO-11,
The
g i v i n g 25.6%
ethylbenzene c o n v e r s i o n w i t h o n l y 0.5% n e t x y l e n e p r o d u c t i o n . The
data
obtained
with
high
ethylbenzene
feed
shows
even
more
d r a m a t i c a l l y t h e e f f i c i e n t c o n v e r s i o n o f e t h y l b e n z e n e t o xylenes o v e r t h e SAPO-11-containing c a t a l y s t .
Thus a t 23.6% ethylbenzene conversion,
a nearly
15
13% i n c r e a s e i n x y l e n e y i e l d i s observed. T h i s i n d i c a t e s t h a t ethylbenzene has been c o n v e r t e d w i t h 75% s e l e c t i v i t y t o xylene isomers. The r e m a i n i n g conversion was t o d i s p r o p o r t i o n a t i o n opening.
products w i t h
a g a i n almost n o r i n g
TABLE V Cg Aromatic I s o m e r i z a t i o n With B i f u n c t i o n a l C a t a l y s t s ~~
Molecular Sieve Component
b
SAPO-5
SAPO-11
SAPO-11
SAPO-11
40 800 185 14 1 17
40 800 185 14 1 17
40 840 250 8.3 2.9 17
40 800 165 14 1 40
Mordenite
a M o l e c u l a r Sieve Content, % Run Pressure, p s i g Run Temperature,"C H2/HC R a t i o WHSV Ethylbenzene i n Feed, % Approach t o p-xylene Equilibrium, % Ethylbenzene Conversion, % Net Xylene P r o d u c t i o n , % a) b)
94.9 67.9 -21.6
97.3 44.6 2.1
94.5 28.2 1.5
50 800 175 8 3.6 15.5
96.2 23.6 12.8
99.3 25.6 0.5
I n a d d i t i o n t o the q u a n t i t y o f molecular sieve l i s t e d , a l l c a t a l y s t s c o n t a i n e d 0.4-0.6% p l atinum. Data o b t a i n e d f r o m U.S. P a t e n t 4,255,606, Example 1.
CONCLUSIONS The medium p o r e aluminophosphate based m o l e c u l a r s i e v e s a r e a c t i v e and selective catalysts f o r reactions.
As
acid
a variety of
catalysts,
i m p o r t a n t hydrocarbon rearrangement
they
promote
olefin
isomerization
and
o l i g o m e r i z a t i o n w h i l e t h e y a r e s i g n i f i c a n t l y l e s s e f f e c t i v e a t t h e competing h y d r i d e t r a n s f e r and c r a c k i n g r e a c t i o n s . I n t h e r e a c t i o n s o f a r o m a t i c s , t h e medium pore aluminophosphates a r e again e f f e c t i v e f o r s k e l e t a l i s o m e r i z a t i o n b u t show low e t h y l group d i s p r o p o r t i o n a t i o n a c t i v i t y . bifunctional
catalysts,
As a c i d components i n
t h e y a r e s e l e c t i v e f o r p a r a f f i n and c y c l o p a r a f f i n
i s o m e r i z a t i o n w i t h low c r a c k i n g a c t i v i t y . These c a t a l y t i c p r o p e r t i e s c o n t r a s t s h a r p l y w i t h t h o s e o f medium p o r e z e o l i t e s such as LZ-105 and w i t h s i l i c a l i t e . considerably
less
active
than LZ-105
for
Thus, medium p o r e SAPO's a r e olefin,
paraffin
and a r o m a t i c
conversions when compared a t t h e same temperature. However, t h e y a r e more s e l e c t i v e f o r o l e f i n and p a r a f f i n i s o m e r i z a t i o n s when e v a l u a t e d a t comparable conversions
.
Not s u r p r i s i n g l y ,
t h e c a t a l y t i c p r o p e r t i e s o f medium p o r e aluminophos-
phates a l s o c o n t r a s t w i t h l a r g e pore aluminophosphate-based m o l e c u l a r s i e v e s of
similar
framework
composition.
With
olefinic
feeds,
the
l a r g e pore
molecular sieves d e a c t i v a t e v e r y r a p i d l y , presumably due t o p o r e p l u g g i n g b y
16
higher molecular weight products. more coke r e s i s t a n t . than 30 minutes,
SAPO-11 I s
With a r o m a t i c feeds, for
The medium p o r e SAPO's a r e d r a m a t i c a l l y
Under c o n d i t i o n s t h a t f u l l y d e a c t i v a t e SAPO-5 a c t i v i t y remains unchanged f o r
i n less
several
hours.
b o t h l a r g e and medium p o r e m o l e c u l a r s i e v e s a r e a c t i v e
a l k y l a t i o n and i s o m e r i z a t i o n .
However,
t h e l a r g e pore molecular sieves
a r e s i g n i f i c a n t l y more a c t i v e f o r t h e d i s p r o p o r t i o n a t i o n o f d i a l k y l a r o m a t i c s , i m p l y i n g t r a n s i t i o n s t a t e shape s e l e c t i v i t y f o r t h e medium p o r e m o l e c u l a r sieves.
I n bifunctional
catalysis
i n v o l v i n g o l e f i n i c intermediates,
large
pore m o l e c u l a r s i e v e s a r e more a c t i v e f o r c r a c k i n g and l e s s s e l e c t i v e f o r skeletal isomerization reactions. The c a t a l y t i c also
p r o p e r t i e s o f t h e aluminophosphate
influenced by
chemical
composition.
The
molecular
sieves are
introduction o f
transition
m e t a l s i n t o framework p o s i t i o n s enhances t h e a c t i v i t y and s e l e c t i v i t y f o r o l e f i n i s o m e r i z a t i o n r e l a t i v e t o t h e silicoaluminophosphates.
The t r a n s i t i o n
metal c o n t a i n i n g alurninophosphates a r e a l s o s u r p r i s i n g l y more s e l e c t i v e f o r
C8 a r o m a t i c rearrangements t h a n t h e c o r r e s p o n d i n g SAP0 m o l e c u l a r s i e v e s , an e f f e c t which can n o t b e a t t r i b u t e d s o l e l y t o improved shape s e l e c t i v i t y . The
enhanced
selectivities
observed
with
medium
pore
silico-
and
metalloaluminophosphates may, t o a l a r g e e x t e n t b e a t t r i b u t e d t o a u n i q u e combination o f m i l d a c i d i t y and shape s e l e c t i v i t y . and c r a c k i n g a c t i v i t y i n o l e f i n - m e d i a t e d acidity.
The
observed r e s i s t a n c e
selectivity t o
para-xylene
to
addition
constituents
to
these
appear
to
coke
deactivation
i n m e t h y l a t i o n and factors,
exert
a
and t h e enhanced
isomerization reactions
is
.
evidence o f shape-sel e c t ive c a t a l y s i s In
The l a c k o f h y d r i d e s h i f t
r e a c t i o n s i s suggestive o f m i l d
however, special
transition
chemical
metal
effect
on
framework catalytic
performance which appears t o be independent o f m o l e c u l a r s i e v e a c i d s t r e n g t h and
spatial
constraints.
manganese-containing
This
molecular
effect sieves
is as
evidenced
by
enhanced
selectivity
ethylbenzene d i s p r o p o r t i o n a t i o n i n t h e presence o f xylenes, para-selectivity size
i n xylene i s o m e r i z a t i o n .
than SAPO-11
as
judged b y
s e l e c t i v e f o r para-xylene to
a uniquely
molecular Alternately, metal,
sieve
for
and b y enhanced
"plug
gauge"
molecules
but
i s f a r more
and f o r ethylbenzene d i s p r o p o r t i o n a t i o n than t h e
'located with
and
Thus, MnAPSO-31 has a l a r g e r p o r e
SAPO-11 under comparable t e s t c o n d i t i o n s . due
cobalt
acid
site
special
and
T h i s enhanced s e l e c t i v i t y may b e
in
the
transition
unexpected
metal-containing
spatial
requirements.
i t may b e due t o a l i g a n d o r e l e c t r o n i c e f f e c t o f t h e t r a n s i t i o n
affecting
the
transition
disproportionation reactions.
states
in
aromatic
isomerization
and
17
REFERENCES 1 S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, J. Am. Chem. SOC. 1982, 1146. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. 2 B.M. Flanigen, J. Am. Chem. SOC. 1984, 6092; U.S. Patent 4 440 871, 1984. 3 E.M. Flanigen, B.M. Lok, R.L. Patton, S.T. Wilson, I n New Developments i n Z e o l i t e Science and Technology; Y. Murakami, A. I i j i m a , J.W. Ward, Eds., Proceedings o f t h e 7 t h I n t e r n a t i o n a l Z e o l i t e Conference; E l s e v i e r , New York, 1986, p.103. 4 R.J. P e l l e t , P.K. Coughlin, M.T. S t a n i u l i s , G.N. Long, J.A. Rabo, U.S. Patent 4 666 875, 1987. Gortsema, R.J. P e l l e t , A.R. Springer, J.A. Rabo, G.N. Long, 5 F.P. Eur. P a t . Appl. 207 133, 1987. Gortsema, R.J. P e l l e t , A.R. 6 F.P. Springer, J.A. Rabo, G.N. Long, Eur. P a t . Appl. 185 329, 1986. 7 D.C. Garska, B.M. Lok, U.S. Patent 4 499 315, 1985. Long, J.A. Rabo, I n New Developments i n Z e o l i t e 8 R.J. P e l l e t , G.N. Y. Murakami, A. I i j i m a , J.W. Ward, Eds., Science and Technology; Proceedings o f t h e 7 t h I n t e r n a t i o n a l Z e o l i t e Conference; E l s e v i e r , New York, 1986, p. 843; 9 S.W. Kaiser, U.S. Patent 4 524 234, 1985. 10 S.k. Kaiser, Arabian J. Sci. Eng., 1985, 10(4),361-6; 1 1 G.N. Lonq, R.J. P e l l e t , J.A. Rabo, U.S. Patent 4 528 414, 1985: 12 S.W. K a i i e r , Eur. Pat; Appl. 142-156, 1985. 13 C.A. Messina, B.M. Lok, E.M. Flanigen U.S. Patent 4 544 143, 1985. 14 S.T. Wilson, E.M. Flanigen U.S. Patent 4 567 029, 1986. 15 R.W. Grose, E.M. Flanigen U.S. Patent 4 257 885, 1981. 16 R.W. Grose, E.M. Flaniqen U.S. Patent 4 061 724, 1977. Flanigen, J.J. - P l u t h , J.V. Smith, I n 17a J.M. Bennett, J.P. CGhen, E.M. I n t r a z e o l i t e Chemistry; ACS Symposium Series No. 218; American Chemical S o c i e t y : Washington, D.C., 1983; pp 109-18. b J.M. Bennett, J.V. Smith, 2. K r i s t . 1985, 171, 65-68. 18 D.E. Walsh, L.D. Rollman, J. Catal. 1979, 195-197. 19 G.A. M i l l s , H. Heinemann, T.H. M i l l i k e n , Oblad, A.G. Ind. Eng. Chem. 1953, 45, 134. 20 R.J. B e r t o l a c i n i , U.S. Patent 4 018 711, 1977. 21 C.M. Detz, L.M. F i e l d , U.S. P a t e n t 4 347 394, 1982. 22 W.O. Haag, D.H. Olson, U.S. Patent 3 856 871, 1974. 23a W.C. Carr, L.M.Polinski, S.G. Hindin, J.L.Kosco, U.S. Patent 4128 591, 1978. b H.F. Tse, U.S. Patent 4 255 606, 1981.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CATALYTIC AND PHYSICAL PROPERTIES OF SILICON-SUBSTITUTED A1P04-5 MOLECULAR SIEVES
K.J. CHAO and L.J. LEU Department o f Chemistry, Tsinghua U n i v e r s i t y , Hsinchu, Taiwan
ABSTRACT The i n c o r p o r a t i o n o f S i i n t o AlP04-5 framework m o d i f i e s t h e a c i d i t y and c a t a l y t i c p r o p e r t i e s o f t h e h o s t w i t h o u t d i s r u p t i n g i t s microporous s t r u c t u r e . Bransted a c i d s i t e s w i t h medium s t r e n g t h were generated b y S i s u b s t i t u t i o n , w h i l e o n l y v e r y weak a c i d i t y was found on t h e o r i g i n a l A1P04-5. B o t h A1P04-5 and SAPO-5 were found t o be a c t i v e i n n o n - o x i d a t i v e dehydrogenation o f ethylbenzene t o s t y r e n e ; t h e a c t i v i t y o f e t h y l benzene c r a c k i n g t o benzene depends on t h e S i c o n t e n t o r B r d n s t e d a c i d i t y on SAPO-5.
INTRODUCTION C r y s t a l l i n e aluminophosphate m o l e c u l a r s i e v e s c o n s i s t o f a l t e r n a t i n g alumina and phosphate t e t r a h e d r a and so a r e e l e c t r o v a l e n t l y n e u t r a l w i t h no e x t r a framework c a t i o n s and no ion-exchange c a p a c i t y ( r e f . 1,2). (E.N.=2.2)
The phosphorus atom
o f h i g h e l e c t r o n e g a t i v i t y may produce a s h i f t i n e l e c t r o n d e n s i t y
away from t h e aluminum atom (E.N.=1.6)
r e s u l t i n g i n a d i p o l a r nature o f t h e
Alp04 s u r f a c e . U s i n g t h e s e m i e m p i r i c a l CND0/2 quantum c a l c u l a t i o n on a c l u s t e r model o f Alp04
, M o f f a t e t a l . ( r e f . 3) found t h a t t h e oxygen atoms possess
n e g a t i v e charge and f u n c t i o n as Lewis base s i t e s , phosphorus atoms w i t h h i g h e r p o s i t i v e charge compared w i t h aluminum atoms a c t as Lewis a c i d s i t e s ; and Brdnsted a c i d s i t e s a r e t h e p r o t o n s a t t a c h e d t o t e r m i n a l oxygen atoms on t h e s u r f a c e o f aluminophosphate. Therefore, t h e r e a r e v e r y few B r d n s t e d a c i d s i t e s and t h e m a j o r i t y o f a c i d s i t e s i s Lewis a c i d s i t e s i n Alp04 m o l e c u l a r s i e v e s . The s p e c t r o s c o p i c r e s u l t s ( r e f . 4) show t h a t t h e a c i d s i t e s a r e m o d e r a t e l y s t r o n g and t h e base s i t e s a r e v e r y weak on A1P04-5 m o l e c u l a r s i e v e which has an u n i d i m e n s i o n a l p o r e system c o n s i s t i n g o f c y l i n d r i c a l channels bounded by 12 membered r i n g s ( r e f . 5). The i n c o r p o r a t i o n o f s i l i c o n i n t o t h e aluminophosphate framework leads t o s i l i c o a l u m i n o D h o s p h a t e m o l e c u l a r s i e v e s (SAPO) ( r e f . 6 ) . The s i l i c o n atoms can m a i n l y s u b s t i t u t e i n t o t h e aluminophosphate framework v i a ( 1 ) s i l i c o n s u b s t i t u t i o n f o r phosphorus, ( 2 ) s u b s t i t u t i o n o f two s i l i c o n s f o r one aluminum p l u s one phosphorus. The s i l i c o n s u b s t i t u t i o n f o r phosphorus r e s u l t s i n a n e g a t i v e l y charged framework which can be coupled w i t h exchangeable
20
c a t i o n s and Brdnsted a c i d s i t e s . The amount o f Brdnsted a c i d s i t e s can be increased by s i l i c o n s u b s t i t u t i o n f o r phosphorus i n t h e aluminophosphate framework. I n t h i s paper, we r e p o r t t h a t t h e i n c o r p o r a t i o n o f S i i n t o t h e n e u t r a l A1P04-5 framework m o d i f i e s i t s p h y s i c a l and c a t a l y t i c p r o p e r t i e s w i t h o u t disrupting
its
microporous s t r u c t u r e .
The framework s i t t i n g o f S i was
confirmed by powder XRD, 2 9 S i MASNMR and EPMA ( e l e c t r o n microprobe a n a l y s i s ) . The temperature programmed d e s o r p t i o n (TPD) o f ammonia from ammonia adsorbed o r ammonium exchanged samples was used f o r c h a r a c t e r i z i n g t h e a c i d i t y and i o n exchange c a p a c i t y o f SAPO-5 and AlP04-5.
The few Brfinsted a c i d s i t e s on AlP04-5
were found t o be a c t i v e i n xylene i s o m e r i z a t i o n and cumene c r a c k i n g ( r e f s . 7 - 9 ) . L i t t l e a t t e n t i o n has been p a i d t o t h e importance o f b a s i c s i t e s on phosphates. As a c a t a l y s t having s u i t a b l e acid-base p a i r s i t e s sometimes shows pronounced a c t i v i t y even i f i t s a c i d o r base s t r e n g t h i s v e r y weak i n t h e form o f simple a c i d o r base. I n t h i s work, t h e c a t a l y t i c a c t i v i t i e s o f A1P04-5 and SAPO-5 were t e s t e d on ethylbenzene conversion. Ethylbenzene was found t o be converted t o benzene by a c i d i c c r a c k i n g on Brfinsted a c i d s i t e s and t o s t y r e n e by nono x i d a t i v e dehydrogenation on dual Lewis acid-base s i t e s . EXPERIMENT PreDaration and C h a r a c t e r i z a t i o n The precusors o f AlP04-5 based molecular s i e v e s (AlP04-5, SAPO-5, BAPO-5 and MgAPO-5) were prepared by r e a c t i n g orthophosphoric a c i d (Verck)
,
pseudoboehmite (Verse1 250)/ and c a t i o n source o f Ludox HS-40 (Du P o n t ) , HgBOg, o r MgC12 (Merck) w i t h t r i p r o p y l a m i n e i n t h e hydrothermal c o n d i t i o n o u t l i n e d by Wilson e t a l . ( r e f . 1 ) . The c r y s t a l l i n e phase was i d e n t i f i e d by XRD on a SCINTAG PADV powder d i f f r a c t o m e t e r and examined by scanning e l e c t r o n microscopy w i t h EPMA on a Jeol Superprobe 733 i n s t r u m e n t and b y MASNMR spectroscopy on a Bruker MSL-200 instrument. The elemental compositions o f products were a l s o analyzed by I C P ( i n d u c t i v e l y - c o u p l e d plasma atomic emission spectrometry). M o d i f i c a t i o n o f A1P04-5 and SAPO-5 samples, which had been c a l c i n e d a t 5OO0C, was made by impregnation w i t h aqueous s o l u t i o n o f 2-5 w t % Mg(CHgC00)2 or F.3803 and MgC12 . The NH4 form o f t h e molecular sieves was obtained by five-times-repeated ion-exchange o f t h e s y n t h e t i c samples w i t h 1N NH4C1 s o l u t i o n ; t h e samples were p r e t r e a t e d i n d r y n i t r o g e n and a i r f l o w a t 50OoC. A f t e r c a l c i n a t i o n a t 5OO0C, samples o f A1P04-5 and SAPO-5 were a l s o cooled t o room temperature i n f l o w n i t r o g e n and t r e a t e d w i t h ammonia f l o w a t 100 o r 200OC. Excess ammonia t h e n was swept o u t from t h e sample a t 100 o r 2DD°C by f l o w i n g d r y N2. The a c i d i t y and c a t i o n exchange c a p a c i t y o f molecular s i e v e products were determined by TPD o f
21
o f ammonia from NH4SAPO-5 o r NHqMgAPO-5 and ammonia adsorbed AlP04-5 o r SAPO-5. The evolved ammonia was trapped i n a s o l u t i o n o f b o r i c a c i d and t i t r a t e d b y s u l f a m i c a c i d ( r e f . 1 0 ) . A h e a t i n g r a t e o f 5oC min-1 and a sweep gas f l o w r a t e o f 30 cm3min-1 were used. The number o f m i l l i e q u i v a l e n t o f ammonia evolved p e r gram sample was c a l c u l a t e d from t h e volume o f t i t r a n t used p e r two minut es vs. temperature o r f o r t h e whole range o f d e s o r p t i o n temperature f rom 100 t o 50OoC. C a t a l y t i c R eac t io n The c a t a l y t i c a c t i v i t i e s on ethylbenzene conversion were determined i n a fixe d -b ed m i c r o r e a c t o r system. A sample o f 0.25 g c a t a l y s t and 1.0 g q u a r t z powder was sandwiched w i t h q u a r t z wool i n a s t a i n l e s s s t e e l tube o f 0.7 cm i n n e r diamet e r. A l l experiments were performed a t atmospheric pressure. P r i o r t o t e s t i n g , each sample was heated f r o m room temperature t o 6OO0C (5'C/min) and c a l c i n e d a t 6OO0C f o r 2h i n a n a i r stream, purged by a h e l i u m stream f o r 30 m i n and t h en c o oled t o t h e r e a c t i o n temperature. Ethylbenzene was f e d v i a a h e l i u m stream which was s a t u r a t e d w i t h ethylbenzene vapor by passage t hrough a b u b b l e r . Reactor f e ed stream and e x i s t stream were analyzed b y FID gas chromatography. A 5% SP-1200/1.75% Bentone on 100/120 S u p e l c o p o r t column was used f o r analyses. As t h e vapor p re s s u r e o f t o l u e n e i s h i g h e r t h an t h a t o f ethylbenzene, t h e c o n t e n t o f t o luene i m p u r i t y was found t o be i n creased t o 2.5 mole% o f t h e f eed. RESULTS AND D I S C U S S I O N A1 1 t he c r y s t a l 1 i n e p r o d u c t s have t h e s i m i l a r XRO p a t t e r n and framework s t r u c t u r e as AlP04-5 ( r e f . 5 ) . The amount o f s i l i c o n i n c o r p o r a t i o n i n SAPO-5 i s low, as shown i n t h e c o m p o s i t i o n o f Al:P:Si=1:1:0.04-0.07
by EPMA and I C P .
The s i m i l a r S i c o n t e n t was found i n b o t h c r y s t a l and b u l k SAPO-5 samples. The absence o f a octahedra 27Al MASNMR s i g n a l t o -0 ppm and t h e presence o f o n l y one 31P MASNMR s i g n a l a t -31 ppm show t h a t t h e r e i s no amorphous m a t e r i a l p r e s e n t i n t h e s y n t h e s i z e d SAPO-5 samples. Two s i l i c a species generat ed i n SAPO-5 have been i d e n t i f i e d by *9Si MASNMR as shown i n F i g . 1. The s i g n a l s w i t h chemical s h i f t s o f -92 and -111 ppm a r e p r o b a b l y c h a r a c t e r i s t i c of S i 4 ' s u b s t i t u t i o n f o r P5+ and t h e c l u s t e r o f s i l i c a i n SAPO-5 ( r e f . 11). The d i s s o c i a t i o n o f ammonia f r o m ammonium exchanged m o l e c u l a r sieves has been e s t a b l i s h e d as t h e mechanism f o r g e n e r a t i n g Bransted a c i d s i t e s . The s t r e n g t h o f t h e s i t e s generated i s r e f l e c t e d on t h e ammonia decomposition temperature. The amount ( i n m i l l i e q u i v a l e n t ) and t h e temperature o f ammonia evolved from NH4SAPO-5 a r e i n agreement w i t h t h a t from NH3 adsorbed on SAPO-5 a t 200°C as shown i n F i g . 2a and Table 1. The a c i d values o f NH4' f o r m o f SAPO-5 a r e 0.7 and 0.5 f r a c t i o n s o f t h e t o t a l s i l i c o n c o n t e n t s i n SAPO-5(A) and SAPO(6) r e s p e c t i v e l y , i n which t h e S i c ont e nt s a r e 0.7 w t % of SAPO-5(A) and 1.2 w t % o f SAPO(6) r e s p e c t i v e l y .
22
T h i s i n d i c a t e s t h a t t h e r e l a t i v e c o n t r i b u t i o n o f S i s u b s t i t u t i n g P decreased w i t h i n c r e a s i n g S i c o n t e n t i n samples. AlP04-5 based m o l e c u l a r s i e v e s were a l s o s y n t h e s i z e d i n t h e presence o f H3BO3 o r MgC12. The p r o d u c t s c o n t a i n e d ~ 0 . 1w t % o f B b y I C P o r -1.0 w t % o f
Hg by I C P and EPMA and were termed BAPO-5 o r MgAPO-5. The a n a l y t i c a l r e s u l t s s u p p o r t t h e o c c u r r e n c e o f a r e a s o n a b l e a d d i t i o n o f B o r Mg t o A1P04-5 c o m p o s i t i o n . The TPD p r o f i l e s o f NH3 on AlP04-5, SAPO-5, BAPO-5 and MgAPO-5 a r e g i v e n i n F i g . 2. The AlP04-5 and BAPO-5 c o n t a i n v e r y few s t r o n g a c i d s i t e s , and t h e m a j o r i t y o f s i t e s i s weak. There e x i s t s a abroad s i t e d i s t r i b u t i o n on SAPO-5 which c o n t a i n s weak a c i d s i t e s as A1P04-5 (Td=200°C) and s t r o n g B r d n s t e d a c i d s i t e s (Td=300°C). The s t r e n g t h o f t h e s e a c i d s i t e s on AlP04-5 and SAPO-5 m o l e c u l a r s i e v e s i s weaker t h a n t h a t on z e o l i t e ZSM-5 (Td=400°C) ( r e f . 10). S u b s t i t u t i o n o f boron i n t o aluminophosphate l a t t i c e does n o t appear t o p r o v i d e s t r o n g a c i d s i t e s , w h i l e t h e magnesium s u b s t i t u t i o n f o r aluminum r e s u l t s i n a a n i o n i c framework and s m a l l member o f s t r o n g B r e n s t e d a c i d s i t e s (Td=4J0°C). B u t extraneous o x i d e s and h y d r o x i d e s i n g e n e r a l do n o t enhance B r d n s t e d a c i d i t y . A f t e r h e a t t r e a t m e n t a t 7OO0C, MgAPO-5 showed p a r t i a l s t r u c t u r a l d e g r a d a t i o n as d e t e c t e d b y X-ray powder d i f f r a c t i o n . The thermal s t a b i l i t y o f MgAPO-5 i s t h e r e f o r e l e s s t h a n t h a t o f A1PO4-5 and SAPO-5 whose decomposition t e m p e r a t u r e
I
- 60
-100 -140 ppm vs. TMS
F i g . 1 . 29Si MASNMR spectrum o f SAPO-5(B)
>lOOO°C.
23
10
5
0
5
0 C
2.5
I
0
,---\
Temperature
F i g . 2.
400
200
600
("C)
TPD o f NH3 f r o m m o l e c u l a r s i e v e s ( a ) SAPO-5, ammonia i n i t i a l l y sorbed
a t 100°C (-.-)
o r 2OO0C (----) and NH4SAPO-5 (-);
i n i t i a l l y sorbed a t 100°C (-) NHqMgAPO-5 (-) t i t r a t i o n data.)
(b) A1P04-5, ammonia
or 2OO0C (----); ( c ) NH4BAPO-5 (---) and
(The TPD p r o f i l e s a r e o b t a i n e d by a l e a s t - s q u a r e s f i t t i n g o f
24
TABLE 1 Composition and a c i d i t y o f m o l e c u l a r s i e v e s
S u r f a c e S i :A1 :P b y EPMA B u l k SI% ( w t ) b I C P S u r f a c e S i % (wtf b y EPMA A c i d amount (m mole/g) by TPD o f NH3 adsorbed sample ( ZOOOC) A c i d amount (T mole/g) by TPD o f NH4 exchanged sample
SAPO-5( A)
SAPO-5( B)
A1P04-5
0.04:l.l:l.O
0.07:l.O:l.O 1.2 1.2 0.22
-0:l.O:l.O 0.1
-
0.7 0.17
-
0.03
0.22
Ethylbenzene c o n v e r s i o n The c a t a l y t i c dehydrogenation o f e t h y l b e n z e n e i s o f i n d u s t r i a l i m p o r t a n c e i n t h e manufacture o f s t y r e n e ( r e f s . 1 2 - 1 8 ) .
I n o x i d a t i v e dehydrogenation,
an acid-promoted mechanism was proposed b y s t u d y i n g t h e r e a c t i o n on Na.Si02.Al203 ( r e f . 12) and SnO.P205 ( r e f . 13) c a t a l y s t s . The c a t a l y t i c a c t i v i t y o f o x i d e s on t h e n o n o x i d a t i v e d e h y d r o g e n a t i o n was c o n s i d e r e d t o be r e l a t e d t o e i t h e r t h e semiconductor p r o p e r t i e s o f m e t a l i o n s i n m e t a l o x i d e s such as C r i n Cr203.Al203 ( r e f . 14) and Fe i n FeO ( r e f . 151, o r t h e a c i d base p a i r s i t e s on t h e mixed o x i d e s as Ti02eZr02 ( r e f . 1 6 ) . R e s u l t s from a c o n t i n u o u s - f l o w f i x e d - b e d m i c r o r e a c t o r a r e g i v e n i n Table 2 . N o n - o x i d a t i v e dehydrogenation t o s t y r e n e and d e a l k y l a t i o n t o benzene were t h e main r e a c t i o n s i n t h e c o n v e r s i o n o f e t h y l b e n z e n e on A1P04-5 based m o l e c u l a r s i e v e s a t 5OO0C, w h i l e t h e amorphous aluminophosphates e x h i b i t e d v e r y low a c t i v i t y on b o t h r e a c t i o n s . W i t h styrene:benzene mole r a t i o > 5 , AlP04-5 i s more a c t i v e i n n o n - o x i d a t i v e dehydrogenation t h a n i n d e a l k y l a t i o n . Due t o t h e presence o f B r d n s t e d a c i d s i t e s , t h e y i e l d o f s t y r e n e was suppressed and t h e p r o d u c t i o n o f benzene was enhanced on SAPO-5. However, e t h y l b e n z e n e was f o u n d t o have no s i g n i f i c a n t dehydrogenation on a l u m i n o s i l i c a t e m o l e c u l a r s i e v e ZSM-5; d i s p r o p o r t i o n a t i o n o r d e a l k y l a t i o n p r o d u c t s o f d i e t h y l b e n z e n e , x y l e n e , t o l u e n e and benzene were o b t a i n e d on t h i s a c i d c a t a l y s t a t 320-500°C. A1P04-5 and SAPO-5 c a t a l y s t s were a l s o m o d i f i e d b y i m p r e g n a t i o n w i t h aqueous s o l u t i o n o f Mg(CH3C00)2~rH3B03 and MgC12 r e s p e c t i v e l y . The i n f o r m a t i o n o b t a i n e d i n MgO and B2O3 m o d i f i c a t i o n concerns t h e r o l e s o f t h e a c i d and base c e n t e r s o f c a t a l y s t s i n ethylbenzene c o n v e r s i o n . Table 2 shows t h a t t h e t o t a l c o n v e r s i o n of e t h y l b e v e n e and t h e y i e l d o f s t y r e n e decrease w i t h t h e a d d i t i o n o f HgBOg and change s l i g h t l y b y t r e a t i n g AlP04-5 w i t h Mg(CH$00)2.
This
25
i n d i c a t e s t h a t t h e b a s i c i t y of t h e c a t a l y s t may p l a y an i m p o r t a n t r o l e i n dehydrogenation r e a c t i o n . The i n c o r p o r a t i o n o f boron i n t o A1P04-5 leads t o BAPO-5 which has a c i d i t y and a c t i v i t y s i m i l a r t o A1P04-5. The s u b s t i t u t i o n magnesium f o r a aluminum i n MgAPO-5 p r o v i d e s s t r o n g Brdnsted a c i d s i t e s and gives h i g h preference f o r benzene p r o d u c t i o n on ethylbenzene conversion. When the Brdnsted protons a r e p a r t i a l l y exchanged by magnesium c a t i o n s , t h e a c t i v i t y o f d e a l k y l a t i o n i s reduced and t h e s e l e c t i v i t y o f dehydrogenation i s s l i g h t l y increased on SAPO-5 (5,Mg).
The i n c o r p o r a t i o n o f S i and Mg i n aluminophosphate
framework leads t o an increase i n t h e t o t a l number o f Brdnsted a c i d s i t e s , w h i l e Lewis a c i d and base s i t e s , which a r e most l i k e l y l o c a t e d on P and 0 atoms r e s p e c t i v e l y , a r e o n l y s l i g h t l y changed. As t h e y i e l d o f s t y r e n e was n o t increased by i n c o r p o r a t i n g S i o r Mg i n A1P04-5,
t h e Brbnsted a c i d s i t e s seem
n o t t o p a r t i c i p a t e i n dehydrogenation r e a c t i o n .
TABLE 2 A c t i v i t i e s o f molecular sieves i n ethylbenzene conversion.a Catalystb A1 Po4-5 A1P04-5 (5,Mg) A1P04-5 (2,B) A1P04-5 (5,B) SAPO-5 ( A ) SAPO-5 SAP@-5 (5,Mg) BAPO-5 MgAPO-5 ZSM-5 (5,Mg) ZSM-5 (5,Mg)'
Ethylbenzene conv. % 5.8 6.1 3.5 0.3 7.5 12.9 6.3 7.3 26.2 91.5 31.2
Benzene 1 .o 0.5 0.9 0.3 2.6 10.8 3.1 1.7 23.1 94.7 22.9
Product d i s t r i b u t i o n ( % b y mole) E thy1 S t rene Toluene benzene Styrene
-
2.0 2.1 1.9 2.5 2.4 1.9 1.6 1.9 2.3 2.2 4.8
91.7 91.4 94.0 97.2 90.5 84.6 91.2 90.2 71.7 1.6 66.3
&
5.3 6.0 3.2 0.1 4.5 2.6 4.0 6.2 2.9 (1 . 5 I e (6.1)
5.4 11 3.5 0.4 1.7 0.3 1.3 3.7 0.1
-
aReaction temperature=500°C, atmospheric pressure; WHSV=O.Ol , time on stream=lhr, feed=97.5 mole% ethylbenzene and 2.5 mole% toluene. b w i t h 5 Wt9: Mg(CH3C00)2 and 2 o r 5 w t % H3BO-j impregnated A1P04-5 samples were denoted as AlP04-5 (5,Mg) and AlP04-5 ( 2 , B ) o r AlP04-5 (5,B); w i t h 5 w t % MgC12 impregnated SAPO-5 and ZSM-5 were denoted as SAPO-5 (5,Mg) and ZSM-5 (5,Mg). The Bransted a c i d values o f SAPO-5, SAPO-5 (A) and SAPO-5 (5,Mg) a r e 0.21, 0.17 and 0.15 m mole/g, r e s p e c t i v e l y . C320OC. dXy1 ene . eDiethylbenzene. The non-oxidative dehydrogenation over CrO3 c a t a l y s t i s based on t h e chemisorption o f hydrogen on surface oxygen w i t h a cleavaqe o f t h e C-H bond and the a l k y l group bounded t o a C r atom ( r e f . 1 4 ) . I n t h e p r e s e n t work,
26
c r y s t a l l i n e microporous A1P04-5 was found t o be a c t i v e i n ethylbenzene dehydrogenation i n the absence o f oxygen. The r e s u l t s o f H3803 doping i n d i c a t e t h a t t h e weak base s i t e s o f t h e c a t a l y s t may p l a y an i m p o r t a n t r o l e i n dehydrogenation. However, t h e b a s i c i t y l o c a t e d on oxygen atoms i s n o t s t r o n g enough, and t h e c o o p e r a t i o n w i t h a d j a c e n t a c i d i c s i t e s i s e s s e n t i a l t o c a t a l y z e t h e r e a c t i o n . A concerted H2 e l i m i n a t i o n mechanism was proposed t o proceed on A1P04-5 and SAPO-5. I n which, two hydrogens o f a- and B- carbons o f ethylbenzene simultaneously a t t a c k t h e a c i d and base s i t e s o f t h e c a t a l y s t . The average bond d i s t a n c e o f P-0 (1.52 A) i s very c l o s e t o t h e C-C bond d i s t a n c e and s h o r t e r than t h e A1-0 (1.72 A) d i s t a n c e ( r e f . 5 ) , and t h i s may p r o v i d e a proper environment f o r acid-base c a t a l y z e d r e a c t i o n . However, t h e Lewis a c i d and base s i t e p a i r s a r e n o t s t r o n g as few Brdnsted a c i d s i t e s on AlP04-5. The y i e l d o f s t y r e n e was n o t increased by r a i s i n g t h e r e a c t i o n temperature above 5OO0C, a t t h i s temperature r e g i o n where Brdnsted a c i d c r a c k i n g became s i g n i f i c a n t . As SAPO-5 and MgAPO-5 e x h i b i t b o t h Brdnsted a c i d s i t e s and Lewis acid-base s i t e s , the adsorbed ethylbenzene on p r o t o n i c species may b l o c k t h e f o r m a t i o n o f dehydrogenation i n t e r m e d i a t e s and i n t u r n reduce the y i e l d o f styrene. CONCLUSION The present r e s u l t s show t h e importance o f b o t h t h e a c i d i t y and b a s i c i t y o f AlPO4 and SAP0 molecular sieves on c a t a l y t i c r e a c t i o n s . I n t r o d u c t i o n o f b i v a l e n t ( i .e. Mg) and t e t r a v a l e n t ( i .e. S i ) c a t i o n s i n t o t h e aluminophosphate framework r e s u l t s i n t h e f o r m a t i o n o f Brdnsted a c i d s i t e s and enhances the acidic catalytic a c t i v i t y .
REFERENCES 1 S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J . Am. Chem. SOC. 104(1982) 1146-1147. 2 E.M. Flanigen, B.M. Lok, L. P a t t o n and S.T. Wilson, i n Y . Murakami ( E d i t o r 1 Proc. 7 t h I n t . Z e o l i t e Congress, Toyko, 1986, E l s e v i e r , Amsterdam, 1986, pp. 103-112. 3 J.8. M o f f a t , R . V e t r i v a l and B. Viswanathan, J . Mol. C a t a l . 30(1985) 171180. 4 G. Dworezkov, G. Rumplmayer, H. Mayer and J.A. Lercher, i n M. Che ( E d i t o r ) , Adsorption and C a t a l y s i s on Oxide Surfaces, E l s e v i e r , Amsterdam. 1985, op. 163-171. 5 S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, Adv. Chem. Ser. 218(1983) 79-119. 6 B.M. Lok, C.A. Messina, R . L . Patton, R.T. Gajak, T.R. Cannan and E.M. Flanigen, J . Am. Chem. SOC. 106(1984) 6092-6093. 7 V.R. Choudhary and D.B. Akolekar, J . C a t a l . 103(1987) 115-125. 8 D . R . Pyke, P. Whitney and H. Houghton. Appl. C a t a l . 18(1985) 173-182. 9 S.G. Hedge, R. Ratnasamy, L.M. Kustov and B.B. Kazansky, Z e o l i t e s B(1988) 137-141. 10 K.J. Chao, B.H. Chiou, C.C. Cho and S . Y . Jeng, Z e o l i t e s 4(1984) 2-4.
21
11 I . P . Appleyard, R.K. H a r r i s and F.R. F i t c h , Chem. L e t t . (1985) 1747-1750. 12 T . Tagawa, T. H a t t o r i and Y . Murakami, J. C a t a l . 75(1982) 56-77. 13 Y . Murakami, K. Iwayama, H. Uckida, T. H a t t o r i and T. Tagewa, J. C a t a l . 71(1981) 257-269. 14 S . Carra and L. F o r n i , C a t a l . Rev. 5(1972) 159-185. 15 E.H. Lee, Catal. Rev. 8(19731 285-300. 16 I . Wang, W.F. Chang, R.J. Shiau, J.C. Wu and C.S. Chung, J. C a t a l . 83(1933) 428-435. 17 T. Yashima, K . Sato, T. Hayasaka and N . Hara, J . C a t a l . 26(1972) 303-310. 18 Monsanto Co., US Patent, 4,115,424(1978).
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ON THE NATURE OF THE CATALYTIC ACTIVITY OF SAPO-5
CH.MINCHEV1, V.KANAZIREV1, V . H A V R O D I N O V A 1 ,
V.PENCHEV1 a n d H. LECHERT2
t l n s t i t u t e o f O r g a n i c C h e m i s t r y , S o f i a 1040
Bulgaria)
2 1 n s t i t u t e o f P h y s i c a l C h e m i s t r y , Hamburg Un v e r s i t y , 2 Hamburg
3 (FRG)
ABSTRACT An a t t e m p t t o e s t i m a t e t h e n a t u r e o f t h e c a t a l y t i c a c t i v i t y o f SAPO-5 was made by means o f t h e " i n n e r s t a n d a r d " a p p r o a c h . SAPO-5 m a t e r i a l s were compared w i t h model m i x t u r e s o f HY i n t r o d u c e d i n t o a n AIPO-5 m a t r i x as w e l l as w i t h HZSM-5 and h i g h l y d e a l u m i n a t e d HY. The s e l e c t i v i t y p a t t e r n s i n m-xylene c o n v e r s i o n s u p p o r t t h e a s s u m p t i o n o f a r e l a t i v e l y homogeneous S i - i n c o r p o r a t i o n , The g e n e r a t e d a c t i v e c e n t e r s a r e s i t u a t e d f a r f r o m each o t h e r as i n t h e c a s e o f t h e h i g h l y d e a l u m i n a t e d f a u J a s i t e .
I NTRODUCTI ON
The new g e n e r a t i o n o f a l u m i n o p h o s p h a t e - b a s e d m i c r o p o r o u s o x i d e s ( A I P 0 4 - n , SAPO-n,
etc.)
( r e f . 1 ) r e v e a l s new p o s s i b i l i t i e s o f c a t a l y s i s by m o l e c u l a r
s i e v e s . The i n c o r p o r a t i o n o f S i i n t h e n o n - a c i d i c a l u m i n o - p h o s p h a t e framework e l i c i t s i n many cases a s i g n i f i c a n t c a r b o n i o g e n i c c a t a l y t i c a c t i v i t y . Recently, remarkable
c a t a l y t i c a c t i v i t y o f SAPO-5 i n n - b u t a n e ( r e f . 2)
and cumene ( r e f . 3) c r a c k i n g , o - x y l e n e
isomerization (ref.3),
alkylation
o f t o l u e n e w i t h methanol ( r e f . 4 ) , o l i g o m e r i z a t i o n o f p r o p e n e ( r e f . 4 ) , c o n v e r s i o n o f decane ( r e f . 5) a n d methanol ( r e f . 6) was r e p o r t e d .
The
c a t a l y t i c a c t i v i t y o f t h e aluminophosphate-type m a t e r i a l s ranges w i t h i n a l m o s t two o r d e r s o f m a g n i t u d e a c c o r d i n g t o t h e c o m p o s i t i o n and p r e p a r a t i o n c o n d i t i o n s ( r e f . 2 ) . Adsorption and I R data
suggest t h e e x i s t e n c e o f
a c i d i c c e n t e r s i n t h e SAPO-5 s t r u c t u r e ( r e f . 3, 5
-
7).
The n a t u r e o f t h e c a t a l y t i c a c t i v i t y o f t h e SAPO-5 m a t e r i a l s i s s t i l l n o t f u l l y understood ( r e f . 7 ) .
O b v i o u s l y , t h e mechanism o f S i i n c o r p o r a t i o n
i n t o t h e l a t t i c e i s o f g r e a t i m p o r t a n c e f o r t h e appearance o f c a t a l y t i c a l l y a c t i v e c e n t e r s . A t t e m p t s were made t o a p p l y some c u r r e n t c o n c e p t s i n t h e
z e o l i t e c a t a l y s i s t o t h e s e new o b j e c t s ( r e f . 7 ) . The g e n e r a t i o n o f non-comp e n s a t e d charges i n t h e a l u m i n o - p h o s p h a t e framework a n d t h e p o s s i b l e c o n t r o l o f t h i s p r o c e s s a r e t h e most i m p o r t a n t d i s c u s s i o n t o p i c s i n t h e r e c e n t l i t e r a t u r e ( r e f . 5-8). I n t h e p r e s e n t work, a n " i n n e r s t a n d a r d " a p p r o a c h has been a p p l i e d f o r
30 e s t i m a t i o n o f t h e p r o p e r t i e s o f SAPO-5 as a c i d c a t a l y s t . The HY z e o l i t e , m e c h a n i c a l l y d i s p e r s e d i n a m a t r i x o f AIP04-5, s t a n d a r d . The physico-chemical
was used as an i n n e r
and c a t a l y t i c p r o p e r t i e s o f t h e so p r e p a r e d
model samples were compared t o those o f SAPO-5, as w e l l as t o ZSH-5 and dealuminated HY w i t h low d e n s l t y o f t h e a c i d c e n t e r s .
EXPERIMENT Samples and c h a r a c t e r i z a t i o n SAPO-5 and A1P04-5 were s y n t h e s i z e d a c c o r d i n g t o ( r e f . 9,lO)
by
u s i n g Pr3N (Herck) and a g e l whose c o m p o s i t i o n i s p r e s e n t e d i n T a b l e 1.
TABLE 1 Composition o f t h e r e a c t i o n m i x t u r e , mol Sample AP SPI SPII
Type AIPO-5 SAPO-5 SAPO-5
Pr3N 1.5 2.0 2.0
Si02
A1203
p2°5
H20
1 .O 1 .o 1 .o
1 .o 1 .o 1 .o
40 50 50
0.4 0.9
Pseudoboehmite f r o m Condea, 85% o r t h o p h o s p h o r i c a c i d and amorphous s i l i c a (Herck) were used as sources f o r Al2O3, P2O5 and SiO2 r e s p e c t i v e l y . The c r y s t a l l i z a t i o n was c a r r i e d o u t under 423-453K
i n T e f l o n tubes i n an a u t o -
c l a v e f o r 6 t o 24 hours. The f i n a l thermal t r e a t m e n t was performed i n a i r a t 823-873K
f o r 6 hours i n a m u f f l e oven.
The samples used f o r comparison were:
HY(Si02/A1203-5.1)
and HZSM-5
(Si02/A1203 -200) o b t a i n e d by t h e u s u a l d e c o m p o s i t i o n o f t h e NHq+-form (ref.lO,as
w e l l as a dealuminated HY, d e s i g n a t e d as US-Ex
prepared a c c o r d i n g t o ( r e f . 1 2 ) . 0.9AP-O.1HY
(Si02/A1203=200)
The model samples 0.95AP-0.05HY
and
were p r e p a r e d by thorough m i x i n g o f t h e A1P04-5 m a t e r i a l w i t h
5 and 10 w t % o f t h e inner s t a n d a r d HY, The p h i s i c o - c h e m i c a l
characteri-
z a t i o n o f t h e samples was made by t h e f o l l o w i n g t e c h n i q u e s : X-ray powder diffraction,CuKkby
a DRON a p p a r a t u s , I R spectroscopy
in the region o f
1400-400 cm-l u s i n g KBr d i s k s , by a Specord 75 I R s p e c t r o p h o t o m e t e r , e l e c t r o n microscopy by JSH-200 and simultaneous t h e r m a l a n a l y s i s (TG-DTA- DTG) up t o 1 2 7 3 ~w~i t h 100/min i n a i r i n MOM a p p a r a t u s . A d s o r p t i o n and c a t a l y t i c measurements The adsorption-thermodesorption measurements were c a r r i e d o u t i n f l o w a t e l e v a t e d temperature.
The a d s o r p t i o n o f NH3(30 kPa,
i n a He s t r e a m a t 423K
and TPD up t o 87310 was measured a 8 a l r e a d y descr i b e d ( r e f . 13). The a d s o r p t i o n and c a t a l y t i c experiments w i t h m-xylene were p e r f o r m e d i n one and t h e same f l o w - t y p e a p p a r a t u s c o u p l e d "on l i n e n w i t h a GC. The c a t a l y s t ' s w e i g h t was 0.49.
The m-xylene was i n t r o d u c e d b y means
31 o f a s a t u r a t o r . H i g h - p u r i t y N2 was used as a c a r r i e r gas. The p r e l i m i n a r y t r e a t m e n t o f t h e c a t a l y s t s was c a r r i e d o u t f o r 5 h o u r s
i n n i t r o g e n a t 773K, f o l l o w e d b y e s t a b l i s h i n g t h e temperature a t 453K and then i n t r o d u c i n g m-xylene from t h e s a t u r a t o r . The a d s o r p t i o n was t r a c e d by t h e f r o n t a l method and chromatographical m o n i t o r i n g o f t h e m-xylene concentration a t the reactor outlet.
The c a t a l y t i c experiments f o r t h e m-xylene
c o n v e r s i o n were performed immediately a f t e r t h e a d s o r p t i o n as f o l l o w s : r a i s i n g up t h e temperature t o 673K; p a s s i n g t h e r e a g e n t t h r o u g h t h e c a t a l y s t u n t i l c o n s t a n t a c t i v i t y was reached and v a r y i n g t h e c o n t a c t t i m e i n t h e range 0 . 3
-
42h by changing t h e f l o w r a t e o f t h e c a r r i e r gas. The a n a l y -
s i s o f t h e r e a g e n t m i x t u r e and t h e r e a c t o r e f f l u e n t was accomplished by GC. The t o l u e n e d i s p r o p o r t i o n a t i o n was c a r r i e d o u t i n a c o n t i n u o u s f l o w reactor (ref.
14) a t 723 and 753K. The WHSV was l . I h - l ,
Ptoluene=27.5kPa and t h e
Hg/toluene molar ratio.2.5.
RESULTS Physico-chemical c h a r a c t e r i z a t i o n o f t h e i n i t i a l and model samples I n accordance w i t h t h e l i t e r a t u r e , t h e r e s u l t s Obtained show t h a t t h e phase p u r i t y o f AIP04-5
( r e f . 1,6) and SAPO-5 ( r e f . 5 )
‘1
is
s t r o n g l y dependent on t h e c o n d i -
t i o n s o f p r e p a r a t i o n . Combined thermal a n a l y s i s proved t o be Very u s e f u l i n p u r i t y c o n t r o l ( r e f . 15), t o g e t h e r w i t h X-ray scopy.
and I R s p e c t r o -
I n F i g . 1 t h e DTG curves o f
*‘pure** AIPO-5 and SAPO-5,
as w e l l
as those o f samples w i t h i n c r e a s i n g amounts o f phase i m p u r i t y ( c u r v e s
I 373. . 573. . .773 . .973 . .1173 I TEMPERATUK,K
b-d)
a r e presented.
les
were p r e p a r e d u s i n g t h e same
nlmpuren samp-
s t a r t i n g g e l by d i f f e r e n t d u r a t i o n s o f c r y s t a l l i z a t i o n ( c u r v e s b-c)
or
b y a d i f f e r e n t way o f homogenizing
Fig.l.
DTG o f as-synthesized:
a,AIPO-5;
b,c,d,
AlPO-5 w i t h i m p u r i t y ; ,
e,
o f t h e r e a c t i o n mixture (curve d ) .
SAPO-5. They were s u b j e c t e d t o thermal ana-
l y s i s a f t e r d r y i n g a t 333K and r e h y d r a t a t i o n . The minimum o f t h e DTG curves a t 453K a r e an i n d i c a t i o n o f t h e presence o f an u n i d e n t i f i e d s e p a r a t e phase which i s n o t s t a b l e a t 823K. I t i s t o be p o i n t e d o u t t h a t X-ray and I R spectroscopy,
which a r e t h e
32 usual c o n t r o l techniques,
are ineffective for establishing
c e n t r a t i o n s such as t h a t
p r e s e n t i n t h e sample c o r r e s p o n d i n g t o c u r v e b,
Fig.l.Taking
i m p u r i t y con-
i n t o c o n s i d e r a t i o n t h a t t h e p r e p a r a t i o n o f monophase a l u m i n i u m
o r t h o p h o s p h a t e s i s q u i t e d i f f i c u l t a n d c a n be a c h i e v e d by a v e r y p r e c i s e m a i n t e n a n c e o f t h e r e s p e c t i v e optimum c o n d i t i o n s f o r each p r o d u c t ( r e f . we recommend a d d i t i o n a l c o n t r o l by t h e r m a l a n a l y s i s ( r e f .
161,
15).
The p u r i t y o f samples c o n t a i n i n g h i g h e r amounts o f u n d e s i r e d p r o d u c t (Fig.l,
c u r v e s c,d)
may be c h e c k e d by I R s p e c t r o s c o p y ( F i g . 2 ) .
The in-
crease o f t h e i m p u r i t y y i e l d can be c l e a r l y i n d e n t i f i e d i n t h e r e g i o n 950-400 a n d 1200-1050 cm-l. The d i f f e r e n c e between t h e I R s p e c t r u m o f t h e p u r e AIP04-5 a n d SAPO-5 i n t h i s range i s n e g l i g i b l e . The complex p h y s i c o - c h e m i c a l c h a r a c t e r i z a t i o n showed t h a t p r a c t i c a l l y p h a s e - p u r e AIPO-5 a n d SAPO-5 w e r e s y n t h e s i z e d , whose d i f f r a c t i o n p a t t e r n s a r e i n v e r y good agreement w i t h t h e l i t e r a t u r e data ( r e f . 9,lO). The s c a n n i n g e l e c t r o n m i c r o g r a p h s c o n f i r m t h e good c r y s t a l l i n i t y o f t h e m a t e r i a l s used i n t h e a d s o r p t i o n and c a t a l y t i c experiments.
The c r y s -
t a s have a h e x a g o n a l c r o s s - s e c t i o n
1200 Fig.2.
800
O f
1
W A V E N U ~ R cm-1 .
The same methods w e r e a p p l i e d
I R spectrum o f as-synthesized:
a, AIPO-5;
c,d,
a b o u t 10-22y.m.
AIPO-5 w i t h i m p u r i t y ;
e, SAPO-5;
in
he c o m p a r a t i v e s t u d y o f t h e mo-
de I samples 0 . 9 5 AP-0.05HY 0 . 9 AP-O.1HY.
and
The X-ray a n d I R spec-
t r o s c o p y d a t a show t h a t c l e a r i n d i c a t i o n s f o r t h e presence o f a z e o l i t i c m a t e r i a l (HY i n t h i s case) c a n be d i s t n g u i s h e d when i t s c o n c e n t r a t i o n i n t h e AIP04-5
i s about 5wt% (Fig.3)
o r h gher (Fig.4)
I
TPD o f NH3 a n d m - x y l e n e a d s o r p t i o n A r e l a t i v e l y w i d e t e m p e r a t u r e r a n g e o f NH3 d e s o r p t i o n , F i g . 5 ,
i s character-
i s t i c f o r a l l samples. The f o l l o w i n g r e g u l a r i t i e s c a n be o b s e r v e d f r o m t h e data presented: -The c o n c e n t r a t i o n o f t h e c e n t e r s r e a c t i n g w i t h ammonia i s h i g h e s t i n HY.
The model m i x t u r e s 0 . 9 5 AP-0.05HY
and 0 . 9 AP-O.1HY
d e s o r b ammonia i n
33
Fig.3.
o f c a l c i n e d AIPO-5 AIPO-5
1OOo 600 WAVENUM)ER , crn"
X-ray d i f f r a c t i o n p a t t e r n
-
0.05HY
( a ) , and 0 . 9 5 F i g . 4 I R spectrum o f c a l c i n e d
(b); t h e arrows
0 . 9 AIPO-5
n o t e l i n e s c h a r a c t e r i s t i c o f HY.
-
AIPO-5-0.05HY
HY(0.4 g SPll
O.1HY
( a ) , 0.95
( b ) , AIPO-5
(C).
1---I
.-
IHZSM-5
SP I
a- SPII
0
F i g . 5 . TPD chromatograms o f NH3 HY(O.4g)-pure
HY;
HY(0.04g)-HY
d i l u t e d w i t h i n e r t rnater i a l .
F i g . 6 . C o n v e r s i o n o f m-xylene as a f u n c t i o n o f t h e m o d i f i e d c o n t a c t time.
34 t h e same temperature range b u t i t s quant t y i s much s m a l l e r compared t o t h e 'non-di l u t e d " HY. -SPI,
0.95 AP-0.05HY and 0 , 9 AP-O.1HY
have v e r y s i m i l a r TPD c u r v e s . The
TPD maximum of' t h e above-mentioned mater a l s i s s h i f t e d t o lower temperatur e s , w h i l e t h a t o f S P I I i s s h i f t e d t o h i g h e r ones, -The TPD chromatograms o f HZSM-5 and US-Ex used f o r comparison
are typi-
c a l f o r z e o l i t e s o f t h i s k i n d . They i n d i c a t e a h i g h e r r e l a t i v e c o n t r i b u t i o n o f t h e s t r o n g a c i d c e n t e r s . T h e number o f t h e s i t e s i n t e r a c t i n g w i t h NH3 i s
v e r y low because o f t h e h i g h S i / A l
o f t h e s e samples.
Comparison o f t h e q u a n t i t i e s o f NH3 desorbed ( T a b l e 2) f o r AIP04-5 and SAPO-5 shows,
i n agreement w i t h ( r e f . 1,3,6 and 71, t h a t t h e former have
lower a c i d i t y . The s h i f t o f t h e maximum r a t e o f ammonia d e s o r p t i o n t o h i g h e r temperatures f o r S P I I compared t o t h e o t h e r aluminophosphate samples i n d i cates a s p e c i f i c e f f e c t o f t h e increased TABLE 2
S i c o n t e n t on t h e a c i d i t y , The d a t a f o r
Thermodesorption o f ammonia,
t h e mixed samples r e v e a l t h e s i g n i f i -
adsor bed a t 420K, P~%=30kPa
c a n t r o l e o f t h e aluminophosphate m a t r i x , which o b v i o u s l y has a c o n s i d e r -
Samples
mmol NH3/g
a b l e number o f c e n t e r s i n t e r a c t i n g w i t h ammonia ( r e f .
17).
AP
0.12
HY
0.96
The m-xylene a d s o r p t i o n r e s u l t s
0.95AP-0.05HY
0.14
recorded i n t h e p r e - c a t a l y t i c region
0.9AP-0. 1OHY
0.16
showed t h a t HY adsorbed 0 . 8 6 mmol/g
SP I
0.22
m-xylene, w h i l e a d s o r p t i o n values f o r
SPI I
0.24
a l l o t h e r samples amounted t o 0 . 0 3 -
HZSM-5
0.08
0 . 1 6 mmol/g.
US-E x
0.04
The above r e s u l t s i n d i -
cate t h a t the differences i n the acide centers'
samples under e x a m i n a t i o n m a n i f e s t themselves i n a
concentration i n the l i k e manner w i t h
r e s p e c t t o t h e model m-xylene substance, which i s used f o r t h e c a t a l y t i c characterization. C a t a l y t i c studies M-xylene c o n v e r s i o n was s t u d i e d a t 673K. The f l o w r a t e o f t h e m-xylene and t h e w e i g h t o f t h e c a t a l y s t s were v a r i e d i n o r d e r t o o b t a i n c l o s e values f o r t h e o v e r a l l r a t e o f c o n v e r s i o n a t d i f f e r e n t c o n t a c t t i m e s . Under t h e e x p e r i m e n t a l c o n d i t i o n s used,
b o t h i s o m e r i z a t i o n and d i s p r o p o r t i o n a t i o n o f
t h e m-xylene proceed. Table 3 r e v e a l s t h e c o n s i d e r a b l e d i f f e r e n c e s i n t h e c a t a l y t i c a c t i v i t y o f t h e samples.Contact t i m e s d i f f e r i n g by about one o r d e r
o f magnitude a r e necessary t o o b t a i n c l o s e degrees o f c o n v e r s i o n u s i n g HY
35 TABLE 3
Conversion o f m-xylene, T ~ 6 7 3 K
Samples
Contact time, h
Total r a t e o f convers ion, %
Products o f i somer i z a t ion, X
41.5
10.0
7.7
0.30
0.95AP0.05HY
0.3 0.4 0.5
25.6 28.1 37.7
12.1 13.9 17.3
0.95 1.01 1-10
0.9APO.1OHY
0.6 0.7 1. O
34.4 37.5 48.5
19.5 20.8 23.6
0.72 0.80 0.97
0.02gHY
0.3 0.4 1 .o
23.9 28.1 44.3
11.8 13.9 19.3
0.96 1.01 1.32
0.40gHY
5.9
63.7
29.5
1.13
5.9
7.0
37.2 43.9
30.6 34.7
0.21 0.26
SP I w
5.9
42.4
37.3
0.14
US-Ex
10.4 19.8 41.5
24.6 40.2 48.0
20.2 29.4 30.2
0.20 0.36 0.59
5.9 10.4
37.7 43.2
37.4 41.9
0.01 0.03
APW
SPI Iw
HZSM-5
S=
Disprop. , % Isom. , X
wThese samples showed a tendency towards d e a c t i v a t i o n
on one s i d e , and S P I , S P I I and US-Ex on t h e o t h e r . T h i s
i s i n good agreement
w i t h t h e above-established differences i n the adsorption center density o f t h e samples. The AP sample e x h i b i t s v e r y low a c t i v i t y . T h e degree o f convers i o n o b t a i n e d even f o r 42h c o n t a c t t i m e i s o n l y 1O.OX.The a c t i v i t y o f t h e m i x t u r e s 0 . 9 5 AP-0.05HY
and 0 . 9 AP-O.1HY
i s m a i n l y due t o t h e HY component.
B y v a r y i n g t h e c o n t a c t times necessary t o o b t a i n c l o s e degrees o f conver-
s i o n , t h e f o l l o w i n g range o f a c t i v i t y i s observed: Alp04
<
US-Ex <
HZSM-5
6 SAPO-5 < <
HY
Most e s s e n t i a l i n f o r m a t i o n about t h e p r o p e r t i e s o f t h e i n v e s t i g a t e d samp l e s can be d e r i v e d f r o m t h e s e l e c t i v i t y d a t a . On t h e b a s i s o f t h e r a t i o between t h e d i s p r o p o r t i o n a t i o n and i s o m e r i t a t i o n p r o d u c t s , S, one can conclude t h a t :
- As expected, d i s p r o p o r t i o n a t i o n proceeds t o a very low degree on ZSM-5
-
HY e x h i b i t s t h o h i g h e s t s e l e c t i v i t y f o r d i s p r o p o r t i o n a t i o n . T h e s i m i l a r
performances o f 0.95 AP-O.05HY and 0 . 9 AP-O.1HY r e v e a l t h e predominant r o l e o f t h e HY component i n t h e m i x t u r e s .
-
d i s p r o p o r t i o n a t i o n proceeds t o a much lower degree on SAPO-5 and US-Ex
samples, compared t o HY. The t h u s e s t a b l i s h e d d i f f e r e n c e s i n t h e d i s p r o p o r t i o n a t i o n a c t i v i t y o f t h e samples under c o n s i d e r a t i o n were c o n f i r m e d i n t h e t o l u e n e c o n v e r s i o n . The observed degrees o f t o l u e n e d i s p r o p o r t i o n a t i o n on HZSM-5 and S P I a t 753K a r e 9.5 and 24% r e s p e c t i v e l y . For HY, even a t a 3OoC lower r e a c t i o n temperature,
t h i s q u a n t i t y was found t o be 49%.
DISCUSSION One o f t h e i m p o r t a n t q u e s t i o n s i n t h e s t u d y o f t h e new s i l i c o - a l u m i n o phosphate molecular s i e v e s today concerns t h e i r a c i d c e n t e r c o n c e n t r a t i o n s . I n p r i n c i p l e , d e v i a t i o n from the e l e c t r o n e u t r a l i t y o f t h e c r y s t a l l a t t i c e and t h e
g e n e r a t i o n o f a c t i v e a c i d c e n t e r s may be expected when a d e f i n i t e
mechanism o f S i i n c o r p o r a t i o n ( S i f o r P) t a k e s p l a c e . However, t h e preparat i o n o f s u f f i c i e n t l y "pure" SAPO as w e l l as i t s a c c u r a t e c h a r a c t e r i z a t i o n i s a d i f f i c u l t and s t i l l i n a d e q u a t e l y a s c e r t a i n e d problem. D i s t u r b a n c e o f t h e balance o f t h e T-elements
o f Si-0-Si
i n t h e s t r u c t u r e may be accompanied by f o r m a t i o n
" i s l a n d s " o r s i l i c a patches ( r e f . 5,7)
and appearance o f e x t r a -
neous A 1 or P ( r e f . 7 ) . The p o s s i b l e f o r m a t i o n o f p o c k e t s w i t h a z e o l i t e c h a r a c t e r (no phosphorus) has a l s o been c o n s i d e r e d ( r e f . 1 8 ) . investigation,
I n the present
t h e use o f HY as an inner s t a n d a r d shows t h a t t h e presence o f
a few p e r c e n t s o f " f o r e i g n "
phases i n AlPO-5 as a model f o r pockets, a c i d
fragments o r o t h e r m i c r o h e t e r o g e n e i t i e s can have a g r e a t e f f e c t as a source o f a d s o r p t i o n and c a t a l y t i c c e n t e r s , e s p e c i a l l y i f t h o s e m i c r o h e t e r o g e n e i t i e s possess a h i g h d e n s i t y o f a c t i v e s i t e s . The s t a r t i n g AlPO-5 sample s t u d i e d e x h i b i t e d a c o n s i d e r a b l e number o f c e n t e r s i n t e r a c t i n g w i t h NH3 and m-xylene b u t a low c a t a l y t i c a c t i v i t y ,
i n agreement w i t h ( r e f . c,3,6).
~ d -
m i x i n g o f o n l y 5wt% HY t o t h i s sample i n c r e a s e d t h e a d s o r p t i o n c a p a c i t y f o r ammonia by 17% and caused a c a t a l y t i c a c t i v i t y g r o w t h by s e v e r a l o r d e r s . A rough e s t i m a t i o n on t h e b a s i s o f t h e a d s o r p t i o n d a t a shows a 4-5 t i m e s lower center-density
i n t h e SAPO-5 samples used, as compared t o t h e HY z e o l i t e .
I n SAPO-5 t h e number o f t h e a d s o r p t i o n s i t e s was 5-6 and 2-3 t i m e s g r e a t e r than t h o s e f o r US-Ex and ZSM-5 r e s p e c t i v e l y . The c a t a l y t i c s t u d i e s c o n f i r m t h e d e c i s i v e importance o f t h e c e n t e r d e n s i t i e s o f t h e i n v e s t i g a t e d Samples f o r t h e i r c a t a l y t i c behaviour.
A good c o r r e -
l a t i o n between t h e c a t a l y t i c a c t i v i t y i n m-xylene c o n v e r s i o n and ammonia ads o r p t i o n was observed when t h e c o n t r i b u t i o n o f t h e aluminophosphate m a t r i x was t a k e n i n t o account ( t h e ammount o f ammonia adsorbed on AIPO-5 was subt r a c t e d f r o m t h a t f o u n d f o r t h e SAPO samples).
I n F i g . 6 t h e m-xylene con-
37 v e r s i o n I S p r e s e n t e d as a f u n c t i o n o f t h e m o d i f i e d c o n t a c t t i m e 2 ' Z g . a where g i s t h e c a t a l y s t ' s mmol/g,
w e i g h t , a,
/F,
i s t h e amount o f ammonia desorbed,
and F i s t h e m-xylene f l o w , mmol/g.h.This
approach i s s i m i l a r t o
t h a t a p p l i e d i n t h e i n v e s t i g a t i o n o f HZSM-5 z e o l i t e s w i t h d i f f e r e n t t e n t ( r e f . 1 1 ) . A s one can see f r o m t h e f i g u r e ,
Al con-
t h e values o b t a i n e d f o r a l -
most a l l samples f i t w e l l t o one and t h e same k i n e t i c c u r v e . On t h e b a s i s o f t h e s e d a t a one can s p e c u l a t e on t h e s i m i l a r p e r f o r m a n c e s o f t h e a c t i v e cent e r s i n t h e p r e s e n t e d s e r i e s o f q u i t e d i f f e r e n t z e o l i t e s o n one s i d e , a n d SAPO-5
on t h e o t h e r . A t t h e p r e s e n t s t a g e o f i n v e s t i g a t i o n we c a n n o t o b t a i n
s u f f i c i e n t p r o o f o f such a h y p o t h e s i s , T h e r e i s no doubt,however, a c t i v i t y p a t t e r n s o f SAPO-5
that the
i n m-xylene c o n v e r s i o n show a c l o s e r e s e m b l a n c e
t o t h o s e o f A I P O - 5 samples c o n t a i n i n g a f e w p e r c e n t s o f HY z e o l i t e .
I t seems
t h a t a r e l a t i v e l y broad range o f a c i d centers a r e e f f e c t i v e i n t h i s r e a c t i o n . Important a d d i t i o n a l i n f o r m a t i o n about t h e c a t a l y t i c p r o p e r t i e s o f t h e samples can be drawn f r o m t h e s e l e c t i v i t y d a t a . W h i l e c l o s e r a t e s o f b o t h d i s p r o p o r t i o n a t i o n and i s o m e r i z a t i o n o f m-xylene a r e t y p i c a l f o r HY and i t s m i x t u r e s w i t h AIPO-5,in t h e c a s e o f SAPO-5,
US-Ex and ZSM-5 samples
t h e i s o m e r i z a t i o n p r e v a i l s . The low e x t e n t o f d i s p r o p o r t i o n a t i o n on t h e shape s e l e c t i v e HZSM-5 r e f l e c t s t h e i n f l u e n c e o f t h e s t r u c t u r a l f a c t o r s when a b i m o l e c u l a r r e a c t i o n p r o c e e d s . Compared t o ZSH-5, e x h i b i t a much more open c r y s t a l s t r u c t u r e , r i n g s . Therefore, SAPO-5
US-Ex a n d SAPO-5
c o n t a i n i n g 12-membered oxygen
t h e r e l a t i v e l y low s e l e c t i v i t y f o r d i s p r o p o r t i o n a t i o n o f
and US-Ex c a t a l y s t s i s d e t e r m i n e d by f a c t o r s o t h e r t h a n t h e t r a n s i -
t i o n - s t a t e s e l e c t i v i t y . T h e geometry o f t h e a c t i v e c e n t e r d i s t r i b u t i o n (number o f c l o s e s t n e i g h b o u r i n g a c i d s i t e s a n d t h e d i s t a n c e s between them) as w e l l as t h e l o c a l e n v i r o n m e n t a l e f f e c t s , may p l a y a p r e v a i l i n g r o l e compared t o t h e o v e r a l l chemical composition e f f e c t ( r e f . 1 9 ) .
The o b s e r v e d S u b t l e
i n f l u e n c e o f t h e a l u m i n o p h o s p h a t e m a t r i x i t s e l f on t h e c a t a l y t i c p r o p e r t i e s o f t h e model m i x t u r e s i s h a r d t o e x p l a i n on t h e b a s i s o f t h e r e s u l t s p r e s e n t l y o b t a i n e d . More i m p o r t a n t l e c t iv i t
f r o m o u r p o i n t o f v i e w i s t h a t t h e se-
P a t t e r n s o f t h e chosen model r e a c t i o n a r e v e r y s e n s i t i v e i n d i s -
t i n g u i s h ng some i n t r i n s i c f e a t u r e s o f t h e SAPO-5 on t h e one hand and s u c h common 1 y used z e o l i t e s as HY and ZSM-5 on t h e o t h e r . F i n a l y , we assume t h a t w i t h t h e p r o c e d u r e u s e d f o r o u r SAPO-5 p r e p a r a t i o n , no c o n s i d e r a b l e f o r m a t i o n o f f o r e i g n phases o r a c i d f r a g m e n t s w i t h i s o l a t e d h e a p i n g o f a c t i v e s i t e s i s t a k i n g p l a c e . The a c i d s t r e n g t h d i s t r i -
b u t i o n i n these m a t e r i a l s i s c l o s e t o t h a t in t h e H Y - f a u j a s i t e b u t t h e d e n s i t y i s many t i m e s l o w e r . T h e s i m i l a r i t y between c a t a l y t i c p r o p e r t i e s a n d e s p e c i a l l y t h e s e l e c t i v i t y o f SAPO-5 a n d US-Ex l e a d s us t o t h e c o n c l u s i o n t h a t t h e S i i n c o r p o r a t i o n i s r e l a t i v e l y homogeneous. The a c i d c e n t e r s gener a t e d a r e s e t a t a distance, s i m i l a r l y t o t h e case o f t h e h i g h l y dealumi-
38
nated f a u j a s i t e , which i s p r o b a b l y t h e reason f o r t h e low d i s p r o p o r t i o n a t i o n c a p a b i l i t y o f SAPO-5.
ACKNOWLEDGEMENTS The a u t h o r s a c k n o w l e d g e t h e B u l g a r i a n Academy o f S c i e n c e s , M i n i s t r y o f c u l t u r e , science and education and t h e Deutsche Forschungsgemeinschaft f o r s u p p o r t of t h i s w o r k . They t h a n k t o D r . U . L o h s e f o r p r o v i d i n g t h e US-Ex sample a n d
Mrs.N.Micheva a n d Mr.V.Minkov
f o r helpful assistance.
REFERENCES 1 E . M . F l a n i g e n , B.M. Lok, R . L . P a t t o n a n d S.T. W i l s o n , i n Y. Murakami A . I i j i m a a n d J. Ward ( E d s . ) , New Developments i n Z e o l i t e S c i e n c e Techn o l o g y , E l s e v i e r , NY, 1986, pp.103-112 2 B.M. Lok, C.M. Messina, R . L . P a t t o n , R.T. G a j e k , T.R. Cannon a n d E . M . Flanigen,J.Am.Chem.Soc., 106, (19841, 6092 3 Xu Qinhua, Y . A i z h e n , B. S h u l i n a n d Xu K a i j u n , i n Y. Murakami, A . I i j i m a
and J.Ward (Eds.),New Developments i n Z e o l i t e S c i e n c e T e c h n o l o g y , E l s e v i e r NY, 1986, pp.835-842 4 R.J. P e l l e t , G.N. Long a n d J.A. Rabo, i b i d , pp.843-849 5 J . A . M a r t e n s , M. M e r t e n s , P . J . G r o b e t , P.A. J a c o b s , i n P. G r o b e t , W, Mort i e r and G. S c h u l z - E k l o f f ( E d s . ) , l n n o v a t i o n s i n Z e o l i t e M a t e r i a l s S c i e n c e , E l s e v i e r , Amsterdam, 1988, pp.97-106 6 N . J . Tapp, N.B. M i l e s t o n e , D.M. B i b b y , i b i d , pp.393-402 7 E . M . F l a n i g e n , R . L . P a t t o n , S . T . W i l s o n , i b i d , pp.13-28 8 R.Khouzami, G. C o u d u r i e r , B.F.Mentzen, J . C . V e d r i n e , i b i d , pp.355-364 9 EP 103117 10 EP 43562 l t V. K a n a z i r e v , T - T s o n c h e v a , C h r . Minchev, Z. Phys.Neue F o l g e , 149, ( 1 9 8 6 ) , 237 447, ( 1 9 7 8 ) , 64 12 U . Lohse, E . A l s d o r f , H.Stach, Z . a n o r g . a l l g . C h e m . , 13 V. K a n a z i r e v , N. B o r i s o v a , Z e o l i t e s , 2, ( 1 9 8 2 ) , 23 14 V . Mavrodinova, Ch. Minchev, V. Penchev, H. L e c h e r t , Z e o l i t e s , 5 , ( 1 9 8 5 ) , 217 15 Ch. Minchev, V . Minkov, V. Penchev, i n p r e p a r a t i o n 16 A . K n i e p , Angew.Chem., 9 8 , (19861, 520 (19871, 115 17 V.R. Choudhary, D . B . A k o l e k a r , J . C a t a l . , l O J , 18 L.S. de S a l d a r r i a g a , C. S a l d a r r i a g a , M . E . D a v i s , J.Am.Chem.Soc., 109, ( 1 9 8 7 ) , 2686 19 D. B a r t h o m e u f , i n F . R i b i e r o , A. R o d r i g u e s , L. R o l l m a n , C. Naccache(Eds.) Z e o l i t e s : S c i e n c e and T e c h n o l o g y , M a r t i n u s N i j h o f f P u b l . The Hague, p p . 31 7-346
H.G. Karge,J. Weitkamp (Editors),Zeolites 0s Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MOLECULAR ORBITAL CALCULATIONS ON THE STRUCTURAL AND ACIDIC CHARACTERISTICS OF ALUMINOPHOSPHATES (AlPO) , SILICOALUMINOPHOSPHATES (SAPO) AND METALUMINOPHOSPHATE (MeAPO) BASED MOLECULAR SIEVES
R.CARSON, E.M.COOKE, J.DWYER*, A. HINCHLIFFE and P.J.O'MALLEY*, Department of Chemistry UMIST Manchester M60 1QD England.
ABSTRACT STO-3G molecular orbital calculations on aluminophosphate units show that the optimised AlOP angle is smaller than that predicted for silicate or aluminosilicate units. The variation in the energy of the AlOP linkage over a range of angles from 100" to 180" suggests that smaller TOT angles are energetically favoured for AlPO structures. Acidity calculations using a 3-21G basis set predict that the ffronsted acid site strength of MgAPO is large compared with bridged zeolite forms. CND0/2 studies of single four ring (S4R) and double four ring (D4R) units suggests a more hydrophobic nature for aluminophosphate based systems compared with silica based systems. An average acid site strength for SAPOs, less than HY and HZSM5. is predicted. INTRODUCTION A large family of novel aluminophosphate molecular sieves has recently been described in the literature 111. These microporous solids not only exhibit properties characteristic of zeolites but also show unusual physicochemical properties ascribable to their unique chemical composition. These new classes of materials were sought for in order to explore new framework compositions of oxides outside of the already known aluminosilicate zeolites and silica molecular sieves. Based on the crystal chemistry of framework oxides and crystal chemical principles, the first elements explored as tetrahedral framework cations were aluminium and phosphorous giving rise to the A1P04-n(A1PO) designation where n denotes a specific structure type. Later substitution of Si to give SAPO, other elements to give MeAPO, silica and other elements (MeAPSO) has given rise to a whole range of new materials consisting of tetrahedral oxide frameworks. More than two dozen structure types have
40 HO ,OH
la1
n
HO ,OH
7;
H
7;
I
HO
c
i I
H H Fig 2 S - 4 R unit used in CNDOl2 calculations
Fig 1 Units used for ab-initio calculations
D-4R unit H, Sila..,Ab, 0-2ITERln H, are charge compensating protons )not shown ITERI are terminating atoms
0
0
= oxygen I T ' atom Si I Al Fig 3 0-4R unit used in CNDO/Z calculations - U , 0
6
E
-lrnt
1Ti -
loo 120 140 160 180 bond angleldegrees fi10X ,fX=Sia All
Fig. 4 . Potential energy curves k r l a l S i O S i . ~ l e (OSiOHAI and IdIAIOP units ISTO 3G MSISl
I11 S
I
In1 P
Al
\
,OHIO3561 51-
Al
4 f03MZIHd
SI
)MfO31021
\A-l
SI
frnl
Irnl
-AI
fP1
Fq 5 CMnpoutKn of S-1R unlts Charges m Lmdglng ?y&gPnS
alven in parenthesis
41
been reported and this includes framework topologies analogous to zeolites, e.g. chabazite (-34. -44, -47). erionite (-17). gismodine (-431, levynite (-35). linde Type A (-42), faujasite (-37) and sodalite (-20) plus a large number of novel structures such as -5, -11, -31 [ 2 ] etc. Pore sizes range from 0.3 nm to 0.8 nm encompassing small, intermediate and large pore structures. Surface selectivity varies from weakly to mildly hydrophilic and depends upon the nature of the incorporated element and the structure type. Incorporation of many of the framework elements generates negatively charged frameworks and acid sites. The use of molecular orbital calculations, both at the ab-initio and semi-empirical level has proven fruitful in obtaining a more thorough understanding of those bonding forces governing structural stability and acidity in zeolites [3,4,51. It is of interest therefore to examine the materials described above via such methods. In this report the stability 2+ and acidic characteristics of AlPO, SAP0 and MeAPO (Me = Mg ) forms are investigated via non-empirical and semi-empirical methods. The models and methods used are similar to thosedescribed previously [5,8]. The units used for the calculations are illustrated in Figs 1,2, and 3. RESULTS AND DISCUSSION Ab-initio Studies Stability of AlOP as a function of /AlOP and Acidity Determination for
SCF-MO calculations employing the STO-3G basis set were used along with the unit of Fig.la for the lAlOP study. First the bridging bond lengths (A10 and PO) and the /AlOP were optimised. The optimised values obtained are given in Table 1 together with the optimised values obtained previously 1121 for ZSiOSiZ
, SSiOAlE and ~SiOHAElinkages.In Fig.4(d)
the variation in the energy of the unit of Fig.la is illustrated as a function of the/AlOP. In this study the PO(br) and AlO(br) were held constant at their optimum values of Table 1. From Table 1 it can be seen that the optimised TOT angle for the aluminophosphate unit is lower than the corresponding silicate and aluminosilicate units. Fig.4 also shows that the flexibility of the TOT angle over a range of values from 140" - 180" is -1 also less for the AlPO unit compared with SSiOSiS and [ZSiOAlE 1 The
.
42
TABLE 1 Optimal bond lengths and Angles calculated using the STO-3G Basis Set
(OH)3SiOSi(OH);
143.7
1.59
1.59
[ (OH)3SiOAl(OH)3]a-'
139 . O
1.57
1.69
(OH)3SiOHA1(OH);
132.0
1.67
1.80
(OH)3 ~ i OH) ~ ~ (
125.0
1.77
1.58
a
b
Values taken from references (12,131 See also ref. [22].
TABLE 2 Average TOT angle for aluminophosphate based molecular sieves and dense phases. Reference System
Angle/"
AlPO-5
150"
AlPO-21
141.9
~191
Low-Quartz A1P04 Low-Cristoballite A1P04
142.5 145.0
[201
Low-Tridymite A1P04
150.0
[201
[201
43
flexibility is however greater than that observed for the ZSiOHAlZ unit. The low optimum TOT angle value of 125" for the aluminophosphate unit indicates that this angle should give rise to the most stable AlPO structures. A selection of A10P angles found for the new molecular sieves and for some dense A1P04 phases is outlined in Table 2. In general the experimental values are larger than the theoretically predicted optimum value.
Inspection of Pig.4 shows however that a certain flexibility in the
AlOP angle is to be expected between 120"
-
180" so that the experimental
angles are within an acceptable range. TOT angle values as low as 121.8' have been reported for AlPO-11 confirming that such small angle values are stable for these materials. A charge density analysis of the A1P04 quartz 2 analogue indicated that the hybridisation of the bridging 0 atom was sp indicating an AlOP angle of 120" [141. It was suggested that the experimentally measured internuclear angle of 182.3" was due principally to the presence of bent bonds. It would be of interest to see if such a situation also exists for the other frameworks of Table 2.
The low optimum
angle for AlOP linkages suggests that small ring systems such as 3 rings and 4 rings should be stabilised for aluminophosphate based systems 1151. Strict alternation of A1 and P prohibits odd-numbered ring systems however. The 3-21G basis set has been used along with the unit of Fig.lb to calculate the acidic characteristics of MgAPO molecular sieves. The calculated partial charge on the bridging proton (qH = 0.5093) and the (COH = 3901 cm -1) can be compared
calculated hydroxyl stretching frequency
with the corresponding value of qH = 0.4727 and
iOH = 3931 cm-l obtained
for the H3SiOHA1H unit [8]. The greater proton partial charge plus the 3 decreased vibrational frequency for the MgAPO unit suggests that strong acidity should be exhibited by MgAPO molecular sieves. The high rate of cracking of n-butane observed over MgAPO is in agreement with this prediction [ 2 ] . For MgAPSO a hydroxyl stretching band at 3595 cm-l is observed in addition to a band at 3621 cm-l 111. The 3621 cm-l band is observed for SAP0 as well and hence can be assigned to 3SiOHA13 type hydroxyls. The 3595 cm-l band is therefore likely to rise due to ZMgOHPi sites. The experimental difference in frequency observed of 26 cm-l is in excellent agreement with the calculated difference of 30 cm-l above.
44
CNDO/2 studies of S4R's and D4R's containing Si. A1 and P. As described in the previous sections ab-initio molecular orbital calculations are successful in predicting local forces governing the stability and electronic properties of molecular sieve frameworks. It is of course to be expected that long range forces can also play a large part in determining molecular sieve properties. For this reason, therefore, it would be useful to examine larger units of molecular sieves via molecular orbital methods. Ab-initio calculations are of limited use for large units because the cost in terms of computer time, even for highly advanced supercomputers, becomes unreasonable. It is necessary to resort to the less accurate semi-empirical methods. Of these semi-empirical calculations CND0/2 has been used for some time to study larger zeolite secondary building units such as single four rings (S4R) and single six rings (S6R) [4,91. In this section CNDO/2 studies are performed on S4R and D4R units containing the T atoms, Si, A1 and P. The geometry of the units is shown in Figs 2 and 3 . In Tables 3 and 4 the electronic charges on the various T atoms are given for the silicate and aluminophosphate S4R and D4R units respectively. In general the polarity of the SiO and A 1 0 are comparable for both the S4R and D 4 R systems. The PO bonds are however predicted to be less polar suggesting that hydrophobic adsorption behaviour should be favoured for A l P O systems. A similar conclusion has been arrived at in reference (22) via use of ab initio SCF calculations at the STO-3G basis set level. The larger units of Figs 2 and 3 also permit us to investigate the correlation between overall chemical composition and the average acid site strengths. Recent experimental studies have compared the acidic characteristics of SAPO materials with zeolites such as HY and HZSM5 [1,161. In general the concentration and strength of the Bronsted acid sites has been found to be lower for the SAPO forms. As outlined previously hypothetical replacement of P by Si gives rise to a bridged
.
hydroxyl group of the form SSiOHA13 Substitution of two silicon atoms for an A1 and a P atom should give rise to a neutral framework charge. Both types of substitutions have been shown to occur 111. The low concentration of acid sites can be ascribed to the low Si content of SAPOs so
far investigated. The acid site strength and its composition dependence
can be studied using the S4R compositions of Fig.5 Taking the charge on the bridging proton as a measure of Bronsted acid strength it can be seen from Fig.5 that the acid site strength of the SAPO S4R unit is predicted to
45
TABLE 3 : Electronic charges calculated for atomic positions in silicate and aluminophosphate S4Rs. Numbering scheme is as given in Fig 2 . (CNDO/2).
Si
I
Si-
- SiI Si
A1
I P
-P - A1i
T1
0.4853
0.5851
T2
0.4853
0.0573
T3
0.4853
0.5851
T4
0.4853
0.0573
-0.2186
-0.1411
-0.2186
-0.1411
O7
-0.2186
-0.1411
a'
-0.2186
-0.1411
H9
-0.1333
-0.0044
H1 0
-0.1333
-0.0044
H1 1
-0.1333
-0.1757
O5 6'
H1 2
*13 H14 H15
H1 6
-0.1333
-0.1757
-0.1333
-0.0044
-0.1333
-0.0044
-0.1333
-0.1757
-0.1333
-0.1757
46
TABLE 4 Electronic charges calculated for atomic positions in silicate and aluminophosphate D4Rs. Numbering scheme is as given in Fig.3.
Si
- Si
P
- A1
1
A<+€’’
I ,A4
;P
P -A1
Atom T1
.4626
0.2518
Atom T2
.4626
0.4114
Atom T3
.4626
0.2518
Atom T4
.4626
0.4114
Atom T5
.4626
0.4114
Atom T6
.4626
0.2518
Atom T7
.4626
0.4114
Atom T8
.4626
0.2518
Atom 01
-.2892
-0.2503
Atom 02 -.2526
-0.2065
Atom 03
-.2892
-0.2503
Atom 04 -.2526
-0.2065
Atom 05
-.2892
-0.2503
Atom 06 -.2526
-0.2065
Atom 07 -.2892
-0.2503
Atom oa
-0.2065
-.2526
Atom 09 -.2560
-0.2085
Atom 010 -.2560
-0.2085
Atom 011 -.2560
-0.2085
Atom 012 -.2560
-0.2085
Atom H1
-.0637
-0.1975
Atom H2
-.0637
0.1996
Atom H3
-.0637
-0.1975
Atom H4
-.0637
0.1996
Atom H5
-.0637
-0.1996
Atom H6
-.0637
0.1975
Atom H7
-.0637
-0.1996
Atom H8
-.0637
-0.1975
be less than that of the aluminosilicate unit with Si/Al = 3 but greater than that calculated for the unit with Si/A1 = 1. Thus the CNDO/Z calculations suggest that, the average acid site strength should be less than HY and HZSM5. The experimental determination of acid site strengths in SAPO-5 and SAPO-11 are in agreement with this finding
[MI. CONCLUSIONS Molecular orbital calculations both at the non-empirical and semi-empirical level have been shown to provide some valuable insights into the nature of the forces governing structural stability and acidity within AlPO, SAP0 and MeAPO frameworks, Combined with previous studies on aluminosilicate systems they show the value of such calculations in attempts to rationalise molecular sieve properties. ACKNOWLEDGEMENTS P J O'M and J D would like to thank BP International plc, Research Centre, Sunbury-on-Thamesfor an EMRA. PJO'M also wishes to acknowledge receipt of a Sir Eric Rideal Bursary from the Society of Chemical Industry.
REFERENCES 1 E.M. Flanigen, R.Lyle Patton and S.T. Wilson, in P.J.Grobet et al. (Editors), Innovations in Zeolite Materials Science, Elsevier. Amsterdam, 1988, p.13. 2 E.M. Flanigen in Y. Murakami et al. (Editors), New Developments in Zeolite Science and Technology, Elsevier, Amsterdam 1988, p.103. 3 G.V. Gibbs, E.P. Meagher, M.D. Newton and D.K. Swanson. in M O'Keefe, et al. (Editors),Structure amd Bonding in Crystals, Academic Press New York, 1981, p.195. 4 G.M. Zhidomirov and V.B. Kazansky, Advances in Catalysis, 34, 1986, 131. 5 P.J. O'Malley and J. Dwyer. Chem.Phys.Letts., 143, 1988, 97. W.J.Hehre, L. Radom, P.V.R. Schleyer and J.A. Pople, Ab-Initio 6 Molecular Orbital Theory, John Wiley & Sons, New York, 1986. 7 W.J. Mortier. J.Sauer, J.A. Lercher and H. Noller, J.Phys.Chem., 88, 1984, 905. 8 P.J. O'Malley and J. Dwyer, J.C.S. Chem.Commun., 1987, 72. 9 S . Beran and J. Dubsky, J.Phys.Chem., 83, 1979, 2538. 10 J.A. Pople and D.L. Beveridge, Approximate Molecular Orbital Theory, McGraw-Hill, New York. 1970. 11 M.F. Guest and J. Kendrick, An Introductory Guide to GAMESS, University of Manchester Regional Computer Centre, Manchester. 12 G.L. Geisinger, G.V. Gibbs and A. Navrotsky, Phys.Chem.Minera1, 11, 1985, 266.
.
48
13 14 15 16 17 18 19 20 21 22
P.J. O'Malley and J. Dwyer, Zeolites, in press. N. Though, D. Schwarzenbach. Acta Cryst., A35 (1979) 658. W.M. Meier, in Y.Murakinani et al. (Editors), New Developments in Zeolite Science and Technology, Elsevier, Amsterdam, 1986, p.3. N.J.Tapp, N.B. Mileston and D. M. Bibby in P.J. Grobet et al. (Editors), Innovations in Zeolite Materials Science, Elsevier, Amsterdam, 1988, p.393 D. Freude, M. Hunger and H. Pfeifer, Chem.Phys.Letts., 128, 1986, 62. J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth and J.V. Smith, Amer.Chem.Soc.Symp.Ser.. 218, 1983, 109. J.M. Bennett, J.P. Cohen. G. Artioli, J.J. Pluth and J.V. Smith, Inorg. Chem., 24, 1985, 188. D. Muller, E. Jahn and G. Ladwig, Chem.Phys.Letts.,volume 1984, 109. P.J. O'Malley, J. Dwyer and A. Hinchliffe, to be submitted to J.Molec. Struct. J. Sauer, H. Haberlandt and W. Schirmer in P.A. Jacobs et a1 (Editors) structure and Bonding in modified Zeolotes, Elsevier, Amsterdam, 1984 p. 313.
H.G. Karge, J. Weitkamp (Editors),Zeolites 0s Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
RELATION BETWEEN PARAFFIN ISOMERISATION CAPABILITY AND PORE ARCHITECTURE OF LARGE- PORE BIFUNCTIONAL ZEOLITES
J.A. MARTENS, M. TIELEN and P.A.
JACOBS
Laboratorium voor Oppervlaktechemie, 8-3030 Leuven, Belgium
Leuven,
K.U.
Kardinaal Mercierlaan 92,
ABSTRACT I n t h i s study long-chain n - p a r a f f i n s i n t h e range from nonane t o heptadecane were converted over b i f u n c t i o n a l z e o l i t e Y, USY, ZSM-3, ZSM-20 and BETA c a t a l y s t s i n t h e presence of hydrogen. n - P a r a f f i n s can be s e l e c t i v e l y converted i n t o branched isomers, provided t h e hydrogenation-dehydrogenation f u n c t i o n i s i n balance w i t h the a c i d f u n c t i o n o f t h e c a t a l y s t . The e q u i l i b r a t i o n o f the two c a t a l y t i c functions depends on t h e c a t a l y s t as w e l l as on the carbon number o f the feed. For each z e o l i t e t h e r e e x i s t s an optimum loading w i t h noble metal which depends on the chain l e n g t h o f the n - p a r a f f i n feed. I f the geometry and dimensions o f t h e z e o l i t e pores are such t h a t hydrocracking i s suppressed by molecular s h a p e - s e l e c t i v i t y , t h e y i e l d o f isomerisation i s improved s u b s t a n t i a l l y . The presence o f mesopores i n z e o l i t e Y c r y s t a l s i s r e f l e c t e d i n the isomerisation y i e l d . INTRODUCTION
Branching o f n - p a r a f f i n s , contained i n petroleum f r a c t i o n s ,
i s desirable
from several p o i n t s o f view and needs no f u r t h e r elaboration.
Noble-metal-
containing f a u j a s i t e s are e x c e l l e n t c a t a l y s t s f o r such isomerisation r e a c t i o n s
[l]. A
p r e r e q u i s i t e f o r o b t a i n i n g high isomerisation y i e l d s
i s that
the
hydrogenation-dehydrogenation a c t i v i t y and the Br0nsted a c i d i t y o f the c a t a l y s t are i n balance [2,3].
Such a c a t a l y s t , according t o Weitkamp,
exhibits 'ideal
b i f u n c t i o n a l behaviour' [ 4 ] . The c l a s s i c a l b i f u n c t i o n a l mechanism, advanced by Weisz [ 5 ] and by Coonradt and Garwood [6] provides a useful d e s c r i p t i o n o f the events during the conversion o f n - p a r a f f i n s over i d e a l b i f u n c t i o n a l c a t a l y s t s
[ 7 ] . According t o t h e c l a s s i c a l r e a c t i o n scheme, the metal phase establishes an e q u i l i b r i u m between p a r a f f i n s and o l e f i n s .
The o l e f i n s d i f f u s e towards the
Brensted acid s i t e s and are protonated t o g i v e alkylcarbenium ions. The r a t e determining step i s the rearrangement and/or s c i s s i o n o f a1 kylcarbenium ions. Although balance,
i n such a c a t a l y s t i t i s considered t h a t both functions are i n it
should
be
realised
that
no
upper
bound
exists
for
the
hydrogenati on-dehydrogenat i o n a c t i v i t y [8]. I n t h i s work i t w i l l be shown t h a t the balance between t h e two f u n c t i o n s i s catalyst-
as
well
as
feedstock-depending.
Moreover,
in
w e l l -balanced
50
bifunctional
zeolites
t h e shape and dimensions
of
the zeolite
pores
are
a d d i t i o n a l parameters which i n f l u e n c e t h e y i e l d o f isomerisation. This w i l l be demonstrated w i t h n - p a r a f f i n s ranging from nonane (n-C,) using z e o l i t e s Y, USY, ZSM-PO,
a l u m i n o s i l i c a t e s t r u c t u r e s a c t u a l l y known. zeolite with
cages,
t o heptadecane (n-Cl7)
ZSM-3 and BETA, which are among t h e most open
comparable t o
ZSM-3
faujasite
s t r u c t u r e o f ZSM-3 i s r e l a t e d t o f a u j a s i t e ,
i s a twelve-membered
supercages
[9].
The
ring
crystal
b u t t h e topology remains unknown
[lo].
ZSM-20 i s a twelve-membered r i n g z e o l i t e w i t h unknown s t r u c t u r e and which Z e o l i t e BETA i s an i n t e r g r o w t h o f seems a l s o t o be r e l a t e d t o f a u j a s i t e [ll]. two z e o l i t e frameworks [12].
The c r y s t a l s are permeated by a tridimensional
A t the channel i n t e r s e c t i o n s there i s l e s s f r e e space than i n a f a u j a s i t e supercage [13,141. Z e o l i t e Y and USY d i f f e r by t h e presence o f s o - c a l l e d mesopores and appreciable q u a n t i t i e s o f extra-framework A1 i n t h e l a t t e r m a t e r i a l [15-171. network
of
twelve-membered
ring
channels
[7,12-141.
EXPERIMENT
ZSM-3 was the ZSM-3(2) m a t e r i a l o f r e f . 9 . The s i z e o f t h e c r y s t a l s was around 1 pm and the S i / A l r a t i o was 1.9. ZSM-20 was synthesized according t o the standard method o f Ernst e t a l . [18]. The S i / A l r a t i o was 5.0 and the c r y s t a l s i z e around 0.5 pm. ZSM-20 was c a l c i n e d a t 673 K i n f l o w i n g oxygen. BETA was synthesized according t o a p r e v i o u s l y reported r e c i p e (B1 i n ref.19) using a K - f r e e g e l . The S i / A l r a t i o o f BETA was 8.0 and t h e c r y s t a l s i z e around 0.2 pm. 1 was a Na-Y z e o l i t e w i t h a S i / A l r a t i o o f 2.45, purchased from Ventron. Usy z e o l i t e was prepared by teaming NH4-Y [20]. The S i / A l r a t i o o f the framework o f USY determined with 2 6 S i M$S NMR was 4.4. The ze l i t e s were converted t o the NH4 -form by c o n t a c t i n g 5 g o f z e o l i t e w i t h 0.5 dm o f a 0.5 M NH4C1 s o l u t i o n under r e f l u x conditions. Subsequently, platinum tetrammine complex was exchanged a t r om temperature i n the z e o l i t e s by c o n t a c t i n g 2 g o f z e o l i t e w i t h 0.1 dm' o f an aqueous s o l u t i o n o f Pt(NH3) C12. The uptake o f platinum complex was assumed t o be q u a n t i t a t i v e . The z e o l i t e powder was shaped i n t o p e l l e t s w i t h a diameter o f 0.3-0.5 mm by compression, crushing and sieving. 0.2 g o f t h e p e l l e t s were charged i n a r e a c t o r tube w i t h an i n t e r n a l d i a t e r o f 1 cm. The c a t a l y s t bed was heated w i t h a t e m era ure r i s e o f 0.1 K s' from 300 t o 673 K under a stream o f oxygen o f 0.33 c a s-' and a pressure o f 0.15 MPa. The temperature was kept a t 673 K f o r one hour. Subsequently, t h e bed was flushed w i t h nitrogen, and hydrogen was admitted f o r one hour, using the same f l o w as f o r oxygen. The a c t i v a t i o n o f Pt/ZSM-3 was performed a t 573 K, i n order t o avoid a l o s s o f c r y s t a l l i n i t y [ 9 ] . The c a t a l y t i c experiments consisted o f converting s i n g l e n - p a r a f f i n feedstocks i n a fixed-bed, continuous-flow microreactor, described e a r l i e r [8]. The Hp/hydrocarbon molar r a t i o i n the feed was 100. The experiments were performed a t v a r i a b l e W/Fo values, i n which W stands f o r t h e amount o f c a t a l y s t and Fo f o r t h e molar f l o w r a t e o f n - p a r a f f i n a t t h e r e a c t o r i n l e t . D e t a i l e d analysis o f t h e r e a c t i o n products was c a r r i e d o u t w i t h GLC using c a p i l l a r y columns coated w i t h Cp-Sil-5. The b i f u n c t i o n a l z e o l i t e c a t a l y s t s w i l l be denoted by t h e z e o l i t e type, preceded by t h e amount o f noble metal ( i n w t - % ) contained i n i t .
5
RESULTS Catal v t i c a c t i v i t y In
Fig.1
0.75Pt/ZSM-3
the
degree
of
conversion
i s p l o t t e d against W/Fo.
of
decane
As the slopes
and
heptadecane
i n Fig.1
over
are i n i t i a l
51
r e a c t i o n rates, an e i g h t f o l d increase o f the i n i t i a l r e a c t i o n r a t e i s observed when decane is rep1 aced w i t h heptadecane. The degree o f conversion o f heptadecane over d i f f e r e n t c a t a l y s t s i s p l o t t e d versus the r e a c t i o n temperature i n Fig.2. Catalysts 0.3Pt/Y and 0.5Pt/Y show the same a c t i v i t y .
The a c t i v i t y o f 0.75Pt/ZSM-3
z e o l i t e Y c a t a l y s t s . lPt/USY,
i s lower than t h a t o f the
lPt/ZSM-20 and lPt/BETA are s u b s t a n t i a l l y more
active.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
WIF, (kg
8
1.8 2.0
mmol-')
lo 320
360
4 0 0 440
480
520
560
temperature( K )
Fig.1. Degree o f conversion o f decane and heptadecane against W/Fo over 0.75Pt/ZSM3 a t 508 K and 0.35 MPa.
Fig.2. Degree o f conversion o f heptadecane over lPt/USY, lPt/BETA, lPt/ZSM-20, 0.75Pt/ZSM-3, 0.3 and 0.5Pt/Y against r e a c t i o n tempera t u r e a t 0.7 MPa w i t h W/Fo=l.l kg s
mmol -1. Hvdrocrac k ino Patterns
The cracked product d i s t r i b u t i o n s o f n-Cg t o n-C17 over 0.75Pt/ZSM-3 i n Fig.3
are s l i g h t l y asymmetric.
For complementary
shown
fragments w i t h carbon
number x and n-x, the l i g h t e s t product i s always t h e most abundant. Central scission o f the hydrocarbon chain i s the major cracking event w i t h nonane and decane,
yielding C4tC5
formation o f c5tc6
and C5
products,
respectively.
i s s l i g h t l y lower than o f c4tC7.
With undecane, Central
the
cracking o f
dodecane i n c6 fragments i s suppressed w i t h respect t o t h e formation o f C4tcs or C5tC7
alkanes.
With tridecane and l a r g e r n - p a r a f f i n s ,
the d i s t r i b u t i o n
curves e x h i b i t d i s t i n c t minima f o r c6 and Cn-6 products. Fig.3 a l s o shows t h a t the formation o f methane, ethane and t h e i r complementary fragments w i t h carbon number n - 1 and n-2, respectively, decreases w i t h increasing chain length, n, o f the feed. The hydrocracking p a t t e r n of heptadecane over d i f f e r e n t c a t a l y s t s i s shown i n Fig.4.
A common feature for a l l c a t a l y s t s i s t h a t t h e formation o f C 6 t c l l
alkanes i s somewhat suppressed w i t h respect t o other fragments. This e f f e c t
52
P Q)
Y
0
z
0
carbon number Fig.3. Hydrocracked carbon number f r a c t i o n s from n - p a r a f f i n s over 0.75Pt/ZSM-3 a t 0.7 MPa. The f u l l curves comprise a l l products with a given carbon number; t h e dashed curves rep esent t h e branched products. W/Fo = 0.5 kg s mmol-I f o r n- , n-C n-C 1 and n-C14, 0.f kg s mmol-l f o r n0.7 kg s ~ n m o l - ~ ~ nf!6;o r ana 1.1 kg s mmol- f o r n-C * T=602 K C12 and n-C f o r n-Cg, 5b$ K f o r n-ClO, 574 K f o r n - C l l , 564 K f o r n-C 563 K 1 h n-Ci3, 560 K f o r n-C14, 558 K f o r n-C15, 558 K f o r n-C16 and 539 K1& n-Ci7.
53
i s l e s s pronounced w i t h l P t / U S Y
and l P t / Z S M - Z O
and i s very s i g n i f i c a n t over
lPt/BETA. I n order t o q u a n t i f y t h i s , t h e molar r a t i o c5/c6 has been p l o t t e d i n Fig.5 against t h e y i e l d o f hydrocracking over t h e d i f f e r e n t c a t a l y s t s . Irrespective o f
t h e hydrocracking y i e l d ,
t h e c5/c6
r a t i o i s highest f o r
lPt/BETA and decreases according t o the f o l l o w i n g c a t a l y s t sequence: lPt/BETA > 0.75Pt/ZSM-3 > 0.3Pt/Y, O,SPt/Y, 1Pt/ZSM-20 > lPt/USY. The composition o f the c6 f r a c t i o n , obtained from heptadecane over l P t / U S Y i n Fig.6 i s shown t o be independent o f the hydrocracking y i e l d from 20 t i l l 80% hydrocracking. The same behaviour was found w i t h t h e other c a t a l y s t s . I n Table 1, the amount o f 2,3-dimethylbutane i n the c6 f r a c t i o n , obtained a t medium hydrocracking y i e l d s ,
i s given f o r d i f f e r e n t feeds and c a t a l y s t s .
For each
c a t a l y s t , i t tends t o increase w i t h increasing carbon number o f t h e feed. From heptadecane i t decreases i n the f o l l o w i n g c a t a l y s t sequence: lPt/BETA > l P t / Z S M - L O > 0.75Pt/ZSM-3 > 0.3Pt/Y,
0.5Pt/Y
> lPt/USY.
carbon number Fig.4. Hydrocracked carbon number f r a c t i o n s from heptadecane lover 0.3Pt/Y, O.SPt/Y, lPt/USY, lPt/BETA and lPt/ZSM-20 a t W/Fo-l.l kg s mnol- and 0.7 MPa; T 4 2 3 K f o r O.JPt/Y, 521 K f o r 0.5Pt/Y, 456 K f o r lPt/BETA, 458 K f o r lPt/ZSM20 and 452 K f o r lPt/USY.
54
so 0.75PtlZSM-3 ~
0
40
1.10
30 20
10
I PtIUSY
0.95
0.90
.
0
*
.
I
20
.
40
'
.
0
"
60
80
100
0
20
60
80
7 100
Y,, (%)
Y,, (96) Fig.5. Molar r a t i o o f c5/c6 products versus hydrocracking y i e l d from heptadecane over 0.3 and 0.5Pt/Y, l P t / U S Y , lPt/BETA and 1 P t / ZSM-20. Reaction c o n d i t i o n s o f Fig.2.
40
Fig.6. Hexane (A) ,2-methylpentane (o), 3-methylpentane (+), 2,Z-dimethylbutane (0) and 2,3-dimethyl butane (A) i n t h e c6 cracked product f r a c t i o n against t h e hydrocracking y i e l d from heptadecane over l P t / U S Y . The r e a c t i o n c o n d i t i o n s a r e those o f Fig.2.
Fig.7. Isomerisation and hydrocracking y i e l d against degree o f conversion o f heptadecane over 0.75Pt/ZSM-3. The r e a c t i o n c o n d i t i o n s are those o f Fig.2. 0
40
20
60
80
100
conversion (%I
TABLE 1 Amount o f 2,3-dimethylbutane i n
C6
f r a c t i o n (%) a t medium hydrocracking y i e l d
catalyst n-Cln 1Pt/USY 0.3Pt/Y 0.5Pt/Y 0.75Pt/ZSM-3 lPt/ZSM-LO lPt/BETA
n-CI1
feed n-C17 n-CIF,
2.0
n-C16 n-C17
-
3.5 -
3.7 3.8
-
3.8 4.4 5.0 9.3
2.0
2.4
2.7
3.1
4.0
4.0
2.3 5.8
-
7.5
-
9.7
-
Maximum isomerisation y i e l d The y i e l d o f i s o m e r i s a t i o n and hydrocracking from heptadecane over 0.75Pt/ZSM-3 i s p l o t t e d i n Fig.7 against t h e degree o f conversion. The former e x h i b i t s a maximum, t h e values o f which are given i n Table 2 f o r t h e d i f f e r e n t n - p a r a f f i n s and c a t a l y s t s . Over lPt/USY, lPt/ZSM-20 and lPt/BETA t h e maximum isomerisation y i e l d decreases w i t h i n c r e a s i n g carbon number o f t h e feed. Over 0.75Pt/ZSM-3 t h e maximum y i e l d o f isomerisation increases w i t h increasing carbon number o f the feed.
55
TABLE 2 Maximum isomerisation yield (%) from n-paraffins catalyst
1 Pt/USY 0.5Pt/Y 0.3Pt/Y 0.75Pt/ZSM-3 lPt/ZSM-20 lPt/BETA
feed n-CQ n-Cln n-CI1 n-C1,
-
52
-
-
56 62 52 63
-
n-Clr; n-CI6 n-C17 50
47 61
59 -
-
62
-
-
66
67
69
57 72 50 51
DISCUSSION Catalvtic activitv. balance between functions and chain lenqth of feed The reactivity of n-paraffins over bifunctional catalysts increases with increasing chain length. For n-C6 to n-C11 alkanes Weitkamp has shown that the initial reaction rate over Pt/CaY increases when the carbon number of the feed is increased [21]. In Fig.1 it is shown that over a 0.75Pt/ZSM-3 zeolite an eightfold increase in activity occurs when n-C17 is used as feed instead of nC10. According to the classical bifunctional mechanism, this increase in reactivity with increasing carbon number can, for obvious thermodynamic reasons, result from an increased partial pressure of olefins formed by dehydrogenation on the noble metal and/or from an enhanced reactivity of these olefins over the acid sites of the catalyst when their chain length increases. The presence of methane and ethane in the hydrocracked products indicates that hydrogenolysis is superimposed on the bifunctional conversion [2]. This CC bond cleavage on the noble metal surface is significant in the conversion of hexane and heptane over Pt/CaY, whereas it is virtually absent when octane or larger n-paraffinic feeds are converted [21]. From the data of Fig.3 it can be seen that over 0.75Pt/ZSM-3 the formation of methane and ethane is highest from nonane, decreases with increasing carbon number of the feed and is negligible for tetradecane and larger n-alkanes. Provided the reactivity of the olefins on the Brensted acid sites increases faster with increasing chain length than does the monofunctional conversion of the corresponding n-a1 kane over the metal particles, then the decreasing contribution of monofunctional metal catalysis with increasing carbon number is explained. For a given bifunctional catalyst, the equilibration of the two catalytic functions clearly depends on the carbon number of the feed. With the 0.75Pt/ZSM-3 catalyst this balance exists for n-CI4 to n-C17 feeds. The present results suggest that for shorter n-paraffins, the amount of noble metal on the catalyst required to obtain a bifunctional conversion must be lower.
56
Isomerisation maximum and chain lenqth of feed The isomerisation yield from decane and dodecane over Pt/USY and Pt/DBY is not dependent on such experimental conditions as partial pressure of He and nparaffin and reaction temperature and consequently the isomerisation yield is a unique function of conversion [7,20]. Therefore, the experimental conditions under which the maximum isomerisation yield is determined are irrelevant, as the isomerisation yield is only conversion dependent. Over 0.75Pt/ZSM-3 the maximum isomerisation yield increases when the carbon number of the feed is increased (Table 2 ) . The contribution of hydrogenolysis decreases in a parallel manner (Fig.3). Therefore, the loss of isomerisation yield can be the result of a loss of feedstock through hydrogenolysis. When heptadecane is converted over lPt/USY, lPt/BETA and lPt/ZSM-20, hydrogenolysi s is absent (Fig.4), while these catalysts are much more active than 0.75Pt/ZSM-3 (Fig.2). This can be explained by the enhanced acid strength per Brfinsted site o f twelve-membered ring zeolites with decreasing aluminium content [22,23]. Over lPt/USY, 1PtIZSM-20 and lPt/BETA, the isomerisation maximum is higher with decane than with heptadecane (Table 2). This, again, is a manifestation of the dependence o f the equilibration of the functions on the carbon number of the feed. If for a given metal function and a given feed the acid function is too strong, higher isomerisation yields may be expected for shorter n-paraffins. In conclusion, for a given catalyst the maximum isomerisation yield seems to depend on the carbon number of the feed in the way qualitatively pictured in Fig.8. It shows a maximum for a given chain length of the feed. For shorter nparaffins, the yield of isomerisation decreases due to a loss of feedstock by hydrogenolysis. For larger feedstocks, the isomerisation yield is decreased by hydrocracking as the acid function becomes too strong, resulting again in an unbalanced catalyst. balance
0Q) , '
.-
t 00
too
strong metal function
strong acid function
C
0
*z m
.-
In
Fig.8. Evolution of the maximum i someri sati on yield with increasing carbon number of the feed.
: carbon number of feed
Over 0.3Pt/Y and 0.5Pt/Y the maximum isomerisation yield from heptadecane is 57 and 61%, respectively (Table 2). Some hydrogenolysis occurs over 0.5Pt/Y
57
(Fig.4),
i n d i c a t i n g t h a t a loading o f 0.5% platinum i s i n excess o f the
For the conversion o f heptadecane over Pt/Y loading must t h e r e f o r e be between 0.3 and 0.5%.
optimum.
t h e optimum platinum
I f i t i s observed for a given c a t a l y s t t h a t i n t h e homologous s e r i e s o f long-chain n - p a r a f f i n s ( s t a r t i n g from octane [24]) t h e i s o m e r i s a t i o n maximum continuously decreases, i t f o l l o w s t h a t t h e two f u n c t i o n s are n o t w e l l e q u i l i b r a t e d and t h a t the hydrogenation-dehydrogenation f u n c t i o n i s too weak. This seems t o be t h e case f o r Pt/CaY (SK-200) [21]. Isomerisation maximum and hvdrocrackina oat t e r n The y i e l d o f isomers from a long-chain p a r a f f i n i s l i m i t e d , even over w e l l balanced b i f u n c t i o n a l c a t a l y s t s . Hydrocracking reactions, which are consecutive t o isomerisation reactions, consume p r e f e r e n t i a l l y t h e d i - and tribranched feed isomers [24]. The isomerisation-hydrocracking network o f n-C8 and l a r g e r np a r a f f i n s over lPt/USY c a t a l y s t i s depicted i n Scheme 1 [24].
n-paraffin
--t
monobranched isomers
dibranched isomers
--t
-+
tribranched isomers
/ a,a-
branched branched 1.2
cracked products Scheme 1. Isomerisation-hydrocracking p a r a f f i n s over lPt/USY [24]. When decane i s converted over Pt/USY, from type A hydrocracking [25]. a, y , v - t r i b r a n c h e d
reaction
network
of
n-
69% o f the cracked products o r i g i n a t e
Type A hydrocracking involves
alkylcarbenium ions.
long-chain
j-scission of
The remaining cracked products are
formed by B1, B2 and C hydrocracking. Over Pt/USY and Pt/CaY t h e formation o f propane and complementary fragments v i a hydrocracking o f feed isomers from n-C8 t o n-CI6
n - p a r a f f i n s does not occur f r e q u e n t l y [26,27].
I n these cases the
s c i s s i o n p r o b a b i l i t y o f a C-C bond increases when i t s l o c a t i o n i s c l o s e r t o the center o f the hydrocarbon chain [26,27].
The data o f Fig.4 i n d i c a t e t h a t there
are deviations from t h i s p a t t e r n w i t h heptadecane over Pt/Y: c6 alkanes are l e s s abundantly formed than C5 products. There i s no mechanistic reason why w i t h heptadecane t h e formation o f C5 fragments should be more abundant than o f c6 [24]. The anomaly i n the d i s t r i b u t i o n o f t h e cracked products a t c6 and cn-6 should o r i g i n a t e from molecular s h a p e - s e l e c t i v i t y imposed by the pore geometry
on t h e rearrangements and/or
p - s c i s s i o n o f t h e intermediate alkylcarbenium
ions. I n any case, t h i s phenomenon i s l e s s pronounced w i t h USY than w i t h Y. I t i s s t r i k i n g t h a t t h e former z e o l i t e contains a secondary large-pore system
58
. The
(mesopores)
r a t i o o f the hydrocracked products from heptadecane over
C5/Cs
0.75Pt/ZSM-3, 1Pt/ZSM-20, 0.3 and 0.5Pt/Y and l P t / U S Y i s d i f f e r e n t (Figs.5) and decreases i n t h e f o l l o w i n g order: BETA > ZSM-3 > Y > ZSM-20 > USY. Thus t h e suppression o f c e r t a i n hydrocracking routes r e s u l t i n g i n c6 fragments by s t e r i c c o n s t r a i n t s exerted by t h e i n t r a c r y s t a l l i n e v o i d volume o f these z e o l i t e s must increase i n t h e opposite order. Weitkamp e t a l . found t h a t the product d i s t r i b u t i o n s from undecane and tetradecane over 0.27Pd/ZSM-20 and 0.5Pt/CaY are very s i m i l a r [28]. The present data show t h a t d i f f e r e n c e s i n the s t r u c t u r e o f t h e void volume o f ZSM-20, Y and USY appear w i t h heptadecane. With [14] and ZSM-3 p a t t e r n sets i n a t
(Fig.3) t h e d e v i a t i o n from t h e r e g u l a r hydrocracking undecane. The exact n a t u r e o f t h i s shape-selective
BETA
suppression o f c6 formation i s n o t y e t c l e a r . Type A hydrocracking takes place i f i n t h e parent feed isomer the side chains are a t a , a ,y - p o s i t i o n s .
Hydrocracking mechanisms B1,
B2 and C are
operative i f s p e c i f i c requirements concerning the c o n f i g u r a t i o n o f the side chains
are
met
(Scheme
1).
2,3-dimethylbutane,
hydrocracking o f 2,3,3-trimethylbranched
e.g.,
is
formed
by
B1
and by B2 hydrocracking o f 2,3,5-
trimethylbranched o r 2,3-dimethyl-5-ethylbranched isomers o f the feed [24]. The content o f 2,3-dimethylbutane i n the c6 f r a c t i o n i s t h e r e f o r e a measure f o r the c o n t r i b u t i o n o f B1 products
from n-C10
and B2 hydrocracking. to
n-C17 n - p a r a f f i n s
Cs
The over
fraction of
lPt/BETA
the cracked
i s r i c h i n 2,3-
dimethylbutane (Table 1). T h i s i n d i c a t e s t h a t over t h i s z e o l i t e a s u b s t a n t i a l number o f the tribranched isomers are converted v i a 61 and B2 hydrocracking instead o f v i a A hydrocracking. Mechanistically, t h i s means t h a t e i t h e r type A p - s c i s s i o n o f a , y , y -tribranched alkylcarbenium ions i s suppressed, o r t h e i r formation
i s hindered.
ZSM-3, ZSM-20 and Y e x h i b i t behaviour intermediate between BETA and USY, r e f l e c t i n g again the existence o f s p e c i f i c d i f f e r e n c e s i n the s t r u c t u r e o f the v o i d volume o f these z e o l i t e s . The c a t a l y s t s 0.75Pt/ZSM-3 conversion o f heptadecane. higher over ZSM-3 (Table 2 ) .
and 0.3-0.5Pt/Y
are p e r f e c t l y balanced f o r the
The maximum i s o m e r i s a t i o n y i e l d i s ,
however, 10% This d i f f e r e n c e should be ascribed t o t h e shape-
s e l e c t i v e suppression o f some o f the hydrocracking routes i n t h i s z e o l i t e . The higher c5/c6
r a t i o and t h e higher amount o f 2,3-dimethylbutane
f r a c t i o n are manifestations o f
this
effect.
lPt/ZSM-PO
and
i n t h e c6
lPt/BETA
adequate pore geometries f o r o b t a i n i n g h i g h isomerisation y i e l d s .
have
Because o f
the h i @ a c i d strength o f these z e o l i t e s , t h e isomerisation y i e l d i s lower than over 0.75Pt/ZSM-3.
59
CONCLUSIONS
The e q u i l i b r a t i o n o f the c a t a l y t i c f u n c t i o n s i n a b i f u n c t i o n a l z e o l i t e c a t a l y s t depends on the chain l e n g t h o f t h e n - p a r a f f i n t o be converted. The required amount o f noble metal increases w i t h increasing chain l e n g t h o f the np a r a f f i n feed. F o r the conversion o f each n - p a r a f f i n t h e r e e x i s t s an optimum noble metal loading. A given metal loading i s adequate o n l y f o r t h e conversion o f a r e s t r i c t e d range o f n - p a r a f f i n s . According t o the c l a s s i c a l b i f u n c t i o n a l mechanism no upper bound e x i s t s f o r t h e hydrogenation-dehydrogenation a c t i v t Y o f the c a t a l y s t . I n p r a c t i c e the hydrogenation-dehydrogenation a c t i v i t y has t o be enhanced by increasing the amount o f noble metal
on t h e c a t a l y s t .
AS
platinum e x h i b i t s hydrogenolysis, there i s an upper bound o f t h e metal load ng above which hydrogenolysis i s superimposed on the b i f u n c t i o n a l convers on pathway. With an optimized combination o f c a t a l y s t and feed the highest isomerisation y i e l d i s obtained. 0.75Pt/ZSM-3 presents an i d e a l c a t a l y s t f o r the b i f u n c t i o n a l conversion o f C14-C17 n - p a r a f f i n s .
For the conversion o f
heptadecane over z e o l i t e Y, the optimum platinum loading i s 0.3-0.5%. Certain hydrocracking routes leading t o t h e formation o f c6 fragments are suppressed i n large-pore z e o l i t e s due t o s t e r i c c o n s t r a i n t s exerted by the i n t r a c r y s t a l l i n e v o i d volume. The enrichment o f 2,3-dimethylbutane
i n the c6
cracked products i n d i c a t e s t h a t type A hydrocracking i s s t e r i c a l l y suppressed. The s t e r i c c o n s t r a i n t s exerted by the i n t r a c r y s t a l l i n e v o i d volume increase i n order:
BETA > ZSM-3 > Y > ZSM-20 > USY.
With well-balanced c a t a l y s t s the
maximum isomerisation y i e l d t h a t can be obtained from a long-chain n - p a r a f f i n should decrease i n the same order. obtaining
high
isomerisation
yield.
BETA has t h e adequate pore geometry f o r A
loading
with
1% o f
platinum
is
i n s u f f i c i e n t t o counterbalance t h e strong a c i d i t y o f t h i s z e o l i t e . The shapes e l e c t i v e suppression o f c e r t a i n hydrocracking routes i s l e s s pronounced w i t h USY than w i t h Y due t o the presence o f mesopores i n the former z e o l i t e . ACKNOWLEDGMENTS J.A.M. and P.A.J. acknowledge t h e Belgian Fund f o r S c i e n t i f i c Research f o r a grant (Research Associate) and a research p o s i t i o n (Research D i r e c t o r ) , r e s p e c t i v e l y . This work has been sponsored by t h e Belgian Government (Diensten Wetenschapsbeleid) i n the frame o f a concerted a c t i o n on c a t a l y s i s .
60
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R E S P E C T I V E INFLUENCES OF THE GEOMETRIC AND CHEMICAL FACTORS I N THE CON-
VERSION OF AROMATICS OVER ACIDIC ZEOLITES
F. FAJULA,M. LAMBRET and F .FIGUERAS L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique Appliquees UA 418 CNRS. ENSCM, 8 Rue de 1 ' E c o l e Normale - 34075 M o n t p e l l i e r Cedex France.
ABSTRACT The c a t a l y t i c behaviour o f a s e r i e s o f mordenites and o f f r e t i t e s w i t h v a r y i n g aluminium c o n t e n t and o r i g i n has been analyzed f o r t h e a l k y l a t i o n o f benzene and t o l u e n e by e t h a n o l and t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e as a f u n c t i o n o f t h e i r A1 m o l a r f r a c t i o n ( m = A l / A l + S i ) . F o r b o t h r e a c t i o n s , t h e a c t i v i t y , expressed on a p e r - w e i g h t b a s i s , passes t h r o u g h a maximum w i t h m=0.1 f o r m o r d e n i t e and m=0.12 f o r o f f r e t i t e . As m decreases, t h e amount o f coke d e p o s i t e d d u r i n g t h e d i s p r o p o r t i o n a t i o n r e a c t i o n decreases, b u t t h e d e a c t i v a t i o n r a t e i n c r e a s e s . A l k y l a t i o n r e a c t i o n s u s i n g e t h a n o l as r e a g e n t a r e c h a r a c t e r i z e d by l o n g s t a b l e l i f e t i m e s o f t h e c a t a l y s t s . The main i n f l u e n c e o f t h e aluminium c o n t e n t was f o u n d t o be on t h e ethylbenzene s e l e c t i v i t y , which reached 70-90% f o r m < 0.12.
INTROOUCT I ON Because o f t h e i r g r e a t i n d u s t r i a l importance,
t h e reactions o f aromatic
hydrocarbons on a c i d i c forms o f z e o l i t e c a t a l y s t s have been t h e s u b j e c t o f e x t e n s i v e s t u d i e s . On t h e o t h e r hand, t h e r e a c t i o n mechanisms a r e now w e l l known and t h e p r o d u c t d i s t r i b u t i o n s s i m p l e enough t h a t a r o m a t i c s u b s t r a t e s may be used as model molecules f o r c a t a l y s t c h a r a c t e r i z a t i o n . The c o n v e r s i o n o f aromatic hydrocarbons has been m o s t l y s t u d i e d e i t h e r on a l u m i n i u m - r i c h , l a r g e - p o r e s t r u c t u r e s (X,Y,MOR
r e f s . 1 - 3 ) o r on s i l i c o n - r i c h ,
m a t e r i a l s (ZSM-5,
Z e o l i t e s o f t h e f i r s t f a m i l y a r e h i g h l y ac-
refs.4-10).
t i v e , even a t r e l a t i v e l y l o w temperatures (20O-30O0C),
shape-selective
b u t c a t a l y s t aging
due t o c o k i n g i s severe, l e a d i n g t o d e a c t i v a t i o n i n a few hours. ZSM-5 based catalysts
a r e used a t h i g h e r temperatures (400-600°C).
They show l i t t l e
d e a c t i v a t i o n w i t h t i m e on stream and t h e i r a c t i v i t y and s e l e c t i v i t y a r e e s s e n t i a l l y c o r r e l a t e d t o s t r u c t u r a l and m o r p h o l o g i c a l f e a t u r e s . Because o f t h e s e l i m i t a t i o n s and d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s , e v a l u a t e t h e e f f e c t o f t h e chemical
it i s difficult to
c o m p o s i t i o n ( a c i d s i t e d e n s i t y and
s t r e n g t h ) o f t h e z e o l i t e s on t h e i r c a t a l y t i c behaviour. I n t h i s work we have used benzene and t o l u e n e a l k y l a t i o n b y e t h a n o l and toluene disproportionation
t o test
c a t a l y s t s w i t h widely varying
the
activity
of
large-pore
zeolite
aluminium c o n t e n t , p r e p a r e d b y d e a l u m i n a t i o n
62
o f mordenites and o f f r e t i t e s . Three aspects o f t h e i r c a t a l y t i c behaviour, namely t h e i r s t a b i l i t y ,
t h e i r a c t i v i t y and t h e i r
selectivity,
have been
p a r t i c u l a r l y analyzed as a f u n c t i o n o f t h e i r aluminium c o n t e n t . METHODS
Catalysts Two s e r i e s o f z e o l i t e s were used : i ) Mordenites w i t h aluminium molar f r a c t i o n s ( m = A l / A l t S i ) r a n g i n g from 0.167 t o 0.025 ( o r S i / A 1 r a n g i n g from 5 t o 40). Samples w i t h m values o f 0.167,
0.129 and 0.099
(Si/A1=5,6.9
and
9.2) were obtained from t h e S o c i 6 t 6 Chimique de l a Grande Paroisse ( A l i t e 1801,
Norton
dealuminated
(Zeolon
100H)
and
s o l i d s w i t h m=0.062,
Union
Carbide,
0.035 and 0.025
respectively.
Three
were s u p p l i e d by t h e
I n s t i t u t FranCais du P e t r o l e . ii1 O f f r e t i t e s w i t h aluminium molar f r a c t i o n s ranging from 0.284 t o 0.059
( S i / A l from 2.52
t o 1 6 ) . Two p a r e n t z e o l i t e s
w i t h m=0.284 and 0.211 ( S i / A l = 2.52 and 3.73 r e s p e c t i v e l y ) were synthesized according t o r e f . 11. Successive hydrothermal and a c i d treatments
(ref.12)
o f t h e ammonium forms o f t h e l a t t e r y i e l d e d dealuminated o f f r e t i t e s w i t h m=0.133, 0.103,
0.083 and 0.059.
Before t h e t e s t , a l l samples were exchanged t w i c e w i t h ammonium n i t r a t e s o l u t i o n s a t r e f l u x temperature, washed, oven-dried, and f i n a l l y c a l c i n e d i n a shallow bed a t 500°C f o r 5h i n f l o w i n g a i r . The r e s u l t i n g c a t a l y s t s were c h a r a c t e r i z e d by XRD, SEM,
v o l umetry and elemental a n a l y s i s ,
as d e s c r i b e d
p r e v i o u s l y ( r e f s . 13,141. Procedures C a t a l y t i c r e a c t i o n s were c a r r i e d o u t i n a f i x e d bed,
down-flow
dif-
f e r e n t i a l r e a c t o r o p e r a t i n g a t atmospheric pressure ( n i t r o g e n f l o w ) on lOOmg o f c a t a l y s t ( i n t h e form o f powder). Products were analyzed by o n - l i n e gas chromatography. The e t h y l a t i o n r e a c t i o n s were i n v e s t i g a t e d i n t h e temperature 220-275°C.
The aromatic hydrocarbon (benzene and toluene,
purity
>
range 99.8%)
was i n t r o d u c e d f i r s t i n t o t h e system. A f t e r f i v e minutes t h e ethanol ( p u r i t y > 99.8%) was added using an independent s a t u r a t o r . Toluene d i s p r o p o r t i o n a t i o n r e a c t i o n s were s t u d i e d i n t h e temperature range 300-400°C. RESULTS AND DISCUSSION Physicochemical p r o p e r t i e s o f t h e a c t i v a t e d z e o l i t e s The
dealumination o f z e o l i t e s by combined steam and a c i d t r e a t m e n t s i n -
duces n o t o n l y a decrease o f t h e i r aluminium and c a t i o n c o n t e n t (Table 1 )
63
CI
r I
-D E
Y
A
U
0
g
0.1
0
-
A
0
r.
m
c
A
C
0
a
8
4
0
I
1
S t a b i l i t y o f the c a t a l y t i c a c t i v i t y .
1 .Benzene a l k y l a t i o n w i t h e t h a n o l .
The advantages o f u s i n g e t h a n o l i n s t e a d o f e t h y l e n e as t h e a l k y l a t i n g agent have been a l r e a d y p o i n t e d o u t
by Chandawar
et
al.
(ref.10)
for
ZSM-5
z e o l i t e s . The most s i g n i f i c a n t f e a t u r e r e p o r t e d by t h e a u t h o r s was t h e l o n g s t a b l e l i f e t i m e o f t h e c a t a l y s t s . Their r e s u l t s support t h e hypothesis o f Anderson e t a l .
(ref.18)
t h a t t h e e t h y l group i n e t h y l b e n z e n e was formed
f r o m ethanol w i t h o u t t h e i n t e r m e d i a t e f o r m a t i o n o f e t h y l e n e .
T h i s would
reduce t h e f o r m a t i o n o f r e s i d u e s t h r o u g h t h e p o l y c o n d e n s a t i o n o f t h e o l e f i n , which i s known t o be t h e main cause o f c a t a l y s t d e a c t i v a t i o n ( r e f s . l , 2 ) . Low d e a c t i v a t i o n r a t e s o f t h e c a t a l y s t s were a l s o observed here,
as
shown by F i g u r e 2 f o r t h e a l k y l a t i o n o f benzene a t 250°C o v e r t h e m o r d e n i t e sample having an A 1 m o l a r f r a c t i o n o f 0.127. Among t h e v a r i o u s f a c t o r s which were analyzed ( t y p e o f z e o l i t e , v a l u e o f m,
experimental c o n d i t i o n s ) t h e one h a v i n g t h e most pronounced e f f e c t on
64
c a t a l y t i c s t a b i l i t y was found t o be t h e h y d r o c a r b o n / a l c o h o l r a t i o . P r o v i d e d t h i s r a t i o be h i g h e r t h a n 7, a l l c a t a l y s t s p r e s e r v e d more t h a n 90% o f t h e i r i n i t i a l a c t i v i t y under o u r t e s t c o n d i t i o n s . I
1 100
200
t i me (min.) Rate o f benzene e t h y l a t i o n a t 250°C
Fig.2.
versus t i m e on m o r d e n i t e ( m
I n f l u e n c e o f t h e benzene : e t h a n o l r a t i o . ( A ) 3.85,
0.127).
2.
To1 uene
At
disproportionation.
a1 1
temperatures
( m ) 7,
( 0 )
studied,
=
9.
the
c a t a l y s t s d e a c t i v a t e d w i t h t i m e on stream. The e x p e r i m e n t a l d a t a were cons i s t e n t w i t h a f i r s t - o r d e r r e a c t i o n and f i r s t - o r d e r The
deactivation -
r=roe k d
t
rate
constants
(kd)
were
catalyst deactivation.
obtained
from t h e equation
where ro and r a r e t h e r e a c t i o n r a t e s a t t = o and t ( r e f s . 1 9 , 2 0 ) .
The values o f k d o b t a i n e d on two o f f r e t i t e s and f o u r m o r d e n i t e s a t 300°C and
400°C and t h e amounts o f carbon measured on t h e spent c a t a l y s t s ( a f t e r 3 hours on stream) a r e g i v e n i n Table 2. The r a t e s o f d e a c t i v a t i o n were found o f t h e same o r d e r o f magnitude f o r both s t r u c t u r e s . On o f f r e t i t e , t h e amounts o f carbonaceous d e p o s i t s formed a t t h e end o f t h e t e s t appeared independent o f t h e aluminium c o n t e n t and t e m p e r a t u r e whereas, on mordenite, t h e y decreased s t e a d i l y w i t h m. The d e t e r m i n a t i o n o f t h e porous volumes,
a c c e s s i b l e t o hydrocarbons (n-hexane and c y c l o h e x a n e ) , o f
t h e used c a t a l y s t s r e v e a l e d t h a t d u r i n g t h e t e s t , t h e samples had l o s t o n l y 20 t o 30% o f t h e i r o r i g i n a l s o r p t i o n c a p a c i t y .
D e a c t i v a t i o n was t h e r e f o r e
due t o t h e coverage o f t h e a c i d s i t e s by s t r o n g l y adsorbed r e s i d u e s r a t h e r than t o a pore p l u g g i n g .
The d e a c t i v a t i o n r a t e s a r e hence expected
i n c r e a s e when m decreases, as i t i s observed f o r b o t h z e o l i t e s .
to
65
I n f l u e n c e o f aluminium c o n t e n t on a c t i v i t y F i g u r e s 3 and 4 p r e s e n t t h e e v o l u t i o n s o f t h e r e a c t i o n r a t e s , expressed on a p e r gram b a s i s , as a f u n c t i o n o f t h e aluminium m o l a r f r a c t i o n f o r t h e a l k y l a t i o n o f benzene a t 250°C and t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e a t
300°C on b o t h z e o l i t e s . F o r t h e l a t t e r r e a c t i o n t h e v a l u e s t a k e n i n t o account a r e t h e i n i t i a l r e a c t i o n r a t e s determined f r o m t h e l n r v e r s u s t p l o t s . I n a l l cases volcano c u r v e s were o b t a i n e d w i t h t h e i r maxima around A t f i r s t glance,
m=0.1-0.12.
such b e h a v i o u r corresponds t o t h a t expected
a c c o r d i n g t o B a r t h o m e u f ' s p r e d i c t i o n s ( r e f .21) r e g a r d i n g t h e c o r r e l a t i o n s between a c i d i t y and c a t a l y t i c a c t i v i t y i n z e o l i t e s . The r a t e o f a r e a c t i o n depends on t h e e f f e c t i v e number strength.
A t h i g h A1 c o n t e n t ,
of
sites
and on t h e i r
the interactions
e f f i c i e n c y and
between A l O i t e t r a h e d r a
decrease b o t h t h e e f f i c i e n c y and t h e s t r e n g t h o f t h e s i t e s . When t h e a l uminium c o n t e n t decreases,
t h e decrease o f t h e number o f s i t e s i s over
balanced by t h e i n c r e a s e o f t h e i r e f f i c i e n c y and s t r e n g t h up t o a l i m i t i n g A1 molar
fraction
below which
none
of
the
A104 tetrahedra
is
in
a
n e x t - n e a r e s t - n e i g h b o u r s i t u a t i o n . Below t h i s t h r e s h o l d , which was c a l c u l a t e d at
m=0.096
for
mordenite
and
m=0.12
for
p e r - w e i g h t b a s i s a r e expected t o decrease, (and
t u r n - over
numbers)
should
remain
offretite,
activities
on
a
whereas t h e a c t i v i t y p e r s i t e constant.
Although
the
former
c o n d i t i o n i s w e l l f u l f i l l e d h e r e and t h e values o f t h e o p t i m a c l o s e l y agree w i t h theory, t h e turn-over
numbers a r e d e f i n i t e l y n o t c o n s t a n t below t h e
v a l u e o f m=0.1, as shown i n f i g u r e s 5 and 6. T h i s means t h a t t h e c a t a l y t i c a c t i v i t y i s n o t determined s o l e l y by t h e a c i d i t y o f t h e samples.
We have
seen e a r l i e r t h a t , upon d e a l u m i n a t i o n , t h e porous volume o f t h e z e o l i t e s was significantly
increased.
Moreover,
the
actual
amount
of
tetrahedrally
c o o r d i n a t e d aluminium i n t h e h i g h l y dealuminated m o r d e n i t e s and o f f r e t i t e s
i s d i f f i c u l t t o e s t i m a t e b y s o l i d s t a t e n.m.r. presence o f s i 1 anol groups ( r e f .22 could
be p u t
1. There a r e indeed s e v e r a l reasons which
f o r w a r d t o e x p l a i n why
c a t a l y s t s deviates from theory.
spectroscopy due t o t h e
t h e behaviour o f
actual
zeolite
It s h o u l d be p o i n t e d o u t , however, t h a t on
t h e same s e r i e s o f mordenites, below t h e v a l u e o f m=0.1,
t u r n - o v e r numbers
were found t o i n c r e a s e i n t h e e t h y l a t i o n o f benzene (Fig.51, i n the disproportionation o f toluene (Fig.6)
and t o decrease
whereas t h e y remained c o n s t a n t
i n t h e isomerization o f orthodichlorobenzene (ref.23).
From t h i s i t can be
s p e c u l a t e d t h a t t h e changes i n A1 c o n t e n t n o t o n l y a f f e c t t h e a c i d i t y , b u t also
strongly
influence the
specific
molecules and t h e a c i d c e n t e r s .
At
interactions present t h e
between
exact
the
reagent
nature o f
i n t e r a c t i o n s and t h e way i n which t h e y a r e m o d i f i e d a r e s t i l l obscure.
these
m m
TABLE 1 : Chemical composition and c r y s t a l l i n i t y o f t h e samples a f t e r exchange-with ammonium n i t r a t e and calcination
OFF
Leo1 it e Si/Ala m At/Alb %Crystc
2.52 0.284 0.62 90
3.7 0.211 0.7, 100
6.5 0.133 0.85 80
MOR 8.7 0.103 0.80 90
12 0.083 0.91 110
16 0.059 0.90 80
5 0.167 0.85 80
6.9 0.127 0.98 90
9.2 0.098 0.99 100
15 0.062 0.98 80
a : Determined from t h e chemical analyses a f t e r d i s s o l u t i o n o f t h e samples (SCA CN RS S o l a i z e ) b : A = K f o r o f f r e t i t e and Na f o r m o r d e n i t e c : p e r c e n t o f c r y s t a l l i n i t y e v a l u a t e d by XRO. * = r e f e r e n c e sample
TABLE 2 : Rate constants ( k i n min-') f o r d e a c t i v a t i o n , and amounts o f carbonaceous deposits i n the dfsproportionation o f toluene
OFF
Zeolite
m = Al/Al+Si a t 300°C
l o 3 kd
wt%C a t 3400 "C 10 kd wt%C
MOR
0.103
0.059
0.127
0.098
0.062
0.035
3.8 2.33
6 2.34
4.7 4.5
8.9 3.28
4.6 1.88
7.7 1.75
2.9 1.93
5.5 2.1
3.3 5.22
-
5.1 1.9
-
27 0.035 0.98 70
39 0.025 0.92 60
67
I n a l l f i g u r e s ( A )M o r d e n i t e (
)
Offretite
0.1
0.2
0.1
m Fig.3. I n f l u e n c e o f t h e A1 f r a c t i o n on t h e a c t i v i t y f o r benzene e t h y l a t i o n a t 250°C.
Fig.4. I n f l u e n c e o f t h e A1 f r a c t i o n on t h e a c t i v i t y f o r t o l u e n e d i s p r o p o r t i o n a t i o n a t 300°C.
A
A n
A
F
0.2
m
n 10. v
& 5
I
z
A
i
:. .:
0 b--
0
W
i I .
0 5 . F
1
0.2
0.1
A
m
I
.
0.1
Fig.5. Turn-over numbers as a f u n c t i o n o f m f o r t h e a l k y l a t i o n o f benzene
A
I. 0.2
m Fig.6. Turn-over numbers as a f u n c t i o n o f m f o r the disproportionation o f toluene
v) N
50t
;
01
F i g . 7 . S e l e c t i v i t y f o r e t h y l benzene a t 250°C as a f u n c t i o n o f m. 0
I
0.2
0.1
m
.
68
From a p r a c t i c a l p o i n t o f view, t h e d a t a o f F i g u r e s 3 and 4 i n d i c a t e n e v e r t h e l e s s c l e a r l y t h a t t h e range o f
aluminium
content
i n which
the
z e o l i t e s develop t h e i r maximum a c t i v i t y i s narrow and does n o t depend on t h e type o f r e a c t i o n considered. I n f l u e n c e o f aluminium c o n t e n t on s e l e c t i v i t i e s Benzene a l k y l a t i o n . A t 250°C e t h y l e n e and mono and d i e t h y l b e n z e n e s were t h e o n l y p r o d u c t s formed
( w i t h a molar
r a t i o monoldiethylbenzene=7-8).
Traces o f d i e t h y l e t h e r were formed on t h e aluminium r i c h o f f r e t i t e s a t 220°C. As i t has been r e p o r t e d by Chandawar e t a l . ( r e f . 1 0 ) t h e s e l e c t i v i t y f o r ethylbenzene i n c r e a s e d w i t h i n c r e a s i n g t h e benzene : e t h a n o l r a t i o and d e c r e a s i n g c o n t a c t t i m e . Under our optimum c o n d i t i o n s and w i t h a benzene : e t h a n o l r a t i o o f 7 , t h e aluminium c o n t e n t had a l s o an i n f l u e n c e ( F i g u r e 7). The s e l e c t i v i t y f o r ethylbenzenes i n c r e a s e d as m decreased f r o m 0.28 t o 0.12 and t h e n reached a n e a r l y c o n s t a n t v a l u e ( 8 0 2 10%) f o r b o t h z e o l i t e s . T h i s e v o l u t i o n c o r r e l a t e s w i t h an i n c r e a s e o f t h e a c i d s t r e n g t h o f t h e s o l i d s (ref.21). The d e h y d r a t i o n o f a l c o h o l r e q u i r e s o n l y a weak a c i d i t y and i s c a t a l y z e d by a l l t h e s i t e s
present i n t h e
zeolite.
Benzene
alkylation
requires
s t r o n g e r a c i d s i t e s ( r e f . 2 4 ) . When m decreases, t h e p r o p o r t i o n o f t h e l a t t e r increases, and below m=0.12 a l l t h e s i t e s a r e s t r o n g enough t o c a t a l y z e t h e reaction. Toluene a l k y l a t i o n .
A t 275"C, w i t h a t o l u e n e / e t h a n o l
r a t i o o f 2,
the
e t h y l a t i o n r e a c t i o n was accompanied by d i s p r o p o r t i o n a t i o n o f t o l u e n e i n t o xylenes and benzene. Three t y p e s o f s e l e c t i v i t i e s c o u l d be d i s t i n g u i s h e d , i t h e alkylation/disproportionation
ratio,
ii ) a p a r a x y l e n e s e l e c t i v i t y and
iii) a p a r a e t h y l t o l u e n e s e l e c t i v i t y .
The i n i t i a l s e l e c t i v i t i e s were l i t t l e a f f e c t e d by m o d i f y i n g t h e a l u m i n i um c o n t e n t . T h i s can be e a s i l y understood s i n c e b o t h t h e a l k y l a t i o n and d i s proportionation
activities
exhibit
parallel
evolutions
(Figs.3
and
4).
Noreover i n i t i a l p a r a s e l e c t i v i t i e s were f o u n d t o depend more on t h e t o t a l c o n v e r s i o n t h a n on m, as i t i s g e n e r a l l y observed on l a r g e p o r e s t r u c t u r e s . The i n f l u e n c e o f t h e A1 c o n t e n t was, by c o n t r a s t , o b v i o u s when f o l l o w i n g t h e e v o l u t i o n s o f t h e p r o d u c t d i s t r i b u t i o n w i t h t i m e on stream. Examples o f such e v o l u t i o n s a r e g i v e n i n F i g u r e s 8 and 9 f o r t h e o f f r e t i t e w i t h m=0.059. I n i t i a l l y t h e main r e a c t i o n was d i s p r o p o r t i o n a t i o n ; among t h e xylenes, t h e para isomer was s l i g h t l y f a v o u r e d . The r a t e o f t o l u e n e d i s p r o p o r t i o n a t i o n decreased w i t h t i m e on stream whereas t h a t o f a l k y l a t i o n i n c r e a s e d , r e a c h i n g r a p i d l y a s t a b l e v a l u e .
As
t h e x y l e n e c o n v e r s i o n decreased t h e p r o p o r t i o n o f t h e p a r a isomer i n c r e a s e d
69
whereas
ethyltoluenes
were
always
produced
near
their
thermodynamic
e q u i l i b r i u m ( r e f . 2 5 ) . The decay o f t h e d i s p r o p o r t i o n a t i o n a c t i v i t y i s due t o the r a p i d poisoning o f the strongest s i t e s o f t h e z e o l i t e . This r e a c t i o n r e q u i r e s s t r o n g e r s i t e s t h a n e t h y l a t i o n ( r e f . 2 4 ) . The same i s t r u e f o r t h e i s o m e r i z a t i o n o f xylenes compared t o t h a t o f t h e e t h y l t o l u e n e s
(ref.26).
Since t h e r a t e o f d e a c t i v a t i o n i s r e l a t e d t o t h e aluminium f r a c t i o n ( t a b l e 2 ) ; t h e s e l e c t i v i t i e s o b t a i n e d a t each t i m e on stream were determined by
t h e c o m p o s i t i o n o f t h e z e o l i t e . Thus a f t e r t h r e e hours o f r e a c t i o n , t h e r a t i o a1 k y l a t i o n l d i s p r o p o r t i o n a t i o n and t h e percentage o f p a r a - x y l e n e i n c r e a s e d from 4 t o 8 and 39 t o 80% r e s p e c t i v e l y when g o i n g f r o m m=0.133 t o 0.059, r e s p e c t i v e l y , on o f f r e t i t e . The same t r e n d was n o t i c e d on m o r d e n i t e .
r -
-.-
I
h
X C
r" 5 0
'0,
0 'c
P r
0
a
Q)
c,
m
a
' 0 100
200
2 00
100
time (mi".)
time (min.) 1
F i g s 8-9. R e a c t i o n r a t e s (mmol.g-".hand s e l e c t i v i t i e s versus t i m e i n t h e a l k y l a t i o n o f t o l u e n e by e t h a n o l a t 275°C on o f f r e t i t e .
CONCLUSIONS I n t h i s work we have t r i e d t o emphasize t h e r o l e o f t h e aluminium cont e n t o f l a r g e - p o r e z e o l i t e s on t h e i r c a t a l y t i c b e h a v i o u r i n t h e c o n v e r s i o n o f a r o m a t i c hydrocarbons. parent z e o l i t e s
I n spite o f differences i n the origin o f the
and i n t h e
structural
and t e x t u r a l
properties o f the
c a t a l y s t s , s e v e r a l common f e a t u r e s h a v e been p o i n t e d out,
a l l o f them b e i n g
r e l a t e d t o changes i n t h e a c i d s i t e d e n s i t y and s t r e n g t h , which a r e s t r i c t l y dependent on m.
The aluminium c o n t e n t
determines
the
stability
i n the
d i s p r o p o r t i o n a t i o n o f t o l u e n e , t h e s e l e c t i v i t y i n t h e e t h y l a t i o n o f benzene and t o l u e n e and, l a s t b u t n o t l e a s t , t h e a c t i v i t y in a l l cases. When t h e r e a c t i o n r a t e s a r e expressed on a p e r - w e i g h t b a s i s ,
they f o l l o w volcano
c u r v e s w i t h t h e i r maxima a t m=0.1 f o r m o r d e n i t e and 0.12 f o r o f f r e t i t e . F o r b o t h s t r u c t u r e s , t h e v a l u e o b t a i n e d s t r i c t l y corresponds t o t h e p o i n t where t h e s i t e s would be i s o l a t e d and f u l l y e f f i c i e n t f o r c a t a l y s i s .
70
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F. Figueras and P . Geneste, F r i s t . I n t e r n Symp. Heter. C a t a l . and F i n e Chem. P o i t i e r s France 1988.
Stud.
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P.A. Jacobs, i n "Carboniogenic A c t i v i t y o f Z e o l i t e s " E l s e v i e r , Amsterdam
25
W.J. T a y l o r , D.D.
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R.H. A l l e n , L.D. Yats and D.S.
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Wagman, M.G.
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H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PARA-SELECTIVITY OF PENTASIL ZEOLITES
Jong-Ho KIM,
S e i t a r o NAMBA. and T a t s u a k i YASHIMA
Department o f Chemistry, Tokyo I n s t i t u t e o f Technology, Ookayama, Meguro-ku, Tokyo 152 (Japan)
ABSTRACT The p e n t a s i l z e o l i t e s m o d i f i e d w i t h o x i d e s , coked, o r s t e a m e d e x h i b i t e d much h i g h e r p a r a - s e l e c t i v i t i e s f o r t h e a l k y l a t i o n o f e t h y l b e n z e n e w i t h e t h a n o l t h a n t h e p a r e n t z e o l i t e s . The m o d i f i c a t i o n s r e s u l t e d i n a r e d u c t i o n o f t h e e f f e c t i v e p o r e dimension, a r e d u c t i o n i n a c i d s t r e n g t h , and an i n a c t i v a t i o n o f t h e e x t e r n a l surfaces. The i n a c t i v a t i o n o f t h e e x t e r n a l s u r f a c e s improved t h e p a r a - s e l e c t i v i t y t o some e x t e n t . The a c i d s t r e n g t h d e t e r m i n e d b y NH3-TPD t e c h n i q u e was m o r e c l o s e l y r e l a t e d t o t h e p a r a - s e l e c t i v i t y t h a n t o t h e e f f e c t i v e p o r e dimension d e t e r m i n e d by o-xylene a d s o r p t i o n experiments. It i s found t h a t t h e improvement i n p a r a - s e l e c t i v i t y by t h e m o d i f i c a t i o n s i s p r i m a r i l y due t o r e d u c i n g t h e s t r o n g a c i d s i t e s on w h i c h t h e i s o m e r i z a t i o n of p-diethylbenzene as a p r i m a r y p r o d u c t proceeds p r e f e r e n t i a l l y .
INTRODUCTION It i s w i d e l y known t h a t ZSM-5 z e o l i t e s m o d i f i e d w i t h o x i d e o f Mg r e f s .
P ( r e f s . 2,3).
3).
o r B ( r e f s . 2.3) e x h i b i t a h i g h p a r a - s e l e c t i v i t :
a l k y l a t i o n o f alkylbenzenes.
Kaeding e t a l .
have proposed t h a t t h e h i g h para-
s e l e c t i v i t y o f m o d i f i e d ZSM-5 z e o l i t e s i s due t o "product s e l e c t i v i t y " , t h e i n t r a c r y s t a l l i n e d i f f u s i v i t y o f para-isomer t h e o t h e r t w o i s o m e r s ( r e f . 2).
1-
for the
namely
i s much h i g h e r t h a n t h a t o f
On t h e o t h e r hand, we h a v e r e p o r t e d t h a t i n
t h e a l k y l a t i o n o f t o l u e n e w i t h methanol and o f ethylbenzene w i t h e t h a n o l on
HZSM-5,
o n l y p a r a - i s o m e r i s a p r i m a r y p r o d u c t and t h a t t h e i m p r o v e m e n t i n
para-selectivity
by t h e m o d i f i c a t i o n w i t h o x i d e s i s due t o t h e r e d u c t i o n o f
s t r o n g a c i d s i t e s which a c c e l e r a t e t h e i s o m e r i z a t i o n o f para-isomer (refs. 1,3).
Moreover, P a p a r a t t o e t a l . have suggested t h a t i n t h e e t h y l a t i o n o f
t o l u e n e on HZSM-5 and HZSM-11, z e o l i t e channels,
para-isomer
i s formed s e l e c t i v e l y i n s i d e t h e
w h i l e t h e i s o m e r i z a t i o n proceeds o n l y on t h e e x t e r n a l
s u r f a c e s ( r e f . 4). I n t h i s paper,
we aim t o c l a r i f y t h e reason why m o d i f i e d p e n t a s i l z e o l i t e s
e x h i b i t the high para-selectivity. w i t h e t h a n o l as a model r e a c t i o n , t o provide c l e a r e r results.
We choose t h e a l k y l a t i o n o f e t h y l b e n z e n e because t h e l a r g e r a l k y l groups a r e expected
12
EXPERIMENT Catalysts
ZSM-5 (Si/A1=96) w i t h a s m a l l c r y s t a l s i z e (30-60 nm), 50) w e r e p r e p a r e d b y p u b l i s h e d p r o c e d u r e s ( r e f s . 1,5).
and ZSM-11 ( S i / A l = These z e o l i t e s c a l -
c i n e d a t 823 K were t r a n s f o r m e d i n t o H-form by s e v e r a l exchanges w i t h l M HC1. The HZSM-5 z e o l i t e s m o d i f i e d w i t h v a r i o u s amounts o f boron,
phosphorus,
and
magnesium o x i d e s were prepared by i m p r e g n a t i o n o f HZSM-5 w i t h aqueous s o l u t i o n s o f b o r i c a c i d , p h o s p h o r i c a c i d , and m a g n e s i u m a c e t a t e , r e s p e c t i v e l y , f o l l o w e d b y d r y i n g a t 373 K and c a l c i n e d a t 823 K f o r 1 8 h i n an a i r s t r e a m . The steaming o f HZSM-5 was c a r r i e d o u t a t 1073 o r 1223 K f o r 1 h under atmosp h e r i c p r e s s u r e o f w a t e r vapor.
HZSM-5
The c o k e d HZSM-5 was p r e p a r e d b y t r e a t i n g
w i t h a t m o s p h e r i c m e t h a n o l v a p o r a t 973 K f o r 1 h.
(Si/A1=9.8)
was k i n d l y s u p p l i e d by Tosoh Co.
Ferrierite
and was t r a n s f o r m e d i n t o H-form
by c a l c i n i n g i t a t 823 K a f t e r s e v e r a l exchanges w i t h 1 M NH4C1. s u r f a c e area o f H - f e r r i e r i t e
Apparatus
and
The s p e c i f i c
i s 390 m 2 /g.
procedure
The apparatus and procedure f o r t h e a l k y l a t i o n o f ethylbenzene w i t h e t h a n o l are described i n t h e l i t e r a t u r e (ref.
3).
The a l k y l a t i o n on s e l e c t i v e l y
poisoned z e o l i t e s was a l s o c a r r i e d o u t i n t h e presence o f 2.4-dimethylquinol i n e (2,4-DMQ)
w h i c h s e l e c t i v e l y p o i s o n e d t h e a c i d s i t e s on t h e e x t e r n a l
surfaces (ref.
6).
The c r a c k i n g o f 1,3,5-triisopropylbenzene
(1,3.5-TIPB)
at
673 K was p e r f o r m e d i n t h e p r e s e n c e o f 2.4-DMQ t o c h e c k t h e a c t i v i t y o f t h e a c i d s i t e s on t h e e x t e r n a l surfaces.
A l l t h e c a t a l y t i c r e a c t i o n s were c a r r i e d
o u t w i t h a continuous f l o w system a t 673 K under atmospheric pressure. The measurement o f temperature-programmed d e s o r p t i o n f o r ammonia (NH3-TPD) was c a r r i e d o u t as f o l l o w s .
A f t e r t h e c a t a l y s t was evacuated a t 823 K f o r 1 h
and t h e n exposed t o 150 T o r r o f ammonia gas a t 423 K f o r 30 min,
the catalyst
was e v a c u a t e d a t 423 K f o r 1 h and t h e n c o o l e d t o room t e m p e r a t u r e . t e m p e r a t u r e was b r o u g h t t o 823 K a t a c o n s t a n t r a t e o f 1 0 K m i n - ’ vacuum.
The under
The r e l a t i v e amount o f ammonia desorbed f r o m t h e c a t a l y s t was d e t e r -
mined by mass spectroscopy. G r a v i m e t r i c measurements o f 0- o r p-xylene a d s o r p t i o n were p e r f o r m e d on a h i g h l y s e n s i t i v e thermal microbalance.
A f t e r z e o l i t e s a m p l e s (0.1 g) w e r e
e v a c u a t e d a t 823 K f o r 2 h u n d e r a p r e s s u r e o f l e s s t h a n
Torr.
the
measurement was p e r f o r m e d a t 393 K and a t an 0- o r p - x y l e n e p r e s s u r e o f 3.6 Torr. RESULTS AND DISCUSSION I n t h e a l k y l a t i o n o f ethylbenzene w i t h ethanol,
diethylbenzenes,
benzene,
73
ethylmethylbenzenes and xylenes were observed as a r o m a t i c products. p r o d u c t s o f t h i s a l k y l a t i o n were p- and m-diethylbenzenes.
The main
The a c t i v i t y and
t h e s e l e c t i v i t y o f t h e HZSM-5 o r HZSM-11 z e o l i t e d i d n o t change w i t h process t i m e f o r 3 h.
I n t h e case o f H - f e r r i e r i t e ,
s i g n i f i c a n t l y w i t h process time.
however,
low.
decreased
The r e s u l t s o f t h e a l k y l a t i o n on HZSM-5,
HZSM-11 and H - f e r r i e r i t e a r e shown i n Table 1. t i o n o f p-isomer
the a c t i v i t y ,
The p a r a - s e l e c t i v i t i e s ( f r a c -
i n t h e d i e t h y l b e n z e n e produced) o f t h e s e t h r e e z e o l i t e s were
The a l k y l a t i o n a c t i v i t y o f H - f e r r i e r i t e was e x t r e m e l y l o w compared w i t h
t h e o t h e r z e o l i t e s , p r o b a b l y because t h e e f f e c t i v e p o r e d i m e n s i o n o f H - f e r r i e r i t e was s m a l l e r t h a n those o f t h e o t h e r z e o l i t e s .
The l o w a c t i v i t y and t h e
l o w p a r a - s e l e c t i v i t y o f H - f e r r i e r i t e s u g g e s t t h a t t h e a1 k y l a t i o n p r o c e e d s m a i n l y on t h e e x t e r n a l surfaces. I n o r d e r t o c l a r i f y w h i c h were t h e p r i m a r y p r o d u c t s i n t h e a l k y l a t i o n o f ethylbenzene w i t h e t h a n o l on HZSM-11 o r H - f e r r i e r i t e , o f each mined.
t h e change i n f r a c t i o n
isomer i n t h e produced d i e t h y l b e n z e n e w i t h d e c r e a s i n g W/F was d e t e r The r e s u l t s on HZSM-11 a t 673 K i n t h e d i e t h y l b e n z e n e y i e l d range o f
24 t o a b o u t 4 % a r e shown i n F i g . l ( a ) .
I n t h e c a s e o f HZSM-11, t h e p r i m a r y
p r o d u c t i s suggested t o be e x c l u s i v e l y t h e p-isomer. t o t h a t o f t h e a l k y l a t i o n on HZSM-5 ( r e f . 3).
This r e s u l t i s s i m i l a r
Therefore, t h e i s o m e r i z a t i o n o f
p-diethylbenzene formed as a p r i m a r y p r o d u c t must be suppressed t o i m p r o v e t h e p a r a - s e l e c t i v i t y o f HZSM-11 a s w e l l as HZSM-5. catalysts, ties,
whose p a r a - s e l e c t i v i t y i s high,
A c t u a l l y , m o d i f i e d HZSM-5
e x h i b i t low isomerization a c t i v i -
and t h e o r d e r o f t h e p a r a - s e l e c t i v i t i e s o f a s e r i e s o f HZSM-5 z e o l i t e s
modified w i t h oxides i s exactly i n reverse order t o t h a t o f t h e i s o m e r i z a t i o n a c t i v i t i e s (ref.
T a b l e 1.
3).
I n t h e case o f H - f e r r i e r i t e ,
T h e a l k y l a t i o n o n HZSM-5,
C a t a 1y s t
HZSM-11
t h e f r a c t i o n s o f m- and o-
and H - f e r r i e r i t e a .
HZSM-5
HZSM-11
43.5
48.4
3.2
Yield /Z Benzene Xyl ene Ethylmethylbenzene Oiethylbenzene
13.7 1.2 1.6 26.3
19.3 2.1 3.2 20.6
1.1 0.1 0.2 1.5
F r a c t i o n /% o-Diethylbenzene m-Diethylbenzene p-Diethylbenzene
1.7 59.1 39.2
3.3 63.5 33.2
19.8 32.7 47.5
Ethylbenzene Conversion /%
-
a R e a c t i o n t e m p e r a t u r e ; 673 K. a p r o c e s s t i m e o f 0 . 5 h.
W/F;
H-ferrieri t e
7.14 g h mol-l.
Data a t
d i e t h y l b e n z e n e s s l i g h t l y decreased, b u t t h a t o f p - d i e t h y l b e n z e n e s l i g h t l y i n c r e a s e d w i t h d e c r e a s i n g W/F as s h o w n i n F i g . l ( b ) .
From t h e s e f a c t s , t h e
p r i m a r y p r o d u c t on H - f e r r i e r i t e c o u l d n o t be s p e c i f i e d . I n o r d e r t o c l a r i f y t h e e f f e c t on p a r a - s e l e c t i v i t y o f t h e a c i d s i t e s on t h e e x t e r n a l surfaces,
t h e a l k y l a t i o n i n t h e presence o f 2,4-0MQ
enter the pores (ref. i n Table 2.
6) was c a r r i e d o u t a t 673 K.
which could n o t
The r e s u l t s a r e summarized
The c o m p l e t e p o i s o n i n g o f t h e a c i d s i t e s on t h e e x t e r n a l s u r f a c e s
was c o n f i r m e d by t h e i n a c t i v a t i o n o f t h e c a t a l y s t s f o r t h e c r a c k i n g o f 1,3,5TIPB w h i c h was a s u i t a b l e t e s t m o l e c u l e f o r d e t e r m i n i n g t h e a c t i v i t y o f t h e external surfaces (ref.
7).
Moreover,
t h e m o d i f i e d HZSM-5 w h i c h e x h i b i t e d a
100
100\
(a) HZSM-11
\
(b)
\
64
ap
\
\
c
0
;.
ti-ferrierite
50
-
c
0
'$ 50' U
U
m L
L rz1 U
U D
# I
I
m
r
\ r
/ 0
F i g . 1. Change i n f r a c t i o n o f 0- (m), m - (A) o r p - i s o m e r (0) i n d i e t h y l b e n z e n e produced w i t h W/F i n a l k y l a t i o n o f e t h y l b e n z e n e w i t h e t h a n o l on HZSM-5 (a) and H - f e r r i e r i t e (b) a t 673 K.
T a b l e 2.
Selective poisoning of
Catalyst
Oiethylbenzene Yield
a c i d s i t e s on e x t e r n a l s u r f a c e s a . F r a c t io n b / %
/x
0-
113,5-TIPB Conversion
1%
m-
P-
51.1 0 0
HZSM-5 HZSM-5' B(l0)HZSM-sd
26.3 16.9 14.2
1.7 0 0
59.1 42.8
0
39.2 57.2 100.0
HZSM-11 HZSM-I 1
20.6 15.3
3.3 0
63.5 58.9
33.2 41.1
59.3 0
1.5 0.4
19.8
32.7 0
47.5 100.0
11.8 0
H-ferrierite H-ferrieritec
0
a R e a c t i o n t e m p e r a t u r e ; 6 7 3 K. W/F; 7.14 g h m o l - l . b F r a c t i o n o f each isomer i n d i e t h y l b e n z e n e produced, ' P o i s o n e d w i t h 2,4-0MQ. d S e e T a b l e 3.
75 p e r f e c t p a r a - s e l e c t i v i t y was c o m p l e t e l y i n a c t i v e f o r 1,3,5-TIPB
cracking.
The
p a r a - s e l e c t i v i t i e s o f HZSM-5 a n d HZSM-11 w e r e t o some e x t e n t i m p r o v e d b y s e l e c t i v e poisoning.
However,
t h e e f f e c t o f improvement i n p a r a - s e l e c t i v i t y
b y p o i s o n i n g t h e a c i d s i t e s on e x t e r n a l s u r f a c e s i s much s m a l l e r t h a n t h a t b y m o d i f y i n g z e o l i t e s w i t h oxides,
indicating t h a t the r o l e o f oxides i s not only
the i n a c t i v a t i o n o f the external surfaces b u t also the suppression o f the i s o m e r i z a t i o n w h i c h p r o c e e d s e v e n i n s i d e t h e pores. case o f H - f e r r i e r i t e ,
On t h e o t h e r hand,
i n the
a p e r f e c t p a r a - s e l e c t i v i t y was a c h i e v e d b y s e l e c t i v e
poisoning o f the external surfaces.
This f a c t indicates t h a t the narrow
c h a n n e l s o f H - f e r r i e r i t e c a n accommodate o n l y p - d i e t h y l b e n z e n e . The p a r a - s e l e c t i v i t y ( f r a c t i o n o f p - i s o m e r changed w i t h W/F.
i n t h e diethylbenzene produced)
I n o r d e r t o compare t h e p a r a - s e l e c t i v i t i e s
o f the pentasil
z e o l i t e s v a r i o u s l y m o d i f i e d , t h e p a r a - s e l e c t i v i t i e s a t an a l m o s t c o n s t a n t y i e l d o f d i e t h y l b e n z e n e (15-20 activity.
X),
t h a t is,
a t an a l m o s t c o n s t a n t a l k y l a t i o n
were d e t e r m i n e d and a r e s u m m a r i z e d i n T a b l e 3.
a l k y l a t i o n a c t i v i t y was a c h i e v e d b y a d j u s t i n g W/F. i m p r o v e d by e v e r y m o d i f i c a t i o n made here.
The a l m o s t c o n s t a n t
The p a r a - s e l e c t i v i t y
Especially,
was
the pentasil zeolites
m o d i f i e d w i t h a l o t o f boron o x i d e e x h i b i t e d an e x t r e m e l y h i g h p a r a - s e l e c t i v i S i l i c a ZSM-5 m o d i f i e d w i t h b o r o n o x i d e d i d n o t e x h i b i t a n y a l k y l a t i o n
ty.
a c t i v i t y under o u r r e a c t i o n c o n d i t i o n s . boron oxide i t s e l f i s ignored.
Therefore,
the alkylation a c t i v i t y o f
I n order t o c l a r i f y t h e reason
modified pentasil zeolites exhibited the high para-selectivity,
why t h e
the adsorption
m e a s u r e m e n t s o f p - a n d o - x y l e n e s a n d NH3-TPD w e r e c a r r i e d o u t .
I n t h e p-
x y l e n e a d s o r p t i o n e x p e r i m e n t s , e v e r y z e o l i t e was s a t u r a t e d w i t h p - x y l e n e w i t h i n 30 m i n .
T h e a m o u n t o f p - x y l e n e a d s o r b e d a t i n f i n i t e t i m e , A(p-X),,
w h i c h corresponds t o a k i n d o f p o r e volume,
i s s h o w n i n T a b l e 3.
The p o r e
volumes o f t h e p e n t a s i l z e o l i t e s were remarkably reduced b y t h e m o d i f i c a t i o n
w i t h o x i d e s , w h i l e s t e a m i n g o r c o k i n g had o n l y l i t t l e e f f e c t . The r e s u l t s o f t h e a d s o r p t i o n measurements o f o - x y l e n e and NH3-TPD p r o v i d e i n f o r m a t i o n o n t h e e f f e c t i v e p o r e d i m e n s i o n o f z e o l i t e s ( r e f . 8) a n d o n t h e a c i d i c properties o f t h e c a t a l y s t s (refs. xylene adsorption velocity,
3,9),
A(o-X)180/A(p-X),
respectively.
The r e l a t i v e o-
was o b t a i n e d f r o m t h e r a t i o o f
t h e amount o f o-xylene adsorbed a t 180 m i n t o t h a t o f p-xylene adsorbed a t i n f i n i t e time.
The r e l a t i v e o - x y l e n e a d s o r p t i o n v e l o c i t y d e c r e a s e d t h r o u g h
the modifications,
i n d i c a t i n g t h a t t h e e f f e c t i v e pore dimension o f t h e penta-
s i l z e o l i t e s was r e d u c e d . a l s o summarized i n T a b l e 3.
The r e l a t i v e o - x y l e n e a d s o r p t i o n v e l o c i t i e s a r e The r e l a t i o n s h i p b e t w e e n t h e p a r a - s e l e c t i v i t i e s
and t h e r e l a t i v e o - x y l e n e a d s o r p t i o n v e l o c i t i e s i s shown i n Fig. 2.
The p a r a -
s e l e c t i v i t i e s are roughly r e l a t e d t o t h e r e l a t i v e o-xylene adsorption v e l o c i ties.
E s p e c i a l l y i n t h e c a s e o f HZSM-5 m o d i f i e d w i t h o x i d e s , a c l o s e r e l a -
T a b l e 3.
E f f e c t o f m o d i f i c a t i o n s on p a r a - s e l e c t i v i t y , Alkylationa
No.
NH3-TPDb
ParaSelectivity
1%
Temperature
/K
~~
2 3 4 5 6 7 8 9 10 11 12 13
m
Adsorption
Catalyst D i e t h y l benzene Y i e l d 1%
1
4
a c i d s t r e n g t h and a d s o r p t i o n p r o p e r t i e s .
HZSM-5 P( 1 )HZSM-5f B(3)HZSM-5; Mg( 18)HZSM-5 P(5)HZSM-5f B(9)HZSM-5; B ( 10)HZSM-5 HZSM-11 B ( ~ O ) H Z S M - i~ f B ( ~ O ) H Z S M -i~ f Stm. (1073)g Stm.(1223)h. C o k e d HZSM-5'
A(P-X),~ /mg g - l
A(o-X)1 /A(p-fYmd
t
e
9i?n
~~
19.8 20.1 18.8 19.9 16.3 18.3 17.8
43.2 49.3 55.3 72.4 89.8 97.1 100.0
563 538 52 3 498 48 3 483 478
118.5 91.4 91.3 74.4 70.0 67. a 30.2
0.639 0.430 0.392 0.247 0.233 0.147 0.007
2.1 12.1 17.4
19.4 18.4 16.7
40.1 89.2 98.7
568 463 473
125.5 45.1 21.9
0.719 0.390 0.046
1.4
15.7 15.1 16.2
68.9 84.5 64.2
493 488 508
100.0 99.2 105.6
0.528 0.420 0.449
4.5 8.5 3.1
a R e a c t i o n t e m p e r a t u r e : 6 7 3 K. W/F: 3.57 g h m o l - ' f o r HZSM-5. P ( l ) H Z S M - 5 . B(3)HZSM-5. Mg(18)HZSM5, a n d H S M 1 1 ; 7.14 g h m o l - ' f o r B(9)HZSM-5. B(lO)HZSMill, B(20)HZSM-ll. a n d C o k e d HZSM-5; 14.3 Parag h m o l - ' for B ( l O ) H Z S M - 5 , a n d S t m . ( 1 0 7 3 ) ; 21.4 g h m o l f o r P(5)HZSM-5. a n d Stm.(1223). s e l e c t i v i t y : f r a c t i o n o f p-isomer i n t h e d i e t h y l b e n z e n e produced. bPeak p o s i t i o n i n NH3-TPD p r o f i l e . CAmount o f p - x y l e n e a d s o r b e d a t i n f i n i t e t i m e . d R e l a t i v e o - x y l e n e a d s o r p t i o n v e l o c i t y ; ( A m o u n t o f o - x y l e n e a d s o r b e d a t 1 8 0 m i n ) / ( A m o u n t o f pxylene adsorbed a t i n f i n i t e time). e tTime t o r e a c h 30 % o f amount o f o - x y l e n e a d s o r b e d a t i n f i n i t e t i m e . Number i n p a r e n t h e s e s i n d i c a t e s t h e a m o u n t o f P. B o r Mg a d d e d f M o d i f i e d w i t h o x i d e o f P, B o r Mg. i n weight percent. g S t e a m e d HZSM-5 a t 1 0 7 3 K. h S t e a m e d HZSM-5 a t 1 2 2 3 K. iHZSM-5 c o k e d w i t h m e t h a n o l a t 9 7 3 K .
77 t i o n s h i p i s observed.
I n t h e c a s e o f t h e s t e a m e d o r c o k e d HZSM-5.
however,
t h e p a r a - s e l e c t i v i t i e s w e r e r e l a t i v e l y h i g h c o m p a r e d w i t h t h e r e l a t i v e oxylene a d s o r p t i o n v e l o c i t i e s . Olson and Haag have r e p o r t e d t h a t t h e p a r a - s e l e c t i v i t i e s o f m o d i f i e d ZSM-5 z e o l i t e s f o r t h e d i s p r o p o r t i o n a t i o n o f t o l u e n e have a c l o s e r e l a t i o n s h i p t o t i m e s t o r e a c h 30 % o f a m o u n t o f o - x y l e n e a d s o r b e d a t i n f i n i t e t i m e , (ref. ty.
8).
t0.3
The t O si3 s a d i r e c t measure o f t h e c r i t i c a l mass t r a n s f e r p r o p e r -
Therefore,
t h e y have proposed t h a t t h e p a r a - s e l e c t i v i t y f o r t h e d i s p r o -
0
1 .o
0.5
R e l a t i v e o-xylene adsorption v e l o c i t y Fig. 2. R e l a t i o n s h i p between t h e p a r a - s e l e c t i v i t y and t h e r e l a t i v e o-xylene a d s o r p t i o n v e l o c i t y . The numbers a r e correspond t o t h o s e o f t h e c a t a l y s t s i n Table 3.
A12
02
0 3
3 Fig. 3. R e l a t i o n s h i p between t h e p a r a - s e l e c t i v i t y and t o 3. The numbers a r e c o r r e s p o n d t o t h o s e of t h e ' c a t a l y s t s i n Table 3.
p o r t i o n a t i o n o f t o l u e n e on m o d i f i e d ZSM-5 z e o l i t e s i s due t o " p r o d u c t s e l e c t i vity".
I n t h i s work,
shown i n T a b l e 3.
t0.3 v a l u e s f o r s e v e r a l z e o l i t e s were d e t e r m i n e d and a r e F o r t h e o t h e r samples,
i t required too long a time t o
o b t a i n t h e a m o u n t s o f o - x y l e n e a d s o r b e d a t i n f i n i t e t i m e , h e n c e tOm3 values c o u l d n o t be determined.
The r e l a t i o n s h i p between t h e p a r a - s e l e c t i v i t i e s
for
t h e a l k y l a t i o n o f ethylbenzene w i t h e t h a n o l and t0.3 values i s shown i n Fig.
I I
I
,--. .
\
'.
Temperature /K
Temperature /K F i g . 4.
323
423
P r o f i l e s o f NH3-TPD.
523 623 723 Temperature /K
823
79 3.
The r e l a t i o n s h i p i s n o t c l o s e .
Accordingly,
i t i s d o u b t f u l t h a t t h e para-
s e l e c t i v i t y f o r t h e a l k y l a t i o n on m o d i f i e d p e n t a s i l z e o l i t e s i s d i r e c t l y caused b y " p r o d u c t
selectivity".
T h e p r o f i l e o f NH3-TPD c o r r e s p o n d s t o t h e s t r e n g t h d i s t r i b u t i o n o f a c i d sites.
F i g u r e 4 shows t h e p r o f i l e s o f NH3-TPD f o r p e n t a s i l z e o l i t e s v a r i o u s l y
modified. peak,
The HZSM-5 and HZSM-11 z e o l i t e s e a c h e x h i b i t e d o n l y one s y m m e t r i c a l
a t 563 and 568 K,
respectively.
Every m o d i f i e d z e o l i t e a l s o e x h i b i t e d
o n l y o n e s y m m e t r i c a l p e a k e x c e p t Mg( 18)HZSM-5 and B(1O)HZSM-11. b i t e d a peak w i t h a s h o u l d e r each. a t u r e s by t h e m o d i f i c a t i o n s ,
The peak p o s i t i o n s h i f t e d t o l o w e r t e m p e r -
as shown i n Fig. 4.
a c i d s t r e n g t h i s weakened b y t h e m o d i f i c a t i o n s .
This fact indicates t h a t the The peak p o s i t i o n s i n NH3-TPD
p r o f i l e s f o r t h e m o d i f i e d z e o l i t e s a r e s u m m a r i z e d i n T a b l e 3. r e l a t i o n s h i p between t h e p a r a - s e l e c t i v i t i e s
B(1O)HZSM-ll
had a s h o u l d e r ,
I n Fig.
5, t h e
and t h e peak p o s i t i o n s i n NH3-TPD
p r o f i l e s f o r t h e v a r i o u s l y m o d i f i e d p e n t a s i l z e o l i t e s i s shown. close r e l a t i o n s h i p i s observed except o f strong acid sites.
They e x h i -
f o r B(1O)HZSM-ll.
An e x t r e m e l y The p e a k
for
i n d i c a t i n g t h a t t h i s z e o l i t e had a s m a l l number
Consequently, t h e p a r a - s e l e c t i v i t y i s r e l a t e d more
closely t o the acid strength than t o the e f f e c t i v e pore dimension o r the c r i t i c a l mass t r a n s f e r p r o p e r t y .
As mentioned before,
t h e p r i m a r y p r o d u c t i n t h e a l k y l a t i o n o n HZSM-5 and
HZSM-11 i s p - d i e t h y l b e n z e n e .
and t h e i s o m e r i z a t i o n o f p - d i e t h y l b e n z e n e
suppressed t o improve t h e p a r a - s e l e c t i v i t y .
m u s t be
I n t h e a l k y l a t i o n o f ethylbenzene
w i t h e t h a n o l a s w e l l as t h e a l k y l a t i o n o f t o l u e n e w i t h m e t h a n o l ,
Peak p o s i t i o n i n NH3-TPD p r o f i l e /K Fig. 5. R e l a t i o n s h i p b e t w e e n t h e p a r a - s e l e c t i v i t y and t h e peak p o s i t i o n i n NH3-TPD p r o f i l e . The numbers c o r r e s p o n d t o t h o s e o f t h e c a t a l y s t s i n T a b l e 3.
the a c t i v i t y
o f m o d i f i e d p e n t a s i l z e o l i t e s i s h i g h enough t o c o n v e r t c o m p l e t e l y t h e a l k y l a t i n g agent under c o n v e n t i o n a l r e a c t i o n c o n d i t T o n s .
The m o d i f i c a t i o n s made
here r e s u l t i n a r e d u c t i o n i n a c i d s t r e n g t h o f p e n t a s i l z e o l i t e s and,
conse-
q u e n t l y , i n t h e s u p p r e s s i o n o f t h e i s o m e r i z a t i o n . p r o b a b l y because i n t h e n a r r o w p o r e s o f p e n t a s i l z e o l i t e s t h e i s o m e r i z a t i o n i s s u p p r e s s e d t o some e x t e n t through " r e s t r i c t e d t r a n s i t i o n - s t a t e s e l e c t 1 v i t y " and r e q u i r e s s t r o n g acid sites.
compared w i t h t h e a l k y l a t i o n .
T h i s i s t h e reason why t h e m o d i f i e d
pentasil zeolites exhibit high para-selectivity
f o r the alkylation.
REFERENCES T. Yashima. Y. S a k a a u c h i and S. Namba, Stud. S u r f . S c i . Catal.. 7 ( 1 9 8 1 ) 739-751. ' W.W. Kaedinq. C. Chu, L.B. Young, B. W e i n s t e i n and S.A. B u t t e r , J. C a t a l . , 67 (1981) 159-174. J.-H. K i m , S. Namba and T. Yashima, B u l l . Chem. SOC. Jpn., 61 ( 1 9 8 8 ) 1 0 5 1 1055. G. P a p a r a t t o . E. M o r e t t i , G. L e o f a n t i and F. G a t t i , J. C a t a l . . 1 0 5 ( 1 9 8 7 ) 227-232. G.T. K o k o t a i l o , P. Chu, S.L. L a w t o n and W.M. M e i e r , N a t u r e , 275 ( 1 9 7 8 ) 1 1 9 120. S. Namba, S. N a k a n i s h l a n d T. Yashima, J. C a t a l . , 8 8 ( 1 9 8 4 ) 505-512. S. Namba, A. Inaka and T. Yashima. Z e o l i t e s , 6 (1986) 107-110. D.H. Olson and W.O. Haag. i n T.E. Whytes ( E d i t o r ) , C a t a l y t i c M a t e r i a l s , ACS Symposium S e r i e s 248, San F r a n c i s c o , 1983, ACS. W a s h i n g t o n , D. C., 1984, pp. 275-307. U. K u r s c h n e r , B. P a r l i t z . E. S c h r e i e r . G. O h l m a n n a n d J. V o l t e r , A p p l . C a t a l . , 3 0 ( 1 9 8 7 ) 159-166.
H.G. Karge, J. Weitkamp (Editors), Zeolites 0s Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HYOROGEN SPILLOVER I N THE CONVERSION OF n-ALKANES ON ZEOLITES
K.-H.
STEINBERG, U. MROCZEK and F . ROESSNER
Karl-Marx-Universitat L e i p z i g , Sektion Chemie, T a l s t r . 35, L e i p z i g , 7010 ( G . 0. R . )
ABSTRACT The hydroconversion o f n-hexane on H - e r i o n i t e takes p l a c e i n a batch react o r a t 493 K with a h i g h y i e l d o f C -, n-C4-,,and n-C - p a r a f f i n s b u t o n l y t r a c 5 es o f methane and ethane. I f P t i s aresent, with l e s s conversion t h e s e l e c t i v i t y changes towards more formation o f methane, ethane and iso-hexanes. To cause these changes i n a c t i v i t y and s e l e c t i v i t y i t i s s u f f i c i e n t t o admix some g r a i n s o f a Pt/alumina c a t a l y s t (about 10 mg/g H - e r i o n i t e ) mechanically i n the H-erion i t e bed. Poisoning o f the P t by CO leads t o r e c r e a t i o n o f t h e o r i g i n a l a c t i v i t y and s e l e c t i v i t y o f the H - e r i o n i t e . The hydroconversion of n-butane i n a p l u g f l o w r e a c t o r a t 633 K leads t o hydrocracking and c r a c k i n g products ( C - t o C - p a r a f f i n s ) o n l y . I n the absence o f P t iso-butane i s formed and fewer c r a c k i n g atoducts are observed. T f P t i s present as some g r a i n s o f a Pt/alumina c a t a l y s t or supported up t o only 0.04 % on the H-mordenite, the highest a c t i v i t y and s e l e c t i v i t y f o r the i s o m e r i z a t i o n are observed. For such a change i n s e l e c t i v i t y i t i s s u f f i c i e n t t o p u t a few g r a i n s o f the Pt/alumina c a t a l y s t as a t h i n l a y e r on top, a t the bottom o r , b e t t e r , i n the c e n t r e o f the H-mordenite bed. The H-mordenite remained w h i t e (no coking) a f t e r a r u n o f 1 . 5 h over a d i s t a n c e o f some m i l l i m e t r e s from t h e Pt/alumina l a y e r with, or even against, the f l o w d i r e c t i o n . The a c t i v e hydrogen generated a t the P t and s p i l l i n g over t o t h e a c i d i c cent r e s o f the H-zeolites regulates t h e r e a c t i o n pathway there: I f t h e supply o f s p i l l o v e r hydrogen i s hjgh, hydrocracking i s favoured; i f i t i s low, the hydroi s o m e r i r a t i o n and the o l i g o m e r i z a t i o n o f o l e f i n i c intermediates are favoured; i f i t i s zero (no P t being present, or N i n s t e a d o f H or P t poisoned), c r a c k i n g and coking dominate. I n b o t h i n z e s t i g a t e d r e a e i i o n s the hydrogen s p i l l o v e r i s a c t i v e over a distance o f about 10 mm.
TNTROOUCTTON B i f u n c t i o n a l z e o l i t e s a r e a c t i v e and s e l e c t i v e c a t a l y s t s i n t h e hydroconversion of n-paraff i n s . P t / H - e r i o n i t e i s the a c t i v e and shape-selective component o f the commercial shape-selective hydrocracking c a t a l y s t LEUNA-Kt. 9562 (ref.
1) f o r gasoline upgrading as w e l l as the backbone o f t h e SELECTOFORMING
process ( r e f . 2). Pt/H-mordenite i s used and claimed t o be a s e l e c t i v e l i g h t p a r a f f i n e i s o m e r i z a t i o n c a t a l y s t ( r e f s . 3 - 8 ) . Usually noble metals h i g h l y dispersed on the z e o l i t e s r e g u l a t e the hydrogenation/dehydrogenation a c t i v i t y , and the a c i d i c centres o f the z e o l i t e s - u s u a l l y protons - a r e r e s p o n s i b l e f o r the C-C-bond rearranging r e a c t i o n s .
82
I n most cases the m e t a l c r y s t a l l i t e s are l o c a t e d o u t s i d e the z e o l i t e c r y s t a l s , e s p e c i a l l y i n narrow-pore z e o l i t e s l i k e e r i o n i t e and mordenite. Theref o r e , even i n the case o f very h i g h metal d i s p e r s i o n a t l e a s t a d i s t a n c e o f a few z e o l i t e u n i t c e l l diameters remains between the metal c r y s t a l l i t e s and t h e a c i d i c centres, which has t o be overcome by t h e hydrogen a c t i v a t e d a t t h e rneta1 t o avoid coking a t the a c i d i c c e n t r e s and/or t o p a r t i c i p a t e i n t h e r e a c t i o n pathway there. This distance can be overcome by gas phase d i f f u s i o n
or by
spillover. To gain some i n s i g h t i n t o t h e p a r t i c i p a t i o n and t h e a c t i o n o f hydrogen s p i l l o v e r i n the r e a c t i o n o f hydroconversion of n-alkanes,
t h e shape-selective
n-hexane conversion on H - e r i o n i t e and t h e i s o m e r i z a t i o n o f n-butane on H-mord e n i t e i n t h e presence 1or absence) o f p l a t i n u m and hydrogen have been s t u d i e d .
EXPERIMENT 0.93 NH4-erionite was prepared by repeated i o n exchange o f s y n t h e t i c Na,Ke r i o n i t e ICKB, B i t t e r f e l d , G.D.R.)
with a Si02/A1203 molar r a t i o o f 6.0.
I t was
a c t i v a t e d i n - s i t u a u t o m a t i c a l l y by t h e program: h e a t i n g i n a i r with a f l o w r a t e o f 3 l / h and with 10 K / m i n up t o 723 K, 3 h a t 723 K , c o o l i n g down i n N2-flow t o 423 K, 6 h a t 423 K, h e a t i n g i n H2 with a f l o w r a t e o f 3 l / h with 10 K/min up t o 723 K, reducing 3 h a t 723 K, c o o l i n g t o t h e r e a c t i o n temperature o f 493 K. Usually 1.0 g of dry H - e r i o n i t e , f r a c t i o n 0.2 - 0.31 mm, was i n v e s t i g a -
t e d i n a q u a r t z glass r e a c t o r w i t h gas r e c i r c u l a t i o n i n t h e conversion o f molar r a t i o o f 1 : 50 a t a t o t a l pressure o f 2 0 . 1 MPa. I n some experiments P t was present, mechanically admixed as 0.5%Pt/)--
n-hexane with an n-hexane/H
A1203 or supported on the H - e r i o n i t e 10.5 % P t by impregnation w i t h [Pt1NH3)4]C12 s o l u t i o n ) . 0.99 NH4-mordenite was prepared by repeated i o n exchange o f s y n t h e t i c Namordenite with a Si02/A1203 molar r a t i o o f 10 (CKE, B i t t e r f e l d , G . O . R . ) . Usually 400 mg o f the dry c a t a l y s t ( f r a c t i o n 0.2 - 0.31 mm) was i n v e s t i g a t e d i n a quartz glass p l u g f l o w r e a c t o r o f 6 mm diameter a f t e r a c t i v a t i o n according t o t h e already described program. The molar H2/butane r a t i o was always 5 : 1. The n-butane contained already 1 5 % o f iso-butane. The GHSV was c o n s t a n t l y
80 v/vh. Besides the H-mordenite, mechanical m i x t u r e s o f some 0.5%Pt/A1203 i n the H-mordenite bed, e s p e c i a l l y arranged as "sandwich" l a y e r s , and Pt-supported H-mordenites (impregnated with H2PtC16 s o l u t i o n ) were s t u d i e d . The product comp o s i t i o n was determined by o n - l i n e gas chromatography.
83
RESULTS AND
DISCUSSION
Hydroconversion of n-hexane The conversion o f n-hexane i n t h e presence o f hydrogen on H - e r i o n i t e a t
-, n-C:4-, and n-C 5 - p a r a f 3 f i n s b u t n o t t o methane and ethane (only t r a c e s ) (Fig. l a ) . With i n c r e a s i n g
493 K l e d , t o a l a r g e e x t e n t , t o t h e formation o f C
r e a c t i o n time the formed n-pentane s t a r t e d t o be converted too. T f P t was supported t o 0 . 5 w t . % on the H - e r i o n i t e ,
the c a t a l y s t was remarkably l e s s a c t i v e
and t h e s e l e c t i v i t y was d r a s t i c a l l y s h i f t e d towards the formation o f iso-hexanes, less butane, no n-pentane, b u t more methane and ethane ( F i g . I b ) . Tn b o t h cases the concentration o f o l e f i n s was lower than 0.2 w t . %.T f t h e P t on t h e H - e r i o n i t e was poisoned by the i n t r o d u c t i o n o f about 1 v o l . % CO i n t h e r e c i r c u l a t i o n system, the a c t i v i t y was increased and the s e l e c t i v i t y o f the H-erion i t e f r e e of P t was observed: no iso-hexanes, b u t n-pentane, n-butane, and propane were the main products ( F i g . 1c).
b
0
n- C6
n- C6
,
4
I
8
4
8
F i g . 1. Hydroconversion o f n-hexane a t 493 K, r e a c t o r with gas c i r c u l a t i o n , dependence of product composition on time. (a) 1 g H - e t i o n i t e . (b) 0.5 % P t / H - e r i o n i t e . ( c ) 1 g 0.5 % P t / H - e r i o n i t e , 1 % CO i n t h e r e c i r c u l a t i o n system.
84
Obviously, t h e presence o f a c t i v e P t d i r e c t s the r e a c t i o n towards t y p i c a l hydroconversion products. I n t h e absence o f P t ( o r P t being poisoned) t y p i c a l c r a c k i n g products (according t o t h e C-numberlare formed. Probably, most o f t h e P t i s s i t u a t e d o u t s i d e o f t h e H - e r i o n i t e c r y s t a l s . The 8-membered pore openings o f t h e e r i o n j t e (0.36 x 0.52 nm) w i l l a t l e a s t h i n d e r the d i f f u s i o n o f [Pt(NH,),I2'
c a t i o n s i n t o t h e e r i o n i t e channels. Even i f t h e r e
were some P t i n s i d e t h e e r i o n i t e , due t o t h e low content t h e m a j o r i t y o f t h e e r i o n i t e would remain f r e e o f P t . Therefore, i f t h e hydrogen a c t i v a t e d a t t h e
P t takes p a r t i n the r e a c t i o n a t t h e a c i d i c ( p r o t o n i c ) c e n t r e s o f t h e e r i o n i t e ,
i t must overcome the d i s t a n c e o f a number o f l a t t i c e u n i t s . To prove t h i s idea we t e s t e d mechanical m i x t u r e s o f a 0.5%Pt/A1203 c a t a l y s t w i t h t h e H - e r i o n i t e (Tab. 1). I n these experiments o n l y some m i l l i g r a m s (same g r a i n f r a c t i o n as the H - e r i o n i t e ) were admixed i n t h e H - e r i o n i t e bed. The data i n Tab. 1 show t h a t already f o u r mg o f t h e Pt/A1203 c a t a l y s t i n t h e bed were s u f f i c i e n t t o cause a c a t a l y t i c behaviour s i m i l a r t o t h a t o f 0.5 % P t supported on the H - e r i o n i t e . The less P t supplied, the more s t r o n g l y t h e c r a c k i n g dominated. A l i the r e s u l t s given i n Tab. 1 show t h a t t h e hydrogen a c t i v a t e d a t t h e P t was a b l e t o overcome l a r g e distances (some mrn) between i t s source and t h e p l a c e o f i t s a c t j o n . That c o u l d happen p r i n c i p a l l y by gas phase d i f f u s i o n o r by s p i l l o v e r .
TABLE 1 Hydroconversion o f n-hexane a t 493 K, r e a c t o r with r e c i r c u l a t i o n , dependence o f product composition on t h e amount o f 0.5 % Pt/A1203 c a t a l y s t mechanically d i s t r i b u t e d i n I g H-erionite,
after 7 h run
0.5 % Pt/A1203 amount, mg
4
10
50
n-hexane conversion, % c r a c k i n g t o C1-C5, % iso-hexanes, %
0.5 6.5 2.0
8.7 4.8 3.9
7.2 1.8 5.4
1
To d i s t i n g u i s h between t h e r e b o t h p o s s i b i l i t i e s we c a r r i e d o u t experiments
with d i f f e r e n t arrangements o f t h e Pt/A1203 c a t a l y s t ( F i g . 2 ) . We p u t 10 mg o f the sample on top o r a t the bottom o f t h e H - e r i o n i t e ,
i n a basket hanging j u s t
over t h e H - e r i o n i t e (no s o l i d c o n t a c t ) , and as a l a y e r separated by a q u a r t z glass powder l a y e r . For comparison t h e r e s u l t on 10 mg Pt/A1203 c a t a l y s t alone i s given.
85
Ili _
1 1
!:..I-.*
..,... .....
arrangement
n-hexane conversion, % 0.5%Pt/A1203
......
...... .... ..
8.7
8.7
H-erionite
48.1
52.5
2.6
quartz glass
F i g . 2 . Hydroconversion o f n-hexane a t 493 K , r e a c t o r with r e c i r c u l a t i o n , dependence o f conversion on arrangement o f 10 mg 0.5%Pt/A1 0 c a t a l y s t and 2 3 1 y o f H-erionite, a f t e r 7 h run
The r e s u l t s show t h a t the surface d i f f u s i o n of the a c t i v a t e d hydrogen ( i t s s p i l l o v e r ) must be the reason f o r the s u r p r i s i n g a c t i o n o f P t being present
f a r away from the a c i d i c centres b u t p a r t i c i p a t i n g i n the hydroconversion o f n-hexane there. Otherwise we should observe t h e same r e s u l t s i n cases a t o d i n F i g . 2. Obviously, the hydrogen s p i l l o v e r takes p l a c e i n and against t h e flow d i r e c t i o n on t h e H - e r i o n i t e b u t n o t through the gas phase ( c ) or on t h e (poor i n surface) S i O
glass (d). 2 I n a l l cases described above t h e maximal l e n g t h o f s p i l l o v e r a c t i o n was
about 10 mm (height o f the e r i o n i t e bed). To check t h e a c t i o n r a d i u s o f t h e hydrogen s p i l l o v e r we c a r r i e d o u t experiments according t o F i g . 3. With a s e t o f s p e c i a l quartz glass sample h o l d e r s described t h e r e i n we
were a b l e t o change
the average distance between the 10 mg Pt/A1203 c a t a l y s t l a y e r on t o p and t h e H - e r i o n i t e without changing the H - e r i o n i t e amount. The r e s u l t s show t h a t the conversion degree o f the n-hexane s t a r t s t o increase i f the H - e r i o n i t e bed h e i g h t i s greater than about 20 mm. That means t h e a c t i o n of the hydrogen s p i l l o v e r becomes less and less as the d i s t a n c e i n t h e H - e t i o n i t e becomes g r e a t e r ; then cracking dominates over hydrocracking.
86
y$LP .....
.',..I.
......
I
I
70
20
30
sample holder
I, mm F i g . 3 . Hydroconversion of n-hexane a t 493
K,
reactor with recirculation,
dependence o f conversion on distance 1 o f t h e 0 . 5 % Pt/A120j
catalyst,
1 g H-erioriite, a f t e r 6 h run
Hydroconversion o f n-butane Under the more severe c o n d i t i o n s o f the hydroconversion o f n-butane i n a p l u g f l o w r e a c t o r a t 550 t o 640 K t h e hydrogen s p i l l o v e r leads t o s i m i l a r e f f e c t s . F i g . 4 shows t h e r e s u l t s on 400 mg H-mordenite (a) and a mechanical m i x t u r e o f 40 mg 0.5%Pt/A1203 c a t a l y s t and 400 mg H-mordenite ( b ) a f t e r 0 . 5 h t i m e on stream, On t h e pure H-mordenite l e s s i s o m e r i z a t i o n b u t more c r a c k i n g
t o C3 hydrocarbons take place, When 40 mg Pt/A1203 c a t a l y s t are admixed withi n t h e H-mordenite bed ( t h e same g r a i n f r a c t i o n s have been used), less n-butane i s converted b u t t h e i s o m e r i z a t i o n i s p r e f e r r e d (and coking is suppressed as
w e l l ) . As i n the n-hexane hydroconversion on H - e r i o n i t e , o n l y the presence o f some g r a i n s o f P t (as t h e Pt/A1203 c a t a l y s t ) i s s u f f i c i e n t t o i n f l u e n c e s t r o n g l y t h e a c t i v i t y as w e l l as t h e s e l e c t i v i t y o f the n-butane hydroconversion. Again t h e a c t i v e hydrogen must s p i l l over from t h e P t onto the H-mordenite's a c i d centres, t a k i n g p a r t t h e r e i n t h e r e a c t i o n .
To c c r r o b o r a t e our observations and i n t e r p r e t a t i o n o f t h e n-hexane e x p e r i ments we i n v e s t i g a t e d a s e t o f d i f f e r e n t m i x t u r e s o f Pt/A1203 and H-mordenite ( F i g . 5 ) and d i f f e r e n t arrangements o f b o t h c a t a l y s t s ( F i g . 6 ) i n t h e f l o w r e a c t o r . F i g . 5 shows t h e r e s u l t of d i f f e r e n t amounts o f t h e Pt/A1203 catal y s t i n the H-mordenite bed. A mechanical m i x t u r e of one p a r t o f Pt/A1203
87
b \
80
s 60 I
' 40
n- c4
ISO- c 4
n - C4
2.4
4
ZG
LJ C,'C
Y
-_
550
590
630
of n-butane hydroconversion on temperature, plug flow 400 my H-mordenite. b ) 400 mg H-mordenite + 40 mg 0.5 % Pt/A1203
F i g . 4. Dependence
reactor. a )
Fly. 5. Influence of the 0.5 % Pt/A120j amount (my) mechanically admixed with 400 ng H-mordenite on the n-butane hydroconversion at 633 K
cracking products
iso-butane
selectivity of the isomerization
88
and 10 p a r t s o f H-mordenite e x h i b i t s the h i g h e s t s e l e c t i v i t y f o r the isomeriz a t i o n a t r a t h e r h i g h t o t a l a c t i v i t y . Less P t i n t h e bed favours c r a c k i n g (and coking, see below); more P t favours n o n s e l e c t i v e hydrocracking.
F i g . 6 . Influence o f d i f f e r e n t arrangements o f 40 mg 0 . 5 % Pt/A1203 c a t a l y s t and 400 my H-mordenite on the ri-butane hydroconversion a t 633 K 40 rng 0 . 5 % Pt/A1203
0c r a c k i n g products
400 mg H-rnordenite iso-butane
s e l e c t i v i t y of the i s o m e r i z a t i o n
The arrangement o f the P t / A l 0 and the H-mordenite i s o f g r e a t importance 2 3 f o r the a c t i v i t y and t h e s e l e c t i v i t y . F i g . 6 shows some examples. If a l a y e r
of 40 my Pt/A1203 i s p u t on top or a t t h e bottom o f t h e H-mordenite, only a weak i s o m e r i z a t i o n b u t an i n t e n s e c r a c k i n g takes p l a c e . D i v i d i n g of t h e
40 rng P t / A l 2 O j
i n t o two l a y e r s improves t h e i s o r n e r i z a t i o n a c t i v i t y , and
d i v i d i n g i n t o four l a y e r s improves i t f u r t h e r .
89 Obviously, t h e r e e x i s t s an o p t i m a l distance between P t and the a c i d i c cent r e s i n the H-mordenite f o r supporting the isomerization. The reason can o n l y be the hydrogen s p i l l o v e r again. Tn the very complex mechanism o f p a r a l l e l and consecutive r e a c t i o n s i n the path o f the hydroconvetsion o f n-butane the unsatu r a t e d intermediates seem t o play a s p e c i a l r o l e f o r t h e coking r e a c t i o n .
F i g . 7. Photos o f the quartz glass r e a c t o r with c a t a l y s t arrangements according t o F i g . 6 a f t e r 1 . 5 h hydroconversion of n-butane a t 633 K, f l o w from t o p t o bottom A
H-mordenite
8
0.5 % Pt/Alz03
F i g . 7 shows photos o f the arrangements B, C, C ' , 0, and E. Arrangement B (H-mordenite o n l y ) shows a s t r o n g l y coked c a t a l y s t bed ( b l a c k ) . I n arrangement C a zone o f about 10 mm i n t h e f l o w d i r e c t i o n remains white/grey.
The s p i l l o v e r
hydrogen i s able - n o t s u r p r i s i n g l y - t o avoid coking even a g a i n s t the f l o w d i r e c t i o n (case
C'). About 5 mm remains white/grey. D i v i d i n g o f t h e Pt/A1203 ca-
t a l y s t i n t o two (0) or four ( E ) l a y e r s improves the r e s i s t a n c e of the H-morden i t e t o f u r t h e r coking. The i n t e r f e r e n c e of the s p i l l o v e r i n t h e r e a c t i o n p a t h o f n-alkane convers i o n on z e o l i t e s needs P t and hydrogen as w e l l . I f the hydrogen i n t h e n-butane conversion i s replaced by n i t r o g e n and t h e o t h e r r e a c t i o n c o n d i t i o n s are k e p t constant, on H-mordenite as w e l l as on a m i x t u r e o f 10 mg 0.5%Pt/AlZO3 and H-mordenite 48.5 % of cracking products are formed, the s e l e c t i v i t y f o r i s o m e r i z a t i o n i s l e s s than 10 %, and f a s t coking takes place ( f o r more d e t a i l s see r e f . 9 ) .
90
CONCLUSIONS S i m i l a r t o the hydroconversion o f n-heptane on H - e r i o n i t e ( r e f . 101, the hydroconversion o f n-hexane on H - e r i o n i t e and o f n-butane on H-mordenite is s t r o n g l y influenced i n i t s s e l e c t i v i t y and a c t i v i t y by the a c t i o n of hydrogen s p i l l o v e r . The a c t i v e hydrogen i s generated a t P t everywhere i n the cat a l y s t bed and s p i l l s over t o the a c i d i c z e o l i t e c e n t r e s . There i t i n f l u e n c e s the mechanjsm v i a c o n t r o l l i n g the c o n c e n t r a t i o n o f unsaturated intermediates such as o l e f i n s , carbenium i o n s and coke precursors. Up t o 630 K t h e hydrogen s p i l l o v e r a c t s over a distance of more than 10 mm. I f the supply o f s p i l l i n g over hydrogen i s high, hydrocracking is favoured; i f i t is low ( l i t t l e P t p r e sent, P t f a r from the a c i d i c c e n t r e s ) , hydroisornerization i s favoured; i f i t
is zero (no P t present, P t poisoned or N2 i n s t e a d o f H ), c r a c k i n g and coking 2 (consecutive r e a c t i o n s o f o l e f i n s and/or carbeniurn i o n s ) dominate.
REFERENCES
1 K.-H. Steinberg, K. Becker, K.-H. N e s t l e r , Acta Phys. Chem. Univ. Szeged 31 (1985) 441 2 5.-0. Burd, J . Maziuk, Hydrocarbon Processing 51 (1972) 97 3 B r i t . Pat. 2 500 072, 10. 7 . 1975 4 B r i t . Pat. 2 712 996, 15. 6 . 1978 5 00 WP 240 503, 2 9 . 8. 1985 6 00 WP C07C/302 673 4 ( a p p l . ) 7 A. Montes, G. Perot and M. Guisnet, React. K i n . C a t a l . L e t t . 13 (1980) 77 8 N. N. Krupkina, A. Z . OorogoEinski and A. L. Proskurnin, I z v e s t . Vyss. UCebn. Sav., N e f t i Gaz 1985, 4 3 9 U. Mrocrek, Thesis, Karl-Marx-Universitat L e i p z i g , 1987 10 A. E l Tanany, G. M. Pajonk, K.-H. Steinberg, S. J . Teichner, Appl. Cat. 39 (1988) 89
H.G. Karge,J . Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
INTERMEDIATES IN THE FORMATION OF AROMATICS FROM PROPENE AND 2-PROPANOL ON H-ZSM-5 ZEOLITES
H. LECHERT’, C.BEZOUHANOVA2, C. DIMITROV2 and V. NENOVA2 Institute of Physical Chemistry, University of Hamburg, Bundesstr.45, $000 Hamburg 13 (Germany)
University of Sofia, Faculty of Chemistry, 1 Av.A.Ivanov, Sofia 1126, (Bulgaria) ABSTRACT The aromatization o f propene and 2-propanol on H-ZSM-5 was studied on samples with Si/Al-ratios of 50, 100, 140, 250, 500, and 1000 at temperatures of the beginning of this reaction at 523 K and 573 K. The aim of these experiments was the detection of possible intermediates of the aromatization. The liquid reaction products contained beside a large fraction of hexane isomers an appreciable amount of substituted cyclohexanes as trimerization products. The first detectable trimeric aromat was 1,2,4-trimethylbenzene from which the other aromatic compounds can be formed via subsequent disproportionation and dealkylation reactions. The formation of the aliphatic trimer is favoured for the propanol feed and for propene in the presence o f water. INTRODUCTION ZSM-5 zeolites are used as highly shape-selective catalysts in different types of reactions. The peculiarities of these catalysts consist of a combination of strong acid sites beside hydrophobic walls and a channel system of specific size leading to special mechanisms of selectivity and very l o w coking. The high Si/Al-ratio leads to high heat stability. Starting from alcohols, aromatics (MTG) or olefins (MTO) are produced. In the recent time a great variety of reactions has been reported starting from alcohols (refs. 1-3). There is a considerable interest in the conversion of propene on H-ZSM-5 zeolites in technical processes and many efforts have been made to find conditions of high selectivity and to get a thorough understanding of the mechanism (refs. 5-9). In studies of Derewinski et al. (ref. 4) on the modification of the acidic properties of pentasils by selective poisoning using pyridine or ion exchange with different alkali metals showed that olefins are formed from alcohols on very weak acid sites. The subsequent transformations to aromatic hydrocarbons via oligomers and cyclopolyenes demand stronger acidity. For the formation of benzene from propene a detailed reaction scheme including subsequent dimers was given by Vedrine et al. (ref. 7).
92
Nevertheless, the aromatization of propene is usually not selective for the formation of benzene. At relatively low temperatures an appreciable amount of C9 products could be observed (ref. 6). In connection with this work, in the present paper we report the catalytic activity of ZSM 5 zeolites with a wide range of Si/Al-ratios connected with a wide range of concentrations of acid sites for the conversion of propene and 2-propanol at 523 K and 573 K where the aromatization is just beginning and precursors of this reaction may be detected. EXPERIMENT The ZSM-5 catalysts were synthesized with Si/Al ratios of s = 50, 100, 140, 250, 500, 1000. The batch composition was generally 14 NaOH * 100 Si02 * lOO/s NaA102 * 5 TPABr * 1000 H20. s is a positive number which is chosen in the experiments from about 20 to infinite. s is equal to the Si/Al-ratio in the batch. As silica source a precipitated product of Merck has been used. The alumina was a sodium-aluminate solution from Riedel de Haen. The water content of the batch for optimal crystallization i s dependent on the silica source and may vary within a factor of 2. The crystallization was carried out in stainless steel, teflon lined autoclaves at 423 K . The time necessary for complete crystallization was 7 hours. The crystallites were analyzed with respect to their Si/Al-ratio. For the samples discussed in this paper the analyzed value was equal to the ratio chosen in the batch. The template was decomposed by heating for 6 hours in an air flow under slowly increasing temperature by 4 K/min up to 723 K. The H-forms were obtained by treating the synthesized zeolites with 0.6 M HC1 aqueous solutions under stirring at room temperature. For the catalytic experiments a fixed-bed flow reactor was used with 1 g resp. 2 ml (1.25 - 2.5 mm fraction) of the zeolite previously pressed, crushed and sieved. The activation and regeneration of the catalysts was performed at 723 K for 3 hours under an air flow. Before the experiments the catalysts were flushed with Ar at the chosen temperature of the reaction. The reaction products were analyzed by GC using a column with benzyl cyanide t silver nitrate on Chromosorb PAW at room temperature for the gaseous products and 3 m column 4mm in diameter containing 12% Benton 34 t dodecylphthalat (1:l) on Celite at 353 K for the liquid products. The latter products were also analyzed by GC-MS using for the separation a 50 m capillary column with Carbowax.
93
RESULTS In preliminary experiments was found that samples of Na-ZSM-5 were inactive for the chosen reactants and conditions. The yields of liquid products from propene and from 2-prOpanOl as a function of the zeolite composition are presented in Fig. 1. The product distribution for propene for the different zeolites is shown in Table 1. The product distribution for 2-propanol is qualitatively similar. Differences can be found in a distinctly increased yield of methyl-cyclopentane and in the fraction of the substituted cyclohexanes. To get an impression of possible changes of the catalyst by the released water the time dependency of the product distribution was studied for the sample with Si/AI = 500 where the highest selectivity effects may be expected. The results are given in Table 2. In all liquid products according to a mass spectroscopic analysis the presence o f C4H7 t (m/z=55) was observed (see also ref. 8). For Si/Al = 1000 for propene no or low conversion could be observed at 573 K in various experiments. This sample has most probably an inhomogeneous distribution of the A1 and will not be taken into account in the further discussion.
1001
I
-7
.... ....'*....'
I
I
I
40 0. -0 .I+
3
20
.z I,,, , , , , , , , , OO 5 10 ,
, , , , , , , ,
15 20 1000#A1/ (SitAl)
,
Fig. 1. Yields of liquid products as a function of the composition of the zeolites: A- propene at 573; 0 - 2-propanol at 523 K;O2-propanol at 573 K.
94
TABLE 1 Propene conversion on H-ZSM-5 Zeolites at 573 K, WHSV = 1.3 hour-', (x 523 K , xx- Propene + Water, molar ratio 1:3), average values from 5 hours stream Si/Al
50'
Liquid.Prod,wt% 36.6 Coke, w t % of feed 1.6 C-6 1 29.8 C-6 2 C-6 3 n-Hexane 23.5 Unidentified 1 Methyl-cyclopentane 4.9 Cyc1ohexane 4.9 Unidentified 2 Unidentified 3 Subst.Cyclohexane 16.7 Benzene 7.6 Unidentified 4 9.8 2.8 Toluene Ethylbenzene p-Xylene m-Xylene o-Xylene 1,2,4-Trimethylbenzene -
'05
50
100
140
250
500
28.9 0.5 10.7
60.0 1.4 3.3 12.2 1.3 6.7 4.8 3.9 6.0 3.7 1.8 4.2 3.0 1.3 17.2 4.1 15.3 3.7 0.8 6.7
44.7 1.8
40.2 2.0
35.4 1.3
43.5
13.7 17.9 8.6
23.5 26.9 17.6
23.1 27.7 23.5
18.5 26.9 21.0
5.7 6.1 4.2
tr 5.8
tr 5.7
tr 6.4
5.5 3.5 1.6 14.2 1.9 10.4 6.7 tr
13.7 3.4 9.0 tr tr
30.8 -
2.5 2.5
-
-
27.2 7.3 18.5
-
-
-
-
-
-
-
-
-
-
30 C-6 1 C-6 2 C-6 3 n-Hexane Methyl-cyclopentane Cyclohexane 2-Propanol Subst.Cyclohexane Benzene Unidentified 1 Toluene Unidentified 2 Ethylbenzene Unidentified 3
1.8 19.9 0.1 32.6 5.9 5.4 18.4 6.8 9.1
-
Time on stream, min 90 120 150 180
tr 7.5
210
-
9.7 3.1 6.2 tr 1.0
13.7 4.4 9.1
-
-
tr -
-
240
270
tr 0.9 0.6 0.6 0.3 tr 1.3 1.7 1.5 0.7 7.0 10.0 14.3 14.5 11.3
0.4 0.7 8.5
2.3 1.9 0.2 1.6 1.7 12.9 31.2 22.1 24.4 26.6 2.9 2.6 3.3 4.2 7.5 0.5 2.9 3.3 4.1 35.8 23.4 19.7 21.6 19.8 15.1 7 . 8 4.9 6.0 6.9 18.9 9.7 17.9 13.0 13.1 2.9 4.3 2.8 3.5 3.9 3.4 2.6 3.5 6.8 7.2 3.1 1.9 2.5 1.4
-
-
TABLE 2 Composition of the liquid products iy wt% from 2-propanol on H-ZSM-5, (Si/A1=500) at 573 K , LHSV = 1 hour , 32,5% liquid products of feed Products
-
2.8 1.5 18.0 21.9 4.0 3.2 3.8 3.3 25.3 19.2 10.8 9.5 19.9 18.3 2.0 5.0 1.1 3.3 3.3 3.3
95
DISCUSSION Under the special conditions of reduced temperature used in our experiments for propene the total amount of liquid products decreased with increasing Si/Al-ratio. As expected, a high amount of dimers was observed as C6 fraction. The relative amount of this fraction is about 15-20% higher for the samples with Si/A1 = 140, 250, and 500 compared with the samples with Si/Al = 50 and 100. Cyclohexane and methyl-cyclopentane are present only in minor quantities. The quantity of substituted cyclohexanes which can be regarded as trimer products increased with increasing Si/Al-ratio. This behaviour may be understood qualitatively from the following wellknown effects. The olefin is adsorbed preferably at the acidic centers. At the centers with the highest acid strength carbenium ions are formed which react with olefin molecules additionally present in the channels. A part of the resulting dimers adds a third propene molecule forming trimeric olefins which undergo a cyclization giving the fraction of the substituted cyclohexanes. Reactions of this kind have been discussed already by Wolthusen et al. (ref. 5 ) . CH ,. -CH=CH2
-
CH3\CH-CH2 -;H-CHj
Cy -tH-CH3
CH3’
Cycloalkanes with 7 - 9 carbon atoms have been found also in (ref. 6). These species are present in the reactants in our experiments to about 10 - 15%, except for the samples with Si/Al = 50 and 100. Exactly for these samples, however, an appreciable amount of toluene and p-xylene could be observed beside some ethyl-benzene and m-xylene. For Si/Al = 50 additionally 6.7% 1,2,4-trimethylbenzene was found which could be detected also in the mass spectroscopic experiments. Other authors describe mesitylene or ethyl-toluenes as products in similar experiments (ref. 15). The amount of benzene in the reaction products is nearly constant at values between 3 and 4.5% for all Si/Al-ratios independent of the formation of other aromat i c s . All these observations lead to the conclusion that substituted aromatics found in the reaction products for the zeolites with Si/A1 = 50 and 100 are
96
not formed from dimerization, dehydrogenation to benzene and subsequent alkylation, but rather from a cyclotrimer of the propene which undergoes dehydrogenation and transalkylation reactions. The source of the constant amount of benzene may be possibly dimers dehydrogenated according to the scheme of Vedrine et al. (ref. 7). To look for the possible source of benzene for Si/Al = 50 an experiment was carried out, trying to convert cyclohexane into benzene under the given conditions at 523 K. Beside some aliphats only a little methyl-cyclopentane was observed. The finding that the aromatization is favoured obviously at lower Si/Alratios seems to suggest the assumption that for the hydrogen shift included in that reaction two activated molecules with not a too large distance or one activated and one strongly sorbed molecule seem to be necessary. If the distance of both is too large the reaction stops at the stage of the cyclotrimer under the temperature conditions of our experiments. For 2-propanol a qualitatively similar product distribution was observed with a reduced activity of the catalyst. The striking difference was the high amount of methyl-cyclopentane and the substituted cyclohexanes being observed in all experiments. The other dimers are drastically reduced in comparison with the data for propene. To get insight into the question whether this effect is due to a change of the catalyst by the released water, for the sample with Si/Al = 500 with the lowest activity the time dependency of the product distribution was studied which is demonstrated in Table 2. Obviously, the water released in the initial dehydration step influences the formation of the oligomers. To prove this, with a sample of Si/Al = 50 an experiment was carried out with a mixture of propene and an excess o f water which is demonstrated in the second column of Table 1. Also in this case the activity is reduced and the concentration of the trimer is increased. The relative concentrations within the C6 fraction are changed compared with the results with pure propene shown in the first column in Table 1. Comparing the results o f Table 2 with the last column of Table 1 a slightly increased activity for the aromatization can be seen from a comparison with column 1 in Table 1. The effect of the role o f the trimer as the origin of the substituted aromats is, however, less pronounced. For the mechanism of aromatization On0 et al.(ref. 9) found, that at 773 K the molar ratio of alkanes to aromatics in the product over H-ZSM-5 was 2.9, so confirming the scheme of Poutsma (ref. 13). An analysis of the hydrogen balance of all identified substances gave no
97
clear picture in our experiments. On Zn-ZSM-5 Ono et al. suggest another mechanism involving dehydrogenation on the zinc species. The final scheme of Ono et al.(ref. 9) for alkene transformation offers two paths of aromatization - via allylic species in the presence of dehydrogenation agents (zinc, gal 1 ium) and direct conversion of the dimers to aromatics and alkanes with the participation of protons in successive hydride transfer and deprotonation of carbenium ions. Our results are consistent with the latter concept with the trimers as the important intermediates. The dehydrogenation may also take place directly on the surface of the walls of the ZSM 5 channels. Because o f this ability the ZSM 5 zeolites are also called "pseudo-transition metals" in (ref. 14). The formed 6 or 5 atom rings from the trimer are quite bulky. Therefore, they probably transform further by methyl migration:
According to a mass-spectroscopic analysis 1,2,4-trimethylbenzene should be the initial aromatic compound formed from propene. Toluene and xylenes are formed from it by dealkylation, disproportionation and isomerization. Thus, C7 and C-8 aromatics are not a result of benzene alkylation, but originate from the discussed trimers. The picture is, in principle, not different in the case of 2-propanol feed. Differences may occur in the formation of the initial carbenium ion. By the contact of the alcohol with the acidic OH-groups of the zeolite the protonation, formation of the oxonium salt and dehydration to the corresponding carbenium ion may be discussed as an alternative way. CONCLUSIONS
The production of aromats from propene on H-ZSM 5 zeolites proceeds mainly via trimers and cyclotrimers. The benzene homologues are formed by further disproportionation and dealkylation of the respective aromatized substances of which 1,2,4-trimethylbenzenecould be detected. The presence of the released water in case of the 2-propanol favours the formation of cyclic compounds and influences also the aromatization reaction.
98
REFERENCES 1 Y. Matsumura, K. Hashimoto and S. Yoshida, J. Catal., 100 (1986) 392-400. 2 C. Bezouhanova, Chr. Dimitrov, H. Lechert, V. Nenova and M. Krusteva, Proc.6th Int. Symp. Heterogeneous Catal., Sofia, 1987, part 2, p. 216. 3 W. Holderich, M. Hesse and F. Naurnann, Angew.Chernie, 100 (1988)232. 4 M. Derewinski, J. Haber and J, Ptaszynski, in Y. Murakami, A. Iijima and J. W. Ward (Editors), New Dev. Zeol. Sci. Tech., Proc. 7 t h Int. Conf., 1986, pp. 957-964. 5 J.P. Wolthuizen, J.P. van den Berg and J.H.C. van Hooff, in B. Imelik et al. (Editors.), Catalysis by Zeolites, Elsevier, 1980, pp. 85-92. 6 K.G. Wilshier, P. Smart, R. Western, T. Mole and T. Behrsing, Appl.Catal., 31 (1987) 339-359. 7 J.C. Vedrine, P. Dejaifve, E.D. Garbowski and E.G. Derouane, in B. Imelik et al. (Editors), Catalysis by Zeolites, Elsevier, 1980, p. 29. 8 S. Bessell and D. Seddon, J. Catal., 105, (19871, 270-275. 9 Y. Ono, H. Kitagawa and Y. Sendoda, JCS Far. I , 83 (1987) 2913-2923. 10 H. Lechert, C. Bezouhanova, Chr. Dimitrov, V. Nenova and G. Taralanska, J.Mol .Catal., 35 ( 1986), 349-354. 1 1 C. Bezouhanova, H. Lechert, Chr. Dimitrov, V. Nenova, J.Molec.Structure, 114 (1984),301-304. 12 G.A.Olah, Uspekhi Khimii, 44 (1975), 793. 13 M.L. Poutsma, in J.A. Rabo (Editor), Zeolite Chemistry and Catalysis, ACS Monograph, Amer.Chem.Soc., Washington D.C. (1976), 14 W. Drenth, W.T. Andriessen and F.B. Van Duijneveldt, J.Molec.Catal.,21 (1983) 291-296. 15 F. Marscheider, Diplome Work, University of Hamburg 1988.
H.G. Karge, J. Weitkamp (Editors ), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - P r i n t e d in T h e Netherlands
ZEOLITE AN0 M A T R I X STRUCTURES AND T H E I R ROLE I N CATALYTIC CRACKING
C.J. Groenenboom Akzo C h e m i c a l s , Research C e n t r e Amsterdam P . O . Box 1 5 , 1000 AA Amsterdam, The N e t h e r l a n d s ABSTRACT Some r e c e n t developments i n t h e s y n t h e s i s and m o d i f i c a t i o n o f Y - z e o l i t e s and t h e i r i n f l u e n c e on t h e p e r f o r m a n c e o f c r a c k i n q c a t a l y s t s a r e r e v i e w e d . In t h e s e c a t a l y s t s a l s o f u n c t i o n a l i n g r e d i e n t s a s a c t i v e a l u m i n a s and m e t a l t r a p s a r e i n c o r o o r a t e d . The e f f e c t o f a l u m i n a on p o r e s i z e d i s t r i b u t i o n and t h u s on t h e c r a c k a b i l i t y o f h e a v i e r f e e d s t o c k s i n d i s c u s s e d . F u r t h e r t h e mechanism o f metal p a s s i v a t i o n i n o u t l i n e d .
I . INTRODUCTION C a t a l y t i c c r a c k i n q o f h y d r o c a r b o n s on s o l i d a c i d s i s a l r e a d y a r a t h e r o l d p r o c e s s . However, ma.jor chanqes have been c a r r i e d o u t d u r i n g t h e y e a r s ( r e f .1.) A t t h e end o f t h e s i x t i e s a r e a l b r e a k t h r o u q h
was r e a c h e d b v t h e a p p l i c a t i o n
o f c r y s t a l l i n e aluminosilicates o r z e o l i t e s (ref.2.3).As
compared w i t h t h e
amOrDhOUS s i l i c a - a l u m i n a s . z e o l i t e c o n t a i n i n g c a t a l y s t s showed a h i q h e r a c t i v i t y , a b e t t e r s t a b i l i t y and a b e t t e r s e l e c t i v i t y f o r g a s o l i n e . The o n l y p e n a l t y t h a t has t o be p a i d was t h e l o w e r p r o d u c t i o n o f o l e f i n e s r e s u i t i n g i n l o w e r o c t a n e numbers. A t t h a t t i m e , however t h i s was n o t considered as a s e r i o u s problem. T h e r e f o r e z e o l i t e s r e p l a c e d a l m o s t c o m p l e t e l y t h e
amorohous c a t a l y s t s . T h i s r e s u l t e d i n a r a t h e r s t a b l e o p e r a t i o n d u r i n g s e v e r a l y e a r s . However, t h e l e a d phase down. f i r s t i n t h e USA and now a l s o i n Europe. has a m a j o r i m p a c t o n t h e r e f i n e r y w o r l d . The c a t a l v s t m a n u f a c t u r e r s r e s p o n d e d on t h a t a n d t h e r e s u l t i s t h a t a l o t o f new c a t a l y s t s were i n t r o d u c e d i n t h e m a r k e t t o i m p r o v e t h e g a s o l i n e o c t a n e s . These c a t a l y s t s d i d n o t o n l y c o n t a i n i m p r o v e d z e o l i t e s . b u t a l s o t h e m t r i x components were m o d i f i e d . I n t h i s p r e s e n t a t i o n some o f t h e m o s t i m p o r t a n t d e v e l o p m e n t s w i l l b e d i s c u s s e d . However. b e f o r e t h a t t h e mechanism o f c a t a l y t i c c r a c k i n q w i l l be o u t l i n e d b r i e f l v . 11. MECHANISM OF CATALYTIC CRACKING D u r i n g f l u i d c a t a l y t i c c r a c k i n q many r e a c t i o n s a r e t a k i n g p l a c e , w h i c h have been w e l l d e s c r i b e a i n some e x c e l l e n t r e v i e w s ( r e f . 1.4.5). Summarizing. t h e mechanism o f c a t a l y t i c c r a c k i n q i s d o m i n a t e d by t h e c h e m i s t r y o t t h e c a r b e n i u m i o n s . w h i c h a r e t o r m e d on t h e a c i d i c s i t e s o f c r a c k i n g catalysts.
100 Reactions o f carbenium-ions t h a t a r e e s p e c i a l l y i m p o r t a n t f o r t h e c a t a l v t i c c r a c k i n g process. a r e . c r a c k i n g , hydrogen t r a n s f e r and i s o m e r i z a t i o n . The f i n a l c o m p o s i t i o n o f t h e c r a c k i n q p r o d u c t s depends on t h e r e l a t i v e r a t e s o f t h e v a r i o u s competing r e a c t i o n s . C r a c k i n g can o n l y proceed on t h e r e l a t i v e l y s t r o n g a c i d s i t e s . On t h e s e s i t e s . t h e s k e l e t a l i s o m e r i z a t i o n o f carbenium i o n s i s a r a p i d p r o c e s s ( r e f . 6 ) . Branched t e r t i a r y carbenium i o n s a r e more s t a b l e t h a n l i n e a r . p r i m a r y and secondary carbenium i o n s . T h e r e f o r e , t h e c o m b i n a t i o n o f c r a c k i n g and i s m r i z a t i o n l e a d s t o t h e f o r m a t i o n o f m a i n l y branched p r o d u c t s . T h i s i s a f a v o r a b l e a s p e c t o f c a t a l y t i c c r a c k i n g , s i n c e branched COmDOUndS have h i g h e r octane numbers
than l i n e a r compounds. Because. lonq c h a i n hydrocarbons a r e more r e a c t i v e t h a n s h o r t c h a i n hydrocarbons, t h e r a t e o f t h e c r a c k i n q r e a c t i o n s decreases w i t h d e c r e a s i n g c h a i n l e n g t h and f i n a l l y becomes v e r y slow, when i t i s no l o n g e r p o s s i b l e t o f o r m s t a b l e t e r t i a r y carbenium i o n s . The c r a c k i n g r e a c t i o n s a r e t e r m i n a t e d by hydrogen t r a n s f e r f r o m hydrogen donors t o t h e carbenium i o n s . V a r i o u s hydrogen donors a r e mentioned i n l i t e r a t u r e , e.g. t r a n s f e r are
n a p h t h e n i c compounds which upon hydrogen
transformed i n t o a r o m a t i c compounds and coke. Also, p a r a f f i n s
can t r a n s f e r a h y d r i d e i o n . I n t h i s way t h e c h a i n o f c r a c k i n g r e a c t i o n s i s propagated. I n g e n e r a l , nydrogen t r a n s f e r r e a c t i o n s a r e much l e s s s e n s i t i v e t o t h e a c i d s i t e s t r e n g t h o f t h e c a t a l y s t t h a n c r a c k i n g and i s o m e r i z a t i o n r e a c t i o n s ( r e f . 7 ) . However, hydrogen t r a n s f e r i s s t r o n g l y i n f l u e n c e d by t h e a c i d s i t e d e n s i t y ( r e f . 8 ) and t h e c o n c e n t r a t i o n o f hydrogen donors ( r e f . 1 0 ) . An u n f a v o u r a b l e aspect of hydrogen t r a n s f e r i s t h a t i t l e a d s t o a l o w e r o l e f i n i c i t y and t h e r e f o r e a 1ow RON o f FCC-gasol ine. N e v e r t h e l e s s , some hydrogen transfer a c t i v i t y isneeded f o r g a s o l i n e s t a b i l i z a t i o n , i n o r d e r t o p r e v e n t e x c e s s i v e o v e r c r a c k i n g ( r e f . 9 , see scheme 1) and t h e f o r m a t i o n o f d i - o l e f i n s .
OLEFIN
BRANCHEDOLEFIN
1I
ISOMERIZATION FAST
HC --+ CARBENIUM ION
T I
BRANCHED CARBENIUM ION
BRANCHED PARAFFIN
SMALLER
HC'S IPAR.
1
KwTRANSFER Ine -DONOR1 RATIO
I
KCRACKING
Scheme 1
+ 0LEF.I
101
I n a d d i t i o n t o t h e r e a c t i o n s mentioned b e f o r e . a l s o c a t a l v t i c coke f o r m a t i o n proceeds v i a carbenium i o n i n t e r m e d i a t e s . I t s h o u l d be n o t e d t h a t t h e u n d e s i r a b i e coke f o r m a t i o n j u s t l i k e hvdrogen t r a n s f e r i s a " b i m o l e c u l a r " r e a c t i o n . which i s enhanced bv a h i g h a c i d s i t e d e n s i t v . w h i l e c r a c k i n q and i s o m e r i z a t i o n r e a c t i o n s a r e monomolecular. For t h i s reason a c a t a l v s t s c r e e n i n q t e s t wasdevelopedin o u r l a b o r a t i e s . which makes use o f a model compound w i t h p r o d u c t d i s t r i b u t i o n . which can d i s c r i m i n a t e between f a v o u r a b l e mnomolecular and u n f a v o u r a b l e " b i m o l e c u l a r " r e a c t i o n s , v i z . o - x v l e n e ( r e f . 10
1.
111. C O M P O S I T I O N OF C R A C K I N G CATALYSTS N o r m a l l v a c r a c k i n q c a t a l y s t i s composed o f a z e o l i t e , w h i c h d e t e r m i n e s t o a l a r q e e x t e n t t h e a c t i v i t y and t h e s e l e c t i v i t y o f t h e c a t a l y s t . Besides t h e z e o l i t e i n manv cases a l s o an alumina phase i s p r e s e n t . T h i s alumina i s e s p e c i a l l v u s e f u l when l a r q e m o l e c u l e s . which can n o t e n t e r t h e z e o l i t e p o r e s have t o be cracked. So. i t demands aluminas w i t h an a p p r o p r i a t e Dore s i z e d i s t r i b u t i o n . F u r t h e r some c l a y i s p r e s e n t . T h i s i s a d i l u e n t . which a l s o helps t o give the c a t a l y s t t h e desired density. I n order t o urepare microspheres w i t h s u f f i c i e n t hardness, a q l u e i s necessary. T h i s q l u e can be s i l i c a ( r e f . 11). s i l i c a - a l u m i n a ( r e f 12. 1 3 ) o r can be c o m p l e t e l v basedonalumina
(ref. 14). On t o p o f t h i s . a l s o o t h e r f u n c t i o n a l i n g r e d i e n t s can be i n c o r p o r a t e d i n t o t h e c r a c k i n q c a t a l y s t . Examples o f t h e s e f u n c t i o n a l i n q r e d i e n t s a r e : m e t a l t r a p s and components which reduce t h e SOx e m i s s i o n o f an FCC u n i t . Both f u n c t i o n s w i l l be d i s c u s s e d l a t e r o n i n t h i s a r t i c l e .
I V . ZEOLITES I N CRACKING CATALYSTS D u r i n g q u i t e a l o n q p e r i o d RE-exchanqed NaY z e o l i t e s have been used i n c r a c k i n q c a t a l y s t s . These t y p e s o f z e o l i t e s a r e r e l a t i v e l y easy t o make(ref.15). C a t a l y s t s c o n t a i n i n g REY z e o l i t e s a r e r e l a t i v e l v s t a b l e ( r e f . 1 6 ) a n d produce a l o t o f q a s o l i n e . r h e r e a r e a l s o s e v e r a l d i s a d v a n t a q e s , i . e . t h e octane numbers o f t h e g a s o l i n e i s r a t h e r low and t h e coke p r o d u c t i o n i s (ref.17).
T h i s can be e x p l a i n e d bv t h e h i g h
rather high
hvdrogen t r a n s f e r a c t i v i t v o f
t h i s type o f z e o l i t e s ( r e f . 18). T h e r e f o r e u l t r a s t a b l e z e o l i t e s which have h i g h e r s i l i c a a l u m i n a r a t i o s ( = S A R I and c o n s e q u e n t l y a l o w e r number o f aluminium s i t e s o e r u n i t c e l l ( =APC) a r e D r e f e r a b l v used nowadays. I t may a l s o be mentioned t h a t an equal SAR o r APC d o e s n ' t n e c e s s a r i l y mean t h a t t h e A l - s i t e d i s t r i b u t i o n s a r e a l s o t h e same. T h i s can be v i z u a l i z e d i n f i g . 1 , where t h e c o n c e n t r a t i o n o f i s o l a t e d a c i d s i t e s (=O-NNN) which r e p r e s e n t s t h e number A l - s i t e s w i t h o u t anv A 1 as n e x t n e a r e s t neiqhbours ( r e f . 8 ) . i s p l o t t e d a g a i n s t t h e APC.
102
T h i s has been done b o t h f o r a random and a w e l l - o r d e n e d d i s t r i b u t i o n . So. i t can be concluded t h a t t h e a p D l i c a t i o n o f t h e most a p p r o p r i a t e s y n t h e s i s and m o d i f i c a t i o n method
can lead t o l a r q e advantages i n oerformance.
U J U I N U UPiR UNIT c n L
(wc)
F i g . 1 . C o n c e n t r a t i o n o f 0-NNN s i t e s p e r u n i t s c e l l f o r a random A 1 d i s t r i b u t i o n and t h e most i d e a l Y z e o l i t e w i t h a maximum number o f 0-NNN s i t e s . A t t h e moment s e v e r a l methods t o o r e p a r e u l t r a s t a b l e z e o l i t e s a r e d e s c r i b e d i n
t h e I i t e r a t u r e ( r e f . 19 1. From these s t e a m c a l c i n a t i o n and isomorDhous subs t i t u t i o n a r e t h e most i m p o r t a n t ones. 'I.S t e a m c a l c i n a t i o n .
T h i s orocess s t a r t s w i t h one o r more ammoniumexchanqe s t e p s , r o l l o w e d bv a s t e a m c a l c i n a t i o n . 'The r e a c t i o n mechanism i s o u t l i n e d i n f i q . 2 ( s e e also ref.20.1 I t may be n o t i c e d t h a t as a r e s u l t o f t h e a p p l i e d method a secondarv p o r e
s t r u c t u r e w i t h a diameter o f ca. 1.5 up t o 10 nm i s formed.
I
-si-
I
-I'
-p- + i O H ~ H O - $ i -51I
P
5 1 -
I
F i g . 2 . Mechanism o f u l t r a s t a b i l i z a t i o n .
t AI(OH)3
103 T h i s may be advantageous f o r t h e c r a c k i n q r e a c t i o n s . F u r t h e r t h e d e a l u m i n a t i o n r e s u l t s i n t h e f o r m a t i o n o f n o n - f r a m e w o r k a l u m i n a s p e c i e s ( = NFA). T h i s NFA can e x n i b i t a c i d i c p r o o e r t i e s and a l s o i n f l u e n c e s t h e z e o l i t e a c c e s s i b i l i t y . R e c e n t l y Shannon e t . a l . ( r e f . 2 1 ) s p e c u l a t e d t h a t d u r i n g d e a l u m i n a t i o n t h e
a I umi n a s p e c i e s "condense" i n t h e zeo I it e s u p e r c a g e s t o f o r m boehmi t e 1 ik e c l u s t e r s as shown c o n c e p t u a l l v i n f i g . 3.
F i q . 3.
A b o e h m i t e - l i k e a l u m i n a c l u s t e r in t h e z e o l i t e Y s u p e r c a g e .
I n q e n e r a l t h e i n f l u e n c e o f t h e NFA o n t h e s e l e c t i v i t y i s n e g a t i v e ( r e f . 17).
M o r e o v e r t h i s e f f e c t i s enhanced s i n c e t h e NFA s p e c i e s a r e r a t h e r m o b i l e and m i g r a t e t o t h e s u r f a c e o f t h e z e o l i t e c r y s t a l s . T h i s h a s been p r o v e n b y XPS measurements c a r r i e d o u t by v a r i o u s r e s e a r c h g r o u p s ( r e f . 2 2 - 2 5 ) . These measurements show t h a t v e r y h i g h c o n c e n t r a t i o n s o f NFA c a n b e r e a c h e d a t t h e surface (see f i q . 4 ) .
-
I
NFA migration 120.
.
2
1,
0"
100.
totat Numinum per unit cell
c_ 3
60
n 3
5 I
,,-.-i--rNay rtartinq material
403r1*-
-/ t
20-
framework aluminum per unit call
0
5
10
15
20
25
30
35
3
ETCHING TIME (min.)
i i g . 4 . A1 d e p t h i n steamed z e o l i t e Y ( U S Y ) ;
based on XPS d a t a .
However. i t i s p o s s i b l e t o remove t h e s e NFA s p e c i e s . f o r i n s t a n c e b v a c i d treatment
of t h e z e o l i t e . T h i s g i v e s r i s e t o a much more f l a t p r o f i l e . I t
mav a l s o be m e n t i o n e d t h a t t h e r a t e o f m i g r a t i o n o f t h e NFA s t r o n g l y depends on t h e RE c o n t e n t o f t h e z e o l i t e . R e c e n t w o r k ( r e f . 2 5 ) showed t h a t REY d e a l u m i n a t e s m o r e s l o w l y t h a n USY and a l s o t h e m i g r a t i o n o f t h e NFA t o t h e s u r f a c e i s h i n d e r e d . The USY z e o l i t e s t a r t s w i t h a A l / S i
r a t i o a t the surface
o f 0 . 5 5 . w h i c h i n c r e a s e s up t o a f i g u r e s l i q h t l y o v e r 1 , a f t e r s t e a m i n q a t 500°C f o r 65 h r s . F o r t h e REY z e o l i t e a r a t i o o f 0 . 5 6 was f o u n d f o r t h e s t a r t i n g m a t e r i a l w h i c h i n c r e a s e s UP t o 0.77 u n d e r t h e same c o n d i t i o n s .
2. Isomorphous s u b s t i t u t i o n . D e a l u m i n a t e d z e o l i t e s can a l s o be p r e p a r e d w i t h e x t r a n e o u s s i l i c a s o u r c e s , f o r i n s t a n c e w i t h (NH4)2 S i F 6 as d e s c r i b e d b y S k e e l s and B r e c k ( r e f . 2 6 ) o r SiC14. a s a p p l i e d by B e y e r e t . a l . ( r e f .
2 7 ) . The f i r s t m e t h o d i s u s e d commer-
c i a l l y now ( r e f . 2 8 ) . A c c o r d i n g t o Rabo e t . a l ( r e f . 2 9 ) . t h i s method g i v e s r i s e t o n e a r l y d e f e c t f r e e z e o l i t e s . They r e p o r t s u p e r i o r t h e r m a l and h y d r o t h e r m a l s t a b i l i t y as compared w i t h a h i g h q u a l i t y ammonium exchanged and steam s t a b i l i z e d Y z e o l i t e (see Table I ) . TABLE 1 C o m p a r a t i v e p r o p e r t i e s of a l u m i n i u m d e f i c i e n t Y z e o l i t e s v e r s u s f r a m e w o r k s i l i c a enriched z e o l i t e Y a f t e r severe nydrothermal t r e a t m e n t . 1
Zeolite NH4Y
Si02/A1203
5.0
’
rel. retention
Na20 ( % )
Ao0)
(%)2
SA
0.36
24.79
25.8
12.5
Y82/USY
5.8
0.17
24.55
59.4
59.1
USY/HCl extract. FSE-Y
9.1
0.33
24.30
23.2
30.3
6.4
0.38
24.69
51.4
59.0
11.7
0.05
24.42
71.7
77.2
1. 16OOOF. 23% steam, 5 h r s . 15 p s i a . 2 . P e r c . r e l a t i v e r e t e n t i o n a f t e r s t e a m i n g v e r s u s unsteamed NH4Y s t a n d a r d . 3. Chemically determined. However. a l o t o f d e b a t e i s g o i n g on a b o u t t h e d i f f e r e n c e s i n p r o p e r t i e s and p e r f o r m a n c e o f b o t h t y p e o f z e o l i t e s . Macedo e t . a l . ( r e f .
30) concluded
f r o m I R and TPD measurements t h a t u l t r a s t a b l e z e o l i t e s p r e p a r e d b y isomorphous s u b s t i t u t i o n posses more a c i d i c s i t e s t h a n t h o s e o b t a i n e d b y c o n v e n t i o n a l steaming.
M o r e o v e r t h e f o r m e r have a h i g h e r p r o p o r t i o n o f B r o n s t e d s i t e s
t h a n t h e l a t t e r , w h i c h may b e c o n s i d e r e d as f a v o u r a b l e f o r t h e c a t a l y t i c cracking reactions.
105
C r e i g h t o n e t a l . ( r e f . 3 1 ) p r e p a r e d c a t a l y s t s c o n t a i n i n g s t e a m s t a b i l i z e d and c h e m i c a l l y d e a l u m i n a t e d ( i . e . ( N H 4 ) 2 S i F 6 ) z e o l i t e s and f o u n d i d e n t i c a l s e l e c t i v i t i e s . However t h i s was a f t e r a t h e r m a l d e a c t i v a t i o n s t e p and one may wonder whether t h i s i s s t i l l r e p r e s e n t a t i v e f o r t h e f r e s h m a t e r i a l s . T h a t t h e f r e s h z e o l i t e s r e a l l y show d i f f e r e n c e s has been p r o v e n by B e y e r l e i n e t . a l . ( r e f . 3 2 ) . These a u t h o r s show t h a t " c l e a n framework" z e o l i t e s ( i . e . p r e p a r e d by i s o a r e low i n a c t i v i t y f o r i s o b u t a n e
morphous s u b s t i t u t i o n w i t h (NH4)2SiF6 cracking
( s e e T a b l e 1 1 ) . They a l s o f o u n d t h a t t h e a c t i v i t y i n c r e a s e d s t r o n g l y
a f t e r a h e a t t r e a t m e n t o f t h e z e o l i t e . T h i s was e x p l a i n e d by t h e f o r m a t i o n o f super a c i d i t y , i . e . an i n t e r a c t i o n between B r o n s t e d a c i d s i t e s and nonframework Lewis s i t e s . TABLE I 1 Comparison o f t h e i s o b u t a n e c r a c k i n g a c t i v i t y o f ammonium h e x a f l u o r s i l i c a t e d e a l u m i n a t e d Y and s t e a m s t a b i l i z e d z e o l i t e s .
Si/All (Chem.ana1) SamDle A . as p r e p a r e d 2 B . a f t e r NH4 exchange o f A LZY 482 USY
'
1. 2. 3. 4.
4.5
Ao
24.54
5.1.
2.7 2.7
Na
(1)
24.56 24.58
(wt %)
Conversion r a t e s 4 ( m o l e / h / g x 10') Total Carb. I o n i-C
% Carb. Ion
1.24
4.1
2.1
51
0.06
12.0
7.0
58
0.15
16.0 36.8
28.1 28.0
ia
-
76
Chemically determined. (NH ) SiF dealuminated. Con!e$tiofial steamstabi 1 i z e d z e o l i t e . Own made USY ( b y s t e a m s t a b i l i z a t i o n ) . O t h e r i n t e r e s t i n g r e s u l t s a r e r e p o r t e d by Corma e t a1 .( r e f . 2 2 ) and K e y w o r t h
et.al. ( ref.23
1, who b o t h show t h a t isomorphous s u b s t i t u t e d z e o l i t e s have a
v e r y steep S i / A I g r a d i e n t (see f i g . 5 ) .
I f i t i s assumed t h a t a m a j o r p a r t o f t h e c r a c k i n g r e a c t i o n s . e s p e c i a l l y with gasoil
take
p l a c e a t t h e o u t e r s u r f a c e o f t h e z e o l i t e , t h i s may a l s o
e x p l a i n t h e low a c t i v i t y f o u n d f o r t h i s t y p e o f z e o l i t e s . F u r t h e r i t may b e n o t i c e d t h a t B e y e r l e i n e t . a l . found t h a t t h e i r m i l d l y s t e a m s t a b i l i z e d z e o l i t e s demonstrate a c i d s i t e s o f equal e f f i c i e n c y
f o r a l l Si/A1 r a t i o s T 5 .
A c c o r d i n g t o t h e a u t h o r s t h i s p o i n t i n t o t h e d i r e c t i o n o f an o r d e r e d distribution o f acid sites ( c f . f i g . l ) .
106
u
60-
0
5
10
15
20
25
30
35
3
ETCHING TIME (rnin )
F i g . 5 . Comparison between steamed and isomorphous s u b s t i t u t e d z e o l i t e Y . T h i s i s i n agreement w i t n t h e r e s u l t s o b t a i n e d by Nieman and vanBroekhoven ( r e f . 10). They i n v e s t i g a t e d t h e a c i d s i t e d i s t r i b u t i o n o f a number o f z e o l i t e s b o t h w i t h model compounds and 2 9 S i
MAS NMR. A M a t h e m a t i c a l model was used t o
t r a n s l a t e t h e NMR d a t a i n t o t h e average number o f A l - s i t e s
(=N"J. Their
measurements i n d i c a t e t h a t t h e a c i d s i t e d i s t r i b u t i o n o f s e v e r e l y steamed z e o l i t e s d e v i a t e s more t r o m t h e i d e a l one t h a n t h e more m i l d l y t r e a t e d materials.
V . A C T I V E MATRICES A f t e r i n t r o d u c t i o n o f t h e z e o l i t e s i n FCC. t h e o l d amorphous s i l i c a - a l u m i n a c a t a l y s t s and so a l s o t h e a c t i v e m a t r i x was o m i t t e d . T h i s r e s u l t e d i n a d r a s t i c s h i f t o f t h e c a t a l y s t p o r e s i z e d i s t r i b u t i o n as can be seen f r o m T a b l e 111. I t i s c l e a r t h a t t h i s s h i f t has a l a r g e e f f e c t on t h e a c c e s s i b i l i t y o f t h e p o r e s f o r l a r g e r hydrocarbons m o l e c u l e s .
TABLE 111 S i z e o f hydrocarbon m o l e c u l e s i n r e l a t i o n t o c a t a l y s t dimensions.
CJlalvsl
j <30APD
30 6 0 8 P O
> 60 A P O 10
60 154
62
I
129
38
I
13
12
10
14
107 Due to the increasing demand for lighter products, like gasoline and light cycle oil, combined with a shrinking market for fueloil there is nowadays a lot o f interest in processing heavier feedstocks, like residual oils. Therefore active matrix components receive again a lot of attention now. What the optimum pore size distribution is, however. is still a matter of discussion. Ruchkenstein et.al. (ref. 33) calculated that the optimum pore diameter will be approximately double the molecule size for the case that the molecules have no difficulty in entering the pores. I f this is not the case, the optimum size may be up to six times larger than the molecule size. The concept of optimum pore size distribution was also stressed by Maselli et a1 (ref. 3 4 ) . but these workers were not able to demonstrate the existence of diffusion limitations in their fixed bed micro-activity test. Young et a1 (ref. 3 5 ) separated cracking catalysts into different size fractions and tested them with a heavy FCC feed in order to detect matrix diffusion limitations. since increasing the particle size would increase the diffusion resistance. They concluded that contrary to theoretical expectations no diffusion limitations were observed. Nevertheless, several authors (ref. 36.37) emphasize the importance of a combination o f a large pore catalyst with a low hydrogen transfer molecular sieve to convert bottoms into valuable products. Another effect that one has to take into account is Pore mouth plugging (ref. 3 8 ) . which can reduce the accessibility of the pores to a large extent. Especially for resid cracking where metal contaminants are present, this effect will probably also play an important role. An interesting model has been presented some time ago by Takatsuka (ref. 3 9 ) who evaluated the performance o f three resid catalysts in their pilot plant using hydroconverted OAO of Arabian Heavy Vacuum Residue as feedstock. In Fig. 6 the product distributions are compared at the conversion level of maximum gasoline yield. The performance o f these catalysts are strongly correlated with the Pore size distribution of the catalyst matrix. The author distinghuishes three different regions o f pore sizes (see fig. 7 ) : A-pores B-pores C-pores
not accesable to large molecules accessable. nigh SA contribution pore s ze 2 to 6 times of molecular size accessable. low SA contribution
However. it may be emphasized that the balance between zeolite and matrix activity is also important for an optimal selectivity (ref. 3 7 . 4 0 ) .
108
1 1 ----- P I-
GAS
GASOLINE
LCO HCO
COKE COMPETITOR 2 COMPETITOR 1 LCO GAS t. GASOLINE C, 430°F HCO
M Z 11X 430 650'F 650°F'
F i g . 6. FCC p r o d u c t s d i s t r i b u t i o n by v a r i o u s c a t a l y s t s ( a t maximum g a s o l i n e y i e l d ) .
F i g . 7 . E f f e c t o f p o r e s i z e d i s t r i b u t i o n on bottoms c o n v e r s i o n .
109 V I . METAL TRAPS The d e t r i m e n t a l e f f e c t s o f m e t a l s on t h e performance of c r a c k i n g c a t a l y s t s a r e w e l l known now: N i i n c r e a s e s t h e gas ( e s p e c i a l l y H and coke p r o d u c t i o n . 2 whereas V l o w e r s t h e c o n v e r s i o n and s e l e c t i v i t y by d e s t r o y i n g t h e z e o l i t e c r y s t a l l i n i t y , which i s a l s o r e f l e c t e d i n an i n c r e a s e d gas and coke make(ref.44. N i p o i s o n i n g can be c o n t r o l l e d more o r l e s s by u s i n g antimony compounds. which a r e added t o t h e f e e d s t o c k ( r e f . 4 2 ) . Another p o s s i b i l i t y t o suppress t h e d e t r i m e n t a l dehydrogenation r e a c t i o n s i s t o use c a t a l y s t s w i t h a s i l i c a r i c h m a t r i x ( r e f . 43, 4 4 ) . F o r V p o i s o n i n g t h e s o l u t i o n i s l e s s c l e a r . Due t o i t s l o w m e l t i n g p o i n t ,
V205 i s v e r y m o b i l e under FCC condit1or.s ( ~ f . 4 5 ~ 4 6and ) can e a s i l y r e a c h t h e z e o l i t e s . T h e r e f o r e , t n e mechanism o f t h e r e a c t i o n between V205 and t h e z e o l i t e has been s t u d i e d e x t e n s i v e l y . From X-ray d i f f r a c t i o n and DTA measurements, PomDe e t . a l ( r e f . 4 7 ) concluded t h a t V205 r e a c t s w i t h RE f r o m t h e z e o l i t e , g i v i n g r i s e t o t h e f o r m a t i o n o f RE-vanadates. T h i s vanadate f o r m a t i o n r e q u i r e s more oxygen p e r RE atom t h a n can be s u p p l i e d b y V205. T h e r e f o r e , t h e a u t h o r s assume t h a t e x t r a oxygen i s t a k e n f r o m t h e z e o l i t e s t r u c t u r e . wbich l e a d s t o d e s t r u c t i o n o f t h e . l a t t i c e . However, such a mechanism i s n o t so v e r y p r o b a b l y under o x i d i z i n g c o n d i t i o n s as a r e p r e s e n t i n t h e r e g e n e r a t o r , as has been p o i n t e d o u t by Maug6 e t . a l ( r e f . 4 8 ) . They c o n d u d e d t h a t t h e d e s t a b i l i z a t i o n o f t h e z e o l i t e i s m a i n l y due t o t h e disappearance o f t h e s t a b i l i z i n g La-0-La b r i d g e s i n t h e s o d a l i t e cage b y r e a c t i o n o f La 0 and V205 under f o r m a t i o n o f LaV04. 2 3 A c o m p l e t e l y d i f f e r e n t t h e o r y was i n t r o d u c e d b y Wormsbecher e t . a l . ( r e f .
46).
These a u t h o r s conclude t h a t under r e g e n e r a t o r c o n d i t i o n s V205 i s c o n v e r t e d i n t o vanadic a c i d , H3V04, which may be c o n s i d e r e d as a s t r o n g a c i d analoqous t o H3P04. T h i s H3V04 can d e s t r o y t h e z e o l i t e b y h y d r o l y s i s o f t h e z e o l i t e Si02/A1203 framework. T h e r e f o r e t h e y propose b a s i c compounds as MgO o r CaO t o n e u t r a l i z e t h e vanadic a c i d . A s i m i l a r concept was developed b y Masuda e t . a l . ( r e f .
4 9 ) . These a u t h o r s a l s o
assume t h a t t h e a c i d i c - b a s i c p r o p e r t i e s o f t h e m e t a l o x i d e s a r e d e t e r m i n i n g t h e i r u s e f u l l n e s s as V c a t c h e r . However, t h e s e t y p e o f conwounds a r e harmful f o r t h e hydrothermal s t a b i l i t y o f the z e o l i t e . Therefore o n l y physical mixtures c o u l d be used i n p r a c t i c e , whereby t h e z e o l i t e and b a s i c compound a r e i n d i f f e r e n t p a r t i c l e s . Consequently, t h e z e o l i t e w i I 1 be l e s s e f f e c t i v e l y p r o t e c t e d a g a i n s t vanadium. A d i f f e r e n t approach was r e c e n t l y developed by G e i s l e r e t . a l . ( r e f .
5 0 ) . who
used compounds i n which t h e m e t a l o x i d e w i t h a h i g h a f f i n i t y t o V205 i s p r e s e n t b u t as a p a r t o f a v e r y s t a b l e s t r u c t u r e .
110
T h i s means t h a t t h e m e t a l o x i d e i s b l o c k e d and d o e s n ’ t h u r t t h e z e o l i t e , whereas r e a c t i o n w i t h V205 under f o r m a t i o n o f h i g h m e l t i n g vanadates remains p o s s i b l e . That t h i s concept worked o u t has been p r o v e n b o t h by performance t e s t s and SEM s t u d i e s . The t e s t r e s u l t s f o r a c a t a l y s t w i t h and w i t h o u t m e t a l t r a p a r e summarized i n T a b l e I V . F r o m t h i s T a b l e i t can be concluded t h a t t h e c a t a l y s t w i t h m e t a l t r a p shows a much b e t t e r performance t h a n t h e c a t a l y s t w i t h o u t t h i s t r a p o r w i t h an o l d e r one. TABLE I V
MAT d a t a o f c a t a l y s t s w i t h an i n t e r m e d i a t e amount o f REY z e o l i t e , an i n e r t m a t r i x w i t h and w i t h o u t m e t a l t r a p s . A. w l t h o u t m e t a l t r a p
no m e t a l s Cow.. X wt ROI
66.5 1.00 1.11
. Y .gas01 i n e a coke
“2
1000 ppm N i b 3000 ppm V
8. m e t a l t r a p I C. metal t r a p I 1 no m e t a l s 1000 ppm Nib no m e t a l s 1000 ppm Nib 3000 ppm V 3000 ppm V
45.2 0.96 2.00 15.9
64.2 0.99 1.01 11.8
62.3 0.98 1.22 6.8
C a t a l y s t p r e t r e a t m e n t : s t e a m d e a c t i v a t i o n f o r 17 h r s a t 75OOC ( 1 b a r , 100 % steam). MAT c o n d i t i o n : r e a c t o r temperature : 483OC Cat on o i l t i m e : 50 s e c . WHSV : 12 h - 1 Feed : K u w a i t VGO a. Re1.Y i s t h e p r o d u c t y i e l d r e l a t i v e t o MZ-7 s t a n d a r d c a t a l y s t a t equal conversion. b . Pore volume i m p r e g n a t i o n .
SEM s t u d i e s on e q u i l i b r i u m c a t a l y s t s c o n t a i n i n g t h i s new m e t a l t r a D were a l s o c a r r i e d o u t . A c r o s s - s e c t i o n o f an FCC m i c r o s p h e r e can be seen i n F i g . 8 . whereas F i g . 9 g i v e s t h e V d i s t r i b u t i o n i n t h a t p a r t i c u l a r particle.WDXA a n a l y s i s shows t h a t t h e V c o n c e n t r a t i o n i n t h e m e t a l t r a p i s v e r y h i g h ( s e e Table V 1. TABLE V R e l a t i v e vanadium d i s t r i b u t i o n o n c a t a l y s t w i t h m e t a l t r a p . CATALYST LAB IMPREGNATED EQUILIBRIUM CAT 3000 pprn . 6084 ppm TOTAL VANADIUM ~~
MATRIX ALUMINA ZEOLITE METALTRAP
~~
1.o 2.6 1.3 26.1
1 .o 3.2 1.7 18.3
111
F i g . 8 . Cross s e c t i o n o f an FCC microsphere w i t h metal t r a p .
Fig.9.
Vanadium d i s t r i b u t i o n o f an FCC microsphere w i t h m e t a l t r a p .
V I I . DESOX I t i s w e l l recognized now t h a t t h e
SOx emissions f r o m r e f i n e r i e s c o n t r i b u t e
t o a l a r g e e x t e n t t o t h e a c i d r a i n problem. T h e r e f o r e , a l l k i n d of r e g u l a t i o n s f o r t h e SOx emissions a r e s e t up now. I n response t o t h i s , c a t a l y s t manufact o r e r s developed adapted c a t a l y s t compositions o r s p e c i a l a d d i t i v e s t o reduce SOx emissions o f FCC u n i t s . With these p r o d u c t s r e d u c t i o n s from ca. 20 up t o 70 %' a r e r e p o r t e d ( r e f . 51, 5 4 ) .
The mechanism o f t h e SOx t r a n s f e r has been o u t l i n e d by Habib ( r e f . 5 5 ) , whereas an e x c e l l e n t r e v i e w o f t h e s t a t e o f t h e a r t was r e c e n t l y p r e s e n t e d by B e r t o l a c i n i e t . a t .
(ref.56).
112
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H.G.Karge, J. Weitkamp (Editors),Zeolites (IS Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
EVALUATION OF NON-COMMERCIAL MODIFIED LARGE PORE ZEOLITES I N FCC E. JACQUINOT, F. RAATZ, A. MACEDO and Ch. MARCILLY
INSTITUT FRANCAIS DU PETROLE, 1-4 r u e du B o i s Preau BP311, 92506 Ruei 1 Malmai son, France ABSTRACT The c a t a l y t i c performances o f H-Y, H-Beta and H-Omega prepared by a steaming-acid l e a c h i n g procedure, have been e v a l u a t e d i n cyclohexene and i n Vacuum Gas O i l ( V G O ) t r a n s f o r m a t i o n . The l e a c h i n g i s a key s t e p , s i n c e depending on t h e o p e r a t i n g c o n d i t i o n s i t can decrease o r i n c r e a s e t h e c o n v e r s i o n l e v e l s . I n cyclohexene t r a n s f o r m a t i o n , t h e a c i d i t y o f t h e s o l i d s : controls the activity and the following ranking is obtained H-Beta)H-Y)H-Omega. I n c o n t r a s t , i n VGO c o n v e r s i o n a c c e s s i b i l i t y o f t h e a c i d c e n t e r s has t o be taken i n t o account which means t h a t H-Y l e a d s t o t h e b e s t r e s u l t s . Moreover, o p t i m i z a t i o n o f t h e a c i d i c p r o p e r t i e s o f H - Y t h r o u g h a s e l e c t i v e a c i d l e a c h i n g improves s i g n i f i c a n t l y t h e c o n v e r s i o n l e v e l s . INTRODUCTION
Y z e o l i t e s m o d i f i e d by two main techniques, (framework
and are
of
or
isomorphous
commercial
FCC
substitution
treatment
(framework
a c t i v e component
only)
hydrothermal
o v e r a l l dealumination), containing o r not containing r a r e earth cations, the
dealumination
i.e.
catalysts
(1,
2,
3).
Indeed,
a
tremendous amount o f d a t a i s a v a i l a b l e i n t h e l i t e r a t u r e on t h e b e h a v i o r o f t h e s e sol i d s . However, o t h e r l a r g e - p o r e z e o l i t e phases a r e p o t e n t i a l c a n d i d a t e s f o r t h i s a p p l i c a t i o n . Depending on whether o r n o t t h e aim i s t o f u l l y r e p l a c e t h e a c t u a l a c t i v e component, and e x c l u d i n g any p r i c e c o n s t r a i n t , two t y p e s o f s o l i d s c o u l d be a p r i o r i considered.
As an a l t e r n a t i v e t o t h e a c t u a l main
a c t i v e component, o n l y l a r g e - p o r e z e o l i t e s w i t h m u l t i d i m e n s i o n a l porous systems should be c o n s i d e r e d as p r o m i s i n g c a n d i d a t e s . zeolite
itself,
(hydrothermal structures
but
submitted
treatment
such
as
followed
ZSM-20
and
to by
a
I n t h i s c a t e g o r y we
conventional acid
zeolite
Beta
z e o l i t e s w i t h monodimensional porous systems,
overall
leaching),
(4).
and
find Y
dealumination other
Deal umi n a t e d
such as Omega and L,
modified
1 arge-pore a r e more
l i k e l y t o be o n l y u s e f u l as secondary a c t i v e components ( 5 ) . L i t t l e i s known about t h e c a t a l y t i c performances i n FCC o f t h e p o t e n t i a l c a n d i d a t e s mentioned above ( 4 , 51, and most o f t h e s t u d i e s a l r e a d y p u b l i s h e d have been devoted t o n o n - m o d i f i e d Beta z e o l i t e s . R e c e n t l y , A.
Corma e t a l .
( 6 - 9 ) have e v a l u a t e d t h e c r a c k i n g p r o p e r t i e s o f Beta z e o l i t e s w i t h v a r i o u s
Si/A1
ratios,
and o f c o n v e n t i o n a l l y dealuminated Y z e o l i t e s .
Two i m p o r t a n t
c o n c l u s i o n s emerge f r o m t h i s work : i ) a t l e a s t f o r n-heptane c r a c k i n g H-Beta i s superior t o H-Y,
and i i )a m i l d a c i d l e a c h i n g o f steamed H-Y g i v e s r i s e t o a
116
decrease o f t h e a c t i v i t y i n g a s - o i l c r a c k i n g . We aim h e r e t o e v a l u a t e t h e performances,
i n a model t e s t r e a c t i o n and i n
t h e c r a c k i n g o f a VGO o f z e o l i t i c phases, which a r e o f p o t e n t i a l i n t e r e s t i n since i t allows a r a p i d
FCC. Cyclohexene has been chosen as a t e s t molecule,
e v a l u a t i o n of t h e hydrogen t r a n s f e r tendency o f t h e c a t a l y s t s , i . e .
formation
o f benzene, cyclohexane and m e t h y l c y c l o p e n t a n e r a t h e r t h a n m e t h y l c y c l o p e n t e n e . I n a f i r s t approach, we w i l l
r e s t r i c t ourselves t o t h e study o f t h e f r e s h
c a t a l y s t s , which means t h a t h y d r o t h e r m a l a g i n g and p o i s o n i n g by metal w i l l n o t be c o n s i d e r e d h e r e .
EXPERIMENT SOL I D S S t a r t i n g f r o m low-sodium ( S i / A l = 14.0)
(NaC400 ppm)
and NH4-Omega ( S i / A l
= 2.91,
NH4-Y
(Si/Al
=
2.81,
dealuminated s o l i d s
NH4-Beta have been
prepared by a t w o - s t e p procedure, i.e. hydrothermal t r e a t m e n t (100% steam) a t v a r i o u s temperatures
(823 t o 1123 K )
s o l u t i o n a t 373 K ( 1 and 3N f o r H-Y,
f o l l o w e d by an a c i d l e a c h i n g i n HC1 1N f o r H-Beta,
0.5N f o r H-Omega). The
sol i d s were c h a r a c t e r i z e d a t each s t e p o f t h e p r e p a r a t i o n procedures by chemical a n a l y s i s , Further d e t a i l s
XRD,
29 S i
MASNMR,
I R spectroscopy
and
N2
adsorption.
on t h e d e a l u m i n a t i o n procedures and t h e c h a r a c t e r i z a t i o n
t e c h n i q u e s can be found i n r e f . ( 1 0 ) . The o r i g i n a l Y z e o l i t e was p r o v i d e d by Zeocat (Z-FlOO),
w h i l e Omega and Beta were s y n t h e s i z e d by J.L.
GUTH e t a1
( E c o l e N a t i o n a l e S u p c r i e u r e de Chimie, Mulhouse, F r a n c e ) . CATALYTIC TESTS MODEL MOLECULE. Cyclohexene t r a n s f o r m a t i o n was s t u d i e d u s i n g a Geomecanique m i c r o t e s t u n i t a l l o w i n g v e r y l o w c o n t a c t t i m e s and a n a l y s i s o f t h e p r o d u c t s a t v a r i o u s t i m e s on stream. The t e s t s were conducted a t atmospheric p r e s s u r e under 5 t h e f o l l o w i n g c o n d i t i o n s : cyclohexene p a r t i a l p r e s s u r e 0 . 2 0 ~ 1 0 Pa, c a r r i e r gas n i t r o g e n , c o n t a c t t i m e s i n t h e 0.25 t o 3.0 s range, t e m p e r a t u r e 643 K, sampling a t d i f f e r e n t t i m e s on s t r e a m ( 2 , 5, 8, 11 m i n ) . VACUUM GAS OIL. Aramco 150 c r a c k i n g was s t u d i e d u s i n g a MAT u n i t a t 755 K under
atmospheric p r e s s u r e . W h i l e k e e p i n g a c o n s t a n t i n j e c t i o n t i m e ( 7 5 s ) , d i f f e r e n t catalyst-to-oil
weight r a t i o s were used (1.2
t o 5.3)
i n order t o vary t h e
c o n v e r s i o n l e v e l s . P r i o r t o t h e MAT t e s t s , t h e z e o l i t e s (20 % i n w e i g h t ) were blended w i t h an aged s i l i c a - a l u m i n a m a t r i x . RESULTS The main physicochemical
characteristics
of
t h e modified zeolites
are
reported i n table 1 together with the operating conditions applied f o r the d e a l u m i n a t i o n procedures.
117 TABLE 1 a ) Chemical and t e x t u r a l c h a r a c t e r i s t i c s o f dealuminated Y z e o l i t e s . * C a l c u l a t e d f r o m t h e u n i t c e l l parameter, u s i n g t h e c o r r e l a t i o n g i v e n i n r e f . (19) ** XRD c r i s t a l l i n i t y , t h e o r i g i n a l NH4-Y i s t a k e n as a r e f e r e n c e . P r e p a r a t i o n procedure Steaming 823 K HCL 1 N HC1 3N Steaming 873 K HC1 1N HC1 3N
8I 4
42:;
T
::;
2 4
57
43
Steaming 1043 K HC1 1N HC1 3N
% DX**
(Si/Al
::: :::
37
Steaming 9 / 3 K HC1 1N HC1 3N
Steaming 1113 HC1 1N HC1 3N
( S i /A1 ) overall
% 69 % 0 %
9 00
1 0 80
%
72 %
11
>loo
69 % 85 % 79 % 93 %
16 41
44
21 42
93 % 78 % 94 %
1:*8 74
> 50 > 50 > 50
91 % 87 % 91 %
K
b ) Chemical and t e x t u r a l c h a r a c t e r i s t i c s o f dealuminated B e t a z e o l i t e s I
I I I I I I
I I I I I I I
1
P r e p a r a t i o n procedure
( Si /A1
Steaming 873 K HC1 1N
1 8
Steaming 973 K HC1 1 N
1 4
Steaming 1073 K HC1 1 N Steaming 1123 K HC1 1 N
overall
62 53 14 29
7 4 19
118 c ) Chemical and t e x t u r a l c h a r a c t e r i s t i c s o f dealuminated Omega z e o l i t e s . * 29Si RMN s i m u l a t i o n a c c o r d i n g t o r e f ( 2 0 ) . 0 The c parameter i s n o t r e p o r t e d s i n c e i t o n l y v a r i e s between 7,51 and 7,55 A . ** XRD c r i s t a l l i n i t y , t h e o r i g i n a l NH4-0mega i s t a k e n as a r e f e r e n c e . ( S i / A l I V ) *% DX**
l p r e p a r a t i o n procedure
I I I I I I
I
I I
a
(i)
Steaming 923 K HC1 0,5 N
T 2 - r l-Kl-31 1 10 94 % ia,io
Steaming 913 K HC1 0,5 N
1 8 85% ia 90 %
Steaming 1013 K HC1 0,5 N
3 068% 18;ar
Steaming 1123 K HC1 0,5 N
loo66% 77-38-
m 18,06
70 %
30
18,Ol
I
I
I I
100
66 %
18,OO
I
As
expected,
temperatures
whatever
leads
to
a
the
structure,
dealumination
the
of
the
steaming framework.
at The
increasing resulting
extra-framework species cause a r e d u c t i o n of t h e N2 a d s o r p t i o n c a p a c i t y o f H-Y and H-Beta z e o l i t e s which do have a m u l t i d i m e n s i o n a l porous system (111, w h i l e a complete b l o c k i n g o f t h e monodimensional porous system o f H-Omega o c c u r s . Concerning t h e e f f e c t o f t h e p o s t - s t e a m i n g a c i d l e a c h i n g two s i t u a t i o n s have t o be c o n s i d e r e d : i ) s e l e c t i v e a c i d l e a c h i n g g i v i n g r i s e t o an e l i m i n a t i o n o f extra-framework species w i t h o u t any f u r t h e r framework d e a l u m i n a t i o n ( H - Y
and
H-Omega
ii)
treated
in
1N
and
0.5N
HC1
solutions,
respectively),
and
n o n - s e l e c t i v e a c i d l e a c h i n g r e s u l t i n g i n a s e v e r e framework d e a l u m i n a t i o n (H-Y and H-Beta t r e a t e d i n 3N and 1N HC1 s o l u t i o n s , r e s p e c t i v e l y ) . I n c o n t r a s t t o t h e steamed H - Y and H-Omega,
steamed H-Beta appeared t o be
very s e n s i t i v e t o a c i d l e a c h i n g , s i n c e even a m i l d l e a c h i n g l e a d s t o v e r y h i g h o v e r a l l S i / A 1 r a t i o s . T h i s i s l i k e l y t o be due t o t h e s i g n i f i c a n t amount o f s t r u c t u r a l defects present i n t h i s z e o l i t e a f t e r t h e synthesis. Without going i n t o t o o much d e t a i l , i t s h o u l d be n o t e d t h a t most of t h e s o l i d s have a h i g h degree o f c r y s t a l l i n i t y , and t h a t f o r each s t r u c t u r e a wide range o f framework and o v e r a l l S i / A 1
r a t i o s a r e covered.
performances o f t h r e e v e r y framework and o v e r a l l S i / A l
important
Thus,
t h e i n f l u e n c e on t h e c a t a l y t i c
parameters namely
the
structure,
the
r a t i o s o f t h e s o l i d s can be s t u d i e d . Since i t i s
n o t always p o s s i b l e t o d e t e r m i n e w i t h good accuracy t h e framework S i / A l r a t i o
o f a l l t h e s o l i d s (H-Beta and a c i d leached s o l i d s c o u l d c o n t a i n many s t r u c t u r a l d e f e c t s ) , we have chosen t o r e p o r t t h e c o n v e r s i o n l e v e l s versus t h e steaming temperature o f t h e o r i g i n a l NH4-form, acid leaching.
even f o r s o l i d s s u b m i t t e d t o a subsequent
119
HY-ZEOLITES
A f t e r steaming, t h e cyclohexene c o n v e r s i o n p r e s e n t s a f l a t optimum versus t h e temperature employed f o r t h e steaming t r e a t m e n t Si/Al
(i.e.
versus t h e framework
r a t i o o f t h e s o l i d s , see t a b l e 1 ) and s e r i o u s l y decreases f o r steaming
temperatures h i g h e r t h a n about 900 K ( f i g u r e 1 ) . The most i m p o r t a n t p o i n t which i s evidenced on f i g u r e 1 concerns t h e e f f e c t o f t h e a c i d l e a c h i n g . Depending on whether o r n o t t h e l e a c h i n g i s s e l e c t i v e ( e x t r a - f r a m e w o r k A1 e x t r a c t i o n w i t h o u t any f u r t h e r framework d e a l u m i n a t i o n , t a b l e 11, an i n c r e a s e o r a decrease o f t h e cyclohexene c o n v e r s i o n l e v e l s i s observed. The same e f f e c t s a r e o b t a i n e d i n VGO transformation ( f i g u r e 2 ) . H-BETA ZEOLITES
Steaming a t i n c r e a s i n g temperatures l e a d s t o a monotonic decrease o f t h e cyclohexene steaming,
conversion
levels
(figure
1).
I t should
be n o t e d t h a t
after
t h e cyclohexene c o n v e r s i o n l e v e l s o f H-Beta a r e h i g h e r t h a n t h o s e
o b t a i n e d f o r H-Y.
The m i l d p o s t - s t e a m i n g a c i d l e a c h i n g r e s u l t s i n a severe
r e d u c t i o n o f t h e s e c o n v e r s i o n l e v e l s , whereas t h e same t r e a t m e n t has a p o s i t i v e e f f e c t f o r H-Y ( f i g u r e 1 ) . As i n t h e case of cyclohexene,
t h e VGO c o n v e r s i o n i s s t r o n g l y a f f e c t e d by
steaming ( f i g u r e 2 ) b u t shows a d i f f e r e n t b e h a v i o r a f t e r a c i d l e a c h i n g . The l a t t e r t r e a t m e n t g i v e s r i s e t o a small i n c r e a s e o f t h e VGO c o n v e r s i o n l e v e l s ( f i g u r e 21, which should be compared w i t h t h e n e g a t i v e e f f e c t evidenced f o r t h e cyclohexene t r a n s f o r m a t i o n ( f i g u r e 1 ) . Moreover, i n VGO c o n v e r s i o n whatever t h e p r e p a r a t i o n procedures
we
used,
H-Beta
gives
lower
performances
t h a n H-Y
(figure 2 ) .
H-OMEGA ZEOLITES
C o n t r a r y t o H-Y and H-Beta,
v e r y low cyclohexene c o n v e r s i o n l e v e l s a r e
o b t a i n e d f o r steamed H-Omega whatever t h e steaming t e m p e r a t u r e ( f i g u r e 1 ) . For t h e s e s o l i d s even a v e r y m i l d and non-optimized a c i d l e a c h i n g l e a d s t o a severe increase o f the conversion l e v e l s . Similarly,
t h e VGO c o n v e r s i o n
level
after
steaming
is
very
low
and
i n c r e a s e s v e r y s i g n i f i c a n t l y a f t e r t h e a c i d l e a c h i n g ( T a b l e 2). However, w h i l e for
the
acid-leached
H-Omega
the
conversion
levels
in
cyclohexene
i t i s no l o n g e r t h e case f o r t h e VGO t r a n s f o r m a t i o n f o r which H-Y appears t o be s u p e r i o r t o H-Omega ( f i g u r e s 1, Table 2 ) .
t r a n s f o r m a t i o n a r e v e r y c l o s e t o t h o s e o f H-Y,
120
75 H-Y
923
STEAMING TEMPERATURE IKI
-7
01
823
1023
873
1123
1
I
261
823
973
1073
I 973
1073
STEAMING TEMPERATURE IK
ti-BETA
I
I
1 on
923
I
873
1123
STEAMING TEMPERATURE (Kl
1w H-OMEGA
I
1
STEAMING TEMPERATURE ( K 1
F i g u r e 2 : Conversion o f VGO versus t h e steaming t e m p e r a t u r e (,steamed + ; steamed and a c i d leached + HC1 1N ; + HC1 3N) R e a c t i o n t e m p e r a t u r e = 755 K ; i n j e c t i o n t i m e = 75 s ; C/O = 3 ; WHSV = 16 h-1 TABLE 2 MAT c o n v e r s i o n and y i e l d s of d i f f e r e n t products o f gas-oil cracking f o r H -Omeg a. Preparation procedure
MAT conv. (wt.%)
Sti;!,"g
25.7
HC1 0.5N
41.2
STEAMING TEMPERATURE (Kl
Fi ure 1 Conversion l e v e l o f c y c l o b e i s u s t h e steaming t e m p e r a t u r e (steamed + ; steamed and a c i d leached +HC1 0,5N ; + HC1 1N ; +HC1 3N) Reaction t e m p e r a t u r e = 643K ; c o n t a c t t i m e = 0,5 s ; t i m e on stream = 2 min.
Gasoline yield (wt.%)
19,8
Gas
;% ! (wt.%) 5.1 14.0
Coke yield (wt.")
1.4 6.0
121
DISCUSSION
CONVERSION. Obviously, t h e framework d e a l u m i n a t i o n o c u r r i n g i n t h e c o u r s e o f t h e steaming procedure i s r e s p o n s i b l e f o r t h e decrease o f t h e cyclohexene and VGO c o n v e r s i o n l e v e l s .
However,
s e v e r a l i m p o r t a n t remarks have t o be made.
F i r s t , a t l e a s t i n t h e case o f H-Y t h e decrease i s non-monotonic,
i.e a f l a t
optimum i s observed near 900 K . Since i t has been shown t h a t t h e s t r e n g t h o f t h e framework a c i d s i t e s i n H - Y i n c r e a s e s w i t h t h e d e a l u m i n a t i o n l e v e l (12,131, t h i s optimum i s l i k e l y t o r e s u l t f r o m two c o m p e t i t i v e e f f e c t s , i . e r e d u c t i o n o f t h e number o f s i t e s and i n c r e a s e o f t h e i r a c i d s t r e n g t h . According t o t h e model advanced by Barthomeuf (141,
such an optimum i s n o t observed f o r z e o l i t e s ,
which a r e s y n t h e s i z e d w i t h t o o h i g h an i n i t i a l
Si/A1
ratio.
This
i s for
example, t h e case w i t h H-Beta. Framework d e a l u m i n a t i o n c o n t r i b u t e s a l s o t o t h e r e d u c t i o n o f t h e c o n v e r s i o n l e v e l s f o r H-Omega, b u t f o r t h i s s t r u c t u r e b l o c k i n g o f t h e channels by extra-framework species p l a y s t h e m a j o r r o l e ( T a b l e 1 ) . T h i s b e h a v i o r i s l i k e l y t o be t y p i c a l o f z e o l i t e s s y n t h e s i z e d w i t h low S i / A l r a t i o s and possessing an u n i d i m e n s i o n a l porous system. The most i m p o r t a n t p o i n t evidenced i n t h i s work i s l i k e l y t o concern t h e e f f e c t o f t h e post-steaming a c i d l e a c h i n g , which c o u l d have a n e g a t i v e b u t a l s o a s t r o n g l y p o s i t i v e i n f l u e n c e on t h e c o n v e r s i o n l e v e l s . Focussing on t h e d a t a p r e s e n t e d here, i t appears t h a t s e l e c t i v e a c i d l e a c h i n g , which o n l y e x t r a c t s extra-framework aluminium species, w i l l l e a d t o an i n c r e a s e o f t h e c o n v e r s i o n levels. giving
The r e v e r s e i s t r u e f o r n o n - s e l e c t i v e rise to
a further
framework
r a t i o n a l i z e d ? Since t h e a c i d accessibility
acid treatments,
dealumination.
How can
leaching could a f f e c t
o f the a c t i v e acid centers,
two
types
c o n s i d e r e d : i ) those possessing a m u l t i d i m e n s i o n a l
the of
i.e.
those
such
behavior
acidity
and t h e
solids
s h o u l d be
porous system,
and ii)
those h a v i n g a monodimensional porous s t r u c t u r e . For s o l i d s w i t h a m u l t i d i m e n s i o n a l porous system ( Y , b e t a ) , o n l y a l i m i t e d r e d u c t i o n o f t h e m i c r o p o r e volume occurs upon steaming, volume i s n o t i n c r e a s e d v e r y s i g n i f i c a n t l y
and t h e m i c r o p o r e
by l e a c h i n g .
Thus,
the positive
e f f e c t on t h e c o n v e r s i o n l e v e l s o f s e l e c t i v e a c i d l e a c h i n g (H-Y t r e a t e d i n 1N HC1 s o l u t i o n ) i s more l i k e l y t o be due t o an i n c r e a s e i n t h e a c i d i c p r o p e r t i e s
o f the s o l i d s r a t h e r than t o a b e t t e r a c c e s s i b i l i t y o f t h e a c i d s i t e s . statement i s s t r o n g l y supported by p r e v i o u s work which has shown, complementary p h y s i c a l techniques,
This
by v a r i o u s
t h a t an i n c r e a s e i n t h e number o f s t r o n g
framework s i t e s occurs t h r o u g h t h e e l i m i n a t i o n , by s e l e c t i v e a c i d l e a c h i n g , o f poisoning
extra-framework
aluminium c a t i o n s
formed
in
the
course
of
the
steaming procedure ( 1 2 ) . Extra-framework aluminium s p e c i e s which e x h i b i t a c i d i c p r o p e r t i e s a r e a l s o e l i m i n a t e d by m i l d l e a c h i n g .
However,
these s i t e s o n l y
possess a moderate Bronsted a c i d i t y ( 1 2 ) and t h u s a r e n o t l i k e l y t o p l a y t h e
122 m a j o r r o l e i n t h e c a t a l y t i c performances o f t h e s o l i d s under t h e c o n d i t i o n s we It s h o u l d be s t r e s s e d t h a t t o b e t t e r r e v e a l t h e b e n e f i t o f a
have employed.
s e l e c t i v e l e a c h i n g o f H-Y i n VGO c o n v e r s i o n , t h e s o l i d has t o be t e s t e d w i t h an a c t i v e m a t r i x so t h a t molecules which a r e t o o l a r g e t o e a s i l y e n t e r t h e z e o l i t e crystals
c o u l d r e a c t more
rapidly.
Otherwise t h e
extra-framework
material
l o c a t e d a t t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e s c r y s t a l would h e l p i n t h e t r a n s f o r m a t i o n o f such heavy molecules,
and t h e s e l e c t i v e a c i d l e a c h i n g would
p o s s i b l y have a n e g a t i v e e f f e c t (8, 9). T h i s p o i n t remains t o be c o n f i r m e d . On t h e
contrary
i n the
case
of
Omega
for
which
the
unidimensional
microporous system i s n e a r l y c o m p l e t e l y b l o c k e d a f t e r t h e steaming s t e p ,
the
p o s i t i v e e f f e c t o f a s e l e c t i v e post-steaming a c i d leaching c l e a r l y r e s u l t s from t h e e l i m i n a t i o n o f t h e extra-framework m a t e r i a l s located i n t h e pores. This does n o t mean t h a t t h e a c i d i c p r o p e r t i e s of t h e framework a r e n o t
m o d i f i e d by
l e a c h i n g , b u t t h e s e changes i n a c c e s s i b i l i t y a r e h e r e predominant. C o n s i d e r i n g now n o n - s e l e c t i v e a c i d l e a c h i n g (H-Y and H-Beta t r e a t e d i n 3N and 1N HC1 s o l u t i o n s r e s p e c t i v e l y ) , i t i s c l e a r t h a t framework d e a l u m i n a t i o n o c c u r r i n g as a r e s u l t o f t h i s t r e a t m e n t i s r e s p o n s i b l e f o r t h e r e d u c t i o n o f t h e conversion
levels.
Indeed,
as
shown
by
physical
techniques,
the
acidic
p r o p e r t i e s o f s o l i d s s u b m i t t e d t o a n o n - s e l e c t i v e l e a c h i n g a r e v e r y low ( 1 2 ) . However,
H-Beta d i f f e r s f r o m H-Y,
since non-selective
leaching leads t o a
s t r o n g decrease o f t h e cyclohexene c o n v e r s i o n , w h i l e a small
increase o f t h e
VGO c o n v e r s i o n occurs. To e x p l a i n such b e h a v i o r t h e r o l e o f a c c e s s i b i l i t y has t o be t a k e n i n t o account. Cyclohexene i s a s m a l l m o l e c u l e f o r which s t e r i c c o n s t r a i n t s a r e n o t v e r y i m p o r t a n t , t h u s i t m a i n l y probes t h e a c i d i c p r o p e r t i e s o f t h e s o l i d s . From t h a t p o i n t of
view,
t h e f o l l o w i n g r a n k i n g i s o b t a i n e d : steamed H-Beta > steamed
H-Yxteamed + a c i d leached H-Omega. T h i s r a n k i n g i s i n q u a l i t a t i v e agreement w i t h t h e a c i d i c p r o p e r t i e s o f t h e s o l i d s as d e t e r m i n e d by p h y s i c a l t e c h n i q u e s , and indeed s t r o n g a c i d s i t e s a r e p r e s e n t i n steamed H-Beta ( 1 5 ) . On t h e o t h e r hand i n VGO c o n v e r s i o n s t e r i c c o n s t r a i n t s cannot
be n e g l e c t e d .
T h i s would
suggest t h a t t h e m u l t i d i m e n s i o n a l channel system o f H-Beta i s n o t as a c c e s s i b l e f o r l a r g e molecules as t h e microporous system o f H - Y .
I n o t h e r words, i n a v e r y
f i r s t approximation, t h e a c c e s s i b i l i t y of t h e a c i d s i t e s r a t h e r t h a n t h e i r a c i d s t r e n g t h c o n t r o l s t h e VGO c o n v e r s i o n
l e v e l s o v e r H-Beta.
This
proposal
is
s u p p o r t e d by a r e c e n t work ( 7 1 , a c c o r d i n g t o which t h e gas o i l c o n v e r s i o n o v e r n o n - m o d i f i e d H-Beta does n o t depend much on i t s Si/A1 r a t i o ,
and by t h e n i c e
s t r u c t u r a l s t u d y o f TREACY e t a l . ( 1 1 ) which s t r o n g l y suggests t h a t t h e open s t r u c t u r e of
z e o l i t e Beta i s
accessibility
considerations
likely would
to
contain
structural
also
explain
why
defects.
acid-leached
These H-Omega
p r e s e n t r a t h e r low VGO c o n v e r s i o n l e v e l s , w h i l e t h e i r cyclohexene c o n v e r s i o n
123
-MAT CONVERSION (wt.%)
CVCLOHEXENE CONVERSION lmol.%l
15 COKE
10 s . c
I
-9 3
Y
>
5
0
25 CVCLOHEXENE CONVERSION lmol.%J
35
45
55
MAT CONVERSION Iw1.X)
STEAMED (923 I0 AND ACID LEACHED ( 0.5 N) H-OMEGA 0
CVCLOHEXENE CONVERSION (mol.%)
F i g u r e 3 : Comparison o f s e l e c t i v i t y curves f o r cyclohexene t r a n s f o r m a t i o n over H-Y, H-Beta and H-Omeaa z e o l i t e s ( I s o m e r i s a t i o n + ; Hydrogen t r a n s f e r + )
F i g u r e 4 : Comparison o f s e l e c t i v i t y curves f o r VGO c r a c k i n g on t h r e e z e o l i t e s = (steamed H - Y (873K) -c : steamed (97310 and a c i d leached (1Nj H-Beta +; steamed (92310 and a c i d leached (0.5N) H-Omega c 1.
124 l e v e l s a r e n o t t o o f a r f r o m t h o s e o f H-Y. SELECTIVITY CYCLOHEXENE. As
described
i n detail
cyclohexene t r a n s f o r m a t i o n o f
elsewhere
m o d i f i e d H-Y
(161,
are
framework Si/A1 r a t i o . H - Y w i t h a u n i t c e l l parameter f a v o r s hydrogen t r a n s f e r r e a c t i o n s (cyclohexane,
the
mainly
selectivities
determined
by
in its
h i g h e r t h a n 24.27
benzene,
methylcyclopentane
formation) r a t h e r than isomerisation (methylcyclopentene..),
while H-Y w i t h a
0
u n i t c e l l parameter l o w e r t h a n 24.27 A i s c h a r a c t e r i z e d by v e r y l o w hydrogen t r a n s f e r s e l e c t i v i t i e s (17, 1 8 ) ( f i g u r e 3 ) . T h i s b e h a v i o r c o u l d be r a t i o n a l i z e d as f o l l o w s . Depending on whether o r n o t t h e s o l i d s c o n t a i n more t h a n about one 0
A1 per supercage ( u n i t c e l l parameter lower o r h i g h e r t h a n about 24.27 A), t h e y
w i l l o r w i l l n o t promote b i m o l e c u l a r r e a c t i o n s such as hydrogen t r a n s f e r . It i s now i m p o r t a n t t o determine how t h e s e l e c t i v i t i e s o f m o d i f i e d H-Beta and H-Omega towards hydrogen t r a n s f e r compare w i t h t h o s e o f m o d i f i e d H-Y.
Considering t h e
s e l e c t i v i t y diagrams r e p o r t e d i n f i g u r e 3, i t appears t h a t t h e s e l e c t i v i t i e s o f H-Beta and H-Omega w i t h r a t h e r low-framework S i / A l t h o s e o f h i g h l y dealuminated H-Y.
r a t i o s are very s i m i l a r t o
T h i s would mean t h a t H-Beta and H-Omega do
n o t much promote b i m o l e c u l a r r e a c t i o n s . I t i s n o t c l e a r a t t h e moment whether such b e h a v i o r d e r i v e s f r o m s t e r i c c o n s t r a i n t s (no cages a r e p r e s e n t i n Omega and i n B e t a ) , o r f r o m a s p e c i f i c A1 s i t i n g . Since non dealuminated H-Mordenite i s a l s o c h a r a c t e r i z e d by a l o w hydrogen t r a n s f e r s e l e c t i v i t y i n cyclohexene c o n v e r s i o n (101, i t i s l i k e l y t h a t s t e r i c c o n s t r a i n t s a r e r e s p o n s i b l e f o r t h e low hydrogen t r a n s f e r
s e l e c t i v i t y o f H-Beta and H-Omega.
The
intermediate
s p e c i e s would be i n t h a t s p e c i f i c case t o o l a r g e t o e a s i l y f o r m i n 12-membered r i n g z e o l i t e s which do n o t possess l a r g e cages. VGO. As shown on f i g u r e 4, t h e s e l e c t i v i t i e s i n VGO c o n v e r s i o n o f H-Beta and
H-Omega v e r y s i g n i f i c a n t l y d i f f e r f r o m t h o s e H-Omega produce v e r y h i g h y i e l d s o f
light
of
gases,
modified H-Y. the
H-Beta
and
s e l e c t i v i t y towards
g a s o l i n e b e i n g r a t n e r l o w . However, H-Beta l e a d s t o a l o w coke l e v e l w h i l e t h e r e v e r s e i s t r u e f o r H-Omega.
From t h e
l o w hydrogen
transfer
tendency
of
H-Omega, one would have p r e d i c t e d a reduced coke f o r m a t i o n , which i s o b v i o u s l y not t r u e .
I n fact,
previously stressed,
i n VGO c o n v e r s i o n over
these
solids,
as
it
has been
a c c e s s i b i l i t y c o n s i d e r a t i o n s a r e o f p r i m a r y importance
which means t h a t performances
o b t a i n e d w i t h model
molecules could not
be
d i r e c t l y t r a n s f e d t o r e a l f e e d . The h i g h s e l e c t i v i t y f o r gases, f o r i n s t a n c e ,
i s l i k e l y t o be due t o an easy o v e r c r a c k i n g o f t h e g a s o l i n e f r a c t i o n i n s i d e t h e n o t s u f f i c i e n t l y open channel system o f H-Beta and H-Omega. CONCLUSION The post-steaming a c i d l e a c h i n g has been shown t o p l a y a major r o l e i n t h e c a t a l y t i c performances o f H-Y,
H-Beta and H-Omega s i n c e i t can i n c r e a s e o r
125
decrease t h e c o n v e r s i o n l e v e l s i n cyclohexene as w e l l as i n VGO t r a n s f o r m a t i o n . I n t h e case o f v e r y open s t r u c t u r e s such as H-Y,
the positive effect o f a
s e l e c t i v e l e a c h i n g m a i n l y r e s u l t s f r o m an i n c r e a s e o f t h e number o f s t r o n g a c i d s i t e s ( e l i m i n a t i o n o f p o i s o n i n g aluminium s p e c i e s ) , w h i l e f o r s t r u c t u r e s w i t h a monodimensional porous system such as H-Omega t h e a c c e s s i b i l i t y o f s i t e s i s o f predominant importance ( i n c r e a s e i n t h e microporous volume). When a model t e s t r e a c t i o n i s employed, a q u a l i t a t i v e r a n k i n g o f t h e s o l i d s a c c o r d i n g t o t h e i r a c i d i c p r o p e r t i e s i s o b t a i n e d . I n t h a t r e g a r d , t h e c a t a l y t i c performances o f H-Beta a r e b e t t e r t h a n t h o s e o f H-Y and H-Omega. However, t h i s r a n k i n g i s no l o n g e r t r u e f o r VGO conversion.
For such a t r a n s f o r m a t i o n o f heavy molecules
a c c e s s i b i l i t y c o n s i d e r a t i o n s have t o be t a k e n i n t o account, which means t h a n t h e most open s t r u c t u r e ,
namely H-Y,
leads t o t h e best
performances.
The
performances o f H-Y can be s i g n i f i c a n t l y improved when t h e a c i d i c p r o p e r t i e s o f steamed H-Y a r e o p t i m i z e d t h r o u g h a s e l e c t i v e a c i d l e a c h i n g . B I BLI OGRAPHY 1 . J . Scherzer, ACS Symposium S e r i e s 248, (19831, 157-200. 2. J.S. Magee, W.E. Cormier and G.M. m t e r m a n , K a t a l i s t i k s ' 6 t h Annual F l u i d Cat. Cracking Symposium, Munich, Germany (19851, May 22-23. 3. Grupo Especializado de Catalisis, Madrid, Spain (19871, September 28-October 1. 4. E.G. Campbell and P.A. Winthrop, European P a t e n t A p p l i c a t i o n , (19871, n"243629. 5. F. Raatz and Ch. M a r c i l l y , European P a t e n t A p p l i c a t i o n , (19861, n o 206871. 6. A. Corma, V . Fornes, J.B. Monton and A.V. O r c h i l l e s , J o u r n a l o f C a t a l y s i s 107, (1987) 288-295. 7. E C o r m a , V. Fornes, F. Melo and J . Perez-Pariente, Symposium on advances i n FCC, American Chemical S o c i e t y , New Orleans meeting, (19871, August 30 September 4. 8. A. Corma, E. H e r r e r o , A. M a r t i n e z and J. P r i e t o , Symposium on advances i n FCC, American Chemical S o c i e t y , New Orleans meeting, (19871, August 30 September 4. 9. A. Corma, V. Fornes, A. M a r t i n e z , F. Melo and 0. P a l l o t a , Symposium on " I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science", Stud. S u r f . S c i . Cat., Vol 37, (19881, 495. 10. E. J a c q u i n o t , PhD, ENSPM, R u e i l Malmaison, France, i n p r e p a r a t i o n . 11. M.M. J . Treacy and J.M. Newsam, Nature, 332, (19881, March 17. 12. A. Macedo, PhD, ENSPM, R u e i l M a l m a i s o r France, (19881 and r e f e r e n c e s there in. 13. R. Beaumont and D. Barthomeuf, J o u r n a l o f C a t a l y s i s , 26, (19721, 218-225. 14. D. Barthomeuf, M a t e r i a l s Chemistry and P h y s i c s , ( l 3 8 7 1 , 49. 15. M. Maache and J.C. L a v a l l e y , i n p r e p a r a t i o n . 16. E. J a c q u i n o t , A . Mendes, F. Raatz, Ch. M a r c i l l y , F.R. R i b e i r o and J. Caiero, a r t i c l e s u b m i t t e d f o r p u b l i c a t i o n . 17. L.A. Pine, P.J. Maher and W.A. Wachter, J o u r n a l o f C a t a l y s i s , (19841, 466-476. 18. R.E. R i t t e r , J.E. C r e i g h t o n , T.G. Roberie, D.S. Chin, C.C. Wear, p r e s e n t e d a t NPRA Annual Meeting, Los Angeles, paper AM 86-45, (19861, March 23. 19. H. F i c h t n e r - S c h m i t t l e r , U. Lohse, G. E n g e l h a r d t and V. Patzelova, Cryst-Res-Tech. 1 1 , (19841, ( 1 1 . 20. A. Macedo, F. RaTtz, R. B o u l e t , J.C. L a v a l l e y and A. Janin, Symposium on " I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science", Stud. S u r f . S c i . Cat, Vol 37, (19881, 375.
11,
g,
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H.G. Karge, J . Weitkamp (Editors 1, Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlands
SURFACE-METALS INTERACTIONS IN FLUID CRACKING CATALYSTS DURING THE UPGRADING OF VANADIUM CONTAMINATED GAS OILS*
M. L. Occelli(’) and J. M. Stencel(2) ’Unocal Corporation, P.O. Box 76, Brea, CA 92621 USA 2Kentucky Center for Energy Research, P.O. Box 13015, Lexington, KY 40512 USA ABSTRACT Laser Raman and x-ray photoelectron spectroscopy (XPS), together with microactivity testing, x-ray diffraction (XRD) and BET surface area measurements have been used to elucidate the interaction o f vanadium with typical constituents of a fluid cracking catalyst. Cracking components 1 ike HY or CREY (calcined rare-earth exchanged zeolite Y) are thermally stable in air at 76O0C/5h even when loaded with 3-4% V. In the presence o f steam, stability is greatly reduced. With less than 1% V, HY collapses with formation of mullite (A1 Si 0 ) and silica (tridymite) whereas CREY collapses with formation 2ed?um orthovanade (CeVO ) . The presence o f lanthanide ions in the zeolite reduces V-toleradce at hydrothermal conditions. Mullite formation was also observed on V-loaded gels. On steam-aged Kaolin, aluminosilicate gels and HY, vanadium is present mainly as bulk V205.
OF
INTRODUCTION In a previous paper (ref. l), interactions between the components (gel, clay and zeolite) of a fluid cracking catalyst (FCC) with a metal contaminant such as nickel have been studied in detail. In the reaction zone (the riser) of a fluid cracking unit, metal-containing porphyrins and similar complexes decompose, leaving Ni contaminants on the cracking catalyst surface. Nickel catalyzes secondary cracking reactions, generating light gases and coke at the expense o f gasoline yields. Luminescence quenching of an artificially Ni -contaminated (zeolite-rich) FCC have indicated that Ni, after deposition, covers the catalyst zeolitic component without affecting its crystallinity (ref.2) Steam aging causes some of the nickel t o migrate, thus restoring part of the zeolite’s original luminescence intensity (ref. 2). Electron micrographs *Based, in part, on a paper presented at the Spring Meeting o f the California Catalysis Society, Menlo Park, CA, April 1988.
128
have shown t h a t steam-aging changes t h e s i z e o f t h e N i c r y s t a l l i t e s present i n a manner c o n t r o l l e d by t h e nature (composition) o f t h e c a t a l y s t surface ( r e f s . 2,3). A f t e r c a l c i n a t i o n , n i c k e l was found t o be present mainly as a N i O - l i k e phase on a l l the FCC components. However, f o l l o w i n g steam-aging, n i c k e l i n t e r a c t s both w i t h t h e c l a y ( k a o l i n ) and w i t h t h e gel t o f o r m surface compounds which are believed t o be l e s s d e l e t e r i o u s t o the cracking process than supported N i O species ( r e f . 1). It i s the purpose o f t h i s paper t o study and r e p o r t t h e i n t e r a c t i o n s o f vanadium w i t h t h e t h r e e basic components o f a FCC.
The o b j e c t i v e o f t h i s
work i s t o o b t a i n a b e t t e r understanding o f those V-surface i n t e r a c t i o n s which can be useful i n designing V - t o l e r a n t cracking c a t a l y s t s . EXPERIMENT Reference ComDounds Preparation A Ce-vanadate (CeV04) sample was prepared by pressing a 2Ce02.V205 mixture a t 10,000 l b s / l min and then heating the r e s u l t i n g wafer i n a i r a t 80O0C/2hr ( r e f . 5). A melt was obtained t h a t gave an x - r a y d i f f r a c t o g r a m consistent w i t h JCPDS p a t t e r n No. 12-757 f o r CeV04; small amounts (51%) o f unreacted Ce02 were a l s o present.
S i m i l a r l y , by h e a t i n g a La203-V205 wafer
a t 80O0C/2hr, a compound w i t h an x - r a y d i f f r a c t o g r a m c o n s i s t e n t w i t h JCPOS
A m i x t u r e ( w i t h an excess) o f g i b s i t e and V205 when heated t o 600'C/air gave a compound w i t h an x - r a y d i f f r a c t o g r a m s i m i l a r t o JCPDS p a t t e r n No. 31-34 f o r A1V04. Other procedures f o r preparing A1V04 have been described elsewhere ( r e f . 6). A l l powder d i f f r a c t i o n measurements were obtained w i t h a Siemens D-500 d i f f r a c t o m e t e r a t a scan r a t e o f l'/min using monochromatic Cu-km r a d i a t i o n . p a t t e r n No. 25-427 f o r LaV04 was obtained.
Catalyst PreDaration and T e s t i n q The amorphous a l u m i n o s i l i c a t e gel
(AAA-alumina) used i n t h i s study
contains about 80.4% Si02, 19.4% A1203 and 0.056% Na20.
Aluminum-27 MAS NMR
has shown t h a t aluminum i s present mainly as Al(V1) and t h a t a f t e r d r y i n g r a t i o i s about 1.2. Mercury (300°C/24h) i n a i r , t h e A l ( V I ) / A l ( I V ) 2 porosimetry measurements have i n d i c a t e d a 479 m /g surface area and 1.67 cc/g pore volume; the average pore diameter o f t h e d r i e d gel was 134 A. The k a o l i n i t e sample was obtained from t h e Georgia K a o l i n Company.
The sample
o f c a l c i n e d r a r e - e a r t h (RE) exchanged Z e o l i t e Y (CREY) and t h e h i g h - a c t i v i t y cracking c a t a l y s t (GRZ-1) were obtained from t h e Davison Company. sample had a Si02/A1203 r a t i o o f 5.0,
contained 7.6% Ce20j,
The CREY
4.0% La20g,
129
2.8% Nd203, 0.9% Pr203, 3.5% Na203 and had a BET surface area o f 749 mL/g. The HY sample (Linde LZY-82) had a Si02/A120, r a t i o o f 5.4 and BET surface 2 area o f 761 m /g. LaY (containing 8.1% La203) was obtained by r e a c t i n g t h i s HY w i t h 1M LaC13 solutions (prepared using LaC13.7H20 c r y s t a l s , Chemicals)
from GFS
.
Catalyst evaluation was performed w i t h a m i c r o a c t i v i t y t e s t (MAT) using conditions described elsewhere (4). Conversions are on a vol% f r e s h feed (FF) basis and have been defined as [ V -V /V ] x 100, where Vf i s t h e volume f P f i s the volume o f product w i t h b.p. > 204'C. P Steam-aging was accomplished by passing steam a t 760'C over the
o f feed and V
c a t a l y s t s f o r 5 hours. used t o metal -load
A s o l u t i o n o f vanadium naphthenate i n toluene was
the various
m a t e r i a l s according
to
an established
procedure ( 7 ) ; the naphthenate was obtained from P f a l t z and Bauer, I n c . and contained 2.9 w t % V. Catal v s t Characterization (i) XPS Measurements.
The FCC and i t s components (AAA-alumina, k a o l i n ,
CREY o r HY) were analyzed by X-ray photoelectron spectroscopy (XPS)
c a l c i n a t i o n (C),
steaming (S) and a f t e r H2-treatment (H2).
after Powders were
pressed i n t o t h i n , 13-mm diameter wafers and then mounted on a heatable sample probe attached t o a Leybold-Hereaus LHS-11 XPS/Auger/ISS instrument. The instrument base pressure was 2 x analyzed a t a pressure o f 2 x exposed t o 25 ml/min
bar.
lo-''
bar; samples were g e n e r a l l y During H2-treatment, m a t e r i a l s were
f l o w i n g H2 a t a pressure o f 2.5
bar and a t
a
temperature o f 400°C f o r periods o f 15 t o 75 minutes i n a h i g h pressure-high temperature r e a c t o r attached t o the side o f t h e LHS-11 a n a l y s i s chamber. A f t e r exposure t o H2, the samples were cooled w h i l e the r e a c t o r . was evacuated t o
bar.
Then the samples were moved i n t o a preparatory
chamber and f i n a l l y i n t o the analysis chamber.
A t y p i c a l time t o cool,
evacuate and move the sample t o the analysis p o s i t i o n was 10 minutes.
All
binding energies reported have been corrected f o r charging by assuming t h a t t h e ubiquitous Cls band i s located a t 284.6eV. (ii)Raman SDectroscooy.
Raman spectra were recorded on a Spex Ramalog 1403 spectrometer (Spec Industries, Metuchen, NJ) equipped w i t h a cooled RCA GaAs p h o t o m u l t i p l i e r tube (CA 31034-02).
The 4880A l i n e o f a model 165 A r t
l a s e r (Spectra Physics, Mountain View, CA) was used t o generate Raman
130
scattered l i g h t .
The l a s e r power i m p i n g i n g on t h e sample was l i m i t e d t o
ap p ro x ima t e ly 50 mW. A l l s p e c t r a were r e c o rded w i t h a s p e c t r a l r e s o l u t i o n o f 5 cm-’ (350 p m s l i t w i d t h ) . S i g n a l p u l s e s f rom t h e p h o t o m u l t i p l i e r t u b e were passed t h r o u g h a model 1182 amp1 i f i e r / d i s c r i m i n a t o r ( P r i n c e t o n A p p l i e d Research, P r i n c e t o n , NJ) and counted by a N i c o l e t 1280 d a t a system ( N i c o l e t I n s t r u m e n t Corp.,
Madison, WI).
Typically,
50-100 scans p e r sample were
averaged i n o r d e r t o o b t a i n s p e c t r a w i t h adequate s i g n a l - t o - n o i s e r a t i o s . d a t a p o i n t was a c q u i r e d e v e r y 0.7 cm-’.
A
Samples were prepared f o r Raman
measurements by g r i n d i n g t h e c a t a l y s t s i n an agat e m o r t a r and p e s t l e . Samples c o n t a i n i n g KN03 ( t h e i n t e r n a l s t a n d a r d ) were prepared by g r i n d i n g 90% c a t a l y s t and 10% KN03 m i x t u r e s . The powdered samples were t h e n pressed i n t o 1 cm d i a m e t e r w a f e r s under a p r e s s u r e o f a p p r o x i m a t e l y 1000 p s i .
Each
waf e r was mounted i n a s p i n n i n g sample h o l d e r and p l a c e d i n t h e sample chamber o f t h e spectrometer.
The a n g l e between t h e i n c i d e n t l a s e r beam and
t h e sample s u r f a c e was 30’; samples were spun a t a p p r o x i m a t e l y 500 rpm t o m i n i m i z e t h ermal m o d i f i c a t i o n s . RESULTS AND DISCUSSION Cat a l v s t T e s t i n q The metal - f r e e , steam-aged amorphous a l u m i n o s i l i c a t e g e l (AAA-alumina) and t h e z e o l i t e - r i c h commercial FCC (Davison’s
GRZ-1)
gave 61% and 84%
co nv ers io n, r e s p e c t i v e l y , when c r a c k i n g t h e same gas o i l under MAT conditions ( r e f . 4). I n b o t h c a t a l y s t s , a c t i v i t y decreased m o n o t o n i c a l l y w i t h V l e v e l s , Table 1 . W i t h 1.5% V, t h e z e o l i t i c FCC l o s t most o f i t s This irreversible u s e f u l a c t i v i t y ; w i t h 2.0% V, i t became i n a c t i v e . d e a c t i v a t i o n c o r r e l a t e s w e l l w i t h s u r f a c e a r e a and c r y s t a l l i n i t y l o s s e s w i t h
V l o a d i n g s , see Table 1 .
Vanadium on t h e steam-aged c a t a l y s t was p r e s e n t
m a i n l y as a r e d u c i b l e o x i d e - l i k e V205.
A t t h e h i g h t emperat ures encountered
TABLE 1 R e t e n t i o n o f S u r f a c e Area and C r a c k i n g P r o p e r t i e s i n t h e Presence o f V ( M a t e r i a l s have been H y d r o t h e r m a l l y Aged) (4,12) % Ret ent ion
Conv ers io n Gasoline S u r f a c e Area C r y s t a l 1i n i t y
0.5 82.5 80.4 100.0
-_
W t % V on Gel 1.0 1.5 2 .o
78.4 69.5 85.4
--
13.8 60.1 76.9
_-
69.7 57.9 69.3
-_
)Jt% V on FCC (GRZ-1 0.5 1.0 1.5 2ta
91.2 88.0 69.0 75.7
77.8 70.4 42.1 51.3
60.5 57.0 21.6 37.8
25.6 24.1 6.0 13.5
131
d u r i n g r e g e n e r a t i o n , V205, because o f i t s l o w (658'C)
m e l t i n g p o i n t , became
a f l u i d c a p a b l e of m i g r a t i n g i n t o t h e c a t a l y s t pores where i t i n t e r a c t e d w i t h t h e z e o l i t e c a u s i n g an i r r e v e r s i b l e r e d u c t i o n i n c r y s t a l l i n i t y
and
cracking a c t i v i t y ( r e f . 12). I n c o n t r a s t , V had l i t t l e e f f e c t on t h e amorphous c a t a l y s t p r o p e r t i e s and a f t e r steaming, t h e 2% V l o a d e d g e l r e t a i n e d -70% o f i t s i n i t i a l s u r f a c e a r e a (and -70% o f i t s i n i t i a l c o n v e r s i o n ) .
Steaming o f t h e m e t a l - l o a d e d g e l
removed o c c l u d e d m a t e r i a l s ( p r o b a b l y s i l i c a ) c a u s i n g t h e p o r e volume o f t h i s catalyst
to
increase s l i g h t l y
t o 0.52
Luminescence as w e l l as EPR d a t a ( r e f . deposition,
vanadyl i o n s (YO2')
cc/g
f r o m 0.46
cc/g
(ref.
1).
13) have shown t h a t a f t e r metal
were formed.
These i o n s a r e q u i t e m o b i l e
( r e f . 14) and a b l e t o m i g r a t e t o charge compensate A l ( I V ) p r e s e n t i n t h e g e l t h e r e b y f o r m i n g S i - 0 - A 1 (1V)-OV02' groups. Then, i n t h e r e g e n e r a t o r , t h e o x i d a t i v e decomposition o f carbonaceous d e p o s i t s forms V205 which c o u l d coat o r b l o c k access t o t h e g e l
a c t i v e s i t e s t h u s c a u s i n g t h e decrease
in
a c t i v i t y seen i n Table 1 . Thermal S t a b i l i t y The x - r a y d i f f r a c t o g r a m o f t h e steam-aged a l u m i n o s i l i c a t e g e l under s t u d y show a weak, broad and f e a t u r e l e s s s h o u l d e r c e n t e r e d a t 28222' t y p i c a l o f X - r a y amorphous m a t e r i a l s .
However, a f t e r l o a d i n g t h e g e l w i t h 2% V and
c a l c i n a t i o n i n a i r ( a t 76OoC/5h), a d i f f r a c t o g r a m s i m i l a r t o JCPDS p a t t e r n No. 15-776 f o r m u l l i t e (A16Si2013) was o b t a i n e d .
The d i f f r a c t i o n l i n e s
almost doubled i n i n t e n s i t y when h e a t i n g ( a t 760°C/5h) was performed i n t h e presence o f 100% steam.
Steaming N i - l o a d e d (1-5% N i ) g e l s d i d n o t f o r m
mull i t e . M u l l i t e has a s t r u c t u r e c o n s i s t i n g o f c h a i n s o f Al(V1)
that
are
p a r a l l e l t o t h e z - a x i s and c r o s s l i n k e d by t e t r a h e d r a c o n t a i n i n g b o t h S i and Al.
I t can be o b t a i n e d by h e a t i n g k a o l i n a t T ,lOOO'C
o r by c a l c i n i n g
a l u m i n o s i l i c a t e g e l s a t temperatures w e l l above 1000°C ( r e f . 1 5 ) . appears t h a t V a c t s as a m i n e r a l i z i n g agent,
Thus, i t
allowing the formation o f
m u l l i t e a t temperatures s i g n i f i c a n t l y l o w e r t h a n t h o s e u s u a l l y r e p o r t e d i n t h e synthesis o f t h i s mineral. structure
and
formation
of
Partial incorporation o f V i n t o the mullite V-0-A1
linkages
could
have
occurred.
S u b s t i t u t i o n o f A1 w i t h Fe and T i i n m u l l i t e i s n o t uncommon, and n a t u r a l i r o n - m u l l i t e s c o n t a i n i n g up t o 6% Fe20j
have been r e p o r t e d ( r e f .
S i l i c a - f r e e m u l l i t e t y p e phases have a l s o been s y n t h e s i z e d
(ref.
16). 17).
Formation o f A1V04 c o u l d n o t be observed b y XRD ( n o r by Raman) on any o f t h e V-loaded a l u m i n o s i l i c a t e samples examined.
132
I n a t y p i c a l FCC,
t h e desired cracking a c t i v i t y
i s introduced by
i n c o r p o r a t i n g HY o r c a l c i n e d r a r e - e a r t h exchanged Y (CREY) c r y s t a l s i n t o a gel-clay
mixture.
In
an
effort
to
investigate
the
mechanism
of
V-deactivation o f a FCC, HY and CREY were metal-loaded w i t h up t o 5% V and then heated f o r 5hr a t 760°C i n t h e presence o f a i r o r steam, see F i g s . 1-8. With as much as 4% V, HY c r y s t a l s were t h e r m a l l y s t a b l e ( a t 760'C) when heated i n t h e presence o f a i r , Figs. 1A-1E. With 5% V, t h e r e was a d r a s t i c reduction i n surface area and c r y s t a l l i n i t y , and a new phase appeared, Fig. 1F. 2A-F.
I n the presence o f steam, these c r y s t a l s became much l e s s stable, Figs. With 1% V, o n l y 45% o f t h e i r i n i t i a l surface area was r e t a i n e d and
w i t h 2% V, evidence o f the presence o f HY c r y s t a l s disappeared, Figs. 2B-C. A new phase was formed which grew w i t h V-loadings, Figs. 2C-F.
Vanadium ( a t
the hydrothermal c o n d i t i o n s used) seems t o have l i t t l e e f f e c t on the HY c r y s t a l l i n i t y up t o t h e 0.6 w t % l e v e l , Figs. 3A-D.
Following t h e collapse
o f the HY s t r u c t u r e , t h e x - r a y amorphous product forms m u l l i t e and smaller amounts o f Si02, Fig. 4. The doublet a t 28 ~ 2 0 ' (d = 4.44A and d = 4.12A) i d e n t i f i e s the Si02 formed t o be o f t h e t r i d y m i t e type. Under hydrothermal conditions, l o s s o f c r y s t a l 1 i n i t y i n metal - f r e e HY c r y s t a l s r e s u l t s from dealumination. Steaming V-loaded HY c r y s t a l s generated a protonated species, probably HqV207 ( r e f . 18) (V205 t 2H20 = H4V207), which accelerated A1 -removal and l a t t i c e collapse.
E x t r a framework
aluminum could then r e a c t w i t h the r e s i d u a l x - r a y amorphous phase t o f o r m m u l l i t e . I n f a c t , an A l ( V 1 ) - r i c h gel l i k e AAA-alumina ( A l ( V I ) / A l ( I V )
ratio
o f 1.2) forms mull i t e when heated i n the presence o f V . L i k e HY, CREY c r y s t a l s w i t h as much as 4% V r e t a i n e d 62% o f t h e i r i n i t i a l surface area and most o f t h e i r o r i g i n a l c r y s t a l l i n i t y when heated a t 760°C/5h i n t h e presence o f a i r ; Figure 5A and 5E.
With 5% V, the c r y s t a l s
l o s t most o f t h e i r surface area and c r y s t a l l i n i t y and a new phase appeared. As w i t h HY c r y s t a l s , heating a t 760'C/5h
i n t h e presence o f steam reduced
the CREY s t a b i l i t y s i g n i f i c a n t l y , Fig.6.
With o n l y 0.2-0.4% V, CREY became
e s s e n t i a l l y amorphous,
Fig. 7.
Thus, the i n t r o d u c t i o n o f r a r e - e a r t h ions
i n t o z e o l i t e s w i t h the f a u j a s i t e s t r u c t u r e decreased the c r y s t a l s ' A t V-loading above 1%, hydrothermal s t a b i l i t y i n the presence o f vanadium. i t was p o s s i b l e t o observe
t h e growth o f a new phase which had been
i d e n t i f i e d t o be cerium orthovanadate (CeV04); lanthanum compounds could n o t be observed, Fig. 8.
133
I
I
Figure 1. X-ray diffractograms o f HY c r y s t a l s heated i n a i r a t 760'C/5hr i n t h e presence o f : (A) 0, (B) 1.0, (C) 2.0, (D) 3.0, (E) 4.0 and (F) 5.0% vanadium.
I l l
"1
1
"
1
'
Figure 2. X-ray d i f f r a c t o g r a m s o f HY c r y s t a l s heated i n steam a t 760'C/5hr i n t h e presence o f : (A) 0, (B) 1.0, (C) 2.0, (D) 3.0, (E) 4.0 and (F) 5.0% vanadium.
i I!
1
I1
Figure 3 . X-ray diffractograms o f HY c r y s t a l s heated i n steam a t 760eC/5hr i n the presence o f : (A) 0, (B) 0.2, (C) 0.4, (D) 0.6, (E) 0.8 and ( F ) 1.0% vanadium.
Figure 4. The formation o f m u l l i t e and t r i d y m i t e a f t e r steam aging HY c r y s t a l s loaded w i t h 5% V.
CREY i s u s u a l l y prepared by r e p e t i t i v e exchanges o f NaY w i t h a s o l u t i o n containing a commercial r a r e - e a r t h (RE) c h l o r i d e mixture.
I n i t i a l l y , the
a i r - d r i e d REY c r y s t a l s contain the various lanthanide ions i n t h e supercages since hydrated lanthanide ions are t o o l a r g e t o d i f f u s e through a 0
six-membered window (2.5A i n diameter) and migrate d i r e c t l y i n t o the s o d a l i t e cage o r i n t o the double-six r i n g u n i t s t h a t connect t h e s o d a l i t e cages. However, on heating, these i o n s l o s e t h e i r water o f h y d r a t i o n and i r r e v e r s i b l y migrate i n t o the small cages where they can have maximum
134
Figure 5 . X-ray diffractograms of CREY crystals heated in air at 760eC/5hr in the presence of: (A) 0, (€3) 1.0, (C) 2.0, (D) 3.0, (E) 4.0 and (F) 5.0% vanadium.
IWO I H E l l (DEGREE1
Figure 7. X-ray diffractograms o f CREY crystals heated in steam
at 760eC/5hr in the presence of: (A) 0, (B ) 0.2, (C) 0.4, (D) 0.6, ( E ) 0.8 and ( F ) 1.0% vanadium.
Figure 6. X-ray diffractograms o f CREY crystals heated in steam at 760'C/5hr in the presence o f : (A) 0, ( 6 ) 1.0, (C) 2.0, (D) 3.0, (E) 4.0 and ( F ) 5.0% vanadium.
W O I H I I A (MGRIE)
Figure 8. Cerium orthovanadate (CeV04) formation after steamaging CREY crystals loaded with 5% vanadium.
coordination with framework oxygens (ref. 19-21). Repetitive exchanges with 1.OM NH4N03 solutions at 60'C did not remove any substantial amounts o f lanthanide ions from CREY, and the RE-oxide content remained unchanged at about 15.0%. At temperatures above 300'C, Ce(II1) oxidized to Ce(1V); the oxidation to the tetravalent state was believed to retard Ce migration (ref. 20). Thus, it is possible that in CREY, La(II1) ions are preferentially located in the small cages and Ce(IV) ions in the supercages where they can more easily interact with vanadium and form stable orthovanadates.
135
I t has been proposed t h a t vanadate f o r m a t i o n r e q u i r e s framework oxygen
t h u s l e a d i n g t o l a t t i c e c o l l a p s e ( r e f . 22). However, a t -7OO"C, t h e apparent lanthanum charge i s 2.38 ( r e f , 19), i n d i c a t i n g t h e presence o f r e s i d u a l La(0H)" i o n s which c o u l d r e a c t (under hydrot hermal c o n d i t i o n s ) w i t h V205 (2 La(OH)t2 t H20 t V205 = 2 LaV04 t 4Ht) g e n e r a t i n g a s t a b l e vanadate and a c i d i t y . I n f a c t , LaY ( c o n t a i n i n g 8.1% La203) when steamed i n t h e presence o f (1-5%) V c o l l a p s e d w i t h LaV04 f ormat ion, F i g . 100. H i g h l y exchanged LaY c r y s t a l s have been shown t o c o n t a i n -La-(OH)-La- t y p e l i n k a g e s between p a i r s o f La i o n s s t r e t c h i n g across t h e s o d a l i t e cages ( r e f s . 23,24). Removal o f t h e s e l a n t h a n i d e i o n s when LaV04 i s formed d e s t a b i l i z e f a u j a s i t e s t r u c t u r e c o n t r i b u t i n g t o i t s c o l l a p s e ( r e f . 24).
the
The CREY c r y s t a l s under s t u d y c o n t a i n almost t w i c e as much Ce t h a n La i n a d d i t i o n t o s m a l l e r amounts o f Nd and P r .
A t t emperat ures above 300°C
( i n a i r ) , c e r i u m has an e f f e c t i v e charge near 2' t h e presence o f an oxycerium complex ( r e f . 20).
which i s c o n s i s t e n t w i t h Since i t i s known f rom EPR
( r e f . 13) t h a t even a f t e r t h e o x i d a t i v e decomposition ( i n d r y a i r ) o f t h e naphthenate, vanadium i n t h e z e o l i t e i s p r e s e n t m a i n l y as vanadyl c a t i o n s i t i s proposed t h a t CeV04 i s formed d i r e c t l y by t h e f o l l o w i n g redox
(VO"),
scheme: [Ce
/O\ \o/ ce1t4
t 2 VOt2
t 4H20 = 2 Ce V04 t 8H'
Thus, removal o f l a n t h a n i d e i o n s d u r i n g vanadate formation,
i n addition t o
d e s t a b i l i z i n g t h e z e o l i t e , generates p r o t o n s which c o u l d enhance framework dealumin at io n. I n f a c t , i n d r i e d LaY ( w i t h 5% V), t h e A l ( V I ) / A l ( I V ) r a t i o inc re as es t o 0.26 f r o m 0.20 upon c a l c i n a t i o n a t 54O0C/10hr i n d r y a i r . Steaming causes a l o s s o f c r y s t a l l i n i t y ; t h e aluminum-27 MASNMR spectrum o f the
steamed
material
is
similar
to
that
a l u m i n o s i l i c a t e g e l s l i k e AAA-alumina ( r e f . 25). w i t h 5% V,
the Al(VI)/Al(IV)
e x t e n s i v e de-a l u m i n a t i o n .
of
V-loaded
amorphous
A f t e r steaming LaY loaded
r a t i o almost doubled i n value,
suggest ing
I t i s proposed t h a t t h e d e s t a b i l i z a t i o n r e s u l t i n g
f rom enhanced d e a l u m i n a t i o n and f r o m f o r m a t i o n o f vanadate i s r e s p o n s i b l e f o r t h e r a p i d ( z i p p e r - l i k e ) ( r e f . 26) s t r u c t u r a l c o l l a p s e observed when CREY (0.2-0.4%)
c r y s t a l s a r e h y d r o t h e r m a l l y heated i n t h e presence o f small amounts o f V, F i g . 7. Raman R e s u l t s The e f f e c t s o f steam-aging (760'C/5h,
100% steam) on V i n t e r a c t i o n s
w i t h a c r a c k i n g c a t a l y s t ( l i k e GRZ-1) and w i t h i t s components a r e shown i n t h e Raman s p e c t r a
i n Figs.
9-11.
On t h e
steam-aged
clay
(kaolin),
136
amorphous aluminosilicate gel and on the lanthanide-free HY crystals, vanadium is present mainly as supported V205, Fig. 9. In contrast, the Raman spectra of the V-loaded CREY crystals exhibit bands at 869, 803, 766, 470 and 370 cm-l which are characteristic of CeV04; see Figs. 108 and 1OC. A surface vanadate species like A1V04 could not be observed. The dependence of CeV04 formation on V loadings was examined by mixing the steam-aged and V-contaminated CREY crystals with known quantities of NaN03. Raman spectra were then obtained in which the intensity due to the NO3 symmetric stretching vibrational band at 1050 cm-l could be compared to the V04 symmetric stretching vibration at 870 cm-l. The increase in
Figure 9. Raman spectra of 5% V loaded on: (A) kaolin and (B) gel after steam-aging. The reference spectra for V205 is given in (C).
Figure 10. Raman spectra o f 5% V on: (A) HY and (B) CREY crystals after steam-aging. Reference spectra for CeV04 and LaVO are shown In (C) and ( D ) , respgctively.
VANADIUM ON CRN (Wn)
Figure 11.
The formation of CeV04 as a function of V loading.
intensities of this vibration as a function of V loading is shown in Fig. 11. The lower limit of CeV04 detection is at approximately 0.7% V; a similar loading is required to observe CeV04 formation by XRD, Fig. 6. Although Raman spectroscopy o f catalysts i s not generally considered quantitative, recent studies have proven that with specially designed and relatively simple experiments, quantitative data can be acquired (ref. 9,lO).
137
The l i n e a r dependence o f CeV04 f o r m a t i o n on V l o a d i n g s a t c o n c e n t r a t i o n s above a p p r o x i m a t e l y 0.7% V i m p l i e s t h a t Ce-V i n t e r a c t i o n s c o n t i n u e t o o c c u r even i n c r y s t a l s i n which XRD d a t a has shown complete c o l l a p s e o f t h e z e o l i t e s t r u c t u r e . Below 0.7% V, t h e s e n s i t i v i t y o f Raman spectroscopy (and XRD) i s somewhat inadequate t o i d e n t i f y CeV04.
However,
s i n c e even s m a l l amounts o f V s i g n i f i c a n t l y reduce CREY c r y s t a l l i n i t y , CeV04 p r o b a b l y forms a l s o a t l e s s t h a n 0.7% V l o a d i n g s .
Alt hough CREY c o n t a i n e d
4.0% La203, LaV04 f o r m a t i o n c o u l d n o t be observed, suggest ing t h a t s i t e s a t
w hic h Ce i s l o c a t e d a r e more a c c e s s i b l e t o V a t t a c k t h a n t hose s i t e s containing
La
and/or
that
CeV04 f o r m a t i o n
i s more
facile
than
LaV04
formation. XPS R e s u l t s
Changes i n Si/A1 and V / ( S i t A l ) r e d u c t i o n ( i n H2) Table 2.
r a t i o s r e s u l t i n g f r o m steaming and/or
o f t h e c a l c i n e d V-contaminated m a t e r i a l s a r e shown i n
Assuming a c o n s t a n t ( S i t A 1 ) s u r f a c e c o n c e n t r a t i o n ,
the V/(SitAl)
r a t i o s s hould i n d i c a t e t h e s u r f a c e o b s e r v a b i l i t y o f V independent o f Si/A1 r a t i o changes.
The V / ( A l t S i )
r a t i o does n o t change
appreciably a f t e r
thermal o r hydrothermal t r e a t m e n t o f t h e v a r i o u s components o f a c r a c k i n g catalyst.
I n c o n t r a s t , steaming a c r a c k i n g c a t a l y s t l i k e GRZ-1 causes a l a r g e decrease i n t h i s r a t i o p r o b a b l y because o f V m i g r a t i o n i n t o t h e open three-dimensional macroporosity o f t h e c a t a l y s t . M i g r a t i o n o f V t o Ce and/or La c e n t e r s has been a l r e a d y r e p o r t e d ( r e f . 22). Decreases i n Si/A1
r a t i o s a f t e r steaming a r e observed f o r t h e g e l ,
k a o l i n and CREY samples.
The S i / A l
r a t i o o f c a l c i n e d CREY i s 40% g r e a t e r
t h an t h e Si/A1 o f c a l c i n e d HY; o n l y upon steaming do t h e Si/ Al t h es e z e o l i t e s become a p p r o x i m a t e l y equal, T able 2.
The Si/ Al
ratios of r a t i o o f HY
c r y s t a l s inc re a s e s upon steaming whereas l i t t l e change i n t h i s r a t i o i s n o t e d f o r GRZ-1. V a r i a t i o n s i n Si/A1 v a l u e s (T able 2) may be a s s o c i a t e d w i t h h y d r o l y s i s o f S i - 0 bonds, A1 m i g r a t i o n , and w i t h t h e presence o f d i f f e r e n t V compounds, A f t e r l o a d i n g 2% V and steaming, HY forms s i l i c a ( t r i d y m i t e ) , m u l l i t e and V205 ( s e e F i g s . 3, 4 and 10) whereas CeV04 and an x-ray
amorphous
component
a r e observed
i n CREY samples.
Thus,
silica
m i g r a t i o n t o t h e s u r f a c e i s p r o b a b l y r e s p o n s i b l e f o r t h e HY c r y s t a l s ' l a r g e i n c r e a s e i n S i / A l r a t i o s whereas m i g r a t i o n o f e x t r a l a t t i c e A1 formed by vanadates and a c i d i t y g e n e r a t i o n c o u l d have decreased t h e S i / A l r a t i o o f t h e V-loaded CREY, g e l , and k a o l i n samples, Ta ble 2.
TABLE 2 Atomic R a t i o s from XPS A n a l y s i s o f Several M a t e r i a l s ( l o a d e d w i t h 2 . W V) and Standard Reference Compounds
TABLE 3 B i n d i n g E n e r g i e s (meV) o f Several Supports (Loaded w i t h 2% V) and Standard Reference Compounds
SanDles Gel Gel Gel Gel
(C. AR) (C. H ) (S, A#) (S. H2)
4.00 4.53 2.18 2.44
0.011 0.008 0.019 0.015
-.
__
.. ..
1.10 1.20 0.82 0.82
0.065 0.057 0.052 0.044
.. .. .. ..
2.26
CREV (S:A$) CREV (S,H2)
2.16 1.51 1.54
0.014 0.008 0.015 0.012
3.14 2.62 1.80 1.18
HV HV HV HV
(C.AR) (C H ) (S:Aa) (S.H2)
0.82 0.90 1.54 1.63
0.034 0.030 0.036 0.038
..
__
GRZ-l IC.ARI
3.25 4.i5 3.75 3.78
0.044 0.037 0.009 0.008
9.7 6.7 4.3 1.5
Kaolin Kaolin Kaolin Kaolin
(C,AR) (C,H ) (S,Aa) (S.H2)
CREY (C,AR) CREV (C H )
GRZ-I [ C ' H j GRZ-1 (S:Aa) GRZ-1 (S,H2) CeVO CeVO:
(AR) (ti2)
LaVO LaVO:
(AR) (H~)
AlVO
(AR) (H?)
AlVO:
.. ..
5.15 4.73 .. ..
__ ..
_.
Gel IC.ARl
..
1.41 1.41
0.43 0.44
.. ..
Kaolin Kaolin Kaolin Kaolin
(i,AR) (C H ) (S:A$) (S,H2)
CREY (C,AR)CREY (C H ) CREV rS:AaI HY HY HV HV
(C,AR) (C H ) (S:Aa) (S.HZ)
GRZ-l GRZ-l GRZ-l GRZ-1 CeVO CeVO:
(C,AR) (C H ) (S:A$) (S,H,) (AR) (H?)
LaVO LaVO:
(AR) (H?)
AlVO AlVO:
(AR) (H,)
u
u
103.1 102.9 103.3 103.2
74.7 74.7 74.9 74.9
103.0 103.2 103.3 103.3
74.6 74.7 74.9 74.9
515.4
103.1 103.1 102.8 102.8
74.9 74.8 74.7 74.8
517.5 517.0; 515.5 517.5 517.2; 515.4
102.9 102.9 103.2 103.2
74.7 74.8 74.8 74.8
516.8 516.8; 515.2 517.2 517.1 517.5 517.5, 515.4
103.3 103.1 103.3
74.7 74.6 74.8 74.7
2p3/2 517.7 517.6; 517.1 517.1; 517.4 517.0; 517.3 516.9; 517.4; 517.2; 517.1 517.0;
516.0 515.2 515.5 515.3 515.6 (w) 515.6
103.2
__
..
-. ..
516.9 516.9, 515.6(w) 517.2 517.2, 515.4
..
..
--
._
.. ..
.. ..
517.2 516.0.
.. ..
.. ..
514.7
TABLE 4 Atomic 4 o f Vt5. Vt4 and Vt3 i n Several Suooorts (Loaded w i t h 24'V) and Reference Compounds' a f t e r R e d u c t i o n w i t h Hydrogen
%fi %Y
Sample
4 x 5
Gel fCI Gel ( S j
59 33
41 67
0 0
CREY (C) CREY (5)
4a 67
52 33
0 0
HY (C) HY ( 5 ) GRZ-1 ( C ) GRZ-1 (5)
48 40
52 60
45 100
55 0
0 0 0
95 61 69 0
5 39 31 70
LaVO
cevo4 A1 VO: V,O,
0
0 0 0 30
139 Binding T a b l e 3.
energies
for
the
V-contaminated
catalysts
are
The range o f t h e S i 2 p b i n d i n g energy i s 102.8-103.3
A12p b i n d i n g energy i s near 74.8 eV.
listed
in
eV w h i l e t h e
The S i 2p b i n d i n g e n e r g i e s range i s
smaller than t h a t previously reported f o r cracking c a t a l y s t s
(ref.
18).
T h i s s m a l l e r range a f f e c t s t h e V2p
b i n d i n g energy p o s i t i o n such t h a t i n 3/2 t h e c a l c i n e d o r steamed m a t e r i a l s i t i s a p p r o x i m a t e l y 517.2 f 0.4 eV. A f t e r
H2 t r e a t m e n t ,
t h e w i d t h o f t h e V 2p
peak v a r i e d a c c o r d i n g t o whether 3/2 m u l t i p l e V o x i d a t i o n s t a t e s ( i n a d d i t i o n t o Vt5) were formed. As a r e s u l t o f enhanced peak w i d t h s (and/or peak envelopes w i t h o b v i o u s s h o u l d e r s ) , t h e V 2p3/2 envelope was f i t t e d t o a c o m b i n a t i o n o f peaks whose p o s i t i o n s a r e
l i s t e d i n Table 3. All
( e x c e p t t h e c a l c i n e d CREY) b e f o r e HZ t r e a t m e n t can be
samples
d e s c r i b e d t o c o n t a i n a s i n g l e t V 2p3,2
peak;
s e n s i t i v e t o d i f f e r e n t V s p e c i a t i o n i n a Vt5 t r e a t m e n t , t h e V 2p
3/2
i t s p o s i t i o n i s not highly oxidation state.
Upon H2
peak i n most m a t e r i a l s ( e x c e p t i n GRZ-1,
V205 and
LaV04) can be r e p r e s e n t e d by two peaks a t a p p r o x i m a t e l y 517.0 eV and 515.5 eV, see Table 3.
R e p r e s e n t a t i v e s p e c t r a o f r e f e r e n c e vanadates and V-loaded
m a t e r i a l s a r e d i s p l a y e d i n F i g s . 12-16.
A f t e r H2 t r e a t m e n t , t h e spectrum o f
steamed GRZ-1 and o f LaV04 c o n t a i n a V 2p
3/2
peak w h i c h i s almost e n t i r e l y
r e p r e s e n t e d by a s i n g l e t a t 517.0 eV, whereas t h e V205 spectrum has a V 2p3/2 peak r e p r e s e n t e d by two peaks a t a p p r o x i m a t e l y 516.0 eV and 514.7 eV. Peak p o s i t i o n s , a t 517.0,
Vt4
and Vt3,
516.0 and 514.7 eV, have been a t t r i b u t e d t o Vt5,
respectively.
The percentage c o n t r i b u t i o n o f Vt5,
peak o f 3/ 2 The V205 samples c o n t a i n Vt4 and
Vt4
H - t r e a t e d samples i s l i s t e d i n Table 4. V
$3 .
I n contrast,
and Vt3
t o t h e V 2p
t h e o t h e r samples a f t e r H2 t r e a t m e n t do n o t c o n t a i n
Vt3 species which suggests t h a t t h e V i s somewhat d i f f e r e n t f r o m b u l k V205. Such XPS-derived r e d u c i b i l i t y d a t a i s , a t f i r s t i m p r e s s i o n , i n disagreement
w i t h Raman r e s u l t s p r e s e n t e d i n F i g s . 9-10.
However, s l i g h t b u t s i g n i f i c a n t
d i f f e r e n c e s i n t h e Raman s p e c t r a o f V-loaded k a o l i n , g e l and HY c r y s t a l s a r e present.
F o r example, t h e 997 cm-l V=O s t r e t c h i n g mode f o r V205 i s narrow
and w e l l d e f i n e d .
I n t h e l a n t h a n i d e - f r e e samples, t h i s band i s broadened
and c o n t a i n s a h i g h - f r e q u e n c y s h o u l d e r .
The Raman bands i n t h e 700-850 cm-’
r e g i o n f o r b u l k V205 a r e t h e r e s u l t o f V-0-V b r i d g e d oxygen s t r e t c h i n g modes ( r e f . 11).
These bands a r e s u b s t a n t i a l l y d i f f e r e n t i n shape and r e l a t i v e
i n t e n s i t y i n V-loaded g e l , k a o l i n and HY. These d i f f e r e n c e s suggest d i s t o r t i o n o f some o f t h e V205 o r f o r m a t i o n o f a s u r f a c e V205 phase which decreases V - r e d u c i b i l i t y under H2 atmosphere.
140
There i s l i t t l e d i f f e r e n c e between t h e r e d u c i b i l i t y o f V i n steamed o r c a l c i n e d HY and k a o l i n .
However, a l a r g e decrease i n V r e d u c i b i l i t y e x i s t s
between steamed and c a l c i n e d GRZ-1 and CREY samples. against
reduction
is
enhanced
in
catalysts
Hence, V s t a b i l i t y
containing
La
and/or
Ce.
Steaming inc re as e s t h e r e d u c i b i l i t y o f V-loaded g e l s , p r o b a b l y because o f V-migration t o t h e g e l surface; i n f a c t , t h e V / ( S i t A l ) 0.019 f rom 0.011 upon steaming, Ta b l e 2.
r a t i o increases t o
I n c o n t r a s t , steaming causes a
l a r g e decreases i n V - r e d u c i b i l i t y i n CREY and i n a CREY-containing c a t a l y s t l i k e GRZ-1, p r o b a b l y because o f vanadate f o r m a t i o n , T a b l e 4. C.VO.
BINDING ENERGY (eV) F i g u r e 12. XPS d a t a f o r t h e V 2p peak o f LaVO , A1V04 and CeV04 i n a s - r e c e i v e d (A&f2and reduced fgrrns
v)
u
BINDING ENERGY lev1
F i g u r e 13. XPS d a t a f o r t h e V 2p peak o f 2% V / k a o l i n , cafC?ned (C) and steamed (S), i n a s - r e c e i v e d (AR) and reduced forms.
BINDING ENERGY lev1
F i g u r e 14. XPS d a t a f o r t h e V 2p peak o f 2% V/CREY, ca?C?ned ( C ) and steamed ( S ) i n a s - r e c e i v e d (AR) and reduced forms.
141
BlNOlNG ENERGY
BlNOlNG ENERGY lev1
F i g u r e 15. XPS d a t a f o r t h e V peak o f 5% V/AAA, c a l c i n e d n d steamed ( S ) , i n asr e c e i v e d (AR) and reduced forms.
&a
The r e f e r e n c e LaV04 c o n t a i n s 5% Vt4
lev1
F i g u r e 16. XPS d a t a f o r t h e V 2p peak o f 2% V/GRZ-1, ca!&ed ( C ) and steamed (S), i n as-received (AR) and reduced forms. +4 whereas CeV04, c o n t a i n s -39% V
a f t e r H2 t re at me n t , Table 4. S i m i l a r l y , steamed CREY (which by Raman a n a l y s i s has CeV04) c o n t a i n s -33% Vt4 a f t e r H2 t r e a t m e n t . The s t a b i l i t y a g a i n s t r e d u c t i o n o f V-loaded CREY resembles t h a t o f CeV04 whereas t h a t o f t h e c r a c k i n g c a t a l y s t more c l o s e l y resembles t h a t o f LaV04, T able 4. T h i s enhanced s t a b i l i t y i n hydrogen i s p o s s i b l y t h e r e s u l t o f d i s t o r t i o n o f t h e vanadates formed w i t h i n t h e z e o l i t e s t r u c t u r e . CONCLUSIONS T y p i c a l c r a c k i n g components (such as HY o r CREY) o f f l u i d i z e d c r a c k i n g c a t a l y s t s e x h i b i t s i m i l a r s t a b i l i t y t o V - d e a c t i v a t i o n when heated i n d r y a i r a t 76OoC/5h. When t h i s thermal t r e a t m e n t i s repeat ed i n t h e presence o f steam, s t a b i l i t y i s d r a s t i c a l l y reduced and HY i s more h y d r o t h e r m a l l y s t a b l e tha n CREY a t t e s t c o n d i t i o n s . I n t h e presence o f steam, occurs
by
two
mechanisms.
the collapse o f the f a u j a s i t e structure In
CREY, i t has been a t t r i b u t e d t o d e s t a b i l i z a t i o n due t o CeV04 g e n e r a t i o n and t o enhanced d e a l u m i n a t i o n caused
by p r o t o n s r e s u l t i n g from
vanadate
f o r m a t i on.
I n HY, p r o t o n a t e d species
(such as H4V20,) promote d e a l u m i n a t i o n and c r y s t a l l i n i t y l a t t i c e c o l l a p s e , m u l l i t e and t r i d y m i t e a r e observed.
losses;
after
Vanadium i m p u r i t i e s have l e s s e r e f f e c t s on t h e s u r f a c e (and c a t a l y t i c ) p r o p e r t i e s o f an a l u m i n o s i l i c a t e m a t r i x ( g e l ) c o n t a i n i n g b o t h A1 ( V I ) and A l(1 V ).
As
with
HY,
V
induces
mullite
formation
at
t emperat ures
s i g n i f i c a n t l y l o w e r t h a n those used i n s y n t h e s i z i n g t h i s m i n e r a l . M u l l i t e f o r m a t i o n was n o t observed when V was r e p l a c e d by N i . Raman spectroscopy and XRD have been p a r t i c u l a r l y u s e f u l vanadate genera t i o n .
i n m o n i t o r i n g phase changes
and
142
ACKNOWLEDGMENTS The many u s e f u l
discussions
and s u p p o r t
r e c e i v e d f r o m t h e Unocal
A n a l y t i c a l Department s t a f f a r e g r a t e f u l l y acknowledged.
Special thanks are
due D r .
E. G o l d i s h and Dr. P. R i t z f o r h e l p i n g w i t h r e f e r e n c e compound p r e p a r a t i o n , x - r a y and Raman measurements. REFERENCES 1 2 3 4
5 6 7 8 9 10
M. L. O c c e l l i and J. M. S t e n c e l , i n "Proc. 9 t h I n t . Congress C a t . , " C a l g a r y , 1988 ( i n p r e s s ) . M. L. O c c e l l i , D. Psaras, S. L. Suib, and J. M. S t e n c e l , App. C a t a l . , 28, 143 (1986). M. L. O c c e l l i , D. C. Kowalczyk, and C. L. K i b b y , A p p l . C a t a l . , 16, 227(1985). M. L. O c c e l l i . S. D. Landau. and T. J. P i n n a v a i a . J. C a t a l .. 90.. 256 (1984) M. Yoshimura, and T. Sata, B u l l e t i n Chem. SOC. Japan, 42, 3195 ( 9 6 9 ) . E. C . S h a f e r , M. W. Shafer, and R. Roy, Z e i t s c h r i f t f u r K r i s t a l o g r . , Bd. 108, 265 (1956). B. R. M i t c h e l l , I n d . Eng. Chem. Prod. Res. Dev., 19, 209 ( 1 9 8 0 ) . E. J. Baran, and I . L. B o t t o , M o n a t s h e f t e f u r Chemie, 108, 311 ( 9 7 7 ) . J. P. B a l t r u s , L. E. Makovsky, J. M. S t e n c e l , and D. M. H e r c u l e s A n a l . Chem., 57, 2500 (1985). R. 8. Q u i n c y , M. H o u a l l a , and D. M. H e r c u l e s , J. C a t a l . , 106, 85 11987). , J. Hanuza, K. Hermanowicz, W. Oganowski and B. J - T r z e b i a t o w s k a , B u l l . , P o l . Acad. S c i . (Chem), 31, 139 (1984). M. L. O c c e l l i , D. Psaras, and S. L. Suib, J. C a t a l . , 96, 2, 363 ( 1 9 8 5 ) . M. W. Anderson, M. L., O c c e l l i , and S. L. Suib, i n p r e p a r a t i o n . S. L. Suib, M. W. Anderson, and M. L. O c c e l l i , i n " P r e p r i n t s Symp. P r e p a r a t i o n and C h a r a c t e r i z a t i o n o f C a t a l y s t s , " Los Angeles, CA 1988 (submitted) S. Ardmaki, and R. Roy, J . Am. Ceramic SOC., 45,5,229 ( 1 9 6 2 ) . W. A. Deer. R. A. Howie. and J. Zussman. i n "An I n t r o d u c t i o n t o t h e Rock Forming Minerals,"'Langman, p. 37 ( 1 9 - 3 ) . A. J. P e r o t t a , and J. E. Young, J. Am. Ceramic SOC., D i s c . and Notes, 57, 9, 405 (1974). M. L. O c c e l l i and J . M. S t e n c e l i n ACS N a t i o n a l Mtg., P e t r . D i v . P r e p r i n t s , New Orleans, LA, 1987. E. F . T. Lee, and L. V. C. Rees, Z e o l i t e s , 7, 143 ( 1 9 8 7 ) . E. F . T. Lee, and L. V. C. Rees, Z e o l i t e s , 7, 446 ( 1 9 8 7 ) . L. V . C . Rees, and Z. Tao, Z e o l i t e s , 6, 234 (1986). R. Pompe, S. J a r a s , and N. Vannerberg, J. A p p l . C a t a l . , l3, 171 ( 1 9 8 4 ) . F . Mauge, J. C . C o u r c e l l e , Ph. Engelhard, G a l l e z o t , P. and Grosmangin, J., Z e o l i t e s , 6,261 (1986). F . Maiige, J. C . C o u r c e l l e , Ph. Engelhard, G a l l e z o t , P. and Grosmangin, J. i n "New Developments i n Z e o l i t e Science and Technology," Y . Murakami, A. I i j i n i a and J . W. Ward, Eds., Kodansha-Elsevier, Tokyo, 1986, p. 803. M. L. O c c e l l i , and P. S. I y e r , t o be p u b l i s h e d . C. V . McDaniel, and P. K. Maher, i n " Z e o l i t e C h e m i s t r y and C a t a l y s i s , ( J . A. Rabo, E d . ) , American Chemical S o c i e t y , Washington, 1976, p . 285.
.
I
11 12 13 14 15 16 17 18 19 20 21 22 23 24
25 26
.
H.G. Karge, <J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders
0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HIGHLY DISPERSED Pt AND Pt-Cr CLUSTERS I N PENTASILS AND THEIR ACTIVITY TRANSFORMATIONS OF LOWER ALKANES
IN
E.S. S H P I R O ~ ,G.J. T U L E U O V A ~ ,v I. Z A I K O V S K I I ~ ,O.P. T K A C H E N K O ~ ,T.V. V A S I N A ~ , O.V. BRAGIN' and KH.M. MINACHEV' lN.0. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, MOSCOW V-334, USSR zInstitute of Catalysis, Novosibirsk, USSR
ABSTRACT Pt and Cr electronic states, reduction degrees and metal dispersion in pentasils have been determined by XPS, TEM, XRD and chemisorption techniques. The formation of Pt(S+) and Pt-Cr clusters significantly affects the catalytic activity in hydrogenolysis of alkanes (C Hs,C,H*) and aromatization: the former facilitats these reactions while the lafter suppresses the activity. INTRODUCTON The study of the regularities of Group VIII metals behaviour in pentasils is of great importance since these systems are rather promising in various catalytic processes, for example. aromatization of lower alkanes (refs. 1-3). T o provide high activity in the aromatization reaction small metallic clusters should be formed in the vicinity of acidic centers which was confirmed by a significant dependence of the Pt/H(Ga)-ZSM-5 activity on preparation and pretreatment conditions (refs. 1 , Z ) . On the other hand, highly dispersed Pt particles in faujasites are known to be stabilized in the presence of other cations, in particular Cr3+ (ref. 4). Alternatively, Cr3+ to CrD reduction and possibly Pt-Cr intermetallic formation, as found on y-AlpO3 (ref. 5 ) . can lead to the same effect. This paper deals with the study of electronic states of Pt and Cr, their dispersion and interaction in Pt, Cr and Pt-Cr catalysts based on pentasil zeolites as a function of preparation and pretreatment conditions and their performance in the transformations (ethane and propane hydrogenolysis and aromatization) of lower alkanes. An attempt was also made to elucidate the relationship between the surface characteristics and catalytic properties of metal zeolite catalysts in these reactions. EXPERIMENT Catalysts containing 0.5. 1.0, 1.75% Pt; 0.75% Cr and 0.5% Pt plus 0.75% Cr were prepared via ionic exchange of NH4ZSM-5 (Si02/A1203=33) with Pt(NH3)4Clz, Cr(N03)3 or (Cr03)x solutions and subsequent impregnation by the rest of these
144
solutions. Along with the initial chromium compounds, the pH and the sequence of Pt and Cr introduction were varied in the bimetallic zeolites (Table 1). Before spectral and catalytic experiments the specimens were exposed to the following treatments: air, 520°C (a); air, 520°C + H2, 400°C (b); air, 520°C + H2, 520°C (c); Hp, 520°C (d). Sample characterization XPS spectra were recorded with an AEI ES 2006 spectrometer according to the technique given in ref. 6. The Pt 4f lines, overlapping with the A1 2p peak and the Cr 2p doublet, were resolved into individual components by a standard peak synthesis routine. The pretreated samples were transferred Into the spectrometer in inert atmosphere according to the procedure described in ref. 1. The electron microscopy measurements were performed with a JEM lOOCX instrument; the resolution was 0.2 nm (ref. 7). CO chemisorption was studied by the pulse chromatography method. X-ray analysis was performed with a powder diffractometer DRON-3 equipped with a high-temperature chamber. Catalrt i c exper iments Ethane and propane hydrogenolysis were studied in a microflow reactor operating in the differential mode at alkane conversions below lo%, hydrocarbon/H2 equal to 1/10. The C2H6 aromatization was investigated in pulse and flow units according to ref. 8. RESULTS AND DISCUSSION 1. The state and dispersion of Dlatinum in monometallic zeolite catalysts. In starting samples platinum was located in the channels and partially on the external surface as [Pt(NH3)4I2+ cations characterized by the Pt 4f7/2 binding energy (B.E.) of 73.9 - 74.1 e.V. Air calcination resulted in the decompositon of the complex cations, Pt2+ migration into channels and partial reduction of Pt2+ Pto with NH3 evolved (B.E. of reduced Pt state equalled to 72.0 72.6 eV and its fraction amounted to 35%). Hp treatment led to additional Pt reduction. Two different states could be evaluated from the spectra: one with a positive shift of 0.8 - 1.2 eV, the fraction of which is maximum for a sample with a low percentage of Pt, and the other close to pure Pt metal. Since at this temperature the degree of Pt reduction is close to 100% (ref. 9) the Pt 4f doublet with higher B.E. could be attributed to small Pt particles (PtS+) which exhibit some electron deficiency due to the interaction with acidic centers of the zeolite. On the contrary, only the state of Pto close to the bulk metal was observed upon direct Hp treatment. The Pt/Si enhancement indicated substantial Pto migratlon onto the external surface in the course of this treatment.
-
145 TABLE 1 Preparation conditions and composition o f c a t a l y s t s studied Wt.%
N
Cr
Pt
1
0.5 0.5 0.5 0 0.5 1.0 1.75
2 3 4 5 6 7
Sequence o f c a t i o n introduction
0.75 0.75 0.75 0.75 0 0 0
P t then C r simultaneously P t then C r
I n i t i a l compound
PH
5 8-8.5 4 4.5 6 6 6
(CrO ) * P t ( N H )4Cl Cr(Nd37;; Pt(N&)&?z Cr(N0 )3; Pt(NH3)dClz (CrO
3
-
PP tt ((Na374cl2 NH3 4c12 Pt( NH3 4c12
The conclusions drawn from XPS studies are i n close agreement w i t h t h e P t d i s p e r s i o n derived from TEM,
CO chemisorption and XRD data (Table 2).
f a c t o r s govern the P t dispersion:
Two main
the mode o f treatment and P t concentration.
The treatment (b) gives r i s e t o widely dispersed Pt"
c l u s t e r s (0.6
-
1.4 nm)
located i n s i d e the s t r u c t u r e , w h i l e treatment (d) r e s u l t s i n P t o m i c r o c r y s t a l l i t e s w i t h a broad s i z e d i s t r i b u t i o n ranging from 2 t o 16 nm. The increase i n P t concentration leads also t o a decrease o f the P t o d i s p e r s i o n although t o a lesser extent than i n the f i r s t case.
2.
The states o f P t and C r i n b i m e t a l l i c z e o l i t e c a t a l y s t s . P t 2 + and P t 4 + were found i n the s t a r t i n g specimens (Table 3).
The l a t t e r
s t a t e probably appeared as the r e s u l t o f incomplete t r a n s i t i o n o f H2PtC16 t o Pt(NH3)4C12 i n the s t a r t i n g solution. Chromium i n c a t a l y s t s 1, 3 and 4 was found as Cr6+ and Cr3+, b i l i t y o f Cr6+ tions.
and i n c a t a l y s t 2 i t e x i s t e d as C r 3 + only. The d i f f e r e n t staions seemed t o be r e l a t e d t o d i f f e r e n t pH values i n the solu-
Surface concentrations o f both elements were higher than i n the bulk,
which i s t y p i c a l f o r metal/pentasil systems ( r e f . 10). When P t and C r were i n troduced from a common s o l u t i o n t h i s e f f e c t became more pronounced.
Probably,
j o i n t d i f f u s i o n o f both components i n t o the channels was more s t r o n g l y hindered. The a i r c a l c i n a t i o n o f Pt-Cr/HZSM-5
r e s u l t e d i n the appearance o f Cr6+
c a t a l y s t 2 and increased i t s f r a c t i o n i n c a t a l y s t 1 w h i l e i n Cr/HZSM-5 Cr6+/Cr3+
in the
r a t i o remained a c t u a l l y the same. Platinum, which was reduced by t h i s
treatment from P t 4 + t o Pt2+,
supposedly s t a b i l i z e d chromium i n i t s h i g h e s t o x i -
d a t i o n state, which i s known t o be r a t h e r unstable and can be reduced under Xr a y i r r a d i a t i o n d u r i n g spectra recording ( r e f .
11). C r ,
i n turn,
stabilized P t
i n i t s i o n i c s t a t e (Tables 2 and 3). The H2 treatment o f Cr/HZSM-5 gave r i s e t o a new chromium s t a t e w i t h B.E. 571.7
-
of
572.6 eV which probably corresponds t o supported Cro ( r e f . 5). The nega-
t i v e s h i f t r e l a t e d t o pure metal (574.0 eV) can be t e n t a t i v e l y explained by two f a c t o r s ( r e f . 5):
an i n t e r a c t i o n w i t h electron-donor s i t e s o f the support and a
TABLE 2 The effect of pretreatment conditions and Pt content on metal state,surface composition and dispersion Catalyst
Pretreatment
B.E.
Pt
Pt 4f7/2. eV
103
Si conditions 0.5 % Pt NH4ZSM
PtO
X
-
ptS+
pt2+
-
73.9(100%) 74.0(64%)
-
1.72
1.05
72.1(29%)
72.9(71%)
-
1.15
0.70
d
72.5( 100%)
-
-
2.22
1.35
-
a
72.6(31%)
C
73.1(56%)
d
72.3(44%) 72.6(100%)
X
-
-
NHqZSM
x
initial
1.18
72.0( 36%)
X
Pt
1.94
-
-
74.1(100%)
2.60
0.84
73.&(69%)
2.58
0.83
-
-
-
Pt dispersion, % _____ (av. crystal size, nm) XRD
a
1.0 % Pt
%
(XPS)
C
NH4ZSM
-1.75 _-
Pt/Si (XPS) Pt/Si(AAS)
4.31
1.39
74.0(100%)
4.70
0.86
73.9(63%)-
a
72.5 (37%)
-
4.12
0.76
C
72.0( 61%)
73.2( 39%)
-
3.33
0.61
d
72.4(100%)
-
-
8.97
1.65
amorph.
-
CO/Pt
61(1.8)
-
TEM -
al(0.8) 16(7.0)
-
amorph.
40(2.8)
-
g(13.2)
ZZ(5.1)
-
B(14.7)
31(3.6)
-
-
-
TABLE 3
The effect of pretreatment conditions on the electronic state of Pt and Cr Catalyst Pretreatment
Cr6+
X
a
39 49
-
d
-
-
X
-
-
a
579.7
22
-
-
579.2 579.4 579.2
20 44 42
C
-
d
-
-
C
2
500.0 500.2
C
d
3
X
4
X
a
pt4+
-
578.0 61 577.2 51 570.0 72 570.0 63 577.7 100 577.7 70 570.1 69 577.0 60 577.4 72 577.4 56 577.4 50 577.5 06 577.6 70
-
-
-
-
573.0 573.0
-
-
-
20 37 76.2 - 75.5
-
-
-
575.9 576.2
31 32 -
-
-
-
-
-
-
76.7
-
-
-
-
-
-
-
572.9 572.6
14 22 -
-
-
-
-
-
76.1 76.1
-
PtO
Pt2+
% B.E.,e.V. % B.E.,e.V. % B.E.,e.V. % B.E.,e.V. % B.E..e.V.
B.E.,e.V.
1
CrO
Cr*+
~r3+
63 23
%
37 77 63 19
55
73-3 73.3 73.2 73.6 73-4 73.5 73.0 73.2 73.5
-
-
-
37 32
-
-
I
-
-
63
60 43 30 45
%
B.E.,e.V.
-
-
-
-
72.0 72.2
37 01
-
-
-
71.8 72.0
57 70
-
-
>
TABLE 4
The catalytic properties of Pt and Pt-Cr/HZSM-5 in C2H6 and C3H8 hydrogenolysis Catalyst
C2H6 hydrogenolysis Pretreatment conditions
C3H8 hydrogenolysis ~
air + HE Rate, Wx103, mol e/gpt 400°C
TON, S-1 400 C
air + H p
H2 Activ. energy,Ea, kJ/mo 1 e 300-400°C
WXlOj, rnole/gpts 400°C
TON, S-'
Ea. kJ/mole
400°C
7.2
1.74b)
SCZH~~) %
300-400°C
0.25
0.30b)
199
-
-
-
1 9 )
-
134
-
-
-
2.9
76
92.5
3.3
1.58')
133
0.19
0. 17')
188
-
-
-
2.4
1.54')
125
0.14
0.34')
120
-
-
-
232
-
-
-
0.28
120
65.7
160
-
-
-
0.55
132
90.3
1.0% Pt
1.75% Pt HmI-50.5%Pt-0.75%Cr HZSM-5 (N 1) 0.5XPt-0. 75%Cr HZSM-5
Ea* kJ/mole
134
2.31')
HZSM-5
mole/gpts 350°C
300-450°C
0.5% Pt
HZSM-5
~~103.
0.36d) 0.45d)
-
a)selectivity was determi d as (C Hg/CH4 + C2H ) x 100 X; b*c)the number of Pt surface atoms was calculated from: TEM (b) or CO chemisorption (c); aTreact. (emperatwe 3 b " C .
149
differential charging effect. A similar signal of Cr 2p with slightly higher B.E. (573.5 eV) was observed for Pt-Cr/HZSM-5 (catalyst 1). These results differ from those known for Cr-zeolite catalysts (ref. 11) which gave evidence in favour of Cr2+ formation under Hp treatment. Similarly. a new Cr 2 ~ 3 1 2line with B.E. of 575.9 eV, observed during reduction of catalyst 2, can be attributed to Cr2+. The differences in precursors of reduced Cr (Cr6+ or Cr3+) as well as their dispersion can cause different reduction behaviour of Cr species in the samples studied. Indeed, TEM data suggest that catalyst 2 contains a substantial portion of Cr as a hardly reduced CrzO3 phase on the external surface (aggregated into blocks larger than 10 nm ) whereas the Cr dispersions in catalysts 1 and 4 seem to be substantially higher. Comparison of the degree of Cr reduction for PtCr/HZSM-5 (catalyst 1) and Cr/HZSM-5 (catalyst 4 ) indicates a prominent Pt-promoting effect similar to that observed for Pt-Cr/y-Alp03 (ref. 5). The difference between Cr 2 ~ 3 1 2B.E. in Pt-Cr and in Cr-zeolites can be related to Pt-Cr interaction which eliminates the negative shifts obtained for supported Cro systems. The less understandable experimental fact is the suppression of the reduction of Pt to Pto in the presence of Cr (Table 3). According to XPS intensity ratios and TEM data, Pto i s , in both types of Pt-Cr samples (catalysts 1 and 2) stabilized, as very fine particles with 0.5 - 1.0 nm size, mainly located inside the zeolite channels. When Pt-Cr samples were directly reduced in H2 flow, the promoting effect of Pt was also observed (Table 3): the degree of reduction in catalyst 1 reached 37% vs. 22% in catalyst 4. Again, as in the case of Pt/HZSM-5 zeolite, Pto migration was more pronounced under these conditions but the Pt/Si increase was not so dramatic. This probably reflects the enhancement of the Pto dispersion in the presence of chromium even under these unfavourable conditions. 3. The effect of Pt state and disDersion on catalytic DroDerties of Pt- and PtCr/HZSM-5 in alkane transformations 3.1. Ethane and propane hydroqenolysis. Pt/HZSM-5 samples pretreated according to (b) possessed rather high and stable activity in C2H6 hydrogenolysis whereas directly reduced catalysts had low activity (Table 4 ) . The reaction rate (per gpt) dropped also when the Pt content increased, but the turnover numbers noticably decreased when dispersion decreased due to a change of the pretreatment mode ("d" instead of "c"). The introduction of chromium into Pt/HZSM-5 strongly influenced the catalytic activity in alkane hydrogenolysis: the reaction rate decreased and the activation energy increased. Taking into account the rather high Pt dispersion for all samples treated in air and H2, the observed changes in activity can be assigned to a Cr doping effect. As seen from Table 4, the changes in activity and selectivity (C3H8 hydrogenolysis) were more pronounced when Cr was reduced to Cro and possibly formed intermetallic compounds with Pt (catalyst 1).
3.2. The activltv in alkane aromatization. The highly dispersed Pt, located in the vicinity of Br0nsted centers, provided the most effective system among Pt-containing catalysts for ethane and propane aromatization (refs. 1-3). One of the factors responsible for such catalytic behaviour i s that small P t o particles can be easily modified under reaction conditions. This might result in an increase of the positive charge on clusters in parallel with an enhanced yield of aromatics upon increasing pulse numbers (ref. 1). Platinum exhibited similar properties on being introduced together with Cr into HZSM-5 (Flg. 1). It should be noted that Cr/HZSM-5 possesses a low aromatization activity, comparable with that of HZSM-5 (ref. 1). Some differences in the activity level were observed as a function o f the preparation procedure (catalysts 1-3). but in all cases the yield of aromatics did not exceed that obtained with Pt/HZSM-5. In a flow reactor, Pt-Cr zeolites were rather efficient in both C2H6 and C3Hg aromatization. Yields of aromatics amounted t o 17 - 22 % (C2Hs) and 5 5 % (C3Hg). As in the case of Pt/HZSM-5 (ref. 1). the activity development in the pulse mode was accompanied by the change of the electronic state o f Pt (Fig. 1). During the first pulses the cationic form of Pt was reduced to Pto, but then a growing positive charge o n Pt clusters was observed as well.
\
v
)
-
U I-
4
12 -
0-
NUMBER
OF
C,HI P U L S E S
F i g . 1. Development o f Pt- and Pt-Cr/HZSM-5 and modification of the electronic state o f Pt in the course of ethane aromatization: a - Pt-Cr/HZSM-5 (Cat. Z), b - Pt/HZSM-5 (Cat. 1 ) . c - Pt-Cr/HZSM-5 (Cat. 3), d - Pt/HZSM-5 (Cat. 5).
These data suggest some correlations between activities in alkane aromatization and hydrogenolysis performed under H2 flow. At first glance this is a rather confusing result, but if we take into account that both alkane dehydrogenation (first stage of aromatization) and hydrogenolysis proceed via the common step of C2Hx intermediate formation (ref. 12). the similarity becomes more understandable. The small Pt clusters with some electron deficiency are the best
151
candidates for this step, and further pathways of C2H, transformations are controlled by reaction temperature, H2 content and acidity of the zeolitic support. The activity in both reactions is likely to depend not only on cluster dimensions but on its structure as well. When located inside the channels such clusters can be stabilized in an icosahedron structure (ref. 13), which possesses highly reactive sites for hydrogenolysis and dehydrogenation. The following changes in local surface structure have been supposed to be responsible for a decreasing hydrogenolysis activity in PtCr zeolites: (a) extremely high Pt dispersion reached in the presence of chromium; (b) formation of Pt-Cr intermetallic compounds leading to a dilution of Pt aggregates by Cr, which probably replaces most reactive Pt centers. Concerning the lower aromatization activity, one can speculate that only PtS+ centers are involved in the catalysis and not Pt-Cr sites. However, this assumption needs further confirmation. CONCLUSION The electronic state, distribution and dispersion of Pt in pentasil-type zeolites depend on the mode of pretreatment, metal concentration and the presence of a second element (chromium). Precalcination of Pt/HZSM-5 leads t o a more uniform Pt2+ distribution in the channels and subsequent Pto stabilization in a finely dispersed form. Chromium provided the same effect via possible formation o f Pt-Cr intermetallic compounds or stabilizing of Pt by Cr3+. A variety of cationic Cr species, viz. Cr(VI), Cr(III), Cr(I1) and Cr(O), were found in pentasil zeolites. The activity in ethane and propane hydrogenolysis was influenced by the state of Pt, Pt-dispersion and its modification by Cr. The correlation between the charge of Pt species and the yield of aromatics observed for both Pt/HZSM-5 and Pt-Cr/HZSM-5 permits the conclusion that at least alkane dehydrogenation is catalyzed by highly dispersed PtS+ species. REFERENCES O.V. Bragin, E.S. Shpiro. A.V. Preobrazhensky, S.A. Isaev, T.V. Vasina. 6.8. Dysenbina, G.V. Antoshin and Kh.M. Minachev, Appl. Catal., 27 (1986) 219231. T. Inui, Y. Makino, F. Maganos and A. Miymoto, Ind. Eng. Chem. Res. and Develop., 26 (1987) 647-652. C.W.R. Engelen, J.P. Wolthuizen, J.H.C. Hoof and H.W. Zundbergen, Proceed. 7th Int. Zeolite Conf., 1986, pp. 709-716. M.S. Tzong, H.J. Jiang and W.M.H. Sachtler, React. Kinet. Catal. Lett., 35 (1987) 207-218. W. Grunert. E.S. Shpiro. R. Felthaus, K. Anders, G.V. Antoshin and Kh.M. Minachev, J. Catal., 100 (1986) 138-148. Kh.M. Minachev, G.V. Antoshin and E.S. Shpiro, Photoelectron Spectroscopy and its Application in Catalysis, M., Nauka, 1981. Yu.A. Ryndin, V.I. Chernyshev, V.I. Zaikovskii, E.N. Yurchenko and Yu.1. Yermakov. React. Kinet. Catal. Lett., 21 (1982) 125-129. O.V. Bragin. A.V. Preobrazhensky and A.L. Liberman, Izv. Akad. Nauk SSSR, Ser. Khim., 12 (1974) 2751-2757.
152
9 T.R. Felthouse and J.A. Murphy. J . Catal., 98 (1986) 411-433. 10 Kh.M. Minachev and E.S. Shpiro, Kinetica 1 Kataliz, 27 (1986) 824-837. 1 1 B. Wichterlova, L. Krajcihova, Z. Tvaruzkova and S. Beran, J . Chem. SOC. 12 13
Faraday Trans. I, 80 (1984) 2639-2645. J.H. Sinfelt, Catal. Rev., 3 (1969) 175-205. B. Moraweck. J . Ckugnet and A. Renouprez, Surf. Sci., 9 1 (1981) 631.
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CONVERSION OF LIGHT ALKANES INTO AROMATIC HYDROCARBONS. 3. AROMATIZATION OF PROPANE AND PROPENE ON MIXTURES OF HZSM5 AND
OF Ga203
N.S. GNEP, J.Y. DOYEMET and M. GUISNET UA CNRS 350, C a t a l y s e en Chimie Organique, U n i v e r s i t S \ de P o i t i e r s , 40, avenue du Recteur Pineau, 86022 P o i t i e r s Cedex, FRANCE
ABSTRACT The a c t i v i t i e s f o r propane a r o m a t i z a t i o n o f p h y s i c a l m i x t u r e s o f HZSM5 ( 2 5 mg) and Ga 0 ( 5 t o 200 mg) a r e much g r e a t e r t h a n t h e sum o f t h e a c t i v i t i e s o f t h e p u r 6 zomponents. T h i s s y n e r g i c e f f e c t i s c h a r a c t e r i s t i c o f a r e a c t i o n i n which c a t a l y t i c s i t e s o f t h e two components ( b i f u n c t i o n a l c a t a l y s i s ) p a r t i c i p a t e . Experiments w i t h model r e a c t a n t s propane, propene, 1-hexene, 1-heptene, methylcyclohexene and methylcyclohexane c o n f i r m t h a t g a l l i u m o x i d e c a t a l y z e s t h e dehydrogenation o f alkanes i n t o o l e f i n s and o f naphthenes i n t o a r o m a t i c s b u t i s n o t a c t i v e f o r o l i g o m e r i z a t i o n and c y c l i z a t i o n r e a c t i o n s . The a r o m a t i z a t i o n o f propane on t h e m i x t u r e o c c u r s i n t h e f o l l o w i n g way : on g a l l i u m o x i d e propane i s dehydrogenated i n t o propene; propene d i f f u s e s f r o m g a l l i u m o x i d e t o HZSM5, on which i t undergoes o l i g o m e r i z a t i o n and t h e n c y c l i z a t i o n ; naphthenic compounds d i f f u s e f r o m HZSM5 t o g a l l i u m o x i d e where t h e y a r e dehydrogenated i n t o C6-C8 aromatics. INTRODUCTION
An i n t e r e s t i n g means o f i n c r e a s i n g t h e v a l u e o f l i g h t alkanes would b e t o transform
them
into
aromatics
t r a n s f o r m a t i o n i n t o C6-Cg
(1,2).
HZSM5
aromatic products (3-5).
is
active
for
propane
A r e a c t i o n a l scheme has
a l r e a d y been proposed t o e x p l a i n t h i s t r a n s f o r m a t i o n ( 4 , 5 ) . The f i r s t stage is
t h e formation
of
propene;
this
olefin,
by
a
series
of
reactions
--01 i g o m e r i z a t i o n , c y c l i z a t i o n , hydrogen t r a n s f e r f r o m naphthenes t o o l e f i n i c compounds--
l e a d s t o aromatic hydrocarbons. The c r a c k i n g o f o l i g o m e r s g i v e s
l i g h t o l e f i n s which, l i k e propene, p a r t i c i p a t e i n t h e f o r m a t i o n o f a r o m a t i c s .
Kitagawa e t a l . (4) have shown t h a t t h e exchange o f HZSM5 by Ga3+ i n c r e a s e s t h e r a t e o f propane c o n v e r s i o n and improves g r e a t l y t h e s e l e c t i v i t y towards aromatics. The o n l y r o l e o f g a l l i u m s p e c i e s would b e t o i n c r e a s e t h e
154
r a t e o f a r o m a t i z a t i o n o f t h e lower o l e f i n s ;
t h e s e species do n o t have a
d i r e c t e f f e c t on t h e a c t i v a t i o n o f propane ( s t e p 1 ) . T h i s i s c o n t e s t e d by Gnep e t a l . ( 6 ) . A c c o r d i n g t o t h e s e a u t h o r s propane a r o m a t i z a t i o n on GaHZSM5 c a t a l y s t s can be c o n s i d e r e d as a b i f u n c t i o n a l
process
i n which
gallium
species c a t a l y z e m a i n l y s t e p 1 (propane a c t i v a t i o n ) and s t e p 5 ( a l i c y c l i c compound a r o m a t i z a t i o n ) w h i l e t h e a c i d s i t e s o f HZSM5 c a t a l y z e m a i n l y s t e p s 2 t o 4. The n a t u r e o f t h e a c t i v e g a l l i u m s p e c i e s i s n o t w e l l known. Indeed, g a l l o s i l i c a t e s w i t h a p e n t a s i 1 s t r u c t u r e a r e a l s o good c a t a l y s t s f o r propane a r o m a t i z a t i o n ( 7 1 , and we have shown i n a p r e l i m i n a r y n o t e ( 8 ) t h a t Ga20j added t o HZSM5 produces e f f e c t s s i m i l a r t o t h o s e produced by exchange o f HZSM5. The aim o f t h i s paper i s t o show how t h e a d d i t i o n o f g a l l i u m o x i d e t o HZSM5 a f f e c t s propane and propene a r o m a t i z a t i o n .
We c o n f i r m ,
namely,
that
g a l l i u m species p l a y no r o l e i n s t e p s 2 t o 4 o f t h e s u c c e s s i v e r e a c t i o n scheme b u t p a r t i c i p a t e i n s t e p s 1 and 5. EXPERIMENT HZSM5 ( S i / A l = 40) was s y n t h e s i z e d a c c o r d i n g t o M o b i l P a t e n t s ( 9 ) . G a l l i u m o x i d e (Ga203, Bform, 99.99 % pure, BET a r e a equal t o 4 m 2 g - l ) was s u p p l i e d b y A l d r i c h Chemical Co. P r i o r t o use, c a t a l y s t s were p r e t r e a t e d f o r 10 h o u r s a t 530°C under d r y n i t r o g e n f l o w . Propane ( 99.95 % p u r e ) and propene ( > 9 9 % p u r e ) were Alphagaz p r o d u c t s . The main i m p u r i t y i n propane was propene. Propene c o n t a i n e d propane ( < 4 0 0 0 ppm), ethane (<1500 ppm) and ethene ( ~ 1 5 0ppm). Propane and propene t r a n s f o r mations were c a r r i e d o u t i n a f l o w r e a c t o r under t h e f o l l o w i n g c o n d i t i o n s : r e a c t i o n temperature, 530°C; propane o r propene p r e s s u r e , 1 b a r ; WWH ( w e i g h t o f r e a c t a n t i n j e c t e d p e r u n i t w e i g h t o f c a t a l y s t and p e r h o u r ) between 0.5 and 80 i n o r d e r t o o b t a i n a wide range o f c o n v e r s i o n r a t e s ( 1 - 4 0 % f o r propane and 1-80 % f o r propene). R e a c t i o n p r o d u c t s were analyzed on l i n e b y gas chromatography; t h e c o n d i t i o n s f o r a n a l y s i s a r e r e p o r t e d elsewhere ( 5 ) . RESULTS 1. Propane t r a n s f o r m a t i o n The c o n v e r s i o n o f propane was c a r r i e d o u t under i d e n t i c a l c o n d i t i o n s ( T = 530°C, p propane = 1 b a r , f l o w r a t e o f propane = 0.018 mole h - ' ) ,
on p u r e
HZSM5 ( 2 5 mg), on p u r e Ga203 (200 mg) and on mechanical m i x t u r e s o f HZSM5 ( 2 5 rng) and Ga203 (5, 25, 75 o r 200 mg). D e a c t i v a t i o n i s always v e r y slow. F i g u r e 1 shows t h a t t h e t o t a l c o n v e r s i o n on t h e mechanical m i x t u r e s i n c r e a s e s a t f i r s t when t h e amount
of
Ga203
i n c r e a s e s and t h e n reaches a p l a t e a u f o r amounts of Ga203 above 75 mg. Except
155 f o r t h e m i x t u r e w i t h 200 mg o f Ga203 t h e t o t a l c o n v e r s i o n i s p r a c t i c a l l y equal
t o the
sum o f
the
conversions
that
can
expected w i t h
be
pure
C6-C8 a r o m a t i c s i s much
components. On a l l t h e m i x t u r e s t h e c o n v e r s i o n i n t o
h i g h e r t h a n t h o s e found on HZSM5 and on Ga203. As i s t h e case w i t h t h e t o t a l conversion, i t i n c r e a s e s a t f i r s t w i t h t h e amount o f Ga203 added t o HZSM5 and t h e n reaches a p l a t e a u ( F i g u r e 1 ) .
2 c.
8e
Y
-
t
c.
x
w
1
x
100
0
200
Ga203 (mg) F i g . 1. Propane t r a n s f o r m a t i o n on mechanical m i x t u r e s o f HZSM5 ( 2 5 mg) and o f g a l l i u m o x i d e Ga 0 . T o t a l c o n v e r s i o n X ( % I and c o n v e r s i o n i n t o a r o m a t i c s XAr ( % ) as f u n c t i o n s 2 0 ? t h e amount o f Ga203. On pure Ga203, t h e p r o d u c t s a r e composed m a i n l y o f propene (70 w t % ) and o f an equimolar m i x t u r e o f e t h y l e n e and methane.
On HZSM5 and on a l l t h e
mechanical m i x t u r e s t h e p r i m a r y p r o d u c t s a r e methane, e t h y l e n e and propene; t h e o t h e r p r o d u c t s --ethane, compounds and C6-C8
butanes, butenes,
a l i p h a t i c o r a l i c y c l i c C5-C6
a r o m a t i c s - - r e s u l t f r o m t h e secondary t r a n s f o r m a t i o n o f
propene and o f e t h y l e n e . F i g u r e s 2 - 4 show,
however,
t h a t t h e r e are great
d i f f e r e n c e s between t h e p r o d u c t d i s t r i b u t i o n s on HZSM5 and on t h e m i x t u r e s . The a d d i t i o n o f g a l l i u m o x i d e t o HZSM5 causes t h e f o l l o w i n g :
-
An i n c r e a s e o f t h e i n i t i a l p r o d u c t i o n o f propene ( F i g u r e 2 ) :
a t low
c o n v e r s i o n ( < l o % ) t h e g r e a t e r t h e amount o f g a l l i u m o x i d e i n t h e m i x t u r e t h e g r e a t e r t h e s e l e c t i v i t y f o r propene. F i g u r e 2 shows a l s o t h a t t h e secondary transformations
o f propene a r e much f a s t e r
with
the
mixtures
than with
HZSM5 : t h e maximum y i e l d i n propene i s l o w e r and o b t a i n e d f o r l o w e r v a l u e s o f conversion; on t h e o t h e r hand,on Ga203 propene does n o t seem t o undergo secondary t r a n s f o r m a t i o n s . - An i n c r e a s e i n t h e s e l e c t i v i t y f o r C6-C8
aromatics:
the greater the
amount o f g a l l i u m added t o HZSM5, t h e g r e a t e r t h e s e l e c t i v i t y ( F i g u r e 3 ) .
Fig. 2
10
0
30
20
0
10
X(%)
20
30 X(%)
F i g s , 2 and 3. Propane t r a n s f o r m a t i o n on mechanical m i x t u r e s o f HZSM5 and o f Ga 0 Percentages o f propene and o f a r o m a t i c s ( w t % ) as f u n c t i o n s o f t h e ; 25 mg HZM5 t 5 mg Ga O3 ( ) ; 25 mg t o ? a ? c o n v e r s i o n X ( % ) , HZSM5 ( 0 HZSM5 t 25 mg Ga O3 ( ) ; 25 mg HZSM5 t 75 mg Ga203 ( A ) ; 1 5 mg HZSM5 t 200 mg ~ a ( +~ I ;02a203 ~ (21).
.
*
*
- A decrease i n t h e s e l e c t i v i t y f o r methane,
ethane, e t h y l e n e ,
butanes,
butenes, a l i p h a t i c o r a l i c y c l i c C5-C6 compounds ( F i g u r e s 4).
-
An i n c r e a s e o f t h e isobutane/n-butane
r a t i o . Moreover, when t h e c o n v e r -
s i o n i n c r e a s e s t h i s r a t i o i n c r e a s e s on HZSM5 and decreases on t h e m i x t u r e s . I n b o t h cases i t approaches i t s e q u i l i b r i u m v a l u e (0.55 a t 530°C
(10)) a t
high conversion.
On t h e c o n t r a r y , on a l l t h e c a t a l y s t s t h e butene d i s t r i b u t i o n i s t h e same as t h e d i s t r i b u t i o n o f t h e thermodynamic e q u i l i b r i u m even a t low c o n v e r s i o n . The d i s t r i b u t i o n o f a r o m a t i c s i s t h e same on a l l t h e c a t a l y s t s when i t i s determined f o r t h e same v a l u e s o f a r o m a t i c y i e l d s . The p r o d u c t i o n o f hydrogen can be e s t i m a t e d f r o m t h e d i f f e r e n c e between t h e hydrogen c o n t e n t o f t h e r e a c t i o n p r o d u c t s and o f t h e r e a c t a n t . F i g u r e 5 shows t h a t t h e y i e l d i n hydrogen i s more s i g n i f i c a n t on t h e Ga203 (200 mg)HZSM5 ( 2 5 mg) m i x t u r e t h a n on HZSM5. About 4.2 moles o f hydrogen a r e formed p e r mole o f a r o m a t i c hydrocarbon f r o m propane t r a n s f o r m a t i o n on t h e m i x t u r e as a g a i n s t about 2 moles on HZSM5.
157
41
'O1 I
i
2
a
Y
0)
c
10 c,
c W
py,
0
10
20 30 X(%)
! + j P :
3
e
a.
1 0
10
20
io
20
30
30
X(%)
0
0
10
20
10
20 30 X(%)
3b
9
J'
Y
L F i g . 4. Propane t r a n s f o r m a t i o n on HZSM5 ( * j , on a m i x t u r e o f 25 mg HZSM5 and 200 mg Ga 0 ( ttt 1 and on 6GaHZSM5 ( A 1. Percentages o f t h e p r o d u c t s ( w t X ) as functions'o? t h e t o t a l c o n v e r s i o n X ( % I * ( a ) methane ; ( b ) ethane ; ( c ) ethene ; ( d ) propene ; ( e l butanes ; i f ) butenes ; ( 9 ) a l i p h a t i c and a l i c y c l i c C5-C6 compounds (non-aromatics C5+) ; ( h ) aromatics.
158
0
100
50
X(%) F i g . 5. Propane t r a n s f o r m a t i o n on HZSM5 (+I,on a m i x t u r e o f 25 mg HZSM5 and as a f u n c t i o n 200 mg Ga 0 ( * ) and on 6GaHZSM5 ( A ) . ( a ) Hydrogen y i e l d ( H o f t h e to?aq c o n v e r s i o n X ( % ) . ( b ) Hydrogen/aromatic m o l a r r a ? i o ( H 2 / A ) as a f u n c t i o n o f t h e c o n v e r s i o n i n t o a r o m a t i c s XAr (%I. 2. Propene t r a n s f o r m a t i o n The c o n v e r s i o n o f propene was c a r r i e d o u t on p u r e HZSM5 ( 2 5 mg), on p u r e Ga203 (200 mg) and on a mechanical m i x t u r e o f HZSM5 ( 2 5 mg) and o f Ga203 (200 mg) under t h e same o p e r a t i n g c o n d i t i o n s as f o r propane c o n v e r s i o n . On Ga203 t h e r e a c t i o n r a t e i s t h r e e t i m e s l o w e r t h a n w i t h propane, t h e main p r o d u c t s b e i n g propane ( 3 0 w t
wt
%I
; small
amounts o f methane,
%I
and C5-C7
ethane,
a l i p h a t i c compounds
ethylene,
butanes,
(30
butenes and
a r o m a t i c s a r e a l s o formed. On HZSM5 and on t h e m i x t u r e propene r e a c t s about 5 0 t i m e s more r a p i d l y t h a n propane. T a b l e 1 shows t h a t t h e a d d i t i o n o f Ga203
t o HZSM5 i n c r e a s e s s l i g h t l y t h e t o t a l c o n v e r s i o n o f propene b u t i n c r e a s e s s t r o n g l y i t s c o n v e r s i o n i n t o C6-C8 a r o m a t i c s . The r e a c t i o n p r o d u c t s a r e t h e same as t h o s e o b t a i n e d i n propane t r a n s f o r m a t i o n b u t t h e d i s t r i b u t i o n s a r e q u i t e d i f f e r e n t . The a d d i t i o n of Ga203 t o H E M 5 has p r a c t i c a l l y no e f f e c t on t h e f o r m a t i o n o f methane, o f ethane, o f e t h y l e n e , o f propane, o f butenes and o f butanes b u t causes a decrease i n t h e p r o d u c t i o n o f non-aromatic p r o d u c t s ( F i g u r e 6 ) and an i n c r e a s e i n t h e p r o d u c t i o n o f C6-C8
C5-C6
aromatics
(Figure 7 ) . On HZSM5 and on t h e m i x t u r e t h e i s o b u t a n e / n - b u t a n e r a t i o i s always h i g h e r t h a n i t s thermodynamic e q u i l i b r i u m v a l u e (0.55 a t 530°C ( 1 0 ) ) . F o r t h e same v a l u e of c o n v e r s i o n t h i s r a t i o i s h i g h e r on t h e m i x t u r e t h a n on HZSM5 : e.g. a t 30 % c o n v e r s i o n 1.6 on t h e m i x t u r e as a g a i n s t
1 on HZSM5;
a t 80 %
c o n v e r s i o n about 2 on t h e m i x t u r e as a g a i n s t 1.4 on HZSM5. No s i g n i f i c a n t d i f f e r e n c e s between t h e two c a t a l y s t s a r e observed i n t h e b u t e n e and i n t h e C6-C8 a r o m a t i c d i s t r i b u t i o n s .
159
F i g s . 6 and 7. Propene t r a n s f o r m a t i o n on HZSM5 ( + 1, on a m i x t u r e o f 25 mg HZSM5 and 200 mg Ga 0 ( ) and on 6GaHZSM5 ( A). Percentages o f non-aromatic C ( w t % ) and o f & h a t i c s (wt % ) as f u n c t i o n s o f t h e t o t a l conversion X
*
(2J.
The y i e l d i n hydrogen i s c l o s e t o zero on HZSM5. On t h e m i x t u r e o f HZSM5 and o f Gap03 i t becomes s i g n i f i c a n t f o r h i g h conversion l e v e l s ; whatever t h e conversion l e v e l , about 1.8 mole o f hydrogen i s formed p e r mole o f aromatic hydrocarbon. TABLE 1 Propene conversion a t 530°C, mol.h-',
p propene = 1 bar, molar f l o w r a t e = 0.018
X = t o t a l conversion, XAr
I I HZSM5 (25 mg) I I Ga203 (0.2 4) HZSM5 (25 mg) t Ga203 (200 mg) I I 6GaZSM5 ( 2 5 mg) I Catalysts
=
conversion i n t o C6-C8
x (%I I 79
4.4 81.3 81.6
I I 1 I I I
aromatics.
(XI
XAr
17.8 0.3 28.8 31.3
3. Conversion o f model r e a c t a n t s on Ga2c3 I n order t o discuss t h e r o l e played by Ga203 on steps 2 t o 5 t h e t r a n s f o r mation o f f o u r model r e a c t a n t s --1-hexene, methylcyclohexene--
o p e r a t i n g c o n d i t i o n s : t = 530"C, about 0.018 mol .h-'.
1-heptene,
methylcyclohexane and
was c a r r i e d o u t on 200 mg o f Ga203 under t h e f o l l o w i n g p hydrocarbon = 1 bar, m o l a r f l o w r a t e o f
160
Table 2 shows t h a t w i t h a l l t h e r e a c t a n t s i s o m e r i z a t i o n and c r a c k i n g are t h e main r e a c t i o n s .
The o l e f i n i c compounds are more r a p i d l y transformed
(conversion between 58-70 % I t h a n methylcyclohexane (conversion o f However, 10 % I , Aromatics are formed f r o m a l l t h e r e a c t a n t s . methylcyclohexane
and,
above a l l ,
methylcyclohexene
lead
to
about only
significant
amounts; i n both cases t o l u e n e i s p r a c t i c a l l y t h e o n l y aromatic product. TABLE 2 Conversion o f model r e a c t a n t s on Ga203 (200 mg) a f t e r 5 m i n u t e s ' r e a c t i o n a t 530°C, p hydrocarbon = 1 bar, molar f l o w r a t e = 0.018 mol.hr-'
I I -1
Reactant
I 67 1-heptene I 58 methylcyclohexane I 9-10 170-72
-1 X : t o t a l conversion, C1-C6
X Isomers
[Products
1-hexene
methylcyclohexene
* I (%I I I I 21 50 I I 5 I 12.5 I
X ( % ) 1X c r a c k i n g
*
I 1 1 I I I
conversion i n t o C1-C5
46 8 3 57.5
(%)I I I I I I I I
X Aromatics
(%I
< 1 < 1 1-2 10-12
products f r o m 1-hexene and i n t o
products from t h e o t h e r r e a c t a n t s .
DISCUSSION The a c t i v i t i e s f o r propane a r o m a t i z a t i o n o f t h e m i x t u r e s o f HZSM5 and Ga203 are much g r e a t e r than t h e sum o f t h e a c t i v i t i e s o f t h e pure components ( F i g u r e 1 ) . T h i s s y n e r g y s t i c e f f e c t i s c h a r a c t e r i s t i c o f r e a c t i o n s i n which t h e c a t a l y t i c s i t e s o f t h e two components ( b i f u n c t i o n a l c a t a l y s i s ) p a r t i c i pate. F i g u r e s 2-7 show t h a t on t h e m i x t u r e Ga203 (200 mg)
-
s e l e c t i v i t i e s f o r propane and f o r propene t r a n s f o r m a t i o n s
HZSM5 (25 mg) t h e are s i m i l a r t o
those obtained on 6GaHZSM5,
a c a t a l y s t w i t h 6 w t % g a l l i u m prepared by impregnation o f HZSM5 w i t h g a l l i u m n i t r a t e ( 6 ) . Therefore, on b o t h c a t a l y s t s
the
gallium
species
participating
i n the
aromatization
are
identical,
probably g a l l i u m oxide. However, we cannot exclude t o t a l l y t h a t t h e r e a r e no r e a c t i o n s between HZSM5 and g a l l i u m o x i d e under t h e o p e r a t i n g c o n d i t i o n s ( h i g h temperature, presence o f hydrogen). The r a t e o f propane t r a n s f o r m a t i o n i s h i g h e r on 6GaHZSM5 than on t h e m i x t u r e . Thus under i d e n t i c a l c o n d i t i o n s t h e conversion o f propane on 25 mg o f 6GaHZSM5 i s equal t o 20 % as a g a i n s t o n l y 8.5 % on t h e m i x t u r e . T h i s can be e x p l a i n e d by a c l o s e r i n t i m a c y between t h e two components o f t h e a r o m a t i z a t i o n c a t a l y s t HZSM5 and g a l l i u m oxide.
161
R o l e o f a a l l i u m soecies G a l l i u m o x i d e causes a s i g n i f i c a n t i n c r e a s e i n t h e r a t e s o f a r o m a t i c and of hydrogen p r o d u c t i o n . It a c t s t h e n as a dehydrogenating c a t a l y s t . T h i s i s shown c l e a r l y
i n propane t r a n s f o r m a t i o n on Ga203:
t h e main p r o d u c t i s
propene. T h i s dehydrogenating e f f e c t i s p r o b a b l y r e s p o n s i b l e f o r most o f t h e i n c r e a s e i n t h e propane t r a n s f o r m a t i o n r a t e caused by t h e a d d i t i o n o f g a l l i u m o x i d e t o HZSM5 ( F i g u r e . 1 ) . Indeed, on HZSM5 t h e t r a n s f o r m a t i o n o f propane i n t o propene which r e q u i r e s s t r o n g a c i d s i t e s i s one o f t h e slower s t e p s o f
( 3 ) . Gallium oxide also catalyzes step 5 o f t h e r e a c t i o n scheme as shown i n t h e t r a n s f o r m a t i o n s o f methylcyclohexane and
propane a r o r n a t i z a t i o n
above a l l o f methylcyclohexene ( T a b l e 2 ) . T h i s e f f e c t o f g a l l i u m s p e c i e s on s t e p 5 e x p l a i n s t h e h i g h e r s e l e c t i v i t y f o u n d f o r propane and f o r propene a r o m a t i z a t i o n and t h e i n c r e a s e i n t h e r a t e o f propene consumption shown i n f i g u r e 2 : t h e maximum y i e l d i n propene f o r m a t i o n i s lower w i t h t h e m i x t u r e o f Ga203
t
HZSM5 t h a n w i t h HZSM5 and i s o b t a i n e d f o r a lower c o n v e r s i o n .
I t i s i n t e r e s t i n g t o e s t i m a t e how g a l l i u m o x i d e on one hand and t h e HZSM5
a c i d s i t e s on t h e o t h e r i n f l u e n c e t h e r a t e s o f steps 1 and 5. F o r s t e p 1 t h i s can be deduced f r o m t h e i n i t i a l r a t e s o f propene p r o d u c t i o n on HZSM5 and on t h e m i x t u r e s : on HZSM5 t h e r a t e i s about 4 t i m e s l o w e r t h a n on t h e m i x t u r e Ga203 (200 mg)
t
HZSM5 ( 2 5 mg) o r on 6GaHZSM5. We can t h u s c o n c l u d e t h a t
about 7 5 % o f t h e propene p r o d u c t i o n comes f r o m Ga203 as a g a i n s t about 2 5 % from t h e acid s i t e s . For step 5 t h e formation o f aromatics through d i r e c t dehydrogenation o f naphthenes i s more s i g n i f i c a n t
than
their
formation
t h r o u g h hydrogen t r a n s f e r . Indeed f o r propene c o n v e r s i o n on t h e m i x t u r e o f Ga203 and HZSM5 t h e p r o d u c t i o n o f one mole o f
a r o m a t i c hydrocarbon
is
accompanied by t h e f o r m a t i o n o f 1.8 mole o f hydrogen w h i l e on HZSM5 t h e f o r m a t i o n o f hydrogen i s n e g l i g i b l e . I f t h e a r o m a t i c s were produced t h r o u g h naphthene dehydrogenation about 3 moles o f hydrogen would be formed p e r ,mole o f aromatic hydrocarbon ( c o n s i d e r e d as t o l u e n e ) . We can t h u s c o n c l u d e t h a t dehydrogenation on g a l l i u m s p e c i e s i s r e s p o n s i b l e f o r about 60 % o f t h e t r a n s f o r m a t i o n o f naphthene i n t o aromatics,
hydrogen t r a n s f e r on t h e a c i d
s i t e s o f HZSM5 b e i n g r e s p o n s i b l e f o r t h e o t h e r 40 %.Another e s t i m a t i o n made from t h e p r o d u c t i o n o f a r o m a t i c s and o f hydrogen d u r i n g propane
transforma-
t i o n g i v e s about 7 0 % f o r dehydrogenation as a g a i n s t 30 % f o r hydrogen transfer
.
G a l l i u m s p e c i e s have no s i g n i f i c a n t e f f e c t on s t e p 4 s i n c e t h e r e p r a c t i c a l l y no p r o d u c t i o n of 1-heptene.
Cracking o f
large
benzene f r o m 1-hexene olefins
such
methylcyclohexane occurs on g a l l i u m o x i d e ;
as
I-hexene,
however,
is
o r o f toluene from 1-heptene
and
we have checked t h a t
t h e s e r e a c t i o n s a r e c o n s i d e r a b l y f a s t e r on H E M 5 ( o v e r 100 t i m e s ) . D u r i n g t h e
162
a r o m a t i z a t i o n o f propane on t h e Ga20g
t
HZSM5 m i x t u r e s o r on 6GaHZSM5, t h e
p a r t i c i p a t i o n o f g a l l i u m s p e c i e s i n c r a c k i n g r e a c t i o n s and t h e n
i n the
r e v e r s e r e a c t i o n (i.e. o l i g o m e r i z a t i o n ) i s t h e r e f o r e almost n e g l i g i b l e ; t h e s e r e a c t i o n s o c c u r m a i n l y on t h e HZSM5 a c i d s i t e s . Scheme o f propane t r a n s f o r m a t i on on Ga/HZSM5 c a t a l y s t s Propane t r a n s f o r m a t i o n on c a t a l y s t s a s s o c i a t i n g g a l l i u m s p e c i e s a c t i v e i n dehydrogenation and HZSM5 a c i d s i t e s can be c o n s i d e r e d as a b i f u n c t i o n a l process such as expressed by Weisz ( 1 1 ) . G a l l i u m s p e c i e s c a t a l y z e s t e p s 1 and 5 of t h e successive r e a c t i o n scheme and t h e a c i d s i t e s o f HZSM5 s t e p s 2 t o 4. Besides t h e s e chemical steps t h e b i f u n c t i o n a l
process r e q u i r e s d i f f u s i o n
s t e p s o f t h e i n t e r m e d i a t e s p e c i e s : propene d i f f u s i o n f r o m t h e g a l l i u m a c t i v e s i t e s t o t h e a c i d s i t e s and naphthene d i f f u s i o n f r o m t h e a c i d t o t h e g a l l i u m s i t e s . The v a r i a t i o n o f t h e c o n v e r s i o n o f propane i n t o C6-C8
aromatics w i t h
t h e amount o f g a l l i u m o x i d e ( F i g u r e 1 ) i s q u i t e c h a r a c t e r i s t i c o f t h i s t y p e o f b i f u n c t i o n a l process :
-
For
small
amounts
of
gallium
oxide,
the
limiting
steps
are
the
dehydrogenation o f propene ( s t e p 1 ) o r t h a t o f a l i c y c l i c compounds ( s t e p 51, and t h e c o n v e r s i o n o f propane i n t o a r o m a t i c s i n c r e a s e s p r o p o r t i o n a l l y t o t h e amount o f g a l l i u m o x i d e .
-
F o r l a r g e amounts, r e a c t i o n s 1 and 5 become f a s t e r compared t o r e a c t i o n s
2-4 which a r e t h e n t h e r a t e - l i m i t i n g steps;
and t h e c o n v e r s i o n o f propane
i n t o a r o m a t i c s no l o n g e r depends on t h e amount o f g a l l i u m o x i d e .
REFERENCES 1 J.R. Mowry, R.F. Anderson, J.A Johnson, O i l and Gas J o u r n a l (1985) 128. 2 J.R. Mowry, D.C. M a r t i n d a l e , A.H.P. H a l l , The A r a b i a n J o u r n a l f o r Science and E n g i n e e r i n g 10 (1985) 367. 3 T. I n u i , F. Okazumi, J . C a t a l . 90 (1984) 366. 4 H. Kitagawa, Y . Sendoda and Y. Ono, J. C a t a l . 101 (1986) 12 5 N.S. Gnep, J.Y. Doyemet, A.M. Seco, F.R. R i b e i r o and M. G u i s n e t , Appl. C a t a l . 35 (1987) 93. 6 N.S. Gnep, J.Y. Doyemet, A.M. Seco, F.R. R i b e i r o and M. G u i s n e t , A p p l . C a t a l . 43 (1988) 155. 7 T. I n u i , A . Miyamoto, H. Matsuda, H. Nagata, Y. Makino, K . Fukuda and F. Okazumi, Proc. 7 t h I n t e r n . Z e o l i t e Conference, Y. Murakami e t a l . (Eds.) Kodansha, Tokyo, 1986, p. 859. 8 N.S. Gnep, J.Y. Doyemet and M. G u i s n e t , J . Mol. C a t a l . 45 (1988) 281. 9 R.G. Argauer and G.R. L a n d o l t , U.S. Pat., 3.702.886 (1972). 10 D.R. S t u l l , E.F. Westrum and G.C. Sinke, The Chemical Thermodynamics o f Organic Compounds, J. Wiley, New York, 1969. 11 P.B. Weisz, Advances i n C a t a l y s i s , D.D. E l e y e t a l . ( E d s . ) , Academic Press, London, 13 (1962) p . 137.
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SHAPE-SELECTIVE CATALYSIS
I N ZEOLITES WITH ORGANIC SUBSTRATES CONTAINING
OXYGEN
R.F. P art on, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs La b ora t o riu m voor Oppervlaktechemie, KU Leuven, 92 K a r d i n a a l M e r c i e r l a a n B3030 Leuven (Heverlee), Belgium
INTRODUCTION AND SCOPE With
the
advent
of
acid
10-membered
ring
(10-MR)
heterogeneous c a t a l y s t s i n hydrocarbon t r a n s f o r m a t i o n s ,
zeolites
as
s e v e r a l new concepts
on s e l e c t i v i t y such as t r a n s i t i o n s t a t e and p r o d u c t shape s e l e c t i v i t y ,
to
name t h e most i m p o r t a n t ones, were developed [ l - 8 1 . I n t h e meantime i t was e s t a b l i s h e d t h a t t h e s e concepts were s u i t a b l e f o r d a t a r a t i o n a l i s a t i o n and property
p r e d i c t i o n n o t o n l y on a c i d c a t a l y s t s
b u t on met al
[9, l o ] ,
b i f u n c t i o n a l [ 8 ] and even b a s i c c a t a l y s t s [ll]o f z e o l i t i c o r i g i n as w e l l . The aim o f t h e p r e s e n t r e v i e w was t o examine whether t hese concepts a r e v a l i d when o t h e r t h an hydrocarbon s u b s t r a t e s a r e c o n v e r t e d on t h e mentioned shapes e l e c t i v e z e o l i t e s and r e l a t e d m a t e r i a l s . The r e a c t i o n s o f molecules c o n t a i n i n g hetero-atoms on z e o l i t e s have a l r e a d y been t h e s u b j e c t o f s e v e r a l r e v i e w s [12-211. S e l e c t i v e chemical c o n v e r s i o n o f o r g a n i c molecules on c l a y m i n e r a l s has been reviewed as w e l l [22]. From t h i s wealth o f information,
i t seems t h a t f o r e v e r y o r g a n i c r e a c t i o n a z e o l i t e and/or c l a y can be found, c a t a l y s i n g i t w i t h a more o r l e s s good a c t i v i t y and
selectivity.
T he r e f o r e ,
the
following
two
editorial
constraints
on
the
p r e s e n t r e v i e w seem j u s t i f i e d : ( i ) o n l y t h e r e a c t i o n s o f oxygenates should be cons idere d, and ( i i ) t h e d a t a s h o u l d be arranged i n a way g o i n g beyond a mere enumeration o f r e a c t i o n s . The p r e s e n t work on t h e c a t a l y t i c c o n v e r s i o n o f oxygenates on z e o l i t e s and c l a y s w i l l , as a r e s u l t , n o t be e x h a u s t i v e b u t o n l y deal w i t h a few s e l e c t e d t o p i c s , on w hich a c c o r d i n g t o t h e a u t h o r s ’ o p i n i o n a c o n s i s t e n t s e t o f d a t a a r e a v a i l a b l e o r which i n t r o d u c e new concepts. ALKYLATION OF PHENOLIC COMPOUNDS P henolic compounds such as c r e s o l s [23-251, [26]
and
alkyl
aryl
ethers
[27-301
find
l o n g c h a i n a l k y l phenols
increasing
industrial
use as
i n t e r m e d i a t e s . Product p u r i t y i s a p r e r e q u i s i t e f o r t h e i r use. F or t h e moment o n l y o - c r e s o l and 2 , 6 - x y l e n o l can be made i n a r a t h e r s e l e c t i v e way by d i r e c t
164
a l k y l a t i o n f r o m phenol u s i n g e i t h e r a b a s i c [31] o r an alumina c a t a l y s t [26!. However, t h e s e l e c t i v e c a t a l y t i c s y n t h e s i s o f p - and m-cresol remains a c h a l l e n g e g i v e n t h e expensive d i s t i l l a t i o n and c r y s t a l l i s a t i o n o p e r a t i o n s needed f o r t h e i r s e p a r a t i o n . As t h e a l k y l a t i o n o f t o l u e n e w i t h methanol o v e r a c i d 10-MR z e o l i t e s i s very s u i t a b l e t o o b t a i n para-enriched a l k y l a t e ,
t h e m e t h y l a t i o n o f phenol
and/or a n i s o l e o v e r t h e same z e o l i t e s i s an obvious way t o achieve enhanced yields
of
para
alkylates.
Most
alkylation
reactions
reported
in
the
l i t e r a t u r e a r e c a r r i e d o u t w i t h phenol i n t h e vapour phase a t atmospheric pr e s s ure and a t t e m p e r a t u r e s between 413
and 673 K u s i n g p e n t a s i l
and
f a u j a s i t e - t y p e z e o l i t e s as c a t a l y s t s . An o verview o f work i n t h i s area i s g i v e n i n Table 1. A l k y l a t i o n r e a c t i o n s o f p h e n o l i c compounds a r e more complex th an t hos e o f t h e c o r r e s p o n d i n g a r o m a t i c hydrocarbons, as n o t o n l y carbon ( C ) b u t a l s o oxygen (0-) a l k y l a t i o n i s p o s s i b l e . It i s , t h e r e f o r e , e s s e n t i a l t o
have a good comprehension o f t h e r e a c t i o n n et work i n t h e a l k y l a t i o n o f phenol and t h e i n f l u e n c e o f t h e z e o l i t e s t r u c t u r e and a c i d i t y on i t . Table 1 D i f f e r e n t a l k y l a t i n g agents used w i t h phenol on z e o l i t e s i n t h e vapour and i q u i d pha: reaction medi um vapour phase
a l k y l a t i n g agent
z e o l i t e type
reference
methanol
X, Y, Mordenite, TSVM, ZSM-5, ZSM-11, X, Y, ZSM-5 X, Y, ZSM-5 X, Y, ZSM-5, ZSM-11 X, Y, Tseokar-2 unspecified
27-29, 32-38, 40-42, 45, 47, 51, 53-54 23, 56 23, 38, 62-64 43-44 57-61 38 30
X, Y, M o r d e nit e, ZSM-5 Y X x, Y, L
39, 46, 55 67 65 66
h i g h e r a1 cohol s o le fins anisole a1 k y l benzenes a1 k y l c h l o r i d e d i a1 k y l c a r b o n a t e
1i q u i d phase
methanol h i g h e r a1 cohol s methyl i o d i d e benzylchl o r i d e
x,
y
1. M e c h a n i s t i c aspects For t h e a l k y l a t i o n o f phenol
Venuto
et
substrate,
al. which
[38,64] is
reported opposite
or benzene w i t h ethene on z e o l i t e REX, a to
decreased their
reactivity
respective
of
the
f ormer
nucleophilicities.
C o m p e t i t i v e a d s o r p t i o n measurements showed an i n c r e a s e d a d s o r p t i o n o f phenol
o f a Rideal type mechanism, c omp l i c a t e d by a c o m p e t i t i v e a d s o r p t i o n o f phenol f o r t h e a l k y l a t i n g agent, was proposed [68,69]. Indeed, an a r o m a t i c compound adsorbed compared t o
benzene
and
ethene
[68].
The
operation
165 on a Brensted s i t e w i l l be p o s i t i v e l y charged and p o o r l y s u s c e p t i b l e t o an e l e c t r o p h i l i c attack.
Such an e l e c t r o p h i l i c aromat ic s u b s t i t u t i o n r e a c t i o n
can be viewed as t h e r e a c t i o n o f an adsorbed e t h y l c a t i o n formed f rom ethene w i t h a f r e e aro m a t i c :
F i g . 1. Mechanism o f t h e a l k y l a t i o n o f phenol w i t h ethene on z e o l i t e s . Recently,
i t was e s t a b l i s h e d [51,70]
t h a t phenol i s more a c t i v e t han
benzene on Y and p e n t a s i l z e o l i t e s w i t h methanol as a l k y l a t i n g agent. seems, t h e r e f o r e , t h a t more p o l a r f a v o u r a b l y w i t h phenol f o r a d s o r p t i o n .
loo
a1 k y l a t i n g
fragments
compete
It
more
I
80 c.
8
Y
E
.-
0
E S 3 A
60
r tc
E m 6 40
s 20
0 0.5
1.1
1
2
s
2.1
5.1
4
MethanollPhcnol (mollmol)
F i g . 2. reac i o n mol-' i n ZSM-5 a t The
Phenol c o n v e r s i o n versus t h e methanol/phenol m o l a r r a t i o i n t h e o f phenol w i t h methanol: (A), on H-K-Y a t 523 K w i t h W/F0=324 kg s t h e vapour phase and a f t e r 60 min ( d e r i v e d f rom [ 47] ) and (6) on H643 K i n t h e l i q u i d phase and a f t e r 70 min ( d e r i v e d f rom [ 46] ). competition
between
substrate
and
a1 k y l a t i n g agent
is
further
evidenced i n F i g . 2. Indeed, u s i n g an i n c r e a s i n g molar excess o f methanol t h e c o nv ers io n in c re a s e s on b o t h p e n t a s i l and Y z e o l i t e s . The e x i s t e n c e o f t h e s t r o n g a d s o r p t i o n o f phenol on p e n t a s i l z e o l i t e s can a l s o be d e r i v e d from T a ble 2. On s i l i c a l i t e ,
phenol and t h e c r e s o l isomers adsorb homogeneously
thro ughout t h e c r y s t a l s as e v e r y p o r e i n t e r s e c t i o n i s occupied by appro x ima t e ly one molecule. On t h e c o n t r a r y , on h i g h - a l u m i n a H-ZSM-5 o n l y a min or amount o f phenol i s adsorbed. with
the
surface
hydroxyl
groups,
Presumably, phenol and
consequently,
interacts strongly relatively
small
q u a n t i t i e s o f phenol a r e a b l e t o b l o c k t h e pores and p r e v e n t complete f i l l i n g o f the intersections.
166
Ta ble 2
Is on p e n t a s i l z e o l i t e s a . adsorbent
adsorbate
H-ZSM-5( 19) s i licali t e s i 1 i c a l it e s i licali t e s i licali t e
phenol phenol p-cresol m-cresol o-cresol
adsorbat e p e r pore i n t e r s e c t i o n 0.15 0.72 0.91 1.03 0.84
2. C a t a l y t i c s t a b i l i t y o f z e o l i t e s SO
-bp
40
IAl
E
0
:
I-
> E
s
20
0
0
20
10
SO
40
Time an dream (h)
F i g . 3. I s o m e r i s a t i o n o f o - c r e s o l on H-ZSM-5 a t 653 K and 6 MPa a g a i n s t t i m e on stream w i t h ( a ) hyflrogen and ( b ) n i t r o g e n as c a r r i e r gas. W/Fo values a r e 339 and 113 kg s m o l - i n p a r t (A) and (B), r e s p e c t i v e l y ( d e r i v e d f rom [so]). The i s o m e r i s a t i o n r e a c t i o n s o f c r e s o l s and t h e a l k y l a t i o n r e a c t i o n s o f p h e n o l i c compounds always s u f f e r f r o m d e a c t i v a t i o n by coke d e p o s i t i o n on t h e zeolite
catalysts,
but
air
regeneration
easily
restores
their
initial
c a t a l y t i c a c t i v i t y [28,40-41,44-45,52-53,561. Some a u t h o r s [27,39-41,47-491 use an i n e r t d i l u e n t f o r t h e feed, w h i l e o t h e r s [28,43-45,50-521 take hydrogen f o r t h i s purpose as i t was found t h a t t h e decay o f t h e c a t a l y s t a c t i v i t y i s c o n s i d e r a b l y s l o w e r i n t h e l a t t e r case [45,50]. This i s i l l u s t r a t e d i n F i g . 3 f o r t h e i s o m e r i s a t i o n o f o - c r e s o l . Such a f a v o u r a b l e e f f e c t o f hydrogen has been r e p o r t e d by Barthomeuf e t a l .
[72-731 i n t h e
i s o o c t a n e c r a c k i n g on Y z e o l i t e s and e x p l a i n e d l a t e r by t h e presence o f i m p u r i t y Fe2+ w i t h h y d r o g e n - a c t i v a t i n g p r o p e r t i e s 1761. I n t h e c o n v e r s i o n o f a c e t y l e n e ov er H-ZSM-5, hydrogen i s a l s o n o t an i n e r t d i l u e n t and can r e a c t
167
w i t h v i n y l c a t i o n s upon f o r m a t i o n o f ethene t h u s a v o i d i n g p o l y m e r i s a t i o n o f a c e t y l e n e and c o k i n g o f t h e c a t a l y s t [74]. Minachev e t a l . [75] observed a f a v o u r a b l e e f f e c t o f carbon d i o x i d e on t h e c onv ers io n o f a n i s o l e on z e o l i t e s . alkaline
earth
than
for
alkali
The e f f e c t i s more pronounced f o r
ion-exchanged
faujasites.
A
similar
enhancement o f t h e i s o p r o p a n o l d e h y d r a t i o n a c t i v i t y was r e p o r t e d and a t t r i b u t e d t o t h e g e n e r a t i o n o f new B r a n s t e d a c i d s i t e s and s u r f a c e carbonate
[761. I n t h e a l k y l a t i o n o f phenol w i t h methanol on z e o l i t e s , Marzewski e t a l .
[34] proved t h a t a t a g i v e n c o n v e r s i o n t h e s e l e c t i v i t y remained unchanged i r r e s p e c t i v e o f whether t h i s c o n v e r s i o n was reached by changing c o n t a c t t i m e or
during
a
decay
experiment.
Consequently,
selectivity
changes
on
a
d e a c t i v a t i n g c a t a l y s t w i t h i n c e r t a i n conversion l i m i t s w i l l allow d e t e r m i n a t i o n o f t h e r e a c t i o n network. T h i s i s an a p o s t e r i o r i v e r i f i c a t i o n o f a procedure used i n o t h e r work [51]. 3. Oxvqen versus carbon a l k v l a t i o n i n t h e Dhenol a l k v l a t i o n w i t h methanol
Typical methanol
p r i m a r y p r o d u c t s o b t a i n e d i n t h e a l k y l a t i o n o f phenol w i t h
are anisole,
0-
and p - c r e s o l .
F o r t h e f o l l o w i n g reasons i t i s
e v i d e n t t h a t a n i s o l e i s formed by 0- and t h e c r e s o l s by C - a l k y l a t i o n :
*
A t l o w c o nv er s i o n s t h e y a r e t h e o n l y p r o d u c t s o b t a i n e d [34,46,51].
*
A t medium c o n v e r s i o n s x y l e n o l s , m e t h y l a n i s o l e s and m-cresol appear [34,51].
As expected f o r an e l e c t r o p h i l i c a r o m a t i c s u b s t i t u t i o n , m-cresol p r i m a r y pro duc t . There alkylation
is
a
quite
requires
general
stronger
belief
acid
sites
among d i f f e r e n t t han
t hose
authors
responsible
i s not a that
C-
for
0-
a1 k y l a t i o n [27,51,67]:
*
I n a s e r i e s o f H-Na-Y z e o l i t e s w i t h v a r y i n g sodium c o n t e n t ,
y i e l d decreases g r a d u a l l y w i t h i n c r e a s i n g p r o t o n c o n t e n t ;
the anisole
a t t h e same t i m e
t h e number o f weak a c i d s i t e s determined b y n - b u t y l a m i n e t i t r a t i o n decreases i n a p a r a l l e l way [47].
*
On a H-Na-ZSM-5 z e o l i t e t h e a n i s o l e s e l e c t i v i t y i s h i g h e r t h a n on sodium
f r e e H-ZSM-5 [46].
*
I n p a r a l l e l t o t h e decreased e l e c t r o n e g a t i v i t y and consequent ly decreased average a c i d s t r e n g t h o f H-K-Y compared t o H-Na-Y, t h e f o r m e r z e o l i t e i s more s e l e c t i v e f o r a n i s o l e f o r m a t i o n [47].
*
As
i n the zeolite literature
i t i s g e n e r a l l y accepted t h a t
pentasil
z e o l i t e s have s t r o n g e r a c i d s i t e s t h a n f a u j a s i t e s , t h e l a t t e r z e o l i t e s should have h i g h e r s e l e c t i v i t y f o r a n i s o l e . There i s ample experiment al evidence f o r t h i s [28,39-42,46,51,53-55].
168
T h i s dependence o f C - a l k y l a t i o n s e l e c t i v i t y upon Brransted a c i d s t r e n g t h has a l s o been observed f o r metal phosphate c a t a l y s t s , as a c o r r e l a t i o n between r i n g a l k y l a t i o n s e l e c t i v i t y and e l e c t r o n e g a t i v i t y o f t h e metal i o n has been e s t a b l i s h e d [77]. I n t h i s r e s p e c t Marczewski e t a l . [ 34] r e p o r t c o n f l i c t i n g evidence, s i n c e i n a s e r i e s o f u l t r a s t a b l e Y z e o l i t e s w i t h de c reas ing alumina c o n t e n t t h e a n i s o l e s e l e c t i v i t y does n o t decrease. The a u t h o r s assume t h a t d e a l u m i n a t i o n decreases t h e b a s i c i t y o f t h e framework and t h a t a d s o r p t i o n o f phenol electrophil i c substitution.
on b a s i c s i t e s o f i n c r e a s i n g s t r e n g t h f avours
As w i l l be e x p l a i n e d l a t e r , t h e l o w a n i s o l e s e l e c t i v i t y observed on ZSM5 [28,40,51] compared t o more open z e o l i t e s [51] i s due t o t h e f a c t t h a t a n i s o l e i s c a t a l y t i c a l l y l e s s s t a b l e i n 10-MR t h a n i n l a r g e p o r e z e o l i t e s [51].
H igh a n i s o l e s e l e c t i v i t i e s on p e n t a s i l z e o l i t e s [27,34,45-461
can be
a t t r i b u t e d t o t h e presence o f e x t r a - f r a m e w o rk aluminium [ 27] o r impregnated phosphor o x i d e s [34,53] and are, t h e r e f o r e , r e l a t e d t o p r o d u c t s e l e c t i v i t y which would f a v o u r t h e d e s o r p t i o n o f l i n e a r molecules [ 34] . 2.50
1
J
100
1.50
60
1.00
40
0.50
¶O
0.00
shape
0
4 7 3 498 623 648 673 6B8 6 2 5 648 6 7 s
Temperature (K)
F i g . 4. Conversion and a n i s o l e / c r e s o l - r a t i o versus r e a c t i o n t emperat ure i t h e r e a c t i o n o f phenol w i t h methanol (A) on H-K-Y w i t h W/Fo= 324 k g s moli n t h e vapour phase and a f t e r 60 min ( d e r i v e d f rom [ 47] ) and (B) on H-ZSM-5 i n t h e l i q u i d phase, w i t h methanol/phenol m o l a r r a t i o o f 2 and a f t e r 70 min ( d e r i v e d f rom [ 46 ] ) .
P
The
following
evidence
clearly
indicates
that
part
of
the
cresol
f r a c t i o n must be o f a secondary n a t u r e as t h e a n i s o l e / c r e s o l r a t i o increases when:
*
t h e r e a c t i o n temperature i s decreased [27-28,35,40-41,46-47,671( t h i s i s t r u e f o r ZSM-5 as w e l l as f o r Y z e o l i t e s ( F i g . 4 ) ) ;
*
t h e c o n t a c t t i m e decreases [28,35,47]
as shown i n F i g . 5;
169
WlFO (kg e mol-1)
WlFO (kg 8 mOl-1)
F i g . 5. Conversion and a n i s o l e / c r e s o l - r a t i o versus c o n t a c t t i m e (W/Fo) i n t h e r e a c t i o n o f phenol w i t h methanol i n t h e vapour phase (A) on H-K-Y a t 523 K and a f t e r 60 min ( d e r i v e d f r o m [ 4 7 ] ) , (B) on H-TSVM a t 623 K ( d e r i v e d from [351) *
*
t i m e on stream i n c r e a s e s and t h u s t h e c o n v e r s i o n decreases f rom h i g h t o
v e r y l o w values [32,45-47,511. The i n f l u e n c e o f an i n c r e a s i n g methanol/phenol r a t i o on t h e p r o d u c t s e l e c t i v i t y and more p a r t i c u l a r l y on t h e a n i s o l e / c r e s o l r a t i o i s shown i n F i g . 6. For Y z e o l i t e s t h i s r a t i o decreases [47],
[28,46,55].
inc re as es
I n the
b u t f o r ZSM-5 z e o l i t e s i t
former case t h i s can be e x p l a i n e d b y t h e
c ons ec ut iv e f o r m a t i o n a t i n c r e a s i n g c o n v e r sions o f c r e s o l s f rom a n i s o l e and x y l e n o l s f rom c r e s o l s . As a r e s u l t o f t h e a l r e a d y mentioned p r o d u c t shape s e l e c t i v i t y i n ZSM-5,
i n c r e a s i n g amounts o f a n i s o l e a r e expected t o desorb
when an excess o f methanol i s used. 1.00
L
100
0.80
- 80
0.60
- 10
0.40
-
40
0.20
-
20
-0
0.00
0.s
1
1.6
2
2.6
5
5.1
4
Yethanol/Phenol (mol/mol)
Fig . 6. Phenol c o n v e r s i o n and a n i s o l e / c r e s o l r a t i o versus t h e methanol/phenol mo lar r a t i o i n t h e r e a c i o n o f phenol w i t h methanol: (A), on H-K-Y a t 523 K w i t h W/Fo=324 kg s mol-' i n t h e vapour phase and a f t e r 60 min ( d e r i v e d from [47]) and (B ) on H-ZSM-5 a t 643 K i n t h e l i q u i d phase and a f t e r 70 min ( d e r i v e d f rom [46]).
170
Thus c r e s o l s a r e n o t o n l y p r i m a r y b u t a l s o secondary product s, whereas a n i s o l e i s o n l y o f primary nature.
T h i s s t r o n g l y suggests t h a t t h e r e i s
f o r m a t i o n o f c r e s o l s f r o m a n i s o l e . I n o r d e r t o c l a r i f y t h e r e l a t i o n between a n i s o l e and c r e s o l s
i n the
r e a c t i o n network,
i s necessary t o have i n f o r m a t i o n about t h e isomer d i s t r i b u t i o n o f t h e c r e s o l s and t h e r e a c t i v i t y it
o f a n i s o l e as s u b s t r a t e . 4. C re s ol isomer s e l e c t i v i t v i n t h e a l k v l a t i o n o f Dhenol w i t h methanol T a ble 3 Comparison o f t h e d i s t r i b u t i o n o f c r e s o l isomers f r o m phenol a l k y l a t i o n w i t h !than01 Catalyst
H-Y H-Y H-USY H-La-Y H-K-Y H-Na-Y P-ZSM-5 P-ZSM-5 H-ZSM-5 H-ZSM-5 H-TsVM Pd - H -TsVM
Ref.
45 34 51 45 47 47 34 53 46 51 35 15
Temp.
Conversion
C r e sol selectivity
ortho
meta
para
(KI
(%I
(%I
(%I
(%I
(%I
60.0 58.6 38.9 52.0 33.0 33.3 65.0 60.9 68.6 65.3 68.9 62.4
22.0 0.0 1.3 8.9 6.0 9.1 0.0 7.0 0.0
18.0 41.4 59.8 39.1 61 .O 57.6 35.0 32.1 31.4 34.1 27.7 34.3
523 473 473 523 523 523 413 723 573 413 573 573
46.9 6.0 20.2 49.7 32.0 66.0 6.0 26.6 13.5 10.2 11.5 16.7
64.2 29.0 63.9 46.3 57.5 70.2 10 .o 43.2 31.3 73.3 30.3 66.5
0.0
3.4 3.3
As t h e h y d r o x y l group o f phenol i s o / p - d i r e c t i n g , m-cresol, alt hough be ing t h e most thermodynamically s t a b l e isomer, can o n l y be formed v i a a secondary 1 , 2 - a l k y l s h i f t . Ta b l e 3 shows t h a t a t l o w and medium conversions t h e m-c re s ol s e l e c t i v i t y i s v e r y l o w indeed. I t f u r t h e r shows t h a t t h e r e i s always a h i g h s e l e c t i v i t y f o r o - c r e s o l , w h ich i n most r e p o r t s i s h i g h e r on p e n t a s i l z e o l i t e s t h a n on z e o l i t e Y. T h i s b e h a v i o u r was unexpected because i n t h e a l k y l a t i o n o f t o l u e n e w i t h methanol l o w o/p r a t i o s i n t h e x y l e n e f r a c t i o n a r e i n v a r i a b l y o b t a i n e d on p e n t a s i l z e o l i t e s [78]. Alt hough t h e d i f f e r e n c e i n k i n e t i c diame t e r between t h e 0 - and p - i s o m e r s i s s m a l l e r f o r c r e s o l s t han f o r xyle nes , t h i s does n o t i m p l y t h e absence o f s h a p e - s e l e c t i v e f o r m a t i o n o f p c r e s o l . As shown i n F i g . 7 f o r t h e i s o m e r i s a t i o n o f m - c r e s o l , z e o l i t e H-ZSM-5 e x h i b i t s a s h a p e - s e l e c t i v e e f f e c t because t h e f o r m a t i o n o f p - c r e s o l i s fa v oure d o v er t h e o-isomer, d e s p i t e t h e f a c t t h a t t hermodynamically t h e pisomer i s about 5 t i m e s l e s s s t a b l e t h a n t h e o-isomer. Moreover, i n t h e i s o m e r i s a t i o n of o - c r e s o l t o m- and p - c r e s o l [49-501 o v e r H-ZSM-5 a d e f i n i t e shape s e l e c t i v e f o r m a t i o n of p - c r e s o l i s e ncount ered ( F i g . 8). The enhanced
171
0 - c r e s o l s e l e c t i v i t y i n t h e a l k y l a t i o n of phenol w i t h methanol must, t h e r e f o r e , be o f o t h e r t h a n j u s t a s h a p e - s e l e c t i v e o r i g i n . On amorphous c a t a l y s t s [24-25,43-44,79-811 u s u a l l y no shape s e l e c t i v i t y i s p r e s e n t and oc r e s o l s e l e c t i v i t i e s as h i g h as 100% can be obt ained. loo[
.
J
60
40
30
60
40
20
-
10
0
0
10
20
SO
40
Time on stream (h)
F i g . 7. I s o m e r i s a t i o n of m - c r e s o l and t h e p / ( o t p ) - c r e s o l r a t i o a g a i n s t t i m e on s ream on H-ZSM-5 a t 653 K and 6 MPa. W/Fo values a r e 339 and 113 kg s mol-' i n p a r t (A) and (B), r e s p e c t i v e l y ( d e r i v e d f rom [SO]). The s o l i d l i n e r e p r e s e n t s t h e thermodynamic e q u i l i b r i u m . 100
i6 0
80
60
40
20
0
10
20
SO
0 40
Time on stream (h)
F i g . 8. I s o m e r i s a t i o n o f o - c r e s o l and t h e p / ( m t p ) - c r e s o l r a t i o a g a i n s t t ime on s ream on H-ZSM-5 a t 653 K and 6 MPa. W/Fo v a l u e s a r e 339 and 113 kg s mol-' i n p a r t (A) and ( B ) , r e s p e c t i v e l y ( d e r i v e d f rom [ 5 0 ] ) . The s o l i d l i n e r e p r e s e n t s t h e thermodynamic e q u i l i b r i u m . I n 1i t e r a t u r e t h e r e appear t h r e e hypotheses which can e x p l a i n t h i s h i g h o r t h o s e l e c t i v i t y , a l t h o u g h o n l y one o f them g i v e s an e x p l a n a t i o n f o r t h e h i g h e r p ara s e l e c t i v i t y on z e o l i t e Y compared t o p e n t a s i l z e o l i t e s .
172
The first hypothesis was advanced by Tanabe et al. [79-811 based on a comparison o f the I.R. spectra o f adsorbed phenolic compounds on MgO and silica-alumina with the catalytic results o f the alkylation of phenol with methanol. On a basic surface such as MgO, phenol is adsorbed perpendicularly, its ortho-position being close t o the surface and alkylation occurring predominantly in this position. On acid catalysts, the B r ~ n s t e d sites will interact with the aromatic nucleus of the adsorbed phenol thus bringing it closer t o the surface and permitting alkylation in the para position, in this way explaining the decreased o-cresol selectivity on silica-alumina. In a second hypothesis advanced by Beltrame et al. [43-441 for acid catalysts, strongly adsorbed anisole, eventually formed in the phenol alkylation, reacts with phenol from the gas phase by an Eley-Rideal mechanism, enabling the phenol OH group t o interact with the activated '0 center of adsorbed anisole and bringing the ortho position o f phenol closer and more accessible t o an attack o f the methyl group than the para position. Such transition state is schematically represented in Fig. 9.
Fig. 9. Possible transition state o f the alkylation o f phenol by anisole. The basic arguments for this hypothesis are the zero and first order in anisole and phenol, respectively, observed in the reaction of anisole with phenol over Y zeolites and gamma-alumina [43-441. Neither hypothesis, however, can predict the lower o-cresol selectivity obtained with Y compared t o pentasil zeolites. Jacobs et al. [51] derived that in the alkylation of phenol with methanol, anisole is an intermediate for the formation of a major part o f the o-cresol, because: * in the anisole conversion, o-cresol is a primary product as it is formed with very high selectivity in the cresol fraction at low conversions [52]; * in the reaction o f anisole with methanol also high o-cresol selectivity is observed [51] ; * the initial selectivity for o-cresol on pentasil and USY zeolites is substantially higher in the alkylation o f anisole with methanol than of phenol [51]; Given ( i ) the direct formation o f o-cresol from anisole, ( i i ) the higher selectivity for o-cresol in pentasils compared t o faujasites, and ( i i i ) the suppression o f the rate of all bimolecular reactions in the reaction network
173 s t a r t i n g f rom phenol and methanol i n p e n t a s i l s [51],
i t i s l o g i c a l t o assume
t h a t t h e t r a n s f o r m a t i o n o f a n i s o l e i n t o o - c r e s o l occurs i n t r a m o l e c u l a r l y [51] and
is
f a v our e d
Mechanistically
in
the
constrained
this
can
be
viewed
environment as
a
of
pentasil
transformation
of
zeolites. an
oxygen
p r o t o n a t e d a n i s o l e t o an oxygen p r o t o n a t e d o - c r e s o l ( f i g . 10).
f i g . 10. I n t r a m o l e c u l a r rearrangement o f a n i s o l e t o o - c r e s o l . A l s o i n homogeneous medium t h e e l e c t r o p h i l i c s u b s t i t u t i o n a t t h e o r t h o p o s i t i o n s o f phenols " i s r a r e l y found t o be a normal r e a c t i o n " [ 82] and o rt h o/ p ara r a t i o s much h i g h e r t h a n t h e s t a t i s t i c a l l y 2/1 a r e found. 5. Z e o l i t e s t r u c t u r e and s e l e c t i v i t y i n t h e a l k y l a t i o n of phenol and a n i s o l e w i t h methanol 5.1. S u m r e s s i o n i n D e n t a s i l z e o l i t e s o f b i m o l e c u l a r r e a c t i o n s A ma jor i n f l u e n c e o f t h e z e o l i t e s t r u c t u r e on t h e p r o d u c t s e l e c t i v i t y
f rom t h e r e a c t i o n o f methanol w i t h a n i s o l e and/or phenol i s t h e suppression o f t h e b i m o l e c u l a r r e a c t i o n s i n p e n t a s i l compared t o Y z e o l i t e s [51].
This
can be i l l u s t r a t e d as f o l l o w s : Z e o l i t e Y i s more a c t i v e t h a n ZSM-5 i n t h e r e a c t i o n o f phenol w i t h methanol
*
because a l l
primary reactions are o f bimolecular nature,
whereas i n t h e
a l k y l a t i o n o f a n i s o l e w i t h methanol t h e o p p o s i t e i s t r u e because some o f t h e p r i m a r y r e a c t i o n s a r e monomolecular ( F i g . 11). 80
60
40
20
0 0
5
10
15
20
25
Time (h)
F ig. 11. Conversion versus t i m e on stream o f (a) a n i s l e and (b) phenol i n t h e a l k y l a t i o n w i t h methanol w i t h W/F = 1736 kg s mol-' and 278 kg s mol- , r e s p e c t i v e l y , on u l t r a s t a b l e Y and ti-Z!M-5 a t 473 K ( d e r i v e d f rom 1511).
174
*
The phenol selectivity in the methylation o f anisole is substantially higher on pentasil than on Y zeolites, whereas for the methylanisole and xylenol selectivity the opposite is observed (Fig. 12). T h e dealkylation o f anisole is o f monomolecular nature, while the alkylation o f both anisole and cresols to methylanisoles and xylenols, respectively, is bimolecular. H-USY
H-ZSY-5
Fig. 12. Pie-chart representation of the product selectivities in the a1 kyl ation of anisole wi h methanol on ultrastable Y and H-ZSM-5 at 473 K with W/Fo= 1736 kg s mol-' (derived from [51]).
* The o-cresol selectivity in the methylation o f phenol and anisole is highest on pentasil zeolites [51]. This has been explained in a previous sect ion. In contrast to what has been claimed recently [48], for all these reasons it is, therefore, not unexpected t o obtain high o-cresol selectivities from anisole on pentasil-type zeolites and at intermediate conversions. 5.2. Para selectivities in the Droduct fractions An obvious way t o enhance the p-cresol selectivity would be the use of 10-MR zeolites with smaller pore dimensions than those of pentasil zeolites. However, it was reported that on H-Ferrierite and H-ZSM-22 the anisole conversion proceeds predominantly on the outer surface of the crystals, thus forming high amounts of o-cresol [52]. Anisole itself undergoes selfalkylation with higher para selectivity on ZSM-5 [52-531 than on faujasitetype zeol i tes [75]. In Table 4 product selectivities for the alkylation o f phenol with methanol are compared over various zeolitic and amorphous catalysts. For all zeolites the para selectivity in the methylanisole and cresol fractions as well as the selectivities of (2,4 t 2,5)-xylenols in the xylenol fraction are substant i a1 ly higher than the corresponding selectivities obtained on amorphous catalysts. Moreover, when the data on ZSM-5 and Y zeolites are
175
compared, for obvious sterical reasons a further enhancement of the selectivity for p-methylanisole and (2,4 t 2,5)-xylenol is observed for ZSM-5. Methylanisoles are formed by C-alkylation of anisole [34] or 0-alkylation of cresols [27] or both together [43-45,511. Table 4 Comparison of the distribution of cresol, methylanisole and xylenol isomers
-~ Catalysts Reference Temoerature(K1 Conversion (%l
ani sol e methylanisoles cresol s xylenol s
MgO AlP04 Si1.- H-ZSM-5 H-USY P-ZSM-! alum.^^39 81 24 55 51 51 53 533 771 523 523 473 473 723 65.9 11.2 19.9 30.0 44.0 56.1 26.6
A1203
65.8 2.5 22.9 8.9
80.7
0.0
9.3 5.5
92.0 8.0
19.3
86.0 14.0
82.3 2.9
98.1 0.9 1.0
95.1
70.5
65.3
para 4.9 29.5 Selectivity in the methylanisole fraction (%1
34.7
ortho meta
100.0
ortho 100.0 meta para Selectivity in the xylenol fraction (% 226 2,4t2,5
53.5a 3.sa 43. Oa 100.0
I 1
22.8 22.7 40.7 13.8
44.0 3.0 41.4
38.9 1.3 59.8
60.9 7.0 32.1
28.3
12.5
71.7
87.5
100.0
selectivities obtained from reaction of anisole with methanol [53] 6. Overall reaction network The overall reaction network for the alkylation o f phenol given in Fig. 13 is valid for all catalysts, including zeolites as well as amorphous solids. Primary products are formed and partially transformed into secondary ones via either C- or 0-alkylation. On less acidic catalysts 0-alkylation dominates, but possibly protonated anisole rearranges on the catalyst surface to o-cresol. This, together with o/p-directing C-alkylation, will result in ortho selectivities higher than the statistical 2/1 o/p-ratios. In zeolites, C-alkylation will be more para-selective when the reaction occurs in a more constrained environment. Parallel to this more o-cresol is formed via protonated anisole and consequently p-cresol selectivity will never be very
176
high. The formation of the more bulky secondary products is strongly influenced by the sterical constraints of the environment and therefore, gives rise to high para-selectivities in zeolites. On amorphous catalysts the ortho-isomers are formed predominantly.
j
3Hc75 T
Fig. 13. Overall reaction network for the alkylation of phenol with methanol. Dashed arrows represent direct o-alkylation. Dominant products on amorphous and zeolite catalysts are surrounded by solid or crenate frames,respectively. There are two major differences between pentasil and faujasite zeolites caused by the more constrained environment o f the former zeolites: Bimolecular reactions as well as diffusion of anisole are suppressed in
177
p e n t a s i l z e o l i t e s , r e s u l t i n g i n a p r e f e r r e d intramolecular rearrangement o f ani sol e t o o-cresol
*
.
For obvious s t e r i c a l reasons, t h e methylation o f the primary products w i l l
be more
para
selective
on
pentasil
zeolites.
This
results
in
higher
s e l e c t i v i t i e s f o r p-methylanisole and 2,4-xylenol. 7. Reaction o f Dhenolic comDounds w i t h bulky a l k v l a t i n s aqents I n the a l k y l a t i o n o f phenolic compounds w i t h b u l k y a l k y l a t i n g agents i n v a r i a b l y a bulky t r a n s i t i o n s t a t e i s required. Thus w i t h z e o l i t e s i t w i l l be p o s s i b l e t o o b t a i n high para s e l e c t i v i t i e s i n t h i s r e a c t i o n . As shown i n Table 5 f o r ZSM-5 the o / p - r a t i o o f i-propylphenol i s more than 20 times lower than f o r cresols. I n t h e same way t h e para s e l e c t i v i t y on Y z e o l i t e s increases i n the f o l l o w i n g order: t-butylphenol > i-propylphenol > cresol
.
The formation o f the more bulky dialkylphenols i s suppressed and o n l y the l e s s bulky isomer i s obtained, which i n the case o f d i - t - b u t y l p h e n o l i s the 2,4-isomer
[67]. For very bulky side chains i t i s even p o s s i b l e t o o b t a i n a
para-enriched a l k y l a t e i n the i n t e r l a m e l l a r space o f a c t i v a t e d clays. Table 5 o/p-Ratio
i n the alkylphenol
f r a c t i o n obtained w i t h d i f f e r e n t a l k y l a t i n g on zeol it e s .
z e o l i t e type
H-Y H-USY H-Na-Y P-ZSM-5 H - ZSM- 5 Tseokar-2 H- ZSM - 5 H - ZSM- 5 H-Y act. clay
a l k y l a t i n g agent methanol methanol methanol methanol methanol cumene i-propano1 propyl ene t-butanol C - 5 alkenes
a1 k v l Dhenol
o/p - r at io
reference
3.33
45 51 47 34 51 57
cresol cresol cresol cresol cresol i-propylphenol i-propyl phenol i-propyl phenol t - b u t y l phenol t-amylphenol
0.65 0.58 1.86 1.88 0.4 0.08 0.08 0.2 0.2
23 23 67 83
When phenol r e a c t s w i t h 1-octanol on 12-MR z e o l i t e s , o n l y p- and oa l k y l a t i o n occurs [84]. As even a t high conversions m-isomers are completely absent, deviations from thermodynamic e q u i l i b r i u m i n t h e d i s t r i b u t i o n o f the products w i l l r e f l e c t t r a n s i t i o n s t a t e shape-selective e f f e c t s . For energetic reasons t h e primary carbenium i o n formed from 1-octanol w i l l isomerise t o t h e more s t a b l e 2-,
3-
and 4 - o c t y l
cations.
c o n s t r a i n t s i t i s n o t expected t h a t
I n the
absence o f geometric
l e s s 4-octylphenol
will
be formed
compared t o the o t h e r isomers. As 4 - o c t y l i s a more b u l k y s u b s t i t u e n t on phenol than 3 - o c t y l , which i n t u r n i s more bulky than 2 - o c t y l , two kinds of
178 shape-selective e f f e c t s can be expected w i t h t h i s a l k y l a t i n g agent:
regio
s e l e c t i v i t y along t h e hydrocarbon chain and p o s i t i o n a l s e l e c t i v i t y on t h e aromatic nucleus.
Fig. 14. S e l e c t i v i t y and p/(pto) r a t i o f o r t h e d i f f e r e n t octylphenols obtained i n t h e a l k y l a t i o n o f phenol w i t h 1-octanol on 12-MR z e o l i t e s (derived from [84]). As shown i n Fig.
14 p r e f e r e n t i a l
para s u b s t i t u t i o n occurs f o r each
octylphenol and a l l 12-MR z e o l i t e s used. For the l e s s b u l k y 2-octylphenol t h e preference f o r t h e para isomer i s about equal f o r H-RE-Y, ZSM-12. For t h e more bulky 3- and 4-octylphenol
H-Mordenite and H-
t h e p/(pto)
r a t i o i s about
0.6 on Y z e o l i t e , whereas 100% para s e l e c t i v i t y i n t h e 3-octylphenols and no 4-octylphenol
i s observed i n H-ZSM-12 and H-Mordenite.
amounts o f 2-,
3- and 4-octylphenols
I n z e o l i t e Y equal
are formed and t h e r e f o r e no r e g i o
s e l e c t i v i t y i s observed, whereas on H-ZSM-12 and H-Mordenite no 4-octylphenol and two
times
more
2-octylphenol
than
3-octylphenol
is
detected.
The
s e l e c t i v i t y d i f f e r e n c e s between H-Mordenite and H-ZSM-12 which apparently are present should be considered w i t h c a u t i o n as no i n d i c a t i o n o f t h e coke content o f t h e two samples i s given. I n any case, i t i s i l l u s t r a t e d t h a t a f t e r proper choice o f a l k y l a t i n g agent and z e o l i t e s t r u c t u r e , r e g i o as w e l l as p o s i t i o n a l s e l e c t i v i t y can be influenced by d i f f e r e n c e s i n environmental c o n s t r a i n t s , zeol it e s . ACYLATION OF AROMATICS The F r i e d e l - C r a f t s
even among 12-MR
a c y l a t i o n o f aromatic hydrocarbons by carboxyl i c
acids o f various chain l e n g t h has been studied by Geneste e t a l . Y-zeolites,
[85-881 on
M o n t m o r i l l o n i t e and AlC13. A summary o f t h e p e r t i n e n t r e s u l t s f o r
t h e a c y l a t i o n o f toluene i s shown i n Fig. 15. On molecular sieves, on clays as w e l l .as w i t h homogeneous c a t a l y s t s , para-acylation i s favoured. The data on t h e p a r a - s e l e c t i v i t y do n o t a l l o w one t o conclude t h a t s i g n i f i c a n t d i f f e r e n c e s e x i s t between AlC13 and A13+Montmorillonite. However, a much stronger preference f o r p - a l k y l a r y l ketones
179
on Ce-Y i s observed. As expected f r o m a l k y l a t i o n r e a c t i o n s on z e o l i t e s t h e s e l e c t i v i t y o f t h e p a r a isomer i n c r e a s e s w i t h t h e c h a i n l e n g t h t o v e r y h i g h v a lues , even t o 100% f o r behenic a c i d (n=20).
n F i g . 15. P a r a - s e l e c t i v i t y and y i e l d o f t h e a c y l a t e d p r o d u c t s i n t h e r e a c t i o n o f c a r b o x y l i c a c i d s (CH3-(CH2),-COOH) w i t h t o l u e n e ( d e r i v e d f rom [85-871).
F i g . 15 shows a second s t r i k i n g d i f f e r e n c e between z e o l i t e s , c l a y s and homogeneous c a t a l y s t s .
I n homogeneous a c y l a t i o n t h e y i e l d i s o n l y s l i g h t l y
dependent on t h e c h a i n l e n g t h o f t h e c a r b o x y l i c a c i d [85-671.
On t h e o t h e r
hand, on c l a y s as w e l l as on Y z e o l i t e s a s t r o n g dependence on c h a i n l e n g t h is
observed.
With
acetic
acid
no
acylation
occurs
[85-871.
On A13+-
M o n t m o r i l l o n i t e t h e y i e l d i n c r e a s e s w i t h c h a i n l e n g t h , whereas on CeY t h e r e i s a maximum y i e l d f o r dodecanoic a c i d . The decrease o f t h e y i e l d f o r l o n g e r c h a i n s can be due t o d i f f u s i o n a l l i m i t a t i o n s on z e o l i t e Y, a l t h o u g h according t o Geneste e t a l . [85] l a r g e c a r b o x y l i c a c i d s would n o t be a b l e t o p e n e t r a t e t h e c a v i t i e s o f z e o l i t e Y . It i s n o t c l e a r why t h e y i e l d i n c r e a s e s w i t h c h a i n l e n g t h because such phenomenon i s n o t encountered i n homogeneous F r i e d e l Crafts catalysis.
SHAPE SELECTIVITY I N THE FORMATION OF ACETOPHENONES 1. F r i e s rearranqement o f Dhenvl a c e t a t e Table 6 ries rearr catalyst
Conversion
(molX) H -Y A1203-F H - ZSM- 5
4.5
6.6 4.2
S e l e c t i v i t y of t h e F r i e s rearrangement ( X ) 75 58 20
o:p-ratio o f hydroxyacetophenones
12.6: 1.O 13.6: 1.8 1.8: 1.5
180
Perot et al. [89] studied the transformation o f phenyl acetate into hydroxyacetophenones. The inter- and intramolecular Fries rearrangement is shown in Fig. 16.
*\ ,c=o
+
Chc=, cf;3
___)
-7
Fig. 16. Inter- (A) and intramolecular ( 6 ) Fries rearrangement o f phenyl acetate. As shown in Table 6 and for obvious steric reasons the selectivity for the Fries rearrangement is higher on Y than on ZSM-5 zeolite. The increase o f the o:p-ratio o f the hydroxyacetophenones in Y compared to alumina can be explained by a suppression o f the bimolecular reactions in the zeolite. However, on H-ZSM-5 the rate o f the bimolecular Fries reaction remains low and the strongly decreased o:p-ratio is exclusively the result of the low rate o f formation of the o-isomer. On the contrary, in the rearrangement o f anisole where a methyl group is involved which is less bulky than an acetyl substituent, the bimolecular reactions are suppressed in pentasil zeolites. 2. Acetoalkoxvlation of phenol. Ortho- (I) and para-methoxyacetophenones (11) in a ratio of 1 t o 10 are formed as acetoal koxylation products of phenol with 1,l'-dimethoxyethane ( 1 1 1 ) on H-ZSM-5 [ g o ] . This ratio is considerably lower than that obtained for the hydroxyacetophenones on the same zeolite (Table 6) and must be the result o f geometric constraints of the intracrystall ine pore volume.
I
I11
181
ZEOLITES AS ENZYME M I M I C S Two d i f f e r e n t approaches a r e r e p o r t e d i n t h e l i t e r a t u r e on t h e use o f z e o l i t e s as mimics f o r enzymes i n o x y g e n a tion r e a c t i o n s . The p r e p a r a t i o n and c a t a l y t i c use o f m e t a l l o p h t a l l o c y a n i n e (MPc) complexes encaged i n f a u j a s i t e supercages has been reviewed [91-961. Fenton's r e a c t a n t was het erogenised i n z e o l i t e s A, X and Y i n an a t t e m p t t o mimic cytochrome P450 [97-981. 1. Encaqed D h t a l l o c v a n i n e s Among o t h e r s [94,95],
an i m p o r t a n t advantage t h a t z e o l i t e s o f f e r over
amorphous s upp o r t s and homogeneous c a t a l y s t s i s shape s e l e c t i v i t y . Indeed, w i t h FePc imm o b i l i s e d i n z e o l i t e Y a pronounced r e a c t a n t shape s e l e c t i v e e f f e c t was r e p o r t e d i n a c o m p e t i t i v e o x i d a t i o n experiment w i t h cyclododecane and cyclohexane as s u b s t r a t e [97,98]. With homogeneous FePc as c a t a l y s t b o t h r e a c t a n t s a r e o x i d i s e d e q u a l l y w e l l . However, w i t h sodium exchanged X o r Y z e o l i t e c o n t a i n i n g encaged FePc,
the r a t e o f oxidation o f the less bulky
cyclohexane i s i n c r e a s e d by a f a c t o r o f about 2 [97]. T h i s shape s e l e c t i v e e f f e c t can be f u r t h e r i n c r e a s e d by exchange o f c a t i o n s w i t h proper dimensions. As shown i n F i g . 17 when t h e s i z e o f t h e c a t i o n increases from
Lit
to
Rb'
the
ratio
cyclohexane decreases
of
the
rates
of
f r o m 1.4 t o 0.14.
oxidition
of
cyclododecane
to
Thus t h e s i z e o f t h e supercage
c a t i o n s i s a v e r y s e n s i t i v e paramater t o t a i l o r s u b s t r a t e s e l e c t i v i t y .
u I a X 0
c
0
*
1.00
\ 0
0
c (D
0
0 '0
0.50
0
-ur
P 0
A
0
0.05
0.10
ionic radius
0.15
0.20
(nm)
F i g . 17. Dependence o f s u b s t r a t e s e l e c t i v i t y i n t h e simult aneous o x i d a t i o n of cyclododecane and cyclohexane on t h e s i z e o f a l k a l i c a t i o n s exchanged i n z e o l i t e X lo ade d w i t h FePc ( d e r i v e d from [ 9 8 ] ) . I n t h e o x i d a t i o n o f n-octane,
r e g i o s e l e c t i v i t y i s e x e r t e d by FePc
immobilis ed i n f a u j a s i t e z e o l i t e s . I n F i g . 17 i t i s shown t h a t w i t h encaged FePc t h e o x i d a t i o n a t p o s i t i o n 2 i s p r e f e r r e d over t h a t a t p o s i t i o n 4. T h i s
182 i s e x p l a i n e d b y t h e e x i s t e n c e o f an o r i e n t i n g i n f l u e n c e o f t h e z e o l i t e framework on t h e s u b s t r a t e , so t h a t t h e more t e r m i n a l carbon atoms a r e more a c c e s s i b l e for t h e a c t i v e i r o n - o x o a c t i v e c e n t e r [97].
F i g . 18. R a t i o o f o c t a n e o x i d a t i o n i n p o s i t i o n 2 t o p o s i t i o n 4 w i t h homogeneous FePc and FePc encaged i n z e o l i t e s X and Y, c o n t a i n i n g 1 and 2 Fe atoms p e r supercage, r e s p e c t i v e l y ( d e r i v e d f rom [ 9 7 ] ) . S t e r e o s e l e c t i v e o x i d a t i o n w i t h FePc i n z e o l i t e Y has been r e p o r t e d by Herron e t a l . [97,98]
and i s i l l u s t r a t e d i n Table 7 . T h i s i s anot her example
o f s t e r e o s e l e c t i v i t y imposed by t h e z e o l i t e o r o f an o r i e n t i n g i n f l u e n c e o f t h e z e o l i t e framework on t h e s u b s t r a t e r e s u l t i n g i n a d i f f e r e n t p r e f e r e n c e f o r o x i d a t i o n a t one o f t h e two d i a s t e r e o t o p i c hydrogen atoms. Table 7 'om [ 97] ) substrate
product r a t i o trans:cis ( I ) -alcohol exo ( I I I ) :endo (11) norborneol
methyl c y c l ohexane norbornane I
FePc-Y
FePc
2 6
9
1.1
183
2 . Completely i n o r q a n i c mimic o f enzymes A c omp let e l y i n o r g a n i c mimic o f cytochrome P450 has been developed by
H erron and Tolman [99-lOO]. The mimic p r o v i d e s remarkable r e g i o s e l e c t i v i t y in
the
partial
oxidation
of
octane
and
substrate
selectivity
i n the
c o m p e t i t i v e o x i d a t i o n o f octane and cyclohexane i n Fe-exchanged z e o l i t e 5A under m i l d c o n d i t i o n s ( Fi g . 19). However, t h e o x i d a t i o n p r o d u c t s a r e s t r o n g l y adsorbed
in
the
intracrystalline
volume
of
the
zeolite.
With
several
treat ment s , t he s e a u t h o r s a r e a b l e t o desorb p a r t l y o r even c o m p l e t e l y t h e o x i d i s e d pro duc t s : w a t e r a d d i t i o n , a c i d d i s s o l u t i o n o f t h e z e o l i t e , p o i s o n i n g w i t h 2,2’-bipyridine
o f t h e acid z e o l i t i c residues a f t e r a c i d d i s s o l u t i o n
( F i g . 19). The r a t i o o f t h e o x i d a t i o n p r o d u c t s f rom oct ane and cyclohexane in c reas es f rom about 1 on s i l i c a - a l u m i n a t o 190 on z e o l i t e 5A.
On Fe-A
z e o l i t e a t e n - f o l d i n c r e a s e o f t h e o x i d a t i o n o f t h e p r i m a r y compared t o t h e secondary carbon atoms o f o c t a n e i s observed w i t h r e s p e c t t o F e - s i l i c a alumina ( F i g . 19). 1.00 C
0
)J
Addition of
I I
/
X 0
10
4: meld + blpyrldlnr
C (A
X 0
c
1
0 V 9. V
%
Q C
a
C
V 0
0.1
catalysts
F i g . 19. S e l e c t i v i t i e s i n t h e o x i d a t i o n a t p r i m a r y t o secondary carbon o f oc t a ne as w e l l as i n t h e c o m p e t i t i v e o x i d a t i o n of oct ane w i t h cyclohexane on (1) F e - s i l i c a - a l u m i n a , (2) Fe-5A a f t e r w a t e r a d d i t i o n , (3) Fe-5A a f t e r a c i d d i s s o l u t i o n and (4) Fe-5A a f t e r a c i d d i s s o l u t i o n and a d d i t i o n o f 2 , 2 ’ b i p y r i d i n e as p o i s o n ( d e r i v e d f r o m [99,100]). Furthermore, i t i s c l e a r f r o m F i g . 20 t h a t a l l r a t i o s o f 1- t o x - o c t a n o l (- o n) a r e about 0.04 f o r t h e amorphous c a t a l y s t , whereas on z e o l i t e A t h i s r a t i o inc re as es s t r o n g l y f o r l a r g e r v a l u e s o f x, when t h e o x i d i s e d carbon i s l o c a t e d more towards t h e c e n t e r on t h e hydrocarbon c h a i n . The same remains t r u e i r r e s p e c t i v e o f t h e c h a i n l e n g t h o f t h e alkane s u b s t r a t e ( F i g . 20). Moreover,
the
relative
reactivity
at
the
terminal
C-atoms
increases
s i g n i f i c a n t l y w i t h t h e c h a i n l e n g t h o f t h e n-alkane. T h i s again i s an example
184
o f a s p e c i f i c zeolite-induced r e g i o - s e l e c t i v e o x i d a t i o n .
I n a competitive
o x i d a t i o n experiment o f 2-methylpentane and n-pentane on z e o l i t e 5A, latter
substrate
is
oxidised
200
times
faster,
indicating
that
the also
s e l e c t i v i t y among alkane substrates e x i s t s . 1.20
I
J
1.20
1.00 -
-..
0.80 -
0.80
-
0.60
0.60
9
I0.40
0.40
~
0.20 -
0.20
0.00 -
0.00
4
2
5
position of oxidissd C atom
Fig. 20. Regio-selective o x i d a t i o n o f alkanes on Fe-A z e o l i t e : T : I . , r a t i o o f terminal t o i n t e r n a l C atoms oxidised; N.R., normalised r a t i o o f terminal t o i n t e r n a l C atoms ( = carbon number/2 - 1) (derived from [99,100]).
SELECTIVE OXIDATION OF HYDROXYARENES D i r e c t o x i d a t i o n o f aromatic compounds f o r cresol s,
hydroquinone
importance.
and
resorcinol
is
of
t h e synthesis o f phenol, considerable
industrial
The i n t r o d u c t i o n o f shape s e l e c t i v e z e o l i t e c a t a l y s t s i n t h i s
area r e c e n t l y proved t o be very successful. s i l i c a l i t e , denoted as TS-1, o t h e r organic substrates
Indeed, a non-acidic t i t a n i u m -
catalyses the H202 o x i d a t i o n o f aromatics and
[101-1101.
TS-1 has already found an i n d u s t r i a l
a p p l i c a t i o n i n t h e production o f hydroquinone and catechol from phenol [102]. 1-b
H202
/
+
Fe2+
0-H*
Fenton's
+
+H+
H-O-9-H H M ,, or MOOH
reagent
peroxonium i o n transition metal peroxo complexes
M
F i g . 21. Schematic representation o f t h e mechanisms f o r h y d r o x y l a t i o n o f arenes w i t h H202. Hydroxylation w i t h H202 can occur v i a t h r e e d i f f e r e n t mechanisms which are shown i n Fig. 21. Surface t i t a n i u m peroxo compounds are proposed t o be
185
t h e actual
oxidants
i n TS-1 c a t a l y s e d
oxidations
[102].
It i s f u r t h e r
s pec ulat e d t h a t t h e t i t a n i u m atoms have a reduced r a t e f o r H202 decomposition due t o t h e i r p e r f e c t i s o l a t i o n from each o t h e r . A Fenton's reagent made o f Fe2+ and Co2+ i s used i n t h e B r i c h i m a process f o r t h e h y d r o x y l a t i o n o f phenol [112]. The RhBne-Poulenc process u s i n g s t r o n g m i n e r a l a c i d s such as HC104 and proceed v i a t h e f o r m a t i o n H3P04 [113], and t h e h y d r o x y l a t i o n on HZSM-5 [lll], o f a peroxonium i o n . 1.00 -
rn
0
0
E
0 N
0.80
Fe2+. C 0 2 + [1121
-
Fa-Y
E
* x
RE-Y
0.60 -
e
*
[991
H3C104, H3P04 11 131
0
P
18-1 (1021
[I101
H-ZSM-6
11111
U
c U
0.40 -
A
+n 0
Y
0.20
-
\
n 0.00 1
2
5
4
5
6
catalysts
F i g . 22. p - S e l e c t i v i t y i n t h e h y d r o x y l a t i o n o f phenol w i t h H202. The p - s e l e c t i v i t i e s o f processes u s i n g H202 f o r t h e h y d r o x y l a t i o n o f phenol a r e compared i n F i g . 22. I t i s r e a d i l y seen t h a t p - s e l e c t i v i t i e s o f 50% and h i g h e r can be o b t a i n e d u s i n g TS-1 z e o l i t e s . side reactions,
Furthermore,
unwanted
such as t h e f o r m a t i o n o f p o l y n u c l e a r aromat ic compounds, are
l i m i t e d . The h y d r o x y l a t i o n o f phenol w i t h H202 was a l s o performed on a c i d z e o l i t e c a t a l y s t s . I n t h e presence o f a f a u j a s i t e , phenol i s c o n v e r t e d t o a m i x t u r e o f hydroxybenzenes and t a r s [110], w i t h a p - s e l e c t i v i t y comparable t o t h a t o b t a i n e d on TS-1 ( Fi g . 22). When t h e r e a c t i o n i s c a r r i e d o u t on Fe-Y, a heterogeneous analog o f Fenton's reagent, a l s o good s e l e c t i v i t y i s obt ained [98].
On H-ZSM-5 almost 100% p - s e l e c t i v i t y
i n t h e dihydroxybenzenes was
r e p o r t e d w i t h H202 [lll]. No t a r f o r m a t i o n was mentioned. Thus i t seems t h a t i n t h e h y d r o x y l a t i o n o f phenol, t h e r e a c t i o n mechanism as w e l l as t h e p o r e a r c h i t e c t u r e a r e s e l e c t i v i t y - d e t e r m i n i n g parameters.
Therefore,
the
reaction
seems
to
be
anot her
example
of
t r a n s i t i o n - s t a t e shape s e l e c t i v i t y . O x i d a t i o n o f 2,6-dimethylphenol
by t - b u t y l h y d r o p e r o x i d e v i a a r a d i c a l
o x i d a t i o n - r e d u c t i o n mechanism occurs i n a s e l e c t i v e way on COX z e o l i t e , i t s poly me ri s a t i o n r e a c t i o n
i n t o p o l y (2,6-dimethy1-1,4-phenylene
ether)
( I 11)
186
being suppressed and its dimerisation into 3,3’,5,5’-tetramethy1-4,4’diphenoquinone (11) being reduced compared t o amorphous supports [114]. The resultant selectivity for 2,6-dimethyl -p-benzoquinone (I) is high but dependent on the partial pressure of the peroxide.
n
ZEOLITES AS CHIRAL HYDROGENATION CATALYSTS Heterogeneous hydrogenation catalysts that produce optically active products contain always an active center next t o an optically active ligand molecule [115]. Dessau reported on the arrangement o f both components in the pores o f ZSM-5 [116]. First a Pt-loaded H-ZSM-5 was prepared and subsequently the chiral template was fixed in the zeolite pores by adsorption of optically active S-( -)-alpha-methylbenzylamine and anchored as the corresponding alkylammonium ion in the pores after its protonation via surface O H groups. Acetophenone can be hydrogenated into optically active 1-phenylethanol . The tailored active site and the hydrogenation reaction are represented in Fig. 23.
El
.......................
H4!-CH3 +NH3
-
//////Al///////////////
Fig. 23. Schematic representation o f the active entities in a ZSM-5 chiral catalyst (A) and hydrogenation o f acetophenone (B) [derived from [116]). SELECTIVE DIELS-ALDER CYCLOADDITIONS Diels-Alder addition of 0-containing double bond compounds (dienophiles) t o 1,4 conjugated dienes has received considerable attention on clays and occasionally also on zeolites [117-1191. In general, with both types o f catalysts cycloaddition rates are very much enhanced with dienophiles that
187 o t h e r w i s e r e a c t s l u g g i s h l y i n homogeneous phase and r e q u i r e e x t r e m e l y h i g h r e a c t i o n pre s s u r e s (up t o 1,500 MPa). Enhanced r e g i o s e l e c t i v i t i e s a r e r e p o r t e d as w e l l . A t y p i c a l example i s t h e a d d i t i o n o f m e t h y l v i n y l k e t o n e t o c y c lopent a diene [ 118,1191 :
EX0
END0 Re ac t io n
selectivities
obtained
in
homogeneous
phase
[120],
with
Fe(II1)- M o n t m o r i l l o n i t e and Cu(1)-Y z e o l i t e a r e compared i n F i g . 24. I n homogeneous
phase
ENDO-addition
is
f a voured
with
cyclic
dienes
for
stereochemical reasons [121]. I n t h e i n t e r l a m e l l a r space o f t h e c l a y and much more i n t h e supercage o f z e o l i t e Y, t h e r e l a t i v e r a t e o f ENDO-addition i s further
enhanced.
Mechanistically,
a
one-electron
transfer
d i e n o p h i l e t o t h e t r a n s i t i o n metal seems t o be i n v o l v e d , reaction
f rom
the
f o l l o w e d by i t s
w i t h t h e d i e n e and r e t u r n o f t h e e l e c t r o n t o t h e r a d i c a l .
The
B r ~ n s t e ds i t e s p r e s e n t on t h e c a t a l y s t s seem t o p r o t o n a t e t h e more p o l a r C=O i n t h e d i e n o p h i l e r a t h e r t h a n t h e d o u b l e bond o f t h e diene, t h u s keeping t h e d i e n o p h i l e more s t r o n g l y i n t h e c o o r d i n a t i o n sphere o f t h e t r a n s i t i o n metal [118], and hence a v o i d i n g i t s secondary i n t e r m o l e c u l a r r e a c t i o n s .
Fa-Yontmorillonite
1
2
catalysts F ig. 2 4 . R a t i o o f t h e END0:EXO c y c l o a d d i t i o n o f m e t h y l v i n y l k e t o n e t o c y c lo pent a diene i n homogeneous medium [120], F e - M o n t m o r i l l o n i t e c l a y [118] and Cu(1)-Y z e o l i t e [117].
188 MISCELLANEOUS SHAPE-SELECTIVE EFFECTS O t h er i n d u s t r i a l l y r e l e v a n t s h a p e - s e l e c t i v e
e f f e c t s o f z e o l i t e s have
been r e p o r t e d as w e l l :
*
The s h a p e - s e l e c t i v e a c i d - c a t a l y s e d PRINS r e a c t i o n between formaldehyde and
is o but e ne,
f o r s t e r i c a l reasons,
drastically restricts
the formation o f
4,4’-dimethyl-1,3-dioxane ( I ) i n f a v o u r o f m e t h y l b u t e n o l s (11), and m e t h y l b u t a n e d i o l s (111) a r e condensed i n p e n t a s i l compared t o Y z e o l i t e s and m i n e r a l a c i d s [122,123].
*
On z e o l i t e s c o n t a i n i n g t h e E r i o n i t e cage, a CAGE EFFECT has been r e p o r t e d
i n t h e a c i d - c a t a l y s e d c o n d e n s a t i o n o f b u t y r i c a c i d i n t o 4-heptanone Only
zeolites
with
this
structural
element
limit
the
[124].
reaction
to
dimerisation,
p r o v i d e d t h e p r o d u c t formed c o n t a i n s 7 o r 8 C atoms. I n a l l o t h e r cases a c i d - c a t a l y s e d a l d o l c o n d e n s a t ion o f t h e ketones o b t a i n e d occurs in
an
uncontrolled
way,
resulting
in
rapid
and
complete
catalyst
deactivation. * S h ape-s e lec t ive base c a t a l y s i s was r e p o r t e d f o r t h e a l d o l condensat ion o f aceton c a t a l y s e d by sodium metal
clusters
encaged i n t h e supercages o f
z e o l i t e Y and t h e p o r e s o f z e o l i t e L [125]. On t h e l a t t e r z e o l i t e l e s s o f t h e b u l k i e r c y c l i c t r i c o n d e n s a t e ( i s o p h o r o n e ) i s formed, r e s u l t i n g i n an enhanced s e l e c t i v i t y f o r t h e d i -condensate ( m e s i t y l o x i d e ) . CONCLUSIONS From t h e s h a p e - s e l e c t i v e e f f e c t s mentioned i n t h e l i t e r a t u r e t o occur m a i n l y w i t h medium-pore z e o l i t e s [124] which were d e r i v e d f o r a c i d - c a t a l y s e d hydrocarbon t r a n s f o r m a t i o n s , r e s t r i c t e d t r a n s i t i o n - s t a t e s e l e c t i v i t y seems t o be t h e dominant e f f e c t when t h e c o n v e r s i o n o f o x y g e n - c o n t a i n i n g s u b s t r a t e s i s concerned. With b u l k y s u b s t r a t e molecules t h e e f f e c t s e x i s t w i t h l a r g e - p o r e z e o l i t e s as w e l l ;
and t h e y a r e a l s o n o t l i m i t e d t o a c i d - c a t a l y s e d r e a c t i o n s
b u t o c c u r w i t h redox and b a s i c c a t a l y s i s as w e l l . From t h e p o i n t o f view o f o r g a n i c c hemis t r y , t h e z e o l i t e , i n F r i e d e l - C r a f t s a1 k y l a t i o n s and a c y l a t i o n s o f aro ma t ic s ,
i s a b l e t o change t h e normal o r t h o : p a r a
enhance t h e y i e l d o f meta-products
s e l e c t i v i t y b u t can
o n l y by secondary a c i d - c a t a l y s i s .
New
s h a p e - s e l e c t i v e e f f e c t s such as r e g i o and even s t e r e o s e l e c t i v i t y e x i s t , which d o n o t oc c u r w i t h t h e usual hydrocarbon s u b s t r a t e s .
189 I t i s f u r t h e r shown t h a t i n t h e case o f phenol m e t h y l a t i o n s , t h e r e a c t i o n network i s e n t i r e l y determined b y t h e a v a i l a b l e i n t r a c r y s t a l l i n e
space.
Z e o l i t e s seem t o be a b l e t o
s e rve as mimics f o r enzymes,
when
m e t a l l o p h t a l l o c y a n i n e s a r e encaged i n t h e cages o f f a u j a s i t e , and even a ze ol i t e - b a s e d c o m p l e t e l y i n o r g a n i c mimic o f cytochrome P540 e x i s t s . ACKNOWLEDGMENTS The aut h ors acknowledge t h e B e l g i a n Government f o r support o f t h i s work i n t h e frame o f a Concerted A c t i o n on C a t a l y s i s . One o f us, PAJ, acknowledges t h e B e l g i a n N a t i o n a l Fund o f S c i e n t i f i c Research (NFSR) f o r a p o s i t i o n as Research D i r e c t o r . DRH i s g r a t e f u l t o NFSR f o r a g r a n t as Research A s s i s t a n t . 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. 28. 29. 30.
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H.G. Karge, J. Weitkamp (Editors), Zeolites (IS Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE USE OF ZEOLITE CATALYSTS FOR THE SYNTHESIS OF NITROGEN-CONTAINING ORGANIC INTERMEDIATES HOELDERICH BASF Aktiengesel lschaft, Amnoniaklaboratorium, D-6700 Ludwigshafen, FRG
W.
F.
ABSTRACT Nitrogen-containing intermediates and fine chemicals play an important role in the chemical industry, e. g. in the synthesis of active substances and the production of dyestuffs as well as solvents. Thus, attempts are continually being made to synthesize new nitrogen-containing compounds and to devise improved routes to known products. In this connection zeolites with their numerous adjustable catalytic properties can be an useful instrument. This article describes in particular the preparation of cyclic and heteroaromatic compounds as well as reactions (e. g. alkylation, isomerization and rearrangement) of these compounds in the presence of zeolite catalysts. INTRODUCTION New and wide-ranging possibilities for using zeolites in catalytic processes are being discovered all the time. Zeolitic catalysts find a broad spectrum o f applications, including acid, non-acid and base-catalysed organic reactions leading to intermediates (ref. 1 - 10). The reasons for the rapid expansion of this field include the easily reproducible production of well defined zeolite surfaces, the considerable advances made in the synthesis o f new zeolitic and nonzeolitic molecular sieves and of isomorphously substituted materials, the high activity, the transition-state and product-diffusion shape selectivity of zeolites, the possibilities of varying and regulating the properties of zeolitic catalysts - including bi- and polyfunctional catalysts - , and last but not least, the better understanding of the nature of the active sites o f such materials. Of all the numerous types of zeolite-catalysed reactions, the topic which has been chosen for this presentation is the synthesis o f nitrogen-containing organic compounds, in particular cyclic and heteroaromatic intermediates. On the basis o f examples taken from reviews of academic studies and from the patent 1 iterature an attempt wi 1 1 be made to examine to what extent zeol it ic catalysts are superior to conventional catalysts, and to which extent the acidity and structural features of such catalysts determine the reaction path. Furthermore, attention will also be directed to the question as t o whether the principles derived for reactions of hydrocarbons on zeolites can also be applied to the reactions of N-containing compounds.
194
NUCLEOPHILIC SUBSTITUTION OF ALCOHOLS WITH AMMONIA Methylamines, which are of considerable technical importance, can be produced from methanol and ammonia by means of nucleophilic substitution. The aim is in most cases to achieve a reaction product having a composition differing from the equilibrium distribution and containing as high a proportion as possible of mono- or dimethylamine (MMA or DMA). Different zeolites have been used to meet this challenge (e. g. ref. 4). The elimination of me3N is related to the pore size of the zeolite. R. D. Shannon and coworkers recently discovered that the small-pore zeolites HRHO, HZK-5 and chabazite show higher DMA selectivities than other zeolites (refs. 1 1 , 12). At 325 "C and WHSV = 2 hr-' up to 63 % selectivity of DMA at 90 % methanol conversion can be obtained. In comparison, the thermodynamic equilibrium predicts only 21 % DMA. H-mordenite apparently has a pore size that allows the formation and movement of the mono- and disubstituted amines, but not the triamine (refs. 13, 20). The Y-zeolite has pores large enough to allow the formation of me3N. Nevertheless, it is not yet possible to control the reaction to give only one of the methylamines, but the composition of the product mixture can be controlled better than in the classical reaction on A1203 (e. g. refs. 14, 15). The production of 2 - 8 C containing amines by vapour-phase catalytic amination of alcohols or ethers occurs only at low conversion (e. g. ref. 16). EtNH2 is formed at 550 "C and 18 bar with a selectivity of 85 % at 6 % conversion. At higher conversion, i. e. at higher temperature, the formation of ethene and other hydrocarbons reduces the selectivity for the amine. The amination of alcohols on zeolite catalysts is not restricted to simple molecules. It has been shown, for instance, that nucleophilic substitution of the OH-group in ethanolamine with NH3 yields ethylenediamine (ref. 17). The reaction involves the utilization o f a dealuminated (HC1-treated Si:Al = 7.2) rare earth or hydrogen ion-exchanged mordenite as the catalyst. With ethanolamine and ammonia in a molar ratio of 1 : 4 as feed and at 300 "C and 4860 psig ethylenediamine is obtained with 84 % selectivity at 15 % conversion. As by-product aminoethylethanolamine was formed exclusively with 16 % selectivity. This behavior contrasts with that of many conventional hydrogenation catalysts such as Co, Ni, Cu/Cr, P t on supports where selectivity to ethylenediamine is sacrificed in favor of conversion. Analogous to cyclic ethers, cyclic or bicyclic amines can be prepared from corresponding educts by elimination of H20 or NH3. Thus, 1,4 diazabicyclo,l2,2,2 1 octane (DABCD) has been prepared, e. g. from N-hydroxy- or N-aminoethylpiperazine, by intramolecular nucleophilic substitution. The industrially used catalysts for that reaction are silica, alumina or tungsten catalysts. Because of the severe reaction conditions (T 3 400 " C ) , the said catalysts promote cracking and condensation reactions of the reactants and the desired
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DABCO, resulting in small yields. Because of the wide variety of undesired products like piperazines, N-alkylpiperazines and pyrazine derivatives, expensive purification of DABCO via distillation or fractional crystallization is required. Therefore it is desirable to find a more selective catalyst. It has been reported (ref. 18) that high-silica zeolites having preferably silica-to-alumina ratios in the range of 35 to 55, such as ZSM-5, ZSM-11, ZSM-12 or ultrastabilized Y-zeolite, provide increased selectivities when the reaction is carried out between approx. 250 "C and 550 OC and at pressure between 0.5 to 2 atm in the vapor phase and contact times between ca. 0.2 and 3 sec. At 400 O C 10 % conversion and 87 % selectivity are achieved using HZSM-5. Under the same reaction conditions using a conventional alumina catalyst only 47 % selectivity but a much higher conversion rate of about 96 % are obtained. The only advantage of the silica-rich zeolite is the reduction in the amount of difficultly separable by-products, because in this case, the only by-product is piperazine. Increasing the conversion rate from 10 to 21 % leads to the formation of not only piperazine (10 % ) but also other by-products ( 3 %). The selectivity for DABCO still remains at 87 %. It is also possible to use ethanolamine as starting material for the preparation of DABCO (ref. 19). At 400 OC and SV = 10 hr-', 64 % yield of DABCO is attained using ZSM-5. This method provides an alternative to the aforementioned route. The synthesis of anilines is of industrial interest. The preparation by reaction of alicycl ic alcohols like cyclohexanol, or ketones like cyclohexanone with ammonia is carried out in the presence of a crystalline silicate catalyst having the structure of ZSM-5 which contains a metal promotor having dehydrogenation activity (ref. 20). Using NiHZSM-5 at 480 OC and 200 psig only 16.2 % aniline selectivity is observed. Higher heterocyclic compounds such as diphenylamine and carbazole are preferentially formed. Since amination and simultaneous dehydrogenation proceed with only moderate yield, it is preferable to choose phenol as starting material for the synthesis of aniline. In the presence of HZSM-5 in an autoclave at 510 OC and 28 atm a phenol conversion o f 92 % and an aniline selectivity of 99 % (ref. 21) are achieved. The reaction is also catalysed by Y-, X- and mordenite zeolites, but these catalysts display a markedly shorter lifetime than HZSM-5, and in the case of X - and Y-zeolites the conversion is lower (refs. 21 - 23). By-products such as diphenylamine and carbazole, which are obtained in the presence of nonzeolitic alumina-si1 ica according to the Halcon-Scientific Design process are suppressed or eliminated by virtue of the shape selectivity of the ZSM-5 catalyst. Amonolysis of anisole on zeolites of the faujasite type yields aniline. The
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conversion is approx. twice as high (60 % at ca. 400 "C) as in the case of phenol. The lifetime of these catalysts is also short (refs. 22, 23). Surprisingly, however, the ammonolysis of anisole in the presence of HZSM-5 at 400 "C leads exclusively t o phenol. The conversion is 95 % (ref. 22). Although the authors point to the markedly higher conversion of anisole (95 % ) compared with phenol (35 % ) , they do not comment on the unexpected formation of phenol instead of aniline. The reaction of chlorobenzene and ammonia to aniline in the presence of zeolite catalysts leads to low conversion rates and short service time because of the formation and deposition of nonvolatile aniline hydrochloride on the catalyst (ref. 22). On the basis of the foregoing results the only suitable route for the synthesis of aniline is the nucleophilic substitution of phenol with ammonia in the presence of HZSM-5. Substitution of conventional heterogeneous catalysts such as non-zeolitic alumina-silica, mixtures of manganese and boron oxides and alumina-titania by shape-selective ZSM-5 catalysts yields advantages with regard to aniline selectivity. In comparison to the homogeneous, Lewis-acid-catalysed Halcon Process (ref. 241, the use of pentasil zeolites opens up an environmentally and energetically more favourable route such as is the case in the Mobil Badger Process (refs. 25, 26). Condensation reactions, in particular of the intramolecular type, are known (refs. 27 - 30) for the preparation of nitriles by means of zeolite catalysts. They can be prepared from amides or from formaldehyde/alcohols in the presence of amines (refs. 27, 28). The preparation of aliphatic dinitriles, such as adipodinitrile, from dicarboxylic acids and ammonia is carried out in the gas phase, for example, on a boron zeolite doped with Na and P (ref. 29). As shown by comparison with a catalyst based on Si02, the content of cyclic by-products is reduced, resulting in an increased selectivity for adipodinitrile - from 83 % to 94 %. The transition-state shape-selectivity of the zeolite, which does not permit the cyclization reaction, is evidently responsible for this. Even HCN as an inorganic compound i s manufactured by reaction of CO with NH3 in the presence of HZSM-5 as catalyst, whereby the water-gas-shift reaction can be avoided (ref. 30). This example also demonstrates how useful zeolite catalysts can be in the synthesis of nitriles.
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REARRANGEMENT REACT1ONS Beckmann rearrangement The most important industrial example of the Beckmann rearrangement is the reaction of cyclohexanone oxime to E-caprolactam. This is a valuable starting material for plastics, in particular for synthetic fibres like Nylon. The classical synthetic route involves the oximation of cyclohexanone with hydroxylamine-sulphate and the subsequent rearrangement of the oxime in concentrated sulfuric acid. Approx. 4 - 5 t (NH4)2S04 per t caprolactam are inevitably obtained as co-product (ref. 31 ). Further problems encountered include hand1 ing a large amount of fuming sulfuric acid and corrosion of the apparatus caused by that acid. In order to eliminate these problems and the formation of the co-product, which can only be taken up by the fertilizer industry to a limited extent, attempts have been made for many years to switch from a homogeneous to a heterogeneous catalytic process. There are a number o f references concerning the use of heterogeneous catalysts like alumina, heteropoly acids, boronphosphate, phosphoric acid or Lewis acids on inert carriers, silica-alumina and boric-acid on alumina for the rearrangement of a variety of ketoximes to amides (refs. 31, 32). As long ago as the 1960s P. S. Landis and P. B. Venuto attempted to use zeolites for this purpose (refs. 1, 26). X - and Y-zeolites as well as mordenite in the H-form or doped with rare-earth or transition metals are employed; e. g. cyclohexanone oxime (30 wt% dissolved in benzene) is converted at 380 "C and WHSV = 1.2 hr-' to E-caprolactam with 76 % selectivity and 85 % conversion during the first two hours. The principal by-product is 5-cyanopent-1-ene together with traces of cyclohexanone and cyclohexanol. As the reaction is continued, the overall conversion decreases to about 30 % after 20 hrs with a drop to 50 % selectivity for caprolactam. According to the authors' findings the optimum reaction temperature for high caprolactam selectivity is between 250 " C and 380 "C with the selectivity increasing with increasing temperature. Above 400 "C caprolactam decomposition is observed. Atmospheric pressure is favored, since at elevated pressure 5-cyano-pent-1-ene is formed as principal product. Non polar solvents such as cyclohexane, benzene or toluene are much more favourabie than the more polar solvents. Nitrogen and particularly carbondioxide as nonbasic carrier gas are very useful for the Beckmann rearrangement. As far as the mechanistic considerations are concerned, a protonic catalysed reaction is assumed, whereby probably an initial adsorption of the ketoxime at a catalyst acid site is involved. Investigations using HNaY-zeolite catalysts with different levels of exchange of Na' show (refs. 33, 34) that the Beckmann rearrangement of cyclohexanone oxime is catalysed by strong Bronsted acid sites of pKa 4 1,5 and that, in
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competition to this reaction, the selectivity-reducing by-product 5-cyano-pent1-ene is formed on acid sites as well as on Na' ions. Furthermore, the catalyst deactivation is assumed to be caused by poisoning of the acid sites by the basic products such as aniline and methylpyridine rather than by coke formation on the catalyst surface, because after reaction the color of the catalyst is still almost white. Generally the greatest limitations of the rearrangement of the ketoxime using Y- and X-zeolites as well as mordenite were the rapid catalyst aging and low selectivity for the caprolactam. It is not possible to avoid these disadvantages even by employing the strongly acidic, hydrophobic HZSM-5 with Si0,/Al2O3 = 78 (ref. 35); a 14 % solution o f cyclohexanone oxime in benzene at 350 "C, 1 atm and LHSV = 1.7 hr-' was nearly quantitatively converted for a period of up to 15 hr. Afterwards the conversion drops rapidly to about 40 % at 21 hr on stream. Therefore, in recent years, considerable interest has focussed upon zeolite catalysts of reduced acidity, in particular on the outer surface and upon other weakly acidic microporous materials (refs. 36 - 40). H. Sato and coworkers demonstrated (refs. 37, 38) that both the catalytic activity and the selectivity of lactam formation increase with increasing Si/Al ratio in HZSM-5 catalysts. By measuring the amount of 4-methyl-quinoline (4-MQ) adsorbed on HZSM-5, the amount of acidic sites on the outer surface of these samples with various Si/Al ratios can be determined. It was found that both the activity and the selectivity increase as the quantity of 4-MQ adsorbed on the outer surface decreases. This and other quoted results indicate that the rearrangement takes place on the outer surface or at the pore mouth of the pentasil zeolites. The relatively large molecular diameter of the cyclohexanone oxime and the E-caprolactam are probably responsible for that. Even HZSM-5 catalysts with a Si/Al ratio of more than 2000 show high activity. Furthermore the service life of the catalysts increases with the increase o f the Si/Al ratio. But even these catalysts lose 50 % of their activity in the course of 12 h. Generally speaking the weaker the acidic centers on the outer zeolite surface the better are the results. The same research group (ref. 39) reduced the acidity of the external surface of HZSM-5 catalysts by treatment with organometall ic compounds such as chlorotrimethylsilane. The comparison, given in Table 1 , of silanated with nonsilanated HZSM-5 shows the advantageous effects of this silanation treatment on the service life of the catalyst and the selectivity for &-caprolactam. The service life can also be improved, but only slightly, by selectively covering the outer surface of a boron pentasil zeolite with Na ions (ref. 36). This is achieved by making the pore volume of the silicate inaccessible with an organic compound, in particular an aromatic hydrocarbon like toluene (saturation the pore volume), and by subsequently contacting such a sample with a deactivating agent such as an alkali metal ion, preferably Na, in a polar solvent. For
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example, a gas mixture o f cyclohexanone oxime, toluene, C02 and H20 in a molar ratio 1:3:7:1 is converted quantitatively at 340 "C and WHSV = 0.8 hr-I to E-caprolactam with 58 % selectivity using such a treated boron zeolite. The service life of this catalyst is more than 15 hr and more than three times longer than that o f an untreated boron zeolite; however, in this case the selectivity is very poor because 5-cyano-pent-1-ene may be formed preferably as by-product on Na' ions according t o A. Aucejo et al. (refs. 33, 34). TABLE 1 Comparison of silanated with nonsilanated HZSM-5 in the reactiona of cyclohexanone oxime to E-caprolactam catalyst si lanatedb
time on stream [hrl 3,3 31 ,O
nonsilanated
3,3 27,O
conversion
selectivity
C%l
C%l
100
95 ,O
98,2
100 95,8
95,O 79,7 89,4
areaction conditions: 8 wt% solution o f oxime in benzene, 350 "C, WHSV = 11.7 hr-1 , 1 atm, C02 as carrier gas, oxime/C02/benzene = 1/5.6/18.3 mol bHZSM-5 with Si/Al :: 1600, treated with chlorotrimethylsilane at 350 "C for 4 hr Pursuing the idea of reducing the acidity of the zeolite in order to get high selectivity and long lifetime of the catalyst, micro-porous materials such as the weakly acidic non-zeolitic molecular sieves (e. g. the middle-pore-sized SAPO-11 or SAPO-41) are used for the Beckmann rearrangement (ref. 40). Using SAPO-11, a 5 wt% solution of cyclohexanone oxime in acetonitrile is converted at 350 "C, atmospheric pressure and WHSV = 10.8 hr-l, to E-caprolactam with 95 % selectivity at 98 % conversion rate. However, also in the latter case, it must be pointed out that the Beckmann rearrangement of cyclohexanone oxime by means of zeolites or other non-zeolitic molecular sieves does not at present, for reasons of service life, constitute an alternative to the homogeneous process now practiced. Generally, the development of heterogeneous catalysts based on middle-pore-sized zeol itic and non-zeolitic molecular sieves for the rearrangement of cyclohexanone oxime indicate that the reaction does not necessarily take place at the acidic centers as active sites, whereas it was generally considered so in former times. Only rather weak acidity, or no acidity, is required in order to achieve high selectivity at high conversion and long catalyst service life. The rearrangement of other ketoximes such as acetone oxime or acetophenone
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oxime to the corresponding amides using HY-zeolites has been reported (ref. 32). For example, a mixture of 20 wt% acetophenone oxime in benzene reacts at 300 OC to acetanilide with 95 % selectivity and N-methylbenzamide with 5 % selectivity. In this case the migration of the phenyl group is favoured over methyl group migration in the aldehyde-ketone isomerization (ref. 3 ) . Rearrangement of ani 1 ines The synthesis of picol ines by rearrangement of aminated arenes constitutes an interesting reaction. There, aniline can be converted at 510 "C, 2860 KPa and WHSV = 1.1 hr-' to d-picoline in the presence of NH3 (NH3/aniline = 1.5 molar) using HZSM-5. Conversion of 13 % and selectivity of 52 % for c(-picoline are obtained (ref. 41). The presence of NH3 is necessary to attain a high picoline yield, because in the absence of NH3 diphenylamine is found as the main product. In a similar way the very interesting rearrangement of 1,3-diaminobenzenes to a mixture o f 2- and 4-amino-pyridines occurs in the presence of acid catalysts (ref. 42). A mixture of 1,3-diaminobenzene and NH3 (molar ratio 1;60) is converted at 350 "C and 190 bar on HZSM-5 to 2-amino-6-methylpyridine with 83 % selectivity at 43 % conversion. A comparison with silica-alumina or alumina under the same reaction conditions (16 - 29 % conversion, 57 - 89 % selectivity) shows the superior properties of the zeolite over other acid catalysts not possessing zeolite structure. This is an useful new route for aminopyridines, compounds which were hitherto only available by complicated reaction of sodium amide with pyridine. With these exciting reactions in mind, it is astonishing that an aromatic ring can be opened at elevated temperature and pressure in the presence o f NH3 and an acid heterogeneous catalyst. CYCLOCONDENSATION WITH AMMONIA Aldol condensations of aldehydes and ketones on zeolitic (refs. 1 , 3, 4, 6, 10) and on non-zeolitic molecular sieves (ref. 43) have been extensively described. Such condensation reactions, which are preferably carried out in the gas phase, lead in the presence of NH3 to pyridine and alkylated pyridines (refs. 44 - 51). For example, acetaldehyde with or without formaldehyde, in the presence of NH3 on HZSM-5 or Cd-HZSM-5 (ref. 44) or Cd-Y-zeolite (refs. 45, 46) at 300 400 O C reacts to form pyridine and picolines in yields of 20 % to 68 % in a relatively unspecific manner. Furthermore, the catalyst deactivates very rapidly. On the other hand, the acid-catalysed reaction o f acetaldehyde with ammonia in the liquid phase at 150 "C - 250 O C yields 2-methyl-5-ethylpyridine (e. g. ref. 46).
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Acetaldehyde, formaldehyde, NH3 and H20 (molar ratio 1:1:1.5:1.7) give an 83 yield of a mixture of pyridine and 8-picoline (molar ratio approx. 2 : l ) when the reaction is carried out at 450 "C on alumino-silicate catalysts of the pentasil type in the acidic H-form (ref. 49). The yield of pyridine in such a reaction can be increased by using ZSM-5 zeolites doped with T1, Pb and Co, respectively (ref. 50); e. g. a mixture of acetaldehyde, formaldehyde and ammonia (molar ratio 2:1:4) was passed at 450 "C and GHSV = 1000 hr-' over TI-ZSM-5 (Si/Al = 90 1 resulting in 63 % yield of pyridine and a 81 % total yield of pyridine and picolines. A synthesis of O-substituted pyridines is based upon the reaction of mixtures of acrolein and alkanals with ammonia in the presence of zeolite catalysts (ref. 51). For instance, if such a mixture of ammonia, acrolein and alkanal R-CH2CH0 in a molar ratio of 3:l:l is allowed to react on an HF-treated borosilicate pentasil zeolite at 400 OC and WHSV = 3 hr-1 , one obtains 0-ethyl-pyridine with 72 % selectivity (R = C2H5), 0-butylpyridine with 78 % selectivity (R = C4Hg) and 8-hexylpyridine with 90 % selectivity ( R = C6H,3). In all cases conversion is complete and catalyst lifetime > 48 hs. The increasing selectivity with increasing chain length of the fi-substituent is surprising, because more extensive cracking of the long-chain alkanals would have been expected. The transition-state shape selectivity of the pentasil zeolite is probably responsible for this effect, since the long-chain alkanals are restricted in their movements within the zeolite channels and adopt definite positions in relation to the other reaction partners. These cyclocondensation reactions of aldehydes and ketones in the presence of ammonia demonstrate once again the superiority of zeolite catalysts over conventional Al2O3- or Si02-based catalysts. - 90 %
O/N-REPLACEMENT IN CYCLIC COMPOUNDS As is known from other acid heterogeneous catalysts, the reaction of cyclic ethers and lactones with ammonia also gives cyclic imines or lactams using zeolite catalysts (refs. 1 , 8, 52 - 57). Tetrahydrofuran in the presence of NH3 (molar ratio 1:7) is converted at 350 "C into pyrrolidine on HL-zeolite with a selectivity of 91 % at 53 % conversion and on HY-zeolite with a selectivity o f 82 % at 61 % conversion. The conversion of THF increases with increasing temperature up to 360 "C, but drops down sharply at higher temperatures. Alumina and silica-alumina are also active in this reaction, but only poor selectivity is obtained. The alkali forms of the L- and Y-zeolite are not active at all, indicating that Bronsted acid sites are responsible for this O/N-replacement (ref. 53). HL- and HY-zeolites are not very suitable for the conversion of tetrahydropyran to piperidine. However, dealumination of these zeolites improves the
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activity and selectivity for this ring transformation (ref. 55); at 460 OC the HL-zeolite (Si/Al = 3.2) gives a 24 % conversion and a piperidine selectivity of 58 %, whereas the dealuminized catalyst (Si/Al = 6) results in 25 % conversion and an 80 % selectivity. The ring conversion, e. g. of THF, is possible with alkylated amines, too. THF reacts with propylamine on HY-zeolite doped with A1 at 360 OC to l-propylpyrrolidinone with 75 % selectivity at 61 % conversion (ref. 52). In contrast, selective O/N-replacement of aromatic compounds such as transformation of furan into the corresponding N-containing compound pyrrole takes place only on zeolites with modest acidity such as NaX-zeolite (ref. 1 ) or Bay-zeolite (ref. 56). In the latter case 14 % conversion and 100 % selectivity for pyrrole are attained. FButyrolactone and NH3 (molar ratio 1:5) are converted in the vapor phase into 2-pyrrolidinone on Cay-, KY- and CuY-zeolites. The highest activity (31 % conversion) and selectivity of 80 % are achieved at 260 "C (ref. 57). For this reaction the alkali cation zeolites are equally as active as the acid zeolites, indicating that acidic sites are not necessarily required. It has been mentioned in the patent literature that caprolactam can be obtained by the reaction of caprolactone and NH3 using HY-zeolites (ref. 58) or HZSM-5 (ref. 59). However, exact data are not disclosed. On the contrary, it was found (ref. 60) that in this reaction no O/N-replacement occurs, but instead there is cleavage of the ring system and dehydration to form hexennitrile. For instance, caprolactone and NH3 (molar ratio 1:7) are converted at 400 OC to hexennitrile with 66 % selectivity using a boronsilicate zeolite and 82 % selectivity using a alumina-silicate zeolite both of the pentasil type and 75 % using a HY-zeolite. If this reaction is carried out in the presence of a conventional catalyst such as A1203, only poor yields of the hexennitrile are obtained because the reaction stops at the intermediate 6-hydroxycapronitrile. The question arises as to why we obtain hexennitrile instead of caprolactam in the conversion of caprolactone with NH3. If this result were in agreement with the idea of shape selectivity, i. e. the less bulky hexennitriles preferentially formed rather than the bulky caprolactam, then caprolactone would have to migrate in the pores of the pentasil type zeolite. On the other hand, it is known from the investigation of the Beckmann rearrangement (refs. 37 - 39) that caprolactam formation takes place at the outer surface because of the bulky compounds. There may be two explanations for the formation of hexennitrile. First, the ring opening of the caprolactone occurs at the pore mouth and then the stretched fragment migrates partially inside the zeolite pore to react with NH3, or second it is just a question of the reaction rate, i. e. that after the ring opening under addition of NH3 the dehydration to hexennitrile occurs faster than the ring closure to form the caprolactam, and shape selectivity is not involved at all.
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OX IDATI O N REACTIONS With hydrogen peroxide and ammon i a The discovery of the titanium-silicate TS-1 has enabled remarkable progress to be made in oxidation reactions with H202 in recent years (refs. 61, 62). Such a titanium-zeolite can also be used for the preparation of cyclohexanone oxime by the reaction of cyclohexane with ammonia and hydrogen peroxide in the liquid phase (ref. 63). Such a mixture i s heated in an autoclave to 60 "C ( - 700 mm Hg above atmospheric pressure), giving 95 % conversion o f the cyclohexanone, 80 % selectivity of the oxime and 15 % selectivity of peroxy-di-cyclohexyl-amine. Surprisingly, using a zirconium-silicalite or a boronsilicate zeolite, analogous results are obtained. This cyclohexanone-oxime formation rate avoids the co-production o f ammonium sulphate, as is obtained in the classical reaction of cyclohexanone and hydroxylammonium sulphate, and hence it can be of industrial interest. With oxvqen and ammonia As has already been shown, the reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence o f oxygen, then pyridine is formed (refs. 64, 65). H. Van Bekkum and coworkers recognized that H-boron zeolite with S i / B = 42, i. e. 2.2 boron atoms per unit cell or Fe-containing ZSM-5-type catalysts, are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H20 and O2 (molar ratio 3:1:6:9) reacts on H-boralite at 330 "C and a WHSV 0.17 hr-' to yield pyridine with 48 % selectivity. The conversion is 24 %. It is possible to improve the conversion by increasing the number of boron atoms per unit cell, i. e. the number of acid sites, or by raising the temperature. At 360 "C the conversion is 81 % but there is increased ethylene formation at the expense of pyridine. Further by-products include diethylether, acetaldehyde, ethylamine and C02. In the case of Fe-containing aluminosilicate ZSM-5 catalyst the conversion increases with increasing Fe content. An FeHZSM-5 with Si/Fe = 189 and Si/Al = 71 leads to a 48 % pyridine selectivity at 28.5 % conversion. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldol ization, reaction with ammonia, cyclization and aromatization can be envisaged, The oxidative ammonolysis of olefines in the presence of e. g. Ag, Cr, Fe or Mn cation-exchanged X - and Y-type zeolites yields unsaturated nitriles (ref. 66). The ammoxidation of toluene and related aromatics to the corresponding nitriles can be achieved by means of X-zeolite (ref. 1) or ZSM-5 zeolites (ref.
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67). Doping with transition metals such as Zn, Cu, Ag and Fe increases the activity. Whereas, for example, ZnX-zeolite deactivates within 7 hrs, the CU-H-ZSM-5 is much more stable and shows also higher selectivity and activity than ZnX. The ion-exchange of H-ZSM-5 to CU-H-ZSM-5 results in a substantial increase of the activity and selectivity. Using CU-H-ZSM-5 with 0.6 wt.-% Cu as catalyst, toluene reacts with NH3 in H20 (mol ratio 1:2:6) at 230 OC and WHSV = 0.17 hr 1 t o benzonitrile with 86 % selectivity at 70 % conversion. Water has a beneficial influence on the activity and selectivity. On the other hand, H-ZSM-5 yields only 35 % selectivity and 7 % conversion. The temperatures required for the ammoxidation are substantially lower in the case of the zeolites than for conventional catalysts like silica-alumina, -zirconia or -titania. Acetonitrile is produced with high yield by ammonolysis o f propane at about 600 "C when a pentasil zeolite with Si02/A1203 = 25 - 200 is employed as catalyst (ref. 68).
OLIGOMERIZATION REACTIONS Oligomerization reactions of olefins on zeolites, which have been the subject of numerous studies, generally yield mixtures of dimers, trimers and tetramers (e. g. ref. 4, 69). On the other hand, oligomerization reactions in the presence of NH3 have received little attention. One of the infrequent examples is the synthesis of dimethylphenylpyridine by cotrimerization of phenylacetylene and acetonitrile in the presence of a Co-doped Y-zeolite which had been specially reduced with NaBH4 in the reaction mixture (ref. 70). At 100 OC an 80 % yield of diphenylmethylpyridine is obtained. In this case the heterogeneous catalyst is superior to the homogeneous monovalent Co complexes for reasons of separation and product work-up. ISOMERIZATION REACTIONS The excellent isomerization capacity of pentasil zeolites is also applicable to aromatic compounds bearing N-containing substituents such as toluidines (refs. 71 - 74) and methylbenzonitriles (ref. 75). A zirconium-doped pentasil zeolite is particularly suitable for the isomerization of 0- or p-toluidine in mixtures of the 0-, m- and p-isomers and is superior to the HZSM-5 catalyst in terms of both product yield and catalyst lifetime (ref. 71). At 430 "C o-toluidine is converted on the zirconium zeolite to an 0 - , m-, p-mixture in a weight ratio of 37:45:15; on the other hand, the HZSM-5 zeolite yields a mixture of isomers in a weight ratio of 52:32:10. Toluidine is produced by the hydrogenation of nitrotoluene mixtures of approximate composition by weight 63 % 0-, 4 % m- and 33 % p-isomers. It is, however, the m-isomer which is of greatest importance as an intermediate for dyestuffs and agrochemicals. In the foregoing nitration-reduction process the
205
toluidine mixture contains only 4 % of the m-isomer. In order to increase the proportion of the desired in-isomer the 0- or p-isomer is isolated from the hydrogenation product, and in a subsequent isomerization process converted on a pentasil zeolite to a toluidine mixture rich in the m-isomer. Surprisingly, in contrast to the isomerization of xylene, this toluidine isomerization does not yield preferentially the para but the meta isomer (e. g. refs. 1 , 4). A compari son of the isomerization of to1 un i tri les and d imethy 1 benzon i tri les shows that, in the case of tolunitrile, the three isomers are distributed in accordance with the thermodynamic equilibrium (ref. 75), whereas in the case of the dimethylbenzonitriles the shape selectivity results in a product distribution such that no 1,2,3-isomersor 1,3,5- isomers are formed. This limitation does not apply to trimethylbenzenes; in this case the 1,2,3-isomer and the 1,3,5- isomer are formed from 1,2,4-trimethylbenzene in a ratio between 1.3:1 and 2.7.1 (ref. 76). ALKYLATION REACTIONS Alkvlation of heteroaromatics The alkylation of the aromatic nucleus (e. g. ref. 4, 26, 77, 78) or of the side-chain (refs. 4, 79 - 82) of alkylbenzenes are competing reactions which have been extensively studied. In contrast, perhaps because such systems are apparently intrinsically more complex, only few references dealing with alkylation of N-containing heteroaromatics are known (ref. 83). Pyridines can be alkylated in the nucleus using faujasites, with the Rposition being preferentially attacked when H-Y or Li-Y-zeolites are used, and the cc- and Zpositions in the case of alkali-earth Y-zeolite catalysts. On H-Y 3 % d-, 12 % R- and 3 % Fpicoline are obtained at 32 % conversion, whereas on Ba-Y 23 % &-, 4 % R- and 8 % Fpicoline are produced at 63 % conversion (reaction conditions: 400 "C, methano1:pyridine = 8 mol, LHSV 1.3 hr-1 , N2). Isomers of lutidine are formed as by-products. On the other hand, when pyridine is treated with methanol in the presence of X- and Y-zeolites ion-exchanged with alkali metals (not Li), side chain alkylation is also oberved as a consecutive reaction. The principal products are ethylpyridine and vinylpyridine as well as isomers of picoline and lutidine. When, for example, Cs-Y-zeolite is used as catalyst (reaction conditions: 450 "C, LHSV = 1.3 hr-', methanollpyridine = 8 mol) the yield of ethylpyridine is approx 27 % at 82 % conversion. In the side-chain methylation of picolines the following order of reactivity is observed: CC > fi, with B-picoline being almost inert. Thus, the maximum yield of 2-ethylpyridine (approx. 30 %, together with 3 % 2-vinylpyridine at 83 % conversion and 450 " C ) is obtained by treating OC-picol ine with methanol on Cs-Y-zeolite. Conversely the yield of 2-vinyl-
r>
206
pyridine is highest (10 %, 425 "C) when Cs-zeolite is used. The above examples indicate how it is possible to suppress or to induce consecutive reactions during the alkylation of pyridine merely by means of the appropriate choice of dopant in the zeolite catalyst. The examples also show how the selectivity can be influenced by the type of zeolite employed. In contrast to the side-chain alkylation of toluene using alkali-earth-doped faujasites, it is characteristic of the side-chain alkylation of picolines that alkylation of the nucleus also occurs (e. g. ref. 4). Alkylation of aromatic amines The alkylation of aromatic amines includes the reaction of aniline with methanol (refs. 84, 85) or with olefins (refs. 86 - 88) in the presence of zeolitic and nonzeolitic molecular sieves. In principle, reaction can take place at the N-containing groups forming N-alkylated compounds or at the nucleus forming C-alkylated compounds. In other words, the methylation of aniline, for example, yields toluidine, N- methylaniline and N,N-dimethylaniline. All are useful intermediates for dyestuffs, agrochemicals and drugs as well as for the organic synthesis. A mixture of aniline and methanol in a molar ratio 1:3 reacts at 350 "C and WHSV = 0.8 hr-' with 51.4 % selectivity for N-methylaniline and 20.1 % for toluidine using HZSM-5 with Si02/A1,03 = 60. The conversion is 70 %. That means the acid HZSM-5 already favours N-alkylation over C-alkylation. By impregnating or moulding with, e. g. MgO, thereby reducing the acidity, the amount of Nalkylate can be increased; N-methylaniline was formed with 86.5 % selectivity at 85.5 % conversion. However, only 3,5 % toluidine selectivity was obtained. The alkylation of aniline with olefines such as propylene can be carried out employing H-mordenite (ref. 86), dealuminated mordenite or Y-zeolite (ref. 87) or SAPO 37 (ref. 88). In contrast to the alkylation with methanol, in all these cases C-alkylation is favoured especially in the ortho-position. Using Hmordenite, aniline and propylene in a 1:l molar ratio are converted at 250 C' and LHSV = 0.35 hr-' to o-alkylate with 84 %, p-alkylate with 4 % and N-alkylate with 15 % selectivity. 32 % conversion is obtained. Using dealuminated mordenite the conversion rate can be increased to 76 % whilst maintaining a similar product distribution, and using SAPO 37 a further increase up to 93 % is possible. In the latter case, however, only 64 % o-alkylate selectivity and 12 % p-alkylate selectivity are achieved. The selectivity for N-alkylate is 21 %. In the author's opinion a concerted reaction between aromatic amine and olefine (ref. 86) is responsible for ortho-alkylation being favoured over para-alkylation.As far as the thermodynamics are concerned the para-alkylates are the most stable, as is demonstrated by the fact that their selectivity increases with higher temperatures demonstrates.
207
CONCLUSION This presentation is intended to underline once again the versatility of zeolites for both synthesis and reactions of N-containing compounds. Some examples demonstrate that existing processes can be improved merely by replacing the conventional heterogeneous catalyst. Other examples show the advantages of the change from homogeneously catalysed processes to heterogeneous catalysis if environmental and process engineering problems occur in the separation and work-up. However, in other cases, like the Beckmann rearrangement to produce caprolactam, the results obtained by homogeneous catalysts can not be achieved by zeolitic or non-zeolitic molecular sieves up to now. Considerable effort is still necessary in order to solve this problem. The principles derived from reactions of hydrocarbons on zeolites can generally be applied to the reactions of N-containing compounds. However, there can be a few differences, too, as the alkylation of pyridine or aniline in comparison t o the alkylation of benzene or toluene has shown. Nevertheless, there are some promising results in the chemistry of N-containing organic compounds which encourage us to proceed further along the road of shape-selective catalysis. REFERENCES 1 2 3 4
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
COMPARISON OF THE ALKYLATION OF ANISOLE AND PHENOL WITH METHANOL ON PENTASIL AND ULTRASTABLE ZEOLITES
RUDY F. PARTON, JULIA M. JACOBS, HEIDI VAN OOTEGHEM and PETER A. JACOBS Laboratorium voor Oppervlaktechemie, KU Leuven, Kardinaal Mercierl aan 92, B3030, Heverlee (Leuven), Belgium
ABSTRACT The alkylation of anisole and phenol with methanol was measured in a microreactor at 473 K and at different contact times, using as catalysts pentasil and H-USY zeolites. Activity and selectivity results showed that monomolecular reactions are favoured on pentasil and bimolecular reactions on large-pore zeolites. The high selectivity o f o-cresol observed on the tenmembered-ring compared to H-USY zeolites is explained by the intramolecular rearrangement of anisole to o-cresol . INTRODUCTION The acid-catalysed a1 kylation of toluene with methanol over tenmembered-ring (10-MR) zeolites to obtain a para-enriched a1 kylate is now well established industrial practice [l-31. The methylation of phenol with methanol [4-91 or anisole [lo, 111 on zeolites gives a mixture of anisole, xylenols and cresols, rich in the ortho-isomer. Evidently, the selectivity of p-cresol is not improved by the use of pentasil-type zeolites. On H-ZSM-5, the o/p-cresol ratio is always higher than one whereas on Y zeolites o/p ratios significantly lower than one have been reported [12-141. On the contrary, a high p-selectivity in the methylanisole fraction is observed in the self-alkylation of anisole on H-ZSM-5 [15] as well as in the methylation of anisole with methanol [14]. In order to make p-cresol selectively by the use of shape selective zeolite catalysts, it is desirable to know the reaction network of the methylation of phenol with methanol and the influence of the zeolite structure on the different pathways. Recently, for A1P04-Ti02 catalysts an overall scheme of the phenol and anisole alkylation was reported [16, 171. On H-ZSM-5 a detailed study of anisole conversion showed three primary pathways [15]. The main pathway of cresol formation is the intramolecular rearrangement of anisole to o-cresol When the influence of the zeolite structure and acidity on the reaction network of the alkylation of phenol with methanol is known it should be possible to select and modify zeolite catalysts in order to obtain the desired isomer with high selectivity.
.
EXPERIMENT M a t e r i a1 s Two batches, (1) and ( 2 ) , o f ZSM-5 and a sample o f ZSM-11 w i t h a Si/A1 r a t i o o f 100 were s y n t h e s i z e d a c c o r d i n g t o e s t a b l i s h e d l i t e r a t u r e procedures [18, 191. A f t e r a i r c a l c i n a t i o n a t 823 K, subsequent ammonium exchange a t r e f l u x c o n d i t i o n s was done. A sample o f H-USY w i t h a framework S i / A l r a t i o o f 20 was r e c e i v e d f r o m Toyo Soda.
A n o t h e r sample o f H-USY was prepared by
steaming o f NH4-Y, purchased f r o m V e n t r o n as Nay, w i t h a Si/A1 r a t i o o f 2.46. Bot h
samples
are
denoted
as
H-USY(T)
framework Si/A1 r a t i o o f USY(V) was 4.3 reaction a l l
and
H-USY(V),
respectively.
The
as det ermined b y 2 9 S i NMR. Bef ore
z e o l i t e s were c a l c i n e d i n s i t u i n f l o w i n g oxygen.
Anisole,
phenol and methanol w i t h a p u r i t y o f a t l e a s t 99% were purchased f rom Janssen Chimica. Reac t io n procedure Time-on-stream (ToS) e x p e r i m e n t s i n t h e vapour phase were c a r r i e d o u t i n hydrogen, u s i n g a f i x e d - b e d c o n t i n u o u s f l o w r e a c t o r a t d i f f e r e n t c o n t a c t times , W/Fo,
and atmospheric p r e s s u r e . W s t ands f o r c a t a l y s t weight and Fo
f o r mo lar f l o w r a t e o f t h e f e e d a t t h e r e a c t o r e n t r a n c e . The z e o l i t e powder was pressed,
crushed,
s i e v e d and t h e 0.25-0.50
mm f r a c t i o n
retained for
f u r t h e r use. The vapour p r e s s u r e o f a n i s o l e , phenol and methanol was 1.3 kPa. The r e a c t i o n p r o d u c t s were analysed o n - l i n e by gas chromatography on a 2 m packed column from Supelco c o n t a i n i n g 0 . 1 % SP-1,000 on Carbopack C . RESULTS and DISCUSSION O v e r a l l a c t i v i t y i n t h e a l k v l a t i o n o f a n i s o l e w i t h methanol 100
-aR
80
.-C0 2 0
60
>
e
0 V
-
40
0 0
.-mC
<
20
0
5
10
15
20
25
l i m e (h)
F i g . 1. A l k y l a t i o n o f a n i o l e w i t h methanol on d i f f e r e n t H - z e o l i t e s a t 473 K w i t h W/Fo= 1,736 kg s mol
-7 .
213
Fig. 1 shows t h a t i n the a l k y l a t i o n o f a n i s o l e w i t h methanol the anisole conversion decreases w i t h ToS f o r a l l c a t a l y s t s investigated, probably as a r e s u l t o f coke deposition. Although these data h a r d l y a l l o w a d i s t i n c t i o n t o be made between i n i t i a l a c t i v i t y ,
the f o l l o w i n g statements can be made: i. t h e r e p r o d u c i b i l i t y o f the conversion i s s u f f i c i e n t as shown f o r t h e two batches o f H-ZSM-5; ii. H-ZSM-5 i s more a c t i v e than t h e u l t r a s t a b l e z e o l i t e s ; iii. the a c t i v i t y d i f f e r e n c e between the two u l t r a s t a b l e z e o l i t e s i s i n l i n e
w i t h the d i f f e r e n c e i n Si/A1 r a t i o o f t h e i r framework; i v . H-ZSM-11 shows i n i t i a l l y a lower a c t i v i t y than H-ZSM-5 b u t seems t o be l e s s susceptible t o deactivation. USY(V) contains more Brensted s i t e s which a r e o f lower strength than i n USY(T). Indeed, the S i / A l r a t i o o f USY(T) i s h i g h e r as w e l l as i t s a c t i v i t y i n the hydroisomerisation o f long-chain alkanes [20]. Therefore, the alkylation o f
phenol
and
anisole
can
p o s s i b l y occur
over
sites
with
r e l a t i v e l y weak a c i d strength. Overall s e l e c t i v i t y i n the a l k v l a t i o n o f a n i s o l e w i t h methanol TABLE 1 Product s e l e c t i v i t y i n the methanol a l k y l a t i o n o f anisole a t 473 K, w i t h W/Fo =
I
1
1,736 kg s mol-' and a f t e r 3 hours on stream. Catalysts Conversion o f anisole
H-USY(T)
H-USY(V)
1
I
H-ZSM-5
H-ZSM-11
(%l
I
30.0
I
58.4
I
I
60.9
49.6
Phenol
51.9
32.5
79.3
84.3
Cresol s
12.6
23.8
15.3
11.6
0.0
6.8
0.0
0.0
35.5
36.9
5.4
4.1
Xyl enol s Methylanisoles
I
30*4
I
I
I
41*4
36.1
Products obtained i n a l l cases are cresols (Cr), methylanisoles and
phenol
(Ph)
(Table
1).
H-ZSM-5
and H-ZSM-11
show
similar
(mAn)
product
s e l e c t i v i t i e s w i t h Ph being present i n l a r g e excess over C r and mAn. Compared t o t h e p e n t a s i l z e o l i t e s , t h e r e i s l e s s Ph and more mAn obtained on H-USY. The decreased Ph s e l e c t i v i t y on H-USY(V) i s t h e r e s u l t o f a conversion effect,
as secondary a l k y l a t i o n products l i k e xylenols
favoured on the more a c t i v e H-USY(V).
(Xy)
and C r a r e
The d i f f e r e n c e i n s e l e c t i v i t y between
H-USY and p e n t a s i l z e o l i t e s cannot be explained by such an e f f e c t . As the
214
methanol conversion t o dimethylether i s h i g h e r on t h e 10-MR z e o l i t e s i t i s p o s s i b l e t h a t the h y d r o l y s i s o f An by water t o Ph proceeds t o a greater e x t e n t on t h e p e n t a s i l z e o l i t e s than on H-USY. Supplementary evidence i s found i n Fig. 2 where i s shown t h a t t h e An conversion i n the presence o f water y i e l d s about 10% more Ph a t t h e expense o f mAn. However, t h e a d d i t i o n o f l a r g e amounts o f water r e s u l t s i n a minor product change.
Therefore,
h y d r o l y s i s alone cannot e x p l a i n t h e d i f f e r e n c e between USY and p e n t a s i l zeol ites. Possibly monomolecular r e a c t i o n s are favoured on 10-MR z e o l i t e s , whereas bimolecular
r e a c t i o n s dominate on H-USY
c o n s t r a i n t s o f the pores.
Indeed,
as
a result
o f differences
in
Ph can be formed v i a a monomolecular
d e a l k y l a t i o n r e a c t i o n , as reported p r e v i o u s l y [15], and should be favoured on the p e n t a s i l z e o l i t e s , whereas mAn can o n l y be formed by a bimolecular r e a c t i o n . Also i n t h e An conversion Ph was formed i n l a r g e excess [15] over pentasi 1 zeol ites.
Time (h)
Fig. 2. Product s e l e c t i v i t y f o r a n i s o l e conversion on H-ZSM-5 i n t h e presence (open p o i p t s ) and absence ( f u l l p o i n t s ) o f water a t 473 K, w i t h W/Fo = 1,736 kg s mol- and a molar anisole/water r a t i o o f 1. Cresol S e l e c t i v i t y i n the a l k v l a t i o n o f a n i s o l e w i t h methanol F i g . 3 shows t h e o-Cr s e l e c t i v i t y against conversion o f An i n the An methylation. I n no case i s m - C r observed. Although t h e data are t r a n s i e n t measurements and do
not
represent
initial
or
steady
state
conversions
obtained a t d i f f e r e n t contact times, t h e f o l l o w i n g conclusions can be s a f e l y drawn: i. o-Cr i s a primary product necessarily formed v i a a monomolecular r e a c t i o n whereas t h e p-isomer i s o f secondary nature and obtained by a bimolecular reaction; ii. t h e bimolecular formation o f p-Cr i s suppressed i n the p e n t a s i l
compared t o t h e Y z e o l i t e s .
I n Fig.
4 i s given the o-Cr
215
s e l e c t i v i t y i n the Ph a l k y l a t i o n w i t h methanol over p e n t a s i l z e o l i t e s . Again no m - C r i s formed, the o-isomer dominates and t h e s e l e c t i v i t i e s are hardly conversion (and ToS) dependent. I n contrast t o what i s observed f o r xylenes,
the
ZSM-5
distribution o f
0-
pores
are
unable
to
influence
0-
and p-
significantly
the
and p-Cr by product shape s e l e c t i v i t y . From Figs. 3 and 4
i t f o l l o w s t h a t t h e o-Cr s e l e c t i v i t y i s lower i n t h e Ph than i n t h e An
alkylation
with
methanol.
All
this
indicates
that
the
intramolecular
rearrangement o f An t o o-Cr i s a major primary pathway. The same was reported f o r t h e An conversion on H-ZSM-5 i n absence o f methanol [15]. 100
90 80 70
A
H-ZSY-6
C
H-USV(V)
0
H-ZSY-11
60 50
40
20
0
60
80
Conversion (%I Fig. 3. o-Cresol selec i v i t y i n the methylation o f anisole a t 473 K and w i t h = 1,736 kg s mol- .
f
W/Fo
100
-;P
80
* c
--
60
-
40
.-0 u
a
0
0
0
0
0
0
Po
0
0
Po 0
t
0
20 0 0
10
20
Conversion
30
40
50
(%)
Fig. 4. C r e s o l - s e l e c t i v i t y from the methylation o f Ph w i t h methanol a t 473 K, w i t h o-Cr (open p o i n t s ) , p-Cr ( f u l l p o i n t s ) and W/Fo = (a) 1,736 and (b) 278 kg s m o l - l .
216
Methvlanisole sel e c t i v i t v i n the a l k v l a t i o n o f a n i s o l e w i t h methanol Table 2 Comparison o f the d i s t r i b u t i o n o f methylanisoles from a n i s o l e a l k y l a t i o n w i t h
I
methanol a t 473 K, 3 hours on stream and w i t h W/F, Catalysts
H-USY(T)
I
=
H-USY(V)
I
1,736 kg s m o l - l . H-ZSM-5
I
H-ZSM-11
1
Conversion o f anisole I%)
I
30.0
S e l e c t i v i t y i n the mAn-fractiona
1
ortho para
20.4 71.6
I
58.4
I
I
30.5 69.5
1
60.9 0.1
99.9
1
49.6
1
0.0 100.0
am-mAn i s n o t detected. As can be seen from Table 2, the p-mAn s e l e c t i v i t y i s almost 100% on p e n t a s i l z e o l i t e s and much lower on z e o l i t e Y.
Thus t h e s e l e c t i v i t y i n the
carbon a l k y l a t i o n r e a c t i o n o f An i s determined by t h e presence o f the s t e r i c c o n s t r a i n t s exerted by the z e o l i t e pore s t r u c t u r e . Overall a c t i v i t y i n t h e a l k v l a t i on o f DhenOl w i t h methanol 60
3
E
.-0
t 2e
40
0
-c 0
0
20
0
c
P
0 0
5
10
15
20
Time (h)
P.
Fig. 5. Phenol conversion i n t h e r e a c t i o n o f phen 1 w i t h methanol a t 473 K and W/Fo = (a) 1,736, (b) 695 and (c) 278 kg s molFig. 5 shows t h a t H-USY(V) i s more a c t i v e than t h e p e n t a s i l z e o l i t e s and H-USY(T), which have the same a c t i v i t y . Only the a c t i v i t y sequence between USY and p e n t a s i l z e o l i t e s i s d i f f e r e n t from t h a t observed i n t h e a l k y l a t i o n o f An.
I n t h e r e a c t i o n o f Ph w i t h methanol
all
primary r e a c t i o n s are
bimolecular and thus favoured on Y z e o l i t e s , whereas i n t h e a l k y l a t i o n o f An
217
some o f the major pathways are monomolecular, intramolecular zeolites.
All
such as t h e d e a l k y l a t i o n and
rearrangement
o f An,
and t h e r e f o r e
favoured on p e n t a s i l
t h i s explains
t h e d i f f e r e n c e i n r e l a t i v e a c t i v i t y o f the
z e o l i t e s i n the Ph and An a l k y l a t i o n . L i k e i n t h e a l k y l a t i o n o f An, H-USY(V) i s more a c t i v e than H-USY(T),
and H-ZSM-11 apparently shows an increased
resistance against deactivation. Overall s e l e c t i v i t v i n the a l k v l a t i o n o f ohenol w i t h methanol Table 3 i n d i c a t e s t h a t the product s e l e c t i v i t y between the two p e n t a s i l z e o l i t e s i s only s l i g h t l y d i f f e r e n t and can be a t t r i b u t e d t o a conversion e f f e c t : when the contact time decreases the An s e l e c t i v i t y increases a t the expense o f C r , Xy and mAn. Xylenols and mAn are secondary products and t h e i r formation i s suppressed a t lower contact times. Cresols are p a r t l y secondary products because they are formed e i t h e r by the a l k y l a t i o n o f Ph o r the intramolecular rearrangement o f An. On the contrary, An can o n l y be formed by a primary reaction.
An
15.3
32.9
22.8
40.9
42.6
9.3
26.7
14.1
23.1
Cr
52.4
58.4
40.7
42.2
43.0
82.3
73.3
79.3
76.9
XY
7.2
4.8
13.8
9.0
8.4
2.9
0.0
1.8
0.0
mAn
25.1
3.9
22.7
7.9
6.0
5.5
0.0
4.8
0.0
For the USY z e o l i t e s Table 3 shows s i g n i f i c a n t d i f f e r e n c e s i n An, mAn and C r s e l e c t i v i t i e s . A t equal conversion and d i f f e r e n t contact times, o r v i c e versa, always more An and l e s s C r i s formed on H-USY(V), i n d i c a t i n g t h a t 0 - a l k y l a t i o n i s p r e f e r r e d over the z e o l i t e w i t h t h e lower Brrnsted a c i d strength. I t i s n o t unexpected t o observe t h i s as the e l e c t r o n d e n s i t y o f the oxygen i n Ph i s higher than on t h e o/p p o s i t i o n s o f t h e aromatic nucleus.
218
Table 3 also shows t h a t a t the same contact t i m e t h e s e l e c t i v i t y f o r C r i s higher on p e n t a s i l than on z e o l i t e s H-USY. For An, mAn and Xy t h e opposite i s observed. A p o s s i b l e explanation could be t h e presence o f stronger a c i d s i t e s on p e n t a s i l z e o l i t e s , favouring carbon a l k y l a t i o n . On t h e other hand, An i n p e n t a s i l s could be p r e f e r e n t i a l l y consumed v i a a monomolecular r e a c t i o n which c o n t r i b u t e s t o t h e formation o f o-Cr. Cresol s e l e c t i v i t v i n t h e a l k v l a t i o n o f DhenO1 w i t h methanol
1.50
1.00
11 t
*
*
0.50
0
20 Conversion
40
60
(%I
Fig. 6 . The o/p r a t i o o f c r e s o l s agyinst conversion a t 473 K w i t h W/Fo = (a) 1,736, (b) 695 and (c) 278 kg s mol-
.
Fig. 6 shows t h a t t h e o/p r a t i o i n t h e C r - f r a c t i o n i s higher on p e n t a s i l than on H-USY z e o l i t e s . Apparently,
o-Cr i s a primary product. However,
it
cannot be excluded t h a t the lower d i f f u s i o n r a t e o f An i n p e n t a s i l z e o l i t e s favours the monomolecular rearrangement o f An t o o-Cr. previous sections,
Indeed, as shown i n
t h e intramolecular rearrangement o f An t o o-Cr proceeds
f a s t e r on p e n t a s i l z e o l i t e s . The o/p r a t i o decreases f o r t h e p e n t a s i l z e o l i t e s w i t h ToS. Possibly t h i s i s t h e r e s u l t o f coke d e p o s i t i o n decreasing t h e e f f e c t i v e pore s i z e e i t h e r by a mechanism o f pore-mouth b l o c k i n g o r by a homogeneous deposition throughout t h e c r y s t a l . I r r e s p e c t i v e o f t h e contact t i m e and t h e amount o f coke, a s i n g l e curve i s found f o r each USY sample. The o/p r a t i o i s determined by t h e conversion and n o t by t h e coke content. On those z e o l i t e s t h e o/p r a t i o increases when
It seems t h a t p-Cr i s p a r t l y a secondary and probably the r e s u l t o f Ph methylation w i t h An. The d i f f e r e n c e i n r a t i o between the two USY samples over t h e whole conversion range explained by t h e i r d i f f e r e n c e i n a c i d strength. Indeed, on H-USY(T),
the conversion decreases.
product t h e o/p can be because
219
of
its
higher
acid
strength,
carbon
alkylation
dominates
over
oxygen
a l k y l a t i o n , r e s u l t i n g i n lower o/p r a t i o s . Methvlanisole methanol
and xvlenol
selectivitv
i n the a l k v l a t i o n o f
Dhenol w i t h
Table 4 Comparison o f t h e d i s t r i b u t i o n o f methylanisole and xylenol phenol a l k y l a t i o n w i t h methanol a t 473 K, d i f f e r e n t contact times.
1
Catalysts
l,&-,
H-USY(T)
1
isomers from
a f t e r 1 hour on stream and a t
1
H-USY(V)
1
H-ZSM-5
H-ZSM-11
(kq s m o l - l l
I
695
Conversion o f phenol
1 ortho
278
I
695
278
I
56.1
30.1
24.7
204 11,736
278 11,736
278
I
7.6
(%l
29.2
13.1
1
44.0
10.2
24.5
21.2
33.1
28.3
33.1
33.0
0.0
0.0
0.0
0.0
meta
4.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
para
74.2
66.9
71.7
66.9
67.0
100.0
0.0
100.0
0.0
100.0
0.0
100.0
0.0
0.0
0.0
0.0 O*O
I
w e l l as on H-USY z e o l i t e s the l e s s bulky isomers o f mAn and Xy are p r e f e r e n t i a l l y formed. As a r e s u l t 2,4-Xy
and p-mAn are almost e x c l u s i v e l y obtained on the 10-MR z e o l i t e s ,
whereas on USY minor amounts o f 2,6-Xy and o-mAn are formed i n a d d i t i o n . Comoarison o f Dentasil w i t h USY z e o l i t e s i n t h e anisole and Dhenol a l k v l a t i o n Based on t h e r e s u l t s obtained, an o v e r a l l r e a c t i o n scheme i s proposed f o r p e n t a s i l and H-USY z e o l i t e s . It i s shown t h a t bimolecular r e a c t i o n s are favoured on H-USY compared w i t h 10-MR z e o l i t e s . Oxygen (1) and carbon (2) a l k y l a t i o n o f Ph are f a s t e r on H-USY z e o l i t e s . Once An i s formed, rearranged
faster
on p e n t a s i l
than on
H-USY
zeolites
it is
by monomolecular
reactions, such as d e a l k y l a t i o n t o Ph (-1) o r intramolecular rearrangement t o o-Cr
(3).
I n t h e An a l k y l a t i o n the
important primary reactions
are o f
monomolecular
nature and t h e conversion on USY compared w i t h p e n t a s i l z e o l i t e s i s suppressed. The o-Cr s e l e c t i v i t y i s improved on 10-MR z e o l i t e s .
Methylanisoles are obtained by t h e a l k y l a t i o n r e a c t i o n o f An (4) and C r (5)
220
and
favoured
on
large-pore
zeolites.
On
pentasil
zeolites
a
high
p-
s e l e c t i v i t y i s obtained, b u t d e a l k y l a t i o n i s favoured on those z e o l i t e s .
lpentasil zeolites1
..'..'
4
..
T
( 4 b&.fMeOH
( 4 ) MeOH
I
..*..'
( 5 ) MeOH
favoured
on H-USY
Q-. /
CONCLUSION
I t i s shown t h a t
bimolecular r e a c t i o n s are
monomolecular r e a c t i o n s on p e n t a s i l z e o l i t e s .
and
I n t h e a l k y l a t i o n o f anisole
w i t h methanol, d e a l k y l a t i o n and i n t r a m o l e c u l a r rearrangement o f a n i s o l e are primary reactions. For t h i s reason t h e conversion on p e n t a s i l z e o l i t e s i s increased i n t h e a l k y l a t i o n o f a n i s o l e w i t h methanol compared t o the a l k y l a t i o n o f phenol. The opposite i s observed on H-USY. The 10-MR z e o l i t e s are l e s s a c t i v e i n t h e phenol conversion than H-USY. As t h e rearrangement o f anisole t o o-cresol i s favoured on 10-MR z e o l i t e s , t h e s e l e c t i v i t y f o r o cresol i s higher on those z e o l i t e s compared w i t h H-USY f o r both a n i s o l e and phenol a l k y l a t i o n .
Differences i n a c t i v i t y and s e l e c t i v i t y between H-USY
z e o l i t e s w i t h d i f f e r e n t framework alumina content i s explained by d i f f e r e n c e s
A higher Brdnsted a c i d s t r e n g t h o f t h e z e o l i t e framework i s responsible f o r more carbon a1 k y l a t i o n . i n acidity.
221
ACKNOWLEDGMENTS The authors are g r a t e f u l t o the Belgian Government, Programmatie Wetenschapsbeleid, f o r sponsoring t h i s research i n the frame o f a "Concerted Action on Catalysis". P.A.J. acknowledges as a p o s i t i o n as Research D i r e c t o r from the National Fund f o r S c i e n t i f i c Research (Belgium). REFERENCES 1 T. Yashima, Y. Sakaguchi and S. Namba, 7 t h I n t . Congr. Catal., Tokyo, Ed. by T. Seiyama and K. Tanabe, E l s e v i e r (Amsterdam), (1980) 739-751. 2 L.B. Young, S.A. B u t t e r and W.W. Kaeding, J. Catal., 76 (1982) 418-432. 3 D.D. Do, Am. I n s t . Chem. Eng. J., 31 (1985) 574-580. 4 R. Pierantozzi and A.F. Nordquist, Appl. Catal., 21 (1986) 263-271. 5 A.A. Agaev and D.B. Tagiev, 2. P r i k l . Kh., 58 (1985) 2734-2735. 6 J. Yamawaki, T. Ando, Chem. Letters, (1979) 755-758. 7 P.D. Chantal, S. Kaliaguine and J.L. Grandmaison, Appl. Catal., 18 (1985) 133-145. 8 M. Renaud, P.D. Chantal and S. Kaliaguine, Can. J. Chem. Eng., 64 (1986) 785-791. 9 M. Marczewski, G. Perot and M. Guisnet, Proc. o f 1 s t I n t . Symp. Heterog. Catal. and Fine Chem., P o i t i e r s , (1988) P80-P89. 10 P. Beltrame, P.L. Beltrame, P. C a r n i t t i , A. C a s t e l l i and L. Forni, Appl. Catal., 29 (1987) 327-334. 11 P. Beltrame, P.L. Beltrame, P. C a r n i t t i , A. C a s t e l l i and L. Forni, Gazz. Chim. It., 116 (1986) 473-474. 12 S. Balsama, P. Beltrame, P.L. Beltrame, P. C a r n i t t i , L. Forni and 6 . Z u r e t t i , Appl. Catal., 13 (1984) 161-170. 13 S. Namba, T. Yashima, Y . I t a b a and N. Hara, Stud. Surf. Sci. Catal., 5 (1980) 105-111. 14 L.B. Young and N.J. Skillman, U.S. Pat. 4,371,714 (1983), assigned t o Mobil O i l Corporation. 15 J.M. Jacobs, R.F. Parton, A.M. Boden and P.A. Jacobs, Proc. o f 1 s t I n t . Symp. Heterog. Catal. and Fine Chem., P o i t i e r s , (1988) C70-C79. 16 J.M. Campelo, A. Garcia, D. Luna, J.-M. Marinas and M.-S. Moreno, B u l l . SOC. Chim. France, (1988) 283-292. 17 J.M. Campelo, A. Garcia, D. Luna, J.-M. Marinas and M.-S. Moreno, Proc. o f 1 s t I n t . Symp. Heterog. Catal. and Fine Chem., P o i t i e r s , (1988) P54-P63. 18 P.A. Jacobs and J.A. Martens, Stud. Surf. Sci. Catal., 33 (1987) 19, r e c i p e 10.b. 19 P.A. Jacobs and J.A. Martens, ibid., 20-21, r e c i p e 11. 20 Unpubl ished r e s u l t s
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H.G. Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
ACIDITY EFFECT OF ZSM-5 7lEOLITES ON PHENOL METHYLATICN RFACI'ION
N.S. CHANG, C.C. CHEN, S.J. CHU, P.Y. CHEN and T.K. CHUAhG
Union Chemical Laboratories, ITRI 321, Kuang F'u Road, Sec.2, Hsinchu, %iwan, R.O.C. ABSrRAm
Vapr-phase phenol alkylation with methanol over ZSM-5 zeolites and modified ZSM-5 zeolites was studied. The main products of the reaction were cresols. ?he shape-selective effect w a s obscure in cresol fonnation, and strong acid sites were responsible for m-cresol fonnation. fithemre, the strong acid sites on the external surface played a major role in cresol isanerization. INTRODUCTION Cresols are important intermediates for producing phosphate plasticizers, phenolic resins and antioxidants in the chemical industry. With increasing demand and limited supplies from petroleum refinery waste and coal tar, the development of new synthesis processes for cresol is increasingly important (ref. 1). The alkylation of phenol with methanol over solid catalyst has been considered to be an interesting process. There were different types of catalysts, including metal oxides and zeolites, t o be investigated in the vapor phase (refs. 2-6). Metal oxides with acid-base properties, such as MgO or FezOs-ZnO, were recently reported to promote the direct ortho-substitution to yield an ortho-methylated product without passing through anisole as an intermediate (ref. 4). However, alkylation of phenol with methanol over zeolite proceeded via 0-methylation and C-methylation to produce anisole and cresols respectively (ref. 2), as follows:
OCH 3
A
bH
224
The present paper compares various kinds of catalyst for phenol methylation and concerns mainly the acid sites that influence the cresol formation. Finally, a reaction pathway is postulated according to the investigation. EXPERIMENT Catalyst preparation (i) Zeolite synthesis: NaZSM-5 zeolites with different SiOz/ A1203 ratios (30-680) and zeolite-@ with SiOz/Alz03 ratio of 30 were prepared in our laboratory, according to the well-known Mobil patents (ref. 7 - 8 ) . Details on the preparation of Na-form and Hform zeolite could be found elsewhere (ref. 9). (ii) Zeolite modifications Impregnation: HZSM-5 zeolite was first soaked in its impregnation solution overnight, then dried and calcined at 550°C for six hours. Ion exchange: HZSM-5 was ion-exchanged with 0.1 M magnesium nitrate or calcium nitrate many times until the maximum exchange capacity was reached and then filtered, washed and dried. Vapor deposition: Tetraethyl orthosilicate (Si(OCzH,)) was employed as modification reagent. 300 vl of Si(OCaH,) was pulsed and vaporized with NZ stream into the reactor at 400°C prior to the reaction test. Catalyst characterization The crystallinity of the prepared zeolite was checked by X-ray diffraction. The chemical composition analysis of the prepared zeolite was carried out following the procedure of Hillebrand et al. (ref. 10). Prior to acidity measurement, all the catalysts were activated under air stream at 550°C for 2 hours and purged with helium for one hour. The total acidity was determined by the temperature-programmed desorption (T.P.D.) of NH3 as shown previously (ref. 11). Catalyst evaluation Reactions were carried out in a fixed-bed reactor (7 mm i.d.) containing 0.5 g of 20-30 mesh catalyst. The catalysts were preheated in a stream of air at 500°C for 3 hours before reaction. Reactants were fed by a syringe pump, vaporized and mixed with preheated carrier gas N Z by flowing downward to the catalyst bed.
225
The reaction effluents were cool and collected in a cold trap The non-condensable gas flow was measured by a wet-test meter and analyzed by gas chromatography using a Porapack Q column. The collected liquid products were analyzed by a HP-5730A gas chromatograph with a Carbopack C/O.l% SP-1000 glass column. The catalytic activity and selectivity were expressed by (moles of reacted phenol/moles of supplied pheno1)xlOO and (moles of reacted product/moles of supplied phenol)x100, respectively. (-5°C).
RESULT AND DISCUSSION Catalytic activitv and selectivitv The alkylation of phenol with methanol was studied initially by comparing various kinds of catalyst. The reaction occurred to give a reaction mixture of cresol isomers as the principal products, and results were summarized in TABLE 1. Methanol was very reactive over zeolites (ref. 12), and Cz-C3 olefin produced in this manner also reacted with phenol to form undesired aromatic compounds. Those would be all lumped in the "other" term specified in TABLE 1. TABLE 1 Catalytic activity and selectivity of various catalysts ~
Catalyst' a ' Acidity (mmoleig cat.) weak sites (70-400"C) strong sites (400-600"C) total sites
s/w PhOH conv. (molX)
~~~
T - A L O ~ --5(35)
Zeolite-C(30) HZSH-5(60) HZSH-5(150)
0.206
0.833
1.029
0.024
0.037
0.301
0.337
0.214
0.230
0.870
1.330
1.082
0.569
0.117
0,044
0.293
0.452
0.603
0.745
0.355
48.8
39.1
45.0
49.5
55.2
Prod. s e l . (molX) 7.4 anisole 58.1 o-cresol a-cresol 0.9 p-creso 1 2.3 17.1 DNP' b , others' 14.1
12.6 37.9 16.1 16.6 11.6 5.2
3.8 35.1 16.6 11.6 12.6 20.3
0.0 26.3 30.1 11.3 13.1 19.2
0.0 26.3 28.2 10.8 18.5 16.6
(a) The molar ratio of SiOs/AlsOa during preparation is shown in the parentheses (b) 2.6-dimethylphenol for 7-AlnOs, 2,4- and 2,5-dimethylphenol for zeolites (c) lnclude methylanisole, alkylbenzenes and DHP other than those described in (b) Condition: reaction temp. 400"C, WHSV 0.98-1.04 hr-' (based on phenol) PhOH/NeOH/HsO = 1/1/5, time on stream 3-4 hrs, carrier Nu 390 Nml/hr
226
NaZSM-5(35) or r-Al~.Oawith relatively weak acid sites, as shown in TABLE 1 , readily catalyzed the methylation of phenol. However, the conversion of phenol did not increase when the number of total acid sites increased and seemed to be proportional to the ratio of strong acid sites to weak acid sites ( S J W ) . This indicated that strong acid sites might be responsible for some kinds of reaction. Presumably, o-cresol and p-cresol were preferable formed in an electrophilic substitution reaction, and the secondary reaction (such as isomerization and transalkylation) was confined due t o the limited number of strong acid sites. On the other hand,the fact that 0- and p-cresol were thermodynamically unfavored (TABLE 2 , ref. 1 3 ) resulted in the reduction of phenol conversion. TABLE 2 Equilibrium concentrations of cresol isomers
T ("lo
298.16
400
500
600
700
800
m-cresal
78.1
69.9
63.2
58.6
54.2
51.7
p-cresol
2.6
4.7
6.5
7.7
9.2
10.0
o-cresol
19.3
25.4
30.3
33.7
36.6
38.3
The selectivity of anisole decreased with increasing S/W ratio which inferred that anisole was also the primary product produced on weak acid sites, as proposed by other workers (ref. 5). The high selectivity of ortho-substituted phenol over Y-A1203 could be well elucidated by a mechanism in which a dissociative adsorption of phenol on the surface was assumed. The ortho-position of the surface phenoxides was easily attacked by the proximately adsorbed alcohol (refs. 14-15). The o-cresol predominance indicated that in our study the shapeselective effect was obscure in cresol formation. Accordingly, a plausible mechanism was considered analogous to the mechanism of toluene methylation over ZSM-5 proposed by Kaeding et al. (ref.16). The oxoniom ion was formed by the protonation of methanol, but somehow both ortho- and para-position of phenol could be attacked without steric hindrance. Nevertheless, 2,4-and 2,5-dimethylphenol had a smaller minimum dimension than the other isomers and were produced in significant excess. This was consistent with the isomers expected for reactions occurring within a restricted space of zeolite.
227
To suppress the formation of anisole under specified conditions, further studies were carried out on the relationship between the selectivity of cresol isomers and acidity of ZSM-5 zeolites. Effect of Si02/A1~03ratio
HZSM-5 zeolites with SiOz/Alz03 ratios from 60 to 680 were employed to afford an insight into the nature of acidity on phenol methylation. Generally, the acid strength of zeolite increased with increasing SiOz/A1~03ratio, and the number of acid sites decreased correspondingly. The crystallite size of ZSM-5 bacically increased with increasing SiOz/A1203 ratio. As a result, the number of strong acid sites on external surface would increase when the A1 content increased. Fig. 1 shows the influence of strong acid sites on the composition of cresol isomers. The mcresol isomers increased from 20.2% to 44.5% by increasing the number of strong acid sites. Only a small change was observed in para-isomer (20.4%-15.9%), and was higher than the equilibrium value (7.7%-9.2%, TABLE 2). It was suggested that m-cresol isomers were formed by the isomerization of o-cresol on the strong acid sites. More details would be discussed later.
0 +J
4o
I
.
r(60)
m-cresol
pcresol
(150).
HZ (255).
0
HZ (100) HZ (150)
HZ (60)
0"
I
I
0.1
I
I
I
0.3
strong acid sites (mnol/g Z. ) '
Fig. 1. Effect of strong acid sites on the formation of cresol isomers. Reaction conditions are described in TABLE 1
228
Effect of modification During our studies with various modified catalysts to enhance p-cresol selectivity, no matter what impregnation or ion exchange method was used, shape-selection was still vague in p-cresol formation except for MgO-HZSM-5 and SiOzfHZSM-5.
TABLE 3 Effect of HZSM-5 catalyst modification t,ata!ysr' amoun?
Imprg
(meta! 0 x 2 2 )
dZSH-5 F.C,/HZ
-
H g O , H Z BLOJ!iiZ
3 1 u t 5 8 . 9 wt% 3 5 ut9,
La,O,/HZ HE-HZ 5.0 wt% -
Ck-HZ
SlO.,'HZ
Ac:d:ty
(mmo1e.g c a t . ) strong s i t e s
G.;14
0.085
0.197
0.216
0.060
0.225 0 . 1 2 3
-
weak sites i4 0 0- 6 0 0*' C )
0.355
0.406
0.746
0.459
0.504
0.779
0.486
-
Hodi f . met hod
-
imprg.
imprg.
imprg.
imprg.
ionexc.
ionEXC.
vapor dep.
58.0
29.9
66.1 63.4 20.7 2 0 . 72 0 . 1
78.7 12.5
(70-400"Cj
P h O H conv.(moli)
Prod. SEl.(mol%) ani so l e cresols DI(pl h i Cresol ccomp. o w . (%) m-c re s o l P-cresol 0-c reso 1
55.2 -E3.1 18.5 43.5 15.9 40.6
59.8
40.5
54.5
53.1
5.2 1.3 63.7 69.0 67.0 15.8 21.7 5 . 817.0 2 1.11
70.1 20.1
24.5 17.3
19.3 24.5
36.9 14.6
58.2
56.:
57.4
5.8
u 37.8 15.4 46.8
48.5
40.1 14.6 45.3
u 39.1 14.8 46.
17.9 27.7 54.4
(a) The molar r a t i o of SiOa/A1.03 i s 150 !b) 2.4- and 2,5-dimethylphenol
Reaction c o n d i t i o n s are described i n TABLE 1
As seen in TABLE 3 , vapor deposition of Si(OCzH,) reduced both the activity of the catalyst and the fraction of m-cresol. It was believed that this material might react with silanol hydroxyls on the external surface to form a surface layer without active sites (ref. 18). As a result, the covering and deactivation of the external surface reduced the isomerization of products formed within the pore structure without significantly blocking the diffusion of molecules in and out the catalyst. The external surface of ZSM-5 could be partially covered by impregnation of MgO to a certain degree as reported earlier (ref. 9). Modification of phosphoric acid was also shown to convert strong acid sites into weak acid sites without changing the overall
229
acid-base properties(ref. 19). All those treatments reduced the isomerization of o-cresol to m-cresol on the external acid sites, and slighly enhanced the formation of p-cresol. However, the zeolite modified by Bz03 exhibited less modification effect than that modified by MgO. This might be attributed to the less amount of Bz03 used and the smaller ionic radius of boron. La203/HZ, Mg-HZ and Ca-HZ catalysts showed similar activity to the parent zeolite, regardless of the S/W ratio. This was probably due to the creation of new active sites by hydrolysis of modified elements under H,O-enriched environment (ref. 20). Further experiments are needed to verigy this point. We believe that strong acid sites on the external surface played a major role in m-cresol formation. Therefore it could be concluded that the alkylation of phenol with methanol over ZSM-5 zeolite proceeded via fast 0-methylation t o anisole and C-alkylation to p-/o-cresol, followed by isomerization to m-cresol on strong acid sites (mostly on the external surface), towards thermodynamic equilibrium.
REFERENCES 1. “Toluene, the Xylenes and Their Industry Derivatives” Chemical
Engineering Monograph 15, 1982. p. 209 2. S. Balsama, P. Beltrame, P.L. Beltrame, P. Carniti, L. Forni and G. Zuretti, Applied Catalysis 13, 161(1984) 3. P. Beltrame, P.L. Beltrame, P. Carniti, A. Castelli and L. Forni, Applied Catalysis 29, 327(1987) 4. K. Tanabe, Catalysis by Acids and Bases, Elsevier, Amsterdam, 1985, pp. 1-13 5. S. Namba, T. Yashima, Y. Itaba and N. Hara, Catalysis by Zeolites, Elsevier, Amsterdam, 1980, pp.105-111 6 . F. Nozaki and I. Kimura, Bull. Chem. SOC. Japan 50, 614(1977). 7. R.J. Argamer, U.S. Patent 3702886 (1972). 8. R.L. Wedlinger, U.S. Patent 3308069 (1967). 9. P.Y. Chen, M.C. Chen, H.Y. Chu, N.S. Chang and T.K. Chuang, Proc. Int. Zeolite Conf., 7th, Tokyo (1986), pp. 739-746 10.W.F. Hillebrand, C.E.F. Lundell, H.H. Bright and J.H. Hoffman, Applied Inorganic Analysis, John Wiley and Sons, New York, 1953 ll.K.J. Chao, B.H. Chiou, C.C. Chu and S.Y. Jeng, Zeolites 4, 2 (1984) 12.C.D. Chang and A.J. Silvestri, J. Catal. 47, 249(1977) 13.J.H.S. Green, Chemistry and Industry Sep. 1 , 1575(1962) 14.S.V. Kannan and C.N. Pillai, Indian J. Chem. 8, 1144(1970) 15.S. Oae and R. Kiritani, Bull. Chem. SOC. Japan 39, 611(1966) 16.W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein and S.A. Butter, J. Catal. 67, 159(1981) 17.N. Takamiya, T. Kondo and S. Murai, Weseda University Report, 21 (1975)
230
18. M. Niwa, S. Morimoto, S. Kato, T. Hattori and Y. Murakami, J.
Chem. SOC. Faraday Trans. 1. 80, 3135(1984) 19. J.A. Lecher and G. Pumplmayr. Applied Catalysis 25, 215(1986) 20. C.J. Plank. Proc. Intern. Congr. Catal., 3rd Amsterdam, 1964, p. 727
H.G. Karge, J. Weitkamp (Editors), Zeolites 0s Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A NEW CATALYST F(x1 MIEX SYNTHESIS --- PALLADIUM
(3N
ZS-5 ZElOLITES
P.Y. CHEN, S.J. CHU, N.S. CHMG, T.K. CHUANG and L.Y. UBN Union Chenical Laboratories, I'IRI 321, Kumg Fu Road, Sec. 2, Hsinchu, Taiwan, R. 0.C.
AEsrRAm
The one-step vapour phase synthesis of methyl isobutyl ketone (MIBK) was extensively studied on supported bifunctional W catalysts. Effects of support acidity and W metal dispersion on both acetone conversion and MIBK selectivity were examined. It was found that ZSM-5 dified by exchange with alkali metal cation, especially C s ions, would enhance the selectivity of MIEX. 'lhe results indicated that MIEX selectivity as high as 82% with the conversion of acetone about 42% at 250°C could be obtained.
INTRODUCTION Methyl isobutyl ketone (MIBK) is commercially produced by two different processes. The conventional three-stage process includes (1) aldol condensation of acetone to form diacetone alcohol which is catalysed by a base, ( 2 ) dehydration of the alcohol to r n c S i t V 1 oxide with an acid catalyst, and ( 3 ) hydrogenation of the unsaturated ketone to MIBK with nickel or copper chromite catalyst(1-3). Recently, i t was reported that the selective one-step synthesis of MIBK was performed on palladium-exchanged resins (4-5), which not only cannot tolerate a temperature higher than 160°C but which also induce polymer formation easily. It is therefore not suitable for practical application. In this study Pd/ZSM-5 catalysts modified by alkali metal were employed for MIBK production. EXPERIMENT 1. Catalyst preperation Zeolite synthesis The ZSM-5 zeolites with different SiOz to AlaO3 ratios were prepared by adding aqueous aluminum sulfate to water glass and tetrapropylammonium bromide solutions. After agitating for three hours, the solution was allowed to crystallize, at 160"C, according to hydrothermal process. The products were then filtered, washed
232
and dried before calcination in a stream of air at 550°C for 16 hours. The crystallinity of ZSM-5 was checked by X-ray diffraction. ZSM-5 modification HZSM-5 zeolite was ion-exchange with 0.1 M alkali nitrate solution ( o r 0.01 M Pd(NH4)C12 solution) many times until the maximum exchange capacity was reached. Subsequently,the material was filtered, washed and dried. Supported Pd catalyst The ZSM-5 zeolite was first soaked in Pd(NH4)C12 solution overnight, then dried, calcined and reduced at 400°C for eight hours in hydrogen atmosphere. 2. Catalyst characterization
The chemical analysis of the prepared zeolites was carried out according to the procedure of Hillebrand et al. (6). The acidity was determined by the temperature-programmed desorption (T.P.D.) of NH3 as shown previously (7-8), and the Pd metal dispersion was measured by chemisorption of CO (9-10) with a Micromeritics pulse chemisorb 2700. 3. Catalyst activity evaluation The reaction was carried out in a fixed-bed reactor at 150-300 "C under one atmosphere pressure. The 3/8" cylindrical reactor contained 1.7 g of 12-20 mesh catalyst. The reactor effluents were cooled and collected in a cold trap. The collected liquid products were analyzed by a HP-5840 gas chromatograph with a 10% carbowax 20 M f 2% Bentone 34 on chromosorb W HP/ 2% KOH column.
RESULTS AND DISCUSSION I. Influence of various catalyst supports The conversion of acetone and the selectivity of each component over various supported Pd catalysts are shown in Table 1. The activity shows the sequence Pd/ r-Al203 > > Pd/NaNZ3!1-5,Pd/NaX > Pd/HZSM-5, Pd/Nap> Pd/USY, Pd/Si02, Pd/TiO2 which indicates that the activity was highly dependent on the acid strength of the support. In order to obtain a good acetone converion , a support with proper acid strength such as y-A1203,NaHZSM-5 and NaX should
'
233
be chosen. The selectivity for MIBK varied between 21.1% and 71.1%. No significant relationship could be found between acid strength and MIBK selectivity. The major by-products were diisobutyl ketone (DIBK), light hydrocarbons and isopropyl alcohol. The highest MIBK selectivity was obtained on Pd/NaHZRr.l-5 and Pd/ HZSM-5, which might be due to the shape effect of ZSM-5 channels (5.2 x 5.6 i).The large molecular by-product DIBK, which was formed by trimeric condensation of acetone, would be minimized due to the narrowness of the ZSM-5 channel. Table 1. Influence of various catalyst supports . -~ Product selectivity, wle % Acid Conv. of Light Catalyst strength of acetone IpA MIBK Mesityl DIBK Oxide hydrocarbons supports mole % 71.6 1% Pd/USY 14.4 7.3 21.1 - 64.7 27.6 1.4 1% Pd/HZSM-5 33.9 22.6 - 27.1 2.3 1% Pd/Nap 69.6 41.0 - 71.3 t 1% Pd/NaHZSM-5 28.7 a 3.8 39.3 1% Pd/NaX 36.9 17.0 39.3 a, m 30.6 1.1 31.6 77.9 3.2 33.7 d 1% Pd/ T-Al207 a, - 15.0 19.1 0 k 39.9 15.9 49.9 1% Pd/ 7 -Al2O3* a, 46.2 a 1% Pd/SiO2 15.1 24.4 29.4 1% Pd/TiO:! t 13.2 8.9 46.6 2 . 8 41.7 * Reaction temperature: 150°C (others were 25OOC); WHSV: 2 hr-l H2/Acetone=l/l; time on stream: 4 hours .F
-
-
11. Catalytic properties of ZSM-5-supported Pd catalyst for MIBK
synthesis (1) Influence of the Al/Si ratio of ZSM-5 supports As shown in Fig. 1,the activity of 1% Pd/NaHZSM-5 was raised from around 646 to 42% by increasing the Al/Sl ratio of NaHZSM-5. The conversion of acetone seemed to be proportional to acidity of NaHZSM-5 while there were enough metal active sites on the catalyst. Generally, the acidity of ZSM-5 increased with increasing Al/Sl ratio. The selectivity for MIBK also showed the same tendency as that observed for the conversion of acetone, whereas the isopropyl alcohol behaved in an opposite manner. It decreased drastically when the Al/Si ratio was higher than 4. This might be due to the fact that in the one-step process condensation, dehydration and hydrogenation proceed as consecutive reactions. Since the
234
acid sites were responsible for the former two reactions, IPA from acetone hydrogenation predominated on the catalyst with fewer acid sites.
Figure I Effect of AI/Si ratio on activity
of I%Pd/NaHZSM -5 Conditions : Calcination temp.: 400'C
i
Calcination time :8hr.i reaciion
temp.:250'CjWHSV:2h~';timeonstream:4hr.; H2/ acetone mole r a t i o : I ; pressure : one atmosphere.
(2) Influence of the amount of Pd loading The properties of supported Pd catalysts are shown in Table 2, 0 where is clear that the average Pd particle size raised from 14 A to as large as 110 f while the Pd loading was increased from 0.1 wt% to 1.0 wt%. However, no appreciable change in the acidity of the support was observed. It is therefore suggested that Pd particles probably were formed on the external surface of the ZSM-5 and/or adjacent to the acid sites in the channels of ZSM-5. The acid sites of ZSM-5 still remained active for condensation of the acetone and dehydration of the diacetone alcohol to the intermediate product of mesityl oxide.
235
Table 2. Effect of Pd content on properties of PdINaZSM-5 catalysts
O.l%ki/Na€ESM-5* 0.2%M/NaHZSM-5 0.546W/Na€ESM-5 0.74&w/NaHZSM-5 1.04&W/NaHZSM-5
0.12 0.21 0.45 0.62 0.94
1.002 0.703 0.828 0.933 0.705 .
*
W particles ave. size metal dis. A %
Amount of Acidity(mnol/g cat.) W laxling weak Strong wt. % (70-400°C) (400-60O0C)
Catalyst
__
0.256 0.225 0.204 0.284 0.184
14 25 49
72.97 39.88 20.53 12.21 9.55
86
106
-
SiO2 to A1203 ratio is 30. The conver-
The effect of Pd metal loading is shown in Fig. 2 .
sion of acetone increased from 33% to 46% initially. However, it decreased to around 41% by further increasing the amount of Pd loading. In addition to conversion, the yield of light hydrocarbons decreased with further increase in Pd metal loading. It is very clear from Table 2 that the Pd metal dispersion decreased rapidly regardless of increasing the amount of Pd loading. The reason for the low conversion at higher Pd loading might be that the large Pd particle was formed on the external surface of ZSM-5, which inhibited the cracking reaction. Furthermore, the fact that mesityl oxide was not observed until the amount of Pd loading was higher than 0.3 wt% suggests that the lower conversion at the beginning might be due to a scarcity of active metal sites so that mesityl oxide could not undergo further reactions.
b*
;' I
Light hydrocarbons
-.\ W v,
AMOUNT OF LOADING , wt % Figure 2 The effect of palladium loading on MlBK synthesis (
The reaction ccsditions are same as Fig. 1
)
236
( 3 ) Influence of reaction conditions
Fig. 3 illustrates the effect of temperature in the range 150300°C. The conversion and selectivity increased gradually with temperature below 250°C; above 25OoC both dropped rapidly. This might be because the rate of coking was enhanced above 2 5 0 ° C . The retardation of acetone conversion might be due to a coverage of the sites of the catalyst by coke.
50 -
100
h
o\o A Q U -
E"
v
-60
L
2c-
> z
/
10-
0 100
k d I
150
b/ M.I.B.K.
/'
0
E
> I-
'Light hydrocar<$'
2 z W l E n
0
3c
-/h
'I\
-JO
-20
I
200
250
3CO
f 0 W -J W
"'
0
350
Fig. 4 shows the effect of acetone space velocity on its conversion at 2 5 0 ° C . Although the conversion decreased with increasin:: space velocity, the change in MIBK selectivity was less pronounced at WHSV higher than 2 . 0 . Below this value, MIBK increased with increasing WHSV. In contrast, the selectivity for DIBK decreased when the WHSV was increased to 2; above this value, DIBK was no longer detected. This finding indicated that acetone would condensate to trimeric species when the contact time was long enough. The optimum range of WHSV is above 2.
231
1 1 1 . Influence of various modified ZSM-5 supports
In order to obtain a good yield of MIBK, the ZSM-5 support was modified by ion exchange with various al!tali metal ions and pallaIt is clear from dium ions. The results are shown in Table 3. Table 4 that all modified Pd/ZSM-5 catalysts (except Pd/HZSM-5) exhibited significantly higher activity, which might be due to a partial replacement of strong acid sites by alkali metal ions. Moreover, although the weak acid sites of Pd/CsHZSM-5 were reduced to almost half of the other catalyst, the high conversion over Pd/ CsHZSM-5 as well as its high MIBK selectivity suggest that basicity might promote the reaction. Further experiments are needed to verify this point.
238
Table 3.
Catalytic properties of modified ZSM-5 supported Pd catalysts
-
&changed CATALYST
cations
w H Z S M 5 1%M/HzsM-5 l%M/NaHZSM-5 1W/KHZsM-5 1%Ri/CsHZSM-5
w Na
K CS
Aver. conv. of selectivity W 5ize acetone MIBK DIBK Others (mole $1 (n:mle/g cat.) ( A ) (mole %) Acidity
weak strong
0.558 0.360 0.688 0.352 0.705 0.184 0.719 0.069 0.343 0
256 206 106 136 250
22.6 27.6 41.0 45.3 41.9
82.3 5.2 65.5 1.5 71.4 6".7 1.4 82.4 2.2
12.6 33.3 28.8 33.4 15.2
CONCLUSIONS The experimental data indicate that the one-step vapor phase synthesis of MIBK from acetone and hydrogen proceeds via a threereaction mechanism, namely, condensation, dehydration, hydrogenation. The proper catalyst may be described as follows: 1. The catalyst should have a weak acid strength and enough active sites of metallic Pd to perform as a bifunctional catalyst. 2. ZSM-5 zeolite, after modification of its acid strength by exchange with alkali metal ions, especially C s ions, could be a good support for MIBK synthesis. 3. The behavior of ZSM-5 supported palladium on thermal stability and selectivity for MIBK is superior to that of a Pd-resin catalyst. REFERENCE 1. S . Kudo, "Formation of Higher Molecular Weight Ketones from Acetone or Isopropanol", J. Chem. SOC. Japan, Ind. Chem. Sect., 58, 1955, pp 785-7 2. JP 46-2009 3. French Patent 1,535 4. "New Solvent Process", Ind. Research, July 1968,PP. 25-6 5. German Patent 1,936,203 6. W.F. Hillebrand, C.E.F. Lundell, H.H. Bright and J.I. Hoffman in "Applied Inorganic Analysis", John Wiley and Sons, New York, 1953. 7. J.C. Post and J.H.C. Van Hoff, Zeolites, 4, 1984, p . 9 8. K.J. Chao, B.H. Chu, C.C. Chu and S.Y. Jeng, Zeolites, 4, 1984. '?. 2
239
9. R . K .
Nand, R.K.
D i t c h a , S . S . Wong, J . B .
Iohen, R.L.
Burwell
a n d J . B . B u t t , J. C a t a l . , 70, 1 9 8 1 , p . 2 9 8 10. R.J. P a r r a n t o , AIChE Symposium S e r i e s , No. 143, V o l . 7 0 , p . 9.
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H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ACETYLENE HYDRATION ON ZEOLITE CATALYSTS: AN I . R .
SPECTROSCOPIC
STUDY OF THE SURFACE S P E C I E S
GY.
ONYESTYAK, J . P A P P J r . a n d D . KALLO
C e n t r a l R e s e a r c h I n s t i t u t e f o r C h e m i s t r y o f t h e H u n g a r i a n Academy o f S c i e n c e s , P.0.Box 1 7 , 1 5 2 5 B u d a p e s t ( H u n g a r y )
ABSTRACT On l a t e t r a n s i t i o n m e t a l - f o r m s o f z e o l i t e s t h e f o r m a t i o n o f adsorbed a c e t a l d e h y d e f r o m a c e t y l e n e and z e o l i t i c w a t e r has been d e t e c t e d by i . r . s p e c t r o s c o p y f r o m 25°C on ( s t e a d y s t a t e c a t a l y t i c r e a c t i o n p r o c e e d s a r o u n d 18OOC). On Cd-X a n d Cd-A t h e c r o t o n i c condensation o f acetaldehyde takes place instantaneously r e s u l t i n q i n t h e f o r m a t i o n o f c o k e r e s i d u e s . A d s o r p t i o n of T-bonded a c e t y l e n e i n c r e a s e s w i t h t h e d e h y d r a t i o n of z e o l i t e samples. F o r zeolites c o n t a i n i n g r e d u c i b l e c a t i o n s a s Cu- a n d A g - c l i n o p t i l o l i t e s a c e t y l i d e formation i s observed. N e i t h e r s i d e r e a c t i o n s n o r a c e t y l i d e f o r m a t i o n a r e d e t e c t a b l e on Cd- a n d Z n - c l i n o p t i l o l i t e s , w h i c h a r e stable catalysts o f high s e l e c t i v i t y f o r acetylene hydration.
INTRODUCTION L a t e t r a n s i t i o n m e t a l i o n s i n aqueous s o l u t i o n o r i n t h e f o r m of o x i d e s ,
phosphates,
tungstates,
acetylene hydration ( r e f .
1).
chromates e t c . a r e a c t i v e f o r
H i g h e r a c t i v i t y and b e t t e r s e l e c -
t i v i t y w e r e a t t a i n e d when Cu2+, Ag', introduced i n X-zeolite
(ref. 2).
Cd",
Zn2+,
Hg2+ i o n s wer
The f a s t d e a c t i v a t i o n o f t h e
c a t a l y s t s was s l o w e d down t o some e x t e n t b y a p p l y i n g m i l d e r r e a c t i o n c o n d i t i o n s a n d b y p r e t r e a t m e n t w i t h ammonia ( r e f . 3 ) .
A
l a r g e v a r i e t y o f l a t e t r a n s i t i o n metal forms o f d i f f e r e n t zeol t e s was t e s t e d i n o r d e r t o e n l i g h t e n t h e s o u r c e ( s ) o f d e a c t i v a t i o n and t o a r r i v e a t s t a b l e and s e l e c t i v e c a t a l y s t ( s ) . C o n c e r n i n g t h e s t a b i l i t y t h r e e groups o f l a t e t r a n s i t i o n metal-forms o f z e o l i t e c a t a l y s t s o f s a t i s f a c t o r y a c t i v i t y can be d i s t i n g u i s h e d ( r e f . 4 ) : ( i ) c a t a l y s t s as Cd-forms o f X - o r A - z e o l i t e s d e a c t i v a t e due t o t h e f o r m a t i o n o f carbonaceous depos t s w i t h a C / H r a t i o o f 1/0.6;
( i i ) Cu-,
Ag-,
Hg-forms o f zeol t e s l o s e t h e i r a c t i v i t y
because t h e a c t i v e c a t i o n s a r e reduced under r e a c t i o n c o n d i t i o n s ; ( i i i ) c l i n o p t i l o l i t e d e r i v a t i v e s conta n i n g i r r e d u c i b l e t r a n s i t i o n metal c a t i o n s , e.g.
t h e Cd- o r Z n - f o r m s
( d e n o t e d as
242
Cd-CLI o r Zn-CLI),
p r e s e r v e t h e a c t i v i t y , and no i r r e v e r s i b l e
changes a r e o b s e r v a b l e f o r s e v e r a l hundred h o u r s . I d e n t i c a l r e a c t i o n m e c h a n i s m s w e r e f o u n d b o t h f o r Cd-X s t a b i l i z e d b y ammonia p r e a d s o r p t i o n ( r e f . 5 ) a n d f o r Cd-CLI ( r e f . 6). Accordingly,
weakly adsorbed a c e t y l e n e and s t r o n g l y
adsorbed water r e a c t i n a r a t e - d e t e r m i n i n g s u r f a c e r e a c t i o n , and the e q u i l i b r i u m desorption o f acetaldehyde exerts a product inhibition. The a b o v e o b s e r v a t i o n o f t h e a c t i v i t y c h a n g e s a s w e l l a s o f t h e c a t a l y t i c c o n v e r s i o n s h o u l d , however, be c o n f i r m e d and some d e t a i l s e l u c i d a t e d . The a i m o f t h e p r e s e n t w o r k i s t o d e t e c t surface species i n connection w i t h the c a t a l y t i c behaviour,
i.e.
t o c l e a r up t h e mode o f a d s o r p t i o n o f t h e c o m p o n e n t s i n v o l v e d , t o s h e d l i g h t on t h e s u r f a c e i n t e r a c t i o n s a n d t o a t t e m p t t o f o l l o w t h e h y d r a t i o n r e a c t i o n and t h e s i d e r e a c t i o n ( s 1 on t h e surface. Typical representatives o f t h e t h r e e groups o f c a t a l y s t s were chosen t o p e r m i t s u i t a b l e comparisons: a n d Cd-A,
f o r g r o u p ( i i ) Cu- a n d A g - C L I ,
f o r g r o u p ( i ) Cd-X
f o r g r o u p ( i i i ) Cd- a n d
Zn-CLI.
EXPERIMENTAL Materials X-zeolite
(Strem Chemicals, USA),
A-zeolite
(Serva, FRG) and
c l i n o p t i l o l i t e as c o n s t i t u e n t o f a r h y o l i t e t u f f f r o m T o k a j H i l l s ( H u n g a r y ) w e r e u s e d . The c o n c e n t r a t i o n o f c l i n o p t i l o l i t e i n t h e r o c k i s 60%; t h e r e s t i s m a i n l y v o l c a n i c g l a s s , c r i s t o b a l i t e a n d some p e r c e n t f e l d s p a r .
quartz,
C l i n o p t i l o l i t e was
p r e v i o u s l y t r a n s f e r r e d i n t o t h e pure sodium-form. The t r a n s i t i o n m e t a l f o r m s w e r e p r e p a r e d f r o m t h e s e z e o l i t e s b y i o n - e x c h a n g e w i t h 0.1 M s o l u t i o n o f t h e n i t r a t e s a t 70°C f o r 3 x 8 h o u r s . The i o n - e x c h a n g e f o r s i l v e r was c a r r i e d o u t i n d a r k n e s s a t r o o m t e m p e r a t u r e . The d e g r e e o f i o n - e x c h a n g e was b e t w e e n 3 0 and 60 p e r c e n t . The f r a c t i o n s o f l e s s t h a n 5 y m i n d i a m e t e r w e r e d r i e d a n d p r e s s e d i n t o w a f e r s , w i t h o u t b i n d e r o f 5 - 8 mg/cm2 f o r i . r . investigations. A c e t y l e n e was a h i g h p u r i t y D i s s o u s gas ( O D V , H u n g a r y ) . A c e t o n e was removed b y t r a p p i n g a t
-
73°C.
A c e t a l d e h y d e was a
Merck ( F R G ) p r o d u c t " z u r S y n t h e s e " grade. C r o t o n i c a l d e h y d e was p r e p a r e d i n o u r l a b o r a t o r y .
243
Methods A P e r k i n - E l m e r 577 d o u b l e - g r a t i n g
i .r. s p e c t r o m e t e r was u s e d .
The w a f e r s were i n s e r t e d i n t o a c e l l p r o v i d e d w i t h h e a t i n g f o r p r e t r e a t m e n t . The c e l l was c o n n e c t e d t o a vacuum s y s t e m a n d a gas d o s a g e a c c e s s o r y . 1.r.
s p e c t r a were r e c o r d e d a f t e r a p r e t r e a t m e n t a t
f o r 1 h o u r u s u a l l y a t 180"C,
i.e.
lo-'
Pa
a t t h e average temperature o f
hydration reaction (curves 1 i n f i g u r e s ) , a f t e r t h e adsorption o f a c e t y l e n e a t 1.3,
2.5 o r 13 kPa, o r a f t e r t h e a d s o r p t i o n o f
a c e t a l d e h y d e a t 0.13 evacuations a t
lo-'
kPa f o r 10 m i n u t e s a t 25°C Pa f o r 2 0 m i n u t e s a t 25°C
h o u r a t 100, 200 a n d 350°C ( c u r v e s 4,
5, 6,
(curves 2),
( c u r v e s 31,
after
for 1
respectively).
The
d i s c u s s e d bands o f t h e s p e c t r a a r e m a r k e d b y a r r o w s . RESULTS A N D D I S C U S S I O N C a t a l y s t s d e a c t i v a t i n g because o f coke f o r m a t i o n A d s o r p t i o n o f a c e t y l e n e on Cd-X ( c u r v e 2 , F i g .
1) can be
d e t e c t e d b y t h e a p p e a r a n c e o f t h e b a n d s a t 3170 c m - l a s s i g n e d t o t h e a s y m m e t r i c k C - H s t r e t c h i n g v i b r a t i o n a n d a t 1940 c m - l a s s i g n e d t o t h e C=C
s t r e t c h i n g v i b r a t i o n . L E C H Y a s y m i s 3287 c m - l
i n the
gas p h a s e ( r e f . 8 ) a n d i t h a s been f o u n d t o d e c r e a s e a f t e r a d s o r p t i o n on Cu-Y t o 3170 ( r e f . 7 ) .
L)CrC c a n
t h e R a m a n - a c t i v e f r e q u e n c y a t 1974 cm-
be c o r r e l a t e d w i t h
( r e f . 8 ) The J C - s y s t e m
o f a c e t y l e n e o v e r l a p s w i t h t h e c a t i o n o r b i t a l s and a " s i d e - o n " a d s o r p t i o n t a k e s p l a c e r e s u l t i n g i n an i . r . a c t i v e c o n f i g u r a t i o n o f l o w e r e d s y m m e t r y . F o r t h e same r e a s o n t h e v e r y weak b a n d o f s y m m e t r i c r C - H s t r e t c h i n g v i b r a t i o n a r o u n d 3250 c m - l c a n b e recognized,
too.
A c e t y l e n e r e a c t s e v e n a t room t e m p e r a t u r e w i t h t h e z e o l i t i c w a t e r bound t o Cd2+ i o n s , a n d a d s o r b e d a c e t a l d e h y d e i s f o r m e d ,
as
i t i s i n d i c a t e d b y t h e c a r b o n y l a b s o r p t i o n band a t 1 6 9 0 cm-'.
S i m u l t a n e o u s l y t h e r e a p p e a r s a n o t h e r b a n d a t 1635 cm-'
assigned
t o a d s o r b e d c r o t o n i c a l d e h y d e . A t 1410 a n d 1360 c m - l t h e a b s o r p t i o n bands o f a s y m m e t r i c a n d s y m m e t r i c C - H d e f o r m a t i o n v i b r a t i o n s i n t h e methyl group o f aldehydes a r e seen ( r e f .
9 ) . The
a s s i g n a t i o n s were c o n f i r m e d by s e p a r a t e measurements c a r r i e d o u t w i t h a c e t a l d e h y d e a n d c r o t o n i c a l d e h y d e . Qc0 f o r a d s o r b e d a c e t a l d e h y d e i s l o w e r b y 3 0 cm-' l o w e r b y 45 cm-'
a n d f o r a d s o r b e d c r o t o n i c aldehyde
t h a n i n t h e f r e e s t a t e ( r e f . 91, w h e r e a s t h e C-H
d e f o r m a t i o n f r e q u e n c i e s a r e t h e same i n g a s e o u s a n d a d s o r b e d p h a s e s . The a l d e h y d e s a r e bound, most probably, by t h e i r carbonyl groups.
244
When t h e s a m p l e i s e v a c u a t e d a t 25'C, disappear,
t h e a c e t y l e n e bands
w h i l e t h e i n t e n s i t y o f b o t h a l d e h y d e bands i n c r e a s e s
I . 4000
3500
2000 1800 Wmvonurnber , crn"
3000
*--
1600
1400
F i g , 1. Cd-X a f t e r e v a c u a t i o n a t 180°C ( 1 1 , a f t e r a c e t y l e n e a d s o r p t i o n a t 2.5 kPa, ( 2 ) f o l l o w e d b y e v a c u a t i o n a t 25'C ( 3 ) , 1OO'C ( 4 ) , 200OC ( 5 ) , 35OCC ( 6 ) . ( c u r v e 3 ) . A f t e r e v a c u a t i o n a t 100cC t h e t r a n s f o r m a t i o n o f
adsorbed
a c e t a l d e h y d e t o c r o t o n i c aldehyde can be observed ( c u r v e 4 ) . t r e a t m e n t a t 200'C
The
r e s u l t s i n t h e disappearance o f acetaldehyde
( c u r v e 5 ) a n d a t 350'C bands emerge a t 1585,
i n s t e a d o f t h e c r o t o n i c a l d e h y d e b a n d new 1480,
1450 c m - l ,
c a r b o n a c e o u s d e p o s i t s ( r e f s . 9,
w h i c h c a n b e a t t r i b u t e d t.o
10).
The i n v e s t i g a t i o n o f t h e b e h a v i o u r o f a d s o r b e d a c e t a l d e h y d e supports these observations. I f Cd-X i s p r e t r e a t e d a t 5OOOC ( c u r v e 1 i n F i g . 2 ) ,
considerably
more a c e t y l e n e i s a d s o r b e d a t 25OC ( c u r v e 2 ) a n d r e m a i n s o n t h e s a m p l e a f t e r e v a c u a t i o n ( c u r v e 3 ) . The a c c e s s i b i l i t y o f Cd2+ i o n s i s e n h a n c e d b y t h e more e x t e n s i v e d e h y d r a t i o n . Owing t o t h e lower water content l e s s adsorbed acetaldehyde i s formed than before (cf.
t h e c o r r e s p o n d i n g b a n d i n t e n s i t i e s a t 1690 c m - ' ) .
In
s p i t e o f t h e l a r g e r amount o f a d s o r b e d a c e t y l e n e i t s s i d e r e a c t i o n p r o d u c t s , e.g.
o l i g o m e r s o r benzene c a n n o t be o b s e r v e d .
On c o n t a c t i n g a c e t y l e n e w i t h Cd-A t h e CEC
stretching vibration
245
band of "side-on" adsorbed acetylene at 1 9 4 0 cm-l is strong, EC-H stretching vibrations are, however, not clearly detectable. The
Fig. 2. Cd-X after evacuation at 500'C ( 1 1 , after acetylene adsorption at 1.3 kPa ( 2 ) , followed by evacuation at 25'C (3).
I
.
4MK)
.
.
.
,
.
3500
.
-
- ' . 3000 2000 -1 Wavrnumbr , em
1000
1600
1400
Fig. 3 . Cd-A after evacuation at 18OOC ( 1 1 , after acetylene adsorption at 13 kPa ( 2 1 , followed by evacuation a t 25°C ( 3 ) . bands of adsorbed acetaldehyde at 1685 cm-' and of adsorbed crotonic aldehyde at 1630 cm-' are very weak, while the dCH
246
bands a t 1450,
1360 cm-l
e v a c u a t i o n a t 25'C
a r e v e r y i n t e n s e ( c u r v e 2,
Fig. 3). After
( c u r v e 3 ) t h e a m o u n t o f a d s o r b e d c r o t o n i c aldehyde 1
1
\ 4000
2000 Wavonumbor, Em-'
3500
3000
1000
MOO
I600
F i g . 4. C u - C L I a f t e r e v a c u a t i o n a t 180'C ( 1 1 , a f t e r a c e t y l e n e a d s o r p t i o n a t 13 kPa ( 2 1 , f o l l o w e d b y e v a c u a t i o n a t 25°C ( 3 ) , 100°C ( 4 1 , 200°C ( 5 ) . i n c r e a s e d somewhat,
JCH bands
while the
diminish,
and t h e band
a r o u n d 1580 cm-l a s s i g n e d t o c a r b o n a c e o u s d e p o s i t s ( r e f . appears;
10)
t h e s a m p l e becomes b l a c k .
Catalysts containing reducible active cations When a c e t y l e n e i s a d s o r b e d o n C u - C L I a t and a s t r o n g asymmetric
=C-H
25OC,
a weak s y m m e t r i c
s t r e t c h i n g v i b r a t i o n bands a r e
d e t e c t e d a t 3305 a n d 3180 c m - l ,
respective1y.R-bonding
of
a c e t y l e n e i s r e f l e c t e d b y t h e band a t 1950 c m - l ( c u r v e 2,
Fig. 4).
The f o r m a t i o n o f a d s o r b e d a c e t a l d e h y d e i s i n d i c a t e d b y t h e band a t 1705 cm-l 2 9 2 0 cm-'
dco
and by t h e OC-H s t r e t c h i n g v i b r a t i o n band a t
(ref. 9).
On p u m p i n g a t 25OC ( c u r v e 3 ) t h e a c e t a l d e h y d e b a n d i n t e n s i t y increases, frequencies,
C-H bands o f a c e t y l e n e a r e s h i f t e d t o w a r d l o w e r
t h e band a t 1950 cm-l
o b s e r v a b l e a t 1810 cm-'.
d i s a p p e a r s a n d a new b a n d i s
Instead o f "side-on"
bound a c e t y l e n e ,
241
a c e t y l i d e appears c h a r a c t e r i z e d by a
Uc3
a t 1810 cm-l a n d b y
l o w e r rl, values ( r e f . 10). =C-H A f t e r e v a c u a t i o n a t 100°C ( c u r v e 4 ) t h e i n t e n s i t i e s o f
-.
'-I
4000
3500
3000 2000 1800 -1 Wavenumber, cm
1600
__ 1400
F i g . 5. A g - C L I a f t e r e v a c u a t i o n a t 180'C ( l ) , a f t e r a c e t y l e n e a d s o r p t i o n a t 13 kPa ( 2 ) , f o l l o w e d b y e v a c u a t i o n a t 25°C (31, 100°C ( 4 ) , 200°C ( 5 ) . a c e t y l i d e b a n d s d e c r e a s e a n d a t 200°C
( c u r v e 5) t h e s e bands
d i s a p p e a r , w h i l e t h e c o n c e n t r a t i o n o f adsorbed a c e t a l d e h y d e does n o t change.
I t d e s o r b s c o m p l e t e l y a t 350°C o n l y .
On A g - C L I t h e a d s o r p t i o n o f a c e t y l e n e a t 25°C r e s u l t s i n t h e
appearance o f one symmetric and two a s y m m e t r i c
rC-H stretching
v i b r a t i o n bands a t 3320 and a t 3260,
respectively
(ref.
7),
3 1 9 0 cm-',
a n d i n t h e f o r m a t i o n o f a d s o r b e d a c e t a l d e h y d e ( c u r v e 2,
F i g . 5 ) . A f t e r e v a c u a t i o n a t 25°C t h e b a n d s a t 3 3 2 0 ,
3260 cm-l
d i s a p p e a r . A t 1OO'C t h e a d s o r b e d a c e t a l d e h y d e f o r m a t i o n i n c r e a s e s a n d t h e i n t e n s i t y o f t h e b a n d a t 3 1 9 0 cm-'
does n o t change
e s s e n t i a l l y . B o t h a c e t y l e n e and a c e t a l d e h y d e d e s o r b above 200°C. No "side-on" adsorbed acetylene b u t some a c e t y l i d e have been detected.
248
C a t a l y s t s of h i g h s t a b i l i t y O n C d - C L I p r e t r e a t e d i n vacuum a t 45OoC ( c u r v e 1 , F i g . 6 ) b o t h t h e a d s o r b e d a c e t y l e n e and t h e f o r m a t i o n of a d s o r b e d a c e t a l d e h y d e a r e o b s e r v e d a f t e r a d m i s s i o n o f a c e t y l e n e a t 25*C ( c u r v e 2 ) and a f t e r e v a c u a t i o n a t 25’C ( c u r v e 3 ) . The p r e t r e a t m e n t a t h i g h e r t e m p e r a t u r e (45OOC i n s t e a d of 180*C) r e s u l t s i n a more band a t 3190 cm-’ and i n t h e a p p e a r e n c e of i n t e n s e 9.C-H,asyrf a t 1930 cm a s a s h o u l d e r a s s i g n e d t o Tr-bonded a c e t y l e n e . QCEC The i n t e n s i t y of t h e O c 0 band a t 1705 cm-l a s s i g n e d t o a d s o r b e d a c e t a l d e h y d e i n c r e a s e s a f t e r e v a c u a t i o n s a t 25°C ( c u r v e 3 ) , a t 100°C ( c u r v e 4 ) and a t 200°C ( c u r v e 5 ) . A t 350’C a c e t a l d e h y d e i s c o m p l e t e l y d e s o r b e d , and n o r e s i d u e s a r e d e t e c t a b l e . A f t e r i d e n t i c a l t r e a t m e n t s i . r . s p e c t r a a r e most s i m i l a r o f Zn-CLI a s of C d - C L I .
. 4000
3500
3000
2000 1000 Wavanumber , cm-’
moo
. 1400
F i g . 6 . Cd-CLI a f t e r e v a c u a t i o n a t 450°C ( 1 1 , a f t e r a c e t y l e n e a d s o r p t i o n a t 1 . 3 k P a ( 2 ) , f o l l o w e d by e v a c u a t i o n s a t 25°C ( 3 ) , 100°C ( 4 ) , 200°C ( 5 ) .
249
CONCLUSIONS The r e a c t i o n o f a c e t y l e n e w i t h z e o l i t i c w a t e r bound t o t r a n s i t i o n metal ions r e s u l t s i n t h e formation o f adsorbed a c e t a l d e h y d e which can be observed a t room t e m p e r a t u r e .
Its
surface concentration increases w i t h increasing temperature u n t i l i t d e s o r b s n e a r 200°C.
On Cd-X a n d Cd-A t h e f o r m a t i o n o f a c e t -
a l d e h y d e i s i n e v i t a b l y accompanied by c r o t o n i c c o n d e n s a t i o n ,
the
p r o d u c t o f w h i c h c a n n o t b e d e s o r b e d a n d t r a n s f o r m s t o c o k e . No c r o t o n i c c o n d e n s a t i o n t a k e s p l a c e o n Cu-,
Ag-,
Cd-,
Zn-clino-
p t i l o l i t e s . The C O b o n d i n a d s o r b e d a c e t a l d e h y d e i s l e s s w e a k e n e d on c l i n o p t i l o l i t e d e r i v a t i v e s t h a n on Cd-X o r Cd-A.
T h e corresponding
f r e q u e n c i e s a r e 1705 a n d 1 6 9 0 cm-’, r e s p e c t i v e l y . Since the degree o f c a t a l y s t dehydration i n f l u e n c e s e s s e n t i a l l y the adsorption o f acetylene,
i n accordance w i t h t h e k i n e t i c s
5 1 , t h e a d s o r p t i o n o f a c e t y l e n e i s much w e a k e r t h a n t h e On Cd- o r Z n - z e o l i t e s a c e t y l i d e f o r m a t i o n c a n n o t b e d e t e c t e d , w h i l e o n Cu- a n d A g - z e o l i t e s i t h a s b e e n r e v e a l e d ( r e f s . 7, 1 0 ) . P r e s u m a b l y t h i s i s t h e f i r s t s t e p o f t h e (ref.
adsorption o f water.
reduction o f these cations found e a r l i e r ( r e f . 4). 1.r.
measurements s u p p o r t e d a l s o t h e o u t s t a n d i n g b e h a v i o u r
o f Cd- a n d Z n - c l i n o p t i l o l i t e ,
on w h i c h none o f t h e u n d e s i r e d
transformations takes place. REFERENCES
1
J.W. C o p p e n h o v e r a n d M.H. B i g e l o w , A c e t y l e n e a n d C a r b o n M o n o x i d e C h e m i s t r y , R e i n h o l d , New Y o r k , 1 9 4 9 ; 2 G. G u t and K. A u f d e r e g g e n , H e l v . Chim. A c t a 57 ( 1 9 7 4 ) 441-452. 3 E . D e t r e k o y , Gy. O n y e s t y a k a n d 0 . K a l l o , R e a c t . K i n e t . C a t a l . L e t t . 15 ( 1 9 8 0 ) 4 4 3 - 4 5 0 . 4 0 . K a l l o a n d Gy. O n y e s t y a k , i n B. D e l m o n a n d G.F. F r o m e n t ( E d i t o r s ) , C a t a l y s t D e a c t i v a t i o n , E l s e v i e r , Amsterdam, 1987, pp. 605-612. 5 Gy. O n y e s t y a k a n d D. K a l l o , A c t a C h i m . Hung. 1 2 4 ( 1 9 8 7 ) 4 7 - 5 5 . 6 D. K a l l o a n d Gy. O n y e s t y a k , t o b e p u b l i s h e d . 7 P . P i c h a t , J . P h y s . Chem. 79 ( 1 9 7 5 ) 2 1 2 7 - 2 1 2 9 . 8 N . T . Tam, R . P . Cooney a n d G . C u r t h o y s , J . Chem. S O C . F a r a d a y T r a n s . 72 ( 1 9 7 6 ) 2 5 7 7 - 2 5 9 1 ; i b i d . 7 2 ( 1 9 7 6 ) 2 5 9 2 - 2 5 9 7 . 9 W.W. Simons ( E d i t o r ) , The S a d t l e r Handbook o f I n f r a r e d S p e c t r a , S a d t l e r , Heyden, 1978. 10 J . H o w a r d a n d Z . A . K a d i r , Z e o l i t e s 4 ( 1 9 8 4 ) 4 5 - 5 0 .
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
P R E P A R A T I O N AND C H A R A C T E R I Z A T I O N OF M o - H Y
ZEOLITES
A . M E S Z A R O S - K I S a n d J.VALYON C e n t r a l R e s e a r c h I n s t i t u t e f o r C h e m i s t r y , H u n g a r i a n Academy o f S c i e n c e s , P.O.Box 1 7 , 1525 B u d a p e s t ( H u n g a r y ) ABSTRACT Mo-HY z e o l i t e s were p r e p a r e d by r e a c t i n g HY z e o l i t e s w i t h t e t r a k i s - H - a l l y l m o l y b d e n u m . The maximum molybdenum l o a d i n g a c h i e v e d was 1 . 3 % b y w e i g h t , i . e . , 1.6 Mo a t o m p e r u n i t c e l l . I R s p e c t r o s c o p y was a p p l i e d t o s t u d y t h e s u r f a c e r e a c t i o n a n d t h e d e c o m p o s i t i o n o f t h e s u r f a c e c o m p l e x upon h e a t i n g i n H o r 0 Upon c o n t a c t i n g t h e z e o l i t e and t h e p e n t a n e s o l u t i o n the o 6 g a n o m e t a l l i c molybdenum compound t h e i n t e n s i t y o f t h e h i g h - f r e q u e n c y OH-band a t 3 6 4 0 c m - l d e c r e a s e s a n d band s h i f t s t o h i g h e r wavenumbers. The l o w - f r e q u e n c y OH-band r e m a i n s p r a c t i c a l l y u n c h a n g e d . The e n t i r e o r g a n i c s u r f a c e c o m p l e x i s decomposed on t r e a t m e n t a t 2OD.C. A b o u t 9 0 % o f t h e h y d r o x y l g r o u p s o r i g i n a l l y p r e s e n t i n HY a r e r e c o v e r e d when t h e s a m p l e i s h e a t e d t o 400’C i n H I f t h e sample i s h e a t e d i n 0 about 50% o f t h e p r o t o n s i n H f ’ i s r e p l a c e d b y Mo. A t 400’C M 6 l H Y p r e p a r a t i o n s p r e s e r v e t h e i r c r y s t a l l i n i t y b o t h i n o x i d i z e d and reduced s t a t e .
.
03
INTRODUCTION
I n r e c e n t y e a r s an i n c r e a s i n g i n t e r e s t i n molybdenumcontaining z e o l i t e c a t a l y s t s has developed ( r e f s .
1-10).
D i f f e r e n t techniques
h a v e been u s e d t o p r e p a r e a n d c h a r a c t e r i z e t h e M o - z e o l i t e s ; however, i m p o r t a n t q u e s t i o n s , o f Mo,
s u c h as d i s p e r s i o n
the structural s t a b i l i t y o f the zeolite,
and d i s t r i b u t i o n
etc.,
remained
unanswered. I n t r o d u c t i o n o f molybdenum i n t o t h e z e o l i t e m a t r i x was attempted b y t h e c o n v e n t i o n a l methods o f i o n - e x c h a n g e a n d i m p r e g n a t i o n f r o m aqueous s o l u t i o n ( r e f s . 2,9,101.
I t i s d i f f i c u l t t o exchange t h e
c a t i o n s o f a z e o l i t e f o r molybdenum i o n s d i r e c t l y b e c a u s e h i g h - v a l e n t c a t i o n i c molybdenum c a n o n l y e x i s t i n v e r y a c i d i c s o l u t i o n where i o n - e x c h a n g e e q u i l i b r i a a r e u n f a v o u r a b l e a n d many z e o l i t e s a r e u n s t a b l e . The a p p l i c a t i o n o f t h e c a t i o n i c d i n u c l e a r molybdenume t h y l e n e d i a m i n e c o m p l e x f o r t h e i o n - e x c h a n g e o f Na-Y p r o d u c e s a u n i f o r m d i s t r i b u t i o n o f t h e m e t a l ; however, a c o m p l e t e removal o f t h e m e t a l f r o m t h e b u l k phase o c c u r s u p o n t r e a t m e n t w i t h O2 a t 300°C ( r e f . 3 ) .
I m p r e g n a t i o n w i t h n e u t r a l and a n i o n i c s p e c i e s
r e s u l t s i n surface loading only.
I n o r d e r t? overcome t h e
252
d i f f i c u l t i e s o f t h e aqueous i o n - e x c h a n g e ,
molybdenum was
introduced i n t o z e o l i t e s b y t h e s o l i d - s o l i d e x c h a n g e m e t h o d . S o l i d s t a t e i o n - e x c h a n g e o f H - z e o l i t e s w i t h MoC15 r e s u l t s i n M o - z e o l i t e a n d HC1 ( r e f s . 1,3). D a i e t a l .
(ref.
1) reported
t h a t z e o l i t e Y l o s e s c r y s t a l l i n i t y i n t h e exchange process p r o b a b l y due t o t h e a t t a c k o f HC1 on t h e z e o l i t e l a t t i c e . T r e a t m e n t w i t h s o l i d o r v a p o u r p h a s e MoC15 c a n b e s u c c e s s f u l l y a p p l i e d i n p r e p a r i n g t h e Mo-form o f t h e a c i d - r e s i s t a n t z e o l i t e s ( r e f . 5).
A s p e c i a l p r o c e d u r e was d e v e l o p e d r e c e n t l y f o r
i n i t i a t i n g c o n t r o l l e d m i g r a t i o n o f M O O ~ ( O H )i ~n t o t h e z e o l i t i c p o r e s f r o m t h e e x t e r n a l s u r f a c e o f M o - i m p r e g n a t e d NaY ( r e f .
10).
H i g h a n d homogeneous d i s p e r s i o n o f molybdenum c a n be a c h i e v e d i n p r e p a r i n g M o - z e o l i t e s t h r o u g h a redox r e a c t i o n between M o ( C O ) ~ complex and t h e h y d r o x y l groups o f t h e z e o l i t e s ( r e f s . 3,4,6-8). The a v e r a g e o x i d a t i o n number o f t h e molybdenum c a n be c o n t r o l l e d by v a r y i n g t h e p r o t o n c o n c e n t r a t i o n o f t h e s u p p o r t .
I n theabsence
o f o x i d i z i n g s i t e s ( p r o t o n s ) thermal decomposition o f adsorbed M o ( C O ) ~ r e s u l t s i n z e r o - v a l e n t Mo d i s t r i b u t e d h o m o g e n e o u s l y w i t h i n t h e p o r e s ( r e f s . 6,7). I t i s w e l l known t h a t o r g a n o m e t a l l i c compounds c a n be a n c h o r e d
t o oxide surfaces (ref.
11). M o - f i x e d c a t a l y s t s o b t a i n e d f r o m t h e
r e a d y r e a c t i o n o f t h e OH-groups o r alumina,
of s o l i d supports,
w i t h molybdenum-n-ally1
extensively (refs.
such as s i l i c a
complexes have been s t u d i e d
11-17). These c a t a l y s t s w i t h w e l l d e f i n e d
s u r f a c e a r e v e r y s u i t a b l e f o r o b t a i n i n g more p r e c i s e i n f o r m a t i o n a b o u t t h e a c t i v e s i t e s . The p r e p a r a t i o n o f m e t a l - z e o l i t e c a t a l y s t s u s i n g t h e r e a c t i o n o f o r g a n o m e t a l l i c compounds a n d z e o l i t e s c o n t a i n i n g r e a c t i v e OH-groups was c o n s i d e r e d ( r e f .
18); h o w e v e r ,
i t has n o t y e t been a t t e m p t e d t o o b t a i n m o l y b d e n u m - c o n t a i n i n g
z e o l i t e f r o m such r e a c t i o n . The p u r p o s e o f t h e p r e s e n t s t u d y was t o examine t h e r e a c t i o n o f " t r u e " HY z e o l i t e s w i t h t e t r a k i s - r ( a l l y l m o l y b d e n u m and t o c h a r a c t e r i z e t h e p r e p a r e d M o - f i x e d HY z e o l it e . EXPERIMENTAL The NaY z e o l i t e was o b t a i n e d f r o m EKA Kemi,
Sweden, a n d c o n t a i n -
e d 56 A1 p e r u n i t c e l l . The ammonium-exchanged Y
zeolite
was p r e -
p a r e d by d o u b l e i o n - e x c h a n g e w i t h 1M NH4N03 a t 70°C. A " t r u e " H Y f o r m o f t h e z e o l i t e was o b t a i n e d b y o u t g a s s i n g NH4Y z e o l i t e [Na,(NH4),,(A102),,(SiD~) 1 3 J i n vacuum a t 400°C f o r 2 h, f o l l o w e d by c a l c i n a t i o n f o r 2 h i n f l o w i n g 02 a n d p u r g i n g i n f l o w i n g He a t
253
400°C f o r 1 h . Gases were p u r i f i e d by c o n v e n t i o n a l methods. M O ( I I - C ~ H ~ was ) ~ p r e p a r e d a c c o r d i n g t o Wilke e t a l . ( r e f . 1 9 ) . The s y n t h e s i s s t e p s a r e a s f o l l o w s : C3H5C1tMg(large e x c e s s )
room t e m p e r a t u r e diethyl ether
-
temperature 4 C 3 H 5 M g C 1 M ° C 1 5 droom iethyl e t h e r , pentang +
C3H5MgC,
M0(a-C3H5I4.
(2)
B e f o r e t h e i r u s e , s o l v e n t s and a l l y 1 c h l o r i d e were r e f l u x e d o n Na w i r e a n d m o l e c u l a r s i e v e 5A, r e s p e c t i v e l y , t h e n d i s t i l l e d . B o t h MoC15 a n d M o ( T C - C ~ H ~ )a~r e e x t r e m e l y s e n s i t i v e t o O 2 o r m o i s t u r e , hence t h e complex was p r e p a r e d under p u r e n i t r o g e n in a n a p p a r a t u s s i m i l a r t o t h a t d e s c r i b e d by Bonnemann ( r e f . 2 0 ) . MoC15 was p r e p a r e d i n s i t u b y r e a c t i n g c h l o r i n e and Mo metal a t 400°C. S o l v e n t s were e v a p o r a t e d from t h e r e a c t i o n m i x t u r e o b t a i n e d from r e a c t i o n ( 2 ) ; t h e n s o l i d r e s i d u e was e x t r a c t e d with p e n t a n e . R e s u l t s of chemical a n a l y s i s c o n f i r m e d t h a t t h e s t o i c h i o m e t r i c c o m p o s i t i o n of t h e e x t r a c t c o r r e s p o n d s t o f o r m u l a
Mo(n-C3H5)4. The H Y z e o l i t e was c o n t a c t e d w i t h t h e d a r k g r e e n s o l u t i o n of t h e Mo(X-C3H5l4 a t room t e m p e r a t u r e . I n o r d e r t o remove t h e e x c e s s Mo complex t h e Mo-fixed z e o l i t e was washed w i t h a copious amount of p e n t a n e . Both f i x i n g r e a c t i o n a n d w a s h i n g were carried o u t under f l o w i n g pure h e l i u m . I n t h e f i x i n g r e a c t i o n t h e e v o l u t i o n of p r o p y l e n e was examined. Gases l e a v i n g t h e r e a c t o r c o n t a i n i n g a b o u t 1 g of z e o l i t e were p a s s e d t h r o u g h a t r a p c o o l e d w i t h l i q u i d N 2 . The v e r y small amount of p r o p y l e n e trapped was d e t e r m i n e d v o l u m e t r i c a l l y , w i t h g r e a t u n c e r t a i n t y ; l e s s than one p r o p y l e n e molecule was e v o l v e d p e r Mo atom i n t r o d u c e d . S e l f - s u p p o r t i n g w a f e r s w i t h a t h i c k n e s s of a b o u t 8 - 1 0 mg.cm-2 were p r e s s e d from t h e homogeneous d i s p e r s i o n of NH4Y powder. Samples were p l a c e d i n a n I R c e l l e q u i p p e d w i t h KBr windows and f u r n a c e r e g i o n i n t o which t h e z e o l i t e w a f e r c o u l d be r a i s e d m a g n e t i c a l l y . The c o n s t r u c t i o n of t h e c e l l a l l o w e d both vacuum a n d g a s - f l o w t r e a t m e n t of t h e w a f e r s . A f t e r deammoniation t h e f i x i n g r e a c t i o n was c a r r i e d o u t by immersing t h e wafer i n t o t h e p e n t a n e s o l u t i o n of M o ( R - C ~ H ~ ) ~ f o r 1 h a n d t h e n i n t o pure p e n t a n e f o r 30 min. The w a f e r was r e p l a c e d i n t h e IR c e l l . The whole p r o c e d u r e was c a r r i e d o u t i n i n e r t g a s t o p r e v e n t t h e c o n t a c t of t h e w a f e r w i t h a i r .
254
The I R s p e c t r o p h o t o m e t e r employed was a N i c o l e t 170 S X Fourier t r a n s f o r m i n s t r u m e n . t . S p e c t r a r e c o r d e d a t room t e m p e r a t u r e were t h e a v e r a g e of 3000 s c a n s a t 2 cm-l r e s o l u t i o n . They were routinely s t o r e d o n f l o p p y d i s c s and computer-enhanced i f n e c e s s a r y . Computer s u b t r a c t i o n of t h e s p e c t r a h e l p e d t o e v a l u a t e c h a n g e s o f absorbance. A b s o r p t i o n c a p a c i t i e s f o r N 2 were measured v o l u m e t r i c a l l y a t -196'C. Samples were r e n d e r e d s o l u b l e i n a m i x t u r e of c o n c e n t r a t e d H F , HN03 and H2S04. The r h o d a n i d e complex o f t h e molybdenum was formed a n d t h e molybdenum c o n t e n t was d e t e r m i n e d b y p h o t o m e t r y . RESULTS
IR s p e c t r a o f H Y and Mo-HY z e o l i t e s a r e s h o w n i n t h e O H - a n d C - H s t r e t c h i n g r e g i o n ( 4 0 0 0 - 2800 c m - ' ) i n F i g . 1 . The c h a r a c t e r i s t i c v ( 0 H ) bands a p p e a r a t 3640 cm-l a n d a t 3540 cm-' ( F i g . 1 , s p e c t r a A and a ) . S p e c t r a B a n d b i n F i g 1 were recorded following the fixation reaction of HY with M o ( ~ T - C ~ H ~ ) ~ ,
t h e p e n t a n e washing and ( f o r e x p e l l i n g p e n t a n e ) a purge w i t h h e l i u m a t 100°C f o r 1 h . The i n t e n s i t y of t h e h i g h - f r e q u e n c y v(0H) band d e c r e a s e s s u b s t a n t i a l l y , and t h e b a n d s h i f t s w i t h a b o u t 8 cm-' t o h i g h e r wavenumbers. The l o w - f r e q u e n c y b a n d r e m a i n s p r a c t i c a l l y unchanged. New C - H s t r e t c h i n g v i b r a t i o n bands a p p e a r between 3000 a n d 2800 cm-'. T r e a t m e n t of t h e w a f e r s i n f l o w i n g H 2 o r O2 a t 200°C f o r 1 h c o m p l e t e l y removes t h e s e bands ( F i g . 1 , s p e c t r a C and c ) . All t h e o t h e r s p e c t r a i n F i g . 1 were r e c o r d e d a f t e r s u b s e q u e n t 1 hour O 2 o r H 2 t r e a t m e n t s a t 300 and 400'C. I t was found t h a t u p o n h e a t i n g Mo-fixed H Y o f t h e same Mo c o n t e n t ( 1 . 3 % by w e i g h t , i . e . , 1.6 Mo p e r u n i t c e l l ) i n H 2 o r 02, s i g n i f i c a n t l y d i f f e r e n t amounts of t h e OH-groups become i n v o l v e d i n b i n d i n g molybdenum. I n t e n s i t y o f t h e v ( 0 H ) bands d e c r e a s e s by a b o u t 10% o n l y when wafer i s t r e a t e d i n hydrogen ( c f . F i g . 1 , s p e c t r a A and E ) a n d a b o u t 50 % upon t r e a t m e n t w i t h oxygen ( c f . F i g . 1 , s p e c t r a a and e l . A t 400'C i n 02, t h e h i g h - f r e q u e n c y OH-band i s a l m o s t of t h e same i n t e n s i t y , b u t t h e l o w - f r e q u e n c y band i s c o n s i d e r a b l y s m a l l e r t h a n t h e c o r r e s p o n d i n g b a n d of H Y . H 2 - t r e a t e d s a m p l e s a r e b l a c k , i n d i c a t i n g t h e p r e s e n c e of r e d u c e d Mo. The w h i t e 0 2 - t r e a t e d s a m p l e s p r o b a b l y c o n t a i n mainly Mo 6 + O x i d a t i o n o f t h e H2-reduced sample a t 400'C r e s u l t s i n a s m a l l ( - 1 0 % ) d e c r e a s e of v ( 0 H ) b a n d s
.
255
]---'I\
I\
13400
I00
2800
-
3400 WAVENUMBERS
4000
WAVENUMBERS
2800
F i g . 1 . I n f r a r e d s p e c t r a i n t h e v ( O H ) a n d t h e v ( C H ) r e g i o n : HY ( A a n d a ) ; c o n t a c t e d w i t h t h e p e n t a n e s o l u t i o n o f Mo(n-C3H514 a t r o o m t e m p e r a t u r e f o r 1 h a n d p u r g e d w i t h He a t 1 O O " C f o r 1 h ( B a n d b ) ; a c t i v a t e d i n f l o w i n g H a t 2 0 0 (C), 3 0 0 ( 0 ) a n d 4 0 0 ' C ( E ) o r i n f l o w i n g 0 a t 2 0 0 ( c ) , 300 ( d ) a n d 4 0 0 ' C ( e ) f o r I h; t h e sample a c t i v a t e $ i n f l o w i n g H2 a t 400'C a f t e r t r e a t m e n t w i t h 0 a t 4 0 0 ' C f o r 1 h ( F ) a n d H a t 400°C f o r 1 h ( G ) ; t h e s a m p l e a s t i v a t e d i n f l o w i n g 0 a t 406.C a f t e r t r e a t m e n t w i t h H a t 4 0 0 ' C f o r 1 h ( f ) . B o t h s a m p l z s c o n t a i n 1.6 Mo-atom p e r u n i t c e ? l . (cf. Fig.
1 , s p e c t r a E a n d F ) , b u t d i s a p p e a r i n g OH g r o u p s c a n b e
r e g e n e r a t e d b y t r e a t m e n t w i t h H2 ( F i g .
1,
No
s p e c t r u m G).
s i g n i f i c a n t c h a n g e o c c u r r e d when we t r i e d t o r e d u c e t h e m o l y b d e num i n t h e s a m p l e w h i c h was h e a t e d u p i n O 2 ( c f .
Fig.
1,
spectra
e and f ) . B E T a d s o r p t i o n i s o t h e r m s o f HY a n d Mo-HY z e o l i t e s f o r N 2 a t
-196'C
were determined a f t e r d i f f e r e n t t r e a t m e n t s .
Sorption
c a p a c i t y o f t h e HY s a m p l e p r e p a r e d b y i n s i t u d e c o m p o s i t i o n o f t h e NH4Y i s c o m p a r a b l e t o t h e s o r p t i o n c a p a c i t y o f t h e M o - f i x e d samples h e a t e d e i t h e r i n O2 o r i n H2.
Similar sorption capacity
v a l u e s w e r e r e p o r t e d f o r NaY a n d HY e a r l i e r ( r e f .
4,21).
A t 1OO'C
p e n t a n e c a n b e q u a n t i t a t i v e l y e x p e l l e d f r o m t h e p o r e s o f HY
256
w h i l e 60% o f t h e p o r e v o l u m e o f t h e M o - f i x e d HY r e m a i n s b l o c k e d f o r a d s o r p t i o n o f N2 a f t e r t h e same t r e a t m e n t ( s e e T a b l e 1 ) . TABLE 1 N i t r o g e n a d s o r p t i o n d a t a o f HY a n d M o - H Y ' ~ ) z e o l i t e s .
Sample a n d t r e a t m e n t
Nitrogen uptake a t 13 kPa a n d -196"C, g/g
Sample 1: H Y z e o l i t e o b t a i n e d f r o m t h e d e a m m o n i a t i o n o f N H 4 Y a t 400°C
0.28
Sample 2 : Sample 1 was c o n t a c t e d w i t h p e n t a n e a n d p u r g e d w i t h He a t 100°C f o r 1 h.
0.26
Sample 3 : Sample 1 was c o n t a c t e d w i t h t h e p e n t a n e s o l u t i o n o f ( M o ( r - C H ) , washed i n p e n t a n e a n d p u r g e d w i t h He h 5 1 6 0 0 C f o r 1 h .
0.12
Sample 4 : Sample 3 was t r e a t e d w i t h f l o w i n g H 2 a t 400°C f o r 1 h .
0.27
Sample 5 : Sample 3 was t r e a t e d w i t h f l o w i n g O 2 a t 4OOcC f o r 1 h .
0.27
( a ) M o c o n t e n t o f t h e s a m p l e i s 1 Mo p e r u n i t c e l l . DISCUSSION
D e h y d r o x y l a t i o n o f z e o l i t e s o c c u r s when p r o t o n s o f t h e z e o l i t e o x i d i z e l o w - v a l e n t m e t a l s . The o x i d a t i o n o f Mo i n t h e Mo(r-C3H514 c o m p l e x by HY c a n b e d e s c r i b e d a s f o l l o w s :
where Z r e p r e s e n t s t h e z e o l i t e m a t r i x . S u p p o s i n g t h a t t h e c o m p l e x w h i c h was n o t f i x e d c h e m i c a l l y was r e m o v e d b y w a s h i n g t h e z e o l i t e w i t h p e n t a n e , one w o u l d e x p e c t a c c o r d i n g t o e q . 3 t h e f o r m a t i o n o f minimum one p r o p y l e n e m o l e c u l e p e r molybdenum i n t r o d u c e d . In fact,
much l e s s p r o p y l e n e was d e t e r m i n e d .
A possible expla-
n a t i o n i s t h a t p r o p y l e n e was s i m p l y t o o s t r o n g l y b o u n d w i t h i n t h e p o r e s t o be e v o l v e d a t room t e m p e r a t u r e .
Nevertheless,
it
c a n n o t be e x c l u d e d t h a t a s u r f a c e o r g a n o m o l y b d e n u m compound c o u l d be f o r m e d w i t h t h e p a r t i c i p a t i o n o f t h e T-0-T g r o u p s i n the reaction.
I t was shown b y Yermakov
s u p p o r t s t h a t =T-C3HS
(ref.
11) f o r n o n - z e o l i t i c
s p e c i e s a r e formed i n such a r e a c t i o n .
The maximum a c h i e v a b l e l o a d i n g i s a b o u t 1.3% Mo b y w e i g h t . U n e x p e c t e d l y a l a r g e p o r t i o n o f t h e a c i d i c OH-groups w h i c h w e r e supposed t o be a c t i v e d i d n o t p a r t i c i p a t e i n t h e f i x i n g r e a c t i o n .
251
S t e r i c h i n d r a n c e may l i m i t t h e maximum M o - c o n t e n t a c h i e v a b l e i n one s t e p .
P r o b a b l y h i g h e r l o a d i n g s c o u l d b e o b t a i n e d i n repeated
reaction-and-decomposition
cycles.
Pentane c a n be e a s i l y removed f r o m t h e p o r e s o f HY. A l l y 1 lig a n d s a r e much m o r e s t r o n g l y b o u n d t h a n p e n t a n e ( T a b l e 1 ) . v ( C H ) b a n d s o b s e r v e d i n t h e 3 0 0 0 - 2 8 0 0 cm-’
region a f t e r treatment a t
100°C c a n b e a t t r i b u t e d t o s u r f a c e a l l y l g r o u p s ( F i g .
B a n d b ) . No b a n d s a p p e a r a t a b o u t 3 1 0 0 c m - l ,
1, spectra
indicating that
surface-bound hydrocarbon has no d o u b l e bond c h a r a c t e r ,
i.e.,
in
t h e s u r f a c e complex a l l y l l i g a n d i s bound t o t h e molybdenum atom by a 6-bond r a t h e r than by a TI-ally1 bond. The hydroxyl band of t h e M o - f i x e d HY a t 3 6 4 8 c m - l
(Fig.
1, s p e c t r a B a n d b ) i s o f much
l e s s i n t e n s i t y t h a n t h a t o f t h e HY ( F i g .
1, s p e c t r a A a n d a ) .
A d s o r p t i o n o f o l e f i n s on d e c a t i o n i z e d Y i n d u c e s s i m i l a r a change i n t h e 3(OH) r e g i o n ( r e f . 2 2 ) .
I t seems l i k e l y t h a t t h e i n t e n s i t y
OH b a n d d e c r e a s e s p r i m a r i l y d u e t o i n t e r a c t i o n
o f t h e 3640 cm-l
o f t h e a l l y l l i g a n d s and t h e hydroxyl groups,
and t h e sample i s
n o t d e h y d r o x y l a t e d t o s u c h a n e x t e n t t h r o u g h o x i d i z i n g Mo. D i s a p p e a r a n c e o f t h e \)(CH) b a n d s i n d i c a t e s t h a t a l l o f t h e
1, s p e c t r a
o r g a n i c s u r f a c e c o m p l e x d e c o m p o s e s b e l o w 200°C ( F i g .
C a n d c ) . The s i m u l t a n e o u s a n d a l m o s t c o m p l e t e r e c o v e r y o f t h e
b a n d a t 3 6 4 0 cm-’
s u p p o r t s t h e c o n c l u s i o n t h a t t h e b a n d was
s h i f t e d and decreased i n i n t e n s i t y through t h e e f f e c t o f t h e a l l y l ligands. The 5 0 % d e c r e a s e o f v ( O H ) i n t e n s i t i e s u p o n t r e a t m e n t i n O 2 a t 400°C c a n p r o b a b l y a c c o u n t f o r a r e a c t i o n s i m i l a r t o t h a t d e s c r i b e d f o r t h e o x i d a t i o n o f r e d u c e d Ag-HY
w h e r e Mont
(ref. 23):
r e f e r s t o a s u r f a c e - b o u n d molybdenum atom and n i s
s m a l l e r t h a n 6.
S u b s c r i p t ‘ s ’ d e s i g n a t e s a s u r f a c e oxygen atom.
I t i s g e n e r a l l y b e l i e v e d t h a t t h e b a n d a t 3 5 4 0 cm-’
hydroxyl groups i n t h e hexagonal prisms.The o f t h e b a n d a t 3 5 4 0 cm-’
represents
i n t e n s i t y decrease
shows t h a t t h e s e h y d r o x y l s a r e p r e f e r -
e n t i a l l y consumed i n t h e a b o v e p r o c e s s . The r e a c t i o n c a n t a k e p l a c e o n l y i f l o w - v a l e n t molybdenum atoms m i g r a t e f r o m t h e s u p e r c a g e s t o t h e more i n a c c e s s i b l e h e x a g o n a l p r i s m s a t h i g h e r temperature. M i g r a t i o n o f t h e Mo a t o m s c a n a l s o o c c u r when t h e s a m p l e i s heated i n Hp.
I n t h i s p r o c e s s p r o b a b l y l a r g e r molybdenum
clusters
258 a r e formed w i t h i n t h e supercages o r on t h e e x t e r n a l s u r f a c e s o f the c r y s t a l l i t e s .
I n H2,
therefore,
i n o n l y a 10 % d e h y d r o x y l a t i o n .
Mo-fixing reaction results
The m o l y b d e n u m c l u s t e r s c a n n o t
be d i s i n t e g r a t e d a n d r e d i s t r i b u t e d b y t r e a t m e n t w i t h O 2 a t 400°C. F u r t h e r s t u d i e s on t h e p r e p a r a t i o n a n d c a t a l y t i c p r o p e r t i e s o f Mo-fixed z e o l i t e s are i n progress. We t h a n k D r . S . H o l l y Hung. Acad.
Sci.,
(Central Res.Inst.
f o r Chem.
o f the
Hungary) f o r making t h e I R spectroscopic
m e a s u r e m e n t s . We a c k n o w l e d g e t h e f i n a n c i a l s u p p o r t o f t h e N a t i o n a l S c i e n t i f i c R e s e a r c h F o u n d a t i o n (OTKA, P r o j e c t No. 782.).
REFERENCES 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23
P . E . D a i a n d J.H. L u n s f o r d , J . C a t a l . , 64 ( 1 9 8 0 ) 1 7 3 - 1 8 3 . E.L. Moorehead, U S P a t e n t 4,297,243 ( 1 9 8 1 ) . M.B. Ward a n d J.H. L u n s f o r d , i n D.H. O l s o n a n d H . B i s i o ( E d i t o r s ) , P r o c . 6 t h I n t e r n . Z e o l . Conf., Reno, U S A J u l y 10-15, 1983, B u t t e r w o r t h , G u i l f o r d , 1984, pp. 4 0 5 - 4 1 6 . S. Abdo a n d R.F. Howe, J. P h y s . Chem., 87 ( 1 9 8 3 ) 1 7 1 3 - 1 7 2 2 . J.R. J o h n s a n d R.F. Howe, Z e o l i t e s , 5 ( 1 9 8 5 ) 2 5 1 - 2 5 6 . Y.S. Yong a n d R.F. Howe, J . Chem. S O C . , F a r a d a y T r a n s . I . , 82 (1986) 2887-2896. Y . S . Yong and R.F. Howe, i n Y . M u r a k a m i , A . I i j i m a a n d J.W. Ward ( E d i t o r s ) , P r o c . 7 t h I n t e r n . Z e o l . C o n f . , Tokyo, Japan, A u g u s t 1 7 - 2 2 , 1986, Kodansha L t d . , Tokyo, 1986. pp. 8 8 3 - 8 8 9 . T . Komatsu a n d T . Yashima, J. M o l . C a t a l . , 4 0 ( 1 9 8 7 ) 8 3 - 9 2 . R. C i d , F.J. G i l L l a m b i a s , J.L.G. F i e r r o , A.Lopez Agudo a n d J . V i l l a s e n o r , J . C a t a l . , 89 ( 1 9 8 4 ) 4 7 8 - 4 8 8 . J.L.G. F i e r r o , J.C. Conesa a n d A . L o p e z Agudo, J. C a t a l . , 108 (1987) 334-345. Yu. I . Yermakov, C a t a l . R e v . - S c i . Eng., 13 ( 1 9 7 6 ) 7 7 - 1 2 0 . Y . Iwasawa, Y . Nakano a n d S . Ogasawara, J . Chem. S O C . , F a r a d a y T r a n s . I . . 74 ( 1 9 7 8 ) 2 9 6 8 - 2 9 8 1 . Y . Iwasawa a n d ' S . Ogasawara, J . Chem. S O C . , F a r a d a y T r a n s . I., 75 ( 1 9 7 9 ) 1 4 6 5 - 1 4 7 6 . Y . Iwasawa, T . Nakamura, K . T a k a m a t s u a n d S . Ogasawara, J.Chem. S O C . F a r a d a y T r a n s . I., 76 ( 1 9 8 0 ) 9 3 9 - 9 5 1 . Y . Iwasawa, H . I c h i n o s e a n d S . Ogasawara, J . Chem. S O C . F a r a d a y T r a n s . I., 77 ( 1 9 8 1 ) 1 7 6 3 - 1 7 7 7 . Y . Iwasawa, Y . S a t 0 a n d H. K u r o d a , J . C a t a l . , 8 2 ( 1 9 8 3 ) 289-298. Y . Iwasawa a n d M. Y a m a g i s h i , J . Catal., 8 2 ( 1 9 8 3 ) 3 7 3 - 3 8 1 . Kh.M. M i n a c h e v a n d Ya. I s a k o v , i n J.A. Rabo ( E d i t o r ) , Z e o l i t e C h e m i s t r y a n d C a t a l y s i s , A C S Monograph 1 7 1 , A m e r i c a n C h e m i c a l S o c i e t y , W a s h i n g t o n , 1976, p p . 5 5 2 - 6 1 1 . G . W i l k e , B . B o g d a n o v i c , P . H a r d t , P . Heimbach, W . Keim, M. K r u n e r , W . O b e r k i r c h , K . Tanaka, E . S t e i n r u c k e , 0. W a t e r a n d H . Zimmermann, Angew. Chem. I n t . Edn., 5 ( 1 9 6 6 ) 1 5 1 - 1 5 2 . H . Bonnen ' i , Angew. Chem., 8 5 ( 1 9 7 3 ) 1 0 2 4 - 1 0 3 5 . W . B r e c k , Z e o l i t e M o l e c u l a r S i e v e s , W i l e y - I n t e r s c i e n c e , New York, 1974. B . V . L i e n g m e a n d W.K. H a l l , T r a n s . F a r a d a y S o t . , 62 ( 1 9 6 6 ) 3229-3243. H . B e y e r , P . A . J a c o b s a n d 3.8. U y t t e r h o e v e n , J. Chem. S O C . , F a r a d a y , I. 72 ( 1 9 7 6 ) 6 7 4 - 6 8 5 .
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
P R OP Y L E NE
ME T A T HE S IS
RE A C T I 0N
0V ER
Mo/
Y - ZE 0 L IT E S
M. tANIECKI Faculty of Chemistry, A. Mickiewicz University, 60-780 Poznafi Poland
ABSTRACT The Mo(C0) loaded Y-zeolites differing in Si/Al ratio and surface acidity have been used to study the activity in propylene metathesis. Both sodium and hydrogen forms of Y-zeolites were applied as supports. Oecom7osition of adsorbed Mo(C0) , during heating,produces a variety of surface Mo species diffgrbng in $ k e degree of decarbonylation and oxidation number (from Mo to Mo 1. The catalytic activity decreased with increasing protonic acidity of the support and oxidation number of molybdenum. U V / V I S transmission spectroscopy was applied to study the influence of the support acidity on the formation of surface species during adsorption and decomposition of MO(CO)~. INTRODUCTION It is well known that one of the most difficult transition metals to ion exchange into the zeolites has been molybdenum, due to the absence of simple salts of this element which are stable under solution-ion-exchange conditions. F o r this reason, the different techniques of Mo implantation into the zeolite framework,including solid-state exchange with MoC15 (1) and M ~ ~ ( e n ) ~ C (2) l ~ has been applied. However, the less direct but m o r e efficient method of molybdenum loading into HY zeolites is based on the saturation of activated zeolite with volatile molybdenum carbonyl. Gallezot et a1.(3) and Ward and Lunsford (2), applying mainly IR spectroscopy, established that Mo(CO)~, initially adsorbed within the supercage of HY zeolite, become oxidized by zeolite protons during thermal activation. The IR and EPR studies by Abdo and Howe (4) cornplemented results reported earlier and indicated that i n the case of Mo(C0)6/NaY the formation o f supported zero-valent Mo is possible. Ward and Lunsford (2),studying normal and ultrastable HY zeolites, found that Mo ions are probably located at SII positions within the large cavities. Although the results presented by the groups of Howe (4), Luns-
260
ford(2) and Yashima(5-7) disclosed some chemistry of the Mo/Y-zeolites, there are few papers concerning the catalytic properties of these systems. Komatsu et al.(6),studying the propylene metathesis over Mo-loaded zeolites as a function of oxidation number, found that molybdenum species responsible for catalytic activity have an oxidation number lower than +4. They also stressed the importance of Mo dispersion on catalytic activity. It was established(7) that atomically dispersed ivlo with oxidation number close to zero is responsible for polymerization and hydrogenation of ethylene. Yong and Howe(8) reported recently the activity of molybdenum zeolites in the Fisher-Tropsch reaction. They established that application of Nay, KY and KL zeolites as supports leads to the formation of active centers in the Fisher-Tropsch process, namely zero-valent molybdenum. The propylene epoxidation reaction has also been studied with Mo-loaded zeolites ( 9 ) . The present paper seeks to contribute to an understanding of the surface chemistry and catalytic activity of molybdenum zeolites.The influence of zeolite composition and thermal pretreatment on the catalytic activity in propylene disproportionation reaction has been studied. EXPERIMENT NaY faujasites with Si/A1 ratio 2.0 and 2.7 were obtained according to the procedure described by Kacirek and Lechert(lO).The equivalent ammonium forms were obtained by triple exchange with 1M solution of NH4C1 at 345 K (for details s e e Table 1). The dehydration of sodium forms as well a s the dehydration and deammoniation of ammonium forms of Y-zeolites was carried out in a stream of purified helium inside the U-shaped quartz reactor at 675, 775 o r 875 K, for two hours. The amount of the support used was always adjusted to 0.25 g o f activated zeolite (for weight losses see Table l).After cooling and one-minute evacuation, the desired amount of doubly sublimed Mo(C016 was transferred at room temperature under vacuum onto the support and kept in a closed reactor for 1 2 hours. A similar procedure for alumina support has been described earlier(l1). The activation o f supported Mo(CO)~ was carried out in helium in 50 K intervals up to 675 K for 60 minutes. The evolved gases were trapped at liq. N2 temperature (for a H2 trap with molecular sieves 5A was used) and, after warming, analyzed with a thermal conductivity detector and 2.5 m
261
column of activated carbon (Mcrck, 25-35 mesh). The propylenc metathesis reaction was studied by a pulse method. The propylone pulses were injected from the gas-sampling vale (1.21 cm3) into the carrier gas (He o r H2) flowing at a rate of 3 0 cm3- n1in-l and carried through the catalyst bed at 335 K . Reaction products were collected in a trap and, after each injection, analyzed with gas chromatography. The average oxidation number ( O . N . ) of Mo was calculated on the basis of oxygen titrations performed at 575 I( with a well calibrated sampling valve. The U V / V I S experiments were carried out in the high vacuum system. The details of the cell, high-vacuum and gas-dosing system have been described elsewhere (12). The self-supporting wafers of zeolite with a thickness of 2 - 3 m g + c m - 2 were employed. The U V / V I S spectra were recorded at room temperature in transmission mode with a Perkin-Elmer U V / V I S spectrometer (Model 556) equipped with baseline corrector. Similarily t o the catalytic experiments, in the spectroscopic studies the zeolite wafers were dehydroxylated under vacuum in the U V / V I S cell at elevated temperatures prior to the Mo(CO)~ admission. In the spectroscopic studies the amount of supported Mo(CO)~ was calculated from the pressure drop of carbonyl vapours, whereas in the catalytic experiments the Mo content was analyzed with atomic absorption spectrometry. TABLE 1 Characterization of zeolites used. Unit cell
Pretr. Weight temp. loss*
IR absorbance**(a.u.) OH vibrations
BPy
(K)
(%)
3640
3550
1545
675 775
30.1 30.4
2.52
1.65
0.88
(NH4)35Na14A152Si1420384 6 7 5 Si/A1=2.72 775 875
28.3 28.5 30.0
-
(NH4)48Na7A162Si1320384 Si/Al=2.11
LPy 1455
-
-
0.04
0.43 1.35
2.52
1.30
0.08
-
1.00 0.13 0.03
0.30 1.46 1.58
~
~~~
*after 2 hours at indicated temperature in N **after 2 hours activation under vacuum follo8ed by interaction with pyridine at 475 K (2h adsorption, lh desorption). 8Py and LPy represent the density o f the Brunsted and Lewis centers, respectively.
262
AND DISCUSSION The application of tIY zeolites with different S i / A 1 ratio shows that the initial composition (and consequently also the initial acidity) influences the decomposition of supported Mo(CO)(,. Figure 1 shows the amount of CI1 and H2 cvolvcd upon heating the Mo(CO16 loaded zaolites, pretreated at 6 7 5 K. In the case of zeolite with higher A 1 content the decomposition of Mo(CO)~ proceeds at lower temperatures than with the sample where Si/A1=2.7. The complete decarbonylation for these samples was achieved at 520 and 6 5 0 K , respectively. RESULTS
3
6 0
1
I \L
2 P rn
0 0
I
w
N
\
62
1 I
I
F i g . 1 . CO and H evolution during decomposftion of supported Mo(CO) HY-1 and HY-2 refiresent supports with Si/A1=2.11 and 2.72 respectively. Pretreatment temperature of the support - 6 7 5 K.
.
0
0
300
LOO 500 600 TEMPERATURE K 1
The quantities of hydrogen released in the reaction of Mo(C0l6 with surface hydroxyl groups are smaller than those found previously ( 2 ) . These amounts, however, can serve a s well as titrations for the estimation of an average oxidation nurnber(0.N.) of molybdenum, because for the samples activated at 575 K a good agreement has been found. The evaluation o f the O . N . was based on the assumption that during oxygen titrations all Mo species are oxidized to Mo'~. It was found for the applied HY zeolites pretreated at 675 K that in every step o f thermal decomposition the 0.11. is always lower f o r the zeolite with lower Si/A1 ratio. F o r example, samples activated at 675 K show the O.N. close to + 4 f o r those with Si/Al=2.1, whereas +6 is shown f o r those with Si/A1=2.7. Similar behaviour has been observed f o r HY supports when pretreated at 775 K. The values of O.N. ( s e e Table 2) for the Mo s p e cies were very much lowered in comparison with pretreatment at 675 K.The decarbonylation process had been also completed at lower temperatures in both cases. As previously, the hydrogen evolution was observed, but only f o r the sample with lower A1 content.
263
When HY zeolite with Si/Al 2.1 was pretreated at 875 K only carbon monoxide was found in the decomposition products. Comparing the influence o f the initial acidity, expressed a s the average density of Ertlnsted and Lewis centers (Table 11, with the average oxidation number (Table 2 ) one can conclude that the surface hydroxyl groups are essentialy responsible for the oxidation process of supported molybdenum carbonyl. Abdo and Howe (4), applying IR spectroscopy, found that adsorbed Mo(C016 interacts with surface hydroxyls even at r o o m temperature and thermal activation causes the complete disappearance of IR bands originating from OH vibrations. In o u r case, while the support is practically dehydroxylated, the molybdenum carbonyl decomposes instantly, releasing only carbon monoxide with parallel formation of Mo species with O.N.close to zero. Such a situation exists when the HY zeolite is pretreated at high temperatures o r when the parent NaY zeolites are applied as supports. I n the absence of an oxidizing agent such a s a hydrogen from OH groups, as well as in the presence of the basic sites present in NaY zeolites, the decomposition of Mo(C0l6 started immediately after deposition onto the support. After one hour of the contact a t room temperature,the species with the average formula M o ( C O ) ~ were formed. Only the activation at elevated temperatures caused the complete decarbonylation of supported Mo(C016. F o r the sample with Si/A1=2.11 the complete decarbonylation was observed above 455 I ( , and above 400 I( for the sample with Si/A1=2.72. The oxygen titrations indicated that application of NaY zeolites a s supports, pretreated at 6 7 5 I ( , leads to the fornation o € zerovalent 140 particles. TABLE 2 Average oxidation number of Support pretreatment temperature ( I( )
675 775 075
Mo
supported on hydrogen Y-zeolites.
Average oxidation number of 110 after activation at indicated temperature ( K ) Si/Al=2.11 Si/A1=2.72 415 575 675 475 515 615 +0.0 -
-
+2.4
+4.0
+1.1 + 3 . 3
+6.0
+0.3
+0.3
+0.4
+0.7
+1.5
+0.1
+0.3
+0.3
-
-
264
Samples differing i n Mo oxidation state and containing two Mo atoms per supercage of Y zeolite were tested i n propylene metathesis reaction. Catalyst activities were determined by passing pulses of propylene over the catalyst kept at 335 K in a stream of helium. Assuming pure disproportionation and equilibrium between the 2-butene isomers, the equilibrium conversion i s 42.8% ( 1 3 ) . Pure disproportionation will yield only 2-butenes and ethylene in equimolar amounts. Analysis up through the hexenes revealed only tiny amounts of 1-butene as an additional product f o r any of the studied samples, showing a selectivity higher than 99.5%. The ratio of ethylene/Z-butenes was usually higher than 1.4, suggesting higher adsorption of 2-butenes on the catalyst. It was also dependent on the pulse number. The ratio of trans2-butene to cis-2-butene was close to the equilibrium value of 2.8. -
I
,s 20
-
I
HY-1
8
U
K?
L$ 10
5 0
5
10 15 PULSE NUMBER
20
Fig.2. Variation of activity with the pulse number for catalysts activated at 575 K . Support pretreatment temperature - 675 K . H Y - 1 , Si/Al=2.11 H Y - 2 , Si/A1=2.72 Nay-1, Si/A1=2.00 Nay-2, Si/A1=2.70 Open triangles represent activity obtained after 6 pulses o f ethylene followed by propylene pulses over HY-1.
The results o f catalytic activity measurements,expressed as propylene conversion, for catalysts pretreated at 675 K are shown in Fig.2. F o r both hydrogen forms of Y-zeolites a low activity after first pulses of propylene was observed. The effect is the reverse of that with the alumina as a support ( 1 4 ) . The maximum activity usually was reachod with 15 p u l s s s , and n z x t a sln1.i ;!::cr>az:? l i a s n:?:;-svzC. ?rli,iy:-.nc conv~rsiari ov?c th-: c a t a l y s t : ; ,.:it:i:ii:1:i::r : < i n t s , i t as in all studied c3s05 always higher than f o r those tdhcro Si/A1=2.7. For iblo-loadcd ;!aY zeolites the activity bias cJnstant f r o m t h c first pulse, however, very much lowered in comparison with HY supports. Very similar results have been obtained €or the catalysts where t l i o sup7orts uere pretreated at 775 o r 875 I<. The initial activity after the first pulses of propylane was at 775 K twice as high and
265
'act.
3ob
a
- 475 K - 575 K
Fig.3. Activity versus average oxidation number (O.N.
HY-1 HY-2
0
2
1.
, ,
Si/Al=2.11 Si/A1=2.72
4
OXIDATION NUMBER at 875 I( over three times as high as for those pretreated at 6 7 5 K. The increase of pretreatment temperature for NaY zeolites above 675 K did not influence the changes in the catalytic activity. In order to establish the influence of the reaction products on the catalytic activity, pulses of ethylene and trans-2-butene were passed over the fresh catalyst before propylene injections. Preadsorption of butene did not change the activity very much, but significant changes were observed for the samples originating from the HY supports pretreated with ethylene. I n a l l cases when ethylene was applied before propylene admission the initial activity increased at least six times that expected for fresh catalyst ( s e e Fig.2.). The explanation for such behaviour can be based on the assumption that carbene-Mo complexes are formed much faster with ethylene than with propylene and butene. This would also e x plain thc presence of an induction period during the first pulses of propylene. It i s known that carbene complexes, which themselves can undergo disproportionation, can be isolated from homogencous olefin metathesis systems(l5,ld). According to the mechanism proposed by Casey and Rurkhardt (15), a carbene complex is initially required in order ts catalyze disproportionation. Figure 3 shows the influence of the value of oxidation number on the catalytic activity, measured after 15 pulses. Similar to the results presented by Komatsu et a1.(6), the activity decreases
266
with increasing oxidation number for HY supports. The low activity o f catalysts originating from NaY zeolites i s due probably to the low dispersion of Moo particles ( 7 ) . In the present paper U V / V I S transmission spectroscopy has been applied to study the adsorption and decomposition o f supported M o ( C 0 l 6 . Figure 4 shows the U V / V I S spectra of molybdenum carbonyl supported over HY zeolite, pretreated at 6 7 3 K . The band positions at 230, 285 and 3 2 5 n m , characteristic for Mo(C016 (171, indicate that at room temperature only physisorption occurs. The changes in V."
Fig.4. U V / V I S spectra of Mo(C016 supported at r o o m temperature over hydrogen form o f Y-zeolite recorded after : a - 5 min.; b - 20 min.; c - 1 hr; d - 2 h r ; e - 4 hr. The amount of Y O ( C O ) ~ introduced 0.0127 mmo1-g- .
0.6
1
2
the absorbance after M o ( C O ) ~ admission suggest a very slow migration of carb0.2 onyl particles inside the zeolite cavities. The decrease of the BrMnsted site 0.0 population (Table 1 ) with increased 200 300 400 pretreatment temperature to 7 7 5 :< WAVELENGTH Inm 1 causes the reaction of introduced W O ( C O ) ~ with the zeolite framework already at room temperature. This was observed a s changes in the main band intensities as well as the ayp:'arance o f th:: ahou1dL.r b a n d s 3 t 7 5 0 a n d 2 7 3 nn. I n a C L i l i o n , t.volutioii
~ i n s ~ : :;i i z
cnszd Zrbnstzrl acidity was cven
inorc
pronounczd 1,ihen the zeolite 'Fig.5.
1
I
300 wx) WAVELENGTH Inm 1
1 500
uv/vIs s!JC?CtrJ
Of
supported at room temperYturc over Nay zeolites. a - after mission of 0.0091 mmo1.g and recorded after 1 min. b - recordod after 30 min. c - after adnissior another 0.0091 mno1.g- , recorded after 1 min. d - recorded after 1 hr.
:40( 30) I
200
;,PI'Jct o f cir3cr-
'
267
was almost completely dehydroxylated at 875 K. The band at 230 nm was present only at the initial stage of adsorption, whereas the bands at 250, 270 and 340 nm increased their intensity.App1ication of the support with Si/Al=2.11 produced the same tendency as in the previous case; however, the decomposition of Mo(C016 started already at room temperature, even if the support was pretreated at 675 K . The instant decomposition of molybdenum carbonyl at room temperature was observed for NaY zeolites. Figure 5 shows spectra after adsorption of Mo(C0)6 over these supports. The prolonged contact of molybdenum carbonyl with NaY caused the appearance of new bands at 260, 320 and 390 nm. Concomitantly,a strong evolution
-300 K - _ _ _ 375 K 175 K
Fig.6. U V / V I S spectra of Mo(C0) adsorbed at room temperature ovgr HY-zeolite and activated at different temperatures. Support pretreatment temp. -675 K Si/Al = 2.72 Amount-pf Mo(CO)~ adsorbed - 0.0138 mmol-g .
of CO was observed. The amount of released CO, both under static and 200 300 Loo 500 dynamic (He flow) conditions suggests WAVELENGTH Inm 1 the formation of Mo(CO)~ ads.species. On the other hand, the bands at 320 and 3 9 0 nm can originate from dimolybdenum-like species (18). The appearance of the same bands has been also noticed for HY supports however, only after activation up to 425 K ( s e e fig.6). Activation at higher temperatures was always associated with the decrease of band quantity and intensity. The results presented suggest that mainly basic sites are involved in the decomposition of supported MO(CO)~. In both cases, of NaY and HY zeolites, the oxygen anions act as basic sites. However, f o r NaY the oxygen ion carries a much higher negative charge than the corresponding oxygen in the H-form of the zeolite. Additional U V / V I S experiments with olefins and Mo-loaded zeolite s indicated the interaction of propylene and butenes with the acidic supports (both protonic and non-protonic) with fast formation o f r - a l l y 1 carbocations (19). Contrary to that, the interaction of ethylene and the zeolite acidic centers was much slower, -1
268
and formation of r - a l l y l i c structures was very much delayed. Such behaviour can explain the increased activity in propylene metathesis after ethylene pulses, as well as the existence of an induction period at the initial stage of the reaction. In the case of propylene the competition reaction of j-r-ally1 complexe formation proceeds first, and next the carbene-Mo complexes are formed. With ethylene the reverse sequence is probably involved, and 7 - a l l y 1 carbocations can be developed after the formation of carbene complexe. CONCLUSIONS 1. The decomposition of Mo(C016 over NaY and HY zeolites occurs on the basic sites. Surface hydroxyl groups of HY zeolites are responsible for the oxidation of supported molybdenum hexacarbony1 at elevated temperatures. 2. In the absence of hydroxyl groups in NaY zeolites the supported metallic Mo can be formed. 3 . The catalytic activity in propylene metathesis is strongly influenced by the value of the oxidation number of supported Mo species. The lower the O.N. of Mo, the higher i s the catalytic activity. Application o f NaY supports leads to the formation of a catalyst with low activity due to the poor dispersion of Moo particl es. 4. Carbene-metal complexes are probably involved in the mechanism of propylene disproportionation over these catalyts. The mechanism via metallocyclobutane intermediate has to be questioned because of its validity on a thermodynamic basis. Besides, no cyclopropane has been observed from propylene, as would be the case if a metallocyclobutane were a n intermediate. ACKNOWLEDGEMENT The author is indebted to Or. H.G.Karge f o r the critical revision o f the manuscript and the provision of the UV/VIS facility which made this work possible. REFERENCES 1 P.E.Dai, J.H.Lunsford, J.Cata1. 6 4 (1980) 173-184 2 M.B.Ward, J.H.Lunsford, Proceed.6th 1nter.Zeolite Conf. July 1983, Reno (D.Olson,A.Bisio Eds.), Butterworth 1984, pp. 405 3 P.Galezot, G.Courdier, M.Primet, B.Imelik, ACS Symposium Ser. 40 (1977) 144-155
269 Ir
5
9
10 11 12 13 14 15 16 17 13 19
S.Abdo, R.F.Howe, J.Phys.Chem. 07 (1983) 1713-1722 and 17221730 T.Komatsu, S.lJamba, T.Yashima, Proceed.Int.Symp.on Zeolite Catalysis, Siofok, blay 13-16, 1985, pp.251 T.Komatsu, S.Namba, T.Yashima, K.Domen, T.Onishi, J.Mol.Cata1. 3 3 ( 1 9 ~ 5 ) 345-356 T.Komatsu, T.Yashima, J.Mo1.Catal. 40 (1987) 83-92 Y.S.Yong, R.F.Howe, Proceed. 7th Inter.Conf.Zeolites, August 17-22, Tokyo 1986 (Y.Murakami et al. Eds.), Kodansha Ltd., 1936, pp.833 t 4 . B . Nard, X .Mizuno, J. H. Lunsford, J. M o l . Catal. 2 7 (1904) 1-10 H.Kacirek,H.Lechert, J.Phys.Chem. 79 ( 1 9 7 5 ) 1587-1593 M.tanieclci, R.L.8urwell Jr.,J.Coll.Interface.Sci. 7 5 (1980) 95-104 H.G.Karge, M.Zidlek, M.taniecki, Zeolites, 7 (1907) 197-202 Selected Values of Properties o f Hydrocarbons and Related Compounds, API Research Project 4 4 , Texas A and M University, College Station, Texas. A.Brenner, R.L.Burwel1 J r . , J.Cata1. 5 2 (1970) 364-374 C.P.Casey, T.J.Burkhardt, J.Am.Chem.50~. 76 (1974) 7000-7810 D.J.Cardin, M.J.Doyle, M.F.Lappert, Chem. Comm. (1972) 927-923 ll.Strohmeier, K.Gerlach, Z.Phys.Chemie, Neue Folge, 27 (1961) 439-442 M.L.Carson, F.W.Moore, 1norg.Chem. 2 (1963) 881-884 M.taniecki, H.G.Karge, in preparation
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H.G.Karge,J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
I. M. Harris,' J. Dwyer,' A.A. Garforth,' C.H. McAteer2 and W.J. Ball2 Chemistry Department, UMIST, P.O. Box 88, Manchester M60 lQD, U.K. BP International Plc, Research Centre, Sunbury-on-Thames,Middlesex, U.K.
'
ABSTRACT
Mo oxide is incorporated into H-ZSM-5. At low loadings, up to 8% oxide, molybdenum is highly dispersed on both internal and external crystalline surfaces. Tetrahedral dioxomolybdenum species are formed by reaction with zeolitic hydroxyls. Molybdenum-oxide-loadedH-ZSM-5 catalysts are evaluated using several reactions and some comparisons are made with gallium counterparts. Catalytic conversion of n-butane over H-ZSM-5 and over zeolites with low loadings of Mo can be explained by an initial activation which involves direct protonation at both C-C and C-H bonds. At higher loadings of Mo oxide and in the presence of Ga oxide an initial dehydrogenation can be involved. As conversion of butane increases secondary products including aromatics are produced. Conversion of butane, butene and 1,7 octadiene demonstrate an order for aromatisation H-<Mo-
The incorporation of transition-metaloxides into zeolite catalysts permits modification of both acidity and diffusivity and can introduce a hydrogen transfer function. Although molybdenum-containing zeolite catalysts have been developed for hydroprocessing there are few reports on the activation of small hydrocarbons by molybdenum
-
containing pentasil
zeolites. This work considers the incorporation and dispersion of molybdenum oxides in H-ZSM-5 and the catalytic consequences particularly in the conversion of n-butane. Results for conversion of butene, 1.7 octadiene and trimethylbenzene are also examined with a view to understanding mechanisms and site location. Experimental results for molybdenumcontaining H-ZSM-5 are compared with results for a gallium-loaded catalyst which has commercial significance (1). 4 lm E m l Zeolite ZSM-5 (Si/Al = 14) was synthesised using a patented procedure ( 2 ) . It was modified in the ammonium form either by impregnation or by direct mixing. The impregnation procedure involved addition of NH4-ZSM-5 to
a solution of ammonium heptamolybdate stirred at 80 "C. The resulting thick paste (pH 5 ) was dried at room temperature and calcined in air at 500 O C (16 hours). Direct mixing involved gentle grinding of NH4-ZSM-5 and Moog to
212
produce an intimate mixture which was then calcined as above.
A Philips PW 1400 instrument was used for XRF chemical analysis and a Philips PW1380 diffractometer for powder diffraction. Mid-infrared spectra were obtained using KBr discs and a Mattson Cygnus 100 FT-IR instrument and the hydroxyl region was investigated using self-supporting discs and a Nicolet 7199B FT-IR spectrometer. TEM and HRTEM images were obtained using a JEOL 2000 EX instrument with point resolution of 2.1
1 and
EDS analyses
were made with a JEOL 2000 PX TEM equipped with a link system analyser. XPS results were obtained using a VG ESCALAB Mk 1 instrument and 27Al and 29Si
MASNMR spectra of fully hydrated samples were produced using a JEOL GNM GX400 FT NMR spectrometer (27Al) or a JEOL FX 200 spectrometer (29Si). For temperature-programmeddesorption of ammonia (TPDA) a VG micromass 6 mass spectrometer was used to monitor desorbed ammonia. RESULTS AND DISCUSSION
Powder XRD of uncalcined mixtures of Moo3 and H-ZSM-5 shows a peak at
8, due to Moo3, which increases linearly with the MOO3 content. However, after calcination this peak cannot be detected in the powder
d = 3.26
pattern at oxide loadings less than 8% (Fig 1). Since XRF analysis (3) shows that MOO is not lost, on calcination, these results imply that Mo is 3 very finely dispersed by calcination. A similar spreading of MOO over 3
alumina and magnesia is reported (4) (5). The excellent dispersion of molybdenum oxide can also be seen by TEM. At 22 oxide loading Mo can be detected analytically but no separate phase is evident. At lo%, and higher loading, surface agglomeration of Mo oxide is observed (Fig 2). Sorption results are consistent with XRD and TEM. A fine dispersion in the pore system of H-ZSM-5 should reduce micropore volume and this is observed. Sorption capacity of p-xylene decreases up to a loading of around 8% oxide and, as expected, the rate of ingress of o-xylene is reduced by incorporation of the oxide ( 3 ) . Fig 3 shows 27Al MASNMR spectra. In NH4-ZSM-5 the single peak, with spinning side bands, at 56 ppm is due to tetrahedral framework aluminium. On calcination to produce H-ZSM-5 a proportion of this is dislodged as shown by the peak at 3 ppm due to octahedral extraframework aluminium. More severe calcination in the presence of molybdenum salts or oxide results in more extensive dislodgement of aluminium as evidenced by an increased signal at 3 ppm and the development of a broad signal associated with extraframework aluminium species of lower symmetry. At a given loading of Mo the amount of dislodged A1 depends on the calcination procedures (3).
FT-IR spectra of the hydroxyl region are shown in Fig 4a. The extensive
273 1
i J
/
mix
I .
I.W
.w
mid-crystal
I
i '0
4
12
6
MO
I
16 OXIDE (%I
!=I, 1.960
Fig.1. Intensity of major (XRD) peak for MoO3.
Fig.2.Dispersh of Mo oxide (EM).
i
la1
lbl
!& i ,-zsns
E
HQSH5
IS m
10 -CMMICU
o
1
-
IW
-S
S H I ~won IN IH,OI,I*/PP.
-
100
o
-m
Fig 3 nAI MASNHR laJNH,/H-ZSM-5 (bl Mo oxde/H-ZSM-5, calcined
/lO%Mo
20 10 0 r
3340 WVENUPBER I c i l l
3900
ml
I
I
WAVENUMBER Ici'l
Fig.b.Effect of calcination with Mo oxide. 1a)hydruxyl region. iblmid I R .
WJ
b-rich &e EM%~v=b!s
I
I
wo
1
0
2
4
6
8 10 Mo oxide (%I
Fig5 Effect of molybdenum oxide content on conversion of n-butane (SOOOC).
274
reduction in the band around 3601 cm-l (%70X),
on incorporation of2X Moo3.
is due to loss of framework aluminium by calcination (Fig 3) and interaction of bridged hydroxyls with molybdenum oxide. The reduced intensity of the band at 3740 cm-l reflects the interaction of terminal silanols with Moo3. Confirmation of this is seen in the mid-infrared spectra (Pig 4b). A band at 870 cm-l is indicative of bulk Moo3 but bands at 915 and 950 cm-' are assigned to molybdenum species in the zeolite. A band at 915 cm-l is previously ( 6 ) assigned to the M=O stretch in tetrahedral Mo species on silica or alumina
and bands at 890 cm-l (7) and 900 cm-l (8) are reported for Mo-modified zeolite Y. Consequently we assign the band at 915 cm-l to the M=O stretch in tetrahedral dioxomolybdenum species and the band at 950 cm-l to T-0-Mo vibration (T = A1 or Si). The absence of absorption at 915 and 950 cm-l in the catalyst prepared by calcining a mixture of MOO and Na-ZSM-5 supports 3
the role of zeolitic hydroxyls in generating these tetrahedral Mo species.
Thus, at lower loadings of Moog in H-ZSM-5 a dispersed layer of dioxomolybdenum species is generated on both internal and external surfaces of the crystals. XPS results show a largely symmetric doublet due to ejection of Mo3dgI2 and M 0 3 d ~ /electrons ~ with binding energies typical of Mo(V1) species (3). However, ESR spectra reveal the presence of some Mo(V) centres after calcination and these increase following reaction with n-butane at 500 OC (3). Relative rates of reaction of n-pentene and 4,4-dimethylpenteneS using a pulsed reactor and a hydrogen stream, show the presence of a hydrogenation function in the presence of Mo oxide (3). Discrimination against 4,4-dimethylpenteneat low loadings of Mo show that Mo in internal sites also possesses a hydrogenation/dehydrogenation function. At higher Mo loadings (10%) and on Moo3 no discrimination is observed (3). The accumulation of Mo oxide on outer crystalline surfaces (Figs 1 and
2) reduces the effectiveness of outer-surface acid sites. The rate of isomerisation of the bulky 1.3,s-trimethylbenzene at 500 "C decreases as Mo loading increases up to about 8% (3) consistent with the formation of a monolayer of dioxomolybdenum species (Pig 4b) by utilising internal and external hydroxyls (Fig 4a). At higher loadings there is build-up of an external phase of MOO with no further decrease in rate. 3 Of interest in this work is the conversion of n-butane which is used here
275
to assess the potential of Mo/H-ZSM-5 in dehydrocyclodimerisation reactions. Continuous-flow experiments using butane in nitrogen (10%) show that small quantities of Mo oxide enhance the rate of conversion at 500 "C, but also lead to increased rates of deactivation ( 3 ) . Over H-ZSM-5 the activation energy is 125 kJ mol
-1
. A pulsed microreactor is used to
estimate activation energies for the Mo/H-ZSM-5 catalysts which deactivate in the continuous mode. Activation energies decrease and then increase as the Mo oxide level is increased ( 3 ) . Consequently, as with the continuous reactor, enhanced activity is observed at lower loadings of Mo.(Pig 5). Changes in activation energy suggest that active sites may be modified by incorporation of Mo oxide or that new sites are produced, perhaps leading to a change in mechanism. In the multistage aromatisation of n-butane over H-ZSM-5 ( 9 ) the ratedetermining step involves the initial activation of the alkane. It is also known that the conversion of olefins over acid zeolites is facile compared with alkane conversion. Consequently, rates of alkane conversion can be enhanced by addition of dehydrogenation centres such as Ni or Zn (10) to the zeolite. Since the Mo/H-ZSM-5 catalysts shows activity in hydrogenation of olefins ( 3 ) it is possible that enhanced activity at low loadings of Mo could result from generation of olefins by dehydrogenation. Reduced rates at higher Mo loadings might then be explained by both a reduction in dehydrogenation sites, resulting from a poorer dispersion of
Mo, and a loss of Bronsted sites to capture olefins produced initially. Separate experiments (3)(11) show that conversion of butene and 1.7 octadiene is facile at the temperatures used, even with higher loadings of Mo oxide, which emphasises the importance of the initial alkane activation in rate factors. To examine this aspect further an intermittent-flowreactor is used, at 450 "C, to investigate product distributions at low levels of conversion. Some typical results are given in Table 1. scheme 1
276
TABLE 1 Conversion of n-Butane at 450 'C. Typical results using an intermittent-flow reactor H-ZSM-5
Conversion
0.40
Mo/H-ZSM-5
Ga/H-ZSM-5
1.08
10.31
5.97
44.9
24.7
5.5
48.8
26.5
26.9
7.7
18.2
28.3
32.2
9.8
12.3
30.3
38.4
14.3
18.8
12.2
76.3
43.4
34.5
10.6
17.8
1.9
6.8
13.7
22.1
6.3
10.4
3.6
1.7
Products (mo1/100 mol/C4H10)
26.9 i -C 4
C;
45.5
c5
0.3
6' B+T
Product H/C
2.49
2.48
1.5
5.3
2.51
2.59
Results at low conversion are similar for both H-ZSM-5 and for low loadings of Mo oxides, and this is emphasised by Fig 6. This suggests that mechanisms over H-ZSM-5 and low-loaded Mo/H-ZSM-5 may be similar, in which case an additional dehydrogenation function does not appear to be necessary to explain initial product distributions over Mo/H-ZSM-5 (2%). A mechanism which can account for these results may be proposed, based on direct protonation of butane according to scheme 1 (a), (b), (c) with minimal contribution from (d).
Reactions (b) and (d) cannot be distinguished on the
basis of products but (d) is presumed to occur on dehydrogenation centres or via radicallradical ion processes (11).
If the carbenium ions formed by the primary reactions (a)
-+
(c) are
desorbed largely as olefins. at very low conversion, then scheme 1 (a),(b)
-
(c) suggests that initial yields of Ca, H2; C;, C2; C;, C1 should be similar and experimental results at low conversion are in reasonable agreement with this (Table 1). Desorbed butenes are found to be in thermodynamic equilibrium (3). in agreement with carbenium ion precursors. Consequently, any olefin produced by dehydrogenation (d) or by other processes such as radical reactions (111, or by Lewis site hydride abstraction, is captured by acid sites prior to desorption. A contribution from processes associated with reaction (d), however, cannot be excluded. Over H-ZSM-5 it is proposed, on the basis of product distributions,
277
that reaction (d) is not extensive and that primary reactions involve direct protonation of C-C or C-H bonds. At low conversion (x = 0.28) over H-ZSM-5 at 450 'C, results (3) suggest that, for conversion of 100 mol n-C4HI0, 48 mol are converted via reaction (b),
25 mol via (c) and 27 mol
via reaction (a). At a conversion (x = 0.4) good agreement with experimental results(Tab1e 1) is found if 45 mol react via (b), 26 via (c) and 29 via reaction (a). These results suggest that protonation of C-C is slightly favoured, over protonation of C-H, over H-ZSM-5 but separate results (12) on H-ZSM-5 activated at higher temperatures suggested more extensive protonation of C-C bonds in n-C4 H10' The initial activation of hydrocarbons by acid zeolites has been a subject of considerable discussion and only recently has direct protonation of C-H (13) or C-C (14) bonds been proposed. The present work, and related studies (12), suggest that, in the case of n-butane, protonation can occur at both centres, the relative extent of these processes depending on catalyst pretreatment and reaction conditions.
If, on the basis of similarity in product distribution at low conversion, we accept that n-butane is initially activated over Mo/H-ZSM-5 (2%) by the same major processes (a), (b), (c) in scheme 1, then we require
an explanation for the enhanced activity observed with low loadings of Mo oxide. An explanation, without recourse to reaction (d), can be provided by considering changes in acidity as reflected in ammonia retention at higher temperatures (Pig 8 ) . Similar results (15) have been obtained for n-hexane conversion over steamed H-ZSM-5 where increased acidity is attributed to the presence of limited amounts of non-framework aluminium which can produce sites of enhanced activity (15)(16)(17).
The 27Al NMR results (Pig
4) show that calcination with Mo oxide does dislodge aluminium from the framework and, as suggested previously (15). an optimal balance between framework and non-framework aluminium may give optimal catalytic activity. However, in the present work the picture is complicated by the presence of molybdenum species which may be involved in active site modification ( 3 ) . At low conversion the carbenium ions produced by primary reactions (scheme 1) are largely in equilibrium with olefinic products, but at higher
conversions there is increased reaction of alkanes and olefins with carbenium ions which cover more of the reaction surface. Consequently, oligomerisation and bimolecular hydride transfer occur to give products which may be desorbed or cyclised to give aromatics depending on the product, the catalyst and the process conditions. As a result the products from the primary processes tend to decrease with increased conversion (Figs 6,7).
278
2
wO
4
6 1 0 conversion 1%)
6
Fig.6.Hydrogen and butenes at low conversion of n-butane (45OOC).
f
CONVERSION I%)
Fig.7. Propane and methane from butane (intermittant flow reactor,450Oc).
/' 2%Mo
/ / /
{21i/ /L:
,
,
;./Mo
c
10 20 30 w) ammonia desorbed at 4OOT [units)
0
r
Fig.8. Catalytic activit and acidity of Mo oxide/H- SM-5.
El
H-ZSK5
f
e 40 8
0
CONTACT TIME Is)
Fig.9.Relative activity of H-ZSM-5 and oxide modified H-ZSM-5.
20
40
60
Mo/H-ZSM-S Ga/H-ZSM-5
0
0 6
Ga
80 100 conversion (%I
Figlo. Aromatics selectivity (BTX) from conversion of n-butane over H-ZSM-5; 0.5% oxide loading (SOO'C).
I
Ga lMo1oxide imprmated in H-ZSM-5
Fig.11. Ammatics selectivity (BTX) at 40@C, 100% conversion (pulse reactor1,from butene or 1.7 octadiene.
279
The Ga/H-ZSM-5 catalysts are much more active than H-ZSM-5 or Mo/H-ZSM-5 (Fig 9) and products at similar conversion also differ. In particular there is much more molecular hydrogen and increased amounts of aromatics and isobutane. These preliminary and limited results for Ga/H-ZSM-5 are probably best explained by dual-function catalysis where both dehydrogenation/hydrogenation and acid catalysis can play a significant role. Initial activation of the alkane is presumably enhanced, in Ga/H-ZSM-5 by effective dehydrogenation [reaction (d) in scheme 11, to provide Ci which is then protonated. The increased amounts of i-C4 and aromatics, in secondary products observed at higher conversion ( 2 to lo%), with Ga as opposed to Mo-loaded catalysts (Table 1) is in keeping with a more effective role for Ga in hydrogen transfer and in olefin cyclisation. Selectivity to aromatics is increased as intermediates are cyclised at the expense of cracking (Fig 10). However, Mo does show some capacity for increased aromatics yield (Fig 10). These aspects are seen more clearly when postulated intermediates are used as reactants. At 400 "C quite low loadings of Ga oxide produce aromatic yields (BTX) close to calculated equilibrium values for the conversion of 1-butene or 1,7-octadiene (Pig 11). Higher loadings of Mo are needed to give significant increases in aromatics. Increased yields of aromatics presumably reflect increased cyclisation, at the expense of cracking, when Ga and Mo centres are present in H-ZSM-5. In addition, at very high loadings, when there is a separate bulk MOO phase, there is 3 possibility of cyclisation of olefins produced by dehydrogenation on coordinatively unsaturated Mo centres which can stabilise aromatic species (18)
.
- A We thank BP International Plc, Research Centre, Sunbury-on-Thames, for an ENRA and BP and SERC for a case award to IMH. We thank D. White and L.
Hayes (B.P. Research Centre) for TEM. REFmmlcEs 1 J.A. Johnson and G. K. Hilder, Proc. Nat.Petrol.Refines Assoc. Annual Meeting, Texas (1984). 2 W.J. Ball and D. G. Stewart, USP 4, 452, 907 (1984) 3 I.M. Harris, PhD Thesis, UMIST (1987) 4 S.R. Stample, Y. Chen, J.A. Dumesic, C. Niu. G.G. Hill, J.Cataly., 105 (1987) 445 5 T.F. Hayden, J.A. Dumesic, R.D. Sherwood. R.T.K. Baker, J.Cataly., 105 (1987) 299 6 Y. Iwasawa, Y.Nakono, S.Ogasawara. J.Chem.Soc.Faraday Trans 1, 74(1978) 2968; Ibid, 1, 75 (1979) 1465
280 7
8 9 10 11 12 13 14
15 16 17
18
R.Cid, F.J. Gil Llambias, J.L.G.Fierro, A. Lopez Agudo, J. Villasenor, J.Catal., 89 (1984) 478 P.E. Dai, J.H. Lunsford, J.Catal., 64 (1980) 173 R.F. Batchelder, H.W. Pennline, R.R. Schehl, D.H. Finseth, Energy Res.Abst., 10 (4) Abst. No.6549 (1985) M.G. Riley and R.G. Anthony, J.Catal., 100 (1986) 322 G.B. McVicker, G.M. Kramer and J.J. Ziemak, J.Catal., 83 (1983) 286 J. Dwyer, R.A. Shigeisi, unpublished work. W. 0. Haag and R.M. Dessau, Proc. 8th 1nt.Cong. on Catalysis, Berlin (1984) Vol 2, p.305, Dechema, Frankfurt-am-Main (1984) A. Corma, J. Planelles, J.Sanchez-Marin and F. Tomas, J.Catal., 93 (1985) 30 A.G. Ashton, S. Batmanian, D.M. Clark, J. Dwyer. F.R. Fitch, A. Hinchliffe and F.J. Machado, in B.Imelik et al. (Editors), Catalysis by Acids and Bases, Elsevier (1985) 101. C. Mirodatos and D. Barthomeuf, J.C.S. Chem Comun. (1981) 39 R.M. Lago, W. 0. Haag, R.J. Mikowsky, D.H. Olson, S.D. Hellring, K.D. Schmitt and G.T. Kerr, in Y. Murokami, A. Iiijima and J. W. Ward (Editors), New Developments in Zeolite Science and Technology, Kodansha-Elsevier (1986) 101 O.V. Krylov, Catalysis by Non-Metals, (1970).
H.G.Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SURFACE AND WALYTIC PR3PERPIEs OF THE NEW Z W L I T E TYPE LZ 132
z.
TVARUZKOVA~, M. WA',
P. JI&, A. NASIRI~,
G. GIORDNW~
and
F. TRIFIRO~
HeymvsW I n s t i t u t e of Physical Chenistry and Electrocknistry, Czechoslovak A c a d q of Sciences, -182 23 Prague 8 (Czechoslovakia)
'J.
2 D t-
of chenistry, university, co~enza( ~ t a l y )
3
Departrru3nt of Idustrial chemistry and Materials, U n i v e r s i t y of ~ologna,
1-40136 Bolcqna (Italy)
ABSTRACT The " s n a l l pore" zeolite LZ 1 3 2 exhibits, in ccmparison with other zeolites, an increased selectivity for the tpnsfonnation of qthanol to ethylene i n the reaction temperature range 35Ck.500 C. A t WEV = 2 h , the weight ratio of C2H4+ tC H to C H i n the reaction products ranges between 1 and 4. Besides the ef2&t of reactant s ~ p selectivity, s this fact may be interpreted as the participation of the asynetrical methoxy groups i n surface and as the presence of protordonor centres of lower acidity. They do not oligamerize ehtylene although they have this effect on the more basic molecule of ppylene. Coke of the type of polyenes is fonned both on the electrowacceptor and protol-rdonor cetres
&-4
.
I
~
I
O
N
The n w type of zeolite, LZ 132 (ref. 1), belongs to the group of "anall pore"
8) which fran the point of view of their selectivity are attractive for the catalytic transfonmtion of methanol to lwer olefins. The aim of this paper is therefore to characterize the surface and catalytic properties of HLZ 132, inclUaing the mechanism of ethylene and propylene oligcnnerizatim and that of the methanol transformation to ethylene and propylene. zeolites (its effective pore radius is 4.3
EXP-
The LZ 132 z e o l i t e was prepared by hyzlrothennal synthesis (ref. 1).The raw
product (Na+-fm, Si/Al = 33, 0.2 w t . % Na) was activated by decQnposing the stmzture-directagent either i n air or successively in N2 and 02, then by performing the ion exchange in 0.5 M NH4N03,ad finally by transforming the
agent into the H+-fonn by heating it a t either 350 or 5OO0C i n vacuo (Table 1). order to canpre the surface and catalytic praperties of the new zeolite with the HZSM-5 zeolite, canparative measuranents were also perfarmeed with a sanple of the latter (Si/Al = 13.6, Ar sorption capacity 5.9 m l e / g zeolite, ratio of the absorbancies OA(549 an-') rations bands i n question).
/ OA(454 an-')
= 0.69 for skeletal vib-
The crystallinity of HLZ 132 was characterized by determining the Ar and H20
TABLE 1
N N W
Surface and catalytic properties of the LZ 132 zeolite Qlm 1
2
LZ 132 Ian
S'D'A. sanple form deoanp. Of
3
4
5
sarption capacity
-le/g zeol. ) Na form Ar(-195OC) HqO(230C)
Activation O A ~ P A ~
H+
H+
invacuo (18 h)
7
8
2
1
0.47 3720
0.40 36l5
6 h
5OO0C
0.28 3740
-
inair 6OO0C 6 h
35OoC
0.46 3720
0.62 3615
5OO0C
0.27 3740
0.22 3615
35OoC
0.48 3720
0.64 3615
5OO0C
0.28 3740
0.21 3620
6OO0C
1) in N2 2) in o2 55OoC
6.34
7.8
7.9
7.72
8.6
9.7
$,A2
10
11
4
ro4 (Inin-5
3
-
0.32 3555
0.12
0.15
0.16
(5) S.D.A.
9
"b "b ".OM
35OoC
in& N a '
6
= structure-directixq agent ("tenplate")
= mrmalized a b d a n c i e s of the bands at 565 and 460 a n ' , resp. initial rate ro of C3H6 oliqanerization at 8OoC and p = 20 torr
0.59 3608sh
0.60 3600sh
-
-
o*61 3555
24.4
0.18 3555 28.2
:$:23.8 (5,6)
283
sorpticn capacities as well as absorban~yratios o%
/
O.2
in the range of s
~
t a l vibrations. The experimenbl details are given in the publications (refs.1-4) quoted in Table 1. The acidobasic properties were characterized by mesurhq IR spectra, i.e. by
the absorbancies of the bands of the structural OH groups of the zeolite as w e l l as the abmrbamies of the bands of adsorts3 d3-acetanitrile, the
de-
adsorption of which was performed a t 25OC wi t h a pressure of 0.27 kPa for 15 m i nutes on sanples which had been previously evacuated d kept a t 35OoC ovanight. The desorption toak place a t 25OC for 15 minutes. ?he measurmmts were carried out i n a high-vacuum IR cwette d with selfsqqortirg pellets (with the tkickness of 10 q / m 2 ); for details see (ref. 5) i n Table 1. The I R spectra were registered with a E F I R spectraneter Nicolet MX 1. The sane type of IR cwett&was used in measurements of the IR spectra of methanol adsorw in quantities ammting to 10m l e / g a t 2 0 ' ~ on self-supporttirq pellets (with thickness of 10 q / m 2 1; the tenperature of activation of the sanple in vacuo before the methanol adsorption was 35OOC. The IR spectra were registered after tempering the plate of the zeolite rapidly to a certain tarcperature i n the range 20-400°C, maintaining the sample a t t h i s tenperature for 30 minutes d then cooling it qui&ly d a ~ nto 20'~. The i n i t i a l ethylene a d promlene oligomerization rates a t 8OoC and polefin= = 2.66 kPa w e r e calculated fran the e x p r i m n t a l l y determined (with a McBain bal-
ance) t h e course of the mass increase of the HLZ 132 s q l e s . For experimental details see (refs. 5,6). The transformation of methanol was performed in a catalytic minoreactor a t
-
4OO0C and MEW = 2 h-' over a catalyst w i t h 0.4 0.6mngrain size. The f e d contained 17 vol.% CH30H, 68 m1.%H20 and 15 vol.% N2 under the total pressure of 101 &a. The reaction products were analyzed with a gas &anatogra& jekt MCH 112).
(Chmpre
RESULTS AND DISCUSSION Preparation and activation of H U 132 The X-ray diffractcgra&ic data of the NaU 132 zeolite correqmrded to those
given i n the literature (ref. 1). ~ 0 t hthe ~r and H ~ Osorption capacities and the a b s o r a ratio o ~ /l
in the range of skeletal vibrations shaw - when canpard with the NaLZ 132 - that the crystallinity of the HLZ 132 zeolite is preserved also in the H+-form. Different corditions of deccmposition of the tenplate (either mare drastic directly by air or m e gentle in N2 and then in a h (Table 1, column 2) have no effect on the crystallinity of the NaLz 132 zeolite and thus indicate the good thennal stability of the LZ 132 zeolite (Table 1, colunns 3-5, 7-9). Increased tenprature + of the activation of the H -form of LZ 132 causes the decrease of the absorbancies
e
284
of the carresponding to three types of OH groups only (Table 1, calm 7-10). These are probably protordonor sites and electmn-acceptor sites created by part i a l dehydmxylaticm, as follaws fran measurerents of I R spectra resulting fran
the interactions of HLZ 132 w i t h d3-acetanitrile (see below). Oliganerization of ethylene and prow1A t 8OoC, HLZ 132 is inactive i n ethylene oliganerization ( r o e 0.2 m i n - l ) . Frun the decrease of the oliganerization i n i t i a l rate (ro) of the more basic propylene (Table 1, column 11) with increasirq degree of dehydroxylation and frat? the resulting changes i n the nunber of both types of acid centres, it f o l l m that prcpylene is oligcmerized on both types of centres. Fran the experbentally determined (fran their man aksorbancies) decrease of the concentration of OH groups and increase of the concentration of Lewis centres with increasing tape-
rature of activation, a decrease of the i n i t i a l oliganerization r a t e fran
(ro)3000 = 24.4 to (ro)sooo = 16.3 may be p r d c t e d (since accoIcLing to (ref. 6) i n the course of dehyamxylatian fran tsm OH p u p one nw Lewis site is formed). This is i n good agreement w i t h the e x p d m n t a l l y founl (ro)5ooo = 15.6 min-1 (sample H in Table 1). he i n i t i a l o l i q m a i z a t i o n rate of ethylene ro-= 0.2 min'l is by orders of magnitude lacrR1: than t h a t of prcpylene (ro = 28.2 min-l) vhich is mre basic. The different behaviour of HLZ 132 i n the oliganerization of C2H4 and C3H6 irdicates that the proton-donor centres (and probably also the electron-acceptor centres) of the zeolite which take p a r t in the oliganerization on zeolites (ref. 6) are less acidic (or weaker) than,e.g., on HZSM-5, wkre the ethylene oligoaneriza-1 tion rate u d e r the smne conditions was ro = 45.1 m i n
.
z+ztivity and. selectivity The distribution of the prcducts of methanol transformation as a function of
(C2-)/T a f t e r 60 minutes on strean a s a function of the reachon temperature is given in Fig. 1. The dependence observed shaws that with increasing tanperature of the reactor the a n a n t of ethylene in the products increases too. With a feed containing no w a t e r w p u r a similar anposition of prcducts of methanol transformation was found, the presence of H20 having only a positive effect on the lifetime of the catalysts (Figs. 2,3). The results presented in Figs. 3 and 4 s h w that HLZ 132 is theunally stable in the course of the catalytic transformation and m y be easily regenerated. The I R spectra registered before and after the catalytic test in the range of skeletal vibrations (Fig. 4) did not charge and have shown that during the cata0 lytic reaction the r a t i o 0 A56w-l / A460m-l of the n o d i z e d a b d a m i e s of the bards i n question (and also the c r y s t a l l i n i t y of the zeolitic catalysts) undement no charqes. the on strean is given in T a b l e 2. The r a t i o of
285
TABLE 2
Catalytic transformation of CH30H over LZ 132 zeolite (Grain size 0.4-0.6 mf reaction terrperature 4OO0Cf WHSV (H20 + cH30H) = 6.5 h-lf input mixture 17 vol.% CH30H + 68 m1.%H20 + 15 vol.%N2. integral microreaCtor)
time on S+XW
onin. 1 15 60 120
total conv. of m3OH
(%I 99.4 96.1 49.1
mnv. of cH30H to hydrocarbons related to total conv. of CH30H to C1 %
c2+5 40.5 51.0 52.8
c3 7.8 3.5 1.7
5
15 0.0 0.0 0.0
31.8 34.5 37.0
- C5
zc4
EG
P 5
1.2 1.2 0.0
11.0 9.6 8.6
7.6 3.7 0.0
In the products no aranates w e r e found
Fig. 1. The hydrocarbon r a t i o the reaction tgnperature.
C2CdTafter 60 min.
on strean as a function of
Catalytic tests over the HZSM-5 zeolite (Si/Al = 1 7 ) , performed under the same reaction conditions, led to a pmduct with a significantly 1content of C2 + c3 olefins. The produd contained in t h i s case 22 wt.% C2 + (1 wt.%CZf 8.3wt.%
5
C3f 26.5 wt.% Csf 37.5 wt.% C4 bons and amnatics.
+ C5 hydrocarbons; the
rest mre higher hydrocar-
286
Fig. 2. The total mnversion of CH (wt.%) deperdirrg on thd time on stream a t 400°C and WlISv = 2 h-'. w i t h water feed without water
A
0
I
I
2
4
I
6 number of cycles
-
Fig. 3. The conversion of CH OH (wt.%) to C ocarbns as a function of the n u b r of working cycles a t aOO°C and WJSV 2-3 , time on stream 1 hour, E a r 5 mrkiq cycle is mnposed f m n the CH OH tgmsfolmaticm for 120 min. and. fran the regeneraticm of HLZ 132 z e o l i t e a s 450 C i n the flaw of O2 f o r 4 hours: feed with w a t e r feed without water
Ey'
Fig. 4. I R spectra of the HLZ 132 zeolite in the frequency range of skeletal vibrations: 1) i n the original state 2) aftgr the catalytic t e s t a t 400 C, 6 working cycles
I
1
3800
I
I
I
cni’
1200
Fig. 5. Sorption of d3-ace&trile on HLZ 132 zeolite after the tramsfomtion of CH OH (6 mrkiq cycles) without regeneration: 1) b e h e the adsorption of d3-acebni.trile 2) after the adsorption of d3-acetonitrile.
288
The results shaw that the yields of C2
+ C3
olefins over HLZ 132 zeolite are
significantly higher than h (C2+Cj
/
e cbtained w i t h FWM-5. The weight r a t i o R = in the products a t t a i n s a t the reaction tanperatwe of 350 5OO0C
-
values of 1 to 4; namely R = 1.6 qt 4OO0C (Fig. 1), i.e. nearly 100 % higher than over HZSI-5 d e r the same reaction conditions, namely R 0.83. Analogious values of R obtained by chang (ref. 7) over " s ~ l l - p o r e "zeolites (erionite, chabasite, ZK-5) are also lawer than thme obtained over HLZ 132 a t the sane reaction
tanperature. Both values of R and the conversion sums to
5 + Cj hydrocarbons over
HLZ 132 are higher than over Nu-3 zeolite (ref. 8 ) which belongs also to the group of zeolites similar to levynite. A t 45OoC after 84 min. on stream the value of R
obtained over HLZ 132 is 2.1, whereas for MI-3 it is 0.92. Supposing that the dif-
ference between the oliganerization rates of C2H4 and C3H6 on the HLZ 132 zeolite is preserved d s o at higher reaction tanperatures, we m y interpret the high value of R i n the CH30H transformation products a s resulting fram the formation of coke fran the greatest p a r t of the C 3 ~ 6formed. man the IR spectra Fig. 5) it f o l l m that this coke on HLZ 132 corresponds only to polyens, as expected. This w a s cow finned by the presence of a strong band a t 1600 an-' (ref. 9 ) , whereas on HZ-5 the cake formed exhibits the structure of both polyenes ard polyarorrratics. Acidobasic praperties and their changes In the murse of the adsorption of acetonitrile the formation of a coordi7ation ccrqlex with el&xon-acceptor mlecules takes place, accarpanied by t h e s h i f t of the fundmental vibration UCCrN) tnwards higher values when canpared with the vibration fresuency of liquid acetonitrile. This e f f e c t is caused by
charges i n the coordination i n the nitrogen 2 s l o n e p a i r o r b i t a l , which are responsible for the strergthenim~of the CN band (ref. 1 0 ) . With regard to the f a c t that the spectnm of liquid acetonitrile exhibits tv.u bands i n the frequency region of the CN bod stretcking vibrations, i.e. the fundamntal band a t 2254 an-' as w e l l as the bard a t 2293 an-' caused by the Ferrni resonaxe between the wobination vibration u3 + u4 and the fundamental vibration u2, it is m e convenient to use d3-ac&nitrile which exhibits anly a single CN vibration. This is especiall y advantqaus i n the case of d3-acetonitrile sorption on zeolites, as i n this case the CW) h o d vibration is affected by the f0llmi.q factors: i) the Lewis centres of electron-acceptor type, ii) the electrostatic f i e l d of the cations, iii) the interaction w i t h the OH g r c u ~ 6of the zeolite which causes the freqency
s h i f t of the CN b w l as d l as the appearance of a hydmgen b a d i n the frequency range of the structural OH group Cref. 10).
The use of d3-acebnitrile in the sorption f a c i l i t a t e s very mch the interpretation of the spectrum. In Figs. 6,7 are present the I R spectra in the (CrNl bond s t r e k h h q vibrations range obtained w i t h d3-acetmnitrile sorption an HZW-5 a d
I
I
I 2600
B
A
I
I
I
.............
2000
2600
I
t
I
1
2000
c m-1
Fig. 6. Sorption of d -acetonitrile on HLZ 132 z e o l i t e before (A) and a f t e r (B) the reaction w i t h Ol$H: 1) evacuation a t 35OoC overnight and the subsequent sorption of d -acetonitrile a t rcun tgnperature, 15 min.; 2) desorption of d3-aceton i t r i l e a2 roam t e n p r a t u r e , 15 m i n .
2600
......... 2000 C l d
Fig. 7. Sorption of d -acetonitrile on €:ZsE2-5 zeglite before (A) and after (B) the interaction w i t h &+31: 1) evacuation a t 350 C overnight and the subsequent sorption of dj-acetonit2ile a t roan temperature, 15 min. ?, 2) desorption of d 3acetonitrile a t rccnn temperature, 15 min.
290
HLZ 132 both b e f o r e and a f t e r methanol adsorption. Table 3 presents the n o d ized absorbancies of the a x r e s p o n d i n g a b s o r p t i o n bards. TABLE 3 Sorption of d 3 - a c e t o n i t r i l e o n HLZ 132 and HZSM-5 before and a f t e r the i n t e r a c t i o n w i t h CH30H 0
0
0
sample
%425 (2500)
’2321
OA2294
HZ.9ul-5 HLZ 132 HZSM-SOCH HLZ 132-CCd3
0.32 0.24 0.24 0.18
1.04 0.45 0.66 0.33
1.00 0.54 0 -82 0.54
0
Am-1
-
OA2266 (2273)
“2113
0.67 0.52 0.46 0.55
0.19 0.10 0.13 0.09
nomlited absorbancies a t the given frequency
Ps follows fran Figs. 6,7, a f t e r the s o r p t i o n i n the range of ( C a ) v i b r a t i o n s three bands a t 2323, 2294 and 2266 an-’,
may be found. I n the case of the LZ 132
z e o l i t e t h e l a s t b a d is s h i f t e d towards the frequency 2273-6 an-’.
bands may be a t t r i b u t e d ( r e f s . 10,111: the k a x l a t 2323 an-’
Fran these
may be attributed to
the interaction w i t h the Lewis centre and t h e band a t 2294 cn-’
to the i n t e r a c -
t i o n of t h e electrostatic f i e l d of the Na+ c a t i o n w i t h d3-acetonitrile. The band
a t 2266 an-’ ( w i t h HZm-5) or 2276-3 an-’
( w i t h IILZ 132) appears i n the spectrum
only when d3-acetonitrile is adsorbed; a l r e a d y a f t e r 1 5 minutes of d e s a r p t i o n a t
roan tarperatwe it either disappears ccrrrpletely or i t s i n t e n s i t y d e c r e a s e s to an i n s i g n i f i c a n t shoulder. T h i s Lard m y be ascrited ( r e f . 10) to the e f f e c t of the OH group of the z e o l i t e on the CN group of d3-acetonitrile. I n the r e g i o n of OH groups s t r e t c h v i b r a t i o n s this e f f e c t is indicate2 by a broad band s h i f t e d t o 3420 an-’ ( w i t h HLZ 132) or 2970 an-’ (with SZSU-5). The bands a t 2425 an-’ (HZSM-5) and 2500 cat-’ are p r o w l y Cwertones of the OH berding v i b r a t i o n ( r e f .lo). As follows fran Table 3 and fran Figs. 6 and 7, after t h e s o r p t i o n d 3 - a c e b nitrile in the I R spectrum a l l three types of the bards mentioned above may be found o n b t h zeolites (i.e. E Z S - 5 and HLZ 132). T h i s w a s the case b o t h w i t h pure sanples previously evacuated and kept a t 35OoC overnight and w i t h samples after their i n t e r a c t i o n w i t h m e t h a n o l a t 40OoC. Differences appear o n l y in the i n t e n s i t i e s of the i n d i v i d u a l bands. When canparing the normalized absorbancies given in Tab. 3 for t h e pure z e o l i t e s , we m y cane to the conclusion that the nlrmber o f Lewis c e n t r e s on LZ 132 mounts to about a half o f that p r e s e n t on !!ZEN-5, t h e n u h e r of Eiqdnsted centres on both z e o l i t e s being a p r o x i m a t e l y the sane. After the s o r p t i o n o f d3-acetonitrile on z e o l i t e sanples after their interact i o n w i t h m e t h a n o l a t 400°C (see Iselow) t h e intensity of the bards a t 2321 and
29 1
2666 an-' decreases by 35% on the average i n the case of the HZW5 z e o l i t e , whereas w i t h the HLZ 132 z e o l i t e only the i n t e n s i t y of t h e bard a t 2321 cn-l (corresponding to t h e Lewis c e n t r e s ) decreases by approximately 30%. I n Fig.5 the I R spectra of the HLZ 132 z e o l i t e a f t e r the c a t a l y t i c test of methanol transformation are given. The coke formed is characterized. by bani% i n
the region 3000-2800 crr-l, 1610-1590 an-'
and 1450-1350 a!!'.Fran t h e i r observed
i n t e n s i t i e s w e may conclude t h a t the coke is of the p l y e n e s type a d more probabl y w i t h hydrogen-saturated c h a r a c t e r ( r e f . 9 ) . The band a t 1540 m-',
which is
typical f o r t h e a r a n a t i c s , is absent. After d 3 - a c e t o n i t r i l e has been sorbed on
appear, t h e band a t
this sanple, t h e very w e a k banis a t 2323 and 2269 cm-'
2293
6'(corresponding
to the i n t e r a c t i o n w i t h the N a '
c a t i o n ) i s absent. The
absence of the band a t 2293 mV1and the presence of t h e hrrls a t 1590 and 1610 an-'
( r e f . 9 ) indicate that n o t only have t h e a c i d i c c e n t r e s been blocked
but a l s o the pores of the z e o l i t e have been f i l l e d . The i n t e r p r e t a t i o n of t h e I R s p e c t r a of d 3 - a c e t o n i t r i l e adsorbed on the HLZ 132 z e o l i t e may be summrized i n the following way: 1) I n the EL2 132 z e o l i t e both types of acidobasic centres (i.e. proton-donor and
electron-acceptor c e n t r e s ) are present. 2) I n c m p r i s o n to HzSG5, t h e n W r of t h e proton-donor centres is approximatel y the sane, whereas the n w b e r of electron-acceptor c e n t r e s on HLZ 132 m u n t s
to h u t a half o f that on HZB1-5. 3 ) Methanol strongly bound on HLZ 132 a f t e r the i n t e r a c t i o n a t 4OO0C i s a l s o
blocking the electron-acceptor
c e n t r e s , whereas the m u n t of proton-clonor
c e n t r e s is approximately the same as on the original HLZ 1 3 2 before its i n t e r a c t i o n w i t h CH30H. The latter c e n t r e Sean t h e r e f o r e to be e a s i l y regenerated a f t e r the i n t e r a c t i o n w i t h methanol a t 4OO0C as w e l l as to be a c t i v e i n t h e methanol t r a n s f o m t i o n . The coke, which is of the polyenes type, is i n t h e
i n i t i a l phase of t h e i n t e r a c t i o n formed f i r s t on t h e electron-acceptor c e n t r e s
and only later also on the proton-donor c e n t r e s . 4 ) Spectra of HLZ 132 f u l l y covered Ly coke shm that t h e latter is localized. not
only on both types of acidobasic c e n t r e s b u t also in the pores of t h e z e o l i t e . Surface canplexes w i t h I n order to g e t an i n s i g h t i n t o t h e causes o f the higher selectivities of t h e HLZ 132 z e o l i t e i n the formation o f l m e r o l e f i n s C2
- C3,
the I R spectra of the
c a t a l y s t i n the frequency range of s t r u c t u r a l OH groups and methanol s u r f a c e complexes i n t h e temperature range 20
-
4OO0C of the i n t e r a c t i o n w e r e registered.
Such r e a c t i o n conditions simulate the i n i t i a l steps of the c a t a l y t i c t r a n s f o m t i o n of methanol. Fig. 8 s h m s t h e I R spectra of the HLZ 132 z e o l i t e i n the frequency range cor-
292
I
I
I
I . . - - - . -...-
I
.
3400
3800
1
I
I
2800
3050
crn-’
Fig. 8.
I R spectra of the HLZ 132 zeolite evacuated a t 4OO0C overnight:
1) before the interaction with methanol, 2) a f t e r the interaction with methanol a t 2Oog f o r 30 min. , 3) a f t e r the interaction with methanol a t 150 C f o r 30 min. , 4) a f t e r the interaction with methanol a t 4OO0C f o r 30 min. responding to the structural OH groups as w e l l a s i n the range correspanding to
the stretching vibrations u(C-H) before and a f t e r the interaction w i t h methanol. f i e corresponding absorbancies are given in Table 4. In the original I R specof the HLZ 132 zeolite four OH barids are present, namely a t 3720, 3615, 3600 and 3555 an-’. After the adsorption of methanol at 2OoC, follwed by a short desorp-
TABLE 4 Interaction of CH30H with HLZ 132 zeolite a t temperatures 20
(in normalized absorbancies temperature C0C,
25 100 150 200 300 400
0
- 4OO0C
- OAm-l) 0
0
0
0
A2987
A2972
A2957
%925
A2855
0.09 0.06 0.05 0.04 0.04 0.03
0.11 0.10 0.09 0.08 0.06
0.23 0.15 0.12 0.11 0.10 0.06
0.03 0.02
0.16 0.12 0.09 0.10 0.09 0.06
-
0.02 0.02
-
293
tion of the gaseous phase a t the sane tgrlperature, the spectrum exhibits two broad bands a t 3710 and 3500 an-' which inclicate the strong interaction of cIi30H w i t h the OH groups (the proton-eonor centres of the zeolite) anzl overlap the band a t 3740 an-'. In the region of the U(C-I;) vibrations two strong bands a t 2957 and 2855 an-' are formed, accompanied by shoulders a t 2987 and. 2925 an-', respectively. When the taperatwe of the interaction is l5OoC, a l l four bands appear i n the spectrum again. The reduction of their intensity i d c a t e s that a part of methanol has desorbd. whereas another part of it has remined solidly bound to the zeolite.
I n the region of the u(C-3) vibrations both bands and their shoulders formed a t ~ O O Cw e r e preserved; only their intensity decreased. In u t i o n , another intense band appeared a t 2972 at-'. M.th the interaction taking place a t 4OO0C a l l four original ban& of OH groups are preserved, their intensity being hithan a f t e r the interaction with n&haml a t 15OoC. A t the latter tanperature of interaction the intensity of a l l four bands i n the range of u (C-E) vibrations (including the shou1ders)is reduced so that even the shoulder a t 2925 cn-'
could not be
registered distinctly.
The decrease of the intensities of the bands i n the frequency range of
U(C-3)
vibrations (as w e l l as that of the calculated absorbancies), acwnpanied by the
increase of the intensities of the OH bards, indicates the cccurence of a reaction between the surface group fonned previously where gaseous hydrocarbon products are formed and the OH groups (the proton-donor centres)are regenerated sirrolltane ously. The coordination of the four above m t i o n e d absorption bands i n the I R spec-
trum w i t h the respective vibration groups provided the possibility of a better understanding of the processes which take part i n the interaction of methanol w i t h HLZ 132 a t higher t a p e r a w e s , i.e. a t 300-400°C i n this case. The bands i n the range of the structural OH groups vibrations as w e l l as their changes clearly correspond to OH groups w i t h prokudonor properties, as follanls also f r a n the I R spectra of HLZ 132 cbtained a f t e r the interaction with acetonitrile. The bands i n the range of the u(C-H) vibration may be - w i t h high prcbability ascribed to two types of surface methoxy canplexes zeolite-ocHj (designated A and El) which are farmed a f t e r the interaction of methanol with the protan-donor centres of the zeolite, similarly as has been described in previous publications (refs. 12-16] i n the case of HX, HY wl E m - 5 zeolites. The bards a t 2955 and 2855 cm" as w e l l as t k shoulder a t 2925 would correspard to the symnetrical zeoliteOCH3 species B. The aqmnetrical ccmplex A i s acrording to I b r r o w (ref. u) characterized by one s t r o q a d two weak C-H bonds. We may adopt the idea t h a t these weak C-I! bonds, when in two neighbowing surface canplexes, f a c i l i t a t e the elimination of water a d the formation of ethylene. Chr results are i n agrement with the suggestions made by Casci and whittam (ref. 8 ) about the role of "shape selectivity" of the zeolite carU=erning the
-
-
-
294
transformation of mthanol to lwer olefins over a similar "small pore" z e o l i t e Nu-3 ard they canplete the suggestions i n the following p i n t s . Over HLZ 132, e t h y l e n e m y be formed as a product of the reaction between either ha reactive surface woplexes of type A or one surface c q l e x of type A and gaseous methanol. T h i s model is i n agreement w i t h the mechan'sm of methanol transformation over ZS4-5 z e o l i t e w i t h dimethyl ether i n t e n n d i a t e , as has been suggested by Derouane ( r e f . 1 7 ) . The experimentally fourd. megroups i n HLZ 132 could s e r v e as a p r e c u r s o r to the formation of d h e t h y l ether or p l a y the role of the latter "in s t a t u M S C ~ "T .h i s would be in agregnent w i t h the mechanism of t r a n s f o r m a t i o n as quoted above. The subsequent transformation of ethylene t o higher o l e f i n s proceeds then to a reduced extent owing to the laver a c i d i t y of the HLZ 132 zeolite. For its h i g h e r a c i d i t y e t h y l e n e is not activate2 by the proton-donor centres a c t i q i n the f o l l w i n g r e a c t i o n , i.e. either the o l i g a n e r i z a t i o n or the r e a c t i o n with another molecule of CH30H. This leads to the lw y i e l d of higher olefins. The content of propylene and further higher olefins (with molecules more basic than that of ethylene) i n the reaction products is f u r t h e r reduced by o l i g a n e r i z a t i o n which leads to the prcgressive covering o f the zeolite by coke of the p l y enes type.
1 Eurcpsn P a t e n t Application, EP 91.048. 2 V. Bosdcek, V. Patzelovd, C. Wbl and Z. Tvaruzkovd, J. C a t a l . 36 (1975) 371. 3 G. Coudurier, C. Naccache ard J. V e d r h , J. Chen. Soc., Chan. Camun. 1982, p. 1413. 4 D.W. Eire&, i n Z e o l i t e Molecular Sieves Pbrqraph, J. W i l e y , N.Y. 1974, pp. 81, 415. 5 Z. Tvaruzkovd, G. C e n t i , P. J h and F. Trifiro, Awl. C a t a l . 1 9 (1985) 307. 6 L. Kubelkavd, J. Novdkovd, V. Bosdcek, V. Patzelovd and. 2. Tvmzkovd, Acta Phys. et Chen. 24 (1978) 189. 7 C.D. Qlarg, Rev. S c i . Erg., 26 (1984) 325. 8 J.L. Casci and T.V. Whittan, Zeolites, B. Drzaj et a l . (rntors), 1985 E l s e v i e r Sci. -1. B.V. 623. 9 L. Kubelkovd, J. Nov&ovd, V. Tupd and Z. T v m z h v a , Prcc. Inter. Symp. on Z e o l . Catal. Siofok 1985, H u q a q , p. 649. 10 C.L. Angel1 and Y.V. Haell, J. Phys. Chen. 73 (1977) 1153. 11 R.E. Sempels ard P.G. Rouxhet, Prcc. of the 49th N a t i o n a l Colloid Symp., Postdam, N.Y. 1975. 12 L. Kubelkova, J. Nwdkovd ard P. J h , Structure and R e a c t i v i t y Of Modified Zeolites, P.A. Jaaobs e t al. (Editors) 1984 Elsevier Sci. Publ. B.V., 217. 13 B.H. fibrrow, JCS, Faraday !trans. 1, 70 (1974) 1527. 14 P. Salvador and W. Kladnig, JCS, Faraday Trans. 1, 73 (1977) 1153. 1 5 V. Bosdcek and 2. T v m b A , Coll. Czech Chem. Cannun. 36 (1971) 551. 16 T.E. F o r e s t e r and R.F. Hawe, J. Am. chgn. SOC. 109 (1987) 5076. 17 E.G. Derouane, J.B. Nagy, P. Dejaifve, J.H.C. van Hooff, B.P. Spekman, J.C. V e d r i n e and C. Naccach, J. Caw. 53 (1978) 40.
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PtRh-DOPED ZEOLITES AS THREE-WAY-CATALYSTS: SIMS ANALYSIS AS A TOOL FOR THE SELECTION O F SUITABLE ZEOLITE TYPES C. PLOG, J . HAAS and J. STEINWANDEL Dornier S y s t e m G m b H , P. 0. Box 1360, D-7990 F r i e d r i c h s h a f e n (FRG)
ABSTRACT Principal q u e s t i o n s i n t h e d e v e l o p m e n t of e x h a u s t c a t a l y s t s a r e r e l a t e d to a n i n c r e a s e of t h e 1 - w i n d o w and a r e d u c t i o n of t h e n o b l e m e t a l c o n t e n t . In t h e s e a r e a s t h e u s e of n o b l e - m e t a l - d o p e d z e o l i t e s i s a d v a n t a g e o u s . T h e s e l e c t i o n of PtRh-doped Y-zeolite and m o r d e n i t e a s t h r e e - w a y - c a t a l y s t s by t h e a d d i t i o n a l use of t h e s u r f a c e a n a l y t i c a l m e t h o d SIMS is p r e s e n t e d . Depth p r o f i l e s s h o w e d t h a t t h e r e e x i s t s a c l e a r c o r r e l a t i o n f o r t h e r e a c t i o n s of t h e c a t a l y t i c c o n v e r s i o n of t h e e m i s s i o n of O t t o - c y c l e e n g i n e e x h a u s t s b e t w e e n c a t a l y s t a c t i v i t y and t h e chemical c o m p o s i t i o n of t h e s u r f a c e of t h e z e o l i t e c r y s t a l s .
INTRODUCTION The r e d u c t i o n of a i r p o l l u t a n t s is of c e n t r a l i m p o r t a n c e t o e n v i r o n m e n t p r o t e c t i o n . C o n s e q u e n t l y , a l l c o m b u s t i o n p r o c e s s e s , p a r t i c u l a r l y of a u t o m o bile e n g i n e s , a r e of c o n s i d e r a b l e i n t e r e s t . F o r e x a m p l e , t h e i n c r e a s e of t o t a l e m i s s i o n of n i t r i c o x i d e s f r o m 2.8 million t o n s p e r year t o 3 . 0 million t o n s p e r y e a r b e t w e e n 1982 and I986 is r e l a t e d e x c l u s i v e l y t o t h e i n c r e a s e of a u t o m o b i l e t r a f f i c . The e m i s s i o n of n i t r i c o x i d e s is t h e c a u s e of a c o n s i d e r a b l e a m o u n t of damage t o trees (Waldsterben). Besides n i t r i c o x i d e s , t h e c o m b u s t i o n p r o c e s s e s i n a n O t t o - c y c l e e n g i n e p r o d u c e h y d r o c a r b o n s ( H C ) a n d c a r b o n m o n o x i d e ( C O ) . w h i c h have a l s o to b e c o n s i d e r e d a s p o l l u t a n t s of i m p o r t a n c e . A s i g n i f i c a n t r e d u c t i o n of t h e e m i s s i -
on of a l l t h r e e t y p e s of p o l l u t a n t s , a t t h e p r e s e n t s t a t e of t h e a r t , is n o t poss i b l e t h r o u g h e n g i n e d e s i g n a n d ignition m o d i f i c a t i o n s . T h e r e f o r e , in t h e FRG ( a n d i n E u r o p e ) t h e t e c h n i q u e of c a t a l y t i c p o l l u t i o n c o n t r o l h a s b e c o m e imp o r t a n t s i n c e 1986. Before t h a t , t h e t e c h n i q u e of c a t a l y t i c p o l l u t i o n c o n t r o l w a s used s u c c e s s f u l l y i n t h e USA and J a p a n . A f t e r a t e s t p h a s e of o x i d a t i o n - . e d u c t i o n a n d d o u b l e - b e d
catalysts
( w h i c h , in p a r t i c u l a r , w a s p e r f o r m e d i n t h e USA), t h e t h r e e - w a y - c a t a l y s t i s now e s t a b l i s h e d w o r l d - w i d e . The t e c h n i q u e is t h e only p o s s i b i l i t y f o r c a t a l y z i n g t h e o x i d a t i o n and r e d u c t i o n r e a c t i o n s s i m u l t a n e o u s l y . T h e r e a c t i o n s a r e l i s t e d in t a b l e 1. C o n c e r n i n g a n a l m o s t c o m p l e t e r e m o v a l of a l l p o l l u t a n t c o m p o n e n t s , however, i t is n e c e s s a r y t o o p e r a t e a t X =1 , e . g. a s t o i c h i o r n e t r i c r a t i o of oxidi-
296
zing a n d r e d u c i n g c o m p o n e n t s in t h e e x h a u s t ga s. T h i s is a c hie ve d in t h e a u t o m o b i l e by using c o m p l i c a t e d m e a s u r e m e n t a n d c o n t r o l p r o c e s s e s ( 1 - p r o b e a n d electronic fuel injection). TABLE 1 S c h e m a t i c s of m o s t t h e i m p o r t a n t r e a c t i o n s in t h e c a t a l y t i c c o n v e r t e r . C,H, C,H,
co H,
+
( m + n / 4 ) 0,
+ m/2 0,
m CO, + n / 2 H,O m C O + n H,O
---0
---0
+ 1/2 0, --Q CO, + 1/2 0, --0 H,O
C O + NO --0 1/2 N, + CO, + 2 ( m + n / 4 ) NO ---0 ( r n + n / 4 ) N, + n/2 H,O) + m CO, C,H, H, + NO ---0 112 N, + H,O
W i t h i n c r e a s i n g o x y g en c o n t e n t ( X > 1 , l ean b u r n r e g i o n ) t h e ve loc ity of poll u t a n t r e d u c t i o n r e a c t i o n s ( n i t r i c o x i d e r e d u c t i o n ) d e c r e a s e s , w hile t h e re a c t i o n velocity o f t h e o x i d at i o n r e a c t i o n s ( C O a n d HC o x i d a t i o n ) i n c r e a s e s . T h e c o n d i t i o n s a r e r e v er s ed in t h e rich b u r n i n g r e gion ( X < l ) . Fig. 1. i l l u s t r a t e s t h e very n a r r o w r e g i o n f o r o p t i n r a m r emo v al of a l l t h r e e p o l l u t a n t c o m p o n e n t s a s a f u n c t i o n of t h e X-value, wh i ch , c o n s e q u e n t l y , is know n a s t h e X-window.
A-window loo n
x
60
h U C
60
U
.al
.-U ry w
40
0.w
0.06
*so
1.06
1.10
A-value Fig. 1. Efficiency o f a t h r ee- way - cat al y s t a s a f u n c t i o n of t h e X-value f o r a l l t h r e e p o l l u t a n t c o m p o n e n t s2)
.
297 N o t o n l ? t h e e f f i c i e n c j of a t h r e e - w a ) - c a t a l l s t ( X = l - t e c h t i i q u e ) , b u t a l s o f u e l c o n s u m p t i o n a n d engine p o l l u t a n t e m i s s i o n d e p e n d o n t h e X-value. T h i s is s h o w n i n Fig. 1. From Fig. 2 . i t f o l l o w s t h a t f o r e c o n o m i c a l r e a s o n s ( f u e l
c o n s u m p t i o n ) and a l s o with r e s p e c t t o r e d u c e d p o l l u t a n t e m i s s i o n , t h e o p e r a t i o n of O t t o - c j c l e e n g i n e s i n t h e lean burn r e g i o n
(X a b o u t 1.2) w o u l d b e d e s i -
r a b l e . However. a t t h i s 1 - v a l u e t h e efficiency of a three-way c a t a l y s t a s t o t h e n i t r i c o \ i d e r e d u c t i o n is e \ t r e r n e l ) u n s a t i s f a c t o r y .
Fig. 2 . I n f l u e n c e of t h e A-value on e \ h a u s t g a s e m i s s i o n a n d f u e l c o n s u m p t i o n of a n O t t o - c j c l e engine').
A l t h o u g h l o n g e \ p e r i e n c e with t h e o p e r a t i o n of t h r e e - w a ) c a t a l j s t s e x i s t s i n t h e U S A a n d J a p a n ( a n d , a l s o i n Europe b e c a u s e of i t s i n t r o d u c t i o n t o mar-
k e t m e a n w h i l e ) . s o m e s i g n i f i c a n t p r o b l e m s remain u n r e s o l v e d . Basic q u e s t i o n s i n r e s e a r c h and d e v e l o p m e n t c o n c e r n i n g e x h a u s t c a t a l ? s t s a r e r e l a t e d t o a n
i n c r e a s e of t h e A-window ( l e a n b u r n r e g i o n ) . a r e d u c t i o n of t h e n o b l e m e t a l c o n t e n t ( a t p r e s e n t a b o u t 1.S g n o b l e m e t a l per l i t e r c a t a l y s t v o l u m e ) . and
f i n a 1 1 j t he d e v e I o p me tit of h i g h - t e m p e ra t u r e - r e s i s t e n t c a t a 1y s t s . Besides t h e l a s t p o i n t . t h e u s e of n o b l e - m e t a l - d o p e d z e o l i t e s is of advant a g e . I n t h e p r e s e n t p a p e r . t h e s e l e c t i o n of s u i t a b l e z e o l i t e s by t h e a d d i t i o n a l use of t h e s u r f a c e anal).tical m e t h o d S l M S is p r e s e n t e d . By using s e c o n d a r y ion m a s s s p e c t r o m e t r j ( S I M S ) it is p o s s i b l e t o a n a l y s e t h e n o b l e m e t a l d i s t r i b u t i o n i n near s u r f a c e r e g i o n s of t h e z e o l i t e c r y s t a l l i t e s .
298 EXPERIMENT As starting materials for t h e preparation of t h e catalysts, commercially
available z e o l i t e s o f mordenite type and Y-zeolites were used (Bayer AG a n d Norton Comp.). The doping o f t h e zeo l i t es with noble m e ta ls w a s p e rfo rm e d by t h e m e t h o d o f ion e x c h a n g e c i t e d i n t h e l i t e r a t u r e . A f t e r p r e p a r a t i o n o f t h e c a t a l y t i c a c t i v e p o w d e r s by ion e x c h a n g e a n d s u b s e q u e n t thermal t r e a t m e n t o r reduction, cordierite honeycombs were c o a t e d
w i t h t h e p o w d e r s by a specially d e v e l o p e d p r o c e s s . T h e g e o m e t r y o f t h e c o r dierite honeycombs corresponded t o t h a t usually used i n t h e automobile indus t r y ( t e s t m o n o l i t e s . 3.15 inch l o n g , I inch d i a m e t e r , 400 c e l l s p e r i n c h ) . T h r e e - w a y - c a t a l y s t s p r e p a r e d by t h e m e t h o d d . a . w e r e t e s t e d in a l a b o r a tory p l a n t t o d e t e r m i n e t h e e f f i c i e n c y o f t h e r e d u c t i o n o f t h e t h r e e r e l e v a n t p o l l u t a n t s N O x , CO a n d HC. A s y n t h e t i c e x h a u s t g a s c o n s i s t i n g o f NO ( f o r
N O x ) a n d p r o p a n e ( f o r HC) a n d CO w a s u s e d . T h e l a b o r a t o r y p l a n t c o n s i s t s o f a gas-mixing s t a t i o n , a gas-heating s y s t e m . a reaction chamber containing t h e
c a t a l y s t , and finally a gas-analysis s y s t e m . The s t a n d a r d composition of t h e s y n t h e t i c e x h a u s t g a s is s h o w n i n t a b l e 2. TABLE 2 Standard composition o f exhaust g a s for t h e present experiments. Gas c o m p o n e n t
Fraction
(Val.-"/.)
---NZ
75.08
CO,
14.0
H2O
8.0
0 2
0.84
co
1 .o
"2
0.33
NO
0.1
C3H8
0.05
By using a g a s c y l i n d e r b a t t e r y a n d e l e c t r o n i c m e a s u r e m e n t a n d control
c o m p o n e n t s it is p o s s i b l e t o vary t h e e x h a u s t g a s c o m p o s i t i o n o v e r a w i d e r a n g e . The s y s t e m p e r m i t s s t a t i c and d y n a m i c m e a s u r e m e n t s , w h e r e t h e s y n t h e t i c e x h a u s t g a s c o n t a i n s a maximum o f 9 d i f f e r e n t c o m p o n e n t s . T h e t e s t p l a n t a l l o w s a maximum g a s v o l u m e s t r e a m u p t o 5 0 I / m i n .
299 The g a s - h e a t i n g s y s t e m a l l o w s e x h a u s t t e m p e r a t u r e s up to 700 OC. Prog a m m a b l e h e a t i n g using s t a n d a r d i z e d t e m p e r a t u r e p r o g r a m s p e r m i t s m e a s u r e m e n t s of t h e s t a r t i n g behaviour of t h e c a t a l y s t s w i t h r e l a t i o n t o t h e d i f f e rent pollutants. The g a s c o m p o s i t i o n d o w n s t r e a m a n d u p s t r e a m in t h e c a t a l y t i c c o n v e r t e r is c o n t r o l l e d by t h e g a s a n a l y s i s s y s t e m . T h e s y s t e m c o n s i s t s o f a GC w i t h FID
a n d HCD d e t e c t o r , and a c h e m i l u m i n e s c e n c e NOx a n a l y z e r . The chemical c o m p o s i t i o n of t h e c a t a l y t i c a c t i v e p o w d e r s w a s a n a l y z e d by EDX, a n d f o r n e a r - s u r f a c e r e g i o n s by SIMS. The i n f o r m a t i o n d e p t h of EDX is a b o u t 1 t o 2 pm. A t a c r y s t a l l i t e s i z e of t h e z e o l i t e s of 1 t o 4 ym, t h e v o l u m e c o m p o s i t i o n is a l s o d e t e r m i n e d by t h e E D X m e t h o d . The SIMS m e t h o d is c h a r a c t e r i z e d by a n i n f o r m a t i o n d e p t h of 1 a t o m i c laye r , t h u s e x a c t i n f o r m a t i o n a b o u t t h e c h e m i c a l c o m p o s i t i o n of t h e s u r f a c e c a n be o b t a i n e d . A s i m u l t a n e o u s s p u t t e r i n g by ion b o m b a r d m e n t a l l o w s a n a l y s i s of d e e p e r a t o m i c l a y e r s and c o n s e q u e n t l y t h e r e c o r d i n g of a d e p t h p r o f i l e . C o n c e r n i n g t h e a n a l y s i s of c a t a l y s t s u n d e r i n v e s t i g a t i o n SIMS m a s s s p e c t r a a t d e p t h s of 0.1, 0.2, 0 . 4 , 1, 2, 4 , 10 nm w e r e r e c o r d e d .
RESULTS A N D DISCUSSION The p o s s i b i l i t i e s f o r u s i n g Y z e o l i t e a n d m o r d e n i t e a s t h r e e - w a y - c a t a l y s t s will b e c o m p a r e d in t h e f o l l o w i n g , w h e r e t h e z e o l i t e s have b e e n d o p e d w i t h p l a t i n u m a n d rhodium in t h e r a t i o S:1. C o m p a r i s o n c r i t e r i a w e r e t h e c o n version c h a r a c t e r i s t i c of t h e p o l l u t a n t s NO, CO a n d p r o p a n e of a new a n d a used c a t a l y s t . T h e new c a t a l y s t s w e r e analyzed by SIMS c o n c e r n i n g t h e d i s t r i b u t i o n of t h e n o b l e m e t a l s . With t h e aim of c h a r a c t e r i z i n g t h e a c t i v i t y of a u t o m o b i l e e x h a u s t c a t a l y s t s it is i m p o r t a n t t o m e a s u r e t h e d e p e n d e n c e of t h e p o l l u t a n t c o n v e r s i o n on t h e 1 - v a l u e a s well a s its d e p e n d e n c e o n t e m p e r a t u r e . I t is p o s s i b l e t o p e r f o r m m e a s u r e m e n t s e i t h e r s t a t i s t i c a l l y o r dynamically ( 1 - p u l s e ) . I n t h e f o l l o w i n g , t h e s t a t i c c o n v e r s i o n c h a r a c t e r i s t i c s a s a f u n c t i o n of t h e e x h a u s t g a s temperature w i l l be presented. Fig. 3. s h o w s a typical c o n v e r s i o n c h a r a c t e r i s t i c of P t R h H - m o r d e n i t e .
A s can be s e e n , c o n v e r s i o n o n s e t f o r a l l t h r e e p o l l u t a n t s is a t a b o u t
2 2 0 OC. Besides t h e HC c o n v e r s i o n a s t e e p i n c r e a s e of t h e c o n v e r s i o n w i t h i n c r e a s e d e x h a u s t t e m p e r a t u r e is f o u n d . At a b o u t 300 OC, nearly 100 % c o n v e r -
300 s i o n f o r a l l t h r e e p o l l u t a n t s is f o u n d . The s o m e w h a t f l a t t e r i n c r e a s e of HC
c o n v e r s i o n is r e l a t e d t o t h e k i n e t i c i n e r t h y d r o c a r b o n p r o p a n e . C o n s i d e r i n g a real e x h a u s t g a s a s t e e p e r i n c r e a s e of H C c o n v e r s i o n d u e to u n s a t u r a t e d h y d r o c arbons would be expected.
A c o m p l e t e c h a r a c t e r i z a t i o n of a t h r e e - w a y - c a t a l y s t is p o s s i b l e o n l y by s i m u l t a n e o u s p l o t t i n g o f t h e c o n v e r s i o n o f a l l t h r e e r e l e v a n t p o l l u t a n t s . In c o m p a r i n g d i f f e r e n t c a t a l y s t s , h o w e v e r , a g r a p h i c view w o u l d l e a d t o conf u s i o n . T h e r e f o r e , t h e e x p e r i m e n t s will be p r e s e n t e d i n t a b u l a t e d f o r m . A s c h a r a c t e r i s t i c t e m p e r a t u r e s , t h o s e c o r r e s p o n d i n g to 50 % c o n v e r s i o n ( s t a r t i n g r e g i o n ) and YO X c o n v e r s i o n ( t y p i c a l o p e r a t i o n r e g i o n of t h e c a t a l y s t ) a r e listed.
loo
'
00 '
60 '
40 '
20
I
200
220
240
260
ZOO
300
Temperature [ "C ] Fig. 3. Typical c o n v e r s i o n c h a r a c t e r i s t i c of a new t h r e e - w a y - c a t a l y s t PtRhH-mordenite f o r t h e p o l l u t a n t s N O , CO and propane.
TABLE 3 C o m p a r i s o n of p o l l u t a n t c o n v e r s i o n o f new PtRh-doped z e o l i t e s . Ty Pe
SO % c o n v e r s i o n
NO
co
00 X c o n v e r s i o n
HC
NO
co
HC
PtRhH-M
289OC
201°C
294OC
290 OC
297 OC
3 0 8 OC
PtRhH-Y
292 OC
3 0 8 OC
341 OC
314OC
321 OC
4OO0C
of type
301 A c o m p a r i s o n of t h e c o n v e r s i o n b e h a v i o u r of t h e t w o PtRh-doped z e o l i t e s
m o r d e n i t e a n d Y-zeolite is s h o w n i n t a b l e 3 , w h e r e i t is s e e n t h a t t h e m o r d e n i t e is c h a r a c t e r i z e d by a s i g n i f i c a n t l y h i g h e r a c t i v i t y c o m p a r e d to t h e Y-zeol i t e . T h i s f o l l o w s f o r t h e r e d u c t i o n of n i t r i c o x i d e a s well a s f o r t h e o x i d a t i o n of c a r b o n m o n o x i d e . In p a r t i c u l a r , t h e high a c t i v i t y of t h e m o r d e n i t e c o n c e r ning t h e o x i d a t i o n of t h e k i n e t i c i n e r t h y d r o c a r b o n p r o p a n e is a n i m p o r t a n t r e s u l t . Y O % c o n v e r s i o n o c c u r s a t a t e m p e r a t u r e 100 OC l o w e r t h a n for t h e Yzeolite. A f t e r t h e e x p e r i m e n t s w e r e p e r f o r m e d , b o t h c a t a l y s t s w e r e a g e d in a t u b e oven a t 800 OC in o x y g e n - and w a t e r - v a p o u r - c o n t a i n i n g a t m o s p h e r e . T h e c o n version c h a r a c t e r i s t i c of t h e aged c a t a l y s t s is s h o w n i n t a b l e 4 . Again t h e h i g h e r a c t i v i t y o f t h e m o r d e n i t e c a t a l y s t c a n be s e e n . W h i l e t h e n o b l e - m e t a l d o p e d m o r d e n i t e s h o w s conversion of t h e p o l l u t a n t s ( b e s i d e s NO) a t t e m p e r a t u r e s s i g n i f i c a n t l y l o w e r t h a n 4 0 0 OC ( 9 0 % c o n v e r s i o n ) , t h e Y - z e o l i t e o n l y c o n v e r t s CO. P r o p a n e c o n v e r s i o n in t h e t e m p e r a t u r e r a n g e u n d e r i n v e s t i g a t i o n ( u p t o 5 0 0 OC) is f a r b e l o w 5 0 %. TABLE 4 C o m p a r i s o n of c o n v e r s i o n of PtRh-doped z e o l i t e s a f t e r o p e r a t i o n . Ty pe
PtRhH-M PtRhH-Y
*)
9 0 % conversion
50 % c o n v e r s i o n NO
co
HC
NO
co
HC
3 4 4 OC 375 OC
342 OC 401 OC
359 OC
-*) -*)
356 OC 4 4 0 OC
3 8 4 OC
-*)
-*)
c o n v e r s i o n b e l o w 5 0 % and 90 % over in t h e t e m p e r a t u r e r a n g e of t h e e x p e -
riments. The very d i f f e r e n t behaviour of t h e t w o z e o l i t e s c o n c e r n i n g t h e c o n v e r s i o n o f t h e t h r e e p o l l u t a n t s i n t h e e x h a u s t g a s of O t t o - c y c l e e n g i n e s s e e m s s u r -
p r i s i n g , t h e m o r e so when t a k i n g i n t o a c o u n t t h a t t h e Y - z e o l i t e a d o p t e d m o r e n o b l e m e t a l t h a n t h e m o r d e n i t e , a l t h o u g h c o n d i t i o n s f o r ion e x c h a n g e w e r e i d e n t i c a l . While t h e Y-zeolite c o n t a i n s 0 . 3 3 m a s s - % of Rh. t h e R h - c o n t e n t of t h e m o r d e n i t e is b e l o w t h e d e t e c t i o n c a p a b i l i t y of EDX. T h e c o n t e n t o f p l a t i -
num is of t h e o r d e r of 0 . 5 4 mass-% f o r t h e Y-zeolite and of 0.16 m a s s - % f o r t h e mordeni t e . Because of t h e higher c o n t e n t of n o b l e m e t a l , t h e Y-zeolite s h o u l d b e e x p e c t e d t o be m o r e a c t i v e t h a n t h e m o r d e n i t e . The z e o l i t e c a n n o t b e r e g a r d e d a s a n inert carrier f o r t h e noble metal, b u t s o m e synergy e f f e c t b e t w e e n noble
302 m e t a l sites a n d acid s i t e s of t h e z e o l i t e t a k e s p l a c e . T h i s e f f e c t p l a y s a n p a r t i c u l a r l y i m p o r t a n t r o l e c o n c e r n i n g t h e c o n v e r s i o n of s a t u r a t e d h y d r o c a r b o n s . However, b e s i d e s t h e s y n e r g y e f f e c t , p l a t i n u m is t h e m o s t i m p o r t a n t c o m p o n e n t in t h e a c t i v i t y of t h e z e o l i t e c a t a l y s t . A s w a s d e m o n s t r a t e d in e a r l i e r w o r k , t h e c o n v e r s i o n of g a s e o u s r e a c t a n t s
is f a v o r e d in n e a r - s u r f a c e r e g i o n s of t h e z e o l i t e c r y ~ t a l l i t e s ~ ’ ~a l’ t~h )o ,u g h t h e o p e n c h a n n e l s y s t e m of t h e z e o l i t e s c o u l d , i n p r i n c i p l e , a l s o i n d u c e t h e reactions. Therefore i t was necessary to investigate w h e t h e r t h e d i f f e r e n c e s in a c t i v i t y of t h e t w o z e o l i t e s can b e r e l a t e d t o a s u r f a c e e f f e c t . Z e o l i t e powd e r s w e r e i n v e s t i g a t e d by u s i n g SIMS, in p a r t i c u l a r c o n s i d e r i n g t h e n o b l e met a l d i s t r i b u t i o n in t h e c r y s t a l l i t e .
In Fig. 4. a P t d e p t h p r o f i l e ( Y - z e o l i t e , o p e n s q u a r e s : m o r d e n i t e , f i l l e d s q u a r e s ) t o a d e p t h of 10 nm is p r e s e n t e d . I n c o n t r a s t t o t h e b u l k v a l u e s t h e mordenite s h o w s a significantly higher Pt c o n t e n t i n near-surface regions. T h i s c o n t e n t is a b o u t t w i c e a s high a s f o r t h e Y-zeolite, a l t h o u g h t h e Y-zeolit e c o n t a i n s a b o u t 3 t i m e s m o r e Pt t h a n t h e m o r d e n i t e in t h e b u l k . M o r e o v e r , t h e P t d e p t h p r o f i l e of t h e m o r d e n i t e s h o w s a p l a t i n u m e n r i c h m e n t a t t h e out e r m o s t s u r f a c e of t h e c r y s t a l l i t e s . In c o n t r a s t t o t h a t , t h e Y - z e o l i t e s h o w s a l m o s t no p l a t i n u m a t t h e o u t e r m o s t s u r f a c e . T h e d i f f e r e n t d e p t h p r o f i l e s of t h e t w o z e o l i t e s c a n b e e x p l a i n e d by t h e very d i f f e r e n t c h a n n e l s t r u c t u r e . The m o r d e n i t e is c h a r a c t e r i z e d by a o n e - d i m e n s i o n a l p o r e s y s t e m . The Y - z e o l i t e , h o w e v e r , is c h a r a c t e r i z e d by a t h r e e dimensional pore s y s t e m with additional cages. The comparably l a r g e platinum c o m p l e x , which w a s used for ion e x c h a n g e , is a b l e t o d i f f u s e a t h i g h e r r a t e s i n t o t h e c h a n n e l s y s t e m of t h e Y - z e o l i t e , r e s u l t i n g i n a h i g h e r p l a t i n u m c o n t e n t . P l a t i n u m c l u s t e r s p r o d u c e d by r e d u c t i o n of t h e c o m p l e x a r e e x p e c t e d t o b e l o c a l i z e d in t h e b i g c a g e s m o r e i n s i d e t h e c r y s t a l l i t e . Due t o t h e n a r r o w m o r d e n i t e p o r e s y s t e m , d i f f u s i o n of t h e p l a t i n u m c o m p l e x is s l o w . T h i s r e s u l t s i n a low P t c o n t e n t . H o w e v e r , t h e p l a t i n u m c o n t e n t in n e a r - s u r f a c e r e g i o n s will b e e n h a n c e d . I t c a n t h e r e f o r e b e a s s u m e d t h a t by r e d u c t i o n P t c l u s t e r s a p p e a r in t h e very o u t e r m o s t s u r f a c e l a y e r s . C o n c e r n i n g t h e m o r d e n i t e , t h e a c t i v e m e t a l is l o c a l i z e d a t t h e l o c a t i o n of t h e r e a c t i o n . T h i s e x p l a i n s t h e c o n s i d e r a b l y h i g h e r c a t a l y t i c a c t i v i t y of t h e m o r d e n i t e c o m pared w i t h t h e Y-zeolite.
303
I\
I C
i)
.-
+,
L iJ
S
a
0
C
0 0 I
v
a
,
10
0 d e p t h C n m l ->
Fig. 4. SIMS Pt d e p t h p r o f i l e o f PtRhH-M ( f u l l s q u a r e s ) a n d PtRhH-Y ( o p e n squares).
A
I
C U
.C
4J
L iJ
C
a
0 C 0 0 I
r
Q'
.
......(I...,.
.... ...............................................................................................................................
LD
rn I
I0
0 d e p t h C n m l -->
Fig. 5. SIMS Rh d e p t h p r o f i l e of PtRhH-M ( f u l l s q u a r e s ) a n d PtRhH-Y ( o p e n
squares).
304 T h e r h o d i u m d e p t h p r o f i l e o f t h e t w o z e o l i t e s s h o w n i n Fig. 5. i l l u s t r a t e s t h e r e v e r s e c o n d i t i o n s a s t h e p l a t i n u m d e p t h p r o f i l e of Fig. 4. T h e Y - z e o l i t e c o n t a i n s m o r e r h o d i u m i n n e a r s u r f a c e r e g i o n s a s t h e m o r d e n i t e . T h i s , in a d d i t i o n . c o r r e s p o n d s t o t h e r a t i o s i n t h e t o t a l v o l u m e a n d c a n b e e x p l a i n e d by a f a s t e r ion e x c h a n g e o f r h o d i u m c o m p a r e d t o p l a t i n u m . T h i s f o l l o w s f r o m t h e a l m o s t i d e n t i c b u l k c o m p o s i t i o n of p l a t i n u m a n d r h o d i u m i n t h e Y - z e o l i t e , a l t h o u g h t h e ion e x c h a n g e s o l u t i o n c o n t a i n e d a five f o l d e x c e s s of p l a t i n u m . T h e i n f l u e n c e of t h e r h o d i u m c o n t e n t in n e a r s u r f a c e r e g i o n s of t h e z e o l i t e c r J s t a l l i t e s is of m i n o r i m p o r t a n c e a s t h a t of t h e p l a t i n u m f o r t h e t w o f o l l o wing r e a s o n s . F i r s t . t h e m e t a l l i c p l a t i n u m p l a y s a much m o r e i m p o r t a n t r o l e c o n c e r n i n g t h e c o n v e r s i o n of t h e p o l l u t a n t s a s a t o t a l t h a n t h e r h o d i u m . S e c o n d . c o m p a r e d t o t h e u s u a l Pt:Rh r a t i o of 5: l t h e r h o d i u m is e n r i c h e d in t h e near s u r f a c e r e g i o n s o f t h e t w o nobel m e t a l d o p e d z e o l i t e s . From t h a t , i t f o l l o w s t h a t an a d d i t i o n a l i n c r e a s e of t h e r h o d i u m c o n t e n t c a n n o t lead t o a f u r t h e r i n c r e a s e of t h e c a t a l y t i c a l a c t i v i t y . T h i s f o l l o w s f r o m a c o m p a r i s o n o f Fig. 4. and Fig. 5. T h e a n a l y s i s of t h e SlMS d e p t h p r o f i l e s s h o w e d t h a t t h e r e e x i s t s a c l e a r c o r r e l a t i o n f o r t h e r e a c t i o n s o f t h e c a t a l y t i c c o n v e r s i o n of t h e e m i s s i o n of O t t o cycle engine e x h a u s t s between c a t a l y s t a c t i v i t j and t h e chemical compos i t i o n o f t h e s u r f a c e of t h e z e o l i t e c r y s t a l l i t e s . I n p a r t i c u l a r t h e e n r i c h m e n t of p l a t i n u m c l u s t e r s f o u n d in near s u r f a c e r e g i o n s o f t h e m o r d e n i t e c r y s t a l l i t e s significantly increases t h e a c t i v i t j , e. g . concerning t h e oxidation o f hydrocarbons. C o n c e r n i n g p l a t i n u m e n r i c h m e n t and d i s p e r s i o n a t t h e m o r d e n i t e s u r f a c e t h e d e m a n d of a r e d u c t i o n of t h e nobel m e t a l c o n t e n t a t c o m p a r a b l e a c t i v i t y w a s p o s s i b l e . While c o n v e n t i o n a l t h r e e - w a y - c a t a l y s t s
c o n t a i n a b o u t 1.5 g
n o b l e m e t a l p e r l i t e r , a m o n o l i t e having a z e o l i t e c o a t i n g is c h a r a c t e r i z e d by a nobel m e t a l c o n t e n t b e t w e e n 0.1 and 0.5 g / l .
REFERENCES
1 2 3 4 5
C h e m i s c h e R u n d s c h a u 22, 03.06.1988, S. 3 8 . O b l a n d e r , K . a n d A . B. Nagel, VDI B e r i c h t e 531 ( 1 9 8 4 ) , S. 39. H e r r m a n n . C . , J . Haas a n d F. F e t t i n g , A p p l . C a t a l . 35, 299 ( 1 9 8 7 ) . H a a s , J . , F. F e t t i n g , C . Plog, W. Kerfin, W. G e r h a r d a n d G. R o t h Appl. C a t a l . 3.5, 311 (1987). H e r r m a n n . C., F. F e t t i n g a n d C . Plog, Appl. C a t a l . 39, 213 ( 1 9 8 8 ) .
H.C. Karge,.J, Weitkamp (Editors1, Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Else& Science Publishers B.V., Amsterdam - Printed in The Netherlands
TRANSFORMATION OF ETHANETHIOL OVER ZEOLITES
M. ZIdtEKl, P. DECYK1, M. 0EREWIdSKI2 and J. HABER2 'Faculty of Chemistry, A . Mickiewicz University, Poznah, Poland 21nstitute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland.
ABSTRACT The hydrogen sulphide elimination reaction of ethanethiol and further transformations of alkyl radicals were studied on sodium and hydrogen forms of X, Y and ZSM-5 type zeolites. BrMnsted acid sites are catalytic active centres in elimination, whereas sodium ions play the role of sites active in transformations of alkyl radicals. The activity of zeolites in elimination increases with the increase of the strength of the zeolite acidity,poisoning with pyridine thus favours the further transformations of alkyl radicals. Adsorption of hydrogen sulphide strongly enhances the transformations to aromatics due to generation of new active sites. INTROOUCTION In hydroprocessing operations petroleum components react catalytically with hydrogen. The most important reaction is hydrodesulphurization leading to the conversion of organic si~lphur compounds into H 2 S and hydrocarbon products (1). The aim of our present study is to investigate the possibility of the application of zeolites to convert thiols vithout the use of hydrogen and to disclose the nature of active sites of these catalysts responsible for the activity and selectivity in the transformations of ethanethiol (Et-SH). EXPERIMENT Catalysts The following catalysts were used: Na-XI Linde Lot N o . 2 5 6 5 3 3 0 , with Si/Al = 1.13; Na-Y, Leuna, with Si/A1 = 2.56; Na-ZSM-5, Ultraset T - l O f l O / J , synthesized in the Institute of Industrial Chemistry Warsaw, with Si/Al = 3 8 and hydrogen forms of these zeolites. Modified forms were prepared by an ion exchange with 0.25M solution of NH4C1. Hydrogen zeolites were obtained by the calcination of ammonium forms in a shallow bed at 673K for 4hr in a flow of pure, dried He. The following catalysts were obtained
306
(degree of exchange in parentheses): and H-ZSM-5 (100%).
H,Na-X (50%);
H,Na-Y (70%)
Acidity measurements The acidity of zeolites was estimated from the Temperature-Programmed Desorption (TPD) of pyridine. The sample of 50 mg of zeolite was activated a t 773K for 1 hr at a pressure o f ~ x I O -Torr. ~ After cooling to room temperature the sample was exposed to 2 0 yl of pyridine for 15 min. and outgassed for 45 min. at R.T. The TPD experiment was carried out in the temperature range 303-1073K a t a heating rate of 10 K/min. Desorbing pyridine w a s analyzed with a Balzers 311 mass-spectrometer. Reaction conditions The conversion of Et-SH was determined in a pulse microreactor, 8 mm in diameter, filled with 0.2 g of the dehydrated form of the zeolite. Zeolite crystallites were pelleted without binder, ground, sieved to a 0.5-1.0 mm diameter range and activated for 4hr at 673K in He flow. The H2S elimination of Et-SH was carried out a t 573, 6 2 3 and 673K. The flow rate of He as the carrier gas was 40 cm 3 /min at atmospheric pressure. Pulses of 1 p1 of Et-SH (Fluka) were introduced at the reaction temperature. Products were analyzed using an on-line gas chromatograph with flame ionization detector and a 4 m column filled with Chromosorb W (60-80 mesh) and silicon oil D C with 5% addition of stearic acid a s the active phase. Catalytic experiments at 623K were also conducted after poisoning the acid centres with pyridine (40 p l ) and after adsorption o f hydrogen sulphide on the activated samples from the flow of hydrogen sulphide ( 1 cm 3 /min) in helium through the catalyst bed at 623K for 1 hr. RESULTS Characterization of the zeolite acidity TPD spectra obtained after pyridine adsorption at R.T. over sodium and hydrogen forms of zeolites are presented in Figures 1 and 2. In the case of sodium forms of faujasite-type zeolites two maxima are observed on the desorption curves. On Fla-Y zeolite the iirst maximum appears a s a shoulder. he spectra have the s a n e characteristics a s those shown in Ref 2.The first maximum originates from physisorbed pyridine and the second from pyridine chemi-
307
Fig.1.
TEMPERATURE
TPO s p e c t r a obtained a f t e r a d s o r p t i o n of p y r i d i n e on sodium forms of zeolites.
IK 1
s o r b e d o n s o d i u m c a t i o n s . The T P D s p e c t r u m of Na-ZSM-5 a f t e r a d s o r p t i o n o f p y r i d i n e ( F i g . 1 ) s h o w s t h r e e maxima: t h e f i r s t , a t t h e l o w e s t t e m p e r a t u r e , i s d u e t o p h y s i s o r b e d p y r i d i n c or p y r i d i n e a d s o r b e d o n t e r m i n a l s i l a n o l g r o u p s ( 3 ) ; t h e s e c o n d i s d u e t o pyr i d i n e chdmisorbed on sodium c a t i o n s ; and t h e t h i r d t o p y r i d i n e c h e m i s o r b e d on t h e f e w Brtlnsted a c i d s i t e s which a r e formed a s a r e s u l t o f t h e t e m p e r a t u r e d e c o m p o s i t i o n of t h e o r g a n i c compounds u s e d i n t h e z e o l i t e s y n t h e s i s . On t h e b a s i s o f t h e p r e s e n t e d
- H,No-X
---- H.No -Y
........ H-ZSM-5
Fig.2.
273
473
673
TEMPERATURE
873
1073
IK ]
TPD s p e c t r a obtained a f t e r a d s o r p t i o n of p y r i d i n e on hydrogen forms of z e o l i t e s .
TPD s p e c t r a t h e f o l l o w i n g s e q u e n c e o f t h e a c i d i c s t r e n g t h o f s o dium c a t i o n s may b e c o n c l u d e d : Na-X Na-ZSM-5 Na-Y H y d r o g e n f o r m s o f z e o l i t e s show t w o TPD maxima ( F i g . Z ) . T h e f i r s t maximum a t t h e l o w e r t e m p e r a t u r e may b e a s s i g n e d t o p h y s i s o r b e d p y r i d i n e a n d t h e s e c o n d t o p y r i d i n e c h e m i s o r b c d o n t h e a c i d i c si-
>
t e s . H-ZSM-5
>
a n d H,Na-Y z e o l i t e s d o n o t s h o w a n y maxima f r o m p y r i -
308 d i n e a d s o r b e d on sodium c a t i o n s . H,Na-X
z e o l i t e i n which the
exchange o f sodium c a t i o n s was o n l y 50% (because of t h e d e s t r u c t i o n o f t h e framework a t h i g h e r d e g r e e s of exchange) shows a b r o a d band a t a b o u t 63QK w h i c h o v e r l a p s w i t h t h e maximum o r i g i n a t i n g f r o m p y r i d i n e a d s o r b e d o n sodium c a t i o n s ( a t 5 8 8 K - c . f . F i g . 1 ) .
I n the
case o f h y d r o g e n f o r m s of z e o l i t e s t h e f o l l o w i n g sequence o f t h e a c i d s t r e n g t h can t h u s be d e r i v e d : H-ZSM-5
> H,Na-Y > H,Na-X
E f f e c t o f temperature F i g . 3A.shows t h e c o n v e r s i o n o f E t - S H o n Na-ZSM-5 d i f f e r e n t temperatures.
zeolite at
The same sequence was o b t a i n e d i n t h e c a s e
o f a l l i n v e s t i g a t e d samples, i . e .
w i t h t h e i n c r e a s e of t h e r e a c t i o n
t e m p e r a t u r e , t h e c o n v e r s i o n o f Et-SH i n c r e a s e s . The m a i n r e a c t i o n p r o d u c t s a r e : H 2 S , e t h y l e n e , d i e t h y l s u l p h i d e and p r o d u c t s of o l i g o m e r i z a t i o n and a r o m a t i z a t i o n . The h i g h e s t amount of E t 2 S i s o b s e r v e d o n Na-X z e o l i t e . F i g . 36 p r e s e n t s t h e a c t i v i t y o f d i f f e r e n t z e o l i t e s a t 573K. F o r sodium f o r m s o f z e o l i t e s t h e f o l l o w i n g sequence o f c o n v e r s i o n
i n t h e f i r s t p u l s e o f E t - S H i s observed: Na-X
> Na-ZSM-5 > Na-Y
and f o r h y d r o g e n f o r m s H-ZSM-5
> H,Na-Y > H,Na-X
,
TIME [ M I N I ,
-&?
* 1qo
2!0*
1-
.
NUMBER
31I0
100
A No-ZSM-5 A H-ZSM-5
200 oNa-X
300
OH,Na-X 0 NO-Y
OF P U L S E S
F i g . 3. E t - S H c o n v e r s i o n on z e o l i t e s : ( A ) Na-ZSM-5 a t d i f f e r e n t on d i f f e r e n t z e o l i t e s a t 573K. t e m p e r a t u r e s , (€I)
309
As seen from Fig. 38 in the case of Na-X zeolite the conversion rapidly decreases with the number of pulses, whereas for other zeolites it decreases only to a small extent o r remains constant, Thus it may be concluded that rapid deactivation of Na-X zeolite takes place in the course of the reaction. Reaction products Tables 1 and 2 represent the initial conversions of Et-SH and the yields of products obtained over sodium and hydrogen forms of zeolites at 623K. The product distributions on sodium forms of faujasites and Na-ZSM-5 are different. The main reaction products over Na-X and Na-Y zeolites are ethylene as the result of elimination, and diethyl sulphide, olefins and aromatics resulting from further transformations of ethyl groups. At variance, Na-ZSM-5 zeolite favours only the elimination reaction, yielding predominantly ethylene. TABLE 1 Conversion of ethanethiol over sodium forms of zeolites at 623K ~
~~~
Catalyst
Na-X
Na-Y
Na-ZSM-5
Conversion ( % )
89.8
19.7
74.7
Yield of products ( % ) Ethylene Butane, butenes Cc5 (olefins,paraffins) Abomat ics Diethyl sulphide
67.0 1.0 0.6 0.5 7.0 12.9
10.1 0.7 1.2 1.2 2.2 4.3
73.4 0.2 0.2
-
0.1 0.6
The exchange of sodium cations for protons causes the increase of the Et-SH conversion in the case of Y and ZSM-5 zeolites (Table 2). A change in the product distribution is also observed. The yield of ethylene becomes higher and that of other products lower. H,Na-X zeolite differs in its behaviour, showing lower initial activity than Na-X; but after exposure to a higher number of Et-SH pulses the activity of both catalysts becomes similar, a s shown in Fig. 36. This may be easily understood if i t i s remembered that the hydrogen-exchanged Na-X zeolite is partially dealurninated and in the acidic atmosphere of reactants slowly decomposes.Indeed, the ir spectra obtained after reaction indicate partial decomposition o f the faujasite framework.
310
TABLE 2 Conversion of ethanethiol over hydrogen forms of zeolites at 623K Catalyst Conversion ( % ) ~~
H,Na-X
H,Na-Y
31.2
30.1
87.8
22.6 0.7 1.1
21.3 0.4 0.2 0.1 1.3
86.3
H-ZSM-5
~
Yield of products ( 9 6 ) Ethylene Butane, butenes cc5 (olefins,paraffins) Agomatics Oiethyl sulphide
-
2.8 4.0
0.3
-
0.1 1.0 0.1
0.8
Influence of pyridine adsorption After pyridine adsorption at reaction temperature the total activity of catalysts distinctly decreases. This is accompanied by considerable changes in the product distribution, as shown in Table 3, in which the selectivity to different products is summarized. The selectivity of Et-SH conversion towards ethylene decreases after adsorption of pyridine,whereas that of the rest o f products - increases. The most distinct effect of the pyridine poisoning is observed on 2 3 4 - 5 type zeolites. TABLE 3 Influence of pyridine poisoning on the activity and selectivity of hydrogen forms of zeolites (TR = 62310 in ethanethiol conversion. H,Na-X pure poisoned
H, Na-Y pure poisoned
pure poisoned
31.2
13.4
30.0
14.2
87.8
21.2
Selectivity to different products (5): Ethylene 72.5 Butanes,butenes 2.2 3.5 c5 C (olefins,paraffins) 9.0 Agomatics Oiethyl sulphide 12.8
61.9 4.5 12.1 0.7 4.5 15.7
91.0 1.3 0.7
81.0 2.8 8.5 0.7
98.3 0.3
64.2 5.2 7.5 1.4 10.4 11.3
Catalyst Conversion ( % )
0.1
4.3 2.6
3.5
3.5
H-ZSM-5
-
0.1 1.2 0.1
Influence of hydronen sulphide preadsorption After preadsorption of H2S the conversion of ethanethiol increases. This effect is most spectacular in the case of Na-X and H,Na-X zeolites. This is understandable in view of the fact that
311
the amounts of H2S adsorbed on Na-Y, H,Na-Y and Na-ZSM-5 are low (Table 4) in comparison to those adsorbed on Na-X and H,Na-X. Table 5 shows the results of catalytic tests on Na-X, H,Na-X and Na-ZSM-5 zeolites before and after H2S adsorption. It can be seen that adsorption of H2S increases the yield of C6 hydrocarbons (olefins and paraffins) and aromatics and results i n a decrease or complete disappearance of Et2S from the reaction products. TABLE 4 H2S adsorption at 623K Catalyst H2S ads.(mmol.g-’)
Na-X
H,Na-X
4.2
2.0
Na-Y
H,Na-Y
0.4
Na-ZSM-5
H-ZSM-5
0.1
-
0.3
TABLE 5 Comparison of activity of pure zeolites with activity after H2S adsorption (TR=623K) Catalyst
Na-X pure
Et-SH conversion ( % ) 89.8 Yield of products,% Ethylene 67.0 Eutane,butenes 1.0 0.6 c5 C (olefins,paraffins) 0 . 5 Agomatics 7.0 Oiethyl sulphide 12.9
H ,Na-X
H,S
ads.
pure
100
31.2
54.5
22.6
-
10.9 34.6 -
0.1
1.1
-
2.0 4.0
H,S
.
Na-ZSM-5 ads.
pure
H,S
ads.
66.1
14.1
81.4
56.2 1.6 3.0 0.2 3.5 2.2
13.4 0.2 0.2
77.1 0.9 0.1 0.6 2.3 0.4
-
0.1 0.6
DISCUSSION Some of the first information concerning the conversion of thiols over zeolites was noted in Ref.4. The similarity in the mechanism of the H20-elimination from Et-OH and H2S-elimination from Et-SH over alumina was discussed by Sugioka e t al.(5).Results of our present study confirm that also in the case of zeolites the same mechanism operates in these two elimination reactions, but further transformations of the resulting hydrocarbon radicals proceed along different pathways. A series of studies (6,7) of the catalytic activity of zeolites poisoned by pyridine showed that in the transformations of alcohols
312
-
consecutive steps in the reaction sequence, viz. alcohol olefinsoligomers cyclopolyenes aronatics/paraffins, are catalyzed by Brtlnsted acid centres of increasing strength, according to the following scheme: very \$leak weak acid centres acid centres alcohol _____f olefins _____j oligomers
-
aromatics strong centres acid
-
/\
strong acid centres cyclopolyenes
strong acid centres paraffins
Analysis of the data summarized in Tables 1 and 2 indicates that on Z S M - 5 zeolites, both sodium and hydrogen exchanged, mainly elimination of H2S takes place, whereas higher yields of the diethyl sulphide and of products of further hydrocarbon transformations are observed on Na-X and Na-Y zeolites. After activation at 673K these zeolites do not contain any acid hydroxyl group (9) and it is the sodium ions which operate as active sites for the reaction of Et-SH. This suggests that two parallel reaction pathways exist,one consisting of simple elimination of H2S on acid hydroxyl groups of the zeolite and the second, in which diethyl sulphide is formed as the first intermediate of the series of consecutive transformations of hydrocarbon radicals, resulting in the formation of higher olefins, paraffins and in particular aromatics. It seems that the f o r mation of diethyl sulphide takes place on sodium cations whose p r e sence in the zeolite framework generates the appearance of Lewis acid-base pairs. Indeed, it is known (8) that in the presence of pairs of Lewis acid-base centres ethanol in an analogous reaction forms diethylether. Na-X becomes rapidly deactivated in the course of Et-SH conversion (c.f.Fig.3). The reason f o r this deactivation could be coke formation. The yield of aromatics was highest over Na-X (Tables 1 and 2). It is known (10) that aromatic and coke have a common precursor and that in the large cavities of faujasite-type zeolites coke i s very easily formed. These conclusions are confirmed by studies of zeolite activity after adsorption of pyridine and HgS. Pyridine adsorbed at 623K poisons mainly Brtlnsted acid sites, and sodium cations are poisoned only partially. TPO o f pyridine has shown the maxima of sodium cations below 623K, but some cations may still retain pyridine molecules even at higher temperatures (Fig.1). Thus, after
313
adsorption of pyridine the simple H2S elimination i s suppressed, and s o is the total activity, whereas the relative importance of the pathway to diethyl sulphide and further transformations of hydrocarbon radicals is enhanced. This effect i s most significant in the case of ZSM-5 zeolites which have the strongest Brtlnsted acid sites. The experiments with the adsorption of H2S provided further support to the postulated r o l e of the sodium cations and acid OH groups in the reaction of Et-SH. On adsorption of H2S on Na-X dissociation of H 2 S occurs, and HS- and H+ ions are formed (11). The dissociative adsorption of H2S decreases with the increase of the Si/Al ratio, and hence the effect of H2S adsorption is most pronounced on X zeolites. HS- ions are adsorbed on sodium cations and generate a dense network of active sites for adsorption o f Et-SH through the formation of a 5-5 bond known to be readily formed and fairly strong. On such "quasi-sulphidic" surface of much lower polarity than the oxide surface, the reactivity of C2H5 radicals may be high, oligomerization and aromatization thus proceeding readily. On the other hand one should note that, in the first step of the dissociative adsorption of H 2 S , hydroxyl groups are formed at sites with a very high electrostatic potential and exhibit a band at 3635 cm-l ( 1 2 ) . Such OH groups are absent in the H,Na-X activated at 673K, but may form upon H2S adsorption and be involved in the aromatization process which takes place on strong acid sites. CONCLUSIONS Reaction of Et-SH over faujasites and ZSM-5 zeolites proceeds along two parallel pathways: (i) simple elimination of H 2 S on Brdnsted acid sites, resulting in the formation of ethylene, and (ii) generation of diethyl sulphide as the intermediate in further transformations of ethyl radicals to form higher olefins, paraffins and aromatics. Formation of diethyl sulphide takes place on sodium cations which in the zeolite framework play the role of L e w i s acid-base pairs. Preadsorption of H 2 S o n Na-X zeolite generates sulphide sites, which strongly enhance the formation of aromatics. ACK N OWL E 0 G E ME N T The authors are greatly indebted to Dr. H.G. Karge for helpful discussion and the critical revision of the manuscript.
314
REFERENCES 1. 8.C. Gates, J.R. Katzer and G.C.A. Schuit, "Chemistry of catalytic p r o c e s s e s " , Mc Graw-Hill Book Company, 1979 p . 390. 2. N. Pesl, Thesis, Technische Universityt Wien, 1978. 3. Nan-Yu Topsoe, K. Pedersen and E.G. Derouane, J . Catal. 70 (1981) 41. 4. C.D. Chang and A.J. Silvestri, J.Cata1. 47 (1977) 249. 5. Masatoshi Sugioka, Takayoshi Kamanaka and Kazuo Aomura, J. Catal. 52 (1978) 531. 6 . S. Dzwigaj, J. Haber and T. Romotowski, Zeolites 4 (1984) 147. 7. M. Derewihski, S. Dzwigaj, J . Haber and G. Ritter, Proc. Intern. Symp. on Zeolite Catalysis, Siofok 1985, Acta Phys. Chem. Szegediensis, Szeged 1985, p. 535. 8. J. Haber and U. Szybalska, Disc. Faraday SOC. 7 2 (1981) 263. 9. M . Zi6lek, I. Bresihska and H.G. Karge, Proc.Intern.Symp. o n Zeolite Catalysis, Siofok 1985, Acta Phys. Chem. Szegediensis, Szeged 1985, p. 551. 10. J.C. Vedrine, P. Dejaifve and E.O. Garbowski, "Catalysis by Zeolites", ed. 8. Imelik e t al., Elsevier 1980, p. 29. 11. H.G. Karge and J . Rask6, J.Colloid Interface Sci. 64 (1978) 522. 12. H.G. Karge, M . Zi6lek and M . taniecki, Zeolites, 7 (1987) 197.
H G. Karge, d. Weitkamp (Editors), Zeolites as Cakdyn$ts,Sorbents and Detergent Rudders 0 1989 Elsevier Science Puhlishers R.V., Amsterdam - Printed in The Netherlands
STUDY OF THE STRUCTURE AND THE REDOX REACTIVITY OF NaX ENCAPSULATED Co(I1)-PHTHALOCYANINE G. Schulz-Ekloff', A. Andreev3
D. Wbhrlez, V. Iliev3, E. Ignatzek' and
'Institut fiir Angewandte und Physikalische Chemie, Universitht Bremen, D-2800 Bremen 3 3 , FRG 21nstitut fiir Organische und Makromolekulare Chemie, Universitgt Bremen, D-2800 Bremen 3 3 , FRG 31nstitute of Kinetics and Catalysis, Bulgarian Academy of Sciences, BG-1040 Sofia, Bulgaria
ABSTRACT The preparation of NaX-encaged cobalt phthalocyanine by thermally activated tetramerization of l,2-dicyanobenzene in cobalt ion-exchanged zeolite results in average loadings < 1 complex per unit cell and some lattice fragmentation. The encaged molecule exhibits restricted CH vibrations and a protonated state in the IR and a twofold symmetry in the EPR spectrum. The encaged chelates catalyze the oxidation of mercaptans and ethylbenzene as quadricyclane to well as the valence isomerization of norbornadiene. The results are compared with those of cobalt phthalocyanine on silicagel and unsupported cobalt phthalocyanine.
INTRODUCTION Zeolite-encapsulated organometallic complexes represent a unique class of immobilized and heterogenized coordination compounds. An especially stable inclusion can be expected for the in-situ prepared metal phthalocyanine molecule accommodated in the supercage of the faujasite structure Crefs.1-31. In the following, new results on the structure of cobalt(I1)-phthalocyanine (Copc, Fig. 3(1)) entrapped inside faujasite X (Copc/X) and its reactivity in the oxidation of ethanethiol, ethylbenzene and in the valence isomerization of quadricyclane to norbornadiene are presented and discussed. In addition, unsupported Copc and Copc on silicagel (Copc/SiOz) was studied to interpret the results.
EXPERIMENT Preparation Self-prepared zeolite NaX (Si/Al = 1.2) was cobalt ionexchanged (cobalt acetate, 0 . 0 2 5 mol dm-3) to a cobalt content of
316
the water-free samples of about 4 wt.t (E 1841 degree of exchange). The COX samples were partially dehydrated (24 h at 513 K, argon), mixed with
(dcb/Co = 4) in a glove box
1,2-dicyanobenzene
(dry
nitrogen), filled in a pressure tube (Duran glass, 100 x 12 mm), sealed at 1 Pa, heated to 573 R (20 I[ min-1) and held at this temperature for 16 h. The reaction product was extracted in a Soxhlet with acetone (4 h) and pyridine until it was color-free to remove excess Copc formed at the external surface of the zeolite crystals [ref. 21. The chosen parameters guaranteed a minimum of unreacted dcb or of the side-product phthalimid. triazine
(Fig. 3,
(2)) can occur
by
special
The formation of variation
of
the
synthesis parameters [ref -41. Large-pore silicagel
300 mzg-1 )
(Riedel-de Haen,
was
loaded
with cobalt by suspending in cobalt chloride solution and saturating with ammonia. The adsorbed cobalt tetra-ammine complex was decomposed reacted with
by
calcination.
The cobalt
(dcb/Co = 4) as
dcb
loaded
silicagel was
described above
resulting in
products (Copc/SiOn ) uniformly dispersed on the carrier [ref -51. Characterization The
quantitative
photometrically
following
loadings the
lattice in concentrated acid
with
Copc
destruction
of
were the
determined faujasite
X
solution or dissolving the chelate
from the surface of the silicagel with sulfuric acid. The samples were characterized by X-ray powder diffraction, light microscopy, scanning
electron
microscopy,
FT-IR
spectroscopy
(in
KBr),
electron paramagnetic resonance (Bruker ER 200-SRC, X-band, 130 K) and nitrogen physisorption capacities at 77 K. Catalysis The oxidation of ethanethiol
(CzHaSH) was carried
out
in a
continuously stirred batch reactor (250 ml. 298 K) containing the dispersed solid Copc/X catalyst (0.12 pmol Copc) and the dissolved thiol (2.7 ymol) in 100 ml heptane as well as air The
conversion
titration with (Ag' / S a -
of
the
thiol
silver nitrate
was using
followed an
by
(=1 mmol
02).
potentiometric
ion-selective
electrode
1.
The oxidation of ethylbenzene was studied in a continuously stirred batch reactor (30 ml, 409 K) containing the dispersed solid Copc/X catalyst
(8 pmol Copc) in
mol). Oxygen was bubbling
12 ml
ethylbenzene
(~0.1
(25 ml min-l) through the liquid phase
via a glass frit. The products were analyzed by gas chromatography
317
(PE F 22, capillary stationary phase).
column,
cyano-methyl-phenyl-silicon
as
The valence isomerization of quadricyclane to norbornadiene was studied in a glass ampoule (10 ml, room temperature) shaken to get a uniform dispersion of the solid Copc/X catalyst (0.1 w o l Copc) in the solvent
(CC14). Quadricyclane
syringe through a rubber
(0.3 mmol)
was added
septum to the oxygen-free
by a
three-phase
mixture. The rate of conversion was followed by sampling and gas chromatography. RESULTS
Infrared spectra The infrared spectra of the Copc/X following to the removal of external excess Copc and of the Copc/SiOz by
subtracting
the
corresponding
(Fig. 1) were evaluated
spectra
the
of
metal-loaded
carriers. The band at 850 cm-1 (Pig. lc) is an artefact.
T R A N
S M
I S
5 I 0 N
!I
0 L
Ffg. 1. FT-IR spectra of cobalt phthalocyanine silicagel (b) and encaged in faujaaite X (c).
450
pure
(a), on
318
The band around 2250 cm-I is assigned to cyano end groups of either unreacted dcb or of formed impurities, e.g. triazine. The spectra of both samples exhibit a new broad band at 1020 cm-1. For Copc/X the intensities of the C-H bands (1050- 1200 cra-l) and of the C-C bands (1200 -1600 cm $ 1 are strongly decreased. Electron paramagnetic resonance spectra The EPR spectra of extracted Copc/X were obtained in air (Pig. 2 a,c) and in vacuo (Fig. 2 b) and were reproduced repeatedly by
C
i
g=2 1
1
1
I
1
1
I
I
1 1
Fig. 2. X-band EPR spectra of Copc/X at 130 K and 1 mPa (b) and at normal pressure in air (a,c). The identified features are inserted. The arrows indicate the increase of the magnetic field A(G) and the abscissa extensions.
319
cycles of removal and addition of air. The vacuum spectrum shows a strong narrow signal at g
= 2, i-e. close to the free electron = 2.4. The addition of air generates a
value, and a broad signal g broad signal at g J 4 and a complex hyperfine structure having spacings around 90
G.
octet centered around g
= 2 and an
Two quintets centered around g = 2.05 are identified.
Copc loading and physisorption of nitrogen The obtained average Copc loadings of the zeolite samples were around 1 wt.4, i.e. about one Copc molecule per three unit cells. The X-ray powder diffraction, giving global informations about the crystallinity of the zeolite samples, did not show a significant loss of the overall crystallinity or a marked change of the lattice dimensions. The nitrogen physisorption capacity is strongly decreased, i.e. often more than 50%, following to the synthesis of Copc/X and to the removal of the external excess Copc. In general, an increase of the nitrogen adsorption is obtained after shaking the samples in
sodium
acetate
solution.
Further
increase
of
the
nitrogen
adsorption is obtained by burning off the organic material in air (773 K, 6 h), resulting in BET values ranging from 50
-
80% of the
initial value of NaX (ca. 700 m'g-'). Catalysis With respect to the missing information about the diffusion coefficients and the mass
transfer coefficients
influencing the
kinetics, semi-quantitative comparisons of catalytic activities r (initial rates of conversion, percent per h) are made, aiguing with orders of magnitude only. The ethanethiol oxidation experiments yielded negligible blank values for the conversion on NaX i.e.
r
(140 mg) and Copc
(0.12 pmol),
< 14 h-1. Relatively high conversions were obtained on
cobalt ion-exchanged NaX (140 mg), i.e.
r
z 104 h-1. A significant
increase of the catalytic activity was observed using the Copc/X samples resulting in initial rates of conversion ranging from 10 10% h-1 for Copc/X from different charges of preparation. Comparable activities were obtained for the Copc/SiOn catalysts (Table 1).
The
supported
Copc
samples
are
2
-
3
orders
of
magnitude more active than the unsupported complex. Repeated use of a catalyst produced no significant loss of activity.
320
TABLE 1 Initial rates of ethanethiol conversion ( & h-l) on Copc/SiOz (0.3 lo-” mol Copc) as a function of the Copc dispersion. 12 7
wt.% Copc on SiOz conversion (Ib h-1)
Decreasing
loading
of
the
0.4 12
6 11
support
with
Copc
0.06 95
gave
increasing
dispersion of the complex, i.e. decreasing thickness of the Copc layers from W-VIS analysis. This eatplains the increase of the rate of conversion (normalized to 0.12 pmol Copc) with decreasing loading The
ethylbenzene
conversions
on
NaX
and
oxidation
tests
CoNaX
the
in
show
blank
no
significant
experiments.
The
efficiencies of the active samples are in the order Copc < Copc/SiOz (6 wt.% Pc) < Copc/X having the relative values Ccpc/Si&
: Copc a 10 and Copc/X : Copc
Maximum
conversions
for Copc/X
= 100.
of 20%
h-’ were
found. The
higher conversions on Copc/X are accompanied by much lower values of the selectivity ratio methylphenylcarbinol/acetophenone. All catalytic samples exhibited rapid and irreversible deactivation. The experiments with the valence isomerization of quadricyclane to norbornadiene give no activities for NaX, CoNaX or unsupported Copc in the blank experiments and an activity ratio for the active catalysts Copc/SiOr : Copc/X = 100. The catalysts Copc/SiOz giving maximum conversions of 40% h-1 usually show deactivation accompanied by changes in the colour, i.e. from blue to green. This colour change was not observed for the Copc/X sample which did not exhibit any significant deactivation, even after contact times of several days.
DISCUSSION Structure The
diameter
of
the
square
planar
cobalt
phthalocyanine
molecule along the metal nitrogen bond axis amounts to 1.25 nm and
(0.75 nm) of the faujasite structure, but can fit into the zeolite supercage of 1.3 nm size rref.61. This means that the zeolite-encaged complex can be obtained only by in-situ synthesis with molecules small enough to diffuse into the zeolite matrix. exceeds the openings of
the 12-ring windows
32 1
The preparation of Copc/X from CoNaX and dcb leads to phthalocyanine molecules which can be either encaged or deposited at the external surface of the zeolite crystals. The external excess Copc is identified by scanning electron microscopy and Xray powder diffraction [ref.2] and can be removed completely by the applied extraction procedures. Shpiro and coworkers [ref. 71 have applied X-ray photoelectron spectroscopy to distinguish between internal and external Copc. The Copc located within the zeolite matrix reduces the nitrogen physisorption capacity. The strong decrease of these values which are found in spite of the relatively low loadings led to the proposal of a fractal distribution of the complex molecules in the zeolite framework [ref. 81. However, the authors did not take into account the contribution of zeolite fragments to the decrease of the nitrogen adsorption. It is highly probable that the material which reduces the nitrogen adsorption following to the removal of the organic compounds by burning off consists of zeolite fragments. The molecule dcb, which has a high tendency to form complexes, might remove aluminum from zeolite framework positions resulting in some subsequent lattice fragmentation. The assumption that this process occurs preferentially in a shell close to the external surface of a zeolite crystal would explain the increase of the adsorption capacity after shaking in sodium acetate solution which might remove some pore plugging material. The FT-IR spectrum shows that the structure of the incorporated Copc has changed significantly. The decrease of intensities of the CH vibration bands might be related to the spatial constraints for the encaged molecule. A band at 1020 cm-1 is found in the metalfree phthalocyanine and is interpreted as NH vibration [refs.9,101. The corresponding band of the Copc/X spectrum indicates a protonation of the inner nitrogen atoms. Protons are formed in the tetramerization of the dcb and take over the charge compensation for the chelated cobalt cations. The hydrogen form of phthalocyanine (Hzpc) has never been identified as a reaction product from zeolite and dcb. The interpretation of the IR spectrum of Copc/X can be supported by comparison with the spectrum of Copc/SiOz. The SiOn support, also containing protons from the tetramerization of dcb, protonates the complex, as is revealed by the generation of a new
322
band at 1020 cm-1 , but does not affect the CH vibrations to such an extent as for the Copc/X. The EPR spectra of the encaged Copc show similarities to the spectra
of
dissolved
the
solid
complexes
crystalline
iref.121.
The
material
Iref.111
appearance of
a
and
the
free radical
signal at g = 2 is observed frequently [refs.ll,l2]. Presumably, the conjugated dianionic macrocycle can generate paramagnetism by electron transfer to an electron accepting impurity or site. Lewis sites in zeolites are known to act as electron acceptors towards organic molecules
[ref.l3].
Therefore the prediction can be made
that the free radical signal will be an inherent constituent of the spectrum of the zeolite-encaged chelate. The broad signal at g a
2.4
(Fig.
2b)
can be
related
to
the
unpaired
3d
electron
associated with the cobalt atom [ref.l2]. The addition of air will, presumably, lead to the chemisorption of dioxygen in axial coordination at the cobalt complexes [refs.14-17].
atom, as is
A signal at g
=
found
for many
cobalt
4. indicating a high-spin
state, can be related to zeolite-coordinated
cobalt ions [ref.l8].
The octet centered at g = 2.05 results from the hyperfine interaction of the unpaired electron with the S9Co nuclear spin I = 7/2 [ref .11].
Interesting new structural information can be gleaned from the existence of the quintets centered at g z 2. A five-component spectrum can be expected if a one-spin system couples with two equivalent nuclear spins I = 1
(S =
1/2; ms =
1/2; ml = 0 , f 1;
f 2; M. = 1; MI = 0 ) . This means, however, that the four nitrogen atoms surrounding the cobalt central ion are not
equivalent,
and
lowered
a
to
that twofold
the
fourfold
symmetry
lowering of the symmetry
symmetry
for
of Copc/X
the
of
the
encaged
complex
chelate.
is The
was also concluded from the
analysis of its W - V I S spectrum [ref -191- Non-equivalent
nitrogen
atoms for the encaged phthalocyanine, in contrast to the external excess complex, are also identified by X.P.S. [ref. 71. Catalysis Cobalt
phthalocyanines
are used
as
catalysts in the Merox-
.
It is proposed that the mercaptans Sweetening process [ref.201 are converted to disulfides via internal redox processes at the cobalt ion [ref.21]. The assumption that the monomer complexes are the active species in the reaction iref.221 is supported by the observed increase of activity with increasing dispersion of Copc
323
on SiOz (Table 1) or in the zeolite. The fact that the oxidation of the thiols proceeds already on cobalt ion-exchanged supports the above-mentioned
zeolites
idea about the decisive role of the
cobalt ion as the active center of the catalyst. The zeolite-encaged Copc exhibits the highest activity in the oxidation of ethylbenzene. This can be related to the high dispersion
of
the
faujasite matrix.
catalytically Presumably,
methylphenylcarbinol proceeds
the
(0-CHOH-CHs)
autocatalytically
via
active
cobalt
complex
oxidation of and
in
ethylbenzene
acetophenone
hydroperoxide
the to
(O-CO-CH3
intermediates
[ref.22]. The usually observed induction periods and irreversible deactivation of the catalysts are also found for Copc/X.
Fig. 3. Structures of metal phthalocyanine and scheme of the valence isomerization norbornadiene ( 3 . The valence (Fig.
3)
plays
(1) and triazine ( 2 ) of quadricyclane to
isomerization of quadricyclane a
role
in
concepts
of
solar
to norbornadiene energy
storage
[ref.24]. Metal phthalocyanines are found to be active catalysts for this reaction [ref.25]. The finding that this isomerization is also catalyzed by zeolite-encaged Copc indicates reaction pathways via transition states fitting into the supercage dimensions. Presumably, the usually observed deactivation processes require more
bulky
intermediate
states,
which
cannot
be
realized
Copc/X, resulting in the maintenance of the catalytic activity.
for
324
CONCLUSIONS Cobalt phthalocyanine molecules which are irreversibly encaged in the cavities of the faujasite lattice are obtained by in-situ preparation.
The
thermally
activated
reaction
results
in
relatively low loadings and undesired side-reactions such as local lattice fragmentation. The chelate is located inside the zeolite matrix, as can be concluded from structural changes revealed by IR, W-VIS, EPR and XPS, and from its contribution to the decrease in the gas adsorption. The encaged complexes are active in various catalytic
reactions,
i.e.
they
are
accessible
for
reacting
molecules. The high catalytic activities for the oxidation of the smaller
molecules
(ethanethiol,
ethylbenzene)
support
a
high
dispersion of Copc in the faujasite matrix.
ACKNOWLEDGEMENT The authors gratefully Deutsche
acknowledge financial support by the
Forschungagemeinschaft.
This
work
is
part
of
a
cooperation between the Institute of Kinetics and Catalysis of the Bulgarian Academy of Science and the Department of Chemistry of the University of Bremen. We are indebted to Dr. U. Hiindorf for preparing and characterizing various samples.
REFERENCES 1
2 3 4. 5 6 7
8 9 10 11 12
a) V.Yu. Zakharov and B.V. Romanovsky, Moscow University Chemistry Bulletin 32,2 (1977) 16, Engl. transl., Allerton Press, New York 1977. b) ibid., 32.2 (1977) 74. G. Meyer, D. Wehrle, M. Mohl and G. Schulz-Ekloff, Zeolites 4 (1984) 30. N. Herron, G.D. Stucky and C.A. Tolman, J.C.S., Chem. Commun. (1986) 1521. E. Ignatzek, Thesis, University of Bremen 1987. D. W6hrle, U. Hiindorf, G. Schulz-Ekloff and E. Ignatzek, Z. Naturforsch. 41b (1986) 179. D. Breck, Zeolite Molecular Sieves, Wiley, New York 1974. E.S. Shpiro, G.V. Antoshin, O.P. Tkachenko, S.V. Gudkov, B.V. Romanovsky and Kh.M. Minachev, in "Structure and Reactivity of Modified Zeolites" (P.A. Jacobs et al., Eds.), Elsevier, Amsterdam 1984; Stud. Surf. Sci. Catal., vol. 18, p. 31. E. Ignatzek, P.J. Plath and U. Hiindorf, 2. physik. Chem. (Leipzig) 268 (1987) 859. H . F . Shurvell and L. Pinzuti, Can. J. Chem. 44 (1966) 125. T. Kobayashi, F. Kurokava, N. Uyeda and E. Suito, Spectrochim. Acta 26A (1970) 1305. J.M. Assour and W.K. Kahn, J. Amer. Chem. SOC. 87 (1965) 207. J.A. debolfo, T.D. Smith, J.F. Boas and J.R. Pilbrow, J.C.S. Faraday Trans. I1 72 (1976) 481.
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13 J.C. Vedrine, A. Auroux, V. Bolls, P. Dejaifve, C. Naccache, P. Wierzchowski, E.G. Derouane, J.B. Nagy, J.-P. Gilson, J.H.C. van Hooff, J.P. van den Berg and J. Wolthuizen, J. Catal. 59 (1979) 248. 14 T.D. Smith and J.R. Pilbrow. Coord. Chem. Rev. 39 (1981) 295. 15 J.H. Lunsford, Catal. Rev.-Sci.Eng. 12 (1975) 137. 16 N. Herron, Inorg. Chem. 25 (1986) 4714. 17 W. Lubitz, C.J. Winscom, H . Diegruber and R. Meseler, Z. Naturforsch. 42a (1987) 970. 18 I.D. Mikheikin, 0.1. Brotikowski, G.M. Zhidomirov and V.B. Kazanskii, Kinetika i Kataliz 12 (1971) 1442. 19 H. Diegruber and P.J. Plath, Z. phys. Chem. (Leipzig) 266 (1985) 641. 20 R.A. Meyers (Editor), "Handbook of Petroleum Refining Processes", McGraw-Hill, New York 1986. 21 J. Zwart, H.C. van der Weide, N. Brbker, C. Rummens, G.C.A. Schuit and A.L. German, J. Mol. Catal. 3 (1977/78) 151. 22 T.P.M. Beelen, C.O. da Costa Gomez and M. Kuijer, J. Royal Netherl. Chem. SOC. 98,lO (1979) 521. 23 A . Kropf, Lieb. Anal. Chem. 637 (1960) 73. 24 H.D. Scharf, J. Fleischhauer, H . Leismann, J. Ressler, W. Schleker and R. Weitz, Angew. Chem. 91 (1979) 696. 25 J. Manassen, J. Catal. 18 (1970) 38.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders
0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SELECTIVE REDUCTION OF NITRIC OXIDE OVER ZEOLITE-SUPPORTED I R I D I U M CATALYST
--
ROAR MYRDALa) and STEIN KOLBOE
Department of Chemistry, U n i v e r s i t y o f Oslo, P.O.
Box 1033, B l i n d e r n ,
N-0315 Oslo 3, Norway
ABSTRACT W i t h t h e goal o f f i n d i n g s u i t a b l e c a t a l y s t s f o r r e d u c i n g NOx b y CO i n oxygen-containing atmosphere, t h e NaX z e o l i t e ( L i n d e 13-X) was i m p r e g n a t e d w i t h i r i d i u m b y i o n exchanging w i t h (Ir(NH3)5C1)C1 2. The c a t a l y s t d i s p l a y e d h i g h c a t a l y t i c a c t i v i t y and s e l e c t i v i t y f o r r e d u c i n g NO a l s o i n t h e presence of oxygen w i t h f pronounced optimum t e m p e r a t u r e c l o s e t o 29OoC a t a space v e l o c i t y GHSV=40000h- where 85% NO c o n v e r s i o n was o b t a i n e d u s i n g a f e e d gas c o n t a i n i n g 200 ppm NO, 1%C O and 3.6 % 02. W i t h o x y g e w f r e e f e e d gas 1 0 0 % NO r e d u c t i o n was o b t a i n e d f r o m 230 OC on. A d s o r p t i o n / d e s o r p t i o n s t u d i e s o f NO, CO and 02 have a l s o been c a r r i e d out.
INTRODUCTION The e q u i l i b r i u m c o n c e n t r a t i o n o f n i t r o g e n o x i d e s NO and NO2 (= NO,)
in air
i s e x t r e m e l y l o w a t o r d i n a r y temperatures. B u t t h e e q u i l i b r i u m c o n s t a n t f o r t h e r e a c t i o n N2 + O2 = 2N0 i n c r e a s e s s t r o n g l y w i t h i n c r e a s i n g t e m p e r a t u r e and a t 2000 K i t i s a l r e a d y 4 ~ 1 0 - ~NO . i s t h u s b e i n g formed d u r i n g a l l h i g h
temperature combustion processes. Besides i t s f o r m a t i o n b y n a t u r a l processes, t h e anthropogenic component has i n c r e a s e d m a r k e d l y d u r i n g t h e l a s t years, due m a i n l y t o i n d u s t r y and t h e widespread use o f cars. It has become i n c r e a s i n g l y c l e a r t h a t NO,
p r o d u c t s due t o NO,
e i t h e r d i r e c t l y o r through
has d e l e t e r i o u s e f f e c t s on p l a n t as w e l l as a n i m a l l i f e
and i s one o f t h e most i m p o r t a n t p o l l u t a n t s ( r e f . 1). The c o n t r o l o f NOx i n t h e atmosphere i s m o s t e f f i c i e n t l y c a r r i e d o u t a t i t s source. Since NOx i s u n s t a b l e one m i g h t l o o k f o r a c a t a l y s t f o r i t s decomposit i o n i n t o N2 and 02, b u t a t p r e s e n t i t appears more p r o f i t a b l e t o l o o k f o r c a t a l y s t s f o r r e d u c t i o n o f NOx b y a r e d u c i n g agent l i k e CO and/or r e s i d u a l hydrocarbons w h i c h a r e a l r e a d y p r e s e n t i n m o t o r exhausts. I n l a r g e i n d u s t r i a l p l a n t s , such as power s t a t i o n s , ammonia i s a t p r e s e n t t h e p r e f e r r e d r e d u c i n g agent, p a r t l y because t h e f l u e gas i s u s u a l l y q u i t e r i c h i n oxygen and p o o r i n
a)
On l e a v e o f absence f r o m The Norwegian Defence Research Establishment.
328
reducing material, making reduction d i f f i c u l t ; and p a r t l y because t h e addition of ammonia t o f l u e gas i s f a i r l y straightforward (ref. 2). I n motor c a r s , ammonia addition i s impracticable and t h e r e a r e already reducing agents present. The oxygen excess i s smaller, thus d i r e c t treatment of the exhaust gases i s t h e preferred method i f oxygen excess can be k e p t small enough. T h u s a main objective in pollution control and the study of c a t a l y t i c reduction of NO by CO i s t o find a c a t a l y s t a b l e t o operate in an oxidizing atmosphere. The two competing r e a c t i o n s a r e (refs. 3-5): 2CO t 2NO + 2C02 t N 2 2CO t 02--2C02. and I t has been reported t h a t I r supported on alumina o r s i l i c a c a t a l y z e t h e NOCO reaction in the presence of oxygen (refs. 3-6). I t i s known t h a t i n some cases metals supported in z e o l i t e pores acquire enhanced c a t a l y t i c p r o p e r t i e s , hence i t was decided t o i n v e s t i g a t e t h e p r o p e r t i e s o f zeolite-supported I r c a t a l y s t s . This c a t a l y s t system has not been t e s t e d on t h e NO reduction and i s l i t t l e studied in general, a p a r t from t h e works of Gelin e t a l . ( r e f s . 7,8). EXPERIMENTAL Catalyst preparation The c a t a l y s t s were made according t o t h e procedure given by Gelin e t a l . ( r e f . 7) by ion exchanging batches of 2.0 g Linde-13X with (Ir(NH3)5C1)C12 (Johnson Mattheys Co) dissolved in 200 ml d i s t i l l e d water. Three d i f f e r e n t batches were made using I r - s a l t m o l a r i t i e s 0.026, 0.005 and 0.001. The ion exchange was c a r r i e d out under stirring (1 h) a t 75OC. The ionexchanged z e o l i t e was then recovered by f i l t e r i n g and f i n a l l y washed with d i s t i l l e d water u n t i l chloride free. Before use t h e c a t a l y s t was a c t i v a t e d by c a l c i n a t i o n in a i r and reduction in hydrogen a t 250 OC.
Characterization The elemental composition of the c a t a l y s t s was analyzed (by AAS and ICP) before and a f t e r ion exchange. 100 mg z e o l i t e was dissolved i n 2.5 ml conc. HF. Saturated boric acid (25 ml) was t h e r e a f t e r added t o bind surplus HF and prevent possible l o s s of v o l a t i l e s i l i c o n fluoride. D i s t i l l e d water was then added t o 100 ml. Stoppered p l a s t i c f l a s k s were used throughout. The i d e n t i t y and c r y s t a l l i n i t y of t h e z e o l i t i c material were checked by Xray d i f f r a c t i o n . Nitrogen adsorption capacity and equivalent surface areas were determined in an ordinary BET type instrument. Crystal morphology and s i z e , before and a f t e r ion exchanging and use a s c a t a l y s t , were studied i n a P h i l i p s SEM 515 scanning e l e c t r o n microscope. Metal dispersion was estimated by H2, COY NO and O2 adsorption and X-ray
329
diffraction line broadening. Adsorption and reaction studies Reaction. The reactants were taken from compressed gas cylinders containing dilute mixtures with main component being nitrogen - supplied and analyzed by Norsk Hydro. Reactant composition and feed rate were controlled by mixing gases from two or three gas cylinders using flow controllers and meters. The experiments were standardized to use about 300 m g catalyst with a feed rate ( reactants t nitrogen carrier gas ) of 200 ml(STP)/min. Prior t o use the Convercatalysts were reduced in flowing hydrogen ( 8 % in He) at 250 OC. sions and rates were obtained from differences in entrance and exit gas streams which were continuously analyzed. Adsorption. Adsorption properties for NO, CO, and O2 of the Ir catalyst was studied by several instruments. Temperature-programmed desorption (TPD) were studied using the flow reactor system. Gravimetric adsorption studies were performed in a Stanton Redcroft STA 785 thermal analysis instrument on 30 m g samples, and in a vacuum balance on 200 m g samples. Gas analysis. The gas (entering and) leaving the reactor w a s analyzed continuously utilizing the following methods and instruments. CO and COP; IR: Leybold-Heraeus Binos 1. (CO; 0-5 % : COP; 0-20%) NO and (NO+N02); Chemiluminiscence: Bendix 8102 NO/NOx analyzer. (0-500 ppm) 02; paramagnetism: Sybron/Taylor Servomex O2 analyzer OA 570. (0-100%) The instruments were periodically checked for drift and possible need for recalibration. Ample time was a1 lowed for stabilization before measurements. RESULTS Ca ta 1 ys ts The main analytical data of the original X-zeolite and the three catalysts which were obtained after carrying out ion exchange are given in Table 1. TABLE 1 Elemental composition (weight percent) o f the 13-X zeolite and the Ir-exchanaed catalystsa). Ca ta 1 ys t desiqnation Ir Si A1 Na Ca Fe 0 18.8 12.9 10.8 0.065 0.012 Na-X Ir-1.36 1.36 17.6 12.1 9.6 Ir-6.41 6.41 16.5 11.3 7.7 Ir-14.4 14.4 15.5 10.6 5.2 a) The analyses are based on zeolites exposed to humid air, i.e. they contain about 23% water.
-
-
-
330
N i t r o g e n a d s o r p t i o n a c c o r d i n g t o t h e BET p r o c e d u r e on t h e Na-X and t h e I r 14.4 ( c a l c i n e d a t 50OoC) gave t h e e q u i v a l e n t s u r f a c e areas 788
and 651 m2/g.
Renewed measurements on t h e Ir-14.4 a f t e r use i n a r e a c t o r t e s t a t up t o 500
OC
f o r 100 h gave 645 m2/g. A c o n s i d e r a b l e p a r t o f t h e area r e d u c t i o n a f t e r m e t a l l o a d i n g i n s i d e t h e p o r e s i s a consequence o f t h e i n c r e a s e d mass o f t h e u n i t c e l l . I t may t h u s be c o n c l u d e d t h a t t h e s u r f a c e a r e a o f t h e z e o l i t e r e m a i n s e s s e n t i a l l y u n p e r t u r b e d b y t h e t r e a t m e n t t o w h i c h i t i s subjected. One purpose o f c a r r y i n g o u t c a t a l y t i c r e a c t i o n s w i t h m e t a l s i m p r e g n a t e d i n z e o l i t e s i s t o o b t a i n a b e t t e r metal dispersion than m i g h t be obtained w i t h o t h e r m e t a l c a r r i e r s . T h i s h i g h d i s p e r s i o n i s b y no means assured, however, and much e f f o r t has been devoted t o f i n d t e c h n i q u e s l e a d i n g t o h i g h m e t a l d i s p e r s i o n s and t o f i n d methods t o c h a r a c t e r i z e i t ( r e f . 9). I t i s n o t t h e o b j e c t i v e o f t h i s work t o go d e e p l y i n t o t h i s problem, b u t i t was deemed i m p o r t a n t t o g a i n a p p r o x i m a t e knowledge a b o u t i n i t i a l m e t a l d i s p e r s i o n s i n t h e c a t a l y s t s and t h e e v o l u t i o n o f t h i s d i s p e r s i o n ( p o s s i b l e a g g l o m e r a t i o n ) as t h e c a t a l y s t underwent t h e v a r i o u s t r e a t m e n t s . The most e x t e n s i v e s e t o f measurements was o b t a i n e d b y a d s o r p t i o n o f H2, O2 and CO a t room temperature. The r e s u l t s a r e g i v e n i n T a b l e 2 w h i c h shows Hatom, 0-atom and CO m o l e c u l e p e r I r - a t o m f o r t h e t h r e e c a t a l y s t s . A n c i l l a r y a d s o r p t i o n e x p e r i m e n t s on Na-X showed z e r o a d s o r p t i o n o f t h e t h r e e gases, t h u s t h e a d s o r p t i o n w h i c h t a k e s p l a c e i s caused b y t h e presence o f i r i d i u m .
TABLE 2 A d s o r p t i o n o f H2, O2 and CO a t room t e m p e r a t u r e on
ir i d i um-zeol it e c a t a l y s t s . Catalyst
Calcinati n temp. /OCa
9
-H
-O C J Ir
Ir
Ir-1.36
250
1.0
0.43
0.59
Ir-6.41
250
1.1
0.42
0.50
500
-
0.49
0.33
0.39
0.18
I1
Ir-14.4
500
Ir
a ) C a l c i n a t i o n i n a i r o v e r n i g h t a t i n d i c a t e d temperature. R e d u c t i o n t o m e t a l l i c s t a t e b y f l o w i n g hydrogen (8 % i n He) a t 250 OC f o r 1 h.
I t has been p o i n t e d o u t t h a t t h e e s t i m a t e s o f m e t a l d i s p e r s i o n b y gas
a d s o r p t i o n easurements have a l a r g e i n h e r e n t u n c e r t a i n t y ( r e f . 10). The e x a c t s t o i c h i o m e t r y o f t h e r e a c t i o n i s n o t known, and m e t a l atoms w i t h i n t h e z e o l i t e
331
cage may n o t be a c c e s s i b l e t o t h e a d s o r b i n g gas. Furthermore, m e t a l c l u s t e r s w i t h u p t o about 23 atoms may have 22 o f them on t h e surface, so t h e a d s o r p t i o n method i s n o t a b l e t o d i s t i n g u i s h c l u s t e r s o f t h i s s i z e and s m a l l e r . L e a v i n g these f i n e r p o i n t s aside, t h e i n f o r m a t i o n o b t a i n e d b y t h e a d s o r p t i o n measurements r e p o r t e d i n Table 2 i s t h a t t h e m a j o r i t y o f t h e i r i d i u m atoms have n o t agglomerated and a r e a c c e s s i b l e t o gas phase molecules. I n f o r m a t i o n a b o u t p a r t i c l e s i z e may a l s o be o b t a i n e d f r o m measurements o f l i n e broadening o f X-ray d i f f r a c t i o n l i n e s b y means o f t h e S c h e r r e r e q u a t i o n ( r e f . 11). The t w o f o r m s o f Ir-6.41
i n Table 2 d i s p l a y e d weak, broad Ir-
d i f f r a c t i o n l i n e s . The broadening l e d t o p a r t i c l e d i a m e t e r e s t i m a t e s o f 4 and 10 nm r e s p e c t i v e l y f o r t h e t w o f o r m s w h i c h had been c a l c i n e d a t 250 and 50OoC. These e s t i m a t e s a r e h i g h e r t h a n t h e r e s u l t s o b t a i n e d f r o m a d s o r p t i o n experiments. Such a d i s c r e p a n c y i s n o t unexpected, however, i f t h e r e i s a pronounced spread i n p a r t i c l e sizes. The m a i n p a r t o f t h e d i f f r a c t i o n l i n e s w i l l b e dominated b y t h e l a r g e s t p a r t i c l e s . The m a i n l y t o t h e base o f t h e d i f f r a c t i o n l i n e
small p a r t i c l e s w i l l c o n t r i b u t e
- i f a t a l l - and t h e i r
contribution
t o t h e l i n e broadening i s u n d e r e s t i m a t e d u n l e s s a f u l l l i n e p r o f i l e a n a l y s i s i s c a r r i e d out ( r e f . 11). The NO/CO --
reaction
The main o b j e c t i v e i n t h i s work i s t o f i n d p o s s i b l y a c t i v e c a t a l y s t s f o r NOx c o n t r o l . I t was t h e r e f o r e decided t h a t t h e r e a c t o r f e e d s h o u l d have a NO
c o n t e n t s i m i l a r t o exhaust gases f r o m c o m b u s t i o n processes. Feeds c o n t a i n i n g 0.02 %
200 ppm on a volume (i.e.
m o l a r ) b a s i s were t a k e n t o r e p r e s e n t a
t y p i c a l combustion gas. Although most r u n s were c a r r i e d o u t w i t h about 1 % CO as r e d u c i n g a g e n t and w i t h oxygen present, i t was t h o u g h t t o be o f i n t e r e s t t o c a r r y o u t a r e a c t i o n s e r i e s w i t h o u t added oxygen and u s i n g a much s m a l l e r c o n c e n t r a t i o n o f CO (500 ppm) t h a n i s u s u a l l y met i n combustion gases. The c o n v e r s i o n s o f NO vs. temperature f o r t h i s r e a c t i o n system and a r e p r e s e n t a t i v e c u r v e f o r a f e e d c o n s i s t i n g o f N O / C 0 / O 2 a r e shown i n F i g . 1.
c 0
4
+,
u
a
U
m L
0
z x
Fig. 1. E f f i c i e n c y o f c a t a l y s t (Ir-1.36) f o r r e d u c i n g NO a t v a r i o u s temperatures. NO i n f e e d 200 ppm. C u r v e A: 500 ppm C O Y No oxygen. C u r v e B: 1.05 % C O Y 3.65 % 02.
100 140 180 220 260 300
Temperature/ O C
332
I t i s seen t h a t i n the oxygen-free system (curve A) a much h i g h e r NO
conversion i s obtained than when oxygen (curve B) i s present. NO i s 100 % reduced t o n i t r o g e n a t 230 OC i n absence o f oxygen, b u t o n l y 15 % when oxygen i s present. Furthermore, i n t h e l a t t e r case conversion never exceeds 85 %. Since 100 % conversion i s e a s i l y obtained i n the oxygen-free case i t i s c l e a r t h a t the r e a c t i o n system i s e s s e n t i a l l y f r e e o f d i f f u s i o n e f f e c t s , and the 85 % conversion l i m i t i n the presence o f oxygen i s n o t caused b y d i f f u s i o n 1i m i t a t i o n s . An Arrhenius p l o t based on the low-conversion p a r t o f curve A gives an apparent a c t i v a t i o n energy o f 130 kJ/mol. The m a j o r i t y o f the measurements were
-
.
I
~
1.oc-
c a r r i e d out i n NO/C0/O2 systems. The
.200
: n
\
C
r e s u l t s o f a s e r i e s o f such measurements w i t h feed gas composition NO;C0;02
= 200
-150
ppm; 1.05 %; 3.65 %; ( b a l a n c e N2) a r e given i n Fig. 2. The d i r e c t l y measured Q50 .
e x i t concentrations o f CO, NO, and NO2
z
from 18 runs a t various temperatures i n the range 200 t o 500
OC
are shown. Care
was taken t h a t t h e measurements should r e f l e c t a steady state. This could f o r
-
¶
O Y , , , , 200 250 300 350 400 450 500
Temperature/
although a f t e r the f i r s t few hours t h e system changed o n l y slowly. A f t e r each change i n r e a c t i o n temperatures 10 '0 20 h was t y p i c a l l y allowed before making temperatures was taken a t random t o
425 'j
low temperatures r e q u i r e more than 30 h,
f i n a l measurements. The sequence o f
z
0
OC
Fig. 2. Composition o f e x i t gases from r e a c t o r i n the temperature range 20050OoC. C a t a l y s t Ir-1.36. Feed: 200 ml/min c a r r i e r gas (N ) c o n t a i n i n g 200 ppm NO; 1.05 % &and 3.65 % 0,. L
ensure t h a t a slow e v o l u t i o n o f c a t a l y s t a c t i v i t y w i t h t i m e d i d n o t v i t i a t e the results. This procedure a t the same t i m e showed t h a t t h e c a t a l y s t a c t i v i t y i s highly stable. It i s worth n o t i n g t h a t CO and NO decreases i n a very p a r a l l e l manner w i t h
increasing temperature u n t i l a l l CO i s o x i d i z e d t o CO2 a t about 275
OC.
A t this
temperature t h e r e i s s t i l l some unconverted NO, and t h e f i r s t traces o f NO2 appears. As t h e temperature increases f u r t h e r the NO l e v e l continues t o f a l l slightly
before an increase s t a r t s . NO2, however, increases s t r o n g l y w i t h
increasing temperature, and a t 300
OC
t h e r e i s 40 ppm i n the e x i t gas. Total
NOx reduction t h e r e f o r e shows a very sharp maximum a t 275-280
OC.
The NO2 concentration has a very broad maximum from 300 t o 400 OC. Above 35OoC t h e components NO/N02/02 are i n v i r t u a l e q u i l i b r i u m . The decreasing NO2
333 c o n c e n t r a t i o n above 400
OC
i s t h e r e f o r e n o t due t o a k i n e t i c process r e l a t e d t o
c a t a l y t i c a c t i v i t y . It s h o u l d be emphasized t h a t NO2 was n o t formed w i t h o u t c a t a l y s t . We b e l i e v e t h a t i t i s no c o i n c i d e n c e t h a t NO2 appears
simultaneously
w i t h t h e complete o x i d a t i o n o f CO. F u r t h e r measurements w i t h o t h e r oxygen c o n c e n t r a t i o n s i n t h e f e e d i n d i c a t e d t h a t i n a l l cases 275-300
OC
were optimum t e m p e r a t u r e s f o r NO,
r e d u c t i o n . A p o s s i b l e dependence upon space v e l o c i t y was n o t i n v e s t i g a t e d . The c a t a l y s t s a b i l i t y t o s e l e c t i v e l y reduce NO b y CO i n t h e presence o f an excess o f oxygen was i n v e s t i g a t e d b y a s e r i e s o f experiments w i t h d i f f e r e n t oxygen c o n t e n t i n t h e feed. A l l t h e s e e x p e r i m e n t s were based on 200 ppm NO and 0.5-1 % CO a t t h e e s t a b l i s h e d optimum temperature, 285
OC.
The
r e s u l t i s d i s p l a y e d i n Fig. 3. I n agreement
0
z
40
x 20
with established practice, the r a t i o R=(202+NO)/C0 ( = o x i d a n t / r e d u c t a n t r a t i o ) i s used t o i n d i c a t e t h e e x t e n t o f oxygen excess ( r e f . 12). The f i g u r e shows c l e a r l y t h e v e r y high s e l e c t i v i t y o f the I r / z e o l i t e cataiyst. Even f o r a t w e n t y f o l d excess o f o x i d a n t o v e r r e d u c t a n t 60 % c o n v e r s i o n i s obtained, and f o r R = 4 85 % c o n v e r s i o n i s o b t a i n e d .
I
I
I
I
12 16 20 OxidarWRaductant r a t i o R 0
4
8
Fig. 3. C a t a l y s t e f f i c i e n c y f o r r e d u c i n g NO; dependence on o x i d a n t / r e d u c t a n t r a t i o R=(202 t NO)/CO. C a t a l y s t : I r 1.36. R e a c t i o n temperature: 285 OC. Feed: 200 m l / m i n N2 c o n t a i n i n g 200 ppm NO, 0.5-1% CO and v a r i a b l e c o n c e n t r a t i o n s o f 02.
A d s o r p t i o n / d e s o r p t i on on c a t a l y s t I n an e f f o r t t o g a i n f u r t h e r u n d e r s t a n d i n g o f t h e c a t a l y t i c process more d e t a i l e d experiments on t h e a d s o r p t i o n and d e s o r p t i o n have been c a r r i e d o u t . -Rate o f a d s o r p t i o n . These e x p e r i m e n t s were c a r r i e d o u t i n a vacuum b a l a n c e
u s i n g ZOO mg o f adsorbent b y r e c o r d i n g t h e w e i g h t change upon i n j e c t i o n o f a small q u a n t i t y o f a d s o r b i n g gas (3 m l STP) l e a d i n g t o a p r e s s u r e o f 2 mbar i n t h e p r e v i o u s l y evacuated w e i g h t chamber. The a d s o r p t i o n t o o k p l a c e a t room temperature under f a i r l y c o n s t a n t p r e s s u r e (COY NO o r 02), a b o u t 2 mbar. The main r e s u l t s are: The a d s o r p t i o n i s f a s t . I n a l l cases 50 % o r more o f t h e q u a n t i t y adsorbed a f t e r 100
- 200 s
i s a l r e a d y adsorbed a f t e r 10 s. Our
p r e v i o u s e x p e r i e n c e w i t h a d s o r p t i o n i n z e o l i t e s t e l l s us t h a t t h e u p t a k e o f adsorbate i n these e x p e r i m e n t s i s a t l e a s t i n p a r t d e t e r m i n e d b y d i f f u s i o n r e s i s t a n c e , so t h e i n t r i n s i c a d s o r p t i o n r a t e i s a c t u a l l y s t i l l higher. A t a l l temperatures where t h e NO/CO r e a c t i o n s were a c t u a l l y m o n i t o r e d (200-5OO0C), t h e a d s o r p t i o n r a t e p e r se was n o t l i m i t i n g .
I n t h e case o f CO o r 02, e q u i l i b r i u m
a d s o r p t i o n was reached i n 100 s ; NO on t h e o t h e r hand d i s p l a y e d a v e r y s l o w
334
adsorption continuing f o r a v e r y long t i m e a f t e r t h e i n i t i a l f a s t adsorption. The e q u i l i b r i u m uptakes a r e r e p o r t e d i n T a b l e 2. The gases CO, NO and O2 a r e a l l s t r o n g l y adsorbed. A t room t e m p e r a t u r e o n l y a s m a l l p a r t w i l l d e s o r b even a f t e r p r o l o n g e d evacuation. A d s o r p t i o n e x p e r i m e n t s a t h i g h e r t e m p e r a t u r e s (250-300°C)
were c a r r i e d o u t
i n t h e thermal a n a l y s i s instrument b y i n j e c t i n g small q u a n t i t i e s o f t h e a d s o r b i n g gas i n t o a c a r r i e r gas s t r e a m (N2). These e x p e r i m e n t s were p e r f o r m e d on t h e Ir-6.41
( c a l c i n e d and reduced i n s i t u ) .
The u p t a k e o f oxygen a t 25OoC was c o n s i d e r a b l y h i g h e r t h a n a t room temperature. A p r e v i o u s l y reduced sample was f o u n d t o adsorb oxygen c o r r e s p o n d i n g t o an a t o m i c r a t i o O/Ir=1.5 a f t e r a few m i n u t e s exposure t o oxygen, a n d t o
i 0.1
o
.
o
~
~
m
E
O/Ir=1.8 a f t e r a more p r o l o n g e d exposure. Due t o e x p e r i m e n t a l d i f ficulties a final equilibrium
2
-
0.1
Y
6 0.2 g
o
25
50
75
Tlns/nln
100
v a l u e was n o t e s t a b l i s h e d , b u t a p p a r e n t l y t h e r a t i o 2 correspond i n g t o I r 0 2 would r e s u l t . Fig. 4 shows t h a t a t 250
OC
o x i d a t i o n / r e d u c t i o n o f t h e Ir p a r t i c l e s was a process t a k i n g
Fig. 4. Weight changes ( l o w e r c u r v e ) o f c a t a l y s t (Ir-6.41; 30 mg; p r e v i o u s l y o x i d i z e d ) upon a d d i t i o n o f hydrogen and oxygen t o c a r r i e r gas a t 250 OC. Upper curve d i s p l a y s t h e simultaneous heat e v o l u t i o n i n t h e c a t a l y s t bed.
p l a c e r a t h e r e a s i l y . I t shows t h e w e i g h t o f a c a t a l y s t sample (30 mg, o x i d i z e d s t a t e ) , and h e a t e v o l u t i o n i n t h e sample as f u n c t i o n s of t i m e . A t 12 min a b o u t 2 m l H2 i s added t o t h e c a r r i e r gas. The h e a t c u r v e shows t h a t a f a s t e x o t h e r m i c p r o c e s s i s t a k i n g place, and a decrease i n w e i g h t i s observed due t o f o r m a t i o n o f w a t e r w h i c h i s desorbed a t t h i s temperature. The s m a l l e n d o t h e r m i c d i p i s caused b y d e s o r p t i o n o f water. One s i n g l e p u l s e o f hydrogen w h i c h g i v e s a hydrogen ambiance f o r a few m i n u t e s i s s u f f i c i e n t f o r a l m o s t c o m p l e t e r e d u c t i o n . F u r t h e r p u l s e s have been found t o have a l m o s t no e f f e c t ,
t h e e x o t h e r m i c peak becoming q u i t e s m a l l .
One s i n g l e p u l s e o f oxygen ( a t 45 min) l e a d s t o a l m o s t c o m p l e t e r e o x i d a t i o n . The oxygen i s s t r o n g l y bound; no decrease i n w e i g h t was observed i n f l o w i n g n i t r o g e n . Hydrogen appears l e s s s t r o n g l y bound. Weighing c o u l d n o t b e done w i t h s u f f i c i e n t p r e c i s i o n , b u t i t was observed t h a t i f a s m a l l p u l s e o f oxygen was added o n l y a few m i n u t e s a f t e r a hydrogen pulse, t h e u s u a l w e i g h t i n c r e a s e was i m m e d i a t e l y observed. B u t i n t h i s case a w e i g h t decrease f o l l o w e d , s i m i l a r t o t h e one observed a f t e r hydrogen a d d i t i o n t o t h e o x i d i z e d system, b r i n g i n g t h e
,
c
335
we ight back t o t h e pre-oxygen value. F u r t h e r work showed t h a t t h e r e a c t i o n 2H2 t O2 -2H20
t o o k p l a c e a l r e a d y a t room temperature.
TPD experiments.
I n many cases TPD i s a method which g i v e s f a r b e t t e r
i n s i g h t i n t o a d s o r p t i o n / d e s o r p t i o n b e h a v i o u r than i s o b t a i n e d f r o m a d s o r p t i o n isotherms i f these measurements a r e c a r r i e d o u t w i t h o n l y moderate accuracy. The equipment used f o r s t u d y o f t h e CO/NO r e a c t i o n was w e l l s u i t e d f o r TPD experiments. A summary o f some r e s u l t s on NO adsorption/TPD a r e g i v e n i n Fig. 5. I t i s seen t h a t t h e r e a r e a t l e a s t t h r e e adsorbed s t a t e s which, w i t h t h e chosen e x p e r i m e n t a l set-up, show peaks a t ca. 100, 1 8 0 a n d 400
OC.
The
d i f f e r e n c e between t h e t h r e e curves i s t h a t c u r v e A was o b t a i n e d a f t e r 1 h a d s o r p t i o n f r o m a 200 ppm NO stream, B was o b t a i n e d a f t e r 2 h a n d C a f t e r 4 h. Curves A and B were o b t a i n e d w i t h o n l y 10 m i n N2 f l u s h i n g b e f o r e t h e s t a r t o f Temperature/'C
Fig. 5. Thermal d e s o r p t i o n s p e c t r a of ppm No in N2) No adsorbed (from a t room t emp era t u r e on Ir-1.36. Curves A, B and C r e l a t e t o adsorpt i o n t i m e s 1, 2 a n d 4 h.
t h e TPD run, whereas C was s t a r t e d a f t e r 1 h f l u s h i n g . A more d e t a i l e d t r e a t m e n t o f t h e above and o t h e r r e s u l t s on TPD w i l l b e g i v e n l a t e r ( r e f . 13).
DISCUSSI O N I ridium-bas ed c a t a l y s t s have been s u r p r i s i n g l y l i t t l e s t u d i e d f o r NO r e d u c t i o n b y CO. Reported s t u d i e s have used a lumina (ref s. 3-5) and s i l i c a support s ( r e f . 6). P re v ious a ut h o r s m o s t l y worked a t h i g h t e mperat ures and a l o w oxygen excess. The NO r e d u c t i o n s e l e c t i v i t y was discussed i n t e r m s o f t h e p a r t i t i o n i n g o f CO between NO and O2 ( r e f . 3,5).
The r e a s o n i n g went along t h e f o l l o w i n g l i n e s :
Based on t h e assumption t h a t NO and D2 r e a c t s w i t h CO a c c o r d i n g t o t h e eqns. d(NO)/dt = -kl(NO)(CO) d(Op)/dt = -k2(Op)(CO) i t i s seen t h a t
(1) (2)
d l n(NO)/dl n(0 2)= kl/k2 (3) T h i s r a t i o t he n d e t e r m i n e s t h e p a r t i t i o n o f CO between NO and 02. Eqn. 3 i s integrated t o
.
= (kl/k2)ln((o,)/(o,),) 1n((NO)/(NO),) ( S u b s c r i p t o r e f e r s t o i n l e t concentrations.)
(4) By v a r y i n g t h e space v e l o c i t y
eqn. 4 can be used t o d e t e r m i n e t h e r a t i o k l / k 2, and a v a l u e o f about 3 was
336
r e p o r t e d ( r e f . 3,5). I n o u r case t h e space v e l o c i t y was n o t v a r i e d , b u t an e s t i m a t e f o r t h e p a r t i t i o n i n g r a t i o can s t i l l be obtained. I n a case l i k e t h e one p r e s e n t e d i n F i g . 2 (02)i s much h i g h e r t h a n (NO), and a l s o much h i g h e r than (CO). Consequently (02) remains e s s e n t i a l l y constant and t h e major r e a c t i o n r o u t e o f CO i s o x i d a t i o n b y 02. Taking t h i s and t h e s t o i c h i o m e t r i c f a c t o r 2 i n t o account, eqn. 2 i s r e w r i t t e n
-
d(CO)/dt = 2k2(02)(CO) Combination o f eqns. 1 and 5 g i v e s
(5)
d(CO)/d(NO) = ( 2 k 2 / k l ) ( ( 0 2 ) / ( N O ) ) T h i s eqn. i n t e g r a t e s t o
(6)
-
-
(7) = ( 2 k 2 / k l ) ( ( 8 2 ) / ( c o ) o ) l n(l-xNO) where x represents f r a c t i o n a l conversion, and (02) t h e mean c o n c e n t r a t i o n XCO
through t h e bed
-
i n this
case m a r g i n a l l y s m a l l e r than ( 0 ~ ) ~ .
Varying conversions were o b t a i n e d by changing t h e r e a c t i o n temperature. By u t i l i z i n g eqn. 7 and t h e CO and NO conversion p o i n t s i n t h e range 200-270°C as given i n Fig. 2, e s t i m a t e s of t h e p a r t i t i o n i n g r a t i o , which increased r e g u l a r l y t o 6.3 a t 27OoC, were obtained. T h i s i n d i c a t e s t h a t t h e
from 3.9 a t 200'
i r i d i u m / z e o l i t e system i s a more s e l e c t i v e c a t a l y s t than t h e Ir/A1203 system. The same a p p l i e s t o a comparison w i t h Knight's r e s u l t s ( r e f . 6). He worked w i t h
Ir/Si02 and obtained 20 % removal f o r R = 8, where t h e I r / z e o l i t e c a t a l y s t removed more t h a n 75 % o f NO. REFERENCES 1 W. Strauss and S. J. Mainwaring, A i r P o l l u t i o n , E. Arnold. London. 1984. DD 38-49. A. G. Clarke and A. Wiliiams, I n s t . Chem. Enq. 2 - Smp. - . Series No 96 (1986) 261. 3 S . J. Tauster and L. L. M u r r e l l , J. C a t a l y s i s 41 (1976) 192. 4 S. J. Tauster and L. L. M u r r e l l . J. C a t a l v s i s 53 (19781 260. 5 K.C. T a y l o r and J.C. S c h l a t t e r , - J . C a t a l y i i s 63 (i980)'53. S.J. Knight, B r i t i s h P a t e n t s p e c i f i c a t i o n 1 581 628 (1980). 6 7 M. Dufaux, P. G e l i n and C. Naccache, i n B. I.I m e l i k , C. Naccache, Y. Ben T a a r i t , J. C. Vedrine, G. Coudurier and H. P r a l i a u d ( E d i t o r s ) , C a t a l y s i s b y Z e o l i t e s , E l s e v i e r , Amsterdam, 1980, pp. 261-271. P. Gelin, G.Coudurier, Y. Ben T a a r i t and C. Naccache, 8 J. C a t a l y s i s 70 (1981) 32. Lemaitre, P. G. Menon and F. Delannay, i n F. Delannay (Editor), 9 C h a r a c t e r i z a t i o n o f heterogeneous c a t a l y s t s , Marcel Dekker, New York, 1984, pp 299-365. 10 P. G a l l e z o t and G. Bergeret, i n P. A. Jacobs, N. I . Jaeger, P. J i r u t and G. S c h u l t z - E k l o f f ( E d i t o r s ) , Metal m i c r o s t r u c t u r e i n z e o l i t e s , E l s e v i e r , Amsterdam, 1982, pp 167-177. 11 P. G a l l e z o t , i n J. R. Anderson and M. Boudart ( E d i t o r s ) , C a t a l y s i s Science and~Technology, Springer, B e r l i n , v o l . ' 5 , 1984; ~ p p2211273. 12 L. L. Hegedus, R. K. Herz, S. H. Oh and R. Aris, J. Catal. 57 (1979) 513. 13 S. Kolboe and R. Myrdal, i n p r e p a r a t i o n .
-
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
METAL-DOPED ZEOLITES FOR SELECTIVE CATALYTIC REDUCTION O F NITROGEN O X I D E S I N COMBUSTION GASES
J . HAAS. J . STEINWANDEL and C. PLOG Dornier S y s t e m GmbH, P. 0. Box 1360, D-7990 F r i e d r i c h s h a f e n (FRG)
ABSTRACT
In t h e p r e s e n t work i t is d e m o n s t r a t e d t h a t t h e a c t i v i t y of z e o l i t e s c o n c e r n i n g t h e S C R - p r o c e s s can be i n c r e a s e d s i g n i f i c a n t l y by m e t a l d o p i n g . T h e SCR c a t a l y s t s p r e p a r e d by m e t a l ion e x c h a n g e o f m o r d e n i t e a n d Y-zeolite w e r e t e s t e d i n a l a b o r a t o r y p l a n t t o d e t e r m i n e t h e efficiency of t h e c o n v e r s i o n of n i t r i c o x i d e s w i t h ammonia. C o p p e r - d o p e d Y - z e o l i t e c a n b e u s e d a s a SCR c a t a l y s t i n a wide t e m p e r a t u r e r a n g e . T h e r e f o r e , t h e o p e r a t i o n is p o s s i b l e a s well in a l o w - d u s t region, s u c h a s behind a d e s u l p h u r i z a t i o n p l a n t . A c o m p l i c a t e d t e m p e r a t u r e c o n t r o l f o r o p t i m u m c o n v e r s i o n is not n e c e s s a r y . A l s o t h e e x c e l l e n t a m m o n i a s t o r a g e c a p a c i t i e s of t h e z e o l i t e c a t a l y s t s c a n be d e m o n s t r a t e d . T h e r e f o r e , c h a n g e s i n t h e ammonia f e e d o r t h e NO c o n t e n t in t h e exh a u s t g a s c a n be c o m p e n s a t e d .
1NTRODUCTlON
Due to t h e p u b l i c d i s c u s s i o n c o n c e r n i n g new t y p e s of e n v i r o n m e n t d a m a g e ( W a l d s t e r b e n ) , a r a p i d d e v e l o p m e n t of t e c h n o l o g i e s h a s s t a r t e d i n t h e FRG c o n c e r n i n g t h e r e d u c t i o n of p o l l u t a n t s f r o m e x h a u s t g a s e s . M e a n w h i l e , p r o c e s s e s c o n c e r n i n g d e s u l p h u r i s a t i o n have b e e n i n t r o d u c e d w i d e l y . In c o n t r a s t , t h e various t e c h n o l o g i e s f o r n i t r i c o x i d e r e d u c t i o n a r e s t i l l u n d e r i n v e s t i gation.
In t h e USA a s well a s t h e FRG, n o t only t h e w e l l - e s t a b l i s h e d ( s i n c e 1970) J a p a n e s e p r o c e s s e s a r e t a k e n i n t o a c c o u n t . T h e J a p a n e s e t e c h n o l o g y o f NO r e d u c t i o n by u s i n g modified TiO, c a t a l y s t s s h o u l d b e r e p l a c e d by a l t e r n a t i v e s o l u t i o n s . C o n c e r n i n g t h e maximum e m i s s i o n v a l u e s ( N O x ) given by g o v e r n m e n t a u t h o r i t y a n d mainly r e l a t e d to l a r g e h e a t i n g p l a n t s (e.g. b l o c k h e a t i n g p o w e r p l a n t s ) , primary p r o c e s s e s d o not lead t o s a t i s f y i n g r e s u l t s . I t is ins t e a d n e c e s s a r y t o d e v e l o p new p r o c e s s e s f o r r e d u c i n g NOx e m i s s i o n s : f o r example wet p r o c e s s e s and reductive processes (using ammonia) m u s t be t a k e n i n t o a c c o u n t . T a b l e 1 s h o w s a s e l e c t i o n of s u c h p r o c e s s e s a l r e a d y u n d e r investigation.
338 TABLE 1 Selection o f secondary t r e a t m e n t s for NOx reduction.
_ _ _ _ _ _ _ _ _ _
* *
Selective noncatalytic reduction (SNCR)
.:
Active carbon process
* * *
Oxidation/Absorption process
* *
Sulphonic a c i d l n i t r i c acid process
Selective catalytic reduction (SCR)
Electron beam process Dry sorption Complex s a l t process
________----__-------------------------------------Oxidative methods produce nitrates or nitric acid, and problems arise concerning the back-transfer of t h e products into t h e product cycle. Therefore, t h e m o s t practicable methods consist of reductive processes, which finally produce nitrogen. For t h e selective reduction of nitric oxides by ammonia a t acceptable temperatures and conversion r a t e s , the use of catalytic processes (SCR) i s essential. A possible reaction scheme is shown i n table 2.
TABLE 2 Selection of possible partial reactions, SCR process Selective Reduction:
4 N H 3 + 6 N 0 ---D 4 NH, + 4 NO + 0,
5 N,
--*
6 H,O 4 N, + 6 H,O
+
Production o f N,O:
2 NH,
+ 2 0, ----0 8 NH, + 12 N O + S 0,
Oxidation of ammonia: 4 NH, + 7 0, --+
4NH,+50, 4NH,+30,
0
--
N,O + 3 H,O 10 N,O + 12 H,O
--+
4 NO, + 6 H,O 4NO+6H20 2N2+6H,O
339 In view of t h e h e t e r o g e n e o u s l y c a t a l y z e d r e a c t i o n s , v a r i o u s t y p e s o f c a t a -
l y s t s have a l r e a d y been d e v e l o p e d . The c a t a l y s t t y p e is mainly r e l a t e d t o t h e r e q u i r e d l o c a t i o n in t h e p r o c e s s . In p r i n c i p l e , t h r e e p o s s i b l e c a t a l y s t l o c a t i o n s m u s t be t a k e n i n t o a c c o u n t , a s s h o w n in Fig. 1. F o r e x a m p l e , p r o c e s s e s a r e k n o w n , whereby nitric o x i d e s a r e r e d u c e d by a n o b l e - m e t a l - c o n t a i n i n g c a t a l y s t a t c o m p a r a b l y l o w t e m p e r a t u r e s , producing n i t r o g e n . H o w e v e r , r e l a t i v e l y high n o b l e - m e t a l c o s t s p o s e s o m e r e s t r i c t i o n s f o r t h e p r o c e s s . In a d d i t i o n , t h e c a t a l y s t s a r e s u b j e c t t o p o i s o n i n g by s u l p h u r d i o x i d e in t h e e x h a u s t g a s . T h e r e a r e c h e a p e r a n d m o r e s t a b l e c a t a l y s t s b a s e d on j a p a n e s e t e c h n o mainly c o n s i s t i n g of vanadium and t i t a n i u m o x i d e s . A g e n e r a l d i s a d v a n t a g e of t h e s e c a t a l y s t s , however, is t h e heavy-metal c o n t e n t of u s e d c a t a -
l y s t s . P r o c e s s e s a r e known which employ z e o l i t e c a t a l y s t s . A n a d v a n t a g e of m o l e c u l a r sieve c a t a l y s t s is t h e s t o r a g e c a p a b i l i t y of a m m o n i a . By t a k i n g adv a n t a g e of t h i s e f f e c t , s h o r t t i m e v a r i a t i o n s of t h e a m m o n i a c o n c e n t r a t i o n c a n be c o m p e n s a t e d . The p r o c e s s e s , however, r e q u i r e r e l a t i v e l y high o p e r a t i o n t e m p e r a t u r e s ( u p t o 48OoC), a n d t h e r e is no c o m p l e t e n i t r i c o x i d e c o n v e r s i o n due t o t h e comparatively reaction-inactive z e o l i t e c a t a l y s t s .
In t h e p r e s e n t work it is d e m o n s t r a t e d t h a t t h e a c t i v i t y of z e o l i t e s c o n c e r n i n g t h e S C R p r o c e s s c a n b e i n c r e a s e d s i g n i f i c a n t l y by m e t a l d o p i n g . T h e r e f o r e , t h e u s e of t h e c a t a l y s t a t l o w e r t e m p e r a t u r e s will b e p o s s i b l e .
Vessel
1 2 3
1
Dust removal
2 -'
Desulph. plant
High-Dust catalyst Low-Dust catalyst LT catalyst
Fig. 1. P o s s i b l e l o c a t i o n s of SCR c a t a l y s t s .
3 8 -
Flue
340 EXPERIMENTAL
A s s t a r t i n g m a t e r i a l s f o r t h e p r e p a r a t i o n of t h e S C R - c a t a l y s t s , c o m m e r c i a l ly a v a i l a b l e z e o l i t e s of m o r d e n i t e t y p e a n d Y - z e o l i t e s w e r e u s e d ( B a y e r A C a n d N o r t o n C o m p . ) . T h e d o p i n g of t h e z e o l i t e s w i t h m e t a l s w a s p e r f o r m e d by t h e m e t h o d of i o n e x c h a n g e c i t e d i n t h e l i t e r a t u r e . S C R c a t a l y s t s can b e used a s b u l k - c o n t a c t honeycomb c a t a l y s t s o r a s c o a t ed h o n e y c o m b c a t a l y s t s . From t h e p o i n t of v i e w of p r o c e s s d e v e l o p m e n t , w e d e c i d e d t o use t h e c o a t e d h o n e y c o m b t y p e . A f t e r t h e p r e p a r a t i o n of t h e c a t a lytic a c t i v e p o w d e r s by ion e x c h a n g e a n d s u b s e q u e n t t h e r m a l t r e a t m e n t o r r e d u c t i o n , c o r d i e r i t e h o n e y c o m b s w e r e c o a t e d w i t h t h e z e o l i t e p o w d e r s by a s p e c i a l d e v e l o p e d p r o c e s s . The g e o m e t r y of t h e c o r d i e r i t e h o n e y c o m b s c o r r e s ponded t o t h a t u s u a l l y used i n t h e a u t o m o b i l e i n d u s t r y ( t e s t m o n o l i t e s , 3.15 inch l o n g , 1 inch d i a m e t e r , 400 S q . c e l l s p e r S q . i n c h ) .
The SCR c a t a l y s t s p r e p a r e d by t h e m e t h o d d e s c r i b e d a b o v e w e r e t e s t e d in a l a b o r a t o r y p l a n t t o d e t e r m i n e t h e e f f i c i e n c y of t h e c o n v e r s i o n of n i t r i c o x i d e s w i t h ammonia. A s a m o d e l s u b s t a n c e f o r t h e g r o u p of n i t r i c o x i d e s , NO w a s used. T h i s can b e j u s t i f i e d b e c a u s e t h e NO c o n t e n t of n o r m a l e x h a u s t g a s is
a b o u t 9 0 %of t h e N O x . The l a b o r a t o r y p l a n t c o n s i s t s of a g a s - m i x i n g s t a t i o n , a g a s - h e a t i n g s y s t e m , a r e a c t i o n c h a m b e r c o n t a i n i n g t h e c a t a l y s t , and f i n a l l y a gas-analysis system. By u s i n g a g a s c y l i n d e r b a t t e r y a n d e l e c t r o n i c m e a s u r e m e n t a n d c o n t r o l c o m p o n e n t s i t is p o s s i b l e to vary t h e e x h a u s t g a s c o m p o s i t i o n o v e r a w i d e r a n g e . T h e s y n t h e t i c e x h a u s t g a s c o n t a i n s a m a x i m u m of 9 d i f f e r e n t c o m p o n e n t s . The t e s t p l a n t a l l o w s f o r a maximum gas v o l u m e s t r e a m u p t o SO I / m i n . The s t a n d a r d c o m p o s i t i o n of t h e s y n t h e t i c e x h a u s t g a s is s h o w n i n t a b l e 3. TABLE 3 S t a n d a r d c o m p o s i t i o n of e x h a u s t g a s f o r t h e p r e s e n t e x p e r i m e n t s .
79.86 10.0
4.0 6.0
0.07 0.07
341 T h e g a s h e a t i n g s y s t e m p e r m i t s e x h a u s t t e m p e r a t u r e s b e t w e e n 150 a n d 450 OC. P r o g r a m m a b l e h e a t i n g using s t a n d a r d i z e d t e m p e r a t u r e p r o g r a m s a l l o w s f o r m e a s u r e m e n t s o f t h e c a t a l y s t behaviour w i t h r e l a t i o n t o t h e d i f f e r e n t e x h a u s t - g a s t e m p e r a t u r e s . T h e g a s c o m p o s i t i o n d o w n s t r e a m a n d ups t r e a m of t h e c a t a l y t i c c o n v e r t e r is c o n t r o l l e d by t h e g a s - a n a l y s i s s y s t e m . The s y s t e m c o n s i s t s o f a GC w i t h FID a n d HCD d e t e c t o r s , a n d a c h e m i l u m i n e s c e n c e NOx a n a l y z e r . T h e chemical c o m p o s i t i o n of t h e c a t a l y t i c a c t i v e p o w d e r s w a s a n a l y z e d by
EDX. The i n f o r m a t i o n d e p t h
o f EDX is a b o u t 1 t o 2 pm. W i t h a c r y s t a l l i t e s i z e
of t h e z e o l i t e s o f 1 t o 4 pm, t h e volume c o m p o s i t i o n is a l s o given by t h e EDX method.
RESULTS A N D DISCUSSION A s a l r e a d y s t a t e d i n t h e i n t r o d u c t i o n , t h e s t a t e of t h e a r t c o n c e r n i n g n o b l e
m e t a l c a t a l y s t s a l l o w s f o r a c c e p t a b l e n i t r i c o x i d e c o n v e r s i o n a t low t e m p e r a t u r e s . A s is a l r e a d y known f r o m work o n t h r e e - w a y c a t a l y s t s ’),
i t is p o s s i b l e ,
by f i n e d i s p e r s i o n of p l a t i n u m i n n e a r - s u r f a c e r e g i o n s of m o r d e n i t e c r y s t a l l i t e s , t o p r o d u c e a c a t a l y s t c h a r a c t e r i z e d by high a c t i v i t y w i t h low n o b l e m e t a l c o n t e n t . I n a d d i t i o n , d u e t o t h e a d s o r p t i o n of a m m o n i a m o l e c u l e s a t t h e acid z e o l i t e s i t e s , t h e r e e x i s t s t h e a d d i t i o n a l e f f e c t of s y n e r g y . T h e r e f o r e , we s t a r t e d t h e i n v e s t i g a t i o n by using P t - d o p e d m o r d e n i t e a s SCR c a t a l y s t , b e c a u s e a r a t h e r low o p e r a t i o n t e m p e r a t u r e in c o n j u n c t i o n w i t h
high c o n v e r s i o n r a t e s s h o u l d t h u s be e x p e c t e d . Fig. 2. s h o w s t h e n i t r i c o x i d e c o n v e r s i o n a s a f u n c t i o n o f t h e g a s t e m p e r a t u r e f o r t w o d i f f e r e n t c a t a l y s t s of t y p e P t H - m o r d e n i t e . C a t a l y s t s w e r e p r e p a r e d f o r i d e n t i c a l Pt c o n t e n t of a b o u t 0.2 g/l.
From Fig. 2. it is s e e n , t h a t f o r b o t h c a t a l y s t s a l m o s t 100%n i t r i c o x i d e c o n version o c c u r s . H o w e v e r , t h e t e m p e r a t u r e r a n g e f o r o p t i m u m c o n v e r s i o n , b u t a l s o i n c r e a s e a n d d e c r e a s e of t h e conversion s l o p e a r e q u i t e d i f f e r e n t . T h i s is d u e t o t h e d i f f e r e n t S i / A l - r a t i o of t h e t w o m o r d e n i t e c a t a l y s t s . T h e m o r d e n i t e o f c a t a l y s t (11) w a s d e a l u m i n a t e d by acid t r e a t m e n t b e f o r e P t - d o p i n g . T h i s
r e s u l t e d i n a m o d u l u s ( m o l a r r a t i o SiO,/AI,O,)
of a b o u t 2 0 . T h e m o r d e n i t e of
c a t a l y s t ( 1 ) w a s n o t modified ( m o d u l u s o f a b o u t 12). By t h e dealumiiiatioii of t h e anionic l a t t i c e of t h e moi-denite in conji:::cti:,-
with a s i n i u l t a n e o u s c h a n g e o f t h e s t e r i c c o n d i t i o n s in t h e p o r e s y s t e m . a c h a n g e i n a c i d i t ) r e s u l t s . C h a n g e s i n s p e c i f i c s u r f a c e w e r e d e t e r m i n e d b)
342 using BET. T h e l o w e r c o n t e n t of n e g a t i v e p a r t i a l c h a r g e s o f t h e l a t t i c e a f t e r d i s s o l v i n g t h e a l u m i n u m r e s u l t s i n a d e c r e a s e in n u m b e r o f acid s i t e s . T h e i r s t r e n g t h , h o w e v e r , i n c r e a s e s . Both e f f e c t s i n f l u e n c e t h e n u m b e r of a d s o r p t i o n c e n t r e s f o r a m m o n i a a s well a s t h e d i f f u s i o n i n t o t h e p o r e s y s t e m . T h i s r e s u l t s i n a much b e t t e r c o n v e r s i o n c h a r a c t e r i s t i c f o r t h e d e a l u m i n a t e d P t H - m o r d e n i t e .
0 1I
150
i
I
200 Temperature
250
300
["Cl
Fig. 2. Activity b e h a v i o u r of P t H - m o r d e n i t e o f d i f f e r e n t m o d u l i a s a f u n c t i o n of g a s t e m p e r a t u r e . C a t a l y s t (1) s h o w s 90%c o n v e r s i o n o v e r o n l y a r e l a t i v e l y n a r r o w t e m p e r a t u r e r a n g e , a n d t h e a c t i v i t y s t r o n g l y d e c r e a s e s a t l o w e r o r h i g h e r t e m p e r a t u r e s . In c o n t r a s t , t h e d e a l u m i n a t e d c a t a l y s t s (11) s h o w s a l m o s t 100%c o n v e r s i o n b e t ween 170 and 200 O C . I n a d d i t i o n , t h e maximum c o n v e r s i o n i s m a i n t a i n e d a l s o a t h i g h e r g a s t e m p e r a t u r e s . T h e d e c r e a s e in c o n v e r s i o n f o r b o t h c a t a l y s t s a t h i g h e r t e m p e r a t u r e s is r e l a t e d t o t h e onset of t h e N O - p r o d u c i n g a m m o n i a o x i d a t i o n . T h i s r e s u l t s in a s i g n i f i c a n t d e c r e a s e of t o t a l NO c o n v e r s i o n . C a t a l y s t (11) s h o w s a h i g h e r
SCR a c t i v i t y a t l o w e r t e m p e r a t u r e s a n d a l o w e r a c t i v i t y
(desired) concerning ammonia oxidation a t higher t e m p e r a t u r e s . A s an a l t e r n a t i v e t o noble metal containing z e o l i t e c a t a l y s t s f o r selective
r e d u c t i o n of n i t r i c o x i d e , z e o l i t e c a t a l y s t s d o p e d w i t h t r a n s i t i o n m e t a l s o f t h e
343 p e r i o d i c s y s t e m w e r e p r o d u c e d . A s b a s i s m a t e r i a l s , Y-zeolite a n d m o r d e n i t e w e r e u s e d . Z e o l i t e s w e r e d o p e d w i t h e i t h e r vanadium o r c o p p e r . F i r s t , vanadium is known a s a c t i v e c o m p o n e n t of t h e T i 0 2 - b a s e d S C R - c a t a l y s t of J a p a n e s e t e c h n o l o g y . C o p p e r , o n t h e o t h e r h a n d , is k n o w n to a c t a s c e n t r a l ion in a m monia c o m p l e x e s , r e s u l t i n g i n a d d i t i o n a l s t o r a g e o f a m m o n i a in t h e z e o l i t e .
n
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!
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i Y
i 3
Fig. 3. C o n v e r s i o n c h a r a c t e r i s t i c of d i f f e r e n t d o p e d z e o l i t e s a s a f u n c t i o n o f t h e gas temperature. Fig. 3. s h o w s a s i g n i f i c a n t d i f f e r e n c e in t h e a c t i v i t y b e h a v i o u r o f t h e t h r e e m e t a l - d o p e d z e o l i t e s a s a f u n c t i o n of e x h a u s t t e m p e r a t u r e . I t c a n b e s e e n t h a t t h e H-mordenite (vanadium-doped) d o e s n o t show significant activity in cont r a s t t o t h e Pt-doped f o r m . Over t h e e n t i r e t e m p e r a t u r e r a n g e 200 t o 400 OC, o n l y NO-conversion r a t e s of 10 X a r e r e a c h e d . Vanadium is p r e s e n t in t h e z e o lite in a c o m p l e t e l y d i f f e r e n t f o r m , a s is t h e c a s e f o r t h e TiO, c a t a l y s t . T h e r e f o r e , vanadium c a n n o t play t h e identical role. In a d d i t i o n , i t is o b v i o u s t h a t t h e H - m o r d e n i t e a l o n e e x h i b i t s e x t r e m e l y l o w a c t i v i t y a s a n SCR c a t a l y s t .
344
I n c o n t r a s t t o vanadium, c o p p e r a s doping metal s h o w s a positive e f f e c t f o r t h e H - m o r d e n i t e . A t t e m p e r a t u r e s up t o 300 OC, t h e r e is n o d i f f e r e n c e b e t w e e n t h e t w o c a t a l y s t s . A t t e m p e r a t u r e s a b o v e 300 OC, h o w e v e r , a s i g n i f i c a n t inc r e a s e i n t h e s e l e c t i v e r e d u c t i o n of n i t r o g e n o x i d e is f o u n d f o r t h e CuHm o r d e n i t e . A t 380 OC a maximum c o n v e r s i o n o f a b o u t 50 % is r e a c h e d . A f u r t h e r i n c r e a s e i n t e m p e r a t u r e , however, r e s u l t s in a s t r o n g d e c r e a s e of NO c o n version. T h i s is d u e t o t h e r a p i d i n c r e a s e o f NH, o x i d a t i o n . Above 400 OC, more ammonia molecules a r e reacting with oxygen t h a n with nitrogen oxide, f i n a l l y r e s u l t i n g in a n i n c r e a s e of NO e m i s s i o n in t h e t o t a l mass b a l a n c e . T h e p a r a l l e l r e a c t i o n o f a m m o n i a o x i d a t i o n c a n b e r e d u c e d by u s i n g a c o p p e r - d o p e d Y - z e o l i t e . a s s h o w n in f i g . 3. T h i s p a r t i c u l a r c a t a l y s t c o m b i n e s t h e c a p a b i l i t y of r e d u c i n g ammonia o x i d a t i o n w i t h a high a c t i v e NO r e d u c t i o n . T h i s l e a d s t o a r e l a t i v e l y high c o n v e r s i o n p l a t e a u o v e r a w i d e t e m p e r a t u r e r a n g e . Between 250 a n d 400 OC NO c o n v e r s i o n is b e t t e r t h a n 80 %.
1
I
I
Fig. 4 . C o m p a r i s o n of c o n v e r s i o n c h a r a c t e r i s t i c f o r d i f f e r e n t l y p r e p a r e d c o p per-doped Y-zeolites.
345
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Fig. 5. C o n v e r s i o n r a t e of t h e r e a c t i o n NO r u p t i o n o f t h e ammonia f e e d .
+
NH, v e r s u s t i m e ( m i n ) a f t e r i n t e r -
T h i s h i g h e r p o t e n t i a l o f t h e c o p p e r - d o p e d Y-zeolite a s SCR c a t a l y s t c a n a l s o b e s e e n f r o m fig. 4 . H e r e , NO c o n v e r s i o n i s p l o t t e d a g a i n v e r s u s g a s t e m p e r a t u r e , and t w o d i f f e r e n t l y p r e p a r e d CuNaY-zeolites a r e c o m p a r e d . I t c a n b e s e e n t h a t by o p t i m i z i n g t h e p r e p a r a t i o n t e c h n i q u e a f u r t h e r i n c r e a s e in c o n v e r s i o n c h a r a c t e r i s t i c s is p o s s i b l e . T h e c a t a l y s t CuNaY(2) s h o w s m o r e t h a n 80 % c o n v e r s i o n over a t e m p e r a t u r e r a n g e 2 0 0 - 400 OC. Between 250 a n d 390 OC a n a l m o s t 100 % c o n v e r s i o n of n i t r i c o x i d e is f o u n d .
-
T h e r e f o r e , t h e c o p p e r - d o p e d Y-zeolite c a n b e o p e r a t e d a s SCR c a t a l y s t a t l e a s t o n a l a b o r a t o r y s c a l e - t o b e u s e d o v e r a wide t e m p e r a t u r e r a n g e . O p e r a t i o n is p o s s i b l e a s w e l l in a l o w - d u s t r e g i o n s u c h a s b e h i n d t h e d e s u l p h u r i z a -
t i o n p l a n t , w h e r e t h e g a s t e m p e r a t u r e is a l r e a d y r e d u c e d s i g n i f i c a n t l y . A c o m -
p l i c a t e d t e m p e r a t u r e c o n t r o l f o r o p t i m u m c o n v e r s i o n is n o t n e c e s s a r y d u e t o t h e wide t e m p e r a t u r e w i n d o w of t h e c a t a l y s t .
346
An a d d i t i o n a l i m p o r t a n t c h a r a c t e r i s t i c of t h e z e o l i t e SCR c a t a l y s t s is s h o w n in f i g . 5. H e r e , T h e o p t i m i z e d C u N a Y - z e o l i t e w a s u s e d . During a l o n g - t e r m o p e r a t i o n a t a n e x h a u s t g a s t e m p e r a t u r e of 300 OC, t h e a m m o n i a f e e d w a s i n t e r r u p t e d , w h i l e t h e NO f e e d w a s m a i n t a i n e d c o n s t a n t . Fig. 5 s h o w s NO c o n v e r s i o n v e r s u s t i m e ( m i n ) a f t e r i n t e r r u p t i o n of t h e a m m o n i a f e e d . T h e e x c e l l e n t a m m o n i a s t o r a g e c a p a b i l i t y of t h e z e o l i t e , in p a r t i c u l a r t h e c o p p e r - d o p e d Y - z e o l i t e , c a n b e d e m o n s t r a t e d . Even u p t o 3 min a f t e r a m m o n i a f e e d i n t e r u p t i o n , a 100 % c o n v e r s i o n o f NO is m a i n t a i n e d . A f t e r t h i s t i m e t h e r e a c t i o n e f f i c i e n c y d e c r e a s e s . H o w e v e r , even a f t e r 10 m i n , t h e r e is still 2 0 % NO c o n v e r s i o n . A f t e r 2 0 min, t h e r e a c t i o n e f f i c i e n c y is z e r o . By u s i n g t h e e x c e l l e n t s t o r a g e c a p a b i l i t y , c h a n g e s in t h e a m m o n i a f e e d o r t h e NO c o n t e n t in t h e e x h a u s t g a s c a n be c o m p e n s a t e d . This h o l d s a l s o f o r s h o r t - t i m e c e s s a t i o n of t h e a m m o n i a f e e d .
T h e d i f f e r e n t l y modified z e o l i t e SCR c a t a l y s t s d e s c r i b e d i n t h i s w o r k c a n be u s e d f o r d i f f e r e n t o p e r a t i n g c o n d i t i o n s . S u c h a d v a n t a g e s a s e c o n o m i c a l basis s u b s t a n c e , excellent activity behaviour ( i n p a r t over wide t e m p e r a t u r e r a n g e s ) a n d , i n p a r t i c u l a r , t h e unique s t o r a g e c a p a b i l i t y f o r t h e r e a c t i o n c o m p o n e n t a m m o n i a have been d e m o n s t r a t e d . I n a d d i t i o n , d u e t o t h e c o m p o s i t i o n of t h e z e o l i t e s , t h e d i s p o s a l of used c a t a l y s t s is l e s s t r o u b l e s o m e c o m p a r e d
w i t h heavy-metal-containing
TiO, c a t a l y s t s .
ACKNOWLEDGEMENTS The a u t h o r s g r a t e f u l l y a c k n o w l e d g e financial s u p p o r t by t h e B u n d e s m i n i s t e r f u r F o r s c h u n g und T e c h n o l o g i e .
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H.G. Karge, J. Weitkamp (Editors ), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION OF NiHZSM-5 CATALYST FUR ISOMERIZATION OF C8 ARaMATICS. SOLIE-STATE INCORFORATION OF NICKEL
B. WICHTEELOV$. R .&~Ikz
S. BERANI. L. KUBELKOVL1, J. NOVhVAl, A . SMI&OVfl. and
J . Heyrovsw Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 182 23 Prague 8 (Czechoslovakia) Research Institute for Oil and Hydrocarbon Gases, 823 17 Bratislava (Czechoslovakia)
A high temperature interaction between ?US?d-5 and solid NiC12. NiS04, Ni(CH3COO)z and NiO was investigated. For NiC12. disaggregation of the salt is complete at 7'70 K with evolution of gaseous hydrochlorid acid, resulting in solid-solid ion exchange of Ni ions into the zeolite cationic sites at the expense of the strong acid skeletal OH groups. The heat treatment of the zeolite with NiS04 above 670 K leads to a decrease in the number of skeletal OH groups to approximately half the original number. Moreover, no evolution of acid or its decomposition products is observed. Ni(CH3COO)2 behaves differently. No ionchange (even in a wet paste) occurs up to the salt decomposition temperature (520K). NiO formed at higher temperatures also does not interact with the skeletal OH groups. The described high-temperature solid-solid ion exchange between HZSM-5 and NiC12 was used successfully for the incorporation of Ni2+ into the zeolite cationic sites. Subsequent reduction of these ions by hydrogen yields well-dispersed metallic nickel, which is active in the isomerization of c8 aromatic hydrocarbons.
Catalysts based on HZSM-5 zeolite with metallic nickel finelly dispersed within the zeolite channels have been found to have excellent hydrogenation activity in isomerization of hydrocarbons, which prevents olefin polymerization and, consequently, zeolite coking. The usual procedure for obtaining well dispersed metallic nickel in the zeolite catalyst involves ion exchange of the Ni2+ ions into the zeolite cationic sites, followed by reduction with hydrogen. According to the patent literature (ref. l), this exchange can be carried out from Ni salt solutions. A further possibility for incorporation of Ni ions into the zeolite cationic sites appears to be solid-solid ion exchange between Ni salts and an H form of the zeolite. Solid-solid ion exchange between metal halides and the H forms of A, X . Y types of zeolites above 570 K was first described by Rabo et al. (refs. 2. 3) and Clearfield et al. (ref.4). Recently, several papers have been devoted to solidstate reactions with the catalytically important H forms of mordenites and ZSM-5 zeolites (ref. 5-8). Evidence based on the B R spectra of the metal cations, IR spectra of OH groups and detection of evolved acids has been given for the disaggregation of the solid salts of Cu, Mn, Ca, Mg, Cr and oxides of Cu. Fe, V and Cr mixed with zeolites; in some cases metal cation exchange with the zeolite protons has been detected.
348
This paper deals with the high-temperature interaction of the solid Ni2+ salts or oxide with the HZSh4-5 zeolite, considering possible use for the preparation of the isomerization catalyst for Cg aromatic transformation. A comparison with NilUSM-5 prepared via Ni2+ ion exchange in aqueous medium is made.
The HzsM-5 zeolites (Si/Al = 13.6 (a) and 22.5 (b)) were synthesized using tetrapropylammonium base according to a procedure given in ref. 9 and then heated to 820 K in an oxygen stream and decationized by 0 . 5 HN%. The NiHZSM-5(b)-w was prepared by the ion exchange of a Ni(NH3)6(CH3C00)2 solution with H'ZSM+5(b) at 298 K and @ 10.8l(Table 1). Ni(CH3C00)2.M+$ and Ni(Q)2.6H@ salts (Merck p.a.) were used for estimation of the level of the ion exchange in an aqueous medium and the amount of Ni salt remaining in solution. HZSM+(a) and HZSM-B(b) and NiC12.6 H20. NiS04.7HS and Ni(CH3C00)2.4HZO and NiO (Merck p.a.) were employed for solid-state interactions between the H Z M zeolites and Ni2+ solid salts or an oxide. The zeolite with a particular salt or oxide (the amounts are given in Table 2) was stirred in an agate mortar to obtain well-powdered and mixed mechanical mixture. This mixture was heated (temperature increase of 5 K/min) to the temperature indicated in Table 2 and then held at this temperature for 6 hours in an oxygen stream. After this treatment, the mixture was equilibrated with atmospheric moisture at ambient temperature (zeolite abbr. NiHZSM-S(a) or (b)-s). In s o m e cases heat-treated mixtures were reduced by hydrogen at 720 K for 4 hours or treated with 1 N NH@% solution at 330 K for 10 hours. The following characteristics were investigated for the heat-treated solid mixtures (modified zeolites) and for the parent zeolites: i) The OH groups were monitored by recording the IR spectra ( ITNicolet MX-1E 7 mg/c&) spectrometer) of the sample in the form of a pellet (thickness of which was evacuated at 670 K for 1 hour. ii) The number of strong acid skeletal OH groups was estimated from the high-temperature peak of the tempetarureprograd desorption of ammonia (TPDA). carried out in the temperature interval from 373 to 670 K (20 K/min) in a stream of dry helium on samples equilibrated with dry ammonia at 373 K. The evolved ammonia was detected by a heat conductivity cell (ref. 10). iii) Mass spectrometric detection (MCH 1302 USSR) was used for determination of gases evolved from the zeolite mixture during its programmed heating in vacuo ( 5 K/min, 1 0 4 Pa). iv) The amount of acid evolved from the zeolite mixture into an oxygen stream was detected by the absorption of the eluent gas in an alkaline solution (0.1 N NaOH) and its back titration with 0.1 N HC1. v) The catalytic activity of zeolites in isomerization of Cg aromatic hydre carbons was tested using a model mixture consisting of 80.0vol.% of 0-xylene and 20.0 vol.% of ethylbenzene in hydrogen (H/M molar ratio 5 . 0 ) . The investigated catalysts in the form of pelletes ( 0 . 5 nnn in diameter, 5 . 0 g) contain 30 w t . % of the zeolite sample and 70 wt.% of alumina (Lachema p.a.). The catalyst was pretreated in a hydrogen stream at 720 K for 4 hours and then in a stream of the model mixture mentioned above (W 3.4h-l) for 100 hours. During the following 5 hours, the conversion and selectivity data (presented in Fig. 2) were found to be constant. Conversion of the initial hydrocarbons and composition of the products were determined by owline chromatographic analysis of gaseous products and off-line analysis of the liquid products, collected for 5 hours in a trap at 273 K.
-
349 TABLE 1 Chemical composition of -5 aqueous medium
zeolite
SiO, wt.%
-5(a) m ( b ) NiHZSM-5(b)-w
93.68 96.04 95.36
and NiHZ'3&6
A1A wt.%
5.90 3.75 3.72
Nag
prepared by ion exchange in an skeletal OH groups (mmol/g) Tm3A chem,anal.
NiO
wt.%
CaO wt.%
0.117 0.046 0.046
0.30 0.30 0.30
wt.%
-
0.76a
1.06 0.62 0.39
0.91= 0.5W xx +
0.29
&.
0.
x xx I
Chemical composition is based on a dry zeolite, a0.102 mmol/g of Ni, reduction in hydrogen at 720 K; Xisomerization activity is presented in Fig. 2; =IR spectra of OH groups are depicted in Fig. 1.
Incorporation of Ni into the HZShf-5 zeolite using an aqueous medium Metal-ion exchange from solutions into the zeolites is a method that is widely used for preparation of their cationic forms. In introducing Ni cations from salt solutions, only a limited degree of ion exchange is achieved, while a considerable amount of Ni remains in solution. For instance. with a suspension of the HZSM-5 (Si/Al = 13.6) in Ni(NH3)6(CH3COO)2 solutions (concentrations 1 7 g Ni/l) for which the Ni/Al molar ratio ranges from 0 . 5 to 0.12 (pH 10.8), a m w i w of 12 % Ni ion exchange is attained at 298 K. Similarly, with the same
,
3745
36\3
36i2cm-1
"0 B
Q
-.
Fig. 1. IR spectra of OH groups of the parent HZSd-5 and NiHiSM-5 zeolites prepared by ion exchange in aqueous medium and in solid-state with NiClZ at 770 K 1- HZSM+(a). 2- HZSM-.5(a) + NiCl2 (0.30mmol Ni/g), 3- HZSM-5(a) + NiCl2 (0.45mmol Ni/g), cf. Table 2, 4- HZSM-5(b), 5- NiHZSd4(b)-w (0.102mmol Ni/g) + reduced in hydrogen at 720 K, & HZSd4(b) + NiC12 (0.102 mmol Ni/g) + reduced in hydrogen at 720 K. cf. Tables 1. 2.
BZsnS(a)
0.91
0.30
Nia2
0.33
-
0.50 0.6dM
0.58
0.90
-5
(a)
0.91
NiCl2.6Et20
0.45
nF
o.oP
0.0P
-5
(a)
0.91
N m 4 . 7E20
0.45
670 770
0.64 0.35
0.27 0.56
-
m,
0.50
Nm2.
0.00
-
mSM-5
m20
0.102
0.00 0.00
HZS4-5 0)
0.50
Ni((cI13ax))2.4€i20
0.102
ESM-Sb)
0.50
Nio
0.40
no no
0 . d
0.oOx
0.50
0.00
concentrations and Ni/A1 ratios the use of solutions of Ni(CH3COO)z or Ni(N03)~ leads, as a consequence of lower pH (5.2and 2.4.resp.) to 8 and 4 % Ni ion exchange, respectively. It is obvious that with N i ( ~ 3 ) 6 ( ~ 3 C ~ ) 2 .Ni(CH$OO)2 and Ni(N%)2 solutions mentioned above, and ion exchange performed at 298 K, at least 5 0 , 70 and 85 %. respectively, of Ni remains in solution. This remaining salt represents a considerable problem in the waste water from preparation of the NiHZWS zeolite. Ni ion incorporation using solid mixture of a zeolite and Ni salt Heating of the mixture of the solid NiCl2 and HZSM-5 in an oxygen stream above 570 K leads to the evolution of gaseous hydrochlorid acid. The amount of evolved HC1 (for mixtures treated at 770 K for 6 hours) corresponds to the amount of Ni in the mixture (Table 2). Simultaneously, a decrease in the number of skeletal OH groups, corresponding also to the amount of Ni in the mixture. is observed by TPDA and IR spectra of OH groups (Table 2, and Fig. 1, band at 3612 cm-l). Moreover. the IR spectra do not detect substantial change in the intensity of the band of the Si4H groups (3745 cm-l); note that the Ni/A1 molar ratios do not exceeds a value of 0.5. Therefore. the above data indicate that the process of solid-solid ion exchange described by the following reaction is complete at 770 K:
Si - 0 - A1
H
I
2 Si - 0 - A 1
+ NiCl2
t Ni
->
+
2 HC1
1 Si - 0 - A1 This conclusion is further supported by the fact that the incorporated chargebalancing Ni ions in NiHZSM-5(b)+iC12 can be re-exchanged with a WQ solution, yielding the H form of the zeolite with practically the same number of skeletal OH groups as the original zeolite (Table 2). Moreover. the reduction of NiHZSd+(b)+iC12 with hydrogen at 720 K again leads to the restoration of nearly all (95 W ) of the skeletal OH roups. The low-intensity band at 3663 cm- observed in the spectra of the heattreated mixture of HZSM-5 and NiCl2 can indicate the presence of a low number of OH groups located on extra-lattice A1 species (ref. 11). These A 1 species should be formed as a result of a leaching of A1 from the zeolite skeleton under the action of hydrochloric acid. However, the low intensity at 3663 cm-l. together with the fact that practically all the skeletal OH groups can be restored by reexchanging or by reduction of Ni2+ ions, indicates that only negligible, if any. zeolite dealumination occurs during solid-solid ion exchange with NiC12. Heating of the mixture of NiS04 with HZSM-5 causes a decrease in the number of structural OH groups. This decrease (increasing with temperature, cf. Table 2) is only roughly h a l f of a possible value given by the amount of Ni ions present in the mixture. In contrast to NiC12. no evolution into the gaseous phase of the corresponding acid or its decomposition products occurs. This indicates that one NiS04 is probably coordinated to one skeletal OH group. Temperatureprograd heating of the physical mixture of HZSM-5 and Ni(CH3COO)z in vacuo. with mass spectrometric detection of the evolved gases. reveals decomposition of the salt above 520 K accompanied by the evolution of CO. q. H2, acetic acid and acetone (as a result of a subsequent reaction occurring on the zeolite). Therefore, formation of NiO can be expected when heating this mixture in a stream of oxygen above 520 K. For physical mixture of NiO and HZSM-5 heated up to 820 K, no incorporation of the Ni ions into the zeolite cationic sites takes place (Table 2). This is most likely due to the low volatility of NiO (melting point, 2173 K). Similarly, ion exchange does not proceed at temperature below 520 K. when urdecomposed Ni(acetate) is present in the mixture; no decrease in the number of structural OBI groups was observed either on heating the mixture in an oxygen stream for 8 hours or on leaving the
f
352 mixture to stand (even in the form of a wet paste) at ambient temperature for several days. Sclllrmarizing, it can be stated that true high-temperature, solid-solid ion exchange occurs quantitatively between HZSM-5 and solid nickel chloride at 770 K. This is not a result of the possible presence of water in the zeolite or in the salt at a temperature below 570 K. HydroRenation, isomerization and dealkylation activity of Ni-modified zeolites Isomerization and dealkylation activity of the original HZSM-5(b) zeolite, depending on the reaction temperature and relative content of ethylene in the C2 fraction of hydrocarbons in the gaseous products. is shown in Fig. 2.
-I
401
I
I
I
/I
ethylbenzene
p-xy lcnc xylenes
590
570
I 610
630
570
590
610
T(K)
Fig. 2. Isomerization of the 0-xylene - ethylbenzene mixture on zeolites depending on temperature (WHSV 3 . 4 h-l. H/cH molar ratio 5.0. pressure 1 m a ) .
A) Conversion
(wt.%) of o-xylene and ethylbenzene based on fed o-xylene and ethylbenzene, respectively, loss (wt.%) of xylenes based on fed o-xylene+ethylbenzene, respectively, and conversion of o-xylen-thylbenzene to pxylene on w ( b ) B) Ethylene to C2 hydrocarbons molar ratio in gaseous products (conversion oi o-xylenetethylbenzene to gaseous products reached a value from 0.1 to 2.8 w t . % depending on temperature). I
1- HZSM-5(b), 2- NiHZEU-5 (b)-w, 3- HZSM-5(b) + NiC12, 4- HZSM-5(b) + Ni(ClI3W)z ; content of Ni is 0.102 nmol Ni/g for all samples, cf. Tables 1. 2.
To test the possibility of using the solid-solid ion exchange for preparation of isomerization catalyst, NiHZSM-5 samples containing the s e u ~amount of Ni (Tables 1, 2) prepared by 1) solid-solid ion exchange with NiC12. ii) decoiw position of Ni(acetate), and iii) standard ion exchange in aqueous medium were compared for their effectiveness in isomerization of the model cg aromatic mixture. It has been folud that isomerization and dealkylation activity of all these NiHZSM-6 catalysts (after their reduction in a hydrogen stream at 720 K) is c o w parable to that of the parent H7S-5 zeolite (Fig. 2). It is in line with the same number of strong acid sites present in the zeolites after reduction of Ni2+ to Nio (Tables 1, 2, Fig. 1). In contrast, the hydrogenation activity of these zeolites , which is important for zeolite coking, differs cansiderably. NiHZSbi+(b)-Ni(acetate), most likely containing Nio outside the zeolite chan-
353 nels, does not exhibit any hydrogenation activity as follows from a content of ethylene in the gaseous products similar to that of the parent zeolite. On the other hand, the hydrogenation activity of NiHZSLM(b)+iClz (solid-ion exchange) is much higher than that of parent zeolite and close to that of NiHZW5(b)w, prepared by standard Ni ion exchange in aqueous medium (Fig. 2B. cf. Tables 1. 2).
It can be stated that the high-temperature solid-solid Ni ion exchange, using the solid nickel chloride and H form of 2 S 5 , proceeds quantitatively without dealumination of the zeolite skeleton and with preservation of the zeolite structure. Metallic nickel formed by subsequent treatment of the zeolite in hydrogen exhibits high hydrogenation activity during isomerization of Cg aromtics. Moreover, the solid-state incorporation of nickel into the zeolite c h a n nels in comparison with ion exchange in aqueous medium has the following advantages : i) The degree of ion exchange can be controlled directly by varying the amount of NiCl2 added to the zeolite. ii) Practically complete Ni ion exchange into the zeolite cationic sites (higher than 90 %) can be achieved. Such high degree of exchange is reached only with difficulty when using solutions of Ni salts. iii) Generally, this pathway of metal ion exchange is especially important for incorporation of metal cations that tend to hydrolyze in aqueous solutions into the zeolite cationic sites. iv) From a technological point of view, the preparation of the cationic forms of zeolites by ion exchange between solids does not produce a large amount of waste water containing metal salts, with detrimental effect on natural envirow ment . However, the solid-solid ion exchange must be carried out with caution to exclude the following: i) the presence of residues of the original salt, by using sufficiently long treatment of the system at temperatures above 670 K; ii) some dealumination or even damage of the zeolite structure by evolved gaseous acid by applying a high velocity of the eluent gas (for this reason, this method can be used only with structurally stable high-silica zeolites like mordenites. ZSd-5 etc.). REFERENCES
1 F.G. Dwyer (Mobil Oil Co.) US patent 4 100 214. 2 J.A. Rabo, M . L . Poustma and G.W. Skeels, Proc. 5th Int. Congr. Catal.. Miami Beach 1972, J.W. Hightower ed.. North Holland Publ. Co. N.Y. 1973, p.1353. 3 J.A. Rabo. Zeolite Chemistry and Catalysis, J.A. Rabo ed., ACS Monograph 171, Washington D.C.. 1976. p. 332. 4 A. Clearfield, C.H.Saldarriaga and R.C. Wlckley, F'roc. 3rd Int. Conf. Mol. Sieves, Zurich, 1973, Hecent Progress Reports, J.B. Uytterhoeven ed.. Univ. Leuven Press. p. 241. 5 A.V. Kucherov and A.A. Slinkin. Zeolites 6 (1986) 175, 7 (1987) 38. 7 (1987) 6 B. Wichterlova. S . Beran. S . Bednarova. K. Nedomova. L. W i k o v a and P. Jiru. Stud. Surf. Sci. Catal. 37 (1988) 199. 7 H.K.Beyer. H.K.Karge and C. Borbely, Zeolites 8 (1988) 79. 8 H.K.Karge. H.K.Beyer and C. Borbely. Catal. Today, 3 (1988) 41. 9 US patent 3 702886 (1972). 10 G.I. Kapustin. T.R. Brueva. A.L. Klyachko. S. Beran and B . Wichterlova. Appl. Catal.. 42 (1988) 239. 11 L . M . Kustov, V.B. Knzansky, S. Beran, L. Kubelkova and P. Jiru. J. Phys. Chem. 91 (1987) 5247.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FORMATION OF CARBOCATIONS FROM C,
COMPOUNDS I N ZEOLITES
Imre K i r i c s i ' , H o r s t F o r s t e r 2 and Gyula T a s i ' ' A p p l i e d Chemistry Department, J o z s e f A t t i l a U n i v e r s i t y , R e r r i c h B. t e r 1, H-6720 Szeged, Hungary ' I n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y o f Hamburg, Bundesstr. 45, 0-2000 Hamburg 13, Federal R e p u b l i c o f Germany ABSTRACT Formation o f u n s a t u r a t e d carbenium i o n s f r o m 1-hexene, cyclohexane, c y c l o hexene, cyclohexadiene and benzene upon i n t e r a c t i o n w i t h t h e H-forms o f z e o l i t e s ZSM-5 and Y was p r o v e d by U V - V I S and I R spectroscopy. W i t h t h e e x c e p t i o n o f benzene, which forms d i e n y l i o n s a t t h e b e g i n n i n g , c a r b o c a t i o n f o r m a t i o n from t h e o t h e r C, compounds s t a r t s w i t h monoenylic species, which t r a n s f o r m i n t o o l i g o e n y l i c i o n s w i t h t i m e o f c o n t a c t . From the c y c l i c hydrocarbons c y c l i c i o n s as w e l l as open-chain a l k e n y l i o n s a r e formed. The i o n f o r m a t i o n c a p a b i l i t y decreases i n t h e sequence c y c l o h e x a d i e n e > c y c l o h e x ene > 1-hexene > cyclohexane > benzene. F o r t h e f i r s t stages o f carbenium i o n development t h e f o r m a t i o n o f a r o m a t i c s u r f a c e species may be excluded.
INTRODUCTION Formation o f a r o m a t i c s i n t h e MTG process i s supposed t o proceed v i a and
alkenyl
carbenium i o n s formed f r o m o l e f i n s as p r i m a r y
t h i s process ( r e f .
alkyl
intermediates
of
1 ) . R e c e n t l y some papers have been p u b l i s h e d d e a l i n g w i t h
g e n e r a t i o n and t r a n s f o r m a t i o n o f a l k y l carbenium i o n s f r o m ethene and
propene
i n zeolites (ref.
olefins
in
zeolites
2 ) . Formation o f a l k e n y l carbenium i o n s f r o m l o w e r
mordenite,
f a u j a s i t e and ZSM-5 has been p r o v e n a l s o
by
UV-VIS
spectroscopy ( r e f . 3 ) . The o b j e c t i v e o f t h i s paper was t o i n v e s t i g a t e i n d e t a i l t h e i d e n t i f i c a t i o n of
unsaturated
zeolites.
s u r f a c e i n t e r m e d i a t e s d e r i v e d f r o m some
cyclohexadiene
+
benzene,
assumed t o be one o f t h e
a r o m a t i c s i n t h e MTG process ( r e f . carbenium
C,
hydrocarbons
S p e c i a l a t t e n t i o n was d i r e c t e d t o t h e t r a n s f o r m a t i o n cyclohexene
ions
4),
pathways
for
in +
yielding
although the formation o f unsaturated
f r o m these c y c l i c compounds has n o t y e t
been
experimentally
proved. EXPERIMENTAL Starting
m a t e r i a l s were z e o l i t e NaY f r o m Union Carbide and z e o l i t e
synthesized i n the laboratory o f Prof.
ZSM-5,
L e c h e r t . D e t a i l s o f sample p r e p a r a t i o n
356 and f u r t h e r t r e a t m e n t a r e g i v e n by K i r i c s i and F o r s t e r ( s e e r e f . cell
compositions
determined
by
neutron
activation
5 ) . The u n i t
analysis
and
atomic
a b s o r p t i o n spectroscopy were HNaY: H, ,Na, ,A1 HNaZSM-5 : T r a n s f o r m a t i o n o f C, 1-Hexene, chased
,Si
.O Z N a O .
,03 0 6 A 1 1 . O ESi
9 4 . go, 9 2 .
hydrocarbons was more t h o r o u g h l y s t u d i e d w i t h t h e l a t t e r .
cyclohexane,
f r o m Merck,
cyclohexene,
Darmstadt,
c y c l o h e x a d i e n e - 1 . 4 and benzene, p u r -
i n a q u a l i t y b e t t e r t h a n 99%, were
used
as
adsorbates. For the spectroscopic i n v e s t i g a t i o n s s e l f - s u p p o r t i n g wafers o f 5-7
mg/cm2
pressed and outgassed a t 770 K o v e r n i g h t
vacuum
thickness
were
under
high
c o n d i t i o n s i n t h e o p t i c a l c e l l , made f r o m f u s e d q u a r t z f o r t h e U V - V I S and f r o m g l a s s w i t h S i l v a c a - g l u e d KBr windows f o r t h e I R e x p e r i m e n t s . The
UV-VIS
spectra
were r u n i n t r a n s m i s s i o n on a
c o n t r o l l e d by a B a s i s 108 computer. background
correction.
deconvolution
of
the
Cary
17
spectrometer
The d i g i t i z e d s p e c t r a were smoothed a f t e r
A n o n l i n e a r l e a s t squares p r o c e d u r e was used f o r e l e c t r o n i c spectra,
fitting
Gaussian
peaks
the
to
the
measured d a t a . I R s p e c t r a were r e c o r d e d on a P e r k i n - E l m e r 225 s p e c t r o m e t e r . The a c i d i t y o f t h e samples was determined a p p l y i n g p y r i d i n e as a p r o b e . The r a t i o o f B r o n s t e d t o Lewis a c i d s i t e s Bpy/Lpy was f o u n d t o be 0.19 and 2 . 2 1 i n case o f HNaY and HNaZSM-5, r e s p e c t i v e l y . RESULTS U n s a t u r a t e d carbenium i o n s have been formed f r o m a l l C 6 compounds gated.
Similarities
investi-
as w e l l as d i f f e r e n c e s were f o u n d i n t h e s p e c t r a o f
v a r i o u s hydrocarbons a f t e r a d s o r p t i o n .
the
While a f t e r cyclohexane admission o n l y
a weak band near 310 nm appeared a t room temperature,
two bands o f v e r y
low
i n t e n s i t i e s a t 373 and 495 nm were f o u n d a f t e r a d s o r p t i o n o f benzene. Generation and t r a n s f o r m a t i o n o f a l k e n y l carbenium i o n s f r o m 1-hexene v e r y s i m i l a r t o those o b t a i n e d f r o m propene i n b o t h z e o l i t e s HNaY and 5.
After
developed, ions,
a d s o r p t i o n and r e a c t i o n o f 1-hexene bands a t 325, which
can be assigned t o t h e mono-,
respectively.
Details
HNaZSM-
380 and
d i - and t r i e n y l i c
o f 1-hexene a d s o r p t i o n and c o n v e r s i o n
comparison o f these U V - V I S r e s u l t s w i t h those o f Maixner e t a l . ,
were
450
nm
carbenium and
the
obtained
by
NMR spectroscopy a r e g i v e n elsewhere ( r e f . 5 ) . Cyclohexene adsorbed i n HNaZSM-5 gave r i s e t o a b s o r p t i o n s a t 298, and 391 nm,
as can be seen i n F i g .
1.
310, 340
W i t h c o n t a c t t i m e t h e 310 and 340
nm
bands i n c r e a s e d and two new a b s o r p t i o n s near 420 and 530 nm became d e t e c t a b l e . Upon e v a c u a t i o n a t 370 K t h e i n t e n s i t y o f t h e 310 nm band was enhanced and additional
band a t 380 nm developed and became d o m i n a t i n g i n t h e whole
an
spec-
357
aI U
aJ U c
n m
n m
n
0
v)
4
m 0
n
4
3 300
Fig.
400
X/nm
Transmission
1.
500
300
500
X/nm
electronic
s p e c t r a o f z e o l i t e HNaZSM-5 loaded w i t h 666 P a cyclohexene. ( a ) A t room temperature, immediately a f t e r admission, ( b ) 1. ( c ) 2, ( d ) 3 h l a t e r . A f t e r evacuation f o r 1 h a t
room temperature, ( f ) 370 K, (9) 470 K and (h) 570 K.
(e)
trum.
400
Fig.
U V - V I S spectra o f z e o l i t e exposed t o 133 Pa cyclohexadiene. ( a ) A t room temperature, s h o r t l y a f t e r admission, ( b ) 30 min, ( c ) 1 h l a t e r . A f t e r evacuat i o n f o r 1 h a t ( d ) room temperature, ( e ) 370 K, ( f ! 470 K and ( 9 ) 570 K.
2.
HNaZSM-5
A f t e r vacuum t r e a t m e n t a t 470 K a b s o r p t i o n s a t 270 ( s h o u l d e r ) , 295 ( t h e
most i n t e n s e band),
380, 420 and 530 nm were observed, w h i l e a t 570 K a
340,
broad a b s o r p t i o n o f o v e r l a p p i n g bands remained. a t 320
S h o r t l y a f t e r admission o f 133 Pa cyclohexadiene v e r y i n t e n s e bands
410, 460, 520 and 570 nm c o u l d be d i s t i n g u i s h e d even by t h e naked
(shoulder), eye (see F i g . at
2);
t h e i n t e n s i t y o f each i n c r e a s e d w i t h t i m e . Upon e v a c u a t i o n
room temperature t h e i n t e n s i t y o f t h e l o w frequency bands
spectrum
d),
while
at
370
K a new band arose a t 590
nm.
decreased
At
a b s o r p t i o n s a t l o n g e r wavelengths almost c o m p l e t e l y disappeared. after
(see
K
470
the
The spectrum
e v a c u a t i o n a t 570 K was s i m i l a r t o those o f cyclohexene under t h e
same
c o n d i t i o n s (compare s p e c t r a h and g i n F i g . 1 and 2 ) . I t was
a general observation t h a t w i t h exception o f
benzene
carbocation
f o r m a t i o n s t a r t s w i t h t h e monoenylic species f o l l o w e d by t h e o l i g o e n y l i c i o n s . In
t h e case o f benzene o n l y t h e development o f t h e d i - and t r i e n y l i c i o n s
is
observed.
As from
i n f o r m a t i o n about t h e s t r u c t u r e o f t h e s u r f a c e s p e c i e s can be the
i n f r a r e d region,
supplementary I R s p e c t r a o f t h e two
most
obtained impor-
t a n t compounds cyclohexene and cyclohexadiene were r u n upon a d s o r p t i o n i n
the
358 zeolites. because
The
IR
s t u d y o f benzene a d s o r p t i o n i n t h e z e o l i t e s
t h i s has a l r e a d y
was
been conducted i n d e t a i l b y Karge and
omitted,
Datka
(ref.
6). The 3.
s p e c t r a o f cyclohexene adsorbed i n z e o l i t e HNaZSM-5 a r e shown i n
The s p e c t r a l changes a r e s i m i l a r t o t h o s e observed by Haber e t
al.
Fig. (ref.
7 ) . Bands a t 1653 and 3025 CN’ a r e c h a r a c t e r i s t i c o f t h e C=C i n t e r n a l c y c l i c double bond and =C-H bond s t r e t c h i n g v i b r a t i o n s , b o t h d e c r e a s i n g w i t h t i m e . The
f a s t e v o l u t i o n o f an a b s o r p t i o n near 1510 cm-’ r e f l e c t s t h e
u n s a t u r a t e d carbenium i o n s . stretching
vibration
No band a t 1478 cm-I,
o f benzene was d e t e c t a b l e .
formation
of
c h a r a c t e r i s t i c f o r the r i n g A f t e r evacuation a t
470
K
to
be
almost a l l bands disappeared. I
al U
c
m
Y
CI .r
E
VI
C
a L
I-
1 3100
2700
2900
1700
1600
I 500
1400
Wavenumber/cm-’
F i g . 3 . IR s p e c t r a o f cyclohexene adsorbed i n z e o l i t e HNaZSM-5. ( a ) Z e o l i t e background spectrum. A t beam temperature ( b ) 2 min a f t e r admission o f 666 Pa o f cyclohexene, ( c ) 1 h, ( d ) 3 h l a t e r . A f t e r e v a c u a t i o n a t ( e ) beam temperature, ( f ) 370 K and ( 9 ) 470 K f o r 1 h. Spectra
obtained
from
c y c l o h e x a d i e n e i n z e o l i t e HNaZSM-5
proved
r a t h e r complex (see F i g . 4 ) . The band a t 3032 cm-’ due t o i n t a c t c y c l o h e x a d i e n e decreased
with
t i m e o f c o n t a c t a t beam t e m p e r a t u r e as a
t r a n s f o r m a t i o n o f t h i s compound. assigned
result
of
surface
Bands d e v e l o p i n g a t 1505 and 1535 cm-’ can be
t o u n s a t u r a t e d carbenium i o n s formed on t h e
zeolite
surface.
Here
again t h e band c h a r a c t e r i s t i c o f adsorbed benzene a t 1478 c d i s absent. S i n c e t h i s band i s u s u a l l y v e r y sharp and i n t e n s e and we c a n n o t f i n d any
indication
359
I
31 00
2900
..
2700
1600
1500
1400
Wavenumber/cm-’ F i g . 4 . IR s p e c t r a o f cyclohexadiene adsorbed i n z e o l i t e HNaZSM5. ( a ) Z e o l i t e background spectrum. ( b ) A t beam t e m p e r a t u r e , 3 min a f t e r admission o f 213 P a o f cyclohexadiene, ( c ) 1 h, ( d ) 2 h, ( e ) 4 h l a t e r . ( f ) A t 370 K a f t e r 1 h. ( 9 ) A f t e r 1 h e v a c u a t i o n a t beam temperature. of
it
in
our
spectra,
the
formation
of
benzene
from
cyclohexene
upon admission t o z e o l i t e s a t low temperatures does n o t
cyclohexadiene
and occur
t o any a p p r e c i a b l e e x t e n t .
DISCUSSION Electronic unresolved
spectra
usually
extremely d i f f i c u l t . t i o n program (see F i g .
as broad
absorptions
rendering t h e i r
due
complete
to
their
analysis
Consequently, a d j a c e n t e l e c t r o n i c t r a n s i t i o n s merge i n t o
broad o v e r l a p p i n g bands, analysis
appear
rovibrational f i n e structure,
which can be deconvoluted a p p l y i n g a d a t a
5),
d e t a i l s o f which and a p p l i c a t i o n t o
o f U V - V I S s p e c t r a w i l l be p u b l i s h e d elsewhere ( r e f .
manipula-
quantitative
8).
Using
the
bands r e s o l v e d by t h i s procedure q u a n t i t a t i v e c o n c l u s i o n s c o n c e r n i n g c a r b o c a t i o n f o r m a t i o n can be drawn. UV-VIS
bands
of
hydrocarbons adsorbed on s o l i d
surfaces
are
generally
360
compared
t o t h e s p e c t r a o f u n s a t u r a t e d carbenium i o n s observed
in
superacid
s o l u t i o n s . T h i s i s t h e most advantageous way f o r a t t r i b u t i n g bands t o d i s t i n c t i o n i c species, as c a r b o c a t i o n f o r m a t i o n has been v e r y e x t e n s i v e l y s t u d i e d superacids ( r e f . 9 ) .
in
a
10.2 Absorbance u n i t s
C
400
300
500
X/nm
I n some cases, rate
of
and
environments not
e.g. f o r t h e appearance o f c e r t a i n carbenium i o n s and t h e i r
formation,
solution only
the
F i g . 5. Deconvoluted s p e c t r a o f ( a ) benzene, ( b ) c y c l o h e x e n e and ( c ) c y c l o h e x a d i e n e adsorbed i n z e o l i t e s . The s p e c t r a were r e c o r d e d a f t e r e x p o s i n g t h e zeol i t e s t o t h e adsorbates f o r 1 h a t room t e m p e r a t u r e .
the
good
correlations
z e o l i t e surface could
or be
similarities
between
ascertained,
even
o f t h e c a r b o c a t i o n s i n b o t h systems a r e d i f f e r e n t and r a t e o f formation b u t also the s t a b i l i t y
of
the
superacid though
the
influence unsaturated
species ( r e f . l o ) . In
other
cases,
l a r g e r d i f f e r e n c e s i n t h e band
maxima
of
carbocations
formed i n s u p e r a c i d s and i n z e o l i t e s were found. T h i s i s demonstrated by Table
1,
in
which
the
wavelengths o f maximum absorbance o f
the
carbenium
i d e n t i f i e d i n a c i d i c s o l u t i o n s a r e compared t o t h o s e observed upon o f hydrocarbons i n z e o l it e s
ions
adsorption
.
I n t h e case o f benzene t h e two bands observed a t 373 and 495 nm do n o t agree w i t h t h e f i n d i n g s o f p r e v i o u s a u t h o r s ( r e f . 1 2 ) , w h i c h means t h a t f u r t h e r i n v e s t i g a t i o n s are required.
The absence o f monoenylic species, p r o v e d by us,
361
is
quite
understandable s i n c e p r o t o n a t i o n o f t h e a r o m a t i c
y i e l d s d i e n y l i c s p e c i e s and,furthermore,
ring
immediately
r i n g - o p e n i n g seems t o be u n f a v o u r a b l e
a t low temperatures. Comparing t h e s p e c t r a o f benzene,
TABLE 1 A b s o r p t i o n maxima superacids.
cyclohexene and c y c l o h e x a d i e n e ( s e e F i g .
o f c a r b o c a t i o n s formed
Carbocations i n s u p e r a c i d s *
CH(
zeolites,
compared
Hydrocarbons adsorbed i n z e o l i t e s
to Ref
maxi''
max'nm CH,\
in
+
C-CH-CH, - - - - - - - CH,
0
305
315
CH, =CH-CH, -CH, -CH, -CH,
325 380 450 298 310 340 391
0
284 324 414 464 517 5 74
0
440, 550 373, 495
325
0
0
330
0 cb
**
**
**
12
**
310
** -
470
13
~
315
450 530 325 357 377 392 415
408
From r e f . 11;
**
12
-14
T h i s work
5 ) i t becomes e v i d e n t t h a t those o f t h e . l a t t e r two resemble each o t h e r . I t i s a l s o obvious t h a t t h e c a p a b i l i t y o f cyclohexadiene f o r i o n f o r m a t i o n exceeds that
o f cyclohexene.
B u t i n t h e i r s p e c t r a t h e r e i s no e v i d e n c e f o r bands
373 and 495 nm c h a r a c t e r i s t i c f o r adsorbed benzene. a
correct
and
unambiguous
assignment o f t h e
at
I t must be mentioned t h a t
UV-VIS
bands
obtained
upon
362
a d s o r p t i o n o f c y c l i c hydrocarbons has n o t y e t been p u b l i s h e d . As
far
as
the
f o r m a t i o n o f benzene
is
concerned,
Gibbs
i n t h e sequence o f t h e r e a c t i o n m e t h y l c y c l o p e n t a n e
formation cyclohexene
+
cyclohexadiene
+
energies cyclohexane
benzene were c a l c u l a t e d f o r d i f f e r e n t
+
of +
tempera-
t u r e s u s i n g t h e thermodynamic d a t a g i v e n i n r e f . 15 (see Table 2 ) . Herefrom i t becomes e v i d e n t t h a t f o r m a t i o n o f benzene and c y c l o h e x a d i e n e i s u n f a v o u r e d low
temperatures.
according
This
is
in
good
agreement
with
experimental
t o which t h e f o r m a t i o n o f a r o m a t i c s f r o m l o w e r o l e f i n s
on
at
results zeolite
HZSM-5 proceeds a t temperatures above 570 K ( r e f . 4 ) . From o u r s p e c t r o s c o p i c r e s u l t s t h e same c o n c l u s i o n s can be drawn. TABLE 2 Gibbs f r e e energy o f f o r m a t i o n o f m e t h y l c y c l o pentane, cyclohexane, cyclohexene, c y c l o h e x a diene, benzene and 1-hexene i n t h e t e m p e r a t u r e range 298-673 K (see r e f . 15).
I
T'K
As
AGf/kJ mol-'
-
MCP
CHAN
CHEN
CHOEN BENZ
298 323 373 473 573 673
35.6 36.6 72.4 123.8 176.8 231.1
31.6 44.7 71.6 127.2 184.4 242.8
106.7 116.3 135.7 176.1 217.8 260.3
186.5 193.0 206.4 234.6 264.0 294.2
1-HEXENE
129.6 133.6 141.8 159.3 177.6 196.5
87.4 98.3 120.6 166.8 214.5 263.2
t h e z e o l i t e s used i n t h i s s t u d y c o n t a i n e d b o t h B r o n s t e d und Lewis
acid
s i t e s , t h e f o r m a t i o n o f t h e f o l l o w i n g s u r f a c e s p e c i e s may be assumed:
b 0 0 0
uv-vis active
*
* ) means a b s o r p t i o n a t h>200 nm This
v e r y s i m p l i f i e d r e a c t i o n scheme shows t h a t f o r m a t i o n
carbenium i o n s can be e x p e c t e d f r o m each compound,
of
unsaturated
i n c l u d i n g cyclohexane,
in
363 t h i s case by secondary t r a n s f o r m a t i o n v i a t h e c y c l o h e x y l carbonium may
be
the
reason
f o r the observation
that
cyclohexane
is
ion.
only
This slowly
UV-VIS a c t i v e s u r f a c e compounds.
converted i n t o
Interconversion o f
carbenium
ions
can
also
take
place
in
zeolites.
T h e r e f o r e t h e bands observed w i t h cyclohexene and cyclohexadiene can be a t t r i buted
to
different
intermediates
formed
upon
transformation
of
primary
generated i o n s . CONLCUSIONS From o u r i n v e s t i g a t i o n s t h e f o l l o w i n g c o n c l u s i o n s must be drawn:
-
From a l l C,
compounds under i n v e s t i g a t i o n u n s a t u r a t e d carbenium i o n s
could
be generated.
-
The c a r b o c a t i o n f o r m a t i o n s t a r t s w i t h t h e monoenyl species, i n t h e case o f benzene w i t h t h e d i e n y l i c carbenium i o n .
- Carbocations cyclohexyl depending sites.
formed and
on
from
cyclohexene and
cyclohexenyl
or
cyclohexadiene
cyclohexenyl
and
may
either
cyclohexadienyl
whether i n t e r a c t i o n t a k e s p l a c e w i t h B r o n s t e d o r
These
be
ions
Lewis
species undergo r i n g - o p e n i n g t o open-chain a l k e n y l
acid
carbenium
ions .
-
Cyclohexane
transforms
in
a b s o r b i n g i n t h e FUV range,
the f i r s t
step
into
saturated
carbocations,
y i e l d i n g e n y l i c carbenium i o n s o n l y a f t e r r i n g -
opening .
-
From
the
formation
spectroscopic capability
sequence cyclohexadiene
-
At
the
i n v e s t i g a t i o n s i t can be
of
>
inferred
t h e hydrocarbons i n v e s t i g a t e d cyclohexene
>
1-hexene
>
cyclohexane
f i r s t stages o f c a r b o c a t i o n development f r o m
C,
that
decreases
>
the
ion
in
the
benzene.
hydrocarbons
the
f o r m a t i o n o f a r o m a t i c s u r f a c e species may be excluded. ACKNOWLEDGEMENT One o f t h e a u t h o r s ( I . K . )
i s g r a t e f u l t o t h e Alexander von Humboldt Founda-
t i o n f o r a r e s e a r c h f e l l o w s h i p . We thank D r . J . t e n Pas f o r t h e s y n t h e s i s o f z e o l i t e ZSM-5. REFERENCES
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2
H.G. Karge,J. Weitkamp (Editors),Zeolites 0s Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
AN
INFRARED AND CATALYTIC STUDY OF ISOMORPHOUS SUBSTITUTION IN PENTASIL
ZEOLITES
M.F.M. Post, T. Huizinga, C.A. Emeis, J.M. Nanne and W.H.J. Stork Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Badhuisweg 3 , 1031 CM Amsterdam (The Netherlands)
ABSTRACT The catalytic sites associated with aluminium-, iron- and gallium-containing silicates of MFI structure have been characterized by infrared (IR) spectroscopy and in catalytic experiments in which n-hexane conversion served as a test reaction. The protonic forms of the metal silicates exhibit characteristic absorptions in the u(0H) region which are assigned to M(0H)Si bridging hydroxyls: Fe, Ga, A1 in order of decreasing frequency. Rate constants for n-hexane conversion were found to increase linearly with the concentration of a particular trivalent metal ion in the silicate material, and this observation, together with the IR studies, provides direct evidence that Al, Ga and Fe can be incorporated in the MFI framework, giving rise to Brplnsted acidity. The catalytic activity expressed per M(0H)Si site decreases in the order Al, Ga, Fe, which is in agreement with the direction of change in IR frequency of the absorption associated'withthe bridging hydroxyl group. A simple but straightforward model is offered to account for the variation in acidity observed. INTRODUCTION Zeolites play an important role in catalysis today, in particular in oil refining. Use is made of their high acidity which, for instance, renders them hundreds of degrees more active than amorphous mixed oxides (ref. 1). Particularly successful has been the use of zeolite Y in catalytic cracking and also the use of mordenite in paraffin isomerization (Shell Hysomer Process, ref. 2). In the late '60s and '70s attention shifted towards the high-silica zeolites. This was triggered partly by the possibilities offered by steam dealumination ("ultrastabilization";for a recent review see ref. 3 ) , but even more by the increasing number of high-silica zeolites that could be synthesized direct using organic templates. Here, of course, the synthesis and application of ZSM-5 (ref. 4 ) was a landmark. From further work it emerged that the all-silica analogues of many zeolites could be synthesized, silicalite being a prime example. This situation allows one to consider zeolites from the point of view of the solid-state chemist and to consider the aluminium sites for activity
-
-
though responsible
as lattice "defects" that can be introduced up to a certain
concentration (isomorphous substitution). In this concept the possibility of
366 introducing a different type of lattice "defect" becomes of interest. Of course one then primarily considers in the first instance other trivalent cations that fit well into a tetrahedral oxygen environment, such as Fe, Ga, and possibly also B. In principle, however, this reasoning can be applied to any lattice "defect",including trivalent ions that prefer octahedral coordination (Cr3+, ref. 5), bivalent ions (Be2+, ref. 6) and tetravalent ions (Ge4+, Ti4+, ref.
7), some of which have been known for some time (ref. 8 ) . Obviously, one - or the highest attainable concentration - for those ions that fit best. High-silica zeolites are generally synthesized hydrothermally from alkaline mixtures, from which ultimately the zeolite is recovered as the solid phase. Since many of the "hetero atoms" such as Fe and Ga will form precipitates of oxides and hydroxides under such circumstances, the first question to be addressed is whether such elements have indeed been incorporated as "defects" in the "zeolite" lattice, or whether they are present as a separate impurity. In the last five years much attention has been devoted to this question, the zeolite phase being studied by a wide variety of techniques, such as X-ray diffraction (ref. 6 ) , infrared (IR) spectroscopy (ref. 9), ion-exchange capacity (ref. 10) , temperature-programmed ammonia desorption ( e .g. ref. 9), and 57Fe Mossbauer spectroscopy (refs. 11 and 12). In many cases it has indeed been concluded that, up to a certain concentration, incorporation into the zeolite lattice had occurred. The next question is the characterization of this "hetero atom" site from a catalytic point of view, since the catalytic activity of the system is generally connected with it, through the charge-compensating bridging hydroxyl. The nature of this hydroxyl group (Si-OH-X, X Al, Fe, Ga, B, . . . ) will depend on X, as has been reported already (ref. 9 ) . In this study we aim to present results from both IR and catalytic studies on the Al, Fe and Ga MFI silicates to arrive at a consistent description. n-Hexane conversion was chosen as a test reaction.
expects the best stability
-
EXPERIMENTAL PART Zeolite syntheses MFI metal silicates with Fe, Ga and A1 were synthesized hydrothermally with tetrapropylammonium hydroxide or n-butylamine as the organic component in the zeolite synthesis mixture. Detailed experimental procedures have been outlined elsewhere (ref. 10): in general, the syntheses were carried out at 423 K for 24 h, with stirring, using teflon-lined autoclaves. In the case of iron and gallium silicates, care was taken to carefully monitor A1 contamination in the product, and also in some experiments to reduce it, e.g. by acid leaching of
367
the amorphous silica base material, or by starting from ultra-pure silica sources. The materials were converted into the hydrogen forms by standard methods: ion exchange with NH4N03 solution (three steps, 10 ml 1 M solution per gram silicate under reflux conditions for 1 h with intermediate washing), followed by drying and calcining at 773 K. All samples were white or almost white (high iron concentration). Details of the composition of the crystalline metal silicates thus prepared are given in Table 1.
TABLE 1 Preparation and composition of the crystalline metal ailicataa studied A. Preparation Starting materials: Amorphous ailica:
-
D = Davinon (jrade 952 Daviaon grade 952 leachod with HCl H = ex Hallinckrodt "3 = obtained by hydrolysis of tetramethoxyailme
LO
NaAlO Fe(N0') Ga (NO31' NaOH Aqueous tetrapropylammonium hydroxide solution (T) n-Butylamine ( 8 )
Molar composition of synthesin mixture: 25 SiO ; p A1 0 ; q Fez03; r GaZ03; n NaZO; t organica; 450 H20 2
2 3
Synthesis conditions: In general: 24 h at 423 K. stirred autoclave Silicates 86, B10: 113 h at 423 K. stirred autoclave Activation procedure: Calcination at 773 K Anmonium exchange at 373 K with 1 M aqueous NE4N03 solution Washing. drying Calcination at 773 K (1 h)
B. Chemical composition of activated (H') Abbreviations defined in Part A
A3 A4 A5 A6
D D D D H LD
A1 A2
0
0.012 0.039 0.063
n
O
0
0.2
n n 0.063
n o
o n
B1
M
O
B2
D
O
n.w
~3
TMS
n
0.063
B4 85 86 87
M M H
O O O
H
O
BB B9
D
O
LD ID
n
Bin
c1 c2
D D
O O
D
LD
n
LO
0
n n
c3 c4
n n
O
o
0
n 0
0
catalysts
3 3 1 1 1 1
1 1 3
n.nm
n
0,125 0.20 0.20 n.167 0.03 0.20
0
1
n n o n n
2.5
1 3
0.063 n.125 0.2 0.33
1 i 1 1
n
3
2.3 2.5
9
T
Q T Q T 9 9
T T
3 T
Q T 9 T 9 T Q T Q T in B 9 T 9 9
T T
in B
T T T o T
8 9 9
0.039 n.129 0.165 0.315 o.on9 0.680
0.004 0.027 0.015 n.nn9 0.007
n.nm 0.024 0.048
n.nn8 n.ni4 n.ns8 0.047
n.nw n.nn8
0.43
1.44 1.84 3.52 0.10 7.7 0.72 0.88 1.00 1.02 1.24 1.72 1.8~
3.91 4.79 5.46
5.57 6.79 9.48
2.60 0.28
10.0 14.5 1.51
1.77
9.8
1.02 1.32 1.60 3.23
4.46
5.79 7.04 14.6
368
Infrared sDectroscoDp Thin self-supporting wafers of the hydrogen forms of various samples were heated for 0 . 5 h at 723 K in vacuo. Infrared spectra were recorded at room temperature (293 K) with a Digilab FTS-15 Fourier transform spectrometer. Catalytic activitv testine Drocedures Rates of conversion of n-hexane were measured in a stainless-steel downflow microreactor containing 2 g catalyst, sized to a fraction having an average diameter of 0 . 4 nun and diluted with silicon carbide particles (diameter 0.2 mm) in a volume ratio of 1/2. To eliminate the possibility of thermal cracking contributing to the overall conversion of n-hexane, we carried out the tests at 723 K (rather than 7 7 3 K, at which the a-test is normally performed; ref. 1)
and at a total pressure of 0 . 5 MPa (helium/n-hexane
- 4/1 mol/mol).
The n-hexane was introduced with a high-precision displacement pump. Helium was mixed with the hydrocarbon stream in the preheater. The conversion was determined by gas chromatographic analysis of the reactor effluent. Conversion of n-hexane (x) was found to obey first-order kinetics; hence the reaction rate constant (k) can be obtained with the aid of the familiar expression for an integral tube reactor: kr
- -ln(l-x),
where
r
denotes the
space time of the reactant, which is a function of its initial concentration, molecular weight, zeolite density and weight hourly space velocity (WHSV). Since in the present study the performance of the various catalysts is compared at equal space velocity (WHSV
- 1 kg/(kg.h)),
it was considered appropriate to
express catalyst activities by the quantity -ln(l-x). RESULTS Catalytic activitv testing For investigation of the effect of isomorphous replacement in crystalline metal silicates of MFI structure on their catalytic activity, many reactions may be considered. Studies reported in the literature include n-decane hydroconversion (ref. 1 3 ) , butane cracking (ref. ll), isomerization of alkyl aromatics (refs. 14 and 15), alkylation of toluene with methanol (ref. 16), conversion of methanol to hydrocarbons (refs. 17 and 18) and catalytic dewaxing (ref. 14). In this work we have confined ourselves to a study of the BrZnsted acidity in the catalytic conversion of n-hexane. This particular hydrocarbon was selected since it is the standard probe molecule in the a-test (ref. 1). Moreover, rates of n-hexane conversion are not disguised by diffusion limitations (refs. 20 and 21) inside coarse-crystalline materials, properly reflecting intrinsic acidity in a series of ZSM-5 materials with different A1 contents (refs. 21 and 2 2 ) .
369 The activities of crystalline aluminosilicates with the MFI structure in the conversion of n-hexane as measured in this study are given in the upper part of Table 2 . As expected, catalyst activity, expressed as -ln(l-x),
increases
linearly with increasing A1 content, which was varied in the range 0.039 0.315 %w (see Fig. l), and these results are fully in line with previous reports (refs. 21 and 22). Apparently, a ZSM-5 material without any aluminium would be completely inactive in the conversion of n-hexane under the reaction conditions chosen. TABLE 2 Conversion of n-hexane over various crystalline metal silicates with the MFI structure Reaction conditions: Feed
T
: n-C /He, 114 mollmol : 723%
p(overal1): 0 . 5 MPa WHSV : 1 kg n-C,l(kg Silicate no.
Al. xw
Fe, xw
-ln(l-x)
Ga, xw overall"
A1 A2 A3 A4
81 82 83 84 85 86
0.018
87 88
0.024
c1
0.058
c2
0.047
0.048
Arb
FeC
0.15 0.20 0.20 0.21 0.25 0.35 0.51
GaC
0.35 1.60 1.79
0.039 0.129 0,165 0.315
0.004 0.027 0.015 0,009 0.007
cat.h)
3.50
0.72
0.1s
0.88
0.50
1.00 1.02 1.24 1.72 1.82 2.60
0.37 0.31 0.33 0.56 0.78 1.02
0.04 0.30 0.17 0.10 0.08 0.20 0.27 0.54
1.70 2.15
0.65 0.53
1.02 1.32
0.48 1.05 1.62
'Overall catalytic activity taken at run hour 1 (expressed as -ln(l-convsrsion); explained in text). The rates of deactivation bware of the order of 2-3 X per h. or less. Contribution of contaminating A 1 to catalyst activity. 'Resulting contribution of Fe or Ga to catalyst activity.
The results obtained with a series of crystalline iron silicates having the MFI structure (samples B1
-
B8) are given in the middle part of Table 2 . Owing
to the different silica sources used in the synthesis, the level of aluminium contamination in the samples varies significantly (from 0.048 down to 0.004 %w). As minor amounts of A1 may contribute to the overall activity of the iron silicate, we first quantified the effect of contaminating aluminium by comparing the activities of samples having similar iron contents but very different levels of A1 contamination (samples B1
-
84). The results, plotted in Fig. 2 ,
show that a highly pure iron silicate material has significant activity, while incorporation of aluminium results in a further gain in activity. The experimental values lie on a straight line which runs parallel to the relation between activity and A1 content for zeolite ZSM-5. Apparently the difference in activity between an Al-contaminated iron silicate and a ZSM-5 material having
370 the same A1 content is independent of aluminium content and reflects the contribution of the iron to overall activity. In other words, the presence of residual A1 in iron silicates does not lead to synergistic effects in the conversion of n-hexane: the contributions of the iron and aluminium in the MFI structure are additive. Hence, the net contribution of the iron to catalytic activity (Table 2 ) was obtained by linearly correcting the overall activities for the contribution of residual A1 by interpolation of the data from Fig. 1. The results, shown in Fig. 3 , indicate the existence of a linear relation between the extent of isomorphous substitution of iron (at least in the range 0.7 - 2 . 6 %w Fe) and catalytic activity. It is also worth while mentioning that
this holds for all the iron silicates tested, which were prepared via different routes. Figure 4 compares the first-order rate constants for n-hexane conversion over materials with MFI structure as a function of concentration and type of trivalent metal ion in the silicate materials. In the figure we have also included data on crystalline gallium silicates (lower part of Table 2 ) . The activities of gallium and iron are net values, i.e., corrected for the contribution of contaminating aluminium.
-tn ( I - x )
- tn(1-x) 0.6r
0
0.1
0.2 0.3 0.4 A t content of ZSM-5, %w
Fig. 1. Conversion of n-hexane over ZSM-5: effect of A 1 content on rate constant. For reaction conditions see Table 2 .
0
0.01
0.02
003
004
0.05
AL content, % w
Fig. 2 . Effect of aluminium contamination on the activity of crystalline iron silicates (MFI structure, -1 %w Fe) in the conversion of n-hexane. For reaction conditions see Table 2 .
371
- Ln(l-x)F, 0.51
.
0
0
I
I
1
I
I
0.5
1
1.5
2
2.5
J 3 Fe,%w
Fig. 3 . Effect of iron content on the activity of crystalline iron silicates (MFI structure) in the conversion of n-hexane. Rate constant is corrected for the contribution of contaminating aluminium to activity. For reaction conditions see Table 2 .
- tn (1-
x ) ~
M=AL
M = Go
M: Fe
0
5
I0 15 M,03/Si02, mmoL/mol
Fig. 4 . Conversion of n-hexane over crystalline metal silicates. Effect of trivalent metal content of the silicate on reaction rate constant. Rate constant of iron and gallium silicates is corrected for the contribution of contaminating aluminium to activity. For reaction conditions see Table 2 .
372 IABLE 3
Absorbonce, log ( l 0 / l )
E f f e c t o f composition o f metal s i l i c a t e s having MFI structur. on tha s t r e t c h i n 5 frequency of H(OR)Si hrid8ing hydroxyl groups
Sample no.
Al,
Fe,
xw
Xw
Ga. Xw
H(OA)Si I4 l"OR], cm
A5 A6
BQ B10 c3
c4
0,009 0.680 0.008
0,014 0.008 0.008
'Terminal
A1
0.28 1.77
Fe Fe 1 . 6 0 Ga 3 . 2 3 Ga
3740' 3610 3830 3630 3615 3615
SiOR hydroxyl groups.
Fig. 5. Infrared spectra of crystalline metal silicates. Bar indicates 0.1 absorbance unit. Sample: a: A 5 ; b: A 6 : c: C4; d: B9.
3800
3700
3600
3500
3400
Wove number, cm-'
Infra-red measurements It has been demonstrated in the past (e.g. ref. 9) that IR spectroscopy is a powerful technique to monitor differences between various crystalline metal silicates. In our work we used it to investigate the effect of isomorphous replacement. The previous studies have shown that shifts in hydroxyl stretching frequencies exist going from ZSM-5 to crystalline gallium, iron or boron silicate. Our results confirm these findings, as indicated in Table 3 and Fig. 5. In this case the aluminium metal silicate has the lowest stretching frequency, followed by the gallium and the iron silicate. DISCUSSION The linear relations observed between the rate constant for n-hexane conver-
sion and the concentration of trivalent metal in the MFI structure provide direct evidence for the fact that the reaction is associated with the trivalent metal ion present in the crystalline metal silicate. This does not necessarily imply that the ions are incorporated into the "zeolite" structure during the synthesis and/or under the conditions prevailing during the catalytic test. It does indicate, though, that the catalytic nature of the sites associated with the trivalent metals in the MFI silicates is independent of the concentration of these sites, at least in the ranges studied in the present work. For it to
be proved unequivocally that trivalent metal is incorporated into the zeolite lattice separate studies are required. A magic-angle-spinning nuclear magnetic resonance (MASNMR) study in which n-hexane cracking activities measured for A1 silicates of MFI structure have been correlated with tetrahedral A 1 signal intensities (ref. 23) is a straightforward example of such a proof for aluminium incorporation. Incorporation of gallium in zeolite structures using Ga MASNMR techniques has been recently demonstrated (ref. 24) and this technique has proved very useful in providing evidence for the incorporation of Ga in MFI structures (ref. 25). Incorporation of iron in the MFI structure to a certain level has been recently demonstrated by means of 57Fe Mossbauer studies combined with electron spin resonance (refs.11 and 1 2 ) and these results are in good agreement with our observations (ref. 2 6 ) . In addition, the IR studies reported here also lend firm support to the idea that A l , Ga and Fe are incorporated in the MFI structure as lattice "defects", giving rise to Br$nsted acidities associated with bridging hydroxyls whose IR frequencies decrease in the order Fe(0H)Si
> Ga(0H)Si > Al(0H)Si.
It is evident from Fig. 4 that there is a marked difference in activity as far as the type of "defect" is concerned. In terms of first-order rate constant the activities of the catalytic sites associated with the Fe, Ga and A 1 ions incorporated are roughly in the proportion 1 : 6 : 25, respectively. It should be mentioned that these results refer to a particular test reaction (i.e., n-hexane conversion) under specific reaction conditions and cannot be generalized in a quantitative way. The activity sequence, i.e., A 1 > Ga > Fe, however, is expected to be encountered generally in Br$nsted acid catalysis. Having established both IR and catalytic measurements it is convenient to plot the relative cracking activity per trivalent metal ion site as a function of the observed hydroxyl stretching frequency, as is done in Fig. 6 . Though the relationship between IR frequency and acid strength in various zeolites and related materials is not unambiguous, there are indications that, in a homologous series of zeolites, the IR frequency shifts systematically to higher wave numbers if the acidic strength decreases (ref. 27). From Fig. 6 we conclude that the bridging hydroxyl groups with the lower wave numbers, associated with the more loosely bonded protons, have the higher catalytic activity per trivalent metal ion. The fact that in this series of IR measurements the zeolite structure is kept constant precludes possible IR shifts due to changes in lattice (refs. 28 and 2 9 ) . A link reported earlier (ref. 9) between IR hydroxyl frequency and NH3 desorption temperature has been established. In ref.
9 the lower IR frequencies correspond to the higher NH3 desorption temperatures, which is in agreement with our findings.
374 R a t e constant, r e l a t i v e units
I I
I \
\
06
\
\ \
04
"1 'y,
0 3610 3620 3630 i-iydroxyl stretching frequency, cm-'
Fig. 6. Relative rate constants for n-hexane conversion over crystalline aluminium, gallium and iron silicates of MFI structure (based on unit concentration of trivalent ion) as a function of hydroxyl stretching frequency.
It is tempting to speculate about the origin of these differences in acidity. In our opinion only quantum-mechanical calculations (such as presently in progress at several locations) will ultimately be able to yield a full understanding of these effects. Meanwhile, though, it is attractive to try and interpret the results with a simple electrostatic model. Thus, it is assumed that all ions have their formal valencies and have their well-known ionic radii (rGa3+ 0.62 , rA13+ 0.51 and rFe3+ 0.64 A; ref. 30). Thus, when we con-
-
-
-
sider the bridging hydroxyl group Si-OH-X, the proton-X distance, at comparable bond angle, will decrease in the order Fe, Ga, Al. In our model this means that the electrostatic repulsion between the proton and X increases in the same way, and that we expect the acidity to increase in the same order. Interestingly, though this model is obviously a gross oversimplification, it does predict the correct acidity ranking. REFERENCES 527-529. 1 P.B. Weisz and J.N. Miale. J.Cata1.. . 4 (1965) . 2 H.W. Kouwenhoven and W. van Zyll Langhout, Chem. Eng. Prog., 67 (1971) 65-70. 3 J. Scherzer, in T.E. Whyte, R.A. Dalla Betta, E.G. Derouane and R.T.K. Baker (Editors), Catalytic Materials: Relationship between Structure and Reactivity, ACS Symposium Series No. 248, 1984, pp. 157-200. 4 US patent 3 702 886 to Mobil. 5 Patent EP 0 021 475 to Shell Internationale Research Maatschappij B.V. 6 J.C. Jansen, E. Biron and H. van Bekkum, in P.J. Grobet, W.J. Mortier, E.F. Vansant, G. Schulz-Ekloff, B. Delmon and J.T. Yates (Editors), Innovation in Zeolite Materials Science: Proc. Int. Symposium, Nieuwpoort, Belgium, 1987, Studies in Surface Science and Catalysis 37, Elsevier, Amsterdam etc., 1988, pp. 133-141. 7 B. Notari, ibid., pp. 413-425. 8 D.W. Breck, Zeolite Molecular Sieves; Structure, Chemistry and Use, Wiley Interscience, New York etc., 1974. 9 C.T.W. Chu and C.D. Chang, J . Phys. Chem., 89 (1985) 1569-1571. 10 Patents GB 1 555 928, GB 2 055 357 and EP 0 030 751, all to Shell Internationale Research Maatschappij B.V.
375
11 R. Szostak, V. Nair and T.L. Thomas, J. Chem. SOC., Faraday Trans. I, 83 (1987) 487-494. 12 G. Calis, P. Frenken, E. de Boer, A. Swolfs and M.A. Hefni, Zeolites, 7 (1987) 319-326. 13 M. Thielen, M. Geelen and P.A. Jacobs, in Proc. Int. Symposium on Zeolite Catalysis, Siofok, Hungary, 1985, pp. 1-18. 14 B.S. Rao, G.P. Babu, V.P. Shiralkar, A.N. Kotasthane and P. Ratnasamy, Actas do 90 Simp6sio Iberoamericano de Catdlise, Volume 2, Lisbon, 1984, pp. 1418-1425. 15 P. Ratnasamy, R.B. Borade, S . Sivasanker and V.P. Shiralkar, Acta Phys. Chem., 31 (1985) 137-146. 16 R.B. Borade, A.B. Halgeri and T.S.R. Prasado Rao, in Y. Murakami, A. Lijima and J.W. Ward (Editors), New Developments in Zeolite Science and Technology, Proc. 7th Int . Zeolite Conference, Tokyo, 1986, Elsevier, Kodansha, Amsterdam, etc., 1986, Studies in Surface Science and Catalysis 28, pp. 851-858. 17 W.J. Ball, J. Dwyer, A.A. Garforth and W.J. Smith, ibid., pp. 137-144. 18 T. Inui, A. Miyamoto, H. Matsuda, H. Nagata, Y. Makino, K. Fukuda and F. Okazumi, ibid., pp. 859-866. 19 S . Sivasanker, K.J. Waghmare, M. Reddy and P. Ratnasamy, Proc. 9th Int. Congress on Catalysis, Calgary, 1988, vol. I, 16-C2, p. 120. 20 V.J. Frilette, W.O. Haag and R.M. Lago, J. Catal., 67 (1981) 218-222. 21 M.F.M. Post, J. van Amstel and H.W. Kouwenhoven, in D. Olson and A. Bisio (Editors), Proc. 6th Int. Zeolite Conference, Reno, Nevada, USA, July 1983, Butterworths, Guildford, Surrey, UK, 1984, pp. 517-527. 22 D.H. Olson, W.O. Haag and R.M. Lago, J. Catal., 61 (1980) 390-396. 23 W.O. Haag, R.M. Lago and P.B. Weisz, Nature, 309 (1984) 589-591. 24 D.E.W. Vaughan, M.T. Melchior and A.J. Jacobson, in G.D. Stucky and F.G. Dwyer (Editors), Intrazeolite Chemistry, ACS Symposium Series No. 218, 1983. . DD. .. 231-242. 25. N.C.M. Alma-Zeestraten et al., to be published. 26. H. Beens et al., to be published. 27. H.G. Karge, J. Ladebeck, 2. Sarbak and K. Hatada, Zeolites, 2 (1982) 94-102. 28. J . Dwyer, in P.J. Grobet, W.J. Mortier, E.F. Vansant, G . Schulz-Ekloff, B. Delmon and J.T. Yates (Editors), Innovation in Zeolite Materials Science: Proc. Int. Symposium, Nieuwpoort, Belgium, September 1987, Studies in Surface Science and Catalysis 37, Elsevier, Amsterdam etc., 1988, pp. 333-354. 29. H.G. Karge and J. Ladebeck, Acta Phys. Chem., 24 (1978) 11. 30. R.C. Weast (Editor), Handbook of Chemistry and Physics, 68th Edition, CRC Press, Boca Raton (Florida), 1987.
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H.G. Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V.. Amsterdam - Printed in The Netherlands
CALORIMETRIC INVESTIGATION OF THE ACIDITY OF DEALUMINATED Y-TYPE ZEOLITES USING VARIOUS BASIC PROBES A. AUROUX, Z . C . SHI, N. ECHOUFI, Y. BEN TAARIT Institut de Recherches sur la Catalyse, C.N.R.S., Conventionne a l'universite Claude Bernard, LYON I, 2 , Avenue A. EINSTEIN, 69626 - Villeurbanne Cedex
-
FRANCE
ABSTRACT Y-type zeolites of various Si/A1 ratios have been obtained by dealumination of NaY zeolites using silicon tetrachloride as the dealuminating agent at various reaction temperatures. All samples were highly crystalline. Microcalorimetric measurements of ammonia, pyrrole, dimethylether and acetonitrile adsorption unveiled various strength distributions among the acid population of these samples. Ammonia proved to be a reliable probe when Bronsted acid sites were investigated. It helped to unveil two site populations whose proportions varied in Si/A1 ratio. These two populations differ by the local environment of the associated A10 tetrahedra. Ammonia, however, failed to reveal the inhomogeneity in one parttcular acid population. Dimethylether, a too weak base, did not appear to be any better than ammonia, whereas pyrrole appeared as a rather acidic probe which helped visualize the basicity difference between the parent material and the dealuminated samples. Acetonitrile proved to be a reliable probe to monitor quantitatively and qualitatively Lewis acidity. INTRODUCTION The huge quantities of zeolites involved in the F.C.C. process account for the
continuous
or
even
growing
interest
manifested
for
the
acidity
investigation of these catalysts. Among the methods which proved to be most suitable and accurate to monitor acidic solids, microcalorimetric methods were recognized as a most powerful tool. Previous studies, however, showed the dependence of the results on the choice of the basic probe and on the experimental conditions of
the calorimetric measurements, especially
the
adsorption temperature and the importance of the base doses with respect to the total number of acid sites of the sample under investigation and their strength distribution. This prompted one of us to use various basic probes to monitor the acid strength distribution within ferrierite samples (1).
In the course of that
investigation other parameters appeared to be crucial with respect to the correct evaluation of the experimental results. In particular, the size of the probe with respect to the channel or cavity openings and the possible existence of cumbersome extraframework species may greatly influence the relevance of the
results to the actual acidity properties. Therefore we selected samples of Y-type zeolites with identical cavity openings which permit the equal diffusion of the selected basic probes.
378
EXPERIMENT AND MATERIALS NaY samples from Union Carbide (LZY 52) were dried overnight at 400°C under flowing oxygen and finally under dry nitrogen. They were subsequently cooled down to the desired temperature to be subjected to the action of SiC14 following the procedure described by Beyer and Belenykaja ( 2 ) . Samples of dealuminated NaY zeolites were thus obtained over a broad range of Si/A1 ratios by varying the dealumination temperature between 250 and 450°C while the Sic1 partial pressure 4 in the nitrogen flow was kept constant by maintaining the SiC14 condenser in a cold trap at 0°C. The flow rate was also kept constant at 1 1.h-'. The average sample weight was within 5 g. The dealumination reaction was carried out for one hour, at which time dry N2 was substituted for the SiC14-N2 flow mixture for an additional hour and the temperature raised to 450°C for yet another hour
so
as
to remove any volatile reaction products from the zeolite. The samples were then progressively cooled down to room temperature and repeatedly washed with deionized water. Ammonium exchange was performed using an aqueous acetate solution. Chemical analysis for aluminium and the residual sodium showed that the exchange level was within 90 X for all samples. The samples were then kept in a dessicator containing a saturated solution of CaCl 2' Analysis. Besides chemical analysis, all samples were analyzed for crystallinity using X.R.D
techniques. The parent NaY (LZY 52) was used as reference. All
samples retained over 98 X crystallinity. The unit cell parameter was determined and used to final the residual framework aluminium content and consequently the Si/Al ratio, assuming the linear relation between the unit cell parameter and the number of aluminium atoms per unit cell ( 3 ) . The
different
Si/A1
ratios
were
also
monitored
using
infrared
spectrophotometry. Anhydrous KBr was thouroughly mixed with the zeolite powder at a content of less than 1 X to allow for better resolution as sharper absorptions were recorded at low zeolite loadings. The frequency shift of the appropriate lattice vibrations was then used to determine the Si/Al ratios and simultaneously to monitor the crystallinity of the zeolite by surveying the intensity of the vibration related to the double 6-ring representative of the hexagonal prisms.
-MAS NMR.
Magic Angle Spinning NMR spectra for "Si
and "A1
were obtained on a
commercial Brucker MSL 300 NMR spectrometer equipped with a MAS probe. The powder samples were placed in cylindrical rotors of the "double bearing" type
379
and typically spun at an average rate of 3400 Hz. Framework Si/A1 ratios were determined on the basis of the relation a
4
using the 29Si NMR spectra ( 4 ) .
27Al NMR was essentially used to monitor the framework or non framework nature of the residual aluminium. Adsorption of pyridine was studied using infrared spectroscopy. Zeolite powder samples were pressed into thin self-supporting disks of 18 nnn in diameter and 8-10 g weight and placed in pyrex or quartz sample holders. These wafers were subjected to heating in flowing O2 up to 350°C or alternatively 760°C and subsequently evacuated at the same temperature for 2hr to a residual pressure of Torr. After cooling they were contacted with purified and thoroughly dessicated pyridine vapour for 2hr. after which excess pyridine was removed by room temperature evacuation. It was subsequently desorbed at 50°C intervals. Following each desorption IR spectra were recorded on a Perkin-Elmer grating spectrophotometer Resolution in the ring-vibration-mode absorption -1
domain was better than 3 cm
.
Microcalorimetry. The zeolite samples were activated by heating in 02 and then in vacuo 100 mg quantities in a specially designed cell in separate activation apparatus and allowed to cool down to room temperature. The cell was then introduced into the microcalorimeter of the Tian Calvet type (from Setaram).
At equilibrium small doses of purified vapour (dried over 4 A molecular sieve and degased by freeze-pump thaw cycles) were allowed into the sample. The evolved heat was recorded continuously together with the equilibrium pressure. RESULTS AND DISCUSSION
Sample Characterization. Dealuminated NaY samples were obtained upon varying the temperature of the reaction of SiC14 over the NaY parent material between 200 and 375°C. All samples showed high crystallinity to X-rays and 27Al NMR confirmed that all
the
residual
aluminium
was
in
tetrahedral
coordination.
Thus
the
dealumination reaction as reported initially by Beyer and Belenkaja (2) apparently proceeds at rather moderate temperatures using SiC14 and anhydrous NaY zeolite. The overall reaction depicted by the following scheme:
380
\
/
( Si
/ \
0
\
/
A1 )
-
Nat t SiC14+NaCl
0 -Ir \ / \ / Si t A1C13 t Si /
/ \
\
/
\
NaAlC14 since nonframework aluminium was detected either before or after washing the samples. seems to undermine the production of
The Si/Al ratios and the residual A1 atoms per unit cell were reported for these various samples in table 1.
TABLE
1.
Structural analysis of zeolites dealumination O C
200 300 350 375 450
Framework A1 per unit cell XRD
IR
NMR
32 24
30 21 17 5 3
37 27 20 7 5
20
2
Si/A1 ratio N M R X R
4.25 6.36 9.49
4.94 7.06 8.32
IR 5.3 8.1 10.6 36.6
37.1
Examination of this table indicates that despite obvious divergence of the three methods used to determine the lattice Si/A1 ratio, the trend exhibited by each pattern of results is self-consistent. All X-ray-derived ratios were consistently lower than those derived via infrared measurements. 29Si NMRderived ratios were fairly close to the former ratios ; however, at low residual aluminium content these are less reliable than those derived via X-rays or infrared measurement in view of the poor accuracy in the determination of the vanishing Si(1 Al) NMR signal. Nature of the acid sites. As indicated by 27Al NMR no extra framework A1 was present following dealumination or activation at 350 to 650°C. t
On the other hand, the IR spectrum of NH4-exchanged dealuminated samples exhibited 9 OH bands at 3650 and 3550 cm-l with slight shoulders at 3600 cm-l whose intensity was barely affected upon varying the activation temperature over the 350-650°C range. In addition, pyridine adsorption as monitored by infrared spectroscopy showed the characteristic pattern of the ring vibrations associated with the presence of pyridinium ions. In particular, the band at 1540 cm- 1 relative to the 19b mode, that at 1630 (8a mode) characteristic of pyridinium ions kept an invariant intensity as the activation temperature of the zeolite sample increased from 350 to 650°C. Only the band at 1450 cm-l and a shoulder at 1620 cm-l could be seen, and these were more pronounced at the highest activation temperatures, probably indicating that Lewis acid sites start to be created beyond activation temperatures of 600°C. The predominant acid sites
381
were, however, unambiguously 'of the Bronsted temperature range.
type over
this
activation
Microcalorimetry measurements. Adsorption of ammonia was conducted as described
in the previous section while the calorimeter temperature was set at 150°C which precluded, at the low pressures. the physical adsorption of this base. Only the strongest acid sites could reasonably interact with this probe.
Samples
dealuminated over the broad range indicated above were activated at 350°C and allowed to interact with small doses of MI3. The evolved differential heats, as a function of the corresponding adsorbed quantities are reported in figure 1.
z
0 In a
0
In 0
* U
0 I-
4 W
I
AmouuT
Fig. 1 : Differential heats of MI dealuminated at 200°C (a), 300°C at 350°C.
N H A ~ DSORBED (cm3.g-1)
adsorption vs coverage at 150°C for samples (c), 375°C (d), 45OOC (el, activated
(g), 350°C
The most striking feature lies in the close similarities of these diagrams. All samples exhibit, to various degrees, two distinct populations whose relative importance of which varies with the extent of dealumination. The least dealuminated samples exhibited the largest important population of the weakest
acid sites corresponding to
the lowest differential heats.
The
dealumination caused the corresponding plateau to shrink progressively and the corresponding heat to slightly increase or remain approximately constant. The most energetic site population was almost unaffected as the residual aluminium content decreased from 56 to within 20
-
16. As the dealumination was
pursued the average strength of the latter sites increased, indicating the sensitivity of the residual sites to the local environment as far as the mobility of the protons is concerned. This is in excellent agreement with earlier reports suggesting that the acid strength of ZSM-5 and mordenite-type
382
zeolites increased with increasing Si/Al ratios (5,6,7).However, several authors have provided evidence for the heterogeneous nature of the acid sites in various zeolites (81, thus it would not seem sensible to consider the observed constant heats as actually indicating a population of equally strong sites. Because the population dealt with was of the Bronsted nature, we felt the use of a less basic probe was necessary to confirm or disprove the apparent homogeneity of these populations. Pyrrole whith pka = 0.4 compared to 9.24 for ammonia should be far more appropriate to detect site heterogeneity. Figure 2 shows the variation of the differential heats of room temperature adsorption of pyrrole.
PVRROLE
Fig. 2 : Differential heats of pyrrole adsorption vs coverage at 23°C for samples activated at 350°C : ITY (a), dealuminated at 350°C (b) - 375°C (c). The histograms show patterns very similar to those already observed in the case of ammonia. However, the trends are totally reversed with respect to the parent
HY zeolite not only exhibited a population of equally strong sites, but these sites appeared to be stronger than those of the dealuminated samples, in constrast to previous observations and the generally accepted view (5.7). Furthermore, the initial differential heats of adsorption of this supposedly weaker base are higher than 140 kJ.mol-' which was measured for ammonia at the same temperature ( 9 ) .
which is rather odd. Thus the question arises as to
whether pyrrole was acting as a basic probe! One cannot indeed reconcile the higher heat observed for pyrrole and the heat trend with dealumination. Therefore it became vital to consider the hypothesis of pyrrole acting as an acidic probe. In fact, this latter compound, in spite of the nitrogen lone pair, was reported in a recent review ( 1 0 ) to behave as an acidic probe. It has a l s o been claimed that this molecule, barely water-soluble, is the least liable, among the secondary "aminas", to accept a proton because of the important loss of stabilisation energy upon formation of the ammonium cation. The lone pair,
383 indeed, contributes, when free, to ensure an aromatic type resonance, which can no longer be ensured upon addition of a proton. On the contrary, the carbanion is usually more stable than the neutral molecule for these very reasons (11).
It
is then concievable that if pyrrole is behaving as an acid probe the differential heats could be higher than those measured for ammonia. It is also quite reasonable that the basic character of
the zeolite decreased with
dealumination as the acidic character increased. Thus the results obtained in the case of pyrrole, while incoherent when considering this probe as a base, are entirely self-consistent and in total agreement with the known pattern of zeolite acidity when this molecule is considered as an acid probe. Dimethylether with a low pka is not suspected to behave as an acid. Therefore it was investigated as an alternate basic probe in the hope that it would help give a more accurate picture of the strength distribution among the acid sites of a given zeolite. Figure 3 shows the differential heats of room temperature adsorption of this molecule on Y zeolites of different Si/A1 ratios. Again, the heats observed for these samples do not show the increase in strength demonstrated via ammonia adsorption. These results probably can be accounted for in terms of the non-specificity of this latter probe towards Bronsted-type sites. Dimethylether was
indeed reported to
adsorb rather
strongly and
specifically on Lewis acid sites and to give rise only to H- bonding with acidic hydroxyl groups ( 1 2 ) .
I.1
CH, 0 CH,
Fig. 3 : Differential heats of CH OCH3 adsorption vs coverage at 23°C for samples activated at 350°C : HY (a), 3dealuminated at 350 (b) 375 (c).
-
Therefore ammonia still appears to be the most reliable probe to monitor the
acid
strength distribution of
solid acids
provided the
appropriate
experimental conditions are chosen in view of the acid surface under study.
384
Especially, extreme care should be taken in choosing the adsorption temperature. Whatever the merits and the drawbacks of the use of ammonia,it helped unveil the fact that dealumination occurs selectively since the population of the lowest acid strength is affected first while the other population is hardly affected by dealumination using silicon tetrachloride vapour. The weakest acid population might be related to the OgH type silanol groups which were shown to react only poorly with pyridine at room temperature and were restored first upon pumping. This population of silanol groups is associated with aluminium centres that are apparently more reactive towards SiC14 or less stable in tetrahedral coordination then those associated with the 0 H groups. 1
However, as the
faujasite-type structure does not provide for more than one crystallographic site, the difference in strength of the populations could be rationalized in terms of local environment : aluminium-centred tetrahedra may be only linked to silicon-centred tetrahedra ; however, the next nearest neigbours could be either aluminium-or silicon-centred. Thus the ratio of the aluminium-centred to the Silicon-centred nearest neighbours is likely to induce sensitive differences in the mobility of the attached protons and give rise to two apparently distinct populations.
Assuming
that
aluminium-centred nearest
the
protons
neighbours
are
having the
the
least
highest mobile,
number
of
progressive
substitution of aluminium by silicon decreases more sensitively these very populations of sites while slightly affecting the population characterised by the least number of aluminium-centred nearest neighbours. This is what is actually observed using ammonia adsorption. However, for reasons that were already emphasized, this method is not sensitive enough to unveil the structural heterogeneity stemming from the effect of the more or less remote "nearest neighbours"
.
o
kJ.rnol-'
1
Ol 0
6
10
16
20
.
v cma.s-'
Fig. 4 : Differential heats of NH adsorption vs coverage at 150°C for sample dealuminated at 350°C and activate2 at 350°C (a) and 760°C (b).
385
The Lewis acid titration is even more difficult to achieve: previous studies ( 1 3 , 1 4 ) have indicated that these are more energetic than the strongest Bronsted
acid sites. Therefore, the relevant heats of neutralisation cannot
but be underestimated when titrated by a base which is not exclusively interacting with such types of sites. In the case of ammonia, as shown in figure 4 , heat treatment at high temperature of a sample containing about 16 to 20
residual aluminium-centred tetrahedra resulted in a significant increase of the initial differential heat of adsorption, confirming that, under these activation conditions, some Lewis acid sites have been formed.
CH,
oi
0
20
cw
40
60
V cm'.g-t
Fig. 5 : Differential heats of' CH3CN adsorption vs. coverage at 23°C for sample dealuminated at 350°C activated at 350°C (a) and 760°C (b). In a quest for a more specific basic probe we investigated the adsorption of acetonitrile, reported to be a good probe for Lewis acid sites, following well documented infrared studies (15, 16). Figure 5 shows, for the effect of
a given sample,
increasing activation temperature on the initial heat of
adsorption of acetonitrile. Clearly, higher heats were evolved when Lewis acid centres were created following appropriate activation. The presence of Lewis sites in the sample activated at 760°C was confirmed on the basis of pyridine adsorption as monitored using infrared spectroscopy and additional 27A1 NMR. The increase of 15 kJ/mole is certainly underestimated, again, because of the predominance of Bronsted sites under these conditions. It is noteworthy to compare the adsorption of ammonia and acetonitrile over the same sample. Figure 6 illustrates the variation of the differential heats of adsorption of these two compounds over the latter sample. Clearly, ammonia seems to indicate the occurrence of two distinct and almost homogeneous populations. The most energetic one could be ascribed to the Lewis acid centres, no matter what their structure ; the least energetic ones could be identified with the residual
386
proton population, in agreement with the results observed with the same sample activated at milder temperatures. Acetonitrile indeed showed less variation of the differential heats, indicating a more heterogeneous nature of the created Lewis centres than would be apparent from the adsorption of a stronger base over even stronger acid sites. Of course the initial heats in this case are lower than those recorded with ammonia. which
is consistent with the acidity
difference. Thus acetonitrile proved to be a more appropriate base than ammonia to discriminate the Lewis centres from Bronsted sites v i a the respective neutralisation heats. These proved to be different enough, to ensure a more accurate representation of the strength distribution among Lewis sites.
0 LJ.mol-1
t
Fig. 6 : Differential heats of NH adsorption vs. coverage at 150°C (a) and of CH CN adsorption vs. coverage at 33.C (b) for sample dealuminated at 350°C and ac%ivated at 760°C. CONCLUSION Ammonia remains
the probe of choice in the titration of Bronsted type
sites. However, it has to be used under the most appropriate conditions, which may vary depending on the solid under study. It is less convenient when both strong Bronsted sites and medium Lewis sites are present within the same sample, since then it may be very difficult to distinguish these sites on the sole basis of the heat values. A less strong base remains to be found to give an accurate picture of the acid strength distribution within a given sample. Acetonitrile would appear to be a reasonable probe to investigate the distribution of rather strong Lewis sites even in the presence of strong Bronsted sites. Pyrrole could hardly be considered as a basic probe. On the contrary it
387
would prove to be convenient acid probe which does not drastically alter the structure of delicate materials such as zeolites. Dealumination via the silicon tetrachloride reaction with thoroughly dehydrated NaY zeolites proved to proceed as a clean reaction with no nonframework aluminium left within the porous structure or at the surface. It produced variously dealuminated crystalline materials whose acidity strength increased as the residual aluminium decreased up to a value of about 5 aluminium atoms per unit cell. Beyond such a limit the acid strength started to decrease again. Two distinct proton populations could be discriminated using ammonia adsorption. The strongest acid population resisted dealumination up to a limit of 16 residual aluminium atoms per unit cell. The difference in strengths between the two populations is interpreted in terms of the nature of the next nearest neighbours. The sites which have a maximum "aluminium" as nearest neighbours are the least strong sites. They are the first to be eliminated upon dealuminating substitution. REFERENCES 1.
2. 3. 4. 5. 6.
7. 8. 9. 10.
11.
12. 13. 14. 15. 16.
A. AUROUX in Innovation in Zeolite Materials Science, P.J. Grobet et a1 Eds, Elsevier, Amsterdam (1987) 385. H.K. BEYER and I. BELENYKAJA in Catalysis by Zeolites, B. Imelik et al. Eds, Elsevier, Amsterdam 5 (1980) 203. J.R. SOHN, S.J. DECANIO. J.H. LUNSFORD and D.J.0' DONNELL, Zeolites 6 (1986) 225. G. ENGELHARDT, U . LOHSE, E. LIPPMAA, M. TARMAK and M. MAGI, Z. anorg. allg. chem. 482 (1981) 49. N. GIORDANO, P. VITARELLI, S. CAVALLARO, R. OTTANA and R. LEMBO, Proceed. of the 6th Intern. Zeolite Conf., D. OLSON and A. BISIO Eds., Butterworths, Guildford (1984) 331. F.R. CHEN, G. COUDURIER and C. NACCACHE in new Developments in Zeolite Science and Technology, Y. Murakami et al. Eds., Kodansha, Tokyo (1986) 773. A. AUROUX, P.C. GRAVELLE, J.C. VEDRINE and M. REKAS, Proceed. of 5th Intern. Conf. on Zeolites, L.V. REES. Ed. Heyden, London (1980) 433. D. BARTHOMEUF, Acta Physica et Chemica 24 (1978) 71. A. AUROUX, Y.S. JIN. J.C. VEDRINE and L. BENOIST, Appl. Catal. 36 (1988) 323. D. BARTHOMEUF, G. COUDURIER and J.C. VEDRINE, Materials Chemistry and Physics 18 (1988) 553. J. MARCH, Advanced Organic Chemistry, Kogakusha Eds., Mc Graw Hill, (1977). 225. J.C. LAVALLEY and J. CAILLOD, J. Chimie Physique 77 (1980) 373. J.C. VEDRINE, A. AUROUX. G. COUDURIER, Catalytic Materials, Relationship between Structure and Reactivity, A.C.S. Symposium Series 248 (1984) 254. A. MELCHOR, E. GARBOWSKI, M.V. MATHIEU and M. PRIMET. J.C.S. Faraday Trans. I, 82 (1986) 3667. J.C. LAVALLEY and C. GAIN, C.R. Acad. Sc. Paris 288 (1979) 177. H. KNOEZINGER and H. KRIETENBRIK, J. Chem. SOC. Faraday Trans I. 71 (1975) 2421.
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H.G. Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V.. Amsterdam - Printed in The Netherlands
THE ACIDITY OF A MODIFIED FAUJASITE STRUCTURE, ZEOLITE CSZ-1
2 2 S . CARTLIDGEl, R.L. COTTERMAN and M.L. HOWES 'Grace GmbH, In der Hollerhecke 1. Postfach 1445, 6520 Worms (W.-Germany) 'W.R.
Grace & Co., Washington Research Center, 7379 Route 32, Columbia,
MD 21044 (USA)
ABSTRACT This work establishes unifying concepts between structure and acidity for Y and CSZ-1 zeolites. Characterisation data are presented which support the proposal that CSZ-1 has a modified faujasite structure. Catalytic cracking of pure hydrocarbons show that CSZ-1 and Y have comparable numbers of acid sites when zeolite aluminium contents are similar but that the acid sites in CSZ-1 are weaker. Differences in acidity can be attributed to structure effects. After hydrothermal treatment and aluminium extraction, acid site strengths are similar for both zeolites.
INTRODUCTION Zeolite acidity plays a key role in determining the performance of fluid catalytic cracking (FCC) catalysts (1). The number and strength of zeolite acid sites determine the extent of hydrogen transfer reactions which, in turn, control gasoline quality and quantity. For example, ultrastable Y (US-Y) zeolites in FCC catalysts enhance gasoline octane in comparison to rare-earth Y zeolites which are used to maximise gasoline barrels. Acid strength of US-Y increases with decreasing sodium levels (2). Bronsted acid sites in US-Y attain a constant maximum acid strength when Si/Al >6 which corresponds to isolated framework aluminium atoms (3). Lewis acidity, associated with non-framework aluminium in US-Y zeolites (4). has important effects on catalyst activity and product selectivity during gas oil catalytic cracking (5).
The conversion of model hydrocarbons under cracking conditions can be used to characterise zeolite structure and acidity. Hexadecane cracking has been widely used to simulate the conversion of petroleum oils over zeolite catalysts (6). Recently activity and selectivity for n-paraffin cracking has been used to investigate the mechanism of catalytic cracking over zeolite Y (7).
390
Acidity is also influenced by zeolite structure ( 8 ) . Modification of the faujasite framework may modify acidic properties and hence alter catalytic properties. Zeolites CSZ-1 (9). ZSM-3 (lo), and ZSM-20 (11) are claimed to have framework topologies related to faujasite. The effect of the structure of near-faujasite materials on acidity can be determined by comparing catalytic properties. The aim of this work is to relate the structural differences between zeolite Y and CSZ-1 to observed acid catalysis.
EXPERIMENTAL Zeolite CSZ-1 was prepared by following the methods given in Examples 1 and 2 of ref ( 9 ) . Sodium silicate, aluminium chloride and sodium aluminate were the sources of silica and alumina. A homogenous seed gel having molar composition 11.94 Si02 : A1203 : 12.7 Na20 : 247 H 0 was prepared and aged overnight. A 2 second hydrogel having molar composition
10 Si02 : A1203 : 0.5 CsCl : 4 NaZO : 1.4 NaCl : 150 H20 and containing 6 mol X A1 0 from the seed gel was heated at 95°C for 16 hours. The CSZ-1 2 3 product was filtered, washed and dried.
Commercial Na-Y and as-synthesised CSZ-1 were ammonium ion exchanged, steamed at 550°C and exchanged again to give US-Y and US-CSZ-1. A further steam treatment of the US-zeolites at 720°C was followed by an acid treatment in aqueous hydrochloric acid to give USE-Y and USE-CSZ-1. Catalytic experiments were conducted in atmospheric pressure, fixed bed reactors at 500°C. Heptanelheptene cracking was carried out over the USzeolites. The WHSV was changed by varying the flowrate of a feed saturated nitrogen stream. The products were collected at 30 second intervals on-stream and analyzed by GC for instantaneous conversions. Hexadecaneltetradecene cracking experiments used kaolin-bound samples steamed at 760°C. The WHSV was varied by changing the catalyst loading. Cumulative liquid and gas products were collected over three minutes and were analyzed by capillary GC for total conversion and product selectivity.
RESULTS AND DISCUSSION Structural considerations Zeolite CSZ-1 is prepared from a synthesis gel which would produce faujasite in the absence of caesium ions. Caesium is quantitatively incorporated into CSZ-1 as an exchangeable cation (Table 1). Modified faujasite synthesis gels have been studied previously (9-13). For example platelet faujasite is prepared from potassium containing sodium aluminosilicate gels. In the presence of lithium, faujasite syntheses are directed towards ZSM-3 (10).
391 Phe hexagonal platelet morphology of CSZ-1 is also shared by ZSM-3 and platelet faujasite. The CSZ-1 crystallites prepared in this work are 0.5 to 3 pm.
PABLE 1 Physical and Chemical Characterisation Data for CSZ-1 and Y Zeolites ~~
~~~~
~~
~
BET N2 Surface Area m2/g
Molar Composition Si02 A1203 Na20 Cs20
Sample
4.9 4.7 18 5.3 5.6 213
csz-1
us-csz- 1
USE-CSZ-1 Y
us-Y USE-Y
ND = not detected
*
1 1
0.78 0.05
1 1 1 1
ND 0.97 0.05 0.05
0.22 0.06 0.04
Framework * Molar Ratio Si02/A1203
4.6 7.4 34 5.6 8.6 46
750 760 750 a20 830 860
-
-
-
Si-29 MASNMR
Zeolites CSZ-1 and Y have comparable Si/A1 ratios (Table 1). The high surface area measured for as-synthesised CSZ-1 (Table 1) indicates a well developed microporous structure. However, the X-ray powder diffraction pattern of CSZ-1 has relatively few sharp and intense diffraction peaks in contrast to faujasite (Fig. 1). The poor quality of the CSZ-1 powder pattern may arise from structural disorder at the unit-cell level. Electron microscopy studies of
Fig. 1 X-ray Powder Diffraction Patterns for As-Synthesised Na-Y (top) and Na.Cs-CSZ-1
!
!
B
D
2
?
s
? c
? 4
N
n
N
B
? 0
? n
B
?
n
392
CSZ-1 (14, 15) and ZSM-3 (16) point towards framework structures which are based on the sodalite cage as the building unit. CSZ-1 may have a faulted faujasite structure created by varying the stacking sequences of single 6-membered rings. A series of such structures has been observed (17) and enumerated (18). Furthermore, the formation of hypercages in Y zeolites by recurrent twinning (19) has been proposed. The identical Si-29 MAS-NMR spectra of as-synthesised Y and CSZ-1 (Fig. 2 atd) show that the two structures are similar on a T-atom scale. It is interesting, however, to compare the Si-29 MAS-NMR spectra of the zeolites after the dealumination. As the extent of dealmination increases, the peak corresponding to Si(0Si) 4 groups increases in intensity for Y zeolite. The ultrastable zeolite structures (Fig. 2 b,e) have very different T-atom distributions. The chemical shifts of peaks in Si-29 MAS-NMR spectra of highly dealuminated zeolites (Fig. 2 c.f) show that primarily Si(OA1). (0SiI3 and Si(OSi)4 sites are present. The rearrangement of framework silicon and aluminium in CSZ-1 with increasing steaming severity is different to that for zeolite Y (Fig. 2 a,b,c cf Fig. 2 d,e,f). In particular the peak corresponding to Si(OSi)4 groups in USE-CSZ-1 (Fig. 2 f) has a shoulder peak at ca. -110 ppm which might represent a second crystallographically inequivalent T atom site. This assignment would explain the structural disorder indicated by X-ray powder diffraction data.
A,dA,
Fig. 2 Silicon-29 MASNMR Spectra for As-Synthesised, Steamed and Acid Extracted CSZ-1 and Y a) Na-Y, b) US-Y, c) USE-Y, d) Na,Cs-CSZ-1. e) US-CSZ-1, f) USE-CSZ-1
b
cAfA
-70 -80 -90 -100 -110 -120 -70 -80 -90 -100 -110 -120
Chemical Shift (ppm)
393
Catalytic Cracking Activity The acidities of CSZ-1 and Y zeolites were probed using two sets of madel compound reactions at cracking conditions. In the first set, heptanelheptene feeds were passed over the US-zeolites; in the second set, hexadecaneltetradecene feeds were converted over the steamed kaolin-bound US and USE-zeolites. The apparent first order rate constants reported in Table 2 were observed to be invariant to ? 2 units for average feed space velocities from 9-25 h-1
.
Rate constants have been normalised with respect to total moles of aluminium in zeolite and are related to acid site turnover numbers.
TABLE 2 Initial Rate Constants for HeptanelHeptene Cracking over US-CSZ-1 and US-Y
*
Feed
n-heptane
Zeo1ite Initial Rate Constant*
us-Y us-csz-1
us-Y
32.2
43.6
6.7
n-heptaneln-heptene us-csz-1 29.5
Rate constant expressed as ( m o l hydrocarbon converted)/ (mol A1 atoms-atm-h).
The low initial first order rate constant for n-heptane conversion over US-CSZ-1 compared to US-Y (Table 2 ) shows that US-CSZ-1 is less active although the total aluminium contents of the zeolites are similar (Table 1). When 5wtX n-heptene is co-fed with n-heptane, the initial rate constant of US-Y increases (Table 2) as expected due to the higher rate of cracking of olefins over acidic catalysts (1)
. However, the four fold increase in initial rate constant for
US-CSZ-1 (Table 2 ) upon heptene addition indicates that additional acid sites are accessible which convert heptene but not heptane. The values of the initial rate constants are determined by the number and strength of catalytically active acid sites. The heptanelheptene conversion data suggest that US-CSZ-1 has a similar number, but weaker, acid sites compared to US-Y. This conclusion is supported by the Si-29 MAS-NMR data for US-Y and US-CSZ-1 (Figures 2 b and 2 e). The framework of US-CSZ-1 contains more aluminium than that of US-Y. Consequently the acid sites associated with framework aluminium atoms in US-CSZ-1 will be weaker.
394 Hexadecaneftetradecene testing of the steamed, kaolin-bound US-zeolites presents further evidence in support of the proposed difference in acidity for US-CSZ-1 and US-Y. The catalytic activity is presented as a function of space time in Figure 3 where the slopes of the lines represent apparent first order rate constants. The increase in average rate constant for the steamed US-Y catalyst when tetradecene is co-fed (Table 3) is similar in magnitude to that observed for the heptanelheptene cracking. The corresponding increase in activity of steamed US-CSZ-1 is not as dramatic as for the heptanelheptene case. This is due to an observed loss of micropore surface area after steaming US-CSZ-1 at 760’C. TABLE 3 Average First Order Rate Constants for HexadecaneITetradecene Cracking over Steamed Kaolin-Bound US and USE-Zeolites Feed
Average First Order Rate Constant* n-C H /n-C14H28 n-C16H34 16 34
CATALYST**
us - csz-1 us-Y
USE-CSZ- 1 USE-Y
* **
07
52 171 119 170
212 169 236
Rate constant expressed as (mol hydrocarbon converted)/ (mol A1 atoms-atm-h). Zeolites kaolin bound and steamed at 760°C
D
I / W H S V ( x 10’)).
hr
Fig. 3: Activity of Steamed, Bound US-CSZ-1 and US-Y for Hexadecanel Tetradecene Cracking
A
A
US-CSZ-1, US-CSZ-1,
[7 US-Y
US-Y
100 2 Hexadecane 95.3% Hexadecane, 4.7% Tetradecene
395
/4+
0.8-
-- 0 . 6 0.4
0.2
0 0.0
'
"
I
"
"
16
'
"
'
"
24
"
"
32
'
I / W H S V ( X l o 3 ) , hr
Fig. 4: Activity of Steamed, Bound USE-CSZ-1 and USE-Y for Hexadecanel Tetradecene Cracking
A
A
USE-CSZ-1 USE-CSZ-1
0
USE-Y USE-Y
100 X Hexadecane 95.3% Hexadecane, 4.7% Tetradecene
The two steamed, kaolin-bound acid extracted zeolites, USE-Y and USE-CSZ-1, show comparable activity for hexadecane conversion (Figure 41, however, the acid strength of USE-CSZ-1 is lower than USE-Y when normalized with respect to aluminium content (Table 3). When tetradecene is co-fed with hexadecane, the increases in catalytic activity are similar for the steamed USE-zeolites (Table
3). This observation strongly suggests that, after extensive dealumination, isolated acid sites in the USE-zeolites have a similar acid strength. The Si-29
MAS-NMR spectra for the USE-zeolites (Figure 2 c and 2 f) show a high degree of dealumination which must be even more extensive after steaming at 760°C.
Catalytic Cracking Selectivity A summary of the product selectivities from hexadecane and hexadecaneltetradecene cracking over steamed, kaolin-bound US-Y and US-CSZ-1 is presented in Table 4. The selectivities to Paraffins, Isoparaffins, Olefins, Eaphthenes and Aromatic hydrocarbons are calculated after identification of ca. 200 components in the C5-C12 fraction. It is important to note that the comparisons between CSZ-1 and Y are made at approximately constant conversion.
396
The gross similarity of the framework topologies of CSZ-1 and Y is confirmed by the almost identical C5-C12 selectivities and nearly identical PIONA distributions (Table 4). The steamed US-CSZ-1 catalyst appears to produce less aromatics and more olefins than US-Y indicating that steamed US-CSZ-1 has poorer hydrogen transfer. This observation is consistent with weaker acid sites.
TABLE 4 Product Selectivities for Hexadecane and HaxadecanelTetradecene Cracking over Steamed, Kaolin Bound US-CSZ-1 and US-Y
Feed CATALYST*
us-Y
WHSV , h-
290
Conversion, wtX Selectivity C5-CI2,X
n-C H /n-C14H28 16 34
n-C16H34 us-csz-1
64.1
83 20 63.7
13.2 32.8 46.0 4.6 3.4
14.1 31.6 47.9 3.8 2.6
21
us-Y 193 38
us-csz-1
60.5
83 34 60.7
11.1 33.2 46.9 4.3 4.6
11.7 31.4 49.4 4.6 3.0
Selectivities (in c5-C12). % Paraffins Isoparaffins Olefins Napthenes Aromatics
*
Zeolites kaolin bound and steamed at 760°C
CONCLUSIONS Zeolite CSZ-1 represents a modification of the faujasite structure in which there appears to be disorder at the unit-cell level. When CSZ-1 and Y zeolites are prepared with similar Si/A1 ratios, large differences can be seen in catalytic cracking activities. The acid sites in CSZ-1 are weaker than those in Y. When CSZ-1 and Y zeolites are dealuminated and acid extracted to high Si/A1 ratios, acid strengths of the two zeolites are similar. This indicates that structural effects on acid site strength are less pronounced when acid sites become diluted.
397
ACKNOWLEDGEMENTS The authors thank W. R. Grace and Co. for permission to publish this work. We also thank Dr. M. P. Shatlock for providing the NMR spectra and D. M. Roberts, D. G. Nelson and A. Siege1 for assistance with experimental work.
REFERENCES 1 B.W. Wojciechowski and A. Corma, Catalytic Cracking, Marcel-Dekker, New York (1986) 65 2 J.W. Ward and R.C. Hansford, J. Catal., l3, (1969) 364 K. Rajagopalan and A.W. Peters, J. Catal., 106, (1981) 410 3 J. Dwyer, "Innovation in Zeolite Materials Science", Studies in Surface Science and Catalysis, P.J. Grobet et al. (Editors), Elsevier 2. (1988) 333 D. Barthomeuf, "Zeolites: Science and Technology", NATO AS1 Series E: Applied Sciences, F.R. Ribeiro et al. (Editors), 80, (1984) 317 4 D. Freude, T. Frohlich, M. Hunger, H. Pfeifer and G. Scheler, Chem. Phys. Letters, (1983) 263 5 S.W. Addison, S . Cartlidge, D.A. Harding, G. McElhiney, in preparation R.A. Beyerlein, G.B. McVicker, L.N. Yacullo and J.J. Ziemiak, ACS Preprint, Div. Pet. Chem. New York, April 1986, 190 6 D.M. Nace, Ind. Eng. Chem. Prod. Res. Dev., 8, (1969) 2 4 7 A. Corma, J. Planelles, J. Sanchez-Marin and F. Tomas, J. Catal. 93
s,
(1985) 30 J. Abbot and B.W. Wojciechowski, J. Catal., 8 9 10 11 12 13 14 15 16 17 18 19
107,
(1987) 451
J. Abbot and B.W. Wojciechowski, J. Catal., 109, (1988) 274 D. Barthomeuf, Mat. Chem. Phys., 17, (1987) 49 M.G. Barrett and D.E.W. Vaughan, U.S. Patent 4, 309, 313, 1982 (W.R. Grace & Co.) J. Ciric, U.S. Patent 3,415,736, 1968 (Mobil) E.W. Valyocsik, European Patent 12,572 Al, 1980 (Mobil) D.E.W. Vaughan and M.G. Barrett, U.S. Patent 4,333,859, 1982 (W.R. Grace & Co.) G.C. Edwards, D.E.W. Vaughan, E.W. Albers, U.S. Patent 4,175,059, 1979 (W.R. Grace) G.R. Millward, J.M. Thomas, S. Ramdas. M.T. Barlow, Proc. 6th Int. Zeolite Conf., D. Olson and A. Bisio (Editors). Butterworth (1984) 793 M.M.J. Treacy, J.M. Newsam, R.A. Beyerlein, M.E. Leonowicz, and D.E.W. Vaughan, J. Chem. SOC., Chem. Commun. (1986) 1211 G.T. Kokotailo and J. Ciric, Adv. Chem. Ser., 101,(1971) 109 J.A. Gard and J.M. Tait, Adv. Chem. Ser., 101,(1971) 230 J.V. Smith and J.M. Bennet, h e r . Mineral, 6 6 , (1981) 777 M. Audier, J.M. Thomas, J. Klinowski, D.A. Jefferson and L.A. Bursill, J . Phys. Chem. &, (1982) 5 8 1
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H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Control of catalytic properties of ZSM- 5 made by fast and template- free synthesis. A . TISSLERI, P. POLANEKZ. U. GIRRBACH3, U. M U L L E R l a n d K . K . LINGER' 1 l n s t i t u t f u r Ano rg an is ch e Chemie und A n al yt i sche Chem i e, J o h a n n e s G u t e n b e r g - U n i v e r s i t a t , D- 6500 M ai n z. FRG; 2 p r e s e n t a d d r e s s : BASF AG. D- 6700 L u d w i g s h a f e n , FRG 3 p r e s e n t a d d r e s s : H o e c h s t AG, D- 6230 F r a n k f u r t , FRG. ABSTRACT Th e f a s t t e m p l a t e - f r e e s y n t h e s i s of ZSM-5 w a s a c c o m p l i s h e d in b o t h st at i c. a nd s t i r r e d a u t o c l a v e s e m p l o y i n g s o d i u m w a t e r g l a s s . s i l i c a h y d r o g e l an t i p) r o g e n i c s i l i c a s a s s i l i c a s o u r c e s . H ig h l y p u r e c r y s t a l l i n e ZSM- 5 w a s o b t a i n e d in t h e s o d i u m w a t e r g l a s s r e a c t i o n s y s t e m in t h e s t i r r e d a u t o c l a v e s w i t h o u t s e e d s w i t h i n 3 h o u r s . A d d i t i o n o f 2 % ( w / w ) of s e e d s r e d u c e d t h e r e a c t i o n t i m e t o 2 h o u r s . T h e c a t a l y t i c a c t i v i t y a n d s h a p e s e l e c t i v i t y o f t h e H - ZS M- S .t est c- d by t h e d i s p r o p o r t i o n a t i o n o f e t h y l b e n z e n e , w a s f o u n d t o b e l a r g e l y co n t r o l l Pci by t h e s y n t h e s i s p a r a m e t e r s a n d by t h e p o s t t r e a t m e n t s . T h e h o m o g r n e o u s a i u inintiin d i s t r i b u t i o n a c r o s s t h e c r y s t a l s a c h i e v e d by t h i s s y n t h e t i s r o u t e r e s u l t e d i n a g r e a t e r shape selectivity compared t o t h o s e with an uneven aluminum d i s t r i b u t i o n b e i n g p r e p a r e d by a d d i n g t e m p l a t e s . V a r i o u s k i n d s o f p o s t t r e a t m e n t s , e.g. "alkaline calcination",led t o an addi t i onal e n h a n c e m e n t of t h e s h a p e s e l e c t i v i t y y i e l d i n g u p t o 99% o f p - i s o m e r in t h e e t h y l b e n z e n e d i s p r o port ion a t i on reac t io n . INTRODUCTION A l t h o u g h t h e t e m p l a t e f r e e s y n t h e s i s o f Z S M - 5 w a s t h e s u b j e c t o f s e v e r a l inv e s t i g a t i o n s ( r e f s . 1,2,3), a n u m b e r o f i m p o r t a n t a s p e c t s s t i l l n e e d t o h e c l a r i f i e d . -
T h e i m p a c t o f t h e s i l i c a s o u r c e o n t h e k i n e t i c s o f t h e ZSM- 5 f o r m a t i o n a n d
-
no detailed s t u d i e s were p erfo rmed on t h e c r y s t a l l i s a t i o n ki net i cs. except
on t h e resulting crystallisation fields was n o t essentially established: t h a t they were reported t o be s l o w
- t h e r e l a t i o n s h i p b etw een c r y s t a l morphology a n d chemical hom ogenei t y of t e m p l a t e - f r e e s y n t h e s i z e d ZSM- 5 a n d its c a t a l y t i c p r o p e r t i e s w a s n o t e x a m i n e d . T h e p a p e r a d d r e s s e s t h e t e m p l a t e - f r e e s y n t h e s i s o f ZS M- 5 in t h e r a n g e o f Si/AI o f 10- 2 0 0 e m p l o y i n g s o d i u m w a t e r g l a s s . s i l i c a h y d r o g e l a n d p y r o g e n i c s i l i c a s a s s i l i c a s o u r c e s . I t f u r t h e r i n v e s t i g a t e s t h e k i n e t i c s o f Z S M - 5 frrrniati on
ti n
d e r c o n c e 11t r a t io 11 a n d t e m p e r a t u r e - g r a d i e n t - f r e e c o n d i t i o 11 s t o p r o v i d r
t h e b a s i s f o r a f a s t s y n t h e s i s p r o c e d u r e . S p e c i a l e m p h a s i s is p l a c e d o n t h e c o r r e l a t i o n b e t w e e n t h e c a t a l y t i c p r o p e r t i e s a n d t h e d eci si v e p a r a m e t e r s o f t h e synthesis aiming a t ZSM- 5 products with optimum catalytic activity and s e l e c t i v i t y . In p a r t i c u l a r , t h e Z S M - 5 c r y s t a l s w e r e s u b j e c t e d t o v a r i o u s p o s t treatments a s a means t o enhance t h e shape-selective d i s p r o p o r t i o n a t i o n of e t h y l b e n z e n e ( r e f . 4 I .
p r o p e r t i e s in t h e
400 EXPERIMENTAL The materials used f o r t h e s y n t h e s i s were commercial sodiumwaterglass
( 2 7 . 7 5 wtX SiO,; 0.11 w t % A 1 2 0 3 ; 7.99 w t % Na,O). a l u m i n u m s u l p h a t e ( M e r c k , GR g r a d e ) , s u l p h u r i c acid ( M e r c k , 9 5 - 97 wtX p.A.1 a n d d i s t i l l e d w a t e r . S t i r r e d s t a i n l e s s s t e e l a u t o c l a v e s of 1.4 1 and 2.4 1 v o l u m e s e r v e d a s r e a c t i o n v e s s e l s allowing on-line
s a m p l i n g d u r i n g t h e r e a c t i o n r u n s . In a d d i t i o n , s t a t i c
T e f l o n - l i n e d a u t o c l a v e s of 140 ml w e r e e m p l o y e d f o r t h e e x p e r i m e n t s with pyr o g e n i c s i l i c a s and s i l i c a h y d r o g e l . The crystallinity o f t h e p r o d u c t s was determined using q u a n t i t a t i v e X-ray p o w d e r t e c h n i q u e s (Philips APD 1 5 ) . Chemical a n a l y s i s of t h e p r o d u c t s was c a r ried o u t w i t h s t a n d a r d m e t h o d s . The a l u m i n u m d i s t r i b u t i o n a c r o s s t h e c r y s t a l s was a s s e s s e d by w a v e l e n g t h - d i s p e r s i v e
e l e c t r o n microprobing
(
Camaca.
Camebax). T h e c a t a l y t i c t e s t s were c o n d u c t e d in a f l o w - t y p e a p p a r a t u s w i t h a s a t u r a t o r f o r e t h y l b e n z e n e and a f i x e d - bed r e a c t o r f r o m g l a s s o n - l i n e - c o n n e c t e d t o t e m p e r a t u r e - p r o g r a m m e d c a p i l l a r y GC.The r e a c t i o n t e m p e r a t u r e w a s 523
RESULTS
K.
A W DISCUSSION
I n t h e w h o l l y i n o r g a n i c r e a c t i o n s y s t e m of ZSM - 5 t h e a l k a l i n i t y a n d t h e a l u m i n u m c o n t e n t ( S i / A I ) w e r e t h e key p e r a m e t e r s i n
(OH-/SiO,)
determining t h e crystallisation. Pure c r y s t a l l i n e ZSM- 5 p r o d u c t s were obtained only in a n a r r o w r a n g e of well d e f i n e d c o n d i t i o n s .
Si/A L 200
100 m
50
1 0
I
I
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Fig.1 Phase d i s t r i b u t i o n in t h e r e a c t i o n
I
1.6 OHXi(
s y s t e m with pyrogenic silicas
401 T h e i n f l u e n c e of t h e s i l i c a s o u r c e on t h e c r y s t a l l i s a t i o n w a s i n v e s t i g a t e d using s t a t i c aut o clav es . The cry s tal li s ati o n r a t e was f o u n d t o b e i ncr eased in t h e s e q u e n c e p y r o g e n i c s i l i c a s t o s i l i c a h y d r o g e l t o s o d i u m - w a t e r g l a s s . In t h e same sequence t h e domain a t which pure ZSM-5 was formed in t h e crystallis a t i o n f i e l d d e c r e a s e d s u b s t a n t i a l l y . As i l l u s t r a t e d in Fig.1 f o r p y r o g e n i c s i l i c a s t h e c r y s t a l l i s a t i o n f i e l d w a s c o m p a r a t i v e l y b r o a d ( S i / A I = 12-200 a n d OH- / S i 0 2 = 0 . 1 5 -
0.6). I t became much s m a l l e r f o r silica hydrogel (Si/AI=
12- 6 0 a n d O H - / S i 0 2 = 0 . 2 -
0.31, a n d i t w a s i m p o s s i b l e t o o b t a i n Z S M - 5
under t h e s e c o n d i t i o n s w i th o u t any i mp u rit i es ( a m o r p h o u s m at er i al . i m p u r e S i 0 2 o r c r i s t o b a l i t e ) w h e n s o d i u m w a t e r g l a s s w a s u s e d . H o w e v e r , w i t h t h e use of s t i r r e d a u t o c l a v e s w i t h very h i g h h e a t i n g r a t e , t h e s o d i u m w a t e r g l a s s s y s t e m
p r o d u c e d p u r e Z S M - 5 in t h e r a n g e o f S i /A I = 12-50 a n d O H - / S i 0 2 = 0.13- 0.17. Hence all f u r t h e r investigations were carried out with t h i s reaction s y s t e m . K i n e t i c s of c r y s t a l l i s a t i o n Crystallisation curves of t h e sodiumwaterglass reaction system obtained w i t h a n d w i t h o u t t h e u s e o f s e e d s a r e i l l u s t r a t e d in F i g .2 . T h e m o l a r c o m p o s i t i o n of t h e o p t i m i z e d r e a c t i o n m i x t u r e a m o u n t e d t o S i / A I = 2 0 . 5 , O H - / S i 0 2 =
0.153, H 2 0 / S i 0 2 = 39. T h e r e a c t i o n t e m p e r a t u r e w a s 4 9 8 K .
I
100 8 a 80 X
60 4..-c Z 40 c ) .
d
v)
% L U
20 0
0
50
100 150 2 00 synthesis- time [ min 1
25(
Fig. 2 C r y s t a l l i s a t i o n c u r v e s w i t h a n d w i t h o u t a d d i t i o n o f s e e d s a t 4 9 8 K
T h e c r y s t a l l i s a t i o n c u r v e s o b t a i n e d w i t h o u t s e e d i n g e x h i b i t e d a si g n i o i d al s h a p e wh i ch is c h a r a c t e r i s t i c f o r p r o c e s s e s i n v o l v i n g t w o d i s t i n c t s t a g e s : an
402
i n d u c t i o n p e r i o d , w h e r e n u c l e i a r e f o r m e d . f o l l o w e d by a c r y s t a l g r o w t h p e r i o d . w h e r e n u c l e i g r o w i n t o c r y s t a l s . When s e e d s w e r e a d d e d , t h e c r y s t a l l i s a t i -
o n c u r v e s s h o w e d n o i n d u c t i o n p e r i o d . The c r y s t a l l i s a t i o n r a t e is d e f i n e d a s t h e r a t e of c r y s t a l l i s a t i o n a t t h e s t e e p e s t p a r t o f t h e c r y s t a l l i s a t i o n c u r v e e x p r e s s e d a s p e r c e n t p e r m i n u t e . Fig.3 s h o w s t h e i n f l u e n c e o f t h e m o s t i m p o r t a n t r e a c t i o n p a r a m e t e r s , Si/AI a n d O H - / S i 0 2 ,
011
t h e i n d u c t i o n period a n d o n t h e
crystallisation rate.
1
OH/SiO2 = 0.15
I
1201
-.c
.Y
901 aJ
aJ
.-E +
I
.-E
60
+ I
60-
aJ +
b
r[)
L
+.
<.U
30 I l l r l I l 10 20 30 40 50 Si/AL
0.1 0.12 0.14 0.16 0.18 0.20 OH ' / S i 02
Fig.3 I n f l u e n c e of Si/AI ( a ) a n d O H - / S i 0 2 ( b ) o n t h e i n d u c t i o n p e r i o d a n d t h e c r y s t a l l i s a t i o n r a t e ( T = 498 K)
The s e n s i t i v i t y of t h e r e a c t i o n s y s t e m t o s m a l l v a r i a t i o n s of' t h e a l k a l i nity is i n d i c a t e d by t h e d i s t i n c t maximum of t h e c r y s t a l l i s a t i o n r a t e n e a r a value OH-/SiO, = 0.155. Only a t t h i s r a t i o was i t p o s s i b l e t o o b t a i n 100% p u r e ZSM- 5 a t a b r o a d r a n g e of t h e Si/AI r a t i o within a b o u t 3 h o u r s (-298 k). The s y n t h e s i s time was s h o r t e n e d t o a b o u t t w o h o u r s (Fig.2) by adding a t l e a s t 2 % ( w / w ) of s e e d s . T h e i n f l u e n c e which t h e a l u m i n u m c o n t e n t of t h e r e a c t i o n m i x t u r e had o n t h e c r y s t a l l i s a t i o n r a t e w a s in c o n t r a d i c t i o n t o r e s u l t s g e n e r a l l y o b t a i n e d in ZSM- 5 s y n t h e s i s in t h e p r e s e n c e o f t e m p l a t e s ( r e f . 5
).
The c r y s t a l l i s a t i o n
r a t e w a s a c c e l e r a t e d w i t h i n c r e a s i n g a l u m i n u m c o n t e n t t o a maximum of Si/AI%13. E m p l o y m e n t o f a r e a c t i o n m i x t u r e of Si/AI=9 y i e l d e d o n l y a m o r p h o u s m a t e r i a l . For r e a c t i o n m i x t u r e s r i c h e r in s i l i c a t h e c r y s t a l l i s a t i o n r a t e d e c r e a s e d r a p i d l y . Exceeding t h e value o f S i / A l
4
50 o n l y ZSM-5 p r o d u c t s w i t h less t h a n
50%c r y s t a l l i n i t y were o b t a i n e d . I t a p p e a r s t h a t w i t h i n c r e a s i n g s i l i c a c o n t e n t t h e MFI
lattice,becoming
more
and
more
h y d r o p h o b i c , is t h e r m o d y n a m i c a l l y
403
u n s t a b l e in t h e a b s e n c e of any organic m o l e c u l e s ( r e f . G 1 which s t a b i l i r e t h e l a t t i c e by r e a l t e m p l a t i n g o r pore f i l l i n g ( r e f . 7 ) . A f t e r a d i s t i n c t r e a c t i o n p e r i o d , which was mainly d e p e n d e n t o n t h e a l u m i n a c o n t e n t a n d t h e t e m p e r a t u r e of t h e r e a c t i o n m i x t u r e , c r y s t o b a l i t e o r
CI
-quartz was formed.
Hence r e a c t i o n c o n d i t i o n s had t o be c h o s e n s u c h t h a t t h e f o r m a t i o n of ZSM-5 was c o m p l e t e d a t t h i s s t a g e . The k i n e t i c s o f c r y s t a l l i s a t i o n was s i g n i f i c a n t l y a f f e c t e d by t h e r e a c t i o n t e m p e r a t u r e . T h e e n e r g i e s of a c t i v a t i o n f o r t h e n u c l e a t i o n En a n d t h e c r y s t a l g r o w t h E, f o r t h e s y n t h e s i s s y s t e m w i t h a Si/AI- r a t i o o f 40 w e r e e v a l u a t e d f r o m t h e A r r h e n i u s p l o t s ( F i g . 4 ) t o E,=59
k J / m o l a n d E,=46 k J / m o l . The
r e a c t i o n t e m p e r a t u r e f u r t h e r m o r e influenced t h e c r y s t a l s i z e . H i g h e r r e a c t i o n t e m p e r a t u r e s r e s u l t e d in bigger c r y s t a l s .
O.!
E, = 4 6
KJ/mol
0 -
I
-
I
r’
\
x
L
- -0.5 c
-1
Fig.4 Arrhenius p l o t s f o r n u c l e a t i o n ( a ) and f o r c r y s t a l l i s a t i o n ( b ) a t Si/A1=40 Catalytic properties The d l s p r o p o r t i o n a t i o n of e t h y l b e n z e n e . i n t r o d u c e d by Karge e t a l . ( r e f . . $ ) . is a s u i t a b l e t e s t r e a c t i o n f o r p r o b i n g t h e a c t i v e and s h a p e - s e l e c t i v e p r o p e r -
t i e s of z e o l i t e s . Unmodified H-ZSM- 5 c a t a l y s t s s y n t h e s i z e d w i t h t e m p l a t e s yielded a p r o d u c t c o m p o s i t i o n c l o s e t o 6 0 1 4 0 ( w / w ) o f m e t a - d i e t h y l b e n z e n e (MDEB) t o p a r a - d i e t h y l b e n z e n e (PDEB)[ref.B). N o o r t h o - i s o m e r w a s f o r m e d u n d e r t h e s e c o n d i t i o n s . This r a t i o of MDEBIPDEB was f o u n d t o be only s l i g h t l y i n f l u e n c e d by t h e s y n t h e s i s p a r a m e t e r s ( r e f . 9 1, e x c e p t f o r l a r g e c r y s t a l s . In c o n t r a s t , t e m p l a t e - f r e e s y n t h e s i z e d ZSM- 5 e x h i b i t e d l a r g e d i f f e r e n c e s in its shape-selective properties, depending on t h e synthesis p a r a m e t e r s t h a t allow one t o tailor c a t a l y s t s with predetermined properties.
404
T h e i n c r e a s e of t h e Si/AI r a t i o o r t h e s y n t h e s i s t e m p e r a t u r e r e s u l t e d in a n e n h a n c e m e n t o f PDEB f o r m a t i o n in t h e e t h y l b e n z e n e r e a c t i o n ( F i g . 5 ) . S a m p l e s w i t h a Si/AI r a t i o o f 13 d i s p l a y e d very h i g h a c t i v i t i y b u t o n l y m o d e r a t e s h a p e s e l e c t i v i t y (35% P D E B ) , w h i l e c r y s t a l s w i t h a Si/AI r a t i o u f 4.0 g a v e l o w e r a c t i v i t y b u t high s e l e c t i v i t y (70% PDEB). T h e s h a p e s e l e c t i v i t y s e e m s t o b e i n f l u e n c e d by t h e S i / A I - r a t i o . T h i s is in a g r e e m e n t w i t h t h e f i n d i n g s o f Karge e t al ( 8 ) a n d Kaeding e t al ( r e f . 1 0 1 u s i n g H-ZSM- S s y n t h e s i z e d w i t h templates.
I
Si/Al= 40
I
- 60
>
c
0
U
-40 m n
A
w
.-t >
a
.-
4-
-20s
U
I0
1
2
3
4
5
6 AL-atoms/unit- cel I
7
I
480 4.90 syn.- temp. [ K I
470
I
500
Fig.5 I n f l u e n c e o f t h e a l u m i n a c o n t e n t ( a ) a n d t h e t e m p e r a t u r e ( b ) of t h e r e a c t i o n m i x t u r e o n t h e a c t i v i t y a n d s e l e c t i v i t y of t h e c a t a l y s t s (activity. % c o n v e r s i o n a t W / F = 300 g . h / m o l : s e l e c t i v i t y - % PDEB a t a c o n v e r s i o n of 2 % ) . T h e c a t a l y t i c a c t i v i t y varied l i n e a r l y w i t h t h e n u m b e r o f a l u m i n u m a t o m s located in t h e zeolite framework over a broad range of t h e Si/Al r a t i o between
20 a n d 5 0 , a s e x p e c t e d a n d d e s c r i b e d e l s e w h e r e (ref.11 ) . T h e m o s t a l u m i n a - r i c h s a m p l e s (Si/Al=13) e x h i b i t e d a n a c t i v i t y a b o u t t w i c e as h i g h a s e x p e c t e d by e x t r a p o l a t i n g t h e v a l u e s o f t h e m o r e s i l i c a - r i c h
samples. I t
s e e m s t h a t t h i s m i g h t b e c a u s e d by t h e g e n e r a t i o n of n e w a c t i v e sites w i t h e n h a n c e d a c t i v i t y which w e r e a l r e a d y d e s c r i b e d by L a g o e t al ( ref.12 1. B e c a u s e p a i r e d a l u m i n u m f r a m e w o r k a t o m s a r e r e q u i r e d For t h e f o r m a t i o n o f t h e s e s i t e s , i t b e c o m e s r e a s o n a b l e t h a t o n l y t h e s a m p l e s w i t h a l o w Si/AI - r a t i o show this effect.
405
The h i g h e r p a r a - s e l e c t i v i t y
of t h e t e m p l a t e - f r e e s y n t h e s i z e d
ZSM- 5
p r o d u c t s is a s s u m e d t o be c a u s e d by t h e more h o m o g e n e o u s a l u m i n u m d i s t r i b u t i o n a c r o s s t h e c r y s t a l s ( F i g . 6 ) . which r e s u l t s in t h e d e c r e a s e of t h e catalytically active s i t e s a t t h e external s u r f a c e of t h e c r y s t a l s compared t o s a m p l e s syr,chesized w i t h t e m p l a t e . I n c r e a s i n g t h e r e a c t l o n t e m p e r a t u r e f r o m 468 K t o 498 K r e s u l t s in b i g g e r c r y s t a l s ( 4 6 8 K e l - 3 p ; 4 9 8 K
5-10 p ) which
ZY
a
a l s o e x p l a i n s t h e e n h a n c e m e n t of t h e p a r a - s e l e c t i v i t y of t h e s e s a m p l e s .
ALLMINIUH
0
2
1
6
8
Fig.6 Aluminum d i s t r i b u t i o n of and w i t h TPA ( b ) .
l o p
0,/J 0
b, b
,
8, 8
~
?.I , ?.I
,
, 16, 16
,e
2Op
ZSM- S c r y s t a l s s y n t h e s i z e d t e m p l a t e - f r e e ( a )
E n h a n c e m e n t of s h a p e - s e l e c t i v e p r o p e r t i e s The r e m a r k a b l e s h a p e - s e l e c t i v e p r o p e r t i e s of
ZSM- 5 a r e c a u s e d by i t s
unique p o r e s t r u c t u r e . T h e r e f o r e , t h e acid s i t e s a s s o c i a t e d with t h e f r a m e work a l u m i n u m o n t h e e x t e r n a l s u r f a c e of t h e c r y s t a l l i t e s d o n o t c o n t r i b u t e t o t h e s h a p e s e l e c t i v i t y . P r o c e d u r e s which a r e u s e d t o e n h a n c e s h a p e s e l e c t i v i t y s e e k t o maximize t h e r e l a t i o n of i n t e r n a l t o e x t e r n a l a c t i v e s i t e s o r t o in-
c r e a s e t h e d i f f e r e n c e in d i f f u s i o n r a t e s b e t w e e n t h e s m a l l ( e . g . p a r a - i s o m e r ) and t h e bulky ( e . g . o r t h o - o r m e t a - i s o m e r ) m o l e c u l e s . I m p r o v e m e n t o f t h e s h a p e - s e l e c t i v e p r o p e r t i e s t h e n c a n be achieved by : g r o w i n g l a r g e r a n d / o r more s i l i c a - r i c h c r y s t a l s (ref.13.14),
-
- s e l e c t i v e removal of aluminum f r o m t h e e x t e r n a l s u r f a c e (ref.15 1, - c o a t i n g t h e c r y s t a l s with a s i l i c e i o u s s h e l l ,
-
modifying t h e c a t a l y s t w i t h m e t a l s a l t s c o n t a i n i n g p a r t i c u l a r l y P a n d / o r M g ( r e f . 1 6 1. T h i s s t u d y r e p o r t s t h e e f f e c t s of t w o s i m p l e p o s t t r e a t m e n t p r o c e d u r e s
showing a n extraordinary enhancement of shape-selective properties especially of
template-free
synthesized
ZSM- 5 s a m p l e s . F i r s t t h e e f f e c t o f a ZSM- 5 s a m p l e s w a s
( t e m p l a t e - f r e e ) s i l l c a c o a t i n g o n a c t i v e (SI/AI=20)
i n v e s t i g a t e d . The c o a t i n g was e f f e c t e d by u s i n g a l a r g e n u m b e r ( 50%) o f c r y s t a l s a s " s e e d s " and s t a r t i n g a s e c o n d s y n t h e s i s in t h e s i l i c a r i c h e n v i r o n m e n t ,
406
This approach produced adequate b u t n o t excellent r e s u l t s . S e c o n d , c a t a l y s t s a m p l e s w e r e t r e a t e d w i t h a s o d i u m h y d r o x i d e s o l u t i o n f o l l o w e d by c a l c i n a t i o n . T h i s s i m p l e p r o c e d u r e h a d a r e m a r k a b l e i m p a c t o n t h e c a t a l y t i c p r o p e r t i e s . It is w o r t h n o t i n g t h a t c a l c i n a t i o n of t h e s a m p l e s a f t e r t h e y w e r e w a s h e d t o
n e u t r a l i t y , did n o t i mp ro v e t h e s h a p e - s e l e c t i v e p r o p e r t i e s . Whi l e t h e c o a t i n g i n c r e a s e d t h e a m o u n t o f PDEB in t h e t e s t r e a c t i o n m e a s u r e d a t 2 % c o n v e r s i o n f r o m a b o u t 36% t o a b o u t 7 0 %, t h e “ a l k a l i n e c a l c i n e d ” c a t a l y s t s g a v e p r o d u c t s w i t h a y i el d o f g r e a t e r t h a n 99% PDEB. T a b l e 1 s h o w s t h e e f f e c t of c a l c i n a t i o n temperature on t h e catalytic properties of t h e treated samples.
Si/AI = 13 activity selectivity
temp.
( K )
Si/AI = 2 0 activity selectivity
-
_ _.... ._ ... . Si/AI = 4.0 activity selectivity - ._ . . - .-. .. . . . .. ~. __
298
8.1
35
3.9
37
3
70
623 773 923
n.m. 19 n.m.
n.m. 53 n.m.
4..5
4 4.
n .m .
3.8 2.8
92 99
n . ti1 . 2.8
n.m.
n.m.
Although t h e “alkaline calcination” had an enormous influence
011
-1
the shape-
s e l e c t i v e p r o p e r t i e s of th e c a t a l y s t s . t h e a c t i v i t y of m o s t s a m p l e s r e m a i n e d a l m o s t unchanged. However, t h e m o s t alumi na- r i ch s a m p l e (Si/AI=13) s h o w e d o n l y a m o d e r a t e e n h a n c e m e n t of t h e s h a p e s e l e c t i v i t y b u t a n e n n r m o u s i n c r e a s e in a c t i v i t y , w h i c h i s a s s u m e d t o be a s s o c i a t e d w i t h t h e f o r m a t i o n o f ad d i t i r i n al c e n t e r s w i t h e n h a n c e d a c t i v i t y . A c a l c i n a t i o n t e m p e r a t u r e of a t l e a s t 7 7 3 h: was required t o achieve t h e optimal catalytic properties. Exceeding t h e calcin a t i o n t e m p e r a t u r e o f 923 K l e d t o a d e c r e a s e d a c t i v i t y if t h e t r e a t e d s a m p l e s . A c t i v i t y a n d s e l e c t i v i t y o f t h e u n t r e a t e d a n d t r e a t e d s a m p l e s a r e s h o w n in Fig. 7. A l l c a t a l y s t s deactivated slowly,which is c o m m o n f o r Z S M - 5 c a t a l y s t . The high s h a p e selectivity of t h e “alkaline calcined” s a m p l e s remained a l m o s t u n c h a n g e d e v e n w h e n w o r k i n g a t h i g h e r c o n v e r s i o n s (F i g . 8 ) .
407
1
I
W/F=300 g.h/mol
C
80 -
b
m 60 W
0
1 2 3 4 5 time on stream [ h l
1 2 3 4 5 time on stream [ h I
Fig. 7 C a t a l y t i c p r o p e r t i e s of u n t r e a t e d ( a 1 , c o a t e d (b) a n d " a l k a l i n e c a l c i n e d " ( r : ) samples a s a function of time on s t r e a m
0
60
n
2
4 6 8 1 0 conv. [%I
Fig. 8 A m o u n t of P D E B in t h e e t h y l b e n z e n e d i s p r o p o r t i o n a t i o n r e a c t i o n u s i n g u n t r e a t e d (a) c o a t e d ( b ) and "alkaline calcined" (c) s a m p l e s a s a ftincticin of t h e c o n v e r s i o n
408
The "alkaline calcined" s a m p l e s possessed a reduced aluminum c o n t e n t c o m p a r e d t o t h e u n t r e a t e d c a t a l y s t . I t is p r o b a b l e t h a t a l u m i n u m w a s r e m o v e d f r o m t h e o u t e r s u r f a c e o f t h e c r y s t a l s . T h e c r y s t a l s w e r e t o o s m a l l t o p er m i t d e t e c t i o n o f t h i s r e m o v a l by m i c r o p r o b e t e c h n i q u e s . " A l k a l i n e c a l c i n a t i o n " o f ZS M - S c r y s t a l s s y n t h e s i z e d w i t h t e m p l a t e s , s h o w i n g a d e t e c t a b l e z o n i n g . produced only a s l i g h t e n h a n c e m e n t of t h e s h a p e - s e l e c t i v e p r o p e r t i e s h u t a r e m a r k a b l e d e c r e a s e i n a c t i v i t y . T h i s is also a h i n t t h a t t h e a l u m i n u m h a s b e e n s e l e c t i v e l y r e m o v e d f r o m t h e c r y s t a l s u r f a c e . A n o t h e r e x p l a n a t i o n is b a s e d upon t h e a s s u m p t i o n t h a t a p a r t of t h e alu mi num s t a y s a s " e x t r a - f r a m e w o r k aluminum" inside t h e channels, influencing t h e diffusion
of t h e p r o d u c t s .
E x t e n s i v e e x a r n i n a t i o n s . e . g . by m e a n s o f s o l i d - s t a t e NMR a n d I R s p e c t r o s c o p y . a r e n e e d e d f o r a more d e t a i l e d u n d e r s t a n d i n g of t h e o b s e r v e d p h e n i ~ i n e n a . ACKNOWLEDGEMENTS F i n a n c i a l s u p p o r t by BMFT ( 0 3 C 236) a n d D e u t s c h e F o r s c h u n g s g e n i e i n s r : h a f t is g r a t e f u l l y a c k n o w l e d g e d .
REFERENCES
a
1 P.A. J a c o b s a n d J . A . M a r t e n s , S t u d . S u r f . S c i . C a t a l . , (1987) 134 2 C . B e l l u s s i e t a l . , S t u d . S u r f . S c i . C a t a l . . 37 (1988) 37 3 A . T i s s l e r , U . M i i l l e r a n d K.K. U n g e r , N a c h r i c h t e n ails C h e m i e . T e c h n i k u n d L a b o r a t o r i u m ,V o I . 3 6 , N o . 6 , 62 4 - 6 3 0 (19 8 8 4. H . C . Kar g e. Z . S a r b a k , K. H a t a d a . J . W e i t k a m p . P.A. J a c o b s . J . C a t a l . 82 11983) 236 5 C h a m a n i a n d L . S a n d . Z e o l i t e s . 3 (1983) 1SS. 6 G . O o m s , R.A. van S a n t e n , R.A. J a c k s o n a n d C.R.A. C a t l o w . S t u d . S u t - f . S ci . C a t a l . 37 (1988) 317 7 H . A . Ar av a a n d B a r r ie M . L o w e . Z e o l i t e s . 6 (1906) 111 8 H . G . Kar d e. Y. W a d a . 1. W e i t k a m o . S . E r n s t . U. G i r r b a c h a n d H . L B r \ e r S t u d . Suyf.. S c i . c a t a i . (1984.) '101 7 U. G i r r b a c h . T h e s i s , J o h a n n e s G u t e n b e r g - U n i v e r s i t y . Mai n z FRC ( 1007) 1 0 W . W . K a e d i n a . 1 . C a t a l . . 9 s (1985) 512 11 D.H. O l s o n , W . O . H a a g , R-M. Lago. J . C a t a l . , l (19801390 12 R . M. Lag o , W . O . H a a g , R . J . M i k o w s k y , D . H. O l s o n , S . D . H e l l r i n g , K.D. S c h m i t t a n d C . T . K e r r , S t u d . S u r f . S c i . C a t a l . , 28 (19861 677 13 M u l l e r , U., D a n n e r . A , , H o l d e r i c h . W . , U n g e r . K.K.: DE P 37284.51.7(1987) 1 4 , T i s s l e r , A . , M u l l e r . U.,D a n n e r , A . , U n g e r , K .K ., Z . K r i s t . , l 8 2 (1988) 2 5 8 - 2 6 0 15 S . Nai n b a, A . l n a k a a n d T . Y a s h i m a , Z e o l i t e s , 6 (1986) 107 16 C h u . C . C . ( t o M o h i l O i l ) . E.P. 38,116 (1981)
m.
H.G.Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent BuiMers 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THEORETICAL STUDIES OF BRmSTED ACIDITY I N ZEOLITES
R. VETRIVELl, C. R. A. CATLOWI, E. A. COLBOURN2 a n d PI. LESLIE3 IDepartment of Chemistry, University of Keele, Keele, Staffordshire, ST5 5BG, U.K. 2Wilton Materials Research Centre, I.C.I. Plc., Wilton, Middlesborough, Cleveland, TS6 8JE., U.K. 3SERC Daresbury Laboratory, Daresbury, Warrington W.44 4AD, U.K.
ABSTRACT A quantum chemical c l u s t e r (QCC) is t r e a t e d explicitly b y a n a b initio technique, a n d is embedded in a point c h a r g e c l u s t e r (PCC). The l a t t e r i s chosen to r e p r e s e n t t h e long-range electrostatic terms in t h e zeolite lattice. Calculations h a v e been c a r r i e d o u t on a s e r i e s of zeolite c l u s t e r s namely, Zeolite-AILTA), ZShI-Il(?lEL), ZSM-5(NFI), Offretite(OFF), hlordenite(FlOR), Faujasite( FAU) a n d Zeolite-L(LTL). The predicted acidic p r o p e r t i e s of t h e s e zeolites, t h e effect of t h e substitution of silicon by aluminium or boron a n d t h e values of t h e vibrational f r e q u e n c y are r e p o r t e d a n d compared with esperimental data. INTRODUCTION Zeolites a r e c a t a l y s t s f o r numerous hydrocarbon conversion reactions, a n d it is h e l l known that t h e i r catalytic behaviour is d e p e n d e n t o n t h e i r acidic Many techniques have been used to s t u d y t h e i r acidic
p r o p e r t i e s (ref.1). characteristics;
these
temperature-programmed (ref.$), n.m.r.(ref.5)] t e s t reactions (ref.6).
include
t h e adsorption
desorption
(ref.J),
of
basic
spectroscopic
molecules
(ref.2),
techniques
[i.r.
and t h e investigation of t h e catalytic activity f o r certain However, each of t h e methods i s limited ( s e e e.g. ref.5).
In t h e p r e s e n t p a p e r , we t h e r e f o r e explore t h e possibility of developing a unified approach based o n theoretical techniques for t h e evaluation of t h e comparative acidities of different zeolites. blETHODOLOGP The basis of
t h e method i s t h e embedding of a quantum chemical
cluster(QCC) in a n a r r a y of point c h a r g e s as illustrated i n Fig.1.
The point
c h a r g e cluster(PCC) containing nearly 80 atoms i s used to r e p r e s e n t t h e more
410
h
\
1-n-p
I
F i g . 1. The p o i n t charge cluster(PCC) models used f o r various z e o l i t e s . (a)-LTA; (b)-MEL; (c)-MFI; (d)-OFF; (e)-MOR; (f)-FAU; (9)-LTL and (h)- a t y p i c a l quantum chemical cluster(QCC)-[HT207].
411
d i s t a n t atoms in t h e zeolite lattice. The number of atoms a n d t h e geometry of t h e c l u s t e r a r e chosen in s u c h a way t h a t t h e Madelung e n e r g y a n d g r a d i e n t of f o r c e s of a n infinite lattice are simulated by t h e PCC,
The quantum
chemical c l u s t e r containing nearly 10 atoms, which is embedded at t h e c e n t r e of t h e PCC, i s treated by a n a b initio technique.
The positions of t h e atoms
in t h e QCC a n d PCC a r e those reported from crystallographic s t u d i e s as shown in Table 1.
Charges of 2 t and 1- a r e assigned f o r silicon and oxygen atoms
in t h e PCC, in view of
t h e partially ionic n a t u r e of zeolites.
A detailed
explanation of t h e procedure f o r generation of t h e PCC f o r ZSM-5 zeolite i s The same p r o c e d u r e i s used f o r all t h e
given in o u r earlier paper (ref.14).
zeolites mentioned in t h e p r e s e n t work. different
zeolites.
The computer
Fig.1 s h o w s t h e PCC model u s e d for
program
used
here
to
perform
a b iriitio
calculations i s t h e G.4IIESS (General Atomic a n d Molecular Electronic S t r u c t u r e System) package as esteiided b y Guest a n d Kendriclc (ref.15).
The caiculations
r e p o r t e d t h r o u g h o u t t h i s p a p e r were c a r r i e d o u t using t h e SV-3-21G
basis
s e t s (ref.16). The following details of t h e calculations should be noted: The substitution of Si b y e i t h e r A1 or B i s effected only in t h e QCC.
i)
The PCC in all t h e zeolites c o r r e s p o n d s to a completely siliceous framework. ii)
The QCC i s chosen so t h a t t h e b r i d g i n g osygen i s i n t h e l a r g e s t r i n g
opening of the evidence
zeolite pore.
available
substitution.
to
fis
Escept f o r Zeolite A, the
preferential
t h e r e i s no conclusive
for
sites
aluminium/boron
Moreover, t h e s i t e s chosen f o r the s u b s t i t u t i o n of Si b y e i t h e r
A1 or B d o not significantly influence t h e results. iii) The geometry of t h e T2O7 c l u s t e r i s not varied since optimisation of t h e T2O7 c l u s t e r in different zeolites leads to t h e same final configuration in all cases.
Hence t h e geometry of T2O7 c l u s t e r i s as r e p o r t e d b y X-ray
diffraction studies.
However, t h e geometry of t h e h y d r o g e n atom i n t h e HT2O.;
c l u s t e r i s optimised. RESULTS AND DISCUSSION The crystallographic information a n d t h e s i t e s chosen f o r t h e p r e s e n t s t u d y a r e summarised i n Table 1.
Results of t h e calculations carried o u t o n
t h e [Si207l6- c l u s t e r f o r different zeolites are given i n Table 2.
The effect of
s u b s t i t u t i o n of silicon b y aluminium or boron on t h e electronic p r o p e r t i e s i s studied b y replacing one of t h e Siat
by A13+ or B3+.
calculations c a r r i e d o u t on [SiA10717- a n d Tables 3 and 4 respectively.
[SiB07I7-
The r e s u l t s of t h e
c l u s t e r s are g i v e n i n
The Bronsted acidic p r o p e r t i e s of t h e s e zeolitic
412
TABLE 1 The c r y s t a l l o g r a p h i c i n f o r m a t i o n f o r t h e selected z e o l i t e frameworks and the d e t a i l s o f t h e c l u s t e r models. Zeolite
Code Space * group
Zeolite-A ZSM-11 ZSM-5 Offretite Mordenite Faujasite Zeolite-L
Fm3c LTA 141112 MEL MFI Pnma OFF P6m2 MOR Pbcn FAU Fd3m LTL P6/mmm
Ref. Number o f c r y s t a l l o - B r i d g i n g oxygen Number o f graphically chosen i n t h e atoms i n the different sites present study PCC c l u s t e r 7 8 9 10 11 12 13
T I
n
2 7 12 2 6 1 2
3 15 26 6 13 4 6
U
T1 T4 T2 T2 T1
-
T1
-
T -
02 012 013 06 07 01 01
- T2 - T5 - T8 - T2 - T1 - T - T1
*The nemonic code adopted f o l l o w i n g IUPAC recommendations (R.M. Pure & Appl. Chem. , g,1091(1979)).
TABLE 2 The calculated properties of clusters of different zeolites
Zeolite
LTA A
m
MFI OFF
MOR FAU
LTL
Electronic energy (a.u.1
-1763.8911 -1790.2582 -1789.2342 -1775.3437 -1791.4322 -1772.6411 -1775.7055
[Si2O7l6- cluster
total energy (a.u.)
-1119.2947 -1121.5762 -1125.6955 -1121.1837 -1121.4406 -1122.2070 -1120.4051
embedded
in
73(Fig.la) 78(Fig.lb) 82(Fiq.lc) 84iFiG.ldj 78(Fig.le) 80(Fig.lf) 85(Fig.lg) Barrer,
point
P a r t i a l charges calculated by Mdliken population analysis Si
O*
Si
t1.76 t1.89 t1.92 t1.85 t1.92 t1.77 t1.77
-1.11 -1.16 -1.22 -1.16 -1.18 -1.11 -1.13
t1.73 +1.93 t1.96 t1.85 +1.83 t1.77 +1.77
charge
-
*bridging oxygen
frameworks a r e studied by replacing the 02- ion sharing the corners of the two tetrahedra by a n OH- ion. the
calculated
proton
Thus the results of the calculations including
affinity carried
out
on [HSi207I5-,
[HSiB07I6- clusters a r e given in Tables 5, 6 and 7 respectively.
[HSia10716-
and
413 TABLE 3 The c a l c u l a t e d p r o p e r t i e s of [Si.U07]7- c l u s t e r embedded i n p o i n t charge c l u s t e r s of d i f f e r e n t z e o l i t e s . Electronic energ>(a.u.)
Zeolite
P a r t i a l charges c a l c u l a t e d by Plulliken population analysis
Total enargj(a.u. 1
~~~
-1700.8916 -1726.0537 -1725.2639 -1711.5069 -1727.2988 -1708,9524 -1711.8380
LT.4 PEL )IF1
OFF PlOR
FX L'I'L
~
Si
-1071.9m -1C74.1698 -1078.3163 -1073.7651 -1074.0401 -1074.8601 -1073.0107
t1.z
t1.90 t1.96 t1.82 t1.92 t1.77 t1.76
A1**
O*
-1.15 -1.18 -1.23 -1.16 -1.19 -1.12 -1.15
t1.o2('r2) +1.21(T4) +1.21(T=) +1.12(12) t1.07(T1) t1.03(1') tI.Ol(T1)
~~~
*bridging oxygen **site a t vhich S i is replaced by A 1 is given i n bi*acltets
TS3LE 4 The c a l c u l a t e d p r o p e r t i e s of [SiB07l7- c l u s t e r embedded i n p o i n t charge c l u s t e r s of d i f f e r e n t z e o l i t e s .
Zeolite
Electronic energy (a.u. )
~
LT.1
-1358.9229 -1375.4298 -1376.2867 -1363.9171 -1377.1815 -1361.9257 -1363.9584
,TI, ?lFI
OFF MOR FAU LrL
Partial charges c a l c u l a t e d by Mullilren population analysis
Total energy (a.u.)
_
_
-855.6375 -857.9309 -862.0686 -855.5207 -857.7857 -858.5699 -836.7358
_
_
Si
O*
t1.67
-1.02 -1.04 -1.07 -1.02 -1.05 -1.00 -1.02
t1.81
t1.88 t1.75 t1.83 t1.69 t1.69
B*
*
to.91 ( T2 ) +1.09(T1) ' t1.07(T2) t1.02(T2) tI.Ol(T1) t0.96(T) to.97(Ti)
*bridging oxygen **site a t which S i is replaced by B is given i n b r a c k e t s
Effect of A1 and B Substitution I t is not possible to fix t h e positions of h y d r o g e n s from crystallographic studies.
Hoxever, t h e r e a r e speculations, made o n t h e basis of I R s t u d i e s
(ref.4), that t h e protons a r e attached t o oxygen ions in t h e vicinity of A13+, which indeed would be espected because t h e p r o t o n s are p r e s e n t as c h a r g e compensators for t h e Al substitutionals a n d will be a t t r a c t e d b y t h e negative c h a r g e of t h e species.
Hence t h e proton was attached to t h e bridging oxygen
and i t s distance from t h e oxygen atom and i t s a n g l e s with t h e 'T' atoms were optimised to yield a minimum e n e r g y configuration.
I t is i n t e r e s t i n g t o note
414
TABLE 5
The calculated properties of [HSi2O7I5- cluster embedded in point charge clusters of different zeolites. Total Partial charges calculated by Optimi sed Proton 0-H affinity energy Mu1 1 i ken population analysis distance (a.u.) (a.u.) si o* Si H (A)
Zeolite Electronic energy (a.u.) LTA MEL MFI OFF MOR FAU LTL
-1786.4069 -1811.2913 -1811.8302 -1797.4488 -1812.5797 -1793.1469 -1796.0577
-1120.1171 -1122.4240 -1126.4814 -1122.0514 -1122.2891 -1123.0563 -1121.2527
t1.77 t1.89 t1.89 t1.86 t1.93 t1.79 t1.79
-1.03 -1.08 -1.13 -1.07 -1.08 -1.04 -1.05
t1.74 t1.94 t1.95 t1.86 t1.82 t1.79 t1.79
t0.44 t0.51 t0.46 t0.48 t0.50 t0.50 t0.49
1.0328 0.9756 1.0040 1.0032 0.9749 0.9712 0.9676
-0.8224 -0.8478 -0.7859 -0.8677 -0.8485 -0.8493 -0.8476
~~
~
* bridging oxygen TABLE 6
The calculated properties of [HSiA107]6' clusters of different zeolites. Zeolite Electronic ** energy (a.u.) ._
LTA(T2) MEL(T4) MFI (T2) OFF(T2) MOR(T1) FAU(T ) LTL(T, )
-1723.4843 -1747.2020 -1747.9627 -1733.7101 -1748.5562 -1729.5246 -1732.2827
Total Partial charges calculated by Optimised Proton 0-H affinity energy Mu1 1 i ken popul at ion anal ysi s distance (a.u.) (a.u.) si O* A1 H (A)
-1073.0435 -1075.2485 -1079.3291 -1074.8657 -1075.1182 -1075.9278 -1074.1082 - -
*
cluster embedded in point charge
.-
_-
__
t1.79 t1.97 t2.01 t1.91 t2.00 t1.86 t1.85 -
-1.04 -1.10 -1.12 -1.08 -1.10 -1.05 -1.05
t1.08 t1.23 t1.20 t1.17 t1.09 t1.06 t1.05 __
t0.43 t0.47 t0.45 t0.46 t0.47 t0.47 t0.46 -
___
1.0222 0.9697 1.0035 0.9988 0.9697 0.9639 0.9616
-1.0467
- 1.0787 -1.0128 -1.1006 -1.0781 -1.0674 -1.0675
bridging oxygen **site at which Si is replaced by A1 is given in brackets TABLE 7
The calculated properties of [HSiBO7I6' cluster embedded in point charge clusters of different zeolites. ___ -Zeolite Electronic Total Part i a1 charges cal cul ated by Optimised Proton energy Mulliken population analysis ** energy 0-H affinity (a.u.) -distance (a.u.) * (a.u.) Si - 0 B H (A) _.. .-
-__
LTA(T2) MEL(T4) MFI(T2) OFF(T2) MOR(T1) FAU(T ) LTL(T1)
-1379.2589 -1394.5589 -1396.7069 -1383.9055 -1396.3953 -1380.5648 -1382.4181
-856.7228 -859.0367 -863.1140 -858.6593 -858.8932 -859.6638 -857.8305
t1.76 t1.90 t1.94 t1.86 t1.93 t1.80 t1.79
-0.96 -0.96 -0.99 -0.97 -0.97 -0.94 -0.92
t0.88 t1.08 t1.00 t1.00 t0.98 t0.95 t0.96
t0.43 t0.43 t0.42 t0.43 t0.42 t0.44 t0.42
* bridging oxygen **site at which Si is replaced by B is given in brackets
1.0155 0.9694 0.9946 0.9979 0.9702 0.9646 0.9606
-1.0853 -1.1058 - 1.0454 -1.1386 -1.1075 - 1.0939 - 1.0947
415
t h a t t h e minimum e n e r g y position f o r h y d r o g e n in all t h e zeolites i s t h e one in which t h e hydrogen atom points towards
t h e l a r g e s t possible pore i n t h e
zeolite. I t i s well known (ref.1) t h a t t h e s e p r o t o n s a r e t h e source of Bronsted For example, t h e correlation between t h e ammonia adsorption capacity
acidity.
and t h e absorbance of t h e band in t h e infra-red region range 3400-3800 cm-1 corresponding to hydrosyl g r o u p vibration h a s been r e p o r t e d (ref.18).
The
of t h e proton magnetic resonance (ref.5) i s also known to
chemical shift (C,)
increase when t h e acid s t r e n g t h of t h e zeolite increases.
The partial c h a r g e s
o n hydrogen calculated f o r t h e different c l u s t e r s a r e also shown in Tables 5-7.
The partial c h a r g e on hj-drogen i s found t o be d e p e n d e n t o n t h e
optimised 0-H distance, t h e dimension of t h e pore in t h e zeolite a n d T-0-T angle.
There
hydrogen.
is a
When
repulsive
interaction
Si4+ i s replaced
between
by A13+
the
or B3+,
'T'
this
atom
and
repulsive
the force
d e c r e a s e s and t h e r e i s increase i n t h e proton affinity (which i s calculated as the difference between t h e values of t h e total e n e r g y of t h e HT2O7 a n d T2O7 c l u s t e r s ) , and a decrease in t h e partial c h a r g e on t h e proton.
The values of
proton affinity for different c l u s t e r s a r e shown i n Tables 5-7.
However it
should be noted t h a t HSi207 i s only a hypothetical c a s e as t h e r e i s n o need f o r the presence of a proton in t h e all siliceous framework.
I t should also
or B3+ s u b s t i t u t i o n s c a u s e t h e introduction of t h e protons
be noted t h a t
a s c h a r g e compensators.
On comparing boron s u b s t i t u t i o n with aluminium
substitution, we find t h a t t h e proton affinity increases with a c o r r e s p o n d i n g decrease in Bronsted acidity which c o r r e l a t e s v e r y well with experimental r e p o r t s based on temperature-programmed desorption s t u d y ammonia and t h e s t u d y of activity of acid-catalysed reactions
(ref.21) of (ref.17) f o r
ZSM-5 zeolite.
F i g 2 shows t h a t t h e proton affinity is i n t h e o r d e r B-zeolites
>
However, t h e differences i n t h e calculated values of proton
Al-zeolites.
affinity f o r different zeolites with same IT' atom (say for 'Al'
substituted
c l u s t e r s shown i n Table 6 ) a r e much smaller. Acidity Correlations The partial c h a r g e s calculated on t h e b r i d g i n g oxygen atoms are a n indication of t h e i r capacity to bind additional protons. charge on
the
bridging
oxygen f o r
d i f f e r e n t zeolite
We compared t h e c l u s t e r s with
their
reported acidity d a t a where available.
The acidities of t h e proton forms of
different
been
zeolites
(except
LTA) have
investigated
by
carrying
out
catalytic t e s t reactions (ref.6) a n d by t h e TPD s t u d y of ammonia (refs. 18-21). These r e s u l t s are summarised in Table 8.
The calculated partial c h a r g e s o n
t h e bridging oxygen f o r different zeolites are plotted a g a i n s t t h e i r a c t i v i t y f o r
416
TABLE 8 R e s u l t s of a c i d i t y esperirnents c a r r i e d o u t on t h e H-exchanged forms of d i f f e r e n t z e o l i t e s . Activity for i sopropanol decomposition (ref. 6) (mi05 sec-i)
Zeolites
FAU
5
LTL OFF MOR PIEL MFI
15
Temperature programmed d e s o r p t i o n of ammonia Ref.
Peak Maxinm temperature (K)
18 18 19 20 21 21
586 618 673 68 1 673 723
21 28 32 39
isopropanol d e h y d r a t i o n (Fig.3) and t h e Tma, in t h e TPD spectrum of ammonia (Fig.4). A linear correlation i s found
to exist.
This s u g g e s t s t h a t
it i s
possible to make u s e f u l predictions of t h e relative acidities of zeolites u s i n g a simple, theoretically derived parameter. The Madelung e n e r g y calculated f o r t h e b r i d g i n g oxygen in 2S?I-5 h a s a l a r g e value
10 eV h i g h e r t h a n f o r t h e o t h e r
(
c o r r e s p o n d e n c e with
its
s t r o n g acidity.
For
all
s t r u c t u r e s examined) i n the
other
zeolites,
the
Madelung e n e r g y o n t h e b r i d g i n g oxygens ha\;e almost t h e same value. The partial c h a r g e o n t h e
bridging
oxygen is f o u n d to b e d e p e n d e n t o n i t s
distance from t h e a d j a c e n t 'TI atom.
When t h e T-0 bond length d e c r e a s e s ,
t h e r e is more t r a n s f e r of electron d e n s i t y from t h e 'T' atom to t h e oxygen. Since t h e oxygen i s a t t a c h e d t o two 'T' atoms t h e T-0 distance i s taken a s t h e a v e r a g e distance of oxygen from both t h e 'T' atoms.
[(Tl-O) t
Hence T-0
(0-T2)]/2, where T I and T2 a r e t h e two t e t r a h e d r a l cations bonded t o the oxygen atom.
The linear relationship e s i s t i n g between t h e T-0 distance a n d
t h e formal c h a r g e o n t h e oxygen atom
is e v i d e n t from Fig.5.
The
two
important r e s u l t s t h a t emerge from t h e a b o v e o b s e r v a t i o n s of t h e calculations are: i ) t h e geometry of t h e local s t r u c t u r e , more specifically t h e geometry of each t e t r a h e d r a l g r o u p in t h e zeolites, influences t h e i r acid s t r e n g t h . ii) when t h e heteroatoms s u c h as A13+ or B3+ replace t h e Silt ions, t h e local s t r u c t u r e will r e a r r a n g e , which i n t u r n will a l t e r t h e acidic properties. These e f f e c t s cannot, however, be investigated i n t h e p r e s e n t s t u d y as we were not a b l e t o s t u d y t h e variation of t h e geometry of o u r QCC o n substitution
of
heteroatoms.
i n v e s t i g a t e t h i s effect.
Future
studies
with
larger
clusters
will
417
I
-1.081
Y
a
3
-1.18 -120
0 -'5 7.0
10
t a
-
2
5
I
g
u
g
u
J
g
4n
6
z
20
30
50
40
TURNOVER FREOUENCY x 10' (s-9
ZEOLITES
Fig. 3. The correlation between the calculated partial charge on the bridging oxygen and the experimental acidity measured as the activity for the isopropanol decomposition for different zeolites.
Fig. 2. The proton affinity values o f zeolites calculated for different cl uster model s .
A-.
-1.04
m -1.08 0
9 -1.12
a
m z
-1.16
0 -1.16
0
w -120
P 4
I
u
-I
n
I 0 -I
-126
560 4
=
-124
-120-
-124.
S
600
640 680 720 TEMPERATURE (K)
c U
760
Fig. 4. The correlation between the calculated partial charge on the bridging oxygen and the experimental acidity measured as the T in the TPD spectrum of ammonia f!Px different zeolites.
Q*
1.56
if
1.60
1.68
1.64
T - 0 DISTANCE
6)
Fig.5. The variation of the ca culated partial charge on the bridging oxygen with T-O(where T-0 = [(TI-O)t( 2-0)1/2 distance.
Legend to Figs. 2-5: 0 - 0 - 0-
S i2O7 c lu s t er,o-0-0-
A-A-A-
SiB07 cluster,
S i A lo7 c luster,
a o i - FAU,
W A - LTL, @@A-OFF,
.OA-MOR,
H@&-MEL, EOA-MFI.
418 Vibrational F r e q u e n c y of t h e Hydrosyl Group Having optimised t h e 0-H d i s t a n c e i t i s possible t o calculate t h e force constant
for
0-H
the
bond
and
hence
its
vibrational
frequency.
The
s t r e t c h i n g vibration of t h e 0-H g r o u p i n zeolites g i v e s r i s e to a b s o r p t i o n in t h e infra-red region at approsimately 3400 t o 3800cm-1 d e p e n d i n g o n w h e t h e r t h e 0-H
bond i s in a silanol g r o u p or i n t h e b r i d g i n g s i t e a n d on t h e
dimension of t h e pore in which t h e hydroxyl g r o u p i s situated.
All zeolites
have more t h a n a single t y p e of hydroxyl g r o u p , a n d t h e o b s e r v e d spectrum i s a n a v e r a g e over t h e various s i t e s with a BWHII (Band Width a t Half Masima) of 200cm-1.
Though t h e deconvolution of t h e s p e c t r a h a s been attempted, t h e
p r e s e n c e of a t least 16 possible cationic sites, e v e n in Faujasite zeolite (ref.22) which contains only one crystallographic
'T'
site, p o s e s
However, t h e vibrational f r e q u e n c y of t h e hydroxyl
g r e a t difficulties.
g r o u p in
possible pore of these zeolites h a s been r e p o r t e d (ref.4).
the largest
These values are
compared with t h e calculated values of t h e vibrational f r e q e n c y f o r different c l u s t e r s shown in Table 9. to experiment.
The calculated v a l u e s are always higher compared
Despite t h e f a c t t h a t t h e vibrational f r e q u e n c y of t h e 0-H
g r o u p a t a single s i t e i s calculated a n d t h e e f f e c t of Si/AI ratio i s neglected in t h e p r e s e n t calculation, t h e calculated values a r e within t h e r a n g e expected from t h e esperimental s t u d i e s , when we note normally have a width of
200 cm-l.
that
t h e esperimental p e a k s
F u r t h e r s t u d i e s , which esamine all
possible sites, are now needed.
TABLE 9 The experimental and t h e c a l c u l a t e d v i b r a t i o n a l frequency of t h e hydros3-1 group i n t h e l a r g e s t pore of d i f f e r e n t z e o l i t e s .
ZEOLITES
V i b r a t i o n a l frequency of 0-H bond ( c m - l ) Experimental reports ( r e f . 4 ) for t h e hydrogen zeolites
LTA
MEL
MFI
OFF MOR FAU LTL
3605 3601 3618 3610 3659 3630
Calculated Value for mi207 cluster
Calculated Value f o r HSi.UO7 cluster
3028 3853 3594 3669 3778 3782 3778
3584 3017 3732 3602 3926 3843 4032
Calculated Value for HSiBO7 cluster 3509 3884 3652 3639 3926 3814 4026
419
SUMMARY The work h a s shown the value of embedded quantum mechanical c l u s t e r calculations in s t u d y i n g t h e acidity of zeolites.
Our r e s u l t s emphasise t h e
importance of the small differences in t h e bond l e n g t h s a n d bond a n g l e s which exist in different zeolites as well as i n different s i t e s of single zeolite.
The
calculated values of t h e proton affinity are s t r o n g l y affected by heteroatom substitution.
bloreover,
the
calculated by t h e llullilten linear
correlation
with
esperiniental techniques.
the
electronic
charge
on
the
bridging
oxygen,
population analysis f o r different zeolites has a acidity
of
different
zeolites
determined
by
Greater difficulty is encountered i n t h e calculation
of t h e vibrational frequency of t h e h y d r o s y l group.
The techniques can be
refined with more sophisticated calculations u s i n g l a r g e r c l u s t e r s . Acknowledgements
We a r e g r a t e f u l to I.C.I. Research Scheme.
Plc. f o r s u p p o r t i n g t h i s work via t h e Joint
We also t h a n k Dr. R. A. Jackson f o r helpful discussions.
REFERENCES 1. J. Dwyer, S t u d . Surf. Sci. Catal., 37(1988) 333-354. 2. H. Lechert, i n F. Riberio, A. E. Rodriques, L. D. Rollmann and C. Naccache (Ed.), Zeolites: Science a n d Technology, l l a r t i n u s Nijhoff Publishers, The Hague, 1984, p.151. 3. I<. Hashimoto, T. Elasuda a n d T. blori, S t u d . Surf. Sci. Catal., 28(1986) 503-510. 4. P. A. Jacobs and W. J. Mortier, Zeolites, 2(1982) 226-230. 5. H. Pfeifer, D. Freude and F1. Hunger. Zeolites, 5(1985) 274-286. 6. P. A. Jacobs, Catal Rev.-Sci. Eng., 24(1982) 415-440. 7. J. J. Pluth a n d J. 1'. Smith, J. Am. Chem. SOC., 102(1980) 4704-4708. 8. G. 'I'.Kolrotailo, P. Chu and S. L. Lawton, Nature, 275(1978) 119-120. 9. D. H. Olson, G. T. Kokatailo, S. L. Lawton and W. M. Meier, J. Phys. Chem., 85(1981) 2238-2243. 10 J. A. Gard a n d J. M. Tait, Acta Cryst., B28(1972) 825-834. 11. J. L. Schlenker, J. J. Pluth and J. V. Smith, Mat. R e s . Bull., 13(1978) 169-174. 12. W. J. Mortier, €1. J. Bosmans and J. B. Uytterhoeven, J. Phys. Chem., 76(1972) 650-656. 13. P. A. Wright, J. M. Thomas, A. I(. Cheetham a n d A. IL Nowak, Nature, 318(1985) 611-614. 14. R. Vetrivel, C. H. A. Catlow a n d E. A. Colbourn, Stud. Surf. Sci. Catal., 37(1988) 309-315. 15. M. F. Guest a n d J. Kendrick, in An i n t r o d u c t o r y g u i d e to GXMESS, University of Manchester Computer Centre, Manchester, 1986. 16. M. S. Gordon, J. S. Binkley, J. A. Pople, W. J. Pietro a n d W. J. Hehre, J. Am. Chem. SOC., 104(1982) 2797-2803. 17. M. G. Howden, Zeolites, 5(1985) 334-338. 18. B. M. Lok, B. K. Marcus a n d C. L. Angell, Zeolites, 6(1986) 185-194. 19. C. Mirodates and D. Barthomeuf, J. Catal., 57(1979) 136-142. 20. C. V. Hidalgo, M. Kato, T. Hattori, M. Niwa a n d Y. Murakami, Zeolites, 4(1984) 175-180. 21. T. Somasundaram, P. Ganguly and C. N. R. Rao, Zeolites, 7(1987), 404-407. 22. W. J. Mortier, Compilation of E x t r a framework s i t e s in zeolites, Butterworth Scientific Ltd., U.K. 1982, p.20.
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H.G. Karge, J . Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
NMR AND IR STUDIES OF ZEOLITES OF THE ERIONITE
TYPE
F . ROESSNERI, K.-H. STEINBERG1, D. FREUDE2, M. HUNGER2 and H. PFEIFER~
lSektion Chemie der Karl-Marx-Universitaet Leipzig, 7010 (G.D.R.) 2Sektion Physik der Karl-Marx-Universitaet Leipzig, 7010 (G.D.R.)
Leipzig, Talstr.35, Leipzig, Linnestr.5,
ABSTRACT Infrared spectroscopy (IR) and proton, aluminium and silicon magic-angle-spinning nuclear magnetic resonance spectroscopy ( L H , 27A1 and 29Si MAS NMR) yield information about the location and concentration of Broensted acid sites, the distribution of aluminium in the framework and the concentration of non-framework species in erionite. Most of the bridging O H groups are formed on the O(1) oxygens located in the double-six-ring between two cancrinite cages. Other OH groups are in the large cavity at O(5) or O(6) oxygens as well as in the cancrinite cages. The differences of the Si/Al-ratios determined by different methods were explained i n term of thermal decomposition of a parasite phase.
INTRODUCTION Among the zeolite catalysts used o n a larger scale in industry, erionite is unique with respect to the smallest available pore diameter (elliptical pore opening: 0.30 x 0 . 5 2 nm). A bifunctional Pt/H-erionite is applied for formselective reforming where n-alkanes are selectively hydrocracked in the feed. A study on the nature and location of hydroxyl groups influenced by the lattice structure m a y contribute to a better understanding of this process.
Figure 1 shows a model of the erionite structure. The cancrinite cages are linked to hexagonal prisms and single six-membered rings. In contrast to faujasite there are six crystallographically different oxygens in the erionite structure: O(1) O(2)
-
connecting the six-rings of the hexagonal prism, shared among the four-ring of the hexagonal prism and the six-ring of the cancrinite cage, O(3) - shared among thc four-ring of the hexagonal prism and the four-ring of the cancrinite cage, O(4) - shared among the four- and six-rings of the cancrinite cage I
422
Fig, 1 O(5)
Model of the erionite structure
-
oxygen of the single six-ring connecting two cancrinite cages, and O ( 6 ) - shared among the single six-ring and the four-ring of the cancrinite cage. Information about the Si/A1 ratio and the aluminium distribution o n nonequivalent tetrahedral T-sites is available from 29.31 M A S N M R spectra. Based o n such results Lillerud (ref. 1) made the assumption of a prefered occupation of the T2-sites ( s e e F i g . 1) by aluminium. For the best f i t between the measured and calculated spectra he found 0 . 1 7 Al/T1 in comparision t o 0.35... 0.45 Al/T2. Taking into account that for erionite T1/T2=2 it c a n be expeated that nearly the same number of bridging OH groups must b e formed i n the environment of aluminium at T 1 and T2 positions. EXPERIMENTAL The ammonium-exchanged erionite w a s prepared by repeated i o n exchange of synthetic NaK-erionite w i t h NH4N03 solution at 363 K. The parent zeolite (Si/A1-3, by the chemical analysis) w a s obtained from VEB Chemiekombinat Bitterfeld, G.D.R. NMR measurements were performed o n a Bruker MSL-300 spectrometer at resonance i r e TABLE 1 Si/Al ratios determined from 27Al and 29Si M A S N M R spectra and concentration of cations determined by chemical analysis Sample
NaK-erionite 0.77NHq-erionite 0.91NH4-erionite 0.93NHq-erionite
Si/Al determined by 2 7 1~ 29si MAS N M R 3.0 3.6 3.6 4.5
2.0 2.3
2.4 2.4
cations per unit cell NHq+
K+
0.0 6.9 8.2 8.4
4.9 1.0 0.8 0.6
Na+ 4.1 0.2 0.04 0.02
423
quencies of 300, 78 and 5 9 . 6 MHz for l H , 27Al and 29Si NMR, respectively. The experimental error of intensities w a s 1 0 X . F o r HNMR measurements shallow-bed activation conditions were used in a glass tube o f 5 . 5 mm diameter containing a 1 0 mm layer of zeolite. The temperature was increased w i t h a rate o f 10 K/h. After maintaining the sam les for 24 h at the final temperature of 670 K they were sealed. 2fA1 and 29Si MAS NMR measurements were generally carried out o n rehydrated samples. The infrared spectra were recorded on a UR-20 spectrometer (VEB Carl-Zeiss-Jena) . The samples (thickness: 5-12 mg/cm2) were a l s o activated in-situ at 670 K and 10-1 P a for 3 0 min. T o increase the transmission all samples were deuterated by repeated treatment with D20 at 373 K .
RESULTS AND DISCUSSION The synthesis of erionite is normally accompanied by the f o r mation of other aluminosilicates (ref. 2). X-ray studies o n the parent NaK-erionite have shown a contribution of zeolite P (ref. 3). Since the H-form of zeolite P is thermally unstable, a comparision of the 27Al NMR spectra of the fresh and 720 K calcined samples must indicate intensity changes of the line at 60 ppm assigned to tetrahedrally coordinated framework aluminium. T h i s assumption is supported by the results shown in Table 2 (see columns 5 and 6). The Si/Al ratio of NaK-erionite before and after calcination remains constant and corresponds to the value obtained by wet chemical analysis. However, the calcined ammonium-exchanged erionites lost approximately 30 % of their framework aluminium. Unlike NMR spectra, the framework vibration bands observed by infrared spectroscopy were not shifted, i.e. the Si/A1 ratio o f erionite was not changed. Both results c a n only b e explained in terms of thermal decomposition of the zeolite P. T h e calculated amounts of zeolite P agree with the calculations based o n NH3t . p . d . and H/D-isotopic exchange (ref. 3). TABLE 2 Concentrations of different species /in lOl9 per gramme/ in the erionite calculated from I H and 27Al M A S NMR SiOH or AlOH Chem. shift /ppm/
re s idua 1 SiOHAl
1.9-2.6
3.9
NH4+
framework A 1 before/after calcination
6.7
601
Zeolite NaK-erionite 0.37 H-erionite 0.77 H-erionite 0 . 9 1 H-erionite 0 . 9 3 H-erionite 0 . 9 3 H-erionite2 IChemical shift of z ' ( A l 2Calcined at 720 K .
14
0
18
54 95 100 105 95
25 23 28 52
M A S NMR.
0 0
0 17 56 5
246
234 214 214 178
246 205 154 154 123
424
In zeolite structures, where each s i l i c o n has four nearestneighbour silicon or aluminium a t o m s , the 29Si NMR chemical shift falls into five ranges w h i c h c a n be correlated with the n u m b e r of aluminium neighbours surrounding a g i v e n silicon (ref. 4): Si(4A1): -80 to 89 ppm -88 to Si(3A1): 97 ppm Si(2A1): -94 to -103 ppm -98 to -107 ppm Si(lA1): Si(OA1): -103 to -114 ppm The ion-exchanged erionites activated at 720 K for 2 h s h o w a complex 29Si M A S spectra (Fig. 2) for different reasons: (i) superposition of signals from the erionite phase and phases of crystalline impurities mainly zeolite P , ( i i ) different lines f o r different number n (n=O 4) of aluminium atoms i n the first s i l i c o n ’ s coordination sphere Si(nAl), and ( i i i ) different lines for Ti and T 2 , i . e . for Si(nAlT1) and S i ( n A 1 ~ 2 ) (ref. 5). Unfortunately, for erionite the effect of the site inequivalence (5.3 ppm) is almost exactly the same as the effect of aluminium in the first coordination sphere (5.5 ppm) (ref. 5). U s i n g a line f i t , Lillerud (ref. 1) demonstrated a favourite occupation of T2 positions f o r aluminium atoms i n erionite. Nevertheless, the splitting procedure dividing the signal into two (or three) parts is very problematic. Our experiments prove the result of Lillerud (ref. 1) that aluminium atoms oocupy preferably T2 positions in NaK-erionite. I n case of H-erionite n o reliable information c a n be derived w h e n we take into account a 10% error in measurement. T h e difference between the spectra of NaK- and H- erionite c a n be explained by thermal decomposition of zeolite P w h i c h contributes a broad unresolved 29Si MAS NMR signal.
-
...
. . .
NmK-mrionlt*
arlonlte
-
I
I
I
-80
-100
-120
S,,,I/ppm
F i g . 2. 29Si MAS NMR spectra of NaK- and 0.91 11-erionite.
425
Information o n the nature and location of hydroxyl groups is available from infrared spectroscopy. In the spectra of a deuterated 0.91 H-erionite activated at 670 K, bands r e m a i n at 2 6 4 0 , 2670, 2 7 1 0 and 2760 cm-l (Fig. 3a). The band at 2760 cm-l, a l s o observed for ion-exchanged Y zeolites and silica. is assigned to S e m i n a l hydroxyl groups located o n the external surfaoe or o n lattice defects. W h e n the activated erionite is exposed to ammonia at 130 P a and room temperature (Fig. 3b), the intensity of the band at 2670 om-I decreases, the shoulder at 2640 cm-l vanishes completely, while the bands at 2 7 1 0 and 2760 cm-1 remain as i n the unloaded Sample. To confirm the location of the hydroxyl groups, a H-erionite caloined in air was ion-exchanged w i t h 0 . 1 M CsC1-solution at room temperature. Comparing the spectra of H- and CsH-erionite deuterated (Fig. 3 a and 3 c ) the shoulder at 2640
VIUn.’
Fig. 3. OD stretching vibration spectra of a 0 . 9 3 H-erionite activated at 670 K (a) and exposed to 1 3 0 P a NH3 (b). and of a CsH-erionite activated at 670 K (c).
cm-l is completely eliminated and the intensity of the band at 2670 cm-I i s drastically reduced. It is known that the Cs+ ion is too large (d-0.338 nm) to enter the hexagonal prism and cancrinite cages through the six-ring nindows (d=0.26 nm). Taking into account these results w e c a n conclude that the bands at 2640 and 2670 cm-l represent acidic bridging OH groups located in the large cavity (gmelenite cage). T h e similarity i n structure between erionite and Y zeolite allows the assignment of the most intensive band at 2670 am-l to O(1)H groups. Hence the band at 2640 cm-1 represents perturbed acidic O H groups vibrating in 6-membered rings /compare ref. 6 /, i.e. they were formed, perhaps, o n O(5) and/or O ( 6 ) oxygens i n the large cavity. Furthermore, the 2 7 1 0 cm-l band should be assigned to hydroxyl groups i n the cancrinite cage (0(2)H, O(3)H and/or O(4)H groups) inaccessible f o r large cations or hydrocarbons (ref. 7). The I H MAS NMR spectra of ammonium-exchanged erionites (Fig. 4 ) contain information about both the nature and the concentration or hydroxyl species (Table 1). In agreement with earlier studies o n zeolites H-Y (ref. 8 , 9 ) , mordenite and ZSM-6 ( r e f . lo), the following assignment is made: the line at 1.9 ppm is caused by nonacidic SiOH groups at the external surface or at lattice defects: the line at 2.6 ppm represents hydroxyl groups associated w i t h non-framework aluminium; the line at 3 . 9 ppm is ascribed to acidic bridging OH groups and the line at 6.7 ppm is due t o residual
426
amounts of ammonium ions. The amount of S i O H and A l O H groups (Table 2, column 2) is a measure of the lattice defects formed by deoomposition of the zeolite P and by dealumination. The intensity of the line at 3.9 ppm c
i
)i
i
i i
iaH/ppm
Fig. 4. 1 H MAS N M R spectra of H-erionite activated at 670 K w i t h a n exohange degree of 0 (a), 37 (b), 7'7 (c), 9 1 (d) and 93 % (0). Spectrum ( f )represents a 0.93 H-erionite activated at 720 K ( * spinning side-bands). depends o n the degree of ion exchange (Table 2). However, the intensity of the line was not changed i n the range of ion exchange between 77 and 93 % , i.e. the OH species formed at higher values of ion exchange have a lower concentration. Indeed, the intensity of the band at 2670 cm-l in the IR spectra assigned to aoidic O(1)H groups was not drastically increased for ammonium exchange higher than 77 X . I n this range the intensity of the band at 2 7 1 0 cm-1 due to O H groups located i n the cancrinite cage increases (ref. 3). The amount of bridging O H groups of the calcined erionite determined by l H M A S NMR is more than 20 % lower than the amount of lattice aluminium determined by 27Al M A S NMR. Besides the residual ammonium, the existence of non-framework aluminium could cause this difference. Furthermore. the acidity of H-erionite is comparable with zeolite Y , taking into consideration the chemical shift of the unperturbed bridging O H groups at 3.9 and 4.2 ppm ( r e f . lo), respeotively. However. the relatively large amount of residual ammonium ion in H-erionite activated i n vacuum at 670 K indicates the presence o f strong acidic sites a s shown by t.p.d. of ammonia (ref. 3). CONCLUSION
(i)
During the synthesis of erionite an parasite phase (zeolite was formed. Its thermal decomposition causes changes i n the 29Si MAS NMR spectra. Different Si/A1 ratios determined by 29Si, 27Al M A S N M R and chemical analysis c a n be explained in terms of thermal de-
P) (ii)
427
composition of the zeolite P and erionite. (iii) Three different bridged hydroxyl groups are detected upon
deammonization of NH4-erionite. Acidic OH groups located in the large cavity are preferentially formed at ion exchange degrees up to 77 % . (v) Potassium ions located in the cancrinite cages are only substituted by multiple ammonium-exchange and intermediate calcination. Furthermore, stronger acidic sites are formed as indicated by the presence of ammonium ions in the erionite activated 670 K. (vi) The existence of OH groups associated with non-framework aluminium was shown. (iv)
REFERENCES 1 2 3
K.P. Lillerud. ZEOLITES, 7 (1987) 14.
D . W . Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. A. Kogelbauer. J . Lercher, K.-H. Steinberg, F. Roessner. A. Soellner and R.V. Dmitriev, ZEOLITES, (in press). 4 C.A. Fyfe, J . M . Thomas and G . C . Gobbi, Anger. Chem.. 95 (1983) 257.
C.A. Fyfe, G . C . G0bbi.G.J. Kennedy, J.D. Graham, R.S. Ozubko, W.J. Murphy, A. Bothner-By, J . Dadok and A.S. Chesnik, ZEOLITES, 5 (1985) 179. 6 P . A. Jacobs and W.J. Mortier, ZEOLITES, 2(1982) 226. 7 F. Roessner. K.-H. Steinberg and S . Rechenburg, ZEOLITES, 7 (1987) 488. 8 H. Pfeifer, D. Freude and M. Hunger, ZEOLITES, 5 (1985) 274. 9 U. Lohse. E. Loeffler. M. Hunger, J . Stoecker and V.Patzelova, ZEOLITES, 7 (1987) 11. 10 D. Freude, M. Hunger and H. Pfeifer, Z. phys. Chem. (NF), 5
152 (1987) 171.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
S P E C I F I C PLATINUM PARTICLES PROPERTIES I N B A S I C ZEOLITES
A. de MALLMANN and D. BARTHOMEUF
L a b o r a t o i r e de R e a c t i v i t e de S u r f a c e e t S t r u c t u r e , UA 1106, CNRS, U n i v e r s i t e P A R I S V I , 4 Place Jussieu, 75252 P A R I S Cedex 05, France.
ABSTRACT The i n f r a r e d wavenumber o f l i n e a r CO adsorbed on P t - f a u j a s i t e s decreases i n t h e o r d e r P t H Y > PtHNaY > PtNaY > PtNaX as t h e z e o l i t e a c i d i t y decreases and t h e b a s i c i t y i n c r e a s e s . New bands a t 1960-2000 cm-1 assigned t o l i n e a r CO a r e formed which a l s o p a r a l l e l t h e z e o l i t e b a s i c i t y . The r e s u l t s suggest a s t r o n g P t / z e o l i t e i n t e r a c t i o n i n v o l v i n g t h e n e g a t i v e l y charged framework oxygen.
INTRODUCTION A l a r g e amount o f work
d e s c r i b e s t h e a d s o r p t i o n o f CO on P t - s u p p o r t e d c a t a -
l y s t s . A major p o i n t i s t o understand whether o r n o t t h e CO bond i s changed upon a d s o r p t i o n on P t ( 1 - 3 ) . I f so,CO c o u l d be used as a probe t o check anymodi f i c a t i o n i n t h e e l e c t r o n i c s t a t e o f P t due t o metal s u p p o r t i n t e r a c t i o n ( 2 - 3 ) . According t o t h e b a s i c o r a c i d i c c h a r a c t e r o f t h e support,one may e x p e c t a d i f f e r e n t i n t e r a c t i o n , w h i c h may be r e f l e c t e d i n t h e CO wavenumber. Upon ads o r p t i o n o f CO i n P t - L z e o l i t e s
s a t u r a t e d w i t h a l k a l i n e c a t i o n s new CO bands
were observed i n a range below 2000 cm-' i.e. a t wavenumbers l o w e r t h a n usual f o r lin e a r l y adsorbed CO and h i g h e r t h a n f o r b r i d g e d CO(4). They were assigned t o P t - C O species formed on b a s i c z e o l i t e s . The p r e s e n t paper r e p o r t s a d e t a i l e d s t u d y o f CO adsot-ptiuti or1 P t - s u p p o r t e d f a u j a s i t e s which c o v e r a range o f a c i d i t y and bas i c i t y . I t w i l l show t h e dependence o f t h e m e t a l / s u p p o r t and P t / C O i n t e r a c t i n n z on t h e s u p p o r t p r o p e r t i e s , i n t h e same z e o l i t e s t r u c t u r e . EXPERIMENT Ma t e r i a1 s Samples o f HY, NaY and NaX p r o v i d e d by Union Carbide were used
. Partial
exchange o f NaY by ammonium a c e t a t e y i e l d e d , a f t e r h e a t i n g , d HNaY sample. P l a t i num was i n t r o d u c e d as i n ( 5 ) by c a t i o n exchange u s i n g a 0.01M s o l u t i o n o f Pt(NH3)4C12, H20 a t 363 K. A f t e r f i l t r a t i o n and washing,the samples were d r i e d a t 353 K o v e r n i g h t . The u n i t c e l l formulae o f t h e m a t e r i a l s were Pt3.1Na5.8(NH4)44 Y, Pt3Na20( NH4)30Y, P t 3 . 2Na43( NH4)6. 6Y and Pt3.4Na70( NH4)9X. T h i s corresponds t o a p p r o x i m a t e l y 4.5 w t % P t . The t r e a t m e n t b e f o r e use c o n s i s t e d o f an o x i d a t i o n t o decompose t h e complex under an oxygen f l o w (30 l / h ) w h i l e t h e temperaturewas and r e d u c t i o n r a i s e d up t o 573 K a t 15 Wh. A f t e r e v a c u a t i o n a t 573 K f o r 3 h
430
.
Upon t h e samples wereevacuated a t 573 K f o r 3h w i t h H2 a t 623 K f o r 15 h , 2+ r e d u c t i o n each P t i o n should have generated two protons which increased t h e number o f protons on each samples by around 6 per u n i t c e l l . A f t e r t h e r e d u c t i o n t h e m a t e r i a l s were r e f e r r e d t o as P t H Y , PtHNaY, PtNaY, PtNaX. The p l a t i n u m p a r t i c l e s i z e was evaluated by e l e c t r o n microscopy. For a l l t h e samples t h e s i z e s were r a t h e r uniform, around 1.2 t 0.2 nm i n diameter b e f o r e o r a f t e r CO adsorption. CO a d s o r p t i o n s t u d i e s
CO a d s o r p t i o n was c a r r i e d o u t on s e l f - s u p p o r t e d wafers (about 15 t o 20 mg)
t r e a t e d i n s i t u as described above.CO was adsorbed a t room temperature a t var i o u s pressures which
determine t h e number, nC0,of CO molecules adsorbed p e r
u n i t c e l l . The r a t i o s nCO/nPt can be c a l c u l a t e d from t h e number nPt o f P t atoms per u n i t c e l l . I n o r d e r t o achieve a good d i s p e r s i o n o f CO on a l l t h e metal p a r t i c l e s t h e waf e r w a s h e a t e d i n t h e presence o f CO f o r 15 h
i n t h e c l o s e d c e l l (2,3). The
spectrawere recorded a t t h a t p o i n t o r a f t e r an evacuation f o r 0.5 h
a t 448
K.
I s o l a t e d CO molecules a r e expected t o occur i n those experimental c o n d i t i o n s (2,3). The i n f r a r e d measurements were performed w i t h a spectrophotometer P e r k i n E l mer 580 B equipped w i t h a Data S t a t i o n . The s p e c t r a l r e s o l u t i o n was b e t t e r than 1 cm-l i n t h e s p e c t r a l range considered. RESULTS
-
Ac id i t y bas ic it y The a c i d i c h y d r o x y l s were c h a r a c t e r i z e d by i n f r a r e d spectroscopy on t h e samples p r e t r e a t e d as above. I t was confirmed by t h e disappearance o f t h e band 61(Ni3) a t about 1350-1380 cm-’
t h a t t h e complex Pt(NH3)4C12 had decomposed. The absor-
bances o f t h e h y d r o x y l s wereranked i n t h e o r d e r PtNaX
%
PtNaY
<
PtHNaY
<<
PtHY
which i s i n l i n e w i t h t h e chemical formulae. The PtNaY and PtNaX e x h i b i t e d o n l y a weak OH band a t 3650 cm-l, i n agreement w i t h t h e low a c i d i t y resulting from pt2+ r e d u c t i o n . + The b a s i c i t y o f oxygen should i n c r e a s e as protons a r e exchanged by Na i o n s and as t h e z e o l i t e A1 c o n t e n t r i s e s (6,7).
The oxygen charge has been c a l c u l a -
t e d u s i n g t h e Sanderson e l e c t r o n e g a t i v i t y e q u a l i z a t i o n p r i n c i p l e ( 8 ) which expresses t h e mean b a s i c i t y expected i n z e o l i t e s (7). The a b s o l u t e values i n c r e a s e from HY t o t h e most b a s i c z e o l i t e NaX ( t a b l e 1 ) . P y r r o l e a d s o r p t i o n s t u d i e s i n NaY and NaX c o n f i r m t h e h i g h e r NaX b a s i c i t y ( 6 ) . CO a d s o r p t i o n CO wasadsorbed a f t e r r e d u c t i o n and evacuation o f t h e wafers as d e s c r i b e d above. Fig. 1 r e p o r t s t y p i c a l experiments f o r PtHNaY and PtNaY. A f t e r adsorp-
431
2200 2000
1800
cm- 1
2200 2000 1900
F i g . 1. CO a d s o r p t i o n on PtNaY ( A ) and PtHNaY (B) ; a : b e f o r e CO a d s o r p t i o n ; b : i n t r o d u c t i o n o f CO a t R.T. (nCO/nPt = 0.29) ; c : H e a t i n g f o r 15 h i n the c l o s e d c e l l a t 573 K and e v a c u a t i o n f o r 0.5 h a t 448 K. TABLE 1 Wavenumber o f adsorbed CO, a r o m a t i z a t i o n a c t i v i t y and b a s i c i t y o f t h e z e o l i t e s . Zeol it e
vCO(cm-l)
S( b,
benzene‘ a)
HF
LF
(%I
PtHY PtHNaY
2063 2060
(c) (C)
PtNaY
2049
no band 1993 (weak) 1993
23.7
3.54
PtKL PtNaX
2060(d) 2033
1960-1980(d) 1968
47.2 54.8
3.50 3.25
O(b) charge
4.095 3.79
- 0.234
-
0.300 0.352
0.356 0.413
( a ) Benzene ( % ) formed a t 773 K f r o m n-hexane ( 1 7 ) ( b ) I n t e r m e d i a t e e l e c t r o n e g a t i v i t y and oxygen charge c a l c u l a t e d f r o m i t ( f r o m reference 7) Z e o l i t e s c o n t a i n i n g p r o t o n s a r e n o t benzene-selective ( 1 7 ) ( d l N o n - i s o l a t e d adsorbed CO molecules ( f r o m r e f e r e n c e 4 )
t i o n o f CO a t room temperature on PtNaY (nCO/nPt = 0.29);spectrum
b i n figure
1A shows a band a t 2076 cm-l assigned t o l i n e a r CO and bands n e a r 1863 and 1802 -1 cm c o r r e s p o n d i n g t o b r i d g e d CO (1-3). H e a t i n g t h e w a f e r f o r 15 h i n the
c l o s e d c e l l a t 573 K i n o r d e r t o r e d i s p e r s e CO on a l l t h e p a r t i c l e s as i n (2,3) and e v a c u a t i o n a t 448 K f o r 0.5 h produced spectrum c. A new band i s formed a t 1993 cm-l i n a d d i t i o n t o t h e band o f l i n e a r CO which s h i f t e d t o 2049 cm-’.
The
b r i d g e d CO bands a r e decreased i n i n t e n s i t y and t h e i r wavenumbers s h i f t e d t o lower values w i t h a broad maximum a t around 1776 cm-’.
A similar set o f treat-
ments f o r PtNaY s h i f t s t h e s t a r t i n g CO band f r o m 2082 t o 2060 cm-l and generat e s a new band a t 1993 cm-l ( f i g u r e l a ) .
432
2063
2049
1
2200
2000
1900
1800
2200
2000
1900
I
crn
I
F i g . 2. CO a d s o r p t i o n a t R.T. on a : PtHY(nCO/nPt = 0.19) ; b :PtHNaY(nCO/nPt = 0.29) ; c : PtNaY(nCO/nPt = 0.29) ; d : PtNaX(nCO/nPt = 0.32).Spectra r e c o r d e d a t 573 K i n t h e c l o s e d c e l l and e v a c u a t i o n f o r 0.5 h a f t e r h e a t i n g f o r 15 h a t 448 K. Note t h e d i f f e r e n t s c a l e f o r a, b and c, d. F i g u r e 2 r e p o r t s t h e changes i n t h e r e l a t i v e absorbance o f t h e bands i n t h e range 1900-2100 cm-l as a f u n c t i o n o f Na and A1 c o n t e n t i n t h e z e o l i t e s . The sodium i n c r e a s e s f r o m c u r v e s a t o c i n Y z e o l i t e s and c u r v e d d e p i c t s PtNaX. The w a f e r s a r e h e a t - t r e a t e d a t 573 K i n t h e presence o f CO i n t h e c l o s e d c e l l and evacuated a t 448 K f o r 0.5 h
The s t a b i l i t y o f t h e bands upon e v a c u a t i o n
a t i n c r e a s i n g temperatures f o r 0.5 h i s shown i n t i g u r e 3 f o r PtNaY (3A) and PtNaX ( 3 8 ) a f t e r s a t u r a t i o n o f t h e w a f e r s w i t h CO. F i g u r e 4 r e p o r t s t h e changes i n t h e s p e c t r a o f adsorbed i s o l a t e d CO m o l e c u l e s on PtNaY upon adsorpt i o n o f e l e c t r o n donor (NH3 o r H20) and e l e c t r o n a c c e p t o r m o l e c u l e s (02). The b e h a v i o r o f PtNaX i s compared i n f i g u r e 4B t o t h a t o f PtNaY i n t h e presence o f CO
+ NH3.
Discussion The r a t h e r good homogeneity o f t h e p a r t i c l e d i a m e t e r s and t h e i r s i z e (1.2 nm), c l o s e t o t h a t o f t h e supercage o f f a u j a s i t e , s u g g e s t s
t h a t they are located i n
t h o s e c a v i t i e s and f i l l c o m p l e t e l y t h e cage volume. The CO bands d e p i c t e d i n t h e i n f r a r e d s p e c t r a can be c l a s s i f i e d i n t o two main c l a s s e s a c c o r d i n g t o t h e i r b e h a v i o r .
First,
t h e u s u a l CO bands observed
i n p l a t i n u m m e t a l s u p p o r t e d on o x i d e s a r e seen a f t e r a d s o r p t i o n ( f i g u r e s 1-3) i n t h e range 2080-2040 cm-l f o r l i n e a r CO and around 1900-1800 cm-l f o r b r i d g e d CO. The bands o f l i n e a r and b r i d g e d CO a r e observed i n t h e f o u r samples s t u d i e d a t a kdvenumber s p e c i f i c f o r each m a t e r i a l . Secondly, b e s i d e s t h o s e bands new bands a r e formed between 2000-1960 cm-l,
i.e. close t o l i n e a r COY i n t h e three materials
w i t h t h e h i g h e r Na content. I t was c o n f i r m e d t h a t t h o s e new bands a r e due t o CO
433
*
I
2083
w
U
z
a
, . . 2200
2000
Fig. 3. CO a d s o r p t i o n (A) : PtNaY ; a : 5.5 e v a c u a t i o n f o r 0.5 h (B) : PtNaX ; a : 4.2
e
f
-
*
1900
1800
I
2000
1900
cm
-1
a t R.T. and t h e n e v a c u a t i o n a t i n c r e a s i n g temperature. t o r r CO ; b : e v a c u a t i o n f o r 0.16 h a t R.T. ; C t o g : a t 423 K ( c ) , 473 K ( d ) , 523 K ( e ) , 573 K ( f ) , 623 K ( 9 ) . t o r r CO ; b : 0.5 h r a t R.T., c t o e : as f o r PtNaY.
F i g . 4. Change i n CO s p e c t r a a f t e r a d s o r p t i o n of NH3(A,B), O,(C) and H20(D) on PtNaY (A,C,D) and PtNaX(B). (A) : NH a d s o r p t i o n on PtNaY ; a : nCO/nPt = 0.29, h e a t i n g a t 573 K f o r 15 h i n t h e c j o s e d c e l l and e v a c u a t i o n f o r 0.5 h a t 448 K. b : i n t r o d u c t i o n o f 8 t o r r NH3 ; (B) : NH3 a d s o r p t i o n on PtNaX ; a : s a t u r a t i o n w i t h CO a t R.T., heat i n g f o r 2 h a t 448 K and e v a c u a t i o n f o r 0.5 h a t 448 K ; b : i n t r o d u c t i o n o f 8 t o r r NH3. (C) : 02 a d s o r p t i o n on PtNaY ; a : as a-A ; b : i n t r o d u c t i o n o f 100 t o r r 02 ; c : h e a t i n g f o r 15 h a t 573 K. (D) : H20 a d s o r p t i o n on PtNaY ; a : as a-A ; b : i n t r o d u c t i o n o f 10 H20 molec u l e s p e r u n i t c e l l ; c : h e a t i n g a t 423 K f o r 15 h.
434
A l l t h e bands were s h i f t e d by about 50 cm-'. The weak and broad bands o f b r i d g e d CO w i l l n o t be s t u d i e d f u r t h e r . The two s e t s o f bands a t 2080-2040 and 2000-1960 cm-' w i l l be considered s e p a r a t e l y and r e f e r r e d t o r e s p e c t i v e l y as h i g h frequency (HF) and low frequency (LF) bands. by adsorbing I 3 C O i n s t e a d o f "CO.
High frequency band The wavenumber o f adsorbed CO i n t h e i n f r a r e d range a t 2040-2080 cm-' depends on several parameters. The d i p o l e - d i p o l e c o u p l i n g between CO molecules (1) and t h e e l e c t r o n t r a n s f e r o f d e l e c t r o n s from t h e metal t o t h e 2n* o r b i t a l s o f CO 10,11) a r e t h e two main e x p l a n a t i o n s i n v o l v e d . The s h i f t o f vco t o lower (2,3, wavenumbers as t h e coverage decreases i s due t o b o t h e f f e c t s ( 1 y 2 y 1 0 ) . The s h i f t r e s u l t i n g from metal-support i n t e r a c t i o n s i s r e l a t e d o n l y t o t h e second one ( 2 ) . I n Pt/A1203 a vco v i b r a t i o n f r e e of CO-CO c o u p l i n g can be reached c l o s e t o e i t h e r by i s o t o p i c d i l u t i o n o f I 3 C O i n "CO a t f u l l CO coverage (1) o r by h e a t i n g t h e m a t e r i a l i n t h e presence o f low CO pressure i n a c l o s e d c e l l 2052 cm'l
which r e s u l t s i n i s o l a t e d CO molecules ( 2 ) . I n a P t f a u j a s i t e t h e wavenumber o f l i n e a r CO i s observed a t 2053 cm-l when t h e i s o t o p i c method i s used. I t corresponds t o vc0 f r e e o f d i p o l e c o u p l i n g and which v e r y l i k e l y i s n o t d i s turbed by metal support i n t e r a c t i o n a t h i g h CO coverage ( 3 ) . I t i s v e r y c l o s e t o t h e value obtained f o r Pt/A1203 ( 2 ) . The experimental c o n d i t i o n s described i n f i g u r e s 1-3
include
heating o f
t h e wafers a t 573 K i n a closed system i n t h e presence o f a small amount o f CO. T h i s should r e s u l t i n i s o l a t e d CO molecules. The vco should n o t be d i s t u r b e d e i t h e r by CO-CO c o u p l i n g o r coverage e f f e c t s (2). The vco d i f f e r e n c e s between t h e f o u r samples c o u l d be regarded as being m a i n l y due t o metal-support i n t e r a c t i o n . From t h e r e s u l t s o f f i g u r e 2 one may rank t h e z e o l i t e s according t o t h e i r v c o which decreases i n t h e o r d e r P t H Y
>
PtHNaY
>
PtNaY
>
PtNaX. T h i s sequence f o l l o w s t h e
decrease i n a c i d i t y expressed by the hydroxyl groups absorbances o r a1 t e r n a t i v e l y t h e increase i n b a s i c i t y e v a l u a t e d from t h e charge on oxygen s i n c e t h e two p r o p e r t i e s v a r y i n o p p o s i t e d i r e c t i o n s (6,7). One may c o n s i d e r t h a t t h e e x t e n t o f e l e c t r o n back-donation from P t t o CO increases as t h e a c i d i c c h a r a c t e r of the zeolite decreases and as simultaneously its basic i t y r i s e s ( t a b l e l ) . This s h i f t s t h e HF band t o lower vCO.The "zero" o r i g i n o f t h e wavenumber o f CO adsorbed on a P t p a r t i c l e w i t h no CO-CO c o u p l i n g and no Pt-support i n t e r a c t i o n i s n o t known. I n a f i r s t approximation i t c o u l d be considered t h a t t h e value o f 2052 cm-l,obtained f o r i s o l a t e d CO adsorbed on Pt/A1203 where t h e metal-support i n t e r a c t i o n s i s weak, would be c l o s e t o t h i s "zero" o r i g i n . I t f o l l o w s t h a t t h e f o u r samplesmay be s p l i t i n t o two groups. For P t H Y and PtHNaY t h e vco value i s h i g h e r than t h i s reference. T h i s would i n d i c a t e a decrease i n back donation from P t t o CO due t o t h e P t - z e o l i t e i n t e r a c t i o n . T h i s
435
would be i n l i n e w i t h t h e a c i d i c c h a r a c t e r o f t h o s e two m a t e r i a l s , t h e m e t a l p a r t i c l e s i n t e r a c t i n g w i t h t h e e l e c t r o n - a c c e p t i n g a c i d i c s i t e s . I n PtNaY and PtNaX t h e CO wavenumber i s l o w e r t h a n t h e r e f e r e n c e a t 2052 cm-'.
T h i s may be
e x p l a i n e d as an i n c r e a s e i n back-donation f r o m P t t o CO as a r e s u l t o f t h e P t i n t e r a c t i o n w i t h e l e c t r o n donor framework atoms, which can be t h e n e g a t i v e l y charged l a t t i c e oxygen. Those b a s i c s i t e s would p l a y t h e same r o l e as e l e c t r o n donor molecules s h i f t i n g t o lower values t h e wavenumber o f CO adsorbed on P t p a r t i c l e s . T h i s was observed w i t h benzene o r ammonia on Pt/A1203 (1,2) o r Pt-zeol i t e ( 3 , l l ) o r w i t h molecules such as e t h y l e n e on Pt/A$03(1,2).
The s h i f t s de-
pend on t h e e l e c t r o n donor molecule. They may be as l a r g e as 30 t o 67 cm-'. s i m i l a r b e h a v i o r i s a l s o observed on tance,the
A
t h e p r e s e n t samples ( f i g u r e 4). F o r i n s -
i n t r o d u c t i o n o f a n 8 t o r r p r e s s u r e o f NH3 on PtNaY o r PtNaX w a f e r s a l -
ready c o n t a i n i n g d i s p e r s e d CO s h i f t s t h e HF band by about 50 cm" t o l o w e r wave1 numbers. The s h i f t i s s l i g h t l y l e s s ( % 14 cm- ) upon w a t e r a d s o r p t i o n on a PtNaY w a f e r w i t h preadsorbed i s o l a t e d CO.These r e s u l t s suggest t h a t i n a d d i t i o n t o t h e e l e c t r o n t r a n s f e r due t o p a r t i c l e i n t e r a c t i o n w i t h t h e b a s i c s i t e s of t h e framework,ammonia and w a t e r a d s o r p t i o n i n c r e a s e s t h e e l e c t r o n back-donation f r o m P t t o CO. The e l e c t r o n a c c e p t o r oxygen m o l e c u l e shows t h e o p p o s i t e e f f e c t on
PtNaY. N e v e r t h e l e s s , i n t h a t case t h e f o r m a t i o n o f CO i s l a n d s s h i f t i n g vco t o h i g h values cannot be excluded (1,2).
I n t e r e s t i n g l y , t h e absorbances o f t h e CO
bands decrease upon O2 a d s o r p t i o n down t o complete disappearance. Simultaneousl y bands o f adsorbed C02 and c a r b o n y l species a r e o b s e r v e d , i n d i c a t i n g t h e
t r a n s f o r m a t i o n o f CO. The l o w frequency band The ease o f f o r m a t i o n o f t h i s band depends on t h e sample. F i g u r e 2 shows t h a t t h e L F band i s n o t formed i n P t H Y even a f t e r h e a t i n g a t 573 K i n t h e presence o f CO. F o r t h e o t h e r t h r e e samples i t s i n t e n s i t y and wavenumber depend on t h e e x p e r i m e n t a l c o n d i t i o n s and f o r s i m i l a r c o n d i t i o n s on t h e z e o l i t e . A f t e r s a t u r a t i o n w i t h CO a t room temperature, t h e band appears i n PtNaY and PtNaX ( f i gure 3) as a s h o u l d e r a t 2029 and 2015 cm-l r e s p e c t i v e l y . It i s s h i f t e d s i m u l t a neously w i t h t h e h i g h frequency band t o l o w wavenumbers as t h e d e s o r p t i o n temperature increases,i.e.
as t h e coverage decreases and as CO i s r e d i s t r i b u t e d on
t h e p a r t i c l e s . When s t a r t i n g f r o m l o w l o a d i n g (nCO/nPt = 0.29 on f i g u r e 1) t h e band i s almost n o t d e t e c t a b l e a t room temperature. I t i s necessary t o h e a t t h e sample t o see f i r s t a s h o u l d e r t h e n a d i s t i n c t band. S i m i l a r s t a t e s a r e o b t a i n ed f r o m b o t h methods
-
high o r low loading
-if
the zeolites i n i t i a l l y saturated
w i t h CO a r e evacuated a t 423 K ( f i g . 3 ) o r i f a small CO amount i s heated w i t h t h e sample i n a c l o s e d c e l l a t 573 K f o r 15 h
( f i g . 2). This indicates simi-
l a r CO c o n t e n t and l o c a t i o n on t h e P t p a r t i c l e s . The thermal s t a b i l i t y o f t h e LF band i s h i g h e r t h a n t h a t o f t h e HF one,as
436
seen i n f i g u r e 3. The l a r g e r decrease o f t h e HF band a t temperatures h i g h e r than about 423 K r e s u l t s i n a l a r g e o v e r l a p p i n g o f t h e two bands which may b r i n g t h e i r wavenumbers a r t i f i c i a l l y c l o s e r . For s i m i l a r experimental condit i o n s , f i g u r e 2 shows t h a t f o r a decrease i n t h e z e o l i t e a c i d i t y (and a s i m u l taneous increase i n i t s b a s i c i t y ) t h e LF band i n t e n s i t y and t h e r a t i o LF/HF i n c r e a s e w h i l e t h e wavenumber o f t h e LF band decreases. The adsorption, on wafers w i t h h i g h l y dispersed CO, o f e l e c t r o n donor molecules, such as ammonia and water s h i f t s t h e LF band t o lower wavenumber simultaneously w i t h t h e usual l i n e a r CO HF band ( f i g u r e 4 ) . I n t h e case o f oxygen a d s o r p t i o n b o t h bands a r e s i m i l a r l y s h i f t e d t o h i g h e r frequencies, and t h e i r i n t e n s i t y decreases a t t h e same time. The p a r a l l e l behavior o f t h e two CO bands w i t h r e g a r d t o a d s o r p t i o n o f those molecules and w i t h evacuation ( f i g . 3) suggest great s i m i l a r i t i e s i n t h e i r o r i g i n . The f o l l o w i n g hypothesis may e x p l a i n t h e presence and t h e p r o p e r t i e s o f t h e LF bands. Adsorption o f CO on b a s i c oxides may l e a d f i r s t t o t h e f o r m a t i o n o f 2 k e t e n e - l i k e s t r u c t u r e s O=C=C02- (12,13). They may be s t a b l e up t o room temperat u r e and g i v e r i s e t o a band near 2033 cm-l i n t h e i n f r a r e d range considered here. Z e o l i t e NaX, w i t h o u t platinum, does n o t y i e l d any CO band i n t h e range above 1600 cm-'.
I f t h e LF band a r i s e s from such species i t should be assumed t h a t p l a t i -
num would c a t a l y s e t h e i r formation. I n a d d i t i o n i t i s n o t c l e a r why these species would r e q u i r e h e a t i n g t o be formed w h i l e they a r e n o t s t a b l e above 300 K on b a s i c oxides. A more 1 i k e l y hypothesis i s t o c o n s i d e r s i m i l a r i t i e s between t h e p r e s e n t LF band and t h e l i n e a r CO bands observed a t 1970-1990 cm-l i n p l a t i n u m C h i n i complexes i n THF s o l u t i o n (14) o r f o r such compounds supported on y-A1203 (15). For aluminas c o n t a i n i n g 1 . 8 t o
2w t
% P t i n t h e form o f supported C h i n i comple-
xes t h e CO i n f r a r e d bands a r e s t a b l e up t o 473-623 K upon evacuation. Those ne2g a t i v e l y charged c l u s t e r s have t h e general formula [Pt3(CO)3( LJ2C0)3]n w i t h n = l t o 5. They a r e found i n s o l u t i o n i n b a s i c media upon r e d u c t i o n o f Pt2+ cat i o n s by CO. Complexes o f t h i s type a r e supposed t o be generated i n unreduced PtNaY t r e a t e d w i t h CO under s p e c i f i c c o n d i t i o n s (16). I n t h e p r e s e n t case i t i s proposed t h a t l o c a l l y some s u r f a c e P t o atoms o f a p a r t i c l e m i g h t be i n t e r a c t i n g so s t r o n g l y w i t h a d j a c e n t and n e g a t i v e l y charged framework-oxygen atoms, f o r i n s t a n c e i n t h e 12-R window, t h a t a s u r f a c e recons t r u c t i o n might occur. Those atoms would be l e s s s t r o n g l y
bonded t o t h e o t h e r P t
atoms o f t h e p a r t i c u l e and could, i n t h e presence o f CO, form some species w i t h CO l i n e a r l y bonded as i n t h e C h i n i complexes. The s t r o n g PtO-0'-
interaction
would f a v o r t h e e l e c t r o n t r a n s f e r t o t h e p l a t i n u m atom, i n c r e a s i n g i n t u r n t h e back-donation t o t h e CO molecule. T h i s would l e a d t o a decrease i n vco g r e a t e r than when CO i s adsorbed on an i n d e f i n i t e P t atom o f a p a r t i c l e . F i g u r e 5 shows a p o s s i b l e scheme f o r t h e d i f f e r e n c e i n P t / z e o l i t e i n t e r a c t i o n g e n e r a t i n g a h i g h o r low CO frequency band. A f t e r f u l l evacuation o f CO a t 573 K t h e readsorp-
437
Pt
HF BAND
p t y PARTICLE
l
T
E
LF BAND
fl F i g . 5. Schemes f o r P t / C O a d s o r p t i o n g i v i n g two forms o f l i n e a r CO. t i o n o f CO
returns
t h e i n i t i a l s p e c t r a showing t h e r e v e r s i b i l i t y o f t h e phe-
nomenon. I t has t o be p o i n t e d o u t t h a t t h e i n t e r a c t i o n p o s t u l a t e d h e r e i s q u i t e d i f f e r e n t f r o m t h e one where P t i s o x i d i z e d by c o a d s o r p t i o n o f gaseous oxygen and CO which generates an i n f r a r e d band a t 2120 cm-l. Correlations w i t h c a t a l y t i c properties A s h i f t t o a l o w e r wavenumber o f CO adsorbed on p l a t i n u m a1 k a l i n e L z e o l i t e s was observed s i m u l t a n e o u s l y w i t h t h e f o r m a t i o n o f new CO bands (LF bands) between 1930-1980 cm-l w i t h a dependence on z e o l i t e b a s i c i t y q u i t e s i m i l a r t o t h e r e s u l t s presented h e r e (4). Those samples e x h i b i t e d a h i g h n-hexane a r o m a t i z a t i o n a c t i v i t y which i n c r e a s e d a s t h e L z e o l i t e s became more b a s i c f r o m t h e L i t o t h e Cs form ( 4 ) o r as p r o t o n s were exchanged f o r K+ ( 1 7 ) . The same r e a c t i o n i s a l s o c a t a l y z e d by PtNaY and PtNaX, t h e y i e l d o f benzene produced b e i n g i n t h e o r d e r PtNaX
>
PtKL
>
PtNaY ( 1 7 ) ( t a b l e 1 ) .
Comparison o f
properties w i t h the platinum behavior w i t h regard
the
catalytic
t o CO i n d i c a t e s a p a r a l l e l
dependence on z e o l i t e b a s i c i t y . S i n c e t h e r e a c t i o n i s m o n o f u n c t i o n a l on t h e p l a t i n u m s i t e s ( 1 7 ) , d i f f e r e n c e s i n c a t a l y t i c p r o p e r t i e s may v e r y l i k e l y a r i s e from m o d i f i c a t i o n o f t h e p l a t i n u m i n t h e z e o l i t e cages. The g r e a t i n f l u e n c e o f z e o l i t e b a s i c i t y on P t p r o p e r t i e s evidenced h e r e by t h e ease o f e l e c t r o n t r a n s f e r f r o m P t t o t h e adsorbed CO m o l e c u l e suggests t h a t t h e most a c t i v e P t s i t e s f o r n-hexane a r o m a t i z a t i o n c o u l d be t h e P t atoms a c t i v a t e d by i n t e r a c t i o n w i t h t h e n e g a t i v e l y charged framework oxygen atoms. The s p e c i f i c e x i s t e n c e o f aromat i z a t i o n a c t i v i t y i n z e o l i t e s which show t h e LF bands a l s o suggests a s t r o n g involvement i n t h i s c a t a l y t i c r e a c t i o n of more h i g h l y d i s t u r b e d P t atoms. CONCLUSION The p l a t i n u m p a r t i c l e s i n systems l i k e Pt/Si02, Pt/A1203 a r e v e r y l i k e l y d i f f e r e n t f r o m t h o s e i n z e o l i t e s . I n t h e f i r s t case t h e m e t a l p a r t i c l e s l i e on
438
r a t h e r f l a t s u rf a c e s , w h i l e i n z e o l i t e s t h e y a r e embedded i n c a v i t i e s where t h e y exp erie nc e t h e i n f l u e n c e o f a l l t h e framework atoms o f supercages. As a consequence t h ey f o r m s m a l l p a r t i c l e s w h i c h a r e more s e n s i t i v e t o any e l e c t r o n t r a n s f e r between t h e metal and t h e support. The s t r o n g i n f l u e n c e o f t h e acid-base p r o p e r t i e s o f t h e z e o l i t e evidenced h e r e on t h e e l e c t r o n i c s t a t e o f p l a t i n u m suggests t h a t such m e t a l atoms would have v e r y s p e c i f i c r e a c t i v i t y i n t h e c a t a l y s i s o f many r e a c t i o n s . References F. Stoop, F.J.C.M. Toolenaar, V. Ponec, J. Catal., 73 (1982) 50. M. P rime t , J. Catal., 88 (1984) 273. M. Primet, L.C. de Menorval, J. F r a i s s a r d , T. I t o , J. Chem. SOC. Far. Trans I, 81 (1985)2867. C. Besoukhanova, J. Guidot, D. Barthomeuf, M. Breysse, J.R. Bernard, J. Chem. SOC. ,Farad. Trans. I, 77 (1981) 1595. H. A r a i , T. Seiyama, M. Harakawa, H. Tominaga, C a t a l y s t D e a c t i v a t i o n (B. Delmon e t a l . , ed.), E l s e v i e r , Amsterdam, 1980, 167. D. Barthomeuf, J. Phys. Chem., 88 (1984) 42. D. Barthomeuf, A. de Mallmann, " I n n o v a t i o n i n Z e o l i t e M a t e r i a l s Science" (P. Grobet e t al. , ed.) ,Stud. Surf. S c i . C a t a l. , E l s e v i e r , Amsterdam, 37 (1988)
365.
W. M o r t i e r , J. Catal., 55 (1978) 138. 10 11 12 13 14
L.C. de Menorval, J. F r a i s s a r d , T. I t o , M. Primet , J. Chem. SOC. Far. Trans, I, 81 (1985) 2855. G.J. B l y h o l d e r , J. Phys. Chem., 79 (1975) 756. P. G a l l e z o t , J. Datka, J. Massardier, M. Primet , B. I m e l i k , Proceed. I n t e r n . Cong. Catal., The Chemical S o c i e t y , London, 2 (1977) 696. M.A. Babaeva, A.A. Tsyganenko, React. K i n e t . C a t a l . L e t t . , 34 (1987) 9. E. G u g l i e l m i n o t t i , S. C o l u c c i a , E. Garrone, L. C e r r u t i , A. Zecchina, J.C.S. Farad. Trans. I, 75 (1979) 96. J.C. Calabrese, L.F. Dahl, P. C h i n i , G. Longoni, S. Martinengo, J.A.C.S.,
96(8) (1974) 2614. 15 M. Ichikawa, Chemistry L e t t e r s , 1976, 335. 16 A. de Mallmann, D. Barthomeuf, t o be p u b l ished. 17 J.R. Bernard, Proceed. F i f t h I n t . Conf. Z e o l i t e s (ed. L.V.C. London, 1980, 686.
Rees), Heyden,
II. SORPTION
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FUNDAMENTAL RESEARCH
AND
MODELING FOR
ADSORPTION OF NORMAL PARAFFINS
A
P PAREX"-PROCESS
TECHNICAL PROCESS OF OF DDR) BY ZEOLITE A
SELECTIVE
W. SCHIRMER, K. FIEDLER, H. STACH AND M. SUCKOW
Z e n t r a l i n s t i t u t f u r physikalische Chemie der Akademie der Wissenschaften der ODR, Rudower Chaussee 5, 1199 B e r l i n , DDR
ABSTRACT A survey i s given o f the experience obtained over 25 years o f modeling o f the technical e x t r a c t i o n o f normal p a r a f f i n s by means o f z e o l i t e 5A (the soc a l l e d "Parex"-process o f ODR). It i s shown t h a t the technical tasks were a cons t a n t challenge f o r the fundamental research, while, on the other hand, any progress i n the research work influenced the technlcal procedure. INTRODUCTON We intend t o g i v e a survey o f 25 years o f experience i n the modeling o f t h e
e x t r a c t i o n o f normal p a r a f f i n s from a gaseous phase by means o f z e o l i t e 5A. These developments were c a r r i e d out I n close cooperation with t h e chemical ent e r p r i s e s o f Leuna and Schwedt. Here we s h a l l not t r y t o f o l l o w the h i s t o r i c a l path o f development, but we want t o show how fundamental research and technolog i c a l p r a c t i c e influenced each other. The technical aim, t h e adsorption-process, was a constant source and challenge f o r the formulation o f new tasks of fundamental research, while any progress i n research allowed us t o complete t h e techn i c a l process. I t i s w e l l known t h a t the production p r o f i l e o f Leuna includes r e a c t i o n s on the basis o f long-chained normal paraffins. A t the beginning o f the s i x t i e s , they were produced i n a Fischer-Tropsch plant, usually i n a high q u a l i t y . But the time o f the p r o f i t a b i l i t y o f the Fischer-Tropsch process was over. We were forced t o look f o r cheaper sources o f n-paraffins, and t h a t was why we d i r e c t e d our a t t e n t i o n t o various types o f crude o i l o f the Soviet Union, which, a t t h a t time, contained between 15 and 20 % o f normal paraffins. Having had some experience i n the fundamental research o f adsorption, we made up our minds t o work i n close cooperation w i t h Leuna, l a t e r on a l s o w i t h Schwedt and w i t h several production plants f o r chemical equipment i n our count r y . The choice o f hydrocarbons (single components and mixtures), e s p e c i a l l y long-chained normal paraffins, as adsorptives f o r fundamental research, represented a c e r t a i n r i s k , f o r we could not know how f a r we should be able t o gener a l i z e the results. By measuring the adsorption behaviour o f d i f f e r e n t types o f hydrocarbons (paraffins, o l e f i n s , and aromatics) on corresponding z e o l i t e s , we
440
became acquainted with the thermodynamic and k i n e t i c p r o p e r t i e s o f these systems. Trying t o meet the requirements o f Leuna where the development o f t h e technical process o f s e l e c t i v e adsorption meanwhile was i n f u l l progress, we were able t o develop t h e f i r s t model o f the so-called "Parex" (paraffin-extract i o n ) process. We were able t o gain time, and our r i s k s turned out t o be an advantage. By our model, we helped t o a t t a i n a scaling-up f a c t o r o f 1:8000 d u r i n g the development o f the Parex process, a r e l a t i o n , which allowed us t o omit one stage i n t h e t r a d i t i o n a l process of technological development ( r e f . 1). The flow-sheet o f the Parex process i s represented by f i g u r e 1 (refs. 2-3). The gaseous raw material, mixed with a carrier-gas, enters t h e p l a n t a t E, passes the adsorbers A(l-3) a t a pressure o f 0.8 - 1.1 MPa and a temperature o f 380°C. Usually, one adsorber A i s working i n t h e adsorption stage, w h i l e 360 two adsorbers are needed f o r the process o f desorption, using ammonia as a de-
-
s o r p t i o n medium. The desorbed product, containing a mixture o f n - p a r a f f i n s and desorption agents, i s condensed i n column K4, where i t i s separated. The soc a l l e d denormal i s a t e passes K1, while the ammonia and the carrier-gas are washed and, f i n a l l y , returned i n t o t h e process. The process i s r a t h e r f l e x i b l e ; i t can be r e a l i z e d a t d i f f e r e n t temperatures i n order t o o b t a i n various f r a c t i o n s o f np a r a f f i n s . The o v e r a l l y i e l d o f n-paraffins amounts t o 95-96% and t h e i r p u r i t y 99.1 %.Special measures are necessary i n order t o separate the coni s 98.5 t e n t s o f approximately 0.5 % o f aromatics. There are now d i f f e r e n t v a r i a n t s o f the process, allowing, f o r instance, the use o f feed-stocks containing o n l y 7 % o f n-paraffins.
-
Fig. 1
P r i n c i p a l f l o w sheet o f the Parex process (ODR), (E-raw material, w-water, A-residual heat, 0-denormal isate, P-n-paraff in)
441
The cooperation between the i n d u s t r i a l enterprises and the i n s t i t u t e was continuous over the past two decades. It was enhanced by a h i g h l y e f f e c t i v e cooperation w i t h the department o f physics o f t h e Karl-Marx-Universtiy o f L e i p z i g (Prof. P f e i f e r ) and w i t h the producer o f t h e zeolites, t h e Chemical Combined Enterprise o f B i t t e r f e l d (Dr. Furtig). The s i n g l e steps o f adsorption, which are t a k i n g place i n columns packed w i t h biporous adsorbents ( z e o l i t e s i n form of p e l l e t s ) , are a) mass transport i n the void space o f the column, i n the macropores of p e l l e t s between t h e c r y s t a l s (molecular and Knudsen d i f f u s i o n ) and i n t h e micropores ( d i f f u s i o n o f the adsorbed phase w i t h i n t h e crystals); b) mass t r a n s f e r through the external surface o f the p e l l e t s , through t h e surface o f crystals, characterized by a barrier;
c) adsorption i n the microporous channel network o f the z e o l i t e f o l l o w i n g a s o r p t i o n isotherm (equi 1ibrium) ; and d) the heat phenomena: conduction through p e l l e t s , owing t o temperature r i s e by adsorption, t r a n s f e r through the external surface o f p e l l e t s , and t r a n s p o r t i n t h e void space o f the column. Under technical conditions,
the most important phenomena are adsorption,
mass transport i n macro- and micropores. and conduction o f heat. We t r i e d , therefore, t o o b t a i n r e l i a b l e data especially i n these f i e l d s . Furthermore, the influence o f temperature and pressure, the replacement data of adsorbed hydrocarbons by water and ammonia, and the c a t a l y t i c behaviour o f a c i d centres t o wards hydrocarbon conversion are phenomena t o be considered. The Drocedure o f modelinq Using fixed-bed adsorbers containing p e l l e t i z e d zeolites, our task was t o develop an optimal model f o r t h i s non-stationary process, where m a t e r i a l and heat transports between a s o l i d and a f l u i d phase and adsorption e q u i l i b r i a p l a y the decisive roles. We were f u l l y aware o f the necessity f o r various s i m p l i f i c a tions. a) We t r i e d t o represent the mixture o f n-paraffins ( i n the range o f CIOCIS) by one s i n g l e component: we chose a f i c t i v e hydrocarbon o f a composition o f approximately 13.5 C-atoms. Thus, we reduced t h e number o f components f o r t h e mass balance t o 2, i = 1 representing the mean n-paraffins component and i = 2 the concentration o f amnonia. A l l the other materials were not adsorbed t o a marked extent; therefore, they can remain disregarded. b) For f u r t h e r s i m p l i f i c a t i o n s we proposed t h a t f o r large-scale conditions the f l o w p a t t e r n i s s u f f i c i e n t l y described by a x i a l l y dispersed plug flow. The material and heat transport i n the void space o f the column i s described by the following equations (assuming the plug f l o w model):
442
(A glossary of the smybols used see at the end of the paper; quantities characterized by top line represent integral mean values in the volume). The mass balance in the macropores of the biporous pellets, for which the adsorption can be neglected, is given by Fick's law: aCi
vc
-8-
+
vq
3
= V, div ( Dc,i grad c i )
(3)
and the mass transfer by convection i s fixed by the equation:
The mass transport in the micropores in the adsorpbed phase is represented by the thermodynamic di ffusion-equat ion:
In this case, the driving force of diffusion is the gradient of the chemical potential. The effective coefficient of micropore diffusion Dp contains the resistance of the surface barrier as well as the resistance of diffusion within the adsorbed phase. The equilibrium of adsorption is given. by a modified Langmuir isotherm:
derived on the basis of statistical thermodynamics and our cellular theory. f (0) denotes the function of replacement depending on the competing adsorption of other components. We assume that the spreading of heat within each zeolite pellet only depends on time. Gradients of temperature only exist between different crystals. Heat conduction within the pellet is then represented by
while the heat transfer on the surface between pellet and surrounding gas phase is given by:
443
The system of the coupled and the partial differential equations (1) - (8) is completed by the choice of appropriate initial and boundary conditions. The parameters for the calculation of the model were determined by data from fundamental research in the institute. The calculation of the model was made for one adsorber, whose performance in ad- and desorption was studied, until it reached a periodic-stationary stage. Finally, we calculated the breakthrough curve of the system. Fundamental research in the laboratory One of the most important tasks we had to undertake in the first period of our research was the exact determination of adsorption equilibria of the corresponding hydrocarbons, sometimes under the influence of water and ammonia. We found that the isotherms of n-decane on a zeolite o f the type 5A are characterized by a general step. As the maximum capacity of one zeolite cavity amounts to two molecules of n-decane, we can distinguish between the adsorption of the first and the second molecules of n-decane. This phenomenon is best realized in the temperature range between 480-520K. By measuring the adsorption isotherms of a system at different temperatures, of course, we had access to very important data: the heat of adsorption of hydrocarbons and their enthalphy and entropy values. Depending on the errors in measuring isotherms, these quantities are sometimes not very precise. We, therefore, paid more attention to the direct calorimetric determination of heats of adsorption and of specific heats of the adsorption systems. We came to the conclusion that calorimetry is an indispensable method in adsorption research. I should like to mention here our efforts to determine parameters, which depend on the zeolitic adsorbent -- for instance, the influence of the binding material necessary to form pellets. We not only observed the blocking of transport-pores by the binder, but also the exchange of ions and a certain influence on the existence of OH groups. The 20 % addition of binding material to the zeolite proved to be a rather active agent, changing the properties of the whole system. It turned out that the stability of the zeolitic material against water is a first-class quality, especially for long-time technical use. The acid centres of the zeolites are very often the source of catalytic decomposition of hydrocarbons, especially for temperatures higher than 570 K. Although the Parex process is in rather a happy situation by the fact that the use o f ammonia "softens" and neutralizes the acid character of the adsorbent, we studied the problems of decompositon of hydrocarbons extensively, with the result that the life-time o f the zeolites (measured by a certain yield of adsorbed n-paraffins) was finally increased by a factor of 5. There are adsorbers, now, which have been operating on-stream, without interruption, for more than two years. Returning to the interdependence between technical practice and theory, we must mention now our efforts to elaborate theoretical approaches, by which we
444
became able t o c a l c u l a t e the most important data necessary f o r t h e model. The experimental determination o f these values i s time-consuming. A general theoret i c a l t o o l would represent a great e f f e c t o f r a t i o n a l i z a t i o n , and hence i t should g i v e us also a more general understanding o f the phenomenon o f z e o l i t i c adsorption. That i s why we developed our c e l l u l a r theory o f adsorption, which considers some c h a r a c t e r i s t i c features o f these systems ( r e f . 4). Using the general expressions o f s t a t i s t i c a l thermodynamics, we took i n t o consideration t h a t each c a v i t y i n t h e z e o l i t i c micropore-system i s separated from other c a v i t i e s by an a c t i v a t i o n b a r r i e r , thus p e r m i t t i n g us t o regard the c a v i t y as a very small independent thermodynamic system. The number o f p o s s i b l y adsorbed hydrocarbons being very small, we have the f u r t h e r advantage t h a t the number o f energy l e v e l s must also be very l i m i t e d . I n other words, a z e o l i t e c a v i t y , f i l l e d w i t h adsorbed hydrocarbon molecules, represents an i d e a l system f o r thermodynamic calculations. S t a r t i n g w i t h the most simple assumptions we developed a Monte Carlo procedure, i n order t o c a l c u l a t e values. The c e l l u l a r theory o f adsorption i s characterized by the f o l l o w i n g approach: The grand p a r t i t i o n f u n c t i o n Z i s devided i n t o a product o f powers o f the corresponding funcitons o f small thermodynamic systems (Nk c e l l s o f type k, k = l ,
....,
...,
k). Each c e l l may have o n l y a few states i (1 = 1, I k ) , characterized by the number n k i o f occupation and the canonical p a r t i t i o n functions Qki. The most important p o s s i b i l i t y f o r evaluation i s the d e r i v a t i o n o f an equation o f isotherm f o r t h e degree o f coverage a: "ki nki Q k i Nk
i=l
(9)
(@k representing the mean number o f occupation i n the c e l l s o f type k, A r=
(p/po)
/ (T/To); Po, To
- normal
pressure, temperature). which we f u l l y described elsewhere ( r e f . 5), we are able t o c a l c u l a t e p r o b a b i l i t i e s o f occupation and, f i n a l l y , a l l thermodynamic functions o f t h e system o f adsorption, t h a t i s t o say: enthalpy, entropy, s p e c i f i c heat, and p o r e - f i l l i n g factors. We f i n a l l y devised t h r e e d i f f e r e n t and independpt ways t o o b t a i n the thermodynamic values needed f o r our By means o f t h i s system of equations,
models: experimental methods, calorimetry, tion.
and the c e l l u l a r theory o f adsorp-
I n t h i s way, we calculated the values needed f o r a homologous s e r i e s o f hydrocarbons as w e l l as f o r d i f f e r e n t types of zeolites. We a l s o obtained data descrlbing the f u n c t i o n o f displacement o f hydrocarbons by water and amnonia.
445
In combining these results with other calculations based on well known Lennard-Jones potential functions, we tried to obtain a deeper insight into the distribution of potentials within the cavities of different types of zeolites. Thus we were able to know where the adsorbed molecules were situated within the cavity and which details of structure may be prevailing. This knowledge about the structure is especially interesting for long-chained hydrocarbons (ref. 5). Another approach, more recently developed, concerns the well known fact that the adsorption as well as the desorption cause some changes of the lattice parameters either by compression or dilatation. The changes of length are within the factor of to of the total crystal dimensions. On the basis of a generalized mechanical compression and dilatation theory, we derived approaches which permitted us to calculate the values of interest. The efficiency of these calculations was demonstrated by the fact that it was possible to find the correct sign - that is, to know whether compression or dilatation are prevailing. Thus we could contribute to the practical performance of the pelletized zeolites by giving suggestions as to their formation as well as to their treatment within the bulk of the adsorber. We were lucky enough to devise a series of modern physical methods of structure determination and analysis. We emphasize here that the application of modern methods of infrared and NMR-spectroscopy i s absolutely necessary in adsorption research, and that concerns not only the fundamental research, but also the development of a technical process. I am going. to give only two examples, characteri st ic of our experience: The use of the photo-electron spectroscopy a1 lows determinations of the distribution of elements of the zeolite lattice between the bulk and the surface o f the crystals. We were interested in knowing whether the elements Si, Al, Na, Mg are homogeneously distributed over the crystal or not. The results, found by ESCA measurements at various depths under the surface of the crystals compared with the bulk concentration determined by chemical analysis, were as follows: a) The element silicon shows the same concentration in the surface as well as in the bulk phase. b) The element A1 is enriched in the surface especially for all dealuminated zeolites. c) The concentrations of sodium are lower in the surface than in the volume for all zeolites of type A, X, and Y, while magnesium is enriched in the surface by a factor of more than two (ref. 6). We observed that the pretreatment of the zeolites had a great influence on the homogeneity of the adsorbent. The fact that the distribution of elements is heterogeneous throughout the crystals must be regarded as normal. The dynamics of adsorption are of great Influence on the technical process. Assuming that 10.000 kg of n-paraffins are daily adsorbed by 1 m3 of adsorbent, we must take for granted that 1017 - 10l8 molecules per second can be adsorbed
446
i n 1 g o f z e o l i t e . Each a v a i l a b l e pore must transport, then,
several dozens o f
molecules per second i n t o the c a v i t i e s o f the microporous system. These data r e present i n a very rough way t h e importance o f the d i f f u s i o n phenomena. We have t o d i s t i n g u i s h between d i f f u s i o n and s e l f - d i f f u s i o n . I n the case o f sorbed molecules, s e l f - d i f f u s i o n represents the random molecular motion o f the sorbed spe-
c i e s i n t h e z e o l i t i c channel network a t s o r p t i o n equilibrium. This process i s described by the E i n s t e i n equation
where D,
(?*), and
t denote the s e l f - d i f f u s i o n c o e f f i c i e n t ,
the mean squared
displacement, and the observation time, respect1,vely. The d i f f u s i o n c o e f f i c i e n t 00 i s given, on the other hand, by F i c k ' s law 4
j, =
- DD
grad c
(11)
with as the d i f f u s i o n f l u x density and grad c as the concentration gradient o f the sorbed molecules i n the c r y s t a l . The r e l a t i o n between 0, and Do i s approximated by the Darken r e l a t i o n
DD =
Ds
d Inc
denoting the adsorbate pressure a t e q u i l i b r i u m w i t h the sorbate concentration c. Up t o 1975, we had measured the d i f f u s i o n data e x c l u s i v e l y by sorption-uptake techniques. A t t h a t time, we began t o use the NMR pulsed-field-gradjent technique i n close c o l l a b o r a t i o n w i t h H. P f e i f e r , Leipzig, and we were g r e a t l y astonished t o f i n d t h a t the Ds-values were up t o 3-5 orders o f magnitude l a r g e r than t h e corresponding data o f Do! A long discussion concerning the correctness o f these NMR values followed. F i n a l l y , the q u a n t i t i e s determined by t h e NMR technique were regarded as correct, and we found ways t o change the conditions o f the sorption-uptake technique, so t h a t nowadays a l l values o f s e l f - d i f f u s i o n and d i f f u s i o n are i n good agreement ( r e f . 7). We now have a f i n a l , very essential task: t h a t i s t o i n t e g r a t e a l l these det a i l e d f a c t s and experiences i n t o a technical adsorption process, working w i t h high e f f i c i e n c y . These problems are o f a technological nature and t h a t i s why we founded a speclal department, "Chemical engineering o f adsorption", under t h e d i r e c t i o n o f t h e l a t e Prof. 0. Gelbin. The model enabled us t o i n v e s t i g a t e t h e dynamic behaviour o f the adsorber. Figure 2 represents the loading p r o f i l e s over the height o f t h e column as a f u n c t i o n o f t h e adsorption-desorption cycle. For d i f f e r e n t phases ( d i f f e r e n t r e l a t i v e time r) o f the adsorption process ( f u l l curves) we observe the formation o f a loading f r o n t . A t T = 0.33 the adsorption i s finished; we see t h a t the concentration o f the adsorbate (n-paraffins) i s
447
high (approximately 0.9 - 0.95 of qlm) near the entrance of the adsorber, descending to values o f nearly 0.05 near the exit.
Fig. 2
Loading profils over column height and adsorption-desorption cycle (x = normalized time)
With the beginning of the desorption (dashed lines), the concentration profiles thoroughly change. At the entrance of the adsorber, the concentration is negligible, while the exit concentration is still considerable. Obviously, the desorption process is slower than adsorption. That is why we chose a relation o f two adsorbers working in the desorption regime to one adsorber on adsorption. We must confirm that this is not the last improvement; It seems to us that there could be other relations with a somewhat higher efficiency. The dashed curve at T = 1 represents the end o f the whole cycle; the exit concentration is now lower than 0.1. a degree of coverage which can be attained under advantageous economic conditions allowing us to repeat the whole cycle once more. Of high value is the measurement of breakthrough curves and their theoretical modeling. Dependent on time of adsorption, the method consists of the deter-
448
mination, easily accomplished, of the exit concentration of normal paraffins. The breakthrough curve for a fresh (not used) sieve I s compared with the breakthrough curves o f aged and used zeolites. We see that the adsorption capacity decreases with increasing time of ageing. In order to be able to calculate these breakthrough curves, we developed a method of calculation, based on the use of the method o f moments, described by us elsewhere. We see that the measured and the calculated curves can be compared, although we must confess that, up to now, we could not solve all problems which hinder an exact comparison between measured and calculated data. One of the decisive economic factors of the Parex process i s the use of a m n i a . It must be regained completely; losses are not acceptable. The amount of desorption gas must be sufficient, of course, to replace the adsorbed hydrocarbons completely. But going beyond an optimum amount leads to a reduction in the yield of desorbed hydrocarbons. By changing the used principle of forwardflow desorption to reverse-flow desorption, the degree of desorption is greater and therefore the yield increases. It seems to be possible to reduce the amount of desorption agent by approximately 50 % without lowering the yield. This would lead to a lower energy input for the whole process. Unfortunately, it has not yet been possible to introduce the principle of reverse flow into the existing plants. It is probable that, under these conditions, the whole adsorptiondesorption cycle must be changed. We calculated models of the Parex process with various circuits of the adsorbing-desorbing regime, and we are convinced that "the final word is not yet spoken" in this direction (ref. 8). We now want to summarize our results and to give a survey of the efficiency of our Parex model. We hope that our intention, to show the interdependence between technical progress and fundamental research, has been elucidated. We were forced here to omit certain developments, especially methods of the NMR technique, and theoretical questions. What is our model now good for? - It allows the simulation of large-scale adsorption-desorption processes on the basis o f model parameters determined in laboratory-scale experiments. - It is not necessary to make an a-posteriori fit o f free model parameters to large-scale measurements. - The model has shown its universal applicability to various types of processes of selective adsorption from the gaseous phase. - The model allows calculations for the variation of: The adsorber geometry, the number of the adsorbers and their switching (circuit of products), the cycle times, the amount of feed and purge gas. the composition of these gases,
.
. . . .
449
.
the type and the state of ageing of the zeolite, the energy consumption of the process. The fact that our chemical enterprises could sell this process dozens of times to other countries and that negotiations for further plants are in progress is the best evidence for its efficiency. The annual production capacity of normal paraffins based on the Parex process, by our present estimate, amounts to more than 1.5 millions of tons.
.
GLOSSARY
1 ) Latln letters:
amount adsorbed, g/g or mmol/g vertical vectors In general transport equatlon matrix in general transport equation equi 1 ibrium constant in Langmuir equation vector in general transport equation operator in general Ized form in transport quation gas concentration In mac o por s, mol m- 9 diffusion coefficient, m5 s-' diffusion coeff i ci ent of axi a1 di spersi on diffusion coefficient under the influence of concentration gradients coefficient of self-diffusion source of material in general transport equation gas concentration In void space of column, mol N3 flux of material, mol s-1 m-3 mass-transfer coefficient from gas ous phase, m s-l adsorbate concentration, mol cm- 9 heat of adsorption, J mol-l radial coordinate of pellet, m radius of pellet, m universal gas constant, J mol-1 ~ - 1 fictive gas concentration in macropores, mol m-3 time o f observation, s or min interstitial gas velocity, m s-1 relative volume of pellet (V ) , of macropores (V,), Micropores(Vq) axial coordinate of the bed gf column, m 2) Greek letters: = (1 overall heat transfer coefficient, W m-* K-l Y = factor of proportionality in general transport equation = volumetric heat capacity of flowing gas, of pe let, J mol-l yg. yp = potential of adsorption in cavities, KJ mol0 = thermal conductivity of flow ng gas, of pellet, W m - l K1 hax,hp = chemical potential, J molP T = normalized time in adsorption-cycle 0 = loading of cavities T = normalized function in adsorption-isotherm 3) Subscripts, indices = property belonging to axial dispersion ax C = property of the macropore-system 9 = property of the void space in column i = component in the gaseous phase P = states in the gaseous phase q = property of the micropore-system P = diffusion in mlcropores Lo = property in the state o f saturation
.
-
.
.
.
-
.
0
-
1
1
.
-
.
450
REFERENCES
K. Fiedler, A., Roethe, K.-P. Roethe and 0. Gelbin, Z. phys. Chem., L e i p z i g 259 11978) 979. K. Wehner; J. Welker and 6. Seidel, Chem. Techn. 2 1 (1969) 548. G. Seidel, J. Welker, W. Ermischer and K. Wehner, Chem. Techn. 31 ( 1979) 405. W. Schirmer, K. F i e d l e r and H. Stach, Thermodynamics o f Adsorption on Zeolites, Molecular Sieves 11, ACS Symp. Ser. 40, Washington, 1977, p. 305. K. Fiedler, U. Lohse, J. Sauer, H. Stach, H. Thamn and W. Schirmer, Proc. 5 t h Int. Conf. on Zeolites, Naples 1980, Heyden (London) p. 490. K.-H. Richter, Workshop "Adsorption o f Hydrocarbons i n Zeolites", Proceedings B e r l i n (DDR) 1979 p. 231. H. P f e i f e r , J. Karger, A. Germanus, W. Schirmer, M. Bulow and J. Caro, Ads. Sci. Technol. 2 (1985) 229. M. Suckow, Thesis 1986, Humboldt-Universitat, B e r l i n (DDR).
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SORBEX TECHNOLOGY FOR INDUSTRIAL SCALE SEPARATION
J . A. JOHNSON AND A. R. OROSKAR UOP I nc . , 25 E . A l g o n q u i n Rd., Des P l a i n e s , I l l i n o i s , 60017, U.S.A. INTRODUCTION
N e a r l y ev ery chemical m a n u f a c t u r i n g o p e r a t i o n r e q u i r e s t h e use o f separat i o n processes i n o r d e r t o r e c o v e r and p u r i f y t h e d e s i r e d product .
I n most
c a s e s , t h e e f f i c i e n c y o f t h e s e p a r a t i o n process has a s i g n i f i c a n t impact on b o t h t h e q u a l i t y and t h e c o s t o f t h e p r o d u c t (1). Because o f i t s s i m p l i c i t y and bro ad a p p l i c a t i o n ,
d i s t i l l a t i o n has become t h e st andard a g a i n s t which O t her s e p a r a t i o n processes
a1 t e r n a t i v e s e p a r a t i o n t e c h n o l o g i e s a r e compared.
ba s e d on f e ed component physicochemical p r o p e r t i e s ,
such as c r y s t a l 1 i z a t i o n
and s o l v e n t e x t r a c t i o n , a r e also used i n t h e chemical i n d u s t r y .
However,
t h e r e a r e many cases t h a t i n v o l v e t h e b u l k s e p a r a t i o n o f c h e m i c a l l y s i m i l a r components o r t h e removal o f components t h a t a r e p r e s e n t a t l o w concentrations.
I n t he s e s i t u a t i o n s t h e t r a d i t i o n a l s e p a r a t i o n t echniques based on
p h y s i c o c h e m i c a l p r o p e r t i e s h a v e l i m i t e d usef ulness and may even be unworkable.
For t h i s l a t t e r c l a s s o f s e p a r a t i o n problems,
adsorption o f f e r s a
s u i t a b l e approach. A d s o r p t i v e s e p a r a t i o n processes o f f e r g r e a t e r degrees o f freedom i n p e r f o r m i n g d i f f i c u l t s e p a r a t i o n problems, because t hey can e x p l o i t d i f f e r e n c e s i n t h e molecular structure,
s t e r i c h i n d r ance and e l e c t r o c h e m i c a l
i n t e r a c t i o n among f e e d components v i s - a - v i s t h e adsorbent.
surface
These small d i f -
f e r e n c e s on t h e m o l e c u l a r s c a l e can be a m p l i f i e d t o p r o v i d e o v e r a l l adsorbent s e l e c t i v i t i e s and c a p a c i t i e s t h a t a r e o f i n t e r e s t f o r p r o d u c t i o n s c a l e economics. A lt h ough t h e investment c o s t o f an a d s o r p t i o n process i s g e n e r a l l y h i g h e r t h an t h a t o f a d i s t i l l a t i o n u n i t w i t h an e q u i v a l e n t number o f t h e o r e t i c a l stages, much h i g h e r s e p a r a t i o n f a c t o r s a r e commonly a t t a i n e d . Thus, a t l o w r e l a t i v e v o l a t i l i t y , an a d s o r p t i o n process becomes t h e more economic o p t i o n ( 2 ) . For p u r i f i c a t i o n processes i n v o l v i n g l i g h t gases where t h e a l t e r n a t i v e i s c ry o g e n i c d i s t i l l a t i o n , a d s o r p t i o n i s p r e f e r r e d even when t h e r e l a t i v e v o l a t i l i t y i s high (3). UOP has s u c c e s s f u l l y a p p l i e d a d s o r p t i v e s e p a r a t i o n on an i n d u s t r i a l
s c a l e v i a t h e Sorbex* process f o r t h e p u r i f i c a t i o n and r e c o v e r y o f b u l k chemicals (4).
The f i r s t commercial a p p l i c a t i o n o c c u r r e d i n 1964 w i t h t h e
452
advent o f the UOP Molex* process f o r recovery o f h i g h - p u r i t y n - p a r a f f i n s . Since t h a t time,
Sorbex technology has been a p p l i e d t o a broad range o f
problems i n c l u d i n g i n d i v i d u a l isomer separations and class separations. applications
a r e c o n t i n u a l l y being i d e n t i f i e d and developed.
New
While a l l
Sorbex a p p l i c a t i o n s share t h e same basic well-proven and e f f i c i e n t process design,
the adsorbent p r o p e r t i e s and t h e performance requirements are d i f -
f e r e n t f o r each type o f separation. O f t h e various m a t e r i a l s a v a i l a b l e f o r use as adsorbents, z e o l i t e s o f f e r a p a r t i c u l a r l y wide range o f parameters t h a t can be manipulated t o make them e f f e c t i v e as adsorbents f o r i n d u s t r i a l scale separations.
I n t h i s paper we g i v e an overview o f e x i s t i n g Sorbex processes, describe t h e d e s i r a b l e c h a r a c t e r i s t i c s o f adsorbents, discuss some o f t h e unique capab i l i t i e s o f z e o l i t e s as adsorbents and suggest p o t e n t i a l a p p l i c a t i o n s f o r t h e future.
ZEOLITES AS PRACTICAL ADSORBENTS The search f o r a s u i t a b l e adsorbent i s g e n e r a l l y the f i r s t step i n the development o f a Sorbex process. There are f o u r primary requirements f o r a p r a c t i c a l adsorbent: s e l e c t i v i t y , capacity, mass t r a n s f e r r a t e and l o n g term stability.
The requirement f o r adequate adsorptive c a p a c i t y r e s t r i c t s t h e
c h o i c e o f adsorbents t o microporous s o l i d s w i t h pore diameters ranging from a few angstroms t o a few hundred angstroms. M a t e r i a l s such as s i l i c a g e l , a c t i v a t e d alumina, a c t i v a t e d carbons, polymeric r e s i n s and molecular sieve z e o l i t e s are, therefore, s u i t a b l e p r a c t i c a l adsorbents. T r a d i t i o n a l adsorbents such as s i l i c a ,
a c t i v a t e d alumina and a c t i v a t e d
carbon e x h i b i t l a r g e surface area and micropore volume.
The surface chemical
p r o p e r t i e s o f these adsorbents make them p o t e n t i a l l y u s e f u l f o r separations by m o l e c u l a r class. However the micropore s i z e d i s t r i b u t i o n i s f a i r l y broad f o r t h e s e m a t e r i a l s , as shown i n Figure 1. This c h a r a c t e r i s t i c makes such mat e r i a l s u n s u i t a b l e f o r use i n separations where s t e r i c hindrances could pot e n t i a l l y be e x p l o i t e d . Polymeric r e s i n s are widely used i n t h e food and pharmaceutical indust r i e s as cation/anion exchangers, f o r t h e removal of t r a c e components and f o r some b u l k separations such as fructose/glucose. They are p r i m a r i l y a t t r a c t i v e f o r aqueous phase separations and o f f e r a f a i r l y wide p o t e n t i a l range o f s u r f a c e chemistries t o f i t a number o f separation needs. For example, they a r e e f f e c t i v e i n p a r t i t i o n i n g by s i z e and molecular weight, and may a l s o be e f f e c t i v e i n i o n exclusion.
453
ACTIVATED CARBON
30 ZEOLITE l b
2
5
10
20
50
PORE DIAMETER, ANGSTROM
, ~, ,
u o ~
Fig. 1. Pore size distribution (Ref. 6). TABLE 1
Molecular sieve pore structures. COMMON NAME
RING OPENING
-
FREE APERATURE
7.4 A 2.9 x 5.7A 6.7 x 7.0 A 7.1 A 5.4 x 5.6 A 5.1 x 5.6 A 3.6 x 5.2 A 4.2 A
FAUJASlTE
MORDENITE
L ZSM-5
ERIONITE A “1 D”
“20”
PORE STRUCTURE
30
1D 1D
2D 3D
“30”
In contrast to these adsorbents, zeolites offer increased possibilities for exploiting molecular level differences among adsorbates. Zeolites are crystalline aluminosilicates containing an assemblage of SiO4 and A104 tetrahedra joined together by oxygen atoms to form a microporous sol id, which has a precise pore structure (5). Nearly 40 distinct framework structures have been identified to date. Table 1 summarizes some of those that have been widely used in the chemical industry. The versatility o f zeolites lies in
454
t h e f a c t t h a t w i d e l y d i f f e r e n t a d s o r p t i v e p r o p e r t i e s may be r e a l i z e d by ap-
S i / A 1 r a t i o and t h e c a t i o n z e o l i t e A, shown i n F i g u r e 2, has a t h r e e dimensional
p r o p r i a t e c o n t r o l o f t h e framework s t r u c t u r e , form.
F o r example,
i s o t r o p i c channel s t r u c t u r e c o n s t r i c t e d b y an eight-membered oxygen r i n g . I t s e f f e c t i v e p o r e s i z e can be c o n t r o l l e d a t about 3A, 4A and 4.5A by exchanging w i t h potassium, sodium and c a l c i u m , r e s p e c t i v e l y . The pot assium f o r m , w i t h 3A pores, i s used f o r removing wat er f rom o l e f i n i c hydrocarbons. The sodium f o rm can be used t o e f f i c i e n t l y remove w a t e r f rom n o n - r e a c t i v e hydrocarbons, such as a l k a n e s . S u b s t i t u t i o n o f c a l c i u m can p r o v i d e a p o r e s i z e t h a t w i l l admit normal p a r a f f i n s , and e xclude o t h e r hydrocarbons.
W)P imia
F i g . 2 . Three z e o l i t e s w i t h t h e same s t r u c t u r a l polyhedron (cubo-octahedron). L a r g e p o r e z e o l i t e s , X, Y and m o r d e n i t es have pores d e f i n e d b y t w e l v e membered oxygen r i n g s w i t h a f r e e d i a m e t e r o f 7.4A. The framework s t r u c t u r e o f X and Y f a u j a s i t e s , sketched i n F i g u r e 2 c o n s i s t s o f a t o t a l o f 192 SiO2 and A102 u n i t s .
The s i l i c a - t o - a l u m i n a r a t i o f o r X i s g e n e r a l l y between 1
and 1.5, whereas f o r Y i t i s between 1.5 and 3 . W i t h s u i t a b l e procedures, Y can be "dealuminated" t o Si/A1 r a t i o s exceeding 100. A d s o r p t i o n p r o p e r t i e s o f f a u j a s i t e s a r e s t r o n g l y dependent on n o t o n l y t h e c a t i o n f orm b u t a l s o t h e Si/A1
ratio.
The f l e x i b i l i t y p r o v i d e d f a n j a s i t e s i n t h e a d s o r p t i o n o f c8
a r o m a t i c s i s shown i n Ta b l e 2. The s e l e c t i v i t y order, f r o m most s t r o n g l y a d s o r b e d t o t h e most r e a d i l y r e j e c t e d , can be changed s i g n i f i c a n t l y by t h e choice o f z e o l i t e p r o p e r t i e s .
455
TABLE 2 S e l e c t i v i t y o f z e o l i t e s i n C8-aromatic system. 1
=
4
= LEAST SELECTIVELY ADSORBED
MOST SELECTIVELY ADSORBED
ADSORBENT
ADSORBENT
ADSORBENT
#l
#2
#3
#4
1 2
3
2 1 3
4
4
3 4 1 2
4 3 2 1
ADSORBENT p XYLENE ~
ETHYLBENZENE m - XYLENE o-XYLENE
WP
itai.?
The s e p a r a t i o n o f f r u c t o s e from g l u c o s e serves t o i l l u s t r a t e t h e i n t e r a c t i o n between t h e framework s t r u c t u r e and t h e c a t i o n i n F i g u r e 3. Ca i s known t o f o rm complexes w i t h sugar molecules such as f r u c t o s e . shows a h i g h s e l e c t i v i t y f o r f r u c t o s e o v e r glucose. However, e x h i b i t any s e l e c t i v i t y f o r t h e hexoses. t i v i t y f o r gluc o s e o v e r f r u c t o s e .
Thus,
Ca-Y
Ca-X does n o t
On t h e o t h e r hand, K-X shows s e l e c -
I t i s t h e p o l a r n a t u r e o f f a u j a s i t e s and
t h e i r unique s h a p e - s e l e c t i v e p r o p e r t i e s more so t h a n t h e m o l e c u l a r s i e v i n g p r o p e r t i e s t h a t makes them most u s e f u l as p r a c t i c a l adsorbents.
I
TIME
C
P
F i g . 3. F ruc t o s e- g l u c o s e s e p a r a t i o n on f a u j a s i t e adsorbents.
456
Mordenites represent another class of large pore zeolites, but in conAlthough trast to faujasites the mordenite pore opening is one-dimensional mordenites can exhibit some interesting adsorption properties, the singledimensional pore structure makes these adsorbents more sensi tive to "poreplugging" by trace feed components, which may be too strongly adsorbed. As a result, no significant commercial uses have been found for mordenite adsorbents. On the other hand, many catalytic uses of mordenite are well known to the chemical industry.
.
Most zeolites are synthesized as 1-10 micron crystals, in an ionic form which is usually most convenient for the synthesis step. This particle size is unsuitable for direct use in liquid or gas phase continuous adsorption processes, and may also 1 imit the commercial scale post-treatment techniques (ion exchange, hydrothermal treatment) that need to be employed to impart the desired adsorption properties. As a result, the microporous zeolite powder is typically combined with a suitable binder and formed into macroporous particles, which meet the hydrodynamic constraints of the process unit. While it is desirable to maximize the volumetric capacity, it is also important to have low macropore resistance to mass transfer and acceptable particle strength. Proper choice o f the binder and the binding method are essential to the formulation of an acceptable adsorbent. Clays have generally been used as binders for zeolitic adsorbents in hydrocarbon service. The adsorbents may be either granular or spherical in shape and should be narrow in size distribution, which is an important factor in the control of axial, convective dispersion.
ADSORPTIVE SEPARATIONS PROCESSING STRATEGIES Industrial scale adsorption processes can be classified according to whether they are batch or continuous (2). In a batch process the adsorbent bed i s saturated and regenerated in a cyclic operation, while in a continuous process a countercurrent staged contact between the adsorbent and the feed and desorbent is established by either a true or a simulated recirculation of the adsorbent. Batch processes generally operate in three regeneration modes: thermal swing, pressure swing and displacement desorption (6). The choice of mode is determined primarily by the feed composition and the adsorbent characteristics. Thermal-swing operation requires long cycle times and thus is suited for the removal of trace contaminants. Pressure-swing operation, on the other hand, allows very low cycle times and i s , thus, effective for bulk
451
s e p a r a t i o n s i n vapor phase.
The c h o i c e o f a desorbent i s c r u c i a l i n a d i s -
placement mode, w h i c h can be used f o r t r a c e as w e l l as b u l k separat ions. Examples o f bat ch a d s o r p t i o n processes i n i n d u s t r i a l p r a c t i c e a r e p r o v i d e d i n T a ble 3 .
TABLE 3 Some examples o f b a t c h a d s o r p t i o n processes (Ref. 3 ) . TRACE COMPONENT SEPARATION H 2 0 I NATURAL GAS, CRACKED GAS AIR, ETC.
TYPE __
ADSORBENT
THERMAL REGENERATION
SILICA, ALUMINA ZEOLITES
THERMAL REGENERATION
C 0 2 I NATURAL GAS, ETHYLENE, ETC. ZEOLITES ORGANICS I VENT STREAM
ACTIVATED CARBON
STEAM REGENERATION
NO,, SO2 I VENT STREAMS
ZEOLITES
THERMAL REGENERATION
ZEOLITES
THERMAL SWING DISPLACEMENT DESORPTION
BULK SE PARATlZS NORMAL PARAFFIN I I S 0 PARAFFIN
SOLVENTS I VENT GASES
ZEOLITES
PRESSURE SWING
CARBON MOLECULAR SIEVE
PRESSURE SWING (KINETIC SEPARATION)
ACTIVATED CARBON
VACUUM DESORPTION
ux tMld
The e f f i c i e n c y o f an a d s o r p t i o n process i s s i g n i f i c a n t l y h i g h e r i n a cont i n u o u s mode o f o p e r a t i o n t h a n i n a c y c l i c b a t c h mode ( 7 ) . I n a b a t c h chromat h e l i q u i d c o m p o s i t i on a t a g i v e n l e v e l i n t h e bed u n d e r g o e s a c y c l i c change w i t h time, and l a r g e p o r t i o n s o f t h e bed do n o t
tographic operation,
p e r f o r m any u s e f u l f u n c t i o n a t a g i v e n t i m e . On t h e o t h e r hand, i n cont i n u o u s o p e r a t i o n t h e c o m p o s i t i o n a t a g i v e n l e v e l i s i n v a r i a n t w i t h t i m e and e v e r y p a r t o f t h e bed performs a u s e f u l f u n c t i o n a t a l l t imes.
The HETP i n a
b a t c h o p e r a t i o n i s r o u g h l y t h r e e t i m e s t h a t i n a cont inuous mode.
For d i f -
f i c u l t s epara t i o n s , b a t c h o p e r a t i o n may r e q u i r e t w e n t y - f i v e t i m e s more ads orb ent i n v e n t o r y and t w i c e t h e desorbent c i r c u l a t i o n r a t e t han a c o n t i n u o u s operation. I n a d d i t i o n , i n a b a t c h mode, t h e f o u r f u n c t i o n s o f a d s o r p t i o n , p u r i f i c a t i o n , d e s o r p t i o n and displacement o f t h e desorbent f rom t h e adsorbent a r e i n f l e x i b l y l i n k e d , whereas a c o n t i n u ous mode a l l o w s more degrees o f f r e e d o m w i t h r e s p e c t t o t h e s e f u n c t i o n s and, thus, tion.
a b e t t e r o v e r a l l opera-
C o n t i n u o u s c o u n t e r c u r r e n t c o n t a c t , however, i n c r e a s e s t h e c o m p l e x i t y o f d e s i g n and t h e d i f f i c u l t y o f c o n t r o l .
One o f t h e f i r s t at t empt s a t con-
t i n u o u s c o u n t e r c u r r e n t a d s o r p t i o n p r o c e s s i n g was t h e H y p e r s o r p t i o n process
458
f o r t h e gas phase recovery o f ethylene ( 6 ) .
T h i s process i n v o l v e d actual
movement o f t h e adsorbent along w i t h a temperature swing desorption.
The
s e p a r a t i o n e f f i c i e n c y was high, but t h e steady a t t r i t i o n o f t h e adsorbent and t h e more complex design u l t i m a t e l y 1 i m i t e d wider commercial a p p l i c a t i o n o f t h i s process. The UOP Sorbex process ( 4 ) , f i r s t commercialized i n 1964, sought t o g a i n t h e t h e o r e t i c a l advantages o f t r u e countercurrent processing w i t h o u t physic a l l y moving t h e adsorbent. This was done through t h e use o f a design concept t h a t simulated t h e countercurrent movement o f t h e 1 i q u i d process streams p a s t t h e s o l i d adsorbent.
By maintaining t h e adsorbent bed s t a t i o n a r y , par-
t i c l e a t t r i t i o n was prevented.
Also,
very importantly,
preserved p l u g f l o w o f l i q u i d past the adsorbent,
t h e Sorbex design
avoiding the non-uniform
f l o w p a t t e r n s and d i s p e r s i o n t h a t would n a t u r a l l y r e s u l t from p h y s i c a l l y moving the adsorbent past t h e l i q u i d . Therefore, t h e i n t r i n s i c separation e f f i c i e n c y o f t h e adsorbent p a r t i c l e could be r e a l i z e d on a macroscopic p r o c e s s scale. The basic Sorbex design has been perfected over t h e years and extended t o many a p p l i c a t i o n s . I t i s used worldwide f o r b u l k separation and recovery o f h i g h p u r i t y products. THE SORBEX PROCESS A schematic f l o w diagram o f the adsorption s e c t i o n of the Sorbex process i s shown i n Figure 4 .
The adsorbent i s d i v i d e d i n t o several beds o f equal
s i z e . A t t h e t o p and the bottom o f each bed i s a l i q u i d d i s t r i b u t o r t h a t i n t u r n i s connected t o a bed l i n e . Each bed l i n e can t r a n s f e r l i q u i d i n t o o r o u t o f t h e adsorbent chamber, but a t any given t i m e o n l y a c e r t a i n number o f t h e bed l i n e s are activated. These bed l i n e s terminate a t t h e r o t a r y valve, t h e i n d u s t r i a l analog o f a m u l t i - p o r t stopcock. The r o t a r y v a l v e i s a l s o connected t o the external process streams:
feed,
desorbent,
e x t r a c t and
r a f f i n a t e . The i n f l u e n t feed and desorbent are d i r e c t e d by the r o t a r y valve t o bed l i n e s t h a t are several adsorbent beds apart. S i m i l a r l y , t h e e f f l u e n t e x t r a c t and r a f f i n a t e streams are withdrawn from bed l i n e s t h a t are several adsorbent beds apart. The r o t a r y valve step time i s constant and s p e c i f i c f o r a given separation.
459
F i g . 4 . Sorbex process, s i m u l a t e d moving bed f o r a d s o r p t i v e s e p a r a t i o n . I n t h e ex a m p l e o f F i g u r e 4 , o n l y f o u r o u t o f t h e t w e l v e bed l i n e s a r e a c t i v e a t any g i v e n t i m e , and t h e i n t e r v a l between each o f t h e a c t i v e l i n e s r e m a i n s f i x e d , d e f i n i n g d i s t i n c t f u n c t i o n a l zones f o r t h e Sorbex process. A d s o r p t i o n t a k e s p l a c e i n t h e beds between t h e f eed and t h e r a f f i n a t e s t r e a m s . R e c t i f i c a t i o n t a k e s p l a c e i n t h e beds between t h e e x t r a c t and t h e f e e d streams. D e s o r p t i o n t a k e s p l a c e i n t h e beds between t h e desorbent and t h e e x t r a c t streams. A f o u r t h zone, l o c a t e d i n t h e beds between t h e r a f f i n a t e and des orb ent streams, d i s p l a c e s desorbent f rom t h e adsorbent pores e n t e r i n g t h e a d s o r p t i o n zone,
w h i l e p r e v e n t i n g t h e cont aminat ion o f t h e d e s o r p t i o n
zone w i t h r a f f i n a t e components. L i q u i d i s pumped from t h e t o p t o t h e b ot t om o f t h e chamber, d i r e c t i o n as t h e r o t a r y v a l v e advances.
i n t h e same
The f l o w r a t e changes whenever t h e
r o t a r y v a l v e causes t h e pump t o c r o s s a new zone boundary.
Steady st epwise
advancement o f t h e r o t a r y v a l v e combines w i t h t h e p a r t i c u l a r n e t l i q u i d f l o w i n e a c h zone t o s i m u l a t e t h e c o n t i n u o u s c o u n t e r c u r r e n t movement o f t h e l i q u i d and s o l i d phases, e s t a b l i s h i n g t h o s e c o n d i t i o n s t h a t a r e most s u i t a b l e f o r t h e f u n c t i o n o f t h a t p a r t i c u l a r zone. I n e f f e c t , t h e r o t a r y v a l v e i s c i r c u l a t i n g t h e s e l e c t i v e and n o n - s e l e c t i v e adsorbent volume a t a c o n s t a n t r a t e , and t h e n e t l i q u i d f l o w r a t e i n any g i v e n zone i s r e f l u x e d a g a i n s t t h a t adso rb ent volume c i r c u l a t i o n r a t e . The c o n c e n t r a t i o n p r o f i l e shown i n F i g u r e 4
460
moves w i t h t h e advancement o f the r o t a r y valve,
w i t h t h e s o l i d and l i q u i d
phases being i n e q u i l i b r i u m w i t h each other. A t y p i c a l steady s t a t e l i q u i d c o m p o s i t i o n p r o f i l e f o r t h e UOP C 4 O l e x * p r o c e s s (8) i s shown on a desorbent-free basis i n Figure 5. The r a t e s o f e x t r a c t and r a f f i n a t e are c o n s t a n t , as are t h e compositions o f these streams going t o t h e desorbent recovery section.
40-
I
F i g . 5. p l ant.
Steady s t a t e l i q u i d concentration p r o f i l e s f o r a C4 Olex p i l o t
As shown i n Figure 4, the e x t r a c t and r a f f i n a t e streams c o n t a i n dew h i c h must be removed i n order t o achieve t h e d e s i r e d p u r i t y . Desorbent must be c a r e f u l l y selected along w i t h t h e adsorbent i n order t o achieve a completely r e v e r s i b l e process. Too strong a desorbent i n t e r f e r e s w i t h t h e adsorption f u n c t i o n , w h i l e a weak desorbent may l i m i t p u r i f i c a t i o n o r recovery. The desorbent must be compatible w i t h t h e feed components and be e a s i l y recovered by evaporation o r simple f r a c t i o n a t i o n f o r t o t a l recycle. sorbent,
The Sorbex process operates i n t h e l i q u i d phase a t a constant temper a t u r e , which i s selected t o maximize mass t r a n s f e r w h i l e maintaining select i v i t y and c o m p a t i b i l i t y w i t h t h e feed components. A t t h e o p e r a t i n g temperat u r e o f choice, t h e r e i s an e q u i l i b r i u m r e l a t i o n s h i p between t h e composition This relationship i s i n t h e l i q u i d phase and t h a t i n t h e adsorbent phase. analogous t o t h a t o f v a p o r / l i q u i d e q u i l i b r i u m i n f r a c t i o n a l d i s t i l l a t i o n . T y p i c a l adsorption isotherms are sketched i n Figure 6. The simplest i s a
461
l i n e a r is ot h erm .
I n t h i s case, t h e p r o p e r n e t zone f l o w r a t e s a r e dependent
s o l e l y upon t h e Henry’s Law c o n s t a n t s and n o t t h e f eed c o n c e n t r a t i o n .
1I WUlD PHASE CONCPNTR 0N
UOP leal4
Fig . 6. T y p i c a l a d s o r p t i o n i s o t h e r m s encountered i n Sorbex s e p a r a t i o n . N o n - l i n e a r a d s o r p t i o n i s o t h e r m s r e s u l t i n n e t zone f l o w r a t e s which a r e a f u n c t i o n o f b o t h t h e Henry’s Law c o n s t a n t s and t h e s l o p e o f t h e isot herms a t t h e f eed c o n c e n t r a t i o n s . Secondary e f f e c t s , such as t h e dependence of s e l e c t i v i t y f o r one component on t h e c o n c e n t r a t i o n o f anot her component, have been n o t e d and pose a d d i t i o n a l problems i n i d e n t i f y i n g t h e most f a v o r a b l e n e t zone f l o w r a t e s .
O v e r a l l , however, t h e Sorbex system has been proven e f -
f e c t i v e f o r a b r o a d r a n g e o f s e p a r a t i o n problems i n t h e pet rochemical, c hemic a l and food/biochemical i n d u s t r i e s . We have r a r e l y been d i s a p p o i n t e d i n t r a n s 1 a t i n g f r o m 1 a b o r a t o r y p r o o f - o f - p r i n c i p l e t e s t i n g t o cont inuous operation. Lik e w i s e , we have been s u c c e s sf ul i n s c a l i n g up f rom o u r p i l o t p l a n t s d i r e c t l y t o commercial p r o d u c t i o n , w i t h adsorbent chamber diamet ers as l a r g e as 6 . 5 meters. S orb ex t e c h n o l o g y i s used f o r a wide range o f i n d u s t r i a l s c a l e separat i o n s , w i t h p r o d u c t i o n c a p a c i t i e s r a n g i n g f rom about 30,000 t o 400,000 t o n s p e r y e a r (9). The v e r s a t i l i t y o f l i q u i d phase s e p a r a t i o n i s shown i n T able 4 , w h i c h l i s t s t h e v a r i o u s Sorbex processes i n use t hroughout t h e world. More t h a n seventy u n i t s have been l i c e n s e d , account ing f o r n e a r l y 8 m i l l i o n I n a d d i t i o n , UOP has developed and t o n s p e r y e a r o f product capacity. commerci a1 i z e d s e v e r a l o t h e r Sorbex processes f o r p r o d u c t i o n o f s p e c i a l t y chemicals a t o u r Shreveport, L o u i s i a n a , l o c a t i o n .
462
TABLE 4
Sorbex for commodity chemicals. PROCESS
SEPARATION
PAREX
p-XYLENEICg AROMAmS
44
MOLEX
n-PARAFFINS/ BRANCHED AND CYCLIC HYDROCARBONS
21
OLEX
OLEFINSIPARAFFINS
6
CYMEX
p or m-CYMENEICYMENE ISOMERS
1
CRESEX
p or mCRESOLICRESOL ISOMERS
1
SAREX
FRUCTOSEI DEXTROSE PLUS POLYSACCHARfDES
-5 78
WIAL PRODUCT CAPACITY
> 8 MILUON TONS IW UOP 1u5.1 uw 1Wl-12
COMPARISON OF ZEOLITE AND RESIN ADSORBENTS The UOP Sarex* process has been used for the separation of high purity
fructose from a mixture of fructose, glucose and polysaccharides. This was the first aqueous phase application of the Sorbex process, with the first commercial unit on-stream in 1978. The Sarex process has employed both zeolitic and polymeric resin adsorbents for the production of high fructose corn syrup (HFCS). The operating characteristics of these two adsorbents are substantially different. A1 though both adsorbents exhibit similar selectivity for fructose over glucose and polysaccharides, we find that the zeolitic adsorbent can be operated at a reduced rotary valve cycle time. This gives a throughput ad vantage for a unit of fixed size or, conversely, a lower adsorbent requirement to achieve fixed capacity. Further constraints are placed on the resin system due to its shrink/swell response to osmotic changes and its compressibility. On the other hand, the resin adsorbent tends to provide higher extract solids concentrations, meaning lower desorbent usage and reduced evaporation requirements. Ho, et al. (10) have compared these two types o f adsorbents in terms o f fundamental characteristics such as capacity, selectivity and adsorption kinetics. Here we can expand on their results and discuss other properties of the two types of adsorbents that are equally important to commercial operability. Elution chromatographic results of Ho, et al., are summarized in Table 5. Particle based adsorption equilibria are similar for both ad-
463
sorbents,
w i t h fructose/glucose
s e l e c t i v i t e s on t h e order o f 1 . 5 - 2 . 4 .
S i m i l a r r e s u l t s have been seen i n UOP t e s t s .
There i s , however,
a large
d i f f e r e n c e i n mass t r a n s f e r c o e f f i c i e n t s between the r e s i n and t h e z e o l i t i c adsorbent i n the data reported by Ho, e t a l . , w i t h values t h a t are 4-5 times slower f o r the r e s i n adsorbent. Analogous t e s t s run by UOP on commercial a d s o r b e n t show t h e same trends, w i t h values t h a t are 2 - 3 times slower f o r t h e r e s i n system tested. These substantial d i f f e r e n c e s i n mass t r a n s f e r r a t e s r e f l e c t i n t h e c y c l e time d i f f e r e n c e s experienced i n continuous operation o f these two adsorbents.
TABLE 5 Adsorption c h a r a c t e r i s t i c s o f z e o l i t e s and r e s i n s (Ref. 10).
SOURCE
MATERIAL
MASS-TAINSFER
EQUlUBRlUM CONSTANT
COEFFICIENT
KFRUCTOSE KQLUCOSE
HO,ET AL.
Ca-Y (300p) DUOUTE (300,~) ZEROUTE (300p )
0.68 0.46 0.49
(1 I KK)F, SK
13 67 61
0.44 0.31 0.21
(1 I KKk;, S6C
12 141 139 UOP leu, 9
As reported by Ho, e t al.,
the z e o l i t e and the r e s i n adsorbents show d i f f e r e n t adsorption isotherm c h a r a c t e r i s t i c s , p a r t i c u l a r l y a t higher concentration.
The r e s i n adsorbents have an isotherm t h a t i s s l i g h t l y concave
upwards, whereas t h e z e o l i t e isotherm i s l i n e a r , o r even s l i g h t l y concave downwards.
Resins, therefore, would have an advantage i n a Sarex operation
t h a t involves a h i g h feed sol i d s concentration. Apart from such fundamental parameters as s e l e c t i v i t y , capacity and mass t r a n s f e r rate, t h e r e are other l e s s t a n g i b l e f a c t o r s t h a t p l a y an important p a r t i n the commercial v i a b i l i t y o f a Sarex adsorbent, namely pressure drop c h a r a c t e r i s t i c s and adsorbent 1 i f e . Pressure O r O D C h a r a c t e r i s t i c s
--
I o n exchange r e s i n s are compressible
and e x h i b i t a c h a r a c t e r i s t i c s t r e s s / s t r a i n r e l a t i o n s h i p as shown i n Figure
7.
I n a d d i t i o n they undergo shrink/swell
as a r e s u l t o f osmotic pressure
v a r i a t i o n r e s u l t i n g from concentration changes.
Z e o l i t e adsorbents are r i g i d
464
an d d o n o t e x h i b i t much s t r a i n w i t h p r e s s ure. sorbents, t h e i m p l i c a t i o n s o f shrink/swell
When r e s i n s a r e used as ad-
and c o m p r e s s i b i l i t y must be con-
s i d e r e d i n o r d e r t o ensure s a f e o p e r a t i o n below t h e d e s i g n p r e s s u r e drop. The p re s s ure d ro p c a n o f t e n become a m a j o r f a c t o r i n t h e d e t e r m i n a t i o n o f maximum t hro ughpu t .
F i g . 7 . C o m p r e s s i b i l i t y c h a r a c t e r i s t i c s o f adsorbents. Adsorbent L i f e
--
Long t e r m s t a b i l i t y under rugged c o n d i t i o n s i s a v e r y
i m p o r t a n t c h a r a c t e r i s t i c o f an adsorbent.
Z e o l i t e s by t h e i r v e r y n a t u r e a r e
n o t v e r y s t a b l e i n an aqueous environment. We have f ound t h a t t h e y have t o be s p e c i a l l y f o r m u l a t e d t o enhance t h e i r s t a b i l i t y i n o r d e r t o o b t a i n s e v e r a l y e a r s o f service. P o l y m e r i c r e s i n s do n o t s u f f e r f rom d i s s o l u t i o n problems. However, t h e y a r e prone t o chemical a t t a c k (11). FUTURE DIRECTIONS FOR SORBEX We h a v e a l r e a d y shown t h e p r o m i n e n t p o s i t i o n o f t h e Sorbex process i n s e v e r a l a p p l i c a t i o n s f o r t h e p r o d u c t i o n o f h i g h p u r i t y chemicals on a comm o d i t y scale.
Many o f t h e s e processes were a t t r a c t i v e when t h e y were f i r s t i n t r o d u c e d t o t h e i n d u s t r y , and c o n t i n u e t o i n c r e a s e i n v a l u e as UOP works t o d e v e l o p improved adsorbents, d e s o r b e n t s and process designs. I n t h e UOP P a r e x * pro c es s alone, t h e r e have s o f a r been t h r e e g e n e r a t i o n s o f adsorbent
465
and f o u r g e n e r a t i o n s o f desorbent.
A t t h e same time,
t h e r e i s reason t o
e x p e c t t h a t t h e Sorbex process can be a p p l i e d t o a much more d i v e r s e range o f p r o b l e m s t h an t h o s e presented i n Table 4 .
T h i s i s even more l i k e l y w i t h t h e
d i s c o v e r y and a v a i l a b i l i t y o f new m a t e r i a l s and z e o l i t e s
--
f o r use as adsorbents.
--
p a r t i c u l a r l y molecular sieves Over t h e y e a r s UOP has i n v e s t i g a t e d
many new a d s o r p t i v e s e p a r a t i o n s as a r e s u l t o f general market s t u d i e s as w e l l as s p e c i f i c c l i e n t i n t e r e s t . The v a l u e o f many chemical p r o d u c t s , from p e s t i c i d e s t o pharmaceut icals t o h i g h performance polymers, i s based on unique p r o p e r t i e s o f a p a r t i c u l a r i s o m e r f rom which t h e p r o d u c t i s u l t i m a t e l y d e r i v e d . I n t h e case o f t r i s u b s t i t u t e d aro ma t i c s t h e r e may be as many as t e n p o s s i b l e geomet ric isomers ( F i g u r e 8), whose r a t i o i n t h e m i x t u r e i s det ermined by e q u i l i b r i u m .
Often
t h e p u r i t y re qu i r e m e n t s f o r t h e d e s i r e d p r o d u c t i n c l u d e an upper l i m i t on t h e c o n t e n t o f one o r more o f t h e o t h e r i s o m e r s . T h i s i s a complicat ed s e p a r a t i o n problem, b u t o f t h e k i n d where numerous p r o p e r t i e s o f z e o l i t i c ads orb ent s o f f e r t h e g r e a t e s t chances f o r success. A
& bB A
C
C
A
& B
& ac A
B @Jc
B
B
B
4 C
F i g . 8. Geometric isomers o f t r i - s u b s t i t u t e d aromat ics. FUTURE ADSORBENT NEEDS A s u r p r i s i n g l y l a r g e number o f i m p o r t a n t i n d u s t r i a l s c a l e s e p a r a t i o n s c a n b e accomplished by t h e r e l a t i v e l y small number o f z e o l i t e s t h a t a r e com-
466
m e r c i a l l y available.
The discovery, c h a r a c t e r i z a t i o n and commercial a v a i l a -
b i l i t y o f new z e o l i t e s and molecular sieves may very l i k e l y m u l t i p l y t h e number o f p o t e n t i a l s o l u t i o n s t o separation problems. I n t h e f u t u r e , we hope t o see a wider v a r i e t y o f pore diameters, pore geometries, hydrophobicity i n new z e o l i t e s and molecular sieves, as w e l l as more p r e c i s e c o n t r o l o f comp o s i t i o n and c r y s t a l l i n i t y i n e x i s t i n g z e o l i t e s . These accomplishments w i l l help t o broaden t h e a p p l i c a t i o n s f o r adsorptive separations,
and may very
l i k e l y l e a d t o improvements i n separations t h a t are c u r r e n t l y i n commercial practice. CONCLUSIONS
The Sorbex process i s one o f t h e most widely used adsorptive separation technologies i n t h e chemical i n d u s t r i e s today. Z e o l i t e s have played a c r u c i a l r o l e i n i t s successful a p p l i c a t i o n , p a r t i c u l a r l y i n t h e petrochemical indus-
try. I n aqueous service, care must be taken i n t h e f o r m u l a t i o n o f z e o l i t i c ads o r b e n t s . The b e n e f i t s o f improved mass t r a n s f e r and p a r t i c l e r i g i d i t y must be balanced against the shape o f t h e adsorption isotherm and feed s o l i d s concentration. The v e r s a t i l i t y o f z e o l i t e s i n the c o n t r o l o f adsorption s e l e c t i v i t y by changing t h e framework, Si/A1 r a t i o and the c a t i o n makes them one o f the most p r a c t i c a l classes o f adsorbents. The widespread use o f Sorbex processes f o r l i q u i d phase adsorptive separation provides a strong basis f o r developing and s c a l i n g up new l i q u i d phase separations i n areas such as biochemicals, f a t s and o i l s and i n d u s t r i a l chemicals. provements i n e x i s t i n g z e o l i t e s , Sorbex applications.
*
The a v a i l a b i l i t y o f new z e o l i t e s , o r i m may s i g n i f i c a n t l y expand t h e number o f
UOP, Sorbex, Molex, Olex, Sarex and Parex are trademarks and/or service marks o f UOP I n c .
461
REFERENCES
1.
S e p a r a t i o n and P u r i f i c a t i o n : C r i t i c a l Needs and O p p o r t u n i t i e s . N a t i o n a l Research C o u n c i l Report, N a t i o n a l Academy Press, 1987.
2.
D. M. R u t h e v e n , " P r i n c i p l e s o f A d s o r p t i o n Processes. John W i l e y and Sons, New York, 1984.
3.
6. E. K e l l e r 11, " I n d u s t r i a l Gas Separat ion" (T. E. Whyte, e t al., Eds.), ACS Symposium S e r i e s No. 223, Washington, DC, (1983).
4.
D. B. B r o u g h t o n , " A d s o r p t i v e Separat ions- L i q u i d s " , K i rk-Othmer, Encyclopedia o f Chemical Technology, Vol. I, 3 r d ed., John W i l e y & Sons, New York, NY, 1978.
5.
R. M. B a r r e r , " Z e o l i t e s and C l a y M i n e r a l s as S o r b e n t s and M o l e c u l a r Sieves," Academic Press, London, England, 1978.
6.
R. T . Yang, "Gas S e p a r a t i o n by A d s o r p t i o n Processes", B u t t e r w o r t h S e r i e s i n Chemical E n g i n e e r i n g , 1986.
7.
D. B. B r o u g h t o n , e t a l . , "The P a r e x P r o c e s s f o r Recovering m - X y l e n e , " Chem. Ens. Proq., 66 (9), 70 (1970).
8.
J. A. Johnson. S. R. Raghuram and P. R. Pujado, "Olex: A Process f o r Producing H i g h P u r i t y O l e f i n s , " Presented a t t h e AIChE Summer N a t i o n a l Meeting, M i n n e a p o l i s , MN, August 1987.
9.
Handbook o f P e t r o l e u m R e f i n i n g Processes, R. McGraw H i l l , New York, NY, 1986.
10.
C . Ho, e t a l . , "A Comparative Study o f Z e o l i t e and Resin Adsorbent f o r t h e S e p a r a t i o n o f Fructose-G1 ucose Mixt ures, " I n d . Enq. Chem. Res., 26, 1407 (1987).
11.
S.A. F i s h e r and G. O t t e n , "Sloughage o f O rganic M a t e r i a l f rom F i e l d D e c r o s s l i n k e d S u l f o n i c A c i d C a t i o n Exchange Resins," Proc. o f t h e 4 2 n d I n t e r n a t i o n a l W a t e r Conf . , Eng. SOC. o f W . Pennsylvania, P i t t s b u r g h (1981).
and A d s o r p t i o n , "
A. Meyers,
ed.,
This Page Intentionally Left Blank
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
GAS O I L DEAROMATIZATION BY ADSORPTION
A . L A K T I C , J.MUHL, I.BECK and M.BEER
INA-Research (Yugoslavia)
and
Development,
Proleterskih
brigada
78,
41000
Zagreb
ABSTRACT
Gas o i l 228/286OC (37.3 w t X o f aromatics) was d i s s o l v e d i n low-aromatic gasoline 76/117OC and dearomatized by adsorption on z e o l i t e 13X. The c y c l i c fixed-bed adsorption was performed under m i l d c o n d i t i o n s , and aromatics were desorbed c o u n t e r c u r r e n t l y by displacement w i t h ammonia a t a higher temperature. The r e f i n e d gas o i l product was c o l o u r l e s s and odourless and meets US FDA requirements f o r technical-grade white o i l s i n a l l wavelength regions o f UV absorption except 300 - 320 nm. The d i f f e r e n c e s o f t h e o u t l e t p t r e a m composition i n r e l a t i o n t o the o r i g i n a l gas o i l were determined by H NMR spectrometry. The r e s u l t s confirmed the s p e c i f i c aromatic a d s o r p t i o n a f f i n i t y on z e o l i t e 13X. The y i e l d s i n r e l a t i o n t o t h e gas o i l consumption were: low aromatic product 29.95 w t % , t o t a l desorbed product 33.52 w t % and p a r t i a l l y dearomatized gas o i l 36.53 w t X . INTRODUCTION Contact w i t h lubricant-based o i l s over an extended time i s associated w i t h a r i s k o f inducing d e r m a t i t i s and s k i n cancer.
Petroleum d i s t i l l a t e s i n t h e
l u b r i c a t i n g o i l b o i l i n g range may c o n t a i n a wide v a r i e t y o f p o l y c y c l i c aromatic hydrocarbons, some o f which a r e known t o be potent carcinogens ( 1 ) . To overcome such hazards these petroleum d i s t i l l a t e s have t o be a d d i t i o n a l l y r e f i n e d . A number o f r e f i n i n g steps, such as e x t r a c t i o n w i t h s o l v e n t s , hydrogenation a t varying s e v e r i t i e s , treatment w i t h s u l f u r i c a c i d and f i l t r a t i o n through b a u x i t e
or c l a y s a r e - c u r r e n t l y i n use. Although t h e c a t a l y t i c hydrogenation i s probably most i n t e r e s t i n g
( 2 , 3 ) , adsorption w i l l p o s s i b l y meet t h e requirements,
if
market demands a r e s m a l l . There a r e some r e p o r t s ( 4 , 5 ) t h a t i t i s p o s s i b l e t o remove small q u a n t i t i e s o f aromatics from gas o i l f r a c t i o n s by a d s o r p t i o n on z e o l i t e 13X. Adsorbed aromatics can be desorbed by d i s p l a c i n g them w i t h p o l a r compounds such as NH3 CO, NO, C02 ( 5 ) .
The i n v e s t i g a t i o n s presented i n t h i s paper were begun, bearing i n mind t h e Yugoslav market demands f o r 3000 applications
.
-
5000 t / y o f low-aromatic gas o i l f o r s p e c i a l
470 EXPERIMENT
Gas o i l 228/286OC, obtained by d i s t i l l a t i o n o f a Yugoslav crude m i x t u r e , was dissolved i n low-aromatic g a s o l i n e 76/11 7 O C , and dearomatized by a d s o r p t i o n on zeolite
13X.
Characteristics
of
gas
oil
228/286OC,
low-aromatic
gasoline
7 6 / 1 1 7 T and the s o l u t i o n are given i n Table 1. TABLE 1 C h a r a c t e r i s t i c s o f gas 0 i l 2 2 8 / 2 8 6 ~ CI l l , low-aromatic gasoline 76/117OC / 2 / and the gas o i l s o l u t i o n / 3 / . 1
Characteristic Gas o i l content, w t %
100
-
S p e c i f i c g r a v i t y a t l5OC
0.8697
V i s c o s i t y a t 4OoC, mnL/s Distillation,
-
0.7460
I S 0 3675
1.18
ASTM D 445
761281
37.3*
0.44**
12.43***
vol%
-
0.50
4.2
-
-
1 A
-
Corrosion, Cu 3h, 5OoC C
***
33.33
76/11 7
H aromatic, %
**
-
Method
228/286
Aromatics, w t %
*
3
0.6929
2.80
OC
2
determined by NMR spectrometry (6) determined by UV spectrometry (UOP 495 c a l c u l a t e d (37.3 x 0.3333) w t %
-
JUS E. H8.028
ASTM D 1319 ASTM D 130-74
75)
Experiments were performed i n the bench s c a l e adsorption equipment, designed and constructed i n was c a r r i e d out
our own workshop ( F i g . 1 ) . The c y c l i c fixed-bed adsorption
from the l i q u i d phase under m i l d c o n d i t i o n s ,
and aromatic
hydrocarbons were desorbed c o u n t e r c u r r e n t l y by displacement w i t h annnonia a t a higher temperature. The
adsorption
step
was
aromatics.The column hold-up displacing
previously
finished
when
the
bed
was
saturated
with
was emptied by n i t r o g e n ; amnonia was added f o r
adsorbed aromatics,
and
because
of
the
better
bed
r e a c t i v a t i o n the r e c i r c u l a t i o n system was a c t i v a t e d a t the same moment as the bed heating up t o 30OoC was s t a r t e d . Nitrogen was used as a r e c i r c u l a t i n g gas ( a c t u a l l y a nitrogen/amnonia m i x t u r e was r e c i r c u l a t e d ) . The system pressure was close t o one atmosphere. During the r e c i r c u l a t i o n two p o r t i o n s o f e f f l u e n t were collected,
the f i r s t up t o l4OoC ( a t the bottom o f the bed) and t h e second
above t h i s temperature. Amnonia desorbing d u r i n g t h e bed heating was absorbed i n an a c i d t r a p a t t h e o u t l e t . The o u t l e t streams were analysed a f t e r amnonia and gasoline s e p a r a t i o n by
471
STREAMS
@
feed
@
product
0 nitrogen
FEED RESERVOIR
@
@ @
amonia recirculating m i x t u r e N2/NH3 column hold-up
;;u;
0 desorbate @
mixture
ROTAMETER
PREHEATER
COLUMN/ ZEOLITE 13X BED
A
RECIRCUL A TING COMPRESSOR
VAL VES
1
-
2
- product -
feed i n l e t outlet i n l e t (emptying t 6 e column) NH3 i n l e t N
- column hold-up o u t l e t - recirculation i n l e t 5 - recirculation outlet 6 - desorbate o u t l e t
LIQUID S€PARA TOR
3
4
@ ---1
m
DESORBA TE
Fig. 1.
Schematic o f experimental apparatus
ABSORP ACID
.
P -E5 3
TABLE 2 Characteristics of low-aromatic gas oil / l / , oartially dearomatized gas oil 121, desorbate <140°C (bottom) /3/, desorbate >140°C (bottom) /4/ and the regenerated solvent /5/ Characteristic
1
Specific gravity at 1 5 O C Distillation, OC Aromatics, vol% H aromatic, % UV Absorption, max/cm (FDA) 280-289 nm
3
4
5
Method
0.8078
0.8342
0.8562
0.8916
0.6942
I S 0 3675
109/287
109/291
101/275
125/289
-
-
-
-
76/129 1.63
JUS. B.HB.028 ASTM D 1319
0.2
2.4
5.4
9.3
-
-
-
-
-
3A
12.53
15.28
6.17
7.85
29.95
36.53
14.75
18.77
(4.0)
3.5
290-299
(3.3)
3.3
300-329 nm
(2.3)
3.2
330-350 nm
(0.8)
0.5
Corrosion Cu 3h, 5OoC Yields, kg/100 kg 13X % wt (from the gas oil consumption)
2
ASTM D 130-74
473
d i s t i l l a t i o n . Their composition was changed i n comparison w i t h t h e o r i g i n a l gas o i l . The differences were determined by
NMR spectrometry through a hydrogen
f u n c t i o n a l group d i s t r i b u t i o n . I t i s p o s s i b l e t o determine t h e hydrogen content for
- 9.0 ppm, Ha,), groups t o aromatic r i n g (2.0 - 3.8 ppm, He I , h,p ... CH, CH2 t o r i n g and p a r a f f i n i c CH, CH2 ( 1 .O - 2.0 ppm, Ha ), and h , p . .. CH3 t o r i n g and p a r a f f i n i c CHJ ( 0 . 5 - 1.0 ppm, H b 1. The aromatic resonance
the following functional
6-alkyl aromatic aromatic
groups:
aromatic r i n g s (6.5
area c o n s i s t s o f monoaromatic-ring proton s i g n a l s (Hir from 6.5 condensed-aromatic-ring possible
to
estimate
monoaromatic
systems.
from 7.05
proton s i g n a l s ( H i ; the
relationship spectra
NMR
between
-
the
were obtained
by
-
7.05 ppm) and
9.0 ppm). Thus i t i s
condensed a
and
the
Varian EM-390
NMR
spectrometer without s o l v e n t . The i n t e r n a l standard was TMS. The aromatic content o f l o w - x o m a t i c product was determined by UV a b s o r p t i o n o f u n d i l u t e d samples, i n t h e wavelength regions d e f i n e d by US FDA requirements f o r technical-grade w h i t e o i l s ( 2 ) , w i t h a UV-VIS Pye Unicam spectrometer. RESULTS AND DISCUSSION The o u t l e t stream c h a r a c t e r i s t i c s a f t e r t h e a m o n i a and g a s o l i n e s e p a r a t i o n by d i s t i l l a t i o n a r e given i n Table 2. I t should be mentioned t h a t s u b s t a n t i a l s o l v e n t losses i n t h e d e s o r p t i o n s t e p were noted. Lowering o f t h e i n i t i a l b o i l i n g p o i n t s o f a l l gas o i l streams, as w e l l as e l e v a t i o n o f the f i n a l p o i n t o f d i s t i l l a t i o n o f t h e regenerated s o l v e n t , a r e caused by the separation procedure. The s p e c i f i c g r a v i t y changes and the aromatic hydrogen content a r e r e l a t e d t o a composition change o f a p a r t i c u l a r stream. A t t e n t i o n should be p a i d t o t h e q u a l i t y changes o f t h e regenerated s o l v e n t , because i t s r e u t i l i z a t i o n could become impossible because o f increased aromatic content and o f products o f c o r r o s i v e nature. The low-aromatic reqirements
i s c o l o u r l e s s and odourless and meets US FDA
f o r technical-grade
absorption except 300 Since
gas o i l
the
structural exclusively
-
w h i t e o i l s i n a l l wavelength regions o f UV
320 nrn.
adsorption
of
the
aromatics
c h a r a c t e r i s t i c changes o f by
the
hydrocarbon
is
the o u t l e t
distribution
essentially
physical,
the
streams a r e caused almost
depending
on
its
adsorption
a f f i n i t y t o z e o l i t e 13X. Z e o l i t e 13X could a l s o a c t c a t a l y t i c a l l y a t increased temperatures causing a c a t a l y t i c c r a c k i n g o f hydrocarbons, but t h i s a c t i v i t y i s l i m i t e d by using ammonia as displacement agent. The
differences
in
structural
characteristics
of
outlet
streams
are
i l l u s t r a t e d by the data presented i n Table 3. As a r e s u l t o f t h e removal o f aromatics,
t h e hydrogen content o f aromatic
* -a P
TABLE 3
Distribution of the hydrogen on functional groups of the gas o i l 228/286OC and the outlet streams of the dearomatization 1 process by H NMR saectrometry
Gas oil 228/286OC Low-aromatic gas oil Partially dearomatized gas o i l Desorbate (140OC (bottom) Desorbate >14OoC (bottom)
4.2
9.3
49.1
37.4
0.2
3.6
52.1
44.2
49
51
0.96
2.4
8.0
50.0
39.6
61
39
1.56
5.4
11.0
46.3
9.3
15.8
42.7
37.3
50
50
1
32.2
43
57
0.75
415
r i n g s f H a r ) and A - a l k y l
I
groups (HA
of t h e low-aromatic gas o i l i s diminished
h,~..
compared t o the values o f t h e o r i g i n a l gas o i l . The hydrogen content o f -alkyl
groups
, H p
(Hfi
a r e increased.
A s i g n i f i c a n t increase
of
the
p r o p o r t i o n Ha /Har
(from 2.2 t o 1 8 ) p o i n t s t o a p r e f e r e n t i a l a d s o r p t i o n o f t h e
less
aromatics,
substituted
which
is
in
agreement
with
their
specific
adsorption a f f i n i t y t o z e o l i t e 13X. S t r u c t u r a l c h a r a c t e r i s t i c s o f t h e p a r t i a l l y dearomatized gas o i l a r e changed w i t h respect
t o t h e o r i g i n a l gas o i l ,
too.
The data show,
n e a r l y 2 - f o l d increase o f t h e p r o p o r t i o n H&/H;; implies
a
change
of
the
aromatic
simultaneously,
( f r o m 0.96 t o 1.561, which
hydrocarbon
nature
(to
prevailing
monoaromatics), and o n l y a l i t t l e increase o f t h e p r o p o r t i o n H a / H a r ( f r o m 2.2 t o 3 . 3 ) . I n s p i t e o f t h a t , i t i s allowed t o use t h e p a r t i a l l y dearomatized process stream as feed i n t h e next adsorption step. and Hd
The increased Har
c o n t e n t , decreased H f i
content, f i x e d H p ,
and
o f t h e desorbate below 14OOC ( b o t t o m ) ,
almost unchanged p r o p o r t i o n H a r /Ha';
i n d i c a t e t h a t , besides t h e monoaromatics, some condensed aromatics a r e desorbed as w e l l . I t i s p o s s i b l e t h a t t h e a l k y l chains o f t h e s u b s t i t u t e d aromatics a r e s h o r t e r than those o f t h e o r i g i n a l gas o i l . The most
significant
changes
are
found
in
the
desorbate
above
( b o t t o m ) . Remarkable increases o f t h e aromatic hydrogen (Har)
&,p...-a l k y l
hydrogen (Hd ) content and decreases o f t h e content,
indicate
the
desorption
of
s u b s t i t u t e d by s h o r t e r a l k y l chains.
the
condensed
140OC
and d - a l k y l
hydrogen
aromatic
(He, H f i
)
hydrocarbons
These data c o n f i r m t h e n e c e s s i t y
for
i n c r e a s i n g the temperature w i t h the a p p l i c a t i o n o f amnonia as t h e displacement agent, and i f e f f e c t i v e regeneration o f t h e z e o l i t e bed i s desired. CONCLUSION
Results
of
the
investigation
proved
the
possibility
of
gas
oil
dearomatization by adsorption from gas o i l s o l u t i o n on z e o l i t e 13X. The r e f i n e d gas o i l product was c o l o u r l e s s and odourless, meeting US FDA requirements f o r technical-grade w h i t e o i l s i n a l l wavelength regions o f UV a b s o r p t i o n except 300
- 320
nm.
The y i e l d s i n r e l a t i o n t o t h e gas o i l consumption were: low-aromatic product 29.95 wt%, t o t a l desorbed product 33.52 wt%, and p a r t i a l l y dearomatized gas o i l 36.53 wt%, which might be used as feed i n t h e next adsorption step. The composition o f o u t l e t streams i n r e l a t i o n t o the o r i g i n a l gas o i l was changed.
The d i f f e r e n c e s
determined by
' H NMR spectrometry confirmed
the
s p e c i f i c adsorption a f f i n i t y o f aromatics on z e o l i t e 13X. The increase o f the aromatic content o f t h e s o l v e n t , i t s c o r r o s i v e nature and
the
substantial
solvent
losses
in
the
desorption
step
suggest
that
476
investigations
of
t h e process
under more severe a d s o r p t i o n c o n d i t i o n s and
without gas o i l d i s s o l u t i o n a r e s t i l l necessary. REFERENCES T.M. Warne, C.A. Halder, Lubr. Eng. 42, 2 (1886) 97-103. W. H i m e l , T . Anstock, R. Spahl, K. Kussner, Erdol und Kohle-Erdgas-Petrochentie v e r e i n i g t m i t Brennstoff Chemie, 39, 9 ( 1 9 8 6 ) 408-414. 3 F.M. Nooy, S.R. Lee, J.R. YOeS, " A p p l i c a t i o n o f K e t j e n f i n e 840ft, Ketjen Cat. Symp '86. 4 M.N. F r i d , 1. V . Borisova, E . V . Zubareva, Neftepererabotka i n e f t e h i m i j a . 9 (1977) 31-33. 5 J.L. Robertson, W.R. Epperly, U S . Pat. 3,476,822 1969. 6 J. Muhl, V . S r i f a , 6 . Mimica, M. Tomadkovid, Anal. Chem., 54 (1982) 1871-1874.
1 2
.
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MODELING DIFFUSION PATHWAYS IN MFI MATERIALS BY TIME-RESOLVED POWDER DIFFRACTION TECHNIQUES
B. F. MENTZEN Laboratoire de Physico-Chimie Min6rale IA, U.A du CNRS No. 116, Blt. 731, UniversitC Claude-Bernard,LYON I, 43 Ed du 11 Novembre 1918. F-69622 Villeurbanne, France. ABsTRAm Interpretation of time-resolved X-ray powder diffraction data (TRXRD) by profile analysis, shows that the spontaneous desorption of a high-coverage MFUp-xylene system (8 sorbate molecules/uc) yielding the low-coverage one (up to 4 sorbate molecules/uc) proceeds via a two-phased system. Similar results are observed for the MFUbenzene system. Rietveld-type refinements of diffraction profiles show that the distributions of the sorbed molecules in both saturated systems are different. Preliminary results on the MFUpyridine and MFUtoluene systems are also presented. INTRODUCTION MFVsorbate systems have been extensively investigated this past decade by using several complementary techniques. Single-crystal(refs. 1,2) or powder diffraction (refs.3.4) and solid-state deuterium NMR (ref.5) or BSi masNMR (ref.6-9) have proven to give valuable information on the nature of some sorbent/sorbate interactionsin these systems. In the case of low-coverage MFVp-xylene (ref.10) and benzene (ref.11) systems (labelled XYLI and BENZI forms respectively), Rietveld-type refinements of X-ray powder diffraction profiles (ref.12) and computer-assisted graphical simulations show that in both systems the organic guest is localized near the channel intersections (4c sites at y=1/4,3/4 of the Pnma space group). Site-occupancy factor refinements yield 2.67(7) p-xylene/uc and 3.55(8) benzenehc (ref. 13). Interpretation of Fourier-differencemaps shows that the p-xylene molecules (labelled xyll') have their long axis parallel to the straight channel (fig.lb) and present partial disorder (hindered rotation), whereas the benzene molecules (labelled benzl') lie on the m mirror plane of the Pnma space group (fig.ld) and present rotational disorder (an almost sphericalrotator). In the case of saturated highcoverage phases containing a eight organic guest molecules per unit-cell (refs.5,14,15) it has been shown that for the MFUp-xylene phase at room temperature (labelled XYLII phase), two distinct xyll and xy12 sorbate molecules are localized near the channel intersections and in the sinusoidal channel junctions (fig.la) with 4.08(6) and 3.92(8) mole/uc respectively (refs.16.17). Neutron powder diffraction on a saturated MFI/benzene system at 77K (labelled BENZIIb phase) shows that there are three distinct locations for the benzene species (refs.4,18) : the benzla and benzlb species form a cluster at the channel intersections, and benz2 sites near the inversion centers (fig.le) in the snaight channels (4b sites at y=O,1/2) with 2.43(3), 2.36(3) and 3.26(4) mole/uc for the three benzene types respectively. Figure 1 represents the
410
schematic hydrocarbon distribution in the XYLYXYLII and BENZUBENZII phases respectively. BENZIIb corresponds to ref.4 and BENZIIa to our results (vide infra).
’ 1’
(d)BENZ I
Fig. 1. Schematic hydrocarbon distribution in the Wp-xylene and benzene systems.
I
( e ) BENZ i I b
It has recently been shown that time-resolved X-ray powder diffraction (TRXRD) might give some complementary information on the structural modifications of sorbendsorbate systems undergoing spontaneous desorption (refs.19.20). Results concerning the desorption of saturated highcoverage MFI/p-xylene, toluene, benzene and pyridine systems are presented. EXPERIMENT The usual experimental conditions are described in refs.19 and 20. The investigated MFI material is a boron-substitueted ZSM-5 (3.2 B/uc). The W s o r b a t e systems have been prepared by pouring directly the anhydrous just-calcined and nearly-cooled material into airtight bottles containing an excess of the liquid aromatic hydrocarbon. Complete sorbendsorbate interaction is ensured after three days. For TRXRDexperiments, step-scannedprofiles have been obtained in the
479
22.5-25 and 44.5-47'213 regions, by using a standard Philips PW1720 diffractometer, which has been automated via an Apple II micro-computer. In order to slow down the benzene desorption at room temperature, the sample has been covered with a 5 pm thick polyester film. The crystallinity of the investigated materials has been checked by using a method described elsewhere (ref.21), and the distribution of the guest molecules in the MFI/sorbate systems determined by recording and refining (ref. 12) some XRD step-scanned profiles (6-48'28 range) corresponding to various MFI/sorbate compositions.
22.5
'2 8
25
c
. . . I
$
-
c)
C
0
4
Jh@)
22.5
'2 8
25
Fig. 2. Time-resolved XRD profiles for the MFI/p-xylene system. (a) 3D plot, (b) five selected profiles and (c) kinetic interpretation. RESULTS Figure 2a represents a 3D-TRXRD plot (22.5-25'28 region) corresponding to the spontaneous XYLII + XYLI desorption reaction. In figure 2b five selected profiles are directly compared. Profile analysis andor peak-height analysis (full details in ref. 20) of the XRD trace corresponding to t = 31 hours, clearly shows that the investigated system is a mixture of two distinct oxthorhombic Pnma phases : in order to simulate the complete spectrum we need ten individual components (the relative intensities and angles in '28 are given in fig.3b). If we just use the five components corresponding to a single 501/051/151/303/133 quintuplet, the simulation cannot be achieved (fig.3~).The peak-shape parameters used for these graphical simulations have been determined by analyzing the isolated 013 peak at 20.325 028 (fig.3a).
480
I000 23185
t-31 h
(b)
360 270 460 105 350 150
2329 2339 2348 2382 2391 2400 140 2410 63 2445 160 24523
.-x
c)
-In
5
20.05
//
\
c
.2e
20.525
22 6
24 as
'2 0 1000 23 190
Fig. 3. Peak-shape analysis. (a) Profile analysis of the 013 reflection (Hk : full width at half maximum in 028, As : asymmetry parameter, n :peak shape parameter of the Pearson VII function
570 375 170 175
x
23470 23915 24055 2451
.-
c)
a
c)
w
(ref.12). r : ul/a2ratio) . (b) graphical simulation of the profile observed at t = 31 h (fig.2b) by using ten components and (c) by using five components.
T
1
,
,
,
22 6
,
,
,
'2 8
24 a5
Similar analysis of all the TRXRD profiles enables one to determine the relative amounts of the XYLII and XYLI phases. Accordingly, the desorption reaction might be formulated as : MFv8p-xyl (XYLII form)
+ np-xyl?
+
(p-xylene gas)
(l-n/4)MFV8p-xyl (XYLII form)
+
n/4MFV4p-xyl (XYLI form)
[l]
If the amounts of the XYLII phase (the x=l-n/4 values) in the desorbing system are plotted versus the square-root of time, the straight line represented in figure 2c is obtained. The average dimensions of the investigated B.ZSM-5 sample being 90x15~15pm, the overall diffusion coefficient Dd=0.4x10-10cm*s-1corresponding to the spontaneaous desorption of an MFV p-xylene system may now be calculated from the line represented in figure 2c, and compared to values reported in the literature (ref.22). The desorption proceeds monotonically. and accordingly, it might be assumed that there are no specific sorbenthbate or sorbate/sorbate interactions during reaction [11. Thamm et al. have observed that there are unusual coverage dependences of the calorimetrically determined molar heats of adsorption of benzene (ref.14) and toluene (ref.23) on silicalite. In order to determine if such unusual behaviour can be evidenced by TRXRD experiments, B.ZSM-5 samples have been saturated with benzene, toluene and pyridine and further investigated.
481
Figures 4a, b and c show some selected time-resolved XRD profiles corresponding to the MFUbenzene (protected), toluene and pyridine systems respectively. The profile at t 4 8 h (figda) corresponds to a monoclinic m n z e n e low-coverage phase which contains slightly less than four benzenehc; accordingly. the monoclinic/orthorhombic phase transition occurs when the benzl' species occupy completely the channel intersections (vide supra fig.1d). All the other individual profiles correspond to orthorhombicsingle or two-phased systems.
2
I
5
.x
c)
! c
c1
I
22 5
'29
25
MFVbenzenu
Fig. 4. Selected XRD profiles. (a) MFI/benzene,(b) toluene and (c) pyridine systems. Interpretation of the individual MFI/sorbate TRXRD profiles by using the above mentioned method shows that for all the investigated systems the spontaneousdesorption seems to proceed via a two-phased system. As previously developed in the case of an Wp-xylene system (refs. 19,20), investigation of another characteristic angular domain (the 44.5-46.5 '20 region where the 10,0,0/804 and 0,10,0 medium intensity peaks appear) yields essentially the same results. The time-dependence of the desorption for the MFI/benzene and toluene systems are reported in figures 5a and b and directly compared to the results given i n ref.14 and ref.23 respectively. For the pyridine system only the unit-cell content versus tln dependence is given (fig&). Compared to the p-xylene one (fig.2c), all the other systems exhibit unusual behaviour. In order to interpret these observations, the localizations of the guest organic molecules in both the high- and low-coverage MFVsorbate systems had to be determined. Hence, the corresponding step-scanned XRD profiles collected in the 6-48'2e range have been exploited by using the direct-characterization method (refs.10,24).Full details concerning the Rietveld refinements will be published elsewhere.
482
i13s P
7
0
3
6 bemene/uc
4 t
0
4
Fig. 5. Kinetic interpretation of the TRXRD experiments. Comparison of kinetic and calorimetric results (ref.14) for the (a) MFI/benzene and (b) toluene systems respectively. (c) the MFUpyridine system.
8
Jtime (h)
For the discussion, some preliminary results are reported in Table 1. Under our experimental conditions, the distribution of the sorbed molecules in the saturated MFI/benzene system corresponds to fig.lc. Critical interpretation of an intermediate data-collection corresponding to a BENZII/BENZI mixture (overall coverage about 7.2 benzenehc), shows that benzeneclusterizarion at the channel-intersections (fig.le) might only be observed for partially desorbed high-coverage phases. A partial plot of the observed/calculated/differencespectra corresponding to the refined (Rp/Rb = lS.Ol7.8%; for the definitions of Rp and Rb see ref. 12) BENZII high-coverage phase is given in fig.6a. For the MFUtoluene system, the hydrocarbon distribution is comparable to that observed for p-xylene (fig.la and lb), and the Rietveld plots for the high-coverage TOLII form (Rp/Rb = 16.6/8.1%) are reported in fig.6b. If the low-coverage TOLI complex is further desorbed, the orthorhombic/ monoclinic phase transition can be observed. The Rietveld refinement (Rp/Rb = 14.6/8.8%) of the profile corresponding to the high-coverage PYRII complex (fig.6~)can be achieved by distributing the pyridine molecules statistically over all the possible sites, i.e., the 4c sites at y=1/4,3/4, the sinusoidal channel junctions, and the 4b sites at y=0,1/2.
DISCUSSION As far as B.ZSM-5 materials are concerned, it is clearly shown that time-resolved XRD might be used as a complementary technique for the investigation of sorbent/sorbateand/or sorbate/sorbate interactions in some MFVsorbate systems.
483
.-x
c)
:
L
I
'20
24
MpUtOlun* ryrl.m
Fig. 6. Rietveld refinements. Observed (full-line), calculated (+) and difference partial plots for the high-coverage (a) m n z e n e , (b) toluene and (c) pyridine systems.
a
"20
24
MFUpyidim ryrbrn
Table 1. Unit-cell parameters for high @) and low (I) coverage B.ZSM-S/sorbate systems (3.2 B/uc)
XYL II* XYL I TOL II TOL I BENZ I1 BENZ I PYR I1 PYR I
20.014(3) 19.908(3) 19.970(1) 19.938( 1) 19.934(3) 20.021(3) 19.984(3) 20.062(3)
19.721(3) 19.812(3) 19.727(4) 19.787(7) 19.875(3) 19.805(3) 19.888(3) 19.859(3)
13.378(2) 1 3.307(2) 13.369(1) 13.319(2) 13.325(2) 13.383(2) 13.358(2) 13.410(2)
5280(2) 5249(2) 5267(2) 5254(3) 5279(2) 5307(3) 5309(2) 5343(2)
+31(4) +13(6) -28(4) -34(4)
* XYL, TOL, BENZ, PYR for p-xylene, toluene benzene and pyridine sorbates, respectively
Interpretation of TRXRD profiles observed for the above-mentioned saturated W s o r b a t e systems shows that the high + low-coverage reaction [l] (spontaneous desorption) may be generalized. The results given in Table 1 show that the sorbate-induced structural changes are different for the X W O L and BENZPYR complexes, i.e., in the first case, the high- --t lowcoverage wansfonnation corresponds to a unit-cell volume decrease (31 and 13 A 3 respectively),
484
whereas in the second case, the reverse situation is observed (28 and 34 A3 volume expansions, respectively). Accordingly, when interpreting calorimetric (refs.14.23) or 29SimasNMR (refs.6,9) results concerning such MFUsorbate systems containing more than four guest molecules/uc, the structural modificationsof two-phased systems have to be considered. Furthermore, in order to get a better knowledge of the desorption mechanisms and elaborate realistic diffusion pathways, the distribution of the individual guest molecules for all the overall loadings between 4 and 8 molecules/uc have to be established. But as already stated in ref.18, Rietveld refinement concerning single-phased MFI materials is not a trivial problem, and refining two-phased MFI systems is far more complex. Nevertheless, by considering a 7.2 benzene/uccontaining system as a singlephase (vide supra), Rietveld refinements show that the XRD profile is better approximated if the benzene molecules adopt the distribution (benzene-clusterization at the channel-intersections)given by Taylor (ref. 18). Accordingly, as far as MFUbenzene and toluene systems are concerned, our results are consistent with those reported in the literature (refs.5,14,18,23). These preliminary TRXRD investigationson some MFI/sorbate. systems have now to be further developed in order to determine if the nuo-phase problem might be generalized for other systems and/or topologies. REFERENCES K.J. Chao, J.C. Lin, Y.Wang and G.H. Lee. Zeolites, 6 (1986) 35-68. 1 2 H. van Koningsveld, H. van Bekkum and J.C. Jansen, Acta Cryst., B43 (1987) 127-132. 3 C. Baerlocher, in D.H. Olson and A. Bisio (Editors), Proceedings of the Sixth International Zeolite Conference, Reno, USA, July 10-15, 1983, Butterworth, UK, pp. 823-833. 4 J.C. Taylor, Zeolites, 7 (1987) 311-318. R. Eckman and A.J. Vega, J. Phys. Chem., 90 (1986) 4679-4683. 5 6 C.A. Fyfe, G.J. Kennedy, C.T. De Schutter and G.T. Kokotailo, J.Chem.Soc., Chem. Commun., (1984) 541-542. 7 G.T. Kokotailo, C.A. Fyfe, G.J. Kennedy, G.C. Gobbi,H. Strobl, C.T. Pasztor, G.E. Barlow, S. Bradley, W.J. Murphy and R.S.Ozubko, Pure & Appl. Chem., 58 (1986) 1367-1374. 8 J. Klinowski, T.A. Carpenter and L.F. Gladden, Zeolites , 7 (1987) 73-78. 9 C.A. Fyfe, J.H. OBrien and H. Strobl, Nature, 326 (1987) 281-283. 10 B.F. Mentzen et F. Vignt-Maeder, Mater. Res. Bull., 22 (1987) 309-321. 1 1 B.F. Mentzen, Mater Res. Bull., 22 (1987) 337-343. 12 D.B. Wiles and R.A. Young, J. Appl. Cryst., 14 (1981) 149-151. 13 B.F. Mentzen, Mater. Res Bull., 22 (1987) 489-496. 14 H. Thamm,Zeolites, 7 (1987) 341-346. 15 C.G. Pope, J. Phys. Chem., 90 (1986) 835-837. 16 B.F. Mentzen, F. Bosselet et J. Bouix, C.R. Stances Acad.Sci.,Ser.B,305 (1987) 581-584. 17 B.F. Mentzen and F. Bosselet, Mater. Res. Bull., 23 (1988) 227-235. 18 J.C.Taylor, J.Chem.Soc., Chem. Commun., (1987) 1186-1187. 19 B.F. Mentzen, F. Bosselet et J. Bouix, C.R. SCances Acad. Sci., Ser. B, 306 (1988) 27-32. 20 B.F. Mentzen, J. Appl. Cryst.. 21 (1988), 266-271. 21 R. Khouzami, G. Coudurier, B.F. Mentzen and J.C. Vedrine. in P. Grobet, W. Mortier, G. Schulz-Ekloff and E. Vansant (Editors), Studies in Surface Science and Catalysis, Vol. 37 (1988), Elsevier, Amsterdam, NL, pp. 355-363. 22 Y.T. Ma, T.D. Tang, L.B. Sand and L.Y. Hou in Y. Murakami, A. Iijima and J.W. Ward (Editors),Proceedings of the Seventh International Zeolite Conference, Tokyo, Japan, August 17-22, 1986, Elsevier, NL. pp. 531-538. 23 H. Stach, H. Thamm, J. Jiinchen, K. Fiedler and W. Schirmer, in D.H. Olson and A. Bisio (Editors), Proceedings of the Sixth InternationalZeolite Conference, Reno, USA, July 10-15, 1983, Butterworth, UK, pp. 225-231. 24 B.F. Mentzen, C.R.Stances Acad. Sci., Ser. B, 303 (1986) 1299-1303.
H.G.Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MEASUREMENT OF INTRACRYSTALLINE DIFFUSIVITIES OF HZSM-5 ZEOLITE AT HIGHER TEMPERATURES AND PREDICTIONS OF SHAPE SELECTIVITY
K. HASHIMOTO, T. MASUDA and M. KAWASE
Department of Chemical Engineering, Kyoto University, Kyoto 606 (Japan)
ABSTRACT A model is developed which describes the activity and the shape selectivity of methylation of toluene to produce xylenes over HZSM-5 zeolite catalysts by taking account of both intracrystalline diffusion and acid strength distributions inside and outside the crystallite. The shape selectivity is strongly affected by the relative rates of intracrystalline diffusions of hydrocarbon molecules. Uptake curves of amounts of benzene, toluene and xyleneisomers adsorbed on high-silica HZSM-5 zeolites, which have no catalytic activities, were measured in the range of temperature from 373-673 K. Effective intracrystalline diffusivities were calculated from the uptake curves over the temperature range. The ratio among the diffusivities of para-, metaand ortho-xylene was about 1 O : l : l in the above temperature range. Each acid strength distribution of acid sites inside and outside the crystallite was measured by combining the two methods developed by us. Rate constants of methylation and isomerizations of xylenes inside and outside the crystallite were estimated by performing these reactions over two kinds of the catalysts: one has acid sites only inside the crystallite, the other only outside the crystallite. Using these values, the proposed model was found to predict well the apparent shape selectivity. A chart based on the proposed model was also presented which provides a guide in preparing more highly selective HZSM-5 catalysts f o r the methylation of toluene.
INTRODUCTION HZSM-5 zeolite catalysts show high shape selectivity, since they have very fine micro-pores located inside the zeolite crystallites, the diameter of which is almost equal to sizes of mono-aromatic molecules. HZSM-5 catalysts are typical solid-acid catalysts, and their acid sites are distributed not only inside but also outside the crystallite. Therefore, the shape selectivity of HZSM-5 catalysts are affected strongly by the size of the crystallite, the magnitude of the intracrystalline diffusivities of hydrocarbons and the acidic properties inside and outside the crystallite (ref. 1). Reactions are usually Although data of carried out at temperatures higher than 5 7 3 K. intracrystalline diffusivities at lower temperatures have been published,, few diffusivities at such higher temperatures have been reported (ref. 2). No data is available for the accurate acidic properties inside and outside the crystallites of zeolites. Wei (ref. 1 ) presented a mathematical model for describing the shape Selectivity of HZSM-5 zeolites, in which the reaction on the outer surface of the crystallite'was ignored, and the validity of his model
486
Fig. 1 Model for methylation of toluene over HZSM-5 zeolite. was not examined experimentally. An improved mathematical model is presented for describing the shape selectivity of HZSM-5 zeolite by taking account of intracrystalline diffusion and acid strength distributions inside and outside the crystallite. The The methylation of toluene to produce xylene-isomers w a s performed. intracryatalline diffusivities at higher temperatures and the acid strength distributions both inside and outside the cryatallite was directly measured, and then these data were used to examine the validity of the proposed model. U-TICAL MODH. FOR SWPE SELECTIVI'K The methylation of toluene to produce xylene-isomers and the isomerization of xylenes occur over HZSM-5 zeolite catalyst. In order t o develop a
mathematical model, follcwing assumptions and simplifications were made: 1 ) The concentration of toluene is large enough to simplify the kinetics and assume the rate of the methylation to be of first-order with respect to methanol. 2) The diffusion of methanol inside the crystallite is so rapid in comparison with the rate of the methylation that its concentration can be regarded as almost constant, C A , inside the crystallite. The validity of this assumption was confirmed by the fact that the modified Thiele modulus for the methylation of toluene inside the crystallite was less than 0.1. 3) The rates of methylations per unit surface area inside and outside the crystallite are approximately the same to each other. The surface area outside the crystallite was less than 1 X of that inside the crystallite. Hence, the methylation occurring outside the crystallite was ignored. 4 ) The resistance to diffusions of xylene-isomers is large inside the crystallite, resulting i n the decrease in the rate of the isomerization of xylenes inside the crystallite. The selectivity of the isomerization of xylenea inside the cryetallite is different from that outside the crystallite. Therefore, the isomerizations of xylenes inside and outside the crystallite were considered. With these considerations, the methylation of toluene with methanol proceeds
487
Methylation of toluene
Isomerizations of xylenes
%ENpm kOm
A : methanol
X p o t R
B: toluene P: xylenes R: water
A+B -P,+R b
PP +R
P P Subscript S is added to all symbols of rate constants for reactions out side cryst allite ) Fig. 2
Stoichiometriea of mL*hylation of toluene and isomerizations of xylene-isomers.
1 ) Diffusion of toluene and methanol into by the following steps (see Fig. 1): the zeolite crystallite. 2) Methylation of .toluene over acid sites inside the crystallite. 3 ) I s o m e r i z a t i o n o f x y l e n e s over acid sites i n s i d e the crystallite. 4 ) Diffusion of xylenes produced inside the crystallite toward the outside of the crystallite. 5) Further isomerisation of xylenes over acid sites on the outer surface of the crystallite. The stoichiometry of the reactions occurring in the above steps 2), 3) and 5) are shown in Fig. 2. On the basis of the physico-chemical steps shown in Fig. 1 , mass balance equations for xylene-isomers were obtained over a d i f f e r e n t i a l catalytic reactor, and w e r e solved analytically. The concentrations of xylenes, (Co,Cm,Cp)out in the exit gas are given by
where
. . D
k=(ko ,km ,kP IT,
H Dm
D=( '0
\, N=( D
P
'
\O
Hm
'), np/
1
kom+kop
K=(
-k
\
om
-k OP
-kmo
-k pm
k +k mo mp -k
mp
k
+k pm
PO
J
Here, K in Eqs. ( 1 ) and (2) represents a matrix of rate constants of the isomerization of xylenes inside the crystallite, whereas Ks is that outside the crystallite. However, Wei (ref. 1 ) ignored this matrix. Subscripts 0, m and p, respectively, represent ortho-, meta- and para-xylene and "out" and l1inl1 refer to the outlet and the inlet gas streams, whereas subscript S means the outside of the crystallite. tI repres,ents a matrix of.Henry constants for adsorption on the outer surface of the crystallite. p is a density of the crystallite, w and vo are the mass of the zeolite crystallite and the flow rate of gas, respectively, and h i 2 (i=1,2,3) are eigenvalues and P represents a matrix of eigenvector. When these values are known, the concentrations of the Then the xylene-isomers in the outlet gas can be calculated by use of Q. (1). para-shape selectivity is easily obtained by Cp/(Co+Cm+Cp). The selectivity
488
Table 1 Physico-chemical properties of IIZSM-5 zeolites used in this work. catalysts
Si/Al
depth of crystallite 2L [WI
HZSM-5(1) HZSM-5(2) HZSM-5(3) SilicaI.ite(1) ~iiicalite(2)
outer surface of crystallite a,,,[ m2/kg 1
25 65
1.9 1.3
1650 2415
20
2.1
1494
m
3.4 2.7
923 1163
shape of crystallite cubic cubic cubic cubic cubic
calculated thus represents an initial differential selectivity under the condition that the conversion of methanol approaches zero. This value is regarded as a representative index of the para-shape selectivity. PREPARATION OF CATALYSTS Several kinds of HZSM-5 zeolites and silicalites (high-silica HZSM-5) listed
in Table 1 were prepared hydrothermally from reactive aluminosilicate gels containing tetrapropylammonium bromide as template in the range of temperature from 4 3 3 to 4 7 3 K . The prepared zeolites were found to have the characteristics of the HZSM-5 zeolites by analysis of X-ray diffraction. The crystal sizes of the zeolites were measured by scanning electron microscopy and are also listed in Table 1. In order to discriminate each contribution of two kinds of acid sites located inside and outside the crystallite to the reactivity and the selectivity of the HZSM-5 zeolite, two modified zeolites, %atalyst-Itt and Ilcatalyst-S", were prepared. ttCatalyst-Itt was HZSM-5 zeolite, acid sites distributed only inside the crystallite, and was prepared by a modification of the method proposed by Beyer et al.(ref.3). Vatalyst-S" was zeolite having acid sites only outside the crystallites, and was obtained by thermal treatment with AlC13 (ref.4). MEASUREMENT OF INTRACRYSTALGINE DIFFUSIVITIES
Silicalite, which has no catalytic activity for the cracking and the isomerization of adsorbates, was used in the diffusion experiment. The intracrystalline diffusivities for xylene-isomers were measured over the temperature range from 473 to 673 K by use of the apparatus shown in Fig. 3. A small amount of sample (25-50 mg) was held in vacuum at 873 K. Pure vapor of adsorbate was introduced, and then the transient change in the total pressure was recorded by use of a piezometric sensor with transducer (Baratron type 221A, MKS), the response of which is first enough to measure accurately the transient change in pressure. To eliminate the influence of several factors (such as mass conductivity between the sorbent and the pressure sensor) on the transient change in pressure, the blank test was performed without zeolite. By comparing the data of the blank test with that with zeolite, an uptake curve of the amount of adsorption was obtained, as shown in Fig. 4. The diffusivity, D, was calculated by comparing the uptake-curve with the following theoretical equation (ref.5):
489
r------------
1
Fig. 3 Apparatus for measurement of effective intracrystalline diffusivity. (1)sample (2)adsorbate (3)helium (6)recorder vacuum ( 5 ) heater
I-::::
1
0.0 10
0.008
1
f0.5
OD0 6
I
aoo 4
aoo z
_ _
- a
0
0
1000
2000
0
Fig. 4 Transient changes in amounts of adsorbed xyleneisomers. para-xylene : equilibrium pressure =110 Pa, M, =1.04~10-~ mol/kg, partition fac or 68.9 meta-xylene : 262, 1.25x10-$ 76.9 ortho-xylene: 520, 3.83~10'~: 81.3
0.2
0.4
0.6
0.0
I
1.2
equllibrkrm pressure C kPal
t [I1
Fig. 5 Linear relationship between adsorption amount and equilibrium pressure.
where the qns are the non-zero positively roots Of
and a=V/(a,wHL), Mt and M, are the amount of adsorption at time t and that after infinite time, respectively, and L represents the half depth of the crystallite. In this experiment, the amount o f sample was taken to be small enough that the temperature rise caused by adaorption could be ignored, and decrease in pressure was kept to within 10 X during adsorption, since equations (3) and (4) were derived under the above conditipns. From the uptake curves of
490 I ' " " " " ' l " " 1
Fig. 6 Arrhenius p l o t s of e f f e c t i v e intracrystalline diffusivities.
t h e amounts of adsorbed xylene-isomers, t h e i r d i f f u s i v i t i e s w e r e c a l c u l a t e d . The t e m p e r a t u r e r i s e s were c a l c u l a t e d as 0.2 t o 0.8 I[ from t h e amounts and were small enough t o f u l f i l l t h e i s o t h e r m a l conditions. The r e l a t i o n s h i p between a d s o r p t i o n amount a n d e q u i l i b r i u m p r e s s u r e was f o u n d t o b e l i n e a r u n d e r t h e c o n d i t i o n s of p r e s s u r e l e s s t h a n 1.47 kPa and t e m p e r a t u r e h i g h e r t h a n 473 K , as t y p i c a l l y shown i n F i g . 5. Hence, t h e d i f f u s i v i t i e s w e r e i n d e p e n d e n t on t h e a d s o r p t i o n amount a n d were e q u a l t o l l s e l f - d i f f u s i v i t i e s " d e f i n e d i n Darken's equation (ref. 7). F i g u r e 6 d e m o n s t r a t e s t h e A r r h e n i u s p l o t s of t h e d i f f u s i v i t i e s . The open k e y s , O , A , u , v , O , r e p r e s e n t t h e d a t a o b t a i n e d i n t h i s work by u s e of two s i l i c a l i t e s w i t h d i f f e r e n t c r y s t a l s i z e s l i s t e d i n Table 1. The each kind of t h e k e y s l i e s on a s t r a i g h t l i n e , i n d i c a t i n g t h a t t h e i n t r a c r y s t a l l i n e d i f f u s i v i t i e s were a c c u r a t e l y measured. Zikanova e t al. (ref. 2) measured t h e i n t r a c r y s t a l l i n e d i f f u s i o n of benzene under t h e c o n d i t i o n s of a s e m i - i n f i n i t e medium. The m e a s u r e d u p t a k e c u r v e c o u l d be e x p r e s s e d by Eq. ( 5 ) jn p l a c e of Eq. ( 3 ) ( r e f s . 5 and 9)
_ --
Mt MQa
W
0
1- r: n=O ( 2 n + 1 )
exp[-D(n+ 'II
1 2 2 t 5 ) m (.2)1
They used, however, a f o l l o w i n g s i m o l i f i e d form of Eq.
_ - 2A D M t- (+Hi) MW
(5)
L
(5):
1/2@2
P
where A p and Vp a r e t h e s u r f a c e and t h e volume o f t h e c r y s t a l l i t e , r e s p e c t i v e l y . Equation (6) can be a p p l i e d only t o t h e uptake-curve for M t / M m < 0.3 ( r e f . 9 ) , i n which t h e t e m p e r a t u r e r i s e i s l i k e t o o c c u r and t o a f f e c t t h e d i f f u s i v i t y . The i n t r a c r y s t a l l i n e d i f f u s i v i t y of benzene a t 33 K c a l c u l a t e d f r o m t h e i r d a t a a t i n i t i a l p e r i o d ( F i g u r e 1 i n r e f . 2) by u s e of Eq. ( 6 ) was
491 1
O.IS
n
1
,
I
0
I
3
.
u
1
- Calalyrl: HZSM-5 - crlcrnol surlacc Inlracryslalllnc - (calculalcd from TPD sprclrum)
I
-
I
.
.
-
'
~
5
--3 nE
0.10
crlcrnal surlocc (Indicalor adsoplin
0.05 - t
0
n
r
---
''"''lT
l
h
--l-+--r-.
o
j
h
-/-. I
I
x
I
-
Fig. 7 Acid strength distribution curves inside and outside crystallite of HZSM-5 zeolite.
, ,
1.9~10-14m 2 / s , and that recalculated from their all data by use of E q . (5) was 8.6~10-l6m2/s (shown by closed circle key, , in Fig. 6. This discrepancy is considered to be resulted from several reasons, such as the error caused by the simplification used for derivation of Eq. (6) from Eq. (5) and the temperature rise occurring at initial period of adsorption, at which Eq. (6) can be applied. Hence, in this work, the intracrystalline diffusivities were calculated from all data measured for long time by use of the exact equation (Eq. ( 3 ) ) . The closed circle key 0 , as well as the closed triangle k e y A measured for para-xylene (ref. 8), is considered to lie on the same line as our experimental data. The diffusivity of para-xylene is the largest among three xylene-isomers. This is considered to be resulted from the fact that the effective molecular size of the para-xylene is the smallest. A difference observed among diffusivities of xylene-isomers is smaller than those which have been supposed. ACIDIC PROPERTIES
Acid sites inside and outside the crystallite were discriminated from each other and their acid strength distribution curves were accurately measured by the indicator adsorption method (ref. 10) and the analysis of temperatureprogrammed desorption spectrum of ammonia desorbing from the catalyst (refs. 12 and 13), as shown in Fig. 7, where HO is Hammett acidity function (ref. 11) and g,(HO) is a density distribution function of acid strength Ho such that gmdHo represents the number of acid sites of acid strength between Ho and HO+dHO per unit mass of catalyst. Although a small amount of acid sites are distributed on the outer surface of the crystallite, these sites 'decrease considerably the para-shape selectivity, because the isomerization of xylenes over these acid sites proceeds under conditions of no steric restriction. Hence, the existence of acid sites on the outer surface of the crystallite cannot be ignored, as discussed in the following sections. ESTIMATION OF RATE PARAMHXFG
Isomerization of xylenes and methylation of toluene occur not only inside but also outside the crystallite, as shown in Figs. 1 and 2. It is obviously difficult to estimate the rate parameters of these reaction all at once. Therefore, two kinds of catalysts were used in the reactions carried out in a
calculaled experimental
1
rnela<30.0)l
para-Xylene(68.5 % ) '
(
28.0)
(65.2)
Fig. 8 Comparison of c a l c u l a t e d shape s e l e c t i v i t y w ith
d i f f e r e n t i a l r e a c t o r a t 673 K t o e s t i m a t e t h e r e a c t i o n parameters: one i n which a c i d s i t e s a r e d i s t r i b u t e d o n l y o n t h e o u t e r s u r f a c e of t h e c r y s t a l l i t e (catalyst-S), t h e o t h e r having a c i d s i t e s only i n s i d e t h e c r y s t a l l i t e (catalyst-I). I s o m e r i z a t i o n o f one of t h e x y l e n e i s o m e r s , f o r e xa m ple p a r a - x y l e n e , was c a r r i e d o u t by u s e of c a t a l y s t - I i n a d i f f e r e n t i a l r e a c t o r . The r e s u l t s o b t a i n e d i n t h e e x p e r i m e n t wer e a n a l y z e d i n t e r m s of a r e d u c e d f o r m (Eq. (7)) o f Eq. (1).
Only t u o p a r a l l e l i r r e v e r s i b l e r e a c t i o n s o cc urre d t o produce o t h e r two xylene i s o m e r s , o r t h o - an d met a- x y l en e, b e c a u s e t h e r e v e r s i b l e r e a c t i o n s and i s o m e r i z a t i o n b e t w e e n o r t h o - an d met a- xyle ne c o u l d be i g n o r e d u n d e r t h e Hence, f r o m t h e v a l u e s of conditions of a d i f f e r e n t i a l reaction. c o n c e n t r a t i o n s of xylene-isomers i n e x i t g as of t h e r e a c t o r , two r a t e c o n s t a n t s w e r e e s t i m a t e d for i s o m e r i z a t i o n of p a r a - x y l e n e t o o r t h o - and m e ta -xyle ne . This k i n d of experiment and a n a l y s i s was r epe a te d for t h e remaining o t h e r two x y l e n e i s o m e r s , y i e l d i n g t h e matrix (IS) c o n s i s t i n g of t h e r a t e c o n s t a n t s of isomerization inside the crystallite. I n a s i m i l a r way t o t h e c a s e of i s o m e r i z d t i o n i n s i d e t h e c r y s t a l l i t e , t h e rate c o n s t a n t s (6s) of i s o m e r i z a t i o n o u t s i d e t h e c r y s t a l l i t e were e va lua te d by u s e of Eq. (8) f r o m t h e r e s u l t s o f i s o m e r i z a t i o n s of x y l e n e i s o m e r s o v e r c a t a l y s t - S.
By u s e o f t h e r a t e p a r a m e t e r s of i s o m e r i z a t i o n s i n s i d e and o u t s i d e t h e c r y s t a l l i t e , t h e m a t r i c e s o f t h e r a t e p a r a m e t e r s a p p e a r i n g i n Eq. ( 1 ) were e s t i m a t e d except t h a t of methylation (k). Methylation of toluene was performed over c a t a l y s t - I . The rate p ar amet er s of m e thyla tion were e va lua te d from t h e e x p e r i m e n t a l d a t a by u s e of a r e d u c e d f o r m (Eq. ( 9 ) ) 'of Eq. (1).
Thus, a l l of t h e r a t e p a r a m e t e r s a p p e a r e d i n E q s . ( 1 ) a nd ( 2 ) w e re e s t i m a t e d . By use of t h e s e v al u es an d i n t r a c r y s t a l l i n e d i f f u s i v i t i e s , s h a p e s e l e c t i v i t y of a c a t a l y s t having a c i d s i t e s i n s i d e and o u t s i d e t h e c r y s t a l l i t e can be predicted. A t y p i c a l comparison between c a l c u l a t e d and e xpe rim e nta l mole f r a c t i o n s of t h e i s o m e r s f o r HZSM-5(3) i n Table 1 i s demonstrated i n Fig. 8, i n d i c a t i n g good agreement between them.
493
-
h
I
v
-
?al !
E
0
.-I v)
aJ
meta-xylene
Fig. 9 Effect of ratio of acid amount outside crystallite to that inside crystallite on fractions of xyleneisomers.
9, C
5 0.5 -
-
-
0
- 0
o4
1
Ic,
para-xylene
-
1i2
1
acid amount outside crystallite acid amount inside cryslallite
4-1
GUIDE TO THE PREPARATION OF CATALPST An increase i n the relative amount, $, of acid sites outside the crystallite decreases the para-shape selectivity. Figure 9 demonstrates the effect of the $ value on the fractions of xylene-isomers calculated by use of the proposed model. The para-shape selectivity was found to increase with the value, namely the decrease in acid amount outside the decrease in the crystallite, because the selectivity becomes to be contro'lled by the intracrystalline diffusion. Under these conditions, the shape selectivity is affected by the crystal size. Figure 10 shows the effect of the crystal size and the $ value on the initial para-shape selectivity calculated by the proposed model. Curves in this figure represent contour lines of the initial para-shape Selectivity. Under conditions of a differential reactor, the maximum and minimum values of the para-shape selectivity are 80 and 40 X , respectively. The area hatched in the figure represents region with maximum selectivity, i.e. 80 X . Two keys, and 'I,represent experimental data points measured for two HZSM-5 zeolites. The figures in parentheses represent the observed selectivities. These values are well predicted by the proposed model. The selectivity of the catalyst (v) approaches almost the optimum value of the selectivity, viz. 80 X . The selectivity of the other catalyst ( 0 ) may be improved by decrease in the value, i.e., decrease in the acid amount outside the crystallite. Thus, the chart shown in Fig. 10 gives a guide to the preparation of highly selective HZSM-5 catalysts.
0
CONCLUSIONS 1) Diffusivities of xyleneisomers inside the crystallitee of pentasil zeolites can be directly measured at higher temperatures 473-673 K. The ratio among the diffusivities of para-, meta- and ortho-xylene was about 1O:l:l.
494
,para-shape Selectivity
1
Fig. 10 Dependence of para-shape selectivity on crystal size and ratio of acid amount outside crystallite to that inside the crystallite.
lC2
1 v
10-8
10crystal size
0
L(m)
3
2) Acid strength distributions were measured inside and outside the
cryatallites of HZSM-5 zeolites. The contribution of acid sites outside the crystallite to the para-shape selectivity can not be ignored. 3 ) An improved mathematical model was developed for describing the initial differential selectivity of HZSM-5 zeolite by taking account of the intracrystalline diffusion and acid strength distributions inside and outside the crystallite. The proposed model was found to predict well the shape selectivities of HZSM-5 zeolites. 4) A chart was presented for illustrating contour curves of para-shape selectivity expressed in terms of crystal size and relative acid amount outside the crystallite, which gives a guide for preparing high-selective catalysts.
REFERENCES J.Wei, J.Catal., 76( 1982)433-439. A.Zikanova, M.Buelow and H.Schlodder, Zeolites, 7(1987)115-118. 3 H.K.Beyer, I.M.Belenykaja, F.Hange, M.Tielon, P.J.Crobet and P.A.Jacobs, J.Chem.Soc.Faraday Trans.1, 81 (1985)2889-2901. 4 C.C.Chan C.T.W.Chu, J.N.Miale, R.F.Bridger and R.B.Calvert, J.Am.Chem. SOC., 1067\984)8143-8146. 5 J.Crank, The Mathematics of Diffusion, Clarendon Press, Oxford,Znd ed., 1 2
1975.
R.M.Barrer and D.J.Clarke, J.Chem.Soc.Faraday Trans. 1 , 70(1974)535-548. 7 M.Buelow, P.Lorenz, W.Mietk, P.Struve and N.N.Samulevic, J.Chem.Soc.Faraday Trans. I, 79 ( 1983) 1099-1 108. 8 Y.H.Ma, T.D.Tang, L.B.Sand and L.Y.Hou, Stud.Surf.Sci.Catal., 28( 1986)5316
538.
9 D.M.Ruthven, Principle of Adsorption 10 11 12 13
& Adsorption Processes, John Wiley & Sons, Tronto, 1984. K.Hashimoto, T.Masuda, H.Motoyama, H.Yakushi j i and M.Ono, 1nd.Eng.Chem. Prod.Res.Dev., 25( 1986)243-250. K.Tanabe, Solid Acids and Bases, Academic Press, New York, 1970. K.Hashimoto, T.Masuda and T.Mori, Stud.Surf.Sci.Catal., 28(1986)503-510. K.Hashimoto, T.Masuda and Y.Takagi, Shokubai (Tokyo), 29( 1987)406-409.
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders
0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MEASUREMENT OF HYDROCARBON DIFFUSION COEFFICIENT I N A CHROMATOGRAPHIC COLUMN OF ZEOLITE CRYSTAL POWDER
NON-ISOBARIC
E. AUST, W. HILGERT and G.EMIG Institut fur Technische Chemie I, Universitat Erlangen-Nurnberg, Egerlandstr. 3, D-8520 Erlangen (West Germany) ABSTRACT Chromatographic columns filled with powder of Linde Na-13X crystals were used to measure diffusions coefficients of benzene and various alkyl benzenes in the temperature range of 553 t o 673 K . The influence o f the carrier gas pressure ( N z ) was tested by increasing the latter t o as much as 140 bar. Finally, the concentration dependence of the diffusion coefficient of benzene was determined at 573 and 598 K. The activation energies of diffusion and the sorption enthalpies at infinite dilution showed a noticable correlation. INTRODUCTION The measurement of the diffusion rate of sorbates or reactants in zeolites is crucial for the design and interpretation of the performance of adsorption equipment and reactors which incorporate molecular sieves. The experimental techniques to measure mass fluxes of sorbed components in these materials are confronted with the following problems: I small diffusion coefficients I highly exothermic sorption process I very small particle size Besides the classical sorption uptake method, chromatographic experiments have evolved as a major source for reliable diffusion data in zeolites. In addition, the NMR-technique has extensively 'been utilized to determine self diffusion coefficients. Unfortunately, one frequently encounters large discrepancies between experimental diffusion coefficients in the literature. Often this flaw is being attributed to unsatisfactory design of experiments or improper measurement conditions.
496
METHODS In this work we have tested the chromatographic method and measured diffusion rates of aromatic hydrocarbons in 13 X zeolite. The temperature range was 553 to 673 K , and the carrier gas pressure ( N z ) was elevated up to 140 bar. We also measured the concentration dependence of the diffusion coefficient D for benzene by partially saturating the carrier gas with this component and injection of the tracer at the column inlet in the usual way. The chromatographic column was filled with zeolite powder with dilution of the zeolite packing by inert material to reduce the retention times. This method ensures a more uniform gas flow as the Zero Length Column (ref.l3).Carrier gas velocity was always high enough to ensure isothermal behaviour of the sorption - diffusion process and exclude limiting external transport resistance at the particle-gas interface. In addition the influence of axial dispersion was eliminated by high gas velocities and the pressure drop along the column axis accounted for in the transport model o f the column. A very similar model for chromatographic experiments with zeolite powder was given by Chiang et al. (ref.1). The Transport Model for the Nonisobaric Chromatoqraphic Column Since we used unpelletized zeolite powder we could describe the concentration distribution in the gas and particle phases by the following equations: gas phase: ( d?X -) at
+ - - (-)ax
D a2X = A*( cg L
at
E ~ L
t
at
3(1-~,; NRC Rc a
5
with
particle phase: ac*
at
2
a2c*
=o/Rf(apz c*
( P I t=O)
(%*) . = ap p=o
P
+
a x aP
=0 0
( 6 ) ( 7 )
(Symmetry)
497
The transport model i s based on the following assumptions: * linear sorption equilibrium at solid-gas interface * ideal gas behaviour in interparticle voids; v-l/p, Da,-l/p (ref.14) * column pressure drop described by Blake-Kozeny equation * Danckwerts-boundary conditions at column inlet and outlet * isothermal column, no influence o f carrier gas on diffusion rate * ideal concentration input function (Dirac) In blank trials was shown that the influence of dead volume ist not significant. The key parameters in eq. (1) and (2) are the adsorption equilibrium constant K and the diffusion coefficient D . Parameters were determined by fitting of numerically computed elution curves to experimental data. The moment method was employed to obtain parameter estimates. Details of the numerical procedure and the nomenclature have been reported before (ref. 2). Experimental Procedure The flow chart of the experimental setup is shown schematically in Fig. 1. The carrier gas flow rate was controlled by a flow controller and maintained at constant pressure by an appropriate backpressure regulator (HITEC). Before entering the steel column with the zeolite powder a highpressure sampling valve (VALCO) injected the content of the sample loop into the carrier gas stream. The column outlet gas stream was split and the smaller part of it injected into the flame of a standard flame ionization detector (DANI). The voltage signal was sampled at regular intervals by the interface of a personal computer and stored on disk for off-line processing. The zeolite crystal powder (Na-13X o f Union Carbide: mean diameter: 2-3 pm)) was mixed with a certain amount of quartz sand (mean diameter: 90 pm) and ratios of up to 100 parts of sand per 1 part of zeolite. The column with inner diameter of 4 to 4.5 mm and lenght between 20 and 40 mm was then filled with the zeolite - sand mixture, sealed with stainless steel HPLC fittings and at last heated at 673 K overnight with continuous flow of nitrogen to remove moisture and impurities from the adsorbent surface. The values for porosity cg were about 0.7. The tracer was saturated in nitrogen at moderate pressures (up to 3 bar) and passed through the sample loop of an injection valve. The typical amount of injected tracer was approximately 3 ng. For determining the concentration dependence of the diffusion rate of benzene, the carrier gas stream was partially saturated with benzene prior to tracer injection.
-
498
1
-
[D\--&-B .rnpllller
- _.healed IltleS
recorder
Fig. 1. Schematic flow diagram of experimental setup for chromatographic measurements at high pressures. EXPERIMENTAL RESULTS Temperature Dependence of Limiting Diffusion Coefficient and Sorption Equilibrium Constant The assumption of constant diffusion coefficient D and linear sorption equilibrium constant K requires very small amounts of injected tracer. Experimentally, this was accomplished by varying the sample volume and checking the variation of peak retention time and peak shape. This procedure is well known for chromatographic experiments and seems essential before further evaluating the collected data (ref.l,3). The mean retention time of the peaks could be influenced by adjusting the proper column bed dilution ratio (less zeolite lowered retention time) and setting the flow rate of the carrier gas. I n most cases experimental peaks appeared between retention times of 30 and 120 seconds, where experiments showed that in this interval1 nonidealities of the input peaks (finite input peak width) and dead volume effects were insignificant in the evaluation of parameters from experimental peaks. Measurement of sorption and diffusion of various aromatic hydrocarbons were thus conducted at infinite tracer dilution within a temperature range o f 553 to 673 K . Fig. 2 and 3 contain plots of the experimentally determined diffusion coefficients D for benzene, toluene, ethylbenzene as well as di-methyl and di-ethylbenzenes. The
499
regression o f the lnD, respectively 1nK vs. 1 / T plots yielded the activation energies of diffusion, Eact, and heat of adsorption, Hads, respectively. Table 1 lists the values of these quantities for each experimental system.
“ t
.
. IC-09 0
, 1 , , 1 1 , , 1.45
1.5
,,,,,,,I,, 1.6
1.55
T
I /
Fig.
a
, , , , I
1.65
11,,,,,1,,
4
1.7
1.8
1.75
1
I ,
iia-3 P I
2. Experimentally determined diffusion coefficients D for benzene, toluene, ethylbenzene and 1-3-di-methylbenzene.
.IE-OB
.5t-09
k-4H 1 -4-Diethylbenzene
1 - 3- Di et hyl be n z e n e
.2E-09
0016
,0017
.001a .on16
,0017
Fig. 3. Experimentally determined diffusion coefficients D for 1 - 2 - , 1-3-, 1-4-di-ethylbenzene.
,0018
500
TABLE 1 Experimental limiting values of heat of adsorption and activation energy for diffusion in Na-13X zeolite ~
Heat of Adsorption AHads [kJ/mole]
- 59.9 - 69.3 - 78.3 - 66.5 - 83.0 - 95.8 - 80.5 - 80.5
Benzene Toluene Ethylbenzene 1-2-Diethylbenzene 1-3-Diethylbenzene 1-4-Diethylbenzene 1-2-Dimethylbenzene 1-4-Dimethylbenzene
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.6 69.4
Activation Energy of Diffusion Eact [ k J / m o l 0 1 I
72.0 43.4 54.2 95.1 80.9 80.5
Benzene Toluene Ethylbenzene 1-2-Diethylbenzene 1-3-Diothylbonaono
1-4-Diethylbenzene 1-2-Dimethylbenzene 1-4-Dimethylbenzene
The magnitude of the diffusion activation energies is quite striking. From the figures in Tab. 1 it seems that there is an unquestionable correlation between Eact and Hads for diffusion of aromatics in Na-13X zeolite. This dependence of Eact with sorption strength rather than molecular size and steric hindrance (compare values for di-ethylbenzenes in Tab. 1) indicates that the passage of molecules through the cage windows is not the limiting step for the diffusion of these molecules. This is in agreement with potential calculations of Ragaini and coworkers (ref.4) who showed that the corresponding activation of ethylbenzene molecules for passing through the window barrier of 13X cages would merely amount to approximately 1 2 kJ/mole. The high values of Eact and Hads in this work are in qualitative agreement with literature data given for sorption/diffusion of aromatic hydrocarbons 'in various zeolites (see Table 2 ) . The combination of high sorption enthalpies and highly temperature dependent diffusion rates pose considerable problems for reliable measurements of the simultaneous sorption and diffusion process. Excellent sorption heat removal is, therefore, essential and renders the chromatographic method superior t o sorption uptake measurements. In addition, the use of unpelletized zeolite powder further facilitated the realization of isothermal conditions within the column.
501 TABLE 2
Literature values of heat of adsorption and activation energy for diffusion of aromatic hydrocarbons Tracer
Zeolite Temperature [I
1
Exp.
Method GC
Na-X
463-623 613-713 613-713 403-477 403-477 403-477 403-477 613-713 406-477 406-477 285-440 400-450 423-463 423-463 423-463 423-463 293-473
Neopentan Na-X Benzene Na-X
187-370 350-400
S
Benzene Benzene Toluene Ethylbenzene
p-Xylene m-Xylene o-Xylene Xylene Xylene Ethylbenzene
Benzene Benzene o-Xylene m-Xylene p-Xylene Ethylbenzene Benzene, Toluene Xylene
*)
CeNa-Y Na-Y Na-Y Na-X Na-X Na-X Na-X Na-Y Na-Y Na-Y 13 X 13 X 13 X 13 X 13 X 13
*,
-AHads
S
56,a 55 63 74 72 74
S
a0
GC GC
s S
GC S S
NMR s GC GC
GC GC
NMR S
Eoct
Author
[ ~ / m o ~ e l motel
76-81 77-91 90 -
46
Choudhary et al./ref.5/ Fomi et al./ref.B/
- * _ 22 27 Goddard et al. 26 26 -I,27 43-144 -"-
hef.61
-,I-
-I,-
-
Goddard et al. hef.101
-
I,
-
Urger et al./ref.Il/
-
19 37 63 103 45 20-25
Germanus et al./ref.lZ/
-
14 25
ioa
_ I -
Ragaini et al. Iref.41 _
_
I
I
_
_
_ * _
B i i l o ~ et al. /ref.7/ BUow et al. /ref.9/
GC : Gaschromatographic method S : Sorptionmethod
NMR: Selfdiffusion measurements (NMR-method)
One of the common problems with zeolite diffusion measurements is the quite frequent discrepancy between values of D determined by different experimental methods and zeolites of different origin and particle size. As for the diffusion results in this work, direct comparison is possible with data for benzene in Linde-13X of Ruthven and Doetsch (ref.15) and also Karger and Ruthven (ref.11) (benzene in large 13X crystals). In both cases diffusion data were collected by the sorption uptake method. Extrapolation of the diffusion coeff icients to the temperatures chosen in reference 15 showed fair agreement (less than one order of magnitude) while the values o f D in (ref.11) with the large 13X crystals were o f f by more than one order of magnitude. It is not clear if this difference is to be attributed to a different origin o f the zeolite crystals o r problems with the
502
diffusion measurements themselves, especially since data given in (ref.11) for two different crystal size fractions showed large deviations i n D. Pressure Dependence of D and K I n some of the above experiments the pressure drop across the zeolite powder bed was considerable and exceded 10 bar. Since the presence of the carrier gas was neglected in the mass balance equations of the trace component, an experimental test of the validity o f this assumption seemed indispensible. We, therefore, extended our experiments into a pressure range that has hardly ever been reached by previous measurements of diffusion in porous solids. In the same fashion, as reported in the preceding section, diffusion coefficients D and adsorption equilibrium constants K were determined for benzene i n 13X in the temperature range of 573 to 663 K and carrier gas pressures of u p to 140 bar (Nz). No significant alteration of the parameters 0 and K was observed. An estimation of the adsorbed amount of nitrogen under these conditions, using a generalized isotherm equation (ref.16) gave pore filling factors for Nz of u p to 10% (ref.17). Since the carrier gas molecules are expected to have a much higher mobility than the more strongly adsorbed benzene molecules, the nonapparent pressure dependence of D does not seem unexpected. Concentration Dependence of D The limitation of the above experiments to highly diluted systems ( i n terms of tracer) is extremely dissatisfying to the engineer who i s concerned with the design of chemical reactors or adsorption equipment. It is, therefore, only logical to extend diffusion measurements into the concentration ranges of practical interest. We adopted a method that has been employed to determine adsorption isotherms by chromatographic experiments, and used it t o evaluate the diffusion coefficient at various concentrations of the trace component in the zeolite (compare with procedure in (ref.18)). The carrier gas was partially saturated with the trace component,’ here benzene, and an additional tracer peak injected on top of this base concentration. The diffusion ceofficient D can be considered c m s t a n t if the ratio of peak height to base concentration height i s sufficiently small. The determination of parameters D and K followed in the same fashion as with pure carrier gas. Fig. 4 contains the concentration dependent diffusion coefficient o f benzene at 573 K up to concentrations of 1.5 molecules of benzene per cage. It can be shown that for s u f f i ciently small peak heights the K parameter is no longer the ratio of c*/cg but describes the slope of the isotherm at the base concentra-
503
tion c*b. After collecting these slope values at various concentrations the isotherm itself can be constructed by numerical integration. I
t
0
I
I
r
.L
.I
I
I
I
.I
.4
.I
I
.I
.I
.a
.9
1.0
1
I
loe
I
1.1
1.2
1.1
i
I
1.4
1.1
Konzentration [mol./csge]
Fig. 4 . The dependence of the diffusion coefficient of benzene on the concentration at 573 K. The experimental isotherms on the other hand can be used to compute the correction factors in Darken's equation: d In p D = Do d In c* where dlnp/dlnc* is obtained from the experimental isotherm. The corrected diffusion coefficients are plotted in Fig. 4 along with the values of 0. The peculiar drop of D and D in the initial concentration range was previously reported for the same experimental system by Ruthven and Doetsch (ref.15) at lower temperatures. After an initial region, the diffusivities exhibit the usual increase with concentration. The thermodynamic correction for the increase of the activity of the sorbate by the Darken equation can, obviously, give no explanation of this unusual concentration dependence. One can therefore argue that sorbate-sorbate interactions are responsible for the observed results. Di'scussion of Results One of the goals of this work was to demonstrate the feasability of chromatographic diffusion measurements in zeolites over a wide range of experimental conditions. Thus, the temperature was varied between 553 to 673 K and the pressure of the carrier gas was increased up to 140 bar. Within this range of conditions and for the different sorbates employed in this work the diffusion coefficients varied over almost three orders of magnitude. In order to avoid unfavorable experimental conditions under which diffusion parameters cannot be determined reliably, column packing dilution with inert material and
504
adjustment of carrier gas flow rate were chosen. The presented method is elegant in that it avoids simultaneous determination of axial dispersion and macro pore diffusion contribution (if bidisperse sorbent was used). The chromatographic method was also shown to be capable of measuring the cotlcentration dependence of the zeolite diffusion and adsorption characteristic. The latter method can be extended to binary diffusion measurements if the tracer is chosen as a substance different from the one that is used to saturate the carrier gas. If component selective detectors were available, these measurements could be extended into wider ranges of concentrations for the binary diffusion system.
REFERENCES 1 Chiang, A.S.; Dixon, A.G.; Ma, Y.M.; Chem.Eng.Sci. 39 (1984) 1451-1459. Aust, E.; Dahlke, K.; Emig, G.; Chem.Eng.J. 35 (1987) 179190. F u , C.-C.; Ramesh, M.S.P.; Haynes, H.W.Jr.; AIChE J. 2 (1986)1848-1857. Ragaini, V . ; Mazzola, E.; Bart, J.C.J.; Z. physik. Chem. Neue Folge 115 (1979) 43-50. Choudhary, V.R.; Srinivasan, K.R.; Chem.Eng.Sci.a (1987) 5 382-385. 275-289 6 Goddard M.; Ruthven D.M.; Zeolites, a(1986) 7 Billow M.; Lorenz P . ; Mietk W.; Struve P . ; J.Chem.Soc Faraday Trans 1,1983,79,1099-1108 8 Forni, L.; Viscardi, C.F.; Oliva, C.; J. Catal. 97 (1986) 469-479. 9 Biilow M.; Lorenz P . ; Mietk W.; Struve P.; J.Chem.Soc Faraday Trans 1,1983,&2457-2466 10 Goddard, M.; Ruthven, D.M.; Proc. of 6th Int. Conf. on Zeolites, Reno(Nevada) (Jul i 1983) , Butterworths , Guildford England, 1984, S. 268-275. 11 Karger, J.; Ruthven, D . M . ; J.Chem.Soc., Faraday Trans. I 11 (1981) 1485-1496. A , ; Karger, J.; Pfeifer, H . ; Samulevic, N.N.; 12 Germanus, Zdanov, S.P.; ZEOLITES 5 (1985) 91-95. 13 Ruthven 0. M.; Eic M.; Zeolites,l988,t3,40-45 14 Carleton F.B.; Kershenbaum L.S.; Wakeham W.A.; Chem.Eng.Sci. 1978,2,1239-1246 Doetsch, I . H . ; AIChE J. 2 (1976) 882-886. 15 Ruthven, D.M.; 16 Wakasugi, Y . ; Ozawa, S.; Ogino, Y . ; J.Colloid Interface Sci. =( 1981) 399-409. 17 Aust, E.; PhD dissertation, University of Erlangen-Nurnberg, 1988 1 8 Kumar, R.; Duncan, R.C.; Ruthven, D.M.; Canad. J.Chem.Eng. 60 (1982) 493-499.
H.G.Karge, J. Weitkamp (Editors1, Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Mr)l-ECULAR MOBILITY OF BENZENE AND P-XYLENE I N MFI' TYPE ZEOLITES
M. BIJLOW, J . CARO, B. W5HL-MJHN and B. ZIBROWILlS C e n t r a l I n s t , i t u t , e of P h y s i c a l Chemistry, Academy of S c i e n c e s t h e G.D.R., DDR-1199 B e r l i n , Rudower Chaussee 5 , (German Democratic R e p u b l i c )
of
ABSTRACT I n f o r m a t i o n c o n c e r n i n g t h e dynamics of benzene and p - x y l e n e i n MFI t y p e z e o l i t e s h a s been o b t a i n e d from s o r p t i o n u p t a k e measure rnents and from a l i n e s h a p e a n a l y s i s o f t h e '8 and 1 3 C NMH. s p e c t r a . F o r b o t h m o l a o u l e s , t h e i n t , r a c ~ y s t alli n e d i f f u s i v i t i e s d e t e r m i n e d by s o r p t i o n u p t a k e and treated by t h e Darken e q u a t i o n d e c r e a s e w i t h i n c r e a s i n g sorbate c o n c e n t r a t i o n . p-Xylcne shows a t h i g h c o n c e n t r a t i o n A much more restricted m o b i l i t y t h a n benzene. From t h e NMR spectra i t f o l l o w s t h a t t h e s o r b e d benzene molecule^^ perform f a s t r e o r i e n t a t i o n s a b o u t t h e i r hexad axes. Superimposcd OII t h i s C6- r e o r i e n t . a t : i o n , t h e benzene m o l e c u l e s jump between intion s i t e s . The mean rc:siclenc:e t , i m e bet.ween two suc:c:eeding j i p p s depends s i g n i f i c a n t l y on tsmperal;ure and s o r b a t e c u n a e n t r a 1.iot-i. W i t h j u s t i f i e d ~ s s u m p t i o n sf o r t,he jump lengt.hs, c I i f f u s i v i - t i e s f o r benzene can be estimated. These da1;a are i n good agree . l i m i t , w i t h t h o s e d e t e r m i n e d by s~.irpI,ioriu p t a k e mmsurc:rricnt.s, ' I
INTRODUCTION MFI t y p e z e c 1 1 i t . e ~show a h i g h cat.alytict acA,ivit,y. Since t h e o a t a l y t i c reac1;im p r o c e e d s w i t h i n t h e z e o l i t ic v o i d volume, a c c u r a t e knowledge of t h e i n t r a c r y : i l . a l l i n e d i f f u s i u n of nrcrl~:cular species i n v o l v e d i n t h o r e a c t i o n is needed t o f u l l y u n d o r s t a m l the mechanisms (of t h e p r o c e s s , and a l s o tm model t h e c!l.JmpleX b e h a v i o u r of t h e c a t a l y t i c system. The s o r p t i o n k i r i n t i c s and iriolecular dynamics of benzene arid p - - x y l e n e have btteri inve:A.,igatxd t o o b t a i n r e l i a b l e i n f o r m a t i o n 011 t h e i r i n t r a c r y s t a l l i n c m o t i o n s .
506 crystal s u r f a c e which t a k e s a c t i v e l y part; i n t h e m a s s t r a n s p o r t and t h e volume of t h e c r y s t a l s , r e s p e c t i v e l y . The o b t a i n e d d i f f u s i v i t i e s , D, were c o r r e c t e d for t h e s o r p t i o n i s o t h e r m c u r v a t u r e by means of t h e Darken e q u a t i o n , D = Do(61n p/61n c ) ~ I. s o t h e r m a l e v a l u a t i o n of t h e d a t a is j u s t i f i e d because t h e i n f l u e n c e of t h e s o r p t i o n h e a t g e n e r a t e d d u r i n g t h e u p t a k e p r o c e s s w a s minimized by an a p p r o p r i a t e c h o i c e of e x t e r n a l t h e r m a l c o n d i t i o n s (Tv To, and To b e i n g t h e t e m p e r a t u r e of t h e s o r p t i o n v e s s e l (zeolite T, crystals) and t h e d o s i n g volume ( g a s e o u s p h a s e p r i o r t o s o r p t i o n u p t a k e ) , r e s p e c t i v e l y ; c f . ( r e f . 1 ) ) . The u p t a k e measurements were c a r r i e d o u t w i t h i n t e m p e r a t u r e and p r e s s u r e r e g i o n s r a n g i n g f c u n 274 t o 423 K and lo-' t o 10 t o r r , respectively. The MFI samples used are l i s t e d i n Table 1 .
>
---
Table 1 ht,umatics/MFI s y s t e m s i n v e s t i g a t e d by s o r p t i o n u p t a k e . .. -
sorbate
sorbent
__
-.......-.I.--._-________
Si / A 1
.c.ry?t.&l-gi---2 z e/p m-. - .--2 - 4(V/A) /cm
b
_.....
(Na,H) -ZSM-5 (a)
benzene
( NFI,
*
H) --zsM--~'
135
*50
(b) i 1ical itc.-l"+> 1000
benzene
(1:)
s i 1i o a l i t c - 1'
benzene*
habit,
.
.
..
3 f r . ~ f 5 _ 10 .~ 3 . 6 2 lo-.7----
polyhedral twinninge
55 x 12__ x 10 2 . 9 7 - 1 0 "T -
polyhedral monocrystn 1s
190 X 56 Z 2 . 5 4 64 *
polyhedral monocryst a 153
.-
I
> 1000
_3>2._5_~-15 s p h e r u l i t i c : 8.79
(d) --
...
--____.
..
p-xy 1m e o-xylene benzene
.*
.
cr y st a 1
~. ..
-
t w inn i n g s ~
__
__
Data p u b l i s h e d i n ( r e f . 4 ) +,. MFI samples used for t h e NMR measurements, c f . (refs. 5 , 6 , 7 ) . Fur t h e q u a l i t y of t h e z e c ~ l i t e sused cf. F i g . 1 as example.
+
Nt.1 H mmsLI ramants.
'I'he '11 NMR expt?rimt?ntr; were pcrforrnecl on 1 . h hotnts- ~ ~ a d NMR e spectrometer lJDRIS (Karl Marx I J n i v o r s i t y of L e i p a i g ) atid on ;3 nIZUKER MSL, 400 :spacl.rometc?r a t a frequency of 1 3 . 8 MIIe nrid 6 1 . 4 MHz, r e s p e c t i v e l y . The s o l i d - l i k e spectra were o b t a i n e d by t h o quniirupole echo p u l s e sequence ( n / 2 , y - 'r -- 3~/2,x; ( r e f . 9 ) ) . 'J'he spectra at low f i e l d are presanl.et1 as h a l f - spectra r e s u l t i n g from F o u r i e r tratisforrnnt inn of the accumulated non-.qiiadraturl: echo decays (ref. 7 ) . In w d e r t-o enlargc: the s p o u t r n l excit.at,iun width, compoeite p u l s e s ( r e f . 10) were i~:;ed i n t h e case 13f ~~--wlene. The best, r e s u l t s w e r e obt.airiccl tly n compcJsite p u 1 ~ ; ~ :
quenc:e ( 1 3 5 3-80 90 133 45 g%i 180 90 1 3 5 ) proposed LY Sirninuvitch a t a l . (ref. 1 1 ) . P e r d e u t e r a t e d benzene and p- x y l e n e - d 4 were used a s s u r h a t e s . ::I
507
A l l measurements of sorbed species w e r e performed w i t h s e a l e d samples. The experimental e r r o r of t h e s o r b a t e c o n c e n t r a t i o n does n o t exceed 10 %. The proton-decoupled 13C NMR s p e c t r a of sorbed benzene w i t h t h e n a t u r a l 13C abundance were a l s o recorded w i t h t h e UDRIS s p e c t r o m e t e r a t a frequency of 22.6 MHz (ref. 5 ) . Most of them were t a k e n u s i n g s i g n a l enhancement by c r o s s p o l a r i z a t i o n , The '%i MAS NMR spectra were measured on t h e MSL 400 at a frequency of 7 9 . 5 MHz u s i n g a s i n g l e p u l s e e x c i t a t i o n and a s p i n n i n g r a t e of about 3 k h .
F i g . 1. "Si MAS NMll spectrum for t h e MFI- ( c ) sample s y n t h e s i z e d arid kindly supplied by D . T . Hayhurst ( r e f . 8 ) . HKSIILTS Benzeno Wig. 2 shows t h e concentmition p a t t e r n s of t h e d i f f u s i v i t y D,, of benzene f o r MFI samples (1)) and (c), r e s p e c t i v e l y . The d i f f u s i v i t i e s i n both samples d e c r e a s e w i t h i n c r e a s i n g c o n c e n t r a t i o n although for s i l i c a l i t e - 1 (MFI-(c)) t h e y are s l i g h t l y e n l a r g e d compared with ZSM-5 ( M F I - ( b ) ) . The data correspond b o t h q u a l i t a t i v e l y and q u a n t i t a t i v e l y t o t h e r e s u l t s published f o r t h e samples MFI (a) and MFI-(d) i n ( r e f . 4 ) . Thus, changing b o t h t h e crystal s i z e (and t h e o r i g i n of t h e samples u s e d ) d i d n o t show s i g n i f i c a n t , i n f l u e n o e on t h e d i f f u s i v i t i e s . From Arrhenius' p1ol.s f o r t h e parameter range:: c o n s i d e r e d , i t f o l l o w s t h a t t h e d i f f u % i o n of benzene i n s i l i c u l i t e - 1 is c h a r a c t e r i z e d n o t o n l y by a h i g h e r m o b i l i t y , b u t a l s o by a lower energy of a c t i v a t i o n (Ea= 27 kJ-mo1"' (ZSM-5); 2 0 kJ-mol ( s i l i c a l i t e - 1 ) ) . The m t i vrAion energy d a t a f o r benzene ore i n s a t i s f a c t o r y agraemcint with t h o s e from model 1ing t h e i n t r a c r y s t a l l i n o t r a n s p o r t by cornputor s i m u l a t i u n meLhOd% ( r e f s . 12, 1 3 ) . J o i n t c o n s i d e r a t i o n of s o r p t i o n u p t a k e and NMR d a t a p r o v i d e s a deeper insight i n t o t h e mir:r.ophysic:al mechanism of inLrar:ryst,nl l i n e molecular m o b i l i t y . Due t o t h e c l o s e s i z e s of benzene molt:!
508
+
423 K 393K A X 363 K r 343 K C 3 323 K 303 K 274 K 0
B x
-
*
I
I
01
0.2
I
I
I
I
I
0.3
0.4
05
06
a7
n/ mmot g‘
F ig . 2 . Concentration dependence of d i f f u s i v i t y d a t a , Do, f o r benaene/MFI( f u l l s y m b o l s ) and (b) MFI-(c) ( c r o s s e s ) .
c u l e s and t h e MFI c h a n n e l d i m e n s i o n s ( r e f . 1 4 ) , sorbed benzene i s e x p e c t e d t o p r o v i d e r a t h e r s o l i d t h a n l i q u i d l i k e NMR s p e c t r a . The l i n e shape:; of tihe ‘H and 1 3 C NMR s p e c t r a are d e t e r m i n e d by e l e c t r i c q u a d r u p o l e i n t e r a c t i o n and by the chemical s h i f t a n i s o - tropy, respectively. Both t y p e s of i n t e r a c t i o n depend on the r e l a t i v e o r i e n t a t i o n of t h e sorbed m o l e c u l e s w i t h r e s p e c t , t o t h e d i r e c t i o n of t h e magnetic f i e l d a p p l i e d . T h e r e f o r e , s e v e r a l k i n d s of m o l e c u l a r motions give rise t o c h a r a c t e r i s t i c l i n e shape patterns. For t h e banzene/MFI--(b) system, t h e dependenoet.; of t h e ‘H NMlZ l i n e s h a p e on b o t h t e m p e r a t u r e and l o a d i n g art: g i v e n i n F i g . 3. From a comparison o f t h e s p e c t r u m € o r h i g h s o r b a t e c o n c e n t r a t i o n ( 7 . 6 molecules per u . c‘. ) and 1 C J W t e m p e r a t u r e ( 3 2 5 K) w i t h t h e theoretical line shapes for different mo%ional statc; ( r e f s . 7 , 1 5 1 , i t h a s ~ C be J c o n c l u d e d t h a t t h e s o r b e d benzene mo1ecu1es perfurrn f a s t r e o r i e n t a t i o n s a b o u t t h e i r C6-axes; even a t 125 K t h e c o r r e l a t i o n L i m e of t h e C6 r e o r i e n t a t i o n is mur:h s h o r t e r t h a n 1 v s . With i n c r e a s i n g t e m p e r a t u r e , t h e 1ine s h a p e which i s c h a r a c t e r i s t i c of a n e x c l u s i v e C6 r e o r i e n t a t i o n d i s appears. A t t e m p e r a t u r e s above 1 7 0 . . . 180 K, a r e c t a n g u l a r c u r v o js found. Apart from t h i s , above 270 K n o q u a d r u p o l e e c h o i s o b s e r v e d f o r t h e used pulse i n t e r v a l of 60 v s . As i t f o l l o w s from l+’i.g. 3, a r e d u c t i o n of t h e s o r b a t e c o n c e n t r a t i o n c a u s e s a similar e f f e c t as a rise i n t e m p e r a t u r e .
509
k
LL 35
concenhation/ molecules per u. c.:
70
0
35
70
[(w
1.6
0
35
70
- w o ) / 2 d /kHz 4.0
7.6
5.6
ZSM-5 Fig. 3 . 2H NMR spectra of perdeuterated benzene sorbed on (MFI-(b)) for various concentrations and temperatures. The spectra were obtained using 3 . 5 CIS n / 2 pulses and a 60 CIS pulse spacing (4096 scans). 13C NMR line shapes of benzene molecules sorbed in the MFI-(d) sample are shown in Fig. 4 . For high sorbate concentrations and low temperatures, powder spectra are obtained, which are characteristic of an axially symmetric shielding tensor. This linc
260 K
200 K
150 K concentration/ molecules per u.c.: 3.2
& 6iso
4 6.0
8.0
Fig. 4 . 13C NMR spectra of benzene sorbed on silicalite-1 (MFI( d ) ) for various temperatures and concentrations. The spectra at 200
K were taken using signal enhancement by cross polarization.
510
shape is known to be generated by a fast mottion of the benzene molecules about their hexad axes of symmetry (ref. 16). With increasing temperature and decreasing loading, t h e 1 ine shape changer. into a Lorentzian one, i.e. an additional molecular mo tion becomes fast enough to average out the chemical shift anisotropy. In the transition range, there are spectra which seem to rwult from a superposition of both line shRpe types. €J_:ULC-E?
'he concentxation patterns of the diffusivity D,, of p-xylem for the MFI-(a) sample are given in Fig. 5. In general, the curves are similar to those for the benzene/MFI system.
A
0
423 K
393 K A 363 K
-
I
m 1
Fig. 5. Concentratinn dependence of diffusivity data, Do, for the p-xylen/MFI-(a) system. But the Do data of p-xylene for the MFI sample considered tend to exceed those for the bensene/MFI system at sorbate concentrations lower than 4 molecules per u.c., cf. Fig. 6 . In contrast, the decrease in Do with increasing concentration (exceeding 4 molecules per u.c.) becomes stronger for p-xylene than for benzene. At room temperature and below, the sorbed p-xylene molecules are highly immobile. This behaviour is also reflected by a concentration dependence of the activation energy of intracrystalline diffusion (25...55 kJ-rnol-' with increasing Ea values at increasing concentration) approximated from Arrhenius' plots. Therefore, the p-xylene/MFI system appears to be more complex than t h e benzene/MFI one.
511
I F i g . 6 . Comparison
1ol'
41
0.2
03
94
45
96
47
n/mmol
Q@
of the concentration dependenix~:: of diffusivity data, D,,, for the systems p-xylenc or! MFI-(a) (A)and benzene on MFI-(b) (H) and MFI-(c) (v),all at 363 K.
S u r p t i o n uptake experiments for o-xylene/MFI-(a) did not allow
determine both equilibrium and intracrystalline diffusiuri data. T h i s finding, cf. Fig. 7, corresponds to that already described in (ref. 1 7 ) . tn
015
I
0.10
n mm0l (1' 0.05
'lh
Fig. 7 . Concentration vs. time dependences for tho o-xylene/MFI-(a) system.
'11 NMR spectra of p-xylene-d4 sorbed on ZSM-5 (MFI-(b)) are shown in F i g . 8. The spectrum obtained by simple 4 2 pulses (Fig. 8a) exhibits a rectangular line shape with clear edges at; 3 7 0 kHa. As distinct from the spectra in Fig. 3 , this line shape is not caused by motional processes but by a non-uniform excitntion of the range of possible resonance frequencies (k140 kHe). According to a relation given in (ref. 20), only 29 % of the truo spectral intensity at (o - wo)/2n = * 7 0 kHe are obtained f o r the applied rf power. Using composite pulses with much larger excitation widths (ref. ll), one obtains a line shape characteristic o f deuterons bonded to carbon in solid state. From the observed quadrupole splitting of about 140 kHe it follows, that all mole-
512
kJ-4 / ,L
F i g . 8. '€1 NMR s p e c t r a of p-xylene -d4 s o r b e d 0t1
ZSM-5
(MFI-(b),
7 . 2 molecules p e r u . c . ,
w u
l
4 .
.
.
,
h
,
.
100000
.
I
. . . .
(
0 HERTZ
'
,
.
.
. .
- 100000
'
"
"
T = 295 K ) ) o b t a i n e d by (a) 5.0 P S J t / 2 p U l s C ? S (16,000 s c a n s ) and (b) composite n/2 p u l s e s of t h e same r f power (32,000 scans). A p u l s e s p a c i n g of 40 p s was used i n both cases.
c u l a r mot,ions have a t room t e m p e r a t u r e c o r r e l a t i o n times much l a r g e r than 1 p s . Taking i n t o account t h e p u l s e i n t e r v a l and t h e d u r a t i o n of t h e composite sc/Z--pulse used ( 4 0 p s and 71 p s , r e s p e c t i v e l y ) , one can o b t a i n a more precise lower l i m i t f o r t h e c o r r e l a t i o n time. A l l m o l e c u l a r motions t h a t change t h e o r i e n t a t i o n of t h e molecular p l a n e with respect t o t h e magnetic f i e l d proceed a t rnaxirnirm s o r b a t e concentrat.ior-i w i t h time c o n s t a n t s l a r g e r than 100 p s . A f a s t r o t a t i o n about t h e p a r a - a x i s of t h u 111 . l e c u l e , deduced from 13C NMR s p e c t r a ( 4 . 8 molecule:: per U . C . ) a t 310 K ( r e f . 1 8 ) , can be excluded. O u r f i n d i n g s are i n a c c o r dance with t h o s e of a p r e v i o u s H' NMR stiidy ( r e f . 1 9 ) f o r a c o n c e n t r a t i o n of 3 . 6 molecules p e r d e u t e r a t e d p-xylene per u. c . Using p-xylene-d4 w e g o t r i d of t h e s t r o n g s i g n a l from CD3, which is i n s e n s i t i v e t o a motion a b o u t t h e p a r a - a x i s and o b s c u r e s t h e l i n e s h a p e of t h e a r o m a t i c d e u t e r o n s . The a d d i t i o n a l i n t e n s i t y i n t h e c e n t r e of t h e spectrum i n F i g . Rb arises most probably from r e s i d u a l methyl d e u t e r o n s . There i s a l s o no e v i d e n c e f o r a f a s t of t h e molecule. I n t h e l i m i t of h i g h exchange r a t e s , 180°-flip such a motion would r e s u l t i n t o a spectrum f o r a n o n - a x i a l l y symmetric f i e l d g r a d i e n t t e n s o r w i t h s t r i k i n g s i n g u l a r i t i e s a t ( w - wo)/2n = i 1 7 kHz. Owing t o the e f f e c t o f motional p r o c e s s e s d u r i n g t h e long composite p u l s e , t h e i n t e r p r e t a t i o n of spectra f o r lower s o r b a t e c o n c e n t r a t i o n s is much more c o m p l i c a t e d .
513
DISCUSSION The i n t r a c r y s t a l l i n e c h a r a c t e r of t h e m o l e c u l a r m o b i l i t y d a t a d e r i v e d from s o r p t i o n u p t a k e e x p e r i m e n t s f o r b e n z e n e a n d pxylene/MFI s y s t e m s i s e n s u r e d by t h e f o l l o w i n g : ( i ) experimental c o n d i t i o n s which s t r o n g l y f a v o u r i n t r a c r y s t a l l i n e d i f f u s i o n under quasi--isothermal c o n d i t i o n s ( r e f s . 4,231; ( i i ) t h e s h a p e o f u p t a k e c u r v e s is t y p i c a l f o r a d i f f u s i o n 1i m i t e d process ; ( i i i ) t h e t i m e c o n s t a n t s of s o r p t i o n u p t a k e s h o w t h e crystal s i z e d e p e n d e n c e expected f o r i n t r a c r y s t a l l i n e d i f f u s i o n , i . e. t h e d i f f u s i v i t i e s are i n d e p e n d e n t o f c r y s t a l s i z e ( a n d of t h e o r i g i n o f t h e samples); ( i v ) t h e r e s u l t s o f s o r p t i o n u p t a k e a r e c o n s i s t e n t w i t h t h o s e of ‘11 and 13C NMR measurements w i t h respect t o b o t h t h e a b s o l u t e v a l u e s and t h e i r d e p e n d e n c e s o t i t e m p e r a t u r e a n d c o n c e n % r a t i m ( c f . below); ( v ) t h e d i f f u s i v i t i e s agree w e l l w i t h d a t a o b t a i n e d i n o t h e r l a h o r a t a r i e s ( c f . Table 2 f i x benzene) ; ( v i f o r b e n z e n e , t h e a c t i v a t i o n e n e r g y v a l u e s which arc: comparable f o r t h e MFI s y s t e m s i n v e s t i g a t e d by v a r i o u s e x p e r i m e n t a l agree w e l l w i t h r e s u l t s f r o m c o m p u t e r s i m u l a t i o n s techniques, ( r e f s . 12,131; ( v i i ) f o r p x y l e n e , t h e informat,iori d e r i v e d f r o m NMR mc~n~~urernerit,:; and cnmputer sirnll1ati0ti work ( r e f , 2 6 ) do n o t c o n t r a d i c t thi: f i n d i n g of a i:c-Jric!erit.ral.icrn-dependftril; e n e r g y of act,ivHt.irm. The latLer p e c u l a r i t y mayl>e i n t e r c o n n e c t e d w i t h t h e phenomannn n P hysteresis in sorption i snt.herms ( c o n c e n t r a t i on-dependent, s o r p t i o n s t a t e s of p - x y l e n e ) repeatedly reported i n literature ( e . g . r e f s . 14,17,27). Despit.t: t h i s c o n s i s t e n c y of d a t a , t h e i r general i n t e r p r e t a t i o n meets s t i 11 d i f f i cu 1t i e ~. ; With cert a i n pre r e q u i s i tes der i ved f rrm appropriatx l i t e r a t u r e , and s u p p o s i n g t h e t r a n s l a t i o n a l mot,ion of aromatic compounds i n MFI t y p e z e o l i t e s to proceed v i a a c t i v a t e d m o l e c u l a r jumps, t h e c o n s t a n t jump l e n g t h model ( r e f . 2 8 ) h a s b e e n sjllg$ested f o r Lhe i n t e r p r e t a t i u n of t h e b e n z e n e d i f f i x l ; i o n d a t a ( r e f . 2 9 ) . Assuming a jump l e n g t h e q u a l t o t h e tli:;t,uncr: between two ad j a o e n t c h a t i n e l i n t e r s c c t i o n s , t h i s model describe:; the b e n z e n e d i f f u s i o n f a i r l y we11 at. c:oric:oritrat.ioris C: 4 m c i l w x ~ l w p e r u .1:. , I : i i ~ t , i L bei:orrie:; i n a d e q u a t e a t h i g h e r l o a d i n g s . For t h i ? 1 attxr ~ ~ ~ i r i ~ ~ ~ r i Ir e, g r~ i o~n ~, i ~it. . i rw~iiild i fcil l o w t h a t , m u l t i p l t ! m o l t : c u l n r jumps a r e t-ieeilei-I %[.] ovt?rl:oriit: i:,he samc f iseci d i s t ; w ~ c t ? . M w : L ~ ~ i - c ~ h a t i l yt.ht+rc , occur:; H uomplex process i n c : l u c l i n ~ r i o t rinly H change o f t h e jump l e n g t h but, also .a v a r i a t i o n n f b o t h 1;tiF: .jum p i n g frequency arid t h o number of ticcc2t;f;ible ocirpt-,iori si1.t.:;. T a k i n g i n t o account, t h e a b o v e p e c u l a r i t i e s o f t h e p . - x y l e n a sorp t i o n , l.ht: i r i ~ i p p il c i r t i i 1i t.y (if t.tw (:~iri~t.iiril. jurnp 1t:ngt.h rncidel w i t h t h e d i f f u s i o n ririta f o r p -sylene/MFI s y s t e m i n :i.t;1.;iightfi.)1.warrlI
514
Table 2 D i f f u s i v i t y d a t a for t h e benzene/MFI s y s t e m MFI sample experimental concentra-T method tion ( s i z e ) /pm3 , . a l./ A l molec. / I A . c. K
,-.
S i licalite-1 ( 3 0 x 2 5 ~ 1 5;)
~ E I - N M Rt r a c e r deSOrptim
D,xlO1'
ref,
c2-s-l
6.0
293
0.5
22
Silicalite-1 p = const. ( 1 0 b x 4 5 x 4 5 ) ; > 1 03 Cahn b a l a n c e
-3.5 -2.0
303 343
0.7 -2
23
v = const. = var.
4.0 1.2
323 363
1 7
>lo3
Silicalite-l (19Ox56x35);>1O3 S i 1ical i t e -1 ( 3 0 x 2 5 ~ 1 5 )>; l o 3 S i l ical i t e - 1 ( 1 ' 1 0 x 5 5 ~ 4 3;)> lo3 Silicalite 1 ( 6 ~ 3 x )2; > 1 O3 t 1-- ZSM - 5 ( 5 . 7 ~ 3 ~;235) ( 1 3 ~ 7 xj6;35
(Na,E1) ZSM 5 (35x15~10);135 (Na,II)- ZSM 5 ( 3 5 x l 5 x 1 0 ) ; 135 (Ma,E1)- ZSM 5 (55X12XlO) ; 5 0
p
p = const. spring balance
-3.5
this paper
303
-6
4
1.0
388
-10
24
4.0
293
0.1
25
4.0
298
1
17
v = const. p = var.
-3.5
363
4
4
p = const. s p r i n g balani:c
-3.5
363
4
4
2.0 2.0
303 363
-6
frequency response p = const,. microbalance
p
= const.
Cahn h a 1a n c e
v = const.. p
= var.
1
this paper
The cwnclusioris on m o l e c u l a r rnobi 1i t y uf s o r b e d b e n z e n e as drawti from 8 o r p t ; i o n u p t a k e are s u p p o r t e d by t h e f i n r i i n g s of t h u 't1 firid I3C NMR. T h e i r r e s u l t s s u g g e s t t h e f o l l o w i n g model for t h e iroleuulilr rnot;iori o f benzene i n MFI t y p e z e o l i t e s ( r e f s . 5 , 7 ) : (i The s o r b e d b e n z e n e rric1lecules perfcmm fas 1. r.ec.JrieritAt.iunr; abIJlAt, t h e i r C6 a x e s . E v e n at 125 K, t h e c o r r e l a t i o n t i m c o f t h i s 111 ' i o n is much s h o r t e r t h a n 1 p s . ( i i ) Superimposed upon t h i s C6 r e o r i e n t a t i o n , t h e r e are jumps of .the benzene r r o l e o u l e s between a 1irnit,rsd number t r f sorpt.ic.m site:; which a l l o w o n l y d i s t i n c t , o r i e n t a t i o n s of t h e hexad axis of tht: lit?tizcrie mco1c:oul e with respcot. 1.0 the c r y s t a l :l;ysl,cm. Tho mean r e s i d e n c e time '6 between t w o s u c c e e d i n g jumps decreases w i t h j r i c r e a s i n g t e m p e r a t u r e arid d e c r e a s i n g l o a d i n g . A m o t i o n a l b e h a v i o u r o f t h i s t y p e explain:: a l s o t h e i n i t i a l l y ' NMR s i g n a l r e d u c t i o n w i t h i n c r e a s i n g u~lint,allibible strong H t e m p e r a t u r e . .Jump m o t i o n s w i t h '6. i n t h e order of t h e p u l s c J
515
i n t e r v a l T of t h e q u a d r u p o l e e c h o s e q u e n c e g i v e r i s e t o "echo d i s t o r t i o n s " accompanied by a s t r o n g d e c r e a s e o f t h e s i g n a l i n t e n s i t y ( r e f s . 3 0 , 3 1 ) . The o b s e r v e d r e d u c t i o n of t h e e c h o i n t e n s i t y beyond t h e d e t e c t i o n l i m i t i m p l i e s a v a l u e o f ' v j i n t h i : o r d e r of 100 p s or less. T h e o r e t i c a l c a l c u l a t i o n s o f 13C NMR spectra show t h a t i n t h e casc of m o l e c u l a r jumps, t h e NMR l i n e s h a p e i s n o t s e n s i t i v e l y d e p e n d e n t on t h e g e o m e t r i c a l a r r a n g e m e n t of t h e s o r p t i o n s i t e s ( r e f s . 3 2 , 3 3 ) . T h e r e f o r e , c o n c l u s i o n s on t h e jumping rate c a n be drawn e v e n i f t h e r e is a l a c k of i n f o r m a t i o n on t h e genuirlc s o r p t i o n s i t e s of benzene i n MFI s t r u c t u r e . S e m i - - e m p i r i c a l p o t e n t i a l c a l c u l a t i o n s ( e .6 . r e f . 3 4 ) have shown t h a t t h e c h a n n e l i n t e r s e c t i o n s as w e l l as t h e p o r e segments i n between car1 a c t OF; s o r p t i o n s i t e s f o r benzene i n s i l i c a l i t e - 1 . From t h e e x p e r i m e n t a l I3C NMR l i n e s h a p e s w e d e r i v e d a rnean r e s i d e n c e t i m e 'I . o f 20 pi; J and 150 pci f o r a c o n c c n t r a t i o n o f 6 m o l e c u l e s p e r U . C . arid a t e m p e r a t u r e of 250 K and 200 K , r e s p e c t i v e l y . Provided t h a t the jumps d e t e c t e d i n NMR s p e c t r o s c o p y arc ~ c ~ ~ ~ r n p a nby i e d a t r a n s l a t i o n a l motion ( J f t h e rnolecule, i t is pos s i b 1e t o e:; t imate i n t rar r y s t a 11i n e s e 1f -d i f f us i v i t i es Do. Assuming t h e d i f f u s i o n p a t h o f a m i g r a k i n g m o l e c u l e as a %urn o f irld i v i dual a c t i v a t e d jumps I f o r i s o t r o p i r: s y s t e m s t h e re1 t i t i on <12i = 6-Do-7 is v a l i d . < I 2 i d e n o t e s ' t h e mean s q u a r e jump lerlgth. With t h e f u r t h e r a s s u m p t i o n t h a t o n l y jumps between a d j a c e n t s i t e s t a k e p l a c e , one obi;ains a rnean s q u a r e jump l e n g t h 7 . 4 - 1 0 - - lc12 ~ ~ f<Jr a l u c a l i z u . l , i o n of sorp-ticin si1.S::; i n the pore: segments ( r e f . 3 5 ) . A p r e f e r r e d a r r a n g e m e n t o f t h e benzene mole cule:; i n oklciriric.1 irit.ersec:tic.itis and the segments o f t h e si.,raighL ctwmnele ( r e f s . ~ , 3 7 , 2 8 ) r e s u l t s i n a v a l u e o f 5 . 5 . 1 0 - - ' ~ in2. Although the a h n i c e uf t h e a r r a n g e m e n t is a l i t - t l o a r t i i t r a r y , t h e rnean d i s t a n c e t j c L w e e n ad jacent; sibs would he i n any case i n t k i c urcler. U.f 1 rim. The c1iffusiviLie:; otitairied i n t h i s way are i n reascjnal-ils agreament w i t h t h e dsLa f rom u p t a k e experirnents . With Lhe c i t ~ c i v u value:; f o r Lhc r i i t w i rwidericx: Lime 'I and the c u r responding a c t i v a t i o n e n e r g y E = ( 1 7 t 4 ) k.J-mo1. l a s e l f d i f f u s i v i t y D, = I . . . ~ - 1 0 - 1 0 cm2-;-is o b t a i n e d at. 303 K f o r a orti ti at.^: r:rincent.ratiori of 6 rnolecules per u . c. Furthermore, t-he: d e c r e a s e of m o l e c u l a r rnrsbility w i t h i n c r e a s i n g s o r b a t e c o n c e n t r a t i o n i s c l e a r l y r e f l e c t m l i n t h e NMR s p e c t r a ( c f . F i g s . 3 arid 4 ) and i:;m be e x p l a i t i e d by at'i iricrea:;e of the mean resiiiwice t i m e I . . 3
p m~.:chttrii:xn i n t h o (:ase o f xylerie, t h e same procedure y i e l d s a t least a n u p p e r l i m i t f o r t h e s e l f d i f f u f i i v i i , y . T h e 1 1 - xylc?rv? mtile(;ules s o r b e d i n MFI a e u l i t e s w e r e l o c a l i z e d b y Lhtwret.ica1 ( r e f . 3 6 ) a: well by XRD s t u d i e : ; ( r e f . 3 9 ) irr 1.ht: clinr~nt:l int.err;ec:t.icjris and thr? p o r e segmont.s of % h e ~j i n u ~ ; u i i i a . lchannel::. WiLh L h i s arr;mgement of sot.pticxi r; i EJ-
516
d e r i v e d lower l i m i t f o r t h e mean r e s i d e n c e time one o b t a i n s Do < lo-'' c m 2 - s - ' a t room t e m p e r a t u r e and f o r a sorbate c o n c e n t r a t i o n of 7 . 2 m o l e c u l e s per U . C . This v a l u e c o r r e s p o n d s w i t h t h e u p t a k e d a t a shown i n F i g . 5. Thus, t h e NMR i n v e s t i g a t i o n s g i v e f u r t h e r e v i d e n c e t h a t the d i f f u s i v i t i e s measured by s o r p t i o n u p t a k e are i n d e e d i n t r a c r y s t a l l i n e d a t a which depend on c o n c e n t r a t i o n . They a l s o allow to d e r i v e a m i c r o p h y s i c a l model f o r t h e intracrystalline molecu 1ar t r a n s p o r t . and
the
(> 100 p s ) ,
ACKNOWLEDGEMENT The a u t h o r s d e d i c a t e t h i s paper t o P r o f e s s o r H . Pfeifer, Karl Marx l l n i v e r s i t y o f L e i p z i g , who s u p p o r t e d t h i s work i n v e r y rnat1y w ~ y s ,a t t h e o c c a s i o n o f h i s 6 0 t h b i r t h d a y . REFERENCES
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( 1 9 7 3 ) 569--590. 17 K . Bes(:hmmn, G . T . Kokotai 1 0 a r d I,. R.it:kart., Chorn. Eng. P r o c e s s . , 22 ( 1987) 223 -229. 18 . I . fl. Naby, E . G . Derowinc, t i . A . Re:sirig rind G. It. Mi 1l e r , J . Phyw. Chem., 87 ( 1 9 8 3 ) 8 3 3 4 3 7 . 19 H.R. Eckmari arid A . . J . 'Jtga, J . Amc:r. Cht:m. SCJI:. , 105 ( 1 9 8 3 ) 4841 -4842. 2 0 M. fllotll, J.11. Davi:d arid M.I. V a l i c , C a r l . . I . Phy:<., 5 8 ( 1 9 8 0 ) 1510 -1517, 2 1 M . Billow, P . S t r u v e , W . M i e t k and M . KoGiFik, .J. Chem. SOP. F a r a d a y T r a r i s . 1, 80 ( 1 9 8 4 ) 8 1 3 - 8 2 2 .
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J . I<#rgur, 11. I ' f e i f e r , L,. R i e k e r t , M. Bulow and Zik&rluv&, J . Cht:m. SCJC:.Ii'ciraday T r a n s . I , sutimittecl. D . B . Shah, D . T . [laytiur:;t, G . Evanina and C. J . Guo, AICI1E .J. , i ri prcvs . N . Vun Pen- Begin, L.V.C. Re@%, J . car^ and M . B i i l o w , Z e o l i t e s , i n press. 1'. Wu, A . Debebe arid Y.H. Ma, Zeiulit.es, 3 (1983) 118 122. A . R . Nuwak and 6 . D . P i c k e t t , p r i v a t e i n f o r m a t i o n t o M. B . R . E . R i c h a r d s and L . V . C . Rees, Z e o l i t e s , 8 (1988) 35-39. R.M. Barrar and D . A . I h b i t s o n , T r a n s . Faraday S m . , 40 ( 1 9 4 4 ) 206 .-2 1 6 . A . Zik&nov&, M. B i i l o w , M. UoEiEik and P. S t r u v e , P r m . 3rd Workshop A d s o r p t i o n i n Micrc,poruus Adeorbents", Eberswalde, G . D.R., October 12-15, 1987, CIPC, Academy o f S c i e n c e s of t h e G . D . R . B e r l i n , Vol.:, pp. 10-19. H . W . Spiess and H. a i l l e s c u , .J. Magn. Reaon., 42 (1981) 381.-
22 C. F B r s t e , A.
23 74
25 26
27 28 29
30
389. 3 1 A . J . Vegcl and Z . Luz, J . Chem. Phys., 86 (198'7) 1803-1813. 32 H . W . S p i e s s , Chern. P h r s . , 6 (1974) 217-225. 33 B. Zibrowius, Ph.D. Theses, Z e n t r a l i n s t i t u t f u r p h y s i k a l i s c h e Chemie d e r Akademie d e r W i s s e n s c h a f t e n d e r DDR, B e r l i n , 1988. 34 J . Ramdas, J.M. Thomas, P.W. BetteridBe, A . K . Cheetham and E.K. Davies, Angew. Chemie ( E n g l . ) , 23 (1984) 671-679. 35 €1. S t a c h , R . Wendt, K . F i e d l e r , B. G r a u e r t , J . J a n c h e n and H . S p i n d l e r , i n 1 C . K . Unger e t a l . ( E d s . ) , C h a r a c t e r i z a t i o n I J f r'orous S o l i d s , E l s e v i e r , Amsterdam, 1988, pp. 109- 118. 36 P. T . Reischrnan and D . H . Olson, P r o c . 60th C o l l o i d & Surf. S c i . Symp., A t l a n t a , U . S . A . , J u n e 15-18, 1986, p , 52. A . Germanus, .J. ICarger, H . P f e i f e r , J . Caro, 37 C. F n r s t a , W . P i l z and A. Z i k k o v A , J . Chem. Soo. Faraday T r a n s . 1,
83 ( 1987) 2301-2309. 38 J . C.. - T a y l o r , Z e o l i t e s , 7 (1987) 311-318. 39 B . F . Mentzen, F. Bosselet and J . Bouix, ( P a r i s ) 305(II) (1987) 581-584.
C. R . Acad. S c i
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
DIFFUSION OF n-HEXANE AND 3-METHYLPENTANE I N H-ZSM-5 CRYSTALS OF VARIOUS S I Z E S
P. VOOGD and H. VAN BEKKUM
Department o f Organic Chemistry, D e l f t U n i v e r s i t y o f Technology, J u l i a n a l a a n 136, 2628 BL D e l f t (The N e t h e r l a n ds)
ABSTRACT A t temperatures below 373 K t h e d i f f u s i v i t i e s o f n-hexane and 3-methylpentane i n z e o l i t e ZSM-5 were determined i n a c o n s t a n t volume v a r i a b l e p r e s s u r e system. The ZSM-5 c r y s t a l l e n g t h was v a r i e d between 20 pm and 150 pm i n o r d e r t o st udy i t s e f f e c t on u p t a k e r a t e s . The e x p e r i ment al r e s u l t s i n d i c a t e t h a t i n t r a c r y s t a l 1 i n e d i f f u s i o n determines t h e s o r b a t e upt ake r a t e p r o v i d e d t h a t c r y s t a l s w i t h h e i g h t s l a r g e r t h a n 40 pm a r e used. Also, t h e e f f e c t o f t h e sorbed amount on t h e d i f f u s i v i t y was s t u d i e d . Near s a t u r a t i o n an i n c r e a s e o f b o t h t h e apparent and t h e i n t r i n s i c d i f f u s i v i t i e s o f n-hexane and 3-met hylpent ane w i t h t h e degree o f p or e f i l l i n g was observed.
INTRODUCTION S inc e
the
first
applications
o f zeolites the determination o f d i f f u s i o n a l
upt ak e r a t e s i n z e o l i t e c r y s t a l s has been a w i d e l y s t u d i e d s u b j e c t o f research. A number o f re v ie w s have been p u b l i s h e d (I-4), c o n s i d e r i n g m a i n l y d i f f u s i o n i n l o w - s i l i c a z e o l i t e s l i k e z e o l i t e A and X and i n n a t u r a l z e o l i t e s l i k e f a u j a s i t e and mo rd enit e .
However,
the
study
o f m o l e c u l a r t r a n s p o r t i n t h e well-known
h i g h - s i l i c a z e o l i t e ZSM-5 appeared t o encounter some d i f f i c u l t i e s . F i r s t l y , t h e synthesis narrow
of
a w o r k a b l e amount o f l a r g e , w e l l - d e f i n e d ZSM-5 c r y s t a l s h a v i n g a
particle
years (e.g.
5,6),
size
d i s t r i b u t i o n has
o n l y become p o s s i b l e i n t h e l a s t few
a l t h o u g h t h e c r y s t a l morphology i s as y e t r a t h e r d i f f i c u l t t o
c o n t r o l . Secondly, some doubt e x i s t s r e g a r d i n g t h e v a l i d i t y o f F i c k ' s e q u a t i o n i n c o n n e c t i o n w i t h d i f f u s i o n i n z e o l i t e ZSM-5. F o r some adsorbates, l i k e 3-methylpentane
and
p-xylene,
Riekert
et
a l . (7,8) s t a t e t h a t t h e s o r p t i o n
k i n e t i c s can r e s u l t f r o m a s u p e r p o s i t i o n o f i n t r a c r y s t a l l i n e d i f f u s i o n and a rearrangement o f s o r b a t e i n t h e ZSM-5 c r y s t a l . The
need f o r
applying
large
ZSM-5
c r y s t a l s can be e x p l a i n e d as f o l l o w s :
d u r i n g a s o r p t i o n u p t a k e experiment t h e g e n e r a t i o n o f h e a t o f s o r p t i o n causes a tem p era t u re r i s e o f t h e sorbent, which may r e s u l t i n an u p t a k e c u r v e s e r i o u s l y deviating
f rom
t h e t h e o r e t i c a l c u r v e f o r an i s o t h e r m a l system ( 9 ) . Because o f
520
the
z e o l i t e ’ s h i g h t h e r m a l c o n d u c t i v i t y ( r a p i d e q u a l i z a t i o n o f t h e t emperat ure thr o ughout t h e c r y s t a l ) t h e r a t e o f h e a t g e n e r a t i o n i s i n v e r s e l y p r o p o r t i o n a l 2 R = c r y s t a l radius, D = t o t h e c h a r a c t e r i s t i c d i f f u s i o n t i m e (tD=R /D, d i f f u s i v i t y ) . Th e r e f o r e , l a r g e c r y s t a l s a r e p r e f e r a b l e t o small c r y s t a l s , e s p e c i a l l y when r a p i d l y p e n e t r a t i n g s o r b a t e s p e c i e s ( l a r g e 0) a r e considered. Another phenomenon which may i n f l u e n c e a s o r p t i o n process i s s u r f a c e - b a r r i e r l i m i t e d d i f f u s i o n (7); sorbate uptake i s l i m i t e d by a t r a n s p o r t r e s i s t a n c e a t the
zeolite
surface. I f t h i s surface b a r r i e r completely c o n t r o l s the t h e c h a r a c t e r i s t i c d i f f u s on t i m e i s p r o p o r t i o n a l t o t h e
crystal
s o r b a t e u pt a k e r a t e c r y s t a l r a d i u s (R), hence m a n i p u l a t i o n o f t h i s parameter i s a u s e f u l approach i n t h i s r e s p e c t as w e l l . An i n t e r e s t i n g i t e m i n z e o l i t i c d i f f u s i o n i s he c o n c e n t r a t i o n dependence o f
as w e l l as i n t r i n s i c d i f f u s i v i t i e s . Th s dependence can be s t u d i e d by
apparent
p e r f o r m i n g u pt a k e measurements, i n t r o d u c i n g a small amount o f s o r b a t e i n t h e system c o n t a i n i n g t h e s o r b e n t which m i g h t a l r e a d y c o n t a i n a c e r t a i n amount of the
sorbate
(interval
method).
Thus
the
fractional
coverage (0) w i l l , a t
a few p e r c e n t s . The d i f f u s i v i t y t h e n d e r i v e d i s o f a d i f f e r e n t i a l
most,change
k i n d . It o b v i o u s l y can be d e t e r m i n e d a t v a r i o u s s o r b e n t coverages. T h i s i s i n c o n t r a s t t o t h e i n t e g r a l d i f f u s i v i t y w h i ch i s det ermined f rom upt ake curves measured ov er a l a r g e c o n c e n t r a t i o n i n t e r v a l , and has by t h a t a mean v a l u e ( 2 ) . The purpose o f t h i s paper i s t w o f o l d . F i r s t l y , t h e i n f l u e n c e o f t h e c r y s t a l r a d i u s on d i f f u s i o n was s t u d i e d ; f o r t h i s purpose we were a b l e t o s y n t h e s i z e ZSM-5 c r y s t a l s o f l e n g t h s r a n g i n g f r o m 20 pm t o 150 pm. Also, t h e c o n c e n t r a t i o n dependence
of
(H-ZSM-5)
the
diffusivity
i n z e o l i t e ZSM-5 was det ermined. The sorbat es
n-hexane and 3-methylpentane. The z e o l i t e was used i n i t s p r o t o n
s t u d i e d were
form.
The
e x p e r i m e n t s were p e r f o rmed i n a c o n s t a n t volume v a r i a b l e
pres s ure system a t t e m p e r a t u r e s below 373
K.
The
s o rb at e s , n-hexane and 3-methylpentane, a r e i n t e r e s t i n g f rom a c a t a l y t i c
point
o f view. Both compounds a r e commonly a p p l i e d i n t h e d e t e r m i n a t i o n o f t h e
so-called
constraint
index
compounds Diffusion
a r e used t o c h a r a c t e r i z e ( a c t i v e ) z e o l i t e s , such as z e o l i t e H-ZSM-5. o f t h e r e a c t a n t s i n t h i s c r y s t a l l i n e m a t e r i a l may be r a t e l i m i t i n g ,
(ll), i n w h i ch
especially
when
large
det ermine
the
d if f u s i v it i e s
experiments pla c e .
at
Rather,
catalytic
conversion
o f t hese
c r y s t a l s a r e used. O b v i o u s l y i t i s v e r y i n t e r e s t i n g t o at
h i g h temperatures dependences
of
reaction
t emperat ure.
However,
sorption
are. u n s u i t a b l e because o f r e a c t i o n s t a k i n g
c a t a l y t i c c o n v e r s i o n s on c r y s t a l s i z e can, by
t r e a t i n g them w i t h t h e T h i e l e concept, r e s u l t i n d i f f u s i v i t i e s o f t h e r e a c t a n t s under c o n s i d e r a t i o n . pub1 ished (10,12).
Some
pioneering
work
i n t h i s f i e l d has been r e c e n t l y
521
EXPERIMENT 1. Method o f S orD t i o n The u pt a k e measurements were c a r r i e d o u t i n a c o n s t a n t volume v a r i a b l e pressure system. and
scheme
A
measured
using
of
re c ord ed u s i n g
situated
the
apparatus
is
shown i n F i g u r e 1. The p r e s s u r e was
a B a r a t r o n 170M-7 c a p a c i t a n ce manometer ( s e n s i t i v i t y 0.113 Pa) a
Kipp
ED41 r e c o r d e r .
The
whole equipment system was
in
a c o n s t a n t - t e m p e r a t u r e room (T = 295 K ) . Uptake r a t e s a t e l e v a t e d
temperatures
were determined w h i l e t h e s o r p t i o n vessel was immersed i n a wat er
bat h w i t h t h e r m o s t a t . To a v o i d t h e p o s s i b i l i t y o f i n t e r c r y s t a l l i n e d i f f u s i o n c o n t r o l l e d s o r p t i o n , a small amount o f z e o l i t e (10-30 mg) was used i n each experiment. The s o r p t i o n v e s s e l had a c y l i n d r i c a l shape (base s u r f a c e = 10
2
cm ) ,
wherein
crystals. permitted In was
each
the
zeolite
sample was arranged i n t h e f o r m o f a monolayer o f
The
volumetric
(interval)
method o f measuring z e o l i t i c d i f f u s i o n
the
st u d y o f t h e i n f l u e n c e o f t h e sorbed amount on t h e d i f f u s i v i t y .
d i f f u s i o n experiment a small percentage (
sorbed.
A
sorption
i s o t h e r m was d e t e r m ined by t h i s method as w e l l , w i t h
d i f f e r e n t sorbed amounts.
F i g . 1. Scheme o f t h e e x p e r i m e n t a l apparatus. 1, st andard volume (78.26 m l ) ; 2, v es s e l c o n t a i n i n g l i q u i d s o r b a t e ; 3, s o r p t i o n vessel (6.99 m l ) ; 4, c o nne c t i o n between s o r p t i o n v e s s e l and d o s i n g volume; 5 , v a l v e ; 6, r o t a r y o i l pump; 7 , p r e s s u r e gauge measuring t h e system pre s s ure ; 8, P i r a n i 668 p r e s s u r e gauge; 9, Penning p r e s s u r e gauge; 10, o i l d i f f u s i o n pump. The c r o s s-hat ched s e c t i o n o f t h e scheme r e p r e s e n t s t h e d o s i n g volume (23.66 m l ) . The
ranges
of
Pa;
amount
sorbed
e x p e r i m e n t a l parameters were as f o l l o w s : prebsure = t 0 . 2
348
K.
-
180
3*10-3- 2*10-2 mmo1.g-l. The t emperat ures were 295 K and I n b l a n k experiments t h e h a l f - t i m e s o f vapor p r e s s u r e expansions a f t e r =
522
valve at
opening were determined. For n-hexane as w e l l as f o r 3-methylpentane and
different
temperatures o f
the
sorption
vessel
t h e h a l f - t i m e s were 0.2
seconds. These data were n o t assimilated i n t h e data evaluation, b u t they are useful when applying t h e so-called method o f moments (18). 2. Z e o l i t e C h a r a c t e r i s t i c s and Treatment Uniform synthesis chemical
samples
of
H-ZSM-5
crystals with
various
sizes
were used. T h e i r
and f u l l p r o p e r t i e s w i l l be reported elsewhere. Some geometrical and characteristics
are
given
i n Table 1. Figure 2 shows SEM (Scanning
Electron Microscopy) photographs o f f o u r c r y s t a l samples, where i t i s seen t h a t all
crystals
have
a
similar
morphology and, consequently, an equal surface
geometry. The c r y s t a l s are non-spherical and twinned. No gel remaining from synthesis was detected during SEM analysis. Broadening o f peaks a t 28= 7.8O and 8.8O i n t h e XRD p a t t e r n s i n d i c a t e d t h a t the aluminium atoms a r e heterogeneously d i s t r i b u t e d among t h e ZSM-5 c r y s t a l s .
Fig. 2. Scanning e l e c t r o n micrographs o f the f o u r ZSM-5 c r y s t a l samples used i n the d i f f u s i o n experiments.
523
Immediately
before
up t o
a s o r p t i o n experiment t h e sorbent was outgassed i n s i t u by
673 K i n vacuum o v e r a p e r i o d o f 10 h r . The sample was k e p t a t
heating t h a t t e mp era t u re f o r 10 h r and c o o l e d s l o w l y t h e r e a f t e r . The p r e s s u r e was t h e n 2.5*10-5 Pa. TABLE 1 Geometrical and chemical c h a r a c t e r i s t i c s o f ZSM-5 z e o l i t e samples used i n t h e d i f f u s i o n experiments.
sample no.
1
2
3
4
mean c r y s t a l l e n g t h , & c - d i r e c t ion (pm) sta ndard d e v i a t i o n
26
61
102
150
o f l e n g t h (pm) mean c r y s t a l h e i g h t # a- o r b - d i r e c t i o n (pm) standard d e v i a t i o n
2.1
3.5
24.9
19
39
56
o f height Si/AIS occurrence o f c r y s t a l
2.7
5.7
13
57
53
54
54
aggl omerates
no
no
no
Yes
*
+ 65
) A l l f o u r samples were phase p u r e a c c o r d i n g t o X-ray a n a l y s i s ( G u i n i e r - De
.
Wol f f ) &) Mean c r y s t a l s i z e s and p a r t i c l e s i z e d i s t r i b u t i o n s o f a sample were determined by image a n a l y s i s o f some 200 c r y s t a l s .
+)
Not determined.
# ) C r y s t a l bre adt h s ( a - a x i s ) were equal t o h e i g h t s ( b - a x i s ) ') As measured w i t h X - r a y f l u o r e s c e n c e .
524
THEORY J . Data Evaluation
For the determination o f the diffusivities the measured sorption uptake curves were fitted with a diffusion equation which is a solution of Fick's law in spherical symmetry for a constant system volume (19),
with qn the nth positive root o f tan qn
qn
=
1
t
2 3.a qn.
and with a = > . K
(3)
v9
The symbols used denote: 7 , relative uptake; t, time; 0, diffusivity; R, crystal radius; V,, sorbent volume; Vg, gas volume; K , dimensionless equi 1 i bri um constant. Equilibrium constants (K) were determined from the sorption isotherms. For that purpose the isotherms were fitted with a relationship derived from t h e DubininPolanij potential theory (20). The equation has the form ca
=
c:. exp [ - _ k . (R.T.ln (p/p0))*]
(4)
b2 with: ca, sorbate concentration in sorbed phase; ,:c saturation value o f ca; k, constant, characteristic of sorbent; /I, affinity coefficient; R, gas constant; T, temperature; p, vapor pressure of sorbate; po, saturation vapor pressure o f sorbate. The fit resulted in an estimation of the values o f the parameters c i and k/b 2 . The equilibrium constant K was calculated from eqs. 4, 5 and 6.
525 Ca
K
= _
P
K
(5)
P
=
K .R.Tv.pz P
w i t h : Tv, t e m p e r a - i r e o f s o r p t i o n v e s s e l ; p, sorbent d e n s i t y ; K sorption P’ e q u i l i b r i u m c o n s t a n t d e f i n e d i n terms o f s o r b a t e pressure. I t i s c l e a r t h a t a i s a f u n c t i o n o f t h e s o r b a t e pressure. D e s p i t e t h e f a c t t h a t
i n the
Henry
pres s ure p,
region o f the
an
i s o t h e r m K i s n o t b e l i e v e d t o be a f u n c t i o n o f
use o f t h e D u b i n i n - P o l a n i j
p o t e n t i a l t h e o r y does cause a
pres s ure dependence. It s h o u l d be s t a t e d t h a t eq. 4 does n o t reduce t o Henry’s a t low pre s s u r e s (20). Conclusions c o n c erning d i f f u s i o n i n v a r i o u s l y s i z e d
law
z e o l i t e c r y s t a l s can o n l y be s a f e l y drawn i f t h e s o r p t i o n e q u i l i b r i a f o r t hese d i f f e r e n t samples a r e i n s u f f i c i e n t agreement. I n o t h e r words, a l l samples must be p h y s i c a l l y and c h e m i c a l l y comparable. Because o f t h e n o n - s p h e r i c i t y o f t h e c r y s t a l s t h e d i f f u s i o n e q u a t i o n was f i t t e d t h e ex perim e n t a l c u r v e f o r 7 5 0.7. A t h i g h e r v a l u e s o f 7 d e v i a t i o n s f rom
with the
theoretical
pronounced. apparent
curve
The
fit
caused
by
a
crystal-shape
resulted
in
an
estimation
R was
taken
diffusivity.
effect of
may
become more
DaPP/R2, w i t h DaPP t h e
h a l f t h e h e i g h t (a- o r b - d i r e c t i o n ) o f a
c r y s t a l . R may be o t h e r w i s e approximated e i t h e r w i t h i n t h e c y l i n d r i c a l geometry or
by
u s i n g t h e r a d i u s o f a h y p o t h e t i c a l sphere w i t h t h e s u r f a c e equal t o t h e
s u r f a c e o f t h e a c t u a l c r y s t a l s . However, d i f f u s i v i t i e s o b t a i n e d by means o f the s e approaches p r o b a b l y do n o t d i f f e r more t h a n p o s s i b l e e r r o r s . E v i d e n t l y , because
of
changes
diffusivities
in
changed.
comp let e ly
determined
intrinsic
diffusivity,
p r e s s u r e d u r i n g an uptake, Error
the
analysis
Q
p r oved t h a t
and hence t h e e s t i m a t e d the
pressure
changes
e r r o r i n the d i f f u s i v i t i e s . Finally, t o obtain the
Dintr
,
a c o r r e c t i o n w i t h Darken’s e q u a t i o n was a p p l i e d
(20) :
2. A n a l y s i s o f Thermal E f f e c t s
I n o r d e r t o g a i n i n s i g h t i n t o t h e p o s s i b l e t hermal e f f e c t s a r i s i n g f r o m t h e gen era t io n o f h e a t o f s o r p t i o n , c a l c u l a t i o n s analogous t o t hose perf ormed by Lee e t that:
a l . (9) have been made. I s o t h e r m a l s o r b a t e upt ake i s assumed, p r o v i d e d
526
with: h.a.R ‘T
=
P.,
Cs.
2 (9)
D
n
L
a R.cn
-AH
B
(-1
=
*
R.T
Explanation
-. e.(i-e) cS
of
symbols:
h,
heat
t r a n s f e r c o e f f i c i e n t ; a, s p e c i f i c e x t e r n a l
surface area; cS, heat c a p a c i t y o f sorbent; AH, s o r p t i o n e n t h a l p y ; 8, f r a c t i o n a l sorbent coverage. I n t h e o r i g i n a l e q u a t i o n f o r fl ( r e f . 9, eq. 11) a simple
Langmuir model
was s u b s t i t u t e d i n o r d e r t o o b t a i n eq. 10.
equation
B
reaches i t s maximum v a l u e a t 0 = 0.5. TABLE 2 Results o f thermal a n a l y s i s on n-hexane and 3-methylpentane d i f f u s i o n a t 295 K i n ZSM-5 c r y s t a l s o f v a r i o u s s i z e s . ~
compound
c r y s t a1
spec. e x t .
height(pm)
n-hexane
3-methylpentane
I n Table
“T
Q,/B
area (m-’)
19
4.4*105
1.3*104
4.2
3. 1*103
39 56
2.8*105 1.8*105
3.4*104
4.2
8. 1*103
4.5*104
4.2
65
1.5*105
5.1*104
4.2
1. 1*104 1. 2*104
19
4.4*105
1.8*105
1.2
1. 5*105
39 56
2.8*105 1.8*105
4.9*105 6.4*105
1.2 1.2
4. 1*105 5.3*105
65
1.5*105
7.2*105
1.2
6.0*105
2 results
methylpentane
B
~~
a r e g i v e n from a thermal a n a l y s i s o f n-hexane and 3 s o r p t i o n a t 295 K i n t h e f o u r s e l e c t e d ZSM-5 samples. Parameters
r e l e v a n t t o t h e c a l c u l a t i o n s have t h e f o l l o w i n g values: n-Hexane:
3-Methylpentane:
-AH = 8.6*10 4 D = l.O*lO-lo c: = 1.3*10-3
(J .mol - )
-AH = 6*10 4
( J .mol-’)
D = 7*10-12
(cm2.sec-l)
(cm2. s e c - l ) (mol . g - l )
527
(mol .g-') - 1 -1 cS = 0 . 8 (J.g * K 1 6 p, = 1.78*10 (g.m-3) - f o r t h e c r y s t a l radius (R) i s taken h a l f the c r y s t a l height =
C:
Z e o l i t e 2SM-5:
-
8.0*10- 4
the s p e c i f i c external surface area (a) has been c a l c u l a t e d based on t h e c r y s t a l shapes shown i n Figure 2, assuming a roughness f a c t o r o f 2.
Furthermore,
the
heat
transfer
coefficient
(h)
has
been
estimated
by
considering a r a d i a t i o n c o n t r i b u t i o n a t 295 K only. Using Stefan's law a value for
h o f 4.6 J.s'1.m-2.K-1
has been calculated. The thermal analysis presented an i n f l u e n c e o f thermal e f f e c t s on the uptake curves.
i n Table
2 rules
out
However,
caution
should
diffusivities which amount
be exerted
with
regard
to
the
values
of
the
given above. I t i s n o t uncommon t h a t they show discrepancies t o several orders o f magnitude ( 2 0 ) . This problem diminishes t h e
o f the c a l c u l a t i o n s shown i n Table 2, because aT s t r o n g l y depends on
clearness
the d i f f u s i v i t y (D). RESULTS AND DISCUSSION The r e s u l t s presented here concern the d i f f u s i o n o f n-hexane a t 295 K and o f 3-methylpentane a t 348 K. Some a d d i t i o n a l work on d i f f u s i o n o f these two compounds w i l l be reported elsewhere. As can be seen i n Figure 3 the s o r p t i o n isotherms o f n-hexane i n H-ZSM-5 are always i n s u f f i c i e n t agreement f o r t h e various
However, t h e e q u i l i b r i u m s o r p t i o n data o f 3-methylpentane show some considerable s c a t t e r i n g ( c f . Figure 4). As t o the q u a l i t y o f the
zeolite
when applying Dubinin-Polanij's equation (eq. 4) on the isotherms, i t
fit
appears model.
samples.
that
the
equilibrium
Nevertheless,
isotherms because
the
most
data
latter
follow
model
was
more c l o s e l y the simple Langmuir not
adopted
to
represent t h e
assumptions on which Langmuir's theory i s based, v i z .
the homogeneity o f s o r p t i o n s i t e s and t h e formation o f a mono-molecular sorbate layer, (20) * The
do
not
meet
concentration
diffusivity
is
r e a l i t y as f a r as s o r p t i o n i n z e o l i t e ZSM-5 i s concerned dependence
shown
of
the
apparent
as
well
as the i n t r i n s i c
-
i n Figures 5 and 6 f o r n-hexane and 3-methylpentane, as
determined w i t h the sample having t h e longest c r y s t a l size, v i z . R
33 pm. I t
appears t h a t near s a t u r a t i o n both the apparent and t h e i n t r i n s i c d i f f u s i v i t i e s o f n-hexane and 3-methylpentane increase w i t h t h e degree o f pore f i l l i n g . Errors i n d i f f u s i v i t i e s appear t o amount t o about h a l f an order o f magnitude,
528
f
Fig. 3. Sorption isotherms of n-hexan6 Fig. 4. Sorption isotherms of 3-mein H-ZSM-5 type zeolites of three thylpentane in H-ZSM-5 type crystal sizes at 295 K: zeolites o f three crystal R-9.6 R.20 pm; 0, R-28 pm. sizes at 340 K: 0, R-9.6 p ; R-20 pm; 0,R-33 jm.
+,
/.ma,
+,
c 1 2 3 4 Number 3 - Methy!?entonc molecules per uni: cel
Number n-h’exonc molecules per unit cel
Fig. 5. Concentration dependence of apFig. 6. Concentration dependence of apparent (x) and intrinsic parent (X) and intrinsic diffusivities (m) of n-hexane at 295 K diffusivities (a) of 3-methylpentane at 3 4 8 K
I
I
10.6
I
I
13-5 R ~ -
1cm21
I
I
10-5
10.6 R2-
13-6
Icrn2)
Fig. 7. Characteristic diffusion time Fig. 8. Characteristic diffusion time of 3-rnethylpentane in H-ZSM-5 as a o f n-hexane in H-ZSM-5 as a function of square of crystal function o f square o f crystal radius at 348 K and at radlus at 295 K and at fractional coverage 0 0.70 fractional coverage 0 0.62
-
529
viz. 5*10-11 cm2/s. The strong concentration dependence o f diffusivity, with respect to zeolites, is not uncommon. Diverse dependences have been reported earlier (2). A simple criterion for intracrystalline diffusion is a linear relationship between the characteristic diffusion time and the square o f the crystal radius (R'), indicating diffusivity t o be independent of crystal size (10). If so, the assumption that no thermal or surface-barrier effects affect the sorbate uptake rate is justified. The possibility o f the intrusion of an intercrystalline diffusion-limited process may then be excluded as well. However, a problem may arise when serious deviations from the linear relation are t o be interpreted, because it is not easy to decide which of the above-mentioned effects is mainly responsible for a deviation. Solutions of extended differential equations (with regard to the simple case of an isothermal, intracrystalline regime), which take account of those phenomena, should then be used t o make proper selections and estimations (20). Figures 7 and 8 show characteristic diffusion times (R2/DaPP) o f n-hexane and 3-methylpentane as a function of the square o f the crystal radius. A linear relationship between the characteristic diffusion time and the square of the crystal radius is indeed observed. However, it appears that a deviation o f the linear relationship occurs at R2= 9.1)*10-~ cm2 (crystal length = 2 6 pm), for both n-hexane diffusion at 295 K and 3-methylpentane diffusion at 348 K. Experiments with crystals having radii larger than 33 pm are planned t o confirm this tentative conclusion. Moreover, in addition t o this approach it is worthwhile to study zeolitic diffusion with equal samples but using different techniques. Karger and Car0 compared results derived from sorption kinetics with self-diffusivities obtained by an n.m.r. pulsed field gradient technique (2, 21). Discrepant values, left unreconciled, have been obtained this way. On the other hand, comparison o f the gravimetric with a chromatographic measuring technique did result in agreeing values, e.g. the diffusivities of 2,2dimethylbutaan in H-ZSM-5 with a very high Si/A1 ratio at high temperatures (448-563 K ) showed excellent agreement (10). As t o n-hexane and 3-methylpentane diffusion in zeolite ZSM-5, in Table 3 a literature survey is given of diffusivities measured with different methods and at various temperatures. Comparison with the results presented here (Table 3) gives rise t o serious contradictions as far as diffusion o f n-hexane is concerned. The results given in Table 3 strongly point t o a need for more studies with different methods using the same zeolite sample.
530
TABLE 3 Literature and present data on diffusion in zeolite ZSM-5 of n-hexane and 3-methylpentane. Compound
3-Methyl pentane 3-Methyl pentane 3-Methyl pentane
n -Hexane n -Hexane
n-Hexane n -Hexane n-Hexane
n-Hexane
Temp.
H-ZSM-5 Si/A1-23 c=6 pm, b=3 pm H- ZSM-5 Si/A1=23 spherul i tes H-ZSM-5 Si/A1=40 c=26 - 150 pm b=19 - 65 pm
296
6.6*10-11
296
7.0*10-12
(K)
c
=
(cm2/s)
..
H-ZSM-5 313 4*1oV9 S i/Al=17 0.5 and 14 pm Na,H-ZSM-5 323 1*10-8Si/A1=34 1*10-~ c=35 pm, b=lOpm 303 7.5*10-12 ? Na-ZSM-5 303 1 .31*10-1° Si/A1=66 7 Pm sil ical ite 373 and 4.2*10-11 473 2 pm (sphere) 1 .33*10-1° resp. 295 5*10-11H-ZSM-5 Si/A1=40 c=26 - 150 pm b=19 - 6 5 pm
*
D
Data on ZSM-5*
crystal length, b
1*10-l0
=
crystal height
Measuring technique
Ref.
Grav i me t ry , constant volume constant pressure Gravimetric, constant volume constant pressure Constant volume variable pressure
8
this work
Grav i met ry
13
Const. volume var. pressure
14
?
Grav i me t ry , const. volume const. pressure Gravimetry, const. volume const. pressure Const. volume var. pressure
8
15 16 17
this work
531 CONCLUSIONS
-
Near s a t u r a t i o n the apparent as w e l l as t h e i n t r i n s i c d i f f u s i v i t i e s o f n-hexane and 3-methylpentane i n H-ZSM-5 increase w i t h t h e degree o f pore filling appears
- It
measure
the
that
ZSM-5
crystals
with
a
h e i g h t o f 20 pm are t o o small t o
i n t r a c r y s t a l l i n e d i f f u s i v i t y o f n-hexane and 3-methylpentane a t
K and 348 K r e s p e c t i v e l y . Experimenting w i t h samples l a r g e r than 40 pm probably ensures i s o t h e r m i c i t y o f t h e sorbents and/or exclusion o f s u r f a c e - b a r r i e r 1i m i t e d sorbate uptake. temperatures
having
of
crystal
295
heights
ACKNOWLEDGEMENTS The authors would l i k e t o thank Prof. J. J. F. Scholten M r . N. van Westen and M r . J. Teunisse o f the Laboratory f o r Chemical Techno ogy f o r t h e use o f the experimental equipment and f o r h e l p f u l assistance. M r J . F. van Lent o f the Laboratory o f Metallurgy i s acknowledged f o r t h e X-ray d f f r a c t i o n analysis. M r . Th. W. Verkroost o f t h e Department o f Mining Engineering i s acknowledged f o r c a r r y i n g out the X-ray fluorescence measurements.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
R. M. Barrer, Adv. Chem. Ser. "Molecular Sieve Z e o l i t e s " , 102 (1971) 1. R. M. Barrer, " Z e o l i t e s and Clay Minerals as Sorbents and Molecular Sieves", Academic Press, London, (1978), Chapter 6. R. M. Barrer, "Zeolites: Science and Technology", F.R. Ribeiro, e t al., Eds., Martinus N i j h o f f Publishers, The Hague, (1984), 261. D. M. Ruthven, Am. Chem. SOC. Symp. Ser., 40 (1977) 320. M. Ghamami and L. 6. Sand, Zeolites, 3 (1983) 155. S. 2. Chen, K. Huddersman, D. K e i r and L. V. C. Rees, Zeolites, 8 (1988) 106. K. Beschmann, G. T. Kokotailo and L. Riekert, Chem. Eng. Process., 22 (1987) 223. D. P r i n z and L. Riekert, B e r . Bunsenges. Phys. Chem., 90 (1986) 413. L. K. Lee and D. M. Ruthven, J. Chem. SOC. Faraday Trans. I , 75 (1979) 2406. M. F. M. Post, J . van Amstel and H. W. Kouwenhoven, Proc. 6 t h I n t . Z e o l i t e Conf. Reno, D. H. Olson e t al., Eds., Butterworths, Guildford, (1983), 517 V. J. F r i l e t t e , W. 0. Haag and R. M. Lago, J. Catal., u ( 1 9 8 1 ) 218. W. . 0. Haag, R. M. Lago and P. 6. Weisz, Faraday Disc. Chem. SOC., 72 11982) 317. J . Heering, M. K o t t e r and L. Riekert, Chem. Eng. Sci., 37 (1982) 229. M. Bulow, H. Schlodder and P. Struve, Adsorpt. Sci. T e c F o l . , 3 (1986) 229 P. Wu, A. Debebe and Y . H. Ma, AIChE Winter Meeting, Orlando, FL, Paper 55a, (1981). Y. H. Ma, T. D. Tang, L. 6. Sand and L. Y. HOU, Proc. 7 t h I n t . Z e o l i t e Conf. Tokyo, Y. Murakami et a1 ., Eds., Elsevier, Amsterdam, (1986) 531. P. Wu, A. Debebe and Y. H. Ma, Z e o l i t e s , 3 (1983) 118. M. K o c i r i k and A. Zikanova, Ind. Eng. Chem. Fundam., 13 (1974) 347. M. Biilow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt and W. Schirmer, J. Chem. SOC. Faraday Trans. I , 76 (1980) 597. D. M. Ruthven, P r i n c i p l e s o f Adsorption and Adsorption Processes, J. Wiley, New York, (1984). J. Karger and J . Caro, J. Chem. SOC. Faraday Trans. I , 73 (1977) 1363.
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H.G.Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
A COMPARATIVE STUDY BY DEUTERON SOLID STATE NMR SPECTROSCOPY OF THE DYNAMICS OF BENZENE AND OLCFINS IN FAUJASITE- AND MORDENITE- TYPE ZEOLITES
B. BODDENBERG, R. BURMEISTER and G. SPAETH Lehrstuhl fur Physikalische Chemie 11, Universitat Dortmund, Otto-Hahn-Str., D-4600 Dortmund 50 (Federal Republic of Germany)
ABSTRACT From the lineshapes of the H' NMR spectra of benzene, ethene and propene sorbed in X-type zeolites the anisotropic and isotropic rotations of these molecules are assess9 and intracrystalline diffusion coefficients obtained. For comparison, the H NMR spectra for these molecules in mordenite are presented. INTRODUCTION In recent years deuteron ( 2H) solid state NMR spectroscopy has proved to be a very powerful tool for the investigation of the microdynamics of
molecules adsorbed on solid surfaces (refs. 1-9)
as well as in the voids
of zeolites (refs. 10-17). This technique is used to study in which way and how fast a deuterium-containing bond axis of a molecule is orientationally moved in space, thus allowing conclusions to be drawn about internal rotations as well as anisotropic and isotropic molecular reorientations. The present contribution addresses the study by *H NMR of the anisotropic reorientations as well as the site exchange motions of benzene, ethene, and propene molecules sorbed in the supercages of X-type faujasites and in the channel system of mordenite. The main emphasis is placed
on the
molecule/faujasite systems, whereas the results obtained for the molecule/mordenite systems serve for comparison purposes. THEORETICAL BACKGROUND The LH-NMR spectrum exhibited by the deuterons (spin I = 1) in specified carbon/hydrogen bonds of like molecules with random orientational distribution in space is displayed in Fig. 1. From the prominent edge splitting of this Pake type powder pattern the deuterium quadrupole coupling constant (DQCC) may easily be derived according to (ref. 18): Av = (3/4) IDQCCI. The coupling constant defines the characteristic NMR time
(1) TNMR =
IDQcCI-l.
534
L
I
Generally, the shape and width of a
H'
spectrum depend on the type of
the rotational motion of the molecule a s well as on the correlation time T
of this rotation in relation to TmR.
Actually, the Pake pattern shape
(Fig. 1) shows up under certain limiting conditions, of which two are of importance in the following. (i) The rate of rotational motion is much lower -1
.
than ( T ~ ~ , )1.e. T >XmR (rigid o r slow orientational exchange case). 2 Then DQcC in eqn. 1 represents the rigid coupling constant e qQ/h. With typi2 cal values of e qQ/h in the range 150 to 200 kHz for deuterons in carbon/hydrogen bonds (ref. 19)
T~~
amounts to several microseconds. (ii) The molecu-
les rotate rapidly about an axis fixed in space, e.g. a molecular symmetry axis. Rapid rotation means that the correlation time T
of
this
rotation
meets the condition T ~ < < (fast T ~ orientational exchange case). Under these circumstances DQCC in eqn. 1 is given by the relation (refs. 18, 20) DQCC
=
2 (1/2) (3 cos A-l)(e2qQ/h)
(2)
where A is the angle between the C-D bond direction and the axis of rotation. It is, however, required that the potential energy as function of the angle of rotation exhibits at least threefold symmetry. I n other cases such as 180' reorientational jumps quadrupole patterns of different shapes, in general, result. If the correlation time T~ of the previously considered uniaxial rotation (and similarly T
of any other type of rotation) becomes the order of T
~
,
rather involved pattern shapes show up, the analysis of which requires much computational effort based on detailed mechanistic models for the molecular rotational processes involved. Pursuing this path with the presently studied systems is hardly feasible because of the lack of criteria to differentiate
535
between motional and heterogeneity effects on the spectrum shapes. If the deuteron carrying molecules rotate isotropically and the correlation time
T~
the 'H-NMR
spectrum consists of a single line of, in general, Lorentzian
of this motion meets the condition of fast exchange ('CR<
shape. The width at half height (6v) of this singlet is (ref. 20) n
6v
= (9n/20)
(3)
(DQCC)L~R -1
under the constraint TR>>wo . DQCC is the rigid quadrupole coupling constant. Eqn. ( 3 ) likewise is valid with DQCC given by eqn. ( 2 ) if the axis around which the molecule is in rapid rotation is reoriented ra$idly isotropically in space, and the condition
R >>.rP is
'I
fulfilled
(ref.
and
17).
The isotropy of the rotational motion is given if the potential energy as function of the solid rotation angle exhibits spherical or cubic symmetry.
EXPERIMENTAL The zeolites used for the present investigation were NaX (13X, Union Carbide) and Na-MOR (Zeolon 900, Norton). The partially (40%) silver exchanged
X zeolite (40AgNaX) was prepared from NaX according to standard procedures (refs. 17, 2 1 ) . The zeolites were put into 10 mm 0.d. NMR glass tubes and slowly heated under vacuum to 43OoC at which temperature they were kept for -5 about 12 hrs. under a final pressure < 10 hPa. Separate samples were loaded with calibrated amounts of benzene-d6 (99.5% D; Sigma, Geisenhofen, Germany), ethene-d4 (99%; Merck, Sharp and Dohme (MSD),
Montreal, Canada),
and pro-
pene-dg (CD3CHCH2, 98%; MSD) to give well defined sorbate concentrations in the range 1 to about 8 molecules/unit cell. Using
the
equipment and
the detailed
procedure described
elsewhere
2
(ref. 17) the H-NMR spectra were measured at the resonance frequency wo/2n = 52.7 MHz by Fourier transformation of coherently added free induction decays and solid state quadrupole echoes (ref. 22) detected in quadrature in cases where isotropically averaged and solid state time domain spectra were observed, respectively. Depending on the loadings of the zeolites and of the widths of the spectra encountered, the number of accumulations was of the order of 100 to 2500 with, in general, 0.5 to 1 s recycle delay time. RESULTS AND DISCUSSION Benzene in NaX, AgNaX and Na-MOR Fig. 2 shows the H'
NMR spectra of benzene sorbed in the zeolites NaX,
40AgNaX (1 molecule/supercage each), and Na-Mordenite (1 molecule/unit cell (u.c.))
at several selected temperatures between 80 and 290 K. In each case
Pake powder patterns of AV = 70 kHz prominent edge splitting show up at low
536
Jc K 25 k n i
~
-
~~
25 k h
T = 291 K
T = 222 K
T = 150
K
T = 7 7 K
T = 292 K
1 = 222 K
T = 100 K
T = 7 7 K
K
Jc
T = 9 8 K
T = 7 7 K
25 kHt
~~
T = 289 K
T = 200 K
25 knl
"
Fig. 2. LH NMR spectra of benzene sorbed in (a) NaX, 1 molecule/supercage, (b) 40 AgNaX, 1 molecule/supercage, and (c) Na-MOR, 1 molecule/unit cell. temperatures. With increasing temperature the LH patterns of benzene in the faujasites retain shape and width up to about 170 K (NaX) and 225 K(AgNaX), but after crossing subsequent 30 K shape transition ranges become completely transformed into Lorentzian-type singlets of halfwidths obeying an Arrhenius-type law with activation energies of 22.5 mol-'
(NaX) and 41 (4OAgNaX) kJ
(Fig. 3 ) . For benzene in mordenite, on the other hand, the patterns
remain practically unchanged up to ambient temperature. The quadrupole coupling constant obtained from the powder patterns is 2 IDQCCI = 93 kHz (eqn. 1) which is just one half of e qQ/h in the range 184 to 187 kHz for the deuterons in the benzene molecule (refs. 23, 2 4 ) . From eqn. (2) A = 90' is obtained, which proves that on the NMR scale ( 2 5 us) the molecules rapidly reorient around the hexad axis with no other rotational type of motion being feasible at temperatures where the quadrupole patterns are observed. In mordenite it is, obviously, the geometric constraint imposed by the 0.58 x 0,70 nm main channel cross section (ref. 25) which permits a benzene molecule to reorient only around the sixfold symmetry axis. This motion proceeds rapidly on the NMR time scale even at 77 K. This conclusion agrees with the results of a recent neutron scattering study of benzene in
537
l*t
bvl Hz
Fig. 3. Temperature dependence of the widths at half height (6u) of the Lorentzian singlets of molecules in NaX (open symbols) and lOAgNaX (filled symbols) at 1 molecule/supercage loading. &A:Ethene. 0,O:Propene. 0,WBenzene. Na-MOR performed at temperatures between 300 and 450 K (ref. 2 6 ) . The translational displacement of the molecules along the unidirectional channel axes 2 H NMR because there
reported in that paper cannot, of course, be detected by
is no change of molecular orientation accompanying this motion. The presently detected rapid C6 rotation motional state of benzene in 2 the faujasites deduced from the Pake shapes of the H patterns confirms previous findings (for NaX) from
NMR line shape (ref. 11) and 1H NMR second
moment (ref. 27) studies. In NaX the anisotropically rotating benzene molecules are considered to be bound to the cations located at the SII crystallographic sites (ref. 27) which are the apices of regular tetrahedra (ref. 21). The collapse of the low temperature quadrupole patterns into singlets (Fig. 2) indicates that a further motion moving the C6 axes of the molecules isotropically in space is operative. The rate of this motion runs through ( TNMR)-l
at some temperature within the previously mentioned shape transition
regions centered at 185 and 240 K for NaX and 40AgNaX, respectively. Applying eqn. (3) to the line width data for benzene in Fig. 3 with IDQcC( = 93 kHz the correlation times T~ for the isotropic motion are evaluated as T~ = 2*10-12 exp(2700/TK-l) s (ER = 2 2 . 5 kJ mol-l) for benzene in NaX, and ‘ I = ~ 1*10-14 exp( 49OO/TK-l) s for benzene in 4OAgNaX. In NaX the isotropic motion being operative is most reasonably explained (ref. 17) as consisting of the jumping of the molecules among
the strongly
538
+
bonding Na ions on SII sites, thereby identifying
T~
with the mean molecular
residence time on such a cation site. This notion implies that according to the relationship
D
=
<12>/6'rR
(4)
the intracrystalline diffusion coefficient D is accessible with <12> being the mean square displacement per elementary site exchange jump. Putting as an e s t i m a t e m = 1.1 nm (refs. 17, 28), which is the supercage distance (ref. 21), diffusion coefficients are obtained that agree excellently with respect to both the preexponential factor and the activation energy with the data obtained with the aid of the proton ( 1H) pulsed field gradient (PFG) technique (ref. 29). At 300 K the diffusion coefficient evaluated from the present data is D(C6D6,NaX) = 1.3*10-11m2s-1. For comparison, the diffusion
* -'
coefficient of benzene at 300 K in Na-MOR is 7*10-11 m s
(ref. 26). Notably the present 2H NMR lineshape-derived diffusion coefficients are obtained
with commercially available zeolite materials of vm as the determination of D by means of the 'H
crystallite sizes where-
PFG technique requires much
larger crystals (ref. 29). The shift by about 60 K to higher temperature of the shape-transition region of benzene in 4OAgNaX compared to NaX reflects the stronger a-bonding t
capability of Ag sions for
T~
t
than Na
ions. Accordingly, the previously derived expres-
of benzene in NaX and 4OAgNaX yield the relation 'rR(40AgNaX)
>>T (NaX) in the temperature range under study, suggesting the identification
R + of T~ (40AgNaX) with the residence time of a molecule on a Ag site (ref.17). = 1.1 run as before, the intracrystalline diffuApplying eqn. ( 4 ) with
sion coefficient may be estimated. At 300 K the resulting value is D(C6D6, 4OAgNaX) = 1.5*10-12m2s-1, which is an order of magnitude lower than in NaX. Ethene in NaX, AgNaX and Na-MOR Fig. 4 shows the 2H NMR spectra of ethene in NaX ( 1 . 2 molecules/supercage) , 40AgNaX ( 1 molecule/supercage) , and Na-MOR (1.4 molecule/u.c.) at several selected temperatures between 80 and 290 K. Like benzene (Fig. 2b),
ethene
in AgNaX exhibits Pake patterns at low temperatures with, however, 130 kHz edge splitting, and singlets at high temperatures. The shape-transition region is centered at about 230 K. For ethene in NaX and Na-MOR similar quadrupole patterns of 130 kHz widths but of non-Pake shape are observed below about
80 K. Whereas in NaX the quadrupole patterns break down into singlets through a shape-transition range centered at about 90 K, the development of the spectrum shapes for ethene in Na-MOR is much more involved. Characteristically,
539
T = 289 K
T = 222 K
T = 119 K
T = 7 8 K
T = 291 K
T = 250 K
T = 167 K
T -77K
~
1' 1' Lv XI T = 284 K
T = 220 K
T = 125 K
T = 8 1 K
Fig. 4. 2H NMR spectra of ethene in (a) NaX, 1.2 molecules/supercage, (b)40AgNaX, 1 molecule/supercage, and (c) Na-MOR, 1.4 molecules/unit cell. in the latter case the spectra continuously change shape and total width but do not attain the Lorentzian singlet shape and rather narrow width ( < 100 Hz) of the NaX sample. The temperature dependence of the singlet linewidths of the ethene/faujasite systems is shown in Fig. 3. The splitting of the Pake pattern edges f o r ethene in 40AgNaX yields IDWC) 2 = 173 Idlz, in agreement with e qQ/h = 175 kHz for deuterons in the ethene molecule (ref. 30). Hence, at these temperatures all rotational types of motion conceivable proceed slowly on the NMR time scale in contrast t o benzene where the hexad axis rotation is fast. Applying eqn. ( 3 ) to the corresponding linewidth data of Fig. 3, the site-exchange correlation time rR is obtained as TR = 7*10-12exp(2900/TK-1) s (ER = 24 kJ mol-l) for ethene in 40AgNaX. The unusual temperature dependences both of the pattern shapes (Fig. 4) and the linewidths 6v (Fig. 3) of the ethene/NaX system are the result of the combined influence of the site exchange (T~) and of the reorientation of molecules around the 7r-electron/cation bond axis (T~) (ref. 17). The more detailed analysis of the low temperature pattern shapes as well as the singlet linewidth temperature dependence going beyond the simple formula given in
540 eqn. ( 3 ) (ref. 17),
yields
tional motion as follows: 'Ip =
2*10-11exp(850/TK-1)
the correlation times for both 'I~ = 3*10-13exp(1700/TK-1)
types of rota-
s (ER = 14 kJ/mol),
s (EP = 7 kJ mol) for ethene in NaX.
Identifying -rR in NaX and 40AgNaX with the mean residence time of an ethene
+
molecule on a Na
m=
and Ag
+
cation site, respectively, and using the estimate
1.1 nm as before, eqn. (4) yields diffusion coefficients the values
of which at 300 K are D(C2D4,NaX) = 2*10-9m2/s-1 and D(C2D4,40AgNaX) = 2*10-12 2 -1 m s The former value is in excellent agreement with the 'H PFG-de-
.
rived value (ref. 32); the latter is in the range of the values obtained with the 1H PFG and adsorption kinetics techniques (ref. 31). The large differences found for the diffusion coefficients at 300 K as well as for the activation energies between NaX and 4OAgNaX clearly reflect the much different strengths of bonding of ethene t o the sodium and silver cations due to the greater n-bonding capability of the latter (ref. 32). The clearest demonstration of this effect is the large shift of the pattern/singlet transition range center from 90 to 230 K corresponding to the large shift of the linewidth curves of Fig. 3. The results presently obtained indicate that the rate of site exchange
+
of ethene bound to Ag is much faster than the rate of anisotropic rotation around the n-electron/cation bond. From T~ and T~ given previously for NaX it is calculated that at about 210 K the rates of these two motions become equal, and at 300 K the site exchange is the faster motion as in the case of the silver-exchanged zeolite. These results support the conclusions from
'H NMR studies (refs. 33, 34) and can hardly be explained with the notion of a free rotation around the a-electron/cation
bond
addition, a Pake-pattern type spectrum with Av = 65 up at T < 80 K
axis
(ref.
32).
In
kHz should have shown
under these circumstances, which is not the case (Fig. 4).
A quantitative explanation of the spectra of ethene in Na-MOR is hardly
feasible at present mainly because the location of the molecules is not known. The saturation capacity of ethene in Na-MOR was found (ref. 35) to be 7.7 molecules/u.c, (O°C
adsorption isotherm) which requires the occupancy not
only of the main channels but also of the side pockets (refs. 21,
36).
Nevertheless, the similarity of shape of the 2H patterns of ethene in NaX and Na-MOR at about 80 K indicates that in mordenite as in NaX the molecules
+
are bound to Na , with the rotation around the n-electron/cation bond axis being in the fast/slow transition regime. The further development with increasing temperature of the mordenite spectra signify site-exchange processes to be operative, the detailed nature of which is presently under study.
541
1207*r '1 JY 20 L H I
T = 270 K
I
T = 200 K
T = 133 K
T = 105 K
1JY 20
201-;IHr
T = 292 K
20 L H t
T = 200 K
knI
T = 167 K
20 LHr
T = 133 K
n- L -KA 20 LHr
T = 292 K
20 LHr
T
--
200 K
20 LHr
T = 132 K
20 LHr
T = 110 K
Fig. 5. *H NMR spectra of propene (CD3CHCH ) in (a) NaX, 1 molecule/supercage, ( b ) 4OAgNaX, 1 molecule/supercage, an% (c) Na-MOR, 3 . 3 molecules/unit cell. Propene in NaX, AgNaX and Na-MOR 2 The H NMR spectra of propene-dg at 1 molecule/supercage loading in NaX and AgNaX (Fig. 5a and b) develop from Pake patterns with Av = 37 kHz
edge
splitting at low temperatures into singlets at high temperatures with the shape transition regions centered at about 115 K (NaX) and 145 K (40AgNaX). The linewidths of the singlets as function of temperature are included in Fig. 3 . In comparison to NaX the 2H spectra of propene in Na-MOR ( 3 . 3 molecules/u.c.) are the same at low temperatures up to about 135 K but exhibit much more complex shapes at higher temperatures. The quadrupole coupling constant IDQCCl = 49 H z derived from the pattern edge splittings is typical for rapidly rotating methyl groups attached to sp2-hybridized carbon atoms (ref. 18). At lower temperatures, therefore, the molecules are seen in slow overall motion with rapidly rotating methyl groups attached to them. Assigning again the breakdown of the quadrupole patterns into singlets to the site-exchange motion, and analyzing the curves in Fig. 3 according to eqn. 3 gives T~ = 2*10-12exp(1800/TK-1) s (ER = 15 kJ mol-l) for propene in NaX, and TR = 8*10-12exp(2050/TK-1) s (ER = 17 kJ mol-l)
542
for propene in 4OAgNaX. Inserting these expressions into eqn. (4), and again using the estimate = 1.1 nm, intracrystalline diffusion coefficients are accessible which at 300 K are calculated to be D(prop.. NaX) = 2.5*10-lo m2 s-' and D(prop., 4OAgNaX) = 2.7*10-11 m s The change of the temperature-independent Pake patterns into spectrum shapes with temperature-dependent widths of propene in Na-MOR sets in at considerably higher temperature (135 K) than in NaX (108 K ) Considering the sodium cations as adsorption sites in both cases, this shift is an indicative of a stronger bonding of propene in Na-MOR in comparison to NaX. The change of spectrum shape and width observed for the propene/Na-MOR sample at temperatures > 135 K is undoubtedly also due to site-exchange motions. In order to obtain quantitative results, spectrum-shape calculations are required which take account of the non-cubic symmetry of Na-MOR. Such work is being undertaken at present.
-' .
.
ACKNOWLEDGEMENTS The authors thank Prof. Dr. F. Fetting, Darmstadt, for the gift of the mordenite zeolite. This work was supported by "Deutsche Forschungsgemeinschaft" and "Fonds der Chemischen Industrie"
.
REFERENCES 1 B. Boddenberg, in G.R. Castro, M. Cardona (Editors), Lectures on Surface Science, Springer, Berlin, 1987, pp. 226-243. 2 H.E. Gottlieb and 2. Luz, J. Magn. Res. 54 (1983) 257. 3 B. Boddenberg, R. Grosse, W. Horstmann and G. Neue, Colloids Surf. 11 (1984) 265. 4 B. Boddenberg, R. Grosse and U. Breuninger, Surf. Sci. 129 (1986) L256. 5 P.D. Majors, T.E. Raidy and P.D. Ellis, J . Am. Chem. SOC. 108 (1986) 8123. 6 B. Boddenberg and R. Grosse, 2. Naturforsch. 41a (1986) 1361; 42a (1987) 272; 43a (1988) 497. 7 R. Grosse and B. Boddenberg, 2. Phys. Chem. NF 152 (1987) 1. 8 W. Horstmann, G. Auer and B. Boddenberg, 2 . Phys. Chem. NF 152 (1987) 23. 9 P.D. Majors and P.D. Ellis, J. Am. Chem. SOC. 109 (1987) 1648. 10 R. Eckman and A.J. Vega, J. Am. Chem. SOC. 105 (1983) 4841; J . Phys. Chem. 90 (1986) 4679. 11 D.L. Hasha, V.W. Miner, J.M. Garces and S.C. Rocke, in M.L. Deviney, J.L. Gland (Editors), Catalyst Characterization Science. ACS Symp. Series. NO. 288, ACS 1985, pp. 485-497. 12 A.J. Vega and 2 . Luz, J. Phys. Chem. 91 (1987) 365. 13 B. Boddenberg and R. Burmeister, Proc. XXIII Congr. Ampere on Magnetic Resonance, Roma, Sept. 15.-19, 1986, Istituto Superiore di Sanita, Roma, 1986, pp. 418-419. 14 Z. Luz and A.J. Vega, J . Phys. Chem. 90 (1986) 4903; 91 (1987) 374. 15 A.J. Vega and 2. Luz, J. Phys. Chem. 91 (1987) 365; Zeolites 8 (1988) 19.
543 16
B. Zibrowius, J. Car0 and H. Pfeifer, J. Chem. SOC. Faraday I 84 (1988) 2347.
17 18
B. Boddenberg and R. Burmeister, Zeolites, in press. R. G. Barnes, in J.A.S. Smith (Editor),Advances in Nuclear Quadrupole
19
H. H. Mantsch, H. Saito and I.C.P. Smith, Progr. NMR Spectrosc.
Resonance,Vol. 1, Heyden, London, 1974, pp. 335-355. 11 (1977)
211. 20
H.W. Spiess, in P. Diehl, E. Fluck, R. Kosfeld (Editors), NMR. Basic Prin-
ciples and Progress, Vol. 15, Springer, Berlin, 1978, pp. 55-214. 21 D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. 22 J.H. Davis, K.R. Jeffrey, M. Bloom, M.I. Valic and T.P. Higgs, Chem. Phys. Letters 42 (1976) 390. 23 F.S. Millett and B.P. Dailey, J. Chem. Phys. 56 (1972) 3249. 24 N. Boden, S. M. Hanlon, Y. K. Levine and M. Mortimer, Mol. Phys. 36 (1978) 519. 25 W.M. Meier, Z. Kristallogr. 115 (1961) 439. 26 H. Jobic, M. Bee and A . Renouprez, Surf. Sci. 140 (1984) 307. 27 H. Lechert and K.P. Wittern, Ber. Bunsenges. Phys. Chem. 82 (1978) 1054. 28 J. Karger and D. Michel, Z. Phys. Chem. (Leipzig) 257 (1976) 983. 29 J. Karger and H. Pfeifer, Zeolites 7 (1987) 90. 30 J. Kowalewski, T. Lindblom, R. Vestin and T. Drakenberg, Mol. Phys. 30 (1976) 1669. 31 A. Germanus, J. Karger and H. Pfeifer, Zeolites 4 (1984) 188. 32 J.L. Carter, D.J.C. Yates, P.J. Lucchesi, J.J. Elliott and V. Kevorkian, J. Phys. Chem. 70 (1966) 1126. 33 G.M. Muha and D.J.C. Yates, J . Chem. Phys. 49 (1968) 5073. 34 G.M. Muha, J. Chem. Phys. 55 (1971) 467. 35 G. Spaeth and B. Boddenberg, unpublished. 36 W. Drachsel and K. A. Becker, Z. Phys. Chem. NF 110 (1978) 85.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands
PROBING THE HYDROGEN SORPTION STATES I N ZEOLITES A BY AND
INFRARED SPECTROSCOPY
LOW-TEMPERATURE GAS CHROMATOGRAPHY SUPPLEMENTED BY
THEORETICAL CALCULA-
TIONS H. F o r s t e r , W . Frede’ and G. P e t e r s ’
I n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y o f Hamburg, Bundesstr. 45, P-2000 Hamburg 13, FRG P r e s e n t Adress: Valvo A p p l i c a t i o n L a b o r a t o r i e s , V o g t - K o l l n - S t r . 30, F-2000 Hamburg 54, FRG P r e s e n t Adress: Reemtsma C i g a r e t t e n f a b r i k e n GmbH, L u r u p e r Chaussee 145, D-2000 Hamburg 50, FRG ABSTRACT The s o r p t i o n s t a t e s o f hydrogen and d e u t e r i u m i n t h e sodium and mixed sodium-calcium forms o f z e o l i t e A, c h a r a c t e r i z e d by t h e s i t e s o f i n t e r a c t i o n and t h e microdynamics performed by t h e adsorbate, have been e x p l o r e d by I R spectroscopy and low-temperature gas chromatography. Bonding t o t h e z e o l i t e i s near t h e l i m i t o f pure p h y s i c a l i n t e r a c t i o n w i t h no e s s e n t i a l charge t r a n s f e r i n e i t h e r d i r e c t i o n . Wells on t h e i n t e r a c t i o n p o t e n t i a l s u r f a c e a r e a t t r i b u t e d t o d i f f e r e n t s u r f a c e compounds i n a c c o r d w i t h s p e c t r o s c o p i c and GC r e s u l t s . The i s o s t e r i c h e a t o f a d s o r p t i o n i s e v a l u a t e d by a s t a t i s t i c - m e c h a n i c a l t r e a t m e n t b e i n g i n good agreement w i t h t h a t e x p e r i m e n t a l l y o b t a i n e d f r o m t h e isotherms.
INTRODUCTION The
application
catalysts
is
o f z e o l i t e s as d r y i n g agents,
selective
based on t h e i n t e r a c t i o n phenomenon i n s i d e
c a v i t y system, whose e x p l o r a t i o n
adsorbents
the
zeolite
and pore-
forms t h e key o f u n d e r s t a n d i n g t h e processes
i n v o l v e d . S o r p t i o n s i t e s and microdynamics o f p h y s i s o r b e d molecules, b a s i c a l l y described
by
potential. can
be
the
sorption state,
calculated.
pattern
as
symmetries,
well
determinable
while
Spectroscopic
from
q u a n t i t i e s a r e band
as band i n t e n s i t i e s c o r r e l a t e d t o
the
and
band
coverage
and
site
p o t e n t i a l and t h e c h a r a c t e r i z a t i o n o f t h e s o r p t i o n s t a t e
correspondence
between c a l c u l a t e d and e x p e r i m e n t a l v a l u e s .
the
z e o l i t e i n t e r n a l s u r f a c e by I R spectroscopy and
operation.
contribute
inferred
t o the enlightenment o f the
gas
is
The o b j e c t i v e
s t u d y was t o i n v e s t i g a t e d e t a i l s o f t h e b e h a v i o r o f s i m p l e to
be
A measure on t h e r e l i a b i l i t y o f t h e s e t - u p o f t h e
our order
interaction
positions
the
adsorbed amounts and h e a t s o f a d s o r p t i o n can
f r o m s o r p t i o n measurements. interaction
are
From t h e l a t t e r b o t h I R s p e c t r o s c o p i c and thermodynamic q u a n t i t i e s
molecules
chromatography,
molecular
sieve
mode
the of on in of
546
Some time ago,
we r e p o r t e d f o r the f i r s t time N I R s p e c t r a o f the overtones
o f adsorbed H2 ( r e f . shift
l),
showing a weak band whose h a l f w i d t h and
w i t h r e s p e c t t o the f r e e molecule pure v i b r a t i o n a l overtone
frequency transition
are t w i c e t h a t o f the fundamental. Comparing fundamental and overtone spectra, the bond d i s s o c i a t i o n energy o f b o t h hydrogen and deuterium i s o n l y n e g l i g i b l y changed i n the adsorbed s t a t e i n agreement w i t h quantum chemical c a l c u l a t i o n s , indicating zeolite
no
e l e c t r o n i c charge t r a n s f e r from t h e adsorbed molecule
skeleton,
so
t h a t a classical set-up o f the
interaction
to
the
potential
seemed t o be j u s t i f i e d . A c q u i r i n g i n f r a r e d a c t i v i t y o n l y i f a d i p o l e moment i s induced by homonuclear d i a t o m i c s p l a y a unique r o l e as adsorbates,
tion,
molecule
does
n o t c o n t r i b u t e t o the spectrum.
Hydrogen
e s p e c i a l l y s u i t e d f o r these k i n d s o f experiments, ties
and
deuterium
as t h e i r molecular
are most p a r t i c u l a r l y e x p l o r e d and t h e i r low mass augments the
v i t y o f these i n v e s t i g a t i o n s .
interac-
while the f r e e are
propersensiti-
As adsorbent z e o l i t e A was chosen, s e r v i n g as a
model adsorbent due t o i t s simple composition and s t r u c t u r e . TABLE 1 Band p o s i t i o n s , frequency s h i f t s w i t h r e s p e c t t o t h e gas phase v - v ( g a s ) and t o the main band v-v(main) o f hydrogen and deuterium adsorbed i n z e o l i t e s Na,Ca,A ( a l l i n cm-’). Assignment o f t h e bands i s g i v e n i n t h e l e f t column (An means the change o f t h e t r a n s l a t i o n a l quantum number n ) . Species Band p o s i t i o n
v-v(gas) Hydrogen
v-v(main)
Fundamentals Gas(O+l) 4161.2
Band p o s i t i o n
v - v ( g a s ) v-v(main) Deuterium
2993.6
A,/Ca
408814092.5
-73168.5
A2/Na
411814124
-43137
0 t30
2939.5
-54
2964
-29.6
0 t24.5
Overtones 8087.1 A,lCa
7940
Sate1 1 it e s Tang. t r a n s l . 413114148 (An=tl) Tang. transl. (An=- 1) Radial
4047 190
5867.1 -147
5760
-107
t43
2973
t33.5
41
2909
-30.5
547 EXPERIMENTAL The experiments were c a r r i e d o u t on t h e z e o l i t e samples Na,,A, Na,Ca,A ion
Na,Ca,A
(A = A l 1 2 S i 1 2 0 2 , +t)h e l a t t e r b e i n g prepared f r o m t h e sodium
exchange w i t h aqueous CaC1, s o l u t i o n s a t room temperature.
spectra
The
and
form
by
infrared
o f s e l f - s u p p o r t i n g w a f e r s ( 5 - 8 mg/cm2 t h i c k n e s s ) i n a c r y o g e n i c
cell
were recorded on a D i g i l a b FTS 20E spectrometer a t 1 cw' r e s o l u t i o n between 20 80 K .
and
D e t a i l s o f t h e p r e t r e a t m e n t and h a n d l i n g o f t h e samples have
g i v e n elsewhere ( r e f .
been
2 ) . The chromatographic equipment m o d i f i e d f o r low-tem-
p e r a t u r e s t u d i e s has been d e s c r i b e d i n a d i f f e r e n t paper ( r e f . 3 ) . INFRARED AN0 GAS CHROMATOGRAPHIC RESULTS The bands observed a f t e r l o a d i n g z e o l i t e Na,Ca,A
w i t h hydrogen o r d e u t e r i u m
and t h e i r assignment a r e summarized i n Table 1. F o r i l l u s t r a t i o n a t y p i c a l low coverage a t 90 K i s d i s p l a y e d i n F i g .
l i t e Na,Ca,A respect
IR spectrum o f H, adsorbed i n zeo1. S h i f t e d downscale by 73 cm' w i t h
t o t h e p u r e v i b r a t i o n a l Raman t r a n s i t i o n o f t h e
free
molecule,
main band c o n s i s t s o f a sharp d o u b l e t w i t h a s e p a r a t i o n o f 5 cm-',
the
approximate-
l y ( r i g h t s i d e ) . Another narrow a b s o r p t i o n appears a t s l i g h t l y h i g h e r frequencies,
with
i n c r e a s i n g i n t e n s i t y as t h e temperature i s l o w e r e d t o 45 K
s i d e ) . The main band i s accompanied by two broad s a t e l l i t e s +42
(left
apart.
CIT'
90 K
i j --.-
.
1
F i g . 1. Low-temperature i n f r a r e d s p e c t r a o f hydrogen adsorbed i n z e o l i t e Na,Ca,A a t d i f f e r e n t temperatures. I s o s t e r i c heats o f a d s o r p t i o n were e v a l u a t e d f r o m GC d e s o r p t i o n experiments r a n g i n g f r o m 50 K t o room temperature as w e l l as f r o m l o w - t e m p e r a t u r e s o r p t i o n isotherms
and w i l l be d i s c u s s e d i n connexion w i t h t h e s t a t i s t i c a l l y
obtained
heats. G e n e r a l l y , on a l l z e o l i t e s i n v e s t i g a t e d o r t h o - p a r a hydrogen i n t e r c o n v e r s i o n
was observed,
so t h a t i n a l l e x p e r i m e n t s p e r f o r m e d an e q u i l i b r i u m m i x t u r e
t h e n u c l e a r s p i n isomers must taken i n t o account,
of
depending on t e m p e r a t u r e i n
t h e adsorbed s t a t e . INTERACTION POTENTIAL aim of t h i s paper i s t o d e s c r i b e t h e s o r p t i o n s t a t e o f H,
The NaCaA.
In
case
of s t r o n g chemical i n t e r a c t i o n i t i s u s u a l l y
zeolites
sufficient
t h e e q u i l i b r i u m geometry o f t h e s o r p t i o n
specify the s o r p t i o n s i t e , the
in
s t r e n g t h of t h e s o r p t i o n bond and i t s i m p a c t s on t h e i n t r a m o l e c u l a r the
of
adsorbed molecules.
however,
the
I f o n l y weak van d e r Waals f o r c e s
to
complex, bond
are
present,
admolecules r e t a i n p a r t o f t h e i r dynamical degrees o f
freedom.
Consequently, i n these cases ( a s f o r t h e H , / z e o l i t e
NaCaA system) t h e dynamics
o f t h e adsorbed molecules have t o be t a k e n i n t o account. The
interaction
sorbent
is
the
potential
key
o f the s o r p t molecules i n
o f understanding
the
the
field
characteristics
of
the
adsorption:
of
Depending
i n g e n e r a l on a l l s i x degrees o f freedom o f a d i a t o m i c ,
i t can
be
expressed
as
center
of
a f u n c t i o n o f the three s p a t i a l coordinates o f
the
t h e two o r i e n t a t i o n a l c o o r d i n a t e s o f t h e m o l e c u l a r a x i s i n space
interaction,
and t h e i n t r a m o l e c u l a r s t r e t c h i n g c o o r d i n a t e . I n t h i s way t h e p o t e n t i a l minima describe
the equilibrium configurations o f the and
sorption
the o r i e n t a t i o n s o f the molecule
complex,
with
i.e.
the
to
the
sorption
sites
surface.
Furthermore, t h e i n t e r a c t i o n p o t e n t i a l p r o v i d e s t h e p o t e n t i a l energy
respect
term o f t h e time-independent Schroedinger e q u a t i o n f o r t h e adsorbed I n t h i s way i t determines t h e r o t a t i o n a l and t r a n s l a t i o n a l
m o l e c u l a r dynamics on one hand and t h e f r e q u e n c i e s o f I R
the
i.e.
transitions
Thus observed I R bands can be compared w i t h t h o s e c a l c u l a t e d
the other. the
molecule.
eigenvalues,
i n t e r a c t i o n p o t e n t i a l t o check t h e r e l i a b i l i t y o f e q u i l i b r i u m
on from
configura-
t i o n s and m o l e c u l a r dynamics o b t a i n e d . As
i t is,
exactly,
one
i n general,
calculations. If sually
impossible t o determine t h e i n t e r a c t i o n
i s dependent on (more o r l e s s ) rough a p p r o x i m a t i o n s
expressed
potential for
model
l o n g range p h y s i c a l i n t e r a c t i o n s a r e p r e d o m i n a t i n g , i t i s uas
a
sum o f
coulomb
and
inductive
electrostatic,
of
d i s p e r s i v e and r e p u l s i v e f o r c e s :
'
'
'
'
(l) = "coul "ind 'disp 'rep F o r t h e e v a l u a t i o n o f t h e e l e c t r o s t a t i c terms, t h e m o l e c u l a r charge d i s t r i b u tion
i s developed i n t o permanent and induced m u l t i p o l e moments t h a t
interact
w i t h t h e e l e c t r o s t a t i c p o t e n t i a l o f t h e s o r b e n t and i t s d e r i v a t i v e s ( r e f . 4 ) . The d i s p e r s i o n i s p u t up a c c o r d i n g t o London ( s e e r e f . 4), w h i l e an e x p o n e n t i a l A*exp(-mr) e x p r e s s i o n i s chosen f o r t h e r e p u l s i v e term. Each term, except
the
i n d u c t i v e one,
i s r e p r e s e n t e d by a sum
over
the
corresponding
549
i n t e r a c t i o n w i t h t h e i n d i v i d u a l l a t t i c e atoms. is
It
always
interactions. for
a problem t o determine
Essentially,
the
parameters
for
the
pairwise
t h i s h o l d s f o r t h e r e p u l s i o n parameters R and
t h e charges o f t h e l a t t i c e atoms ( i n t h e e l e c t r o s t a t i c term) as
well
m,
as
f o r t h e i r p o l a r i z a b i l i t i e s ( i n t r o d u c e d by t h e London f o r m u l a ) . I n e a r l i e r estimations o f physical i n t e r a c t i o n s i n z e o l i t e s A ( r e f . c a t i o n s were g i v e n t h e i r nominal charges, at
balanced by -0.25 e l e m e n t a r y
each o f t h e 48 oxygens b e l o n g i n g t o one a-cage and n e g l e c t i n g t h e
and
aluminum
atoms.
5) the
This r e s u l t e d i n f a r too h i g h i n t e r a c t i o n
units silicon
energies
at
s o r p t i o n s i t e s near c a t i o n s ( r e f . 6 ) . A l t h o u g h n o t y e t f i t t e d f o r t h e c a l c u l a tion
of
method
a b s o l u t e ( i n t e r a c t i o n ) energy values, is
t h e quantum
capable t o produce r e l i a b l e charge d i s t r i b u t i o n s
chemical and
SCC-Xa
equilibrium
c o n f i g u r a t i o n s f o r t h e system under c o n s i d e r a t i o n , even i f as much as 50 atoms are involved ( r e f . calculations
Xa
7 ) . Consequently, a charge d i s t r i b u t i o n determined by SCCf o r unloaded NaA and CaA s t r u c t u r e s was
finally
introduced
i n t o t h e c l a s s i c a l i n t e r a c t i o n p o t e n t i a l , w h i c h a r e l i s t e d i n Table 2. TABLE 2 Parameters and f o r m u l a s f o r t h e i n t e r a c t i o n p o t e n t i a l c a l c u l a t i o n s . ~~
Term
Vrep
=
Parameter
Na
Ca
A1
Si
16.39 44.7 0.175
8.55 35.6 0.131
11.79 41.4 0.223
12.63 35.6 0.195
1.4
1.0
1.2
vreP
A [ l o 7 Jmol-’I m [ nm-‘] a
13.25 34 0.225
Vel
q [ u n i t s of e l
-0.725 0.7
A [ l + 1 . 4 4 5 < v l ~ ~ ~ > ] e - ~ ~ [ l + a P , ( cOo) s] 5
Vdisp = C[a+yP,(cos
Vel
0
o)/31/r6
= ( - l ) l q M 1 P l (cos O ) / r l t l
q = atomic charge
= s t r e t c h i n g coordinate
P = Legendre p o l y n o m i a l
a,y = p o l a r i z a b i l i t y
r = d i s t a n c e m o l e c u l e - l a t t i c e atom
M1 = l t h m o l e c u l a r moment
0 = a n g l e between m o l e c u l a r a x i s
and t h e j u n c t i o n o f m o l e c u l a r c e n t e r o f g r a v i t y w i t h t h e r e s p . l a t t i c e atom D i s p e r s i o n and p a i r w i s e r e p u l s i o n parameters f o r t h e v a r i o u s l a t t i c e have
been
determined by a l e a s t square f i t t o
sorption
c a l c u l a t e d w i t h a quantum chemical ab i n i t i o method ( r e f . Table 2 . The H,-specific
complex
atoms
geometries
8 ) and a r e g i v e n i n
parameters a r e m a i n l y t h e permanent m u l t i p o l e moments
and t h e d i p o l e moment p o l a r i z a b i l i t i e s , depending i n g e n e r a l on t h e i n t r a m o l e cular
s t r e t c h i n g coordinate 6 .
compared
As the i n t r a m o l e c u l a r v i b r a t i o n i s v e r y
t o t h e e x t e r n a l m o t i o n s ( r o t a t i o n and t r a n s l a t i o n ) t h e
e x p e r i e n c e t h e average o v e r a number o f p e r i o d s o f t h e f o r m e r .
latter Now
fast only
dependent
550
only
on
the
represented
v i b r a t i o n a l quantum number
v,
the
molecular
parameters
by the corresponding expectation values over a v i b r a t i o n
are
period,
which were t a k e n f r o m r e f . 9. A f i r s t and e s s e n t i a l l y q u i t e good a p p r o x i m a t i o n t h e v i b r a t i o n a l f r e q u e n c y s h i f t o f I R bands i s t h e d i f f e r e n c e between
to
i n t e r a c t i o n p o t e n t i a l s i n t h e two s t a t e s covered by t h e t r a n s i t i o n ,
the
i.e.
v=l
and v=O i n case o f t h e fundamental AV
= Vsorb-vfree
= V(V=I)-V(V=O)
(2)
THEORETICAL RESULTS AND DISCUSSION t h i s paper we w i l l c o n f i n e o u r s e l v e s t o t h e d i s c u s s i o n o f
In
o b t a i n e d on z e o l i t e Na,Ca,A. potential
the
results
A pseudo 3-dimensional d i s p l a y o f t h e i n t e r a c t i o n
s u r f a c e i n t h e ( 1 1 0 ) - p l a n e o f z e o l i t e Na,Ca,A
i s shown i n
Fig.
2.
F i g . 2. I n t e r a c t i o n p o t e n t i a l s u r f a c e ( i n pseudo t h r e e - d i m e n s i o n a l p r e s e n t a t i o n ) o f a (110) p l a n e i n t h e supercage o f z e o l i t e Na,Ca,A ( l e f t ) c o n t a i n i n g two body d i a g o n a l s as w e l l as two Na(S1) and Ca(S1) i o n s . Each
data p o i n t represents the (x,y)-position
i n t h e p l a n e and t h e
energy v a l u e o f t h e most f a v o u r a b l e o r i e n t a t i o n o f t h e Deep
pertinent
H, m o l e c u l a r a x i s .
p o t e n t i a l minima o f a b o u t -920 cm-’ have been found on a s e c t i o n o f
spherical
s h e l l t h a t i s 256 pm a p a r t f r o m t h e
Cazt-ions.
The
a
corresponding
minima i n f r o n t o f t h e sodium i o n s a r e o n l y v e r y s h a l l o w deepenings ( - 4 7 0 cnr’) in
an e q u i p o t e n t i a l s p h e r i c a l s u r f a c e (-435 cm-’) around t h e c e n t e r o f t h e
a-
cage, 355 pm a p a r t . A d j a c e n t a-cages a r e s e p a r a t e d b y oxygen 8 - r i n g s o f a b o u t 400 pm diameter, g i v i n g r i s e t o a p o t e n t i a l w e l l o f V=-150 c d o n l y . The p o t e n t i a l energy a t t h e c e n t e r o f t h e c a v i t y i s -120 cm-’. Observed those
frequency
calculated
s h i f t s f o r H,
and 0,
( s e e Table 1 ) a r e
according t o equation ( 2 ) f o r s o r p t i o n s i t e s
compared in
front
Ca and Na i o n s and on t h e s p h e r i c a l s h e l l 355 pm a p a r t f r o m t h e c e n t e r o f
with of the
551 supercage,
respectively,
i n Table 3.
H e r e i n energy v a l u e s a t t h e
potential
minima have been a p p l i e d and t h e o n l y d i f f e r e n c e made between H, and D, was t o use t h e i r i n d i v i d u a l m o l e c u l a r c o n s t a n t s .
Experimental and t h e o r e t i c a l s h i f t s
o f t h e f o u r i n f r a r e d bands agree remarkably w i t h o u t any f u r t h e r a d j u s t m e n t
of
t h e i n t e r a c t i o n parameters produced by t h e l e a s t square f i t f o r t h e H , - l a t t i c e atom i n t e r a c t i o n s d e s c r i b e d i n t h e f o r e g o i n g s e c t i o n . The n e x t s t e p i s t o ask f o r t h e e q u i l i b r i u m geometries o f t h e s o r p t i o n complexes. I n f r o n t o f t h e c a t i o n s t h e m o l e c u l a r a x i s i s p e r p e n d i c u l a r t o t h e c a t i o n - m o l e c u l e i n t e r s e c t i o n , w h i l e p o i n t i n g towards t h e oxygen atoms f o r molecules on t h e sphere around t h e c e n t e r o f t h e c a v i t y . TABLE 3 Observed and c a l c u l a t e d frequency s h i f t s o f t h e p u r e l y v i b r a t i o n a l t r a n s i t i o n s ( i n cm-I). Species
Av(eXp.) Av(ca1c.) Hydrogen
A,/Ca A,/Na
+
species C
Av(eXp.) Av(ca1c.) Deut e r ium
- 73
-72
-54
-50
-43
-42
-29.6
-28.5
B u t t h e complexes a r e n o t expected t o be v e r y r i g i d . of
The p o t e n t i a l
energy
molecules e x a c t l y on t h e t h r e e f o l d symmetry a x i s i n f r o n t o f c a l c i u m
(V=-900 cm-'),
f o r example,
m o l e c u l a r a x i s i n t h e p l a n e p e r p e n d i c u l a r t o t h e symmetry the
ions
i s almost independent f r o m t h e o r i e n t a t i o n o f t h e axis.
Consequently
molecules a r e a b l e t o r o t a t e f r e e l y i n t h e p l a n e w h i l e p e r f o r m i n g o u t
of
plane l i b r a t i o n s ( t o r s i o n a l v i b r a t i o n s ) . On
the
exactly
symmetry
a x i s mentioned t h e
V,(r)P,(cos
O),
orientational
potential
is
where r i s t h e d i s t a n c e f r o m t h e c a t i o n ,
almost is
0
the
a n g l e between t h e m o l e c u l a r and t h e symmetry a x i s . I n t h e p o t e n t i a l minimum V, w i t h t h e p o s i t i v e s i g n f o r c i n g t h e p e r p e n d i c u l a r arrangement o f
i s + l o 0 0 cm-',
t h e molecules. R o t a t i o n a l e i g e n v a l u e s have been c a l c u l a t e d f o r t h e v i b r a t i o n a l ground and f o r t h e f i r s t e x c i t e d s t a t e (see r e f . transitions
from
1).
The p u r e l y
vibrational
t h e two l o w e s t r o t a t i o n a l l e v e l s a r e s e p a r a t e d by
with
t h e low frequency t r a n s i t i o n b e i n g t h e more i n t e n s e
main
band s p l i t t i n g was 4.5 cm-' (see F i g .
1).
one.
I n case o f D,
4.4
The the
c d
observed calculated
s p l i t t i n g o f t h e d o u b l e t i s l e s s than 2 cm-' and o b v i o u s l y n o t r e s o l v e d . The
translational
separated
into
t a n g e n t i a l one.
a
motion
high
o f molecules i n f r o n t o f c a l c i u m
frequency r a d i a l
and
a
degenerate
ions low
can
be
frequency
While no a t t e m p t has been made t o c a l c u l a t e t h e l a t t e r , which
t h e two s a t e l l i t e s i n F i g .
1 a r e i m m e d i a t e l y assigned t o ,
t h e second
radial
d e r i v a t i v e o f t h e p o t e n t i a l i n t h e minimum g i v e s a v i b r a t i o n a l c o n s t a n t o f 190 cm-'.
Already i n 1980 we observed a weak FIR t r a n s i t i o n a t 190 c d and a l r e a d y
552
assigned i t t o a r a d i a l t r a n s l a t i o n a l v i b r a t i o n o f m o l e c u l e s adsorbed i n f r o n t o f c a l c i u m i o n s (see r e f . 10). From
the
foregoing
d i s c u s s i o n i t i s o b v i o u s t h a t we have
to
deal
with
s e v e r a l s p e c i e s o f adsorbed hydrogen molecules
localized
translations,
in
front
librations
of
towards
calcium the
ions,
cations
performing
and
almost
frustrated free
planar
r o t a t i o n s ( s p e c i e s A), molecules
weakly bound t o t h e sodium i o n s ,
p e r f o r m i n g t h e same
kinds
of
motions ( s p e c i e s B), molecules r o t a t i n g w i t h t h e i r c e n t e r o f mass on a sphere around t h e o f the cavity, the
center
simultaneously performing t r a n s l a t i o n a l v i b r a t i o n s
and v i c e v e r s a ( s p e c i e s C) ( T h e i r end-over-end
center towards
rotations
are
weakly h i n d e r e d . ) , molecules
delocalized
system
moving t h r o u g h t h e whole p o r e
from
a-cage
t o a-cage ( s p e c i e s D ) , and molecules c a p t u r e d w i t h i n a c e r t a i n a-cage and d e l o c a l zed t h e r e i n ( s p e c i e s
El. An e l e c t r i c f i e l d s t r e n g t h o f 62 esu/cm2 on t h e s o r p t on s i t e i n f r o n t Ca(S1) i o n s was determined f r o m t h e i n t e n s i t y o f t h e main band, than
that
55 esu/cm2 o b t a i n e d f r o m t h e
of
Furthermore,
deuterium
being
spectrum
of
larger
(ref.
10).
t h e r e l a t i v e l y h i g h i n t e n s i t i e s o f t h e h i g h - f r e q u e n c y bands
may
be e x p l a i n e d by c o n t r i b u t i o n o f t h e two s p e c i e s B and C . As with
the the
estimated Thus
e x p e r i m e n t a l f r e q u e n c y s h i f t s and band p a t t e r n s calculated from
ones,
the
rotational
and
agree
remarkably
translational
t h e i n t e r a c t i o n p o t e n t i a l a r e supposed t o be
eigenvalues
very
re1i a b l e .
i t i s i n p r i n c i p l e p o s s i b l e t o use t h e i n t e r a c t i o n p o t e n t i a l minima
and
the eigenvalues f o r s t a t i s t i c a l c a l c u l a t i o n s . Finding multiple
out
t h e most p r o b a b l e c o n f i g u r a t i o n r e g a r d i n g
occupation immediately y i e l d s Fermi-Dirac o r
either
single
Bose-Einstein
or
statis-
t i c s . T h e r e f o r e we have a p p l i e d
-
Fermi-Dirac
-
Bose-Einstein
statistics
with a partition function
G!ZN/(G-N)
N!
for
the
l o c a l i z e d s p e c i e s ( A and B ) , s t a t i s t i c s w i t h the p a r t i t i o n
function
( G t N - 1 ! ZN/ (G- ) ! N !
f o r t h e d e l o c a l i z e d species and
-
Maxwell-Boltzmann
G
is
the
statistics
for
number o f p o t e n t i a l w e l l s ,
molecular p a r t i t i o n f u n c t i o n .
the
gas phase.
N t h e number o f p a r t i c l e s
and
2
the
Each s p e c i e s i s b r o u g h t s e p a r a t e l y i n t o e q u i l i -
b r i u m w i t h t h e gas phase. W i t h t h i s method a p p l i e d t h e number o f adsorbed molecules, t h e i r d i s t r i b u t i o n o v e r t h e s e v e r a l species, and t h e i n t e r n a l energy o f t h e s e s p e c i e s can be
553 c a l c u l a t e d . The a b s o l u t e amount o f gas adsorbed a t d i s t i n c t t e m p e r a t u r e s i s w i t h i n t h e range o f e x p e r i m e n t a l accuracy (see F i g . 3), hence r e s u l t i n g i n g i n a q u i t e good agreement between c a l c u l a t e d and measured h e a t s o f a d s o r p t i o n .
500
1
1500
2000 p/Nl"-2
7
5
3
1000
Moleculesluni t cell
F i g . 3 . Comparison o f s t a t i s t i c a l and e x p e r i m e n t a l r e s u l t s f o r hydrogen. A) D i s t r i b u t i o n o f t h e hydrogen molecules o v e r t h e d i f f e r e n t adsorbed and E. species A,B,C,D B ) Comparison o f a d s o r p t i o n i s o t h e r m s a t 100 K s t a t i s t i c a l l y and exper i m e n t a l l y obtained. C) Coverage dependence o f c a l c u l a t e d energy o f a d s o r p t i o n AU, e n t h a l p y o f a d s o r p t i o n AH and i s o s t e r i c h e a t o f a d s o r p t i o n Qst w i t h t h e exp e r i m e n t a l l y achieved i s o s t e r i c h e a t o f a d s o r p t i o n . As can be seen from Table 4, t h e r e i s good agreement o f t h e p r e d i c t e d w i t h i s o s t e r i c heats o b t a i n e d by gas chromatography. Thermal d e s o r p t i o n experiment5 s t a r t i n g from 50 K prove a t l e a s t 3 o f t h e p r e d i c t e d s p e c i e s (see F i g . 4 ) . TABLE 4 Comparison o f t y p e and i s o s t e r i c heats o f a d s o r p t i o n o f t h e d i f f e r e n t s o r p t i o n complexes. Spec ies
A
BtC DtE
Type
predicted
Langmui r l i n e a r isotherm Volmer, concave i s o t h e r m
QSt[kJmol
1
Type
Q t[kJ/mol 1
experimen t a
7.5-8
Langmui r
4- 5
symm. peak f o r m
1.5
concave i s o t h e r m
f
7-8 ca.5
1.2-1 8
554
F i g . 4 . Low-temperature thermal desorpt i o n o f deuterium from z e o l i t e Na,Ca,A s t a r t i n g from d i f f e r e n t l o a d i n g s . The CI peak i s a t t r i b u t e d t o s p e c i e s D+E, t h e B peak t o s p e c i e s B+C and t h e y peak t o s p e c i e s A.
I
,
,
,
.
,
,
I00
0
200
100
T, K
CONCLUSIONS
-
Physisorption
of
simple
m o l e c u l e s i n z e o l i t e s may
e l e c t r o s t a t i c i n t e r a c t i o n model,
be
described
p r o v i d e d a r e a l i s t i c charge
by
an
distribution
o v e r t h e z e o l i t e s k e l e t o n , d e t e r m i n e d quantum c h e m i c a l l y , i s t a k e n as a ba-
-
sis. H e r e w i t h i n t e r p r e t a t i o n of I R s p e c t r a i s p o s s i b l e , even q u a n t i t a t i v e l y .
-
T h i s method y i e l d s b e s i d e s l o c a l i z e d a l s o d e l o c a l i z e d s p e c i e s .
-
Far-reaching
-
On
e l e c t r o s t a t i c i n t e r a c t i o n s s h o u l d be
included,
additionally
r e g a r d i n g t h e dynamics o f t h e a d s o r b a t e . can
the
b a s i s of i n t e r a c t i o n p o t e n t i a l c a l c u l a t i o n s
a
statistical
be developed y i e l d i n g t h e thermodynamical p r o p e r t i e s o f
the
model
sorption
system, a l s o q u a n t i t a t i v e l y . ACKNOWLEDGEMENT We thank t h e Deutsche Forschungsgemeinschaft f o r c o n t i n u i n g s u p p o r t t o t h i s work o v e r a number of y e a r s . REFERENCES 1 H. Bose, H. F o r s t e r and W . Frede, Chem. Phys. L e t t . , 138 (1987) 401-404. 2 H. F o r s t e r and W. Frede, I n f r a r e d Phys., 24 (1984) 151-156. 3 H. F o r s t e r and G. P e t e r s , J. C o l l . I n t e r f a c e S c i . , i n p r e s s . 4 A . D . Buckingham, Adv. Chem. Phys., 12 (1967) 107-142. 5 E. Cohen de Lara and T. Nguyen Tan, J. Phys. Chem., 80 (1976) 1917-28. 6 0. Zakharieva-Pencheva, H. Bose, H. F o r s t e r , W. Frede and M. G r o d z i c k i , J. Mol. S t r u c t . , 122 (1985) 101-114. 7 M. G r o d z i c k i , 0. Zakharieva-Pencheva and H. F o r s t e r , J. Mol. S t r u c t . , 175 (1988) 195-201. 8 W. K u t z e l n i g g , V . Staemmler and C. H o h e i s e l , Chem. Phys., 1 (1973) 27-44. 9 M. S c h u l d t , Ph.D. Thesis, Hamburg, 1979. 10 H. F o r s t e r , W . Frede and M. S c h u l d t , i n L.V. Rees ( E d i t . ) , Proc. V t h I n t . Conf. Z e o l i t e s , Naples, 1980, Heyden, London, 1980, pp. 458-467.
H.G.Karge,J. Weitkamp (Editors1, Zeolites 0s Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
FOURIER-TRANSFURM INFRA-RED PHOTOACOUSTIC SPECTROSCOPY,
USEFUL
A
TECHNIQUE FOR THE STUDY OF STRONGLY PHYSISOHBED MOLECULES.
J.
Philippaerts. E.F. Vansant and Y. A. Yan.
Department of Chemistry, University of Antwerp (U.l.A.J, Universiteitsplein 1 , 8-2610 Wilrijk, Belgium.
ABSTRACT The pore-size moditication or Na-mordenite by implantation of gas-volumetric boron-nitrogen compounds is investigated by sorption experiments and by FTIH-photoacoustic spectroscopy (FTIR-PASJ. The P A S spectra of a Na-mordenite after physical sorption of diborane show the specific characteristics of the physisorbed borane groups. These reactive species are further treated with ammonia and methylamine at different reaction conditions. The ultimate porosity is tested by adsorption of test gases such as K r and X e tat 273 KJ, and the BN-compounds, formed inside the channels of the substrate, can be characterized with I . R. -spectroscopy. INTRODUCTION Structural modifications of the mordenite-type zeolite are the even now the synthesis of new
subject of several investigations; zeolites widely
other
reported.
study its
with
pore sizes and
In this work,
other
characteristics
a Na-mordenite has been used to
the reactions between boron and nitrogen compounds channels.
sites.
Modification
could
in
inside
Much work has already been done in this field
the H-form mordenite where the acidic OH groups are the resulted
reactions
is
with the hydrides SiH. and
a controlled pure size
reduction.
Thjs
on
reactive
BaH.
knowledge
be used for the preparation of substrates for gas
separa-
tion and inclusion of gases (refs. 1-2).
+Si OH
+
SiH.
2
jSi-O-SIHs
+
Ha
(2)
The reactive silane or borane groups chemisorbed to the structure cou d be further saturated with HzO, i , 3 0 ):
MeOH or amines, e.g.
(refs.
+Si-O-BHa
The
>-
NH,
+
implantation
+Si-O-BHn+NHa
of these groups could
molecules like A r from the structure.
even
exclude
small
Recently, the modification
reactions were studied using FTIH-spectroscopy w i t h photoacoustic detection (FTIR-PAS). This detector can be loaded i n a controlled atmosphere
which
allows the study of all types of
solid
mate-
rials, especially zeolites, without the need for difficult sample pretreatments.
The
consuming techniques of
time
grinding
and
pressing self-supporting wafers are evaded.
Spectral information
over
even changes in
the full l.R.-region
disks. that
can be obtained,
the
vibrations can be investigated without making use of KBr
lattice
Scattering various
effects have no influence on the
spectra
sizes of crystallites can be investigated
problems of baseline changes.
so
without
Because of these properties, spec-
tral substractions yield difference spectra of very good quality. W i t h this photoacoustic detection technique the reactivity of the
different agents
types
of structural OH groups towards
was demonstrated ( r e f .
products
4).
the
modifying
The intermediate
and
final
formed by hydroIysis and amination reactions could
determined.
The
mechanisms of most of these modification
I.R.
tions are explained using troth the photoacoustic
be reac-
spectra and
the results of adsorption and chemisorption experiments, obtained in a volumetric sorption apparatus. photoacoustic detection technique f o r solids can only
The used
when the samples do not evolve gases that show an
absorption such as H a O ,
CO,,
NHa,...
infrared
These evolved gases would
a photoacoustic signal that is about 100 times more
give
be
intens
than the signal of t h e solid material. Only gases such as He, Ne,
Ar, Nz, O a , . . . H-form
can be used inside the chamber of the detector.
zeolites,
the structure by covalent bonds. shown
of
P A S spectra
In this work,
Na-mordenite successively treated with BaH.
(NH, and CHaNH,).
In
the modifying agents are irreversibly bound to
In Na-mordenite,
and
are amine
only a physical sorption can
take place since no acidic OH-groups are present.
A dissociation
of
the diborane molecules into BHs groups occurs on the
surface
of
the
bridges
Na-mordenite.
Addition of BHa on the
siloxane
557 in a strong physical sorption t n d t can be measured
results
with
F T I R - P h S without tne disturbance o t the signal b y aesorted
dibo-
rane molecules.
sorbed tarane groups can react further with Nh,
The the
or amines
The free electron pair or the nitrogen competes with
(refs. 5 , 7 , .
or the siioxane oridges and
electrons
boron-nitrogen
com-
pounds are rormed inside the zeolite channels. These BN-compounds studied in detail by FTIR-PAS a s a function of the
are
reaction
amount o r evolved Hz and the intluence o t
temperature.
The
BN-compounds
o n tne pore size of the zeolite
the
are
investigated.
The Na-mordenite was supplied b y the Norton Co.
and the modi-
EXPERIMENTAL fying agents (BzH.,
was diluted in H2 (95%) to prevent polymerisation reac-
Diborane
It was purified b y destillation in high vacuum.
tions.
g a s e s Ar.
were
NH,) were from U C A H (Union Carbide Belgium,.
used without further purirication.
ments
T h e test
hr, Xe and Nz came from A i r Liquide (Belgium, and they
were
carried out in a volumetric
The adsorption sorption
experi-
apparatus
as
discribed in tref. 1).
FTIR
The
spectrometer
with a prototype of the
acoustic detector designed b y J.F. velocities acoustic minutes
were
used (0.08
saturation effects. and
McClelland.
0.64 cmlsec.)
FTIR-
MTEC-100
photo-
Different mirror
to check t o r
photo-
Data collection times of 20 to
were necessary.
and the single beam spectra,
ratioed the
-
averaging of 300 to 1000 scans,
signal intensitity, used,
DXB
spectra were collected with a Nicolet 5 equipped
depending
on
Happ-Genzel epodization w a s obtained in this
way,
against the PAS spectrum of carbon black to correct
source characteristics.
50 the
The samples t o r the 1 . R .
were prepared in the volumetric sorption
apparatus,
were for
analyses
transterred
to a glove b o x that was purged with ultra dry nitrogen and loaded into the P A S detector. T h e detector was, in addition, purged with dry He to increase the signal intensity.
658 R E S U L T S A N D DISCUSSION Boranation reaction The
in
hours and a n a l y s e d for its original s o r p t i o n
17
over
(3 gram) w a s outgassed a t 723 K
Na-mordenite
vacuum
characte-
ristics with A r , K r and X e a t 273 K ( k i n e t i c r u n s ) and with N a at
K (isotherms).
77
The s u b s t r a t e was t h e n equilibrated at 273
brought into c o n t a c t with 300 Torr (40 k P a ) of
and
different
contact periods.
K at
Outgassing was performed in a s t a t i c
way by freezing t h e weakly physisorbed B t H .
77
diborane
in a c o o l i n g trap
at
or in a dynamic way by e v a c u a t i o n of the s a m p l e t h r o u g n a
k;,
cooling trap in which t h e d i b o r a n e is f r o z e n out. amount of d i b o r a n e could be d e t e r m i n e d by
The
expanding
the
T h e amount of B o h b held by
frozen gas in the volumetric system.
the sample after various s o r p t i o n and o u t g a s s i n g periods is shLswn In general t h e s o r p t i o n r a t e of BIH.
in T a b l e 1 .
mostly overnight contact is applied.
that
is rather low so
T h e a e s o r p t i o n method
and time is of great importance s i n c e the s o r p t i o n is based o n physical vacuum
interaction. ot
A s expected.
t h e a p p l i c a t i o n of a d y n a m i c
only a tew m i n u t e s c a n reduce t h e amount or
B 2 H L drastically.
ments, the sample w a s saturated with dry temperature
retained
After outgassing, t h e s a m p l e was cooled d o w n to
For t h e 1 . R .
to avoid turther a e s o r p t i o n o t BIH..
77 k
a
and loaded into the glove
lqI,
box.
measure-
equilibrated at room
l.k.
The
spectra.
obtained a t 3 diiferent mirror velocities, a r e s h o w n i n i i g u r e la A broad OH band
to lc.
(3100
around the exchangeable Na'
36UO cm-'J
molecules is located at LS.?O cm.'
these
this temperature, cules. aiso
-
is attriouted to water
cations. T h e d e r o r m a t i o n vibration ot Atter a denydration at
it is observed that s o m e a c l s o r b r d water
probably around t h e cations,
a r e still present.
mole-
T'his
WAS
confirmed b y the results of a DSC, a n a l y s e s of the lua-mor,de-
nite. The presence of water m o l e c u l e s was e v e n observed o n 14-form mordenites
where
very small a m o u n t s ot Na'-cations
present d u e to incomplete ion-exchange. these carions
did
nut
react
with diborane.
lsutupe exchange procedure. while
demonstraces
all
tref.
still
bv
substrates revealed that the water m o l e c u l e s held
observed rrom the I.R.-spectra
ted
were
A boranation reaction or
3 J .
e v e n not at
A l s o in the
425
the k
as
deuterium
t h e s e water molecules & r e not a t r s c
the o t h e f hvdroxvl g r o u p s
are
exchanged.
~
Tnis
their special proper ties ot s t r o n g ci~ooroination
1.0
TABLE
.
1
Amount of BiHL sorbed as a function of
sorption
and
outgassing time ~~
amount
outgassing
sorption
time
time
Bi
Hc
(minutes1 static vacuum
(hours)
mmol/g.
dynamic vacuum
0.091
1.5
30
17
180
3
0.255
17
10
5
0.244
17
60
0.344
20
240
0.318
60
180
0.345
Temperatures higher than about 800 K are
the cations.
necessary
to evacuate these water molecules. In
photoacoustic spectra of Na-mordenite.
the
treated
with
aiborane. smal1 bands of B-H stretching vibrations ot chemisorbed -BHn
g r o u p s are seen at 2573 and 2492 c m - 1 due to the reaction or
The B-0 stretching
B2Ha with silanol groups and water molecules.
vibrations of these chemisorbed groups can be seen at 1580 - 1400 These assignments can be supported with the frequency
cm-1.
the
B-H
vibration band in the 1 . R .
mordenite L . P .
spectrum of a boranated
(figure 1 d ) (ref. 3-4).
of 0-
The 0 D bands at 2 7 6 0 and
li'v0 c m - ' are still present arter a reaction at room
temperature
which illustrates their l o w reactivity (see also ref.
4J.
In the Na-mordenire, an additional broad B-H vibration band is seen
at 2L,:6i, c m - '
bridges. wnere
EIH,
srrr.
5 ! .
does
A
whicrl we assign to BHr groups at the
comparabie rrequrncy is round for the is
mine-buranes
stabilized by the electron pair of
tne
nitrogen
In t h e spectra a. b and c. the shape or the I . f i . -bands
not change with increasing scan speed wnicn means
spectral
silohanr
region i s photoacoustical ly saturated
cnaracteristic
structure
',
vibrations
or mordenite
and around 1 V W cm"
Letween auij t.o 13ilu
i>m-
a u e to overtclrles or
lattice vibrations,
I b ~ J Vc m - ! .
!ref.
can
De
are
a broad
that G).
no The
iocateu uoublet,
noticed. At about
a small and sharp vibration band can be observed which
560
u0
fm
5 0)
m U I
U 0
1
c
3900.0
PSOO.0
3400.0
P400.0
1000.0
1400.0
BOO.00
400.00
uAvtiwuseAs tcu-1)
Fig.
(a) t o (c) : PAS s p e c t r a of borrnated Na-mordonite a t
1.
s c a n s peed 0.32
, 0.16
and 0.08 cm/reo r e s p e c t i v e l y ;
(d) : boranated H/D-mordmnitm a t ncalr s p e e d 0.08 cm/sec.
is attributed to desorbed diborane.
Nevertheless,
ducible
even after several hours
spectrum
can be obtained,
storage in the P A S detector. spectra time
in
no
a very reproor
The data collection time t o r the
Figure 1 was more than Z hours and even
changes i n the spectra were observed.
From
atter
3
that
voiumetric
sorption measurements i t was calculated that only 0.1 2 u.u3 mmol H?,y.
ii
was evolved due to the reactions with silanol groups. The
value,
which is tna ratio ot H,
smaller than 0.5
.
evolved to k h .
sorbed,
was
561
Reactions with Amines After equilibration of the boranated mordenite to room rature,
an
borane
amount
equal to the amount
NH,
of
adsorption
took
physisorbed
substrate.
place within a few seconds while
amount of HZ was evolved.
small
of
groups was brought into contact with the
tempe-
only
a
The very
This indicates that most or the
compounds tormed contain a dative bond trets. 4-51 :
\
I0
+
BHj
NHj
+
/
\
->
IUI
HxBtNHj
t
/
Only a few groups reacted further according to : -'
H31u+Uh,
H- N'EiH?
t
1-13
W
94
rn
8111
m 4 I
a a 1 I I
V
3900.0
3400.0
2900.0
2400.0
1900.0
WAVENUMBERS
Fig. NH,
2.
1400.0
900.00
400.00
EM-1)
FTIR-PAS spectra of Na-mordenite modified with B Z H L and
at (a): 298 K .
( f ) : 543 K,
tg):
(b): 353 K ,
573 K ,
( c ) : 382 K,
(h): 623 K .
(d): 423 K ,
te): 493 K
662
Further reactions or these BN-compounds.
tollowed b y a release
or H 2 . can be expressed in terms ot a relative k value 1 h 1 . 1 . This vaiue.
detined
amination was
by
reaction and the value arter the Goranat ion to be V.16 2 L..ul.
determined
sample
rhe Jirrerrnce between the I? value artrr 1.k.
reaction,
spectrum
or
figure L a shows NH-stretching and oerclrmat ion
ox
tions at 33-1 and 1616 cm-'. band
The
respectively.
A
the
new B-rl
tne
vinra-
vibration -
at 2350 cm-l and a small 6-N vibration band at 13.65
luUC3
cm-'
can be noticed. however, the frequency and shape ot the m a i n
6-H
vibration
tiana
does
not crlanqe very
explained b y the type or bond : interaction is
This
can
b y the nitrogen.
amine-boranes.
The compounds formed
donor)
are
called
F r o m the adsorption ~ i n e t i c s it can be seen
tnat
boron-nitrogen compounds completely block the channels f o r
molecules such as Xe while for hr an adsorption capacicy CRti 7
oe
i t is an electron donor-acceptor
in which the structural oxygen (the original
replaced
these
much.
of the original capacity is obtained atter 1 hour
X
time (iigure
of
sorption
4).
Thermal treatment After was
the reaction w i t h NHs at room temperature,
heated
bands,
the
properties.
as
a
the
stepwise in order to follow the change of evolution of Ht and the variations or Figure
function
the
sample I.H.
the
sorption
5 shows the increase of the relative I3 value
of the temperature
for
both
NH,-
and
CHINH1-
treated samples. T h e curves show a sharp increase ot the Hr vaiue between
343 k: and 423 K for NHs and between 373 K and 423
CHsNH,.
This
indicates
further reactions of the
I(
for
amine-boranes
into amino-boranes acoording t o :
The pound
higher thermal stability of the
methylamine-borane
com-
can easily be explained by the inductive effect of the CH,
group on the dative bond ( r e f s . 5 , 7 ) . The I . R . the
spectra of an NHs-treated sample shows that most
B-H and N-H vibrations decreased in intensity after heating.
A thermal treatment at 423 K caused a shift of the N-H band
of
vibration
to 3440 cm-' and the B-H vibration was split into two broad
bands at 2 5 3 7 and 2 5 5 2
C I I I - ~ .During
this treatment i t is clearly
663
computed ditference spectra (figure 3 ) that
trom
seen
vibration
band
shirts to nigher rrequencies tup to
the
1500
w h i c n indicates a higher bond order ( s e e reaction 9).
b-N ern-')
but i t
is
also noticed that a low-frequency k i - N vibration band appears, and
this is attributed to conjugation of B - N bonds.
A polymerisation
taken p l a c e which thermodynamically f a v o r s the stability
has
the amino-boranes. tncir H atoms,
all
of
This efrect prevents these products releasing and above 423 k only a small increase in the
relative k value is observed ( f i g u r e 51. Arter
a thermal treatment at 623 K .
vibration bands disappeared. nitride type krom
trir
3900.0
Of
kinetics
2900.0
it
2400.0
c a n be noticed that.
1900.0
WAVENUMBERS
Fig.
the N -H and
B-H
boron-
compound is formed inside tne channels.
s.arption
3400.0
nearly R I I
Thus w e can conclude that a
1400.0
the
900.00
amino-
400.00
(CM-1)
3. Difference spectra of Na-mordenite modified with BzH6 and NHs and thermally treated
(2) : 423 K
-
382 K
,
( 1 ) t 382
(3) : 493 K
K
-
-
352 K
423 K
.
,
564 borane polymers are less e f f e c t i v e in blocKing the z e o l i t e nels
since
the
r,r uptake increases
with
increasing
cnan-
reaction
temperature. This i s causea b y shortening ot t h e ti-lu bond d i s t a n a n a possibly b y reorientation ot t h o s e c o m p o u n d s inside
ces
the
z e o I i te pores. should
It
be
mentioned that a c h a n g e in
drastically changes the a m i n a t i o n reactions.
in
the
heating
rate
T n i s is illustrated
tigure 1; where spectrum c a ~s h o w s t h e c h a r a c t e r i s t i c s ot
the
zeolite atter a slow heating u p to 623 L.; in vacuum
and
mociiried
spectrum ( b l atter a thermal shock treatment a t the same t e m p e r a ture
in vacuum.
T h i s o b s e r v a t i o n indicates that
porymerisation
c a n occur before H1 is split off. T h e s p e c t r u m is very similar t o o r a s a m p l e treated a t 423 li a t a
that
effecrs
IOU
heating
rate.
These
are e v e n more pronounced when t h e s a m p l e is treated with
a thermal shock up to 675
I.;
in a static vacuum a s shown in f i g u r e
6c. The t i - N stretching vibration shifts to higher f r e q u e n c i e s ( u p
to
1470
cm-', while still a significant a m o u n t ot 6 - H
N-H
and
groups a r e present a s indicated by their intense vibration bands.
Fig.
0.8
P
K r u p t a k e a t 273
4.
after
different
tion steps, uptake
of
sample
(=
mordenite)
0.8
*
0.4
+ 0.2
ORIGINAL +
BPH8
-e
313 K
A
443K
-EJ-
513 K
+
NH3
-+ 688K + HYDROLYSIS
0.0 0 1 2 3 4 5 6 7 8 9
time expV2
K
modifica-
relative to the the
original
outgassed
Na-
565
ELATIVE R-VALUE Fig. 5. Relative R value
of the reac-
tion between nated and
bora-
Na-mardenite
as
amine
a
function
of
reaction
tempera-
the
ture.
* NH3 -A. CH3NH2 273
373
473
673
673
773
Temperature (K)
bl
0
z 4
: 0 VI
m
d I
d
a' 1 I I
v
SSOO.0
3400.0
2900.0
2400.0
1900.0
WAVCNUMBERS
Fig.
6.
FTIR-PAS
spectra
1400.0
900.00
400.00
(CM-1)
of ( a ) and ( b ) :
Na-mordenite
with
diborane and ammonia heated at 623 K respectively at low and high heating rate in vacuum; in static vacuum.
( c ) heated at 673 K at high heating
rate
566
However, at
298
K
the formed EN-compounds c a n be partially in contact with water.
As shown
adsorption capacity for K r is again 80 although
the
X
in
hydrolyzed
figure
4,
the
of t h e original capacity
Xe-adsorption is still controlled by
a
diftusion
process. CONCLUSION Intra-red
studies
o n solid materials such a s
their interaction with gases, very
important
sample be
eorbents,
and
liquias or even other solids,
are
when the samples can
be
characterized
without
In this w o r k it is proven that F T I H - P A S can
preparation.
used to elucidate the mechanisms ot the reactions inside of a zeolite.
channels physicallv ration, with
It i s shown that sorbents
with
sorbed species d o not cause problems r o r signal gene-
so that the sorbent-sorbate interactions c a n be
PAS.
the
strong,
The
I.H.
studied
spectra or the Na-mordenite moditied
various types of bN-compounds help t o explain the results ot
s o r p t i o n ex per 1 men t s
.
with the
REFEkENC€S 1
2 3 4
5 6
7
A. 'Thijs. G. Peeters. E . F . Vansant and 1 . Verhaert. Journ. Cnem. SOC.. Faraday Trans. 1. 1 9 W . 2821. A. Thijs, G. Peeters. E.F. Vansant and I. Verhaert, Journ. ihem. SOC.. Faraday rrans. 1 , 2,l Y t i . 3 . 28.35. J. Fhilippaerts, E . F . Vansant, i;. Peeters and E. Vanderneydrn. Anal. Chim. Acta, 165, 1967, 237. J . Philippaerts, E .F. Vansant, in D . E . Levden (Editor,. Silanes, Surfaces and lntertaces U o l . 2 . 1963 ( i n p r e s s ) . W. Sawodny, J. Goubeau. Leitschr. Fur Phvsic. Chem. Neue Folge, bd. 44, 1965, 227. A . hosencwaig. Photoacoustics and Photoacaustic s p e c ~ r o s c o p ~ . Wiley Interscience. New 'iork , 1680. h.C. Taylor. A d v . in Chemistry 4.Z, 1964, 59.
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ADSORPTION PROPERTIES OF LARGE CRYSTALS OF ZSM-5 ZEOLITE AS A FUNCTION OF THE
DEGREE OF DEALUMINATION
J. KORNATOWSKI,
M. ROZWADOWSKI, A. G U T S Z E ~AND K.E.
WISNIEWSKI
I n s t i t u t e o f Chemistry, Nicolaus Copernicus U n i v e r s i t y , Gagarina 7, 87-100 Torun (Pol and) 'Department (Pol and)
of
Biophysics,
Medical Academy,
Karlowicza 24,
85-092
Bydgoszcz
ABSTRACT Large c r y s t a l s o f ZSM-5 z e o l i t e o f mean length o f about 300 pmwere grown. The z e o l i t e samples were dealuminated t o various extents by both hydrothermal and chemical a c i d and s a l t s o l u t i o n s treatments. Their adsorption isotherms f o r benzene and methanol were examined as a f u n c t i o n o f the degree o f dealumination. The benzene isotherms were described w i t h equations o f t h e Polanyi-Dubinin pot e n t i a l theory. Using the determined parameter values, the f u n c t i o n s c h a r a c t e r i z i n g the adsorption p o t e n t i a l s were p l o t t e d . INTRODUCTON Presentation o f a c o r r e c t mechanism f o r an adsorption process i s one o f t h e
most important aims o f adsorption theory. An explanation o f t h e mechanism o f ads o r p t i o n on ZSM-5 z e o l i t e s i s s t i l l an unsolved t h e o r e t i c a l problem. The few papers dealing w i t h the adsorption on ZSM-5 ( r e f s .
1-3) are fragmentary and they
show e x p l i c i t l y t h e considerable d i f f i c u l t i e s w i t h i n t e r p r e t a t i o n o f t h e adsorpt i o n measurements. I n adsorption
investigations,
the
Pol anyi -Dubi n i n p o t e n t i a1 theor
' (PO) ( r e f s . 4, 5), developed by Dubinin ( r e f . 6) as the theory o f volume f i l i n g o f
micropores,
(TVFM) i s p a r t i c u l a r l y important.
I t s fundamental equation i s t h e
Dubinin-Astakhov equation (DA) ( r e f . 7):
W = W oexp I-(AIPEU)nI = Wo exp I - k(AIP)"/ where W represents the volume o f the l i q u i d - l i k e adsorbate present i n micropores a t temperature T and r e l a t i v e presssure p/ps; Wo i s the t o t a l volume o f t h e m i cropores, A i s t h e d i f f e r e n t i a l molar f r e e enthalpy o f adsorption, i l a r i t y f a c t o r r e f l e c t i n g the nature o f the adsorbate, energy o f adsorption o f standard vapour f o r which
p=
P
i s the sim-
Eo i s the c h a r a c t e r i s t i c 1, k i s the s t r u c t u r a l pa-
rameter c o r r e l a t e d w i t h the micropore dimensions, and n i s the parameter charact e r i z i n g the shape o f the adsorption p o t e n t i a l d i s t r i b u t i o n i n microporous adsorbent.
568
The volume filling of micropores in A and X zeolites during adsorption of different vapours of relatively small molecules, e.g. water, n-pentane etc. was already proved experimentally (refs. 8, 9). Recently, the applicability of eqn. (1) for studies of adsorption on zeolites was proven in the papers of refs. 10-13. ZSM-5 and the other zeolites of the pentasil family are still intensively studied because of their adsorptive and catalytic properties (ref. 14). However, to date only little attention was paid in the literature to the dealumination of those zeolites (refs. 15-20), In 1984, Scherzer (ref. 21) presented an extensive review of both the preparation of aluminium-deficient zeolites and their characterization including structural and physico-chemical characteristics (adsorption, thermal stability, NMR, IR, acid and catalytic properties) for Y and mordenite zeolites. The preparation methods were divided into three groups: a) thermal and/or hydrothermal dealumination, b) chemical dealurnination (reactions with acids, salts, chelating agents and volatile compounds), and c) combination of thermal and chemical dealumination. Each of them can produce a different dealumination effect. The dealurnination of zeolites influences significantly the behaviour of the zeolite structures (refs. 15-21). Furthermore, the properties of zeolites depend on the dimensions of zeolite crystals (refs. 22-25) due to the role of diffusion of adsorbate molecules. Therefore, we decided to search for a correlation between adsorption properties and the degree of dealumination in large ZSM-5 crystals where the extraction of aluminium ions from the lattice forms aluminiumdeficient centers of a disturbed structure. EXPERIMENT Large crystals of ZSM-5 zeolite were grown according to the procedure described in ref. 26. Their crystallinity was examined by XRO techniques (SovietUnion-made ORON) and by scanning electron microscopy (NOVOSCAN 30). The amount of aluminium in the samples was determined from both the elemental analysis and the titration of solutions after dealumination treatment with 0.002 M EDTA solution, using chromazurol S as the indicator (ref. 27). The parent sample (K4) was prepared by calcination of as-prepared ZSM-5 zeolite in a quartz tube at about 830 K first under flowing air for 24 hrs and next under flowing oxygen for 72 hrs. Samples A02 and A05 were dealuminated by leaching of the K4 with 1.25 M HCl at the boiling point for 2 and 5 hrs, respectively, and fivefold washing with water at the boiling point for 15 minutes. The sample US2 was prepared by leaching of the K4 with 1.25 M HC1 followed by steaming in a quartz tube at about 1070 K for 6 hrs; the flow rate of water vapour was about 70 g h-l. After a final leaching with 1.25 M HNO3 at the boiling point for 5 hrs the samples were washed as before. The sample US3 was prepared from the K4 by ion exchange in 2 M NH4N03 solution at room temperature for
.
569
70 hrs followed by steaming at about 990 K for 65 hrs, leaching with HNO3 and washing as with the US2 sample. The SD sample was prepared by threefold leaching of the K 4 with 0.3 M AlCl3 solution for 10 hrs at room temperature and intensive agitation. Adsorption isotherms for spectroscopically pure benzene and methanol vapours were determined at a temperature of 298.2 K in a vacuum device equipped with a McBain balance. The pressure within the range to 5 * kPa was measured with a resistance vacuum meter rescaled for the adsorbates studied by means of the compensation method (McLeod vacuum meter). Higher pressures were determined with a differential vacuum meter. Before the adsorption measurements, the zeolite samples were heated under stationary vacuum of about kPa at 700 K to constant mass (about 8 hrs) Thermogravimetric analyses (TGA) were carried out for 100 mg samples within the temperature range 295-1273 K at a heating rate of 5 K/min, under flowing air atmosphere. The equipment was from the company MOM, Hungary.
.
RESULTS AND DISCUSSION The synthesized zeolite was fully crystalline and a pure ZSM-5 phase (ref. 26). X-ray spectra of the zeolite exhibited lines in positions typical for ZSM-5. SEM photomicrographs showed a regular columnar habit of the crystals with very rare twins and intergrowths. Most of the crystals had a mean length of about 300 pm within the range 280-320 pm. The full range of dimensions was 200 to 350 pm. The results of dealumination are listed in Table 1. Isotherms of benzene adsorption on the samples studied are presented in Fig. 1.
1.5
-p
UI
'L
1.0
n' u ' W
4
a 0
0.5
U
0.0 0.0
0.2
0.4 0.6 0.8 1.0 R E L A T I V E P R E S S U R E , P/PI
1.2
Fig. 1. Isotherms of benzene adsorption on dealuminated samples of large ZSM-5 zeolite crystals: 1 - K4, 2 - AD2, 3 - AD5, 4 - US2, 5 - US3, 6 - SD.
570
The applicability of eqn. 1 (DA) for description of benzene isotherms (Fig. 1) is supported by the values of the determination coefficients OC (ref. 28) presented in Table 2. The determination coefficients are defined by N
c (Y,-$P
i=l
DC=?=l
N
2
(yl
-yP
i=l
where r represents the correlation coefficient, yi the dependent variable, ji the value of the dependent variable approximated by the method of least squares, .. y the arithmetic mean value of the dependent variable, and N is the number of pairs of experimental data. TABLE 1 Characteristics of dealuminated samples of large ZSM-5 crystals. -
Sample
-
- - . -_
--
Composition. wt % Fi02 A1203 Nap0
_ --
-_ K4 AD2 AD 5 us2 us3 SD
97.7 98.9 99.1 99.3 98.7 98.5
2.10 0.90 0.70 0.60 1.05 1.25
Deal umi nat ion, % e 1 ement a1 analysis -
.
Si02 *l2O3 -
-
0.200 0.020 0.020 0.005 0.030 0.020
. ... ..
-
79.1 186.8 240.7 281.4 159.8 134.0
57 67 71 50 40
TABLE 2 Values of the determination coefficients (DC) for the descriptions of the isotherms of benzene adsorption on dealuminated ZSM-5 samples calculated with use of eqns. 1, 3, 4 (see below). Determination Coefficients for
Sample
K4 AD2 AD 5 us2
us3 SD
eqn. 1
eqn. 3
0.98031 0.981 39 0.94265 0.95640 0.92834 0.98976
0.99908 0.99933 0.99998 0.99988 0.99981 0.99989
eqn.
4
0.99992 0.99939 0.99998 0.9999 1 0.99982 0.99990
571
A more general equation resulting from the PD theory is the Dubinin-Radushke-
vich-Stoeckli equation (ref. 29) which for zeolites, where the parameter n from the DA equation i s usually greater than 2, can be written in the following form (ref. 28):
where A is the half-width o f the distribution function f (k) and ko represents the maximum of the function of micropore volume distribution with respect to k. When the value o f parameter k is included, which corresponds to the maximum dimension o f the micropores, i.e. kmax (ref. 30), eqn. 3 is transformed into the following isotherm equation:
where Y = The data from Table 2 show that eqns. 3 and 4 are applicable to describe the isotherms of benzene adsorption on the studied zeolites. In accordance with the PD theory, the adsorption is determined by the adsorption potential. As is inferred from refs. 13. 31-33, the distribution of the adsorption potential is closely connected with the heterogeneity of microporous adsorbents. This distribution has the following forms, corresponding to eqns. 1, 3 and 4, respectively. X ( A ) = - dW/dA = ( n W , k An-'/ 0") expl- k (AIflPI
where v = e r f l k , / A m
+ erflk,,,,
-kOlAm.
(5)
572
The functions of the adsorption potential distribution X(A) for the isotherms of benzene adsorption (Fig. l ) , obtained from eqns. 5 and 7, are presented in Figs. 2 and 3, respectively.
Fig. 2. Functions of the adsorption potential distribution obtained from eqn. 5: 1 - K4, 2 AD2, 3 - AD5, 4 - US2, 5 - US3, 6 - SD.
-
Fig. 3. Functions of the adsorption potential distribution obtained from eqn. 7: 1 - K4, 2 - AD2, 3 - AD5, 4 - US29 5 - US3, 6 - SD.
The curves demonstrate a pronounced effect of the ZSM-5 zeolite dealumination on the adsorption potential. Short acid treatment (sample AD2, Table 2, Figs. 2, 3) causes relatively small dealumination and decreases the adsorption without a change of the potential. This can be caused by leaching o f A1 atoms from the center of the crystal and deposition close to the outer shells, which finally hinders benzene molecules from penetrating the channels. Longer acid treatment (sample AD5, Table 2, Figs. 2, 3) causes much higher dealumination and increases adsorption with parallel increase of the adsorption potential. This indicates that new micropores through Al-vacancies are created and the mentioned hindrance for benzene penetration is removed. Acid leaching followed by steaming and a second acid leaching (sample US2, Table 2, Figs. 2, 3) causes the highest dealumination and the greatest decrease of the adsorption potential. This might suggest a considerable increase of the adsorption volume of micropores. Steaming without prior acid leaching (sample US3, Table 2, Figs. 2, 3) leads to lower dealumination, which does not change the adsorption potential but increases the
573
adsorption at lower relative pressures. This could indicate very easy penetration of benzene molecules. Salt dealumination (sample SO, Table 2, figs. 2, 3) leads to the lowest dealurnination. resulting in effects similar to but lower than the short acid deal umi nat ion. Investigations of methanol adsorption did not show any significant influence of the dealumination on the adsorption properties of the zeolite. The above equations did not provide a correct description of the isotherms. XRD analysis of the series of differently dealurninated ZSM-5 zeolite samples revealed complete maintenance of the crystal 1 ine structure. The dimensions of the crystals remained also unchanged. TGA measurements supported the conclusions derived from the adsorption data about changes of micropore structure. The detailed XRD and TGA investigations will be published separately. ACKNOWLEDGEMENT The work was partially supported by the Polish Ministry of National Education within the Project CPBP 01.06. REFERENCES 1
Y.H.
Ma, T.D. Tang, L.B. Sand and L.H. HOU, Proc. 7th Int. Zeolite Conf., Murakami et al, Eds.), Kodansha, Tokyo, 1986, pp. 531-538 and refs. 2-6 therei n. Y.H. Ma. Proc. Eng. found Conf. 1983, (A.L. Myers and G. Belforz, Eds.), New York. 1984. pp. 315-324. A.N. Kotasthane, V.P. Shiralkar, S.G. Hegde and S.B. Kulkarni, Zeolites 6 (1986) 253. M.M. Dubinin, Chemistry and Physics of Carbon, (P.L. Walker, Jr., Ed.), Marcel Dekker, New York, 1966, Vol. 2, pp. 51-120. M.M. Dubinin, Progress in Membrane and Surface Science, (D.A. Cadenhead, Ed.), Academic Press, New York, 1975, Vol. 9, pp. 1-70. M.M. Dubinin, Zh. Fiz. Khim. 39 (1965) 1305. M.M. Dubinin and V.A. Astakhov, Izv. Akad. Nauk SSSR, Ser. Khim (1971) 5. M.M. Dubinin, J.G. Zhukovskaia and K.O. Murdmaa, Izv. Akad. Nauk SSSR, Ser. Khim (1966) 620. M.M. Dubinin, J.G. Zhukovskaia and K.O. Murdmaa, Izv. Akad. Nauk SSSR, Ser. Khim (1966) 802. U. Lohse, 6. Engelhardt and V. Patzelova, Zeolites 4 (1984) 163. U. Lohse, G. Engelhardt, E. Alsdorf, P. Kolsch, M. Feist and V. Patzelova, Ads. Sci. Technol. 3 (1986) 149. U. Lohse, J. Richter-Mendau and V. Patzelova, Ads. Sci. Technol. 3 (1986) 173. M. Jaroniec, J. Piotrowska, M. Bulow and 6. Finger, Adsorption in Microporous Adsorbents-Workshop 111, Academy of Sciences of the GDR, 1987, Vol. 2, pp. 41-48. M. Bulow, H. Schlodder, L.V.C. Rees and R.E. Richards, Proc. 7th Int. Zeolite Conf., (Y. Murakami et al., Eds.), Kodansha, Tokyo, 1986, p. 579 and refs. therein. P.A. Jacobs, M. Tielen, J.B. Nagy, 6. Debris, E.G. Derouane and 2. Gabelica, Proc. 6th Int. Zeolite Conf., (D.H. Olson and A. Bisio, Eds.), Butterworth, Guildford, 1984, p. 783. J.A. Lercher, 6. Rumplmayr and H. Noller, Proc. Int. Symp. on Zeolite Catalysis, Siofok, 1985, p. 71. R. von Ballmoos, Dissertation, Verlag Sauerlander, Aarau, 1981, pp. 155-177. S. Namba, A. Inaka and T. Yashima, Zeolites 6 (1986) 107. (Y.
2
3 4 5
6 7 8
9 10 11 12 13 14 15 16 17 18
19 6. Engelhardt, H.G. Jerschkewitz, U. Lohse, P. Sarv, A. Samoson and E Lippmaa, Zeolites 7 (1987) 289. 20 H. Hamdan and J. Klinowski, Chem. Phys. Lett. 139 (1987) 576. 21 J. Scherzer, Catalytic Materials, ACS Symp. Series 248 (1984) 157-200. 22 L.B. Sand, Proc. 5th Int. Zeolite Conf.,(L.V.C. Rees, Ed.), Heyden, London. 1980. p. 1. 23 A. Nastro and L.B. Sand, Zeolites 3 (1983) 57. 24 R. Mostowicz and L.B. Sand, Zeolites 3 (1983) 219. 25 P. Ratnasamy, G.P. Babu, A.J. Chandwadkar and S.B. Kulkarni, Zeolites 6 (1986) 98. 26 J. Kornatowski, Zeolites 8 (1988) 77. 27 M. Theis, Z. anal. Chem. 144 (1955) 106. 28 M. Rozwadowski and R. Wojsz, Carbon 22 (1984) 363. 29 M.M. Dubinin and H.F. Stoeckli, J. Colloid Interface Sci. 75 (1980) 34. 30 M. Rozwadowski, R. Wojsz, K.E. Wiiniewski and J. Kornatowski. Zeolites, submitted for publication. 31 H. Marsh and B. Rand, J. Colloid Interface Sci. 33 (1970) 101. 32 J. Choma, M. Jaroniec and J. Piotrowska, Carbon 26 (1980) 1. 33 M. Jaroniec, Langmuir 3 (1987) 795.
H.G. Karge, J. Weitkamp (Editors),Zeolites (IS Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands
-
SELF-CONSISTENT-CHARGE Xa CALCULATIONS ON SORPTION COMPLEXES OF NITROUS
OXIDE
ATTACHED TO T R A N S I T I O N METAL OCCUPIED ZEOLITE CLUSTERS 0. Zakharieva-Pencheva',
M. G r o d z i c k i ' and H. F o r s t e r 3
' P h y s i c a l F a c u l t y , U n i v e r s i t y o f S o f i a , S o f i a 1126, B u l g a r i a '1. I n s t i t u t e o f T h e o r e t i c a l Physics, U n i v e r s i t y o f Hamburg, 0-2000 Hamburg 36, West Germany 3 1 n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y o f Hamburg, D-2000 Hamburg 13, West Germany ABSTRACT Quantum chemical SCC-Xa MO c a l c u l a t i o n s were c a r r i e d o u t on an a l u m i n o s i l i c a t e s i x - r i n g c l u s t e r occupied by n i c k e l o r copper i o n s , approached by a n i t r o u s o x i d e molecule i n N- o r 0 - f a c e d i n t e r a c t i o n i n o r d e r t o f a c i l i t a t e t h e assignment o f i n f r a r e d bands. I n a l l cases s t a b l e s o r p t i o n com l e x e s a r e found w i t h s t r o n g e r bonding i n t h e case o f n i c k e l and t h e M +-NNO s p e c i e s . E l e c t r o n i c s t r u c t u r e and charge d i s t r i b u t i o n a r e analyzed, r e v e a l i n g an N-N t o N-0 bond charge r e d i s t r i b u t i o n f o r b o t h o r i e n t a t i o n s on n i c k e l , w h i l e a loss o f e l e c t r o n i c charge f o r t h e copper species r e s u l t s . The s p l i t t i n g o f t h e wI and t h e frequency s h i f t o f t h e w 3 fundamental band o f N20 on n i c k e l i o n exchanged z e o l i t e A may be e x p l a i n e d by t h e d i f f e r e n t o v e r l a p p o p u l a t i o n s o f t h e bonds.
R
INTRODUCTION The a n a l y s i s o f i n t r a z e o l i t i c compounds can be c o n s i d e r a b l y f a c i l i t a t e d quantum chemical assumed
c a l c u l a t i o n s y i e l d i n g i n f o r m a t i o n on i )
d i f f e r e n t a d s o r p t i o n geometries,
orbitals
the
i i ) the constitution
i n v o l v e d i n t h e a d s o r p t i o n process,
and i i i ) t h e
by
stability of
of
molecular
strengthening
weakening o f t h e adsorbate i n t e r n a l bonds i n comparison t o t h e f r e e
or
molecule.
The s p e c t r a l changes upon a d s o r p t i o n become i n c r e a s i n g l y complex w i t h a
grow-
i n g number o f atoms i n t h e adsorbate. Even w i t h t r i a t o m i c s such as C02 and N20 i n t e r p r e t a t i o n o f t h e r e c o r d e d s p e c t r a t u r n s o u t t o be r a t h e r
the Thus,
difficult.
f o r an u n d e r s t a n d i n g o f t h e s p e c t r a l changes, quantum chemical i n v e s t i -
g a t i o n s a r e compulsory due t o t h e a m b i g u i t y o f t h e e x p e r i m e n t a l
results.
The
o b j e c t i v e o f t h i s paper was t o g i v e some a s s i s t a n c e t o t h e assignment o f bands observed i n t h e i n f r a r e d s p e c t r a o f t r a n s i t i o n - m e t a l ion-exchanged z e o l i t e s
A
when t h e y were exposed t o N20 as a probe. PROCEDURE SCC-Xa c a l c u l a t i o n s were performed on a z e o l i t e s i x - r i n g c l u s t e r ,
by
e i t h e r a Ni2+,
either
by
Cu2' o r Cut i o n ,
approached by a n i t r o u s
i t s t e r m i n a l n i t r o g e n o r oxygen atom (see F i g .
oxide
l ) , as
occupied molecule has
been
576 described i n d e t a i l i n r e f s .
2)
1 and 2. A c c o r d i n g t o o u r p r e v i o u s r e s u l t s ( r e f .
a l i n e a r arrangement w i t h t h e c a t i o n was assumed.
sorption
i n f r o n t o f S1 cations i n zeolite A o r N20 molecule and t h e b a r e c l u s t e r a r e
complex
faujasites.
This c l u s t e r
The
S2
free
2-M-NNO
models cations
calculated
a
in for
2-M-ONN
F i g . 1. SCHAKAL drawing o f t h e z e o l i t e s i x - r i n g w h i c h i s o c c u p i e d by a t r a n s i t i o n metal i o n and used as a model c l u s t e r f o r quantum chemical c a l c u l a t i o n s . The c l u s t e r i s approached by an N20 m o l e c u l e i n -NNO o r -0" end-on o r i e n t a t i o n . THEORETICAL RESULTS Quantum chemical
calculations
s t r u c t u r e o f the molecular o r b i t a l s ,
a l l o w c o n c l u s i o n s t o be
about
the
t h e charge d e n s i t y d i s t r i b u t i o n and
the
a d s o r p t i o n geometry by comparing r e l a t i v e t o t a l e n e r g i e s .
drawn
Our f i n d i n g s can be
summarized as f o l l o w s . Table 1. Percentage o f a t o m i c c o n t r i b u t i o n s t o t h e 30, 40 and 2n o r b ' t a l s o f N20, f r e e and adsorbed on t h e model c l u s t e r o c c u p i e d w i t h N i
E+ .
30 E(eV)
Ni-NNO Ni-ONN NNO 25.54 27.46 20.30
........................... Nc 2s 2P N t 2s
0
2p 2s
2P N i 4s 4P 3d
-
31 56 3
-
1
6
1 4 3 26 57 1 2 5
5 3 6 1 42 43
40 E(eV)
Ni-NNO Ni-ONN NNO 19.13 17.02 17.46
--_-_--_-__________________ Nc 2s
2P
3 3
N t 2s 2P 0 2s 2p
2 40 52
-
2 1 46 49 1
2 38 52 1 7
1. The
strongest i n t e r a c t i o n s i n a l l c o n f i g u r a t i o n s take place v i a
(symmetry
2n) o f N20 ( c f .
Table 1).
I n the
gas
phase
predominantly
oxygen c h a r a c t e r ,
the
n i t r o g e n (Nt) c o n t r i b u t e a l m o s t u n i f o r m l y
terminal
a d d i t i o n , oxygen
this
MO has
whereas i n t h e adsorbed s t a t e oxygen to
this
and
MO.
In
atoms f r o m t h e z e o l i t e s k e l e t o n as w e l l as 3dxz and
o r b i t a l s o f N i 2 + p a r t i c i p a t e s i g n i f i c a n t l y i n t h i s MO, d e s t a b i l i z a t i o n of
HOMO
the
3dyz leading t o a s l i g h t
t h e r e s p e c t i v e N i 3d l e v e l s (see F i g .
2).
The
la
MO
shows a s i m i l a r b u t l e s s pronounced s t r u c t u r e .
E 4'.
-10,:
- 11.0
-11.j
Ni-zodite
cluster -NNO
Ni-zeolite cluster
Ni-zeolite Cluster-ONN
F i g . 2. N i c k e l 3d o r b i t a l s i n t h e b a r e zeol i t e c l u s t e r and t h e i r change upon i n t e r a c t i o n w i t h n i t r o u s o x i d e i n a l i n e a r -NNO and -0" arrangement. The
p a r t i c i p a t i o n o f 3dxz and 3dyz o r b i t a l s i n t h e case o f a d s o r p t i o n
via
nitrogen i s considerably higher than i n the reversed o r i e n t a t i o n . This i s a h i n t t o a s t r o n g e r Ni-adatom i n t e r a c t i o n i n Z-Ni-NNO compared w i t h Z-Ni-ONN i n t e r a c t i o n and w i l l be confirmed by subsequent charge d e n s i t y a n a l y s i s .
2. I n f r e e n i t r o u s oxide,
f r o m t h e two h i g h e s t occupied a MOs, 30 and 4a, t h e
f i r s t one i s an almost i d e a l oxygen sp h y b r i d ,
w h i l e t h e second one i s
an
approximate sp h y b r i d o f t h e t e r m i n a l n i t r o g e n atom ( c f . Table 1). A s t r o n g r e o r g a n i z a t i o n o f these l e v e l s , depending on t h e o r i e n t a t i o n o f N20, o c c u r s in
t h e adsorbed s t a t e due t o t h e s t r o n g i n t e r a c t i o n
with
N i 3dZz. Accor-
d i n g l y t h e N i 3dZ2 l e v e l i s d e s t a b i l i z e d by a p p r o x i m a t e l y 1 eV f o r adsorp-
578
579
t i o n v i a b o t h Nt and 0. However, f o r m a t i o n o f an N-bound s p e c i e s s t a b i l i z e s the
nitrogen
interaction, oxygen
sp
sp
8
h y b r i d (40 i n t h e f r e e m o l e c u l e ) by
eV
due
r e s u l t i n g i n an i n t e r c h a n g e w i t h t h e ( s l i g h t l y hybrid
(3u i n t h e f r e e
observed w i t h t h e 0-bound species,
molecule).
The
to this
destabilized)
analogous
effect
is
s t a b i l i z i n g t h e 30- MO by 7 eV w h i l e t h e
40 l e v e l remains p r a c t i c a l l y u n a f f e c t e d .
Hence,
t h e 3a o r b i t a l i s
always
p a r t o f t h a t atom f a c i n g t h e n i c k e l i o n . be
t h e n e x t s t e p t h e charge d i s t r i b u t i o n f r o m SCC-Xa c a l c u l a t i o n s w i l l
In
analyzed.
can
The most i m p o r t a n t r e s u l t s a r e d i s p l a y e d i n Tables 2 and 3 and
be expressed as f o l l o w s :
1. D i f f e r e n c e s i n t h e e l e c t r o n i c s t r u c t u r e o f t h e Ni2' caused
by
4s,
4p and 3d e l e c t r o n s .
i o n are
In particular,
simultaneously
a transfer
e l e c t r o n s f r o m 3d t o 4p s h o u l d be mentioned due t o t h e h i g h e r
of
0.13
coordination
i n t h e s o r p t i o n complex. 2. The
0-
t o t a l charge Qmol o f t h e N20 m o l e c u l e becomes more p o s i t i v e i n t h e
bound compound. 3. P o s i t i v e o v e r l a p p o p u l a t i o n s between n i c k e l and t h e a d j a c e n t atom a r e q u i t e w e l l pronounced i n b o t h cases o f o r i e n t a t i o n and a r e l a r g e r compared t o ads o r p t i o n i n f r o n t o f c a l c i u m (see r e f . 3 ) . Moreover, t h e N i - N t o v e r l a p pop u l a t i o n exceeds t h a t o f N i - 0 by 20%, p r o v i n g a s t r o n g e r bonding v i a n i t r o gen.
Correspondingly,
t h e p o t e n t i a l c u r v e s show t h e same t r e n d ,
i . e . the
minimum t u r n s o u t t o be deeper f o r t h e Ni-NNO c o n f i g u r a t i o n ( c f . T a b l e 2 ) . 4. A
bond-bond charge t r a n s f e r t a k e s p l a c e w i t h i n t h e adsorbed m o l e c u l e
t h e Nt-Nc f e r s from
t o t h e Nc-0 bond i n b o t h cases o f attachment. t h a t found w i t h t h e Ca c l u s t e r ,
from
This f i n d i n g
i n which case a
charge
diftrans-
f e r was observed always f r o m t h e a d j o i n i n g t o t h e t e r m i n a l bond. Concerning with the
the
copper-exchanged z e o l i t e s t h e v a r i o u s
molecular . o r b i t a l s
predominant N20 p a r t i c i p a t i o n show e s s e n t i a l l y t h e same f e a t u r e s case o f n i c k e l .
differences
as
Compared t o n i c k e l t h e r e a r e s i m i l a r i t i e s b u t a l s o
i n t h e charge d i s t r i b u t i o n s ( c f .
Table 4 and
5),
yielding
in some the
following results:
1. Approaching t h e Cut s i x - r i n g c l u s t e r b y N20 two s t a b l e s o r p t i o n complexes i n a Z-CU-NNO and Z-CU-ONN a l i g n m e n t may be formed, as can be d e r i v e d f r o m t h e minima o f t o t a l energy a t Cu-Nt and Cu-0 d i s t a n c e s o f 1.64 i. 2. I n
accordance w i t h t h e Ni2'-exchanged
c l u s t e r the larger
charge
transfer
f r o m N20 t o Cu t a k e s p l a c e i n t h e Z-CU-ONN complex. 3. I n
b o t h species t h e adsorbed molecules a r e p o s i t i v e l y charged,
nouncedly while
f o r Z-CU-ONN where a l l adsorbate atoms
lose
more
i n case o f t h e Z-Cu-NNO t h e l o s s o f charge on t h e 0 and Nc atoms
p a r t l y compensated by t h e g a i n o f charge on t h e Nt atom.
pro-
e l e c t r o n i c charge; is
580
TABLE 4 E f f e c t i v e atomic and m o l e c u l a r charges Q, e l e c t r o n o c c u p a t i o n numbers AOs xs and x o v e r l a p p o p u l a t i o n x and t o t a l e n e r g i e s E o f 2-CU-NNO. P'
of
581
4. F o r b o t h s o r p t i o n complexes p o s i t i v e o v e r l a p charges were o b t a i n e d Cu and t h e a d j a c e n t atom of N20.
o f Z-CU-NNO
between
The o v e r l a p charge i s l a r g e r i n t h e
case
i n accordance w i t h a s l i g h t l y deeper minimum o f t o t a l energy.
5. The c r u c i a l d i f f e r e n c e i s t h e m i s s i n g bond-bond charge t r a n s f e r w i t h i n adsorbate m o l e c u l e . comparison
The o v e r l a p popu a t i o n s o f b o t h N20 bonds decrease
w i t h t h e gas phase values
F o r t h e N-N bond a more
the in
pronounced
r e d u c t i o n i s observed. INTERPRETATION AND D I S C U S S I O N On t h e b a s i s of t h e quantum chemical r e s u l t s and normal c o o r d i n a t e a n a l y s i s we attempted t o i n t e r p r e t t h e observed i n f r a r e d s p e c t r a o b t a i n e d (see r e f . 4), applying the f o l l o w i n g strategy:
-
F i r s t we t r i e d t o f i n d quantum c h e m i c a l l y s t a b l e s o r p t i o n complexes o f
N-
and O-bound n i t r o u s o x i d e .
-
Next
we examined which v i b r a t i o n a l c o o r d i n a t e s and f o r c e
constants i n f l u -
75
1350
1230
2300
21 50
Frequency /cm- 1 F i g . 3. I R s p e c t r a o f n i t r o u s o x i d e adsorbed i n z e o l i t e s Nii.,NatefA and Cu3 1Na5 8A i n t h e r e g i o n s o f t h e u, and v3 f u n amen a s.
582
ence most s t r o n g l y t h e v i b r a t i o n a l mode under c o n s i d e r a t i o n .
-
F i n a l l y we sought t o f i n d t h e e x t e n t o f change i n t h e o v e r l a p p o p u l a t i o n o f t h e i n v o l v e d bonds upon a d s o r p t i o n compared t o f r e e N20. Analyzing
at
first
in
t h e u 1 band o f n i t r o u s o x i d e
the
Ni2+-exchanged
z e o l i t e , one peak i s observed a t 1304 cni' and t h e second as a s h o u l d e r a t 1290 cm-'
(see F i g . 3 ) , compared w i t h t h e gas phase v a l u e o f 1285 c d . The s p l i t t i n g
of
the
vl
band may
be due t o t h e
two
sorption
complexes
in
different
I n t h e L modes o f t h e v1
o r i e n t a t i o n o f t h e adsorbate (Z-Ni-NNO and Z-Ni-0").
fundamental v i b r a t i o n t h e h i g h e s t p a r t i c i p a t i o n c o e f f i c i e n t i s t h a t o f t h e N-0 s t r e t c h i n g coordinate, participation
which i s more t h a n t w i c e as l a r g e as t h e c o r r e s p o n d i n g
coefficient
o f t h e N-N
elements i n d i c a t e t h a t aXl/aFNo ding
aX,/aF"
stretching
coordinate.
The
Jacobian
i s about f o u r times h i g h e r than the correspon-
and a l s o t h a t ahl/aFNiN i s n e a r l y t w i c e as h i g h
as
aX,/aF". Ni-NNO
t h e h i g h - f r e q u e n c y band a t 1304 cm-' must be a s s i g n e d t o t h e
Therefore
and t h e 1290 cni' a b s o r p t i o n t o t h e N i - O N N species, a l t h o u g h i n t h e case o f t h e latter ref.
no
i n n e r charge r e d i s t r i b u t i o n i s observed as i s f o u n d w i t h
2).
(see
I n b o t h o r i e n t a t i o n s t h e v a l u e s o f t h e o v e r l a p charges 0-N and
are almost i d e n t i c a l ,
N-N
hence t h e d i f f e r e n c e i n t h e r e s p e c t i v e f r e q u e n c i e s
due t o t h e d i f f e r e n t N i - 0 and N i - N o v e r l a p c h a r g e s .
be
Ca
may take
Here we have t o
i n t o account t h e l a r g e e i g e n v e c t o r o f t h e Ni-adatom i n t e r n a l c o o r d i n a t e t o t h e L
mode
o f this vibration,
as w e l l as t h e l a r g e v a l u e o f
the
corresponding
Jacobian element. u p s c a l e f r e q u e n c y s h i f t o f t h e v 3 band w i t h i t s main peak a t 2257
The
may be i n t e r p r e t e d o n l y by t h e N i - N o v e r l a p p o p u l a t i o n .
CN'
I n t h e L mode o f
the
v 3 v i b r a t i o n t h e p a r t i c i p a t i o n c o e f f i c i e n t o f t h e N-N s t r e t c h i n g c o o r d i n a t e i s
t h e l a r g e s t , f o l l o w e d b y t h a t o f t h e N-0 c o o r d i n a t e . O n l y i n t h e case o f Z - N i -
NNO
configuration
Furthermore, from
their
this
L mode.
does t h e Ni-adatom c o o r d i n a t e p a r t i c i p a t e i n t h e
i s c o n f i r m e d by t h e c a l c u l a t e d d i s p l a c e m e n t s o f t h e
equilibrium positions.
I n t h e case o f t h e Z-Ni-ONN
atoms
species
displacement o f t h e t e r m i n a l 0 atom i s small w h i l e t h a t o f N i i s n e a r l y This
i s a l s o r e f l e c t e d i n t h e J a c o b i a n elements o f t h e v 3 v i b r a t i o n .
case
of
force
t h e Z-Ni-ONN complex t h e v 3 mode i s n e a r l y i n s e n s i t i v e t o
element (aX3/aFNio
= 0).
the
oxygen atom,
just
In the
t h e o p p o s i t e i s t h e case.
be
i.e. v i a the nitrogen or
as t h i s v i b r a t i o n r e p r e s e n t s e s s e n t i a l l y t h e m o t i o n o f
n i t r o g e n w i t h r e s p e c t t o t h e t e r m i n a l atoms,
the FNiO
Furthermore the v 3 v i b r a t i o n turns o u t t o
i n s e n s i t i v e t o t h e mode o f a t t a c h m e n t t o t h e c a t i o n , central
the zero.
w h i l e f o r the
the
v 1 mode
Compared t o v l t h e h i g h e r f r e q u e n c y s h i f t
t h e v 3 mode i n b o t h o r i e n t a t i o n s o f N20 a d s o r p t i o n can be u n d e r s t o o d f r o m
of the
l a r g e v a l u e s o f t h e c o r r e s p o n d i n g e i g e n v e c t o r s and t h e J a c o b i a n elements. The s i t u a t i o n w i t h t h e copper-exchanged z e o l i t e s i s r a t h e r complex,
as
it
583 was
found
by
CO
probing
t h a t the
primarily
exchanged
Cu2'
during
5 ) . This i s
p r e t r e a t m e n t i s e s s e n t i a l l y auto-reduced t o Cut and Cuo ( s e e r e f . in
accordance
w i t h t h e c a l c u l a t e d r e s u l t s on t h e Cu2+-loaded
the
cluster
which
cannot c o n s i s t e n t l y e x p l a i n t h e e x p e r i m e n t a l s p e c t r a . T h e r e f o r e an a t t e m p t was made t o t e n t a t i v e l y a s s i g n t h e d o u b l e t a t 1289 and 1282 cm-' t o t h e v 1 bands o f n i t r o u s o x i d e adsorbed i n f r o n t o f a Cut i o n i n -NNO and -0"
alignment,
the
while
d o u b l e t a t 2245 and 2223 cm-' t o t h e v j fundamental mode,
overlapping
subbands
under
i n t e r a c t i n g w i t h Na' appreciable
the
cations.
shift
of
envelope were
attributed
N20
species
The s p l i t t i n g o f t h e v1 i n t o two bands
without
t h e c e n t e r may be e x p l a i n e d by t h e
to
and
adjacent
existence
of
two
p a r t i c i p a t i o n o f t h e Cu-0 or CU-N i n t e r n a l c o o r d i n a t e i n t h e L mode o f
the
opposite e f f e c t s : - d i m i n i s h i n g o f t h e N-0 o v e r l a p p o p u l a t i o n and
-
v 1 fundamental.
Concerning t h e v g mode, o n l y f o r t h e Z-Cu'-NNO
species can t h e b a n d s h i f t t o
a h i g h e r frequency (2245 cm-') be reasonably understood f r o m t h e l a r g e v a l u e o f the
overlap
CU-N
population i n combination w i t h the
vibration.
(Only
coordinate
participate
in
the in
Cu-NNO o r i e n t a t i o n
eigenvectors
does
the
Cu-adatom
t h e L modes o f v g . ) F o r
the
Cu+-ONN
of
this
internal complex
a
downscale s h i f t compared t o t h e gasphase f r e q u e n c y s h o u l d o c c u r . The
h i g h e r i n t e n s i t y o f t h e v, HF band may be e x p l a i n e d by t h e l a r g e r
N-0
bond d i p o l e moment i n t h e case o f t h e Z-M-NNO s o r p t i o n complex. CONCLUSIONS Quantum chemical sorption
complexes
assignment
of
the
calculations complex
overlapping
demonstrated by t h e copper example, give
supplemented by
vibrational
analyses
o f t r i a t o m i c s i n z e o l i t e s o f f e r a h e l p f u l guide bands
usually
success i s n o t guaranteed
impetus t o more r e f i n e d experiments,
-
on
for
the
observed.
As
which
s i n c e more s o p h i s t i c a t e d
should calcula-
t i o n s do n o t seem p o s s i b l e a t p r e s e n t . ACKNOWLEDGEMENT This
work was supported b y t h e U n i v e r s i t i e s o f Hamburg and S o f i a
Fonds d e r Chemischen I n d u s t r i e .
and
the
584 REFERENCES 1. 0. Zakharieva-Pencheva, H. Bose, H. F o r s t e r , W. Frede and M. G r o d z i c k i , J . M o l . S t r u c t . , 122 (1985) 101-114. 2. M. Grodzicki, 0. Zakharieva-Pencheva and H. F o r s t e r , J. M o l . S t r u c t . , 175
(1988) 175-201. 3. M. Schumann, M. Grodzicki, H. Bose, H. F o r s t e r and 0. Zakharieva-Pencheva, i n D. Shopov e t a l . (Eds.), Heterogeneous C a t a l y s i s , Proc. S i x t h I n t . Symp. S o f i a , 1987, Volume 2, P u b l i s h i n g House o f the B u l g a r i a n Academy o f S c i ences, Sofia, 1987, pp. 193-198. 4. H. F o r s t e r and M. Remmert, 3. M o l . S t r u c t . ,174 (1988) 357-362. 5. H. F o r s t e r and U. Witten, Z e o l i t e s , 7 (1987) 517-521.
H.G. Karge, J. Weitkamp (Editors), Zeolites [IS Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
I R STUDY OF THE ADSORPTION OF BENZENE ON HZSM5
Andreas J e n t y s and Johannes A. Lercher I n s t i t u t f u r P h y s i k a li s c h e Chemie, Technische U n i v e r s i t a t Wien Cetreidemarkt 9, A-1060 Vienna, AUSTRIA
ABSTRACT The a d s o r p t i o n o f benzene on a s e r i e s of HZSM5 samples (1-4.5 aluminum atoms p e r u n i t c e l l ) was s t u d i e d by means o f i.r. sp ec roscopy. The sam p l es were e x p o s e d t o b eq ze n e v a p o r a t p r e s s u r e s b e t w e e n lo-’ a n d 1 mbar. Benzene was adsorbed on Na c a t i o n s and on hydroxyl groups l e a d i n g t o f o u r d i f f e r e n t f o r m s of hydrogen-bonded molecules: adsorbed on SiOHAl groups, hydrogen-bonded SiOH i n s i l a n o l n e s t s , f r e e SiOH g r o u p s , and t w o m o l e c u l e s a d s o r b e d p e r SiOHAl group. The s t r e n g t h o f t h e i n t e r a c t i c g o f benzene d ecr eased i n t h i s sequence. The s p e c t r a a f t e r e q u i l i b r a t i o n a t 10 mbar b e n z e n e p r e s s u r e w e r e co m p ar ed q u a n t i t a t i v e l y f o r a l l samples s u g g e s t i n g t h e a d s o r p t i o n s t o i c h i o m e t r y b ei n g s t r i c t l y one f o r a l l s i t e s and samples. The a d s o r p t i o n e q u i l i b r i u m c o n s t a n t o f benzene and, hence, t h e a c i d s t r e n g t h s of a l l a c i d s i t e s d o n o t v ar y w i t h t h e aluminum c o n t e n t p e r u n i t c e l l . INTRODUCTION Benzene i s a f r e q u e n t l y proposed and used probe molecule f o r t h e e v a l u a t i o n of t h e a d s o r p t i v e p r o p e r t i e s o f z e o l i t e s ( 1 - 5 ) a n d , i n p a r t i c u l a r , f o r t h e s t r e n g t h of t h e hydroxyl groups (6-8) by means o f v i b r a t i o n a l spectroscopy. Rec e n t l y , adsorbed benzene was used f o r e v a l u a t i o n o f t h e b ase s t r e n g t h o f a l k a l i e xch an g ed z e o l i t e Y (9-11). A s a c o n s e q u e n c e , s i g n i f i c a n t i n f o r m a t i o n on t h e l o c a t i o n o f b en ze n e i n z e o l i t e s , e s p e c i a l l y i n X a n d Y t y p e (12-14) a n d ZSM5, exists.
For
HZSMS, Mentzen (15) r e p o r t s benzene t o r o t a t e on a n axis a t t h e m i r r o r
plan e and/or t o be p r e s e n t i n two symmetric l o c a t i o n s a t t h e channel i n t e r s e c t i o n s , t h e l a t t e r p o s s i b i l i t y s u p p o r t e d by h i s own X-ray powder d i f f r a c t i o n m e a s u r e m e n t s o f p - x y le n e on b o r a l i t e , a b o r o n - c o n t a i n i n g a n a l o g u e o f HZSM5 (16,17). S i m i l a r c o n c lu s io n s , i.e.,
t h e l o c a t i o n o f a p a i r o f benzene molecules
a t t h e c h a n n e l i n t e r s e c t i o n and o n e s i n g l e b en zen e m o l e c u l e i n t h e s t r a i g h t c h a n n e l , were r e a c h e d by T a y l o r on t h e b a s i s o f p o w d er n e u t r o n d i f f r a c t i o n measurements of deuterobenzene i n HZSM5 (18,19).
The i m p o r t an ce o f c a t i o n s as
l o c a l i z e d a d s o r p t i o n s i t e s was emphasized by Thamm (20) and F o e r s t e (21) sugg e s t i n g a d s o r p t i o n a t t h e c h a n n e l i n t e r s e c t i o n o n B r o n s t e d an d L e w i s a c i d s i t e s , respectively. Similarly,
t h e a u t h o r s u s i n g v i b r a t i o n a l sp ect r o sco p y as
586
a n a l y t i c a l means s t r o n g l y suggest t h a t a t l e a s t a t low and moderate coverages d i r e c t l o c a l i z e d a d s o r p t i o n of benzene predominates (1-11). I n t h i s p a p e r we i n t e n d ( i )t o d e s c r i b e t h e i n t e r a c t i o n s o f benzene w i t h HZSM5 (1 t o 4.5 A 1 per u n i t c e l l ) a t e q u i l i b r i u m p r e s s u r e s ranging from t o 1 mbar benzene, ( i i ) t o d e s c r i b e t h e changes of t h e a d s o r b a t e s a s a f u n c t i o n of the aluminum content a t an e q u i l i b r i u m p r e s s u r e of benzene a t which i n t e r a c t i o n s of benzene w i t h s i l i c a i m p u r i t i e s can be neglected and (iii)t o charact e r i z e t h e v a r i a t i o n of t h e a c i d s i t e s of HZSM5 a s a f u n c t i o n of t h e aluminum c o n t e n t . T i m e - r e s o l v e d i.r. s p e c t r o s c o p y i s u s e d t o check w h e t h e r t r a n s p o r t phenomena a f f e c t t h e i.r. s p e c t r a measured. EXPERIMENTAL Materials The ZSM5 samples were synthesized according t o t h e method o f I n u i e t a1 (22) using w a t e r g l a s s (28 mol% s o l u t i o n i n water), aluminumsulfate and t e t r a p r o p y l ammoniumbromide as reagents. ZSM5 was t h e o n l y c r y s t a l l i n e phase d e t e c t e d by
XRD. Subsequently, t h e samples were ion-exchanged w i t h ammoniumnitrate solut i o n . The c o m p o s i t i o n o f t h e HZSM5 s a m p l e s , t h e c o n c e n t r a t i o n of s t r o n g and weak Bronsted a c i d s i t e s and t h e concentration of t e t r a h e d r a l l y and octahedrall y coordinated aluminum a r e compiled i n Table 1.
TABLE 1 Unit c e l l composition and a c i d i t y of HZSM5 samples Sample
Unit cell composition
H
A1
Si
strong H'1U.C.
weak
H'1U.C.
Si/A1
Na'1U.C.
tetr. oct. A13+/U.C.
0
HZSM5-1
4.5 4.5 91.5 192
3.28
0.27
20.2
0.4
3-28
1-22
HZSM5-2
3.7 3.7 92.3 192
1.70
0.18
25.2
0.4
1-70
2.0
HZSM5-3
2.1 2.1 93.9 192
1.64
0.23
45.8
0.3
7-64
0.46
HZSM5-4
1.5 1.5 94.5 192
1.39
0.16
0.4 0.6
1.39 0.78
0.11 0.52
0.3
0.97
0.03
HZSM5-5
1.3 1.3 94.7 192
0.78
0.42
63.0 73.6
HZSM5-6
1.0 1.0 95.0 192
0.97
0.09
101.0
1.r. and temperature-programmed desorption (t.p.d.) measurements Self-supporting d i s c s of t h e HZSM5 samples were analyzed by t r a n s m i s s i o n
-
a b s o r p t i o n i.r. s p e c t r o s c o p y (BRUKER IFS 88, 4 cm-l r e s o l u t i o n ) . The d i s c was p l a c e d i n a s a m p l e h o l d e r i n t h e c e n t e r o f a small f u r n a c e i n t h e i.r. beam. The bands of t h e l a t t i c e v i b r a t i o n s b e t w e e n 2090 and 1740 cm-l w e r e used as s t a n d a r d s t o normalize t h e i n t e n s i t i e s of all bands. For a c t i v a t i o n and t.p.d.;
581 t h e sample was heated i n s i t u w i t h a r a t e of 10 K.min-’ 873 K a t pressures lower than
t o a temperature of
mbar. The p r e s s u r e changes were determined
by means of a Balzers 311 quadrupole mass spectrometer.
The amounts desorbed
were q u a n t i f i e d using a reference sample o f HZSM5 (23). RESULTS Adsorption of benzene HZSM5 a c t i v a t e d a t 873 K e x h i b i t e d f i v e bands due t o OH v i b r a t i o n s a t 3745 (SiOH), 3726 ( f r e e SiOH a t d e f e c t s i t e s ) , 3698-3670 (water on e x t r a l a t t i c e
c-.
m a t e r i a l ) , 3610 (SiOHA1, b r i d g i n g h y d r o x y l g r o u p ) and a r o u n d 3500 cm-l ( p e r t u r b e d SiOH a t d e f e c t s i t e s ) . These bands w e r e o b s e r v e d w i t h a l l s a m p l e s i n v e s t i g a t e d ; t h e i r r e l a t i v e i n t e n s i t i e s , however, v a r i e d s i g n i f i c a n t l y .
0.0325 A
v!
35bo
L
3000
2500
2000
moo
mo
\i’
3000 2wo 2000 M A V W B E R S MI-i
id0
( 7 y v E H y B E R B CM-1
10
Fig. 1. D i f f e r e n c e between t h e i.r. s p e c t r a o f t h e a d s o r b a t e w i t h benzene a t t h e equilibrium pressure i n d i c a t e d and t h e spectrum of t h e a c t i v a t e d HZSM5.
588 The d i f f e r e n c e between t h e adsorbate s p e c t r a and t h e spectrum of a c t i v a t e d HZSM5-1 a r e compiled
i n Fig. 1. The d i f f e r e n c e s p e c t r a between two subsequent
equilibrium p r e s s u r e s a r e compiled i n Fig. 2. Bands p o i n t i n g upwards increased, t h o s e p o i n t i n g downwards d e c r e a s e d i n i n t e n s i t y compared t o t h e s u b t r a c t e d spectrum. Increasing t h e benzene p r e s s u r e up t o cm-'
mbar caused t h e OH band a t 3610
t o d e c r e a s e , i n d i c a t i n g i n t e r a c t i o n w i t h t h e bridging hydroxyl groups. A
broad band o f p e r t u r b e d OH g r o u p s a p p e a r e d a t 3350-3250 cm-l, w h i l e bands o f adsorbed benzene were observed a t 3093, 3074, 3039 (CH s t r e t c h i n g v i b r a t i o n s ) , 2000, 1973, 1892, 1832 (combination bands of out-of-plane CH deformation vibrat i o n s ( 4 and r e f e r e n c e s t h e r e i n ) ) and a t 1477 cm-'
(C-C
I
T
II
stretching vibration).
'
1
T
0.Om A
Fig. 2. Changes i n t h e i.r. s p e c t r a i n d u c e d by t h e s t e p w i s e i n c r e a s e of t h e benzene equilibrium pressure.
589 and
At
mbar b e n z e n e p r e s s u r e , t h e b an d s a t 1971 a n d 1 8 3 2 cm-l
i n c r e a s e d i n I n t e n s i t y . P r i m a r i l y , t h e band a t 3610 cm-l d ecr eased i n i n t e n s i t y and o n l y a v e r y small f r a c t i o n o f t h e i n t e n s i t y o f t h e h y d r o x y l band a t 3 7 2 6 cm-l d e c r e a s e d . Two o v e r l a p p i n g b a n d s o f p e r t u r b e d h y d r o x y l g r o u p s w e r e ob1 se r v ed a t 3350 cm-l and 3240 - 50 cmWe n ot ed a somewhat h i g h e r wavenumber
.
f o r t h e l a t t e r band i n a r e c e n t s t u d y o v e r HZSM.5 ( 2 4 ) , a n d we t h i n k t h a t t h e d i f f e r e n c e may be due t o s u b t l e s t e r i c d i f f e r e n c e s a t t h e benzene a d s o r p t i o n site. The presence of two p e r tu r b e d bands s u g g e s t s two t y p e s o f hydrogen bondi n g an d / o r t w s t y p e s o f a c i d s i t e s (25).
After e q u i l i b r a t i o n a t a p r e s s u r e o f
10-1 mbar benzene, t h e OH band 3726 cm-l d e c r eased i n i n t e n s i t y and new bands 1
appeared a t 3612, 3250 w i t h a s h o u l d e r a t 3174 cm-l and a t 1954 and 1811 cm-
.
Upon i n c r e a s i n g t h e b e n z e n e p r e s s u r e t o 1 mbar t h e b a n d s a t 3 6 1 2 , 3 1 7 4 , 1 9 5 4 and 1811 cm-l f u r t h e r i n c r e a s e d i n i n t e n s i t y . The b an d s a t 3350 cm-l a n d a t 1971 a n d 1832 cm-l d e c r e a s e d s l i g h t l y i n i n t e n s i t y . W i t h i n c r e a s i n g b e n z e n e p r e s s u r e t h e bands a t 3093, 3074, 3039 and 1477 cm-l d i d n o t change i n wavenumb e r s , b u t i n c r e a s e d i n i n t e n s i t y . The i n t e g r a l i n t e n s i t y o f t h e band a t 1 4 7 7
cm
-1
,
p l o t t e d as a f u n c t i o n o f t h e e q u i l i b r i u m p r e s s u r e (see Fig. 31, seems t o
f o l l o w a Langrnuir-type isotherm. The a d s o r p t i o n of benzene i s co m p l et el y r e v e r s i b l e a t ambient temperature. 0.x)
No(*uuzEDA88Q118ANQ.
B€"E 1 8 0 1 2 an4 URB. U N m
I
prwure [rnbar]
Fig. 3. I n t e g r a l i n t e n s i t y of t h e band a t 1477 cm-l of benzene on HZSM5-1 as a function of the equilibrium pressure. The t i m e d e p e n d e n c e o f t h e i.r. a d s o r b a t e s p e c t r a o f b en zen e o n HZSM5-1 a f t e r a s t e p up o f t h e p r e s s u r e f r o m
to
mbar b e n z e n e i s p l o t t e d i n
Fig. 4. Note t h a t a l l b a n d s i n c r e a s e d i n p a r a l l e l . S u ch b e h a v i o u r s u g g e s t s t h a t l o c a l ad s o r p t io n - d e s o r p t io n e q u i l i b r i u m e x i s t s durbng t h e t r a n s i e n t , like the situation prevailing i n s i t u a t i o n d es cr i b e d by Rieck and
much
590
TIE
[el
1000
0.12
5
500
0.085
0.05
f 0.015
-
-0.02
1 3500
F i g . 4.
3000 2500 2000 WAVENUMBERS CM-1
1500
1.r. d i f f e r e n c e s p e c t r a d u r i n g e q u i l i b r a t i o n o f b e n z e n e o n HZSM5-1
a f t e r a s t e p w i s e i n c r e a s e o f t h e benzene p r e s s u r e f r o m
to
mbar.
The
i.r. spectrum o f t h e a c t i v a t e d sample i s s u b s t r a c t e d f r o m each o f t h e s p e c t r a .
0.019
0.01C5 v)
I-
H
z 3
W
y
0.002
a rn
I-
U
0 v)
m 4
-0.006
1
:
I
I
I
-0.015 3500
3000
2500
2000
1500
WAVENUMBERS CM-1 Fig. 5. Benzene on tIZSF15-3
....
s p e c t r u m measured;
____
sum o f b o t h bands
mbar).
- s i m u l a t e d b a n d s a t 3350 and 3240 cm-’;
591 Influence of t h e Si/A1 r a t i o We i n v e s t i g a t e d benzene a d s o r p t i o n a t a p r e s s u r e o f
mbar o f b e n z e n e ,
because under t h i s condition t h e bands of SiOH v i b r a t i o n s a r e h a r d l y a f f e c t e d by t h e presence of benzene. The most s i g n i f i c a n t d i f f e r e n c e s between a l l samples a r e t h e v a r i a t i o n of the r e l a t i v e i n t e n s i t i e s of t h e perturbed OH band and t h e decrease of t h e i n t e n s i t y of t h e bands due t o adsorbed benzene w i t h decreasing concentration of aluminum. Typical d i f f e r e n c e s b e t w e e n t h e i.r. s p e c t r u m of t h e a d s o r b a t e and t h a t of t h e a c t i v a t e d s a m p l e t o g e t h e r w i t h t h e two deconi n Fig. 5. Fig. 6 shows t h e v a r i a t i o n s 1 Fig. 7
voluted perturbed OH bands a r e compiled
of t h e i n t e g r a t e d i n t e n s i t i e s of t h e bands a t 3350 and a t 3240-50 cm-
,
t h e concentration of strong and weak Bronsted a c i d sites determined by t.p.d. of pyridine as a f u n c t i o n of t h e aluminum content of t h e u n i t c e l l . The average c o n t r i b u t i o n of t h e a c i d s i t e s f o r a d s o r p t i o n o f benzene on t h i s s e r i e s of HZSM5 s a m p l e s i s b e s t i l l u s t r a t e d i n Fig. 8. I t shows t h e dependence o f t h e concentration of adsorbed benzene upon t h e concentration of aluminum and sodium p e r u n i t c e l l . The l i n e a r f u n c t i o n p a s s e s t h r o u g h t h e o r i g i n , s u g g e s t i n g t h a t indeed we account f o r a l l p o s s i b l e adsorption s i t e s . DISCUSSION
I t i s accepted (4-11) t h a t two groups of i.r. bands can be used t o characteri z e a d s o r b e d benzene: ( i )bands of u n p e r t u r b e d and p e r t u r b e d OH g r o u p s and ( i i )bands of c o m b i n a t i o n s of CH out-of-plane d e f o r m a t i o n v i b r a t i o n s b e t w e e n
2100 and 1800 cm-l, t h e i r wavenumber d i f f e r e n c e (25,271 and t h e i r wavenumber i n c r e a s i n g with t h e i n t e r a c t i o n strength. I n t o t a l , four perturbed hydroxyl bands were observed (3612, 3350, 3250 and 3170 cm-l).At low pressures (up t o
mbar) only those a t 3350 and 3250 cm-’,
a t h i g h p r e s s u r e s , i n a d d i t i o n , t h o s e a t 3612 and 3170 cm-l. Below benzene two pairs of CH out-of-plane 1892 and 1973, 1830 cm-l.
mbar
deformation bands were observed a t 2000,
The f i r s t p a i r o f bands i n d i c a t e s t h e s t r o n g e s t
adsorption on H E M 5 and was a t t r i b u t e d t o benzene on r e s i d u a l Na
+
c a t i o n s (10).
The h i g h s t a b i l i t y o f t h i s a d s o r b a t e i s a l s o r e f l e c t e d i n t h e h i g h e r t h e r m a l s t a b i l i t y of benzene on NaZSM5 a s compared t o benzene on HZSM5 (27).
The
i n t e n s i t y of t h e o t h e r two bands i n c r e a s e d i n p a r a l l e l w i t h b o t h p e r t u r b e d h y d r o x y l bands (3350 and 3240-50 cm-l). We a t t r i b u t e t h e s e bands t o benzene adsorbed on Bronsted a c i d i c hydroxyl groups. From t h e CH out-of-plane
d e f o r m a t i o n v i b r a t i o n s o f benzene, t h e r e i s no
i n d i c a t i o n of t h e p r e s e n c e o f two t y p e s of B r o n s t e d a c i d s i t e s , i n d i c a t i n g r a t h e r similar s t r e n g t h s of i n t e r a c t i o n . The s e p a r a t i o n of t h e two perturbed OH bands ( a p p r o x i m a t e l y 100 cm-l; J a c o b s r e p o r t i n g a A Y o f 50 cm-I
(8)) i s ,
however, v e r y l a r g e compared t o t h e d i f f e r e n c e s i n t h e s h i f t s (rnax. 17
ern-')
592
NORMAUZEDABSORBANCE OFTHE PERTURBEDy OH We. UNITS)
SITES I U.C.
sT:wl
SITES
31 b
0.1 .
0.0
*.." -
0
*.._':,
,,~3250,
3 4 Al I UNIT CELL
1
2
5
0
1
2
3 4 Al I UNIT CELL
5
Fig. 6. Normalized i n t e g r a l i n t e n s i t y
Fig. 7. Concentration of t h e s t r o n g and
of t h e perturbed OH bands a s a func-
weak Bronsted a c i d s i t e s as a f u n c t i o n
t i o n o f A13'/U.C.
of Al3'/U.C.
OAOO
0.006
OXHO
0x116
NORMALIZED ABSORBANCE, BENZENE, 1516-1412 (ARB. UNITS)
0.020 UII~
Fig. 8. Normalized i n t e g r a l i n t e n s i t y of t h e band a t 1477 cm-' f u n c t i o n of t h e sum of t h e t o t a l amount of A13'
of benzene a s a
and Na'.
reported (7). I f t h i s s p l i t t i n g were due t o two hydroxyl groups w i t h i d e n t i c a l p o s i t i o n s of t h e u n p e r t u r b e d OH g r o u p s , t h e d i f f e r e n c e i n t h e s t r e n g t h of these
hydroxyl groups should be extremely high. I n t h a t c a s e one expects two
experimental observations: ( i )t h e r e should be a p r e f e r e n t i a l a d s o r p t i o n on t h e s i t e o f t h e h i g h a c i d s t r e n g t h a t low e q u i l i b r i u m p r e s s u r e s and ( i i )a more
593
b a s i c m o l e c u l e (e.g. t o l u e n e ) s h o u l d p r o d u c e a l a r g e r wavenumber d i f f e r e n c e between t h e two p e r t u r b e d OH bands. N e i t h e r e x p e c t a t i o n was f u l f i l l e d by o u r experiments. We d i d not observe a s i g n i f i c a n t p r e f e r e n t i a l a d s o r p t i o n a t low equilibrium pressures, o r a l a r g e r wavenumber d i f f e r e n c e of t h e two perturbed OH bands i n p r e l i m i n a r y experiments of toluene adsorption. If t h e two perturbed OH bands indeed o r i g i n a t e from Briinsted a c i d s i t e s of
comparable s t r e n g t h , t h e bands of t h e f r e e hydroxyl groups must have a spacing i n wavenumbers similar t o t h e perturbed ones, Thus, we propose t h a t t h e band a t 3350 cm-’ 3500 cm-l
stems from t h e OH band a t 3610 cm-l and that a t 3250 cm-l from t h a t a t
,
t h e former possessing strong, t h e l a t t e r weak Briinsted a c i d i t y (28).
S u p p o r t f o r t h i s a t t r i b u t i o n i s g i v e n f r o m t h e t h e p a r a l l e l i n c r e a s e of t h e concentration of t h e s t r o n g BrBnsted a c i d s i t e s and t h e i n t e g r a l i n t e n s i t y of t h e band a t 3350 cm-l and t h e c o n s t a n t v a l u e s f o r t h e c o n c e n t r a t i o n o f weak Bronsted a c i d s i t e s and f o r t h e i n t e n s i t y of t h e band a t 3240-50 cm-’
(see Figs.
6 and 7). The two perturbed OH bands a t higher equilibrium p r e s s u r e s of benzene can be a t t r i b u t e d t o t h e i n t e r a c t i o n of benzene with S i O H groups (3612 cm-’)
( 4 ) and t o
t h e adsorption of a second benzene molecule on a s t r o n g Bronsted a c i d i c hydroxyl group.
The l a t t e r adsorbate s t r u c t u r e was a l s o suggested by Taylor (18,191 and
i s supported by t h e s l i g h t decrease i n i n t e n s i t y of t h e bands a t 1971 and 1832 cm -1 c h a r a c t e r i s t i c f o r adsorption on BrBnsted a c i d s i t e s . I n both forms of ads o r p t i o n benzene e x h i b i t s CH o u t - o f - p l a n e d e f o r m a t i o n v i b r a t i o n s similar t o l i q u i d b e n z e n e , s u g g e s t i n g o n l y weak i n t e r a c t i o n s w i t h t h e s u r f a c e . The i d e n t i c a l wavenumbers of t h e perturbed OH bands a f t e r benzene a d s o r p t i o n on a l l HZSM5 samples suggest e i t h e r t h a t t h e s i t e s do n o t vary i n s t r e n g t h as a f u n c t i o n of t h e aluminum c o n t e n t o r t h a t benzene i s t o o i n s e n s i t i v e a probe molecule t o d i f f e r e n t i a t e . Because of t h e l i n e a r dependence of t h e concentration of adsorbed benzene upon t h e sum of t h e concentration of t o t a l aluminum and Na’, we c o n c l u d e t h a t t h e a d s o r p t i o n s t o i c h i o m e t r y i s s t r i c t l y one a t
mbar
benzene, T h i s s u g g e s t s t h a t even s i l a n o l n e s t s c r e a t e d by d e a l u m i n a t i o n o n l y adsorb one benzene molecule per s i t e . Extrapolation of t h e concentrations adsorbed t o zero aluminum content and Fig.8 demonstrate t h a t Na
t
a r e not located
a t t h e c a t i o n exchange p o s i t i o n o f t h e z e o l i t e . Note t h a t t h e s e Na
t
cations
c o u l d n o t be removed by r e p e a t e d i o n exchange. Because of t h e s t r i c t l i n e a r dependence of t h e amount of benzene adsorbed per u n i t c e l l on t h e concentration of aluminum, w e f u r t h e r conclude t h a t t h e a d s o r p t i o n e q u i l i b r i u m c o n s t a n t f o r a l l samples i s i d e n t i c a l and hence t h e a c i d s t r e n g t h of t h e s i t e s i s independent
of t h e u n i t c e l l composition. I n c o n c l u s i o n , we have shown t h a t benzene i s a d s o r b e d on l o c a l i z e d s i t e s 1
which can be i d e n t i f i e d and q u a n t i f i e d up t o an equilibrium p r e s s u r e of 10-
594 mbar. A t lower p r e s s u r e s p r e f e r e n t i a l a d s o r p t i o n on Na' t a k e s p l a c e followed by a d s o r p t i o n on Bronsted a c i d s i t e s . Only a t p r e s s u r e s of lo-'
mbar benzene and
h i g h e r do s i g n i f i c a n t a d s o r p t i o n on SiOH groups and m u l t i p l e a d s o r p t i o n on one s i t e o c c u r . While b e n z e n e i s n o t a v e r y good p r o b e m o l e c u l e t o d i f f e r e n t i a t e between s u b t l e d i f f e r e n c e s of Bronsted a c i d s t r e n g t h , i t i s w e l l s u i t e d t o c h a r a c t e r i z e a d s o r p t i o n on z e o l i t i c m a t e r i a l q u a n t i t a t i v e l y and t o d i f f e r e n t i a t e between c l a s s e s o f a c i d s t r e n g t h . ACKNOWLEDGEMENT This work was supported by t h e ttFonds zur Forderung d e r W i s s e n s c h a f t l i c h e n F o r s c h u n g " u n d e r p r o j e c t P5757. We a r e g r a t e f u l t o Dr. H. Mayer f o r t h e X R D measurements. REFERENCES 1 L. H. L i t t l e , I n f r a r e d S p e c t r a o f Adsorbed S p e c i e s , Academic P r e s s , London, New York (1966) and r e f e r e n c e s t h e r e i n . 2 J.J. Freeman, and M.L. Unland, J. C a t a l . 54 (1978) 183. 3 M.L. Unland, and J.J. Freeman, J. Phys. Chem. 82 (1978) 1036. 4 M. P r i m e t , E. G a r b o w s k i , M.V. M a t h i e u and B. I m e l i k , J. Chem. SOC. Faraday Trans. I 76 (1980) 1942. 5 B. Coughlan, W.M. C a r r o l l , P. O'Malley and J. Nunan, J. Chem. SOC. Faraday Trans. I 77 ( 1 9 8 1 ) 3037. 6 P.G. Rouxhet and R.E Semples, J. Chem. SOC. Faraday Trans. I 77 (1972) 2021. 7 P.A. Jacobs, C a t a l . Rev. S c i . Eng. 24 (1982) 415. 8 P.A. J a c o b s , J . A . M a r t e n s , J. Weitkamp and H.K. B e y e r , D i s c u s s . Faraday SOC. 72 (1981) 353. 9 A. de Mallmann and D. Barthomeuf, J. Chem. SOC. Chem. Comm. (1986) 476. 10 A. d e Mallmann a n d D. B a r t h o m e u f , S t u d . S u r f . S c i . C a t a l . 2 8 (1986) 609. 11 D. Barthomeuf and A. d e Mallmann, S t u d . S u r f . S c i . C a t a l . 3 7 ( 1 9 8 8 ) 365. 1 2 A.N. F i t c h , H. J o b i c a n d A. Renouprez, J. Phys. Chem. 9 0 ( 1 9 8 6 ) 1311. 1 3 A.N. F i t c h , H. J o b i c a n d A. R e n o u p r e z , J. Chem. SOC. Chem. Comm. ( 1 9 8 5 ) 284. 14 V.Yu. Borovkov, W.K. H a l l and V.B. K a s a s a n s k i , J. C a t a l . 51 ( 1 9 7 8 ) 439. 1 5 B.F. Mentzen, Mater. Res. Bull. 22 (1987) 337. 1 6 B.F. Mentzen, F. B o s s e l e t a n d J. Bouix, C.R. Acad. S c i . P a r i s , t. 305, S e r i e I1 (1987) 581. 17 B.F. Mentzen, F. B o s s e l e t and J. Bouix, C.R. Acad. S c i . P a r i s , t. 306, S e r i e I1 (1988) 27. 18 J.C. Taylor, J. Chem. SOC. Chem. Comm. (1987) 1186. I 9 J . C . Taylor, Z e o l i t e s 7 (1987) 311. 20 H. Thamm, H.G. Jerschkewitz and H. S t a c h , Z e o l i t e s 8 (1988) 151. 21 C. F o e r s t e , A. Germanus, J. K a e r g e r , H . P f e i f e r , J. C a r o , W. P i l z a n d A. Zikanova, J. Chem. SOC. Faraday Trans. I 8 3 (1987) 2301. 22 T. I n u i , 0. Yamase, K. Fukuda, A. I t h o , J. Turumoto, N. M o r i n a g a , T. H a g i w a r a and Y. Takegami, Proc. 8 t h I n t . Congr. Catal., B e r l i n , ( 1 9 8 4 ) 569. 23 C. H a l i k , J.A. L e r c h e r and H. Mayer, J. Chem. SOC. F a r a d a y T r a n s . I , 84 (1988) i n p r e s s . 24 A. J e n t y s , G. Warecka and J . A . Lercher, J. Mol Catal. submitted. 25 M.L. Hair and W. H e r t l , J. Phys. Chem. 74 (1970) 91. 26 J.S. Rieck and A.T. B e l l , J. C a t a l . 85 (1984) 143. 27 M. Derewinski, J. Haber, J. Ptaszynski, J.A. Lercher and G. Rumplmayr, Stud. S u r f . S c i . Catal. 28 (1986) 957. 28 A. J e n t y s , G. Rumplmayr and J.A. Lercher, Applied C a t a l . (1988) submitted.
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ADSORPTION SEPARATION OF METHYLNAPHTHALENE ISOMERS ON X AND Y ZEOLITES
1 1 2 V. SOLINAS , R. MONACIl, E. ROMBI and M. MOREIDELL1 'Dipartimento d i Scienze Chimiche, 09124 C a g l i a r i ( I t a l y ) .
Universita d i Cagliari,
V i a Ospedale 72,
L
Dipartimento d i Ingegneria Chimica e M a t e r i a l i , U n i v e r s i t a d i C a g l i a r i , Piazza d'Armi, 09124 C a g l i a r i ( I t a l y ) .
ABSTRACT The separation o f 1-methylnaphthalene (1-MN) and 2-methylnaphthalene (2-MN) was studied on X and Y z e o l i t e s exchanged w i t h a l k a l i n e and a l k a l i n e - e a r t h c a t i o n s , i n a l i q u i d phase a t 293 K. The t o t a l adsorbed amount depends on t h e volume o f t h e c a t i o n i n t h e z e o l i t e framework. The z e o l i t e s exchanged w i t h t h e l a r g e r c a t i o n s showed h i g h e r s e l e c t i v i t y . The system e x h i b i t s a non-ideal behaviour showing a s t r o n g dependence o f select i v i t y on composition. INTRODUCTION The z e o l i t e adsorbents a r e commercially used i n v a r i o u s s e p a r a t i o n processes, such as n- p a r a f f i n , and i n xylene isomers separation processes ( r e f . 1 ) . I n t h i s work we a r e seeking s u i t a b l e z e o l i t e s on which one o f two methylnaphthalene isomers i s s e l e c t i v e l y adsorbed i n t h e c o m p e t i t i v e a d s o r p t i o n o f t h e isomeric mixture, with t h e aim o f developing a separation process. The technique o f c o m p e t i t i v e adsorption i n l i q u i d phase u s i n g an i n e r t s o l v e n t has been r e p o r t e d by Namba e t a l . ( r e f . 2 ) . The separation o f 1-methylnaphthalene (l-MN) and 2-methylnaphthalene (2-MN) was s t u d i e d on X and Y z e o l i t e s exchanged w i t h a l k a l i n e and a l k a l i n e - e a r t h c a t i o n s . The molecular dimensions o f MN, as maximal c r o s s - s e c t i o n a l s i z e , were 0
6.2 A f o r 1MN and 5.8 A f o r 2MN ( r e f . 3). These c r i t i c a l dimensions o f aroma0
t i c molecules a r e comparable w i t h 7 A C a u j a s i t e nominal pore openings. EXPEHIMENT
Absorbents X-
and Y-zeolites
v a r i o u s l y exchanged were prepared from commercially
596
a v a i l a b l e powder (Nal3X and LZY-52 from Union Carbide). c a r r i e d out by t r e a t i n g 10 g o f the commercial
The exchange was
z e o l i t e w i t h 150 cm3 o f a
b o i l i n g aqueous s o l u t i o n o f a l k a l i n e o r a l k a l i n e - e a r t h c h l o r i d e (1M) f o r 2 h. The procedure was repeated f i v e times. The exchanged z e o l i t e was then washed, d r i e d a t 373 K f o r 3 h and calcined a t 773 K f o r 12 h. The
adsorption
runs were
performed on
zeolite
as
powder.
The main
c h a r a c t e r i s t i c s o f the z e o l i t e s employed are summarized i n Table 1.
Adsorbates and solvents 1- and 2-methylnaphthalene and n-octane were obtained from a commercial source.
They
were
high-purity
reagents
and
were
used
without
further
purification.
Methods A l l t h e runs were performed i n a closed vessel i n which 0.5 g o f z e o l i t e ,
degassed a t 773 K f o r 5 h, were contacted w i t h 2 m l o f the s o l u t i o n o f 1- and 2-methylnaphthalene i n a-octane.
The i n i t i a l concentration o f each isomer was
4 wt%. The amount o f each isomer adsorbed was obtained from t h e concentration o f each isomer i n l i q u i d phase, which was determined by gas chromatographic analysis. The e f f i c i e n c y o f t h e z e o l i t e s i n separating the methylnaphthalene isomers was measured by t h e s e l e c t i v i t y (S 1 defined by t h e f o l l o w i n g formula: 2MN/ 1MN
%/1
=
2-MN adsorbed on z e o l i t e 1-MN adsorbed on z e o l i t e
X
1-MN i n s o l u t i o n
2-MN i n s o l u t i o n
(11
RESULTS AND DISCUSSION I n t h e competitive adsorption o f methylnaphthalene isomers on X and Y zeolites,
the amount o f each isomer
adsorption time (Fig. 1 ) .
adsorbed
increased monotonously w i t h
597
Fig. 1. Amount o f methylnaphthalene isomers adsorbed ( r ) on KY vs. adsorption; ( A 1 1-MN, ( 0 I 2-MN.
time o f
The adsorption e q u i l i b r i u m was a t t a i n e d w i t h i n 24 h. The e q u i l i b r i u m was confirmed by the coincidence o f the amounts o f each isomer adsorbed a f t e r 24 and 48 h o f adsorption. Table 1 shows the t o t a l amount o f methylnaphthalene isomers and t h e select i v i t y (S J , determined a f t e r 24 h o f adsorption. 2/1 I t can be seen t h a t t h e t o t a l q u a n t i t y o f isomers adsorbed i s u s u a l l y higher i n the Y z e o l i t e s than i n the corresponding X z e o l i t e s . This may be due t o the higher value o f the S i / A l r a t i o o f t h e Y z e o l i t e s , and thus t o a smaller population o f c a t i o n i c s i t e s .
I
I
From a comparison o f the s e l e c t i v i t y values f o r t h e Me X and Me Y z e o l i t e s
I
( w i t h Me = L i , Na, K, Rb, CsJ i t can be seen t h a t they vary i n q u i t e a small range, between 1 and 2 and between 0.9 and 2.4,
respectively.
598
TABLE 1 Adsorption of methylnaphthalene isomers on X and Y exchanged zeolites. Adsorbent
% exchangeda
Li X b NaX KX RbX csx M9X CaX Bax Li Yc NaY KY
71
RbY CSY
CaY Bay
/
92 64 82 72 98 96
61 / 89 73 75 76 91 78
Amount of methyl naphthalene isomers adsorbed (mg/g zeolite) 141.81 122.42 b3.40 69.07 81.98 173.32 167.93 50.71 207.66 206.91 143.5b 116.22 99.95 169.36 168.66 153.01
s2/ I
1.16 1.18 1.60 1.61 1.98 U.8b
1.28 2.38 0.93 0.94 2.23 2.30 2.37 0.73 0.74 2.45
a
Ion-exchange degree with respect to the original form; b13X powaer, Si/Al = 1.4; 'LZY-52 powder, Si/A1 = 2.4. Amount o f adsorbent = 0.5 g., amount of adsorbate = 2 ml of solution (4% W/W of both 1MN and 2-MN).
I1 I1 On examining the Me X and Me Y zeolites (with ME"
=
Mg, Ca, Ba) the
tollowing considerations can be made: the selectivity increases about threefold from Mg to Ba; it is affected by the acidity in the structure caused by the exchange of the bivalent cations; it is similar for BaX and BaY zeolites. I1 Particularly for Me Y, an inversion of the selectivity (S
2/ I
)
for the
more acid zeolites (Mg and Ca) can be observed, with values approaching 0.7. This inversion was confirmed when using a totally decationated HY zeolite, which presents a low adsorption capacity (50 mg/g) and a selectivity of S
2/ 1
=
u.5. The high selectivity found for the BaY zeolite may be due to the fact that the Ba introduced comes out of the channel wal
I
(ref. 2).
figure 2 (a and b) shows the amount of methylnaphthalene isomers adsorbed 3 versus the volume (r ) of cations in the zeolite structure. X- and Y- zeolites show a similar trend: the total adsorbed amount decreases generally, on increasing the volume of the cations. It can be noticed that the
599
amounts adsorbed on Mg-X, Ca-X versus L i - x , Na-X a r e d i f f e r e n t and i n c o n t r a s t w i t h t h e c o r r e s p o n d i n g s e r i e s on z e o l i t e Y . 200
250 1
I
r 160
150 2oo'.
\
b
Li
Na
-. Mg
120
100 '.
80
40
cs
Ba O*'
0
2 (r3) 3
1
4
o
5
~ ( ~ 33 )
1
F i g . 2 . Amount of methylnaphthalene isomers adsorbed ( t h e exchanged c a t i o n . ( a ) X - z e o l i t e s , ( b ) Y - z e o l i t e s .
r 1 vs. volume
4
5
(r3 1 o f
On t h e o t h e r hand, t h e s e l e c t i v i t y i n c r e a s e s on i n c r e a s i n g t h e volume o f t h e c a t i o n s i n F i g u r e 3 ( a and b ) . 2.5
2.5
2.0
2.0
s2/1 1.5
%/1 1.5
1 .o
1 .o
0.5
0.5
0
1
2 (r3) 3
F i g . 3. S e l e c t i v i t y (S2,,) x-zeol i tes, (b) Y-zeol it e s .
4
vs.
volume
4
Li
3
N
Mg
0
5
I--
b
( r 1 of
Ca
1
2 (r3) 3
4
t n e exchanged c a t i o n .
5 (a)
600
Selectivity can also be correlated with the Sanderson electronegativity of the framework in Figure 4 (a and b).
2.1
a
2*I %/1 2.1
1 .I
1.
0.
3
3.3
Sint
3.3
3.6
3.6
Sint
3.9
Fig. 4. Selectivity (S2/1 1 vs. intermediate Sanderson's electronegativity (S. 1 I nt (a) X-zeolites, (b) Y-zeolites.
It
can be seen that the selectivity increases when Sint decreases, i.e. when
the structure becomes more basic. There are two sets ot results, one for the Y and one for the X zeolites. For each of these types the correlations are very simi 1 ar. Unsaturated hydrocarbons, which have
II
electrons capable o t a strong in-
teraction with the surface, can cause changes in the surface field properties. The difference noted between the X and Y zeolites may be related to nonidentical charge densities (linked to the aluminium atom content) in the two materials, producing dissimilar fields. Barthomeuf (ref. 4) shows that the specific field of the cations has a
601 small i n f l u e n c e a t h i g h coverage. The o n l y changes o r i g i n a t e f r o m t h e d i f ference i n aluminium atom content, which m o d i f i e s t h e t o t a l number o f charges i n t h e framework and hence t h e z e o l i t e f i e l d .
Ihe n i g n e r t h e aluminium content,
t h e lower t h e i n t e r a c t i o n energy between c a t i o n s i t e and adsorbed molecule. I t i s known t h a t t h e e f f e c t o f t h e z e o l i t e f i e l d due t o A10- on a d s o r p t i o n
4 i s b e t t e r s t u d i e d a t h i g h s u r f a c e coverage, w h i l e t h e s p e c i f i c c a t i o n i n f l u e n c e i s b e t t e r s t u d i e d a t low coverage. The above f e a t u r e s reveal a complex v a r i a t i o n o f t h e separation f a c t o r , showing no obvious c o r r e l a t i o n w i t h t h e charge, volume and p o l a r i z i n g power o f t h e exchangeable c a t i o n s o r t h e d i p o l a r moments o f t h e aromatic molecules. A t h i g h loadings i n t h e l i q u i d phase, t h e s e l e c t i v i t y i s determined by t h e e t f e c t s o f sorbate-sorbate i n t e r a c t i o n s ( r e f . 5). I n order t o v e r i f y t h i s hypothesis, runs were c a r r i e d o u t w i t h d i f f e r e n t r e c i p r o c a l concentrations o f t h e two isomers. Table 2 shows t h e s e l e c t i v i t i e s obtained. The s e l e c t i v i t y changes s i g n i f i c a n t l y w i t h t h e c o n c e n t r a t i o n o f t h e
i n i t i a l s o l u t i o n , and t h i s f a c t confirms t h e n o n - i d e a l i t y o f t h e b i n a r y m i x t u r e o f methylnapnthalene isomers. Table 2 S e l e c t i v i t y and amount adsorbed on KY a t 293 K upon changing t h e c o n c e n t r a t i o n o f methylnaphthalene isomers 1-MN (%w/wJ 2
2-MN (%w/w) 6 5
3
4 6
4
2
s2,, .I .50
1.63 2.23 2.80
r
1
r. z
36.3 42.8 63.6 90.9
r
95.0 82.3 80.0
39.2
total 131.3 125.1 143.6 130.1
x1
x2
0.277 0.342 U.443 0.69Y
0.723 U.658 0.557 0.301
Adsorption E q u i l i b r i a I n order t o b e t t e r understand t h e e q u i l i b r i u m behaviour o f t h e system under examination,
a more d e t a i l e d a n a l y s i s o f t h e adsorption e q u i l i b r i u m isotherms
was developed i n t h e case o f z e o l i t e KY. The pure-component e q u i l i b r i u m d a t a a t 293 K are shown i n F i g u r e 5 t o r both 1- and 2-methylnaphthaiene, c o n c e n t r a t i o n i n t h e adsorbed phase b u l k l i q u i d pnase C.
i n terms o f
r as a t u n c t i o n o f c o n c e n t r a t i o n
I n t h e same t i g u r e ,
i n the
t h e experimental d a t a a r e compared
w i t h curves c a l c u l a t e d through t h e Langmui r equi 1ibrium model
r / r"
= KC/l+KC
(2)
602
1
6
0.05
0.01
0
0.10
1
c
0.18
Fig. 5. Experimental data o f concentration i n t h e adsorbed phase, 2-MN. concentration i n the bulk phase, C(mol/l); ( 0 ) 1-MN, ( A
r (mg/g), vs.
using t h e f o l lowing values o f the equi 1ib r i um parameters, as obtained through t h e usual 1east-squares-estimati on tecnni que ,
K (l/mol)
rm (mg/g)
1-MN
2-MN
37.2
86
168
166
The observation t h a t two pure components e x h i b i t almost i d e n t i c a l values f o r the amount adsorbed a t s a t u r a t i o n conditions, r
m
, seems t o i n d i c a t e t h e
p o s s i b i l i t y o f d e s c r i b i n g b i n a r y e q u i l i b r i u m data through t h e multicomponent Langmuir e q u i l i b r i u m model, which i n t h e case o f two adsorbable components reduces as f o l l o w s :
r
-
- ir
KiCi
1+K.C.+K2C2
,i=
1,2
(3)
1 1
Such a model would p r e d i c t t h e f o l l o w i n g composition-independent expression f o r the binary s e l e c t i v i t y ( 1 )
which i n the case under examination leads t o S
2/1
= 2.31.
However, t h i s conclusion does n o t agree w i t h experimental f i n d i n g s , which, as shown by t h e data summarized i n Table 2, s e l e c t i v i t y upon composition.
i n d i c a t e a strong dependence of
603 This behaviour i n d i c a t e s t h e presence o f s t r o n g i n t e r a c t i o n s among adsorbate molecules, which are r e s p o n s i b l e t o r t h e observed d e v i a t i o n s f r o m i d e a l aehaviour. Thus, t h e m o d e l l i n g o f b i n a r y e q u i l i b r i u m data r e q u i r e s e x p l i c i t accounting f o r d e v i a t i o n s from i d e a l i t y i n t h e adsorbed phase.
l o t n i s aim t h e t h e o r y o f
adsorption from r e a l s o l u t i o n s , as f i r s t developed by Myers and P r a u s n i t z ( r e f . 6 ) i n t h e c o n t e x t o f gaseous m i x t u r e adsorption and subsequently extended t o l i q u i d mixtures ( r e f s . 7, 81, o f t e r s t h e most convenient framework. A t e q u i l i b r i u m c o n d i t i o n s t h e f u g a c i t i e s o f each i - t h component i n t h e l i q -
u i d and i n t h e adsorbed phase ( i n d i c a t e d w i t h a prime symbol) a r e equal
-
-
f. = f 1
1
’.
w i t h o u t going i n t o t h e d e t a i l s o t t h e e v a l u a t i o n o f t h e above f u g a c i t i e s , and r e c a l l i n g t h a t t h e system under examination i s a t low pressure,
one can
w r i t e as t o l l o w s ( r e f . I ) : T O ’ ’ ( T ) Y1. X1. = foYa(T,O)y;x; i i where
Y.
1
(5)
i n d i c a t e s t h e a c t i v i t y c o e f f i c i e n t , foYa(T,o) t h e pure-component 1
f u g a c i t y i n t h e adsorbed phase a t t h e same temperature, immersion values as i n t h e b i n a r y mixture.
and
0
f r e e energy o f
When t h e e q u i l i b r i a o f t h e pure
components a r e described through t h e Langmuir model , t h e q u a n t i t y foYa(T,o) can 1
be r e a d i l y evaluated from t h e tiibbs isotherm. This leads t o t h e f o l l o w i n g r e l a t i onshi p:
x
i
K .x
= - Y;X;
K.
1
c.
J
J j -
(61
yj
where i d e a l behaviour f o r t h e l i q u i d phase i s assumed ( i . e . y
= 1). From i eq(6) t h e f o l l o w i n g new expression f o r t h e b i n a r y s e l e c t i v i t y can be obtained:
(7) which depends on t h e composition o f t h e adsorbed phase through t h e a c t i v i t y c o e t f i c i e n t s . Such a f u n c t i o n can be q u a n t i t a t i v e l y evaluated by t h e use o f a model f o r t h e excess f r e e energy o t t h e adsorbed phase. F o r example, w i t h t h e n i l d e b r a n d model ( r e f . 91 t h e f o l l o w i n g expressions are obtained:
604
where the parameter A
i n d i c a t i n g t h e i n t e r a c t i o n between the two adsorbed 12’ species, can be considered as an adjustable parameter and estimated by d i r e c t
comparison w i t h the experimental data. This i s achieved through the f o l l o w i n g l i n e a r i z a t i o n r e l a t i o n s h i p : ,2 2 = A l p (x, x; 1 I n Y;/Y;
-
(Y)
This leads t o t h e l i n e a r p l o t shown i n F i g . 6, when t h e experimental values o f Table 2, the Langmuir constant values estimated above and eq(7) are used. From F i g . 6 the value A12 = 143 i s obtained.
I I n Y;/Yi
0
-0.5 0
-0.5
( x i 2 - x i2)
t0.5
Fig. 6. C o r r e l a t i o n o f experimental data from equation (9).
Tnus, i n conclusion, the binary s e l e c t i v i t y can be obtained by s u b s t i t u t i n g eq(8) i n t o eq(71, as f o l l o w s : 2 K2 exp(A12x; /r-1 = 2 K1 exp(A12x; /P)
-
(10)
The values obtained by t h i s r e l a t i o n s h i p are compared w i t h experimental data i n Figure 7. As f u r t h e r support f o r t h e conclusion t h a t non-ideal adsorbate-adsorbate interactions
are
dominant
in
tne
system
under
examination,
the
binary
e q u i l i b r i u m data may be r e p l o t t e d according t o the s t a t i s t i c a l approach r e p o r t ed by Ihm and Lee ( r e f . 10). I n p a r t i c u l a r , t h e f o l l o w i n g r e l a t i o n s h i p i s considered : - I n S2,1
=
KO
t
(1
- 2xi)cw/2kT
(I11
605
where K" i s a c o n s t a n t r e l a t e d t o t h e s u r f a c e t e n s i o n o f t n e p u r e component, c
is t h e number o f t h e n e a r e s t neighbour s i t e , w h i l e w r e f e r s t o t h e v a r i o u s i n v o l v e d m o l e c u l a r i n t e r a c t i o n e n e r g i e s as f o l l o w s :
w = U' 1-1
(12)
0.5
0
xi
I
F i g . 7. C o r r e l a t i o n o f e x p e r i m e n t a l d a t a f r o m e q u a t i o n ( 1 0 )
and t h e s u r f a c e coverage X ' can be c a l c u l a t e d f r o m t h e 2/ 1 1 s o r p t i o n data. The c o n s t a n t K O and cw/EkT can be o b t a i n e d f r o m t h e i n t e r c e p t The s e l e c t i v i t y S
vs.(l-2xi). 2/ 1 The e s t i m a t e d v a l u e cw/2kT = 0.75, which i s s i g n i f i c a n t l y d i f f e r e n t f r o m
and s l o p e o f t h e l i n e a r p l o t s o f - l n S
zero, c o n f i r m s t h a t t h e system e x h i b i t s a n o n - i d e a l behaviour, w i t h U i 2
+ Uilor
F u r t h e r s t u d i e s a r e i n p r o g r e s s along t h e s e l i n e s , w i t h d i f f e r e n t z e o l i t e s and temperatures, a d s o r p t i o n process.
i n o r d e r t o b e t t e r understand t h e n a t u r e o f a s e l e c t i v e However some h i g h s e l e c t i v i t y values,
determined i n t h e
p r e s e n t study, suggest c l e a r l y t h a t t h e s e p a r a t i o n of t h e s e two isomers i s p o s s i b Ie
.
ACKNOWLEDGEMENTS We w i s h t o thank Mr. A. R i v o l d i n i f r o m t h e I s t i t u t o d i G i a c i m e n t i M i n e r a r i , U n i v e r s i t i d i C a g l i a r i , f o r t h e atomic a d s o r p t i o n analyses on t h e z e o l i t e s .
606
REFERENCES 1 O.M. Ruthven, i n P r i n c i p l e s o f Adsorption and Adsorption Processes, John Wiley and Sons ( t d i t o r s ) , New r o r k 1984. z S. Namba, Y. Kanai, H. Shoj and T. Yashima, Z e o l i t e s , 4 (1984) 77. 3 D. Fraenkel, M. Cherniavsky, 8. I t t a h , M. Levy, J. Catal., 101 (1986) 273. 4 D. Barthomeuf, B.H. Ha, J. Chem. SOC. Farad. Irans. I , 7 (1913) 2158. 5 D.M. Ruthven, i n Olson and B i s i o ( E d i t o r s ) , Proceedings o f t h e s i x t h I n t e r n a t i o n a l Z e o l i t e s Conference, Butterworths, 1983, p. 31. 6 A.L. Myers, J.M. Prausnitz, A.I.Ch.E.J., 11 (1965) 121. 7 C. Minka, A.L. Myers, A.I.Ch.E.J., 19 (1973) 453. 8 O.G. Larionov, A.L. Myers, Chem. Eng. Sci., 26 (19/1) 1025. 9 J.H. Hildebrand, J.M. Prausnitz, R.L. Scott, i n r e g u l a r and Related Solut i o n ; Van Nostrand Rheinhold ( E d i t o r ) , New York, 1970. 10 S.K. Ihm, H.S. Lee, i n Murakami, L i j i m a , ward ( E d i t o r s ) , New Oevelopments i n Z e o l i t e s Science and technology, E l s e v i e r Amsterdam 1986, p. 571.
H.G. Karge,J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlanda
GEXwlRD REISS
Bayer AG, PK-A Zeolithe, D-5090 Leverkussen, F.R.G.
ABSTRACT Pressure swing systems for direct adsorptive production of oxygen from air are described and the costs of production are calculated and compared.
INTRODUCTIOK In the past ovygen production from air was mainly carried out by means of cryogenic systems. Since 1970 a new technique has been developed whereby ovygen is separated from air by the adsorption process using molecular sieve zeolites. Up to now the main fields of application of oxygen produced by these adsorption separation plants have been for water treatment and in the smelting of scrap metal. In some areas it has replaced over 50 % of the original oxygen market "(ref. 1)". Comparison of the adsorption system with conventional oxygen supply shows that the former has distinct advantages. Adsorptive production of oxygen from air The separation of oxygen from air with ?IS zeolites is based on the fact that nitrogen (quadruple moment) has a higher affinity to MS zeolites than oxygen. Ca-Zeolite A, .ha-Zeolite X and Ca-Zeolite X are normally used. Therefore H,O, CO, and N, are preferentially adsorbed and the required product, oxygen (plus argon), less so (see Fig. 1). Two different adsorption processes are usually applied in the market: pressure swing adsorption (PSA) with ail adsorption pressure of 3 - 6 bar and a desorption pressure of 1 bar (ref 2 ) and vacuum swing adsorption (VSA) with adsorption at 1.0 to 1.5 bar and desorption at 200 - 300 mbar (ref 3 ) .
In both processes the H,O, CO, and nitrogen in the air are adsorbed in one step. Desorption is carried out b? purging the bed with the oxygen produced or by evacuating the ?IS zeolite. Oxygen produced by PSA/VSA has already been used in various fields e.g. - -. . Y
-S
medicine ( 0 . 2 Nm3 02/h) (ref 4 ) and steel lancing (3.000 Nm O,/h)
(ref 5)
608
Pressure, mbar Fig. 1. Adsorption isotherms I of Ca-Zeolite !
Fig. L . Two-bed PSA u n i t ( a ) a i r compressor, (b) 0, product, (c) purge, ( d ) pre ssufe e q u i l i z a t i o n
Adsorption urocesses Pressure swing adsorption systems mainly d i f f e r i n the number of adsorberbeds used. Depending on t h e required oxygen r a t e , u n i t s with two, th r e e o r , l e s s common, four beds a r e used. The key t o the best pe rformance is maximum oxygen product recovery which reduces t h e s i z e of the a i r rompressor needed. T h i s is achieved by using small beds and pre ssure e q u i l i z a t i o n a f t e r t h e adsorption period. The simplest PSA process, and hence a l s o tho most favourable from t h e point of view of investment, is a two-bed u n i t with cy cl es of between 10 and 80 seconds per bed and a maximum adsorption pressure of 6 bar ( a b s ) . After adsorption, pressure is equalized a t the O2 product end of the u n i t . Air i s used f o r r ep r es s u r i zat i o n . A t t h e desorption s t a g e the nitrogen adsorbed
is desorbed or displaced by some of t h e 0, product. In orde r t o obta in a constant 0, product stream, a b u f f er must be used (se e F i g . 2 ) .
609
Since 0 , recovery capacity is limited in a two-bed unit, three-bed PSA systems are used for larger quantities of 02. The higher investment costs are offset by the lower energy consumption of the air compressor. All three-bed PSA systems are based on an old patent (ref 2) from Feldbauer (see Fig. 3 ) . In more advanced processes, the oxygen yield is increased by producing part of the 0 , during air repressurization and co-current pressure release (ref. 6 ) . Here the cycle should last 3 x (45-60) seconds. il
d a
Fig. 3 . Three-bed PSA unit (a) 0,-product, (b) repressurization + equflization, (c) purge, (d) air, (el waste
Fig. 4 . Three-bed VSX (a) air blower, (b) vacuim pump, (c) 0,-product, (d) repressurization
Vacuum swing adsorption plants generally consist of three adsorbent beds and have a process sequence of adsorption, evacuation and repressurization with the oxygen produced. The aim of this process is to ensure low energy consumption of the vacuum pump. The cycle lasts 3 x 60 seconds (see Fig. 4 ) . The energy consumption of the vacuum pump depends on the adsorption-desorption properties of the zeolite and the process data. In the last few years, we have made improvements in commercial zeolites f o r the 0, VSA process. The nitrogen adsorption capacity of the selected CaZeolite A has been increased by 100 8 . We noticed that i n the 0 , VSA process the O2 rate and hence also the energy consumption of the vacuum purr~p decrease as N2 adsorption increases.
610
The energy consumption shown in Fig. 5 was measured on comniercial VSX plants. A further reduction in energy consumption was achieved !)y optimizing the vacuum pumps (ref. 7) and by a purge-and-filling step after evacuation (back-fill purge process = BFP process) (ref. 8).
q//
160
* 140
Il2,
ii 100 d
/*
$O
80 1 0
7
8 9 10 11 12 13 14
N i t r o s e n load,
10
100
1000
Nlhg
Fig. 5 . Performance of an 0, vsa plant to N, capacity (at 1 000 mbar, 25OCf of Ca-Zeolite A . (a) 0 production rate, (b) 0, yigld, (c) horsepower of vacuum pump
Fig. 6 . (a) Staff and ( 0 ) maintenance costs of PSA/VSA oxygen plants
Comparison of adsorptive separation processes PSA/VSA processes can be compared from the point of view of capital cost, operation costs, flexibility of capacity turn-down and tinally oxygen production costs. From the market survey and experience it has to be concluded that two-bed PSA systems are limited to approximately 100 3m3 O,/h, three-bed PSA units are built for gas rates of between 100 - 500/600 Nm3 -O,/h for one 3 train and VSA plant with capacities up to 2 000 UI O,/h per train. Y
We have endeavoured to calculate the production costs or the price of oxygen produced by the three adsorptive processes described above. Personnel and repair costs are estimates (see Fig. 6 ) . The electricity requirements of the two- and three-bed PSA system are based on a medium adsorption pressure of 3 bar (abs) and a 2 9 % and 39 % oxygen yield.
611
The energy consumption of 3-bed PSA systems shown in Fig. 7 agrees with the ialues given in the literature (ref. 9 ) . The specific. energy consumption for inygen produced hy t h e VS4 process was determined with existing plants, taking into account the BFP process. The costs of investment in oxygen PSA/VSA plants (see. Fig. 8) a r e based on our experience of the market.
2-O:
*fi
P II
I 2
\ d
.
0.5
13
100 1000 02-capacity, Nn'/h (93 %) ' I
Fig. 7. Energy consumption (T! of oxygen PSA/VSA systems ( a ) ?-bed PSA, ( b ) 3-bed PSA,
3-bed VSA, (d) cryogenic systenls (c)
I
10
Fig.
8.
-
L
'
'
I
.
'
'
'
1000 100 02-capacity, Nn3/h (93 %)
Capital cost (DM/!h3
O J h ) of 0, PSAIVSA systems (93 % 0,) ( a ) ?-b6d PSA, ( h ) 3-bed PSA,
(c) 3-bed VSA
The price of oxygen produced by adsorptive processes can be seen from the calculations in table 1.
612
Table 1. Calculation of the costs of producing oxygen (93 % vol.) from air using the PSAIVSA proyess. Example: VS.4 process, 1 000 Nm /h 0, -; electricity: 0.1 DM/KW
Investment : DM 2.8 x 106 Quantity of MS zeolite : 75 000 kg Engergy consumption : 0.48 KWh/Nm 0, Annual operation: 8.200 h Personnel and repair costs: DY 2 9 000 ” Depreciation : 10 years DM 17 000 MS service life: 10 years 7
Tied-up capital Calculation of net assets required f o r operations, consisting of electricity costs, investnients, repairs, MS zeolites. Electricity : 0.48 x 1 000 x 0.1 Capital spending: Repairs Idsorbent =
N
8 200
Net assets
= = = =
DM 393 600 DM 2 800 000 DM 17 000 DM 525 000 DM 3 735 600
Calculation of production costs DM/h Energy Depreciation + Repairs + Personnel
a
=
Production costs
48.0
34.1 2.07 2.68
86.85 DM/h
Calculation of operating c o s t s DM/h Production costs + 13 % of net assets/operations/8 900 h + 30 % of production costs = Operating costs
86.85 59.22 17.38 163.51 DM/h
613
Production costs can be reduced greatly as the s ~ L \ . of the plant increases (see Fig. 9). The most cost-effective adsorptive method of producing oxygen is the V S A process. This h a s been confirmed by our knowledge of the market. Using the VSA, 100 to 2 000 Xm 3 oxygen per hour and per train is alieadl twiiig produced, with several plants in Europe, Japan, Korea, India and Taiwan.
0.6
fi
k
0.5 0.4
t
I 10
I
1
I
I
100
-& I
I
I
I
f
1000
Fig. 9. Costs of oxygen, supplied by PSA/VSA Electric current price (1): 0,15 DM/KW ( 2 ) : 0,l
DM/KW
(a): 2-bed PSA, (b): 3-bed PSA (c): 3-bed VSA Comparison of adsorptive separation processes with cryogenic plants Small oxygen adsorption systems producing up to 300 N I3 /h can compete with liquid oxygen, depending on the standard required. Typical applications include welding, fish-farming, glass-blowing and small waste-water treatment plants, With larger quantities of O 2 gas, oxygen produced in VSA systems can compete with cryogenic plants. The energy consumption of cryogenic plants (ref. 101, shown in Fig. 7, reverts the advantage of the VSA process as far as energy is concerned. Other advantages are ease of operation, the possibility of an u p to 10 - 20 % turn-down in nominal load and the availability of oxygen a few minutes after start-up of the plant.
614 REFERENCES Gas Review Nippon, No. 143 US Patent 3.338.030 G. ReiB, Chem. Ind., 35 (1983), 689-693 Brochure of Buderus AG, Wetzlar Gas Review Nippon, Summer 1985, pp. 6-7 US Patent 3.636.679 DOS 3.413.895 DOS 3.144.012 Leaflet of Linde AG, Werksgruppe TVT Mtinchen, Sauerstoffverbrauch durch PSA-Anlagen 10 H. Springmann, Wirtschaftlichkeitsgrenzen fkr den Bau von Luftzerlegungsanlagen, 3. Arbeitstagung der Linde A G , Oct. 14.-16., 1975 in Munich
H.G. Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
ADSORPTION AND DIFFUSION OF DIFFERENT HYDROCARBONS IN MFI ZEOLITE OF VARYING CRYSTALLITE SIZES D.H. LIN, V. DUCARME, G. COUDURIER and J.C. VEDRINE(*) Institut de Recherches sur la Catalyse, C.N.R.S., ConventionnO B 1'UCB. LYON I, 2 Avenue Albert Einstein, 69626 - Villeurbanne (France) ABSTRACT MFI zeolite samples have been synthesized with a crystallite size varying in a range of 1 to 220 um and with aluminum, boron or silicon as substituting elements for silicon in the framework. The rate of hydrocarbon adsorption at temperatures from 18 to 120°C has been measured gravimetrically with a micro-balance. N-hexane and para-xylene have been observed to adsorb too rapidly for small crystallites to give reliable information with such a technique. 3-methyl-pentanehas been on the contrary, found to be suitable, over the whole range of sizes. The rate of 3-methyl-pentane adsorption has been observed to decrease with increase of the particule s i y i , wtp.. allows us to calculate a diffusion coefficient equal to ca 1.1 x 10 cm s The chemical feature of the materials, mainly the presence of more or less strong Bronsted acid sites, has been shown to have an effect on the adsorption rate, but only of minor importance.
.
INTRODUCTION The importance of zeolitic-type materials is well established and need not be emphasized in terms of sorptive and catalytic properties. The concept of shape selectivity has also been widely discussed and assigned to a particular fitting between
the
reactant and/or product molecules
and
the
zeolite
geometrical aspects : pore size, interconnecting channel structure, etc. The role of adsorption and diffusion of molecules within the pores is obviously important for shape-selectivity properties. Several review papers have already been devoted to the subject, particularly by Barrer (ref. 1) and Ruthven (ref. 2). The article by Palekar and Rajadhyaksha (ref. 3 ) reports interesting results with special emphasis on catalytic reactions. Various experimental techniques have been employed in trying to measure diffusivities. The most commonly used method is conventional gravimetry, where sorption kinetics is measured by means of a microbalance. The most serious limitation is in terms of the response time of the equipment and the heat transfer effect. Other techniques include chromatography (ref. 4 ) . tracer diffusion (ref. 51, neutron diffusion (ref. 6 ) and pulse field gradient NMR
(*)
to whom queries should be sent
616
spectroscopy (ref. 7 ) .
The former two involve adsorption and diffusion with a
concentration gradient (driving force) whiie the latter two involve motion of the adsorbates within the pores. This difference results in large discrepancy in diffusion coefficient values, by 2 to 4 orders of magnitude. More recently Ruthven et al. (ref. 8) have tried to eliminate the intrusive effects of adsorption heat and mass transfer by using a chromatographic technique
(ZLC
zero-length column).
It
consists
of
analyzing
by
gas
chromatography the desorption of an adsorbate under sudden purge with an inert carrier. To try to provide some insight to the difficult problem of adsorption and diffusion we have studied gravimetrically the adsorption of hydrocarbons on samples exhibiting a large domain of crystallite sizes (1 up to 220 pm) and Bronsted acid sites of different strength and concentration. Special care was taken to synthesize well crystallized samples. Note that we have chosen only samples with well defined shape (usually parallelepipedic twinned single crystals) and narrow size distribution. EXPERIMENT
A Sartorius microbalance was used for measuring the adsorption and desorption rates. The samples (ca. 10 mg) were outgassed at 400°C overnight down to
torr (1.3 x 10-2Pa) and then cooled down to room temperature for 10 to
15 minutes. The samples were placed at the desired temperature and further contacted with the hydrocarbon vapor at a given pressure measured by a Barocell gauge. The adsorbate molecules were outgassed by the freeze-pump-thaw technique several times. The weight changes were directly recorded with a Sefram recorder. MPI-type materials were synthesized either in our laboratory in the classical way in basic medium or in professor R. Wey's laboratory in Mulhouse in fluoride medium (ref. 9). Chemical analyses of the samples were performed in our laboratory by atomic absorption of the elements other than Si, namely aluminum, boron and sodium. The morphology of the crystallites was determined by scanning electron microscopy from CAMECA with the aim of measuring the size distribution. EXPERIMENTAL RESULTS AND DISCUSSION The chemical composition of the samples is given in table 1. All samples are well crystallized as evidenced by IR spectroscopy (ref. 10). X-Ray diffraction and adsorption capacity values ( 8
n hexane, 6
0.5 for 3-methyl-pentaneand 8
2
0.5 mol per
0.5 for p-xylene).
U.C.
for
617 TABLE 1 Chemical composition of the various samples calculated from chemical analysis of Al, B and Na assuming 96 TO per U.C. the proton content is calculated assuming tetrahedral framework for A? and B. Samples
Composition (per u.c.)
~
A Silicalite
H0.3
A1O. 06 A1O. 32
si95.68
'192
H1.4 H3.56
A10.02
B1.38
si94.6
'192
Na0.05
A13.61
A10.02
B3.18
si92.39 si93.4
'192
H3.2
H4.a3
A14.80
B0.03
si91.17
'192
H3. 14
A13.11 A10.11
BO. 03
si92.86
'192
Na0.11
si95.89
'192
3
A14.1
si91.9
'192
05
B H-A1, ZSM-5 C H-B, ZSM-5
D H-A1, ZSM-5 E H-B, ZSM-5 F H-A1, ZSM-5 G H-A1, ZSM-5 H Silicalite
I H-A1, ZSM-5
H3.8
si95.94
0192
'192
TABLE 2 Average crystallite sizes of the various samples calculated from SEM data Size (urn>
Samples
A/V* (pm-')
~~
A
220 x 40 x 40
0.11
B
75 x 40 x 30
0.14
C
90 x 15 x 15
0.29
D E F
38 x 20 x 12
0.32
30 x 12 x 8
0.48
17X7X5
G
2.5 x 0.8 x 0.6
H
5 x 1 x 0.3
I
1 x 0.6 x 0.35
0.80
6.6 9.0
11.0
fc A and V correspond respectively to the surface and volume of the crystallites deduced from SEM data (shape and size of crystallites). The accuracy in A/V values obviously decreases with decreasing crystallite size. The average size was determined by analysis of 20 to 30 particles for homogeneous size distribution and more than 200 for slightly heterogeneous size distribution.
The adsorption of n-hexane, 3-methyl-pentane and p-xylene was studied at room temperature for all samples. The rate of adsorption was observed to decrease sharply with increasing crystallite size (ref. 11). The rate was too fast for n-hexane and para-xylene in the case of small crystalite sizes to give reliable information by the gravimetry technique (adsorption within a few seconds for sample I, for instance). On the other hand, the adsorption rate
618 appeared to be reliably measurable for 3-methyl-pentane within the whole crystallite size range (fig. 1). The adsorption rate may be expressed using the Crank relationship (ref. 12): m
6
m,
n2
-t-- 1 - -
- 1 E - 2 exp n=l n
[-
4nnADt 2V2 2 2
I
For small adsorption time this relationship may be written :
3 . c m
[-]
A D 112
m - =
2
-
t1/2 = kt1l2
V n
where A is the total external surface, V the total volume of the cristallites, and mt and m,
the weight gain at time t and at equilibrium. Figure 2 gives the
variations of k values for 3-methyl-pentane adsorption at room temperature versus A/V for the various samples within the whole crystallite size range. Several conclusions may be drawn : (i) We were unable to synthesize samples with intermediate crystallite sizes (A/V between 1 and 7 pm-'). (ii) For large crystallite sizes (A/V
< 1 Vm-')
a linear relationship
between k and A/V exists (Fig. 2B). For small crystallite sizes (A/V > 7 prn-') one may also consider a linear relationship. However, for the whole range of crystallite size some curved relationship appears to exist, as shown in Fig. 2A, which may correspond to some surface barrier phenomenon for small crystallite sizes, as discussed below. (iii) For large crystallite sizes it seems that A1-ZSM-5 samples correspond to a smaller SlOp8, i.e. lower adsorption rate than for B-ZSM-5 and to a larger extent than for Si-ZSM-5. This trend also exists for smaller crystallite sizes, but the effect is not very important. It is known (ref. 13) that Bronsted acid site strength follows the'relation Si
< B << Al. It may thus be suggested that Bronsted acid sites hinder
3-methyl-pentane adsorption commensurate with the strength of the sites. The effect is not very important but does exist. The adsorption rate depends on the adsorbate concentration (ref. 14-17). The diffusion coefficient can then be corrected using Darken's equation : D = Do (dLn p/dLn 9). Using different concentration gradients, i.e. different partial pressures, it was possible to calculate D Experimental data are given in Fig. 3 for 0' sample A for three different adsorption temperatures.
619
8
E \
c,
E
2
0
6 t
(s
I /2)
10
Figure 1. 3-methyl-pentane uptake at room temperature for different crystallite sizes (in urn) of MFI-type samples.
D
: 38 x 20 x 12 um G : 2 . 5 x 0.8 x 0 . 6
E:30x12x8um
F : 1 7 x 7 x S u m
um
A
B
A e
0.4.
N \
-I
0.3.
v)
v
0.2.
0.1.
Figure 2 . Variations of k values versus A/V for the different samples. A : Within the total crystallite size range,
B
: Enlargement of the part corresponding to large crystallite sizes. The symbols correspond to samples in tables 1 and 2, and the elements in parentheses refer to the substituting element for Si in the framework.
620
20
1
0 m t x I 02 (9.9Figure 3. Variations of k ('0). 80 (0) and 120 ( x ) " C
2
II
versus the amount of 3-m
hyl-pentane adsorbed at 40 J D for sample A with k2 = 2 - . V
n
Figure 4. Variations of the m,/moo values as a function of t1l2 at 18 (0).40 80 (t) and 120 ( x ) " C for sample H.
(o),
62 1
Diffusion 1.1 x 0.5
-
coefficient Do values were found to be equal to cm’s-l in the whole range of crystallite sizes at 4OOC. The
adsorption of 3-methyl-pentane was studied as a function of temperature from 18 to 120°C. as shown in Fig. 4 for sample H. The activation energy value could -1 therefore be determined and was found in the range 10 to 21 kJ.mol for 3-methyl-pentane, the larger values corresponding to the larger crystallite sizes. Catalytic properties are known to be influenced by diffusion features (ref. la),
particularly for shape selectivity. The catalytic behaviour involves
several steps :
(i) Adsorption at the crystallite surface. Its probability is rather low since the surface is mainly composed of pore openings in MFI structure. (ii) Entrance through the pore openings which may involve a surface barrier phenomenon. This should increase when the crystallite size decreases since the number of openings increases sharply. (iii) Diffusion of reactant molecules within the pores. (iv) Adsorption of reactant molecules on active sites and desorption of product molecules. It depends obviously on the number and density of active sites and on the space available with respect to intermediate species size. (v) Diffusion of the product molecules. (vi) Exit of the product molecules. Catalytic properties have been studied for A1-ZSM-5 samples for methanol conversion at 370°C and toluene alkylation with methanol at 400°C. Si- and BZSM-5 are inactive under the same conditions. The selectivity in para-xylene with respect to the other two isomers has been observed to increase noticeably with crystallite sizes. For instance, it varies from 55-60 X up to 95 Z when the crystallite size increases from 1 to 220 pm. However. the deactivation rate in such reactions was found to increase greatly with increasing crystallite size (ref. 19). This presumably stems from the corresponding sharp lowering of the number of pore openings, deactivation being primarily due to pore blocking. CONCLUSION Adsorption rate measurement appears to be a good way to elicit information about diffusivity phenomena in zeolite matrice insofar as an appropriate adsorbate is chosen. Mass transfer and heat transfer consecutive to adsorption are two serious limitations one has to take into consideration when using the thermogravimetry technique. 3-methyl-pentane turns out to be a suitable adsorbate for studying diffusivity in MFI zeolite over a large range of crystallite sizes ; n-hexane
622
and p-xylene are suitable only for large crystallite sizes in order for the adsorption rate to be low enough for meaningful measurement. Strong Bronsted acid sites associated with A1 are observed to slightly hinder diffusion of the adsorbate molecules by comparison with weak or non-acid sites associated with B and Si respectively.
A limitation to diffusion due to a surface barrier effect, as often suggested (e.g.
ref. 20, 211, does not appear to be very efficient for
3-methyl-pentane,at least for large crystallite sizes. As a matter of fact, the fast initial adsorption involves many more molecules of adsorbate than the number of surface pore openings. Such an effect may explain Fig. 2A for low crystal size, particularly because the number of pore openings is then not negligible. Nevertheless, we are of the opinion like Ruthven (ref. 8) that it is possible to implicate this effect in the behaviour between crystallites of small size in regard of crystallites of large size and perhaps it is also a limiting step even for crystallites of large size but to a lower extent. In fact, if the diffusivities so calculated are not actually representative of the true intracrystalline diffusion, as compared to values obtained by field gradient N.M.R. or neutron diffraction studies, they may be more representative of the catalytic activity, since both adsorption and diffusion phenomena are involved. REFERENCES
1 2 3 4 5 6
7 8 9
10 11 12
R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, chap. 6, Academic Press, London, 1978, pp. 256-337. D.M. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley, New York, 1984. M.G. Palekar and R.A. Rajadhyaksha, Catal. Rev. Sci. Eng. 28 (1986) 371-429. D.M. Ruthven, K.F. Loughlin and R.I. Derrah, Molecular Sieves, Adv. Chem. Ser., Vol. 121, W.M. Meier and J.B. Uytterhoeven, Ed., Washington (1973) 330-344. L. Forni, C.F. Viscardi and C. Oliva, J. Catal. 97 (1986) 469-479. L. Forni and C.F. Viscardi, J. Catal. 97 (1986) 480-492. A.N. Pitch, H. Jobic and A. Renouprez, J. Phys. Chem. 90 (1986) 1311-1318. D.L. Hasha, V.W. Miner, J.M. Garces and S.C. Rocke. Catalyst Characterization Science, ACS Symp. Ser., Vol. 288, M.L. Deviney and J.L. Gland, Ed., Washington, (1988) 485-497. J. Karger, H. Pfeifer, E. Ride1 and H. Winkler, J. Colloid Interf. Sci. 44 (1973) 1870. D.M. Ruthven and M. Eic, Perspectives in Molecular Sieve Science, ACS Symp. Ser. Washington, Vol. 368, W.H. Flank and T.E. Whyte, Ed., (1988) 362-375. J.L. Guth. H. Kessler and R. Wey, Proceed 7th IZC, Tokyo, Y. Murakami et al.. Ed., Elsevier, Amsterdam, 1986, p. 121. G. Coudurier, C . Naccache and J.C. Vedrine, J. Chem. SOC., Chemical Corn. 1982. pp. 1413-1415. J.C. Vedrine, G. Coudurier and B.P. Mentzen, Perspectives in Molecular Sieve Science, ACS Symp. Ser., Vol. 368. W.H. Flank and T.E. Whyte. Ed., 1988, pp. 66-84. J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1976.
623
13 14 15 16 17 18 19 20 21
G. Coudurier and J.C. Vedrine, Pure and Appl. Chem. 58 (1986) 1389- 396. M. Goddard and R.M. Ruthven. Zeolites 6 (1986) 283-289. A. Cointot, G. Joly and V. Perperas, J. Chirn. .Phys. 80 (1983) 213-2 8. A.P. Vavlitis. D.M. Ruthven and K.F.J. Loughlin, J. Colloid Interf. Sci. 84 (1981) 526. M. Biilow, P. Struve and Ch. Redsus, Proceed 5th IZC, Napoli, L.V.C. Rees, Ed., Heyden, London, (1980) pp. 580-591. W.O. Haag, R.M. Lago and P.B. Weisz, Discussion Faraday SOC. 72 (1981) 317-330. V. Ducarme and J.C. Vedrine. Appl. Catal. 17 (1985) 175-184. J. Karger and H. Pfeifer, Perspectives in Molecular Sieve Sciences, ACS Sympos. Ser., Vol. 368, W.H. Flanck and T.E. Whyte, Ed., Washington (1988) pp. 376-396. E.G. Derouane, J-M. Andre and A.A. Lucas, J. Catal. 110 (1988) 58-73.
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H.G.Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HIGH RESOLUTION SORPTION STUDIES OF ARGON AND NITROGEN ON LARGE CRYSTALS OF ALUMINOPHOSPHATE A1P04-5 AND ZEOLITE ZSM-5
u.
MULLER',
K.K. UNGER',
Y. GRILLET',
F
DONGFENG
PAN^,
A. MERSMANN'
. ROUQUEROL3, J. ROUQUEROL3
'Institut fur Anorganische Chemie und Analytische Chemie, P. 0. Box 3980, Johannes Gutenberg-Universitat, D-6500 Mainz, F.R.G. 21nstitut fur Verfahrenstechnik der Technischen Universitat, Lehrstuhl B, Arcisstr. 21, D-8000 Miinchen 2, F.R.G. 3Centre de Thermodynamique et de Microcalorimetrie du C.N.R.S., 26 Rue du 141e R.I.A., 13003 Marseille, France ABSTRACT High resolution adsorption (HRADS) with argon and nitrogen at 77 K in the pressure range of c p/po < 0.5 were performed on large crystals of zeolite ZSM-5 (180 pm) and aluminophosphate AlP04-5 (150 pm) using a novel volumetric device. Multi-step isotherms of both adsorptives on ZSM-5 could be observed for the first time. The adsorption followed by low temperature microcalorimetry resulted in distinct exothermic signals at the steps in the adsorption isotherms. Based on the results of atom-atom potential energy calculations (AAP) as well as independent model building it was shown that 24 'kinetic' adsorbate molecules can be filled into a ZSM-5 unit cell. Experimental results are reasonably interpreted assuming a primary filling of narrow channels and a secondary adsorption in the wider channel intersections. Localized adsorption is understood as a possible filling mechanism. In ZSM-5 and nitrogen as adsorbate there is evidence for a transition of fluid-like to solid-like adsorbate phase. AlPOq-5 behaves as a homogeneous sorbent with a micropore capacity of four molecules per unit cell for argon and nitrogen. For both adsorptives in the molecular sieves under investigation the initial isosteric heat of adsorption for nitrogen was found to give values comparable to the enthalpies of adsorption derived from the temperature dependence of experimentally determined HENRY constants. HENRY constants and the initial isosteric heat of adsorption are indicative of a stronger adsorption of nitrogen compared to argon which is thought to be due to additional interactions between the nitrogen quadrupole moment and the crystalline molecular sieve framework. INTRODUCTION Zeolite ZSM-5 and aluminophosphate AlP04-5 have attracted considerable interest as microporous model adsorbents. Both materials are crystalline molecular sieves with strictly regular pore systems, viz. intersecting straight and zigzag 10-membered ring channels (ultramicropores) connected by larger cavities (micropores) in ZSM-5 111, and unidimensional 12-membered ring tubes (micropores) in A1POq-5 12 1 . Based on novel synthesis concepts 13,41 the growth of large uniform crystals at high yields and with narrow particle size distribution has been achieved. Employing large monocrystals in sorption investigations, no erroneous contributions by interparticle vapour condensation and external surface area
626 effects to adsorption occur as on polycrystalline powders. Thus, the intrinsic sorptive properties can be reliably determined with high precision over a wide range of coverage 8 . Gravimetric sorption experiments with nitrogen at 77 K on large ZSM-5 crystals led to the discovery of a sharp hysteresis loop at low pressures of p/po = 0.15 15-71. Furthermore, in contrast to the BDDT-model 181 which predicts a LANGMUIR-type of isotherms for microporous adsorbents, both argon and nitrogen revealed distinct multi-step isotherms on large ZSM-5 crystals whereas no deviation from type-I isotherms was seen for AlP04-5. This study presents experimental results obtained by combining high resolution adsorption techniques (HRADS) with low temperature microcalorimetry. The calculated thermodynamic properties like HENRY constants, heats of adsorption and atom-atom approximation for adsorbate-adsorbent interactions strongly suggest localized adsorption on energetically enhanced crystallographic sites. In addition, a transition of the adsorbate to a denser phase at higher coverage is assumed to explain the sorption behaviour of nitrogen on large ZSM-5 crystals. EXPERIMENTAL Large crystals, 600 pm, of AlPO4-5 131 were prepared by optimization of a procedure described elsewhere 191. A sample consisting of 150 pm hexagonal rods was used in this study. Monocrystals of HZSM-5 (180 pm with Si/A1 = 1000) were synthesized according to a process described in the literature 141. The products were characterized by scanning electron microscopy SEM. x-ray diffraction, thermal analysis and electron microprobe analysis. Additionally the HZSM-5 crystals were analyzed by IR and 29Si-MAS-NMR spectroscopy. Adsorption isotherms with argon and nitrogen at 77 K were recorded on a novel dynamic volumetric device (Omnisorb 360, OMICRON Corp., U.S.A.). pressure of p/po =
Data acquisition started at a relative
and was continued up to p/p" = 0 . 5 , thus collecting se-
veral hundred quasi-equilibrium points of the adsorption isotherm. Nitrogen adsorption isotherms at 303 to 430 K were determined gravimetrically at a relative sensitivity of lo6 (ultramicrobalance 4433, Sartorius, F.R.G.)
. Continuous calori-
metric measurements were performed on a reversible isothermal microcalorimeter
of Tian-Calvet type (C.N.R.S. Thermodynamique et Microcalorimetrie, Marseille, France). A detailed description of the microcalorimeter is given in the literature 1101. Prior to all experiments samples calcined at 823 K were degassed for 12 hours at 473 K and at a vacuum of
mbar.
CALCULATIONS Calculations of the intermolecular interactions of argon adsorbed on ZSM-5 and AlP04-5 respectively, were performed using atom-atom approximation for the potential energy (AAP) and Lennard-Jones 6:12 potentials. Only contributions of the framework oxygen atoms were taken into account, based on determinations of the refined structures of ZSM-5 111,121 in space group Pnma and P6cc for
627 A1P04-5
121. V a l u e s f o r t h e p o l a r i z a b i l i t y of a r g o n (1.63.10-3
nm3) a n d n i t r o g e n
(0.88-10-3 nm3) a s w e l l a s t h e i r r a d i i were t a k e n from l i t e r a t u r e d a t a 131. Cov-
e r a g e s €I were c a l c u l a t e d from t h e e x p e r i m e n t a l i s o t h e r m s as m o l e c u l e s of a d s o r bate per
n i t c e l l ( m o l e c / u c ) , u s i n g STP v a l u e s f o r a r g o n (1.784.10-3
nitrogen
1.251.10-3
g/cm3).
g/cm3) a n d
D a t a d e r i v e d by g r a v i m e t r y were c o r r e c t e d f o r buoy-
ancy w i t h t h e s o r b a n t c r y s t a l l i n e d e n s i t i e s . I s o s t e r i c h e a t s o f a d s o r p t i o n , qST, a r e t a k e n from t h e e x p e r i m e n t a l h e a t f l u x V(dp/dt)
4 by
t h e equation:
-4
qST =
(1)
dn/dt w i t h d n / d t as c o n t i n u o u s a d s o r b a t e u p t a k e r a t e a n d a c o r r e c t i o n V ( d p / d t ) f o r t h e h e a t l o s s d u r i n g t h e e x p a n s i o n o f gas i n t o t h e s a m p l e b u l b . The i s o s t e r i c h e a t of a d s o r p t i o n , qST, c a n b e compared w i t h t h e v a l u e of t h e d i f f e r e n t i a l e n t h a l p y of a d s o r p t i o n , q d i f f s
( q d i f f = qST - R T ) . From t h e i n i t i a l s l o p e of t h e i s o t h e r m s
t h e HENRY c o n s t a n t s KH were c a l c u l a t e d a c c o r d i n g t o €I = K H
-
p. The h e a t of ad-
s o r p t i o n , -AH, was d e r i v e d from HENRY c o n s t a r i t s u s i n g KH = Kfi e x p (-4H/RT). RESIJLTS and DISCUSSION I s o t h e r m s a n d isosteric h e a t of a r g o n a n d n i t r o g e n on AlP04-5 Type-I i s o t h e r m s o f a r g o n a n d n i t r o g e n (see F i g . 1, shown f o r N2) on large A1P04-5 c r y s t a l s w e r e o b s e r v e d f o r b o t h gases a t 77 K. A p p l y i n g LANGMUIR-plots as w e l l as t h e method of DUBININ-RADUSHKEVICH, a m i c r o p o r e f i l l i n g c a p a c i t y of
3.9
0.2 molec/uc f o r n i t r o g e n a n d 3 . 8 2 0 . 2 molec/uc f o r a r g o n w a s c a l c u l a t e d .
Dense a d s o r b a t e p h a s e s f o r n i t r o g e n a n d a r g o n i n AlP04-5 were assumed. I s o s t e r i c h e a t of a d s o r p t i o n , qST, r e a c h e d a maximum a t a c o v e r a g e o f a b o u t 3.5 m o l e c / u c ( 1 4 . 5 kJ/mol N2 and 1 3 . 2 kJ/mol A r ) a n d d e c l i n e d when t h e s a t u r a t i o n c a p a c i t y was e x c e e d e d . W i t h i n t h e e x p e r i m e n t a l e r r o r t h e i n i t i a l i s o s t e r i c h e a t , qST, f o r ads o r p t i o n o f n i t r o g e n (13 f 0.6 kJ/mol a t 0 e O . 1 ) c a n b e compared t o t h e h e a t o f adsorption, -AH,
c a l c u l a t e d from t h e t e m p e r a t u r e d e p e n d e n c e o f t h e HENRY c o n s t a n t
KH ( 9 . 9 2 3.3 k J / m o l ) . A t 77 K t h e l i n e a r s l o p e o f t h e n ' i t r o g e n i s o t h e r m a t
p / p o c 10-5 y i e l d e d v a l u e s of KH = 13.1 molec/uc'mbar a n d KH = 1 4 . 2 molec/uc.mbar r e s p e c t i v e l y . The l a t t e r was c a l c u l a t e d u s i n g t h e v i r i a l e q u a t i o n of BARRER 1141. F i g . 1. High r e s o l u t i o n a d s o r p t i o n (HRADS) i s o t h e r m s a t 77 K of a r g o n a n d n i t r o g e n o n large c r y s t a l s of a l u m i n o p h o s p h a t e A1P04-5 ( 1 5 0 pm) a n d z e o l i t e ZSM-5 ( 1 8 0 bm a n d S i / A 1 = 1000).
628 Isotherms and isosteric heat of argon and nitrogen on ZSM-5 Multi-step isotherms were observed for nitrogen on ZSM-5 at 77 K. Steps in the adsorbate uptake were monitored at coverages of 20, 22, 24 and 30.5 molec/uc, respectively (see Figs. 1,2). Standard deviations amounted to 0.8 molec/uc in the uptake values, based upon statistical certainties of 95%. The initial isosteric heat of adsorption of nitrogen was calculated to 16.5 kJ/mol. The exothermic peaks of 6.4 kJ/mol clearly coincide with the observed steps in the isotherm as illustrated in Fig. 2. The exothermic contributions at coverages of 20 to 22 and 24 to 30.5 molec/uc respectively, were comparable to the sum of the heats of
condensation (5.56 kJ/mol) plus solidification (0.72 kJ/mol) being required for phase transitions in tridimensional bulk nitrogen. At low coverage (0e0.1) values of qST (16.8 2 0.2 kJ/mol) equal the enthalpy change derived from van't HOFF plots 1.7 kJ/mol). HENRY constants for nitrogen on ZSM-5 at 77 K were computed
(18.2 to KH
=
830 molec/uc'mbar, using the virial equation at p/po*
decreased to KH
=
this value was
594 molec/uc.mbar.
Fig. 2. Nitrogen isotherm and isosteric heat of adsorption on large ZSM-5 crystals at 77 K. The argon adsorption on ZSM-5 occurred in steps of 20 and 24 molec/uc (Fig. 1). At these steps in the isotherm an exothermic increase (1.7 kJ/mol) of the isosteric heat of adsorption, qST, (see Fig. 3)occurred; however, the heat curve as well as the isotherm were less pronounced compared to the N2/ZSM-5 system. Potential energy distribution in A1P04-5 and ZSM-5 Results of atom-atom approximation (AAP) calculations for argon atoms adsorbed on A1P04-5 are summarized in Fig. 4. Six minima of the potential energy were found above the evenly distributed TO4 6-rings in the channel walls. Model-building with localized 'kinetic' gas molecules (0.38 nm diameter) at these minima, indicated a preferred population of three argon atoms per radial TO4 six-ring
629 channel segment. This represents a micropore filling of 4 molec/uc and is in close agreement with the experimentally observed data from sorption isotherms and enthalpy curves.
1 -7
-
.. .
.
... .,
Fig. 3. Argon isotherm and isosteric heat of adsorption on large ZSM-5 crystals at 77 K.
From AAP-calculations and independent model-building 24 minima of the potential energy in an unit cell of ZSM-5 were registered. Eight minima are situated at the channel wall 6-rings in the straight tubes (see Fig. 6) and twelve in the zigzag channels (see Fig. 7). Additional four ones are located at the wider channel intersections, thus giving a total of 24 rninima/uc. Note that in the argon and nitrogen isotherms steps occur at fillings of 20 and 24 molec/uc, respectively.
Fig, 4. Potential energy distribution (AAP approximation) for argon across a channel in A1P04-5.
630 Sorptive properties of large AlP04-5 crystals Adsorption studies of microcrystalline AlPOq-5 have been described in the literature 115-181. Unusual type-V isotherms of the system H*O/AlP04-5 have been reported and were attributed to crystal hydrate formation 1161. Studies of STACH and coworkers I17 I at ambient temperatures identified A1P04-5 as energetically homogeneous sorbent. At low temperatures type-I isotherms f o r oxygen and nitrogen are reported by WILSON et a1 1151 and BOND et a1 1181, respectively. However, these isotherms showed 30 to 40% of the final uptake in a relative pressure range of 0.05
of preadsorption of n-hexane on AlP04-5, previously investigated 131, clearly showed that the observed hysteresis was not caused by micropores in AlPO
5. 4The comparatively low heats of adsorption of argon and nitrogen obtained in
this study (see Fig. 5) support the view that AlP04-5 exhibits a mildly hydrophilic sorbent character. At low pressures the micropore filling capacity for kinetic gas molecules (4molec/uc) is reasonably predicted by theoretical AAPcalculations and model-building.
0.1
AlP04-5 I77 K1
I¶ I -
P/OO
kd-1
0.4
I¶
0.a 10
0.1
¶
0.1
0.0
0
l
a
a
4
1
Fig. 5. Nitrogen isotherm and isosteric heat of adsorption of argon and nitrogen on large AlP04-5 crystals at 77 K. Sorptive properties of large ZSM-5 crystals Data for nitrogen adsorption on polycrystalline powders of ZSM-5 habe been already reported in the literature 119,201. In general, the overall sorption capacities agree fairly well with this study. However, most of these measurements collected only few isothermal points (5 to 20) and were not extended into the low pressure range p/poe
A geometrical model of JACOBS et al. 1201 pro-
poses a closest-possible packing of nitrogen molecules in the larger channel intersections of ZSM-5. In contrast to this model, theoretical calculations of gas-
631 solid interactions by EVERETT and POWL 1211, recently adapted to zeolites by DEROUANE I22 I , clearly indicate an enhancement of adsorption energy in smaller pores of molecular diameter (e.g. 0.55 nm channels in ZSM-5). According to DUBININ 123 I these channel 'ultramicropores' are primarily filled due to overlapping force fields from opposite walls of the narrow micropores. With increase of the pore size up to 2-3 molecular diameters (i.e. 0.8 to 1.0 nm for argon and nitrogen and close to the space available in a ZSM-5 channel intersection) the superimposed enhanced adsorption energy deciines, and cooperative secondary rnicropore filling might occur 1241.
Fig.' 6. Potential energy distribution for argon in a straight channel of ZSM-5 (arrows: channel minima; asterix: minima at the intersection).
I ,-,, ,o..B/..-ciIl
Fig. 7. Potential energy distribution for argon in a zigzag channel of ZSM-5 (arrows: channel minima; asterix: minima at the intersections). For argon and nitrogen AAP calculations and model-building yielded a channel population of 20 molec/uc which is in full agreement with the first step in the adsorption isotherms (see Fig. 1). Further filling of the four larger intersec-
tions with one argon atom in each site is attributed to the second plateau in the argon isotherm, giving a final coverage of 24 molec/uc. At this stage all channels and intersections are occupied by kinetic gas molecules. The exothermic increase (1.7 kJ/mol) during the intersection filling (20 up to 24 molec/uc) might be due to adsorbate-adsorbate interactions (e.g. the heat of fusion for bulk liquid argon amounts to 1.21 kJ/mol). As already shown (figs. 1-3) the situation is more complex for the nitrogen in ZSM-5. Although the kinetic molecular diameter of argon (0.35 nm) and nitrogen (0.38 nm) are quite similar, nitrogen as a diatomic molecule offers a considerable
quadrupole moment which permits additional adsorbate-adsorbent interactions. Hence, the steps in the nitrogen isotherms are more pronounced and the isosteric heat of adsorption is enhanced (1.8 kJ/mol) compared to argon. After initial uptake of 20 molecules in the channels (i.e. first step in the isotherm) a further coverage up to 22 molec/uc follows. The latter connects the channel adsorbate (2 nm) to the overall crystal adsorbate macroscopic phase (at least 30,000 nm,
as taken from the crystal dimension along the (100) or (010) axis). At this point, monitored as a small substep in the isotherm, the adsorbate phase loses significantly in kinetic energy and mobility, as is obvious from Fig. 2. At a coverage of 24 molec/uc, corresponding to a filling of all channels and intersections, the largest step in the isotherm up to 30.5 molec/uc takes place. This unusual behaviour is known to be correlated with a sharp type-A hysteresis loop between the adsorption and desorption branch of the isotherm 15,61. It has independently been observed even on smaller ZSM-5 crystals 171. The energy which is dissipated when tridimensional nitrogen is transformed from a gaseous to a solid phase is close to the value of 6.3 kJ/mol which has been experimentally observed during the densification of the fluid adsorbate in ZSM-5. The uptake ratio of 24/30.5 molec/uc before and after the inflexion of the isotherm at p/po
=
0.15 is identical with the ratio of the density of liquid (0.808g/cm3)
to solid nitrogen (1.027 g/cm3). Freezing of adsorbed 2D nitrogen at 77 K is a well documented phenomenon occurring on homogeneous graphite or boron nitride as adsorbents 110,251, and complex phase diagrams have been calculated 1261. Abrupt transitions of dense fluid-like to solid-like adsorbate in slit-shaped pores was theoretically predicted over a range of 1 - 2 up to 3 - 4 adsorbate layers 1271. Adapted to ZSM-5, this situation occurs at the completion of fluid filling
in the wider intersections, where
the 2D channel adsorbate is contacted with a
3D adsorbate phase. Furthermore this result can be explained by the 'site-andbond' network hypothesis of MAYAGOITIA, which assumes cooperative filling processes when half of the network sites (i.e. intersections in ZSM-5 at N
=
22
molec/uc) are filled 1281. Monte Carlo simulations of gases in narrow pores suggest that phase transitions and metastable states are formed which should show up
in experimental sorption isotherms as hysteresis effects 1291. This is con-
633 sistent with earlier observations on the existence of a hitherto unknown lowpressure hysteresis loop in the nitrogen ZSM-5 system 15-71. CONCLUSION The results of both high resolution adsorption and microcalorimetry at low temperatures provide clear evidence that the stepped isotherms of argon and nitrogen on large ZSM-5 crystals can be rationally explained by localized adsorptive molecules at the channel walls and the channel intersections. The findings are consistent with theoretical predictions. Heats of adsorption on A1P04-5 are lower compared to ZSM-5. Further investigations on unidimensional zeolitic materials (e.g. high silica ZSM-12) should show whether this behaviour is caused by different surface force fields of aluminophosphates and zeolites or if it is due to the different pore sizes of 10-membered and 12-membered micropore channels. At least nitrogen adsorbed at higher loadings in the ZSM-5 network should be considered as an immobile dense phase. Low temperature NMR and neutron diffraction experiments are in progress to further elucidate the adsorbate structure of gases in microporous adsorbents. ACKNOWLEDGEMENTS We are grateful to Dr. G. Engelhardt, Konstanz, and Dr. H.-G. Karge, Berlin,
for 29Si-MAS-NMR and IR-spectroscopic characterization of the ZSM-5 sample. This work is financially supported by Deutsche Forschungsgemeinschaft. REFERENCES 1 D.H. Olson, G.T. Kokotailo, S.L. Lawton, W.M. Meier J. Phys. Chem., 85 (1981) 2238-2243. 2 J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, J.V. Smith Am. Chem. SOC., Symp. Ser., 218 (1983) 109-118. 3 U. Muller, K.K. Unger, Z. Kristallogr., 182 (1988) 190-2. 4 U. Miiller, K.K. Unger, Zeolites 8 (1988) 154-156. 5 U. Miiller, K.K. Unger, Fortschritte Mineralogie, 64 (1986) Beiheft 1, 128. 6 U. Miiller, K.K. Unger, Stud. Surf. Sci. Catal., 39 (1988) 101-8. 7 P.J.M. Carrott, K.S.W. Sing, Chem. Ind., (1986) 786-7. 8 S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, J. Amer. Chem. SOC., 62 (1940) 1723-32. 9 S.T. Wilson, B.M. Lok, E.M. Flanigen, US Pat. 4,385,994 (1981). 10 J. Rouquerol, S. Partyka, F. Rouquerol, J. Chem. SOC., Faraday Trans. I, 73 (1977) 306-314. 11 H. van Koningsveld, H. van Bekkum, J.C. Jansen Acta Cryst., B43 (1987) 127-132. 12 H. Lermer, J. Steffen, M. Drager, K.K. Unger Zeolites 5 (1985) 131-4. 13 A.V. Kiselev, A.A. Lopatkin, A.A. Shulga, Zeolites 5 (1985) 261-7. 14 R.M. Barrer, J.A. Davies, Proc. Roy. SOC. London, A320 (1970) 289. 15 S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, Am. Chem. SOC., Symp. Ser. 218 (1983) 79-106. 16 U. Lohse, M. Noack, E. Jahn, Ads. Sci. Techn., 3 (1986) 19-24.
634 17 H. Stach, H. Thamm. K. Fiedler, B. Grauert, W. Wieker, E. Jahn, G. Ohlmann, Stud. Surf. Sci. Catal., 28 (1986) 539-546. 18 G.C. Bond, M.R. Gelsthorpe, K.S.W. Sing, C.R. Theocharis, J. Chem. SOC., Chem. Comm., 1056-7 (1985). 19 E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R . L . Patton, R.M. Kirchner, J.V. Smith, Nature, 271 (1978) 512-6. 20 P.A. Jacobs, H.K. Beyer, J. Valyon, Zeolites 1 (1981) 161-8. 21 D.H. Everett, J.C. Powl, J . Chem. SOC., Faraday Trans. I, 72 (1976) 619-36. 22 E.G. Derouane, J.M. And&, A.A. Lucas, Chem. Phys. Lett., 137 (1987) 336-340. 23 M.M. Dubinin, J . Coll. Interface Sci., 23 (1967) 487-499. 24 D. Atkinson, P.J.M. Carott, Y. Grillet, J. Rouquerol. K.S.W. Sing, in A.I. Liapis (Ed.), Proc. Eng. Found., 2nd Conf., Fundamentals of Adsorption, 1987, 89-98. 25 Y. Grillet, F. Rouquerol, J. Rouquerol, J. Coll. Interface Sci., 70 (1979) 239-244. 26 A.V. Vernov, W.A. Steele, Langmuir, 2 (1986) 219. 27 M. Schoen, D . J . Diestler, J . H . Cushman, J. Chem. Phys., 87 (1987) 5464-76. 28 V. Mayagoitia, F. Rojas, I. Kornhauser, J. Chem. SOC., Faraday Trans. I, 81 (1985) 2931. 29 B.K. Peterson, J.P.R.B. Walton, K.E. Gubbins, in A.I. Liapis (Ed.), Proc. Eng. Found., 2nd Conf., Fundamentals of Adsorption, 1987, 463-71.
H.G.Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A NEW POTENTIAL LARGE-SCALE APPLICATION OF ZEOLITES AS FIRE-RETARDANT MATERIAL H.
K. BEYER',
G. BORBiLY',
P. MIASNIKOV'
and P. RbZSA3
'Central Research I n s t i t u t e f o r Chemistry o f the Hungarian Academy o f Sciences, P.O. Box 17, 1525 Budapest (Hungary) ' I n s t i t u t e o f B u i l d i n g Science, P.O. Box 71, 1502 Budapest (Hungary) 3Department o f Mathematics, Technical U n i v e r s i t y o f Budapest, Muegyetem rkp. 3-9, 1111 Budapest (Hungary) ABSTRACT
The heat f l u x i n systems c o n s i s t i n g o f coaxial l a y e r s o f f i r e - r e t a r d a n t mater i a l s and b u i l d i n g structures has been simulated and mathematically modelled. The f i r e - p r o t e c t i v e e f f e c t o f z e o l i t e s was calculated and experimentally determined. Compared w i t h conventional f i r e - r e t a r d a n t m a t e r i a l s z e o l i t e s are more e f f e c t i v e i n the temperature range up t o 570 K, thus applicable e s p e c i a l l y f o r the p r o t e c t i o n o f heat-sensitive b u i l d i n g s t r u c t u r e s and goods against the effect of fire. INTRODUCTION
The o b j e c t i v e o f f i r e - r e t a r d a n t m a t e r i a l s i s t o i n h i b i t as e f f e c t i v e l y as possible the f l o w o f heat towards the p a r t o f the b u i l d i n g o r b u i l d i n g s t r u c t u r e t o be protected. I n s t a l l e d i n f i r e - p r o t e c t i v e constructions such as f i r e -doors, movable o r f i x e d f i r e - w a l l s , f a l s e c e i l i n g s , etc., they slow down the flow o f heat through these b u i l d i n g s t r u c t u r e s and thus a l s o the spread o f f i r e w i t h i n the b u i l d i n g . Covering the surface o f b u i l d i n g structures, f i r e - r e t a r d a n t m a t e r i a l s p r o t e c t them against the e f f e c t o f f i r e . This p r o t e c t i o n i s of g r e a t s i g n i f i c a n c e i n the case o f supporting constructions, e.g. p i e r s , beams and s t r u t s , since they lose t h e i r s t a t i c a l s t a b i l i t y and load-bearing capacity a t a c e r t a i n temperature. Last b u t n o t l e a s t , the enclosure o f storage containers w i t h f i r e - p r o t e c t i v e m a t e r i a l may p r o t e c t heat-sensitive goods (e.g.
valuable
papers, magnetic records, f i l m materi a1 ) against damage from f i r e . Heat i n s u l a t i o n and absorption o f heat by the p r o t e c t i n g m a t e r i a l i t s e l f p l a y a d e c i s i v e r o l e i n f i r e r e t a r d a t i o n . A l a r g e heat capacity o f the s t r u c t u r e t o be protected may be achieved by making i t heavy. However, weight and dimensions o f b u i l d i n g s t r u c t u r e s are generally l i m i t e d by constructional considerations. Moreover, increasing the bulk density o f f i r e - r e t a r d a n t m a t e r i a l s g e n e r a l l y r e s u l t s i n a d e t e r i o r a t i o n o f t h e i r h e a t - i n s u l a t i o n properties. Therefore techniques have been developed t o apply m a t e r i a l s which by phase i n v e r s i o n from s o l i d t o l i q u i d s t a t e (e.g. Glauber's s a l t ) o r by an endothermic chemical r e a c t i o n (e.g. dehydroxylation o f g i b b s i t e ) "absorb" heat energy i n l a t e n t form. The o b j e c t i v e of the present paper i s t o i n v e s t i g a t e whether z e o l i t e s may be applicable as f i r e - r e t a r d a n t m a t e r i a l due t o t h e i r endothermic dehydration
636
process. Heat f l u x and temperature p r o f i l e s i n l a y e r s o f f i r e - p r o t e c t i v e m a t e r i a l s and constructions are simulated and mathematically modelled i n order t o compare the p r o t e c t i n g e f f e c t s o f z e o l i t e s and conventional m a t e r i a l s . Heat conduct i v i t y c o e f f i c i e n t s were experimentally determined and heat capacity data needed f o r these c a l c u l a t i o n s were taken from the l i t e r a t u r e . Temperature p r o f i l e s i n z e o l i t e layers were measured and compared w i t h c a l c u l a t e d ones. METHODS
Mathematical model l i n g o f temperature p r o f i l e s i n f i r e - r e t a r d i n g m a t e r i a l s I n order t o describe t h e heat t r a n s f e r through f i r e - r e t a r d a n t l a y e r s and implement the p r o t e c t i o n o f c o n s t r u c t i o n s against the e f f e c t o f f i r e , the homogeneous problem o f heat conduction i n a composite medium c o n s i s t i n g o f L l a y e r s o f coaxial c y l i n d e r s being i n contact i s considered ( r e f . 1). The c o n s t r u c t i o n i s assumed t o be o f i n f i n i t e length, thus the heat i s t r a n s f e r r e d i n r a d i a l d i r e c t i o n only. L e t the thermal c o n d u c t i v i t y o f t h e 1 - t h l a y e r be denoted by kl(T),
the d e n s i t y by pl(T),
and the s p e c i f i c heat by cl(T) expressing t h e f a c t
t h a t they are dependent on the temperature T. The temperature d i s t r i b u t i o n T ( r , t ) has t o be determined as a f u n c t i o n o f t h e r a d i a l coordinate r and the time t, thus t h e one-dimensional coupled system o f heat conduction equations
a (kl(T)
kl ( T I aT1 ( r , t )
aT1 ( r , t )
)+-
ar
ar
ar
r
aT1 ( r , t ) =
P1(T)C1(T)
(1)
at
has t o be solved i n the domains
,
rl-l 6 r 2 r
t > 0
,
1=1,2,...,L
(ro=O)
s a t i s f y i n g the conditions o f coupling
1=2,3,
...,L
t h e boundary conditions Tl(o.t)
<
m
,
TL(rL,t) = T L ( t )
and the i n i t i a l conditions Tl(r,o)
:0
,
rl-l s r s rl
,
1=1,2,...,L
(ro=o)
.
637
The c o n d i t i o n s ( 2 ) imply t h e c o n t i n u i t y o f temperature o r p e r f e c t thermal c o n t a c t between two consecutive l a y e r s . The c o n d i t i o n s (3) s t a t e t h a t t h e h e a t f l u x i s continuous a t t h e i n t e r f a c e s . For the numerical s o l u t i o n o f the system o f p a r t i a l d i f f e r e n t i a l equations
(1) t h e method o f f i n i t ;
d i f f e r e n c e s w i l l be a p p l i e d . Making use of t h e Crank-
Z), a system o f a l g e b r a i c equations w i l l be
-Nicolson i m p l i c i t method ( r e f .
obtained. Since t h e c o e f f i c i e n t s kl,
p1 and c1 depend on t h e temperature,
the
c o e f f i c i e n t s o f t h e a l g e b r a i c system a r e f u n c t i o n s o f t h e unknown values o f t h e temperature as w e l l , thus t h e system o f equations w i l l be s o l v e d by i t e r a t i o n . I n o r d e r t o o b t a i n t h e a l g e b r a i c system, l e t t h e v a r i a b l e s t and r b e d i s c r e t i z e d by s u b d i v i d i n g them i n t o i n t e r v a l s A t and Arl,
respectively,
according t o t ( n ) = nAt,
r(i) =
1-1 C m (Ar p=l p p
f o r i = m, m t l , . . . ,
m =
1. - 1C m
..
n=1,2,3,.
-
Arl)
mtml
l=l,Z,
m=l P
t
and
in
iArl
rl-l 5 r 5 r
1
where
...,L
i s i n t r o d u c e d f o r s i m p l e r n o t a t i o n (m=O i f 1=1). F o r t h e d i s c r e t e values o f t h e f u n c t i o n s t o b e determined l e t us i n t r o d u c e T,(
r ( i ) , t ( n ) ) = Ti ( ’ I n
for
i=m, m t l , . . . ,
mtml
,
1=1,2
,... ,L
I n a s i m i l a r way t h e corresponding values o f heat c o n d u c t i v i t y and t h o s e o f t h e product o f d e n s i t y and s p e c i f i c h e a t w i l l b e denoted as i=m, mtl,.,.,
mtml
The values o f h e a t c o n d u c t i v i t y corresponding t o t h e m i d d l e o f t h e i n t e r v a l s [ r ( i ) , r ( i t 1 ) l w i l l be denoted as
Making use of these n o t a t i o n s , t h e i m p l i c i t method o f Crank-Nicolson l e a d s t o t h e f o l l o w i n g system o f a l g e b r a i c equations:
638
For i=0,mlymltm2,mltm2tm3,..
.,
L C m p= I P'
the boundary c o n d i t i o n s ( 4 ) and the
conditions o f coupling (2) and ( 3 ) have t o be taken i n t o consideration. S u b s t i t u t i n g the d i s c r e t i z e d form o f these c o n d i t i o n s and a f t e r rearranging t h e system (5), i t can be w r i t t e n i n t h e f o l l o w i n g concise form:
where
Dn
=
i s a diagonal m a t r i x , t h e elements o f which a r e
639
K(p)= [a. .I 1.I
i s a t r i d i a g o n a l m a t r i x o f t h e thermal c o n d u c t i v i t i e s w i t h
elements a.i,i
kl;hn
ai,i =
(’In (’1’ ki-1/2 + k i t 1 / 2
f o r i=O
n
n
ai,i
‘i-1
‘it1
a.1 ,i-1 =
- u ? - ~,
where
ai,i+l
=
for m =
1-1 1 c m < i < c m = m t ml p=l p=l
I=I,Z,
...,L
f o r i =
1-1 ~m = m p=l p
1=2,3,
...,L
-u?+~
for
1-1 i =Z m p=l
= m
1=2,3,
...,L
640
Tn
i s a column vector o f the unknown temperature,
where L q-1 =
C m
-1
and
9-1/2 =
p=l
L C m -1/2, p=l
and E i s the q - t h u n i t vector. q- 1 Since elements o f t h e c o e f f i c i e n t m a t r i x i n (6) depend on t h e unknowns
T,!')"',
t h e system (6) can be solved by the f o l l o w i n g i t e r a t i o n :
Assuming the i t e r a t i o n t o be convergent, we get
l i m rtl(j)
=
T"+'
j-
S t a r t i n g w i t h t h e i n i t i a l c o n d i t i o n , i . e . f = 0, the i t e r a t i v e s o l u t i o n o f equation ( 7 ) gives temperature d i s t r i b u t i o n s T1.T2.. . ,Tn f o r the
.
d i s c r e t i z e d time v a r i a b l e n. Determination o f m a t e r i a l - s p e c i f i c c o e f f i c i e n t s needed f o r t h e c a l c u l a t i o n o f temperature p r o f i1es Density, heat capacity and thermal c o n d u c t i v i t y of s t a i n l e s s s t e e l , mineral wool, z e o l i t e 13X and g i b b s i t e considered i n t h e present study as c o n s t r u c t i o n and f i r e - p r o t e c t i v e materials, resp., a r e sumnarized i n Table 1. The thermal c o n d u c t i v i t y o f z e o l i t e 13X (extrudates, diameter: 2 mm, water adsorption capa c i t y : 260 rng-g-' dehydrated product) , g i b b s i t e (commercial pel l e t s , diameter: 2-4 nun) and y-A1203 (obtained from the g i b b s i t e sample by heat treatment a t 870 K) was determined by measuring t h e e l e c t r i c energy needed t o m a i n t a i n a temperature g r a d i e n t of 12 K.cm..'
i n a 5 cm t h i c k l a y e r o f t h e m a t e r i a l . The
t e s t device i s described i n r e f . 3.
641
TABLE 1 Values of coefficients used f o r t h e c a l c u l a t i o n o f temperature p r o f i l e s mater ia 1
therma 1 conduct iv i ty W. m- 1. K-1
stainless steel A I S I 347 ( r e f . 4)
0.135
+
14.8
-
heat capaci ty J kg-1. K-1
.
0.22.T t 401 f o r 293
-
bulk d e n s i t y kg m-3
.
7830
0.00017.T 0.001 mineral wool f o r 293
0.67-T + 700 f o r 293
150
hydrated 13'
0.047 ( a t 360 K)*
see Fig. 1
832
gibbsite
0.32
see Fig. 1
1150
y-A1203
0.075 ( a t 360 K)*
0.75.T + 462 725 f o r 293
0.000072.T t 0.004 f o r 293
0.000036.T t 0.995 f o r 293
air ( r e f . 3)
( a t 363 K)*
*regarded as independent o f T
353.T-1
The c o n t r i b u t i o n o f t h e s p e c i f i c heat t o the o v e r a l l heat capacity a t d i f f e r e n t temperatures were c a l c u l a t e d t a k i n g i n t o
h
d
consideration t h e temperature dependence
I
Y
o f the s p e c i f i c heat o f the i n i t i a l mate-
b-4
I
cn
r i a l and the product formed as w e l l as
Y
T
conversion degree c a l c u l a t e d from the
v
3
.r
U
xm
10
-
thermogravimetric curve. The c o n t r i b u t i o n of dehydration and r e a c t i o n heat was obtained i n t h e f o l l o w i n g way. I n t h e
U
c,
m aJ c
6 -1 I
2
case o f gibbsi t e t h e DTA curve was
t
I
g r a p h i c a l l y integrated, q u a n t i f i e d using I\
t h e value o f 1890 kJ.kg-l ( r e f . 6 ) f o r
? a heats o f water desorption reported by Fig. 1. Temperature dependence o f the o v e r a l l heat capacity o f g i b b s i t e D z h i g i t e t a]. ( r e f . 7). I n F i g . 1 the ( f u l l line) and z e o l i t e 1% (dashed sum of both c o n t r i b u t i o n s i s p l o t t e d . line).
642
The f i r s t maximum a t about 590 K i n t h e g i b b s i t e curve i s due t o a p a r t i a l conversion t o boemithe, the decomposition o f which i s revealed by t h e endothermic peak a t about 830 K. Test device The t e s t device used consists o f a s t a i n l e s s s t e e l tube ( i n n e r diameter
45 mm, length 20 cm) i n which a second tube ( i n n e r diameter 15 mm, l e n g t h 14 cm, w a l l thickness 2 . 5 mm) i s centred. Heat f l u x i n a x i a l d i r e c t i o n i s prevented by a l a y e r o f mineral wool f i l l i n g o u t t h e space between the ends o f both tubes. I n t h e middle, mantled thermocouples a r e brazed onto both these tubes. The mat e r i a l s t o be tested a r e contained i n space A i n s i d e the i n n e r tube and/or space B between t h e tubes. The device i s placed i n a t u b u l a r furnace heated up t o a temperature between 770 and 900 K and t h e temperatures o f both tubes a r e registered. RESULTS AN0 DISCUSSION Heating curves o f the o u t e r (a) and inner (b) tube of the t e s t device cont a i n i n g z e o l i t e 13X i n space A a r e shown i n F i g . 2. Curve ( c ) represents t h e temperature increase o f the i n n e r tube as a f u n c t i o n o f time f o r t h e same case c a l c u l a t e d according t o the numeric method described i n the previous section. I n c o n t r a s t t o the c a l c u l a t e d data (curve c), t h e temperature experimental determined approaches 373 K a f t e r a s h o r t time and remains constant over a r e l a t i v e l y long time i n t e r v a l . This d e v i a t i o n can be explained by progr e s s i v e desorption o f water vapour the z e o l i t e l a y e r and i t s recondensat i o n a t t h e cooler i n n e r tube. This h
process contributes t o the heat f l u x
Y v
through the z e o l i t e l a y e r o n l y i n t h e
W
L =I
temperature range up t o 373 K regard
CI
m L a W E
l e s s of the f i r e r e t a r d a t i o n aspect and, therefore, has n o t been taken
F
i n t o consideration i n the mathemat i c a l model. A t higher temperatures both the experimentally determined
30( 10
20
Time (min) F i g . 2. Measured (a,b) and calcu ated ( c ) temperature of the o u t e r ( a ) and i n n e r (b,c) tube of the t e s t dev ce described i n the t e x t .
and t h e c a l c u l a t e d curve a r e i n sat i s f a c t o r y agreement. The s l i g h t d e v i a t i o n may be due t o a minor h e a t f l u x i n a x i a l d i r e c t i o n . Experimental and c a l c u l a t e d r e s u l t s were a l s o i n good agreement when space A and/or B
643 c o n t a i n e d m i n e r a l wool, z e o l i t e 13X and g i b b s i t e i n v a r i o u s c o m b i n a t i o n s . Since c a l c u l a t i o n s gave r e 1 i a b l e r e s u l t s , temperature p r o f i l e s o f d i f f e r e n t v a r i a n t s of a s i m p l e model c o n s i s t i n g o f a h e a t - i n s u l a t e d s t e e l t u b e have been c a l c u l a t e d i n o r d e r t o compare t h e e f f e c t i v e n e s s o f z e o l i t e s and c o n v e n t i o n a l f i r e - r e t a r d a n t m a t e r i a l s . Some t y p i c a l r e s u l t s o f such c a l c u l a t i o n s a r e i l l u s t r a t e d i n F i g s 3-5. Only t h e temperature o f t h e t u b e i s given, w h i c h i s p r a c t i c a l l y c o n s t a n t i n r a d i a l d i r e c t i o n because o f t h e h i g h h e a t c o n d u c t i v i t y . The i n n e r space of t h e t u b e i s empty o r c o n t a i n s m a t e r i a l s consuming h e a t i n l a t e n t form. A c o n s t a n t ambient temperature o f 1073 K i s supposed. Curve b i n F i g . 3 i l l u s t r a t e s t h e h e a t - r e t a r d i n g e f f e c t o f z e o l i t e 13X compared t o t h e e f f e c t o f a m i n e r a l wool l a y e r ( a ) o f t h e same t h i c k n e s s . S i n c e t h e weight o f structures i s generally t h e l i m i t i n g f a c t o r i n b u i l d i n g , curve c repr e s e n t s t h e temperature i n c r e a s e o f t h e s t e e l t u b e surrounded b y g i b b s i t e , t h e w e i g h t o f which corresponds t o t h a t o f t h e z e o l i t e l a y e r . Up t o a b o u t 570 K z e o l i t e p r o v i d e s t h e b e s t p r o t e c t i o n ; however, a t h i g h e r temperatures g i b b s i t e i s more e f f e c t i v e . F i g . 4 demonstrates t h e e f f e c t o f combined a p p l i c a t i o n of z e o l i t e and h e a t - i n s u l a t i n g m a t e r i a l s . The t u b e i s assumed t o b e i n s u l a t e d w i t h a 3 cm t h i c k l a y e r t h e i n n e r s h e l l o f which c o n s i s t s o f z e o l i t e 13X ( t h i c k n e s s r z ) and t h e o u t e r one o f m i n e r a l wool ( t h i c k n e s s rm). Heat r e t a r d a t i o n approaches a maximum when r z / ( r z t r m )
= 0.66.
Combined a p p l i c a t i o n o f z e o l i t e and a conven-
t i o n a l f i r e - i n s u l a t i n g m a t e r i a l r e s u l t s n o t o n l y i n a s l i g h t improvement o f t h e e f f e c t i v e n e s s b u t a l s o i n a decrease o f w e i g h t .
h
E .r
E
v
P
ir
L 1 0.5 Time (min) F i g . 3. Temperature o f a s t e e l t u b e ( o u t e r 0 9, i n n e r 0 7 cm) vs t i m e . Ambient temp.: 1073 K. I n s u l a t i o n : (a) 3.9 cm m i n e r a l wool, ( b ) 3.9 cm z e o l i t e 13X, ( c ) 3 cm g i b b s i t e .
rz/(rz+rmw) F i g . 4. Time r e q u i r e d t o h e a t an i n s u l a t e d s t e e l t u b e ( o u t e r 0 9, i n n e r 0 8 cm) a t 1073 K ambient temp. up t o t h e i n d i c a t e d temperature vs r Z / ( r Z + r m ) .
644 The e f f e c t o f heat-consuming m a t e r i a l s i n t h e inner p a r t o f the tube i n s u l a t e d o u t s i d e w i t h a 3 cm l a y e r o f mineral wool i s i l l u s t r a t e d i n F i g . 5 (curves a-c). Compared t o t h e empty tube (a) a z e o l i t e f i l l i n g considera b l y r e t a r d s the temperature increase (b); however, a t higher temperature again g i b b s i t e i s more e f f e c t i v e ( c ) . The most e f f e c t i v e p r o t e c t i o n provides a g i b b s i t e f i l l i n g combined w i t h z e o l i t e 13X ( 2 cm) and mineral wool ( 1 cm) surrounding the tube (curve d ) . Curve ( e ) represents the heating curve when
40
80
120
i n the combination (d) z e o l i t e i s replaced by g i b b s i t e on weight basis.
Time (min) Fig. 5. Temperature o f a s t e e l tube (outer 0 9, i n n e r 0 8 cm) vs time. Ambient temp.: 1073 K. For f u r t h e r d e t a i l s see t e x t . CONCLUSIONS Z e o l i t e s a r e more e f f e c t i v e i n f i r e r e t a r d a t i o n than conventional heat-insul a t i n g m a t e r i a l s . Enclosi’ng b u i l d i n g s t r u c t u r e s o r constructions t o be protected against damage from f i r e , they e x h i b i t an e x c e l l e n t e f f e c t i v e n e s s i n the tempera t u r e range up t o about 570 K. Thus they e s p e c i a l l y provide p r o t e c t i o n f o r i n flammable and heat-sensitive b u i l d i n g s t r u c t u r e s and goods. F i l l e d i n t o e x i s t i n g o r created hollow p a r t s w i t h i n s t r u c t u r e s t o be protected, z e o l i t e s e x h i b i t an a d d i t i o n a l p r o t e c t i v e e f f e c t . However, g i b b s i t e proved t o be more e f f e c t i v e as f i r e - r e t a r d a n t m a t e r i a l i n t h e temperature range above 600 K. Combined a p p l i c a t i o n o f z e o l i t e s , g i b b s i t e and conventional h e a t - i n s u l a t i n g m a t e r i a l s ensures the b e s t p r o t e c t i v e e f f e c t . Mathematical modelling o f simple f i r e - p r o t e c t e d s t r u c t u r e s provides t h e p o s s i b i l i t y t o c a l c u l a t e the optimum choice o f m a t e r i a l and l a y e r thickness i n c o n f o m i t y with constructional considerati ons. REFERENCES 1 M.N. Ulzisik, Heat Conduction, Wiley, New York, 1980. 2 D.M. Young and R.T. Gregory, A Survey o f Numerical Mathematics, AddisonWesley, Reading, Massachusetts, 1972. 3 R. Doring, Koch und H. Zeltner, Wametechnische I s o l i e r u n g , VEB Fachbuchverlag, Leipzig, 2nd ed., 1981. 4 H.Y. Wong, Heat Transfer f o r Engineers, Longman, London, 1977. 5 C.H. Shomate and ELF. Naylor, J. Am. Chem. SOC. , 67 (1945), 72-75. 6 Tran-huu Th’e, i n P. Pascal ( E d i t o r ) , Nouveau T r a i t 6 de Chimie Minerale, Vol. 6, Masson, Paris, 1961, p. 582. 7 O.M. Dzhigit, A.V. Kiselev, K.N. Mikos, G.G. M u t t i k and T.A. Rahmanova, JCS Trans. Far. SOC. I, 67 (1971), 458-467.
111. ION EXCHANGE AND DETERGENT BUILDING
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H.G.Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
INDUSTRIAL PRODUCTION OF ZEOLITES
E. ROLAND
Degussa AG, Inorganic Research Department, Rodenbacher Chaussee 4,6450 Hanau 11, FRG
ABSTRACT
The manufacture of zeolites is exemplified by the industrial synthesis of NaA zeolite, NaY zeolite and ZSMJ. Chemical, technical and economic factors are considered. Dealumination using Sic14 and a shaping process are described as additional steps in the zeolite product design.
INTRODUCTION
The beginning of the industrial production of zeolites dates back to the year 1954, when Union Carbide Corporation first commercialized Linde molecular sieves. However, much pioneering work was still necessary at that time to scale-up the syntheses. A report by MiZton (ref. 1) gives an impression of the many difficulties that had to be overcome before the first manufacturing plant could be put into operation later in 1957: "Major problems solved before this plant was constructed were: filtration of the fine particle zeolite, continuous drying and activation, reduced crystallisation times and raw material utilisation efficiency." Since then, zeolites have been developed into key materials in the chemical industry for a wide field of applications ranging from ion exchange to the catalysis of petrochemical processes. In 1987, the production of synthetic zeolites had grown to an estimated 460,000 metric to& worldwide, and this from not much more than a few hundred tons per year in the late fifties and about 60,000tons in 1975 (refs. 2-4). The manufacturing technology had to reach a high standard to make possible the extensive use of zeolites indicated by these figures. However, in spite of the importance of this aspect of zeolite chemistry, it has been'only rarely reviewed (refs. 5-7). It cannot be the purpose of this contribution to provide a comprehensive treatise on the industrial production of zeolites, due space limitations. Rather, an attempt is made to describe some typical zeolite production processes on the basis of the experience reported in the patent literature. Since the commercial importance of the zeolite material was a criterion for its choice as an example, a brief survey covering the economic situation of different types is provided, followed by a technical discussion of their industrial synthesis.
646
COMMERCIALA S P E W OF ZEOLllT PRODUCTION
As a result of vigorous research programs in academia as well as in industry, more than three hundred types of zeolites or related molecular sieves (ref. 8) based on about sixty different structures are known at the moment. However, only few of these types are actually produced. Table 1gives an estimation of zeolites manufactured in amounts of more than one hundred tons per year. The figures in this table are based on published data (refs. 3,4). For example, the amount of zeolite Y was estimated from the quantity of FCC catalyst sold worldwide (ref. 4). TABLE 1
Synthetic Zeolites Produced in Bulk Amounts type
estimated annual 1987 usage worldwide (thousands of metric tons)
zeolite A
-
400
major fields of application
detergents adsorption
zeolite Y
50-60
zeolite X
2
adsorption
ZSM-5 and analogues
1
catalysis of petrochemical processes
mordenite
0.3
C, /CI isomerization dewaxing
FCC
The zeolites listed in Table 1 can be divided in two groups:
- The "commodity" zeolites A, X and Y First introduced in the chemical industry between 1954 and 1960, these zeolites now represent more than ninety-nine percent of all the zeolites produced. Much development work has been invested in adjusting these zeolites to the requirements of the different applications. This inlcudes: - chemical modifications, for instance by ion exchange and dealumination procedures; - variations in particle size and crystal morphology; - forming. Even after more than thirty years of development,many patent applicationsare filed every year dealing with the synthesis and the modifications of these zeolites. This clearly shows that improvements are still possible.
- The "specialty" zeolites ZSM-5 and large-port mordenite Because of their great potential for catalysis in petrochemistry and for the synthesis of organic
647
intermediates, industrial research programs mainly focus on these and similar materials at the moment. The production capacities are still comparativelylow for two reasons: - Applications requiring bigger quantities are still under development. - Manufacturingprocesses are more complicated than for the "commodity"zeolites, as the syntheses have to be done under elevated pressure and often in the presence of structuredirecting agents.
PRODUCTION TECHNOLOGY OF ZEOLKES The manufacturing of zeolite powders is only one step in a sequence of processes by which materials for very specific applications are designed. In many cases additional treatments are necessary to modify the properties of the zeolite by changing the cations located in the pore systems, or even the frameworkcomposition.Finally, a formingprocedure is included,since most applicationsrequire shaped materials. It is essential that the different steps are specifically matched according to the properties of the desired product. Thus, the sequence of these steps can be varied. In particular, shaping can be done before an ion exchange. In some processes for the preparation of binderless pellets, the starting materials are shaped and converted to a zeolite afterwards. The different parts of the industrial product design - synthesis of zeolite powders, modification and shaping - are discussed below, taking specific examples for illustration.
Molecular sieve zeolites are produced in industry either from aluminosilicatehydrogels or by conversion of clay minerals. The hydrogels can be prepared from various sources of silica and alumina, which are listed in Table 2. Depending on the choice of these starting materials as well as on the requirements of the synthesis, bases or mineral acids have to be added. For some preparations a structure-directingagent is needed as well. The kinds of starting materials, the method of mixing them, determine the structure of the resulting gel. The nature of the gel, in turn, influences the rate of the subsequent crystallization, the particle size distribution, and the formation of impurities. Production processes for NaA zeolite, NaY zeolite and E M - 5 are discussed to demonstrate how the synthesis of a zeolite can be directed by the conditions of the gel preparation and the crystallization to afford materials for the applications envisaged, and how it can be done in an economical way at the same time.
648
TABLE 2
Silica and Alumina Sources in Zeolite Manufacturing Processes
sodium silicate solutions sodium aluminate solutions amorphous solid silica: various precipitated silicas, blast furnace slag, fly ash from the production of silicon silica sol
aluminium salts (sulfate, chloride)
colloidal alumina
c Iays (raw kaolin, metakaolin, halloysite) volcanic glasses
By far the greatest amount of NaA zeolite is consumed in the manufacturing of detergents, since it was decided in many countries to replace phosphate as the builder component responsible for the reduction of the water hardness: in 1987, 375,000metric tons of NaA zeolite were produced worldwide only for the detergent industxywith a forecast of 500,000 metric tons in 1990 (ref. 3). This commercial success of zeolite chemistry could only happen because it was possible to develop a zeolite with properties which are expected of a detergent builder: - high Ca binding capacity and rate; - good dispersability; - low sedimentation tendency; - low abrasiveness. Translated into the language of a zeolite chemist, this means: A NaA zeolite suited as a detergent builder should be free of impurities (in this case, hydroxysodalite is a possible impurity, if the reaction conditions are not chosen properly); it should have a small average particle size (approximately4 q~ and less) and a narrow particle size distribution. In addition, the zeolite crystals must not have sharp edges or comers. Fig. 1shows an electron micrograph picture of a zeolite meeting these requirements. It will be outlined how such a zeolite can be synthesized on an industrial scale by reaction of a sodium silicate and a sodium aluminate solution. This combinationof educts is the most commonly used for the manufactureof detergent-grade
NaA zeolite, as in the processes of HENKEL, DEGUSSA and PQ, for instance. Sodium silicate solutionsare easily handled and inexpensivestarting materials. For the synthesisof NaA zeolite,
649
Fig. 1. Electron micrograph showing crystals of detergent NaA zeolite. various technical qualities are available, which differ in the content of Na2O and Si02: - a sodium silicate solution prepared by the fusion of selected sands with soda ash, having a content of 8 percent Na2O and 27 percent Si02 (Na2O/SiOz = 1 :3.4); - a sodium silicate solution synthesized via hydrothermal decomposition of sand by caustic, containing 14 percent Na2O and 26 percent Si02 (Na2O/SiOz = 1 : 1.9). For the preparation of the sodium aluminate solution, hydrated alumina is usually digested by caustic. The clay route (HUBER, ETHYL)is less often applied. There can be a disadvantage of this method, in that iron impurities in metakaolin cause a coloring of the product zeolite. By addition of iron complexingagents (ref. 9) or by special grinding of the metakaolin (ref. 10) the brightness can be improved. The MIZUSAWA process, which starts from an acid-treated clay, has been described elsewhere (ref. 11). The reaction mixture for the synthesis of zeolite NaA (Na2OA1203'2Si02'4.5H20) can be varied within wide boundaries of the overall composition. For the efficiency of the industrial process, however, it is necessary to find the best composition in terms of product quality, yield and synthesis time. A possible composition for the production of NaA zeolite meeting these requirements is represented by the ratios 1 (ref. 12). An excess of alumina is employed in this example. 2.11 Na2O :A1203 : 1.74 Si02 :77.6 H20
I As mentioned above, the average particle size is avery critical parameter of the zeolite product. It is mainly influenced by
- the Na20/H20 ratio in the reaction mixture: high basicity favors the formation of small particles (ref. 13);
650
- the mixing sequence and the addition rate of the different components;
- the temperature of the gel precipitation; - the stirringhhearingenergy applied during the gel preparation; - seeding (ref. 6). Considering these factors, it becomes quite clear that the gel preparation is the crucial step with regard to the particle size in the synthesis of a detergent zeolite. In Fig. 2, the flow diagram for the preparation of detergent NaA zeolite is shown as an example (refs. 14, 15). Usually, the gel precipitation temperature is between 50 and 80 "C, the temperature during the crystallizationbeing slightly higher (80-90 "C).The time required for the gel preparation may vary from 0.5-1.5 hours, whereas 1.0-2.0 hours are necessary for the zeolite crystal1ization.
Fig. 2. Process for the production of NaA zeolite.
As indicated in Fig. 2, after the crystallization the zeolite suspension is filtered with a filter press, a rotary filter or a band filter. The filtrate containingthe mother liquor and the wash water is usually characterized by low concentrations of sodium hydroxide. It can be used for the neutralization of acidic effluents. If the reaction mixture contained an excess of Al203,as in compo-
651
sition 1,there is also some sodium aluminate present. In this case, the filtrate is concentrated and recycled to the digestion of the hydrated ,4203, for example (ref. 15). For economic and environmental reasons, complete utilization of the mother liquor is essential in the manufacturing of zeolites. Many detergent manufacturers prefer a zeolite slurry instead of a zeolite powder. Water and a suspension stabilizer, which prevents rapid sedimentation, are then added to the filtered zeolite. Otherwise, the zeolite is spray-dried. It should be stressed that in most cases the entire process is run semi-continuously. Whereas it is possible to prepare the gel in a continuous way in zeolite syntheses (ref. 16), the crystallization is usually carried out batch-wise in reaction vessels having a size of sometimes more than 100 m3. The typical zeolite yield obtained this way is about 120 kg/m3 (ref. 7).
Zeolite Y is mainly employed as a component of FCC catalysts in a proportion of 15 to 40 percent. For this application, the "standard" NaY zeolite is subjected to rare earth and ammonium ion exchanges as well as to calcination steps. This process has been discussed in detail in the literature (refs. 17,18). The following properties are expected of a NaY zeolite suited as a starting material for the zeolite component of a FCC catalyst: - high purity: the presence of the zeolites P,gmelinite and phillipsite, which are thermodynamically more stable and are possibly formed as byproducts, is to be avoided; - good crystallinity; - high surface area; - high thermal and hydrothermal stability, which correlates with a high SiWAl2O3 ratio in the zeolite. The enhancement of the SiOdAl203 ratio has been the subject of intensive research efforts. High-quality NaY zeolites obtained by direct synthesis are characterized by si02/A1203 ratios between approximately 5.0 and 5.6. Values up to 7.8 are mentioned in the literature (ref. 19), but the corresponding materials are difficult to obtain: It has been found that the SiOdAl203 ratio in the zeolite product is highly sensitive to the excess alkalinity in the batch, which is defined as [(NaOH)-(NaAlO2)ySiO2 (ref. 20). Lower values of this excess basicity result in higher SiodAl203 ratios in the zeolite prepared (ref. 21). Concurrently, however, the crystallization time has to be multiply prolonged. A tendency to the formation of more siliceous Y zeolites can also be observed, if the SiOdAl203 ratio in the reaction mixture is increased considerably: to raise the SiodAl203 ratio in the zeolite from 4.2 to 6.5, it has to be enhanced by a factor of 5 in the gel (ref. 22). Thus, the direct synthesis of Si02rich Y zeolites is not economical. Zeolites of this kind are better manufactured by dealumination of a "standard" NaY zeolite.
652
Fig. 3. Process for the production of NaY zeolite (ref. 24).
It is evident from a comparison of the patent literature that most of the reaction mixtures yielding NaY zeolites with SiOdAl203= 5.0-5.6have the compositions2. Various combinations of the starting materials mentioned in Table 2 are used as reactants.
(2.5-3.5) Na2O : A1203 : (8.0-10.0)Si02 : (120-180) H20
2 In the first syntheses of NaY zeolite the gel slurry had to be aged for a day or more at ambient temperature. Seed crystals (of zeolite X, for instance) could be added to the ready gel instead, but at the moment the addition of various seeding mixtures is in common use. The latter presumably contain particles having at least partially a zeolite structure (ref. 23).The chemical compositions of these nucleation center slurries, which are described in the examples of the patent literature, are specified by the ratios 1.The exact way these seeding mixtures are prepared and aged strongly influences the rate of the NaY zeolite crystallization.
653
Depending also on the synthesis mixture and the kind of gel prepared, the time necessary to obtain a well crystallizedNaY zeolite varies from 12 to 30 hours. The reaction temperatures are in the range 90 and 100 "C.Vigorous stirring may cause some problems, as it often results in the formation of gmelinite and phillipsite as impurities. A problem can also arise from sequential crystallizationyielding mainly zeolite P.This can be avoided by dilution of the zeolite slurry to reduce the temperature or by quick filtration. Fig. 3 shows a process published in a GRACE patent (ref. 24) as an example for the synthesis of NaY zeolite. A characteristicof this process is the utilization of the mother liquor. In the preparation of NaY zeolite, this is a weak sodium silicate solution. By addition of an aluminium sulfate solution a silica-alumina hydrogel is formed, which is used as a starting material in the next synthesis. It is also possible to precipitate amorphous silica from the mother liquor. A direct recycling of unprocessed crystallization filtrate is critical, however, because the mother liquor possibly contains nucleation centers of impurities.
Whereas zeolite Y is synthesized for one major catalytic application, ZSMJ can act as a catalyst for petroleum refining as well as for many different petrochemical processes (Table 3), in which it shows the unique property of shape selectivity(refs. 25,26).Several additionalprocesses are currently in the development stage. TABLE 3
Major Commercialized Processes Catalyzed by ZSM-5 FCC (Octane Enhancing) Xylene lsomerization Dewaxing
MTG (Methanol to Gasoline) Ethylbenzene Synthesis (Mobil-Badger) p-Methylstyrene Synthesis
Each of these processes requires a speciallydesigned catalyst:the zeolite component may vary in the siOz/A1203ratio, the number and strength of acid sites, the crystal size and morphology.
In addition, for the final molded catalyst, the nature and amount of binder, the size and the porosity of the agglomerates are important. The result is a catalyst with very special properties regarding catalytic activity and life-time.This also means that for each application a very special type of ZSM-5 has to be synthesized.The ability to select the kind of ZSMJ best suited for a
654
certain process is part of the know-how of the catalyst manufacturer. The development of a ZSM-5 type zeolite in this way becomes all the more difficult as the number of synthesis parameters is substantially increased compared to the preparation of the "commodity" zeolites by the need for a structure-directing agent ("template"). Some templatefree laboratory preparations are described in the literature (ref. 27), but this author is not aware that a corresponding industrial production process has been realized so far. Among the many possible templates the tetrapropylammonium cation is the most favored (ref.27). According to the the targeted zeolite, the syntheses are performed in autoclaves having volumes from 0.5 m3 (pilot plant size) up to about 10m3at temperatures between 100and 180"C, corresponding to an autogenous pressure of 1-10 atm. If high concentrations of tetrapropylammonium salts and high temperatures are used, decomposition to amine and propene occurs, effecting a rise of the pressure and increasing the number of possible templates present. The time necessary for the synthesisvaries from 20 hours to several days, depending on the gel composition and the reaction conditions. Especially the choice of the silica source has a great influence on the crystallization rate (ref. 27). After crystallizationand drying, the zeolite is calcined in air or oxygen at approximately 600°C to remove the organic template from the pore systems. The effluent air contains C02 and NOx, which can cause pollution problems. This is one of the reasons why the production of ZSM-5 in the absence of templates is highly desirable.
As indicated above, the as-synthesizedzeolite can be subjected to additional treatments,which change its properties. The methods for the chemical modification can be classified in two groups of reactions: those, which leave the zeolite framework unchanged (exchange of non-framework cations, silanation), and - those, which affect the framework composition (dealumination).
-
In the chemical industry, four dealumination methods are currently practiced (refs. 28-31): - acid treatment; - high-temperature steam treatment (ultrastabilization) of NH4Y zeolite with optional subsequent acidification; - reaction of NH4Y zeolite with an aqueous solution of (NH4)zSiFa ; - reaction of NaY zeolite with gaseous Sic14 Dealurninationprocedures were originally developed to improve the thermal and hydrothermal stability of zeolite Y in FCC catalysts. More recently, however, the hydrophobic properties of Sia-rich Y zeolites have attracted much interest for the removal of organic components from humid effluent air or fromwater. The followingproperties are expected of a Y zeolite suited for this purpose:
655
- excellent crystallinity to ensure high adsorption capacities: the framework must not undergo partial destruction during dealumination; - high hydrophobicity: Si02/&03 ratios > 50 are requisite. Dealumination using Sic14 is the best method for reaching this goal, as very high SiWAl2O3 ratios can be obtained from NaY zeolite in one step without loss of crystallinity. A process based on this reaction (equation 1) is under development at the moment.
A flow chart (Fig. 4) shows the different steps of this procedure. The most critical part is definitely the reaction between Sic14 and calcined NaY zeolite at temperatures higher than 300 OC, as reaction (1)is very exothermic. In the next step the formed NaAlC14 must be carefully washed out, before the product can be filtered and dried.
Fig. 4. Process for the dealumination of NaY zeolite with SiC14.
656
ShaDine Zeolites are commerciallyavailable in a variety of different shapes: microgranulates, spheroidal agglomerates, extrudates and molded bodies with a honeycomb-like structure, for instance. The different forming procedures applied constitute a science in itself (ref. 32). Here the forming of NaA zeolite into a granulate, which can be used as a desiccant in insulating glass, is given as an example. The world market for such a zeolite product is about 8000 metric tons per year at the moment. A product best suited for this purpose would have the following characteristics: - high adsorption capacity for water, but exclusion of gases, which are components of air or which are applied for insulating (SF6 for instance); - narrow agglomerate size distribution (diameter: 1-2 mm); - high attrition resistance. The first of these requirements can be met by a zeolite A having a pore size of 3 k As a consequence, NaA zeolite is partially potassium-exchanged and granulated. Possible binders are clays, such as a bentonite. At DEGUSSA, a more direkt method is used, which is based on the choice of sodium silicate as a binder for 4 A NaA zeolite (ref. 33). By this "molecular glue" the pores of the NaA zeolite are partially plugged, creating the 3 A characteristics required. The shaping process is illustrated in a simplified way in Fig. 5: In a mixer a sodium silicate solution is sprayed onto NaA zeolite powder. The resulting mixture, which contains the agglomeration nuclei, is transferred to a balling pan. There, the particles are enlarged in size and attain a spheroidal shape of high strength. Afterwards the green pellets are classified, dried and calcined.
Fig. 5. Process for the production of a NaA zeolite granulate having 3 A Characteristics.
657
CONCLUSION: Future Developments There is no doubt that in the next years the production technology of zeolites will concentrate mainly on highly sophisticated materials. For example, the growing demand for ZSM-5 and related zeolites (for instance borosilicates) will be an incentive to improved manufacturing processes. In particular, it will be necessary to produce these materials in a more inexpensive way. In this context it would be a major achievement if the manufacturingcould be done in the absence of structure-directingagents. Lower prices for these zeolites will make them competitive for applications still covered by other materials which are less expensive at the moment. Besides these topics, the modificationof the "commodity"zeolites to refined materials for new applications will continue to be the subject of development efforts. In addition, other molecular sieves, which were for a long time neglected and stored on the laboratory shelves, will be re-discovered for new applications. For instance, there is - more than twenty years after its first preparation - currently a growing interest in zeolite Beta as a catalyst in oil refining (ref. 34). In the long run, some members of the Alp0 family, where by now a whole variety of new materials is available on the laboratory scale (ref. 8), could open totally new areas of application for molecular sieves. A spectacular example in this regard is the recently discovered aluminophosphate-based molecular sieve VPI-5,which has a free-pore-sizediameter of more than 10 A (ref. 35). Economic success, however, will mostly depend on the prolific interaction of research, development, application engineering, manufacturing and commercialization, which has been so efficient in the past.
REFERENCES R.M. Milton, in Molecular Sieves, Society of Chemical Industry, London, 1968, pp. 199-202. European Chemical News, 31 (Nov. 25,1977), 35. C. Dietrich and W. Leonhardt, Tenside Surfactants Detergents, 24 (1987) 322-327. N.Y. Chen and T.F. Degnan, Chem. Eng. Prog., 84 (1988) 32-41. D.W. Breck, Zeolite Molecular Sieves, Wiley-Interscience,New York, 1974, pp. 725-755. C.W. Roberts, in R.P. Townsend (Ed.), The Properties and Applications of Zeolites, The Chemical Society, London, 1980, pp. 103-120. 7 D.E.W. Vaughan, Chem. Eng. Prog., 84 (1988) 25-31. 8 E.M. Flanigen, R.L Patton and S.T.Wilson, in PJ. Grobet, WJ. Mortier, E.F. Vansant and G. Schulz-Ekloff (Eds.), Innovation in Zeolite Materials Science (Stud. Surf. Sci. Catal. 37). Elsevier, Amsterdam, 1988, pp. 13-28. 9 R.C. Fitton, Ger. Patent Appl. 2743597 (Mar. 30,1978), assigned to J.M. Huber Corp. 10 A.P. Ferris, Ger. Patent Appl. 2823927 (Dec. 14,1978), assigned to English Clays Lovering Pochin & Co. Ltd. 1 2 3 4 5 6
658
11 I. Yamane and T. Nakazawa, in Y. Murakami, A. Iijima and J.W. Ward (Eds.), New Developments in Zeolite Science and Technology (Stud. Surf. Sci. Catal. 28), Kodansha/Elsevier, Tokyo/Amsterdam,1986, pp. 991-1000. 12 H. Furtig, Thesis, Universitat Halle-Wittenberg, 1964. 13 B. Latourrette, Europ. Patent Appl. 149929 (July 31,1985), assigned to Rhone-Poulenc. 14 a) H. Strack, W. Roebke, D. Kneitel and E. Pam, Ger. Patent 2660722 (Mar.l2,1987),
assigned to Degussa AG. b) H. Strack, W.Roebke, D. Kneitel and E. Parr, Ger. Patent 2660725 (Mar. 26,1987), assigned to Degussa AG. 15 L.E. Williams, Ger. Patent Appl. 2633304 (Feb. 17,1977), assigned to J.M. Huber Corp. 16 J. Arika, M. Aimoto and H. Miyazaki, Europ. Patent Appl. 128766 (Dec. 19,1984), assigned to Toyo Soda Manufacturing Co., Ltd. 17 J.S. Magee and J.J. Blazek, in J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis (ACS Monograph 171), American Chemical Society, Washington,D.C., 1976, pp. 615-679. 18 P.B. Venuto and E.T. Habib,Jr., Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Decker, New York, 1979, pp. 30-49. 19 W.B. Wilson, Brit. Patent Appl. 1431944 (Apr. 14, 1976), assigned to Shell Research B.V. 20 H. Lechert, in P.A. Jacobs, N.I. Jaeger, P. Jid, V.B. Kazansky and G. Schulz-EMoff (Eds.), Structure and Reactivity of Modified Zeolites (Stud. Surf. Sci. Catal. 18), Elsevier, Amsterdam, 1984, pp. 107-123. 21 G.C. Edwards and R.L. Chiang, Ger. Patent Appl. 3347123 (July 5,1984), assigned to W.R. Grace & Co. 22 P.A. Jacobs and J.A. Martens, Synthesisof High-Silica Aluminosilicate Zeolites (Stud. Surf. Sci. Catal. 33), Elsevier, Amsterdam, 1987, p. 343. 23 S. Kasahara, K. Itabashi and K. Igawa, in Y.Murakami, A. Iijima and J.W. Ward (Eds.), New Developments in Zeolite Science and Technology (Stud. Surf. Sci. Catal. 28), KodanshaElsevier, Tokyo/Amsterdam, 1986, pp. 185-192. 24 C.H. Elliott, Jr., US Patent 4164551 (Aug. 14,1979), assigned to W.R. Grace & Co. 25 S.M. Csicsery, Pure Appl. Chem., 58 (1986) 841-856. 26 J. Weitkamp, S. Erst, H. Dauns and E. Gallei, Chem.-1ng.-Tech., 58 (1986) 623-632. 27 P.A. Jacobs and J.A. Martens, Synthesis of High-Silica AluminosilicateZeolites (Stud. Surf. Sci. Catal. 33), Elsevier, Amsterdam, 1987. 28 J. Scherzer, in T.E. WhyteJr., R.A. Dalla Betta, E.G. Derouane and R.T.K. Baker (Eds.), Catalytic Materials (ACS Symposium Series 248), American Chemical Society, Washington,D.C., 1984, pp. 157-200. 29 D.W. Breck and G.W. Skeels, US Patent 4503023 (Mar. 5,1985), assigned to Union Carbide Corporation. 30 H.K. Beyer and I. Belenykaja, in B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud (Eds.), Catalysis by Zeolites (Stud. Surf. Sci. Catal. 5), Elsevier, Amsterdam, 1980, pp. 203-210. 31 H. Strack and P.Kleinschmit, Europ. Patent Appl. 72397 (Feb. 2,1983), assigned to Degussa AG. 32 C.E. Capes, Particle Size Enlargement, Elsevier, Amsterdam, 1980.
659
33 H. Strack, P. Kleinschmit and E. Parr, Europ. Patent Appl. 72396 (Feb. 2,1983), assigned to Degussa AG. 34 R.L. Wadlinger, G.T. Kerr and E.J. Rosinski, US Patent 3308069 (Mar. 7, 1967), assigned to Mobil Oil Corporation. 35 Chemical & Engineering News, 66 (March 21,1988), 22-24.
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H.G.Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CALCIUM AND MAGNESIUM EXCHANGE
LOVAT V.C.
I N Na-A, Na-X AND THEIR PRECURSOR GELS
REES
P h y s i c a l Chemistry L a b o r a t o r i e s , I m p e r i a l C o l l e g e o f Science and Technology, London SW7 2AY ,(England) ABSTRACT Na-Ca and Na-Mg b i n a r y and Na-Ca-Mg t e r n a r y exchange i s o t h e r m s have been determined i n z e o l i t e A . The e f f e c t o f t e m p e r a t u r e and n o r m a l i t y o f t h e s o l u t i o n phase on t h e exchange e q u i l i b r i u m has been s t u d i e d . Standard f r e e e n e r g i e s , e n t h a l p i e s and e n t r o p i e s have been c a l c u l a t e d f r o m t h e b i n a r y i s o t h e r m data. The b i n a r y isotherms have a l s o been determined i n samples o f z e o l i t e A and X g e l s as a f u n c t i o n o f c r y s t a l l i z a t i o n t i m e and t h e s t a n d a r d f r e e e n e r g i e s o f exchange have been c a l c u l a t e d . The k i n e t i c s o f t h e b i n a r y exchange r e a c t i o n s have been measured i n t h e s e amorphous g e l samples and compared w i t h t h e k i n e t i c s i n t h e c o r r e s p o n d i n g c r y s t a l l i n e z e o l i t e s . I O N EXCHANGE I N ZEOLITE A
C r y s t a l l i n e z e o l i t e A i s used ( r e f . l ) , w h i l e z e o l i t e X has been suggested ( r e f . 2 ) as a b u i l d e r i n d e t e r g e n t f o r m u l a t i o n s t o r e p l a c e phosphates which a r e banned, o r whose c o n c e n t r a t i o n s a r e s e v e r e l y 1i m i t e d , i n many c o u n t r i e s throughout t h e world.
Z e o l i t e b u i l d e r s exchange t h e Ca2+ and Mg2+ i o n s p r e s e n t i n i o n s r e s i d e n t i n t h e z e o l i t e s . The s e l e c t i v i t y o f
hard waters w i t h t h e Na'
Na-A towards t h e i n g o i n g d i v a l e n t i o n i s v e r y l a r g e as can be c l e a r l y seen i n t h e exchange isotherms i n F i g . 1 where Ca,
and Mg,
and Ca,
and Mg,
represent
the respective equivalent cation fractions o f the ingoing divalent ions i n the z e o l it e phase, z, and s o l u t i o n phase, s ( r e f . 3 ) .
The c o r r e c t e d s e l e c t i v i t y
c o e f f i c i e n t , Kc, can be expressed i n a c o n v e n i e n t f o r m f o r use w i t h t h e i s o t h e r m d a t a by eqn.1. 2 Ca, (Na,) K C z-2. - 2N (Naz) Ca,
.
4 YtNaC1 .. r Y+CaC1
where N i s t h e t o t a l n o r m a l i t y o f t h e s o l u t i o n phase and Y+- i s t h e a c t i v i t y c o e f f i c i e n t o f t h e i n d i c a t e d s a l t i n t h e m i x e d s a l t s o l u t i o n phase. The ~
c o r r e c t i o n i n t r o d u c e d by t h e l a s t two terms i n eqn.1 removes t h e s e l e c t i v i t y o f t h e exchange i n t r o d u c e d by t h e s o l u t i o n phase.
Thus, t h e c o r r e c t e d s e l e c t i v i t y
c o e f f i c i e n t i s a q u a n t i t a t i v e r e p r e s e n t a t i o n o f t h e s e l e c t i v i t y o f t h e exchange Eqn.1 q u a n t i f i e s t h e c o n c e n t r a t i o n -
a s s o c i a t e d w i t h t h e z e o l i t e phase o n l y .
v a l e n c y e f f e c t which i s c l e a r l y demonstrated i n Fig.1.
Decreasing s o l u t i o n
phase n o r m a l i t y r e s u l t s i n i n c r e a s i n g s e l e c t i v i t y i n the exchange r e a c t i o n towards t h e i n g o i n g d i v a l e n t i o n .
662
EXPERIMENTAL The chemical a n a l y s i s o f t h e z e o l i t e A used i n these s t u d i e s gave an almost i d e a l u n i t c e l l composition o f Nal
112. O O A l O2
. 12 .08Si02]
26 .8H20
A t l e a s t 5 days were allowed f o r each p o i n t t o come t o e q u i l i b r i u m i n t h e isotherms a t 65OC and 10 days a t 25OC. RESULTS The c o r r e c t e d s e l e c t i v i t y c o e f f i c i e n t s , Kc, were c a l c u l a t e d f r o m t h e i s o t h e r m data i n Fig.1 u s i n g eqn.1 and f r o m these t h e loglOKc p l o t s i n Fig.2 and 3 r e s p e c t i v e l y were constructed.
vs Ca,
o r Mg,
As i n d i c a t e d above these
p l o t s i n d i c a t e t h e s e l e c t i v i t y o f t h e z e o l i t e phase f o r t h e r e s p e c t i v e i n g o i n g d i v a l e n t i o n as a f u n c t i o n o f l o a d i n g .
Logl0KC
values g r e a t e r than z e r o
i n d i c a t e preference f o r t h e d i v a l e n t i o n over t h e Nat i o n by t h e z e o l i t e phase. Fig.2 and 3 show t h a t Na-A p r e f e r s Ca2'
i o n s up t o l o a d i n g s g r e a t e r t h a n 90%
and Mg2' i o n s up t o %40%. Thus Na-A i s a b e t t e r b u i l d e r towards Ca2+ t h a n 2t These f i g u r e s , a l s o show t h e enhancement i n s e l e c t i v i t y f o r b o t h Mg d i v a l e n t i o n s as t h e temperature increases because o f t h e endothermic n a t u r e
.
o f t h e exchange r e a c t i o n . a polynomial
The experimental p o i n t s i n Fig.2 and 3 were f i t t e d t equation o f t h e form
(2) (where A, = Ca, o r MgZ). The r e s u l t i n g polynomials a r e g ven i n Table 1 and t h e isotherms c a l c u l a t e d from these polynomials a r e drawn as continuous l i n e s
i n Fig.1 which c l e a r l y show t h e g o o d n e s s - o f - f i t o f t h e polynomial i n r e p r e s e n t i n g t h e experimental isotherms. TABLE 1 Polynomial Equations o f loglOKc Exchange Reaction Nat
+
$Ca2 t
vs A,
Temp( O C ] 25
Polynomi a1 Eqn loglOKc
-+
3Mg2'
25 65
6.03Caz t 11.5CaZZ
-
9.11CaZ 3
5.28CaZ
6.32CaZ2
-
4.01Caz 3
= 1.46
-
+
6.34MgZ2 3.57MgZ 2
+ 5.08Mgz 3
= 2.61
= 3.41
65 Nat
-
= 2.83
loglOKc
2.66MgZ 7.74Mgz
t
-
From these polynomials t h e c o r r e c t e d s e l e c t i v i t i e s , Kc, a r e 676 and 2570 f o r Ca,
=
0 and 28.8 and 407 f o r Mg,
= 0 a t 25 and 65OC r e s p e c t i v e l y .
These values
i n d i c a t e t h e v e r y h i g h i n i t i a l s e l e c t i v i t i e s f o r Ca2+ a t 25 and 65OC r e s p e c t i v ely.
663
Fig. 1. Na/Ca and Na/Mg b i n a r y exchange isotherms 0 0.2N; X 0.1N; 0 0.05N; 0 0.01N; 0.005N
1.6
\
0.0
20
b%K, 0 1.5
-0.8 1.0
00 '
-1.6
0.5
0
- 2.4
0
-
0
. 0.2
Fig. 2 . Loglo
5
L
0.4 0.6 0.8
Kc v s Ca,
(Symbols same as i n Fig. 1 )
0.2 0.4 a6 08
Fig. 3. Log10 K, vs Mg, (Symbols same as in F i g . 1)
664
From these loglOKc
p l o t s o r t h e polynomials i n Table 1 i t i s p o s s i b l e t o
c a l c u l a t e t h e standard f r e e energies, e n t h a l p i e s and e n t r o p i e s o f these exchange reactions (ref.3).
The standard q u a n t i t i e s so o b t a i n e d a r e l i s t e d i n Table 2.
TABLE 2 Standard Thermodynamic Q u a n t i t i e s f o r Exchange i n Z e o l i t e A Exchange Reaction
A G ~
Temp('C)
TA S~
AH&
kJ(g equiv)-l t
Nat
-+
4Ca2
Nat
+.
$Mg2'
The TAS*
25 65
-2.68 -4.69
12.2
14.9 16.9
25 65
3.26 1.20
18.6
15.5 17.4
values a r e very s i m i l a r f o r a l l f o u r exchanges l i s t e d i n Table 2.
The n e g a t i v e AG& values f o r t h e Ca2' exchange and t h e corresponding p o s i t i v e values f o r t h e Mg2+ exchange a r i s e , t h e r e f o r e , from t h e l a r g e r endothermic e n t h a l p i e s f o r t h e Mg2+ exchange.
The d i v a l e n t i o n s have t o shed water molecul-
es associated w i t h t h e i r h y d r a t i o n s h e l l b e f o r e t h e y can e n t e r t h e channel network o f z e o l i t e A .
The endothermic d e h y d r a t i o n energy must be some 50%
g r e a t e r f o r Mg2' i o n s t h a n f o r Ca2'
i o n s and i t i s t h i s i n c r e a s e i n d e h y d r a t i o n
enthalpy which produces t h e p o s i t i v e f r e e energy t o t h e Mg2' exchange r e a c t i o n . When z e o l i t e A i s used as a b u i l d e r i n detergency, one i s d e a l i n g , a t i t s s i m p l e s t concept, w i t h a t e r n a r y exchange r e a c t i o n .
A l l such t e r n a r y
exchange r e a c t i o n s behave i n a manner s i m i l a r t o t h a t shown by Fig.4.
Ternary
isotherms d i s p l a y a h i g h s e l e c t i v i t y f o r b o t h d i v a l e n t i o n s a t l o w Mg,
and Ca,
loadings and i n t h i s r e g i o n o f t h e i s o t h e r m l i t t l e o r no d i v a l e n t i o n s remain i n t h e s o l u t i o n phase a t e q u i l i b r i u m .
A l l divalent ions, i n i t i a l l y i n the
s o l u t i o n phase a r e exchanged i n t o t h e z e o l i t e which r e s u l t s i n a s t r a i g h t l i n e z e o l i t e phase composition curve, as seen i n Fig.4, a t low d i v a l e n t i o n l o a d i n g s . When t h i s l i n e i s e x t r a p o l a t e d t o Na,
= 0 i t i n t e r s e c t s t h e Ca-Mg a x i s a t a
value equal t o t h e Ca:Mg r a t i o i n t h e i n i t i a l s o l u t i o n phase.
The c o n c e n t r a t i o n
o f d i v a l e n t i o n i n t h e s o l u t i o n phase g r a d u a l l y b u i l d s up w i t h i n c r e a s i n g d i v a l e n t i o n l o a d i n g o f t h e z e o l i t e phase u n t i l , f i n a l l y , t h e Na, v a l u e approaches zero and t h e Cas/Mgs r a t i o approaches t h a t o f t h e i n i t i a l s o l u t i o n phase.
The Mg,
value a t t a i n s i t s i n i t i a l s o l u t i o n phase value l o n g b e f o r e
t h e Ca, value a t t a i n s i t s i n i t i a l value. Mg,
passes through a maximum i n a l l t e r n a r y isotherms measured i n our
laboratories.
The magnitude o f MgZ a t t h e maximum depends on t h e Ca/Mg r a t i o
o f t h e i n i t i a l s o l u t i o n phase b u t t h e maximum always occurs a t a Na, v a l u e o f The value o f Mg, i s found t o i n c r e a s e from . ~ 0 . 1 9 f o r a Ca/Mg r a t i o a 0.33. o f 2 : l t o ~ 0 . 3 8f o r a Ca/Mg r a t i o of 1:2. t h a t Mg,
It i s a l s o i n t e r e s t i n g t o n o t e
never increases above a value o f a0.4,
t h e maximum found i n t h e b i n a r y
665 Ca
Na
5 6
0.2
\,-4
0.4
0.6
0.8
Fig. 4. Na/Ca/Mg ternary exchange isotherm at 65OC. Initial solution phase 0.05N and Ca/Mg ratio o f 1:l. 0 Zeolite phase 0 Solution phase Equilibrium compositions in each phase have the same number.
m
3
R e a c t i o n Tlme(hours)
Fig. 5. Ion exchange and water capacities o f zeolite A and X gels as a function o f crystallization time.
666
exchange o f Ca-A w i t h Mg2' ions (ref.3) confirming t h e great d i f f i c u l t y o f loading z e o l i t e A w i t h more than 2Mg2+ ions per u n i t c e l l when there are Ca2' ions a v a i l a b l e t o the z e o l i t e . The rates o f exchange o f t h e Na' i o n s i n Na-A by Ca2', mixtures o f these ions have been determined.
Mg2' and b i n a r y
The d i f f u s i o n c o e f f i c i e n t s
c a l c u l a t e d from these r a t e s are given i n Table 3.
This t a b l e shows t h a t t h e
r a t e o f pure Ca2' exchange i s 10 f o l d f a s t e r than the corresponding Mg2' exchange r a t e ( r e f . 4 ) .
The presence o f Mg2' ions i n t h e b i n a r y mixtures does
n o t decrease t h e r a t e o f exchange found w i t h pure Ca2+ s o l u t i o n s when the s o l u t i o n phase contains an i n i t i a l Ca/Mg r a t i o o f 2 : l .
However, when t h i s
r a t i o decreases t o 1:2 the r a t e o f d i v a l e n t exchange i s considerably reduced. Mg2+ ions, i n t h i s l a t t e r case, must block t h e eight-membered windows c o n t r o l l i n g access t o the channel network o f z e o l i t e A and slow down the ingress o f the more r a p i d l y d i f f u s i n g Ca2' ions. From these r a t e measurements the time constants f o r 50% exchange o f
lpm
radius Na-A c r y s t a l s ( t h e s i z e o f c r y s t a l used i n detergent formulations) by Ca2' and Mg2' ions could be calculated. respectively.
They were found t o be 9s and 250s
The slowness o f the big2' exchange must be ascribed, once again,
t o the need t o s t r i p water molecules from the h y d r a t i o n s h e l l before access t o the channel network o f z e o l i t e A can be achieved.
The slowness o f t h e exchange
r e a c t i o n allows the Mg2' ions t o r e a c t w i t h o t h e r species present i n the detergent formulation. TABLE 3
K i n e t i c s o f I o n Exchange i n Z e o l i t e A I n i t i a l S o l u t i o n Phase Pure Ca Ca:Mg Ca:Mg 2:l 1 :1 Diffusion Coefficient
1.67
1.70
1.38
Ca:Mg 1 :2 0.62
Pure Mg 0.18
0/1 0-1 5m2s-1 I O N EXCHANGE I N PRECURSOR GELS
I n the above studies c r y s t a l l i n e z e o l i t e A has been shown t o be an e x c e l l e n t b u i l d e r f o r t h e removal o f Ca2'
ions.
However, t h e performance o f
the z e o l i t e as a b u i l d e r f o r t h e removal o f Mg2' ions i s poor e s p e c i a l l y when Ca2' ions are present i n the s o l u t i o n phase. This poor performance towards Mg2' i s associated w i t h need t o reduce t h e e f f e c t i v e s i z e o f t h e hydrated Mg2' i o n before i t can e n t e r t h e channels o f z e o l i t e A. We decided, therefore, t o study the exchange behaviour o f t h e precursor gels o f z e o l i t e s A and X as a f u n c t i o n o f c r y s t a l l i z a t i o n time ( r e f . 5 ) .
P r i o r t o c r y s t a l l i z a t i o n the g e l s
should have more open s t r u c t u r e s w i t h l a r g e r water contents than the f i n a l c r y s t a l 1i n e product.
667
EXPERIMENTAL The z e o l i t e A g e l s had m o l a r compositions o f Si02/A1203 o f 2 . 0 0 ; Na20/Si02 o f 2.50 and H20/Na20 o f 40 w h i l e t h e z e o l i t e X g e l s had c o r r e s p o n d i n g r a t i o s o f 3.0, 1.2 and 40 r e s p e c t i v e l y .
The g e l s were d i v i d e d e q u a l l y between
t e n s e a l e d p o l y p r o p y l e n e b o t t l e s which were t h e n heated i n an oven a t 8OoC. The t e n samples were a l l o w e d t o c r y s t a l l i z e o v e r r e a c t i o n t i m e s o f 0 t o 14 hours f o r t h e z e o l i t e A g e l s and 0 t o 24 hours f o r t h e z e o l i t e X g e l s .
R e a c t i o n was
stopped by f i l t r a t i o n f o l l o w e d by washing w i t h c o l d , d i s t i l l e d w a t e r u n t i l t h e pH o f t h e washings f e l l below 9.
The samples were exchanged t w i c e w i t h
c o n c e n t r a t e d NaCl s o l u t i o n s , washed w i t h d e i o n i s e d w a t e r a t pH 10 (pH a d j u s t e d w i t h NaOH) u n t i l c h l o r i d e f r e e and t h e n a i r d r i e d . The exchange c a p a c i t i e s o f t h e s e a i r d r i e d samples were determined by i s o t r o p i c d i l u t i o n u s i n g 0.1 N NaCl s o l u t i o n s tagged w i t h 22Na.
The w a t e r
c a p a c i t i e s o f t h e samples were o b t a i n e d f r o m t h e w e i g h t l o s s on h e a t i n g o v e r n i g h t a t 50OoC.
The c a l c i n e d samples were r e h y d r a t e d i n a d e s i c c a t o r o v e r
s a t u r a t e d NaCl s o l u t i o n and t h e i r w a t e r c a p a c i t i e s r e d e t e r m i n e d as above. I o n exchange i s o t h e r m s were determined u s i n g “Na atomic a b s o r p t i o n a n a l y s i s .
tagged samples and, a l s o by
E q u i l i b r a t i o n t i m e s o f 10 days p e r p o i n t were
used i n a l l cases. RESULTS The samples were d e s i g n a t e d A o r X t o i n d i c a t e z e o l i t e A and X g e l s r e s p e c t i v e l y and t h e number f o l l o w i n g t h e l e t t e r i n d i c a t e s t h e c r y s t a l l i z a t i o n t i m e i n hours, e.g. A4 i n d i c a t e s a z e o l i t e A g e l heated a t 8OoC f o r 4 hours. Xray a n a l y s i s showed t h a t samples AO, A1 and A2 were amorphous w h i l e A3 was p a r t l y c r y s t a l l i n e .
Samples A4 t o A14 were c o m p l e t e l y c r y s t a l l i n e w i t h
much sharper d i f f r a c t i o n l i n e s t h a n sample A3.
Samples XO t o X4 were amorphous,
X6 p a r t l y c r y s t a l l i n e w h i l e samples X8 t o X24 were c o m p l e t e l y c r y s t a l l i n e w i t h sharp d i f f r a c t i o n l i n e s . The w a t e r c a p a c i t i e s o f t h e t e n , as-synthesised, z e o l i t e A and X samples a r e g i v e n i n F i g . 5 and, a l s o a f t e r r e h y d r a t i o n f o l l o w i n g c a l c i n a t i o n a t 50OoC. The severe decrease i n w a t e r s o r p t i o n c a p a c i t y a f t e r c a l c i n a t i o n o f t h e amorphous Ad), A1 and A2 samples and t h e c o r r e s p o n d i n g XO t o X4 samples c l e a r l y demonstrates t h e l a c k o f thermal s t a b i l i t y o f t h e amorphous samples. Fig.5 a l s o i n c l u d e s t h e i o n exchange c a p a c i t i e s o f t h e s e samples based on b o t h h y d r a t e d and dehydrated mass o f t h e samples.
The exchange c a p a c i t y o f t h e
z e o l i t e A g e l s i n c r e a s e w i t h i n c r e a s i n g c r y s t a l l i n i t y whereas t h e c a p a c i t i e s o f t h e z e o l i t e X g e l s a r e l e s s dependent on c r y s t a l l i n i t y .
The enhanced
exchange c a p a c i t y o f c r y s t a l l i n e z e o l i t e A compared w i t h z e o l i t e X i s c l e a r l y seen i n Fig.5.
668
I n Fig.6 the Na/Ca and Na/Mg exchange isotherms o f samples AO, A1 , A2 and A14 are presented and i n Fig.7 the corresponding isotherms o f samples XO, X1, X2, X3 and X24. A l l f o u r sets o f isotherms c l e a r l y demonstrate the decreasing s e l e c t i v i t y f o r the ingoing d i v a l e n t i o n w i t h increasing c r y s t a l l i z a t i o n time, This t r e n d continues i n the case o f z e o l i t e A gels up t o the c r y s t a l l i n e product. However, an i n t e r e s t i n g reversal o f t h i s trend occurs w i t h z e o l i t e X gels on going from sample X3, which i s s t i l l amorphous t o Xrays, t o X24 which i s 100% crystalline.
Secondly, the change i n s e l e c t i v i t y w i t h c r y s t a l l i z a t i o n time i s
greater w i t h the z e o l i t e X samples than w i t h the z e o l i t e A samples. I n Fig.8 and 9 the respective logl0KC i n the z e o l i t e A gel samples a r e presented.
p l o t s f o r Na/Ca and Na/Mg exchange The corresponding p l o t s f o r t h e
z e o l i t e X gel samples are given i n Fig.10 and 11. These p l o t s q u a n t i f y the s e l e c t i v i t y trends seen i n the isotherms as a f u n c t i o n o f c r y s t a l l i z a t i o n time. I n Fig.8 and 10 the much higher s e l e c t i v i t i e s f o r Ca exchange, i n both types o f z e o l i t e gels, compared w i t h the s e l e c t i v i t i e s f o r Mg exchange i n Fig.9 and 11 can be seen. The change i n s e l e c t i v i t y w i t h c r y s t a l l i E a t i o n time i s s l i g h t l y more pronounced i n the case o f the Mg exchanges. The experimental p o i n t s i n the above logloKc
p l o t s were f i t t e d t o eqn.3
and the r e s u l t i n g set o f polynomials are l i s t e d i n Table 4. i n the Co values ( i . e . t h e value o f loglOKc
The f l u c t u a t i o n s
a t AZ = 0) w i t h c r y s t a l l i z a t i o n
time a r e due t o the s p a r c i t y and s c a t t e r o f t h e experimental p o i n t s i n these loiloKc
p l o t s as AZ * 0.
However, the standard f r e e energies o f exchange,
AG , i n d i c a t e (a) the preference f o r Ca2' over Na' i n both z e o l i t e systems ( b ) the opposite r e s u l t f o r Mg2' and ( c ) a decreasing s e l e c t i v i t y f o r the d i v a l e n t i o n w i t h increasing c r y s t a l l i z a t i o n time a p a r t from t h e reversal i n t h i s t r e n d w i t h the X24 sample. Preliminary k i n e t i c s t u d i e s have been c a r r i e d o u t w i t h the amorphous A2 sample.
This sample had a spread i n p a r t i c l e s i z e over t h e range 1.9 t o
23.7~11w i t h an average o f 8.2um.
Because o f t h e amorphous nature o f t h e
sample and the wide d i s t r i b u t i o n o f p a r t i c l e s i z e i t i s d i f f i c u l t t o o b t a i n accurate d i f f u s i o n c o e f f i c i e n t s f o r Ca and Mg exchange i n t h i s sample. I f t h e p a r t i c l e s are assumed t o be spheres o f 4.lum r a d i u s then the d i f f u s i o n c o e f f i c i e n t f o r Mg exchange i s the same as t h a t given i n Table 3 f o r Mg exchange i n c r y s t a l l i n e z e o l i t e A w h i l e t h e d i f f u s i o n c o e f f i c i e n t f o r Ca exchange i s a t h i r d o f t h a t given i n Table 3 f o r Ca exchange i n c r y s t a l l i n e z e o l i t e A. Thus, i n t h e amorphous samples the r a t e of exchange w i t h Ca2+ i o n s i s only three times f a s t e r than t h a t f o r Mg2' ions compared w i t h the t e n f o l d d i f f e r e n c e w i t h c r y s t a l l i n e z e o l i t e A.
669
CALCIUM
MAGNESIUM
Ca
0
Fig. 6. Na/Ca and Na/Mg binary exchange isotherms of zeolite A gels a t 25OC. Solution phasg 0.05N. AO, A l , A2 and A 1 4 represent gels which had been reacted a t 80 C f o r 0, 1 , 2 and 14 hours respectively. CALCIUM
MAGNESIUM
0
F i g . 7. Na/Ca and Na/Mg binary exghange isotherms of zeolite X gels a t 25 C. Solution phase 0.05N. XO, X1, X2, X3 and X 2 4 represent gels which had been reacted a t 8OoC f o r 0, 1, 2 , 3 and 24 hours respecti ve ly .
Fig. 8. LoglO Kc vs Ca, plots for z e o l i t e A gels. (Symbols and conditions as defined in Fig. 6 ) -1.0
J
670 3.0
2.0 1 .o
1 .c
Y, m 0 -1
0.C
0.c
.o 0 x2 x3
0
ex24
0 2
-1.c
F i g . 10. Loglo
- 1 .I
Kc v s Ca,
plots for zeolite X
gels. (Symbols and conditions as defined i n Fig. 7 )
K vs MgZ p l o t s for z e o l i t e A F i g. 9. Log 10 c gels. (Symbols and condit ions as def ined i n Fig. 6 )
F i g . 11. LoglOK
C
v s Mg,
plots for z e o l i t e X
gels. (Symbols and condit ions as def ined i n Fig. 7 )
671
TABLE 4 LoglOKC
polynomials and Standard Free Energies o f Exchange Zeol it e A Samples
Na-Ca Exchange Sample A0 : LoglOKc Sample A1
: LoglOKc
Sample A2
: LoglOKc
=
2.20-3.29Cazt7.30Caz2-5.17Caz3
= 2.47-5.55Cazt11.91Ca 2-8.09Ca = 2.48-5.07Caztll .31CaZz2 -8.37Ca:3
Sample A14 : LoglOKc
= 2.95-10.
Na-Mg Exchange Sample A0 : LoglOKc
=
Sample A1
: LoglOKc
= 1 .72-4.82Mgzt7.43Mgz2-5.80Mg:3
Sample A2
: LoglOKc
= =
Sample A14 : LoglOKc
AG8
AG*
AG*
12Cazt23.93Caz2-17.61CaZ3
1 .30-2.15Mgzt2.90Mgz2-3.5OMg
AG*
A&
= -3.61kJ/g e q u i v . = -3.45kJ/g equiv. = -3.38kJ/g equiv. = -2.94kJ/g
equiv.
= 0.33kJ/g equiv.
A G ~= 0.28kJ/g equiv.
AG* 0.93t4.46Mgz-17.45Mgz 2+10.53Mg 1 .59-0.06MgZ-10.83Mgz 2t7.74MgZ3 AG*
= 1.30kJ/g equiv.
= 1.57kJ/g equiv.
Z e o l i t e X Samples Na-Ca Exchange Sample XO : LoglOKc
=
1 .97-5.15Caztll .91Ca 2-8.75Caz3
= -2.14kJ/g
equiv. equiv.
= -1.99kJ/g
2-11 .60CaZ3 AG*
= -1.5lkJ/g
equiv.
1 .70-5.07Cazt13.66Caz2-12.07Ca:3 AG* = 2.04-5.72Cazt14.28Caz z2-11.51CaZ3 A&
= -0.76kJ/g = -1.81kJ/g
equiv. equiv.
Sample X1
: LoglOKc
Sample X2
: LoglOKc
= 1 .90-5.56Cazt14.23Ca
Sample X3
: LoglOKc
=
Sample X24 : LoglOKc
AG* *GO
2.19-7.13Cazt16.81Ca~2-12,38Ca
Na-Mg Exchange Sample XO : LoglOKc
=
0.82t0.64Mgz-1 .75Mg '-2.19MgZ3
Sample X1
: LoglOKc
=
Sample X2
: Logl0KC
0.92-0.17Mgzt1 .17Mg:2-6.18Mgz3 = 2.16kJ/g equiv. 2+5.45Mgz3 A G ~= 3.92kJ/g equiv.
Sample X3
: LoglOKc
Sample X4
: LoglOKc
AG* AG*
= 1.23kJ/g equiv.
= 0.76t3.87Mgz-15.00Mgz = 0.30t8.98MgZ-39.10Mg = 0.88t1 .16Mgz-6.42Mgz
2+27.16Mg
f t0.19Mgz3z
AG*
AG*
= 5.40kJ/g equiv. = 3.04kJ/g equiv.
CONCLUSIONS These s t u d i e s show t h a t t h e s e l e c t i v i t y f o r Ca2' and Mg2+ i o n s i s g r e a t e r i n t h e more open and more h i g h l y hydrated amorphous a l u m i n o - s i l i c a t e gel samples than i n t h e z e o l i t e A and X products which f i n a l l y c r y s t a l l i z e o u t from these gels.
The exchange c a p a c i t i e s o f these g e l samples a r e q u i t e l a r g e
and n o t g r e a t l y d i f f e r e n t from those o f t h e corresponding c r y s t a l 1i n e z e o l i t e . The d i f f e r e n c e in t h e r a t e s o f exchange o f Ca2+ and Mg2+ i s much reduced i n t h e amorphous z e o l i t e A gel samples compared w i t h t h e t e n f o l d d i f f e r e n c e found i n c r y s t a l l i n e z e o l i t e A. Thus, amorphous z e o l i t e A and X gel samples seem t o have e x c e l l e n t b u i l d i n g p r o p e r t i e s f o r b o t h Ca2+ and MgZt ions. REFERENCES 1 A.C. Savitsky, Soap Cosmet. Chem. Spec., 53 (1977) 29. 2 G.H. Kuhl and H.S. Sherry, "Proc. F i f t h I n s t . Conf. Z e o l i t e s " (Ed. L.V.C. Rees) Heyden, London, 1980, p.813. 3 S . A . I . B a r r i and L.V.C. Rees, J. Chromatog., 201 (1980) 21.
672
4 D. Drummond, A. De Jonge and L.V.C. Rees, J. Phys. Chem., 87 (1983) 1967. 5 V.C. Mole and L.V.C. Rees, "Recent Developments i n I o n Exchange" (Ed. P . A . W i l l i a m s and M.J. Hudson), E l s e v i e r Appl. Science, London, 1987, p.264.
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FUNDAMENTALS OF PHOSPHATE SUBSTITUTION I N DETERGENTS BY ZEOLITES
-
c o b u i l d e r s and o p t i c a l b r i g h t e n e r s
M.J. Schwuger, M. Liphard Henkel KGaA, P.O. Box 1100, 4000 Dusseldorf 1 (Federal Republic o f Germany)
ACSIP_ncI
This study i s concerned w i t h the physico-chemical background t o t h e s u b s t i t u t i o n o f phosphates i n laundry detergents, w i t h s p e c i a l emphasis on combinat i o n s o f z e o l i t e A w i t h other detergent i n g r e d i e n t s , e s p e c i a l l y water-soluble c o b u i l d e r s and o p t i c a l b r i g h t e n e r s . The most important a c t i o n mechanisms o f z e o l i t e A / c o b u i l d e r systems were i n v e s t i g a t e d f o r a v a r i e t y o f washing cond i t i o n s . I t i s shown t h a t the b u i l d e r performance o f z e o l i t e A i s o f t e n r e i n forced by small amounts o f water-soluble substances such as p o l y c a r b o x y l a t e s . These detergents a r e a b l e t o compete, i n terms o f performance, w i t h products c o n t a i n i n g a h i g h p r o p o r t i o n o f phosphate. F u r t h e r , the i n t e r a c t i o n o f z e o l i t e A w i t h f l u o r e s c e n t whitening agents i n detergent powders was s t u d i e d . I t i s shown t h a t the photophysical p r o p e r t i e s o f a t y p i c a l o p t i c a l b r i g h t e n e r are n e g a t i v e l y i n f l u e n c e d by i o n i c b u i l d e r s and s u r f a c t a n t s . I n c o n t r a s t t o sodium triphosphate, z e o l i t e A can enhance t h e appearance o f powders even i n t h e p r e sence o f a n i o n i c s u r f a c t a n t s . 1._ -_INTRODUCTION A t the beginning o f t h e 60's the l a r g e p a r t played by phosphates i n the
e u t r o p h i c a t i o n o f staynant and s l o w l y f l o w i n g s u r f a c e waters l e d t o worldwide research i n t o the problem o f f i n d i n g s u i t a b l e s u b s t i t u t e s f o r phosphates i n detergents. The s o l u t i o n t o t h i s problem was complicated by the f a c t t h a t such a s u b s t i t u t e not o n l y had t o measure up t o s i g n i f i c a n t e c o l o g i c a l and t o x i cological c r i t e r i a ,
i t a l s o had t o s a t i s f y p u r e l y p r a c t i c a l c o n s i d e r a t i o n s o f
use and performance, as w e l l as be economically v i a b l e , o f course. Z e o l i t e A, a w a t e r - i n s o l u b l e , f i n e l y dispersed,
i o n exchanger, met a l l these r e q u i r e -
ments. By now, z e o l i t e A has been incorporated i n t o detergents i n many count r i e s , e s p e c i a l l y i n those where e x i s t i n g o r planned l e g i s l a t i o n r e g u l a t i o n s o r v o l u n t a r y agreements r e q u i r e a r e d u c t i o n or t o t a l ban on phosphates i n laundry detergents. I n 1983, t h e f i r s t phosphate-free,
powdered detergent con-
t a i n i n g z e o l i t e A was launched on the market i n the Federal Republic o f Germany. I n 1986 the percentage o f z e o l i t e A-based products increased markedly, and i n March 1988 t h e i r share o f the heavy-duty laundry detergents had reached 70%.
When z e o l i t e A i s t o be incorporated i n t o detergent f o r m u l a t i o n s i n s t e a d o f sodium triphosphate,
i t i s necessary t o i n v e s t i g a t e the common and the
674
d i f f e r e n t properties o f both b u i l d e r s . Subsequently the differences i n t h e i r i n t e r a c t i o n s w i t h other detergent ingredients have t o be taken i n t o account, both i n s o l u t i o n , f o r a v a r i e t y o f washing conditions, and i n the powder itself. The use of sodium aluminium s i l i c a t e s i n the washing process has been examined i n a series o f basic investigations ( r e f s . 1-10). This study i s concerned w i t h the physico-chemical background t o two p r a c t i c a l aspects o f z e o l i t e A-based laundry detergents: combination o f z e o l i t e A w i t h o p t i c a l brighteners and w i t h water-soluble cobuilders, w i t h special emphasis on polycarboxylates.
2.
EXPERIMENT
Most o f the methods have been described elsewhere ( r e f s . 1, 9). Concentrat i o n determination o f polycarboxylates was c a r r i e d o u t w i t h a microprocessor ion-analyzer 901 (Orion Research) by potentiometric t i t r a t i o n w i t h c e t y l benzyl-dimethyl ammonium chloride, which forms water-insoluble complexes w i t h long-chain polycarboxylates. The t i t r a t i o n curve was obtained v i a a surfact a n t - s e n s i t i v e electrode ( r e f . 11). Fluorescence spectra of the o p t i c a l brightener were made w i t h a ZMF4 / PMQ
I 1 1 Spectralfluorimeter (Zeiss). The degree o f whiteness (Berger) was obtained w i t h a spectroreflectometer RFC 3-24 ( Z e i s s ) . The composition o f the powders used was i n accordance w i t h the composition o f t y p i c a l laundry detergents. The powders were produced by d i s s o l v i n g or dispersing the ingredients i n water, then mixing and subsequently d r y i n g them. For easier handling the powders were ground. The f o l l o w i n g substances were used: Industrial-grade z e o l i t e A ( S a s i l Q f r o m Henkel KGaA, DUsseldorf), w i t h an ion-exchange capacity o f 162 mg CaO/g. Other b u i l d e r s and cobuilders: N a - t r i phosphate (STP), hydroxyethane-1,l-diphosphonic a c i d (HEDP), ethylene diamine t e t r a k i s (methylene phosphonic acid)(EDMP), Na-salt o f maleic a c i d / a c r y l i c acid copolymer (AC) w i t h a mean molecular weight of 70.000 g/mol ("Sokalan CP 5" from BASF AG,
Ludwigshafen). Optical brightener: b i s - t r i a c i n y l deriva-
t i v e of 4,4'-diaminostilbene-2,2'-disulfonic a c i d (BM, "Blankophor MBBH" from Bayer AG, Leverkusen).
3 * RESU_L_TS-!N_Q -QI_ScUs_s_!ON_ Ll.,Dete_r-gen_cy p e r f ormance of bu i1ders ...-~e_ne_rrl-a~e_c~~L Builders i n laundry detergents are o f great importance f o r detergency performance, which includes s o i l removal i n a s i n g l e washing cycle, as w e l l as long-term e f f e c t s such as i n h i b i t i o n o f t e x t i l e i n c r u s t a t i o n s . Table 1 gives the c h a r a c t e r i s t i c functions o f water-soluble and water-dispersible b u i l d e r s
675 i n t h e washing process. Sodium triphosphate i s the most important waters o l u b l e b u i l d e r , b u t t y p i c a l c o b u i l d e r s l i k e phosphates, NTA, EDTA and other complexing agents, as w e l l as polycarboxylates, belong t o t h i s type. Z e o l i t e A i s a r e p r e s e n t a t i v e example o f a w a t e r - d i s p e r s i b l e b u i l d e r .
Water-soluble b u i l d e r s (ex.: sodium triphosphate) ~~
Binding o f m u l t i v a l e n t c a t i o n s (complexation, counter-ion condensation)
Water-dispersible b u i l d e r s (ex.: z e o l i t e A ) _____
~~
~~
I o n exchange o f m u l t i v a l e n t c a t i o n s , e s p e c i a l l y Ca2+
Inhibition of precipitate formation (concentration and temperature dependent)
Reduction i n p r e c i p i t a t e formation (mainly k i n e t i c a l l y controlled)
Dispersion o f s o i l and p r e c i p i t a t e s by s p e c i f i c adsorption
C r y s t a l l i z a t i o n surface f o r s p a r i n g l y s o l u b l e compounds
Heterocoagulation w i t h s o i l particles One o f the main f u n c t i o n s o f b u i l d e r s i s t h e i r a b i l i t y t o s o f t e n t h e washing l i q u o r , i . e . they s t r o n g l y lower t h e calcium and magnesium concentrat i o n . Table 1 shows how t h i s laads t o an increase i n s o i l removal. A proper detergent b u i l d e r softens by processes such as i o n exchange o r complexation, and n o t by p r e c i p i t a t i o n , l i k e calcium carbonate i n soda-based laundry d e t e r gents. I n a d d i t i o n , s p a r i n g l y s o l u b l e compounds should not form d e s p i t e t h e s o f t e n i n g by the b u i l d e r . P r e c i p i t a t e s may o r i g i n a t e from t h e r e a c t i o n o f calcium o r magnesium ions w i t h detergent i n g r e d i e n t s such as a n i o n i c surfactants,
soda, phosphate and water glass. CaC03 can a l s o be formed from
tap water t h a t contains hydrogen carbonate, due t o an a l k a l i n e r e a c t i o n w i t h the detergent i n t h e washing l i q u o r (Table 2 ) . Most o f these s p a r i n g l y s o l u b l e p r e c i p i t a t e s are deposited on f i b r e s and the h e a t i n g elements o f washing machines, which shortens t h e l i f e t i m e o f machines and c l o t h e s . E a r l i e r detergents c o n t a i n i n g a h i g h concentration o f
676 phosphate solved t h i s problem by p r o v i d i n g a s u f f i c i e n t l y h i g h concentration of triphosphate anions i n the washing l i q u o r . These q u i c k l y formed strong, water-soluble calcium and magnesium complexes. When z e o l i t e A i s incorporated i n t o detergents there are two main differences: the e l i m i n a t i o n o f d i v a l e n t cations from the washing l i q u o r by insoluble i o n exchangers i s somewhat slower than by water-soluble b u i l d e r s , and due t o the pore s i z e o f z e o l i t e A, Mg2'
is
exchanged more slowly than Ca2+, e s p e c i a l l y a t low temperatures ( f i g . 1) ( r e f . 1 ) . I f both types of ions are present, Ca2* w i l l be exchanged preferent i a l l y (ref. 12).
TABLE 2
Main sources o f p r e c i p i t a t e s leading t o deposits on f i b r e s and machine elements tap water
HCO
-
Ca2', Mg2' detergent
soda anionic surfactants,soap water glass phosphates (low dosage)
1',..' /-•
1.0 0.8 0.8
,ARoAIM**
0.6
01.
0.2 0.2p
I 10 20 30 50 60 70 LO
t Iminl Fig. 1. Ion exchan e by z e o l i t e A as a function o f time. 5.36*10-3 mol/l Caq+ or Mg2', 1 g / l z e o l i t e A, 25 OC Q t = amount exchanged a t time t, Q70 = amount exchanged a f t e r I 0 minutes.
677 I n order t o compete w i t h triphosphate, z e o l i t e A i s t h e r e f o r e v e r y o f t e n used i n combination w i t h small amounts o f water-soluble c o b u i l d e r s ( r e f s . 9, 13, 1 4 ) . Reinforcenent o f s o i l removal as w e l l as i n h i b i t i o n o f t e x t i l e
i n c r u s t a t i o n i n these systems w i l l be discussed.
3.2.
Zeal it e ...A.. -.cqbu i,ldr.__s~stemslS~L!.-r:~sx.al
The h i g h s e l e c t i v i t y o f z e o l i t e A f o r calcium ions can be considered a p o s i t i v e f a c t o r i n s o i l removal, F i r s t , the concentration o f magnesium i n tap water i s g e n e r a l l y much lower than t h a t o f calcium. Secondly, a c e r t a i n r e s i d u a l concentration o f magnesium i o n s can increase t h e detergency o f a n i o n i c s u r f a c t a n t s by counter-ion e f f e c t s , as f o r instance i n the s o l u b i l i z a t i o n o f o i l and dyes ( r e f . 7 ) . Table 1 shows t h a t s o i l d i s p e r s i o n can be an important f u n c t i o n o f cob u i l d e r s , i n combination w i t h z e o l i t e A,
i n s o i l removal. I n a d d i t i o n , as
water-soluble compounds, they are able t o b i n d calcium ions f a s t e r than z e o l i t e A.
O / t ImJ/sl
61
Fig. 2. Heat evolved o r absorbed d u r i n g t i t r a t i o n o f z e o l i t e A (1.5 g / l ) and z e o l i t e A/AC mixtures w i t h a CaC12 s o l u t i o n . T i t r a t i o n range: 10 min, f i n a l Ca2+ concentration 3 . 1 3 ~ 1 0 - m ~ o l / l . Curves were corrected f o r heat o f d i l u t i o n . ( 1 ) no AC, ( 2 ) 60 mg/l AC, ( 3 ) 120 mg/l AC, ( 4 ) 180 mg/l AC, ( 5 ) 240 mg/l AC F i g . 2 shows t h a t i n z e o l i t e A / AC mixtures, f o r instance, Ca2+ i s f i r s t bound t o the polycarboxylate. (Nowadays AC i s an important c o b u i l d e r f o r z e o l i t e A i n European detergents.)
I n t h i s c a l o r i m e t r i c measurement, the i o n
exchange by z e o l i t e A was observed v i a t h e corresponding heat e f f e c t d u r i n g
678 t i t r a t i o n o f a z e o l i t e A d i s p e r s i o n w i t h a CaC12 s o l u t i o n ( c u r v e 1 ) . When d i f f e r e n t concentrations o f AC were added t o the z e o l i t e A s l u r r y , t h e i o n exchange was delayed by t h e time necessary t o p r o v i d e the maximum amount o f Ca2+ ions t h a t cart be bound t o the c o b u i l d e r (curve 2 - 5 ) .
The o v e r a l l heat
e f f e c t i n the mixtures, however, d i d n o t depend on t h e amount o f AC added ( t a b l e 3 ) . This means, t h a t calcium ions f i r s t become bound t o AC and a f t e r wards most o f t h e calcium ions are exchanged by z e o l i t e A.
I n t h i s way the
p r e c i p i t a t i o n o f AC i n the presence o f excess calcium ions i s suppressed and the s o l u b l e Na o r mixed Na/Ca s a l t s o f the p o l y c a r b o x y l a t e s a r e l e f t . This can be shown by c o n c e n t r a t i o n measurements o f w a t e r - s o l u b l e AC i n the presence and absence o f z e o l i t e A i n hard water ( t a b l e 4 ) . S i m i l a r r e s u l t s have been obt a i n e d i n soda / z e o l i t e A / AC systems ( s e c t i o n 3.3.2..
zeolite A (g/l)
AC (mg/l)
1.5 1.5 1.5 1.5 1.5
60 120 180 240 0 60 120 180 240
fig.
7).
dH/J 0.15 0.20 0.31 0.45 1.87 1.86 1.81 1.83 1.a4
TABLE 4 E f f e c t o f z e o l i t e A on t h e r e s i d u a l c o n c e n t r a t i o n o f AC. i n i t i a l c o n c e n t r a t i o n AC (ma/l)
300 200 100 40
r e s i d u a l c o n c e n t r a t i o n AC (ma/ll AC o n l y +29/1 z e o l i t e A (precipitation) 0 280 0 180 0 100 0 40
water hardness: 3.75.10-3mol/l
Ca2+, 2 5 V
The s o l v a t e d AC s a l t s a r e thus now a v a i l a b l e f o r a d s o r p t i o n on s o l i d surfaces such as pigments o r f i b r e s , and can t h u s support t h e removal o f s o i l i n z e o l i t e A-containing systems ( f i g . 3 ) . The same holds f o r other s o l u b l e c o b u i l d e r s l i k e the phosphonates EDMP and
679
1
2
3
6
5 6 builder 19/11
Fig. 3 . Detergency performance o f b u i l d e r systems. S o i l i n g : clay/sebum on cotton, washing conditions: 95'C. 30min., 285ppm water hardness, launderometer. ( 1 ) z e o l i t e A, ( 2 ) z e o l i t e A/AC 9:1, ( 3 ) z e o l i t e A/EDMP 9:1, ( 4 ) z e o l i t e A/HEOP 9:1, ( 5 ) STP
reflectance 1%1
75-
73-
71-
nlln 0
l0
15
Fig. 4. Detergency performance o f laundry detergents w i t h 4% AC and d i f f e r e n t z e o l i t e A content. S o i l i n g : clay/sebum on cotton, washing conditions: 90eC, 450 ppm water hardness (Ca:Mg=5:1), 8g/l detergent, European drum-type washing mach ine
HEOP, and f o r small amounts o f STP i n combination w i t h z e o l i t e A ( r e f . 1 ) .
This may happen by dispersion o f s o i l by s p e c i f i c adsorption on the s o i l p a r t i c l e or by subsequent desorption o f the soluble Ca/Na s a l t or complex, and consequent loosening o f calcium bridges between s o i l and f i b r e .
Fig. 3 shows that by p a r t i a l l y s u b s t i t u t i n g z e o l i t e A w i t h a cobuilder, almost the same l e v e l o f detergency as w i t h sodium triphosphate can be reached. This holds not only f o r model systems but a l s o f o r a complete detergent formulation. Fig. 4 shows how the detergency of z e o l i t e A i s r e i n f o r c e d by AC i n a heavy-duty laundry detergent. I t a l s o shows t h a t the improvement i n detergency cannot be a t t a i n e d by the cobirilder alone. The more complex calcium binding processes and the mode of a c t i o n o f AC i n complete washing l i q u o r s
w i l l be discussed below.
-
.__.______..___I-___.__.__..__.._.__ 3.3. Z e o l i t e A cobuilder
systems
/
Textile incrustation
As the a c t i o n mechanism o f phosphonates has already been discussed ( r e f s . 9, 13), t h i s study i s confined t o polycarboxylates, namely copolymers o f maleic and a c r y l i c acid. A t present these polymers are widely used i n zeol i t e A-based detergents because of t h e i r good ecotoxicological p r o p e r t i e s ( r e f s . 15-17). The mechanisms o f i n h i b i t i o n o f t e x t i l e i n c r u s t a t i o n s are s t i l l under discussion by manufacturers o f detergents and polymers ( r e f . 16, 18-22). This i s a r e s u l t o f t h e i r manifold possible modes o f action. Small amounts o f polycarboxylates can r e t a r d the p r e c i p i t a t i o n o f sparingly soluble calcium s a l t s such as CaC03 ("threshold e f f e c t " ) . As anionic p o l y e l e c t r o l y t e s they are able t o bind cations (counter-ion-condensation) where m u l t i v a l e n t cations are s t r o n g l y preferred ( r e f . 23). Whereas the pure calcium s a l t o f the polymer i s nearly i n s o l u b l e i n water, mixed Ca/Na-salts are soluble, i . e .
only over-
stoichiometric amounts o f calcium ions can cause p r e c i p i t a t i o n . Polycarboxylates are also able of dispersing many s o l i d s i n aqueous s o l u t i o n . Both dispersion and threshold e f f e c t s r e s u l t from the adsorption o f the polymer on p a r t i c l e surfaces.
The predominant mode o f a c t i o n i n a detergent thus depends on many f a c t o r s . To understand the mechanisms, i t i s therefore necessary t o take the various laundering conditions,
such as water hardness, b u i l d e r composition o f the
detergent, dosage and washing temperature,
i n t o account. Table 5 shows the
t y p i c a l b u i l d e r composition o f a German z e o l i t e A-based laundry detergent and the corresponding conditions f o r a t e x t i l e i n c r u s t a t i o n t e s t . The study concentrates on t h i s p o i n t o f emphasis.
681 TABLE 5. B u i l d e r composition of a t y p i c a l German z e o l i t e A-based heavy-duty laundry detergent, and concentration ranges i n the washing l i q u o r
zeolite A polycarboxylates water glass soda (HCO3- from tap water)""
composition 20 25 % 2 - 4 % 1.5 - 4 % 5 -10%
-
concentration ranae* 1.2 2.5 g/1 0.12 0.40 g/1 0.3
-
1.0
0.374 g / l
g/1
(as soda)
typical t e s t conditions: dosage* 6 - 10 g / l , medium HC03- concentration i n tap water: 215 mg/l**, 25 wash cycles, 90°C, water hardness: 2.28 - 5.38 m o l / l Me2+, Ca2+: Mg2+ = 5 : l 3.3.1.
Threshold e f f e c t
The s t a b i l i z a t i o n o f s p a r i n g l y s o l u b l e s a l t s such as CaC03 i n a c o l l o i d a l s t a t e i s one o f t h e p o s s i b l e mechanisms discussed f o r polycarboxylates i n detergents. The advantage o f such a mode o f a c t i o n i s t h e f a c t t h a t , i n cont r a s t t o i o n exchange o r complexation, t h e concentration o f t h e c o b u i l d e r can be much lower than t h e calcium concentration i n t h e washing l i q u o r . This means t h a t , i n p r i n c i p l e , small amounts o f t h r e s h o l d - a c t i v e compounds c o u l d be used as cobuilders even i n soda-based laundry detergents.
F i g . 5. P r e c i p i t a t i o n i n h i b i t i o n ( " t h r e s h o l d e f f e c t " ) o f CaC03 by AC as a f u n c t i o n o f temperature and soda concentration. 3.04*10-3mol/l Ca2+, ( 1 ) 105 mg/l AC, ( 2 ) 210 mg/l AC
682 The e f f e c t , however, i s s t r o n g l y dependent on the experimental ( o r washing) conditions,
i.e.
temperature, soda and cobuilder concentration. Fig. 5 shows
washing t e s t r e s u l t s i l l u s t r a t i n g the range o f effectiveness o f AC i n a carbonate-containing system f o r t y p i c a l c e n t r a l European conditions o f water hardness (3.04
*
mol/l Ca2+). The r e s u l t s are based on t u r b i d i t y measure-
ments. The appearance o f CaC03-particle larger than aproximately 0.2
pm
within
30 minutes was taken as an i n d i c a t o r o f whether or n o t the polycarboxylate ex-
h i b i t e d a threshold e f f e c t under the various conditions. The soda concentrat i o n s i n the t e s t include the hydrogen carbonate content o f the tap water as well as the soda content o f the detergent ( t a b l e 5 ) . The r e s u l t s show that, f o r t y p i c a l German, phosphate-free, heavy-duty detergents, AC i s no longer threshold-active a t temperatures above 40 OC. This holds even more so f o r higher carbonate concentrations,
0.8-
4.e. purely soda-based detergents.
2
0.60.L
-
0.2-
0-
Fig. 6. Temperature dependence of the influence of AC on the formation of prec i p i t a t e s > 0 , q m i n soda and soda/zeolite A systems. Z e o l i t e A 1.5g/1, soda 0.6/1, AC 0.3g/1, 3.75*10-3 mol/l Ca2+ ( 1 ) soda, ( 2 ) soda + AC, ( 3 ) soda + z e o l i t e A, ( 4 ) soda t z e o l i t e A t AC For z e o l i t e A and soda-containing products the p a r t i c i p a t i o n o f z e o l i t e A i n the e l i m i n a t i o n of calcium ions during the washing process has t o be taken i n t o account as shown i n f i g . 6. For t y p i c a l t e s t concentrations,
the amount
of coarsely dispersed CaC03 i s reduced i n the presence of z e o l i t e A over the
683 whole washing temperature range. The e f f e c t o f AC on the t o t a l amount o f prec i p i t a t i o n i s s t r o n g l y dependent on whether or not the system contains zeol i t e A. When only soda i s used, p r e c i p i t a t i o n i s i n h i b i t e d o n l y below 40
OC
as
would be expected from the range o f threshold a c t i v i t y shown i n f i g . 5. With increasing temperature, p r e c i p i t a t i o n increases strongly and exceeds the maximum possible formation o f CaCO3. I n t h i s case, on a d d i t i o n o f CaC03, AC i s p r e c i p i t a t e d as i t s calcium s a l t , as can be seen from the respective measurements o f the residual concentrations o f water-soluble AC ( f i g . 7 ) . I n cont r a s t , the amount o f p r e c i p i t a t i o n which occurs i n the presence o f z e o l i t e A
and AC
i s n e g l i g i b l y low, and the residual concentration o f water-soluble AC
i s as high as i n z e o l i t e A / AC systems without soda ( f i g s . 6 / 7 ) . These r e s u l t s can be explained by the binding o f calcium by z e o l i t e A and AC i n i t s water-soluble form. The polycarboxylate i s able t o function l i k e t h i s because the calcium ion concentration of the water i s lowered by z e o l i t e A. Thus, Ca2+ i s no longer i n excess of AC and the formation o f the insoluble calcium s a l t o f the polycarboxylate i s no longer possible.
AC I
300
200
100
!
0
Fig. I. Residua Z e o l i t e A 1,5g/ o f AC: 300 mg/l
Ii 1 2 :
concentration of AC i n soda and soda/zeolite A systems. 3.15 * mol/l Ca2+, i n i t i a l concentration ( 1 ) soda, ( 2 ) z e o l i t e A t soda, ( 3 ) z e o l i t e A.
, soda 0.6g/1,
I t has been assumed t h a t c o l l o i d a l CaC03 i s forned even i n the case o f
understoichiometric amounts o f Ca2+ r e l a t i v e t o the number o f carboxylate groups i n the polymer, due t o the low s o l u b i l i t y product o f CaC03. The mode o f action o f the polymers i s then c a l l e d "CaC03 dispersing capacity" ( r e f . 9 ) . Corresponding c a l c u l a t i o n s are based on the assumption o f a "complexation constant",
defined j u s t as a normal complexing agent. This does not apply f o r
polycarboxylates, not o n l y because o f the inhomogeneity i n molecular weight o f the products used, but also because o f the d i f f e r e n t mechanism o f calcium binding (Counter-ion condensation, r e f . 23).
I f , however, the polymer has t o compete w i t h carbonate f o r the calcium ions, i t s effectiveness i n the washing process should depend on i t s a b i l i t y t o bind calcium, i.e.
on degree o f counter-ion condensation o f the p o l y e l e c t r o -
l y t e s . This o n l y depends on the distance between the anionic ( i . e .
carboxy-
l a t e ) groups ( r e f . 2 4 ) , i f the polymer i s s u f f i c i e n t l y long. The shorter the distance, the higher the degree o f counter-ion condensation. I f the molecule i s too short, i t loses p a r t o f i t s p o l y e l e c t r o l y t i c character and behaves more l i k e a low-molecular-weight e l e c t r o l y t e , i . e .
i t dissociates more strongly,
Corresponding r e s u l t s have been obtained f o r z e o l i t e A-based laundry detergents ( r e f . 20). T e x t i l e i n c r u s t a t i o n decreases w i t h increasing molecular weight (most s t r o n g l y i n the low-molecular-weight
range) and w i t h increasing
share o f maleic acid i n acrylic/maleic a c i d copolymers. I t i s worth n o t i n g t h a t the same sequence i s obtained f o r the so-called "CaC03 dispersing capacity"
.
Two more aspects have t o be taken i n t o consideration
,
i . e . which kinds o f
deposits on f i b r e s can be reduced, and t o what degree can t o t a l t e x t i l e i n c r u s t a t i o n be reduced. An important p r e c i p i t a t e i s CaC03, which leads t o a high percentage o f CaO i n the ashes o f laundered t e x t i l e s . This share, i n turn, depends on the amount o f CaC03 p r e c i p i t a t e formed under the s p e c i f i c washing conditions. Fig. 8 shows the amount o f coarse CaC03 i n z e o l i t e A-containing systems a t 90
OC
f o r two d i f f e r e n t calcium concentrations and two d i f f e r e n t
dosages. Both can be considered t o be t y p i c a l t e s t conditions f o r heavy-duty laundry detergents ( t a b l e 5 ) . A t lower calcium concentrations and lower dosages ( f i g . 8, curve l b ) , the amount o f CaC03 i s low. I n view o f the carbonate concentration r e s u l t i n g from tap water only, i t can be concluded t h a t no CaC03 i s formed i n t h i s case. Only when the detergent i t s e l f contains soda i s CaC03 formed. This has been proved by washing t e s t s . Using a heavy-duty laundry detergent w i t h 25 % z e o l i t e A, no CaO ash was formed f o r 6 g / l detergent dosage (corresponds approximately t o curve 1 b ) . When 5 % soda was put i n t o the detergent, CaO ash was observed under i d e n t i c a l t e s t conditions. This could be
685 reduced by 88 % i n turn, when the product was given an AC content o f 3%. However, the ( r e l a t i v e l y low) amount o f MgO ash o n l y dropped by about 4 %.
i
2
Na,co, 19/11 Fig. 8. P r e c i p i t a t i o n of CaC03 (>0.2/~m) i n the presence o f z e o l i t e A as a function of soda concentration. Conditions: 90°C, 2Omin. ( l b ) 1.75g/l z e o l i t e A, 3.04~10-3mol/l Ca2+; (2b) 2.40g/l z e o l i t e A, 5.36.10-3mol/l Ca2+; (la,2a) no z e o l i t e A, calculated maximum amounts o f CaC03 The higher the water hardness, the greater the amount o f CaC03 formed, even i f the dosage o f the detergent i s increased ( f i g . 8, curve 2 b ) . I t has t o be taken i n t o account t h a t higher dosage means an increase i n both z e o l i t e A
and
soda concentration. Fig. 9 shows the reduction of t o t a l ash by 4 91 AC under conditions closer t o those given i n curve 2b. AC i s very e f f e c t i v e against CaC03 i n the presence of 25 9; z e o l i t e A, considering t h a t a large p a r t o f the non-reduced ash does not contain CaO. The measurements ( r e f . 13) a l s o show t h a t i f the amount o f p r e c i p i t a t e i s increased by lowering the z e o l i t e A content o f the detergent, the polycarboxylate loses i t s e f f i c i e n c y , as has been shown previously ( f i g . 6). The p o s i t i v e e f f e c t o f AC as a cobuilder i s o n l y found i n combinat i o n w i t h z e o l i t e A, not w i t h soda only.
686
n 25 L
-zeolite A [%I AC 1%1
F i g . 9 . E f f e c t o f z e o l i t e A and AC on i n h i b i t i o n o f d e p o s i t s on c o t t o n i n a heavy d u t y laundry detergent. Conditions: 25 wash c y c l e s , 90°C, 450ppm water hardness, Ca : Mg = 5 : 1 , 8g/1 detergent w i t h 5% soda content, European drumtype washing machine. 4. OPTICAL BRIGHTENERS I N DETERGENT POWDERS
This chapter i s concerned w i t h t h e i n f l u e n c e o f z e o l i t e A on t h e w h i t e n i n g o f detergent powders by a t y p i c a l c o t t o n whitener (EM). T h i s was s t u d i e d by measuring the fluorescence spectra and the degree o f whiteness o f powders cont a i n i n g d i f f e r e n t b u i l d e r s as w e l l as combinations o f b u i l d e r s and s u r f a c t a n t s . F i g 10 shows t h e fluorescence emission spectrum o f t h e pure, o p t i c a l b r i g h t e ner i n comparison t o those o f powders c o n t a i n i n g z e o l i t e A, STP and Na2S04. The water-soluble,
i o n i c b u i l d e r s l e a d t o a bathochromic s h i f t i n t h e spectrum
whereas z e o l i t e A leads t o a hypsochromic s h i f t . Though t h e i n t e n s i t y o f t h e emission i s h i g h e r i n the case o f STP and Na2S04, t h e w h i t e n i n g e f f e c t o f BM i s stronger i n t h e z e o l i t e A-based powder. Table 6 shows t h a t t h e degree o f whiteness c o r r e l a t e s w i t h t h e p o s i t i o n o f t h e maximum i n t h e emission spectrum. A s h i f t t o higher wavelengths causes t h e powder t o a c q u i r e a y e l l o w i s h t i n g e , and t h e degree o f whiteness decreases.
687
XInml F i g . 10. I n f l u e n c e o f b u i l d e r s on the fluorescence spectra o f ( 1 ) O M , ( 7 ) BM + zeci:ite A, ( 3 ) BM t Na2S04, ( 4 ) BM + STP
BM i n powders.
Table 6 . I n f l u e n c e of b u i l d e r s and t a l l o w f a t t y a l c o h o l s u l f a t e (TA) on the s h i f t o f the maximum i n the fluorescence spectrum o f EM, and r e l a t i v e degree o f w h i t e ness o f powders powder i n g r e d i e n t
zeol it e A z e o l i t e A t TA STP
STP t TA Na2S04 Nags04 + TA
rel.degree o f whiteness
100 % 91 %
a7 % 73 % 91 % 80 %
I I
A h a x ( nm)
-
_. t o
-
-
5 5 5 10
The p o s i t i v e i n f l u e n c e o f z e o l i t e A on t h e photo-physical p r o p e r t i e s o f t h e o p t i c a l b r i g h t e n e r can be supported by n o n i o n i c s u r f a c t a n t s , whereas a n i o n i c s u r f a c t a n t s may lead t o negative e f f e c t s ( f i g . 1 1 ) .
As can be seen from t a b l e 6
the r e s p e c t i v e bathochromic s h i f t and the decrease i n whiteness depend on the type o f a n i o n i c s u r f a c t a n t used and are more pronounced i n t h e presence o f STP and Na2S04. The r e s u l t s can be discussed on the b a s i s o f an aggregation model which i s a l s o used f o r the e x p l a n a t i o n o f whitening e f f e c t s on f i b r e s ( r e f s . 25, 2 6 ) . I n s o l u t i o n , a n i o n i c f l u o r e s c e n t b r i g h t e n e r s can be present i n t h r e e d i f f e r e n t forms: d i s s o c i a t e d , as i o n p a i r s and as aggregates.
688 The e q u i l i b r i u m i s influenced by the other solvated substances i n the solut i o n . Whereas i n water w i t h low e l e c t r o l y t e concentration s i n g l e molecules o f the o p t i c a l brightener are preferred ( d i s s o c i a t e d or ion p a i r s ) , high e l e c t r o l y t e concentrations may cause aggregation. I t has been assumed that large aggregates (micelles) may be formed between a _single whitener molecule and nonionic surfactants
,
whereas an anionic whitener/anionic surfactant aggre-
gate i s thought less l i k e l y t o occur ( r e f s . 2 6 , 2 7 ) .
Loo
Fig. 11. Influence o f surfactants on the fluorescence spectra o f BM i n zeol t e A-containing powders. (1) alkylbenzene sulfonate; ( 2 ) t a l l o w f a t t y alcohol, ethoxylated (5EO); ( 3 l i n o l e y l / l i n o l e n y l alcohol, ethoxylated ( 1 0 E O ) . (enlargement X 1 0 ) . Whitener aggregates or microcrystals should have new and d i f f e r e n t o p t i c a l properties. Fig. 12 shows t h a t the emission spectra o f BM i n s o l u t i o n s w i t h high STP or Na2S04 concentration show bathochromic s h i f t s ,
i.e.
the f l u o r -
esence emission o f the aggregates takes place a t longer wavelengths than t h a t o f the s i n g l e molecules. I t may be assumed t h a t the formation o f aggregates i n the detergent s l u r r y i s responsible f o r the d e t e r i o r a t i o n i n the appearance of the spray-dried powder. Z e o l i t e A as a water-insoluble b u i l d e r does n o t lead t o aggregate formation and thus improves the o p t i c a l p r o p e r t i e s o f the detergent.
689
I
F i g . 1 2 . I n f l u e n c e o f b u i l d e r s on t h e fluorescence s p e c t r a o f BM i n aqueous s o l u t i o n , pH 10. ( 1 ) water, ( 2 ) z e o l i t e A, ( 3 ) STP, ( 4 ) NapSO4.
s.:-co?Jcc!SreM Z e o l i t e A/polycarboxylate b u i l d e r combinations a r e v e r y e f f i c i e n t i n reduci n g calcium-containing t e x t i l e i n c r u s t a t i o n s on f i b r e s . Under German heavyd u t y laundry c o n d i t i o n s , t h i s e f f e c t i s caused by t h e b i n d i n g o f c a l c i u m ions by t h e polymer i n a water-soluble form. This i s o n l y p o s s i b l e i n the presence o f z e o l i t e A; otherwise, the water i n s o l u b l e calcium s a l t o f t h e polymer w i l l p r e c i p i t a t e o u t . According t o t h i s mechanism, polymers w i t h h i g h c a r b o x y l a t e content and r e l a t i v e l y h i g h molecular weight should be used i n combination w i t h z e o l i t e A. I n detergents based on soda o n l y t h i s mode o f a c t i o n i s n o t e f f e c t i v e . The same holds f o r t h e t h r e s h o l d e f f e c t a t h i g h washing temperatures. The o p t i c a l appearance o f detergent powders w i t h t y p i c a l c o t t o n whiteners i s s t r o n g l y i n f l u e n c e d by b u i l d e r s and s u r f a c t a n t s . I t i s shown t h a t t h e degree o f whiteness c o r r e l a t e s w i t h t h e p o s i t i o n s o f t h e maximum i n t h e f l u o r e s cence emission spectrum. Water-soluble i o n i c species l i k e STP, Na2S04 and a n i o n i c s u r f a c t a n t s lead t o a bathochromic s h i f t i n t h e spectrum o f the o p t i c a l b r i g h t e n e r , causing t h e powder t o a c q u i r e a y e l l o w i s h t i n g e . Z e o l i t e A and nonionic s u r f a c t a n t s cause a hypsochromic s h i f t leading t o a b e t t e r powder appearance. This behavior i s explained on t h e b a s i s of an aggregationdeaggregation model o f the o p t i c a l b r i g h t e n e r .
690
Summarizing the o v e r a l l r e s u l t s obtained w i t h detergents containing z e o l i t e A,
i t can be postulated t h a t under normal washing conditions such detergents
are equivalent t o conventional products containing STP. Under c e r t a i n circumstances z e o l i t e A-based products are even safer than conventional ones, due t o new, special, modes o f action.
1) 2) 3) 4)
M.J. Schwuger, H.G. Smolka, H.G. Smolka, M.J. Schwuger, M.J. Schwuger, H.G. Smolka, M.J. Schwuger, H.G. Smolka,
.
C o l l o i d Polymer Sc 254 (1976) 1062-1069 C o l l o i d Polymer Sc 256 (1978) 270-277 C o l l o i d Polymer Sc , 26 (1978) 1014-1020 C.P. Kurzendorfer, Tenside Detergents 13
.
(1976) 305-312 5 ) C.P. Kurzendorfer, M.J. Schwuger, H.G. Smolka, Tenside Detergents 16 (1979) 123-129 6 ) H.G. Smolka, M.J. Schwuger, Tenside Detergents 4 (1977) 222-228 7 ) M.J. Schwuger, H.G. Smolka, Tenside Detergents 16 (1979) 233-239 Schwuger, Ber.Bunsenges.Phys.Chem. 83 (1979) 8 ) H. NuRlein, K. Schumann, M.J. 1229-1238 9 ) C.P. Kurzendorfer, M.Liphard, W.von Rybinski, M.J. Schwuger, C o l l o i d Polymer Sci. 265 (1987) 542-547 10) M.J. Schwuger, E.J.Smulders, i n W.G.Cutler, E.Kissa ( E d i t o r s ) , Detergency, Theory and Technology, Marcel Dekker, New York 1987 pp.371-439 11) C.P.Kurzendorfer, M.Schlag, i n : Dechema Monographien 102, VCH Verlags-. gesellschaft 1986,pp.561-574 12) K.R.Franklin, R.P.Townsend, JCS Faraday I 81 (1985) 1071-1086 13) H.Andree, P.Krings, H.Upadek, H.Verbeek, i n : A.R.Baldwin ( E d i t o r ) Proceedings of the Second World Conference on Detergents, Montreux 1986, Amer.Oil.Chem.Soc. 1987 p p. 148-152 14) P.Berth, M.Berg, K.Hachmann, Tenside Detergents 20 (1983) 276-282 15) H.G.Opgenorth, Tenside Surfactants Detergents 24 (1987) 366-369 16) A.Hettche, W.Trieselt, P.Disse1, Tenside Detergents 23 (1986) 12-19 17) G.Jacobi, Angew.Makromol.Chem.l23/124 (1984), 119-145 18) P.Zini, SBFW 113 (1987) 45-48 19) F-Richter, E.W.Winkler, Tenside Surfactants Detergents 24 (1987) 213-216 20) J.Perner, H.W.Neumann, Tenside Surfactants Detergents 24 (1987) 334-340 21) J.A.McDonel1, A.Liu, JAOCS 64 (1987) 769-775 22) M.J.Nagarajan, JAOCS 62 (1985) 949-955 23) F.Oosawa, Polyelectrolytes, Marcel Dekker, New York 1971 24) G.S.Manning, Acc. Chem. Res. 12 (1979) 443-449 25) J.Wegmann, Melliand T e x t i l b e r . 48 (1967) 59-69 und 183-190 26) P.S. Stensby, i n : W.G.Cutler, R.C.Davis, Detergency Theory and Test Methods, p a r t , 111 Marcel Dekker, New York 1981, pp.730-806 27) J.R. Aspland, S p e c i a l i t i e s , November (1966) 3-12
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ZEOLITE A
-
A BUILDER FOR LIQUID DETERGENTS ?
W. LEONHARDT and B.-M.
SAX
Degussa AG, ZN Wolfgang, AC-AT2, R o d e n b a c h e r Chaussee 4 , D-6450 Hanau/Main, F . R . G .
ABSTRACT The stability behaviour of liquid detergents containing zeolite A as a builder is described. The sedimentation behaviour of this insoluble builder is examined using various parameters, such as variation of the particle diameter of zeolite A in the 8.6 um - 1 pm range, and also changes in the viscosity of the external phase. The storage stability of the samples was determined at room temperature and at 4O0 C, the viscosity by calculating the thixotropic index and measuring the flow times. INTRODUCTION Differences can be observed in the development of liquid detergents in Europe and the U.S.A. caused by a variety of factors such as legal measures and washing practices. Whilst there has been a noticeable upwards trend in the use of liquid detergents in the U.S.A. since the middle of the 1 9 7 0 ' ~a~significant change in growth was only evident in Europe from the beginning of the 1980's (refs. 1-2). In principle, it is possible to differentiate between two types of liquid system: the most widespread liquid systems are the so-called "unbuilt liquids". These contain no builders in the normal sense of the word, but soften the wash liquor using soap. Lime soap, which is difficult to dissolve, is formed, trapped in micelles with the aid of surfactants and then dispersed. This prevents the lime soap from penetrating or being deposited on the fibres of the fabric . The proportion of surfactants amounts to between 40 and 505, including 10-20Z of soap (ref. 3 ) . In comparison with commercial powder detergents, liquid detergents contain 2 to 3 times the amount of surfactants, and hence organic components. Initial calculations referring to the degradability led to COD and TOC values three times higher than with powder detergents, which demonstrates that the use of inorganic builders is advantageous from an ecological aspect (ref. 4 ) . These kinds of system make up the second category of liquid detergents and are known as "built liquids". They contain coaplexing agents such as phosphates and citrates, which are common in powder detergents (refs. 5-71. However,
692
liquid detergents of this nature are not widespread in Europe and are mainly used in the U.S.A. The application of zeolite in liquid systems could also be of interest to prevent the eutrophication of lakes and rivers caused by phosphates. ZEOLITE A IN LIQUID SYSTEMS Zeolite A differs from all other builders particularly as a result of its insolubility in alkaline media. When zeolite A is incorporated into a liquid, a suspension is formed which is not thermodynamically stable under normal conditions (ref. 8 ) . As a result of the differing specific density of the solid and the liquid, the zeolite settles to the bottom of the liquid as sediment, causing the suspension to break up. The stability of this kind of system is influenced by a diversity of factors. STOKE'S Law (eqn. 1) describes in idealized form the sinking behaviour of a ball in a liquid medium under the influence of gravity and interprets the connections between the individual factors. v = 2 r2 v =
.
AQ
.
g I 90
(1)
sinking velocity of particle
r = radius of particle g = gravitational constant Ap= Q =
difference in density of phases viscosity of liquid phase
The particle size plays an important role in the sedimentation behaviour due to the radius of the particle having a quadratic effect on the sedimentation velocity. The viscosity of the liquid phase also affects the sedimentation velocity, though this must not be so high as to hinder the pouring of the liquid detergent. The liquid detergent must be easily flushed out of the detergent dispenser into the drum of the washing machine. At the same time, however, it must be ensured that the good detergency properties of a liquid detergent of this nature be maintained when compared to previous systems. The final parameter which should be mentioned is the variation of the pH value. In order to prevent the zeolite from being decomposed, the pH value of the liquid system should always be greater than 8. The effect of zeolite A with different particle sizes on the stability and on the viscosity of the liquid system is examined be1.0~. EXPERIMENTAL SECTION Zeollte The zeolite grade NaA (formula: Na10 A1203 . 2Si02 . 4.5H20) was used here. Zeolites with particle sizes of 3.2 pm and 8.6 p m were prepared synthe-
693
tically to conform with the customary specifications (refs. 9-10]. Starting with a 3 . 2 urn zeolite, a 1 . 8 p m zeolite was produced by grinding the initial product in an air-jet mill; the 1 . 1 grade was wet-ground in a colloid mill, dried and then ground in a pin mill. The particle size was measured using a Cilas granulometer 715 E 627. The calcium and magnesium binding power (CaBP/ HgBP) were determined by complexometric titration of the remaining hardness of Ca- and Hg-ions following contact times of 15 minutes. TABLE 1 Calcium and magnesium binding power of the examined zeolites as a function of the particle size. Particle Size [pml
8.6 3.2 1.8 1.1
-
CaBP [ mgCaO/g 1
157 169 170 157
MgBP [ mgCaO/g 1
12 29 32 50
The following surfactants were used: sodium alkyl benzenesulphonate, HOLS; C13-Cl5 oxoalcohol 7 EO, BASF; further ingredient: Defoaaer Wacker S132, WACKER CHEMIE . Viscositv HeThe viscosity was determined using a Brookfield viscometer RVT at 5 and 50 r.p.m. and spindle 4. For this purpose, the suspension was poured into a 100 ml beaker into which the spindle was dipped up to the mark and read after 3 minutes. 3 minutes waiting time was allowed between measurements made on the same system. Test to Detemine the Flow Tipe This is used to determine the length of tine required by 100 ml of liquid to flow out of a flow cup with a nozzle of 6 mm in diameter without pressure (ref. 11).
%bruLT& In order to be able to make a statement on the storage stability, samples of 100 ml were stored in closed glass bottles for 1 week and 1 month respectively at 22 and 40 OC. The filling height was 50 m. The phase behaviour was judged visually.
-
694
The raw materials are weighed into a 250 ml flask with stop cock, with zeolite being added as the last component. The mixture is dispersed using a dissolver (Ultra Turrax, 9 m/sec) under water jet vacuum for 15 minutes. RESULTS AND DISCUSSION Simple systems consisting of surfactant, zeolite A and water were used to test the influence of surfactants and surfactant mixtures on the viscosity behaviour and on the stability. Zeolite contents of 15, 25 and 35% were used together with 20% of surfactant. Tests were carried out on pure surfactants and on mixtures in a ratio of 1 : 2 , 1 : l and 2 : l . Pure anionic surfactants and surfactant mixtures with a high anionic content lead to a high degree of viscosity, which is particularly noticeable with zeolite contents of 25 and 35% (see Fig. 1 ) . Although the high viscosities lead to an improvement in the storage stability, applicational characteristics such as pouring and flushing properties are negatively influenced as a result. The flow times give a good indication of the two properties mentioned last (see Fig. 2). Good flow times together with acceptable viscosities were attained with mixtures of LAS (Lincar Alkylbenzene Sulfonate) and oxoalcohol in a ratio of 2 : l and 1 : l .
12000
%
+ 15
X. 5
rpm
-+-.25
X. 5
rpm
-.c. 25 X . 50 rpm 35 X . 5 rpm
+ 8000
-V-
I
\ -\ \ \
I
\
3 5 X . 50 rpm
Y
40
I
I
6/20
5/15
l0)lO Nonlonic/Anlonlc
1g/5
2d/o
[XI
Fig. 1. Viscosity at 5 and 50 r.p.n. as a function of the zeolite content and surfactant mixture, particle size 3 . 2 vm.
695
*
--
200
\
; \
\ 150
\ \
\
E
F
x
- c . 25 X -m- 35 x
\
n
s
+ 15 \
\ \
h
100
Fig. 2. Flow times of the suspensions examined in Fig. 1.
3 -c 15 X
.
-&
--c
0/20
5/20
10/10
15/5
-
25 X 35 71
20/0
7: Nonlonlc/Anlonic
Fig. 3. Thixotropic index TI of the suspensions shown in Fig. 1; TI = rl ( 5 r.p.a) (50 r.p.n).
696
The term thixotropic index (TI) is used to define the quotient of two viscosities measured at different shear rates (rotational speed ratio 10:1); this value indicates the structural viscosity of the systems. The higher the TI is, the more structured the system and the more favourable the stability behaviour . Taking into consideration the viscosity and flow times, the system containing 13% LAS and 7% oxoalcohol 7 ED was found to be the most effective (see Fig. 3). The system containing 13% LAS and 7% oxoalcohol EO was used to examine the stability and flow behaviour of zeolite A as a function of the particle size. Particularly the zeolite with an average particle size of 1.1 vm demonstrates well-defined thickening effects in its viscosity behaviour (Fig. 4 ) . Systems containing 25\ and especially those with a zeolite content of 35% lead to paste-like suspensions. The other zeolites result in more liquid suspensions, whereby the zeolite with an average particle size of 1.8 vm shows the least thickening effects with increasing zeolite content and is therefore said to demonstrate relatively "neutral' behaviour. The low viscosity is an advantage for the applicational properties, because these suspensions can be poured easily. This is also apparent from the flow times which are virtually constant at around 45 seconds for suspensions containing 25 and 35% zeolite (1.8 p a ) , (see Fig. 5).This independence is reflected in the thixotropic index (Fig. 6). Values from 1.2 to 1.3 are attained with the 1.8 VI zeolite; all of the other zeolites - with the exception of the zeolite with an average particle size of 1.1 m, which resulted in values between 3 and 4 - lie in the range from 1.3 to 2.0.
Fig. 4. Viscosity of the suspensions as a function of the particle size and zeolite content; left: at 5 r.p.m.; right: at 50 r.p.m.
697
--c 1.1 urn .-c-1.8 prn
--+-
1
3.2
pm
-
8.6 urn
15
25 Zeolite
35
[%I
Fig. 5. Flow times of the suspensions examined in Fig. 4
I 15
25
35
Zeolite [ X I
Fig. 6. Thixotropic index TI of the suspensions shown in Fig. 4.
698
Storage tests permit an assessment of the role played both by the viscosity and the particle size (see Tab. 2 ) . The particle size has the greatest influence on the stability. The most favourable values, from a point of view of viscosity ( ~ and 5 TI), are achieved with the zeolite with a particle size of 3 . 2 urn, yet the stability is greatly inferior to that of the 1 . 8 pm zeolite particularly when the values of the suspensions containing 25% zeolite are coapared. As already mentioned above, the 1 . 1 vm zeolite leads to a welldefined thickening effect, which is an advantage for the stability of the system but not acceptable from an applicational aspect. An almost complete phase separation becomes apparent within a few days with the 8 . 6 pm zeolite, resulting in a breaking up of the suspension.
TABLE 2 Storage test carried out on suspensions containing zeolite (13% LAS, 7% C13/C15 oxoalcohol 7 EO, 15-25-35,. zeolite 4 A , 0.1% defoamer, remainder H 2 0 ) . The degree of separation was determined as the amount of clear phase related to the filling height and expressed as a percentage. Particle Size [ pml
Zeolite Content [%I
Separation [%I a RT
4 0 OC
0
0
1.1 1.1 1.1
15 25 b 35 b
1.8 1.8 1.8
15 25 35
3.2 3.2 3.2
15 25 35
50 1 50 I 3 1 6
8.6 8.6 8.6
15 25 35
60 1 30 1 12 1
-
-
-
22 1 10 1 1 1 2
-
55 1 50 1 3 1 8 68 1 40 1 20 1
-
a 1st number I 2nd number = separation following 7 days I following 28 days. If the separation was more than 10% after 7 days, the value for 28 days was not
determined. b Storage test was not carried out here, as these were pastes and not liquid suspensions.
699
CONCLUSION The experiments described above have shown how the particle size of the zeolite being examined influences the stability and flow behaviour of liquid detergents. Particle sizes in the 2 Ma range were found to be favourable; this value is far below the one which is currently considered to be optimum for detergent zeolites - approx. 4 um. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
Chemical Marketing Reporter, 25 (1988) 30,39. W. Budek, Seifen, Ole, Fette, Wachse, 113 (1987) 359-363. DE-OS 3516091 ( 1 9 8 6 ) . L. Huber, Seifen, Ole, Fette, Wachse, 113 (1987) 393-397. EP 0203660 ( 1 9 8 6 ) . EP 0200264 ( 1 9 8 6 ) . DE 2916656 ( 1 9 8 0 ) . Th. Tadros, Advances in Colloid and Interface Science, 12 (1980) 141-261. DE 2517218 ( 1 9 8 6 ) . DE 2660726 ( 1 9 8 6 ) . DIN 53 211 ( 4 / 1 9 7 4 ) .
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent BuiMers 0 1989 Elsevier Science Publishers B.V., Amsterdam- Printed in The Netherlands
DEVELOPMENT AND PERFORMANCE OF ZEOLITE-A-BUILT NON-PHOSPHATE DETERGENTS
H. UPADEK and P. K R I N G S Henkel KGaA, P.O.
Box 1100, H e n k e l s t r a s s e 67, D-4000 D u s s e l d o r f (West-Germany)
ABSTRACT Today's non-phosphate brand powder d e t e r g e n t s c o n t a i n z e o l i t e A as t h e main b u i l d e r component. I n a d d i t i o n , p o l y c a r b o x y l a t e s and sodium c a r b o n a t e s u p p o r t t h e a c t i o n o f z e o l i t e . Reduction o r replacement o f z e o l i t e i n non-phosphate p r o d u c t s r e s u l t s i n unacceptable t e x t i l e i n c r u s t a t i o n s . From dosage e x p e r i m e n t s i t i s concluded t h a t non-phosphate d e t e r g e n t s based on z e o l i t e a r e s a f e r t h a n p h o s p h a t e - c o n t a i n i n g p r o d u c t s , as r e g a r d s t h e i n h i b i t i o n o f d e p o s i t s on t e x t i l e s and on washing machines. INTRODUCTION E s s e n t i a l p r e r e q u i s i t e s f o r a good wash r e s u l t o f a d e t e r g e n t a r e t h e q u a l i t y and q u a n t i t y o f t h e b u i l d e r s . They e x h i b i t n o t o n l y s p e c i f i c e f f e c t s b u t a l s o e x e r t a d e c i s i v e i n f l u e n c e on t h e a c t i o n o f t h e o t h e r d e t e r g e n t components. U n t i l t h e e a r l y s e v e n t i e s , t h e t e r m " b u i l d e r " was synonymous w i t h sodium t r i p h o s p h a t e (STP). I t s c o n t r i b u t i o n t o t h e wash r e s u l t i s l a r g e l y based on t h r e e key f u n c t i o n s :
-
e l i m i n a t i o n o f c a l c i u m and magnesium i o n s by s e q u e s t r a t i o n ;
-
d i s p e r s i n g power.
s p e c i f i c detergency performance; When d e t e r g e n t s c o n t a i n i n g phosphates came under c r i t i c i s m , t h e search f o r
a l t e r n a t i v e s l e d t o z e o l i t e A ( r e f s . 1 - 3 ) . Comparison o f t h e c h a r a c t e r i s t i c s o f STP and z e o l i t e A shows t h a t , s u r p r i s i n g l y , t h e y possess many i d e n t i c a l p r o p e r t i e s , s o t h a t sodium aluminium s i l i c a t e can be used t o p a r t i a l l y o r w h o l l y e l i m i n a t e t h e phosphate c o n t e n t i n d e t e r g e n t s ( r e f s . 4-5). Since 1976, t h e r e f o r e , combinations o f z e o l i t e A and STP have been i n c r e a s i n g l y i n c o r p o r a t e d i n European d e t e r g e n t s . As a r e s u l t , p r o d u c t i o n c a p a c i t y o f z e o l i t e A was r a p i d l y expanded, and i s c u r r e n t l y above 400 000 t o n s p e r y e a r worldwide
.
102
PHOSPHATE SUBSTITUTION BY ZEOLITE/CO-BUILDER SYSTEMS The improvement i n d e t e r g e n t f o r m u l a t i o n s b r o u g h t about by t h e use o f z e o l i t e A f i n a l l y l e d t o t h e p r o d u c t i o n o f non-phosphate powder d e t e r g e n t s , which were f i r s t b r o u g h t o n t o t h e European m a r k e t i n 1983 ( r e f . 6 ) . T h i s new p r o d u c t c a t e g o r y was made p o s s i b l e by t h e use o f s p e c i a l c o - b u i l d e r s , which i d e a l l y complemented t h e b u i l d e r p r o p e r t i e s o f z e o l i t e A ( r e f s . 7-8). N i t r i l o t r i a c e t i c a c i d (NTA) was, and i n S w i t z e r l a n d and The N e t h e r l a n d s s t i l l i s , used as such. As a s o l u b l e o r g a n i c complexing agent, NTA has p r o p e r t i e s s i m i l a r t o STP, and i n c o m b i n a t i o n w i t h z e o l i t e i t a c t s as a c a r r i e r f o r t r a n s f e r r i n g c a l c i u m i o n s from t h e d e p o s i t o r s o i l t o t h e i o n exchanger z e o l i t e A. However, t h e e c o l o g i c a l d e b a t e on NTA i n some European c o u n t r i e s l e d t o t h e use o f a n o t h e r b u i l d e r c o m b i n a t i o n , T h i s c o n s i s t s o f t h e t e r n a r y b u i l d e r system z e o l i t e A, p o l y c a r b o x y l a t e and sodium carbonate. While sodium c a r b o n a t e m a i n l y a c t s as an a l k a l i z i n g , d e t e r g e n c y - s u p p o r t i n g component, t h e p o l y c a r b o x y l a t e s p e r f o r m a number o f f u n c t i o n s which c o n t r i b u t e t o t h e wash r e s u l t ( r e f s . 9-11).
These sodium s a l t s
o f homopolymers o r copolymers o f a c r y l i c a c i d have m o l e c u l a r w e i g h t s between
2 000 and 120 000. One t y p e c u r r e n t l y w i d e l y used i n Europe i s a copolymer o f a c r y l i c a c i d w i t h m a l e i c anhydride. The mechanism by which t h i s new c l a s s o f d e t e r g e n t i n g r e d i e n t s i n h i b i t s i n c r u s t a t i o n v a r i e s , depending on t h e t y p e o f polymer, c o m p o s i t i o n o f t h e b u i l d e r system, t e m p e r a t u r e and w a t e r hardness. The p o l y c a r b o x y l a t e s can e i t h e r produce a t h r e s h o l d e f f e c t , i. e . t h e y can i n h i b i t , i n small unders t o i c h i o m e t r i c amounts, c a l c i u m s a l t s f r o m p r e c i p i t a t i n g ; o r , by b i n d i n g c a l c i u m , t h e y can p r e v e n t t h e f o r m a t i o n o f d e p o s i t s . They a l s o enhance t h e detergency performance and due t o t h e i r d i s p e r s i n g power p r e v e n t redeposi t i o n o f s o i l p a r t i c l e s on t h e f i b r e s .
BUILDER VARIATION I N NON-PHOSPHATE DETERGENTS The z e o l i t e - A - b a s e d b u i l d e r system d e s c r i b e d h e r e i s now a p a r t o f t h e most i m p o r t a n t non-phosphate heavy-duty powder d e t e r g e n t s i n Europe. The average amounts o f z e o l i t e A (22.0 %), sodium c a r b o n a t e (9.7 %) and p o l y c a r b o x y l a t e (3.5 %) c o n t a i n e d i n 27 analysed d e t e r g e n t s r o u g h l y correspond t o t h e o p t i m a l p r o p o r t i o n s o f t h e s e b u i l d e r components needed t o o b t a i n good p r i m a r y and secondary wash r e s u l t s ( F i g . 1). I f t h e p r o p o r t i o n s a r e v a r i e d , e . g. i f z e o l i t e i s p a r t i a l l y o r c o m p l e t e l y r e p l a c e d by sodium c a r b o n a t e , as i s t h e c a s e i n some non-phosphate p r o d u c t s on t h e American market, t h i s r e s u l t s i n p o o r e r
performance under European wash c o n d i t i o n s . T e x t i l e i n c r u s t a t i o n i s p a r t i c u l a r l y a f f e c t e d . T h i s has been demonstrated i n washing machine t e s t s w i t h a number o f f o r m u l a t i o n v a r i a t i o n s .
103
'I F i g . 1. Average percentages o f z e o l i t e A, sodium carbonate and p o l y c a r b o x y l a t e s i n non-phosphate powder d e t e r g e n t s .
To make t h e r e s u l t s c l e a r e r , washing was i n t e n t i o n a l l y performed under
c r i t i c a l t e s t c o n d i t i o n s , i. e. r e l a t i v e l y h i g h water hardness and l o w dosage (see Table 1).
TABLE 1 Builder variation T e s t i n g equi pment Washing p r o g r a m Wash l o a d Water hardness Dosage Detergent base Number o f wash c y c l e s Incrustation
-
Test c o n d i t i o n s European drum t y p e washing machine 90 'C ( w i t h o u t prewash) 3.5 kg o f l a u n d r y b a l l a s t , 2 c o t t o n f a b r i c s , a r t i f i c i a l l y s o i l e d swatches 450 ppm (Ca:Mg = 5 : l ) 160 g o f d e t e r g e n t i n 20 1 o f w a t e r Non-phosphate heavy-duty d e t e r g e n t (12.5 % s u r f a c t a n t s , 22.5 % p e r b o r a t e t e t r a h y d r a t e ) 25 D e t e r m i n a t i o n by e x t r a c t i o n o f t h e c o t t o n f a b r i c s w i t h ethylenediamine t e t r a a c e t i c a c i d
Under t h e s e c o n d i t i o n s , a 40 % r e d u c t i o n o f t h e z e o l i t e c o n t e n t ( D e t e r g e n t B, see F i g . 2 ) l e d t o a moderate i n c r e a s e i n t e x t i l e i n c r u s t a t i o n a f t e r 25 wash
c y c l e s ; when no z e o l i t e was p r e s e n t ( D e t e r g e n t C) t h e r e was a d r a m a t i c i n c r e a s e i n d e p o s i t s . The s u b s t i t u t i o n o f sodium carbonate f o r z e o l i t e ( D e t e r g e n t D ) a l s o l e d t o a c l e a r i n c r e a s e i n i n c r u s t a t i o n . It i s o b v i o u s t h a t , under t h e
104
chosen c o n d i t i o n s , t h e p o l y c a r b o x y l a t e cannot p r e v e n t t h e p r e c i p i t a t i o n and d e p o s i t i o n o f c a l c i u m c a r b o n a t e w i t h o u t t h e s u p p o r t of o t h e r b u i l d e r s . A c o n t r o l wash t e s t under almost t h e same c o n d i t i o n s showed t h a t , even when t h e polymer c o n t e n t was doubled t o 8 %, f o r m u l a t i o n D was u n a b l e t o match t h e i n c r u s t a t i o n - i n h i b i t i n g e f f e c t o f d e t e r g e n t A. On t h e o t h e r hand, comparison o f f o r m u l a t i o n s A and
E (see F i g . 2) s u b s t a n t i a t e s t h e e f f e c t i v e n e s s o f t h e
polymers when s u f f i c i e n t z e o l i t e i s p r e s e n t .
Incrustation
1%1 7 6 5
4
3 2 1
25 4 5
15 4 5
0 4 5
0 4 30
25 0 5
[% Zeolite A [YO]Polycarboxylate [%] Sodiumcarbonate
Detergent
F i g . 2. I n f l u e n c e o f b u i l d e r v a r i a t i o n on f a b r i c i n c r u s t a t i o n ( Z e o l i t e A = Sasil
@
, calcium
exchange c a p a c i t y : 160 mg CaO/g)
.
The o p t i m i z a t i o n o f t h e non-phosphate b u i l d e r system i s a p r e r e q u i s i t e f o r t h e f i n e t u n i n g o f t h e o t h e r d e t e r g e n t components. By u t i l i z i n g s p e c i a l s u r f a c t a n t combinations and a d a p t i n g t h e b l e a c h system, i n p a r t i c u l a r by i n c o r p o r a t i n g b l e a c h a c t i v a t o r s , non-phosphate heavy-duty d e t e r g e n t s c a n . a c h i e v e t h e same l e v e l o f performance o v e r t h e e n t i r e t e m p e r a t u r e range as t h e t o p phosphate-containing products. INFLUENCE OF
DOSAGE
ON
THE
PERFORMANCE
OF
NON-PHOSPHATE
DETERGENTS
Under b o r d e r l i n e c o n d i t i o n s , however, t h e new non-phosphate d e t e r g e n t s show c h a r a c t e r i s t i c s which c l e a r l y d i f f e r e n t i a t e them from p h o s p h a t e - c o n t a i n i n g p r o d u c t s . T h i s i s e s p e c i a l l y t r u e i f the. dosage i s t o o low. As an example, a non-phosphate d e t e r g e n t X and a p h o s p h a t e - c o n t a i n i n g heavy-duty d e t e r g e n t Y were t e s t e d with r e g a r d t o t h e i r p r i m a r y d e t e r g e n c y performance and t o secondary e f f e c t s a f t e r s e v e r a l washes. D e t e r g e n t X c o n t a i n e d t h e non-phosphate b u i l d e r system w i t h 25 % z e o l i t e ; d e t e r g e n t Y c o n t a i n e d 2 1 % STP i n a n S T P / z e o l i t e c o m b i n a t i o n ( T a b l e 2).
705
TABLE 2 B u i l d e r composition o f two powder d e t e r g e n t s Non-phosphate [ X ] a
Phosphate-containing [ Y
25.0 % Z e o l i t e Ab 4.0 % P o l y c a r b o x y l a t e C 7.5 % Sodium carbonate
21.0 % 14.0 % 1.2 %
]a
STP Z e o l i t e Ab Polycarboxyl ateC
aBoth d e t e r g e n t s c o n t a i n r o u g h l y t h e same amounts o f s u r f a c t a n t s , p e r b o r a t e and b l e a c h a c t i v a t o r . b S a s i l @c,a l c i u m exchange c a p a c i t y : 160 mg CaO/g. C M a l e i c / a c r y l i c a c i d copolymer.
TABLE 3
Dosage v a r i a t i o n
Test c o n d i t i o n s
T e s t i n g equi pment Washing programm Wash l o a d
90 'C ( w i t h o u t prewash)
Soi 1s Water hardness Dosage Number o f wash c y c l e s
European drum-type washing machine
3.5 kg o f s o i l e d l a u n d r y , a r t i f i c i a l l y s o i l e d swatches Sebum, d u s t , l i p s t i c k , r e d wine, t e a , c o f f e e , c u r r a n t (on c o t t o n and c o t t o n / p o l y e s t e r b l e n d ) 290 ppm (Ca:Mg = 5:l) 100, 160 and 220 g o f d e t e r g e n t i n 20 1 o f w a t e r 1 / 25
The t e s t s were c a r r i e d o u t under n e a r - p r a c t i c e c o n d i t i o n s w i t h 3 dosages, whereby 220 g correspond t o t h e recommended h a r d water dosage, 160 g t o a moderately low, and 100 g t o a v e r y low dosage ( T a b l e 3 ) . When t h e recommended dosage was used, b o t h d e t e r g e n t s gave t h e same p r i m a r y detergency r e s u l t s ( s e e F i g . 3 ) . T h i s was t r u e n o t o n l y f o r t h e 90 'C t e s t shown here, b u t a l s o f o r analogous t e s t s a t 40 and 60 'C.
As t h e dosage was
reduced, t h e detergency performance decreased, as expected, whereby t h e d r o p i n t h e performance o f t h e non-phosphate p r o d u c t , e s p e c i a l l y a t v e r y l o w dosages, was s l i g h t l y g r e a t e r t h a n t h a t o f t h e phosphate c o n t a i n i n g product. The d i f f e r e n c e between t h e two b u i l d e r systems shows up more c l e a r l y when t h e f a b r i c i n c r u s t a t i o n i s examined a f t e r 25 washes ( s e e F i g . 4 ) . W i t h t h e non-phosphate z e o l i t e - c o n t a i n i n g d e t e r g e n t , s i m i l a r i n c r u s t a t i o n was produced a t a l l t h r e e dosages under t h e chosen wash c o n d i t i o n s , whereas w i t h t h e phosphate c o n t a i n i n g p r o d u c t t h e r e was a c l e a r i n c r e a s e i n d e p o s i t i o n as t h e dosage was reduced. Analyses o f t h e f a b r i c showed t h a t i n case o f t h e p h o s p h a t e - c o n t a i n i n g d e t e r g e n t s p a r i n g l y s o l u b l e c a l c i u m phosphate i s a l m o s t s o l e l y r e s p o n s i b l e f o r t h e heavy i n c r u s t a t i o n s .
706
Rellectance
"1 60'
(Mean values from all soils)
I
100
1
I
Dosage
160
220
[gl
F i g . 3. I n f l u e n c e o f dosage on o v e r a l l detergency performance o f non-phosphate and p h o s p h a t e - c o n t a i n i n g d e t e r g e n t .
lncrustalion 1%1
765-
---.--. -.-.-.
Phosphalecantaining
4-
32-
(Cotton)
.
-.-.-- -_ ---
-- ---<
Nan-phosphate
' 1 0
100
160
220 Dosage
Is1
F i g . 4.
I n f l u e n c e o f dosage on f a b r i c i n c r u s t a t i o n a f t e r 2 5 wash c y c l e s w i t h non-phosphate and p h o s p h a t e - c o n t a i n i n g d e t e r g e n t .
I n a f u r t h e r t e s t , 50 wash c y c l e s were c a r r i e d o u t a t 90 'C w i t h t h e same d e t e r g e n t s , t o i n v e s t i g a t e v a r i o u s f a c t o r s i n f l u e n c i n g d e p o s i t i o n on machine p a r t s , i n p a r t i c u l a r on h e a t i n g elements. The r e s u l t s o b t a i n e d were s i m i l a r t o t h o s e o b t a i n e d i n t h e f a b r i c i n c r u s t a t i o n t e s t s , i. e. s t r o n g l y i n c r e a s e d d e p o s i t i o n w i t h l o w dosages o f t h e p h o s p h a t e - c o n t a i n i n g d e t e r g e n t (see F i g . 5 ) .
707
,
Deposits
Is1
50 Wash-cycles,
4i
Non-phosphate
F i g . 5. I n f l u e n c e o f dosage on d e p o s i t s on h e a t i n g c o i l s a f t e r 50 wash c y c l e s w i t h non-phosphate and p h o s p h a t e - c o n t a i n i n g d e t e r g e n t .
G e n e r a l l y , w i t h a l l p h o s p h a t e - c o n t a i n i n g d e t e r g e n t s t h e r e i s a danger t h a t , a t l o w dosages, s p a r i n g l y s o l u b l e c a l c i u m phosphate w i l l be formed. The consequences a r e premature wear and t e a r o f t h e wash l o a d and t h e machine, and impairment o f t h e d e s i r e d a p p l i c a t i o n c h a r a c t e r i s t i c s ( r e f . 1 2 ) . T h i s i s e s p e c i a l l y i m p o r t a n t i n view o f t h e i n c r e a s i n g general t r e n d towards low dosages, brought about by economic and e c o l o g i c a l c o n s i d e r a t i o n s . The dosage experiments show t h a t , on t h e whole, non-phosphate d e t e r g e n t s based on z e o l i t e a r e s a f e r than p h o s p h a t e - c o n t a i n i n g p r o d u c t s , w i t h r e s p e c t t o t h e i r e f f e c t s on t e x t i l e s and washing machines. ECOLOGICAL ASPECTS
AND
MARKET
TRENDS
I n v e s t i g a t i o n s i n t h e Federal R e p u b l i c o f Germany have shown t h a t consumers want d e t e r g e n t s t o have good environmental c o m p a t i b i l i t y as w e l l as good wash performance. The non-phosphate d e t e r g e n t s b u i l t up o f z e o l i t e A , p o l y c a r b o x y l a t e and sodium carbonate f u l f i l l t h e s e r e q u i r e m e n t s ( r e f s . 13-15). T h e i r good e n v i ronmental compati b i 1 it y , t h e i r good d e t e r g e n t p r o p e r t i e s , and t h e i r p r i c e , which i s comparable t o t h a t o f p h o s p h a t e - c o n t a i n i n g p r o d u c t s , a l l h e l p e d them t o break through o n t o t h e market. As a consequence t h e r e has been a d r a s t i c s h i f t i n t h e s h a r e o f t h e v a r i o u s b u i l d e r components i n heavy-duty d e t e r g e n t s . T h i s i s i l l u s t r a t e d by a r e p r e s e n t a t i o n o f t h e changes i n volume o f d e t e r g e n t b u i l d e r s i n t h e Federal R e p u b l i c o f Germany ( r e f . 16, F i g . 6 ) . It can be seen t h a t , f o r example, t h e
phosphate c o n t e n t of heavy-duty powder d e t e r g e n t s has f a l l e n by 70 % s i n c e 1975. A t t h e same t i m e , t h e c a l c u l a t e d amount o f phosphate i n t h e s u r f a c e waters o f West Germany f e l l f r o m 42 % t o 17 %, as f a r as t h e c o n t r i b u t i o n f r o m d e t e r g e n t s and c l e a n s e r s i s concerned ( r e f . 16).
F i g . 6. Change i n volume o f d e t e r g e n t b u i l d e r s from 1975 t o 1987 i n t h e FRG.
0
Heavy-Duty Powder Detergents
(End of 1987)
F i g . 7. R e l a t i v e market shares o f non-phosphate and p h o s p h a t e - c o n t a i n i n g heavy-duty powder d e t e r g e n t s i n d i f f e r e n t European c o u n t r i e s (A = A u s t r i a , CH = S w i t z e r l a n d , 0 = F e d e r a l R e p u b l i c o f Germany, NL = The N e t h e r l a n d s ) .
709 The use o f non-phosphate d e t e r g e n t s i s on t h e i n c r e a s e i n s e v e r a l European c o u n t r i e s ( F i g . 7 ) . While i n t h e Federal R e p u b l i c o f Germany non-phosphate heavy-duty powder d e t e r g e n t s had a market share o f 63 % by t h e end o f 1987, i n S w i t z e r l a n d , f o r i n s t a n c e , a l l d e t e r g e n t s a r e r e q u i r e d by l a w t o b e f r e e o f phosphates. I n t h e meantime, t h e s e p r o d u c t s have a l s o won a c o n s i d e r a b l e share o f t h e market i n such c o u n t r i e s as A u s t r i a and The Netherlands. The success o f z e o l i t e - c o n t a i n i n g d e t e r g e n t s t h r o u g h o u t Europe proves, d e s p i t e a l l i n i t i a l s c e p t i c i s m , t h a t z e o l i t e A c o u l d be a s u b s t i t u t e f o r phosphate: t h i s raw m a t e r i a l can be s a i d t o have proved i t s e l f i n t h e 15 y e a r s s i n c e i t s d i s c o v e r y . I n view o f t h e growing need f o r e n v i r o n m e n t a l p r o t e c t i o n , i t s f u t u r e would seem t o be assured. REFERENCES 1 DE 24 12 837 82, Henkel, 1974. 2 DE 24 12 838 82, Henkel, 1974. 3 P. B e r t h , G. J a k o b i , E. Schmadel, M.J. Schwuger and C.H. Krauch, Angew. Chem. 87 (1975) 115. 4 P. B e r t h , J. Amer. O i l Chem. SOC. 55 (1978) 52. 5 M.J. Schwuger and E.J. Smulders, i n W.G. C u t l e r and E. K i s s a (Eds.), Detergency Theory and Technology, Marcel Dekker, New York, 1987, Ch. 6, pp. 37 1-439. 6 P. B e r t h , P. K r i n g s and H. Verbeek, Tenside Deterg. 22 (1985) 169. 7 C.P. K u r z e n d o r f e r , M. L i p h a r d , W. von R y b i n s k i and M.J. Schwuger, C o l l o i d & Polymer Sci 265 (1987) 542. 8 H. Andree, P. K r i n g s , H. Upadek and H. Verbeek, i n A.R. Baldwin (Ed.), Proc. 2nd World Conference on Detergents, Montreux, 1986, p. 148. 9 W. W i r t h , I b i d . , p. 138. 10 M.K. Nagarajan, J. Amer. O i l Chem. SOC. 62 (1985) 949. 11 P. Z i n i , S e i f e n l l l e F e t t e Wachse 113 (1987) 45. 12 A. Lohr and K. Pracher, Tenside Deterg. 25 (1988) 36. 13 Umweltbundesamt, M a t e r i a l i e n 4/79, D i e Prufung des Umweltverhaltens von Natrium-Aluminium-Silikat Z e o l i t h 4A a l s P h o s p h a t e r s a t z s t o f f i n Wasch- und R e i n i g u n g s m i t t e l , E r i c h Schmidt (Ed.), B e r l i n , 1979. 14 L. Huber, S e i f e n U l e F e t t e Wachse 113 (1987) 393. 15 H.J. Opgenorth, Tenside Deterg. 24 (1987) 366. 16 P. B e r t h and P. K r i n g s , i n G e s e l l s c h a f t Deutscher Chemiker (Eds.), Kompendi um Auswi rkungen d e r Phosphathochstmengenverordnung f u r Waschmi t t e l a u f K l a r a n l a g e n und i n Gewassern, Sankt Augustin, 1988, i n press.
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0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
S I M U L T A N E O U S S E P A R A T I O N OF S U S P E N D E D S O L I D S , A M M O N I U M A N D P H O S P H A T E IONS FROM WASTE WATER B Y M O D I F I E D C L I N O P T I L O L I T E
J. O L A H Y 1 J. PAPP,2 A.MESZAROS-KIS,’
G Y . MUCSI’ and D.
KALL02
‘ R e s e a r c h C e n t r e f o r W a t e r R e s o u r c e s D e v e l o p m e n t , 1453 B u d a p e s t , P.O.Box 2 7 ( H u n g a r y ) n
L Central Research I n s t i t u t e f o r C h e m i s t r y , H u n g a r i a n A c a d e m y o f S c i e n c e , H - 1 5 2 5 Budapest, P.O.Box 17 ( H u n g a r y )
ABSTRACT A municipal s e w a g e t r e a t m e n t t e c h n o l o g y has been d e v e l o p e d based o n t h e r e c o g n i t i o n t h a t ( i ) z e o l i t e - c o n t a i n i n g p o w d e r (i.e. c r u s h e d z e o l i t i c r o c k ) o f a p p r o p r i a t e g r a i n s i z e a d d e d t o w a s t e w a t e r i n c r e a s e s the biological a c t i v i t y o f the s l u d g e , ( i i ) removal o f s u s p e n d e d s o l i d s is i n c r e a s e d a n d ( i i i ) t h e p h o s p h a t e removal e f f i c i e n c y o f t r i v a l e n t c a t i o n s ( F e 3 + , A l J + ) is h i g h e r in the presence o f zeolite. A f t e r biological t r e a t m e n t t h e r e m a i n i n g a m m o n i u m and dissolved or c o l l o i d o r g a n i c s u b s t a n c e s c a n be removed in a c o l u m n f i l l e d w i t h z e o l i t e (e.g. c l i n o p t i l o l i t e ) s e l e c t i v e f o r a m m o n i u m ions, INTRODUCTION A f t e r biological t r e a t m e n t w i t h a c t i v a t e d s l u d g e t h e o u t l e t sewage mostly c o n t a i n s c o n t a m i n a n t s in u n a c c e p t a b l e a m o u n t s . In o r d e r t o r e m o v e t h e r e m a i n i n g i m p u r i t i e s chemical m e t h o d s a r e usually a p p l i e d , e.g. s u s p e n d e d s o l i d s can be c o a g u l a t e d by adding a l u m i n i u m s u l f a t e or a s y n t h e t i c p o l y e l e c t r o l y t e s ; p h o s p h o r u s a s p h o s p h a t e s a r e precipitated w i t h a l u m i n i u m o r i r o n s a l t s o r w i t h l i m e ; a m m o n i u m removal proceeds via i o n - e x c h a n g e o r t h r o u g h n i t r i f i c a t i o n f o l l o w e d by d e n i t r i f i c a t i o n (ref. 1). C o n s i d e r i n g t h e p u b l i c a t i o n s o f p r o c e s s e s a p p l i e d it s e e m s t h a t the i m p r o v e m e n t o f s e w a g e t r e a t m e n t t e c h n o l o g i e s is still o f c u r r e n t interest. As a r e s u l t s o f t h e a b o v e t r e a t m e n t s with e x p e n s i v e c h e m i c a l s , w a s t e s a r e f o r m e d wich c o n t a m i n a t e t h e e n v i r o n m e n t . T h e u s e o f natural m a t e r i a l s 1 ike z e o l i t e s a s i o n - e x c h a n g e r s a n d / o r adsorbents s e e m s r a t h e r promising, s i n c e they a r e much c h e a p e r and t h e i m p u r i t i e s a r e r e m o v e d in l e s s h a z a r d o u s forms.
712
Among n a t u r a l z e o l i t e s , c l i n o p t i l o l i t e o c c u r s m o s t frequently, e.g.
i n U.S.A.,
Japan,
t h e S o v i e t U n i o n and Hungary ( r e f . 2 ) .
Ames was t h e f i r s t o b s e r v t h e s e l e c t i v e ammonium i o n - e x c h a n g e f o r c l i n o p t i l o l i t e (ref. 3);
thereafter the a p p l i c a b i l i t y o f
c l i n o p t i l o l i t e f o r ammonium r e m o v a l f r o m r a w a n d w a s t e w a t e r s was extensively investigated.
Koon a n d Kaufmann s t u d i e d i n d e t a i l
t h e ammonium r e m o v a l w i t h c l i n o p t i l o l i t e i n a f i x e d - b e d c o l u m n and d e t e r m i n e d t h e r e g e n e r a t i o n c o n d i t i o n s ( r e f . 4 ) . experts found t h a t p h i l l i p s i t e i s applicable, o f ammonium f r o m w a t e r b y i o n - e x c h a n g e ( r e f .
too,
5).
Italian
f o r removal
Union Carbide
p r o d u c e d a z e o l i t e s p e c i a l l y s e l e c t i v e f o r ammonium i o n s ( r e f . 6 ) . Similarly,
i n f i x e d - b e d o p e r a t i o n L i b e r t i and c o - w o r k e r s
(ref.
7,8) u t i l i z e d c l i n o p t i l o l i t e f o r ammonium r e m o v a l a n d an a n i o n - e x c h a n g e r e s i n f o r p h o s p h a t e r e m o v a l i n sewage t r e a t m e n t p i l o t p l a n t s o f 10 a n d 240 m 3 / d c a p a c i t i e s . When z e o l i t e A ,
came i n t o u s e as a d e t e r g e n t b u i l d e r i t s r o l e
i n sewage t r e a t m e n t was i n v e s t i g a t e d . N e i t h e r C a r r o n d o e t a l . ( r e f . 91, n o r Holman a n d H o p p i n g ( r e f .
10,
1 1 ) f o u n d any u n f a -
vourable e f f e c t o f z e o l i t e A during the treatment o f laundry effluents. I s h i i and K a j i p u b l i s h e d ( r e f . 1 2 1 , t h e e q u a t i o n s o f a d s o r p t i o n i s o t h e r m s o f ammonium a n d p h o s p h a t e i o n s on s u s p e n d e d c l i n o p t i l o l i t e doped w i t h a l u m i n i u m s u l f a t e . The authors have succeeded i n i n c r e a s i n g t h e r e m o v a l o f s u s p e n d e d solids,
p h o s p h a t e a n d t o some e x t e n t ammonium i o n s f r o m m u n i c i p a l
sewage b y t h e u s e o f c l i n o p t i l o l i t e s u s p e n d e d i n c o n c e n t r a t e d aqueous s o l u t i o n o f a F e 3 + s a l t .
The r e m a i n i n g o f t h e ammonium
c a n t h e r e a f t e r be removed b y i o n - e x c h a n g e i n a f i x e d - b e d c o l u m n f i l l e d w i t h c l i n o p t i l o l i t e . The a i m o f o u r f u r t h e r i n v e s t i g a t i o n s was t o c o n f i r m t h e e a r l i e r r e s u l t s t h r o u g h l a r g e - s c a l e experiments a n d f u r t h e r m o r e t o c l a r i f y up t h e i n f l u e n c e o f a c l i n o p t i l o l i t e s u s p e n s i o n on t h e a c t i v i t y a n d p r o p e r t i e s o f a c t i v a t e d s l u d g e . EXPERIMENT Materials
A 6 3 - 1 8 0 I.cm f r a c t i o n o f a r h y o l i t e t u f f c o n t a i n i n g a b o u t 5 0 % c l i n o p t i l o l i t e ( f r o m R a t k a , T o k a j H i l l s , H u n g a r y ) was s u s p e n d e d i n d i f f e r e n t a m o u n t s o f aqueous i r o n c h l o r o - s u l f a t e s o l u t i o n c o n t a i n i n g 200 g F e 3 + / l . S u s p e n s i o n s w i t h d i f f e r e n t c l i n o p t i l o l i t e / /Fe3+ r a t i o s were used.
713
The o p t i m a l g r a i n s i z e o f t h e t u f f was e x p e r i m e n t a l l y determined: ( i ) t h e s m a l l e s t p a r t i c l e s must n o t escape t h e s e t t l i n g t a n k , ( i i ) the e f f i c i e n c y o f t h e l a r g e s t p a r t i c l e s should n o t essent i a l l y decrease because o f t h e s h o r t e r s e t t l i n g t i m e and t h e slower d i f f u s i o n - c o n t r o l l e d processes, To e n s u r e f a v o u r a b l e o p e r a t i n g c o n d i t i o n s , 0 . 5 - 2 . 0
mm g r a i n
s i z e f r a c t i o n o f t h e a b o v e t u f f was u s e d i n t h e i o n - e x c h a n g e column. The c l i n o p t i l o l i t e - c o n t a i n i n g t u f f i s c h a r a c t e r i z e d as f o l l ows. C l i n o p t i l o l i t e c r y s t a l s o f s i z e 1 - 1 0 ,urn a r e i r r e g u l a r l y embedded i n t h e r o c k , w h i c h c o n t a i n s q u a r t z , montmorillonite,
crystobalite,
f e l s p a r a n d some 1 0 % v o l c a n i c g l a s s . The
minerals form a texture w i t h a convenient pore-size d i s t r i b u t i o n .
No s w e l l i n g i s o b s e r v a b l e i n w a t e r , i . e . t h e s i z e o f p a r t i c l e s The m a i n p h y s i c a l - c h e m i c a l d a t a o f t h e u s e d t u f f a r e summarized i n T a b l e 1.
does n o t change.
TABLE 1 The p r o p e r t i e s o f t h e c l i n o p t i l o l i t e - c o n t a i n i n g r o c k Chemical Composition i n w t . % : Si02
2'3
69.50
11.65
Fe203
Na20
K20
MgO
CaO
loss ign.
1.06
0.44
4.44
0.59
1.83
10.53
Pore-size d i s t r i b u t i o n r
1.6 10
d1.6
-= r PP .= 1 0
-
Volume o f p o r e s i n f u n c t i o n o f p o r e s i z e s
nm
0.1 c m 3 / g
nm
0.1
cm3/g
<
r ~ 7 5 0 0nm 0.5 c m 3 / g P P o r o s i t y : 40-50 % ( t h e r a t i o o f t h e t o t a l p o r e volume and t h e
volume o f t h e p o r o u s m a t e r i a l ) S u r f a c e a r e a f r o m benzene a d s o r p t i o n : 20-30 mz/g S u r f a c e a r e a f r o m n i t r o g e n a d s o r p t i o n : 400-500 mz/g I o n - e x c h a n g e c a p a c i t y : 1.1 meq/g The c h a r a c t e r i s t i c s o f sewages a r e l i s t e d i n T a b l e 2 t o g e t h e r w i t h t h e mean v a l u e s o f e x p e r i m c n t a l d a t a ,
s i n c e d i f f e r e n t sewages
w e r e f e d i n d i f f e r e n t p l a n t s . The a m o u n t o f s u s p e n d e d s o l i d s ,
COD
( C h e m i c a l O x y g e n Demand) t o t a l p h o s p h o r u s a n d o r t h o - p h o s p h a t e , ammonium a n d n i t r a t e c o n t e n t s w e r e d e t e r m i n e d , M o h l m a n n i n d e x ( a datum f o r c h a r a c t e r i z i n g t h e s e d i m e n t a t i o n p r o p e r t i e s o f t h e suspended s o l i d s ) and b i o l o g i c a l a c t i v i t y o f t h e s l u d g e were m e a s u r e d , a s we1 1 .
714 TABLE 2 The m a i n p a r a m e t e r s o f sewage t r e a t m e n t s i n d i f f e r e n t p l a n t s Location, Year (Type 1
Amount o f sewage
m3 /d
Biological 1oadi ng
Age o f sludge
kg BOD d
k g - l d-’ D u n a k e s z i , 1985
48
0.25
2.3
1850
0.08
15.5
,
0.04
25
( p i l o t plant)
Zalaegerszeg,
1987
(large scale) Bal atonbereny , 1987
Character o f sewage
municipal 50%+canning f a c t o r y 50%
municipal 60% food i n d u s t r y 40% m u n i c ip a l
(large scale) Equipment and Methods C o n t i n u o u s e x p e r i m e n t s were c a r r i e d o u t i n a p i l o t p l a n t and i n l a r g e - s c a l e p l a n t s . T y p i c a l t e c h n i c a l d a t a o f sewage treatment a r e g i v e n i n Table 3. The a r r a n g e m e n t o f t h e t e c h n o l o g i c a l u n i t s ( s c r e e n , g r i t chamber, a e r a t o r ,
s e c o n d a r y s e t t l i n g t a n k ) w e r e t h e same f o r a l l
i n s t a l l a t i o n s . The z e o l i t e s u s p e n s i o n was a d d e d t o t h e sewage i n t h e c h a n n e l j u s t b e f o r e t h e a e r a t o r , w h e r e a p e r f e c t m i x i n g was achieved. loading,
The t e c h n o l o g i c a l p a r a m e t e r s ( e . g . etc.)
residence time,
and t h e s i z e o f equipment were i d e n t i c a l f o r b o t h
t h e e x p e r i m e n t a l a n d c o n t r o l l i n e s . The p i l o t p l a n t ( i n D u n a k e s z i ) operated w i t h high loading,
t h e o t h e r s i n a t o t a l o x i d a t i o n mode.
RESULTS A N D D I S C U S S I O N A v e r a g e s o f a g r e a t amount o f e x p e r i m e n t a l d a t a d e t e r m i n e d b o t h i n p i l o t p l a n t and i n l a r g e - s c a l e p l a n t s a r e summarized i n T a b l e 3 . The l a t t e r p l a n t s a r e l o c a t e d i n t h e w e l l p r o t e c t e d a r e a o f Lake-Balaton.
W
c,
nc,
ru r
0
c,
mc1 E a J
n s n 1
o v T c,
L
0
K
0 .r
m
V
c,
0 J
I
m
m c
o
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m -
m 7
c
m
m m
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m
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09CO
m m
o
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7
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I1 I1 I1 I1 II I1
I1
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W I I nil XI1 W I I I l l I1
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'7
211I1
w I1 m II m II c I1 v I1 I1 -11 411 c, I1 K I1
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m11 m
0 I1
I1 I1
N
N I1 I1 I1 I1
N
11 I1 II I1 I I I I I I
II I1
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N I1 II
It
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III 1
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I1 II I1 11
I1 I1 I1 I1 I1 I1
mii m 11
c
m m
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m w m e c
m m
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715
716 Removal o f P h o s p h o r u s D e p e n d i n g on t h e amount o f c o n t a m i n a n t s 15-30 mg F e 3 + a n d
30-100 mg p o w d e r e d c l i n o p t i l o l i t e c o n t a i n i n g r o c k i s a d d e d t o one l i t r e o f sewage. The r e m o v a l o f p h o s p h o r u s i n t h e p i l o t p l a n t ( i n D u n a k e s z i ) as a f u n c t i o n o f t i m e on f l o w i s i l l u s t r a t e d i n Fig.
1. I n most p r o t e c t e d d i s t r i c t s o f Hungary (around t h e g r e a t
l a k e s ) t h e p e r m i t t e d l i m i t o f p h o s p h o r u s c o n t e n t i n emmissions i s
1.8 mg P/1 (5,5 mg P O i - / l ) ,
w h i c h c a n r e l i a b l y be a t t a i n e d w i t h
t h i s treatment.
5
10
15
20
25
50
30
55 60 time, day
F i g . 1. O r t h o - p h o s p h a t e c o n c e n t r a t i o n as a f u n c t i o n o f t i m e on f l o w i n t h e s e t t l e d sewage i n l e t (l), i n t h e c o n t r o l sewage o u t l e t ( w i t h o u t additives, 2 ) , w i t h a d d i t i o n o f a suspension c o n t a i n i n g 18.3 mg F e 3 + / l + 65 mg z e o l i t e / l ( 3 ) On a d d i n g 1.16-1.95 i r o n i o n s t o g e t h e r , z e o l i t i c r o c k t o 1 atom phosphorus, P/1 c a n be g u a r a n t e e d .
w i t h the suspension o f
a d e p h o s p h o r i z a t i o n t o 1.8 mg
W i t h o u t z e o l i t e a d d i t i v e 1.7-2.5
i r o n ions
a r e needed f o r c o m p a r a b l e p u r i f i c a t i o n . Our o b s e r v a t i o n s , p o r t e d by t h e f i n d i n g o f I s h i i and K a j i ( r e f .
sup-
12), a r e s i m i l a r
f o r A12(S04)3 a d d i t o n w i t h and w i t h o u t z e o l i t e s . Consequently, t h e e f f i c i e n c y o f phosphate removal w i t h t r i v a l e n t c a t i o n s increases i n the presence o f z e o l i t e ; 1 g o f z e o l i t i c rock i t s e l f removes 6 . 4
mg p h o s p h o r u s o w i n g t o t h e m o b i l e Ca a n d Mg ( F e ) i o n s
i n t h e z e o l i t e phase which f o r m phosphate p r e c i p i t a t e
717
a f t e r e x c h a n g e f o r ammonium i o n s f r o m t h e sewage. The o b s e r v e d e f f e c t o f c l i n o p t i l o l i t e i s 3-6 times g r e a t e r t h a n t h e v a l u e o f 6.4 mg/g.
A f t e r an o p e r a t i o n p e r i o d f o r some weeks t h e p h o s p h a t e
removal e f f i c i e n c y
i s preserved i n the z e o l i t e containing aerator,
when t h e dosage i s s t o p p e d e v e n f o r o n e d a y . The r e a s o n f o r t h i s e f f e c t i s t h e r e t e n t i o n o f i r o n i n t h e z e o l i t i c r o c k i n t h e a e r a t o r . The r e s i d e n c e t i m e o f z e o l i t i c r o c k w i t h i r o n s a l t occluded i n the pores o f d i f f e r e n t s i z e i s l o n g e r t h a n t h a t o f n o n - f i x e d i r o n s o l u t i o n . The r e t a r d e d d e s o r p t i o n o f i r o n during a longer residence time r e s u l t s i n a b e t t e r utilization o f t h e Fe3+ introduced. This i s i n q u a n t i t a t i v e c o r r e l a t i o n w i t h the experimental findings. Removal o f Suspended S o l i d s I n each case t h e c o n t e n t o f suspended s o l i d s i n t h e e f f l u e n t i s l e s s when t h e b i o l o g i c a l t r e a t m e n t o f t h e sewage i s c a r r i e d out with z e o l i t e / i r o n - s a l t addition than without i t (c.f. " e x p e r i m e n t a l " a n d " c o n t r o l " d a t a i n T a b l e 3 ) : e.g.
i n a large
s c a l e p l a n t ( Z a l a e g e r s z e g ) t h e e f f l u e n t c o n t a i n e d 35 m g / l s u s p e n d e d s o l i d s w h i c h d e c r e a s e d t o 18 m g / l b y z e o l i t e / i r o n - s a l t addition.
The z e o l i t e p a r t i c l e s p r e f e r a b l y a d s o r b c o l l o i d s ,
and
c o a g u l a t i o n s e e d s a r e t h u s f o r m e d . The h y d r o x i d e s o f m e t a l s f o r m e d f r o m t h e i n t r o d u c e d s a l t s e x e r t an a d d i t i o n a l c o a g u l a t i o n effect
.
As a r e s u l t o f z e o l i t e t r e a t m e n t ( i ) t h e d e n s i t y o f a c t i v a t e d s l u d g e i n c r e a s e s f r o m 1 5 g / l t o 30 g / 1 i n t h e s e c o n d a r y s e t t l i n g tank,
( i i ) the d e w a t e r a b i l i t y improves, i . e .
the
c a p i l l a r y s u c t i o n t i m e decreases f r o m 30 s t o 10-15 s , and ( i i i ) t h e Mohlmann i n d e x d i m i n i s h e s s i g n i f i c a n t l y . The E f f e c t o f Z e o l i t e on t h e B i o l o g i c a l A c t i v i t y o f t h e S l u d g e An i n c r e a s e d b i o l o g i c a l a c t i v i t y i s r e f l e c t e d b y t h e C O D v a l u e s o f t h e f i l t e r e d e f f l u e n t s ( T a b l e 3 ) s i n c e w i t h t h e same residence times,
they are r e g u l a r l y lower a f t e r z e o l i t e treatment
than i n the c o n t r o l l i n e . The d i g e s t i o n a c t i v i t y o f t h e s l u d g e c a n be b e t t e r e x p r e s s e d i n terms o f COD g - l h - l
. Pilot
p l a n t experiments (Dunakeszi,
showed an a c t i v i t y o f 52 C O D 9 - l h - l g-'h-'
i n t h e case o f z e o l i t e i n t r o d u c t i o n .
v a l u e s i n an o p e r a t i n g p l a n t ( B a l a t o n b e r e n y , 44 C O D g - l h - ' ,
respectively.
1986)
i n t h e c o n t r o l l i n e and 65 COD The c o r r e s p o n d i n g 1 9 8 7 ) w e r e 35 a n d
These e x a m p l e s ( a n d a s e r i e s o f
o t h e r d a t a ) p o i n t t o an i n c r e a s e o f t h e b i o l o g i c a l a c t i v y b y a b o u t 2 5 % due t o z e o l i t e i n t r o d u c t i o n .
Upon z e o l i t e a d d i t i o n a s i g n i f i c a n t i n c r e a s e o f n i t r i f i c a t i o n has b e e n o b s e r v e d a s w e l l . a b o u t 60%, i . e . i n the effluents
For instance,
i n Zanka ( 1 9 8 6 ) i t was
53 m g / l a n d 85 m g / l n i t r a t e c o n t e n t s w e r e f o u n d o f c o n t r o l and e x p e r i m e n t a l l i n e s r e s p e c t i v e l y .
The c o r r e s p o n d i n g v a l u e s i n B a l a t o n b e r e n y ( 1 9 8 7 ) w e r e 24 m g / l a n d 57 m g / l ,
respectively.
The i n c r e a s e o f b i o l o g i c a l a c t i v i t y e f f e c t e d t r o u g h z e o l i t i c a d d i t i v e c a n be e x p l a i n e d as f o l l o w s : ( i ) Z e o l i t e p a r t i c l e s a r e seeds f o r b a c t e r i a f l o c k s . F l o c k s a r e t h u s f o r m e d i n g r e a t e r number a n d o f s m a l l e r s i z e t h a n i n t h e absence o f z e o l i t i c g r a i n s . s m a l l e r t h a n 0.3 mm; a n d 2 mm.
With z e o l i t e s the f l o c k s are
w i t h o u t z e o l i t e s t h e i r s i z e i s b e t w e e n 0.4
The t r a n s p o r t o f o x y g e n a n d n u t r i e n t s i s f a s t e r i n t h e
s m a l l e r f l o c k s t h a n i n l a r g e r ones. ( i i ) Z e o l i t e p a r t i c l e s s o r b ammonia w h i c h i s a c c e s s i b l e f o r n i t r i f i c a t i o n b a c t e r i a c o n c e n t r a t e d on z e o l i t e c r y s t a l s ,
thus the
n i t r i f i c a t i o n accelerates. ( i i i ) The b i o l o g i c a l c o m p o s i t i o n o f a c t i v a t e d s l u d g e c h a n g e s f a v o u r a b l y i n t h e p r e s e n c e o f z e o l i t e . The " p r e d a t o r c i l i a t e s " (Litonotus, Loxophillum,
Vorticella,
Convallaria,
Opercularia,
C o a r e t a t a ) m u l t i p l y . Nematodes a l s o a p p e a r i n t h e s l u d g e .
These
a n i m a l s f e e d on t h e f r e e - s w i m m i n g b a c t e r i a i n l i q u i d phase. ( i v ) The b i o l o g i c a l s t a b i l i t y o f t h e s e p a r a t e d a n d z e o l i t e containing sludge i s greater than without z e o l i t e : observable a n a e r o b i c d i g e s t i o n proceeds w i t h i n f i v e days i n t h e absence o f z e o l i t e w h e r e a s t h e s l u d g e r e m a i n s u n c h a n g e d e v e n f o r t h r e e weeks u n d e r t h e same c o n d i t i o n s ,
i f i t c o n t a i n s z e o l i t e . The phenomenon
i s being investigated i n d e t a i l . ( v ) Contrary t o usual sludges, the desiccated sludge c o n t a i n i n g z e o l i t e i s o f h i g h v a l u e because a f t e r composting i t c a n be u s e d a s a f e r t i l i z e r o f s u f f i c i e n t n u t r i e n t v a l u e s . When i n s t e a d o f i r o n s a l t an a l u m i n i u m s a l t i s u s e d f o r t h e treatment, h o w e v e r , t h e s l u d g e c a n h a r d l y be u t i l i z e d i n a g r i c u l t u r e . F i n a l Treatment A f t e r b i o l o g i c a l treatment o f increased a c t i v i t y the r e s t o f t h e ammonium c a n be r e m o v e d c o n v e n t i o n a l l y b y i o n - e x c h a n g e w i t h zeolite,
p r e f e r a b l y w i t h c l i n o p t i l o l i t e because o f i t s h i g h
s e l e c t i v i t y f o r ammonium.
The ammonium c o n t e n t i n t h e o u t l e t c a n be
kept below 4 mg/l. The o p e r a t i o n p a r a m e t e r s f o r o u r p i l o t p l a n t w e r e o p t i m i z e d . The f i l l i n g c a n be r e p e a t e d l y r e g e n e r a t e d w i t h a 2 w t . %
719 KCl/NaCl
s o l u t i o n o f pH
=-
1 0 . The ammonia was r e m o v e d e i t h e r b y
s t r i p p i n g w i t h a i r and a b s o r b i n g , solution,
e.g.
i n a phosphoric a c i d
o r b y p r e c i p i t a t i o n i n t h e f o r m o f MgNH4P04. We
observed t h a t t h e f i r s t s e c t i o n o f ion-exchanger works as a filter,
where d i s s o l v e d o r c o l l o i d ( m a i n l y o r g a n i c ) c o n t a m i n a n t s
n o t a l r e a d y removed a r e bound. A f t e r t h e c o u n t e r c u r r e n t r e g e n e r a t i o n they can be s e t t l e d from t h e r e g e n e r a t i n g s o l u t i o n . REFERENCES 1
Water T r e a t m e n t Handbook, V . E d i t i o n , Degremont 1979, pp. 57-74, 141-144. 2 Y . M u r a k a m i , A . I i j i m a a n d J.W. Ward ( E d i t o r s ) : New Development i n Z e o l i t e Science and Technology. E l s e v i e r S c i e n t i f i c P u b l . Co. A m s t e r d a m , 1 9 8 6 . 3 L . L . Ames: Amer. M i n e r a l o g i s t 4 5 ( 1 9 6 0 ) 6 8 9 - 7 0 0 . 4 J.H. Koon a n d W.J. K a u f m a n : O p t i m a t i o n o f Ammonia Removal b y I o n E x c h a n g e U s i n g C l i n o p t i l o l i t e . U S R e p o r t f o r t h e EPA, 1 9 7 1 . 5 P. C i a m b e l l i , P . C o r b o , C . P o r c e l l i a n d A . R i m o l i : Z e o l i t e s 5 ( 1 9 8 5 ) 184-187. 6 US P a t e n t 3 723 3 0 8 7 L. L i b e r t i , G. B o a r i a n d R. P a s s i n o : Wat. S u p p l y 1 ( 1 9 8 3 ) , 169-176. 8 L . L i b e r t i , N. L i m o n i , A . L o p e z , R . P a s s i n o a n d G . B o a r i : Water Research 20 ( 1 9 8 6 ) 735-739. 9 M.J.T. C a r r o n d o , R . P e r r y a n d J.N. L e s t e r : J. Wat. P o l l u t . C o n t r o l Fed. 52 ( 1 9 8 0 ) 2 7 9 6 - 2 8 0 6 . 1 0 W.F. H o l m a n a n d W.D. H o.p.p i n q- : J. W a t . P o l l u t . C o n t r o l F e d . 52 (1980) 2887-2905. 1 1 J.E. K i n g , D.W. H o p p i n g a n d F.W. H o l m a n : I . J . Wat. P o l l u t . C o n t r o l Fed. 52 ( 1 9 8 0 1 2 8 7 5 - 2 8 8 6 . 1 2 M . I s h i i a n d K. K a j i : . G y p s u m a t L i m e 186 ( 1 9 8 3 ) 8 - 1 6 .
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IV . M0DIFICATI0N AND CHARACT ER IZ AT I0N
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H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FRAMEWORK AND NON-FRAMEWORK A1 SPECIES IN DEALUMINATED ZEOLITE Y P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs Laboratorium voor Oppervlaktechemie, Kathol ieke Universitei t Leuven, Kardinaal Mercierlaan 92, B-3030 Leuven, Belgium ABSTRACT The hydrothermal dealumination of zeolite Y was followed by "Si and 27Al MAS NMR. From deconvoluted MAS NMR spectra the amount of the different silicon and aluminium s cies, formed during steaming, could be determined. Next to well documented signals at 60 ppm of tetrahedral framework A1 and at 0 ppm of octahedral Al, lines at 50 ppm, 30 ppm and in the region of 4-0 ppm are observed, the chemical nature of which is discussed. Zeolite Y with framework Si/Al ratio from 3.5 to 17 contains non-negligible amounts of extra-framework silicon and aluminium species, which complicate the determination of the framework Si/Al and non-framework:framework A1 r ios. A considerable amount of extra-framework aluminium is missing in the "A1 MAS spectra. This 'NMRinvisible' aluminium, assigned to oligomeric alumina species, can be made 'NMRvisible' by complexation with acetylacetone under specific conditions. INTRODUCTION "Si and 27Al MAS NMR is a powerful technique to provide direct information pertaining the local environment of these nuclei in zeolite structures (ref. 1). Knowledge of the coordination and quantification of the different A1 species is essential in understanding the catalytic activity (ref. 2). In principle, from "Si MAS NMR spectra the si1icon:aluminium ratio of the framework (SiF/AIF) can be determined and from 27Al spectra the nonframework:total A1 ratio (AINF/A1). With hydrothermally dealuminated faujasites incorrect AINF/A1 are obtained (refs. 3-6) due to the presence of so-called 'NMR-invisible' aluminium which resides in non-framework environments of low symmetry subjected to large electric field gradients; the corresponding large quadrupole effect on the 27Al nucleus results in a broadening of the NMR signal beyond the detection limit. In a previous paper (ref. 7) we reported on a technique to visualize all NMRinvisible Al. The method consists of converting non-framework aluminium with acetylacetone (acac) into A l ( a ~ a c ) ~complexes, the 27Al MAS NMR signals of which can be re1 iably used for quantitative interpretation. In this report we show that our method is applicable to weakly as well as strongly dealuminated faujasites. The application of our acac impregnation method is discussed and compared to former efforts (refs. 8-10). In steamed faujasites different non-framework aluminium and silicon species are observed. Their chemical nature and quantification will also be elucidated.
122
EXPERIMENTAL M a t e r i a l s . NaY from Ventron w i t h Si/Al=2.46 was exchanged 90% w i t h NH4' and steamed i n various ways. Samole 1 was obtained by self-steaming hydrated NH4-Y i n deep-bed geometry (height x diameter=3x3 cm) a t 873 K f o r 4 h. SamDle 2 r e s u l t e d from t h e same treatment a t 823 K f o r 24 h, followed by NH4-exchange, a second c a l c i n a t i o n a t 973 K, and a f i n a l NH4-exchange. SamDles 3. 4 and 5 were obtained from Sample 2 by a treatment under 0.1 MPa o f steam a t 823 K f o r 1 h, a t 973 K f o r 3 h and a t 973 K f o r 7 h, r e s p e c t i v e l y . Hydrated samples were obtained by immersing t h e z e o l i t e s i n water and d r y i n g a t room temperature. S i l ica-alumina w i t h Si/A1=7.7 was prepared by h y d r o l y s i n g t e t r a e t h y l o r t h o s i l i c a t e and aluminium isopropoxide. A1 ( a c a c l j comolex was purchased from Janssens Chimica. Acac treatment. A 1 g q u a n t i t y o f hydrated z e o l i t e was soaked i n 2 m l 38 v01.-% acac i n ethanol a t 293 K. A f t e r 1 h t h e ethanol and t h e excess acac was evaporated a t 293 K under a f l o w o f dry a i r . MAS NMR spectra were recorded 24 h a f t e r t h e impregnation. NMR measurements. The NMR measurements were performed using a Bruker 400 MSL spectrometer w i t h a magnetic f i e l d o f 9.4 T. The "Si MAS NMR experiments were r u n a t 79.5 MHz, w i t h a pulse l e n g t h o f 4 p s , a pulse i n t e r v a l o f 5 s, a spinning r a t e o f 3 kHz and 10,000 scans. 27Al MAS NMR was performed a t 104.2 MHz, w i t h a pulse l e n g t h o f 0.6 ps, a radiofrequency f i e l d s t r e n g t h o f 5 mT, a pulse i n t e r v a l o f 1 s, u s u a l l y a spinning frequency o f 5 kHz and 3,000 scans. The r a t h e r strong and very s h o r t pulse i s necessary t o s a t i s f y t h e c r i t e r i a o f uniform, quadrupole-interaction-independent e x c i t a t i o n o f t h e c e n t r a l 27Al t r a n s i t i o n ( r e f . 11). The d i f f e r e n t l i n e s i n the "Si and 2 7 A l spectra were deconvoluted using the Bruker GLINFIT program ( r e f . 12). The best f i t s o f the 29Si and 2 7 A l spectra were obtained w i t h Gaussian and Lorentz l i n e form, respectively
.
RESULTS 29Si and 27Al MAS NMR spectra and t h e i r deconvolution are shown i n Figs.1-5
f o r t h e hydrated and acac-treated
Samples 1-5.
The 29Si
spectra encompass
S i ( O A l ) , S i ( l A l ) , Si(ZA1) and Si(3A1) l i n e s and a l i n e a t -110 ppm, denoted w i t h ' a ' . The Si(OA1) and Si(lA1) l i n e are o f t e n asymmetric and were f i t t e d w i t h a double l i n e s t r u c t u r e . The 2 9 S i spectra o f t h e acac-treated samples and o f the hydrated samples are very s i m i l a r . The 27Al spectra i n Figs. 1-5 o f Jwdrated Samples 1-5 e x h i b i t a l i n e T a t 60 ppm. I n t h e 4-0 ppm region a second l i n e , 0', i s observed. An a d d i t i o n a l l i n e , p o s i t i o n e d between -5- -15 ppm i s present i n t h e spectra o f Samples 2-5 and i s denoted a l s o w i t h 0'. A weak, sharp l i n e , 0, i s superimposed on the broad l i n e s 0' a t 0 ppm i n t h e 27Al spectrum o f Sample 4 (Fig. 4). Besides the 0' and T l i n e s , i n t h e 27Al MAS NMR spectrum o f Samples 1 and 2 a l i n e i s observed a t 50 ppm, denoted w i t h T' (Figs. 1 and 2). Samples 3 and 4 e x h i b i t an a d d i t i o n a l l i n e T" a t 30 ppm (Figs. 3 and 4). Sample 5 shows o n l y a T" l i n e , w h i l e T' i s absent (Fig. 5).
123
29si NMR
"Yd 2fd
3fl
I
.
.
d
.
1
.
*
.
.
-90
Acac
1Al
n
1
.
.
.
,
-100
1
.
.
.
.
-110
PPH
-90
-100 PPH
NMR
T T'
?'
-110
Acac
T T'
Fig. 1. Experimental (A) and deconvoluted (B) "Si and *'A1 MAS NMR spectra o f hydrated (Hyd) and acac-treated (Acac) Sample 1. Side bands o f l i n e T are i n d i c a t e d w i t h '*'.
724
~ Q SNMR I 2AI
3AI I
1Al
OAl
a
I
I
1
,
100
,
,
,
0'
1
,
,
,
,
PPH
2AI
I
I
1
,
0
I I
.
.
.
l
,
.
1Al
OAl
a
Acac T T'
h
I
50
3AI
2 7 NMR ~ ~
HYd T T'
Acac
0' 0"
,
-50
Fig. 2. Experimental (A) and deconvoluted (B) 29Si and 27Al MAS NMR spectra o f hydrated (Hyd) and acac-treated (Acac) Sample 2. Side bands o f line T and 0" are indicated with '*' and 'm', respectively.
725
29Si NMR 3f1
2f1
if1
0 4
Acac 3AI
a
I
I
100
T"
I
l
l
0'
PPM
I 1
-50
100
n
7
Acac T T' T"
h
8
58
i
I
2 7 NMR ~ ~ T T'
1Al OAl
2AI
50
1
?'$"
8 PPH
-50
Fig. 3. Experimental (A) and deconvoluted (B) 2 9 S i and 27Al MAS NMR spectra o f hydrated (Hyd) and acac-treated (Acac) Sample 3. Side bands o f l i n e T and 0" are i n d i c a t e d w i t h '*' and 'm', r e s p e c t i v e l y .
726
2 2T1
3Al I
I
l
T" l
Acac
1A1
OAla
3AI
2AI
1Al
OAl a
I
nl
I
I
I
ni
2 7 NMR ~ ~
HYd T T'
9 NMR ~ ~
0'0
Acac T T' T" I 1
I
Fig. 4 . Experimental (A) and deconvoluted [S) 29Si and 2 7 A l MAS NMR spectra o f hydrated (Hyd) and acac-treated (Acac) Sample 4 . Side bands of l i n e T and 0" are indicated with '*' and 'm', respectively.
121
2% NMR
HYd 2fl
1Al I
OAl I
Acac
a l
2AI
l
I
ltl
l
(
a I
G
A
I 0
-90
2
-100 PPH
7 NMR ~ ~
-110
Acac T I
1"
0 0" I r n
Y
Fig. 5. Experimental (A) and deconvoluted (B) *'Si and 27Al MAS NMR spectra o f hydrated (Hyd) and acac-treated (Acac) Sample 5. Side bands o f line T and 0" are indicated with and 'a', respectively. I*'
128
The 27Al spectra of the acac-treated samples show T signals for Samples 1 5, T' lines for Samples 1-4, T" lines for Samples 3-5, a signal 0' in the 4-0 ppm region for Samples 2-5, and for all samples three additional lines at -2, 7 and -12 ppm, denoted with 0" (Figs. 1-5). The 27Al MAS NMR spectra of Sample 1 and 2 did not change after a second treatment with acac. For Samples 4 and 5 the relative intensity of the 0" lines with respect to the other signals increased after a second acac treatment. Further acac treatments did not change the spectra. 2 7 NMR ~ ~ T I
do
T
0 ' I
I I
I
Fig. 6. 27Al MAS NMR spectra of Sample 4 with (A) and without (8) NHq-exchange.
2 g NMR ~ ~
2 7 NMR ~ ~ T' T"
0'
h
1
.
1
.
I
.
-100
PPM
'
"
'
-120
l . . . . . . . . . I . . . . . . . . . I . . . . . . . . . l . .
200
180
PPti
0
-100
Fig. 7. Experimental "Si and experimental (A) and deconvoluted (B) 27Al MAS NMR spectra of silica-alumina. The side bands of line T'are indicated with '*' and I * * ' .
729
2%1
,
1
NMR
1
10
1
1
I
,
I
5
#
6
I
I
I
0
I
1
1
1
l
-5 PPM
I
L
1
,
I
-10
*
,
I
I
I
-15
,
,
,
,
I
,
-20
Fig. 8. 27Al MAS NMR spectrum of Al(acac)j. The 27Al spectra of Sample 4 with and without NH4-exchange are compared in Fig.6. These NMR spectra were obtained using a long delay time in order to enhance the resolution of the broad 0' signals and the sharp line 0. Line 0 disappears after NH4-exchange. Silica-alumina exhibits 27Al signals at 50, 30, 0 and -15 ppm and a very broad 29Si line with maximum intensity at -108 ppm shown in Fig. 7. The 27Al spectrum of A l ( a ~ a c ) shown ~ in Fig. 8 is a spectrum with three maxima at - 2 , -7 and -12 ppm. DISCUSSION NMR-visible extra-framework aluminium soecies in hydrated fau.iasites The assignments of the 27Al line at 60 ppm to aluminium, tetrahedrally coordinated in the zeolite Y framework, and at 0 pprn to octahedral nonframework aluminium is generally accepted (ref. 1). In hydrothermally dealuminated faujasites additional 27Al lines in the range of 50-30 ppm have been observed by several authors. Gilson et al. found next to lines at 61 and 0 ppm a resonance at 34 ppm in zeolite Y with SiF/AlF=23, obtained by steaming at 1033 K in 100% steam (ref. 13). Samoson et al. observed 60, 50, 30 and 0 ppm lines in the 27Al spectrum of zeolite Y, dealuminated under 0.1 MPa of steam successively at 873 and 1123 K and with SiF/AlF=20 (ref. 11). Freude et al. (ref. 4,5) reported 60, 50 and 0 ppm 27Al signals in zeolite Y with SiF/AlF=26, obtained by hydrothermal dealumination at 1043 K. From these data it seems that 50 and/or 30 ppm 27Al lines are typical for highly dealuminated faujasites, but it is not clear which factors govern their appearence. The data of Figs. 1-5 shed more light on the conditions under which the species that give rise to 30 and 50 ppm signals are formed. Samples 1 and 2 exhibit a 50 ppm signal and no 30 ppm signal (Figs. 1-2). These zeolites are self-steamed and are weakly dealuminated. For Samples 3, 4 and 5, which were steamed under 0.1 MPa of steam, the intensity of the 50 ppm signal decreases while that of the 30 ppm signal increases with increasing steaming severity
730
(Figs. 3-5). The species which g i v e r i s e t o t h e 50 ppm s i g n a l seem t o be unstable under t h e steaming c o n d i t i o n s applied, w h i l e t h e ones responsible f o r the 30 ppm seem t o be formed under such conditions. Thus t h e 30 ppm l i n e has t o be assigned t o non-framework aluminium. Signals a t 50, 30 and 0 ppm were observed w i t h s i l i c a - a l u m i n a ( r e f . 13). Before steaming s i l i c a - a l u m i n a shows one s i n g l e signal a t about 50 ppm, assigned t o aluminium, t e t r a h e d r a l l y coordinated i n an amorphous s i l i c a m a t r i x , w h i l e a f t e r steaming a d d i t i o n a l s i g n a l s appear a t 0 and 30 ppm ( r e f . 13). I n the spectrum o f s i l i c a - a l u m i n a shown i n Fig. 7 t h e main s i g n a l s are a t 50 and 0 ppm and weaker l i n e s are positioned a t 30 and -15 ppm. Corma e t a l . ( r e f . 14) suggested t h a t upon dealumination o f z e o l i t e Y amorphous s i l i c a - a l u m i n a w i t h strong B r ~ n s t e da c i d i t y i s formed. It i s found now t h a t 2 7 A l s i g n a l s a t 50, 30, 0 and -15 ppm, observed w i t h s i l i c a - a l u m i n a (Fig. 7), appear i n t h e spectra o f
steamed z e o l i t e Y (Figs.
1-5).
I n view o f t h i s evidence,
we are tempted t o
the 50 ppm signal t o aluminium, t e t r a h e d r a l l y coordinated amorphous s i l i c a - a l u m i n a . The 50 ppm l i n e i s denoted as T' i n order
associate
in to
d i s t i n g u i s h i t from the framework T l i n e . Freude e t a l . a s c r i b e t h e 50 ppm l i n e t o t e t r a h e d r a l non-framework aluminium however,
i n a d i f f e r e n t environment,
v i z . AlOOH associated w i t h two framework oxygens ( r e f s . 4,5). Samoson e t a l . using LD-spectroscopy found t h a t t h e apparent l i n e p o s i t i o n o f t e t r a h e d r a l l y coordinated aluminium can vary between 70 and 30 ppm, due t o second-order quadrupole e f f e c t s ( r e f . 11). I n t h a t work 27Al l i n e s a t 30 and 50 ppm i n spectra o f dealuminated f a u j a s i t e were both assigned t o T'.
Gilson e t
a l . a t t r i b u t e d the 30 ppm l i n e t o penta-coordinated non-framework aluminium (ref.
13),
based on t h e l i n e p o s i t i o n o f
penta-coordinated
aluminium
in
andalusite. Our work provides evidence t h a t t h e species responsible f o r the 30 ppm signal are chemically d i f f e r e n t from those o f t h e T' l i n e as t h e i r r e l a t i v e i n t e n s i t y v a r i e s w i t h t h e s e v e r i t y o f t h e steaming. Although t h e n a t u r e of i t s coordination i s n o t c l e a r ,
t h e 30 ppm l i n e w i l l
be considered f u r t h e r as
t e t r a h e d r a l and denoted w i t h T" i n Figs. 1-5. Mobile octahedral A ~ ( H z O ) ~ ~complexes + l o c a t e d i n t h e z e o l i t e pores on c a t i o n i c s i t e s g i v e r i s e t o 0 ppm s i g n a l s ( r e f . 3). A narrow 0 ppm l i n e i s observed i n hydrated Sample 4, next t o broader l i n e s w i t h s l i g h t l y p o s i t i v e chemical s h i f t a t 4-0 ppm and s l i g h t l y negative s h i f t a t -15 ppm (Fig. 4). The 0 ppm l i n e disappears a f t e r NH4-exchange (Fig.6), w h i l e t h e broad l i n e s are n o t influenced. Therefore,
t h e narrow l i n e a t 0 ppm i s caused by exchangeable
aluminium c a t i o n s and t h e broader l i n e s by unexchangeable species. Due t o t h e i r d i f f e r e n t nature the 0 ppm l i n e w i l l be f u r t h e r denoted by 0, and t h e 4-0 and - 5 - -15 ppm l i n e s by 0',
r e s p e c t i v e l y . I n Samples 1-3 and 5 l i n e 0 i s n o t
observed. An assignment o f l i n e s 0' t o s p e c i f i c non-exchangeable species i s d i f f i c u l t . Several s t r u c t u r e s w i t h 6-coordinated A1 show 27Al NMR l i n e s i n t h e
731
15 t o -15 ppm range ( r e f . 15). Perhaps the broad l i n e s 0' are due t o polymeric aluminium species framework by
left
i n the
pores
a f t e r dealumination
steaming.
Support
t o this
assignment
d i f f r a c t i o n measurements o f Shannon e t a l .
is
of
the
provided
zeolite by
X-ray
16); i n steamed Y they
(ref.
observed octahedral A1 i n a boehmite topology. r a t i o from 2 9 S i MAS NMR sDectra
SiF/AIF
The S i F / A I F r a t i o o f a f a u j a s i t e can be c a l c u l a t e d from t h e 29Si spectrum using the i n t e n s i t y o f the i n d i v i d u a l Si(nA1) l i n e s ( r e f . 1). For dealuminated faujasites
a c o r r e c t determination o f t h e Si(nA1)
l i n e i n t e n s i t i e s can be
complicated due t o the presence o f signals o f non-framework s i l i c o n ( S i N F ) . I n spectra o f Sample 1-5 a signal appears a t -110 ppm (Figs. 1-5). This
t h e "Si
p o s i t i o n i s t y p i c a l f o r a Si(OA1) amorphous environment ( r e f . 17). As discussed i n previous section, amorphous s i l i c a - a l u m i n a i s a l s o present i n the dealuminated samples. The corresponding 29Si signals, which are expected t o be broad (Fig.
7), could not be resolved (Figs. 1-5). The amount o f amorphous
s i l i c a i n t h e samples ( S i N F / S i ) deconvoluted "Si the SiNF/Si
and t h e S i F / A I F
r a t i o c a l c u l a t e d from the
spectra o f Figs. 1-5 are given i n Table 1. The S i F / A I F
and
r a t i o s o f the d i f f e r e n t hydrated and acac-impregnated samples are,
w i t h i n experimental
error,
the same (Table
l), i n d i c a t i n g t h a t t h e acac-
treatment does not modify the z e o l i t e s . TABLE 1 S i N F / S i and AINF/Al r a t i o o f hydrated and acac-treated Sample 1-5
SiF/AIF, Sample No.
29si NMR
2 7 ~ 1NMR
S i F / A I F S i N F / S i AINF/Al(a)
AINF/Al(b)
(%I
(%I
(%I
~i NF/AI
A1 F/Al
T' TI1 0' 0 oll(e,f) (%) (%) (%) (%) (%) (%)
T(f) (%))
20
86 74
-
66 44
1 1 A(d)
3.49 3.47
8 6
29 28
35 33
1 4 7 26 6 -
2 H 2A
4.8 5.1
6 7
48 49
51 54
34 56
9
- 2 5
8
-
3 H 3A
6.3 6.0
17 16
61 58
67 65
42 60
5 4
4 3 3 3
-
4H 4A
7.4 7.7
20
66 68
72 73
52 68
8 16 27 6 13
1
19
10 7
81 85
83 86
47 81
-
-
5H 5A
13 17
19 28 13
7
-
-
48 53
58 40
-
48 38
-
49
68
53 19
a, from eqn. 1; b, from eqn. 3; c, hydrated; d, acac-treated; e, i n c l u d i n g 0'; f, i n c l u d i n g associated side bands; t h e average accuracy on t h e i n t e n s i t y o f the l i n e s i s about 10%.
13 2
AINFIAI ratio from 29si MAS NMR sDectra From the deconvoluted 29Si NMR spectra and the ratio of silicon to aluminium of the sample (Si/Al), AINF/Al (X) can be calculated from the following equation (ref. 7,ll):
For steamed faujasites for reasons explained above Si/AIF is different from SiF/AIF, derived from the 29Si spectrum. Indeed, Si=SiFtSiNF. Therefore, eqn. 1 can be written as:
or else:
The Si/A1 ratio of the starting material can be used to calculate AINF/A1 from eqn. 1 if no aluminium or silicon was leaving the sample during steaming and NH4-exchange. The data of Fig. 6 indicate that during the latter treatment exchangeable aluminium species, responsible for 27Al line 0, can be lost. Nevertheless, the intensity o f line 0 in Sample 4 is low and non-existing in Sample 1-3 and 5. A loss of silicon is highly improbable. For the calculation of AINF/Al of Sample 1-5, a Si/Al value of 2.49 was used, determined for NaY with "Si NMR and in excellent agreement with the value of 2.46 from chemical analysis. SiNF/SiF was the ratio o f the intensities of line a (-110 ppm) to that of the Si(nA1) lines. In Table 1, AINF/Al was calculated from eqn. 1 using SiF/AIF, and according to eqn. 3. It can be seen from Table 1 that the correction for non-framework silicon is significant. Visualization of NMR-invisible extra-framework aluminium soecies For hydrated Samples 1-5, AINF/A1 calculated as the 27Al (T'tT"tOtO')/ (TtT'tT"tOt0') line intensity ratio is given in Table 1. The values of AINF/A1 thus obtained are systematically lower than those obtained from the 29Si spectra (Table 1). This is the well-known problem of invisible aluminium in the 27Al spectra of dealuminated zeolites explained in the introduction and which cannot be solved even when optimized instrumental conditions (short, strong excitation pulse, high spinning and resonance frequency) are used (ref. 1). To detect the NMR-invisible aluminium and to quantify the total amount of octahedral extra-framework A1 in Samples 1-5 the acac method (ref. 7) was applied. The acac-treated Samples 1-5 exhibit 27Al signals at -2, -7 and -12 ppm. These three lines are also found in the spectrum of solid Al(acac)j under
733
MAS c o n d i t i o n (Fig.
8). The s p l i t t i n g o f t h e 0" s i g n a l i s caused by the
presence o f the quadrupolar 27Al nucleus i n an e l e c t r i c f i e l d g r a d i e n t w i t h a x i a l symmetry ( r e f . 18), corresponding t o t h e symmetry o f Al(acac)3.
From t h e
p o s i t i o n o f t h e maxima i n the 27Al NMR spectrum o f Al(acac)g a value o f 3 MHz was obtained f o r t h e quadrupole coupling constant, Cp. For acac-treated Samples 1-5, AINF/A1 c a l c u l a t e d as t h e 27Al (T'+T"tO'tO")/ l i n e i n t e n s i t y i s given i n Table 1. The AINF/A1 r a t i o thus obtained f o r Samples 1-5 i s i n r e l a t i v e l y good agreement w i t h t h e values (TtT'+T''tO'tO")
measured by "Si
NMR (Table 1). The (T'tT")/T
r a t i o w i l l g i v e an i n d i c a t i o n o f
the r e a c t i v i t y o f t h e T' and T" s i t e s towards acac. From Table 1 one can d e r i v e that
for
every
sample hydrated and
acac-treated
this
ratio
is
Therefore, t h e a d d i t i o n o f acac does n o t a f f e c t t h e population o f T' s i t e s and consequently complexes o n l y formerly NMR-invisible aluminium.
similar. and T"
One can speculate on the possible s t a t u s o f t h e NMR-invisible aluminium species, which should reside on s i t e s w i t h very strong quadrupole i n t e r a c t i o n (Cq>>3 MHz). I n t h e 27Al spectra o f the acac-treated samples l i n e 0' i s present (Figs.
1-5),
i n d i c a t i n g t h a t a t l e a s t p a r t o f t h e alumina polymers are not
converted i n t o A1 ( a ~ a c ) ~ As . a l l NMR-invisible aluminium i s complexed w i t h acac,
i t probably i s l e s s polymerised.
The NMR-invisible aluminium can be
assigned t o oligomeric alumina species i n the z e o l i t e pores. Complexation o f extra-framework aluminium w i t h acac proves t o be a r e l i a b l e and general method f o r the s e l e c t i v e q u a n t i f i c a t i o n o f NMR-invisible e x t r a framework aluminium. Statements t h a t q u a n t i f i c a t i o n o f non-framework aluminium on hydrated samples i s possible, as proposed e.g.
i n r e f . 11 and 19,
should be considered w i t h great caution. The procedure t o perform t h e complexation w i t h acac i s very c r i t i c a l . The impregnation should be performed a t 293 K. Acetylacetone can e x t r a c t aluminium from t h e z e o l i t e Y l a t t i c e ( r e f . Z O ) , c e r t a i n l y a t h i g h temperature. We experienced w i t h Sample 1 t h a t a t 333 K dealumination does indeed occur. Prolonged contact o f the sample w i t h acac s o l u t i o n can be another cause of dealumination ( r e f . 8). Ray e t a l . ( r e f . 10) proceeded by adding t h r e e drops of acac t o the sample i n the MAS r o t o r . According t o t h a t procedure f o r a USY z e o l i t e w i t h Si/A1=4.8 the f u l l i n t e n s i t y o f the 0" l i n e i s obtained o n l y a f t e r f i v e days. According t o our method, 24 h i s s u f f i c i e n t . Bosacek e t a l . ( r e f . 8) and Freude e t a l . ( r e f . 9) performed s t a t i c 27Al NMR experiments on z e o l i t e Y, soaked w i t h 38 v o l - % acac i n ethanol i n a sealed-off sample holder. A narrow 27Al signal from A l ( a ~ a c ) dissolved ~ i n ethanol was observed next t o signals
from s o l i d s t a t e aluminium species. I n t h a t procedure more acac than necessary t o complex the i n v i s i b l e aluminium i s l e f t
on the sample, leading probably t o a
slow a d d i t i o n a l dealumination o f the z e o l i t e framework ( r e f . 7).
734
ACKNOWLEDGMENT We acknowledge t h e support o f t h e Belgian National Fund f o r S c i e n t i f i c a Senior Research Associateship Research f o r a Research D i r e c t o r s h i p t o P.A.J., t o P.J.G. and a Research Associateship t o J.A.M. T h i s work has been sponsored by t h e Belgian Government i n the frame o f a concerted a c t i o n on c a t a l y s i s . We are very much indebted t o J.B. Uytterhoeven f o r h i s a c t i v e c o n t r i b u t i o n i n o b t a i n i n g funds f o r t h e NMR f a c i l i t y (IIKW p r o j e c t no 4.0002.84) and t o Synfina-Oleofina f o r f i n a n c i a l support.
REFERENCES
1 G. Engelhardt and D. Michel, High-Resolution S o l i d - s t a t e NMR o f S i l i c a t e s and Z e o l i t e s , John Wiley 8 Sons, Chichester, 1987. 2 W.O. Haag, R.M. Lago and P.B. Weisz, Nature (London) 309 (1984) 589. 3 J. Klinowski, C.A. Fyfe and G.C. Gobbi, J. Chem. SOC., Faraday Trans. 1, 81 (1985) 3003. 4 D. Freude, M. Hunger and H. P f e i f e r , Z. Phys. Chem. NF 152 (1987) 171. 5 D. Freude, E. Brunner, H. P f e i f e r , D. Prager, H.-G. Jerschkewitz, U. Lohse and 6. Oehlmann, Chem. Phys. L e t t . 139 (1987) 325. 6 E. Brunner, H. Ernst, D. Freude, M. Hunger and H. P f e i f e r , Stud. S u r f . Sci. Catal. 37 (1988) 155. 7 P.J. Grobet, H. Geerts, J.A. Martens and P.A. Jacobs, J. Chem. SOC., Chem. Commun. (1987) 1688. 8 V. Bosacek, 0. Freude, T. Frohlich, H. P f e i f e r and H. Schmiedel, J. C o l l o i d I n t e r f a c e Sci. 85 (1982) 502. 9 D. Freude, T. Frohlich, H. P f e i f e r and 6. Scheler, Z e o l i t e s 3 (1983) 171. 10 G.J. Ray, B.L. Meyers and C.L. Marshall, Z e o l i t e s 7 (1987) 307. 11 A. Samoson, E. Lippmaa, G. Engelhardt, U. Lohse and H.-G. Jerschkewitz, Chem. Phys. L e t t . 134 (1987) 589. 12 Bruker ABACUS Catalog n o ABA078. 13 J.-P. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, T.G. Roberie and M.P. Shatlock, J. Chem. SOC., Chem. Commun. (1987) 91. 14 A. Corma, V. Fornes, A. Martinez, F. Melo and 0. P a l l o t a , Stud. S u r f . Sci. Catal . 37 (1988) 495. 15 D. Muller, W. Gessner, H.-J. Behrens and G. Scheler, Chem. Phys. L e t t . 79 (1981) 59. 16 R.D. Shannon, K.H. Gardner, R.H. Staley, 6. Bergeret, P. G a l l e z o t and A. Auroux, J. Phys. Chem. 89 (1985) 4778. i7 R. Dupree, D. Holland and D.S. Williams, P h i l o s . Mag. 850 (1984) L13. 18 A. Kentgens, K. Scholle and W. Veeman, J. Phys. Chem. 87 (1983) 4357. 19 C. Fernandez, F. Lefebvre, J . B.Nagy and E.G. Derouane, Stud. Surf. Sci. Catal . 37 (1988) 223. 20 R. Beaumont and D. Barthomeuf, J. Catal 27 (1972) 45.
.
H.G.Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V.,Amsterdam - Printed in T h e Netherlands
CHARACTERIZATION OF CALCINED FAPO-5 S. Schubert', H.M. Ziethen', Martens3 and P.A. Jacobs3
A.X. Trautweini, F. Schmidt*, Auong-Xin Li3, J.A.
'Inst. f. Physik, Med. Universitllt, Ratzeburger Allee 160, D-2400 Labeck (FRG) 2SOd-Chemie AG, Katalyse-Labor, Waldheimer Strape 13, D-8206 BruckmUhl-Heufeld (FRG) 3Laboratorium voor Oppervlaktechemie, K.U. Leuven, 92, Kardinaal Mercierlaan, 8-3030 Leuven (Belgium) ABSTRACT FAPO-5 samples were synthesized according to a patent method and characterized by chemical analysis, differential thermal analysis, scanning and transmission electron microscopy, electron spin resonance, X-ray diffraction, MBssbauer spectroscopy and magnetic susceptibility measurements. The as-synthesized material contains about 70% of Fe(II1) and about 30% of Fe(I1) with structures similar to those of the Fe(II1)- and Fe(I1)-phosphates. Ki impurities are enriched in cracked sphere-shaped particles, whilst the Ki -free particles exhibit a hexagonal leaf-type morphology. Calcination at 973 K of the as-synthesized molecular sieve finally yields three different iron phases as identified by Mossbauer spectroscopy, 1.e. a-Fe2Or (25%), Fe(II1)-phosphate clusters (40%) which are probably occluded within the calcined FAPO-5, and Fe(II1) (35%) in a site with tetracoordination and high structural symmetry. According to X-ray diffraction studies the latter phase corresponds to AlPO4-tridymite, with Al(II1) being partially substituted by Fe(II1). INTRODUCTION The aluminophosphates are an important new class of molecular sieves (ref .1). The crystallization from the gel is (25OOC) in
the presence
of an
affected between
373 K (100OC) and 523 K
organic template under hydrothermal conditions.
For applications as catalysts or adsorbent
materials, the
resulting solid must
be calcined to remove both template and water. The crystal structure of
one member
of this family, designated AlPO-5, was
reported some years ago (ref.2). Four-coordinated aluminum and phosphorous oxide was found with occasional secondary coordination of some of the aluminum with non-framework species such as HzO or OH- occluded in the cavities. Iron-containing molecular-sieve FAPO-5 belongs
to a
class of metal-alumino-
phosphates denoted by the acronym MeAPO (ref.3). They can crystallize as a largepore variant with AlPO-5 structure, as an intermediate-pore system with AlPO-11 structure and as a small-pore sieve with AlPO-34 structure (ref.1). FAPO-5 was synthesized according to Messina et al. (ref.4). We already reported on the synthesis and characterization of a FAPO-5 sample (ref.5). Fe(I1) salt was used as a source for iron and TEAOH as a template. The synthesis gel
736
was doped by K'. Under the synthesis conditions approx. 66 wt.% of the iron is oxidized and substituted in the framework, and 34 wt.% remains in the divalent state. Whether Fe(1I) is partly or totally incorporated in the framework is not completely clear. Two types of morphologies were found: hexagonal leaf- and ball-shape particles. The latter crystals are ten times smaller by volume. TEA' is the main charge-compensating cation in the leaf-type morphology and K* in the small balls. The aim of this work was to study the thermal stability of FAPO-5 and to elucidate the behaviour of the iron-containing phases during calcination in air. EXPERIMENT Materials A FAPO-5 synthesis gel was prepared using the procedure described in example 3 of ref.4. Fe(I1) chloride tetrahydrate was from Janssen Chim., pseudo-boehmite from Vista, tetraethylammonium hydroxide (TEAOH) 40% in water (containing 1.25 wt% of K20 as impurity) and 85% phosphoric acid from Fluka. The gel with molar composition (TEAOH) : (FenO3)O.l : (Al203)O.g : (PzOs) : (Hz0)40
(1)
was crystallized at 423 K for 24 hours. A reference AlPO-5 material was synthesized in the same way in the absence of iron. Some of the material was adjusted for constant humidity and then used for measurements (FAPO-5/0). The remaining part was calcined at 673 (400), 773 (5001, 873 (600) and 973 K (700OC) over 24 h in air and then again adjusted to constant humidity. The latter samples are denoted as FAPO-51400, FAPO-5/500, FAPO-5/600 and FAPO-5/700, respectively. In order to characterize the structure of FAPO-510 and its calcined forms several spectroscopic techniques were applied. Methods The FAPO-5 material was characterized using differential thermal analysis (DTA) in an inert atmosphere (ref.61, electron spin resonance (ESR) (ref.71, X-ray diffraction (XRD) (ref.81, scanning and transmission electron microscopy (SBM, T E N , Mbssbauer spectroscopy (ref.9,10) and magnetic susceptibility measurements (ref.11). The chemical analysis of the FAPO-5 samples, after their dissolution in RC1, was made with a PE ICP 5,500 instrument. The unit cell dimensions of the samples were determined in the hexagonal system with 4/3 (h2+hk+ka)/a2 + 12/c2
-
4 sins 8/22
by a least-squares fit of hk1 reflections (ref -12). 8 is the and 3 the wavelength of the incident X-rays.
(2) diffraction angle
131
u .
~
~~~
273
773
T I K l
Fig. 1. DTA of FAPO-5 in air.
RESULTS AND DISCUSSION Differential thermal analysis Differential thermal gravimetry (DTG) in He has been reported and discussed in a previous paper (ref.5). In contrast to DTG in He and DTA of AlPO-5 (ref.13) the DTA measurements in air (Fig. 1) show an additional strong exothermic peak in the range between 673 K and 1043 K. The exothermic peak is related to a phase transition from the AlPO-5 structure to the AlPO4-tridymite structure. X-ray diffraction XRD measurements (Table 1) have been performed with FAPO-510, calcined at increasing temperatures. The pattern of FAPO-5/0 is comparable to that of FAPO-5 (ref .4). The pattern of FAPO-51400 exhibits only slight changes compared to that of FAPO-5/0. The changes of FAPO-5/500, FAPO-5/600 and FAPO-51700 are considerable. FAPO-51600 and FAPO-51700 are identical and represent the tridymite structure of bulk aluminophosphate. Electron microscopy SEW: measurements have been performed with FAPO-510, FAPO-5/400 and FAPO-5/700. The images of FAPO-5/0 show two morphological types, one hexagonal leaf-shape (which is similar to that of AlPO-5 (ref.14)) and one ball-shape with large pores (which is 3 times smaller in diameter than the hexagonal crystals (ref.5)). It is interesting to note that the crystal structure of both morphologies is the same, as evidenced from XRD. During calcination the two types of crystal shape remain. Fig. 2 shows images of FAPO-51700. However, as mentioned above, the XRD pattern, i.e. the crystal structure, changes on going from 673 K to 973 K.
738
TABLE 1 dbkl [nm] versus I/I.OX of FAPO-5/0,..,/700 and AlPO4-tridymite (ref.15). Reflexes are counted only when I/Imax ) 10%. FAPO-5/600 dbkl
I/Imax
[nml 1.173 0.676 0.587 0.444 0.419 0.393 0.339 0.305 0.295 0.264 0.258 0.239
a
drri [nml
:/Imar
llb
1.179 0.685 0.448 0.421 0.395 0.343 0.307 0.296 0.266 0.259 0.238
86b 27b 38’ 45b 100b 3gb 26b 25b llb 26b llb
29b 76b 61b lOOb 35b 23b 29b llb 25b 15b
I/hx
:/Imax
[nml
80b
dbkl
I FAPO-5/700 I
1.179 0.681 0.448 0.435 0.421 0.413 0.395 0.385 0.341 0.307 0.296 0.266 0.259 0.252 0.239
lOOb 26b 35b 22c 50b 18c 85b 13C 40b 21b 31b 10b 24b 1Oc llb
0.438
0.413 0.387 0.300 0.252 0.233 0.211
lOOC 76c 64C 21c 24c 11c 10c
0.435 0.413 0.385 0.300 0.252 0.233 0.211
lOOC 6gC 65c 22c 24c 1lC loc
Tridymite
I
dbkl I / I m a x [nml 0.431 0.413 0.386 0.328 0.320 0.300 0.255 0.233 0.211
lOOc lOOc 85c 30C
10c 3OC 50c 50c 5OC
Unit cell dimensions were calculated by a least-squares fit of the hkl reflections using eq.(2), which yields a=1.367 nm and c=0.845 nm. AlPO-5 phase. AlPOa-tridymite phase.
Fig. 2.
SEN images of FAPO-5/700 with hexagonal leaf-shape crystals (a) and ball-shape crystals (b).
TEM was applied to selected samples. Fig. 3 shows a TEN image of FAPO-5/0 showing the (100) lattice planes. The distance between these lattice planes amounts to 1.18 nm. The corresponding diffraction image of this area of the sample shows lattice spacings of 0.23 nm, 0.403 nm and 0.43 nm, respectively. These values are in agreement with the XRD data.
739
Fig. 3.
Mossbauer
TEH image of FAPO-5/0 showing the (100) lattice planes.
and
ESR
spectroscopy of
FAPO-5
samples
calcined
at
increasing
temperatures Hossbauer spectra have been recorded between 4.2 K and for FAPO-5/0
room temperature (RT)
calcined at different temperatures. The spectra have been analyzed
with a least-squares procedure using Lorentzian parameters are summarized in Table 2.
lines. The
resulting Mdssbauer
In the case of FAPO-5/0, Fe(II1) species contribute about 70% and Fe(I1) species about 30% to the measured Hossbauer _ (1) _ The as-sunthezised sample.
spectral area
(Table 2 ) .
The characterization
from the measured isomer shifts. Part
of these species is unambiguous
of Fe(II1) in FAPO-5/0
similar to
is identified as
that of
bulk Fe(1II)POd but with
Fe(II1) ions being magnetically diluted and
forming a
single-phase solid solu-
tion of
spectra of this sample at RT and at
framework iron with coordination (Al,Fe)P04. For
completeness, ESR
2.2 K are presented in Fig. 4a and 4b. The resonance at g=4.3 is due to tetracoordinated Fe(III)-ions in framework positions while that at g=2 is due to Fe(II1)-oxy or -hydroxy products occluded in the cavities. (11) Calcination at 673 K (400OC). After calcination in air of Fe(1I) species
are no
longer detectable
FAPO-5/0, the
in the Ndssbauer spectrum
at RT and
4.2 K (Fig. 5a and 5b). The RT spectrum does not exhibit magnetic broadening. The 4.2 K spectrum of FAPO-5/400 exhibits a broad and unresolved magnetic fea-
is interesting to note that about the same amount of iron was originally present as Fe(I1) in FAPO-5/0. The remaining 6511 of the absorption area corresponds to paramagnetic Fe(II1) ions, the same amount which was already present in FAPO-5/0. Thus from comparison of the Hossbauer spectra of the FAPO-5/0 and FAPO-5/400 samples, it is clear ture of about 35% of the total absorption area. It
740 that calcination at 673 K converts Fe(I1) into Fe(II1) species. This tentative picture is supported by ESR and XRD data: a) At 2.2 K and at RT the ESR spectra of FAPO-5/400 show additional Fe(II1) species compared to FAPO-5/0, and b) the RT XRD pattern of FAPO-5/400 exhibits essentially the same lines as FAPO- 510. (iii) Calcination at 773 K (500oC). The analysis of the Mdssbauer spectrum at 4.2 K (Fig. 6a and 6b) of FAPO-5/500 yields about 55% of the absorption area corresponding to paramagnetic Fe(II1) ions. The decreasing amount of paramagnetic Fe(II1) ions from 65% for FAPO-5/400 to 55% for FAPO-5/500 indicates that part of the FAPO-5 structure has collapsed to a new phase in FAPO-5/500 with iron ions being closer together and hence exhibiting cooperative magnetic properties at low temperature. The ESR spectra (Fig. 7a and 7b) recorded at RT and at 2.2 K, respectively, show that the g=4.3 signal which corresponds to framework Fe(II1) ions has decreased in intensity compared to the situation of FAPO-5/400. In addition, the XRD pattern shows that part of the FAPO-5 structure has been destroyed. (iv) Calcination at 873 K (600oC). The Mdssbauer spectrum of FAPO-5/600 at 4.2 K (Fig. 8a and 8b) exhibits a magnetic pattern with total absorption area of about 60% consisting of two subspectra. One is due to small a-Fe~O3 particles and the other to magnetically inhomogeneous Fe(II1) species whose hyperfine parameters are close to Fe(II1)-phosphate. Comparing the amount of the paramagnetic Fe (111) species of FAPO-5/400 (65%), FAPO-515OO (55%) and FAPO-51600 (40%),it is tempting to assume that FAPO-5 undergoes a structural phase transition with increasing calcination temperature. From the XRD pattern it is obvious that the original FAPO-5 structure was transformed into a tridymite-type structure when calcined at 873 K. The remaining 40% of the absorption area is due to a broad and a sharp single line, which will be explained below. (v) Calcination at 973 K (700oC). The RT and 4.2 K Mdssbauer spectra of FAPO-5/700 (Fig. 9a-9d) show that about 25% of the absorption area is due to a-Fe~03, characterized by a Morin transition (compare the quadrupole splittings of sub-spectra I11 in Fig. 9a-d). About 75% of the absorption area at RT corresponds to Fe(II1) species, which are further characterized as follows. Their microscopic iron-ligand structure is to a considerable extent homogeneous, as exemplified by the sharp quadrupole doublet at RT; however, this doublet is superimposed by a single line which becomes visible only below 77 K. The spectrum at 77 K (not shown) has a complex structure, as part of the Fe(II1) species exhibit hyperfine field distribution at this temperature.
741 g-values
3.0
1.0
2.0
% /--I , , v , , v l , , , , , l , , , l , , , ~ L I
.
1000
500
0
H [mTl ESR spectra lor FAPO-5/0 recorded at RT ( a ) and 2.2 K (b).
Fig. 4 .
1 .ooo
c
.-0
.-4 .995
.ooo .995
.990
.985
-10
0
10
Velocity [mm/sI Fig. 5.
Hdssbauer spectra for pAPO-5/400 recorded at RT ( a ) and 4.2 K (b).
742
Q,
.-2 1 . 0 0 0 t
a a, a d
.995
.905 *ggO*
.
10
0
-'I u
Velocity [mm/s]
Fig. 6. Mdssbauer spectra for FAPO-5/500 recorded at RT (a) and 4.2 K (b),
g-values 3.0
1.0
2.0
-
500
1000
H [mT] Fig. 7.
ESR spectra for FAPO-5/500 recorded at RT (a) and 2.2 K (b).
743
~1.000 IQ,
.-> .995 d
a,
LT
.990
10
-10
Velocity [mm/s]
Fig. 8.
Mdssbauer spectra for FAPO-5/600 recorded at RT (a) and 4.2 K (b).
The spectrum
at 4.2 K (Fig. 9b) consists of two resolved, magnetically split
subspectra (111) and (IV), a single line (11) and an unresolved broad absorption line (I). Undoubtedly subspectrum (111) can be attributed to a-FezOa (see above). Subspectrum (IV), because of its hyperfine parameters, is attributed to "bulk"
FePO4 (ref.16).
lost
likely
this
Fe(II1)-phosphate is not a separate
macroscopic crystalline phase but is occluded as FePO4 clusters within the calcined FAPO-5. This picture is supported by the SEH images of FAPO-5/700 (Fig. 2 ) , which do not show macroscopic already present
in FAPO-5/0.
crystalline
phases
except
antiferromagnetism (ref .16,17). Therefore they exhibit a specific intensities
of
for those
Both a-FezOt and the FePO4 clusters reveal canted
their magnetic
hyperfine
ratio of peak
pattern when exposed to an external
magnetic field, parallel and perpendicular to
the gamma-beam.
These ratios are
3:4:1 and 3:2:1, respectively (Fig. 9c,d and Table 2). The remaining 35% of the absorption area at 4.2 K (Fig. 9b) due to subspectra I and I1 yield a broad and a sharp
single line with identical isomer shifts (0.3 mm/s). We attribute these
two subspectra to a paramagnetic Fe(II1) magnetic relaxation
at this
species, which
exhibits intermediate
low temperature. From magnetic susceptibility mea-
surements between 7 K and 170 K using a Faraday balance it is obvious that this paramagnetic iron is in the ferric high-spin state (p=6.05 t0.35 Bohr magnetons) (ref.11). Under the applied field, parallel and perpendicular to the gamma-beam,
744
this species yields a six-line pattern with sharp lines and zero magnetic anisotropy. The latter follows from the specific ratio of peak intensities of 3:O:l and 3:4:1, respectively, in parallel and perpendicular field geometry (see subspectra I and I1 in Fig. 9c,d). This behaviour, together with the moderate internal field of 54 T, the relatively small isomer shift of 0.3 mm/s and the zero quadrupole splitting, suggests that Fe(II1) of this species occupies a site with high structural symmetry and tetracoordination by oxygen ligands. Therefore it most likely corresponds to framework iron of FAPO-5/700. This framework mainly represents the tridymite structure of Alp04 (ref.15). It is therefore reasonable to conclude that Fe(III1, corresponding to subspectra I and 11, substitutes Al(II1) in a formed phase of AlPO4-tridymite. The individual phases identified by N6ssbauer spectroscopy remain undetectable by XRD, because of the small iron content. CONCLUSIONS The PAPO-5 sample, synthezised in the presence of ferrous iron according to procedures in the literature, is thermally unstable. One should note that the organic template was contaminated by small amounts of K+. The role of the potassium is still not clear. The as-synthesized sample contains 70% Fe(II1) and 30% Fe(I1). While the theoretical degree of rubstitution of the iron in the AlPO-5 framework is 101, it can clearly be seen from H6ssbauer spectroscopy that only part of the Fe(II1) is present as framework iron. The remaining part is occluded as Fe(II1)- oxy- or hydroxy-species in the pores. At increasing calcination temperatures the Fe(I1) species are rapidly oxidized to Fe(II1). At the calcination temperature of 773 K the micropore-struct w e starts to collapse to an AlPOa-tridymite form. At 973 K the phase transition is completed. During the calcination process, the extra-framework iron sinters to small a-FenO:, particles not detectable by XRD. Part of the FAPO-5 framework iron goes through the phase transition and finally occupies an AlPOa-tridymite framework site. The remaining part is identified as small FePO4 clusters occluded within the calcined material.
745
1.00
c 0 .-
.98
ln
.-v) E
4
vl c
a
.
1.000 W
.-> +
d
.995
d
W
fx
.990 .985
0 Velocity [mm/sl
-10
10
Fig. 9a,b. Hdssbauer spectra for FAPO-5/700 recorded at RT (a) and 4.2 K (b).
I
-10
Fig. 9c,d.
1
1
1
1
1
1
1
4
1~
0 Velocity [m/sl
1
1
10
Hbssbauer spectra for FAPO-5/700 recorded at 4.2 K and U.~t=6.21 I X-beam (c) and 4.2 K and 8.~;=6.21 l’d-beam (d).
T
TABLE 2 Isomer shift (I.S.), quadrupole splitting (Q.S.), line width (L.W.), internal magnetic field ( H i n t ) and relative absorption area (A) at various temperatures for FAPO-5/0 samples. Isomer shifts are given relative to a-Fe at RT. The numbering of the subspectra is the same as in the figures. Standard deviations are of the order of 5%.
-
I.S. Q.S. L.W. Fig. Subsp. [mm/sl [mm/sl Cmm/sl APO-5/0 (a) RT
APO-51400
APO-5/500
I" I
0.55
0.5 1.23 0.5
0.87 2.76 -0.25
0.49 0.47 0.52
0.3
-
74.3 12.6 13.1
-
5i.2
1
54.4 33.8 11.9
I
0.37
1.00
0.88
4.2
5b
I I1 I11 IV
0.41 0.38 0.39 0.39
0.97 0.39 0.07 -0.11
0.97 0.58 1.16 1.16
I
0.34
1.05
0.74
I I1 I11 IV
0.41 0.38 0.39 0.39
0.97 0.39 0.07 -0.11
0.97 0.58 1.16 1.16
8a
I I11
0.36 0.39
0.74 0.99
0.49 4.51
8b
I I1 I11 IV
0.38 0.38 0.46 0.49
0.0 0.0 0.08 0.12
5.3 0.7 0.51 0.65
52.7 49.0
I I11
0.37 0.36
0.76 -0.22
0.38 0.3
51.6
I I1 I11 IV
0.31 0.33 0.46 0.47
0.0 0.0 0.04 0.26
5.31 0.52 0.45 0.45
53.1 49.0
26.4 10.2 23.1 40.3
I+II I11 IV
0.37 0.5 0.5
-0.03 -0.04 0.27
0.38 0.42 0.7
54.4 53.6 47.8
34.9 25.2 39.9
3:0:1 3:4:1 3:4:1
I+II I11 IV
0.35 0.43 0.53
-0.04 0.00 0.29
0.33 0.43 0.92
54.1 53.6 47.4
34.6 25.4 40.1
3:4:1 3:2:1 3:2:1
6a RT
RT 4.2
'APO-51700 RT
6b
9a
4.2
9b
(C)
0.3
5a
-
(b)
0.73 2.14 2.97
RT
--
4.2
'APO-51600
0.4 0.98 1.2
Ratio of peak intensity (d)
4.2
9c
i;;
9d
,
~
~
43.0 50.8
13.0 22.1
-
42.6 11.6 24.6 21.2
-
43.0 50.8
-
-
-
-
I
65.1 34.9 24.5 18.6 16.8 40.2 75.4 24.6
~~
~
(a) Taken from (ref.5). (b) External magnetic field 6.21 T, parallel to gamma-beam. (c) External magnetic field 6.21 T, perpendicular to gamma-beam. (d) Counted from outer to inner lines. Line intensity ratio for least-squares fit of experimental spectrum.
747
ACKNOWLEDGEMENTS HXL acknowledges the KU Leuven for a Ph.D. grant. JAM and PAJ are grateful to NFWO-FNRS for a Senior Research Associate Fellowship and a position as Research Director, respectively. PAJ also acknowledges the Belgian Government for financial support in the frame of a concerted action. AXT and SS appreciate financial support from the Deutsche Forschungsgemeinschaft. We are grateful to Kontron GmbH, Munich, and to Philips, Eindhoven, for carrying out some of the SEM and TEM measurements.
REFERENCES 1
2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17
E.M.Flanigen, B.M.T.K.Lok, R.L.Patton and S.T.Wilson, Pure and Appl. Chem. 58 (1986) 1351. J.M. Bennett, J.P. Cohen, E.H. Flanigen, J.J. Pluth and J.V. Smith, ACS Symp. Ser. 218 (1983) 109. E.M. Flanigen, B.M.T.K. Lok, R.L. Patton and S.T. Wilson, Stud. Surf. Sci. Catal. 28 (1986) 103. C.A. Messina, B.M.T.K. Lok and E.M. Flanigen, EPA 131, 946 (19841, assigned to Union Carbide Corp. Hong-Xin Li, J.A.Martens, P.A.Jacobs, S.Schubert, F.Schmidt, H.M.Ziethen and A.X.Trautwein, Stud. Surf.Sci.Catal.37 (1988) 75. J. Perez-Pariente, J.A. Martens and P.A.Jacobs, Appl. Catal. 31 (1987) 35. L.R.M. Martens, P.J. Grobet, W.J.M. Vermeiren and P.A. Jacobs, Stud. Surf. Sci. Catal. 28 (1986) 935. J.A. Martens, M. Mertens, P.J. Grobet and P.A. Jacobs, Stud.Surf. Sci. Catal. 37 (1988) 97. N.N. Greenwood and T.C. Gibb, "MBssbauer Spectroscopy", Chapman & Hall, London 116 (1971) 134. G.J. Long, J.G. Stevens, "Industrial Application of the Mdssbauer Effect", Plenum Press, New York (1986). S. Schubert, Diplom-Arbeit, Universit(Lt Hamburg (1987). W.J. Mortier, unpublished results. X. Qinhua, D. Jialu, Y. Aizhen and J. Changtaiin "Proceedings of the Int. Symp. on Zeolite Catal.", Siofok, Hungary (1985) 99. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, Intrazeo. Chem., ACS Symp.Ser. 218 (1983) 79. Inorganic Index to the Powder Diffration Pile, ASTM (1972). V.Beckmann, Y. Bruckner, W. Fuchs, G. Ritter, H. Yegener, Phys. Stat. Sol. 29 (1968) 781. R.L. Nininger, D. Schroer, J. Phys. Chem. Solids, 39 (1978) 137.
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H.G. Karge, J. Weitkamp (Editors ), Zeolites as Catal.ysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Y. F. chul, C. F. Keweshanl and E. F. Vansant2 lEngeuard Corporation, Menlo Park CN-28, Edison, N.J. 08818, USA 2university of ~ntwerp(UIA) , Deparbnent of hemi is try, Universiteitsplein 1, B-2610 Wilrijk, Belgium
AEnRACT A simple methcd to control the inlet pore dimter of H-zeolite (HZ) by chemical vapor deposition (CVD) is described. ?he technique involves batch contacting of 'IMOS with Hz through a manifold system with sanple weight change s controlled reduction i gravimetrically monitored. The degree of pore by repeated deposition and calcining cycles; to reduce unwantd polymerization ard irwxease the efficiency of 1M)6 deposition. High acidity and law deposition temperature favor 1M)6 deposition, but at l m tenprature physical adsorption of 1M)6 occuzs which could lead to near polymerization. For ZSM-5, both tennhal silanol gmups (with IR f 3740 m-l) and aluminum-related framework hydrcocyl g m q s (at 3 6 l O X o n the zeolite external surface play i m p r t a n t mles in the IM3s deposition. Mild Steaming of silicalite generates mre terminal silanol groups at 3740 an-l and contributes to TMOS deposition. HF treatment generates mre internal hydroxyl grcprps which are inaccessible for 1M3s reaction.
rtmammIcN control of the pore-opening size of zeolites can be an important capability in performing shap-selective catalytic reactions. prnper reduction of the Opening size of the zeolite pore channels auld increase the shape selectivity of catalysts without adversely affecting inherent activity. Wre size reduction agents, such as auld be accamplished by coating the zeolite with mdifyh-c~ P IDS (refs. 1-2) and methyl-chlorosilanes (ref. 3) which are wlecules larger than the zeolite pore-opning size, so that only the zeolite externdl surface reacts. "he reactions are believed to h l v e ard the zeolite external silanol groups (ref. 2):
In addition, polymerization reactions of the organosilanes on the zeolite surface could also take place (ref. 4). Calcining h air at 538% m e s the
750
organics resulting i n a thin layer of Si% and regenerates silanol grcups for further readion. Modifying agents containiq S i are nonwdly used since the silica formed is relatively inert u d e r most m c t i o n conditions. %modified catalyst could be used for mny reactions that requke shape selectivity ard high activity. Examples have been given of the selective production of p dialkylbmzene (i.e. xylene ard diethylbenzene) and the selective cracking of paraffins, etc. over both modified mordenite ard ZSM-5 (refs. 1,5). lb CVD technique is normally used to uniformly coat silica since monolayers of can be added stepwise. ?he method involves a m e n t i o n a l adsorption system which incorporates a quartz-spring balance to monitor the in-situ weight gain. small Samples of 1 Fpn or less are usually contacted ard vacum of about 0.13 to 0.67 Pa is used for sample degassing. 'Ihe pqmse of this paper is t o describe a s*le technique which allows effective preparation of a larger sample, to be used for both catalyst characterization anl catalytic studies. It u t i l i z e s an inexpasive amunexcially available Airless-ware manifold system for both CVD ard probe molecule uptake to detennhe the extent of pore s i z e reduction. Moreover, since 'IM3s deposition is affected by temperature of deposition ard zeolite acidity, experimmts were also undertaken to study their roles and ways t o increase D ' m deposition rate. DcPEmmm
?he apparatus used for this study cansists of a ccrranercially available
Airless-ware manifold obtained fran Kontes, for the contact of "IMX w i t h 5-10 gm of z e o l i t e in stand2ud 100 m l flasks (see Fig. 1).
-F.:' Drier
Bubbler
Fig.1 Shawing the setup for CVD of "IMX on zeolite sanples usmanifold system.
a swle
In contrast t o the in-situ quartz-sprhq balance system used i n the cawentional adsorption systern to d t o r the sanple weight m e , the flasks in the present system r~.ereeasily remDved frap Me d f o l d f o r weighing on an analyti d balance. me use of t h i s sinple systen enabled u s to deterrmne ' ,inavery short t i m e , the pmper conditions mquimd for rpauCing the zeolite pore-
751
opening sizes. Abut 5-10 qm of the acid form of a zeolite was introduoed i n t o a t a r e d d r y flask which was then placed in a heatmantle. &atwas applied while the flask was open to the atmqhere to drive off most of the water. when evidence of Steaming had ceased, the flask was closed with the teflon valve assembly and installed on the manifold. Degassing was carried cut for 1-2 hr a t 320% and pa. The flask was then oooled, anl dry N w e n was
achnitted after cbtained. ?he flask assanbly was re-conneded w i t h the manifold and re-evamated a t 320%. ?he system was then isolated f m 12-13.3
which the dry sample weirplt was
the pmp, and the previously evacuated flask containing the was apened t o the manifold. The i n i t i a l pressure in the manifold, 2 . w 0 . 2 7 kpa, rose to abwt 3 . e 0 . 2 7 kpa over a 20 to 40 minute period. Since the reactions cited above generate methanol which can further react to form methyl ether, water and hydrocarbons, the pressure rise is expe&ed. ?he sample was then evacuated, cooled and reweighed. Following this, intermittent a i r calchation of the sample a t 538OC was n o m l l y carried cut before another deposition. This
cycling contact procedure was repeated until the desired weight gain w a s
dstained. After CVD, the zeolite n o m l l y changed frun an o f f a t e to a b m color (due to carkon deposition) as the TMX was deposited. Haever, after a i r m . 'Ihe resulting S ~ H Zwas calcination, it returned t o its original a then subjected t o probe-moleculeadsorption analysis us* toluene, n-hexane and water. The same experimental technique as the CVD was used for the probe adsorption study. Hawever, the pressure over the SiHZ a t roan temperature was kept below the pmbe vapor pressure, usually 2.420.27 kF5. After the probe samples w e r e evacuated and subjected to a second CVD of 'IMOS analysis, the S% and then calcined and prabe analyzed. Further poreapening reduction occurred as monitored by probe molecule adsorption. Table 1 lists the dimensions of the chemical species involved in the study.
TABLE 1 kpa
Species
H-Mordenite H-234-5 'IMOS
H20
n-Hexane Toluene
Critical Dimension (mi) Vapor Press.@ 25% 0.67 0.54 0.89 0.32 0.49 0.67
x x
0.70 0.56
ca 1.07 3.20 17.7 3.60
m e zeolites used i n this study included mordenite and zm-5.
camercial
mrdenite samples with silica/alumina mlar ratio of -20 we.re obtained f m the m e g e l h a r d
Corporation and used for developnent of the new methcd since
752
mrdenite has higher activity for IM)6 deposition than ZSM-5. H-ZSM-5 (silica/almina mlar ratio -70) was Kepared according to Ref. 6 and prekeate-3 according to pblished pmcedwes (ref. 7). H-silicalite (i.e. ZSM5 with a silica/alumina ratio of abart 440) was cbtained f m Union m i d e . 'IMS deposition on these samples was carried cut in a conventional adsorption unit since only mall samples were available for sbxly. "he infrared (IR) spectra of various samples were cbtained via F"IR-PE (Fhoto Accustic SpectroscoW) or DFUFT (Diffuse Reflectance I n f h Fcurier Transform) techniques. Note that all IR Spectra of the sanples with IM)6 deposition were obtained after the samples were evacuated at xxxn tenperature to remwe htqxetation of the difference spectra Fhysically adsorbed m. -re, cbtained for silicalite must be carefully considered since the siqnals are relatively weak and could be misls&irq. RIisJLm
Effectiveness of the atmaratus The inlet size of the mrdenite porediameter is controlled by the m t of Si% deposited on the surface. Tb detennine if the zeolite pore q e n h g d d be modified by just coating the right amcunt of Si% on the zeolite surface without intennittent air calchtion, two graups of sanples (i.e. A & B vs. C ) w e r e prepared and studied. Sample A (4.0 gm) and sample B (10.1 gm) were allawed to accumulate SiO2 weight by cycling betwen TbC6 deposition and evacuation without frequent calcinations, but sample C (5.8 gm) was calcined mre frequently between depositions. ?he d a t i v e TbC6 deposition t h used were 120 rnir~utesfor A (i.e. 6x20 min) and 266 m i n u t e s for B (i.e. 1lx20-40 &), but the contact time for c was limited to two 10 m i n u t e periods. Fig. 2 shows the deposition rate by percent wight gain of 'IMS versus time for these three samples. It is cbserved that the initial rate of deposition for in spite of the fact that sample B the A & B samples is identical (2.9 %-), is 2.5 tinis larger. ?he significantly higher rate of C (9.9 %/hour) canpared to A & B indicates that the deposition of TMX on the surface of HM was very fast initially and that additiondl contact time pmhbly only mtribute-3to polymerization, as will be discussed further below. After a c h i n g , sample A was left with 3.6% added Si% and sample B with 8.1%. Both sanples were then subjected to a further CVD, ea& with a total deposition time of 130 minutes. The deposition rate for the second (ND on both samples was markedly 1and the respective rates differed significantly f m n each other: the rate of A' was 1.29 %/hour while that of B' was 0.40 %/hcur. Calcining of these samples resulted in a total of 5.3% added Si% on A' and 8.6% on B'. Calcinirq C resulted in 2.0% SiO2 added to the surface. C was subjected to two additional depositions (at deposition rates of 6.9 %/hour for C' and 9.7 %/hourfor C"
753
respectively) of 10 minutes duration with each aeposition follawed by calcining. It was abserveed that the CVD rates f o r C'(2.7% acWd Si%) and C" (3.7% added S i q ) do not d i f f e r significantly f m that f o r C. ?his is in
sharp Contrast to the differmce in rates cbserved between A-A' and B-B', indicating again that a different mecharu'm (i.e. polywrization) had prcbably ooavred d u r k the longer contact: pericds of A & B.
A. SJU SILICA
C*C.C"
I
A'
C A
8'. 8.8s SILICA 0
CONT*CI
nYE UN.
coWT*cI TIME. YlN
Fig. 2 showing that TMOS deposition rates are Fig. 3 a w i n g the effect of silica coating on
affected by intermediate calcination rtepa. Samples C-C" have the highest rate.
Bilicamodified mordenite samples. Sample A' k, better coated than sample 9.
The s i z e of the inlet diameter of the pore is monitored by the sample's
a b i l i t y to exclude p r o b m o l d e s of a specific critical dimension. S i m e the s i z e of the toluene molecule is near the s i z e of the mrdenite pore, it wnuld be the most sensitive of the probes used to indicate initial pore reduction.
Fig. 3 i l l u s t r a t e s the effect of pore restriction thruqh the exclusion of toluene for A - A'& B - B' SiHM samples. It is evident that the twu depositions of A' (i.e. 5.3% S i q ) are more effective than the single deposition sample of B (i.e. 8.1% Si02). As discussed above, it is possible that polymerization occurred t o a greater eXterrt during the CVD of B in whi&
case the
sample w a s exposed for a longer mntact time thereby prcduciq more nonselective deposition. ?his is further evidenced by the data for the C sample sham in Table 2, where the shorter deposition time was u t i l i z e d in conjunction
w i t h more frequent calcining. RLis resulted i n a much more efficient S i q coating than either A o r B samples. ?herefore, repeated depositions of IM)6 w i t h short contact times and frequent intermittent calcination between each depasition is fllggested for controllhg the inlet pore diameter efficiently. To determine i f the adsorption capacity is mintained after the deposition,
examhhg a l l of the prcbes applied to a sample, one can infer the approximate s i z e of the pore s i z e apenirrgs. Table 2 surmuaH20 is selected as a probe. By
754
H-M~dedte 3.6%SiHM (A) 5.3%Sm (A')
0 1/ 120 2/ 130
9.2 8.1 3.0
6.4
-
13.6
3.0
10.4
1/ 266
10.8
0.5-0.6 C0.5
13.4 12.0 10.0
0.5-0.6 0.5-0.6 C0.5
O/
8.1%SW (B) 8.6%SiHM (B')
2/ 130
5.2 0.5
5.7 1.1
2.0%SiHM ( C ) 2.7%SiI-lM (C') 3.7%SiHM (C")
1/ 20 2/ 10 3/ 10
6.1 3.0 0.7
4.1 0.8
d
-
0.67 -0.65 0.5-0.6
rizes the key observations. Note that while the adsorption of toluene and nhewne is significantly reduced for a l l sarrples, the adsorption of H S is not greatly affected. It is concluded that even though the inlet pore s i z e of the m o d m i t e was reduced to <0.5m a t least 80% of the internal capacity maintained.
Ihe nature of 'IMX dewsition
Ixlring the
c(xuse
of the study, it was observed t h a t the rate of
depo-
sition semd to be deperdent on zeolite acidity. nJ0 -1of ZSM-5 w i t h different silica/alunina molar ratios (i.e. 70 vs. 440) werx, therefore, studied a t r w m temperature and 105%. The acidity of the sanple was rnoniton& by the intensity of the I R signals of the Brcplsted sites of -5 near 3610 a-1(ref. 8). The lawer silica/alumh ZSM-5 has higher acidity, as indicated by the strunger IR signal i n that region. Ikwwer, both sarrples seem to have similar Concentrations of teminal silanol grrups the IR 3740 m-l fnquency region (see Fig. 4), even thcugh the nystal s i z e of silicalite is abmt 4 t i m e s that of the ZSM-5 sanple (i.e. 2 vs. 0.5 um). N o t e also that prior to deposition, a l l the samples were deh-td wemight a t 450%. The w t s show that both lm temperature and higher acidity favor the TICS uptake rates (see Fig. 5). Morearer, silicalite (i.e. low acidity -5) has a very small TIC6 adsorption capacity a t 105%. f i l . Effect of termerature. Temperature plays an important role in TICS deposition. The higher 'IMas adsorption of -5 a t man tenperature relative t o that a t 105% could be attrilmtd to @ysical adsorptim, s h abaxt 70% of the adsorbed species could be removed by 1 hr evacuation a t the me tenperature. In contrast, the lK6 chanisorbed a t 105% cuild mt be removed s a -
755
w
0
z a
m
a
0 v)
n
4
x:, 'jcm-;lc
AFTER TMOS DEPOSITION PARENT
3740 c m
DIFFERENCE
1 I
W
4c
0
z
a m a
53P
a
p '
3 7 4 0 cm
AFTER TMOS DEPOSITION PARENT
-'
3610 cm - 1
3700
3600
DIFFERENCE
3700 HUVENUMBERS. cm
-
3000
-'
the FTIR-PAS spectra for the effect of 'MB & p c s i t h l Q1 as (a) ht at 105%. (c) 440/1 (a) 70/1 Sioz/A1203 ZSM-5 at 25%. (b) Sio2/A1203 294-5 at 25%. (a) same as (c) ht at 105%. Fig. 4 s h & q
156 40
%
30
G A I N
25
,
0
I
I-
[ ~
70 Z S M - I . 2 6 %
-*- 70 Z S M - I , 106.C
-s- M O z ~ r - 5 . IOS'C
'
Y
SAFTEREVAC.
2ok
0
+AFTEREVAC.
15
0
S AFTER EVAC.
x
0 AFTER EVAC.
1
5
S 0
2
4
6
JWZ,MINO.?
8
10
Fig. 5 &wing the effects of tenperatme and zeolite acidity on adsorption on 234-5 at 25 and 105%.
12
lMls
larly. m e fraction of m which was adsorbed at roan temperature and remained after evacuation is still higher than that obtained by depostion at 105% (see Fig. 5 campare X and * points). The extra 'IM3s seem to be a result of lK6 adsorption at the 3610 an-l framemrk hydroxyl groups. The adsorption is slightly stronger than physical adsorption h t is relatively weak and the a d s o m species d d be removed by heating up to 105*. ?his can be seen by amparing Figs. 4a and 4b which shw that for the adsorption at rocm temperature the intensity of the 3610 an-l signal was reduced but that for the 105% sample was not affeded. For silicalite the effect is less pronaunced due to its l w acidity. jii). Effect of zeolite acidity. 'Ihe higher adsorption of TPSX on higheracidity ZSM-5 is -; for if only k m h a l silanol groups (i.e. 3740 m-l) are important for lK6 deposition then both ZSM-5 and silicalite samples, which seem to have similar amxlTlt of the 3740 m - l groups, shculd have the same nc6 activity. A close examination of the FTIR-PAS difference spectra of the parent an3 the samples after deposition (see for exanple, Figs. 4b and 4d) reveals that there is a greater reduction of the 3740 c W 1 sigml of the higher acidity ZSM-5, inaicatimg that it has mre terminal silanol ~lmupsnxcted during the TPDS deposition. !the nedmusn ' is not clearly understood, h t it is possible that the framework hydruxyl gmups (i.e. 3610 m-l, even thcqh few in number on the external surface) play an important mle in the readions of with the terminal silanol grarps. Thenfore, the cbervation here only partially agrees with the observation of Niwa and m r e T s (refs. 1-2) that external silanol groups existing on the zeolite surface are primarily responsible for the TPDS deposition.
157
Activation of hydmxy1srouDs for 'DES deDosition Since it is determind that zeolite external 1 h grarps are responsible for the lMX deposition on mrdenite and ZSM-5, it is desirable to explore ways to activate these groups and their effects on !lMX deposition. Mild Steaming has been known to be effective for incxeasing the acidity of ZSM-5 (ref. 9). The 2234-5 sample, with a Sioz/A1203 ratio of 440, was mildly steamed at 260% for 2 hours and studied using a DRIFT technique. An examination of the IR Spectrum shm only a slight increase in the intensity of the signal at 3740 can-l (see Fig. '3);and a small hmxse !lMX uptake rate was observed. In addition, a fresh ZSM-5(440) sanple was also mildly treated with dilute HF solutions to generate additional hydruxyl grarps. Interestingly, instead of the signal at 3740cm-l, the signal of silicalite at 3720 an-l was enhanced, and that signal seems to increase with increased severity of HF treatment (see Fig. 6b). Unfortunately, there was also no enhancement in 'DES uptake rate, h signal belongs to an confirming the assumption that the 3720 an-l 1 internal hydroxyl grcep, which &ts in high-silica ZSM-5 and is inaozessible for lMX reaction. The assignment of this hydrmcyl signal is the subject of a separate paper.
MILDLY STEAMED
DILUTE HF TREATED
Fig. 6 &awing the GRIFT spedra for the effects of mild s t e a n h g and dilute HF treatment an the generation of 1gmups on silicalite ( m - 5 , Sio2/A120+40)
3100
3600
3600
3700
WAVE N u M B E R S, c rn-
'
DIS
5. I. Wang and C.L. Ay, m-oceeding of the 8* International Oxqress on mtalysis, Calgary, canada, 1988, Vol 1, 324-331. 6. R.J. Aryaur and G.R. Landolt, US P a m 3,702,886 (1972). 7. D.H. Olson, W.O. Haag and R.M. lago, J. Qtdl. 61 (1980) 390-396. 8. J.C. vedrine, A. ~uroux,v. Bolis, P. Dejaifve, C . Naccache, P. Wierzchmki, E.G. Derouane, J.B. Naqy, J-P. Gilson, J.H.C. Van Hooff, J.P. van den E e q and J. Wolthuizen, J. C a m . 59 (1979) 248-262. 9. Y.F. Chu and A.W. olester, U.S. Patent 429,176.
H.G.Karge, J. Weitkamp (Editors), Zeolites us Catalysts, Sorbents and Detergent BuiMers 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A NEW STRUCTURAL-MODIFICATION T E C H N I Q U E F O R ZEOLITES: CHEM I S O R P T I O N OF S i t HI
Yongan Yan,J.Verbiest,J.Philippaerts,E.F.Vansant University of Antwerp tU.1.A.
and P.De
Hulsters
Department of Chemistry,
),
Laboratory of Inorganic Chemistry, 8-2610 wilrijk, Belgium.
Chemisorption causes
two
homogeneous
disilane o n H-mordenite small of
structural
intracrystalline
modification.
applied
reaction
process
causes
of
show
treatment.
second
capacity a
high
or
acidity.
resistance
to
on
the
without
The
obtained
a
steaming
The properties of t h e s e modified z e o l i t e s h a v e
evaluated b y c h e m i s o r p t i o n kinetics,
been
external
modification
a n efficient pore s i z e reduction
sorption
substrates
pore entrance and
The
1)
additional
T h e obtained t y p e depends
conditions.
port
modification:
Implantation of
and/or 2 ) preferential
groups surface
loss
of
types
gas a d s o r p t i o n
experiments and F T I R - P A S spectroscopy.
INTRODUCTION
The the
methods by which
additional g r o u p s a r e chemisorbed into
zeolitic framework a r e well established techniques for
tuning of gas separators, catalysts
shape-selective
T h e s e modification techniques are based o n
I 1 31.
i m p antat ion
internal
gas encapsulators and
of electrophilic groups
which
causes
stable narrow ng of the pore dimensions and a distinct c h a n g e elect.rical During
f
reaction reactivity
rate or
grad ent
eld
chemical
fine
within the zeolite
modif cations of can
be
controlled
b y slow diffusion of
the channe I s. Therefore
micro-porous or
limited
channel
an a in
system.
substrates
the
either
low
the modifying agent
by
through
b y choosing suitable modifying a g e n t s of
different molecular sizes and reactivities, or by controlling t h e modification conditions,
the desired modified s u b s t r a t e s c a n
obtained. T h i s can result in external
be
s u r f a c e or intracrystalline
modification.
In
this
work
we have studied t h e
mechanisms
chemisorption on small port H-mordenite.
is
investigated
of
disilane
T h e resulting
porosity
by a d s o r p t i o n experiments using
with different molecular s i z e s and shapes. are
characterised
by Fourier T r a n s f o r m
Spectroscopy (FTIR-PAS).
controlled
gases
T h e modified z e o l i t e s Infrared
We have demonstrated
suitable modifying agent for H-mordenite. be
various
Photoacoustic
that d i s i l a n e is a
the chemisorption
by either the reactivity or a d i f f u s i o n
can
process,
and the final products, a s anticipated, show different structural properties.
EXPERIMENT
The
synthetic
Chemique
Na-mordenite
SP,
supplied
de la G r a n d e P a r o i s s e ( F r a n c e ) ,
NH.-form
by a conventional
content
of the zeolite was 2.36 mmole/g.
La
SociCtC
ion-exchange procedure.
The
ammonium
A thermal treatment of
at 7 3 3 K in vacuum resulted in t h e f o r m a t i o n of
NH,-mordenite H-form
by
w a s converted into t h e
zeolite
0.221 c m s / g ,
(deam.).
a s measured
T h e void volume of this
substrate
by n i t r o g e n s o r p t i o n at 7 7 K .
the is
Disilane,
obtained from UCAR (Belgium), w a s purified by destillatlon. T h e structural modification of the z e o l i t e was c a r r i e d out in a
dynamic gas adsorption a p p a r a t u s [ 1 , 2 1 , with a porous f r i t was used.
reactor
circulation
The
react ion.
the
react ion
Adsorption evaluate
kinetics
the
progress was fo I lowed b y measuring
the
of Ar,
spectra
continuous during
were
sorbent
chemisorbed at different
Kr.
Xe.
the sorption c h a r a c t e r i s t i c s
Infrared
fixed-bed
This permitted
gas phase t h r o u g h the
of H n evolved and S i l H I
amounts
step.
of
in which a
N,
and O 2 were
after
recorded o n a
each
times. used
to
modification
Nicolet
SDXB-FTIR
spectrometer equipped with an M T E C - 1 0 0 photoacoustic cell.
761 RESULTS AND DISCUSSION S t r u c t u r a b M o d i f i c a t i o n Process The
structural
modification
was studied by
kinetics of disilane chemisorption o n H-mordenite. the
evolution
11)
and
the
Figure 1 shows
of the reaction during chemisorption
(Region I ) ,
temperature
following at
ambient
the secondary reaction at 3 7 3 K
(Region
the hydrolysis reaction at 423K
(Region
111).
For
a
period of 100 hr a dose of disilane, with initial pressure of 128 Torr
was
contacted
temperature.
with
The reaction
1.7
grams
of
zeolite
at
ambient
was monitored by trapping the gaseous
and physically sorbed disilane at 77K and measuring the amount of HZ produced
during
7 ,
the reaction.
I
Within the
first
hour,
I
6
5
4
4
B i
3
2
1
0
0 2 4 6 8 1 0 2 4 /'t hr'
2
4
6
8
Fig.1. Chemisorption of SiaH. on H-mordenitr SP and mmole S i z H . chemisorbed; 0 mmole further reactions. H a evolved. 0 R the ratio of H a / S i a H . . The steps are: I. 18;';hr chemisorption at 2 9 8 K . 11. After evacuation of disilane 25 hr secondary reactions at 373K. I l l . After introducing excess H z O 49 hr hydrolysis at 4 2 3 K .
the
largest
amount of disilane is adsorbed,
equilibrium
is reached.
evolved to the SilHI chemisorbed, indicating
a
strong
and after 16
the R value,
However,
is much lower than 1
physisorption of disilane in
stages.
As the reaction progresses,
becomes
1
after about 100 hours.
hours
an
the ratio of HI (=0.42),
the
initial
the R value increases
and
This suggests that each chemi-
sorbed disilane molecule forms one covalent bond with the zeolite framework.
Under
these
reaction
conditions
the
modification
process is controlled by the relatively low reactivity. is
allowed to diffuse within the intracrystalline
uniformly
Disilane
channels
distribute itself along the entire pore system
chemisorption
A t the end of the reaction,
occurs.
and
before
0.85
mmol/g
disilane was chemisorbed. uniformity of the modification was further
The
demonstrated
when the disilanated zeolite was heated at 3731(, after evacuation of
the
remaining gaseous and physically sorbed
dose of HZ was liberated, 1.
S i 2 H L . Another
as can be seen in region I I of
figure
During this treatment the chemisorbed hydrides i n the zeolite
react
with
the
available
structural
OH-groups,
rather
than
dehydrogenating between each other C 5 1 . About 1.72 mmole/g of O H groups
reacted w i t h disilane. at
disilane
Therefore,
the chrmisorption
ambient temperature is associated with
an
of
implan-
tation p ocess throughout the entire volume of the channels. A
s milar
higher
procedure was followed to carry out reactions
temperatures.
The
initially heated to 373K,
dehydrated
423K,
zeolite
4731.;and 52.3k:,
samples and allowed
at
were to
stabilize for 30 minutes before contact with identical amounts of disilane.
This
treatment results in a dramatic decrease of
amount of chemisorbed disilane, when ambient temperature,
as shown in f i g .
the
compared to the reaction at
2.
At higher temperature
equilibrium is reached within 30 minutes, and no m o r e disilane is chemisorbed
after
a
longer
period
structural
hydroxyl
groups
which
of react
time. with
The
decreases as a function of the reaction temperature. the increasing R-values.
number
disilane
of
also
in spite of
763
1
1.0
t
2.0 n
d
-1.6
- 1.2
- 0.8 0.0
!I
1
1
0.4
0.0 313
213 M
c
573
113
m m a t u r e tK1
Fig.?. Intluence of the reaction temperature o n the chemisorption process. disi lane chemisorbed; 0 the amount of reacted OH-groub)s in H-mordeni te.
The
observed
creased
phenomena can be explained in terms of a n
reactivity
conditions,
the
at
higher
entering
temperatures.
Under
inthese
disilane molecules immediately
react
with OH-groups on the external surface and at the pore entrances. These chemisorbed molecules prevent the other disilane from
diffusing
lower
into the internal pore system.
accessibility
of
the
internal
volume,
molecules
Because a
of
the
preferential
reaction
w i l l occur at the channel entrances and o n the external
surface.
Furthermore,
the chemisorption at temperatures between
373 and 523K gives rise to the formation of 2 t o 3 covalent bonds
per
disilane
concluded
molecule with the zeolitic structure.
that,
It
when these results are compared to the
can
be
charac-
teristics of the implantation process, the reaction conditions a s described here w i l l After
hydrolysis
HnO-vapour, dicates of
lead to a n external
R
values
(surface) modification.
of the disilyl groups
at
423K-523K
between 6 and 7 a r e obtained,
which
that at least a portion of the incorporated Si-Si
disilane
molecules a r e broken during this
treatment.
with inbonds After
764 dehydration reaction
733K.
at
with
the
disilane.
modified z e o l i t e s T h i s behaviour
blocking of the z e o l i t e c h a n n e l s
show
is d u e to
no 1)
further complete
b y t h e modifying groups and, 2 )
the inert character or the modified s u r f a c e of the zeolite.
Characterization of the Disilanated Z e o l i t e s
In
order
sorption and
to
confirm the r e l a t i o n s h i p
temperature and
to allow an e s t i m a t i o n of
these
treatments,
between
the
chemi-
the different m o d i f i c a t i o n mechanisms, the c h a n g e s in pore
size
during
a d s o r p t i o n e x p e r i m e n t s have been carried
using various test gases with different kinetic diameters. 3
shows the a d s o r p t i o n rate of X e a t 273K o n m o r d e n i t e
modified
out
Figure
samples,
with different a m o u n t s of S i n H I at a m b i e n t temperature.
A f t e r chemisorption of more than 0.5 m m o l e / g of disilane. the Xe
0
2
4
6
8
1
0
1
2
t+/.in4 Fig.3. Adsor tion kinetics modified H - m o r d e n parent o n the zeolites. , 0h c . 5e 2 m i s o r b e d O.67;'&:*0.78; symbols, res ectively correspond to t h e a d s o r p t i o n of X e before and afrer hydrolysis-dehydration. Qe=1.96 mmole- g - ' .
A
v
765
5, U
u
0.4 0.2
0.2
0.0
0.6
0.4
1.0
0.8
The amounta of chemiaorbed Sip6 / m m l
Fig.4. The va iation of relative adsorption amounts of Ar* K r ; o n modified zeolites chemisorbed with’:%dferent’ amounts of disilane. Qe(02 )=0.160; Qe(Ar)=0.174; Qe(Kr)=0.796 m m o l e . g - * .
6
A
adsorption increase lower
in
rate
of Xe.
Hydrolysis of t h e
further
A
the amount of chemisorbed disilane causes
sorption
groups, of
becomes controlled by a diffusion process.
an
even
residual
Si.H,
results in a further reduction
followed by dehydration,
the pore sine so that more pronounced diffusion processes
Xe and K r were observed.
of
Moreover, the incorporated obstructions
cause a significant decrease in the adsorption capacity for small a s c a n be seen in figure
molecules such a s O n and A r , results
are
in
agreement
with the
hypothesis
These
4.
of
a
uniform
disilane implantation throughout the zeolite channel system. Figure samples, but
shows the kinetic plots of Xe o n
5
two
H-mordenite
containing the same amount (0.675 mmolelg) of
chemisorbed at different reaction temperatures.
disilane
The
sample
treated at 353K exhibits a much lower adsorption rate compared t o a
substrate modified at ambient temperature.
On the other hand,
the sorption capacity after 1 7 hours contact time is larger after a
This effect is even more
high temperature modification.
nounced
after
dehydration.
hydrolysis
Considering
of the
the
residual
these experiments show that the effective pore 6.2A
[El,
Si-H
kinetic diameter of
was reduced to about 4 A .
size,
bonds Xe
proand
(3.96A).
originally
766
I
1.0
0.8
0.6
a"
\
If--
:/
~ ~ ~ 0 . 4
0.2
0.0
0
4
2
,.
6
8
1
0
1
2
3
2
t%.ink
,o
Fig.5 Xe adsorption at 273K o n parent sam le after chemisorption of 0.67 m m o l e l g disilane 0 at %Sb and at 3 7 3 K ; after hydrolysis of t h e two respective samples
.
-1
4 z Q
H
ul I
f
n I
I V 3800.0
P700.0
P800.0
WAVMUWERS
PSOO.0
(CM-1)
F i g . 6 FTIR-PAS spectra of modified H-Mordenite SP after deuterium exchange: a ) parent sample; b ) 0.67 mmole/ disilane chemieorbed at 353K; c ) 0 . 5 2 ; d ) 0 . 6 7 ; e) 0.8g mmole/g disilane chemisorbed at 298K.
These results c a n be correlated with observations from infrared spectroscopy. The acidic hydroxyl groups which absorb a t 3605 cm-I
(at 2664 cm-' after deuterium-exchange) in
mordenite
are
the reactive centra for chemisorption reactions 15-71. The intensity of this vibration band decreases a s a function of the amount of
chemisorbed
hand,
at
disilane at ambient temperature.
well
a s the infrared data,
channel entrances,
loss
great
is
(figure 6). T h e adsorption experiments, confirm
higher temperatures causes a preferential the
other
indicates that most of the OH-groups inside the
which
channels a r e not affected
without
the
higher reaction temperatures only a small decrease
observed, as
On
that
chemisorption
at
reaction of disilane at
thus enhancing the pore-blocking
in adsorption capacity and acidity
effect of
the
zeol i te.
CONCLUSION Chemisorption temperatures resulting
on
H-mordenite
SP
at
causes a homogeneous modification of the
on
the pore entrances,
substrate for
causing
pore narrowing but only a small decrease in
capacity and acidity. great potential in
ambient the
A t more elevated temperatures the chemisorption
preferentially
profound
disilane
in a pore-narrowing effect and capacity loss
used test gases. occurs
of
a
more
adsorption
These results indicate that disilane has a
zeolite modifications f o r gas separation
and
shape-selective catalysis. REFERENCES 1. 2. 3. 4.
5.
6. 7. %.
R. M. Barrer, E. F. Vansant and G. Peeters, J. Chem. SOC., Faraday I , 74, (1978), 1871. A . Thijs, G T e e t e r s , E. F. Vansant, 1 . Verhaert and P. De Bihvre, J. Chem. Sac., Faraday I , 7 9 , (1983). 2821. E. F. Vansant, "Studies in Surface Science and Catalysis", Elsevier (Amsterdami. 37. (1987), 143. Y. A. Yan, J , Verbiest and E. F. Vansant tin reparation). J.Phi 1 ippaerts, E.F.Vansant, G.Peeters,?and E.Fanderheyden. Analytlca Chimica Acta. 195, ( l Y 8 7 ) , -37. J. Philippaerts and E.F.ansant, "Silanes Surface and Interface, Vol. Z " , D. E. Leyden Ed., (1987).(in press) J. Phili paerts. E. Vansant and Y.A.Yan. Proc. Int. m osium on Zeorite Catalysts. Sorbents and Detergent 'iuflders. WIirzbur FRG, Sept. 4-@, (19HEIJ. tsubmi:ted for ubl ication) D. i;eck. "Zeolite Molecular Sieves , W i ley Pnterscience.
This Page Intentionally Left Blank
H.G. Karge,J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
DEALUMINATION OF THE ZEOLITES OFFRETITE AND ERIONITE STUDIED BY SOLID-STATE zgSi- AND Z7A1-MAS NMR SPECTROSCOPY
Karl Petter Lillerudl and Michael Stockerz, 'Department of Chemistry, University of Oslo, P.O.Box 1033 Blindern, N-0315 Oslo 3, Norway. ZDepartment of Hydrocarbon Process Chemistry, Center for Industrial Research, P.O.Box 124 Blindern, N-0314 Oslo 3 , Norway. ABSTRACT The ZeSi- and 27A1-MAS NMR spectra of the zeolites offretite, offretite/(lO%)erionite, offretite/(30%)erionite and erionite were recorded including their dealuminated samples. The interpretation of the NMR spectra was made by computer simulation assuming a binomial distribution of the aluminium atoms, and a graphic comparison of the calculated and measured spectra. It is shown that aluminium was mainly removed from the Ti sites of the offretite lattice during the dealumination process.
INTRODUCTION Zeolites have been applied as heterogeneous catalysts during the last few decades. Their unique channel dimensions (pore diameter less than 1 0 A ) and the existence of exchangeable cations, which permits introduction of cations with various catalytic properties (ref. l), makes them particularly useful to the oil and petrochemical industry. Much effort has been spent to elucidate the zeolite structure and its relation to activity and selectivity in chemical reactions. Shape selectivity and acid strength of zeolites are important aspects of their catalytic activity. Offretite and erionite are zeolites of increasing interest, e.g. as catalysts in t.he methanol conversion to light olefins. Industrial application of zeolites requires usually a hiqh catalyst lifetime, a problem which miqht be solved by dealumination of the zeolites, i.e., partial removal of aluminium atoms from their lattice positions. Solid-state 29Si- and 27A1-MAS NMR spectroscopy should be a powerful tool to follow possible structure modifications due to dealumination effects.
770
Since it was shown by Lippmaa et al. (ref. 2 ) that ZgSi-MAS NMR-spectra were sensitive to the nature and chemical environment of the atoms, considerable knowledge has been gained during the last years about the structures of zeolites from the study of their 2gSi- and 2”A1-MAS NMR spectra (ref. 3 ) . The Z9Si-MAS NMR spectra of zeolites with one crystallographic T site exhibit up to five lines which correspond to the five possibilities for the linkage of an SiOa-tetrahedron through oxygen bridges to n AlOa-tetrahedra (n = 0 - 4 ) : Si (n Al) (ref. 4 ) . In zeolites with multiple T site positions up to five resonance lines per T site could be observed. Since chemical shift dispersion due to crystallographic inequivalent sites often is of the same order of magnitude as the shift due to the above-mentioned different number of aluminium atoms in the first coordination sphere (ref. 51, severe resonance line overlap may result. In addition, we are dealing with solid-state NMR which means lower resolution compared with high-resolution NMR. The assignment of signals may, therefore, be more complex (ref. 6 ) . Furthermore, if Lowenstein’s rule is obeyed, the A1 atoms always have four Si atoms in the first coordination sphere, and the 27A1-MAS NMR spectra show, therefore, only one resonance line per crystallographic position. ZrA1-MASNMR chemical shifts allow discrimination between the different framework positions (TI and Tz, tetrahedrally coordinated) and non-framework (octahedrally coordinated) aluminium (ref. 7 ) . 27A1-MAS NMR spectra of zeolites exhibit only one central line per T site corresponding to the transition between the energy levels. However, this line is broadened by quadrupolar interactions. The chemical shifts of framework aluminium in zeolites cover only a narrow range between 53 and 62 ppm (ref. 7 ) , whereas octahedrally coordinated A1 is observed around 0 ppm. The Z7A1-NMR linewidth (AH), measured at half height of the NMR resonance line, is the best parameter to investigate the chemical environment. The quadrupolar nuclei are very sensitive to the coordination number. the nature of ligands and the symmetry of substitution (ref.7). In the present study. we report the 29Si- and (partly) 27A1-MAS
771
NMR spectra, together with the computer-simulated spectra, for the following zeolites (dealuminated to various extent): Off retite Offretite/(
112
are occupying two different types of tetrahedral sites (TI and T2) in this zeolite. Each type of tetrahedral site gives rise to a maximum of five resonance lines, separated by 5 - 6 ppm, which may be correlated with the number of A 1 atoms surrounding a given Si atom (Si(n Al), with n = 0 - 4 ) . Since in the offretite structure there are twice as many TI-atom sites as Tz-atom sites, the total intensity of the TI spectrum should be twice that of the T2 spectrum. The spectra are superimposed in such a way that a Si(n Al) atom occupying a TI tetrahedral site has a chemical shift very close to a Si ((n + 1) All occupying a T2 tetrahedral site (ref.9). The A1 atoms are assumed to be randomly distributed among the TI or TZ tetrahedral sites in such a way that LBwenstein's rule is obeyed, i.e. no A1-0-A1 linkages are present, whereas A1 distribution between TI and TZ sites may not be random. The intensity of an individual resonance may be calculated according to (ref. 9)
IsiT (n All: Intensity of the resonance resulting from silicon atoms occupying Si TI type or Tz type tetrahedral sites and having n A1 atoms as nearest neighbours (n = 0 - 4). Itot : Total intensity. P : Probability that the n-th neighbour is a Si atom. q : 1-p The compuker-generated spectra are calculated according to equations (1) and ( 2 ) and compared with the experimental spectra The single resonance lines are represented as Gaussian curves. RESULTS AND DISCUSSION The tetrahedral framework of offretite is composed of interconnected double and single six-membered rings. Each unit cell contains 12 tetrahedral atoms (TI atoms) located in double six-membered rings and 6 tetrahedral atoms (Tz atoms) located in single six-membered rings, resulting in an overall ratio of two
773
TI atoms for each Tz atom in offretite (ref.
9 ) . In erionite, each
second Tz site is turned around 60° compared to the first TZ site (see Figure 1). In the case of offretite a number of different assignments of the resonance lines in the observed aqSi-MAS NMR spectra have been made to bring the Si/A1 ratios from chemical analysis into agreement with those calculated from the NMR spectra (refs. 6 and 9 and references cited therein). The assignment of the lines in an observed spectrum to their respective crystallographic sites and number of aluminium atoms in the first coordination sphere is well understood. This allows us a detailed interpretation of the Si and A1 distribution in the framework structure of zeolites by solid-state NMR spectroscopy.
Fig.1. Structure of offretite (upper figure) and erionite (lower figure).
114
-:iS:2
MAS- NMR spectLa The ZqSi-MAS NMR chemical shifts for the different Si (n All atoms range from about - 8 5 ppm to about -112 ppm. As shown in Figures 2 - 4 , the 29Si-MAS NMR spectra can be deconvoluted in terms of ten overlapping Gaussian curves (dashed lines: simulated patterns). The parameters adjusted to obtain the best agreement in the simulations were the total Si/A1 ratio (values from the chemical analysis are used as total Si/A1 ratios), the relative distribution of A1 between the Ti and T2 sites and the line width. The Si/A1 ratios resulting in the best fits are reported in the Figures 2 - 4 , shown for the zeolites offretite (Fig. 2 1 , erionite (Fig. 3 ) and offretite/(30%) erionite (Fig. 4 ) . A general trend was observed for the offretite zeolites: during the dealumination process aluminium is mainly removed from the TI sites of the zeolite lattice. This is valid for offretite and the two offretite-erionite intergrowths (see also Figs. 2 and 4 , no figure for offretite/(
115
TI sites results in a decrease of T t intensity followed by a relative increase of the intensity of the T2 line. This is well demonstrated by the formation of a clear shoulder in the spectrum of the most dealuminated sample (see Fig.5). The experimental findings are confirmed by our simulated spectra. The difference in chemical shift for the TI and Tz sites used in our simulations is larger than expected for offretite ( = 13 ppm) compared to that observed by Fyfe et al. (ref. 6 ) for the zeolite omega ( = 6-7 ppm). On the other hand, our 27A1-MAS NMR line widths (AH) are smaller (between 940 and 1820 Hz) than those measured by Nagy et al. (ref. 7) for offretite (2800 Hz), erionite (3500 Hz) and T-zeolite (2200 Hz).
Sl/Al r e t l o : - T o t a l = 2.1
Dsalumlnated onc 0. Sl/Al r a t l o : -Total- 5 . 2
I
--f
S V A 1 ratlo:
,'
t
',
\ \
---,I
'A' \
Dsalumlnated f o u r t,.l%tbs. IT S ioItAa l - r 0e .t 0l o :
'
I Sl/Al r a t l o : Total- 10.0
.
T, - 1 0 . 0
,
80
90
100
110
120
-ppm
Fig. 2. Si-MAS NMR spectra of offretite.
80
ao
-.-- . . ,--a
100
110
I
120
-ppn
Fig. 3 . Si-MAS NMR spectra of erionite.
776
Not daalumlnDte A1 -nnr
Si/A1 ratlo:
Daalunlnatad onca. Al-nnr . T,: e a ~i T,: 372 A1
D a ~ l u n l n ~ t a onca. d S i / A 1 ratio: Total. 4.e
__-I
eo
ao
', '\. _--' , \
100
110
120
-Pm
Fig. 4. Si-MAS NMR spectra of offretite/(30%)erionite.
Fig. 5. A1-MAS NMR spectra of erionite/(30%)erionite.
The Z'Al-MAS NMR chemical shifts range from 51 to 58.6 ppm for tetrahedrally coordinated Al. Only small amounts of octahedrally coordinated A1 were found in the dealuminated samples (see Fig.6). CONCLUSION From the examination of our results we conclude that 2SSi- and Z'Al-MAS NMR spectroscopy can be used to monitor the removal of A1 from a specific lattice site during dealumination of zeolites. It was shown that aluminium is mainly removed from the TI sites of the offretite lattice. The same conclusion could be drawn from the Z'Al-MAS NMR measurements. In addition, high-field Z'Al-MAS NMR
investigation could yield better-resolved TI and Tz resonance lines for detailed interpretation of P7A1-NMR spectra.
~
150
100
K)
0
-50pprn
Fig. 6. 27Al-MAS NMR spectrum of erionite (dealuminated once).
ACKNOWLEDGEMENT The authors are indebted to the Royal Norwegian Council for Scientific and Industrial Research for financial support of this investigation and to Prof. J.B. Nagy (University of Namur) for recording the NMR spectra. REFERENCES 1 S.M. Csicsery, Zeolites, 4 (1984) 202. 2 E. Lippmaa, M. Magi, A. Samoson, M. Tarmak and G. Engelhardt, J. Am. Chem. S O C . , 103 (1981) 4992. 3 C.A. Fyfe, J.M. Thomas, J. Klinowski and G.C. Gobbi, Angew. Chem., 9 5 (1983) 257. 4 J.B. Nagy, 2. Gabelica, G. Debras, P. Bodart, E.G. Derouane and P . A . Jacobs, J. Mol. Catal., 20 (1983) 327. 5 K.P. Lillerud, Zeolites, 7 (1987) 14. 6 C.A. Fyfe, G . C . Gobbi, G.J. Kennedy, J.D. Graham, R.S. Ozubko, W.J. Murphy, A. Bothner-By, J. Dadok and A.S. Cheenick, Zeolites, 5 (1985) 179. 7 J.B. Nagy, Z. Gabelica, G. Debras, E.G. Derouane, J.-P. Gilson and P.A. Jacobs, Zeolites, 4 (1984) 133. 8 K.P. Lillerud and J.H. RiEder, Zeolites, 6 (1986) 474. 9 J.H. Rader, Zeolites, 4 (1984) 311. 10 R.J. Mikovsky, Zeolites, 3 (1983) 90.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands
Galliation and '*O-exchange Reactivities of ZSM-5 and ZSM-11
A. Endoh, K. Nishimiya, K. Tsutsumi' and T. Takaishi Toyohashi University of Technology, Toyohashi 440, Japan
ABSTRACT The "0-exchange between COa and ZSM-5 and ZSM-11 was measured, along with their galliation. There was a strong correlation between the reactivities €or these two processes, and they markedly depended upon the sources of starting materials. The reactivities stem from defects the concentration of which may depend upon the know-how of synthesis of the suppliers. About 6 €ramework oxygen atoms became reactive for exchange per Ga atom inserted, which suggests that Ga occupies a corner o€ the 4-membered oxygen ring. The initial heats of adsorption of NHB, calorimetrically determined, were ca. 160 kJ.mol-' in H-Ga-ZSM-5 and ZSM-11. The catalytic activity on galliated zeolites is due to the fact that one acid site is produced by each Ga atom in the framework. INTRODUCTION The isomorphous substitution of T-site atoms ( T = Si or A1 ) of the zeolitic framework by Ga attracts wide attention in connection with its catalytic activity. Usually, zeolite containing Ga is directly synthesized by the addition o€ a gallium source into the reactant mixture. Recently, a new method for the substitution was discovered, that is, galliation of highly siliceous zeolites could be achieved by dipping them in sodium galliate solution (ref. 1). To understand their catalytic properties of Ga-zeolites, further knowledge is required about the nature and location o€ acid sites produced in the framework. In the present paper, we investigated the reactivity for galliation of ZSM-5 and Z S M - 1 1 supplied €rom several source, because such a reactivity may depend upon the concentration of de€ectu contained. Furthermore, the '*O-exchange reactivities between COz and these zeolites were measured to estimate the concentration of defects in the starting zeolites, and to search the location of Ga in the framework.
780
EXPERIMENT ZSM-5 and ZSM-11 zeolites, in highly siliceous or galliated forms, were supplied from several sources. Their compositions are listed in Table 1. TABLE 1 Compositions of ZSM-5 and ZSM-11, and abbreviated notations for various samples studied. Sample
Si/Al( Ga ) ratio
Abbreviation*
Na-ZSM-11 Na- Ga- ZSM- 11
1066 9.8 ( by c.a. )** 15.4 ( by n.m.r. ) 9.8 ( by c.a. ) A1 trace 685 10.7 ( by c.a. ) 10.7 ( by c.a. )
N a- 11-C Na- Ga- 11- C
H- G a-Z S M- 11 Na- Z S M- 11 Na- Z SM-5 Na- Ga-Z SM- 5 H-Ga-ZSM-5 H-ZSM-5
* *
-
0S9
H- Ga- 11-C Na-11-M Na-5-M Na-Ga-5- M H-Ga-5-M H-5-M
C and M denote sample suppliers, Cambridge school and Mobil groups, res pee tivel y. * c.a., chemical analysis; n.m.r., 29Si m.a.s. n.m.r. spectroscopic analysis ( These values were determined by the supplier ). For details concerning the samples refer to the supplier's paper (ref. 1).
The standard procedure for galliation was used as follows. One gram of zeolite was added to SO ml of the aqueous solution of Gas03/ NaOH ( 0.0278 rnol4-l Gaz03, 0.10 mo1.l-l NaOH). The mixture was heated to 575 K under stirring and kept at this temperature for 24 hr. The product was filtered, washed with distilled water and dried in air a t 555 K. The amount of Gaa03/ NaOH solution used. for 1 g of zeolite, was varied in some cases from 5.5 to 120 ml, and the treating time was also changed from 24 to 48 hr. The degree of galliation was checked by i.r. and "Si m.a.9. n.m.r. NH4+ exchange of the samples was carried out by the usual procedure.
781
High - Temperature Adsorption Calorimetry Differential molar heats of adsorption of NH3 on the zeolites were measured at 47s K using a twin-conduction-type micro-calorimeter after the sample waa evacuated at 773K. The apparatus and the procedure of measurement were described in a previous paper (ref. 2). Exchange of Oxygen Isotopes between CO1 and the Zeolites The apparatus used for the reaction was the same as that described in a previous paper (ref. 3). The zeolite, ca. 40 mg, was completely dehydrated at 775 K for 2 0 h r under a vacuum of lo-' Pa before exchange reaction. The reactions were carried out at a pressure ca. 1850 Pa of C O z and a t temperatures between 073 and 775 K. RESULTS AND ANALYSIS Figure 1 shows 29Si m.a.s. n.m.r. spectra of H-5-M as-received and its products treated with G a z 0 3 / NaOH solution. The products gave almost the same chemical shifts and peak-intensities as the starting zeolite. 1.r. spectra, in the region of the framework vibration, did not change by the treatments. This tendency prevailed even if the treating time was prolonged to 4 8 h r and the amount of solution was increased to 120 ml. It is concluded that under the present conditions H-5-M was not galliated at all. The same w a s the case with Na-11-M. Na-5-M, on the other hand, was galliated. Figure 2 shows i.r. spectra of the product ( abbreviated hereafter as Na-Ga-5-M ) and the starting zeolite. In this case, the peaks referred to the vibration of the framework were shifted to the low-wavenumber side after the treatment. This means that a change has occurred in the force constant of the T-0 bond by the insertion of Ga into the framework (ref. 1). Figure 5 shows m.a.9. n.m.r. spectra of the samples. Slight changes were observed in the shapes and chemical shifts in these spectra. However, the change was too small to calculate the Si/Ga ratio. Chemical analysis of Na-Ga-5-M, on the other hand, showed that the sample contained 7.7 Ga atoms and 0.5 A1 atoms per unit cell, that is, a large amount of gallium oxide impurity existed in the channels. Figure 4 shows the differential molar heats of adsorption of NH3, q d , f f , on the samples. The initial heats on Na-11-C and Na-5-M were ca. 50 kJ.mol-', and amounts of ammonia adsorbed were very small. By galliation, the initial
782
heats increased to ca. 90 kJ.mo1-I in Na-Ga-11-C and Na-Ga-5-M. A further drastic increase to ca. 180 kJ.mol-' was achieved by protonating of the galliated zeolite t o H-Ga-11-C and H-Ga-5-M, and amount of
2.0111.
L -105
l*,
I -110
I\
1
-115
1
1
1
1
1
I -120
1
1
8ym. atretch
Aaym.
1
1
0-rlng 1-0
I -125
1400
ppm from TMS
Fig.1 29Sim.a.s. n.m.r. spectra of H-5-M and products treated with GazO3/ NaOH solution.
1200
1000 800 600 Wavenumber 1 cm-'
400
Fig.2 1.r. spectra of Na-5-M and galliated zeolite ( Na-Ga-5-PuI ) in the region of the framework vibration. Spectra were recorded by a conventional KBr disc technique in air at room temperature.
Na-Ga-6-M
-
H-Ga-1 1-C
-10s -110 -11s -120 -12s
ppm from TMS
F i g 3 29Sim.a.s. n.m.r. spectra of Na5- M and NaGa-5-M.
I
0
I
1000 NH3 Adsorbed j
I
2poo II
I
00
mo1.g-
Fig.4 Differential molar heats of adsorption of NH3 on the zeolites at 475 K.
NH3-adsorption markedly increased. The high initial heats, ca. 160 kJ.mol-', clearly correspond to the acid-base interaction of the acidic site with NH3. Tsutsumi et al (ref. 4 ) suggested that the adsorption with a small q d r t f . , less than 80 kJ.mol-', is referred to the interaction between the framework and 0 10 20 30 NH3, and the adsorption with a greater Reaction Time / hr heat to the interaction with an acid site, either Bronsted or Lewis acid. Since no A1 was detected in H-Ga-11- \ C P) C (ref. l ) , the acid sites must result 0 % from the presence of Ga atoms in the 25 framework. The adsorbed amount with -0 0) higher q d , f f . was ca. 800 pmo1.g-l in P Q r H-Ga-11-C; namely, there are flve acid 0 X sites per unit cell, which is well related w 0 10 20 30 to the number of Ga atoms, 6.7, deReaction Time / hr duced from 29Si m.a.9. n.m.r.. The difference amounting to ca. 1.7 is nonnegligible and may be attributed to amorphous impurities coating the acid sites. The initial high heats of adsorption observed in H-Ga-ZSM-5 may be due to a small amount of Ga atoms as well as A1 atoms in the fkamework. Figure 5(a) shows the degree of 10 20 30 exchange of oxygen between ZSM-11 Reaction Time / hr samples and COz at 775 K. Na-11-C had a much higher exchange reactivity Fig.5 180-exchange reactivities bethan Na-11-M. Furthermore, Na-11-C tween CO2 and the zeolites. was galliated rather easily, while Na(a) ZSM-11 series (b) ZSM11-M was not. Na-11-C and Na-11-M 6 series ( c ) Temperature decould not be distinguished macroscoppendences of the reactivities ically, and the above difference in their in Na-Ga-5-11.
784
reactivities may be attributed to the content of defects. When Na-11-C was galliated to Na-Ga-ll-C, the '*O-exchange reactivity was drastically increased. The same was the case with a pair of ZSM-5 zeolites ( Na-5-M and Na-Ga-5-M )( Fig.S(b)). Fig.5( c) shows the temperature dependence of the reactivity in Na-Ga-11C.
DISCUSSION The reactivities in galliation and '*O-exchange had a good correlation with each other. For example, Na-11-C is easily galliated up to 6.7 Ga atoms per unit cell, and showed high reactivity in isotopic exchange. Na-11-M exhibited no reactivity for either galliation o r isotopic exchange. Hence, we propose the following model for defects to explain the present results. If one Si on a T-site of the framework is removed, then four silanol groups are formed. Oxygen atoms in these silanol groups may have a much higher reactivity for isotopic exchange than the normal framework oxygen atom. When the sample is treated with sodium galliate solution, Ga is easily inserted into this position. Thus, the galliation is controlled by the concentration of defects in the framework. S,i 0
I I
-Si-O- Si- O-Si-
Ti
Si
0
0
H -%OH
HO-SiH I
0
0
s'i
$i
I
-Si- 0-Ga- 0-Si-
I
0 ii
Now, we adopt the following approximation to analyze kinetics data of NaGa-11-C, and search for the location of G a i n the framework. The oxygen atoms in the crystal are classifled into three kinds, that is, reactive and less-reactive framework-oxygen atoms ( specifled by the subscripts 2 and S, respectively ), and oxygen atoms in amorphous oxide or silanol groups ( specifled by the subscript 1 ), and they should have very different rate constants,
where k is the rate conetant.
185
In a later stage of the reaction, the first and second kinds of oxygen atoms are in equilibrium with the gas phase, and the rate of exchange is determined by the third kind of oxygen atoms. Then, according to eqn.(O) in the preceding paper (ref. S ) . we have
-
where n g and n, ( i = 1 S ) denote the number of moles of the oxygen atoms in the gas phase and those of the i-th kinds, respectively, y, the fraction of l a 0 in C 0 2 and yJ0 its initial value at t = 0 . Plots of
against t becomes linear apart from the initial portion near t = 0 , as shown in Fig.B(a). From the slope of the linear part the value for lc3(n1+ n2 n3 n g ) is obtained. The total amount of oxygen, (n1 + n 2 + n 3 ) , is known, and hence the value for k3 is obtained. The intersect of the linear part with the ordinate gives the value for
+ +
n3n& (nl
+ n2 + n g ) ( n l+ n2 + n3 + n g )
and hence the values for n3 and ( n l + n 2 ) are determined. Next, in the intermediate stage of the reaction, the flrst kind of oxygen atoms is in equilibrium with the gas phase, and the third dose not participate in the exchange because of its small rate constant, and hence the rate of exchange is determined by the second kind of oxygen atoms. Then, eqn.(2) is replaced by the following equation,
786
-
f
QO
01
+, -45
I
m >. -1.0 m
Y
0
20
0
60
40
t/h
-
0 ,
om
Q-. C
I
-
O
x
m
-1.0
0
0
I
10
t/h Fig.6 Plots of the degree of exchange us. time, based on eqns.(2) and (3) a t 723 K. vgo = 0.90; amount of oxygen atoms in gas ( nJ), 0.251 mmol; in solid ( n, ), 1.168 mmol.
+
where the value of (nl 7x2) has already been obtained from Fig.B(a). Fig.B(b) shows plots of eqn.(3), and unknown parameters n1, na and k3 are determined by a method similar to the above. There were small variances in the values for n l , n2 and n3 determined from data at four different temperatures. Their average values are,
787
+ n2 + n3 ) = 0.11 n1
nl
(nl
(
nl
::zz
0.01,
+ + n3)
= 0.20 f 0.01,
n3)
= 0.69 f 0.01.
+
+
Temperature dependences of kZ and k3 gave the activation energies of l.22eV and 0.82eV, respectively. If one Ga atom introduced produced u exchange-reactive framework- oxygen atoms, then 8 . 7 oxygen ~ atoms per unit cell must easily be exchanged. On the other hand, the number of easily exchangeable framework oxygen atoms, n2, is 45 (= 192 x nZ/(n? 713)). Therefore we have
+
u
= 4316.7 = 6 4
(4)
Since Ga atom has a larger radius ( 0.061 nm ) than Si ( 0.040 nit1 ), i t distorts neighbouring oxygen atoms, which are then displaced in order to relax the stress induced. The oxygen atoms displaced alqo push their bridging Si and neighbouring oxygen atoms, which are also a little displaced. Thus, the stress induced by Ga substitution propagates over a wide range, and it decays with the increasing distance from Ga. Oxygen atoms directly neighboured to Ga may be in an unstable state and highly reactive to the isotope exchange. Next-neighboured oxygen atoms may become reactive to some extent. According to eqn.(4), four oxygen atoms directly neighboured to Ga and two next-neighboured are easily exchanged. In the framework of ZSM11, the T-site has twelve next-neighbouring oxygen atoms. Thus, two out of twelve oxygen atoms next-neighboured to a Ga atom have special properties o r geometry. The T-site on a corner of the 4-membered oxygen ring has such two special next-neighboured oxygen atoms. The stress in the framework may decay with the increasing distance, and the second neighbour in the 5-ring may experience only a small stress, but the stress may not be relaxed in the closed 4-ring. Thus, we arrived at the conclusion that Ga atoms are selectively inserted at the corner of the 4-ring. This result supports Kraushaar et al.'s view on the silylation of ZSM-5 (ref. 5). In conclusion, the "0-exchange and galliation reactivities are mainly related to defects, and strongly depend upon the quality of the crystal. There-
fore, one must carefully characterize such qualities. by using the present techniques and the silylation method, to clarify catalytic properties of ZSM-5 and 11. We thank Prof.J.M.Thomas, Dr.X.S.Liu, University of Cambridge, Dr.A.W.Chester, Mobil Research and Develop., and Mobil Cat. Corp. of Japan, for the supply of the samples.
REFERENCES J.M.Thomas and X.S.Liu, J. Phys. Chem., 90 (1986) 4843-4851. K.Tsutsumi, S.Hagiwara, Y.Mitani and H.Takahashi, Bull. Chem. SOC. Japan, 5 5 (1982) 2572-2575. T.Takaishi and A.Endoh, J. Chem. Soc., Faraday Trans. I, 83 (1987) 3 411-424. 4 K.Tsutsumi and K.Nishimiya, Thermochemica Acta, in press 5 B.Kraushaar, L.J.M.Van de Ven, J.W.de Haan and J.H.C.van Hoof", International Symp. on Innovation Zeolite Materials Science, Belgium, Sept. 1987, pp.167-174. 1 2
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THERMAL DECOMPOSITION OF IRONPENTACARBONYL I N ZEOLITES OF FAUJASITE TYPE. A STUDY OF THE INFLUENCE OF ARGON, H2, H2/C0 GAS MIXTURE AND VARIOUS S i / A l RATIOS USING MUSSBAUER, ESR AND MASS SPECTROSCOPY
H.M.
Ziethen and A.X.
Trautwein
I n s t . f . Physik, Med. U n i v e r s i t a t , Ratzeburger A l l e e 160, D-2400 Lubeck, FRG ABSTRACT The thermal decomposition o f Fe(C0)5 i n z e o l i t e o f t h e F a u j a s i t e t y p e depends on h e a t treatment, on the a p p l i c a t i o n o f s t a t i c o r continuous vacuum, on p a r t i a l pressure and composition o f gas m i x t u r e s i n t h e r e a c t o r , and on S i / A l r a t i o . Decomposition products i n s i d e t h e z e o l i t e have been analyzed by temperature-dependent Mossbauer and ESR measurements. Desorption products were invest i g a t e d by mass spectroscopy.
INTRODUCTION Thermogravimetric measurements have shown t h a t the thermal decomposition of Fe(C0)5 i n NaX z e o l i t e occurs i n two steps, w h i l e i n NaY i t i s a one-step process / r e f . I/. I n b o t h z e o l i t e s the thernial decomposition under continuous vacuum (?.10-3Pa) y i e l d s c r y s t a l 1 i n e a-Fe p a r t i c l e s which a r e considerably l a r g e r than t h e z e o l i t e c a v i t i e s and which are l o c a t e d o u t s i d e t h e z e o l i t e m a t r i x . Under t h e s p e c i f i c c o n d i t i o n t h a t the decomposition o f ironpentacarbonyl i n NaX takes place under s t a t i c vacuum, i . e . w i t h o u t removing t h e decomposition products from t h e r e a c t o r , a subcarbonyl i s formed above about350K. This subcarbon y l , Fe(C0I3, o r i t s polymerized form, (Fe(C0)3)n,
i s probably anchored t o t h e
z e o l i t e frame i n s i d e NaX, thus forming (Fe(C0)3)n...NaX.
I t i s s t a b l e up t o ca.
600K, and i t stays i n s i d e the z e o l i t e c a v i t i e s even under continuous vacuum conditions /ref.
2 / . The f o r m a t i o n o f t h i s subcarbonyl e x p l a i n s t h e stepwise decom-
p o s i t i o n o f ironpentacarbonyl i n NaX. F u r t h e r thermal decomposition o f (Fe(C0)3)n NaX under continuous vacuum up t o ca. 800K y i e l d s m e t a l l i c i r o n p a r t i c l e s
...
w i t h a r e l a t i v e l y broad d i s t r i b u t i o n o f magnetic h y p e r f i n e f i e l d s b u t w i t h a r e l a t i v e l y narrow s i z e d i s t r i b u t i o n . The superparamagnetic behavior o f these part i c l e s i n d i c a t e s t h a t t h e i r diameter i s comparable t o t h a t o f t h e z e o l i t e c a v i t i e s / r e f . 2/. We have supplemented t h i s i n v e s t i g a t i o n ( i ) by s t u d y i n g t h e thermal decomposition o f ironpentacarbonyl i n NaX under the presence o f A r o r H2 w i t h i n the i n s i t u c e l l , ( i i ) by u s i n g z e o l i t e s w i t h d i f f e r e n t S i / A l r a t i o f o r t h e decomposition under
790
argon, and ( i i i ) by r e c o r d i n g Mossbauer s p e c t r a o f i r o n p a r t i c l e s which have been exposed t o a H2/C0 gas m i x t u r e and by m o n i t o r i n g r e a c t i o n p r o d u c t s v i a mass spectroscopy. MATERIAL AND METHODS The a n a l y t i c a l c o m p o s i t i o n o f t h e e l e m e n t a r y u n i t o f t h e z e o l i t e s under s t u d y i s g i v e n by a S i / A l - r a t i o o f ( i ) 1.18 (NaX) with Na74.5
(A'88.6
( i i ) 1.32 (NaX) w i t h Na(H)82.8
Si103.4 0384), SilOgs2
0384), and
( i i i ) 2.48 (Nay) w i t h Na43.7 H,1.5
Si136.8 0384). F o r t h e a d s o r p t i o n o f Fe(C0)5 i n t h e v a r i o u s z e o l i t e s and t h e subsequent
thermal decomposition, we have combined a gas-dosing system d e s c r i b e d by B e i n / r e f . 3/ and an i n - s i t u g l a s s c e l l / r e f .
2/. L o a d i n g o f t h e z e o l i t e s w i t h two
Fe(C0)5 molecules p e r supercage was achieved by l o w - t e m p e r a t u r e a d s o r p t i o n . Thermal decomposition o f Fe(C0)5 under A r and H2 was c a r r i e d o u t under an i n i -
5
t i a l p r e s s u r e o f ca. 1 0 Pa. Maximum d e c o m p o s i t i o n t e m p e r a t u r e Tmax was reached w i t h a h e a t r a t e o f 0.5Kmin-'.
A f t e r h a v i n g a r r i v e d a t Tmax t h e m a t e r i a l was
c o o l e d down t o t h e Mossbauer measuring t e m p e r a t u r e w i t h i n a b o u t 10 min. The A r gas wasdeprived from O2 by a BTS c a t a l y s t ( f r o m BASF) and f r o m H20 by e i t h e r a L i n d e 4A m o l e c u l a r s i e v e o r a c o l d t r a p . The r e m a i n i n g O2 c o n t e n t was <1 ppm. Mossbauer and mass s p e c t r o s c o p i c measurements and t h e i r analyses c o r r e s p o n d t o those d e s c r i b e d e a r l i e r / r e f s .
1, 2, 4/.
RESULTS AND DISCUSSION Thermal decomposition under argon A f t e r thermal decomposition o f Fe(C0)5 i n NaX up t o Tmax=433K, 503K, and 673K, Mossbauer s p e c t r a were r e c o r d e d i n t h e t e m p e r a t u r e range between 4.2K and room temperature (RT). Here we show t h e 4.2K s p e c t r a ( F i g . 1 ) and t h e c o r r e s ponding Mossbauer parameters ( T a b l e 1 ) which were d e r i v e d f r o m l e a s t - s q u a r e f i t s of experimental s p e c t r a u s i n g L o r e n t z i a n 1 i n e s . C o n t r a r y t o t h e s i t u a t i o n where Fe(C0)5 i s c o m p l e t e l y decomposed a t 433K und e r vacuum, we f i n d
i n t h e p r e s e n t s t u d y , under argon, s t i l l ca. 27% o f i n t a c t
i r o n p e n t a c a r b o n y l ( S 5 , F i g . l a and T a b l e 1 ) . As decomposition p r o d u c t s we i d e n t i f y ca. 58% ( S 1 ) i r o n s u b c a r b o n y l s , (Fe(C0)3)n, and ca. 15% (S2-S4)
Fe2+ s p e c i e s
a c c o r d i n g t o t h e i r Irlossbauer parameters ( T a b l e 1 ) as d e s c r i b e d e a r l i e r / r e f . By mass spectroscopy we have observed d u r i n g t h e thermal decomposition o f
2/.
Fe(C0)5 i n NaX under A r a r e l a t i v e l y h i g h amount o f H2 gas ( p r o b a b l y f r o m r e s i d u a l h y d r o x y l groups o f NaX), w h i c h g i v e s r i s e t o t h e o x i d a t i o n o f z e r o - v a l e n t i r o n t o Fe2+ / r e f . 5, 6/. A f t e r r e a c h i n g decomposition temperature Tmax=503K, Fe(C0)5 i s c o m p l e t e l y
791
g- values
M
2
1.
b 0
d x' dB -5
B lmTl Velocity Irnrn/sl
F i g . 1 ( l e f t ) . Experimental Mossbauer s p e c t r a o f t h e thermal decomposition p r o d u c t s o f Fe(CO)S i n NaX under a r g o n w i t h maximum decomposition t e m p e r a t u r e of a ) 433K, b ) 503K and c ) 673K. The Mossbauer measurements were performed a t 4.2K. F i t parameters o f c a l c u l a t e d s p e c t r a ( s o l i d l i n e s ) a r e summarized i n Table 1. F i g . 2 ( r i g h t ) . ESR s p e c t r a measured a t a ) RT and b ) 2K o f thermal d e c o m p o s i t i o n p r o d u c t s o f Fe(COIs i n NaX under argon w i t h Tmax=673K. decomposed ( F i g . I b ) , and a t h i g h e r temperature (Tmax>673K) t h e i r o n s u b c a r b o n y l (Fe(CO)3)n f u r t h e r decomposes under f o r m a t i o n o f a magnetic species ( b r o a d subspectrum S6 i n Fig. I c ) . Isomer s h i f t 6*0.lmms-l a t 4.2K as w e l l as f e r r o m a g n e t i c resonance b e h a v i o r a t RT and 2K ( F i g . 2 ) i n d i c a t e t h a t we a r e concerned h e r e w i t h magnetic i r o n p a r t i c l e s . The Fe3+ c o n t r i b u t i o n s , v i s i b l e i n t h e ESR s i g n a l a t 9=4.3,
a r e m i n o r and a r e t h e r e f o r e n o t d e t e c t a b l e i n t h e Mossbauer spectrum.
Thermal decomposition up t o 733K y i e l d s decomposition p r o d u c t s t h e Mossbauer s p e c t r a o f which a r e shown i n F i g . 3. C h a r a c t e r i s t i c o f t h e s e s p e c t r a i s t h a t t h e magnetic s p e c i e s e x h i b i t more s t r u c t u r a l i n f o r m a t i o n t h a n t h e b r o a d sub-
2+
spectrum S 6 i n F i g . l c . The 4.2K spectrum ( F i g . 3 c ) c o n s i s t s o f 20% o f Fe s p e c i e s (6>0.7mms-')
and 10% o f a-Fe p a r t i c l e s (6~0.1mms-1 and Bint=34
Tesla).
The r e m a i n i n g 70% o f i r o n correspond t o t h e " s t r i p p e d " spectrum i n F i g . 4a,
792
TABLE 1 Isomer s h i f t s 6 ( i n mm/s),
quadrupole s p l i t t i n g s AE
Q
( i n mm/s), l i n e w i d t h s
r
( i n mm/s), and r e l a t i v e abundances A o f subspectra ( i n %) f o r v a r i o u s thermal decomposition products o f Fe(C0)5 i n NaX under argon. Measurements were performed a t 4.2K. Figure
s1 s2 la
s3 s4 s5
1b
s1 s2 s3 s4 S1
lc
r
A
0.47
0.43
58.3
0.90
0.65
0.46
7.6
1 .oo
2.10
0.40
3.3
AE
subspectrum
s2 s3 s6
Q
0.02
1.19
2.60
0.40
4.3
-0.06
2.47
0.28
26.5
0.00
0.60
0.49
73.0
0.97
0.65
0.30
6.8
0.94
2.10
0.40
9.3
1.34
2.60
0.40
10.9
0.02
0.56
0.37
18.5
0.97
0.65
0.48
3.8
1.30 0.09
2.55
0.75
11.9
0
9.65
65.7
( a ) 6 g i v e n r e l a t i v e t o a-Fe a t RT.
which was c a l c u l a t e d w i t h t h e h y p e r f i n e f i e l d d i s t r i b u t i o n p a t t e r n drawn i n F i g .
4b. The d e v i a t i o n of t h i s p a t t e r n from t h e b u l k - v a l u e o f a-Fe o f Bint=34
Tesla
and i t s broad d i s t r i b u t i o n around 30 Tesla i n d i c a t e s t h e inhomogeneous composit i o n o f t h i s species. A p a r t from n o n c r y s t a l l i n e i r o n f o r m a t i o n and s u r f a c e l a y e r s of c r y s t a l l i n e i r o n p a r t i c l e s , which have been t h e main sources f o r t h e Bint d i s t r i b u t i o n discussed i n r e f . 2, p o s s i b l e c o n t r i b u t i o n s t o Bint
may a l s o a r i s e
from i r o n - c a r b i d e s and surface-chemisorption o f CO by i r o n p a r t i c l e s . The s o l i d l i n e of Fig. 3c represents a successful f i t w i t h Fez+ species (20%), a-Fe p a r t i c l e s (10%) and magnetic species w i t h inhomogeneous composition (70%). The s i n u l a t i o n s a t 77K and RT (Fig. 3a, b ) , i n a d d i t i o n , t a k e care o f t h e superparamagn e t i c r e l a x a t i o n of the magnetic species / r e f .
7/. The r e s u l t i n g superparaiag-
n e t i c blocking-temperature TB o f 230K i s used t o e s t i m a t e t h e average p a r t i c l e volume V o f t h e magnetic species by comparing magnetic a n i s o t r o p y energy w i t h thermal energy, KVskTB, w i t h t h e a n i s o t r o p y c o n s t a n t K b e i n g 7.10 5Jmq3 f o r small iron particles /ref.
8/. The e s t i m a t e f o r V y i e l d s as average p a r t i c l e diameter
t h e value 201, which o b v i o u s l y i s l a r g e r than t h e diameter o f t h e z e o l i t e super-
793
-10
i
-6
4
10
$I 60
70
Imm/sl
Velocity
.005 c
404
5$ ,003 U 0
.-> .992L’
’
-10 -8
’
1
.
. -. !
-6 - C - 2 0 2 Velocity Imm/sl
,002
z ,001 &
’
C
.
6
’ 0
’
10
OI
10
20
30
w)
Hyprfinefield B [Teslnl
F i g . 3 ( l e f t ) . Experimental Mossbauer s p e c t r a o f t h e thermal decomposition p r o d u c t s o f Fe(CO)s i n NaX under argon w i t h maximum decomposition t e m e r a t u r e o f 733K. The Mossbauer measurements were performed a t a ) RT, b ) 77K, and c ) 4.2K. The sol i d 1i n e s r e p r e s e n t t h e o r e t i c a l s p e c t r a i n c l u d i n g k e 2 + s e c i e s i.e. 5% w i t h 6=0.82mm~-~, A E -2.18mm~-~, 5% w i t h 6 = U . 7 4 m m ~ - ~ , and 10% w i t h 6=1.27mm~-~, A&=2.81mms-’, i.e. 10% w i t h 6=0,1mms-1, A E 0, B i n t = 3 4 Tesla, and magnetic species, i.e. 70% w i t h 6=8f25ms-’, AEQ=O, and t h e B i n t d i s t r i b u t i o n as shown i n F i g . 4b. The c a l c u l a t e d s p e c t r a i n c l u d e , i n a d d i t i o n , t h e superparamagnetic r e l a x a t i o n beh a v i o r o f a-Fe and o f t h e magnetic species w i t h inhomogeneous c o m p o s i t i o n , i . e . t h e 4.2K spectrum i s i n t h e s l o w and t h e RT spectrum i s i n t h e f a s t r e l a x a t i o n 1 i m i t.
A’, &,
F i g . 4 ( r i g h t ) . a ) Mossbauer spectrum o f F i g . 3c which has been s t r i p p e d f r o m Fez+ and a-Fe c o n t r i b u t i o n s . The s o l i d l i n e r e p r e s e n t s a c a l c u l a t e d spectrum w i t h 6=0.25mms-’, AEQ=O, and t h e Bint d i s t r i b u t i o n as shown i n b ) . cages (13A). Since d u r i n g thermal decomposit.ion n e i t h e r was a m e t a l - m i r r o r observed i n t h e i n - s i t u g l a s s c e l l n o r a desorbed i r o n d e t e c t e d i n t h e mass spec-. trum, we conclude t h a t t h e i r o n p a r t i c l e s remained i n s i d e t h e z e o l i t e m a t r i x . I t i s very l i k e l y t h a t t h e anisotropy constant i n our estimate i s t o o small; surf a c e a n i s o t r o p i e s o f i r o n p a r t i c l e s , due t o c h e m i s o r p t i o n o f CO f o r example,may y i e l d an i n c r e a s e o f t h e a n i s o t r o p y c o n s t a n t by 30% / r e f . 9/. I t n i g h t a l s o be p o s s i b l e , as suggested i n c o n n e c t i o n w i t h t h e f o r m a t i o n o f P t p a r t i c l e s i n NaX / r e f .
l o / , t h a t t h e i r o n p a r t i c l e s a r e i n d e e d l a r g e r i n s i z e t h a n 1 3 1 due t o
794
pressure [Pa)
10
0
20
30
50
50
60
CO - ---- ------/-----10+
.yo
8: 6CH4
90
-
F
4: 2
C
t
L l
*
-10
-8
I
-6
I
-4 -2
!
.
.
0 2 4 Velocity Imdsl
a
6’
I
.
8
10
7
:
:
I
:
mass
Fig. 5 ( l e f t ) . Experimental Mossbauer s p e c t r a o f t h e thermal decomposition p r o ducts o f Fe(COI5 i n NaX under H, w i t h maximum decomposition temperature o f a) 433K and b ) ,c) 573K. The Mossbauer measurements were performed a t a ) ,c) 4.2K and b ) RT. S o l i d l i n e s correspond t o c a l c u l a t e d s p e c t r a u s i n g t h e parameters summarized i n Table 2. F i g . 6 ( r i g h t ) . Mass s p e c t r a o f desorbed gases a r i s i n g from thermal decomposition o f Fe(CO)S i n NaX under H, w i t h maximum decomposition temperature a) 433K and b ) 573K.
s t r u c t u r a l rearrangement o f t h e z e o l i t e m a t r i x d u r i n g h e a t treatment. Transmiss i o n e l e c t r o n microscopy s t u d i e s a r e i n progress t o e l u c i d a t e t h i s behavior. Thermal decomposition under H2 Complementary t o t h e s i t u a t i o n under i n e r t gas ( A r ) , described i n t h e previous s e c t i o n , we have a l s o s t u d i e d t h e thermal decomposition o f Fe(C0)5 i n NaX under H2. The Mossbauer s p e c t r a o f decomposition products, o b t a i n e d a f t e r heating up t o TmaX=433K and 573K a r e shown i n F i g . 5. Comparing t h e decomposi-
-
t i o n under A r (Fig. l a ) and under H2 ( F i g . 5a, Table 2 ) i t i s obvious t h a t Fe(COI5 has completely vanished a t Tma,=433K
under H2, c o n t r a r y t o t h e s i t u a t i o n
795
under A r . The c o r r e s p o n d i n g mass spectrum ( F i g . 6 a ) i n d i c a t e s t h a t CO i s desorbed from t h e z e o l i t e . Flossbauer s p e c t r a o f decomposition p r o d u c t s correspondi n g t o Tmax=573K ( F i g . 5b,c,
Table 2) r e v e a l t h a t t h e i r o n s u b c a r b o n y l has been
decomposed c o m p l e t e l y : a c o n s i d e r a b l e amount o f i r o n (ca. 30%) has been o x i d i z e d t o Fez+ (S4, F i g . 5 c ) , and a s i g n i f i c a n t p o r t i o n o f i r o n (ca. 70%) has formed magnetic components (S5, F i g . 5c). The l a t t e r e x h i b i k a broad magnetic p a t t e r n even a t RT ( F i g . 5b); t h i s o b s e r v a t i o n i s a s s o c i a t e d w i t h i r o n p a r t i c l e s w i t h c o n s i d e r a b l e magnetic a n i s o t r o p y , which p r o b a b l y a r i s e s because o f c h e m i s o r p t i o n of CO and H2 a t t h e i r s u r f a c e o r because o f t h e i r i n t e r a c t i o n s w i t h t h e z e o l i t e frame.
Due t o t h e l a c k o f Fe peaks i n t h e mass spectrum o f t h e desorbed gases
of t h i s probe ( F i g . 6b) we s t i l l b e l i e v e t h a t t h e s e i r o n p a r t i c l e s a r e enclosed i n t h e c a v i t i e s o f t h e m o l e c u l a r s i e v e . The mass spectrum o f F i g . 6b i n a d d i t i o n r e v e a l s t h a t , besides t h e c o n t r i b u t i o n s a l r e a d y i n h e r e n t i n F i g . 6a, methane was formed. Methane i s i d e n t i f i e d by i t s c h a r a c t e r i s t i c fragments / r e f . 11/ a t masses 1 4 ( 1 . 6 ~ 1 0 - ~ P a ) ,15(2.7*10-6Pa) and 16(3.3.10-6Pa).
I t i s tempting t o
assume t h a t a t e l e v a t e d temperature t h e f o r m a t i o n o f CH4 and Fez+ has o c c u r r e d v i a t h e r e a c t i o n s 3H2+C0 -+ CH4+H20 and Feo+H20 + FeO+H2.
TABLE 2 Isomer s h i f t s 6 ( i n mm/s), quadrupole s p l i t t i n g AE
Q
( i n mm/s),
l i n e widths
r
( i n mm/s), and r e l a t i v e abundances A ( i n % ) o f s u b s p e c t r a f o r thermal decompos i t i o n p r o d u c t s o f Fe(CO)5 i n NaX under H2. Measurements were performed a t 4.2K.
Figure
5a
5c
su bspec t r u m
s2
&(a)
AEQ
r
A
0.01
0.43
0.46
78.5
0.95
0.65
0.31
7.3
s3
0.78
2.10
0.42
4.1
s4
1.31
2.61
0.50
10.1
s4
1.27 0.1
2.69
0.50
30.0
0
7.0
70.0(b)
s5
( a ) 6 g i v e n r e l a t i v e t o a-Fe a t RT. ( b ) The broad magnetic p a t t e r n has a l s o been s i m u l a t e d by u s i n g a Bint d i s t r i b u t i o n , T h i s d i s t r i b u t i o n i s s i m i l a r t o t h e p a t t e r n shown i n F i g . 4b; i t i s , however, c e n t e r e d h e r e around ca. 25 T e s l a compared t o ca. 30 T e s l a i n F i g . 4b.
796
Reaction products under H2/C0 atmosphere Thermal decomposition products o b t a i n e d from Fe(C0I5 i n NaX under A r a t Tmax=733K (corresponding t o t h e s p e c t r a shown i n F i g . 3 ) were exposed a t 523K t o a gas m i x t u r e w i t h composition 3H2tC0 f o r 16h. The i n i t i a l pressure o f t h e
5
H2/C0 m i x t u r e i n t h e g l a s s - i n s i t u c e l l a t RT was ca. 10 Pa. The r e s u l t i n g react i o n products y i e l d Mossbauer and mass s p e c t r a as shown i n F i g . 7a,b and F i g . 8, r e s p e c t i v e l y . From t h e quadrupole d o u b l e t w i t h 6.~1.3mm/s i t i s obvious t h a t about 70% o f t h e i r o n (Table 3 ) appears as Fe2+ species (S1). Only about 30% o f t h e i r o n (S2) c o n t r i b u t e s a t 4.2K w i t h a broad unresolved magnetic p a t t e r n ( F i g . 7b), which i s h a r d l y d e t e c t a b l e a t 77K (Fig. 7a), i n d i c a t i n g t h a t S2 r e p r e s e n t s small magnetic p a r t i c l e s . A comparison o f t h i s s i t u a t i o n w i t h t h e p r e c u r s o r ( F i g . 3 ) o f the r e a c t i o n products r e v e a l s t h a t most o f t h e z e r o - v a l e n t i r o n has been o x i d i z e d by t h e H2/C0 gas m i x t u r e a t 523K t o f e r r o u s i r o n . Because o f t h i s f i n d i n g and due t o t h e appearance o f methane, ethane and C02 i n t h e mass spectrum ( F i g . 8 ) we suspect t h a t t h e d i s s o c i a t i v e a d s o r p t i o n o f CO a t t h e s u r f a c e o f iron particles /ref.
12, 13/ i s f o l l o w e d by t h e f o r m a t i o n o f hydrocarbons
and by t h e o x i d a t i o n o f Fe" t o FeO and o f CO t o C02. To f u r t h e r c h a r a c t e r i z e t h e magnetic p a r t i c l e s which a r e represented by t h e broad l i n e ( S 2 ) i n F i g . 7b, we have s i n t e r e d the r e a c t i o n products a t 823K i n continuous vacuum (ca. 10-3Pa) f o r I h r and subsequently recorded a Mossbauer spectrum a t 4.2K ( F i g . 7 c ) . T h i s 2t spectrum c o n t a i n s ca. 70% o f c o n t r i b u t i o n s from Fe species (S1), w h i l e t h e remaining ca. 30% ( S 2 ) o f t h e a b s o r p t i o n p a t t e r n undoubtedly r e p r e s e n t s a-Fe p a r t i c l e s (Table 3 ) . T h i s r e s u l t p r o v i d e s f u r t h e r evidence t h a t t h e above mentioned r e a c t i o n s i n c l u d e zero-val e n t i r o n p a r t i c l e s
.
Thermal decomposition i n z e o l i t e s w i t h d i f f e r e n t Si/A1 r a t i o A l t e r n a t i v e l y t o t h e thermal decomposition o f Fe(C0I5 i n NaY under c o n t i n u ous vacuum, which has y i e l d e d r e l a t i v e l y l a r g e c r y s t a l l i n e a-Fe p a r t i c l e s o u t -
s i d e t h e z e o l i t e m a t r i x /ref.
I/,we have i n v e s t i g a t e d t h e f o r m a t i o n o f decom-
p o s i t i o n products under argon. F i g . 9 shows t h e corresponding CO d e s o r p t i o n from 300 t o 503K. From t h e f i n a l CO p a r t i a l pressure o f 2.87.103Pa a t 503K and t h e o r i g i n a l amount o f carbonyls o f 9.44.10-4mol
we e s t i m a t e t h a t 9424% o f t h e over-
a l l amount o f CO has been desorbed a t 503K. The r e s u l t i n g decomposition products y i e l d Mossbauer spectra ( F i g . 10a,b) w i t h a d i s t r i b u t i o n o f i n t e r n a l f i e l d s ( F i g . 1Oc) which i s p r a c t i c a l l y i d e n t i c a l a t 4.2K and a t RT. T h i s o b s e r v a t i o n i n d i c a t e s t h a t Fe(C0j5 has been completely desorbed from NaY and decomposed i n t o CO and r e l a t i v e l y l a r g e i r o n p a r t i c l e s o u t s i d e t h e z e o l i t e m a t r i x . Only about 10% o f t h e i r o n forms c r y s t a l l i n e a-Fe p a r t i c l e s w h i l e t h e remaining 90% o f t h e i r o n forms amorphous i r o n , i r o n c a r b i d e s o r oxycarbides and t o a m i n o r e x t e n t (<5%) i r o n oxides. Comparison o f t h e thermal decomposition o f Fe(C0)5
in
NaY under argon a t 503K ( F i g . l o b ) w i t h t h e s i t u a t i o n i n NaX under t h e same
I91 1.00
.99
.90
pressure ( P o l
1.00 c
.-YI
',.
.9s
w
e
c
.98
> .-+
-e
10 - 5
LT 01
1.00
0
.99
10
20
30
40
50
60
moss
J
-10
-5
0 Velocity Imm/sl
5
10
F i g . 7 ( l e f t ) . a,b) Experimental Mossbauer s p e c t r a o f r e a c t i o n p r o d u c t s o b t a i n ed by exposing t h e m a t e r i a l c o r r e s p o n d i n g t o F i g . 3 t o a gas m i x t u r e w i t h composition3H2+C0, p r e s s u r e ca. lO5Pa, and temperature Tmax=523K f o r 16h. The Mossbauer measurements have been performed a t a ) 77K and b ) 4.2K. c ) Experimental Mossbauer spectrum a t 4.2K o f t h e m a t e r i a l c o r r e s p o n d i n g t o a,b) a f t e r s i n t e r i n g a t 823K i n continuous vacuum ( ~ a l O - ~ P af )o r 5h. S o l i d l i n e s i n a ) - c ) r e p r e s e n t c a l c u l a t e d s p e c t r a w i t h t h e parameters summarized i n Table 3. F i g . 8 ( r i g h t ) . Mass spectrum o f desorbed gases o b t a i n e d b y exposing t h e m a t e r i a l corresponding t o F i g . 3 t o a gas m i x t u r e w i t h c o m p o s i t i o n 3H,+CO, p r e s s u r e ca. lOSPa, and temperature Tmax=523K f o r 16h under s t a t i c c o n d i t i o n s . conditions (Fig. l b ) again reveals t h a t ironcarbonylsare s t a b i l i z e d i n z e o l i t e s w i t h small Si/A1 r a t i o (1.18 f o r NaX) compared t o t h o s e w i t h l a r g e Si/A1 r a t i o w i t h t h e s u g g e s t i o n / r e f s . 14, (2.48 f o r Nay). T h i s r e s u l t may be a s s o c i a t e d (i) 15/ t h a t t h e acid-base i n t e r a c t i o n between t h e Na+ c a t i o n s o f t h e z e o l i t e (Lew i s - a c i d ) and t h e oxygen atoms o f t h e c a r b o n y l s (Lewis-base) i s a s t a b i l i z i n g f a c t o r , and ( i i ) w i t h t h e f a c t t h a t t h e number o f Na+ c a t i o n s i n NaX i s ca. two times as l a r g e as t h a t i n Nay. I t i s i n t e r e s t i n g t o n o t e t h a t an a d d i t i o n a l measurement u s i n g a z e o l i t e w i t h S i / A l r a t i o 1.32 s u p p o r t s t h i s r e a s o n i n g . The Mossbauer spectrum a t 4.2K o f t h e thermal decomposition p r o d u c t s o f Fe(C0)5, ob-
798
300 -10
-5
0
5
10
1
350
LOO
450
500
decomposition temperature lK1
V e l o c i t y Imm/sl
I
I
-10 Hyperfinefield
B ITeslal
-5
Q
5
10
Velocity I m d s l
F i g . 9 ( t o p r i g h t ) . CO-desorption c u r v e r e c o r d e d f r o m mass s p e c t r a of t h e t h e r mal decomposition o f Fe(C0)S i n NaY under a r g o n (ca. 105Pa). F i g . 10 ( l e f t ) . a,b) E x p e r i m e n t a l Mossbauer s p e c t r a o f thermal decomposition p r o d u c t s of Fe[CO), o b t a i n e d i n NaY under argon (ca. IO’Pa) a t Tmax=503K. The Mossbauer measurements were performed a t a ) RT and b ) 4.2K. S o l i d l i n e i n b ) r e p r e s e n t s c a l c u l a t e d spectrum which c o n s i s t s o f 10% o f a-Fe and 90% o f t h e B i n t d i s t r i b u t i o n shown i n c ) . The f i e l d o f 34 T (a-Fe) corresponds t o 6=O.lmm/s, f i e l d s of <40 T correspond t o 6=0.25mm/s and >40 T t o 6=0.45mm/s. F i g . 1 1 ( b o t t o m r i g h t ) . Experimental Mossbauer spectrum a t 4.2K o f t h e thermal decomposition p r o d u c t of Fe(C0)’ o b t a i n e d i n a z e o l i t e w i t h S i / A l r a t i o 1.32 und e r argon (ca. 105Pa) a t Tmax=433K. Subspectra Si-Ss c o r r e s p o n d t o t h e a s s i g n ment i n F i g . l a . The c o n t r i b u t i o n o f Fe(C0)5 amounts t o 12%.
t a i n e d i n t h i s z e o l i t e under argon a t Tmax=433K ( F i g . 111, r e v e a l s t h a t i r o n pentacarbonyl c o n t r i b u t e s o n l y 12% t o t h e o v e r a l l a b s o r p t i o n . I n NaX w i t h Si/A1 r a t i o 1.18 under t h e same c o n d i t i o n s t h i s c o n t r i b u t i o n i s 27% ( F i g . l a ) . Thus i t i s obvious t h a t a l r e a d y a s m a l l v a r i a t i o n i n S i / A l r a t i o i s r e f l e c t e d by a s i g n i f i c a n t change o f z e o l i t e - F e ( C 0 ) 5 i n t e r a c t i o n . CONCLUSIONS Thermal decomposition o f Fe(C0)5 i n NaX under c o n t i n u o u s vacuum ( c a . 10-3Pa)
799
TABLE 3 Isomer s h i f t s 6 ( i n mm/s),quadrupole ( i n mm/s),
s p l i t t i n g s AE
r,
( i n trun/s), l i n e w i d t h s
Q
r e l a t i v e abundances A ( i n %), and i n t e r n a l magnetic f i e l d Bint
(in
T e s l a ) o f subspectra c o r r e s p o n d i n g t o F i g . 7. Measurements were p e r f o r m e d a t 77K (7a) and 4.2K (7b,c), Figure
subspectrum
respectively. ,(a) A,EQ
7a 7b
s1 s2
7c
s1 s2
r
A 100
1.27
2.66
0.68
1.27
2.85
0.5
71.9
0.18
0
4.5
28.1
1.32
2.80
0.71
66.3
0.14
0
0.43
33.7
8int
34.3
( a ) 6 g i v e n r e l a t i v e t o a-Fe a t RT.
y i e l d s i r o n p a r t i c l e s o u t s i d e t h e z e o l i t e . Under s t a t i c vacuum and under gas atmosphere (argon, H2, H2/C0 m i x t u r e ) t h e i r o n c a r b o n y l remains i n s i d e t h e NaX m a t r i x d u r i n g thermal decomposition. However, upon i n c r e a s i n g t h e S i / A l r a t i o f r o m 1.18 (NaX), o v e r 1.32 t o 2.48 (Nay), t h e acid-base i n t e r a c t i o n between t h e Na' c a t i o n s o f t h e z e o l i t e and t h e c a r b o n y l s o f Fe(C0I5 fades because t h e number o f Na' c a t i o n s decreases. Thus, i r o n p e n t a c a r b o n y l i s s t a b i l i z e d i n z e o l i t e s w i t h small Si/A1 r a t i o and i t i s e a s i l y desorbed f r o m z e o l i t e s w i t h enhanced Si/A1 ratio. Formation o f i r o n s u b c a r b o n y l s , (Fe(C0)3)n, occurs i n NaX under s t a t i c vacuum o r gas atmosphere a t moderate decomposition temperatures. They a r e s t a b l e up t o %570K under vacuum, up t o Tmax670K under A r and up t o Tmax~500Kunder H2. Tmax These i r o n s u b c a r b o n y l s a r e anchored t o t h e NaX frame; hence, f u r t h e r decomposit i o n a t h i g h e r temperatures i s p o s s i b l e even under c o n t i n u o u s vacuum (ca. l t ~ - ~ P a ) w i t h o u t d e s o r p t i o n o f i r o n from t h e NaX m a t r i x . Thermal decomposition o f i r o n s u b c a r b o n y l s i n NaX under argon a t e l e v a t e d temperature (Tmax%730K) y i e l d s r e l a t i v e l y l a r g e n o n c r y s t a l l i n e i r o n p a r t i c l e s , which a r e l o c a t e d undoubtedly i n s i d e t h e z e o l i t e m a t r i x . A t p r e s e n t i t i s n o t c l e a r whether these p a r t i c l e s e x h i b i t l a r g e s u r f a c e a n i s o t r o p i e s due t o chemis o r p t i o n o f CO o r shape a n i s o t r o p y o r a r e indeed l a r g e r i n s i z e t h a n t h e supercages (13A) due t o s t r u c t u r a l rearrangement o f t h e z e o l i t e m a t r i x d u r i n g h e a t t r e a t m e n t . T h i s b e h a v i o r w i l l be f u r t h e r t e s t e d by e l e c t r o n t r a n s m i s s i o n m i c r o s copy s t u d i e s . Thermal decomposition o f i r o n s u b c a r b o n y l s i n NaX under H2 and exposure o f n o n c r y s t a l l i n e i r o n p a r t i c l e s t o H2/C0 gas m i x t u r e l e a d s t o t h e d i s s o c i a t i v e
800
a d s o r p t i o n o f CO a t t h e s u r f a c e o f i r o n p a r t i c l e s w i t h t h e subsequent f o r m a t i o n o f hydrocarbons and the o x i d a t i o n o f Fe" t o Fe2+ and o f CO t o C02. Our f u r t h e r i n v e s t i g a t i o n o f t h e i r o n - z e o l i t e system i n c l u d e s s t u d i e s w i t h Si/A1 r a t i o between 1.3 and 2.4, m u l t i p l e a d s o r p t i o n and decomposition steps, and exposure t o v a r i o u s gas m i x t u r e s .
ACKNOWLEDGEMENT This work was supported by t h e Deutsche Forschungsgemeinschaft. D i s t r i b u t i o n s o f h y p e r f i n e f i e l d s were o b t a i n e d w i t h a procedure d e s c r i b e d by G. Le Caer and J.M. Dubois, and spectra i n c l u d i n g superparamagnetic r e l a x a t i o n were c a l c u l a t e d
by u s i n g a r o u t i n e w r i t t e n by H. Winkler; t h e use o f these programs i s g r a t e f u l l y acknowledged.
REFERENCES 1 F. Seel, B. Wolf, U. Gonser, R. K l e i n , G. Doppler, E. B i l l and A.X. T r a u t wein, Z e i t s c h r i f t f u r anorganische und allgemeine Chemie 534 (1986) 159. 2 H.M. Ziethen, G. Doppler, A.X. Trautwein and F. Schmidt, C a t a l y s i s Today 3 (1988) 83. 3 T. Bein, F. Schmidt and P.A. Jacobs, Z e o l i t e s 5 (1985) 240. 4 G. Doppler, E. B i l l , U. Gonser, F. Seel and A.X. Trautwein, H y p e r f i n e I n t e r a c t i o n s 29 (1986) 1307. 5 D. Ballivet-Tkatchenko and G. Coudurier, I n o r g a n i c Chemistry 18 (1979) 558. 6 T. Bein, P.A. Jacobs and F. Schmidt, Studies i n Surface Science and C a t a l y s i s 12 (1982) 111. 7 H.M. Ziethen, D i s s e r t a t i o n , Lubeck (1988). 8 P.H. Christensen, S . MBrup and J.W. Niemantsverdriet, J. Phys. Chem. 89 (1985) 4898. 9 S. MBrup, B.S. Clausen and H. Topsbe, J. Physique Colloq. 41 (1981) C1 331. 10 A. K l e i n e , P.L. Ryder, N. Jaeger and G. S c h u l z - E k l o f f , J. Chem. SOC., Faraday Trans. I,82 (1986) 206. 11 A. Cornu and R. Mussot, Compilation o f Mass S p e c t r a l Data, Heydon & Son Ltd., London (1975) p. 1 A . Niemantsverdriet, C.F.J. F l i p s e , A.M. van der Kraan and J.J. van L o e f , 12 J.W. A p p l i c a t i o n s o f Surface Science 10 (1982) 302. 13 F. Schmidt, I n d u s t r i a l A p p l i c a t i o n o f t h e Mossbauer E f f e c t , G.J. Long and J.G. Stevens (eds.), Plenum Press, N.Y. (1986) p. 667. 14 D. Ballivet-Tkatchenko, G. Coudurier and H. Mozzanega, S t u d i e s i n Surface Science and C a t a l y s i s 5 (1980) 309. 15 T. Bein, S.J. McLain, D.R. Corbin, R.D. F a r l e e , K. M o l l e r , G.D. Stucky, G. Woolery and D. Sayers, J. Am. Chem. SOC. 110 (1988) 1801.
H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders
0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Investigation of Ultra Stable Y by Differential Thermal Analysis Injection of Water Vapour
after
A.YCSHIDA and .K.INOUE Government Industrial Research Institute, Kyushu; Shuku-machi, Tosu-shi, Sagaken 841 Japan
ABSTRACT Differernt.ia1 thermal analysis(DTA) of ultra stable Y(USY) with preinjection of water vapour was performed with an improved DTA unit. The exotherms with water vapour were compared with the results of the traditional steaming test. The peak shap and temperature were affected by the rate of rising temperature and amount of water pre-injected into a t h m l chamber. However, the pre-injection temperature of water vapour did not affect them rerrarkably. The collapse of the pore system was confirmed by XRD and nitrogen gas adsorption. The correlations between the intersection temperatures of maximum rising gradients of peaks relative to the base lines and the results of the steaming test at 75OoC were comparatively good.
Introduction Zeolite Y has been used as an active ingredient in fluid catalytic cracking (Fcc) catalysts(ref.1). One of the most important properties of zeolite Y used in FCC catalysts is its hydrotherml stability(ref.2-10). As sham by McDaniel et al.(ref.2), zeolite Y can be ultrastabilized by a conventional ion exchange and heat treatment. The dealumination of zeolite Y was also achieved by treatment with HC1 solution(ref.3-4). The hydrothermal stability of USY has been studied by means of the steaming test (ref.5-6) Bremer et al. (ref.7), Skeels et al. (ref.8) and Li et al. (ref.9) carried out investigations using convenient DTA method. Hmver, in DTA the sample is usually exposed to an atmsphere of air or inert gas flow. The stability of zeolite Y under 100% steam is different from that in an inert gas or in air. To solve this problem, DTA with pre-injection of water vapour was conceived.
.
METHODS
Zeolite sodium Y samples were prepared from colloidal silica, silica sol and sodium aluminate. LZ-Y52, LZ-Y82(U.C.C.) and 3 reference catalysts, JEC-ZY4.8(Y4.8) , JK-Z-HY4.8(HY4.8) and JEC-Z-Y5.6(Y5.6) supplied from the Catalysis Scciety of Japan were used too. NH4NaY samples were prepared by the
conventional cation-exchange procedure and heated in a capsule or with steam. After the second cation exchange, HNH4Y samples(=USY) were prepared. Sane of
802
them were again heated in a capsule or with steam, and treated with 0.096N and 0.95N HC1 solutions. The improved DTA unit is sham in Fig.1. Until the injection temperature 60 cm3 min-' of helium gas was was reached, with a heating rate of 2OoC min-', introduced through the water vapour effluent without the pin. Then, 1 to 10 cm3 of water were injected through the penetrating pipe at the bottom of the furnace at the rate of 1 cm3 min-1, ax^ evaporated. The programed temperature was held at the injection temperature for 15 min. From the injection temperature up to 100O-143O0C, a small amount of water vapour continued to evaporate. At an appropriate temperature, the furnace was removed and quenched i n air. It was checked by XRD arid nitrogen gas adsorption to determine to what extent the sample had retained its crystallinity and pore system. The exotherms of DTA and the results of the steaming test (with 100% steam, for 6 hours) were anpared. Some of the zeolite samples were chemically analyzed by a wet process.
Fig.1 Apparatus for DTA with pre-injection of water vapour l=vacuum compound, 2=penetrating injection pipe, 3=acrylic resin, I=silicon ruber, 5=refractary,6=furnace, 7=alumjna, 8=sample, 9 = t h m l chanber, lO=pin or tube, ll=water vapour effluent, 12=Mohr pipette, 13=micro-tube punp
803
RESULTS AND DISCUSSION As the amunt of water injected increased, the exotherm of HY4.8 shifted t o 3 laver temperature, and equilibrated(Fig.2). Five cm of water evaporated t o 3 about 24.4 dm a t 800°C, i.e. about 1000 times that of the thermal chandxr. Hence, we supposed t h a t the atmosphere i n the themdl c h a h x reached 100 % stem a f t e r injection of 5 cm3 of water. The exotherms were affected by the heating rate a f t e r injection of water. However, the injection temperature had no remarkable e f f e c t when enough water was pre-injected(Fig.3). In the case of 2OC m i n . - l and l 0 C min.-', the peak temperatures increased slightly, because the real heating rates were s l i g h t l y
n
u
W
0
~
1c m
2! W
~ 3
3 -
-
b
=
1 I
x
I-
3
960
9
4
Q
0
U
C
W
800
Fig.2
700
900 1000 1100 Temperature('C) The e f f e c t of the amount of
water pre-injected on the exot h m s of HY4.8 a t heating rate of 10°C min-l.
In] ection temperature=860'C
800
900
1000
Injection Temperature(OC) Fig.3
The e f f e c t s of heating rate arad
injection t e n p r a t u r e on the exother-
ma1 peak temperature of HY4.8. 3 Amount of H2C=7 rn lwithout 'injection, 2=200cmin.-l -1 -1 3=1OoC min. , 4=5'C min.
.
6=2*C min.-',
6=loC min.-'
804
850
950
1050
1150
1250
Temperature(OC)
Fig.4 Differential therrrcgrams of zeolite Y under 100 % steam. 1,2:NH4NaY(LZ-Y52), 1=Na20=3.8%; 2=2.9%; 3:NH4NaY(3-3-103) Na20=2.9%; 4:HNaY(LZ-Y52), U.D.=2.449 nm, 5:HNHqY(HY4.8), U.D.=2.450 run; 6:HY(LZ-Y52; 2 cycles of ibn exchange and heat treatment), U.D.=2.428 7:HY(LZ-Y52) after treatment with 0.096N HC1 solution, U.D.=2.428 run; 8:HY(Y5.6) after treatment with 0.95N HC1 solution, U.D.=2.426 nm.
nm;
805
higher than the programed one. The amount of zeolite sample did not change the exothemal peak temperature remarkably. The shapesof exothenns of the zeolite samples are shown in Fig.4. In the case of NH4NaY, the peak shape and the exothermal peak temperature were affected by the Si02/A1203 molar ratio of the original sodium Y. After the second ion exchage, the hydrothermal stability was inproved remarkably, though the unit cell dimension did not shrink, because of the decrease of Na' ions. Mter the second heat treatment and subsequent treatment with 0.096N HC1, the unit cell dimension shrunk to 2.428 nm, and the content of sodium oxide decreased to ca. 0.1%; hawever, the exothermal peak temperature did not shift to higher temperature. HY treated with 0.096N HC1 contains ca. 12% of A1203 while HY treated with 0.95N HC1 contains only ca. 1.5% of A1203. These results shaw that the dealumination improved the hydrothermal stability of zeolite Yr as shown by Patzelova et al. (ref.10). For NH4NaY(3-3-103)r HY4.8 and HY(LZ-Y52, two cycles of ion exchange and heat treatment) after the treatment with 0.95N HCl
" 850
950 1050
900
lo00
1000
1100 1200 1300
Temperature ("C) Fig.5 The changes of M!A curves, XRD intensities and specific surface areas during the course of degradation of zeolite Y samples. A;NH4NaY(3-3-103), B;HNH,Y(JK-Z-HY4.8) , C;HY(JRC-Z-Y5.6) after treatment with 0.95N HC1 solut.ion. I=sum of peak heights of 6 strong lines in the range of 18-32. I,=sum of peak heights of 6 strong lines of LZ-Y52(sodium form).
806
-
0 / 2 -4
948 950 952 9%
956 958
T ('C 1 Fig.6 Correlations betwen T and the retained crystallinity after the steaming test.of USY (heating rate of DTA=lO°C min.-') Blank marks=75OVCfor 6 hours, Solid marks=810°C for 6 hours, 0 a:LZ-Y82, 0 :USY(LZ-Y52), V :HY4.8, A A :USY(Y4.8) 0 .:prepared from silica sol, 0 :prepared from colloidal silica Iniection temp.=710°C, amount.of water=8 cm3 =Ibefore steaming test
.
+
1
.
.
0
1
m
3 0.8 .-?l
5 0.6
a,
c
m
& 0.4 c w
b
0
CI
0.2
\
n -924 926 928 930 932
934 936
TP'C) Fig.7 Correlations between T and the retained crystallinity after the steaming test (heating rate of DTA=2'C min.-'). Symbols and experimental conditions are the same as Fig.6 except for the heating rate of ETA.
807
solution, the change of X-ray intensities and BFT surface aeas(SSA) of mounted samples in the pt cell of the DTA unit are shm in Fig.5. In the case of NH4NaY(3-3-103), XRD intensity and SSA decreased sharply before the rising of the second peak c, so that the first broad peak(a) shaws the destruction of crystalline structure and the disappearance of the pore system in the zeolite. After the second peak(c), no crystalline phase was detected. Hawever, in the case of NH4NaY(LZ-Y52, Na20=2.9%), the sample quenched at 1060 'C(d in Fig. 4) contained a small amount of crystalline phase, whose dIn the cases of spacings are 0.547, 0.345, 0.288, 0.270 and 0.254 m. NH4NaY(LZ-Y52, Na2G=2.9% and 3.8%), the second small peaks seems to overlap with a large third peaks. Further investigations seem to be necessary to clarify these pheIxxnena. In the case of HNH4Y(=UsY, B in Fig.5) and HY after 0.95N HC1 treatment(C in Fig.51, the decreases in intensities of XRD were slightly faster than those of SSA. Here the second sharp peak(b) shows the collapse of the pore system. The results &awn in Fig.5 indicate that about 40-50% of of the pore systems in U S and HY treated with HC1 solution were destroyed before the emergence of second peaks. The correlations between the intersection temperature of the maximun rising gradient of the peaks to the base (T in Fig.4) and the results of the steaming test at 750 'C were
"70
750
790
830
Temperature(T
870
1
Fig.8 Correlations between the crystallinities after steaming test and the steaming tenprature. The symbols are the same as in Fig.6. lbtted lines shaw the crystallinities before the steaming test.
808
comparatively good, but not as good as the results obtained at 810°C(Fig.6,7). Fig.8 shaws the retained crystallinities after the steaming tests. These results are similar to those in Fig.5, though the initial slopes of the degradation have very slight gradients. The exotherms seem to corespond to the change between A and B in Fig.8. The results at 75OoC are the data before the final complete destruction, and T in Fig.4 means the begining of the final destruction. Then, as for USY, these two values seem to correlate with each other. However, the results at 810°C correlate with intermediate states or final states of complete destruction. we examined the correlations between the exothermal peak temperatures and the results at 810°C. They were, however, not so good. Finally, we confirmed the relationships between the crystallinities before DTA, the unit cell dimensions and T (Fig.9). The results shaw that the decrease of crystallinity, which was led by the formation of defect sites during the ultrastabilization, decreases T remarkably. Furthermore, the samples with high crystallinities and with large shrinkage of unit cell dimensions show the highest hydrothermal stabilities. However, it seems that the effect of shrinkage of unit cell dimension is not so great as the crystallinity of USY. The future aim of this work is to
I
U. D. ( n m )
Fig.9 Correlations between the crystallhities before DTA, dimensions and T of USY samples.
the unit cell
809
predict the static degradation curve such as Fig.8 from the exothenns with different heating rates. For this purpose, further investigations are necessary. References 1 C.J. Plank and E.J. Rosinski, U.S.P. 3,140,253 (1964) 2 C.V. &Daniel and P.K. Maher, "Molecular Sieves", Society of Chemical Industry, London, 1968, pp. 186-195. 3 J. Scherzer, J. Catal., 54 (1978) 285-288. 4 V. Basacek, V. Patzelova, Z. Tvaruzkova, D. Freude, U. LOhse, W. Schirmer, H. Stach and H. Thamn, J. Catal., 61 (1980), 435-442. 5 J.M. Ward, U.S.P. 3,929,672 (1975). 6 H. Nakamoto, T. Matsuda, T. Ida, K. Shirono, M. ogata, Y. Nishimura, "New Developnents in Zeolite Science and Technology", Preprints of Poster Papers The 7th International Zeolite Conference. Japan Association of Zeolite. 1986, pp. 77-78. 7 H. Bremer, W. Morke, R. Schodel and F. V c q t , ACS 121 (1973), pp. 249-257.
Breck, "Proceedings of The Sixth International Zeolite Conference", Butterworths, 1984, pp. 87-96. 9 Cai-Ying Li and L.V.C. Rees, Zeolites, 1986(6), 60-65. 10 V. Patzelova, U. Lohse, E. Engelhardt, E. Altsdorf and P. Koelsch, Ropa Uhlie, 28 (1986), 343-350. 8 G.W. Skeels and D.W.
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H.G. Karge, J. Weitkamp (Editors ), Zeolites as Catalysts, Sorbents and Detergent Builders 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PROPERTIES OF HYDROTHERMAL LOW-DAMAGEO 5A AND 1OX ZEOLITES
R. SCHOLLNER' and H. SIEGEL
IOepartment of Chemistry , Karl-Marx-University
,
Talstr
. 35,
2VEB Chemieanlagenbaukombinat Leipzig-Grimma, Eahnhofstr
.
L e i p z i g , GOR-7010
3/5,
Grimma, GRD-7240
ABSTRACT Treatment o f A and X z e o l i t e s under hydrothermal c o n d i t i o n s leads t o framework a l t e r a t i o n . New adsorption s i t e s with weaker Md+-adsorbate i n t e r a c t i o n s a r i s e because o f t h e formation o f
5 Si-0l3
(-)
(2Si-O)*
(-)
(-)
(-)
9 - 0 ) Al(0)OH and A 1 O(OH)? and other s i m i l a r species i n and o u t s i d e o f t h e z e o l i t e cages. This simulates a d e f i c i e n c y o f e f f e c t i v e c a t i o n i c adsorption s i t e s . (
A1-OH,
A1.0,
( 2
INTRODUCTION
5A and 1OX z e o l i t e s are cormonly employed as adsorbents i n pressure-swingadsorption (PSA) processes f o r gas separation e s p e c i a l l y f o r p r o d u c t i o n o f hydrogen from a v a r i e t y o f feedstocks and oxygen or n i t r o g e n from a i r . The values o f s t a t i c and dynamic adsorption c a p a c i t i e s as a f u n c t i o n o f part i a l pressures and heats o f adsorption o f t h e i n t e r e s t i n g components are c r i t e r i a f o r the s e l e c t i o n o f u s e f u l adsorbents. We have shown t h a t commercial 5A and 1OX z e o l i t e s are p a r t i a l l y damaged /l/.
The hydrothermal degradation process o f the z e o l i t i c framework takes p l a c e duri n g t h e i o n exchange a t an unfavourable pH-value and a t thermal dehydration i n t h e temperature range from 350 t o 820 K. The hydrothermal degradation o f t h e z e o l i t i c framework, e s p e c i a l l y f o r zeol i t e s o f type A, i s o f course a t t a i n a b l e when t h e z e o l i t e i s t r e a t e d with water o r steam a t higher temperatures f o r a longer time /5/. EXPERIMENT
(1) Measurements with the gases N2, CH4 and CO adsorbed on 5A and 10 X zeol i t e s without a d d i t i o n a l hydrothermal damage were c a r r i e d o u t i n a microbalance o f t h e Satorius-Verke AG/FRG.
Samples o f about 100 mg were evacuated a t 0.1 Pa
f o r 1 h, then heated f o r 6 h a t 725 K with an i n i t i a l h e a t i n g r a t e o f 1 K/min. (2) Oehydration c o n d i t i o n s ( f o r the r e s u l t s i n f i g u r e 2) The z e o l i t e samples were d r i e d a t 323 K, than pressed, crushed t o a g r a i n s i z e o f 0.2 t o 0.25 mm and f i l l e d i n t o a U-tube o f g l a s s with i n n e r diameter o f 4 mm.
812
Sample amount: 1 g z e o l i t e (dehydrated); f i l l i n g level 150 mn, g a s stream: 4 l / h (3000 v/vh) a ) measurements without hydrothermal damage (see experiment (1)) b ) h e a t i n g of t h e samples f o r 2.5 h t o 575 K i n a i r , f u r t h e r h e a t i n g f o r 2 h a t 575 K , then f o r 3 h t o 725 K i n dry i n e r t g a s c ) h e a t i n g of t h e samples f o r 1 h t o 475 K i n a i r , f u r t h e r h e a t i n g f o r 2 h a t 475 K , then i n dry g a s stream f o r 1 h t o 725 K d ) h e a t i n g of t h e samples f o r 2 h t o 725 K i n dry inert g a s e) h e a t i n g of t h e samples f o r 1 h t o 475 K w i t h a i r , f u r t h e r h e a t i n g f o r 2 h a t 475 K , t h e n f o r 1 h t o 725 K without g a s s t r e a m f ) h e a t i n g of t h e samples f o r 2 h t o 725 K without g a s stream. (3) For thermal a n a l y t i c a l measurements a microthetnoanalyzer (Setaram, Lyon, France) was used. This a p p a r a t u s a l l o w s combined TG-OTG-DTA i n v e s t i g a t i o n s . Experiments were c a r r i e d o u t by h e a t i n g approximately 10 mg of loaded z e o l i t e powder ( < 0.1 mm) i n an argon flow of 50 m l min-l a t a h e a t i n g r a t e of 10 K min-'. The g a s stream was d r i e d with z e o l i t e A. P r i o r t o t h e i r measurement, t h e a c t i v a t e d z e o l i t e powders were s t o r e d f o r 10 days i n a d e s i c c a t o r over a l i q u i d of benzene o r n-butanol. The following z e o l i t e s were i n v e s t i g a t e d : Ca4, 4NaA I n d u s t r i a l l y prepared zeolite 5A (VEB Chemiekombinat B i t t e r f e l d Wolfen, G.D.R.). Ca4.4NaA-DB Air-dried CaNaA thermally a c t i v a t e d a t 873 K i n a muffle f u r n a c e under deep-bed c o n d i t i o n s (08). RESULTS AND DISCUSSION I n f i g u r e 1 a r e shown t h e measured s t a t i c a d s o r p t i o n c a p a c i t i e s o f N2 a s a f u n c t i o n of t h e p a r t i a l p r e s s u r e a t 293 K of 5AZ powder a f t e r ion exchange, of g r a n u l a t e d 5AZ and of a 5AZ commercial product (VEE Chemiekombinat B i t t e r f e l d Wolfen, G.D.R.) a f t e r dehydration and c a l c i n a t i o n . For comparison N2 v a l u e s f o r commercial 5A z e o l i t e s a r e given i n t a b l e 1. TABLE 1 N2 amount adsorbed a t 293 K and 0.1 MPa on commercial 5A z e o l i t e s Zeolite
N2 amount (mg/g>
5A UCC 5A B a v l i t h K 254 5A Grace 5285 5A CKB 5AZ CKB
11.2 11.5 11.2 10.0 11.2
813 The d i f f e r e n c e between an undamaged and a s t r o n g l y h y d r o l y t i c a l l y t r e a t e d
1OX z e o l i t e , c h a r a c t e r i z e d by values o f s t a t i c adsorption c a p a c i t i e s o f N2, i s enormous ( f i g u r e 2).
I n our measurements we have used a method o f thermal a c t i v a t i o n (dehydration and c a l c i n a t i o n ) c o n s i s t i n g of t h e a d d i t i o n o f s m a l l amounts o f ammonia i n t o t h e gas stream of e i t h e r a f l u i d - b e d r e a c t o r o r a fixed-bed r e a c t o r a t h i g h r a t e s o f streaming d r y i n e r t gas or a i r a t temperatures up t o 820 K / 3 , 4/. The s t a t i c and dynamic adsorption c a p a c i t i e s o f N2, CH4 and CO o f t h e new 5AZ(KMU) and lOX(KMU) z e o l i t e s were measured up t o 1.0 MPa. They a r e s i g n i f i c a n t l y higher than t h e c a p a c i t i e s o f t h e c o n e r c i a l l y a v a i l a b l e 5A and 1OX zeol i t e s (see t a b l e 2). The presence o f ammonia under t h e c o n d i t i o n s o f dehydrat i o n and c a l c i n a t i o n changes t h e pH-values i n t h e c a v i t i e s and t h e e q u i l i b r i u m o f t h e s a l t h y d r o l y s i s . The formed protons a r e n e u t r a l i z e d by amnonia i n t h e presence o f water, and NH;
i o n s are produced. These NH;
i o n s a r e s t a b l e up t o
a temperature o f 560 K. A t higher temperature t h e d i s s o c i a t i o n o f NH;
i n t o NH3
and protons takes p l a c e and the h y d r o l y t i c degradation process can proceed again.
I n the 1OX z e o l i t e s most o f t h e water i s removed up t o 560 K, hence i t i s p o s s i b l e t o prepare undamaged 1OX(KMU) z e o l i t e s . I n t h e case o f 5A z e o l i t e s a s m a l l amount of water i s s t i l l adsorbed between 560 and 700 K, and t h e formed protons can a t t a c k Si-0-A1 bondings. Therefore o n l y s m a l l damaged 5AZ(KMU) zeol i t e s can be produced. Such hydrothermal low-damaged z e o l i t e s are u s e f u l f o r PSA processes. TABLE 2 Values o f s t a t i c adsorption c a p a c i t i e s zeolite pellets
with 25 % binder
N2 (mg/g) 1.0 MPa ~~~~
5AZ 5AZ 1OX 1OX
CKB KMU CKB KMU
CH4 (mg/g)
0.1 MPa
11.2 17.0 10.8 20.5
~~
44.0 58.0 37.0 52.0
~
0.1 MPa
1.0 MPa
10.5 15.0 8.5 14.5
41.0 48.0 32.0 40.0
~~~
In t h e presence o f water i n 4A, 5A, 13X and 1OX z e o l i t e s h y d r o l y s i s procesor Ca(OH)2 i n
ses a r e s t a r t e d and protons on t h e framework and NaOH, Ca(OH)+
d i f f e r e n t amounts i n t h e water phase a r e formed. This f i r s t s t e p of h y d r o l y s i s , t h e so-called s a l t - h y d r o l y s i s , i s dependent on t h e type of framework and t h e k i n d o f cations. I n A z e o l i t e s t h e framework c o n s i s t s o f an a l t e r n a t i o n o f A104 and Si04
tetrahedrons. The p r o t o n form of t h i s z e o l i t e i s n o t s t a b l e . 4A z e o l i t e s repre-
814
Figure 1
Figure 2
I 35
mg'g
i'"
-
35 N a C a ( 8 9 ) X
/5?-
\30 deln v d r a t e d h n d e r different. I -
30 -
conditions/
-25
25 -
-
h
C
20 -
15
pellets u n a c t i v a t e d
10 5
5AZ-CKB commercial prod. I
I
I
I
I
1
:;* -
100 200 300 400 500 600
100 200 300 400 500 600 700
N2-Isotherms at 293 K
N2-Isotherms at 293 K
Figure 3
2 S i- O,(-)
Mechanism of hydrolysis during thermal dehydration
Me'
SSi-0-Al-0-SifL 0 SSi-0
-
-Si-O,(-)
700
Salt hydrolysis Met
S S i - 0 - Al- 0 H+HO-Si 0 3 Si-0
H 3Si-0 > S i - O p A\l - O - S i $ 3Si-0 +Me OH M =:
I/z
N i , Ca(0H):
, SSi-OH qA -
*'
1,
one component 5 per c a v i t y = 2 d i s i l i c i c a c i d per c a v i t y = 67 % m o n o s i l i c i c a c i d
f
HO-Si
Me+
>Si-O-Sig
-= At
=Si-OJA,),OH SSi-0' +
I-)
37 S i - 00'L A l
(3)
-0
Md+)
(1)
At Si-OH Me' S i - O H (-1 O H / si-0 - A\ -OH 'OH
1
At
T /
Cat'
-
components and are produced d u r i n g dehydration process depending on temperature (as a f u n c t i o n of temperature: one component 1 per c a v i t y = 1 d i s i l i c i c a c i d per c a v i t y = 83 % m o n o s i l i c i c a c i d
Met 3si-0, 3Si-OpAI-OHtHO-Si SSi-0
H20
HO-Si At
-
SSi,o SSi'
i
(4)
s (+)
Me
N O
(2) HO-Si
S i - O H t A L (OH),,Me Si-OH
(-),OH
ssi - 0 -A1
(4)
Si-OH
HO-Si
f
tL\t - H20
2 3Si-0-Si
(-1
(t)
At O , ( O H ) ~ - Z ~ Me
1s)
815 sent a s a l t between a weak polybasic a c i d and a s t r o n g base. 5A z e o l i t e s are s a l t s o f a weak polybasic a c i d and a weak base. Oepending on t h e S i / A l r a t i o ,
1OX z e o l i t e s are conceivable as a s a l t o f a stronger p o l y b a s i c a c i d r a t h e r than i n t h e 5A form and a weak base. For t h e X z e o l i t e s i t i s known t h a t t h e a c i d s t r e n g t h increases with t h e % / A 1
r a t i o . I n t h e presence of water, a r e v e r s i b l e
e q u i l i b r i u m e x i s t s between b o t h forms of a c i d s i t e s o f t h e framework (see f i g u r e 3).
I n t h e case o f 5A z e o l i t e s t h e number o f a c i d s i t e s i s h i g h e r than i n t h e 4A z e o l i t e s and t h e r e f o r e t h e h y d r o l y t i c s t a b i l i t y o f 5A z e o l i t e s i s smaller.
Depending on t h e amount o f a c i d s i t e s i n t h e presence o f water and a t temperat u r e s higher than 370 K, f u r t h e r h y d r o l y s i s products a r e formed by decomposition o f Si-0-A1 bondings a t t h e a c i d s i t e s o f s a l t h y d r o l y s i s . Such a h y d r o l y s i s process can only be explained when t h e a t t a c k o f water i n t h e presence o f protons takes p l a c e a t Si-0-A1 bondings i n t h e neighbourhood o f a c i d s i t e s i n a s t a t i s t i c a l manner. I n such a case A l - s i t e s are damaged step-by-step a t t h e s u r f a c e and a l s o i n t h e b u l k . The f i r s t evidence o f t h i s h y d r o l y s i s process i s the f o r m a t i o n o f e x t r a framework aluminate species. We were able t o show t h i s by X-ray s t r u c t u r a l i n v e s t i g a t i o n s on deep-bed-activated and rehydrated 5A z e o l i t e s . I n t h e hydrated form o f hydrothermally t r e a t e d z e o l i t e s Al(0H)i species were found /2/.
Freude e t a1 . / 6 /
discuss a l s o t h e formation o f Al(0H)i anions a f t e r deep-bed a c t i v a t i o n and f o l l o w i n g r e h y d r a t i o n by means o f t h e 27Al-MASNMR method. Besides A l ( 0 H ) i anions, on t h e other hand, other s i t e s with p a r t i a l l y hydrolysed Al(OH)-containing
sites
are generated by t h e a t t a c k o f water (see f i g u r e 3, compounds 3 and 4). During the c a l c i n a t i o n process t h e OH groups a t t h e A 1 atoms and t h e Si-OH groups are changed by desorption o f water, and Si-0-Si bondings are formed. A f t e r t h i s h e a l i n g process t h e a l t e r a t i o n o f the A 1 s i t e s i s i r r e v e r s i b l e . The existence o f A l ( 0 H ) i anions, i n c l u d i n g the other p a r t i a l l y hydrolysed A 1 s i t e s , and the statement t h a t two Si-OH groups depending on geometric f a c t o r s can c r e a t e no more than one S i - 0 - 5 group form t h e b a s i s f o r t h e q u a n t i t a t i v e explanation f o r t h e r e s u l t s o f t h e molybdate method. The molybdate method used t h e formation o f the yellow-coloured D-dodecamolybdo s i l i c i c a c i d complex on r e a c t i o n between o r t h o s i l i c i c a c i d and molybdic a c i d / 9 , 10/. A f t e r the a c i d i c d i s s o l u t i o n o f 4A z e o l i t e s ( w i t h o u t any process o f thermal dehydration) o n l y m o n o s i l i c i c a c i d s a r e detectable. I n t h e case of pure r n o n o s i l i c i c a c i d the formation o f the y e l l o w complex i s completed a f t e r 2 . 1 min a t 298 K and with pure d i s i l i c i c a c i d under the same c o n d i t i o n s o n l y a f t e r 5 min. The measured k i n e t i c curves p e r m i t an i n t e r p r e t a t i o n o f t h e r e s u l t s . The formation o f t h r e e - s i l i c i c or other o l i g o m e r i c s i l i c i c acids i s p o s s i b l e when two A 1 s i t e s i n the neighbourhood are hydrolysed. The molybdate method i s
816
a proof f o r t h e degradation process. With d e c r e a s i n g c o n t e n t of m o n o s i l i c i c a c i d an e n l a r g e d h y d r o l y s i s of Si-0-A1 bondings and a h i g h e r number of damaged A 1 s i t e s is i n d i c a t e d . The p a r t i a l l y damaged A 1 s i t e s have an i n f l u e n c e on t h e a d s o r p t i o n p r o p e r t i e s of 5A and 1OX z e o l i t e s . I n a l l c a s e s t h e s t a t i c a d s o r p t i o n c a p a c i t i e s of such molecules a s N 2' O2' CO, CH4, A t and o t h e r nonpolar s p e c i e s a r e diminished, depending on t h e k i n d s of i n t e r a c t i o n f o r c e s between adsorbed molecules and t h e changed z e o l i t e framework. The s t a t i c a d s o r p t i o n c a p a c i t i e s of p o l a r molecules, l i k e H20 and C02, a r e a l s o dependent on t h e degree of d e g r a d a t i o n . Water has a dipolemoment and is a molecule with an e l e c t r o n - d o n a t i n g s i t e . It i n t e r a c t s with d i p o l e s of t h e z e o l i t e s u r f a c e and e s p e c i a l l y with e l e c t r o n d e f i c i e n c y sites, t h e c a t i o n s i n t h e A and X z e o l i t e s . The formation of d i f f e r e n t Ca-alurninate and Ca-aluminos i l i c a t e s p e c i e s a f t e r dehydration of t h e hydrolysed A 1 sites (see f i g u r e 3 ) modifies t h e i n t e r a c t i o n p r o p e t t i e s of t h e Ca i o n s . I n t h e o t h e r surroundings t h e s t r e n g t h of i o n i c bonding of t h e Ca ion w i t h t h e new anion s p e c i e s is less. With i n c r e a s e d damage t h e i n t e r a c t i o n f o r c e s , and t h e r e f o r e t h e amount of adsorbed water, d e c r e a s e s . Nitrogen is a very s e n s i t i v e molecule f o r t h e i n t e r a c t i o n of t h i s h y d r o l y s i s process. The i n t e r a c t i o n f o r c e s between t h e quadrupole moment of N2 and t h e f i e l d g r a d i e n t of Ca i o n s a r e r e s p o n s i b l e f o r t h e number of molecules adsorbed a t room temperature. By means of t h e NMR method i t is p o s s i b l e t o compare t h e v a l u e s of i n t e r a c t i o n f o r c e s between t h e c a t i o n s ( l i k e Na' and Ca2+) and adsorbed N2 molecules / l / . Ca i o n s have a more i n t e n s i v e i n t e r a c t i o n with N2 t h a n Na i o n s because of t h e higher i o n i c s t r e n g t h a t n e a r l y t h e same ion diameter. With t h e formation of Ca-aluminate o r Ca-aluminosilicate s p e c i e s t h e i o n i c p a r t of bonding is s m a l l e r than t h e Ca-framework bonding i n z e o l i t e , and t h e r e f o r e t h e f i e l d g r a d i e n t is s i g n i f i c a n t l y s m a l l e r . Consequently t h e amount of adsorbed N2 a t 293 K and 0 . 1 MPa d e c r e a s e s s t r o n g l y with t h e p r o g r e s s i v e d e g r a d a t i o n p r o c e s s . I n t a b l e 3 t h e amount of adsorbed water and n i t r o g e n and t h e amount of monos i l i c i c a c i d determined by t h e nolybdate method a r e l i s t e d f o r a NaL,Mg4A zeol i t e hydrothermally t r e a t e d under t h e same c o n d i t i o n s . The results of t h e methods used show t h e same tendency. The p r o g r e s s i v e degree of d e g r a d a t i o n of t h e A 1 sites i n t h e framework l e a d s t o s m a l l e r s t a t i c a d s o r p t i o n c a p a c i t i e s of wa-
ter and n i t r o g e n and s m a l l e r amounts of m o n o s i l i c i c a c i d . The measured amount of adsorbed N2 is a u s e f u l v a l u e f o r t h e c h a r a c t e r i z a t i o n of t h e degree of framework degradation i n t h e f i r s t s t a g e when t h e framework of A or X z e o l i t e is damaged but undestroyed, i . e . , no amorphous m a t e r i a l was formed. The dec r e a s e of N2 adsorbed and t h e d e c r e a s e of m o n o s i l i c i c a c i d i n d i c a t e t h e same process.
817 TABLE 3 Behaviour of a Na4Mg4A z e o l i t e a f t e r d i f f e r e n t hydrothermal treatments thermal treatment
N capacity 263 K , 0.1 MPa
time
(%)
monosilicic acid (%)
21.5
100
35.8
26.5 26.0 25.3
90
dry nitrogen
(3 c y c l e s ) ( 6 cycles) ( 9 cycles)
30.1 29.4 27.3
air
(3 c y c l e s ) ( 6 cycles) ( 9 cycles)
25.2 24.5 24.3
H2°
capacity
untreated
85 80 80 80
75
(ng/g)
27.0 26.8 23.1
Conditions: 1 g z e o l i t e powder were a c t i v a t e d a t 670 K f o r 2 h i n a r o t a t i n g quartz tube with d r y n i t r o g e n o r a i r i n counter-current (1.2 l / h ) . A f t e r rehyd r a t i o n a new a c t i v a t i o n process was s t a r t e d . With i n c r e a s i n g content o f Ca c a t i o n s i n t h e 4A z e o l i t e t h e amount o f adsorbed N2 i s increased (see f i g u r e 4). Depending on t h e degree o f Ca2+-ion exchange t h e amounts o f adsorbed A t , CH4 and CO increased t o o ( f i g u r e 5). The h i g h e r t h e i n t e r a c t i o n f o r c e s a f t e r t h e exchange o f one Ca2+ i o n t o two Na+ ions, t h e stronger the ascent of adsorbed molecules, depending on number o f Ca2+ ions per l a r g e c a v i t y i n t h e A z e o l i t e ( A r has o n l y d i s p e r s i o n i n t e r a c t i o n f o r c e s , CH4 a d d i t i o n a l p o l a r i s a t i o n i n t e r a c t i o n forces, N2 a d d i t i o n a l quadrupolf i e l d g r a d i e n t i n t e r a c t i o n forces, and CO a d d i t i o n a l d i p o l e - d i p o l e i n t e r a c t i o n forces).
A comparison o f t h e heat o f adsorption as a f u n c t i o n o f t h e l o a d i n g between two NaMgA z e o l i t e s with d i f f e r e n t degrees o f hydrothermal degradation shows t h a t d u r i n g the thermal h y d r o l y t i c process t h e primary Me2+ z e o l i t i c p o s i t i o n s are changed, probably by formation o f other chemical compounds. This simulates a d e f i c i e n c y o f Me2+ ions. I n t h e case of g r a d u a l l y damaged NaMgA z e o l i t e by deep-bed a c t i v a t i o n t h e decrease o f a d s o r p t i o n heat i s r a p i d ; only a small number of unchanged A 1 s i t e s are present and t h e r e f o r e o n l y a s m a l l number o f Mef+ c a t i o n s e x i s t s i n t h e i r o r i g i n a l p o s i t i o n s . On t h a t account t h e adsorption heat, dependent on Nli
loading, decreases r a p i d l y (see f i g u r e 6 ) .
T.t i s concluded t h a t d u r i n g t h e hydrothermal damage o f A z e o l i t e s new Ca
2+
bondings are formed with weaker i o n i c i t y , as w e l l as a h m i n o s i l i c a t e anions with p r o p e r t i e s other than t h a t o f t h e o r i g i n a l framework and e x t r a l a t t i c e Al-spec i e s o u t s i d e t h e framework. This a l t e r a t i o n can be s t u d i e d by desorption o f molecules with s p e c i f i c i n t e r a c t i o n f o r c e s t o these new s i t e s . We s e l e c t e d water, methanol, ethanol, n-propanol, n-butanol, tetrachlorrnethane, dioxan and benzene f o r these measurements.
818
Figure 4
Figure 5 a (mglg) Gases adsorbed on No &,A-Zeolites
X-78 Zeolite No C a l X ) A
/
10
5 Torr
100 200
z 1
300 400 500 600 700
N2-Isotherms at 293 K
\
\
-1
'\
20
~
It I
NaMg(80lA deep-bed dehydrated
-C a N a A - -- Ca No A - D B
\ \
22
\
N a A dehydrated without damage
mglg
I
5
10
4
Figure 7
K J l m o 'NO M g (80)A dehydrated without damage
\\
3
15
lsostere adsorption heat
5
6
Number of Ca-ions per Large cavity
Figure 6
t
2
DTG -curves
819 The best r e s u l t s were obtained w i t h n-propanol, n-butanol and benzene because the possible i n t e r a c t i o n s o f t h e OH-groups o r o f the aromatic system t o the Ca (-
ions o f the o r i g i n a l framework and o f the new (-
(
3 Si-0) A?(O)OH,
(
3Si-O),
A]-OH,
(
2
(-
Si-O)2 AI=O,
(-
AI(0)(OH)2 anions and o t h e r s i m i l a r species i n our o u t s i d e
the framework are d i f f e r e n t . I n a l l cases t h e s p e c i f i c i n t e r a c t i o n forces between t h e e l e c t r o n d e f i c i e n c y s i t e s , the o r i g i n a l Ca2+-ions i n the z e o l i t i c framework and t h e electron-donat i n g s i t e s (OH groups of the a l c o h o l molecules o r the aromatic benzene c y c l e s ) are the h i g h e s t . A l l other i n t e r a c t i o n forces i n t h e hydrothermally damaged 5A z e o l i t e s , espec i a l l y t h e i n t e r a c t i o n forces with t h e new Ca species, are weaker. The f a c t t h a t more than 4 molecules o f n-butanol a r e adsorbed per l a r g e c a v i t y f a c i l i t a t e s t h i s i n t e r p r e t a t i o n . I n t h e case o f Ca4.4NaA z e o l i t e each Ca i o n i n t e r a c t s with one OH group o f a n-butanol molecule ( f i g u r e 7). After hydrothermal treatment o f t h i s z e o l i t e t h e desorption behaviour o f n-butanol i s markedly changed ( f i g u r e 7).
REFERENCES 1 R. Schollner, Sitzungsber. der Adll der DDR, 10 N (1985) 38. 2 H. Siegel, R. Schollner, 8. Staude, J. J . van Dun, W. J . M o r t i e r , Z e o l i t e s 7 (1987) 372. 3 R. Schollner, C. Bode, H. Siegel, R. Broddack, E. Petzold, I. F o r s t e r , 6. Kulbe, H. Herden, R. Kunze, M. Jusek, OD 239533, 26. 7. 1985. 4 R. Schollner, C. Bode, H. Siegel, R. Broddack, M. Jusek, 8. Kulbe, H. Herden, R. Kunze, DO 239534, 2 6 . 7. 1985. 5 W. Lutz, 8. Fahlke, K. Lohse, R. Seidel, Chem. Techn. 35 (1983) 35. 6 0. Freude, J. Karger, H. P f e i f e r , Proc. I n t . Symp. Z e o l i t e C a t a l y s i s , Siofok, Hungary, 1985, p. 89. 7 W. Oehme, W. M e i l e r , J . R. Lochmann, H. Siegel, Z. Chem. 27 (1987) 431. 8 H. Siegel, Thesis 8, Karl-Marx-University L e i p z i g , 1986. 9 E. T h i l o , W. Wieker, H. Stade, Z. anorg. a l l g . Chem. 340 (1965) 261. 10 H. Stade, Z. anorg. a l l g . Chem. 441 (1978) 29, 446 (1978) 5.
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H.G. Karge, J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PHASE TRANSFORMATIONS AND CIlANGES IN LATHCE PARAMETERS OF ZSM-5 AS A FUNCTION OF Al CONTENT AND TEMPERATURE G.T. KOKOTAILO*, L. RIEKERT and A. TISSLER Institut fur Chemische Verfahrenstechnik, UniversitEt Karlsruhe, D-7500 Karlsruhe * present address: 98 North American Street, Woodbury, NJ 08096, USA
ABSTRACT Very high quality Z S M J with Si/Al ratios from 35 to 12000 were synthesized and converted to the H-form. Their lattice parameters were determined from high resolution X-ray data as a function of temperature up to 585°C. The nionoclinic phase with p angle reduced from 90.6" for ZSM-5 with a Si/AI ratio of 12000 to 90.090 is evident in ZSM-5 with a SVAI ratio of 35 at room temperature (25°C). The monoclinic-onhorhombic phase transition is dependent on Si/AI and temperature. For highly silicious ZSM-5 the temperature range for this phase transition was found to be greater than that determined by MASNMR. Lowering the temperature of the monoclinic phase tends to increase the p angle. INTRODUCTION
ZSM-5, a high-silica zeolite with a unique framework and pore system, was synthesized by Argauer and Landolt (ref. 1). The framework topology of ZSM-5 was detemiined (ref. 2) and found to contain an unique bui!ding block consisting of eight 5 membered rings. The uncalcined ZSM-5 is orthorhombic, space group Pnma and lattice parameters a=19.92, b=20.07, c=l3.42A (refs. 3-5). It has been reponed (ref. 6) that high silica ZSM-5 on calcination undergoes a phase transformation from orthorhombic Pnma to monoclinic, &i/n. The angle p is a function of occluded material but attains a maximum of 90.6" for highly dealuminated ZSM-5. Nakamoto and Takahashi (ref. 7) reported that the ease of transfomiation of the orthorhombic to the monoclinic phase is a function of Si/AI increasing with decreasing Al content and does not appear for SVAI less than 80. Fyfe (ref. 8) has shown that the MASNMR spectrum of as synthesized high silica ZSM-5 exhibits 12 resonances characteristic of the orthorhombic form. On calcination and removal of the organic material in the pore system the NMR spectrum exhibits 24 resonances characteristic of the monoclinic phase. Sorption of organic molecules induces a change from the monoclinic to the orthorhombic phase (ref. 9). This phase transformation is also temperature-sensitive and occurs at 80 - W C as indicated by MASNMR (ref. 8). Hay and Jaeger (ref. 10) observed the same effect with X-ray diffraction but at a lower temperature, 44 - 5 2 C . The effect of SVAI ratio and temperature on the symmetry and lattice parameters of ZSM-5 are discussed in this report. EXPERIMENTAL Very high quality ZSM-5 with SVAI ratios of 35, 100, 800 and 12OOO were synthesized by the method of Argauer and Landolt (ref. l), as very good crystalline samples are required in order to obtain high resolution X-ray diffraction patterns. The samples were calcined 4 hours at 500°C in a muffle furnace and ion exchanged twice for 1 hour at 100°C in 1 molar W C I solutions, filtered, washed and dried, and calcined at 5OOOC for 4 hours to obtain the hydrogen form. X-ray diffraction patterns were obtained with a Philips diffractometer and a Huber 4-circle Guinier camera (631) with CuKa radiation, a Johannsen monochromator, a Si internal standard and temperature controller. The samples for the Guinier camera were packed in 0.5 mm silica glass capillaries to avoid preferred orientation. A series of X-ray film patterns were obtained by advancing the film 0.5 cm and increasing the temperature from 25°C to 725°C in 100°C intervals for Si/AI = 35.50" intervals for
a22
WAl = 100.75" intervals for Si/AI = 800 and 12000. The films were read with a Nonius comparator with a resolution of 0.05 mm or 0.015" 20 and lattice parameters were determined to f .oOO6nm. The d spacings were extrapolated from the (1 1l), (200) and (31 1) lines of Si and the parameters a, b. c and p were obtained using computer programs GIVER,REFINE and THEOWIE courtesy of the Instifit fur Anorganische Chemie, Universitlit Karlsruhe. The films were microphotometered, courtesy of the Astrophysics Dept.. Princeton University. RESULTS AND CONCLUSIONS The plot of the change in angle p or the degree of separation of the 313 and 313 lines for HZSMJ at room temperature (25°C) is given in fig. 1.
90.6
-
-a
90.4
-
.-0C .-00
90.2
-
90.0
,
3 9)
m,
K c
3 I
I
I
I
Figure 1: Monoclinic angle in H-ZSM-5 samples with different Si/AI ratios at room temperature The angle decreases with Si/AI ratio but does not reach 900 for the lowest Si/AI ratio measured. The a and b parameters drop to a minimum at Si/AI = 100 and then gradually increase with increasing Si/AI while the c parameter increases with Si/AI up to 100 and then tends to level off, fig. 2. This is unexpected as with increasing Si/Al ratio the average T-0 distance would decrease and the lattice parameters would follow suit. The increase in silanol groups and or the change in the ratio of ionic and covalent bonding may with increased Si in the structure have an affect on the lattice parameters and the unit-cell volume. The plot of angle p as a function of temperature T and Si/AI as shown in figure 3 indicates a decrease in p with temperature. This effect becomes more pronounced with increasing Si/AI ratio; there is a marked increase for Si/AI = 12OOO. The lattice parameters as a function of temperature and Si/AI ratio are given in Table 1. There is very little variation in a, b and c with only p varying at low temperatures. The angle p approaches 9W with decreasing Si/AI ratio and increasing temperature. It would be expected that a highly siliceous ZSM-5 would be orthorhombic at high temperatures. It has been shown by von Ballmoos and Meier (1 1) that the Al distribution i n ZSM-5crystiils is non uniform. with a high alumina outershell. Nitrogen adsorption isotherms in ZSM-5exhibit hysterisis loops low at temperature and pressures (3,12,13). Mfiller and Unger (12) have shown variations in A1 distribution with Si/AI ratios. The shape of the nitrogen adsorption isotherms is affected by the Si/Al ratio and the variation in A1 distribution in the crystals (12,13). The quality of the crystals. the Al distribution in the crystals, the presence of faults and errors in the determination of the lattice parameters may be the contributing factors in the p angle not reaching 900 at high temperatures with
823
E
2.02 r 2.01
- 2.00 c
!!
aJ
c.
f 1.99
E a aJ
I
p
1.98
1.33 I
0
3 Log(SVAI)
2
5
4
Figure 2: Lattice parameters in H-ZSM-5 samples of different Si/AI ratio a; o h;O'c
90.6
4
n
90.0 0
I
I
I
I
I
100
200
300
600
500
n 600
Temperaturr[oC) Figure 3: Change of the monoclinic angle in H-ZSM-5 samples of different Si/AI ratio by heating, 0 SVAI = 35; ISi/AI = 100, Si/AI = 800; o Si/AI = 12000
024
L
w I
1
10
9
I
0
I 7
20 Figure 4: Microphotometer trace of ZSM-5 (Si/AI = 35) spectrum 7 -10' 28
*
the exception of the Si/AI = 100 sample. The phase transition monoclinic to orthorhombic occurs over a small temperature range below 100°C (14) and is dependent on the Si/AI ratio decreasing with A1 content. High resolution X-ray diffraction and good crystals are required to determine whether 313 and 313 reflections are doublets or whether the angle p is 9P, a single sharp line indicating orthorhombic symmetry. The crystallinity of the low SUAI = 35 ZSM-5 sample and the resolution of the Guinier camera are clearly shown by the resolution of the 200 and the 020 doublet in fig. 4. The X-ray diffraction pattern of this sample at mom temperature (25°C) indicates the 31 3,313 and the 501,501 doublets in a microphotometer trace of that section of the Guinier film pattern, fig. 5(a). The shoulders disappear and the peaks sharpen up at 125"C, fig. 5b, indicating that ZSM-5 with Si/AI = 35 at 25°C has monoclinic symmetry.
825
4
v\
0
a
b -I+
0 0
v\v\
I
25
I
I
24
20
1
I
25
23
1
24 20
-
23
Fig. 5: Microphotometer traces of ZSM-5 spectra, 21 - 25'28, (a) at 25°C (b) at 125°C TABLE 1 Lattice Parameters as a Functionof Si/AlRatioand Temperature
SVAI
T("C)
a(m)
Mnm)
c(m)
P
35 35 100 100 800 800 800 12000 12000 12000 12000 12000
25 447 25 575 25 287 587 25 90 250 450 575
1.992 I .993 I .986 1.99 1 1.987 I .986 1.986 1.989 1.991 1.990 1.991 1.990
2.01 1 2.008 2.004 2.004 2.008 2.007 2.003 2.010 2.010 2.007 2.008 2.007
1.336 1.338 1.340 1.342 1.338 1.336 1.335 1.340 1.340 1.336 1.338 1.338
90.09 90.07 90.13 90.00 90.18 90.12 90.09 90.52 90.23 90.07 90.05 90.05
826
The Guinier X-ray film pattern of €1-ZSM-5 with a Si/Al ratio of 100 indicates the monoclinic phase with the 313 - 313doublet evident. At -173°C the separation of this doublet has increased, indicating a litrger p angle. This demonstrates that lowering the temperature of the monoclinic form of ZSM-5 results in the increase of the p angle. Thus, the Si/AI ratio of ZSM-5 determines the magnitude of the p angle in the monoclinic phase which persists for a SVAI ratio of: at 25°C. The displacive phase transformation, monoclinic-orthorhonibicand orthorhombicmonoclinic, is temperature-dependent. The presence of sorbate or extraneous material also affects the phase transformation. ACKNOWLEDGEMENTS We wish to thank Prof. E. Althaus and G.Ott of the Institute of Mineralogy, Dr. H. Henke of the Institut flir Chemische Verfahrenstechnik, Universi6t Karlsruhe and S. Honenbach of Astrophysics Dept. Princeton University, for their help. G.T. Kokotailo acknowledges the Alexander von Humboldt Senior US Scientist Award.
LITERATURE 1.
2. 3. 4. 5.
6.
7. 8. 9. 10.
11.
12. 13. 14.
US Patent 3,702,886, 1972 G.T. Kokotailo, S.L. Lawton, D.H. Olson and W.M. Meier, Nature, 272, 56, 52, 437, 1978 D.H. Olson, G.T. Kokotailo, S.L. Lawton, W.M. Meier, J. Phys. Chem. 8.r. 2238. 1981 H. Lermer, J. Steffen, M. Draeger and K.K. Unger, Zeolites 1, 57, 1985 K.T. Chao, J.Ch. Lin, Y.Wang, G.H. Lee, Zeolites 6,35, 1986 E.L. Wu, S.L. Liiwton, D.H. Olson, A.C. Rohrman Jr. and G.T. Kokotailo, J. Pliys. 2777, 1979 Chem. H. Nakamoto iind 11. Takithitshi, Chemical Society of Japan, Chem. Letters 10 13, 19x1 C.A. Fyfe, G.J. Kennedy, G.T.Kokotailo, J.R. Lyerla and W.W. Fleming, J. Chem. So Chem. Conim. 740, 1985 C.A. Fyfe, G.J. Kennedy, C.T. De Schutter and G.T.Kokotailo, J. Chem. Soc. Chem. Comm. 541, 1984 D.G. Hay and H. Jaeger, J. Chem. Soc. Chem. Comm. 1433, 1984 R.Von Ballmoos, W.M. Meier, Nature 289,78, 1981 K. Muller, K.K. Unger, Characterization of Porous Solids, Elsevier Science Publ., B.V. Amsterdam 1988 p. 101 K. Beschmann, G.T. Kokotailo, L. Riekert, ibid 1988, p. 355. C.A. Fyfe, H. Strobl, G.T. Kokotailo, G.J. Kennedy and G.E. Barlow, JACS, 110, 3373, 1988
a,
H.G.Karge,J. Weitkamp (Editors),Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands
The Effect of Sorbates and Elevated Temperatures on the Structures of Some Zeolite Catalysts C.A. Fyfe. G.T. Kokotailo, H. Strobl, H. Gies, G.J. Kennedy, C.T. Pasztor, and G.E. Barlow Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Y6
The catalytic properties of zeolite catalysts are dependent on their unique structural features, such as pore geometry, distribution and concentration of T-atoms and adsorbed species, presence of defects, and temperature. Solid state NMR is highly sensitive to changes in the local environments of T-atoms, and when used in conjunction with X-ray diffraction provides a more complete description of zeolite framework structures. This combined use of XRD and MAS NMR to determine the changes in the framework of zeolite catalysts due to temperature and organic molecule adsorption will be discussed in detail. The importance of zeolites as catalysts, catalyst supports, adsorbents, and ion exchangers is well established. The properties of a particular zeolite are dependent on the topology of its framework, the size of its free channels, the location of charges, the size of the cations within the framework, the presence of faults and occulded material, and the ordering of the T-atoms. Therefore, structural information is necessary in order to understand the sorptive and catalytic properties of zeolites ( E l . The unavailability of large crystals suitable for single crystal studies has necessitated the use of powder diffraction techniques to resolve the structures with the more recent use of synchrotron radiation (6-8) and the Rietveld x-ray powder profile method for refining structures (9). A number of other techniques have been used to characterize zeolite catalysts. X-ray photon spectroscopy has been used to determine the composition of surfaces (10) and the use of neutron scattering has made it possible to identify OH groups (11). Electron diffraction (121, contrast line transmission electron microscopy (13) and lattice imaging have been used to determine the presence of faults in zeolites. Since Si and A1 have essentially the same scattering factors the Si and A1 sites may be distinguished by the T-0 bond lengths
828
-
(Si-0 1.62 A and A1-0 = 1.73 A) if there is long range Si and A1 ordering throughout the crystal, although this is seldom the case. ZSM-5 crystals tend to grow with a high A1 content outer shell (14) and it is not known whether A1 occupies certain sites preferentially (15) and whether some of it is not in the framework. Recently, powder X-ray diffraction has acquired a complimentary technique in high-resolution MAS NMR spectroscopy (14-19) and their combined use has made it possible to obtain a more complete elucidation of zeolite structures, and how they are affected by sorption of organic species, temperature and the presence of defects. In these studies, X-ray diffraction is used to determine long range order while MAS NMR is sensitive to changes in local environments and short range order. Solid State NMR The direct dipole-dipole interaction between nuclei dominate the NMR spectra of abundant nuclei in solids, yielding spectra with broad lines and negligible resolution. The interactions are orders of magnitude larger than the chemical shifts and spin-spin couplings which are related to the chemical and molecular structure as observed for species in solution where the dipolar interactions average out to exactly zero. The situation is simplified by "diluting" the nuclei in the sample by observing nuclei with low natural isotopic abundance or by physical dilution in the system as homonuclear dipolar interactions drop off very rapidly with internuclear distance. Heteronuclear interactions may be removed by dipolar decoupling and the remaining interaction, the chemical shift anisotropy, can be averaged by "magic angle" spinning (19) about an axis inclined at 54'44' to the magnetic field and is reduced to the isotropic component of the anisotropic tensor, just as for random molecular motion in solution. Cross polarization may also in general be used to enhance the dilute nucleus magnetization from that of protons in the system (20). In zeolites, protons are not covalently bonded into the solid matrix so that a9Si and "A1 spectra may be obtained using magic angle- spinning alone. These experiments are best performed at the highest possible field strength and a conventional high-resolution spectrometer may be used. Since the initial work of Lippmaa and Engelhardt (16) a number of investigations of a9Si and "A1 spectra of low Si/A1 ratio have been reported and for structures with a single independent T atom there is a maximum of 5 resonances for a Si/A1 > 1, indicating that the spectrum is sensitive to the local Si environment or the composition and distribution of Si and A1 in the first coordination sphere. The resonances are broad due to a distribution of local environments. A typical "Si spectrum of analcite and the chemical shift ranges of the 5 possible local silicon environments are shown in figures 1 and 2. It has been shown that the main contributing factor to line broadening in agSi MAS NMR spectra of a zeolite with a random distribution of A1 in the framework is the distribution of local Si environments due to the non-ordered neighbouring Si and A1 sites. Zeolite A which has a completely ordered Si (4A1) framework gives a sharp spectrum consisting of one line due to Si (4A1) as seen in
829
Fig. 3a. A high Si A (ZK-4 with Si/A1=3) yields a characteristic 5 resonance spectrum, Fig. 3b, with the Si(4A1) line very weak due to the high Si/Al racio. The lines are broad due to a distribution of local envircnments. The ZK-4 sample (Fig. 3b) was highly dealuminated by passing water vapor over the hydrogen form at atmospheric pressure and 973*K for 48 hrs. (21). Its a*SI MAS NMR spectrum consists of a single sharp line (Fig. 3) which can be assigned to Si(OA1) indicating the complete removal of A1 from the framework with the integrity of the structure preserved as shown by the XRD patterns in Fig. 3. The upfield shift of the Si(OA1) lines is evident and is consistent with the reduction in chemical shift distribution arising from the disordering of Si,Al in the local environment ( 2 2 ) .
-
-8 0
-90
-
-100
ppm from T M S
-
Fig. 1. SOSi HAS spectrum of the zeolite analcite showing the resolution of the different silicon environments indicated (ref..17)
Si( 1Al)
w
m3
-s(-si)
=uSi(4Al) I
-80
I
I
-90
I
I
-100
1
I
1
-110
Fig. 2. The five possible local environments of a silicon atom together with their characteristic chemical shift ranges. The dashed lines show the shift values of zeolite ZK-4 as indicated. The inner boxes represent the shift ranges suggested in the earlier literature (ref. 16) while the outer ones reflect more recent data (ref. 1 7 ) .
830
Thus, the removal of lattice aluminum yields very narrow 19Si resonances all of which are due to Si(OA1, 4Si) and which correspond to crystallographically independent Si atoms in the structure (23). The number, relatlve intensities and shifts of the resonances yield direct informatioi as to the structure of a zeolite catalyst and are very sensitive to subtle and small changes in the lattice due to defects or distortion. It should be emphasized again that the XRD and MAS NMR techniques are complimentary, the former determining long range order while the latter is sensitive to local, short range order.
2 9 ~ 1
&
,
I
-80
0
mas n m r
1
-100
,
,
.
I
X rd.
.
L
-120
p.pm from Hr'Si
Fig. 3. "Si (79.6 MHz) HAS NMR spectra of (A) zeolite A, (B) zeolite ZK-4, (C) completely siliceous zeolite A, and (D-F) the corresponding powder XRD patterns (ref.21)
.
831
ZSM-5 ZSM-5 (24) is of particular intsrest because of its high catalytic activity and its unique shape selective and sorptive properties which enable 4.t to convert methanol to hydrocarbons, (25) in the gasoline range to selectively produce p-xylene ( 2 6 ) and hydrodewax (27). ZSM-5 is the end member of the pentasil family of zeolites the other being ZSM-11 (28b). The structure of the low Si/A1 ratio ZSM-5 is orthorhombic with space group Pnma, (28d) while that of the high Si/A1 phase is monoclinic with space group P2,/n (29). The orthorhombic phase has 12 independent T atoms and the monoclinic 24. This was confirmed by SSSi MAS NMR (30, 31). Template-loaded ZSM-5 regardless of Si/A1 ratio is orthorhombic with Pnma space group. Recently, extremely pure samples of a very highly crystalline ZSM-5 were synthesized and completely dealuminated. Combined with careful optimization of all NMR experimental variables this enabled us to obtain ultra high resolution a@Si MAS NMR spectra of ZSM-5 (Fig. 4) in which 21 of the 24 possible resonances are clearly resolved (32, 3 3 ) . The line widths are approximately 5Hz (x 0.06 ppm) which is an improvement of an order of magnitude over original spectra (obtained in 1982). The deconvolution of the spectrum is straightforward. The spectrum obtained at 295K has no resolution enhancement. Powder XRD patterns using a Rigaku X-ray diffractometer confirmed the highly crystallhe nature of the sample which was subsequently used to determine the effect of temperature and/or sorbate. The x-ray diffraction samples were covered with an amorphous collodion film to prevent any change in
I
- 108
I
1
I
I
PPM
1
FROM
I
TMS
I
I
I
I
- 118
Fig. 4. SOSi MAS NMR spectrum of highly crystalline ZSM-5.
832
sorbate content. The experimental PAS NMR variables were optimized as previously desxibed (32,331 and it was found that the high degree of resolution is essentially maintained at both elevated temperatures and in the presence of sorbed species (33). Temperature and sorbed specips induce a phase transformation of the monoclinic to the orthorhombic with a decrease in the independent atoms from 24 to 12 (30, 31). The effect of p-xylene adsorption at ambient temperature on the 29Si MAS NMR spectrum of highly siliceous ZSM-5 as a function of concentration is shown in Fig. 5. At low loading (0.4 molecules /unit cell) the effect on the spectrum is minimal, with small shifts in individual peaks which increase with increased loading. At 1.6 molecules/unit cell the change in the spectrum is complete and it now shows only 12 resonances indicating a phase transition to the orthorhombic (30) in agreement with XRD data.
I
n
10
8
.
2.0
4
1.6 I
t
G
.7
1 .4
1.2
Fig. 5 .
Effect of p-xylene on ZSM-5 at ambient temperature (ref. 33)
833
The NMR spectra indicate both orthorhombic (12 T atoms) and monoclinic (24 T atoms) phases are present in the intermediate loadings in different proportions. The change in relative intensity of some of the better resolved resonances indicates the midpoint of the transition a t 1 molecule/ unit cell with a complete transition at 2 moleculedunit cell. As in the case of sorbed species, the effect of raising temperature is to induce a phase transformation from monoclinic to orthorhombic. Detailed spectra at 10" intervals fig. 6 show gradual shifts of individual resonances up to 353'K with a rapid change between 353 and 363°K. This is confirmed by synchrotron X-ray diffraction analysis of the phase transformation as a function of temperature ( 3 4 ) . which also shows that above the transition temperature only a single phase (the orthorhombic) persists. The temperature and range over which the phase transition takes place is a function of framework A1 content decreasing with increasing Al. A TEMP
I
6
(K)
393
Fig. 6 .
Effect Of temperatureon ZSM-5 (ref. 33).
I
834
The combined effect of temperature and rorbate shown in Fig. 7 is to lower the phase transition temperature and increase its range. In the phase transition temperature range, both phases now coexist with the material being crystalline. Above the transition temperature, there are only minor changes reflecting lattice expansion due to temperature within the orthorhombic (12 T atom) phase ( 3 3 ) .
tamp 363 K
353 K
343 K
333 K
* 363
553
h
343
dk
333
t 323
K
0
323
313 K
313
t 303 K
303
105 K
205
Pig. 7. Combined effect6 of temperature and p-xylene on the "Si Pus NHR 6peCttum of 2811-5 at the conditions indicated (ref. 33).
835
ZSM-1 1 ZSM-11, the other end member of the pentasil family of zeolites was synthesized by Chu (35). ZSM-11 crystallizes in the tetragonal system with space group I4m2 (36). It is very difficult to synthesize pure ZSM-11 without intergrowths with ZSM-5 or stacking faults, so that for hkl reflections, h + k + 1 = 2n and the doublets merge into singlets. '9Si MAS NMR spectra of a highly dealuminated and phase- pure ZSM-11 sample (37) have been obtained as a function of temperature. As in the case of ZSM-5 where elevated temperatures promote a transition to a phase of higher symmetry, it was found that at a temperature of 373"K, Fig. 8 , the spectrum is resolved into six sharp Lorentzian peaks with relative intensities 1:(2+2):2:1:2:2 which is consistent with the proposed structure, tetragonal with space group I4m2. with the six resonances due to the 7 independent atoms in the ideal framework structure [16(T,,T,,T,,T,,T,)8(T, ,T6)]. At room-temperature, the symmetry is lower and the limiting structure is reached only at 253°K.
I
I
l
l
l
,
105
-420
20°
1
-105
1
1
(
,
1
,
,
1
1
1
1
1
-
120
Fig. 8 . 2BSi MAS NMR spectra from ZSM-11 (a)293OK (b) 373%. At 373°K the expected seven resonances are observed, but at 293°K the spectrum shows additional resonances, indicative of a lowering of the local symmetry.
836
Synchrotron X-ray powder diffraction data were obtained using beam-line X13 A at the Brookhaven National Synchrotron Light Source ( 3 8 ) . Rietveld refinement (39) shows that the high-temperature structure is exactly that previously proposed and is currently being applied to deterrine the structure of the low temperature from (41). Initial results indicate the much greater sensitivity of MAS NMR to detecting phase transitions in this system and additional synchrotron X-ray data collected at 253OK may be needed to resolve this problem.
I
I
1 410
Fig. 9.
1
1 -114
1
1
1
1
1
-118
MAS NMR spectra obtained at 79.5 MHz. highly siliceous zeolite ZSM-11, and this material after sorption of, B. 25 p1 quinoline, C. 25 p1 acetylacetone, and D. 20 p1 n-octane. (In all cases, 250 mg of zeolite was used.) a's1
A.
837
The effect of sorbed species on the "Si MAS NMR spectra of ZSM-5 has been clearly shown (32.33). The syrmoetry of ZSM-11 being higher than that of ZSM-5, smaller changes in local environment are to be expected due to the presence of sorbed species. As in the case of ZSM-5, the effect of sorbed molecu-es on the ZgSi MAS NMR spectra is thought to be a modification of the framework structure which alters the local T-atom environment. This is confirmed by the small but significant changes observed in the XRD patterns. which are reversible on desorption. The spectra in Fig. 9 show the effect of 25 p1 of quinoline, acetyl acetone and 20 p1 of n-octane on 250 mg of highly dealuminated ZSM-11 (42). Unlike the case of ZSM-5, the changes induced do not appear to lead to a clearly defined highly crystalline state and may involve a degree of disruption of local geometry. It is interesting to note that the room temperature spectrum due to the sorption of 20 p1 of n-octane is modified to be very similar to that of ZSM-11 at 273°K which indicates the n-octane sorption increases the symmetry of the ambient temperature phase of ZSM-11. ZSM-39 The highly siliceous zeolite ZSM-39 (431, is the silicate analog of the 17A cubic gas hydrate. Crystals of ZSM-39 tend to have octahedral habit. Its structure was determined by Schlenker et a1 (44a) and refined from single crystal data in space group Fd3 by Gies et al. (44b). X-ray powder patterns of the room temperature form however show a few very weak extra reflections indicating a space group synnnetry lower than face centered cubic
Fig. 10. Framework structure of ZSM-39. synnnetry. There are 3 independent atoms in the ideal cubic face centered structure of ZSM-39, 8(T,), 32(T,) and 96(T,). The "Si MAS NMR spectra of four different ZSM-39 samples (Fig. 11) (a-d) show varying degrees of line broadening which cannot be due to A1 distribution as the Si/Al ratios of 3 of the samples (11. a-c) are very high and the fourth was synthesized with aminopropane as template (sample courtesy of D.M. Bibby) (45). In the spectra b and c the lines narrow and extra resonances are resolved. In apectrum 2d the T, resonance is split into 3 resonances of equal intensity making the relative spectral intensities 1:4:4:4:4. This splitting is probably due to the removal of the 3-fold symmetry
838
w i s . The same distortion appears in the other spectra (a b and c) to varying degrees due to a distribution of local environments and consequent line broadening. The a9Si MAS NMR spectra of the ZSM-39 (aminopropane base) seen at ambient temperature in Fig. 11 d, vyere obtained over a temperature range 373-383'K. Fig. 12. As the temperature is raised, all the resonances remain sharp but there is a clear change over a 10' temperature range to a new spectrum of three resonances with relative intensities 1:4:12 clearly indicating cubic face centered symmetry at least on a local basis on the NMR time scale. The changes are completely reversible and also apply to the samples with less well resolved spectra. The nature of these high temperature phases is somewhat ambiguous. The spectra may well be reflecting much more flexible frameworks for zeolites in general than hitherto anticipated or may indicate changes to new but relatively higher symmetry lattices. However, investigations of the phase transition temperature of silicon ZSM-39 (dodecasil 3C) synthesized in the presence of different quest molecules (45) showed that it varies. This demonstrates the influence of nonframework constituents on the symmetry of the silicon framework even for more dense structures as ZSM-39.
Fig. 11. a9Si MAS NMR spectra of zeolite ZSM-39 with differing Si/Al ratios; (a) Si/A1 = 285, (b) Si/Al = 2400, (c) Si/A1 = 310, and (d) from aminopropane base synthesis.
839
I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
-100
PPM FROM TMS
-125
Fig. 12. "Si MAS NMR spectra of ZSM-39 recorded at the temperatures indicated.
(1) Bragg; W.L.; Claringbull, C.F., The Crystal Structure of Minerals, Bill and Sons, London, 1965. (2) Meier, W.M., in Molecular Sieves, SOC. Chem. Ind. 1968. p. 10. (3)
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Breck, D.W., Zeolite Molecular Sieves, Wiley Interscience. 1974.
(4) Smith, J . V . , Zeolite Chemistry and Catalysis, Ed. Rabo, J.A., ACS Monograph. 171, (1976) p. 3. (5)
Barrer, R.M., Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978.
(6) McCusker, L.B.; Baerlocher. C. Proc. Sixth Int. Conf. Zeolites, Reno, 1983.
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(7) Eisenberg, P.; Newman, J.B.; Leonowicz, M.E.; Vaughan, D.E.W., Nature, 1984, 309, 45. (8) Toby, B.H; Eddy, M.M.; Fyfe, C.A.; Kokotailo, G.T.; H.; Cox, D.E., In Press. Nature. (9) Rietveld, H.M., J. Appl. Cryst., 1969.
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(11) Bennett, J.M.; Cox D.E.; and Blackwell, C.S.; J. Phys. Chem. 1983, 87, 3783, 1983. (12) Bennett, J.M.; Gard, J.A., Nature, 1967, 214, 1005. (13) (a) Kokotailo, G.T.; Sawruk, S., Lawton, S.L., Am. Mineral, 1972 57, 439. (b) Thomas, J.M., Proc. Fifth, Int. Con. Catal., Berlin 1984. Verlag Chemie. Vol. 1, p. 31. (14) Von Ballmoos, R.; Meier, W.M., Nature, 1981, 289, 78. (15) Fyfe, C.A.; Gobbi, G.C.; Kennedy, G.J.; Graham, J.D.; Ozubko, R.Z.; Murphy, W.A.; Bothner-By, A.; Dadok, J.; Chesnick, A.S., Zeolites, 1985, 5 , 179. (16) (a) Lippmaa, E.; Magi, M.; Samoson, A.; Grimmer, A.R.; Engelhardt, G., J. Am. Chem. SOC., 1980, 102, 4889. (b) E. Lippmaa, M. Magi, A. Samoson, M. T a d . G. Engelhardt, J. Am. Chem. SOC. 1981, 103,4992. (17) (a) Fyfe, C.A.; Thomas, J.M.; Klinowski, J.; Gobbi, C.G.; Anuew. Chem., 1983, 95, 257, and Angew, Chem. Int. Ed., 1983, 22, 259. (b) Fyfe, C.A.; Kokotailo, G.T.; Kennedy, G.J.; Gobbi, G.C.; DeSchutter, C.T.; Ozubko, R.S., and Murphy, W.J., Proc. Int. Symp. Zeolite 85, Elsevier, Ed. Draj, B.; Hocevar, D.; and Pjenovich, S., p. 219, 1985. (c) Kokotailo, G.T.; Fyfe, C.A.; Kennedy, G.J,; Gobbi, G.C.; Strobl, H.J.; Pasztor. C.T., Barlow, G.E.; Bradley, S.; Murphy, W.J., Ozubko, R.S.; Pure and Applied Chemistry, 1986, 58, 1367. (18) Englehardt, G.: Michel, D., High Resolution Solid State NMR of Zeolites and Related Systems, John Wiley h Sons, to be published.
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(22) Kokotailo, G.T.; Fyfe, C.A.; Gobbi, G.C.; Kennedy, G . J . ; DeSchutter, C.T.; Ozubko. R.S.; and Murphy, W.J., Zeolites 85, Elsevier, Amsterdam, p. 219. 1985. (a) Fyfe, C.A.; Gobbi. G.C., Murphy, W.J.; Ozubko, R.S.; and Slack, D.A., Chem. Lett. 1983, 1547. (b) Fyfe, C.A. ; Gobbi, G.C. ; Murphy, W.J. ;Ozubko, R.S. , and Slack, D.A., Chem. Lett. 1983, 1547. (c) Fyfe, C.A.; Gobbi, G.C., Murphy, W.J., Ozubko, R.S., and Slack, D.A., J. Am. Chem. SOC. 1984, 106, 4435. (24) U.S. Patent 3, 702, 886 (25) Mersil. S.L.; McCullough, J.P.; P.B., Chem Tech 1976, 6, 86.
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H.G. Karge, J. Weitkamp (Editors ), Zeolites as Catalysts, Sorbents and Detergent Builders 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE EFFECT OF TEMPERATURE AND SORPTION OF p-XYLENE AND BENZENE ON THE STRUCTURE OF ZSM-5 G.T. KOKOTAILO, L. dIEKERT and A. TISSLER Institut filr Chemische Verfahrenstechnik, Universitft Karlsruhe, D-7500Karlsruhe
ABSTRACT The effect of p-xylene and benzene adsorption on the structure of ZSM-5 with varying SUAI ratios, temperature and pressure has been determined. High quality ZSM-5 with SQA1 ratios of 35, 100 and 800 were synthesized and converted to the hydrogen form. The lattice parameters were accurately determined from data obtained with a Guinier 4 circle camera. Room temperature sorption of p-xylene and benzene promoted the monoclinic-to-onhorhombic phase transition with some changes in the a. b and c parameters especially in the case of benzene sorption; where the a and b parameters become equal on full loading. Low pressure p-xylene and benzene flow over ZSM-5 samples maintained at various temperatures caused the monoclinic to orthorhombic phase transition with changes mainly in the a and b parameters and unit cell volume as a function of temperature. INTRODUCTION ZSM-5 and ZSM-11are high silica zeolites which have been of particular interest in recent years because of their high catalytic activity, adsorptive properties and unique pore geomery. Their selective accessibility to sorbates and reactant molecules is an unique feature of these zeolites. It was only recently that the interaction between sorbed species and the zeolite framework was demonStrated (1,2) by high resolution MASNMR for sorbed species in highly siliceous ZSM-5.The 2% MASNMR spectra of high Si ZSM-5and ZSM-I 1 are modified by the sorption of organic molecules with related changes in corresponding x-ray diffraction patterns. The spectra arc characteristic for the sorbed molecule. As the concentration of the sorbed molecule is increased the character of the NMR spectrum is altered until a limiting spectrum is obtained which with further increases in concentration does not change except for a loss in resolution. This indicates that structural changes arc occurring which are characteristicDf the'sarbate while the integrity of the framework is maintained. These changes are completely reversible. Temperatun has an effect on the rate at which these changes take place. The effect of p-xylene and acetylacetone on the structure of highly siliceous ZSM-5 as determined by NMR has been described (3). In this report the effect of pxylene and benzene sorption on the structure of ZSM-5has been extended to ZSM-5 with lower SVAI ratios, and the effect of subjecting the samples to reactor conditions of flow at various temperatures and pressure will be discussed. EXPERIhENTAL
ZSM-5with SVAI ratios of 35,100 and 800 were synthesized by the method of Argauer and Landolt (4) and reduced to the H-form by calcining 4 hours at 5oO.C. ion exchanging with 1 molar W C I for 1 hour at 1oO.C.washing twice, filtering. drying and calcining for 4 hours at 5WC. The x-ray dihction patterns were obtained at room ternpaatwe with a Philips diffractometer and CuKa radiation, Samples with sorbed species wcrc c o d with a collodion film to prcvent evaporation. Samples were also packed in silica glass capillaries to avoid prefemd orientation and sealed to prevent evaporation. X-ray patterns for these samples
844
were obtained with a lluber 4 circle Guinier camera (631) with CuKa radiation. ii Jo1l;lnnscn monochromator and a silicon internal standard. A capillary microreactor was designed using 0.6 mm silica glass tubes with 0.05 nini wall thickness in which the zeolite samples werc packed and a flow of p-xylene and benzene a t 1-2 mlhr and prcssurcs of 12700 and 1270 Pa resp. with nitrogen as a carrier gas were maintained. X-ray diffractionfilm patterns we^ obtained at various temperatures up to 525°C using a Guinier 4 circle camera (5). The films ~ r exposed c for 5 hours at each temperature with a total exposure of up to 80 hours. The temperatures were determined from the (1 1 I ) reflection of a silver internal standard. The films were read and the d spacings and lattice parameters determined as previously described (5). RESULTS AND CONCLUSIONS Room temperature sorption of p-xylene on ZSM-5with SVAI ratios of 35, 100 and 800 has very little effect on the a, b and c parameters after an initial increase on adsorption of 0.7 rnoVuc but the p angle decreases to 90”with sorption of nbout 3 molecules per unit cell, Table 1. This indicates that sorption of 3 molecules per unit cell causes nionoclinic-to-onhorhombic phase transition. In the case of benzene the b pmmeter tends to increase with sorption and a phase transition occurs with a sorption of about 4 molecules/uc as indicated by the change in 0 angle to 90”.Table 2. The anomaly is the sample with a Si/AI ratio of 800 where the final transition is to the pseudo-tetragonal phase with a=b. The effect of sorption on the lattice parameters is given in fig. I .
n
2.01
E
ff
2.00 A
v
Y
$
1.99
m Q
1 . 9 8 b
8 1.35
‘J
4
1.34
0
0
n v
C
2
4 6 MdAJ.C.
8
Figurc 1: Lattice Parameters of H-ZSM-5 with Si/A1=800 as a Function of Benzene Concentration
845
Table 1 Effect of p-Xylene on Lattice Pmmeters of Z S M J with Si/AI Ratio 35,100 and 800 at Room Temperatures % wt. MoVuc
0 1.35 2.69 5.38 6.85 10.76 15.00 0 3.86 5.38 10.76 15.00 0 2.69 5.38 7.46 10.76 15.00
0 .7 1.4 2.8 3.6 5.7 8 0 2 2.8 5.7 8 0 1.4 2.8 4 5.7 8
SVAl
35 " " " " "
100 " " " I '
800 "
" I '
" I '
a 1.992 nrn 1.997 2.002 2.000 2.000 1.995 1.993 1.986 1.994 1.997 2.000 1.995 1.987 1.991 1.999 1.993 1.997 1.984
b
2.01 I nm 2.015 2.014 2.014 2.015 2.016 2.018 2.004 2.008 2.008 2.010 2.007 2.008 2.010 2.007 2.008 2.008 2.012
C
P
1.336 nm 1.342 1.347 1.351 1.349 1.348 1.346 1.340 1.342 1.343 1.349 1.346 1.338 1.337 1.342 1.340 1.341 1.343
90.09O 90.08 90.09 90.01 90.00 90.00 90.00 90.13 90.09 90.02 90.00 90.0 90.18 90.02 90.05 90.00 90.00 90.00
VOl.
uc
5.349(nm)3 5.400 5.430 5.442 5.437 5.422 5.406 5.332 5.372 5.381 5.410 5.388 5.340 5.350 5.382 5.361 5.375 5.358
Tiihlc 2
lo wt
Mol/uc
0 2.80 5.60 8.00 1 1.00 0 3.20 5.10 8.10 I 1 .oo 0 2.00 3.26 4.00 4.26 7.60
0 2.3 4.6 6.6 9.0
Effect of J3cn7snc on Iiiitice 1';irnincters of ZSM-5 with SVAI k i h s 35, 100 and XO() ;it Room Tempcr;iture SVAI n b C
P
35 'I
" I'
"
100
2.7
"
4.2
I'
6.7 9.0 0 1.64 2.7 3.3 3.5 6.3 11.00 9.0
'I
8()0 'I
: : 'I
1.992 nm 1.998 1.998 1.999 2.000 1.986 1.997 1.996 2.001 1.999 1.987 1.995 1.999 1.998 1.995 1.998 1.995
2.01 1 nm 2.014 2.018 2.019 2.014 2.004 2.0 15 2.013 2.017 2.0 13 2.008 2.016 2.109 2.015 2.015 2.001 1.995
1.336 nni 1.342 1.341 1.349 1.347 1.340 1.345 1.344 1.348 1.350 1.338 1.341 1.346 1.344 1.343 1.343 1.345
90.09 90.00 90.1 1 90.00 90.00 90.13 90.02 90.00 90.00 90.00 90.18 90.09 Y0.07 90.05 90.03 90.00 90.00
Vol. tic
5.349 (11m)-1 5.400 5.430 5.444 5.423 5.332 5.4 10 5.402 5.440 5.433 5.330 5.392 5.3OX
5.412 5.399 5.364 5.353
846
The effect of p-xylenc, flowing tinder pressure over ZSM-5. on the I;ittice pirratiieicrs i \ shown in fig. 2. The a and b parirmetcrs drop off wiih temperature while the decrease is les\ for c. The change in the unit cell volume for ZSM-5with SVAI ratios of 35 and I(X) is shown in fipn 3; it also decreases with tempenturc. The lattice parameters and unit cell volume at ambient-tempenture are higher than for the unloaded sample. This is true for flow as well as static sorption. The interaction of the sorbate with the framework and cations is evident from the changes in the MASNMR spectr;! and the x-ray diffraction patterns.
i 0
g 2.00 C
Y
0
a,
cm
T
0 I
OI
I
I
100
200
300 T(C)
P
'p
400
500
Figure 2: Lattice Parameters of 11-ISM-5 Samples with p-Xylene Flaw as a Function of Temperature for a Si/A1=35 and o Si/AI=100.
0
100
200
300
400
500
T (C) Figure 3: The Volume of the Unit Cell of H-ZSM-5 Samples with p-xylene Flow RS n function of temperature for 0 Si/AI=3S and o Si/AI=IOo.
The effect of flowing benxne on the structure of ZSM-5at elevated temperatures is shown in figure 4. The samples with SVAI ratios 35, 100 and 800 were run up to temperatures 200,425 and 525°C resp. The b arameter decreases at a greater rate than the a to 425°C for the sample wilh SVAI ratio of 8 and the c parameter remains essentially constant. For the 35 and 100 SVAI ratio samples the a and b parameters decnase with temperature while the c parameter remains relatively constant. The initial effect of benzene flow through ZSM-5 with varying SVAI ratios causes an increase in the lattice parameters but not as large as for p-xylene.
d
The change in lattice pmmeters is significant as film distance are measured with a resolution off 0.05 m m or 0.015"20 and lattice parameters determined to & .006mm.
2.01
0
-.
+
-+
I
* *0
0
0
I+
~
0
100
300
200
400
500
T(C) Figure 4: Lattice Parameters of fi-ZSM-5 with Benzene Flow Through as a Function of Temperature for * Si/AI = 35,o SVAI = 100, + Si/AI = 800 The temperature at which the phase transition, monoclinic to onhorhombic, takes place is lowered with gas flow and increasing temperature. Benzene and p-xylene flowing over ZSM-5 at 12700 and 1270 Pa pnssures, resp. and elevated temperature cause the phase transition, monoclinic to onhorhombic. Elevated temperatures tend to decrease the lattice parameter and the unit cell volumes are reduced. Whether this reduction is due to coking which was observed remains to be determined. It is evident that ZSM-5with varying alumina content is in the onhorhombic phase under these siniuhted catalytic conditions. Whether the lattice pmmeters ;ue related to catalytic activity is not i I S yet known. The effect of sorption and tcmpcriitun: as determined by x-ray diffraction confirms MASNMR studies (3) of highly siliceous zeolites and extrapolates these results to lower SdAl
848
ratios which are in the catalytic range. As in the NMR studies, the effect of adsorption and temperaturr on the ZSM-5 structure is characteristicof the sorbed molecule and also the A1 content of the zeolite. This work is proceeding with other sorbants and reactor conditions. ACKNOWLEDGEMENT We wish to thank Prof. E. Althaus and Dr. G.011, of the Institute of Mineralogy. for the use of their x-ray diffraction equipment and valuable discussions.
G.T.Kokotailo wishes to acknowledge the Alexander von Humboldt U.S. Sr. Scientist Award. REFERENCES (I)
(7) (3) (4)
(5)
C.A. Fyfe. G.J.Kennedy, C.T. DeSchutter and G.T. Kokotailo. J. Chem. SOC. Chem. Comni.. 541, 1984. G.W. West, Aust. J. Chem.. 37, 455. 1984. C.A. Fyfe, H. Strohl. G.T.Kokotailo, G.T. Kennedy and G.E. Barlow. J. Am. Chem. Soc. in press. U.S. Patent, 3,702,886, 1972. A. Tissler, L. Riekert, G.T. Kokotailo. this volume.
H.G. Karge, J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ThE CYARACTERIZATION OF MODIFIED ZSM-5 CATALYSTS PREPARED V I A A SOLID STATE REACTION FOR PROPANE AROMATIZFITION
YANG Yashu, GUO Xiexiari, DENG Maicun, WANG Limin and FU Zaihui Dalian Institute of Chemical Physics, Chinese FIcademy o f Sciences P.O. SOX 100, Dalian, P.3.CHINA
I.iESTRAZT This paper dsscribes a simple method o f solid state reaction for preparation o f Zn-, V o - , a;>d Cr-ZSM-5 catalysts instead o f ion exchange. NH,-TPD, I R , TPR, ESR and UPS techniques x e r e used to characterize the interaction o f HZSM-5 with Znn, %C1:.I and CrC.. ahich leads to introduction o f cations into the channels o f zeo1i:cs. Zn-ZSM-5 is more active f o r propane conversion and gives the better BTX selectivity. Over Mo-ZSM-5, propane mainly undergoes cracking into methane a:id ethane, J X ~ the loading o f CrT’ o f ZSM-5 enhances the propar@ dehydrogenation t o propsne.
iXTPODUCT1ON The
aromatiiation o f liqht hydrocarbons h a s been investigated since the
eal-ly ;;eventies ’. 1 - i ’ )
.
Interest in studies of modified ZSM-5 catalysts
propane aromatizatian was greatly stimulated developed by UC)P 3nd prepared via
the
RP.
by
the Cylar
for
jointly
Process
in yeneral, the modified high-silica zeolites a r e
introduction o f metal cations by meaiis o f an ion exchange.
This DrrJccss requires a lor,g period of refluwing in the ion-exchange solution, is iI!convenient in industry. Thus the introduction
:ghiih
higti-iilica . : e o I i t e s .via
a
of
metal ions into
s o l i d state reaction deserves attmtion from both
the.7retiral a n d zractical p o i r t s of view.
Thjs paper dESCi-ihS the characteri-
z a t i o n of Zn, ?‘u and C r ior% introduced by a solid state reaction C~si t:a;is in 2 3 - 5 z ~ o iltes. TPR,
!a,
ESR and XPS
Ohto
EdtiiJniC
Chemical and physical nethods such as T P D - W ~ ,
;,Ere used.
4s the
test reaction, propane aromatization was
c a r r i e d o u t in a fired--Sedpulse reactor.
EXPERIYFNT M a T a 1a t 5 Catdly3ts of Z11(2.0wt%l-, Ma(3.5wtXI- and Cr(l.3vct%;-ZSM-5 wore prepared by mixing
and grinding the powder o f HZSM-5 (obtained f r o m Nan Gai University, SiOc/
A1.Ua=34)
wjth
ZnO, MoCl- and CrO-. respectively.
crus’led t o 40-60 mesh aiid calcined
3%
CIfter
shown in Tab:e 1 .
pelleting, they were
850
TABLE 1 Ka t e r i a l s
CATOLYSTS
Preparation Conditions
ZnO
H e a t i n g a f Zn a c e t a t e a t 540C i n a i r f o r 6 h r
CrO::
H e a t i n g o f Cr(NOsr:., a t 550°C i n a i r f o r 4 h r
roc1:.
Y9.99%
Zn/ZS#-5
Calcination of
Zn/ZS?l-S(i)
I o n e x c h a n g e o f 29 o f NH,,ZS?I-5
d
m i x t u r e o f ZnOtHZSM-5 i n He a t 980nC f o r 1 h r w i t h 2 0 0 ml o f 0.1M O F Zn(NO:,):..
s o l u t i o n f o r 16 h r a t 8O'.,C, t h o r o u g h w a s h i n g w i t h w a t e r t o f r e e d r i e d o v e r n i g h t a t l 2 O r ' C , c a l c i n e d i n a i r a t 540DC f o r 3 h r
NO:,,
Mo/ZSM-5
C a l c i n a t i o n o f a m i x t u r e o f MoCI:;+HZSM-J
Cr /ZSM-5
Calcination o f
d
ii? Ar a t 450'X f o r 4 h r
m i x t u r e o f CI-C:,*ICSM-~i n s i r a t 54cS'T f o r 4 h r
F r o p a n e h a s 99.0% p u r e a n d f r e e d f r s m t r a c e s o f 0:: arid w a t e r p r i o r t o use.
CATFLYST C;H~RAC:TERIZATION NH-?-TPD s p e c t r a were measured w i t h a c c n v e n b i o n a l TP3 a p p a r a t u s .
NH-.:-TPD
About 0.29 o f s d s p l e m a t e r i a l w i s p l a c e d i n a q u a i t z r e s c t o i - a n d a d s o r p t i o n s a t u r a t e d by ptjlses o f dry ammonia a t lEO<'C.
TPD *as c a r r i e d o u t f r o m 120°C
t o 5OO.C w i t h
3
h e a t i n g r a t e o f @ - 3 2 T / m i n arid w i t h H e ( 3 0 nrl/min) as t h e
c a r r i e r gas.
. W e t o t a l NH., u p t a b e was d e t e r m i n e d b y a t h e r m a l c o n d u c t i v i t y
d e t e c t o r , c o l n p 3 r i n g tbe i r ? t e g r a t e d a r e a b e l o w t h e c u r v s w i t h t h a t o f a known v o l m e o f NH3. T h e s a m p l e s were p r e s s e d i n t o s e l f - s u p p o r t i n g wafers and
19 SPECTROSCCPY
placed i n a quartz i n - s i t u I R c e l l .
The s a m p l e s here p r e t r e a t e d a t 580'C
i n He
(30 m l / a i n ) f a r 1 h r , evacu.ited a t 50rJr'C t o 10"" T o r r f o r 4 lir, c o o l e d t o 230r'C and e x p o s e d t o
d
s a t u r a t e d v a p o r p r e s s u r e o f p y r i d i w o f 2rJ,C, a f t e r 1 h o f dd-
s o r o t i o n . t h e e x c e 5 s a c d w e a k l y a d s o r b e d p y r i d i n e was 1-evoked by e v a c u a t i o n a t :he sime t e m p e r a t u r e f o r 30 min t~ S e l o w 10-" T o r r t a pyi-idioe on
the s u r f a c e .
Ie3've
only chmisorbed
1R s p e c t r a luere r e c o r d e d o n a Per t i ri-E 1rner 580
s;>ecCrometer .at r o o m t e m p e r a t u r e . X-RAY
XPS s p e c t r a * e r e r e c a r d e d o n a PHI 550
PHOTOELECTRON SPECTROSCOPY
;pectrometcr
u s i n g Al-K6
X-rays,
r e f e r e n c i n g t o t h e BE o f C,,,
b i r d i n g e n e r g i e s (EE) wer
(?86.4
EV).
E!
i o r r x t e d by
Pie s u r f a c e a t o m i c r a t i o s o f S i , A I ,
a n d 21-1 were c a l c u l a t e d f r o m t h e i n t e G r a t e d XPS s i g n a l i n t e n s i t y c o r r e c t e d f o r atomic s e n s i t i v i t y f a c t o r s " + ) .
F o r t h e XPS s t u d i e s ? s e l f - s ~ ; p p o r t e dwafers were
l o a d e d i n t o a s t a i n l e s s steel h o l d e r and e v a c u a t e d t o lo-" Torr a t 25-500'-'C f o r 1 hr.
TPR
The TPR e x p x i m n t a l p r o c e d u r e u s e d :*as e s s e n t i a l l y s i m i l a r t o t h a t d e s -
c r i b e d by McNicol'r.".
An i n - s i t u a d e o r b e n t t r a p h o u s e d a h e a d o f ttie r e a c t o r
was u s e d t o remove the l a s t traces o f H,JJ and O:.. From che c a r r i e r q a s .
861
a b o u t 0.29 o f c a t a l y s t was p l a c e d i n a q u a r t z r e a c t o r , p r e t r e a t e d a t 590°C i n d r y He (30 m l l m i n ) f o r 1 h r and c o o l e d i n He t o room t e r p e r a t u r e . The gas s t r e a m was t h e n s w i t c h c d t o a mixture of 6% Ii,: i n Or b e f o r e r u n n i n g t h e TFR. The f l o w r a t e of t h e r e d t i c i n 5 gas was 30 ml/cnin and t h e h e a t i n g r a t e was i6"C/
min.The t o t a l !ir:u p t a k e bas !neiisured b y i n t e g r a t i n g t h e a r e a below t h e c u r v e and c a l i b r a t i n g w i t h a known volume o f He. ESR SPECTROSCOPY
T h e c a t a l y s t s a m p l e s (40-hO mesh) were l o a d e d i n t o a n ESR
t u b e , h e a t e d i n Ye o r a i r (30ml/min) a t 5O0-55OFC f o r 1 h r and t h e n e v a c u a t e d a t the stlme t e m p e r a t u r e t o lo-.'' Tori- f o r 4h p r i o r t o ESR measurements. s p e c t r a Hiere r u n on a JES-FEZSG s p e c t r o m e t e r a t 20-C.
The ESR
The g f a c t o r s f o r t h e ESR
s i g n a l were d e t e r m i n e d I - e l a t i v e t o DPPH w i t h g-2.0036. CFITNYTIC KTIVITY
y l s c m i c r o r e a c t o r was used w i t h 0.29 o f c a t a l y s t .
Thi?
p u l s e volume o f 0.4Bml o f p r o p a n e was c a r r i e d through t h e c a t a l y s t bed by cle a t a f l o w r a t e o f 35 ml/min. and 5OOrsC f o r 1 h r .
The c a t a l y s t was p r e t r e a t e d in He a t 580 C f o r 30 min
R e a c t i o n p r o d u c t s u e r e a n a l y z e d o n l i n e u5iiv.l a gas chromato-
g r a p h w i t h a ttrermal c o n d u c t i v i t y d e t e c t o r .
A porapak
I)
column programmed from
20 t o 200'>C N J S used , The e l u t i o : ) t i m e was a b o u t 60 min f o r Cv a r o m a t i c s .
RES!JL'TS FIN3
D I SCtJSS I Chi
I ) In/ZStI-J
NHv-TPD and IR s p e c t r o s c o p y ai-e t h e nost p o t e n t methods f a r i n v e s t i g a t i n g t h e 3 c i d p r o p e r t i e s o f ZSM-5 t y p e z e o l i t e s . E M - 5 a r e shown i n F i q . 1 .
T y p i c a l TPD p a t t e r n s o n HZSM-5 and Zn/
T h e r e 31-8 two p e a k s o f t h e 1 ( o r @aboLt 220°C) and h
( o r l a b o u t 430eC) s t a t e s , i n acjreement w i t h Topsfie e t a l . ( * ' .
The amounts o f NH,:
d e s o r b e d froin t h e s e c a t a l y s t s arid t h e rlraxima t e m p e r a t u r e s a r e summarized i n T a b l e 2. TASLE 2
NH.!-TPD RESULTS
H2SM-5
Zn/Z5M-5
Max imum load ii-q (mmo 1/g 1
0.92
0.90
FIctual loadingOnmol/g)
1.37
1.10
Cat a 1 y s t
Peak maxima (T,."C)
1 h %
a r e a under t h e peak 1 h
215
229
439
lt30
53
63
47
37
852
100
200
300
400
600
500
TEMPERATURE ["C]
-Figure 1
TPD spectra of NH3 desorbing from HZSM-5 and ZnnSM-5;
B = 8'Umin.
1
1
1
1
1
I
7
w
x
4
k 3 n
I
--
I \ I I I,/
1900
3500
1600
IR spectra, obtained at 2O"C, of HZSM-5 and ZnRSM-5 (a) evacuated at 500°C, followed by pyridine adsorption at 2OO"C, then evacuated at (b) 200"C, (c) 450°C.
1
1
600
1
--
1
1
800
TEMPERATURE ["C]
WAVENUMBER [crn"] Figure 2
,
400
Figure 3
TPR spectra of ZnO and ZnRSM-5 0 = 16"Umin; sample weight: 0.2 g; flow rate of H Ar gas mixture: &rnl/rnin.
853
The NHS TPD r e s u 1 t s . m t h e a b o v e c a t a l y s t s show t h a t ( 1 ) t h e t o t a l NH3 u p t a k e &a5
lowered by 20% on Zn/ZSM-5 w i t h t h e h p e a k b e i n g p r e f e r e n t i a l l y d e c r e a s e d
rathec- t h a n t h e 1 peak.
( 2 ) t h e Tm o f t h e 1 peak i n c r e a s e d and t h a t o f t h e h
I R s p e c t r a on Zn/ZSM-5 and HZSM-5 are shown
peak d e c r e a s e d by 5 c C o n Zn/ZSM-5.
The i n t e n s i t i e s o f t h e b a n d s a t 3600 cm-'
i n Fig.2a.. Zn/ZSM-5.
and 3740cm'-l a r e l o w e r on
When p y r i d i n e was a d s o r b e d and e v a c u a t e d a t 20OC8C, t h e 3600cm-1 band
,
d i s a p p e a r e d more q u i c k l y t h a n t h e band a t 3740cm-' ( B r k n s t e d a c i d s i t e s ) , a t 1455cm-'(Lewis
s i t z s ) appeared
acid sites
and new b a n d s a t 1 5 4 3 ~ m - - ~ )
rnd a t 1 4 8 8 c r n - ' ( t o t a l
acid
a s shown i n F i g 2b. The r e s u l t s o f p y r i d i n e a d s o r p t i o n a r e sum-
T h i s shows t h a t t h e i n t e n s i t y c f I , at: 1543 cm-'' was
m a r i z e d i n T a b l e 3.
lowered f r o m 6.21 on HZSM-5 t o 3.30 on Zn/ZSM-5, avd I l . a t 1455cm.' was i n c r e a s e d c o r r e s p o n d i n g l y f r o m 1 . 3 6 t o 3.85 w i t h l i t t l e l o s s o f t o t a l a c i d s i t e s a t 1WBcm-l. IR INTENSITIES OF PVRIDINE ADSOSPTION
TABLE 3
Cata l y s t
HZSM-5
Evacaat ion t e m p e r a t u r e
230
1.36
1,
4.57
1l./:* NH.,-TPD
7n:ZSM-5
?00
450
450
3.39
3.05
1.21
6.46
0.86
1.CG
and I R s t u d i e s o f t h e a c i d i c p r o p e r t i e s o f Z5M-5 z e o l i t e s h a d e b e e n
d e s c r i b e d i n v a r i o u s ~ a p e r s ( ' ~ - ~ 'I .t is w ? l l known t h a t the a c t i v e a c i d s i t e s a r e c h a r a c t e r i z e d by ~n IR bar:d a t 3600 c m ' ' arid s t a t o a r o u n d 670-770k.
d
d e s o r p t i o n peak o f t t : e h o r
Weaker a c i d s i t e s a r e c h a r a c t e r i z e d by a 3724-3740rm.'
band w h i c h i s c o r r e l a t e 9 w i t h d e s o r p t i o n o f YlW, a t a h o u t X O I : f o r t h e 1 state.
The Si-OH-Rl
r
g r o u p s a t 3hDOcm-* arid thF? hvdro.cy1 group:;
shown t o b e B r t n s t e d a c i d s i t e s .
T ~ EI R
01'
0
a t 3 7 2 0 c 1 n - ~i r e
and NH..-TPD e:!perini;+ntal results des-
c r i b e d a b o v e a g r e e i n i n d i c a t i n g t h a t the Br.#risted a c i d s i t e s on HZSII-5 r e a c t w i t h Zinc ions, i.e,
t h e Zne'
ions s e l e c t i v e l y r e d u c e t h e number o f s t r o n g a c l d
s i t e s , and c h a n g e t h e a c i d d i s t r i b u t i o n , l , - / I , .
, and
T,,
as w e l l .
TPR and XPS d e s c r i b e t h e 280 d i s t r i b b t i o n i n t h e channels o f ZSM-5 z e o l i t e s . The a f f e c t o f Z n z e o l i t e i n t e r a c t i o n on t h e TPD p r o f i l e o f n o n - s u p p o r t e d
ZnO
and s u p o r t e d 2-10 i r e shown in F i g . 3 , and the t o t a l Ii!>u p t a k e i s s u n r n a r i z e d i n T a b l e 4.
The t o t a l HI* u p t a k e d u r i n g TPR is much h i g h e r o n Zn/iSM-5
f o r which t h e r e l a t i v e r e d u c i b i l i t y is 5.4%.
t h a n c n ZnO
T h s H:; t i p t a k ~ so n t h e 2nIZSM-5
c a t a l v s t s a r e n e a r l y c o n s t a n t f o r d i f f e r e n t Zn l o a d i n g s f;-on 0.4 t o 2.0% a r d f o r
catalv5t5 prepared hv ion exchange. This implies that ZnO is widely dispersed in the channels o f the zeolites. Measurements of H, consumption indicate that the TABLE 4
TPR" DATA F3R ZnO AND Zn/ZSM-J peak temp.
He uptake
re1 at i ve
mmol/q-Zn
reducibility X
Sample
Zn(wt%)
pretreatment
1.7I
T"
ihO
80.3
S40r,C,Air,6 hr
386
675
0.14
Zii/ZSM-S
2.3
58OC'C!He, 1 hr
347
623
2.55
ZnIZSM-5
0.4
580"C,He, 1 hr
515 700
2.26
88.6
L'n/XM-SD
3.L
540°C. a i r
420 680
2.37
09.R
,I
5.4
100
( a ) sample weight 0.29 ( t ) prepared b y ion exchange method
extant o f reductioii o f Zn". to Zn is abaut I & % , i.e.
it
is lot4 charge density
of Zn+-' ions and n o t Zn metdl that are formed during 7PR MI exhibits trio peaks:
. The TPR profile of
low temperature peak T m j o f 386% attributes to
easily reduced senall particles, and h i g h temperature peak T,,+: 60 large particles mors difficult to reduce.
of 675 C rslates
The low teniperature peak. rapidly
decrca?;es f o r ZilO supoorted on ZSM-5 zeolites, when the 211 loading is reduced fr.om 2.0 to O.G%, t L r e pc;k
the tiiqh temperature peak splits into two and the low terrpera-
disappears.
The change in the TPR profile is due ta an interaction
between Zn iors and the zeolites on ZnIZSM-5 which gives rise to the high reducing temperature around !r20-70O0C.
%PS of
XPS data are listed in Table 5. These show that the surface
Si/Al ratio
H X M - 5 is higher than the n m i n a l hulk composition o f S i / A l =17, and that
this surface Si/P1 ratio increases from 25.0 to 30.3 Khen the evacuation temp e r a t u r e is increased from 25°C t a 500'.C. nf
This suggests that thare is a loss
5urface A i atoms from tiZSM-5 under v d c w m even at 25lSC, end this 105s is
increased at elevated temperatures.
En Zn/ZSM-S, the surface Si/Al r a t i o shows
only a modest increase t o 19.2 after eracuation at 2S"C and to 2G.4 dfter c t l J t i o n at 500°C.
L'VI-
The higher eracuation temperature causes increases in t h e
Zn/51 and Zn/Al ratios from 4.BxlO-e to 6 . 7 ~ 1 0 -.md ~ 0.92 to 1.37 respectively. The Zn/Si and Zn/A1 data from Zn/ZSM-5 is further evidence that the solid state reaction of HZSM-5 w i t h ZnO causes the migration of Zn ions from the outer stirface into the chanriels of the zeolites even at room temperature, with more extensive migration at the hipher temperature.
Zn ions located near R 1 atoms
protect the A1 atom; f r a m escaping from the surface. CATALYTIC K T I V I T Y
The catalytic activity for propane conversion is Shown in
Tabje 6. This shows a higher activity and selectivity for propane aromatilation on Zn/ZSM-5 prepared either by sol id calcination or liouid ion-exchange, than
855
200
400
600
TEMPERATURE ["C] Figure 4
TPD spectra of NH3 desorbing from HZSM-5 and MoESM-5; fl = 32'Umin.
gl'= 1.99
100G H H-
Figure 5
ESR spectra of M o (V)at 20°C; mixture of H-ZSM-5and MoC15 calcined in He at 450°C.
Figure 6
ESR spectra of Cr (V); CrESM-5 calcined in He at (a) 500°C. (b) 550°C
856
on HZSM-5.
Zn ions enhance the rate of dehydrogenatioi of propane and t.he transformation of intermediates into aromatics. With regard t o Zn state and Zn*'-ZSM-5 bifunctional action on Zn/ZSM-5 catalysts will be discussed elsswhere (:C).
TC\BCE 5
XPS CATA ~
Ca tal yst E v a c . Temp.( C )
El. ( O v : , e v )
~~
~~
~~
25
500
532.8
~
~~
;7n/ZSM-S
H-2SM-5
25
532.1
532.0
500 532.0
En (4lry. , e v )
74.c
74.4
74.3
74.4
Es (Sir:,. ,av)
103.6
103.2
103.0
iC3.1
1021.5
1021.3
503.3
502.9
Et. ftnTw , e v )
E,,,, :Zn,ev)
Surface atomic ratio
O/Si
1.91
Si/FI1
1.84
2.13
30.3
I .91
13.2
20.4
Z d S i ( x lo+)
4.8
6.7
Zn//S:
0.92
1.37
Catalyst
25.0
Zn/ZSM-5
H-2SM-5
?I (at % )
Mo/ZSM-S
C r IZSM-J
2.0
1.5
Coiiversion(C%)
17.4
96.4
79.9
81.4
70.4
16.5
BTX
27.1
50.7
42.0
37.1
a.6
3.6
i7.0
33.7
I 9
se1ec.X
1 * 5 4 < i , 3.5
Producu t d i 5 tr ihu t ion(C% )
.
methane
16.8
e thy1 ene
24.5
1.1
1.7
1 .D
3.b
13.9
ethane
11.3
27.5
36.0
42.0
54.3
1.9
propyl ene
a.
1.3
.-..
bu tene
2.6
butane
15.8
14.4
17.0
1.5
2.0
--.. 1.0
_....I
0.4
----
-..1.2 --.- -
- _ I
?O.? 0.b
0.9
1.2
4.2
0.4
3.1
- ---
h.7
1.8
C!,..
1 .b
3.3
benzene
8.7
39. I
30.6
28.3
toluene
13.1
10.E
11.0
8.2
1.3
1 .r7
xylmes
5.3
0.8
0.4
0.6
0.6
0.6
0.29 catalyst calcined at 580C for 0.5 hr in He prior- to r e a c t i o n :
Reaction Temp. 500C, flow rate o f He 35ml/rnin. (except f o r Znc,, )
867
2 ) Mo/ZSM-5 and Cr/ZSM-5 ESR s p e c t r a h a v e b e e n a p p l i e d t o s t u d y t h e s o l i d s t a t e r e a c t i o n o f h i g h -
si1:ca
Calcination of a
z e o l i t e s w i t h t h e compounds o f MoCl?. a n d CrOo"l'.
m i x t u r e o f MoCI,; and H%SM-J(in He), CrOn and HZSM-5 ( i n a i r , f o l l o w e d b y e v a c u a t i o n ) r e s u l t s i n t h e a p p e a r a n c e o f i n t s n s e e.5.r.
s i g n a l s of M o ( v )
( g =1.93, g =1.89) and C r ( v ) fg=1.967) shown i n F i g . 4 a n d F i g . 5 r e s p e c t i v e l y . s p e c t r a shown i n F i g . 6 i n d i c a t s s t h a t t h e h i g h t e m p e r a t u r e p e a k is
NH,,-TPD
very
I t is b e l i e v e d t h a t t h e r e a c t i o n b e t w e e n HZSM-5
weak o n M o C S M - 5 .
and M C C I , ~ , Cr03 i n t h e s o l i d s t a t e l e a d s t o t h e i n t r o d u c t i o n o f M o ( v ) , C r ( v ) ions onto c a t i o n i c p o s i t i o n s o f the high-silica
zeolites.
The h i g h t e m p e r a t u r e
treatment of Ci-/ZSM-S shows t h a t t h e b i n d i n y o f C r ( v ) t o c a t i o n p o s i t i c n s o n the z e o i i t e framework is s t r o n g e r t h a n t h a t o n t h e s u r f a c e o f s i l i c a - a l u m i n a
a t 550-C i n a i r l e a d s t o a c o n s i d e r a b l e
c a l c i n a t i o n o f :he a i x t u r e (HZSM-S+CrO.,) d e c r e a s e i n t h e i n t e n s i t y o f t h e e.5.r. of hyperfine s p ! i t t i n q
s i g n a l o f c r ( v ) i o n s and t h e a p p e a r a n c e
a s shown i n F i q . 5 .
(h.f.s.)
A t atoms a r e h i p h l y d i l u t e d j n t h e z e o l i : e
A 1 atonis is l a r p e .
In h i g h - s i l i c a
zeolites the
framework, and t h e d i s t a n c e b e t w e e n
+
+
T h e r e f o r e , i t is s u a g e s t e d t h a t i t is CrOn, M o C l , a n d n o t
j s o l a t e c ! i o r . 5 ( C I - ~ + , Mo5:,')
t h a t are coordirated t o t h e z e o l i t e c a t i o n i c posi-
t is t h e c o o r d i n a t e d C r ( v ) , V o ( v ) large molecc;lcs t h a t s h i e l d s
tions.
because
acid
s i t e s r:eecled f o r o l i g o m e r i z j t i o n and c : ~ c l l z a t i o n , a n d l e a d s t o a low s e l e c t i v i t y f o r DrGpane a r o m a t i z a t i o n and a 9 i g h a c t i v i t y f o r p r o p a n e c r a c k i n g o n Mo/ZSt?-lj
and p r o p a n e d e h ~ ~ d r o g e n a t i oonn Cr/XM-5, as shown i n TPblc- 6. Evidently, t h e Dossibilty o f c a t i o n introduction i n t o high-silica
zeolites
by a s o l i d s t a t e r e a c t i o n is d e t e r m i n e d by p h v s i c a l p r o p e r t i e s o f metal o x i d e s , s u c h as m e l t i n g p o i n t , s u b l i m a t i o n p o i n t , e t c . and t h e p r u s e n c e of a c i d c i t e s z e o l i t e s kchirh may b e c o n r i d e r e d 9 s p o w e r f u l t r a p s f o r t h e m i g r a t i n g S i n c e PioCI,, : 2 t B * ' C ) and C r O n ( 1 6 7 ° C ) a r e low-me1 tii-q-point-compounds, z r c h a n p takes p l a c e s m m t t i l y 1-1;5;1-5
+ mctal-ccmpounds
iii
ion-
the c o n d i t i o n s of c a l c i i m t i o n of t h e ini*tur€
s t a b o v e 500 C.
The m e l t i n g poin!
P ~ o t h e rf a c t o r i s r e p o r t e d
(1978"'C).
On
CatiJii5.
o f 3 0 is h i g h
t o e x p l a i n t h e ion-exchange
o c c u r e d i n the m i x t u r e HZSM-5 + ZnO t h r o u g h s o l i d s t a t e r e a c t i o n a b o v e 500
c.
REFERENCES
1. S.M. C s i c s w y , J. C a t a l . 17(1970)207 2 , M . S . SCLJlfell, Appl. C a t ; l l .
32(1987)1
3. J.A. J o h n s o n a n d G . K . Miller, P a p e r OM-84-45, NFRA Rrin. M e e t i n g , March 1904,
San O n t o n i o . 4. C.D.
W a ~ n e r , L.E.
Davis, V.V.
Z e l l e r , J.A.
T a y l o r , R.H.
Raymond a n d L.H.
Gale,
Surface and Interface Analysis
3(1981)211
5. B.D. Mcnicol, J. Catal. 46(1977)438 6. N.Y.
Topsoe, Karsten Pedersen, and E.G. Derauane, J. Catal. 70(1981)41
7. J.R. Anderson, K. Foger, T. Mole, R.A. Rajadhyaksha, and J.V. Sander, J . Catal. 50(1979)114
8. K.H. Rhee, V.U.S.
Rao, J.M. Stencel, G . A .
Melson and J.E. Crawford
ZEOLITES 3 (19833337 9. E.M.
L o k , B.K. Marcus, and C.L.
Angell,
ZEOLITES
6(1986)185
10. Y.Yang, Fu Zaihui and Guo Xiexian, submitted to J. Catal.(chinese)
1 1 . A.V.
Kucherov and A.A Slinkin,
ZEOLITES
7(1987)38
12. Y.Yang, Fc; Zaihui, Wang Limin, Deng Maicun and Guo %iexian, submitted to
J. Catal. (Chinese)
859
AUTHOR INDEX A Auroux, A.
315 377
Aust, E.
49 5
Andreev, A.
B Ball, W. J. Barlow, G. E. Barthomeuf, D. Beck, I. Beer, M. Bekkum, H. van Ben Taarit, Y. Beran, 5. Beyer, H. K. Bezouhanova, C. Boddenberg, B. Borbely, G. Bragin, 0. V. Bulow, M. Burrneister, R.
271 827 429 469 469 519 377 347 635 91 533 635 143 505 533
C Caro, J. Carson, R. Cartlidge, 5. Catlow, C.R.A. Chang, N. 5. Chao, K. 1. Chen, C.C. Chen, L. Y. Chen, P. Y. Chu, 5. J. Chu, Y. F. Chuang, T. K. Colbourn, E. A. Cooke, E. M. Cotterman, R. L.
505 39 389 409 223,231 19
223 23 1 223,231 223,231 749 223,231 409 39 389
Coudurier, G. Coughlin, P. K
61 5 1
D Decyk, P. Deng, M. Derewinski, M. Dimitrov, C. Doyemet, J. Y. Ducarme, V. Dwyer, J.
305
849 305 91 153 615 39,271
t
Echoufi, N. Ernig, G. Erneis, C. A Endoh, A.
377 495 365 779
F Fajula, F. Fiedler, K. Figueras, F. Forster, H. Frede, W. Freude, D. Fu, 2 . Fyfe, C. A.
61 439 61 355,545,575 545 42 1 849 827
G Garforth, A. A.
271
Geerts, H. Gies, H. Giordano, G. Girrbach, U. Gnep, N. 5. Grillet, Y.
72 1 827 281 399 153 625
Grobet, P. J. Grodzicki, M.
72 1 575
860
Groenenboom, C.J. Guisnet, M. Guo, X. Gutsze, A.
99 153 849 567
Haas, J .
295,337
Haber, J. Harris, I.M. Hashimoto, K. Hilgert, W. Hinchliffe, A. Hoelderich, W.F. Howes, M. L. Huizinga, T. Hulsters, P.De Hunger, M. Huybrechts, D. R.
305 271 48 5 49 5 39 193 389 365 759 42 1 163
315 315 80 1
J Jacobs, P. A. Jacquinot, E. Jentys, A. Jiru, P. Johnson,J. A.
163,211 49,163,211,721,735 115 585 281 45 1
K Kallo, D. Kanazirev, V. Kawase, M. Kennedy, G. J. Keweshan, C. F. Kim, J.-H. Kiricsi, I. Kokotailo, G. T. Kolboe, St.
Laktit, A.
Lambret, M. Laniecki, M. Lechert, H. Leonhardt, W. Lercher, J. A. Leslie, M. Leu, L. J. Li, Huong-Xin Lillerud, K. P. Lin, D.H. Liphard, M.
469 61 2 59 29,91 69 1 585 409 19 735 769 615 673
M
I
Jacobs, J. M.
567 70 1 347
L
H
Ignatzek, E. Iliev, V. Inoue, K.
Kornatowski, J. Krings, P. Kubelkova, L.
241,711 29 485 827 749 71 3 55 82 1,827,843 327
Macedo, A. Mallmann, A. de Marcilly, Ch. Martens, J.A . Masuda, T. Mavrodinova, V. McAteer, C. H. Mentzen, B. F. Mersmann, A. Mbsztiros-Kis, A. Miasnikov, P. Minachev, Kh. M. Minchev, Ch. Monaci, R. Morbidelli, M. Mroczek, U. Mucsi, Gy. Muhl, J. Muller, U. Myrdal, R.
115 429 115 49,72 1,735 485 29 27 1 477 62 5 251,711 63 5 143 29 595 595 81 71 1 469 399,625 327
861
N Namba, 5 . Nanne, J. M. Nastro, A. Nenova, V.
365 28 1 91
Nishimiya, K. Novakova. J.
779 347
71
0 Occelli, M. L. Olah, 1.
127 71 1
O'Malley, P. J. Onyestyak, Gy. Ooteghem, H. v. Oroskar, A.R.
39 241 21 1 45 1
P Pan, Dongfeng Papp Jr., J. Parton, R.F. Pasztor, C. T. Pellet, R. J. Penchev, V. Peters, G. Pfeifer, H. Philippaerts, J. Plog, c. Polanek, P. Post, M. F. M.
62 5 241,711 163,211 827 1 29 545 42 1 555,759 295,337 399 365
Rabo, J. A. Rees, L.V.C. ReiB, G. Riekert, L. Rohl-Kuhn, B. Roeher, F. Roland, E.
115 1 66 1 607 82 1,843 505 81,421 645
595 625 62 5 635 567
5 Sax, 8.-M. Schirmer, W. Schmidt, F. Schollner, R.
439 735 81 1
Schubert, 5. S~h~lz-Ekl~ G.f f , Schwuger, M. J.
735 315 673
iebik, R. Shamshoum, E. 5. Shi, Z.C. Shpiro, E.S. Siegel, H. SmieSkov6, A. Solinas, V. Spaeth, G. Stach, H. Steinberg, K.-H. Steinwandel, J. Stencel, J. M. Stocker, M. Stork W. H. 1. Strobl, H. Suckow, M.
R Raatz, F.
Rombi, E. Rouquerol, F. Rouquerol, J. R6zsa, P. Rozwadowski, M.
69 1
347 1 377 143 81 1 347 595 533 439 81,421 295,337 127 769 365 827 439
T Takaishi, T.
Tkachenko, 0. P.
779 355 49,72 1 399,82 1,843 143
Trautwein, A. X. Trifir6, F.
735,789 28 1
Tasi, Gy. Tielen, M. TiBler, A.
862
Tsutsumi, K Tuleuova, G. J. Tvarurkova, 2. TupB, M.
779
W
143 281 281
Wang, L. Wichterlovd, B. Wisniewski, K.E. Wohrle, D.
U Unger, K.K. Upadek, H.
399,625 70 1
v Valyon, J. Vansant, E. F. Vasina, T. V. Vedrine, J.C. Verbiest, J. Vetrivel, R. Voogd, P.
251 555,749,759 143 61 5 759 409 519
849 347 567 315
Y Yan, Y. Yang, Y. Yashima, T.
555,759 849 71
Yoshida, A.
80 1
2 Zaikovskii, V.I. Zakharieva-Pencheva,0. Zibrowius, B. Ziethen, H. M. Zidlek, M.
143 575 505 735,789 305
863
SUBJECT I N D E X * A Ab initio calculations 409 Acetophenones, shape selective formation of 163 Acetylacetone 72 1 Acetylene 24 1 Acidity 19,39,305,377 Active centers 29 389,409,485,585 Acid sites Acylation Adsorption
-
of aromatics
163 439,451,469,585, 595,811,843 843
-,calculation of potentials of
567
-, concentration dependence of coefficients
-,dynamics of -,gas chromatigrophic investigation of
-, heats of -,isotherms technica processes Of of
495 115 545 779 567
439 607 -,selective 645 -, theory 439 -,vacuum swing 607 Alkaline X and Y zeolites 595 Alkyl amines 193 Al kylation 71,211 -, of aromatics 163 -, of aromatic amines 193 -,of heteroaromatics 193 -, of phenolic compounds 163 2 7 ~ MAS 1 NMR 271,721 Aluminas 99 Aluminophosphate 39 -I
-, pressure swing
*
-,AlP0,-5 ,-based catalysts Anisole Aromatic, skeletal rearrangementsof Aromatic hydrocarbons Aromatisation -, of alkanes -, of propane
625 1 21 1 1 495,505,843 91,271 143 849
B Basicity Beckmann rearrangement of anilines Benzene -,adsorption of Beta Bifunctional catalysis Bimetallic pentasil catalyst Boron-nitrogen compounds Brensted acidity Builder systems Butane, conversion of Butene, conversion of
429 193 533 585 115
49,153 143 555 409 673 271 27 1
C ,C ,
olefins
C, aromatic isomerization
Calorimetry Carbocations 13C NMR Chemical vapor deposition Chiral hydrogenation catalysts Chromatographic experiments Clinoptilolite Clay
Note: page numbers refer to the first page of the respective contribution
281 347 377 355 505 749 163 495 71 1 163
864
co
429
Co(ll)-phthalocyanine, NaX encaged Co-builder Computer simulation Concentration dependence Cresol formation C rystaI Iite size Crystallinity of zeolite catalysts
F
315 70 1 769 495 223 615 127
Cyclocondensations
193
FAPO-5 Faujasite Faujasite-typezeolites -,as adsorbents Ferrierite Fire retardant materials Fluidized cracking catalysts (FCC) Fries rearrangement FTIR-PAS
D Dealumination
61,377, 389, 645,72 1,769
-,effect on adsorption on ZSM-5 -, Of ZSM-5 Degradation model Dehydrosulphurization Desorption Diels Alder synthesis Diffusion Diffusion coefficients Diffusivities Dimerization Disilanation Disproportionation DTA
567 567 81 1 305 477 163 505,519,615 495 533 91 759 61 80 1
E Enzyme mimic 163 EPR spectroscopy 315,735,789 Erionite 39,81,421,769 -, as adsorbent 451 Ethanethiol 305 -,oxidation of 315 Ethylation 61 Ethylbenzene dehydrogenation 19 Ethylbenzene oxidation Exhaust emission
315 295,337
735 115,429,533 789 451 71 635 99,127 163 759
G GalH-ZSM-5 catalyst 153,271 Galliation 779 Gas chromatographic investigation of adsorption states 545 Gas oil 469 Gas sorption 555,759 Gibbsite 63 5
H Heat conductivity 635 Heating plants 295 Heats of adsorption 779 Hexadecane cracking 389 n-Hexane, conversion of 365 High resolution adsorption 625 H-mordenite 759 505,533 2HNMR Hydration on transition metal zeolites 241 - of acetylene 24 1 Hydrocarbons, transformation into carbocations 355 Hydroconversion 81 Hydrogen 545 Hydrogenation activity of nickel 347 Hydrogen transfer 115 Hydrothermal stability 127,801 Hydrothermal treatment 81 1
865
Hydroxyarenes, oxidation of 163 Hydroxylation 163 Hydroxyl groups 42 1 H-zeolites 749 H-ZSM-5Zeolite 91,485,519 acid sites of 585
-.
I Infrared spectroscopy
39,355,365, 429,545,585 -, of hydration on transition 241 metal zeolites -,of Mo-fixed HY 251 Intermediates of aromatization 91 Intracrystal line diffusion 485 Ion exchange 66 1 Iridium /zeolite X catalysts 327 Iron carbonyls 789 IR spectroscopy 39,355 lsomerisations 193 lsomorphoussubstitution 365
L Laser Raman spectroscopy Lattice structure Laundry detergent Liquid detergent Low temperature microcalorimetry LZ 132 zeolite
127 42 1 673 69 1 625 281
M Mass spectroscopy Mechanism, of alkylation of phenolic compounds -,of aromatization of propane and propene -, propylene, ethylene and methanol reaction over LZ 132 Metal doped zeolites Metal traps Metha no1transformation
789 163 153 281 295 99 281
Metathesis 2 59 Methylation of toluene 485 Methylnaphthalene isomers 595 3-Methyl pentane 519,615 MFI structure 365,505 M FlIsorbate systems 477 MlBK synthesis 23 1 Microcalorimetry, acidity measurements by 377 (n-C H I, , reaction Mo with 251 Model of degradation 81 1 Model reactions 153 Modeling of technical separation processes 439 223,231 Modified ZSM-5 Mossbauer spectroscopy 735,789 Mo IHY 251 Mo 1 H-ZSM-5 catalyst 27 1 Molecular dynamics 533 Molecular orbitals 39 Molecular sieves 439,607 Molybdenum carbonyl 2 59 Mordenite 16,81,533 -,as adsorbent 45 1 Mullite and orthovanadates formation 127 m-xylene conversion 29
k-t
N n-Alkanes 81 n-Hexane 519 Nickel incorporation 347 NiHZSM-5 catalyst 347 Nitrogen-containing organic intermediates 99 Nitrogen oxide removal 295 Nitrous oxide 575 NMR spectrometry 271,421,469,827 -, of '3c 505 -,of2H 505,533 Nobel metal doped zeolites 337
866
NO, selective reduction of NO, pollution control
327 327
0 1,7-0ctadiene, conversion of 271 '*O-exchange 779 Offretite 61,769 Olefins 533 -,skeletal rearrangementsof 1 Oligomerisations 193 Omega 115 O/N replacement reactions 193 Optical brighteners 673 Oxidaton reactions 193 -,of hydroxyarenes 163 Oxygenates 163 Oxygen-containing substrates 163 Oxygen, separation from air 607
66 1 80 1 607 377 91 153 91
Q Quadricyclane norbornadiene, valence isomerization of 315 Quantum chemical calculations 575 Quantum mechanical calculations of acidic properties 409
R Rietveld refinements Resins, comparison with zeolites
477 45 1
s
P Palladium / ZSM-5 Paraffins, skeletal rearrangements of n-Paraffins, extraction of Paraffin isomerisation Parex, separation process Particle size Phase transition Phenol -,methylation of Phenolic compounds, alkylation of Photoacousticspectroscopy Phosphate Platinum /zeolite systems, acidity and basicity of -,state of Pt in PoIyca rboxylates Pore-opening reduction Potential energ distribution [AAP) PdRSM - 5
Precursor gels Pre-injection of water Pressure swing adsorption Probe molecules in microcalorimetry 2-propanol Propane, aromatization of Propene
23 1 1 439 49 439 69 1 82 1 21 1 223 163 555 701,711 429 143 673,701 749 625 231
SAPO-5
-, catalytic activity of -, comparison with
19 29
dealuminated H-Y 29 Secondary building units 39 Selective extraction 439 Self-diffusion 505 Separation of methylnaphthalenes 595 439,451 Separation technology Sewage treatment 71 1 Shape selective catalysts 163,749 Shape selectivity 71,163,399,485 Shaping of zeolite materials 645 MAS NMR 271,721 Si incorporation 19 SIMS 337 Solid-state NMR 769 Solid-state reactions in zeolites 347,849 Sorbex, separation technology 451 Sorption 519,827
867
Sorption complex Spillover
575
81 Statistical thermodynamics 545 389 Steaming 69 1 Storage test 555,759,843 Structure modification 39 Structure stability 337 Surface analysis
T Technical white oil TPD, of NO from I r o n X-zeolite Template-free zeolite synthesis Thermal analysis The r ma I stab iIit y Terminal silanol groups Thermogravimetry Theoretical studies of zeolite acidity Three-way-catalyst Time-resolved XRD Transition metal forms Trimerization
469 327 399 735 127 749 615 409 337 477 575 91
2 Zeolite A 545,661,673,691,701 -, as adsorbent 45 1
-, heat conductivity of
71 1 45 1
Zeolite mordenite -, as adsorbent Zeolite NaA Zeolite NaY Zeolite Omega Zeolite Ultrastable Y
555 45 1 645 645 115 21 1
Zeolite X 305,661 -, heat conductivity of 63 5 Zeolite Y 49,259,305,595,721 -,hydrogen form, reaction with Mo (n-C,H,), 49 Zeolite ZSM-3 49 Zeolite ZSM-5 71,211,223,305, 399, 61 5,625,645,779,821,843,849
-, as adsorbent
-,characterization by IR
U
uSY -, catalytic activity of UV-VIS spectroscopy
80 1 21 1 355
Vacuum swing adsorption Vanadium contaminants
607 127
VGO cracking Viscosity
115 69 1
W 711
X XRD X ray photoelectron spectroscopy
45 1 585
-,control of catalytic activity of
-, large crystals of - modified by alkaline
399 567 231
-,tem late-free
W
Waste water
63 5 49 389
Zeolite BETA Zeolite CSZ-1 Zeolite/Fe (111) composite material Zeolite L, as adsorbent451
80 1 127,143
E
synt esis of Zeolite ZSM-1 1 Zeolite ZSM-20 Zeolite 13X Zeolites, commercial importance of Zeolites, production technology of
399 71,211,779 49 469 645 645
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STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Louvain, Louvain-la-Neuve,Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.
Volume 1 Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417,1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet Volume 2 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 Volume 3 Preparation of Catalysts 11. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t6 de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13- 15, 1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyiie, September 29October 3, 1980 edited by M. LazniEka Volume 10 Adsorption a t the Gas-Solid and Liquid-Solid Interface. Proceedings of an InternationalSymposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouquerol and K.S.W. Sing Volume 1 1 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-1 6, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jird and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Volume 14 Vibrations a t Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
870 Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 16 Preparation of Catalysts Ill. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12-1 6, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirb, V.B. Kazansky and G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Volume 20 Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 24 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Driaj, S.HoEevar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Volume 27 Catalytic Hydrogenation edited by L. Cerveng Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 3 0 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Volume 3 1 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Volume 32 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 34 Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment
87 1 Volume 35 Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chaney, R.F. Howe and S.Yurchak Volume 37 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-1 7, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 38 Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 39 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Volume 40 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 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. PBrot Volume 42 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pa61 Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Volume 44 Successful Design of Catalysts - Future Requirements and Development. Proceedingsof the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Volume 46 Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an InternationalSymposium, Wurzburg, September 4-8, 1988 edited by H.G. Karge and J. Weitkamp
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