Studies in Surface Science and Catalysis 60 CHEMISTRY
OF MICROPOROUS CRYSTALS
Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1 9 9 0
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis Advisory Editors : B. Delmon and J. T. Yates Vol. 60
CHEMISTRY OF MICROPOROUS CRYSTALS PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON CHEMISTRY OF MICROPOROUS CRYSTALS, TOKYO, JUNE 26-29, 1990 Edited by Tomoyuki lnui
Kyoto University
Seitaro Namba
Tokyo Institute of Technology
Takashi Tatsumi
University of Tokyo
ELSEVIER
KODANSHA 1991
Tokyo
Amsterdam-Oxford-New York-Tokyo
Copublished by KODANSHA LTD., Tokyo and ELSEVIER SCIENCE PUBLISHERS B. V., Amsterdam exclusive sales rzghts in Japan KODANSHA LTD. 12-21, Otowa 2-chome, Bunkyo-ku, Tokyo 112, Japan
U.S. A. and Canada ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas, New York, N. Y. 10010, U. S. A.
for the
for the rest of the world
ELSEVIER SCIENCE PUBLISHERS B. V. 25 Sara Burgerhartstraat, P. 0. Box 211, 1000 AE Amsterdam, The Netherlands
ISBN 0-444-98702-9(V0l. 60) ISBN 0-444-41 8 0 1 -6(Series) ISBN 4-06-2051 61 -3(Japan)
Copyright 0 1991 by Kodansha Ltd.
All rights reserved No part of this book may be reproduced in any form, by photostat, microfilm, retrieval system, or any other means, without the written permission of Kodansha Ltd.(except in the case of brief quotation for criticism or review)
PRINTED I N JAPAN
Organization
Organizing Committee CHAIRMAN: Professor H. Tominaga (University of Tokyo) GENERAL SECRETARY: Professor T. Inui (Kyoto University) COMMITTEE: Professor A. Iijima (University of Tokyo) Professor E. Kikuchi (Waseda University) - Symposium Site Arrangements Dr. S. Namba (Tokyo Institute of Technology) - Program and Publications Dr. Y. Nishimura (Catalysts and Chemicals Industries Co.) - Finance Professor Y. Ono (Tokyo Institute of Technology) - Program and Publications Professor K. Segawa (Sophia University) - Symposium Site Arrangements Professor T. Tatsumi (University of Tokyo) - Secretary and Program Professor K. Tsutsumi (Toyohashi University of Technology) Dr. K. Usui (Mizusawa Industrial Chemicals, Ltd.) - Finance Professor T. Yashima (Tokyo Institute of Technology) - Treasurer
Local Arrangements: COMMITTEE: Dr. T. Matsuda (Waseda University) Dr. S. Nakata (Chiyoda Corp.) Dr. S. Ogihara (University of Tokyo) Professor E. Suzuki (Tokyo Institute of Technology) Dr. Y. Watanabe (Geological Survey of Japan)
This Page Intentionally Left Blank
List of Contributors
Numbers in parentheses refer to the pages on which a contributor's paper begins.
Abe, K. (21) Mizusawa Industrial Chemicals, Ltd., 1-21, 4-Chome, Nihonbashi-muromachi, Chuo-ku, Tokyo 103, Japan Alberti, A. (107) Instituto di Mineralogia, Universit2 di Ferrara, Italy Bulow, M. (199) Central Institute of Physical Chemistry, Academy of Sciences of the G. D. R., Rudower Chaussee 5, Berlin-Adlershof, 1156, Germany Chao, K. J. (123) Department of Chemistry, National Tsinghua University, Hsinchu 30043, Taiwan, R. 0. C. Chen, J. (63) Department of Chemistry, Jilin University, Changchun, China Chen, S. H. (123) Department of Chemistry, National Tsinghua University, Hsinchu 30043, Taiwan, R. 0. C. Chen, S.-y. (165) Institute of Coal Chemistry, Academia Sinica, Taiyuan, Shanxi, 030001, P. R. C. Davis, M. E. (53) Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U. S. A. Derouane, E. G. (11, 29) Facult& Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles, 5000-Namur, Belgium Dessau, R. M. (255) Mobil Research & Development Corp., Princeton, New Jersey, U. S. A. Dewaele, N. (29) Facult6s Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles, 5000-Namur, Belgium Dumont, N. (11) Facultes Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles, 5000- N amur, Belgium
viii List of Contributors
Endo, T. (189) Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aoba, Sendai, Miyagi 980, Japan Farfan-Torres, E.M. (97) Unit&de Catalyse et Chimie de MatQiaux DivisCs, UniversitE Catholique de Louvain, Place Croix du Sud 2, boite 17, 1348 Louvain-la-Neuve, Belgium Feng, S. (63) Department of Chemistry, Jilin University, Changchun, China Fukuoka, A. (335) Catalysis Research Center, Hokkaido University, Sapporo 060, Japan Fukushima, T. (37) Chemical Research Laboratory, Tosoh Corporation, 4560 Tonda, Shinnanyo, Yamaguchi 746, Japan Gabelica, 2. (11,291 Facult& Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles, 5000-Namur, Belgium Giordano, G. (29) Dipartimento di Chimica, Universit2 della Calabria, 1-87030 RENDE (CS), Italy Grange, P. (97) Unit&de Catalyse et Chimie de Materiaux DivisEs, Universit6 Catholique de Louvain, Place Croix du Sud 2, boite 17, 1348 Louvain-la-Neuve, Belgium Haag, W. 0. (255) Mobil Research & Development Corp., Princeton, New Jersey, U. S.A. Hanaoka, T. (303) National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan Hatakeda, K. (81) Government Industrial Research Institute, Tohoku, Nigatake 4-2-1, Miyagino-ku, Sendai 983, Japan Hattori, M. (89) Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, HigashiHiroshima 724, Japan Hibino, T. (151) Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Hidaka, S. (159) Central Research Laboratories of Idemitsu Kosan Co. Ltd., 1280 Kamiizumi, Sodegaura, Kimitsu, Chiba 299-02, Japan Hong, S. B. (179) Division of Chemistry, Korea Institute of Science and Technology, P. 0. Box 131, Cheongryang, Seoul, Korea
List of Contributors ix
Huybrechts, D. R. C. (225) K. U. Leuven, Dept. Biotechnische Wetenschappen, Laboratorium voor Oppervlaktechemie, Kardinaal Mercierlaan 92, B-3030 Heverlee (Leuven), Belgium Hwang, B. W. (179) Division of Chemistry, Korea Institute of Science and Technology, P. 0. Box 131, Cheongryang, Seoul, Korea Ichikawa, M. (335) Catalysis Research Center, Hokkaido University, Sapporo 060, Japan Igawa, K. (37) Chemical Research Laboratory, Tosoh Corporation, 4560 Tonda, Shinnanyo, Yamaguchi 746, Japan Iino, A. (159) Central Research Laboratories of Idemitsu Kosan Co. Ltd., 1280 Kamiizumi, Sodegaura, Kimitsu, Chiba 299-02, Japan Imafuku, S. (21) Mizusawa Industrial Chemicals, Ltd., 1-21, 4-Chome, Nihonbashi-muromachi, Chuo-ku, Tokyo 103, Japan Inaoka, W. (37) Chemical Research Laboratory, Tosoh Corporation, 4560 Tonda, Shinnanyo, Yamaguchi 746, Japan Ione, K. G. (311, 319) Institute of Catalysis, Novosibirsk 630090, USSR Ito, T. (11) Tamai Sangyo Co., Ltd., Zenibako 3-chome, 524-11, Otaru 047-02, Japan Iwamoto, M. (327) Catalysis Research Center, Hokkaido University, Sapporo 060, Japan Iwamoto, R. (159) Central Research Laboratories of Idemitsu Kosan Co. Ltd., 1280 Kamiizumi, Sodegaura, Kimitsu, Chiba 299-02, Japan Iwamoto, T. (3) Department of Chemistry, College of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan Iwasaki, T. (81) Government Industrial Research Institute, Tohoku, Nigatake 4-2-1, Miyagino-ku, Sendai 983, Japan Izumi, Y. (371) Department of Synthetic Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan Jacobs, P. A. (225) K. U. Leuven, Departement Biotechnische Wetenschappen, Laboratorium voor Oppervlaktechemie, Kardinaal Mercierlaan 92, B-3030 Heverlee (Leuven), Belgium
x List of Contributors
Karge, H. G. (213) Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, Faradayweg 4-6,1000 Berlin 33, Germany Kasahara, S. (37) Chemical Research Laboratory, Tosoh Corporation, 4560 Tonda, Shinnanyo, Yamaguchi 746, Japan Kato, M. (141) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Kawashima, Y. (151) Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Kikhtyanin, 0. V. (319) Institute of Catalysis, Novosibirsk 630090, USSR Kikuchi, E. (377) Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169, Japan Kim, S. J. (179) Division of Chemistry, Korea Institute of Science and Technology, P.O.Box 131, Cheongryang, Seoul, Korea Kimura, T. (335) Catalysis Research Center, Hokkaido University, Sapporo 060, Japan Kumar, R. (43) National Chemical Laboratory, Pune 411 008, India Kurusu, Y. (73) Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102, Japan Kusterer, H. (281) Engler-Bunte-Institute, University of Karlsruhe, Kaiserstra Be 12, 7500 Karlsruhe, Germany Lago, R.M. (255) Mobil Research & Development Corp., Princeton, New Jersey, U. S. A. Liu, S.B. (123) Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 10764, Taiwan, R. 0. C. Lu, G.-m. (165) Institute of Coal Chemistry, Academia Sinica, Taiyuan, Shanxi, 030001, P. R. C. Malysheva, L. V. (319) Institute of Catalysis, Novosibirsk 630090, USSR
List of Contributors xi
Matsuda, T. (377) Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169, Japan Matsuzaki, T. (303) National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan Meriaudeau, P. (267) Institut de Recherches sur la Catalyse, CNRS, 2, avenue A. Einstein, 69626 Villeurbanne Cedex, France Murakami, Y. (151) Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Naccache, C. (267) Institut de Recherches sur la Catalyse, CNRS, 2, avenue A. Einstein, 69626 Villeurbanne Cedex, France Nagy, J. B. (11, 29) Facult& Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles, 5000-Namur, Belgium Nakamura, I. (159) Central Research Laboratories of Idemitsu Kosan Co. Ltd., 1280 Kamiizumi, Sodegaura, Kimitsu, Chiba 299-02, Japan Namba. S. (171) Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Neuber, M. (291) Engler-Bunte-Institute, University of Karlsruhe, Kaiserstra Be 12, 7500 Karlsruhe, Germany Newsam, J. M. (133) Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801, U S A . NieBen, W. (213) Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, Faradayweg 4-6,1000 Berlin 33, Germany Nishimiya, K. (141) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Niwa, M. (151) Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Notari, B. (343) ENI-Ricerca a Sviluppo, 20097-San Donato Milanese, Milano, Italy Occelli, M. L. (353) Unocal, Brea, CA 92621, U. S. A.
xii List of Contributors
Ogawa, M. (21) Mizusawa Industrial Chemicals, Ltd., 1-21, 4-Chome, Nihonbashi-muromachi, Chuo-ku, Tokyo 103, Japan Okamoto, M. (363) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguroku, Tokyo 152, Japan Onaka, M. (371) Department of Synthetic Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Ono, Y. (363) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguroku, Tokyo 152, Japan Onodera, Y. (81) Government Industrial Research Institute, Tohoku, Nigatake 4-2-1, Miyagino-ku, Sendai 983, Japan Parton, R.F. (225) K. U. Leuven, Departement Biotechnische Wetenschappen, Laboratorium voor Oppervlaktechemie, Kardinaal Mercierlaan 92, B-3030 Heverlee (Leuven), Belgium Paukshtis, E. A. (311) Institute of Catalysis, Novosibirsk 630090, USSR Peng, S.-y. (165) Institute of Coal Chemistry, Academia Sinica, Taiyuan, Shanxi, 030001, P. R. C. Ratnasamy, P. (43) National Chemical Laboratory, Pune 411 008, India Romannikov, V. N. (311) Institute of Catalysis, Novosibirsk 630090, USSR Sapaly, G. (267) Institut de Recherches sur la Catalyse, CNRS, 2, avenue A. Einstein, 69626 Villeurbanne Cedex, France Schulz, H. (281) Engler-Bunte-Institute, University of Karlsruhe, Kaiserstra fie 12, 7500 Karlsruhe, Germany Segawa, K. (73) Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102, Japan Shimada, M. (189) Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aoba, Sendai, Miyagi 980, Japan Shiraishi, A. (141) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan
List of Contributors xiii
Stencel, J. M. (353) Kentucky Center for Energy Research, Lexington, KY 40512, U. S. A. Sugi, Y. (303) National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan Sugiyama, A. (73) Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102, Japan Suib, S.L. (353) University of Connecticut, Storrs, CT 06268, U. S. A. Suzuki. E. (363) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguroku, Tokyo 152, Japan Takaishi, T. (141) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Takeuchi, G. (303) Research and Development Laboratories, Nippon Steel Chemical Co., Ltd., Kitakyushu, Fukuoka 804, Japan Takeuchi, K. (303) National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan Tokita, T. (21) Mizusawa Industrial Chemicals, Ltd., 1-21, 4-Chome, Nihonbashi-muromachi, Chuo-ku, Tokyo 103, Japan Tokoro, T. (303) National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan Toktarev, A. V. (319) Institute of Catalysis, Novosibirsk 630090, USSR Torii, K. (81) Government Industrial Research Institute, Tohoku, Nigatake 4-2-1, Miyagino-ku, Sendai 983, Japan Tsutsumi, K. (141) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Uh, Y. S. (179) Division of Chemistry, Korea Institute of Science and Technology, P. 0. Box 131, Cheongryang, Seoul, Korea Usui, K. (21) Mizusawa Industrial Chemicals, Ltd., 1-21, 4-Chome, Nihonbashi-muromachi, Chuo-ku, Tokyo 103, Japan Weitkamp, J. (291) Institute of Chemical Technology I, University of Stuttgart, Pfaffenwaldring 55, D-7000 Stuttgart 80, Germany
xiv List of Contributors
Xu, R. (63) Department of Chemistry, Jilin University, Changchun, China Yamagishi, K. (171) Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Yamanaka, S. (89) Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, HigashiHiroshima 724, Japan Yashima, T. (171) Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Yeom, Y. (179) Division of Chemistry, Korea Institute of Science and Technology, P. 0. Box 131, Cheong. ryang, Seoul, Korea Young, D. (53) Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U. S. A. Zhao, S.(281) Engler-Bunte-Institute, University of Karlsruhe, KaiserstraBe 12, 7500 Karlsruhe, Germany
Contents List of Contributors ......................................................................................................... Preface ........................................................................................................................
vii xix
I . Synthesis Clay-like and Zeolite-like Structures Built of Polymeric Cyanocadmate (T.Iwamoto) ...........................................................................
3
129Xe-NMRStudy of the Crystallization of SAPO-37 (T. Ito, N. Dumont, J. B. Nagy, 2. Gabelica and E. G. Derouane) ..............................
11
Application of RDF to Synthesis of Zeolite (K. Usui, K. Abe, T. Tokita, S.Imafuku and M. Ogawa) ...............................................................
21
Synthesis of Zeolite ZSM-48 with Different Organic and Inorganic Cations (G. Giordano, 2. Gabelica, N. Dewaele, J. B. Nagy and E. G. Derouane) ..............................................................................
29
Synthesis and Characterization of Zeolites (W. Inaoka, S. Kasahara, T. Fukushima and K. Igawa) ...............................................................
37
Synthesis and Characterisation of Ferrisilicate Zeolites (R. Kumar and p. Ratnasamy) ..............................................................................
43
Further Studies on the Synthesis of VPI-5 (M. E. Davis and D. Young) ...............................................................................................................
53
New Families of M(III)X(V)O,-Type Microporous Crystals and Inclusion Compounds (R. Xu, J. Chen and S. Feng) ....................................
63
Molecular Design of Two-Dimensional Zirconium Phosphonate Catalysts (K. Segawa, A. Sugiyama and Y. Kurusu) ......................... Mesoporous Materials Produced from Hydrothermally Synthesized Hectorites (K. Torii, T. Iwasaki, Y. Onodera and K. Hatakeda) .........................................................................................................
81
Clays Pillared with Ceramic Oxides (S. Yamanaka and M. Hattori) ...............................................................................................................
89
Zirconium Pillared Montmorillonite: Influence of Reduced Charge of the Clay (E. M. Farfan-Torres and p. Grange) .......................................
97
xvi Contents
11. Structure
Crystal Chemistry of Si-A1 Distribution in Natural Zeolites (A. Alberti) ...............................................................................................................
107
NMR Studies of Cation Location in Zeolites (K. J. Chao, S. H. Chen and S. B. Liu) ..........................................................................................
123
Developments in X-ray and Neutron Diffraction Methods for Zeolites (J. M. Newsam) .................................................................................
133
Effects of Structural Disorder on the Generation of Acidic Sites in Zeolite L (K. Tsutsumi, A. Shiraishi, K. Nishimiya, M. Kato and T. Takaishi) .............................................................................................
141
111. Modification
Growth of Silica and its Controlling of Pore-opening Size on CVD Zeolites (T. Hibino, M. Niwa, Y. Kawashima and Y. Murakami) .........................................................................................................
151
New Method of Modifying Y - type Zeolite-Fe Supported Zeolite (R. Iwamoto, S. Hidaka, I. Nakamura and A. Iino) .....................
159
Modification of HZSM-5 by Diazomethane (G.-m. Lu, S.-y. Chen and S.-y. Peng) .......................................................................................
165
Preparation of Metallosilicates with MFI Structure by AtomPlanting Method (T. Yashima, K. Yamagishi and S. Namba) ...............171 Chemical Interactions of Aluminophosphate Molecular Sieve with Vanadium Oxide (S. B. Hong, B. W. Hwang, Y. Yeom, S. J. Kim and y. S. Uh) ..........................................................................................
179
Optical Properties of Dyes Incorporated into Clay (T. Endo and M. Shimada) ......................................................................................................
189
IV. Diffusion Molecular Mobility of Single Components and Mixtures on Zeolites (M. Bulow) ..........................................................................................
199
Investigation of Diffusion and Counter-diffusion of Benzene and Ethylbenzene in ZSM-5-type Zeolites by a Novel IR Technique (W. NieBen and H. G. Karge) ........................................................................
213
Contents xvii
Catalysis
Zeolites as Partial Oxygenation Catalysts (D. R. C. Huybrechts, R. F. Parton and p. A. Jacobs) .....................................................................
225
Kinetics and Mechanism of Paraffin Cracking with Zeolite Catalysts (W. 0. Haag, R. M. Dessau and R. M. Lago) ......................
,255
Dual Function Mechanism of Alkane Aromatization over HZSM-5 Supported Ga, Zn, Pt Catalysts: Respective Role of Acidity and Additive (P. Meriaudeau, G. Sapaly and C. Naccathe) .....................................................................................................................
267
Autocatalysis, Retardation, Reanimation and Deactivation during Methanol Conversion on Zeolite HZSM5 (H. Schulz, S.Zhao and H. Kusterer) .............................................................................................
281
Shape Selective Reactions of Alkylnaphthalenes in Zeolite Cata]ysts (J. Weitkamp and M. Neuber) ............................................................
291
Alkylation of Biphenyl Catalyzed by Zeolites (Y. Sugi, T. Matsuzaki, T. Hanaoka, K. Takeuchi, T . Tokoro and G. Takeu-
..................................................................................................................... 303
Correlation between Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites and Selectivity of Conversion of Alkylbenzenes (V. N. Romannikov, E. A. Paukshtis and K.G. Ione) .................................
311
N- and C-Methylanilines Formation on Zeolites with Different Structural and Acidic Properties (0.V. Kikhtyanin, K. G. Ione, L. V.Malysheva and A. V. Toktarev) .........................................................
319
Copper Ion- exchanged Zeolites as Active Catalysts for Direct Decomposition of Nitrogen Monoxide (M. Iwamoto) ..............................
327
Ship-in-Bottle Synthesis of Sterically Crowded Fe-Phthalocyanines in NaY Zeolite Hosts and Their Catalytic Behavior in Regioselective Oxidation of Alkanes (M. Ichikawa, T. Kimura and A. Fukuoka) .............................................................................................
335
Titanium Silicalite: A New Selective Oxidation Catalyst (B. Notari) ...............................................................................................................
343
The Effects of Iron Impurities on the Cracking Properties of Pillared Clays (M. L. Occelli, J. M. Stencel and S. L. Suib) .................. 353 Catalysis by Hydrotalcite in Liquid-phase Organic Reactions (Y. One, E. Suzuki and M. Okamoto) ...............................................................
363
xviii Contents
Iron-exchanged Montmorillonite as an Efficient Acid Catalyst in Liquid-Phase Organic Synthesis (Y. Izumi and M. Onaka) ...............371 Influence of Pore Structure on the Catalytic Behavior of Clay Compounds (E. Kikuchi and T. Matsuda) ...................................................
377
This volume is a collection of 14 plenary lectures and 25 invited and contributed papers presented at the International Symposium on Chemistry of Microporous Crystals (CMPC) held at Sophia University in Tokyo, Japan, June, 26-29, 1990. The symposium was organized by the Japan Association of Zeolite in collaboration with twelve major academic Japanese societies dealing with the chemistry of microporous crystals. The symposium was attended by over 250 researchers from 13 countries. The objective of the symposium was to present new horizons and developments in chemistry and application of natural and synthetic crystalline materials having microporous structures. At this meeting various trends were noted: - new possibilities for highly selective oxidation of hydrocarbon and synthesis of fine chemicals using modified zeolites and metallosilicates; - sophisticated syntheses of some valuable hydrocarbons such as 2,6dimethylnaphtalene and styrene which could not be obtained successfully by conventional catalysts; - detailed mechanism of decomposition and aromatization of paraffinic hydrocarbons on zeolitic catalysts; - methanol conversion on zeolite catalysts; - syntheses of novel wide pore aluminophosphates and their isomorphously substituted porous crystals; - datailed analysis on the state of cations in zeolites and metallosilicates; - application to direct decomposition of nitric oxide; - dynamic behaviors of molecules in zeolite pores; - chemistry and reaction performance of clay minerals. Besides the 39 papers included in this volume, about 50 poster papers were presented at the symposium. They were intended to reflect the wide interest in the new wave of zeolite and clay material researches. The editors thank the authors for the superior quality of their presentations and for contributing to this volume. The editors also thank the referees for their conscientious review to ensure the high scientific level of this volume. Thanks are also extended to the Organizing Committee and all the chairpersons of the sessions for willingly giving their time and expertise to the symposium. Special thanks are due Professor H. Tominaga (Chairman of the Symposium), Professor A. Iijima (President of the Japan Association of Zeolite), and Professor K. Luhmer (Chairman of the Board of Trustees, Sophia University) ; without whose invaluable efforts this important symposium could not have
xx Preface
been held. Grants from the Commemorative Association for the Japan World Exposition and the Asahi Glass Foundation for Industrial Technology are deeply appreciated. October 1990 Tomoyuki Inui Seitaro Namba Takashi Tatsumi Editors
I
Synthesis
This Page Intentionally Left Blank
3
Clay-like and Zeolite-like Structures Built of Polymeric Cyanocadmate
T o s c h i t a k e Iwamoto Department o f Chemistry, C o l l e g e o f A r t s and Sciences,, The U n i v e r s i t y o f Tokyo, Komaba, Meguro, Tokyo 153, Japan
ABSTRACT A number o f c l a y - l i k e 2D l a y e r e d and z e o l i t e - l i k e 30 framework h o s t s t r u c t u r e s have been m a t e r i a l i z e d u s i n g isopolycyanocadmate as t h e l i n k a g e u n i t . The 2D l a y e r e d and t h e 3D framed h o s t s accommodate a v a r i e t y o f o r g a n i c guest molecules and onium c a t i o n s o f d i f f e r e n t g e o m e t r i c a l characters. INTRODUCTION I n o u r l a b o r a t o r y we have o b t a i n e d a number o f i n c l u s i o n compounds u s i n g cadmium( 11) cyanide o r isopolycyanocadmate( 11) as t h e h o s t s and s e v e r a l o r g a n i c molecules as t h e guests.
The h o s t s t r u c t u r e s so f a r determined by s i n g l e
c r y s t a l d i f f r a c t i o n experiments have been c l a s s i f i e d i n t o t h r e e groups i n general,
b c r i s t o b a l i t e - l i k e [ I ] , clay-like,
Cadmium cyanide, Cd(CN)2,
and z e o l i t e - l i k e
[2].
i s analogous t o Si02 w i t h r e s p e c t t o t h e AB2 compo-
s i t i o n , t h e t e t r a h e d r a l c o n f i u g r a t i o n o f A, t h e b r i d g i n g behavior of B between a p a i r o f A atoms, and t h e a b i l i t y t o b u i l d a three-dimensional c a v i t i e s o f m o l e c u l a r s c a l e a r e formed.
framework i n which
Cadmium caynide i t s e l f c r y s t a l l i z e s i n
a c u b i c system o f t h e a n t i c u p r i t e type, i n which two i d e n t i c a l & c r i s t o b a l i t e l i k e frameworks i n t e r p e n e t r a t e each o t h e r w i t h o u t any cross-connection: c a v i t y formed i n one framework i s f i l l e d by t h e o t h e r .
the
When we r e p l a c e one o f
t h e frameworks by a p p r o p r i a t e guest molecules such as those o f CC14, CCl3CH3. etc.,
we may o b t a i n a novel c l a t h r a t e s t r u c t u r e w i t h an adamantane-like c a v i t y ,
as shown i n Fig. 1 [ I ] .
Our r e s u l t s i n c l u d i n g those r e c e n t l y o b t a i n e d a r e
summarized i n Table 1. The most remarkable and c h a r a c t e r i s t i c d i f f e r e n c e between Cd(CN)2 and S i O frameworks i s seen i n t h e A-B-A o f Si-0-Si.
span length: ca. 5.5 A o f Cd-CN-Cd
2 and ca. 3.2 A
The i n c l u s i o n o f such haloalkane guests, being b u l k i e r t h a n a l k a l i
and a l k a l i n e e a r t h c a t i o n s , i s e s s e n t i a l t o b u i l d up t h e 8 - c r i s t o b a l i t e - l i k e
4 T.Iwamoto
cadmium cyanide h o s t s t r u c t u r e w i t h t h e adamantane-like c a v i t y . o f the 8-cristobalite-like
A modification
s t r u c t u r e has r e c e n t l y been r e p o r t e d by Hoskins and
Robson [ 3 ] , whose work i s i n t e r e s t i n g f o r t h e i n t r o d u c t i o n o f a n e g a t i v e charge i n t o t h e h o s t [ C U Z ~ ( C N ) ~ - ] u s i n g Cu(1) and Zn(I1) as t h e t e t r a h e d r a l centers. Table 1.
Cd(CN)2-G i n c l u s i o n compounds and t h e i r l a t t i c e parameters.
a
G
a/A
G
12.597(2)
CC14
12.668( 2 )
C H C l 2CHC1
12.691 (2)
12.611(3)
CC13CH3
12.6701 5)
CC12FCC1F2
12.704(3)
CHC12CH3
12.623( 2)
cyclo-C6H1
12.685( 2)
CC13CF3
12.733( 2)
CH3CHClCH2CH3
12.646( 2)
(CH3)3CC1
12.688(2)
cyclo-C6H1 1CH3
12.74( 3)
G
afA
CHC13
CHC12CH2Cl
a.
afA
A l l t h e c r y s t a l s belong t o t h e face-centered c u b i c space group Fd%.
Fig. 1. Adamantane-like c a v i t y formed i n t h e 6 - c r i s t o b a l i t e - l i k e cadmium( 11) cyanide host. Unlike Si(IV),
framework o f
Cd(I1) o f t e n takes an o c t a h e d r a l c o n f i g u r a t i o n as w e l l as a
t e t r a h e d r a l one: i t i s r a t h e r d i f f i c u l t f o r S i ( 1 V ) t o t a k e an o c t a h e d r a l c o n f i g u r a t i o n under ambient c o n d i t i o n s .
The isopolycyanocadmate( 11) systems
b u i l t o f t h e CN-linkages among t e t r a h e d r a l and o c t a h e d r a l Cd atoms can mimic t h e s t r u c t u r e s o f s i l i c a t e m i n e r a l s composed o f t h e t e t r a h e d r a l S i and o t h e r octahed r a l cations. Our isopolycyanocadmate( 11) systems a r e comprised m a i n l y o f t h e heptacyanotricadmate(I1) u n i t f o r the z e o l i t e - l i k e structures.
The u n i t i s b u i l t o f a
Structure of Polymeric Cyanocadmate
l i n e a r l i n k a g e o f two t e t r a h e d r a l Cd (Cd-t)
-(NC)-(Cd-t)-CN-(Cd-0)-NC-(Cd-t)-(NC)-, i n a t i o n between t h e N- and C-terminals
and one o c t a h e d r a l Cd (Cd-o)
atoms,
i n which t h e c r y s t a l l o g r a p h i c a l d i s c r i m i s sometimes d i f f i c u l t o r i m p o s s i b l e f o r
t h e c y a n i d e group i n t h e parentheses c o n n e c t i n g t h e l i n k a g e u n i t s . I n t h e c l a y - l i ke l a y e r e d s t r u c t u r e s , we have seen o c t a c y a n o t r i c a d m a t e ( II),
hexacyanodiiodotricadmate( II),and hexacyanochloroaquatricadmate( 11), as we1 1 as heptacyanocadmate(I1).
as t h e b u i l d i n g u n i t s .
For t h e l a s t , a c a t i o n i c l i g a n d
3-dirnethylammoniopropylamine i s i n v o l v e d i n t h e h o s t s t r u c t u r e .
Hence, t h e
b u i l d i n g u n i t o f t h e l a y e r e d s t r u c t u r e s has t h e g e n e r a l c o m p o s i t i o n Cd3(CN)6L2, i n which t h e s i x c y a n i d e groups p a r t i c i p a t e i n c o n n e c t i n g t h e u n i t s and two u n i d e n t a t e L l i g a n d s a r e f r e e from t h e c o n n e c t i o n t o c o n s t r u c t t h e l a y e r b u t protrude from t h e surface o f t h e layer.
FORMATION AND PREPARATION The c l a y - l i k e and t h e z e o l i t e - l i k e i n c l u s i o n compounds a r e p r e p a r e d f r o m CdC12, K2[Cd(CN)4], g u e s t species.
a r e l e v a n t o r g a n i c amine o r ammonium h a l i d e , and an o r g a n i c
T’he h a l i d e a n i o n o f t h e tetramethylammonium s a l t , c h l o r i d e o r
i o d i d e , i s sometimes i n v o l v e d i n t h e h o s t s t r u c t u r e o f t h e c l a y - l i k e compounds: water m o l e c u l e may a l s o behave as t h e l i g a n d .
On t h e o t h e r hand, t h e ammonium
c a t i o n i s entrapped i n t h e i n t e r l a y e r space as t h e g u e s t s p e c i e s as w e l l as t h e n e u t r a l g u e s t molecule.
An e x c e p t i o n a l case has been seen f o r t h e t r i m e t h y l s u l -
fonium c a t i o n , which i s accommodated i n a c a v i t y formed i n t h e l a y e r s t r u c t u r e itself. amine,
When we use N,N-dimethyl-1,3-diaminopropane
(dmtn) as t h e o r g a n i c
i t i s p r o t o n a t e d a t t h e m e t h y l a t e d amino group t o g i v e dmtnH and behaves
as a u n i d e n t a t e c a t i o n i c l i g a n d t o t h e o c t a h e d r a l Cd atom i n t h e l a y e r e d Cd3(CN)7 h o s t . c a t i o n i c guest.
I n t h e z e o l i t e - l i k e Cd3(CN)7 host, however, dmtnH behaves as a Tetramethylammonium c a t i o n always behaves as t h e c a t i o n i c g u e s t
i n b o t h c l a y - l i k e and z e o l i t e - l i k e s t r u c t u r e s . C r y s t a l s o f t h e i n c l u s i o n compound a r e formed a t t h e i n t e r f a c e between t h e aqueous s o l u t i o n o f t h e m e t a l s p e c i e s and t h e o r g a n i c amine o r ammonium s a l t and t h e o r g a n i c phase of t h e n e u t r a l g u e s t s p e c i e s by s t a n d i n g f o r a few days o r weeks i n a r e f r i g e r a t o r o r a t ambient temperature.
The p r o d u c t s a r e g e n e r a l l y
n o t so s t a b l e under ambient c o n d i t i o n s t h a t t h e y l i b e r a t e t h e n e u t r a l g u e s t molecules g r a d u a l l y .
The specimens s u b j e c t e d t o s i n g l e c r y s t a l X-ray e x p e r i -
ments s h o u l d be coated w i t h epoxy o r a c r y l i c r e s i
i n o r d e r t o p r e v e n t sponta-
neous decomposition. CLAY-LIKE 2D LAYERED HOST STRUCTURES Our f i r s t c l a y - l i k e s t r u c t u r e was demonstrated f o r an i n c l u s i o n compound o f
f lurorobenzene, heptacyano( 3-dimethylammoniopropy
amine)tricadmate( 11)-fluoro-
5
6 T. Iwamoto
benzene(l/l): [Cd3(CN)7*(CH3)2NH(CH2)3NH2]*C6H5F [2]. The strcuture of the neutral layer is illustrated in Fig. 2. The building unit o f the layer is the ) ~ : of the cyanide groups linkage o f N C - C ~ ( ~ ) - C ~ ( ~ ) - C ~ ( O ) - N H ~ ( C H ~ ) ~ N H ( C H ~one coordinating to the terminal Cd(t) is unidentate, and the cationic ligand dmtnH coordinated to the other terminal o f Cd(o) extends its skeleton on the layer. A hydrogen bond is formed between the dimethylammonio group o f the dmtnH in the layer and the N-terminal o f the unidentate cyanide involved in the adjacent layer. The guest fluorobenzene molecule is accommodated between the layers pillared by the hydrogen bonds with its aromatic plane almost parallel to the layers so
Fig. 2. Structure o f the neutral layer in the clay-like inclusion compound [Cd3(CN)7 '(CH3)2NH(CH2)3NH2I *CgHgF. Table 2.
Clay-like layered structures
host composition guest onium guest molecule crystal system space group Cd3( CN),dmtnH Cd3(CN)7dmtnH Cd3( CN)7dmtnH
none none none
Cd3(CN )8 Cd3(CN 18 Cd3(CN 16 12 Cd3 (CN I6C 1(H20)
2NH(CH3)3
2N(CH3I4 2N(CH3I4 S ( CH3 1
cc1 ( CH3)2CHCH20H
CgHg 2C12C=CC12 C12C=CC12 none
tricl inic tricl inic tri cl ini c monoclinic monoclinic monoclinic orthorhombic
P i P i p i
Cm Cm Cm Pnam
Structure of Polymeric Cyanocadmate
t h a t t h e s t r u c t u r e can be seen as a model o f a p i l l a r e d i n t e r c a l a t i o n compound. S i m i l a r h o s t s t r u c t u r e s have been observed f o r t h e carbon t e t r a c h l o r i d e and t h e i s o b u t y l a l c o h o l i n c l u s i o n compounds.
The c l a y - l i ke l a y e r s t r u c t u r e s so f a r
demonstrated by s i n g l e c r y s t a l s t r u c t u r e analyses [ 4 ] ,
i n c l u d i n g those mentioned
above. a r e l i s t e d i n Table 2. ZEOLITE-LIKE 30 STRUCTURES As l i s t e d i n Table 3, a number o f i n c l u s i o n compounds o f t h e Cd3(CN)7 h o s t
have been o b t a i n e d w i t h t h e general formula [ C d 3 ( C N ) 7 ] * [ ~ n i u m ] * [ g u e s t ] where t h e onium c a t i o n i s always accommodated as t h e guest i n t h e z e o l i t e - l i k e 30 framework s t r u c t u r e [51. Table 3.
Z e o l i t e - l i k e framework s t r u c t u r e s
t y p e onium
guest
c r y s t a l system
space group
CH2C1CH2C1
orthorhombic
PnZlrn
The h o s t s t r u c t u r e s so f a r known can be c l a s s i f i e d i n t o f i v e t y p e s according t o t h e way o f connection among t h e linkage units.
-(NC)-Cd(CN-)2-CN-Cd(NC-)4-NC-Cd(CN-)
-
2
The u n i t i s i n common comprised o f two t e t r a h e d r a l Cd (T) and
one o c t a h e d r a l Cd (0) atoms.
The -T-0-T-
u n i t , i n c l u d i n g t h e b r i d g i n g cyanide
groups, makes a 1D i n f i n i t e c h a i n on a plane w i t h a p e r i o d i c a l sequence o f bending s t r u c t u r e .
There have been observed s u b s t a n t i a l l y t h r e e ways o f bending
f o r t h e i n f i n i t e chains, ( 1 ) TOT-trans,TT-trans, TOT-cis,TT-trans,
as shown i n F i g . 3.
( 2 ) TOT-trans,TT-cis,
and (3)
A c h a i n on a p l a n e i s l i n k e d w i t h t h e
cyanide groups o u t o f t h e p l a n e a t e v e r y Cd atom t o t h e , c h a i n s i n t h e a d j a c e n t planes above and beneath: T i n one p l a n e i s connected t o 0 i n t h e a d j a c e n t
7
8 T. Iwamoto
I
I
0
\
I
I
r4 I
I
Fig. 3. Ways o f bending f o r i n f i n i t e c h a i n o f -T-0-Tl i n k a g e : ( 1 ) TOT( 2 ) TOT-trans,TT-cis; ( 3 ) TOT-cis,TT-trans. Circle: tetrahedral trans,TT-trans; Cd; square: o c t a h e d r a l Cd; open: on a plane: s o l i d : on a d j a c e n t plane: s o l i d l i n e : cyanide group on t h e plane; broken l i n e : cyanide group o u t o f t h e plane. planes and v i c e versa. structure.
The d i s t a n c e between t h e planes i s t h e same i n a c r y s t a l
According t o t h e way o f connection, t e t r a - ,
penta-,
and hexagons a r e
formed between t h e planes, t h e polygons which a r e cornered by T and 0 atoms and edged by CN groups.
C a v i t i e s thus formed i n t h e three-dimensional
framework a r e
surrounded by t h e polygons whose edges a r e t h i c k enough t o h o l d a guest i n s i d e due t o t h e + e l e c t r o n s
on t h e CN t r i p l e bond.
I n t y p e I, TOT-trans.TT-trans
c h a i n s a r e arranged i n p a r a l l e l on t h e same
plane, b u t t h e d i r e c t i o n o f bending i s reversed on a d j a c e n t planes.
The i n t e r -
c h a i n connection g i v e s pentagons which a r e c r y s t a l l o g r a p h i c a l l y independent o f b u t g e o m e t r i c a l l y i d e n t i c a l t o one another i n t h e i d e a l i z e d s t r u c t u r e .
I n type
11, t h e d i r e c t i o n o f bending i s a l t e r n a t e l y reversed on t h e same p l a n e f o r TOTtrans,TT-trans
chains; a t e t r a g o n , a pentagon, and a hexagon a r e given.
111, TOT-trans,TT-trans
I n type
chanis a r e arragned i n p a r a l l e l t o each o t h e r so t h a t
t h e t r i g o n a l u n i t c e l l s a r e r a t h e r p r e f e r a b l e t o t h e orthorhombic ones adopted f o r types I and 11. l e l e d TOT-trans,TT-cis hexagon a r e formed.
A t e t r a g o n and a hexagon a r e formed.
I n type I V , paral-
c h a i n s make t h e hexagonal u n i t c e l l s ; a t e t r a g o n and a I n t y p e V, TOT-cis,TT-trans
chains a r e p a r a l l e l e d on t h e
same p l a n e b u t t h e d i r e c t i o n o f bending i s reversed i n a d j a c e n t planes; a tetragon. a pentagon, and a hexagon a r e formed.
Although t h e r e a l c r y s t a l
Structure of Polymeric Cyanocadmate 9
Illustration of the idealized structures for type I through V of the zeolite-like 3D frameworks. Notations for atoms are the same to those in Fig. 3. The chains of solid circles and squares are at c = 0.5 if the c-axis is taken vertical to the sheet: those of open ones are at c = 0 or 1. A selection of the unit cell, containing net two layers, is outlined by thin lines for each structure. Fig. 4.
structures are considerably distorted from the images, the idealized structure of each type is illustrated in Fig. 4. CONCLUSION The present -cristobalite-like, clay-like, and zeolite-like series of cadmium cyanide or polycyanocadmate structures have never been discovered in nature as minerals. They are not the products obtained from naturally-occurring structures through chemical modification nor replacement of moieties. The author proposes the term "mineralomimetic chemistry" as a field of chemistry to develop mineral-like multi-dimensional inorganic structrues using materials quite different from those occurring in nature. The Cd-CN-Cd span, being longer than Si-0-Si, is essentially accompanied by an inclusion structure in the mineralomimetic systems. In other words, we can expect a variety of microporous
10
T.lwamoto
s t r u c t u r e s from t h e a r t i f i c i a l design of metal cyanide systems. The a u t h o r would l i k e t o acknowledge h i s colleagues, P r o f e s s o r Reiko Kuroda,
Dr. S h i n - i c h i N i s h i k i o r i , Plr. Takafumi Kitazawa, M.A., B.Sc.,
and Mr. Hidetaka Yuge, B.Sc.,
M r . Motoyasu Imamura,
f o r t h e i r c o o p e r a t i o n i n t h i s work.
REFERENCES 1. T. Kitazawa, S. N i s h i k i o r i , R. Kuroda, and T. Iwamoto, Chem. L e t t . , 1729. 2. T. Kitazawa, S. N i s h i k i o r i , R. Kuroda, and T. Iwamoto, Chem. L e t t . , 459. 3. 3 I - F . Hoskins and R. Robson, J. Am. Chem. SOC., 112(1990), 1546. 4. T. Kitazawa e t al., under p r e p a r a t i o n . 5. T. Kitazawa e t al., under p r e p a r a t i o n .
(1988) (1988)
11
129Xe-NMRStudy of the Crystallization of SAPO-37
T. Ito 1, N. Dumont 2, J. B.Nagy 2, 2. Gabelica 1
and E. G. Derouane 2
Tarnai Sangyo CO., LTD, Zenibako 3-chorne, 524-1 1, Otaru 047-02 (Japan)
2 Facult& Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles,
5000-Narnur (Belgium).
ABSTRACT The different stages of SAPO-37 crystallization have been characterized by NMR of 129Xe adsorbed on a series of intermediate phases isolated during synthesis. The nucleation of SAPO-37 occurs within the amorphous phase; the gel then undergoes a preliminary structuration through the formation of large cavities during the aging period. Upon heating at 2OO0C, the gel transforms into welldefined crystals of SAPO-37. The crystallinity increases with the synthesis time and reaches a maximum after 32h. For longer synthesis times, a crystalline sidephase, SAPO-40, develops in the liquid phase. SAPO-40 has a narrower pore structure than SAPO-37, possibly limited by 12 T puckered rings. INTRODUCTION The NMR chemical shift of 129Xe adsorbed on molecular sieves reflects all the interactions between the electron cloud of the xenon atoms and their environment in the intracrystalline void volume [l]. This nucleus therefore proved to be an ideal probe for investigating various zeolitic properties such as pore dimensions [2, 31, location of the countercations [4, 51, distribution of adsorbed or occluded phases [6-81 and framework polarisability [8, 91. In this study, we used the 129Xe-NMR technique to examine the behavior of gaseous xenon adsorbed at different pressures on a series of intermediate phases isolated during the crystallization of a Faujasite-type silicoaluminophosphate, SAPO-37. Such a method has already proved successful in defining the different steps that successively occur during the crystallization process of zeolites Nay, ZSM-5 and ZSM-20 [ l o ] : gel restructuration, increase of the crystallinity of the
12 T. [to, N. Dumont, J. B. Nagy, Z. Gabelica and E. G. Derouane
zeolite,
or
its further transformation
into thermodynamically
more stable
products. Concerning the study of the crystallization of a gel that typically yields SAPO-37, this method is expected to be helpful in many respects: 1)
2)
3)
It is able to detect the presence in the gel of very short-range ordered phases, thus providing more information than the conventional X-ray diffraction technique, the applicability of which is limited to the study of crystals of 50 A in size at least. It is an appropriate means to determine the composition of a mixture of SAPO37 and SAPO-40. Such an estimation is rather difficult to achieve by X-ray diffraction because most of their respective diffractogram peaks overlap. Finally, it complements adsorption data of n-hexane and thus yields some information on the porous structure of SAPO-40 which has not yet been elucidated.
EXPERIMENTAL
Svnthesis The intermediate and final SAPO-37 phases were prepared according to the general procedure described in the patent literature [ l l ] and optimized in our laboratory [12]. A mixture of molar composition 1.0 A1203 : 0.9 P2O5 : 0.4 Si02 : 0.86 (TPA)20 : 0.023 (TMA)20 : 50 H 2 0 was aged by stirring at 20°C for 48h. Several aliquots of this gel were poured into rotating Teflon-lined stainless steel autoclaves and heated at 200°C for various periods of time (0 to 32h). As the formation of crystalline SAPO-40 is known to be promoted by longer synthesis times [12], one of these aliquots was heated for 149h in order to obtain a mixture of SAPO-37 and SAPO-40. For this study, we selected the more representative samples among the various intermediates isolated during the synthesis course, namely those formed after 0, 5, 10, 32, and 149h of heating. After cooling, the solid and the mother liquid of each intermediate phase, including the non-heated gel, were separated by centrifugation. The solids were washed with water, dried overnight at 90°C, and checked for nature, purity and relative crystallinity by The powder X-ray diffraction (Table 1) and scanning electron microscopy. intermediate phases are referred to as Pn, where n denotes the crystallization time. Pretreatment Water and organic molecules occluded during the synthesis were removed from the intracrystalline volume as follows. The solids were slowly heated (5"C/min) in a N2 flow up to 550°C and held at this temperature for 2h. The coke deposit resulting from the non-oxidative degradation of the organics was then
'"Xe-NMR
of Crystallization of SAPO-37
13
oxidized with dry air at 550°C for another 3h. The samples were rapidly cooled to 20°C. The SAPO-37 lattice emptied from its original organic molecules is known to be unstable in wet atmosphere [8,13]. A known amount of each calcined sample was then introduced in an NMR tube and immediately evacuated up to a final pressure of 10-5 Torr at 200°C to remove any trace of moisture.
Adsorotion and NMR exoerimentg The adsorption isotherms of xenon were measured at 34°C using a classical volumetric apparatus. The 129Xe-NMR measurements were performed at the same temperature on a Bruker CXPPOO spectrometer operating at 55.3 MHz. The nhexane adsorptions were conducted at 90°C on a Stanton Redcroft STA-780 thermoanalyzer. The samples were submitted to a preliminary calcination under dry air up to 650°C with a heating rate of 10"C/min.
Table1 : Nature, relative xenon adsorption capacities and relative crystallinities of the intermediate phases obtained at different stages of the SAPO-37 crystallization.
I
I Sample(a)
Nature (XRD)
I Relative Xe adsorption capacities
Pn
Relative
c r y s t a l I i nit ies
(%) (c)
XRD
l*gXe-NMR
0
(d) (d)
(b)
amorphous phase amorphous phase SAPO-37 SAPO-37 SAPO-37+SAPO-40
PO p5 p10 p3 2 P149 I
18 18 81 100 135 (e) I
0 78 100 (f)
83 100 55
I
n denotes the crystallization time (h). Relative xenon adsorption capacities calculated from the isotherms with respect to the most crystalline SAPO-37 phase (P32) arbitrarily considered to have a 100% adsorption capacity. Weight percentage of SAPO-37 in each intermediate phase determined with respect to the most crystalline SAPO-37 phase arbitrarily considered to be 100% crystalline (P32). The crystallinity of Po and P5 cannot be determined by this technique with respect to P32 because these two phases do not contain well-defined SAPO-37 crystals. Xe isotherms only allow calculation of the global Xe adsorption capacity of the mixture (SAPO-37+SAPO-40). It is not possible to estimate the crystallinity of SAPO-37 in the mixture P i 4 9 by X-ray diffraction, as most of the XRD peaks of SAPO-37 and SAPO-40 overlap.
14 T Ito,
N Dumont. J B Nagy, Z Gabelica and E G Derouane
RESULTS AND DISCUSSION
Adsomtion isotherms The quantities of xenon adsorbed by each Pn sample are plotted against the equilibrium pressure, in a double logarithmic scale (Fig.1). All intermediate phases show fully linear isotherms over the pressure range investigated (10 to 900 Torr). No saturation is observed below 900 Torr, even for Po. P10 and P32 have parallel isotherms, as expected for samples of identical structure. The isotherms of Po and P5 have the same slope as those of Pi0 and P32 which are rather well crystalline. However, this observation is a necessary but not a sufficient condition to affirm that these materials, amorphous to X-rays, already contain microcrystallites having a Faujasite structure. At a given pressure, the amounts of xenon adsorbed by the different phases allow one to estimate their relative crystallinities, with respect to the most crystalline sample, P32. These results are presented in Table 1. The isotherms of Po and P5 overlap, which implies that both intermediates have the same adsorption capacity. Although the X-ray diffraction and scanning electron microscopy techniques do not detect any crystalline material in Po and P5, the xenon adsorptions reveal that these samples, formed during the first stages of the synthesis, are already porous. Their adsorption capacity is not negligible (about 20% of the maximum observed for the 100% crystalline SAPO-37, P32) and is probably due to the presence of large cavities (about 25A in diameter, see below) formed in the amorphous phase during the aging period. The crystallinity of P i 0 deduced from the xenon adsorption results is in very good agreement with that obtained by comparing the areas of the characteristic XRD peaks of P i 0 and P32. On the other hand, it seems that the adsorption capacity of the mixture of SAPO-37 and SAPO-40 occurring in P i 4 9 is larger than that of pure SAPO-37 (P32) (Fig.l), clearly suggesting that SAPO-40 has a larger xenon adsorption ability than SAPO37. According to our results (see below), SAPO-40 involves a smaller total pore volume than SAPO-37, limited by narrower pores. We therefore attribute the fact that SAPO-40 adsorbs more xenon than SAPO-37 to "confinement effects" [14-161. Indeed, according to the model recently proposed by Derouane et al., the heat of sorption of a molecule in a zeolitic pore is proportional to the parameter Wr(s) describing the channel geometry and size. As Wr(S) is expected to be. larger for SAPO-40 than for SAPO-37, because of its narrower pores, the sorption equilibria favor the sorption of the xenon atoms in SAPO-40, where they are better confined. 129Xe- N M
R
Xenon adsorbed in samples Po and P5 gives rise to a broad NMR signal (line C) distinguishable from the peak characterizing the gaseous phase (line G) located at
Iz9Xe-NMR of Crystallization of SAPO-37 15
1021
1O2O
m
> E,
c,
a a¶
K
=
10’’
10l8
PXe (Torrl Fig. 1. Isotherms of xenon adsorbed on samples Po ( A ) , P5 (+), P i 0 (01, p32
( x ) and p149 ( 8 ) .
I\
‘32
A
A
u 2 -
. ,
1
150
. I . -
ioo
PPM
*
’
50
, . . I .
0
Fig. 2. Typical 129Xe-NMR spectra for samples Po, P5, PIO, P32 and P14g.
16 T. Ito, N. Dumont, J. B. Nagy. 2. Gabelica and E. G. Derouane
0 ppm (Fig. 2). We therefore conclude that the gel aged at room temperature for 48h already contains a preliminary void structure consisting of large cavities
which allow a continuous exchange between adsorbed xenon atoms and the gaseous phase. The degree of connection between these cages is large and their sizes are uniform enough to be characterized by a rather defined chemical shift. The xenon atoms adsorbed in a zeolitic pore experience different perturbations which contribute additively to the experimental chemical shift, tiobs 111:
60 is the reference chemical shift of gaseous xenon extrapolated to zero pressure. 6~ accounts for the interactions between the xenon atoms and the electric field of the compensating cations. The presence of some magnetic species in the zeolite (e.g. paramagnetic cations) gives rise to the 6~ contribution. &, the chemical shift of adsorbed xenon extrapolated to zero pressure, characterizes the Xe-zeolitic pore walls interactions. The smaller the channels and the cavities or the larger the diffusional constraints, the larger the 6s. 6xe is due to Xe-Xe interactions in the adsorbed phase. It is proportional to the local density of the gas in the void volume and increases as a function of the equilibrium pressure, if the zeolite is very crystalline. If the cavities in the gel were open and not connected, the probability of exchange between the xenon atoms located in the cages and the gaseous phase should increase with the pressure. Therefore, the slope of the straight line 6obs = f(PXe) should be negative. For samples Po and Pg, the chemical shift is quasi constant, whatever the xenon pressure (Fig. 3). This suggests that the degree of connection between the cages is small. The value of 6s (about 45 ppm) allows us to estimate that the average diameter of these cavities is about 25 A [17-181. The width of the NMR signal illustrates the dispersion of the sizes and of the degrees of connection of the cages. It decreases from Po to P5, which can be explained by a homogenization of these two parameters as the crystallization proceeds. Line A observed for the intermediates Pio, P32 and PI49 (Fig. 2) characterizes the interactions of the xenon atoms in the supercages of a well-crystalline Faujasite-type framework. The overlap of the straight lines 8=f(PXe) of the three samples proves that they have the same structure (Fig. 3). The value of 6s is 56 ppm. The difference with respect to the isostructural zeolite Y (64 ppm) stems from their different framework polarisability and composition [8]. For too long synthesis times, the SAPO-37 network starts deteriorating, as suggested by the broadening of signal A for Pi49 (P32: 200 Hz, P14g: 625 Hz at 250 Torr). On the other hand, a second NMR signal (line B) appears at lower fields, illustrating the presence of the other crystalline phase, SAPO-40. The structure of the latter has
lz9Xe-NMR of Crystallization of SAPO-37 17
140
c .
E
E E
120
-
100
-
80
-
.,
60
-/u-
40
-
/
/
. 1
0
/
A
I
200
.-*a-
x-----@-~-
CE-8 +
w
+
a
t
400
+
I
600
A
+
I
800 PXe [Torrl
1000
Fig. 3. Dependence of the chemical shift on the xenon pressure for samples Po ( A ) , p5 (+It PIO( o ) ,P32 (S) and Pi49 (m).
120 c .
-c P
100
80 Fig. 4. Dependence of the chemical shift on the number of xenon atoms adsorbed on samples PO (A), p5 (+I9 PI0 (01, p32 ( X I and PI49 (.I.
60
40 d
1.1oZ0 axe
5.10 It@lllS/#
18 T. Ito, N. Durnont, J.
B. Nagy, 2. Cabelica and E. G. Derouane
not yet been elucidated.
However, its 6s value (95 pprn), larger than that
characterizing the 12 T windows of SAPO-37 (56 ppm), suggests that SAPO-40 involves a narrower pore structure, possibly limited by 10 T windows or puckered 12 T rings, as previously suggested [19]. Thermal analysis data and n-hexane adsorption confirm this assumption. Indeed, we have shown that SAPO-40 contains about 30 wt.% of organic compounds less than SAPO-37, and that its porous volume is about 60% of the void volume defined by the supercages of SAPO-37 [20]. The n-hexane sorption capacity of SAPO-40 (10.9 g/g) compares quite well with In that of ZSM-5 (10.6 g/g), which possesses a three-dimensional structure. contrast, this capacity is much larger than that characterizing the onedimensional networks of zeolites ZSM-12, ZSM-48 and Mordenite [20]. This observation, combined with the fact that SAPO-40 adsorbs xenon and n-hexane more rapidly than these one-dimensional zeolites suggests that the SAPO-40 framework consists of interconnected channels in which small molecules can diffuse quite easily. On the other hand, SAPO-40 and ZSM-5 are characterized by a similar parameter Ss whereas xenon was shown to diffuse much more rapidly thorough SAPO-40 than in ZSM-5. These observations suggest that either the mean diameter of the SAPO-40 pores is larger than that characterizing the ZSM-5 channels or that the general "tortuosity" of the pore system in SAPO-40 is lower than in ZSM-5 that contains both straight and zig-zag channels. Xenon adsorption experiments performed on the mixture P i 4 9 suggest that SAPO-40 contains channels with a diameter involving less then 12 T atoms, i.e. smaller than or equal to 10 T atoms. On the other hand, if the SAPO-40 pores were limited by 10 T rings, we should admit a very tortuous channel system to justify the Ss value equal to that of ZSM-5, which is in contradiction with the rapid diffusion of xenon in this material. The only way to explain the similar 6s values measured for ZSM-5 and SAPO-40 is to suppose that the latter involves a trivial channel tortuosity, e.g. a straight interconnected channel network, the pores of which are limited by 12 Tmembered, but highly puckered windows. Such a geometry is quite in line with the typical tetragonal morphology of SAPO-40 single crystals (121. Using the adsorption isotherms, the equilibrium pressures can be converted into amounts of xenon atoms per gram of calcined and dehydrated samples, which allows us to plot the straight lines presented in Fig. 4. In the case of the mixture P i 49, the total xenon concentration at each pressure is shared between SAPO-37 and SAPO-40 by taking into account the percentage of xenon atoms in each phase given by the intensity ratio of lines A and B. For the same amount of adsorbed xenon, less crystalline samples show a larger local density in the void volume. Therefore, the Xe-Xe interactions increase and 6Xe increases Consequently, the slope of the straight line S=f(nXe/g) increases as the crystallinity decreases. The ratios between the slopes of the intermediate phases Pie, P32 and Pi49 can be
1Z9Xe-NMRof Crystallization of SAPO-37 19
used to calculate their relative crystallinities (Table 1).
The value obtained for
P i 0 is in excellent agreement with those obtained from the XRD data and from the xenon isotherms. If we accept the hypothesis, which is confirmed by the SEM micrographs, that PI 4 9 does not contain any amorphous material, the "crystallinity" measured for this product actually corresponds to the weight percentage of SAPO-37 crystals in the mixture (55%). By XRD, it is impossible to determine accurately the composition of a mixture of SAPO-37 and SAPO-40 because most of the peaks of their respective diffractograms overlap. Although SAPO-37 is the major product in P i 49, the intensity ratio of lines A and B indicates that 55% of the xenon is adsorbed in SAPO-40. This observation can be rationalized by supposing that the narrower (puckered) channel structure of that material favors the adsorption of xenon due to confinement effects (see above).
CONCLUSION The different steps occurring successively during the crystallization process of SAPO-37 can be summarized as follows. SAPO-37 stems from a direct gel restructuration: large cavities form in the amorphous phase during the aging period at ambient temperature. The degree of connection between these cages is small and they do not present any regular ordering, which explains why they cannot be detected by XRD. This preliminary void structure does not develop much during the first 5h of heating: only a homogenization of the sizes of the cavities or an increase in their degree of connection is observed. The crystalline phase formed after 10h contains about 80% well-defined SAPO-37 crystals. The crystallinity increases with the synthesis time and reaches a maximum (arbitrarily chosen as 100%) after 32h. The diameter of the crystals is about 15pm and the electron micrographs do not reveal the presence of any amorphous phase. For longer synthesis times, the SAPO-37 framework starts deteriorating whereas another crystalline material, SAPO-40, forms. This latter has a narrower intracrystalline void space than SAPO-37 and may involve straight interconnected channels limited by 12 T puckered rings. An interesting observation is that the yield of SAPO-37 still increases when SAPO-40 starts forming [20]. This suggests that, in contrast to SAPO-37 which starts organizing its structure in the amorphous phase, SAPO-40 nucleates from the liquid phase (mother liquor)-gel interface. Indeed, the liquid phase undergoes a marked change in composition when crystalline SAPO-37 is formed [20]. This hypothesis was confirmed by separating the mother liquor from phase P32 and by continuing to heat it separately in an autoclave, at 200°C for 74h. A mixture of SAPO-5 and SAPO-40 was obtained. These results are more thoroughly discussed elsewhere.
20 T. Ito, N. Dumont, J. B. Nagy, 2. Gabelica and E. G. Derouane
ACKNOWLEDGMENT N. Dumont thanks the Belgian National Science Research Foundation (FNRS) for an "Aspirant" position. T. Ito acknowledges financial support from the Laboratory of Catalysis at FUNDP, Namur. REFERENCES 1 J. Fraissard and T. Ito, Zeolites, & (1988)350. (1987)314. 2 J. Demarquay and J. Fraissard, Chem. Phys. Lett., L. Maistriau, T. Ito and E.G. Derouane, Zeolites, 14 (1990)310. 3 4 T. It0 and J. Fraissard, Zeolites, Z (1987) 554. A. Gedeon, J.L. Bonardet, T. Ito and J. Fraissard, J. Am. Chem. SOC., 5
(1989)
2563. 6 7 8 9 10 11
12
A. Gedeon, T. Ito and J. Fraissard, Zeolites, (1988)376. T. lto, J.L. Bonardet, J. Fraissard, J. B.Nagy, C. Andre, Z. Gabelica and E.G. Derouane, Appl. Catal., 43 (1988)L5. (1989)L1. N. Dumont, T. Ito and E.G. Derouane, Appl. Catal., E. G. Derouane and M. E. Davis, J. Mot. Catal., 48 (1988)37. T. Ito, J. Fraissard, J. B.Nagy, N. Dewaele, Z. Gabelica, A. Nastro and E.G. Derouane, Stud. Surf. Sci. Catal., 4!&(1989) 579. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, U.S. Patent 4, 440, 871 (1984). L. Maistriau, N. Dumont, J. B.Nagy, 2. Gabelica and E.G. Derouane, Zeolites, 111
(1990) 243. 13
14 15 16 17
M. Goepper, F. Guth, L. Delmotte, J.L. Guth and H. Kessler, Stud. Surf. Sci. Catal., &lE (1989)857. E.G. Derouane, J.- M. Andre and A.A. Lucas, J. Catal., 11p (1988)58. E.G. Derouane, Chem. Phys. Lett., 142 (1987)200. E.G. Derouane, in "Guidelines to the Mastering of Zeolite Catalysts", D. Barthomeuf, E.G. Derouane and W. Hoelderich, eds., Plenum Press, New York, in press. J. Fraissard, T. Ito, M. Springuel-Huet and J. Dernarquay, Stud. Surf. Sci. Catal.,
28 (1986)393. 18 19 20
E.G. Derouane and J. B.Nagy, Chem. Phys. Letters, XXZ (1987)341. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. SOC.,m(1984) 6092. N. Dumont, T. Ito, Z. Gabelica, J. B.Nagy and E.G. Derouane, to be published.
21
Application of RDF to Synthesis of Zeolite
Usui, K. Abe, T. Tokita, S. Imafuku and M. Ogawa Mizusawa Industrial Chemicals, Ltd. 1-21, 4-ChomeY Nihonbashi-muromachi, Chuo-ku, Tokyo 103 Japan
K.
ABSTRACT The radial-distribution function (RDF) is an effective means for obtaining information on the structure of amorphous material. RDF was applied to the aluminosilicate hydrogels, which were formed by mixing and reacting sodium silicate solution with sodium aluminate solution under stirring, in order to produce NaA-zeolite crystal. It seems that all aluminosilicate hydrogels formed at the mixing, aging and heating process have similar structures comprised mainly of a 4-member chain. INTRODUCTION The radial-distribution function (RDF) specifies the density of atoms or electrons as a function of the radial distance from any reference atom or electron in the system. It can be applied to both crystalline and amorphous materials, and is especially effective for amorphous material. Previously in 1989, we reported the application o f RDF to various zeolites At that time we were interested in the and dealuminated products therefrom [I]. structural changes or decomposition of crystalline zeolites by weak acidtreatment or dealumination of the crystalline zeolites. The present study deals with the application o f RDF to the sodium aluminosilicate hydrogels, which were formed by mixing, aging and heating of sodium silicate solution with sodium aluminate solution under stirring, in order to produce NaA-zeolite crystal. PREPARATION OF SODIUM ALUMINOSILICATE HYDROGELS Raw mater i a1 s The chemical analyses of raw materials, namely sodium silicate solution and sodium aluminate solution, are as follows;
22 K. Usui. K. Abe, T. Tokita, S. Imafuku and M. Ogawa
Sodium s i l i c a t e s o l u t i o n Na20
7.82%
Si02
23.07%
Sp. G r .
Sodium a l u m i n a t e s o l u t i o n Na20 2'3 Sp. G r .
1.324
Mol r a t i o
22.81% 1.516
Mol r a t i o
Si02/Na20
Preparation
17.30%
3.04
of hydrogel
Na20/A1203
1.25
samples
0 3 The sodium s i l i c a t e s o l u t i o n was heated t o 50 C, and 350cm o f t h e s o l u t i o n
was p u t i n t o a beaker o f ca. Idm i n a water-bath. Under s t i r r i n g w i t h a s t i r r e r 3 made o f s t a i n l e s s s t e e l , ca. 260cm o f t h e sodium aluminate s o l u t i o n , which was 0
heated t o 50 C p r e v i o u s l y , was added t o t h e s i l i c a t e s o l u t i o n .
After mixing f o r
5 minutes, an a p p r o p r i a t e volume o f t h e m i x t u r e was taken o u t and f i l t e r e d immediately b y Buchner-funnel under vacuum. 0
The cake was washed w i t h about two
0
volumes o f h o t water (40 -50 C) t o t h e volume o f t h e m i x t u r e on t h e Buchner funnel.
The wet cake o r hydrogel was sample A.
The r e s t o f t h e m i x t u r e was
0
aged f o r 3 hours a t 50 C, a f t e r aging s i m i l a r l y an a p p r o p r i a t e volume o f t h e m i x t u r e was taken o u t and f i l t e r e d . as sample A.
The f i l t e r cake was washed b y t h e same way
The wet cake was sample B.
The r e m a i n i n g m i x t u r e was heated t o
0
90 C i n two hours.
A l s o a f t e r t h e heating, an a p p r o p r i a t e volume o f t h e m i x t u r e
was taken out, and sample D was prepared b y t h e same procedure as samples A and 0
Holding t h e temperature a t 90 C f o r one hour, t h e m i x t u r e o f t h e hydrogel
B.
was changed t o c r y s t a l l i n e NaA z e o l i t e , i.e.
sample D-1.
Furthermore conversely, t h e heated sodium s i l i c a t e s o l u t i o n was added i n t o t h e heated sodium aluminate s o l u t i o n .
A f t e r m i x i n g f o r 5 minutes, corresponding
t o sample A, t h e hydrogel sample was prepared as sample C. Each hydrogel sample was s t o r e d i n an a i r - t i g h t vessel under r e f r i g e r a t i o n 0
( 7 C ) u n t i l i t was dispatched t o t h e Toray Research Center f o r t h e measurement o f RDF. The c o n d i t i o n s f o r t h e s y n t h e s i s o f NaA z e o l i t e were as f o l l o w s ; Molar r a t i o f o r t h e r e a c t i o n was Na20/Si02 1.20, H20/Na20
Si02/A1203 2.00 and
40.00.
RESULTS Samples A, B, D and D - I were observed under a scanning microscope (SEM). SEM photographs o f t h e s e samples a r e shown i n F i g s . 1 through 4. The chemical analyses o f t h e hydrogels and c r y s t a l sample are shown i n Table 1.
The
Application of RDF to Synthesis of Zeolite 23
Chemical analyses (%) of t h e h y d r o g e l s and c r y s t a l
Table 1.
Sample A Na20
15.77
Si02 2'3 H20(+) H20(-) IIO'CX~~
Si02/A1203
*
Sample B
*
Sample D
16.46
16.12
Sample D - I
Sample C
17.54
15.68
44.76
42.22
41.77
36.09
43.91
27.55
29.18
28.73
30.68
28.87
11.92 68.98(1)
12.14 65.87(1)
13.38 66.80
13.92
11.54 74.55
65.31(2)
66.00(2)
2.757
2.455
2.467
-
1.996
2.581
Molar r a t i o
F i r s t , i n o r d e r t o i n v e s t i g a t e changes i n hydrogel samples d u r i n g RDF measurement over a l o n g p e r i o d o f ca. 5 hours, samples A and B were measured f o r RDF t w i c e s e q u e n t i a l l y ; t h e moisture, H20(-), was a l s o determined t w i c e , i .e.
b e f o r e RDF and a f t e r RDF, t h e former i s s u f f i x e d as ( 1 ) and t h e l a t t e r i s s u f f i x e d as ( 2 ) .
The m o i s t u r e ( % ) o f samples A and B are shown i n Table 1; no
s i g n i f i c a n t changes a r e recognized.
Also no s i g n i f i c a n t d i f f e r e n c e i n RDF i s
shown among these samples ( 1 ) s and (2)s, as shown i n F i g s . 5 and 6. The m o l a r r a t i o o f Si02/A1203 o f hydrogels and c r y s t a l are i n reasonable agreement with p r e v i o u s works
(3, (3)
and (4).
The r a d i a l - d i s t r i b u t i o n f u n c t i o n curves o f a l l samples a r e shown i n F i g s . 5 through 10. other.
The RDF p a t t e r n s o f samples A, B, C and D a r e v e r y s i m i l a r t o each
Every hydrogel shows a s i g n o f t h e Si(A1)-0 bond a t t h e r a d i u s o f 1.6-
1.7A as a t r a c e o f a shoulder i n Figs. 5 through 10. Since t h e main peaks a r e 3.25A,
4.2-4.6A
and 7.OA,
t h e most reasonable u n i t
s t r u c t u r e o f t h e sodium a l u m i n o s i l i c a t e hydrogel i s t h e 4-member c h a i n i l l u s t r a t e d i n F i g . 11. shown i n F i g . 12.
Many 4-member chains e a s i l y f o r m s o d a l i t e cages as
I n t h e case o f a s i m p l e r s i l i c a s t r u c t u r e , t h e double 4-
member r i n g (cube) was recognized b y Sakka e t a1 (5). I t i s e n t i r e l y unexpected t h a t even a f t e r h e a t i n g , i n o t h e r words o n l y one
hour b e f o r e c r y s a l l i z a t i o n , no s i g n o f c r y s t a l l i z a t i o n appeared.
I n our n e x t
s t u d y we would l i k e t o f i n d t h e m i s s i n g l i n k between t h e hydrogel and t h e crystal.
24 K. Usui, K. Abe. T. Tokita, S. Irnafuku and M. Ogawa
SEM p h o t o g r a p h s o f sample A
Fig.1.
Fig.
2.
SEM p h o t o g r a p h s o f sample
B
Application of RDF to Synthesis of Zeolite 25
F i g . 3.
SEM photographs o f sample D
F i g . 4.
SEM photographs of
sample D - I
26
K.Usui, K. Abe, T. Tokita, S. Imafuku and M. Ogawa
Z
0
H
I-
u
Z 3
Fig.
5
RDF
of A ( I ) / A ( 2 )
LL Z 0 n I3
m
H
a
I-
m
H
0 _I
6 H
0
a a
z
0 I 1
t-
u
z
3 LL 2 0 I-i
+ 3
RDF o f B ( 1 ) a n d B(2)
Fig.
6
m
H
a
t-
Ln 1-4
0
-I
a 1-1
0
a
Z 0 H
t-
u
z
3 LL
z
0 H
t-
RDF o f A ( 1 ) ,B( 1 ) m i Fig.
7
(0 t o 10A)
3
m
H
U I-
m t-I
0
A
Q
H
0 Q
a
Application of RDF to Synthesis of Zeolite 27
z
0
6 00,
b-I
I
Cl 2
3
4.001
!I.
z
n
I-I
2.001
1-
8
RDF o f
2 1-1
,B l)'D-l t o 20A)
0 POI
CC I 1
0
-2 001
,I
-
- 4 000
4 000
.
I ow
J
.
I 2 wo
I
I I6
z
0
I--I
I-
9
RDF o f
$ I4
sample C
CII I-
cn 1-1
0
z
0 H
t-
u Z
3
LL
Z 0 H
Fig.10
RDF o f
s amp e D
t
2 I4
oc in L1-1
0 J
<
CI
0 -=I U
I
0
wo
i
28 K. Usui, K. Abe, T.Tokita. S. Imafuku and M. Ogawa
F i g . 11
Scheme o f 4-membered c h a i n
F i g . 12 S o d a l i t e cage formed by t h e 4-membered c h a i n
CONCLUSION
Every sodium a l u m i n o s i l i c a t e hydrogel formed a f t e r mixing, aging and even a f t e r h e a t i n g o f aluminate i n t o s i l i c a t e s o l u t i o n o r v i c e versa has a s i m i l a r u n i t s t r u c t u r e c o m p r i s i n g a four-membered c h a i n i l l u s t r a t e d i n F i g . 11. hydrogel showed s i g n o f a S i ( A 1 ) - 0 bond a t t h e r a d i u s o f 1.6-1.7A,
Each
as a t r a c e of
a shoulder i n F i g s . 5 through 10. REFERENCES 1. K. Abe, S. Kojima, M. Ogawa, K. Usui and T. Nakazawa, The a p p l i c a t i o n o f r a d i a l d i s t r i b u t i o n f u n c t i o n t o v a r i o u s z e o l i t e s and dealuminated p r o d u c t s therefrom, Recent Research Report, ZEOLITES FOR THE N I N E T I E S , 8 t h I n t . Z e o l i t e Conf., Amsterdam, J u l y 9-14, 1989, p.231/2 2. M. Ogawa, K. Abe, K. Usui and T. Nakazawa, Nippon Kagaku K a i s h i , 1989 (1989) 358/63 3. C. L. Angel1 and W. H. Flank, M o l e c u l a r Sieves-11, Am. Chem. SOC. (1977) p.194 4. D. W. Breck, Z e o l i t e , John W i l e y & Sons I n c . (1974) p.337 5. I. Hasegawa and S. Sakka, S i l i c a t e Species w i t h Cagelike S t r u c t u r e i n S o l u t i o n s and Rapid S o l i d i f i c a t i o n w i t h Organic Q u a t e r n a r y Ammonium Ions, Z e o l i t e Synthesis (ACS Symposium S e r i e s 398) Am. Chem. SOC., Washington, DC 1989, p.140/51
Synthesis of Zeolite ZSM-48 with Different Organic and Inorganic Cations
G. Giordanol , Z. Gabelica, N. Dewaele, J. B.Nagy and E.G. Derouane Laboratory of Catalysis, Facultbs Universitaires N.D. de la Paix, rue de Bruxelles, 61, 8-5000 NAMUR (Belgium) 1 Dipartimento di Chimica, Universita della Calabria, 1-87030 RENDE (CS), (Italy).
ABSTRACT The specific influence of various organic and inorganic cationic species on the crystallization rate of zeolite ZSM-48 and on its final (meta)stability in the hydrogel medium, has been examined. Three series of hydrogel systems involving (i) various diamines, Na+ and Al at various concentrations, (ii) Na+ and TMA+ cations admixed either with propylamine or octylamine, in presence and in absence of A1 and (iii) hexamethonium ( H M + + ) ions in presence or in absence of Na+ and Al, have been systematically studied. C8 - monoamines and - diamines achieve a good pore volume filling of ZSM-48 and therefore accelerate its crystallization. If TMA+ is present in the hydrogel, ZSM-48 is formed only when an alkylamine is added, otherwise the system yields pure ZSM-39. In systems involving HM++ ions, fairly well crystalline ZSM-48 is formed provided the Na+ and Al initial concentrations are adequatly adjusted. Higher Al contents favour the formation of Al-richer EU-1 framework that is also well stabilized by HM++ ions. INTRODUCTION High-silica ZSM-48 zeolite having 10 membered ring channels and medium pore size can be synthesized in presence of many organic and inorganic cations. Previous patent and paper literatures have reported the synthesis of ZSM-48 zeolite in presence of propylamine or octylamine with tetramethylammonium ions [l-31, diamines [4, 51 and hexamethonium ions [6-81. Among inorganic cations, Na+, Li+, K+ and NH4+ have been used in presence or in absence of aluminium [7, 81. The role played by the organic cations as structure directing agents [9] or as stabilizing pore fillers [ l ,
30 G. Giordano. Z. Gabelica, N. Dewaele, J. B. Nagy and E. G. Derouane
101 is very important in the zeolite synthesis. In many cases, they were shown to compete with alkali ions for the stabilization of the negatively charged framework [9, 111. On the other hand, inorganic cations influence the zeolite nucleation process as structure directors [ l l , 121 or as stabilizing mineralizers during the growth process [12-141, thereby influencing the final size, the morphology of the crystals as well as the crystallization yield. They also act as counterions to the aluminosilicate framework negative charge. The present study consists in examining critically the specific influence of the various organic and inorganic cationic species on the formation and crystallization of ZSM-48 zeolite. Three different reaction systems leading to the formation of ZSM-48 zeolite have been investigated. The first series involved Na+, diamines and Al, the second involved Na+, T M A + and/or alkylamines and Al, and the last series involved Na+, hexamethonium ions and Al. More particulary, for each organic admixture, we have examined the influence of Na+ and/or Al concentrations on the ZSM-48 (meta)stability and on the nature of the final crystalline product that possibly co-crystallize under these conditions. EXPERIMENTAL Several series of hydrogel having the following molar composition have been prepared:
Type 1 system:
x Na20 20 RN y A1203 60 Si02 3000 H20
where RN stands for 1,6 diaminohexane (C6H16N2) or 1,8 diaminooctane (C8H20N2), x=O or 15 and 01 y 11; Type 2 system: 15 Na20 x TMAX y RN z A1203 10.8 H2S04 60 Si02 3258 H20 where TMA stands for tetramethylammonium and X =CI or Br; x is equal to 0 or 15.6 or 33; RN stands for n-propylamine (PrNH2) or n-octylamine (OcNH2) and y is equal to 81.6 for the system with PrNH2 and to 80.4 for OcNH2 and, finally, 01 z 11.02; and Type 3 system: x Na20 y HMBr2 z A1203 60 Si02 3000 H20 where HMBr2 stands for hexamethonium bromide and 01 x 110, 0 . 5 1 ~15, and
01 2 12. Typically, type 1 hydrogel was prepared by mixing the appropriate
Synthesis of Zeolite ZSM-48
31
amounts of the following commercial ingredients: fumed silica (Aerosil Serva), aluminium hydroxide (Serva), sodium hydroxide (Riedel de Haen), 1,6 diaminohexane (Janssen) or 1,8 diaminooctane (Fluka) and distilled water. After homogenization, the gel was transferred into a 60 ml Teflon-lined Morey-type autoclave, and heated at 180+2 OC, under autogeneous pressure, in stirred conditions. The synthesis procedures of type 2 and 3 were described previously [l, 81. The identification of the solid phases and the evaluation of their crystallinity were performed by X-ray powder diffraction while the pore volume of the ZSM-48 materials was evaluated by isothermal (90 "C) sorption of n-hexane, followed in the thermobalance (Stanton Redcroft ST780 cornbinled TG-DTA-DTG thermoanalyser). RESULTS AND DISCUSSION 1) Svste ms with diamine3 Type 1 systems involving Na+ ions and diaminohexane and/or diarninooctane have been left to crystallize without aluminium, in order to better define the role of Na+ ions. Table 1 shows the nature of the crystalline products obtained in the Al-free system. The presence of sodium favours the formation of dense phases, such as a-quartz, that was shown to be more stable than ZSM-48 [7]. In a second series of experiments of type 1 , the influence of the nature of the diamine on the rate of ZSM-48 crystallization has been examined (Table 2). Compared with the hydrogel involving 1,6 diaminohexane, ZSM-48 crystallizes more rapidly when 1,8 diaminooctane is present in the hydrogel. Probably the lenght of the diaminooctane chain is better accornodated into the channels of ZSM-48 zeolite as to achieve a more complete pore volume filling. Indeed, the channel length per unit cell of Table 1. Nature and crystallinity of the products obtained from the system: x Na20 20 RN y A1203 60 Si02 3000 H20. RN
x (mole)
Nature of the products
0
ZSM-48
15
a-quartz
0
ZSM-48
15
a-auartz
(C6H16N2)
(CsH20N2)
32 G. Giordano, Z. Gabelica, N. Dewaele, J. B. Nagy and E. G. Derouane
Table 2. Nature and crystallinity of ZSM-48 obtained in presence of various diamines, as a function of heating time. RN
heating time at 180 "C (days) 2
C ry st al Iin ity
(YO)
19
(C6H16N2)
(C8H20N2)
6
97
2 6 21
66 100 25 + cristobalite
ZSM-48 is 16.8 A while the length of 1,6 diaminohexane and 1,8 diaminooctane is 11.51 and 14.02 A respectively. The zeolite therefore prefers diaminooctane as pore filler. Figure 1 shows the crystallization kinetics of ZSM-48. A good agreement is shown between the crystallinity evaluated by X-ray and adsorption of n-hexane. These kinetic curves confirm the metastability of ZSM-48 zeolite. Indeed the conversion of ZSM-48 into cristobalite, a dense and stable phase, occurs for long reaction times. The difference between the two curves at start reaction times is due to the presence of hydrated silica (Aerosil) that also adsorbs n-hexane.
0
,e/
2
6
.
10 14 Time (days)
18
Figure 1. Crystallization kinetics of ZSM-48 synthesized from the system: 20 C8H20N2 60 Si02 3000 H20 0 = % of ZSM-48 evaluated by X-ray A = % of ZSM-48 evaluated by n-hexane adsorption A = Yo of crystallinity of cristobalite
Synthesis of Zeolite ZSM-48
33
Table 3. Nature and crystallinity of the products obtained from the system: 20 C8H20N2 x A1203 60 Si02 3000 H 2 0 x (mole)
Synthesis time (days)
Nat ure [crystalIi nity] of the products
0
3
ZSM-48 [ l OO%]
5
ZSM-48 [lOYO]+ amorphous
10
ZSM-48 [30%] + KZ-2
5
amorphous
15
ZSM-5/ZSM-11 [20°/0]
5
amorphous
11
amorphous
0.15
0.5
1 .o
+ amor.
The nature of crystalline products obtained in presence of aluminium is reported in Table 3. A complete crystallization of ZSM-48 zeolite is rapidly obtained in Al-free systems (Fig. 1). Traces of Al in hydrogel drastically decrease the crystallization rate and for x=0.15 (A1203/Si02 = 0.0025), zeolite KZ-2 co-crystallizes with ZSM-48. Zeolite KZ-2 is a member of TON family, with a framework similar to that of ZSM-48 [15]. This material can effectively be synthesized in presence of diamines, under similar conditions [16]. A further addition of Al results in the formation of a pentasil phase, namely an intergrowth of ZSM-5 and ZSM-11 zeolites, in agreement with a previous work [17]. In this case it is not possible to estimate the percentage of each pentasil phase in the intergrowth because of their low crystallinity. Very high A l 2 0 3 / S i 0 2 ratios only yield amorphous products, even for long reaction times. It can be observed that the diamines do stabilize the zeolitic channels and they are incorporated intact in the ZSM-48 framework, as ascertained by 13C-NMR data [18].
2! Systems with TMA and aIkylamines, Table 4 summarizes the experimental conditions and the nature of the solid phases obtained from the system involving Na+ and TMA+ ions, npropylamine or n-octylamine in presence or in absence of Al. The source of Si and Al only influences the crystallization kinetics but not the nature of
34 G. Giordano, Z. Gabelica, N. Dewaele, J. B. Nagy and E. G. Derouane
the crystalline products (Table 4, samples 5 and 6). On the other hand, the synthesis procedure strongly influences the nature of the crystalline products. Indeed a different procedure favours the formation of 100% crystalline ZSM-39 when a hydrogel containing 33 mole TMACI, 81.6 mole P r N H 2 and 0.15 mole Al is used [ l ] . For the crystallization of ZSM-48 zeolite the presence of an alkylamine is necessary, since in presence of TMA+ ions only the reaction leads to the formation of ZSM-39. ZSM-48 formed after heating for 21 days a gel that only contains TMACI but no propylamine (sample 3, Table 4) shows a lower crystallinity (45%) than the ZSM-48 synthesized under equivalent conditions from a gel containing both TMACI and PrNH2 as organics, which is 100% crystalline (sample 2). It can be assumed that the low crystallinity results from a poor pore filling by PrNH2. Indeed, it was shown [ l ] that a complete pore filling in ZSM-48 can be adequately achieved when Na+, TMA+ ions are present together with PrNH2 in the initial gel. In the same gel containing octylamine, ZSM-48 also rapidly crystallizes in presence of small amount of A1 (Table 4, samples 4 and 5). However, for larger amounts of At, a poorly crystalline ZSM-48 is obtained after 7 days heating, which for longer crystallization times, completely transforms into ZSM-22, another zeolite involving a linear channel structure (sample 6, Table 4). This latter is probably formed upon a
Table 4. Nature and crystallinity of the products obtained from the system: 15 Na20 x TMAX y RN z A1203 10.8 H2S04 60 Si02 3258 H20
X
Y
Z
Sy nt he s is Nat ure [cryst a IIinity ] time (days) of the products
1 2 3
33 33 0
0 81.6 81.6
0 0 0
21 21 21
ZSM-39 [95%] ZSM-48 [lOOoA] ZSM-48 [45'/0]
4 5
15.6 80.4 15.6 80.4
0.48 0.48
6
15.6 80.4
1.2
29 7 15 7 15
ZSM-48 ZSM-48 ZSM-48 ZSM-48 ZSM-22
Sample
(mole)
[85%] [85%] [94%] [49%]
RN=PrNH2 and X=CI for samples 1,2 and 3; and RN=OcNH2 and X-Br for samples 4,5 and 6. In the samples 5 and 6 the source of Si and Al is different and respectively Silica Aerosil (Serva) and Aluminium hydroxide (Serva)
Synthesis of Zeolite ZSM-48
35
redissolution of the pre-formed ZSM-48 in the Al-free gel (amorphous phase) still present after 7 days heating.
3) Svstems involvina hexamethonium ions Table 5 reports the experimental conditions and the nature of the crystalline products obtained from gels containing HMBr2. A fairly well crystalline ZSM-48 is obtained when adequate amounts of HMBr2 and Na20 are chosen (sample 1 and 2). The optimum content of HM++ ions is 2.5 mole per 60 moles of Si02 [8]. For smaller concentrations of HM++ (e.9. 0.5 mole of HM++ per 60 moles of Si02 - sample 4), ZSM-5 is the only crystalline phase detectable. Its crystallization is actually expected to occur in a high silica hydrogel containing both Al and Na+ ions, the latter being known to initiate the formation of 5-1 SBU [19, 201. Indeed, for low N a 2 0 concentrations, even in presence of larger amounts of HM++ ions, only amorphous phase is detected (sample 5). In contrast, high N a 2 0 concentrations initiate the formation of dense phases such as cristobalite or a-quartz that contaminate ZSM-48 (sample 7), as already observed in the case of diamines (Table 1). Finally, similarly to the other reaction systems, Al-rich hydrogel lead to the formation of different zeolites. In our case, zeolite EU-1 is formed. Note that dense silica polymorphs or silica-rich zeolites (ZSM-5, KZ-2, ZSM-22) are formed at the expense of metastable ZSM-48 for long reactions times in monoamine and diamine bearing systems (see above), indicating that excess of Al in the initial hydrogel does not play a particular role in the formation of these phases. In contrast, a higher Al content in the Table 5. Nature and crystalline of the products obtained from the system: x Na20 y HMBr2 z A1203 60 Si02 3000 H20 mole
Sample X
8
Y
Z
5
5
0
5 5 5 1 2.5 10
5 0.5 0.75 0.5
5
Synthesis time (h)
0.5
0.5
5 5 5
0.5 0.5
0.5
48 66 66 66 120 93 115
5
2
144
Nature [cry sta I I inity] of the products ZSM-48 [97Yo] ZSM-48 [81Yo] ZSM-48 [91%]+ cristob. ZSM-5 [l O%]+amorph. amorphous ZSM-48 [48Yo]+amorph. ZSM-48 [76Y0]+c risto b . + a-quartz EU-1 [l 5%]+amorphous
36 G. Giordano, Z. Gabelica, N. Dewaele, J. B. Nagy and E. G. Derouane
hydrogel is a predominat factor favouring the formation of differently arranged Al-richer EU-1 framework, provided HM++ ions are present. The stabilizing role of HM++ ions as counterions and pore fillers in the case of EU-1 is discussed elsewhere [8]. REFERENCES N. Dewaele, Z. Gabelica, P. Bodart, J. B.Nagy, G. Giordano and E.G. 1 Derouane, Stud. Surf. Sci. Catal., 37 (1988) 65. 2 P. Chu, U.S. Pat. 4,397,827 (1983). M.K. Rubin, E.J. Rosinski and C.J. Planck, U.S. Pat. 4,086,186 3 ( 1 978). L.D. Rollmann and E.W. Valyocsik, U.S. Pat. 4,423,021 (1983). 4 A. Araya and B.M. Lowe, J. Catal., 85 (1984) 135. 5 J.L. Casci, B.M. Lowe and T.V. Whittam, Brit. Pat. Applic. 2,077,709 6 (1981). 7 G.W. Dodwell, R.P. Denkewicz and L.B. Sand, Zeolites, 5 (1985) 153. 8 G. Giordano, J. B.Nagy, E.G. Derouane, N. Dewaele and Z. Gabelica, in M.L. Ocelli and H.E. Robson (Eds.), Zeolite Synthesis (ACS Symposium Series 398), Am. Chem. SOC.,Washington DC, 1989, p. 587. 9 see e.g. B.M. Lok, T.R. Cannan and C.A. Messina, Zeolites, 3 (1983) 252. 1 0 C. Pellegrino, R. Aiello and Z. Gabelica, in M.L. Occelli and H.E. Robson (Eds.), Zeolite Synthesis (ACS Symposium Series 398), Am. Chem. SOC., Washington DC, 1989, p. 161. 1 1 Z. Gabelica, N. Blom and E.G. Derouane, Appl. Catal. 5 (1983) 227. 1 2 A. Nastro, Z. Gabelica, P. Bodart and J. B.Nagy, Stud. Surf. Sci. Catal. 19 (1984) 131. 1 3 Z. Gabelica, E.G. Derouane and N. Blom, in T.E. Jr. White, A. Della Betta, E.G. Derouane and R.T.K. Beker (Eds), Catalytic Materials: Relationship between Structure and Reactivity (ACS Symposium Series 248), Am. Chem. SOC., Washington DC, 1984, p. 219. 1 4 A. Nastro and L.B. Sand, Zeolites, 3 (1983) 57. 1 5 P.A. Jacobs and J.A. Martens, Synthesis of High-Silica Aluminosilicate Zeolites (Stud. Surf. Sci. Catal., 33), Elsevier, Amsterdam,l987. 1 6 L.M. Parker and D.M. Bibby, Zeolites, 3 (1983) 8. 1 7 E. Moretti, S. Contessa and M. Padovan, La Chimica e L'lndustria 67 (1985) 21 and references cited therein. 1 8 G. Giordano, N. Dewaele, Z. Gabelica, J. B.Nagy and E.G. Derouane, in preparation. 1 9 A. Nastro, C. Colella and R. Aiello, Stud. Surf. Sci. Catal. 24 (1985) 39. 2 0 G. Bellussi, G. Perego, A. Carati, U. Cornaro and V. Fattore, Stud. Surf. Sci. Catal. 37 (1988) 37.
37
Synthesis and Characterization of Zeolites
W.
Inaoka, S. Kasahara. T. Fukushima and K.
Igawa
Chemical Research L a b o r a t o r y , Tosoh C o r p o r a t i o n , 4560 Tonda, Shinnanyo. Yamaguchi 746, Japan ABSTRACT W i t h o u t u s i n g o r g a n i c templates, h i g h s i l i c a z e o l i t e s , ZSM-5, mordenite. f e r r i e r i t e , z e o l i t e L, z e o l i t e o f f r e t i t e / e r i o n i t e , and z e o l i t e Y, have been s y n t h e s i z e d as a s i n g l e phase from t h e r e a c t a n t m i x t u r e s i n Na20-K20-A1 O3 -Si02-H20 system. The c o m p o s i t i o n range o f t h e r e a c t a n t m i x t u r e s s u i t a b l e f o r t h e c r y s t a l l i z a t i o n o f each z e o l i t e spec es was c l a r i f i e d . The s t i r r i n g c o n d i t i o n d i r i n g c r y s t a l l i z a t i o n was c r i t i c a l f o r o b t a i n i n g p u r e z e o l i t e . INTRODUCTION We d e f i n e z e o l i t e s w i t h Si02/A1203 mo a r r a t i o > 5 as h i g h s i l i c a z e o l i t e s . These,
t y p i c a l l y e x e m p l i f i e d by ZSM-5,
catalysts.
p l a y an i m p o r t a n t r o l e as i n d u s t r i a l
S t u d i e s t o a p p l y them as hydrophobic adsorbents
have a l s o been
conducted. From a commercial p o i n t o f view,
i t i s important t o f i n d the conditions f o r
s y n t h e s i z i n g h i g h s i l i c a z e o l i t e s w i t h o u t u s i n g any o r g a n i c templates. a l s o i m p o r t a n t t o s t u d y t h e z e o l i t e f o r m a t i o n mechanism i n v o l v e d [1.2].
It i s For
t h e p r e p a r a t i o n o f z e o l i t e c a t a l y s t s , d e a l u m i n a t i o n i s t h e main t e c h n i q u e . New pores w i t h a d i a m e t e r o f 40-200
8
have been induced i n d e a l u m i n a t i n g z e o l i t e Y
by a h y d r o t h e r m a l t r e a t m e n t . The p r e s e n t paper d e s c r i b e s s y n t h e s i s s t u d i e s f o r t h e h i g h s i l i c a z e o l i t e s w i t h o u t u s i n g o r g a n i c t e m p l a t e s as w e l l as t h e i r characterization. EXPERIMENTAL
SYNTHESIS For each s y n t h e s i s , a r e a c t a n t m i x t u r e was p l a c e d i n a s t a i n l e s s a u t o c l a v e w i t h an a g i t a t o r . 90'-200°C Zeolite
for was
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 i n a t e m p e r a t u r e range
20-72 h. identified
The s o l i d p r o d u c t was f i l t e r e d , by
powder X-ray
washed and d r i e d .
d i f f r a c t i o n measurement.
a n a l y s i s was conducted by atomic a d s o r p t i o n s p e c t r o m e t r y .
Chemical
38 W. Inaoka, S. Kasahara, T. Fukushima and K. Igawa
DEALUMINATION NaY of Si02/A1203 = 5.6 was dealuminated by ion exchanges i n NH4N03 solution, calcination and HC1 treatment. The final Si02/A1203 ratio was 680 by chemical analysis. The pore size distribution was measured by N2 adsorption, Hg porosimetry and electron microscopy. The details have been described previously [ 3 ] . RESULTS AND DISCUSSION High silica zeolites, ZSM-5, mordenite, ferrierite, zeolite L, zeolite offretite/erionite, and zeolite Y, could be crystallized as a single phase. Clinoptilolite and dachiardite, which have almost the same composition as natural mordenite, were not crystallized. Zeolite L and zeolite offretite/erionite were crystallized not in Na' but in a bialkali Na+-Kt system. Fig.1 is a triangular diagram of the Na20-A1203-Si02 system which shows the reactant composition range for each zeolite species as a single phase. The composition of mother liquor of zeolite slurry was found to be almost Na20.3SiO2.nH20. The vertical line shows the Si02/A1203 ratio of zeolite. Therefore, it is possible to calculate the Si02/A1203 ratio of zeolite crystallized from the reactant composition, that is, the value on the vertical line where the vertical line and the line passing through the reactant composition and Si02/Na20 = 75/25 intersect. The maximum Si02/A1203 ratio of zeolite was 50 and the solid product with Si02/A1203 > 50 was crystalline silicates such as kenyaite and quartz. The starting material, especially the silica source, and the stirring condition during crystallization were found to affect not only the rate of crystallization but also the crystallization area of high silica zeolites, while water content in a reactant mixture was found not to be critical. ZSM-5 ZSM-5 was the most siliceous zeolite synthesized in this experiment and its Si02/A1203 ratio could be varied widely from 20 to 50 by changing the reactant composition, mainly the Si02/A1203 ratio. ZSM-5 was crystallized under stirring condition not under static condition. Typical synthesis conditions for ZSM-5 were as follows: the composition of reactant mixture is 6Na20'A1203'50Si02'1250H20 and the crystal 1 ization occurred at 165OC for 72 h under stirring with peripheral speed of 1 m/s. The apparent activation energy for synthesizing ZSM-5 with SiO2/Al203 = 25 was 16.9 kcal/mol for nucleation and 21.9 kcal/mol for crystallizatioh, compared with 25.6 kcal/mol and 19.4 kcal/mol synthesized by using tetrapropylammonium (TPA) ion, respectively [ 4 ] .
Synthesis and Characterization of Zeolites 39
F i g . 1. z e o l it e s .
Composition diagram f o r t h e r e a c t a n t f o r s y n t h e s i s o f h i g h s i l i c a
SiO2
A
7
VAIratio of
20
\
coexisting phase: quartz, sodium polysilicate, Z S M - ~
coexisting phase: analcime
Na2O
__t
F i g . 2. Composition diagram f o r t h e r e a c t a n t and s u i t a b l e t e m p e r a t u r e f o r s y n t h e s i s o f h i g h s i l i c a mordenite.
40 W. Inaoka, S. Kasahara, T. Fukushima and K. Igawa
The difference in activation energy for nucleation suggests that there are many more nuclei created in the Na' system than i n the TPA' system. MORDENITE Mordenite with Si02/A1203 ratio of 10-20 was synthesized and crystallization conditions were studied in detail [ 5 ] . Fig. 2 is a triangular diagram showing reaction conditions and products. The circled area designates the region in which pure mordenite is obtained, mordenite of a given composition is produced along the dashed line, and figures on the dashed line give Si02/A1203 ratios. Temperature affects not only the rate of crystal1 ization but also the crystallization area of high silica mordenite. Mordenite was crystallized irrespective of stirring and static condition. The crystal size was controlled from 0.05 to 3 um in diameter by changing stirring condition and temperature.
FERRIERITE Ferrierite with but under stirring ferrierite is very be extended by the the 'K utilization Na' ion.
Si02/A1203 ratio of 12-20 was crystallized not under static condition. The region of reactant composition suitable for limited in the Na20-A1203-Si02 system (Fig. 1). but it can copresence of 'K ion in the reactant mixture. Table 1 shows factor and ferrierite has a higher affinity for K+ than for
Table 1. Reactant and ferrierite composition reactant mixture ferrierite 'K utilization Si02/A1203 Na/A1 K/A1 % Si02/A1203 Na/A1 K/A1 19.6 19.6 19.4 18.5
1.41 1.24 0.70 1.04
0.35 0.82 1.05 0.69
Synthesis conditions : H20/Si02
18.2 17.4 17.9 17.2 =
0.65
0.22 0.21 0.31
0.37 0.79 0.91 0.69
100 96 87 100
20, 18OoC for 72 h under stirring condition.
ZEOLITE L AND ZEOLITE OFFRETITE/ERIONITE 'K ion is necessary for the crystallization of pure zeolite L and of pure offretite/erionite (o/e), because their framework structure is based on the ratio in the reaccancrinite cage occupied by the 'K ion [ 6 ] . The )'KtaN(/ tant composition was 0.6-0.8 for obtaining pure zeolite L and 0.2-0.8 for zeolite o/e. Si02/A1203 ratio of zeolite L and of o/e was 5-7 and 6-10, respectively. The intergrowth of offretite and erionite in zeolite o/e was confirmed by electron diffraction patterns.
Synthesis and Characterization of Zeolites 41
ZEOLITE Y High s i l i c a z e o l i t e Y has been b e l i e v e d t o be a t y p i c a l z e o l i t e which can be c r y s t a l l i z e d o n l y under s t a t i c c o n d i t i o n w i t h seed c r y s t a l s o r by a g i n g t h e r e a c t a n t m i x t u r e a t low temperature [ 7 ] . process shown i n Fig.3, a l u m i n o s i l i c a t e gel,
However,
according t o t h e Tosoh
u s i n g c l e a r aqueous n u c l e i s o l u t i o n and homogeneous
h i g h s i l i c a z e o l i t e Y w i t h a Si02/A1203 r a t i o o f up t o
6.2 c o u l d be c r y s t a l l i z e d under s t i r r i n g c o n d i t i o n w i t h i n a s h o r t t i m e [ 8 ] . I t has been observed by TEM, NMR and a d s o r p t i o n measurements t h a t z e o l i t i c n u c l e i 0
w i t h a f a u j a s i t e s t r u c t u r e (50 A ) a r e formed i n c l e a r aqueous s o l u t i o n l e a d i n g a l u m i n o s i l i c a t e g e l t o h i g h s i l i c a z e o l i t e Y. hypothesis
that
zeolite
crystallization
These r e s u l t s s u p p o r t t h e
originates
from
liquid
phase
aluminosilicates. DEALUMINATION OF ZEOLITE Y
Dealumination i s an i m p o r t a n t process t o i m -
prove t h e thermal s t a b i l i t y and r e s i s t a n c e t o a c i d o f z e o l i t e . T h i s i s one o f t h e main t e c h n i q u e s f o r p r e p a r i n g z e o l i t e c a t a l y s t s (US-Y).
New p o r e s
(mesopores) have been i n t r o d u c e d d u r i n g hydrothermal t r e a t m e n t ( F i g . 4 ) . were d i r e c t l y confirmed b y e l e c t r o n microscopy.
which
The d e n s i t y o f mesopores de-
pended on t h e degree o f d e a l u m i n a t i o n and t h e s i z e d i s t r i b u t i o n o f mesopores 0
w i t h t h e i r maximum d e n s i t y a t about 100 A i n diameter being deduced from t h e images agree approximately w i t h those o b t a i n e d by Hg p o r o s i m e t r y measurement. Mesopores may be c r e a t e d from t h e m i g r a t i o n o f S i atoms from t h e framework o f z e o l i t e toward A1 vacancies, which
makes s t r u c t u r a l d e f e c t s . The i d e a i s sup-
p o r t e d by t h e f i n d i n g t h a t mesopores were n o t observed i n Y dealuminated w i t h SiC14.
REFERENCES 1 F. Y. Dai, M. Suzuki, H. Takahashi and Y. Saito. Proc. 7 t h I n t e r n . Z e o l i t e Conf., (1986)223. 2 V. P. S h i r a l k a r and A. C l e a r f i e l d , ZEOLITES, 9(1989)363. 3 H. H o r i k o s h i , S. Kasahara, T. Fukushima, K. I t a b a s h i , T. Okada, 0. Terasaki and D. Watanabe, Nippon Kagaku K a i s h i , No.3(1989)398. 4 A. Erdem and L. B. Sand, J. Catal., 60(1979)241. 5 K. I t a b a s h i , T. Fukushima and K. Igawa, ZEOLITES, 6(1986)30. 6 D. W. Breck, Z e o l i t e M o l e c u l a r Sieves, John Wiley and Sons, New York (1974)77. 7 H. K a c i r e k and H. Lechert, J. Phys. Chem., 79(1975)1589. 8 S. Kasahara, K. I t a b a s h i and K. Igawa, Proc. 7 t h I n t e r n . Z e o l i t e Conf., (1986)185.
42 W. Inaoka. S. Kasahara, T. Fukushima and K. Igawa
Iflltrstlo;.]
[Sodium hydroxide aq. 801.1 Sodium aluminate aq. 801.
I Granular sodium aluminoailicate gel Na~0,A1~0~,10Si0~,aH~0
70min. under atirrina
LOOC,
4
Aquaous nuclei solution
Sodium aluminosilicata gel slurry 0.86 (Na2O,Al2O3,lOSiO2,92H20)
0.14 (1 5.3Na~O,A1~0~,10SiO~,183H~O)
3Na~0,A1~0~,10S10~,105K~O
under stirring
L__[__1 Filtration, Washing
F i g . 3.
Flow diagram f o r s y n t h e s i s o f h i g h s i l i c a z e o l i t e Y.
0 Pore
dlsmeter
(nm)
Pore s i z e d i s t r i b u t i o n s deduced f r o m Nz a d s o r p t i o n measurements. F i g . 4. 0 : HY prepared f r o m NaY by t r a n s f o r m a t i o n 0 : NaY w i t h SiO2/A12O3 = 5.6, i n t o NH4Y, f o l l o w e d by steam a t 750 OC.
43
Synthesis and Characterisation of Ferrisilicate Zeolites
R. Kumar and P. Ratnasamy National Chemical Laboratory, Pune 411 008, India
ABSTRACT Ferrisilicate zeolites wherein iron ions replace silicon in the lattice framework have potential as catalyst in various conversion processes. During the past decade ferrisilicate analogs of sodalite, MFI, MEL, MTT, EUO, M'IW, FAU, BETA, MOR and LTL have been synthesised and characterised by various physicochemical techniques as well as catalytic reactions. After a review of the general synthesis procedures a list of criteria is presented to confirm the location of Fe in the zeolite framework. Examples are provided to illustrate the utility of the various characterisation techniques. INTRODUCTION The first isomorphous replacement in the zeolite framework was reported by Goldsmith in 1952 in the synthesis of a germanium containing thomsonite wherein Ge replaced Si in the lattice [l]. Later, Barrer et al. [2] reported a number of Ga- and Ge- bearing zeolites. In the past decade the isomorphous 3+ substitution of many tri-, tetra- and pentavalent cations (B%, Fe%, Ga , Ge4+, Ti4+ and P5+ ) in various zeolite frameworks has been reported. How does the isomorphous substitution of A13+ or Si3+ in the zeolite lattice by other ions affect their structural stability, acidity and catalytic performance ? The present paper deals with ferrisilicate analogs of various medium (10-ring) and large (12-ring) pore zeolites. The ionic radii of Si4+, A1% and Fe% are 0.039, 0.057 and 0.067 nm, respectively. In addition, Fe3+ can also undergo a change in its oxidation state thereby leading to a lowering of the stability of the crystal structure. The isomorphous substitution of Si by Fe in the lattice structure of ZSM-5 [3-61, ZSM-23 [7], sodalite [8], beta [9] and FAU [lo] has been reported. More recently, we have synthesised and characterised the ferrisilicate analogs of ZSM-11, EU-1, ZSM-12, L and mordenite. We describe herein the general procedures for synthesising and characterising the ferrisilicates.
44 R. Kurnar and P. Ratnasarny
EXPERIMENTAL Synthesis During the synthesis of ferrisilicates in aqueous systems, the following equilibria prevail : [Fe(H20>6I3+
+
Si(OH>4
[Fe(H~0)6]~~t H20
[FeSiO(OH)3Izt
Hyd t H30t
(1)
+ H30+
(2)
[Fe(OH>(H20>512t
The objective of the synthesis would be to maximise the first reaction (leading to ferrisilicates) and suppress the formation of hydroxides of iron by the latter reaction by operating at low pH, using aluminium free source of Si and adjusting the reaction conditions to maximise the concentration of monomeric/short chain silicate species. Usually the monomeric/short chain silicate species is added to an acidic solution containing the Fe3+ ions to form the ferrisilicate complex.
The organic
base, as a template, is added after the formation of the ferrisilicate gel. After adjusting pH to the desired value, the amorphous gel is converted into the crystalline zeolite by crystallisation in an autoclave at elevated temperatures. Ferric nitrate is the usual source of Fe. Tetraethylorthosilicate (TEOS) is a preferred source of Si even though sodium silicate, silica gel and silica sol can also be used. By way of illustration the synthesis of the ferrisilicate analog of ZSM-11 is given below : 40 g of
TEOS was added slowly to a solution containing 2.0
g Fe(N03)3.9H20,
30 g doubly distilled water and 6 . 2 g H2S04 under stirring. To the above mixture a solution of 7.6 g 1,8 diaminooctane in 40 g water was added. Finally, 4.3 g NaOH dissolved in 25 g water were added under vigorous stirring. The resulting white gel was stirred at 298 K for 1 h before transferring it into a stainless steel autoclave (200 ml capacity). The crystallisation was carried out statically at 433 K for 8 days. The as-synthesised zeolite was carefully calcined at 753 K (heating rate ZO/min) first in dry nitrogen for 8 h and then in air for an additional period of 8 h. The protonic form of the zeolite was obtained by repeated ion exchange with 1N aqueous solution of NH4C1 (tNH40H) (pH=7-8) at 343 K for 2 h, drying and calcining.
Characterisation The chemical analyses were done by a combination of wet chemical, atomic absorption (Hitachi 2-800) and ICP (JY-38 VHR) methods. The crystalline phase identification was carried out by XRD (Philips PW-1710 Cu Ka). The zeolites were further characterised by scanning electron microscopy (Cambridge, Stereoscan 400), thermal analysis (Netsch, Model STA 490), ESR
Synthesis and Characterisation of Ferrisilicates 45
(Bruker E-2000), MASNMR (Bruker MSL-300), FTIR (Nicolet 60SXB) and Mijssbauer spectroscopies, magnetic susceptibility (Cahn Ventron), adsorption (McBain balance) and catalytic measurements. The procedures have been fully described in our earlier publications [3,6,7,9,10,12-161. RESULTS AND DISCUSSION Synthesis and characterisation Ferrisilicate analogs of zeolites are of potential utility as monofunctional (acidic) or bifunctional catalysts. Analogous to A1%, the replacement of Si4+ by FeZI. generates Bronsted acidity [7,14]. The use of ferrisilicate zeolites as bifunctional catalysts is due to the fact that under severe steaming conditions part of the Fe can be removed from the lattice framework and dispersed as finely divided iron oxide particles within the zeolitic pore system [17]. In this case, the material can function both as an acidic catalyst (due to that part of Fe3+ still in the lattice framework) and also as a redox catalyst due to the presence of finely dispersed Fe203/Fe304 in its pore system [17]. Table i summarises the list of ferrisilicate zeolites that have been prepared to-date by direct synthesis in basic media. Ferrisilicate pentasil zeolites have also been synthesised hydrothermally in an acidic, fluoride-containing medium [ 18,191. Such samples, however, sometimes, exhibit lower Bransted acidity (than those prepared in basic media). This may be due to the simultaneous replacement of 02- by F- thereby eliminating the need for charge-balancing cations like Na+ or H+ [19]. When ferrisilicate analogs of known zeolites are made it is essential to establish the presence of Fe3+ in the lattice framework. Towards this objective we have used a variety of techniques each one of which gives specific structural/textural information about the sample's characteristics (Table 2). Table 3 lists physical properties of some of the synthetic ferrisilicates. Among the known ferrisilicate zeolites, (Fe)-ZSM-5 is the most documented [ 3-6,18,19,21-271. The material has been synthesised from both basic [ 31 and fluoride-containing media [ 191. Incorporation of Fe in lattice positions has been established by XRD [4], framework IR [4], Massbauer [25], ESR [ 3,271, and uv-vis [ 191 spectroscopies, ESCA [ 31, ion exchange [ 251, magnetic susceptibility [ 261 and catalytic activity [9,15] measurements. We have recently synthesised the ferrisilicate analog of pure ZSM-11 (free from ZSM-5) [ll]. The location of Fe3+ ions in the MEL lattice has been confirmed by all the above techniques. For example, the increase in the unit cell parameters of the MEL lattice on Fe incorporation is shown in Fig. 1.
46 R. Kumar and P. Ratnasamy
Table 1. Synthesis conditions for ferrisilicate analogs of zeolites. Zeolite
Source of Si
MFI MEL
NagSiO3
MTT
Silica NagSiOg -do-
EUO FAUa MORa
nos
9
TEOS
MTW
Na2SiOg TEOS Silica
BETAa
LTLb
Organic template
Temp.
(K) Triethylbutyl amm. bromide TBA-OH/1, 8-diaminooctane Pyrrolidine Hexamethonium dibromide (Seeds) TEA-Br Methyltriethyl amm. bromide TEA-OH
453 443
Time (d)
3 9 2
Ref.
6 11 7
453 443
4
12
373
1 4
4
10 13 11
12 5
9 11
443 443 393 443
aFe source was ferric sulphate; for other zeolites ferric nitrate was used. bThe Si/A1 in L and Y zeolites varied in the range 4-5 and 2.3-3 respectively. The other ferrisilicates were substantially free from Al. Table 2 .
Techniques for characterising ferrisilicate zeolites.
Techniques
Relevant information
1 Color 2 Chemical analysis 3 XRD
4 Electron microscopy/ EDAX 5 Adsorption 6 Thermal analysis
7 IR spectra
White color indicates the absence of bulk hydroxides/oxides of iron Fe and A1 content Crystallinity/phase purity; lattice expansion due to Fe incorporation Absence of amorphous matter outside the crystalline phase; distribution of Fe. Absence of amorphous matter within the pores of the zeolite Temperature of crystal collapse; pattern of evolution of organics
:
Hydroxyl bands Framework 8 ESR
9 Magnetic moment 10 Mdssbauer spectra 11 ESCA 12 Ion-exchange capacity 13 Acidity 14 Phosphorescence 15 UV-VIS spectra 16 29% MASNMR 17 Catalytic activity in acid catalysed reactions
Shift to higher wave-numbers of bridged hydroxyls (Fe-OH-Si) Shift to lower wave-number of sym. and asym. stretching frequencies (Fe-0-Si instead of Si-0-Si) Peak at g = 4.3 due to distorted Td Fe% Insensitivity of peak to reduction conditions Bohr magneton 5.6-6.0 IS = 0.3-0.4 mm/s at 4.2 K and IS = 0.2-0.3 mm/s at 298 K Absence of extra Ols peak due to Fe-oxides Quantitative criterion in TPD of ammonia [lS] Lower,,T 5000 A : F e k in Td;7000 A : Fek in [20] IODq = 7500-8500 cm-l(see ref.19, p.91) Shortening of spin-spin relaxation time [ 103 Quite useful, provided the ferrisilicates are A1 free
Synthesis and Characterisation of Ferrisilicates 47
Table 3.
P h y s i c a l p r o p e r t i e s of f e r r i s i l i c a t e analogs of z e o l i t e s
Property
MFI
MEL
MTT
ELI0
MTW
MOR
BETA
FAU
LTL
Si/Fe ESR, g value Mag. moment p BM, RT 97 K Mbssbauer IS m/s, RT 4.2 K Ion-exchange K+/Fe02, % Adsorptiona W t . %, water n-hexane Cyclohexane
36 4.3
35 4.3
58 4.4
18 4.4
65 4.3
09 4.3
17 4.4
17 4.3
10 4.4
5.8 5.6
5.8 5.6
5.9 5.5
5.7 5.5
5.8 5.7
5.8 5.6
0.24 0.34
0.22 0.32
28.0
18.9
0.25 0.35
0.26 0.33
79
82
86
76
9.8 11.0 5.1
8.5 11.3 5.2
5.8 8.5 3.5
9.4 11.0 5.7
75 7.0 7.5 6.0
7.2 4.1 4.8
23.6 18.0 18.7
.
aP/Po = 0.5 (except i n t h e case of LTL where PIPo was 0.8) T = 298 K
A 5
10
DELAY
5
(TI m
10
Sec.
Fig. 2. Spin-spin decay of 29Si s i g n a l s of (A):(Al)-FAU and (B): (Fe)-(A1)-FAU z e o l i t e s . Lines 1-4 r e f e r t o S i (3Al)-Si (OAl) signals respectively.
Fe 1 (Fe t Si 1 Fig. 1.
Unit c e l l parameters as a f u n c t i o n of Fe-content of (Fe)-ZSM-ll zeolites.
48 R. Kumar and P. Ratnasamy
(Fe)-ZSM-23 free from aluminium has been synthesised [7] and the presence of Fe in the lattice framework confirmed by spectroscopic (XRD, IR, ESR and XPS), DTA/TG, magnetic susceptibility, ion exchange and catalytic activity measurements. (Fe)-beta has also been similarly prepared and characterised [9]. Isomorphous substitution of Fe in the faujasite lattice has been demonstrated [lo] using various techniques including solid state MASNMR. NMR spin-echo experiments indicated that the spin-spin relaxation time of 29Si is shortened due to the presence of Fe in the FAU lattice framework
[lo].
In
this
experiment,
the
180'
pulse
refocuses
the
inhomogeneity effects contributing to the line broadening. Thereafter, any contribution to Si line-width due to susceptibility effects (arising from occluded Fe203 or Fe304, for example) will be refocused and the decay of the 29Si spin echoes will be determined only by spin-spin relaxation of Si nuclei. Fig. 2 presents the decay of the spin echo Si signal intensity for (A1)-FAU (Fig. 2A) and (Fe)-(A1)-FAU, the latter containing Fe and A1 ions in the framework (Fig. 2B). The T2 (spin-spin relaxation time) for the sample containing Fe was only 2.2 compared to 7.8 msec. for the Al-analog. This lower value comes from the Si-Fe nuclear-electron coupling and provides conclusive evidence that Fe is in the framework. It may be mentioned here that when Fe3+ ions are introduced by ion exchange (for Nat ), the resulting samples do not exhibit the shortening of T2 values. This is because Si-0-Fe "through bond interactions" are present when Fe is in the lattice and are absent when Fe is present only as a counter-ion outside the lattice framework. In all these zeolites, DTA studies revealed that the ferrisilicates have lower thermal stability compared to their Al-analogs.
Al-free ferrisilicate analogs of mordenite have recently been
synthesised using tetraethyl ammonium bromide [13] and the presence of Fe in lattice positions demonstrated by various techniques. Senderov et al. [26] have also reported the synthesis of mordenite containing both A1 and Fe. The synthesis, characterisation and catalytic properties of (Fe)-EU1 have also been described [12,29]. Fig. 3 shows a plot of reciprocal gram susceptibility against temperature for (Fe)-EU-1. The data could be fitted to the Curie-k'eiss Law and the Weiss temperature was close to 0 K indicating the absence of significant interaction between Fe9 ions. These results confirm the high dispersion of FeZC ions (probably in lattice positions) in these materials. 57Fe Mossbauer measurements on the as-synthesised samples of (Fe)-EU1 and (Fe)-beta zeolites at 298 K and 4.2 K and at 4.2 K with externally applied magnetic field (4.13T) are presented in Fig. 4 (curves A,B and C,
Synthesis and Characterisation of Ferrisilicates
49
(Fe)- EU- I 75
A
! AS -SYNTHESIZED
0
n
-0 x
CALCINED
50 -
25 -
0II -50
I
0
I
I
100
50
150
I
200
I 300
I
250
TEMP, K
FIG.3 ! R ECIPROCAL
GRAM
SUSCEPTI BlLlTY OF ( F e )
- E U- I
z
-
0 v)
G z
v)
z a
K
I-
w
> I-
a
-I W
0:
- 10
0
10 -10
0
m
ISOMER S H I F T ( 6 1 , m m I S
FIG.4-MOSSBAUER
SPECTRA OF ( F e ) EU-I AND (FePBETA ZEOLITES
50 R. Kumar and P. Ratnasamy
respectively). The values of the isomer shifts (0.26 and 0.22 at 298 K and 0.33 and 0.32 at 4.2 K, for (Fe)-EU-1 and (Fe)-beta, respectively) and quadrupole splitting (0.00 mm/sec) are indicative of tetrahedrally coordinated Fek species [ 301 having insignificant distortion in the local tetrahedral surroundings of ferric ions. The Mossbauer spectrum at 4.2 K in the presence of externally applied magnetic field (perpendicular to they-rays) (Fig. 4, curve C) shows the characteristic paramagnetic hyperfine structure arising due to the 3-crystal field split states +5/2> +3/2)+1/2) of 6S5/2Fe3t ions. The average value of the internal magnetic field (Hint = 46.8 T) lies well within the range specified for tetrahedrally coordinated ferric ions [31]. Catalytic properties Changes in shape selectivity due to the isomorphous substitution of A1 by the larger Fe has not, so far, been unequivocally been established. However, differences in catalytic activity, selectivity and stability between alumino- and ferrisilicate zeolites arising from the presence of weaker acid sites in the latter [14] have been noted [3,7,19,21,22]. In the conversion of methanol to olefins [21], for example (Fe)-ZSM-5 yields more C2-C4 olefins than the Al-analog (Table 4). In the hydrodewaxing of gas oil, (Fe)-ZSM-5 has a lower activity than the Al-analog (as seen from the higher temperature (641 vs 623 K) required to dewax the oil to the same pour point level [21]). However, the larger C5+ yield observed over the less acidic ferrisilicate was probably due to the lower secondary cracking over it. The lower acid strength of the ferrizeolites has implications also in the relative rates of deactivation (vis-8-vis the Al-analogs) in those reactions where bulky polyalkylaromatics formed within the pore system can Table 4. Conversion of methanol to olefins over H-(Al)-ZSM-5 and H-(Fe)-ZSM-5. Feed : 80 % (v/v) methanol in water; Temp. : 723 K; WHSV : 2.2 h-l Press. : atm; Methanol conversion : 100 %; Dimethyl ether : 0.0 %.
SiO2/M2O3 Average crystal size, p Hydrocarbons, w t . % Ethylene Propylene Butenes C1-C4 alkanes
c5+
H-(Al)-ZSM-5
H-(Fe)-ZSM-5
86 2-3
72 2-3
3.1 4.6 1.o 45.7 45.6
10.3 21.6 15.5 15.5 37.0
Synthesis and Characterisation of Ferrisilicates 51
lead to deactivation of the catalyst. Fig. 5 illustrates the relative deactivation rates of (A1)- and (Fe)-ZSM-ll in the disproportionation of ethylbenzene to benzene and diethylbenzenes. Even though the initial activity of the Al-analog was higher the catalyst deactivated faster. It may be mentioned here that the distribution of ,the three (para, meta and ortho) diethylbenzene isomers was similar on both the catalysts. Hence, the shape selectivity of both the Al- and Fe- zeolites was similar. The observed differences in their deactivation characteristics is probably due to differences in the strength of their acid sites. Ferrizeolites can exhibit bifunctional catalytic behaviour when part of the Fe3+ ions are removed from framework positions (by hydrothermal treatment, for example). In such samples, finely dispersed iron oxide particles coexist with Fe3+ ions in lattice positions. While the latter can take part in acid-catalysed reactions (like the disproportionation of ethylbenzene to benzene and diethylbenzenes), the former can give rise to redox activity, for example, in the dehydrogenation of ethylbenzene to styrene.
t
0
40.
: 648
3*5 7.0
0 ; 648
i
-
WFe) -ZSM-II
0
0
I
I
a
1
I
I
4 6 2 TIME ON S T R E A M ( T O S ) , h
-
I
Fig. 5. Ethylbenzene conversion against time-on-stream : -0: H-(Fe)-ZSM-11, and *:H-(Al)-ZSM-ll
I
8
I
A0
52 R. Kumar and P. Ratnasamy
ACKNOWLEDGEMENT This work was partly funded by UNDP. REFERENCES 1 J.R. Goldsmith, Min. Mag., 29 (1952) 952. 2 R.M. Barrer, J.W. Baynham, F.W. Bultitride and W.M. Meier, J. Chem. SOC., (1959) 195. 3 P. Ratnasamy, R.B. Borade, S. Sivasanker, V.P. Shiralkar and S.G. Hegde, Acta. Phys. Chem., 31 (1985) 137. 4 R. Szostak and T.L. Thomas, J. Catal., 100 (1986) 555. 5 G. Calis, P. Frenken, E. deBoer, A. Swolfs and M.A. Hefni, Zeolites., 7 (1987) 319. 6 R.B. Borade, Zeolites., 7 (1987) 398. 7 R. Kumar and P. Ratnasamy, J. Catal., 121 (1990) 89. 8 R. Szostak and T.L. Thomas, J. Chem. SOC. Chem. Commun., (1986) 113. 9 R. Kumar, A. Thangaraj, R.N. Bhat and P. Ratnasamy, Zeolites., 10 (1990) 85. 10 P. Ratnasamy, Pk.ii. Kotasthane, V.P. Shiralkar, A. Thangaraj and S. Ganapathy, in M.L.Occelli and H.E. Robson (Eds), Zeolite Synthesis (ACS Monograph 398), Am. Chem. SOC., Washington DC, (1989) p.405. 11 R. Kumar and P. Ratnasamy, (unpublished results). 12 R. Kumar, A. Thangaraj, R.N. Bhat, M.J. Eapen, S.K. Date, E. Bill and A. Trautwein, J. Catal., (submitted). 13 A.J. Chandwadkar, R.N. Bhat and P. Ratnasamy, Zeolites (in press). 14 L.M. Kustov, V.B. Kazansky and P. Ratnasamy, Zeolites, 7 (1987) 79. 15 A.N. Kotasthane, V.P. Shiralkar, S.G. Hegde and S.B. Kulkarni, Zeolites, 6 (1986) 253. 16 R. Kumar, S.K. Date, E. Bill and A. Trautwein, Zeolites (in press). 17 V. Nair, Ph.D thesis, Georgia Inst. Tech., (1987). 18 J. Patarin, J.L. Guth, H. Kessler, G. Condurier and F. Raatz, French Patent 17711 (1986). 19 D. Lin, Ph.D thesis No.126-89 (1989), University of Claude Bernard, Lyon, France. 20 B.D. McNicol and G.D. Pott, J. Catal., 25 (1972) 223. 21 P. Ratnasamy, React. Kinet. Catal. Lett., 35 (1-Z), (1987) 219. 22 S. Sivasanker, K.M. Reddy, K.J. Waghmare, S.R. Harisangam and P. Ratnasamy, in "Proc. XI Symp. Iberoamer. Catal., Mexico (1988) 741. 23 B. Wichterlova, S. Beran, S. Bedanarova, K. Nedomova, L. Dudiwova and P. Jiru, Stud. Surf. Sci. Catal., 37 (1987) 199. 24 G. Dopplern, R. Lehnert, L. Marosi and A.X. Trautwein, Stud. Surf. Sci. Catal., 37 (1987) 215. 25 R. Szostak, Molecular Sieves, Principles of Synthesis and Identification, Reinhold (1989) 230-238. 26 A. Meagher, V. Nair and R. Szostak, Zeolites, 8 (1988) 3. 27 G.P. Handreck and T.D. Smith, J. Chem. SOC. Faraday Trans. I., 85 (1989) 319. 28 E.E. Senderov, A.M. Bychkov, I.M. Miskhin, A.L. Klyachko and H.K. Ekyer, Stud. Surf. Sci. Catal., 49 (1989) 355. 29 I.S. Dring, D.H. Hall, R.J. Oldman, J.L. Casci, W.N.E. Meredith and R.P. Tooze, Physica B, 158 (1989) 167. 30 R.L. Garten, W.N. Delgass and M. Boudart, J. Catal., 18 (1970) 90. 31 V.G. Bhide and S.K. Date, Phys. Rev., 172 (1968) 345.
53
Further Studies on the Synthesis of VPI-5
M. E. Davis and D. Young Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.)
ABSTRACT New synthetic procedures used to crystallize VPI-5 are described. Mixtures of amines and quaternary ions are utilized to crystalize pure VPI-5. A low cost, high yield preparation involves the use of triisopropanolamine and tetramethylammonium hydroxide. Some samples of VPI-5 can be transformed into AlPO4-8 upon certain calcination conditions. Extensive washings of the aforementioned, as-synthesized VPI-5 yields a product which does not transform into AlP04-8. INTRODUCTION Virginia Polytechnic Institute number 5 (VPI-5) is a family o f aluminophosphate based molecular sieves (refs. 1-5) which share a common three-dimensional topology and contain 18-membered rings (refs. 1-8). The extra-large pores of the VPI-5 materials are unidimensional channels circumscribed by rings containing 18 tetrahedral atoms and possess free diameters of approximately 12-13 A. Recently, MCM-9 (ref. 9) has been shown to be a mixture o f Si-VPI-5 and SAPO-11 (refs. 10-12). However, it is not clear what the framework silicon concentrations are in either Si-VPI-5 (ref. 5) or MCM-9 (ref. 12). If one assumes the maximum levels of substitution from bulk chemical analysis, the silicon concentrations are low. Low element substitution was shown to be true for Co-VPI-5 (cobalt containing VPI-5) (ref. 5) as well. AlP04-8 (ref. 13) appears to contain VPI-5 from X-ray powder diffraction data. However, we conclusively showed that no VPI-5 is contained in AlPO4-8 (ref. 10). Here, we will demonstrate that certain samples of VPI-5 can be transformed to AlPO4-8. It is not surprising that MCM-9 and AlP04-8 have relation to VPI-5 since they are synthesized using di-n-propylamine (DPA) and tetra-butylammonium
54 M. E. Davis and D. Young
hydroxide (TBA), respectively, and that DPA and TBA are the organic agents initially described for the preparation of VPI-5 (ref. 4). The purpose of this work is to report new synthetic procedures for VPI-5. METHODS Svnthesi s Pseudoboehmite alumina (Catapal-B) and 85 wt% H3PO4 were used as the a1 uminum and phosphorus starting materials. Aqueous (55 wt%) tetra-butylammonium hydroxide (TBA) and (25 wt%) tetramethylammonium hydroxide (TMA) w e r e purchased f r o m Alfa. D i - n - p r o p y l a m i n e ( D P A ) and triiso-propanolamine (TIPOA) were obtained from Aldrich. A typical synthesis procedure involves the following steps: (i) alumina is slurried in water, ( i i ) phosphoric acid is diluted in water, ( i i i ) the phosphoric acid solution is added to the alumina slurry, (iv) the aluminophosphate precursor mixture is aged stirring at ambient conditions for 2 hours (static-TBA), (v) organics are added to the precursor mixture to form the final reaction mixture which is aged stirring at ambient conditions for 2 hours, (vi) the reaction mixture is charged into autoclaves (15 ml homebuilt or 25-600 ml Parr) and statically heated at autogenous pressure in forced convection ovens. Products are recovered by slurrying the quenched autoclave contents in water, decanting off the supernatant liquid, filtering the white solid, and drying the crystals in ambient air. Analysis The pH values of quenched autoclave contents are recorded prior to dilution with water for product recovery. Thermogravimetric analyses (TGA) were performed in air on a DuPont 951 thermogravimetric analyzer. A Siemens I2 X-ray diffractometer was used to collect X-ray powder diffraction data with CuKa radiation. Magic angle spinning 31P NMR spectra were recorded on a Bruker MSL 300 spectrometer. The 31P NMR spectra were taken at a frequency o f 121.496 MHz and a spinning rate of 3 - 5 kHz. Chemical shifts are reported relative to 85 wt% H3PO4. REVIEW OF VPI-5 CRYSTALLIZATIONS In our initial paper on the synthesis of VPI-5, we described preparation methods that involved the use of TBA and DPA (ref. 4). Subsequently, we studied further aspects of the crystallization process using a VPI-5 gel (ref. 5) which contained TBA. The results from these experiments lead to the following description for a TBA mediated crystallization o f VPI-5. When the
Synthesis of \'PI-5
reaction mixture is completely formulated, the solid fraction contains TBA and an excess of A1 over P. We speculate that at this point phosphoric acid has diffused into the pseudoboehmite (which may contain remnants of the layered boehmite structure) and has coated its surface. Upon heating, the TBA as well as some of the solid phase A1 and P are dissolved into the liquid phase. Within the solid phase, A1 and P react to form A1-0-P bonds in the absence of detectable amounts of TBA. With reaction time, the solid Al/P decreases while the liquid Al/P increases. As the A1 dissolves into the aqueous phase, the pH rises until it reaches near neutral. The solid phase appears to form a layered aluminophosphate intermediate which may "cross-1 ink'' to ultimately form VPI-5. The 27Al and 31P magic angle spinning NMR spectra of the solid phases obtained from the crystallization of VPI-5 are consistent with the above description (ref. 5). Very recently, we crystallized VPI-5 in situ in a magic angle spinning rotor and collected 27Al and 31P NMR spectra during the crystallization process (ref. 14). The results obtained from the in situ NMR crystallization are the same as those shown previously from dried solids (ref. 5). Thus, the in situ experiments are consistent with the description given above for the crystallization process. All of our observations lead us to believe that the crystallization of VPI-5 involves a solid phase reordering process. This solid phase transformation does not involve TBA as either a template or a space-filler. TBA may be present in the solid phase in very low concentrations that are below detection limits by I R and NMR and may still play some role in the crystallization mechanism. Also, it may serve to moderate the pH of the reaction mixture during its formulation and heating such that certain aluminophosphate precursor species are present in the solid phase. Here we will explore these two possibilities in order to further understand the crystallization process for forming VPI-5. VPI-5 SYNTHESES In our previous papers (refs. 4,5) we described syntheses of VPI-5 using DPA and TBA. Since that time we have been able to crystallize VPI-5 using many other organics such as diisopropylamine, dipentylamine (DPentA), triethanolamine (TEOA), triisopropanolamine (TIPOA), cyclopentylamine, and cyclohexylamine at synthesis temperatures from 120 to 150°C. The wide variety of organics able to crystallize VPI-5 suggests further that the organic does not act as a template and from chemical analyses it is clear that it does not act as a space-filler.
55
56 M. E. Davis and D. Young
Earlier (ref. 2), we pointed out the similarities in the X-ray powder diffraction patterns of VPI-5 and the aluminophosphate hydrate H1 (ref. 15). Figure 1 shows the X-ray powder diffraction pattern of VPI-5 with the reflections common to H1 darkened for comparison. H1 is synthesized at low pH
90-1
I
4I
16.53
FFF
8.23
F F
6.16
16.43 9.49 8.23 6.21 5.48
Fig. 1. X-ray powder diffraction pattern of VPI-5. Reflections common to H1 are darkened. in the absence of organic agents. Unlike VPI-5, it converts to the aluminophosphate analog of tridymite upon heating to 110°C. This is an effect which we have observed for layer structures. After many attempts to synthesize H1 we obtained a sample which contains approximately 20% H1 with 80% H2 (ref. 15). This sample loses the X-ray line at 16.4 A upon heating to 350°C in vacuum and has no microporosity. On the other hand, a physical mixture of 20% VPI-5 and 80% H3 (ref. 16) shows microporosity after heating to 350'C in vacuum. Thus, H1 behaves very similarly to the solid intermediate obtained during the crystallization of VPI-5 (ref. 5). These results suggest that the organic may function as a pH moderator. To test this premise we performed many syntheses with the gel compositions xNH4+ 0 A1203 0 P2O5 0 40-50 H20 where x was varied and the source of NHqt was NH40H. Our premise was that if the organic serves only to moderate the pH, the NH40H should work as well as TBAOH in crystallizing VPI-5. At no time were we able to synthesize VPI-5
Synthesis of VPI-5 57
w i t h NH40H. I n s t e a d we s y n t h e s i z e d a broad v a r i e t y o f c r y s t a l l i n e s o l i d s which appear t o n o t be microporous. These r e s u l t s a r e n o t s u r p r i s i n g i n view o f t h e numerous c r y s t a l l i n e phases r e p o r t e d p r e v i o u s l y ( r e f s . 17, 18) f r o m ammonium aluminophosphate r e a c t i o n m i x t u r e s . Thus, t h e o r g a n i c agents used f o r t h e c r y s t a l l i z a t i o n o f VPI-5 most p r o b a b l y have o t h e r f u n c t i o n s t h a n j u s t pH moderators. The VPI-5 samples prepared u s i n g TBA show g r e a t e r thermal s t a b i l i t y t h a n t h o s e s y n t h e s i z e d w i t h DPA ( r e f . 3). A l s o , t h e TBA s y n t h e s i z e d V P I - 5 i s s t a b l e i n i t s mother l i q u o r w h i l e t h e DPA s y n t h e s i z e d VPI-5 i s n o t ( r e f s . 4, 5). The d i f f e r e n c e s i n s t a b i l i t y a r e n o t due t o pH v a r i a t i o n s between TBA and DPA m o t h e r l i q u o r s . Thus, t h e q u a t e r n a r y i o n a p p e a r s t o a s s i s t t h e c r y s t a l l i z a t i o n process i n some manner t h a t i s n o t p o s s i b l e w i t h an amine. U n f o r t u n a t e l y , TBAOH i s v e r y expensive compared t o most amines. I n a t t e m p t s t o i m p a r t physicochemical p r o p e r t i e s o f a " q u a t " s y n t h e s i z e d V P I - 5 t o an "amine" s y n t h e s i z e d VPI-5, we conducted s e v e r a l e x p l o r a t o r y s y n t h e s e s u s i n g m i x t u r e s o f amines and q u a t s . Some o f o u r r e s u l t s a r e i l l u s t r a t e d i n T a b l e 1. The q u a t e r n a r y ammonium h y d r o x i d e s were added j u s t a f t e r t h e a d d i t i o n o f t h e amine u s i n g t h e s y n t h e s i s procedure o u t l i n e d above. F i r s t , i t i s a p p a r e n t t h a t a s m a l l amount o f a " q u a t " can i n f l u e n c e t h e s y n t h e s i s o f VPI-5. When u s i n g DPA, 1/80 o r 1/60 TMA extends t h e t i m e t h a t VPI-5 i s s t a b l e i n t h e mother l i q u o r . Also, 1/40 TMA i n h i b i t s t h e f o r m a t i o n o f VPI-5. Q u a t s o t h e r t h a n TMA appear t o p e r f o r m t h e same t a s k s as TMA. S i n c e TMAOH i s inexpensive, we c o n t i n u e d t o e x p l o r e t h e use o f TMAOH w i t h o t h e r amines. The d a t a i n T a b l e 1 show t h a t TMAOH can be used w i t h amines o t h e r t h a n DPA t o c r y s t a l l i z e VPI-5. To d a t e TIPOA appears t o g i v e t h e b e s t VPI-5 p r o d u c t which f o r t u n a t e l y a l s o i s o b t a i n e d i n h i g h y i e l d . The r e s u l t s g i v e n i n T a b l e 1 show t h a t s m a l l amounts o f a q u a t s i g n i f i c a n t l y a f f e c t t h e c r y s t a l l i z a t i o n p r o c e s s o f VPI-5. In order t o understand f u r t h e r these e f f e c t s , s e v e r a l amine-quat syntheses were s t u d i e d as a f u n c t i o n o f t i m e . T a b l e 2 shows d a t a f r o m samples o b t a i n e d f r o m h e a t i n g g e l s w i t h c o m p o s i t i o n DPA 0 yTMA A1203 0 P2O5 0 40 H20 t o 142°C a t autogenous p r e s s u r e . These r e s u l t s r e v e a l t h a t t h e a d d i t i o n o f TMAOH: (i) slows t h e c r y s t a l l i z a t i o n process, (ii)i n h i b i t s t h e f o r m a t i o n o f H3, and (iii)i n c r e a s e s t h e s t a b i l i t y o f t h e VPI-5 p r o d u c t i n t h e mother l i q u o r . To f u r t h e r i l l u s t r a t e t h e e f f e c t s o f TMAOH on t h e s y n t h e s i s o f VPI-5,
TIPOA c o n t a i n i n g g e l s were used t o s y n t h e s i z e VPI-5. The advantages o f TIPOA l o w c o s t , (ii)no phases o t h e r t h a n H3 a r e formed a t 142'C ( w i t h DPA, are: (i)
~ l P O ~ - can l l be formed a t 142°C; w i t h TEOA, A1P04-5 can be formed a t 142'C; o n l y a t h i g h e r temperatures do we observe AlP04-5 w i t h TIPOA), and (iii)t h e
58 M. E. Davis and D. Young
TABLE 1 Amine-quat syntheses o f V P I - 5 a t 142OC. Gel composition: R 1 Re A1203 P2O5
Experiment
F G
R1
-
R2
--
Sol i d Obtained
VPI-5
DPA DPA DPA OPA
1/100 TMAa 1/80 TMA 1/60 TMA
VPI-5 VPI-5 VPI-5
DPA
1/40 TMA
unidentified crystal1 i n e s o l i d
DPA
1/60
DPA DPA
TEA^
1/60 TPAC
H I
DPentA
1/60 TBA 1/60 TMA
J
TEOA
1/60 TMA
K L M
40H20
TIPOA TIPOA T I POA
1/60 TMA 1/40 TMA 1/20 TMA
VPI-5 t
~1po4-11 VPI-5 VPI-5 VPI-5 VPI-5
+
A1 PO4- 5 VPI-5 VPI-5 VPI-5
aTetramet hyammoniurn hydroxide bTetraethyl ammoni um hydroxide CTetrapropylammoni um hydroxide v i s c o s i t y o f t h e f i n a l r e a c t i o n m i x t u r e i s much l o w e r t h a n w i t h o t h e r amines. F i g u r e 2 i l l u s t r a t e s t h e change i n pH as a f u n c t i o n o f h e a t i n g t i m e f o r s e v e r a l TIPOA syntheses o f VPI-5. A l l f o u r r e a c t i o n m i x t u r e s y i e l d e d VPI-5. Thus, h i g h e r amounts o f TMAOH can be used w i t h TIPOA t h a n w i t h DPA. The s y n t h e s i s w i t h no a d d i t i o n o f TMAOH y i e l d s VPI-5 with H3 c o n t a m i n a t i o n a t a l l r e a c t i o n t i m e s . W i t h t h e i n c l u s i o n o f 1/60 TMA, t h e TIPOA r e a c t i o n m i x t u r e g i v e s VPI-5 w i t h no H3 a t 4 hours. However, w i t h f u r t h e r h e a t i n g times, H3 appears i n t h e c o l l e c t e d s o l i d . A t a TIPOA/TMA r a t i o o f 40, t r a c e amounts o f H3 a r e n o t observed i n t h e p r o d u c t u n t i l 44 hours o f h e a t i n g . F i n a l l y , w i t h TIPOA/TMA = 20, no H3 i s obtained. As w i t h t h e DPA syntheses, t h e e f f e c t s o f TMAOH on t h e TIPOA syntheses are: (i)t h e r e d u c t i o n i n c r y s t a l l i z a t i o n r a t e ( f i n a l pH v a l u e s between 7-8 were observed f o r a l l syntheses) and (ii)t h e suppression o f H3 f o r m a t i o n .
Synthesis of VPI-5 59
TABLE 2
Properties of samples collected as a function of heating time. Heating time (hours)
PH
-
2
3.7 6.1
4
6.5
10
6.6
29 50 116 215
0
y = 1/60
Y = O
Products
PH -
--
3.8 4.9
VPI-5, trace H3
Products
-VPI-5, amorphous
VPI,-5, trace H3
6.6
vpi-5
VPI-5, trace H3
6.6
vp1-5
6.7
VPI-5, more H3
6.9
vp1-5
6.8 6.7 6.8
VPI-5, H3 VPI-5, H3 unknown
6.8 6.9 6.9
vp1-5 vp1-5 vp1-5
'
The products o b t a i n e d from TBAOH syntheses a r e o f t e n contaminated w i t h small amounts o f H3. The a d d i t i o n o f TMAOH i n t h e r a t i o TBA/TMA = 20 appears t o e l i m i n a t e t h e formation o f H3.
'i 4
Fig, 2.
TIPOA
A TIPOCI/TM mTIPUUM T I W M
-
-
80
40 20
pH versus time f o r syntheses o f V P I - 5 .
60 M. E. Davis and D. Young
The sample from experiment L (Table 1) was heated in air to 600°C and then cooled to room temperature. The X-ray powder diffraction patterns of these solids are illustrated in Figure 3 and indicate that VPI-5 is beginning to transform into AlP04-8. Figure 4 shows the 31P NMR spectra from VPI-5 and AlP04-8. The 31P NMR spectrum of hydrated VPI-5 is very unique (refs. 3,5) and contains three resonances at -22.8, -26.7 and -33.2 ppm. The explanation of these peaks is given elsewhere (refs. 3,5). The 31P NMR spectrum of AlP04-8 gives a single resonance at -29.4 ppm which is at a completely different chemical shift than any of the peaks in the VPI-5 spectrum. Thus, by inspection of the 31P NMR spectrum as well as the X-ray powder diffraction pattern, one can ascertain whether VPI-5 samples have partially or wholely transformed to AlPO4-8. Sample L became a light yellow after heating which indicates the presence of extra-framework material. 50 g of VPI-5 (sample L) were stirred i n 250 ml of boiling water for 12 hours in an attempt to remove any occluded material. After heating the washed solid to 600"C, the X-ray powder diffraction pattern (Figure 3) indicates that VPI-5 has not transformed to AlP04-8. Also, the solid remained white. The 31P NMR spectrum of the
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
Two-Theta
Fig. 3. X-ray powder diffraction patterns. Middle: as-synthesized VPI-5 heated to 600'C. 600°C.
Bottom: as-synthesized VPI-5. Top: Washed VPI-5 heated to
Synthesis of VP1-5 61
n
1
I
I
40
20
0
I
-29.4
I
-40 -20 PPM
I
-60
I
-80
Fig. 4. 31P solid-state NMR spectra of pure VPI-5 and pure AlP04-8. washed sample appears the same as the as-synthesized material and the TGA's of these solids are nearly the same (shape is the same and final weight losses are within 0.5 wt%). The data given in this paper show that pure VPI-5 samples can be prepared from synthesis gels containing a mixture of amines and quaternary ions. These organics appear to perform roles other than pH moderation. A small amount of TMA is shown to greatly affect the crystallization process. Although the organics do not serve as templates or space-fillers, a small amount of residual material appears to remain occluded within the VPI-5 crystals and requires extensive washings for removal. Extraction of the residual material is necessary for obtaining VPI-5 which does not transform to AlP04-8 upon We are currently investigating the nature of the heating to at least 600'C. occluded material by using a TGA in which the off-gas is sent to a mass spectrometer. Attempts to record a 13C NMR spectrum have yielded only noise
62 M. E. Davis and
D. Young
as m i g h t be expected because o f t h e l o w c o n c e n t r a t i o n o f r e s i d u a l o r g a n i c s . Elemental a n a l y s e s f o r C and N ( b o t h w e l l below 0.5 w t % ) c o n t a i n e r r o r s u f f i c i e n t t o r e n d e r t h e C/N r a t i o meaningless. REFERENCES 1
2 3 4 5 6 7
8 9 10 11 12 13 14 15
16 17 18
M. E. D a v i s , C. S a l d a r r i a g a , C. Montes, J. Garces, and C. Crowder, Nature, 331 (1988) 698-699. M. E. D a v i s , C. S a l d a r r i a g a , C. Montes, J. Garces, and C . Crowder, Z e o l i t e s , 8 (1988) 362-366. M. E. Davis, C. Montes, P. E. Hathaway, J. P. Arhancet, D. L. Hasha, and J. M. Garces, J. Am. Chem. SOC., 111 (1989) 3919-3924. M. E. D a v i s , C. Montes, and J. M. Garces, ACS Symp. Ser., 398 (1989) 291 -304. M. E. D a v i s , C. Montes, P. E. Hathaway, and J. M. Garces, Stud. Sur. S c i . C a t a l . , 49A (1989) 199-214. C. E. Crowder, J. M. Garces, and M. E. Davis, Adv. X - r a y A n a l y s i s , 32 (1989) 503-510. J. W. Richardson Jr., J. V. Smith, and J. J. P l u t h , J. Phys. Chem., 93 (1989) 8212. P. R. Rudolf, and C. E. Crowder, Z e o l i t e s , 10 (1990) 163-168. E. G. Derouane, and R. von Ballmoos, U. S. Pat. 4,673,559 (1989). M. E. Davis, P. E. Hathaway, and C. Montes, Z e o l i t e s , 9 (1989) 436-439. R. Szostak, 1. L. Thomas, and D. C. Shieh, C a t a l . L e t t . , 2 (1989) 63-70. E. G. Derouane, L. M a i s t r i a n , Z. G a b e l i c a , A. Tuel, J. B. Nagy, and R. von Ballmoos, Appl. Catal., 5 1 (1989) L13-L20. S. 1. Wilson, B. M. Lok, and E. M. Flanigen, U. S. Pat. 4,310,440 (1982). M. E. Davis, B. D. Murray, and M. Narayana, ACS Symp. Ser., i n p r e s s . F. d’Yvoire, B u l l . SOC. Chem. (1961) 1762. J. J. P l u t h , and J. V. Smith, Nature, 318 (1985) 165-166. J. P. Smith, and W. E. Brown, Amer. Miner., 44 (1959) 138. J. F. Haseman, J. R. Lehr, and J. P. Smith, S o i l S c i . SOC. Amer. Proc., 15 (1950) 76.
63
New Families of M( III)X( V )O,-Type Microporous Crystals and Inclusion Compounds
Ruren X u , Jiesheng Chen and Shouhua Feng Department of Chemistry, Jilin University, Changchun , China ABSTRACT Three new M(N)X(V)O,-type families, designated GaPO‘ s , AlAsO‘ s and GaAsO’ s respectivel y , with microporous or layered framework structures have been synthesized hydrothermally using various amines and quaternary ammonium as templates. Unlike AlPO‘ s, almost all known structures of these families possess M(N) bonded to more than four 0 atoms. The micropores of the 3D frameworks are comparable in diameter to the intermediate and small ones of AlPO’ s. The templates capable of entering the MXO, framework contain not more than 8 carbon atoms for GaPO‘ s and not more than 4 for AlAsO’ s and GaAsO‘ s. Larger primary amines lead to the formation of layered structures of AlAsO’ s and GaAsO‘ s. INTRODUCTION The concept uzeolites” conventionally served as the synonym for aluminosilicates with microporous “host” lattice structures. Upon removal of the “guest” water, zeolites demonstrate adsorptive property at the molecular level; as a result they are also referred to as “molecular sieves. ” Crystalline zeosils, AlPO‘ s , SAPO’ s , MAPO’ s (M=metal)
, expanded clay minerals and Werner com-
pounds are also able to adsorb molecules vitally on repoval of any of the guest species they occlude and play an important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure; their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. Of the zeolitic materials, AlPO’ s cut a conspicuous figure because of their structural diversity and the incorporation of other elements into their frameworks. The recently developed VPI-5 (refs. 2 ,
3) announced the feasibility of synthesis of micoporous structures with windows comprising rings of over 12-T. All AlPO’ s , SAPO’ s and MAPO’ s form a family of microporous structures constructed by or essentially by Al(N) and P(V). Some of them are isostructural with zeolites but a majority have novel structures. The primary building units (PBU) centred by P ( V ) are invariantly PO, whereas those centred by Al(lU) are A10, in most cases and A105 or even A106 in a few cases. So far all AlPOI s , SAPO’ s and h4APO’ s have been synthesized exclusively in the presence of amines or
64 R. Xu, J. Chen and S. Feng
quaternary ammoniums , that is, the so-called templates. Without a template only dense polymorphs of AlP04 crystallize from the synthetic systems. In 1985, a gallophosphate with a porous framework structure was reported by Parise (ref. 4). This was followed by the systematic synthesis of the GaPO family (refs. 5 , 6 ) in our laboratory and the structural characterization of several single crystals of the compounds (refs. 7-11). Recently, we also focused attention on the synthesis of aluminoarsenates and galloarsenates with open framework structures (refs. 12-14).
While the aluminophosphates are labeled as AlPO’ s ,
the gallophosphates , aluminoarsenates and galloarsenates can be designated as AlPO’ s , AIAsO‘ s and GaAsO‘ s respectively. It is expected that due to the discrepancy between Al(IU) and Ga(IU) as well as that between P(V) and As(V)
, the four
families should behave differently to a certain degree in both structure and
property. This paper will attempt to explore the structural features and the crystal chemistry of
M(IU)X (V)Or-type microporous crystals and/or inclusion compounds on the basis of the new families GaPO’ s, AlAsO’ s and GaAsO’ s in combination with the well known AlPO’ s. PREPARATION AND PRODUCT COMPOSITION All the syntheses are carried out in the presence of a template. The templates used include the following : butylamine (BuNH2)
, cyclohexylamine
(CHA) , 1,2-diaminopropane (DAP)
, dimethylamine
, dipropylamine (DPA) , ethanolamine (EAN) , ethylamine (EtNH,) , ethylenediamine , hexanediamine (HDA) , isopropylamine (iPrNHz) , propylamine (PrNH2) , tetraethylam-
(DMA) (EDA)
monium hydroxide (TEAOH ) , tetramethylammonium hydroxide ( TMAOH ) and triethylamine (Et3N). GaPO’ s are synthesized hydrothermally from a reaction mixture with batch composition (0. 5-
3)R
: GazO3 :
(1-1. 5)Pzos : (25-100)HzO,
where R represents various amines or quaternary
ammoniums. To obtain the crystal, typically a gel formed by mixing GaOOH, H3P04 and water with stirring at 80 ‘C followed by adding a template is heated in PTFE-lined autoclaves. Depending on various structures, the crystallization temperature and time vary within 150- 190 ‘C and 72-
144 hours respectively. AlAsO’ s are obtained by heating at 200 ”C for 4-10
days a homogeneous
mixture of A1(OC$17)3, HdAs207, water and an amine (quaternary ammonium) with batch composition (0. 6-3)R
: A1203 :
(1. 0-1.
5)ASzOs
:
(25-100)H~O.
The reactants for GaAsO’ s syn-
thesis are GaOOH, HdAszO,, an amine (quaternary ammonium) and water. In some cases, HF is used in order to facilitate the crystallization. The crystallization temperature is 200 *C while the reaction time varies within 5-15
days depending on structures.
The as-synthesized GaPO‘ s have essentially a neutral framework, that is, the P/Ga ratio is invariantly around unity. Their empirical composition can be expressed as sR * GazO3 * 1. 0 f
0. lPzOs yHzO. Table 1 lists the x and y values of various as-synthesized GaPO’ s obtained in our laboratory. It is also found that the As/Al ratio in AlAsO’ s and As/Ga ratio in GaAsO‘ s are around unity.
1. Of 0. 1ASzOs nHzO and pR GazO3 1. Of 0. lAszOs pHzO with the details being presented in Table 2 and Table 3 respectively.
Similarly, their compositions are written as mR
A1203
M(III)X( V )O,-Type Microporous Crystals 65
Table 1. Compositions of as-synthesized GaPO' s crystals
R
X
Y
GaPOI-C1
TMAOH
0.32
GaPO,-C2
HDA EtaN
0.40
0 0 0. 30 0 0. 22 0. 36
code"
GaPO,-C3
0. 24
GaPO,-C4
EAN
0.58
GaP0,-C5
iPrNH,
0.36
GaPO,-CG
HDA
0. 62
coden
GaP0,-C7
R
Y
z
b
GaP04-C8
DPA
0.82
0.34
GaP04-C9
HDA
0.26
0.34
GaPO,-C10
CHA
0.64
0.66
GaPO,-C11
DMA
0.88
0
GaPO,-C12
C
a GaP04-C4 and -C5 are isostructural with AlPOd-21 and -14 respectively;
b the template used is PrNHz whereas the as-synthesized GaP04-C7 has a composition of GaP04 0. 5NH3 1. 5H20 0. 08PrOH (see ref. 11); c GaPOd-Cl2 is obtained by the calcination of GaP04-C4 at 500 "C for 2 hours and is isostructural with A1PO4-25. Table 2. Compositions of as-synthesized AIAsO' s crystals
R
R
m
n
0. 50 0. 52 0.70
1.80
m
n
code
AIAsOi- 1
EAN
0.98
AIAsO,-7
a
AIAsOi-2 AlAs0,- 3
TMAOH
0.60
0. 56 0. 48
AIAsOn-5
EDA
EtNHz BuNHz CHA
0.60
1.00 1.50
AIAsOl- 6
EDA
0. 80 0. 80
AIAsOd-8 AIAsO,-9 AIAs0,- 10' AIAs0,- 11'
iPrNHl
DAP DAP
0 0 0. 54 0. 82 1. 52 1. 20
AIAs04-12'
HDA
0.62
2.00
code
AIAsOi- 4
1. 34
~~~
a The template used is DMA but the as-synthesized AIAsOr-7 has a composition of A1As04 0.30NH3 2. OOHzO; b AlAsOd-10, -11 and -12 are of layered structure.
Table 3. Compositions of as-synthesized GaAsO' s crystals code
GaAs0,-1
R TMAOH
P 0.38
P
code
0
GaAsO,-7
R
Q
EtNH,
P 0.74
1. 02
GaAsO,-2
DMA
a
GaAsO,-8
PrNH,
0.42
2.34
GaAs04-3
DAP
0. 58
0. 46
GaAsO,-Sb
iPrNH,
1. 4 4
1. 98
GaAs0,-4 GaAsO,-5
EDA EAN
1. 06 1. 42
1. 78 1. 64
GaAsO,-l OD GaAs0,-llb
BuNH~ CHA
1.82 1.46
1.92 1.98
GaAs0,-6
EAN
0. 46
2. 46
GaAs0,-1 2b
HDA
0.94
1.36
a H F is also used during the synthesis and the GaAs04-2 obtained has a composition of GaAs04
0. 59DMA O.32HF
0.30HzO; b GaAs04-9, -10, -11 and -12 are of layered structure.
CRYSTAL CHEMISTRY
1. Selected single crystal structures Among the over thirty framework compounds, at least 6 have been structurally determined by means of single crystal X-ray diffraction. These include GaP04-C3, -C4, -C7, AlAsO4-1, -2 and GaAs04-2. The details of the cell parameters and the primary building units (PBU) of their structures are given in Table 4.
66
R. Xu, J. Chen and S. Feng Table 4. Crystallographic parameters and primary building units (PBU) of selected MX0,-type structures
Gap043
12.267
-
16. 746 -
P63
POI, Gaol. GaO,(OH )
9
GaP04-C4
8.688
17.952
9.104
108.27
PZ1/n
PO4 ,Ga04. GaO, (OH)
10
GaP04-C7
9.681
9.657
9.762
102.90
PZ,/n
AIASOd-1
8. 781
10.262
20. 433 -
Pca b
13
AIAsOi-2
9. 168
19.382
9.779
115.30
P2i/n
15
GaAs04-2
18. 011
10. 466
19. 035
113. 98
Pz1/n
AsO,,GaO,(OH), GaOdF,GaO,(OH)FI
14
The structure of GaPO4-C3 has OH groups each bridging among three adjacent Ga atoms. Two thirds of the Ga atoms are circumscribed by four framework 0 atoms connected with four P atoms respectively, whereas the other one third each is located in a distorted trigonal bipyramid formed by four framework 0 atoms and a hydroxyl group. Ignoring the OH groups, the linkages between alternating Ga and P atoms generate a (4,2)-3D framework with 8-ring channels running along the b axis. The template triethylamine is encapsulated in the channel in protonated form. GaP04-C4 has a monoclinic structure analogous to AlPO4-21. Of the three types of Ga atoms, one is centred in a tetrahedron of framework 0 atoms and each of the other two is situated in a distorted trigonal bipyramid formed by four framework 0 atoms and an OH group. On the other hand, each OH group bridges two Ga atoms in trigonal bipyramids. Ethylenediamine molecules are in pairs located in the open 8-ring channels (Fig. l a) . On calcination at elevated temperature the encapsulated ethylenediamine is removed and GaP04-C4 tranforms into GaP04-C12, which is isostructural with A1P04-25, the small-pore aluminophasphate exclusively constructed by A1O4 and PO4 tetrahedra. The recently reported GaP04-C7 is obtained by using propylamine as a template. It is interesting that what the assynthesized species occlude is the fragment NHf group instead of the PrNH2. All P atoms in GaP04C7 are 4-coordinated by framework 0 atoms whereas each Ga atom is 6-coordinated by four framework 0 atoms and either two OH groups or an OH group and a terminal H20 molecule. Chemical analysis indicates that propanol is also encapsulated (but disorderly) in GaP04-C7. By linking the adjacent Ga and P atoms, a three-dimensional framework with 3-, 4- and 8-rings is formed (Fig. lb ) . The odd ring arises from the bridging of Ga atoms by an OH group. The framework structure possesses three-dimensional 8-ring channels running along [loo],
[Ol O] and [OOl] respectively.
AlAs04-1 has a microporous framework structure with occluded ethanolamine molecules. As for P in AlPO' s and GaPO' s , the As in AlAsO4- 1 is also tetrahedrally coordinated by framework 0 atoms. In contrast, each A1 atom is located in either a tetrahedron of framework 0 atoms or an octahedron comprising four framework 0 atoms and two OH groups of ethanolamine molecules. In turn, each of the OH groups bridges two equivalent A1 atoms. The linkages between alternating A1 and As give a 3D framework (Fig. l c ) related to APD (ref. 16). All ethanolamine molecules are in pairs situated in the &ring channels along the c axis. AIAsOd-2 (ref. 15) is constructed by ASO4, A104
M(I1I)X ( V ) 0,-Type Microporous Crystals 67
and A104(OH) of 3
:
1 : 2. Each framework 0 atom links an A1 with an As while each OH group
bridges two non-equivalent A1 atoms. The 3D framework (Fig. I d ) contains two-dimensional 8ring channels in the directions [loo] and [ O O l ] respectively. The TMA cations are located in these channels with one CHJ foot of each cation being rooted in the channel wall. GaAs04- 2 , the only structure- known framework galloarsenate, occludes protonated dimethylamine molecules, OH groups and F anions. The primary building units in it are AsO,, G a 04(O H ), G a 0 4 F and Ga04(OH)F with a mole ratio of 3
:
1 : 1 : 1. Each OH group and F anion bridges be-
tween two Ga atoms. The 3D framework consisting of Ga and As nodes has 10-ring channels running along the b axis (Fig. l e ) and 8-ring channels along the a and the c axes respectively. The template DMA molecules are shown in these channels.
a
b
C
Fig. 1. Stereoview plots of 3D frameworks: ( a ) GaP04-C4,(b) GaP04-C7and (c) AIAs04-1 along the a axis.
68 R. Xu,J. Chen and S. Feng
d
e
Fig. 1 (continued). (d) AIAs04-2 along the a axls, and (e) GaAs04-2 along the b axis.
2. Characteristics related to structures 2. 1 Coordination number In most AlPO’ s , both A1 and P are tetrahedrally coordinated by 0 atoms. So fa r, it has been found that A1P04-12, -17, -21 and -EN3 contain 5-coordinated Al; A1P04-15 contains 6-coordinated A1 and A1P04-14 contains both (refs. 17,18). For all these AlPO’s with 5- or 6-coordinated Al, the largest window of the 3D frameworks does not exceed an 8-T ring and the templates used during their synthesis are relatively small. X-ray structural analysis indicates that the as- synthesized GaPO’ s including GaP04-C3, - C4
( Z l ) , -C5(14), -C7 and GaP04-12 (ref. 7 ) all contain Ga with coordination number exceeding 4. The extra ligands are often OH groups and occasionally (such as in the case of GaP04-C7) H20 molecules. The situation is the same for AlAsO’ s (ref. 19). *’A1 MAS NMR spectroscopy demonstrates that almost all other AlAsO’ s besides A1As04-1 and -2 possess 5- or 6-coordinated A1 as well. AIAs04-3, -5, -6, -7 and -12, contain only 6-coordinated Al; AlAsO4-4, -10 and -11 contain both
4- and 6-coordinated A1 while A1As04-8 and -9 contain both 4- and 5-coordinated ones. Meanwhile, I3C MAS
NMR spectroscopy suggests that the amines occluded in AIAsO’ s are protonized to
varying degrees. This implies that the extra ligands are, as for AlPO’ s and most GaPO‘ S , hydroxyl groups. By means of X-ray photoelectronic spectroscopy, it has been discovered (ref. 19) that the
M( 11I)X ( V )Ol-Type Microporous Crystals 69
Ga atoms in most as-synthesized GaAsO’ s are 5- and/or 6-coordinated rather than 4-coordinated. 13C MAS NMR spectroscopy also indicates the protonation of the occluded amines in GaAsO‘ s , sug-
gesting that it is OH groups instead of H 2 0 molecules which extra-coordinate to the Ga atoms. In summary, the coordination situations of the M( III) in various MX04-type framework compounds are presented in Table 5. The variability from AlPO’ s through GaAsO’ s is attributable to the smaller charge/radius ratio of Ga(llI) and As(V) in comparison with Al(lU) and P(V) respectively, although a quantitative assessment of this remains to be done. Table 5. The amount of MXO, in each family containing certain coordination units (or PBU) unit A104 AIOl
AIPO’ s majority minority
+AIO, (OH)
6-coordinated Al‘
few
GaO, GaO,+GaO,(OH)
-
6-coordinated Ga“
-
a Including 6-
f 4- or 6-
GaPO‘ s
-
AlAsO’ s
GaAsO’ s
no or few minority
-
majority
-
-
few majority
-
no or few
few
-
vast majority
few
+ 5-coordinated AI(Ga).
2. 2 Framework window AlPO’ s have open frameworks with channels circumscribed by 8-rings through 18-rings. As a result, the calcined species can adsorb molecules of various sizes. The smallest pore is 3. 0 and the largest (VPI-5) is over 10
A
in diameter. In contrast, the framework windows of known GaPO’ s
never exceed a lo-ring. Measurements in our laboratory show that the adsorption isotherms of calcined GaPO‘ s are similar (refs. 5 , 6 ) to those of zeolite molecular sieves. Moreover, all samples exhibit a reversible adsorption-desorption feature. Some so-called “plug gauge’’ molecules with known diameters have been used to determine the micropore sizes of the calcined GaPO’ s. Table 6 presents the pore diameters and the window rings of various GaPO’ s. One can see that the pores in size correspond to the intermediate and small ones of AIPO’ s. The largest, those of GaP04-C2 and -C6 obtained by using the larger HDA as the template, have a kinetic diameter of 6. 0
A.
As mentioned previously, either AlAs04-1 or AIAs04-2 possesses a framework with 8-ring windows. Except for the layered ones, all A1AsO‘ s are obtained by using relatively small amines as their templates. It is reasonable that their 3D frameworks contain windows of no more than 10-rings. Another important characteristic of AlAsO’ s is that all of them have poor thermal stability. Calcination at elevated temperature to remove the template invariably leads to destruction of the framework. It is technically difficult to measure the pore sizes of the 3D frameworks of AlAsO’s. GaAs04-2, the open framework galloarsenate, has 10-ring windows in its 3D structure. Nevertheless, it should be considered that F anions, besides OH groups, are bonded to the Ga and consequently contribute to sustaining the whole framework. All GaAsO’ s other than layered ones occlude small-sized templates1 therefore, it is expected that their framework windows do not exceed a 10ring either. Like AlAsO’ s , GaAsO’ s are unstable at elevated temperatures.
70 R. Xu, J. Chen and S. Feng
Table 6. Pore diameters and window rings of GaPO’ s
GaP0,-
c1
pore diametersck)
window
estimated
ring of T-atoms determined bv X-ray -
c3
3. 6-4. 3 6. 0 4. 3-6. 0
8 o r 10
8
c4
-
8
8
c5
2. 6-3. 6. 0 3. 6-4. 4. 3-6. 4. 3-6. 3. 6-4. 3. 6-4. 3. 6-4.
6 or 8
8
c2
C6 c7 C8
c9 c10
c11 c12
6
8
10
10
-
3
a
8
0
8 or 1 0
-
0
8 or 10
3 3
8
3
8
8
3. Role of templates We have tried to synthesize GaPO’ s , AIAsO‘ s and GaAsO’ s using various templates from the smallest primary amine methylamine through the largest quaternary ammonium tetrabutylammonium hydroxide. It has been found that templates containing more than 8 carbon atoms are not able to enter the GaPO, framework, and for the AIAsOl and GaAs04 3D frameworks, templates with more than 4 carbon atoms are ineffective. With larger primary amines such as BuNHz, CHA and HDA, layered AlAsO’ s and GaAsO’ s are obtained. The layered GaAs04-9 is acquired even with iPrNH2, a relatively small template. In contrast, the AIP04 3D framework is able to occlude a template as large as tetrabutylammonium hydroxide (ref. 20). These results lead to the conclusion that the ability of the 3D frameworks to occlude larger templates is in the order AlP04>GaP04>AlAsOl> GaAs04. This may be explained by the fact that the weaker G a - 0 and As-0 cannot sustain 3D frameworks which are much more open. While the large-pore AIPO4-5 is synthesized with twenty-three various templates (ref. 21), no AIAsO’ s and GaAsO’ s can be obtained with two or more different templates. A few GaPO’ s each can be synthesized with more than one template similar to each other in size and shape. For example, GaP04-C3 has been obtained in our laboratory with TMAOH , EkN, TEAOH and diethanolamine ; GaP04-C4 with EDA and EAN; and GaP04-C11 with DMA and EtNH2. Generally, large-pore 3D frameworks favor the accommodation of templates of varying sizes and shapes. Every small-pore 3D framework of AIAsO’ s and GaAsO’ s can only accommodate one template because of its structural constraint. The cell dimensions of a 3D structure vary with the occluded template. Table 7 shows that the unit cell of GaPO-C3 is enhanced distinctly with increasing template size. There appears for AIP04-15 (refs. 2 2 , 2 3 ) , GaP0,-C7 and AIAsO,-7 an interesting phenomenon that during synthesis the amines (1,4-diaminopropane ,PrNHZ and DMA respectively) are fragmentized into N H t
, which enters the MX04 framework as a template. No satisfactory explanation for
this has been reported to date.
M(III)X(V )O,-Type Microporous Crystals 71
Table 7. Cell variation with template for GaP04-C3 template
dimensions( A )
volume( A 3,
TMAOH
a=12.299, b=16.707
2188.75
Et3N
a=12.346, b=16.838
2222.60
TEAOH
a=12.595, b=16.870
2317.66
Some amines , especially diamines such as EDA ,DAP and HDA , each can lead to two or more 3D frameworkstructures with the same M(III) and X(V). A1P04-12, -21 and -EN3 are all obtained by using EDA as a template. EDA also results in the formation of GaP04-12, -C4(21), A1AsOl-5 and
-6. GaPOI-C2, -C6 and -C9 are all synthesized with HDA, while both AlAsO-3 and - 4 are formed in the presence of DAP. It seems that the variation of the 3D structures is caused by the optical iscmers of these diamines (ref. 17). Last but not least, in most cases, with the same template, different MXO, can form different 3D frameworks. AlP04-20, GaP0,-C1, AIAsO,-2 and GaAs0,- 1 are not isostructural at all with one another, although they all occlude the same TMA cation. In a few cases, the opposite follows. For instance, AIP04-12 and GaP04-12 (ref. 7 ) crystallize in the presence of the same template EDA, while A1P04-1 4 and GaP0,- 1 4 (C5) the Same iPrNH2. CONCLUSION The successful synthesis of the three new families GaPO’ s , AIAsO’ s and GaAsO’ s with microporous frameworks or layered structures reveals the diversity of M (N)X (V)04-type crystal chemistry. According to their thermal stability and adsorptive feature, to the first order GaPO‘ s can be classified as molecular sieves whereas AlAsO’ s and GaAsO’ s should belong to inclusion compounds. With the increase of the radii of M(III) and X(V)
, MXOl tend to occlude smaller templates. While a few
GaPO’s are analogues of several known AIPO’ s , no AlAsO or GaAsO, to our knowledge, is isostructural with any of known AlPO’ s (SAPO’ s and MAPO’ s ) or GaPO’ s , reflecting the difference between Al(III) and Ga(III) as well as P(V) and As(V). We thank the National Natural Science Foundation of China for financial support of this work. REFERENCES 1. R. M. Barrer, Proc. 7th Intl. Zeolite Conf. , 3 (1986). 2. M. E. Davis, C. Saldarriage, C. Montes, J. Garces and C. Crowder, Nature, 331, 698 (1988). 3. M. E. Davis, C. Saldarriage, C. Montes, J. Garces and C. Crowder, Zeolites, 8, 362 (1988). 4. J. B. Parise, J. Chem. SOC. , Chem. Commun. , 606 (1985). 5. S. Feng, Doctoral Thesis (Jilin University), (1986). 6. S . Feng ad R. Xu, Chem. J. Chinese Univ. (Chinese Edition), 8 , 867 (1987). 7. J. B. Parise, Inorg. Chem. , 24, 4312 (1985). 8. J. B. Parise, Acta Crystallogr. , Sect. C , C42, 1 4 4 (1986). 9. G. Yang, S. Fengand R. Xu, J. Chem. Soc.. Chem. Commun., 1254 (1987). 10. S. Feng, R. X u , G. Yang and H. Sun, Chem. J. Chinese Univ. (English Edition), 4 , 1 (1988). 11. T. Wang, G. Yang, S. Feng, C. Shang and R. Xu, J. Chem., Chem. Commun., 948 (1989). 12. J. Chen and R. Xu, J. Solid State Chem. , 8 0 , 149 (1989).
72 R. Xu, J. Chen and S. Feng
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
G. Yang, L. Li, J. Chen and R. Xu, J. Chem. SOC.,Chem. Commun., 810 (1989). J. Chen, L. Li, G. Yang and R. Xu, J. Chem. SOC., Chem. Commun. , 1217 (1989). L. Li, L. W u , J. Chen and R. Xu, Acta Crystallogr. , Sect. C, in press. J. J. Pluth and J. V. Smith, Nature, 318, 165 (1985). J. M. Bennett, W. J. Dytrych, J. J. Pluth, J. W. Richardson and J. V. Smith, Zeolites, 6, 349 (1986). J. V. Smith, Chem. Rev., 88, 149 (1988).
J. Chen, Doctoral Thesis (Jilin University), (1989). M. E. Davis, C. Montes and J. M. Garces, ACS Symp. Series, 398, 291 (1989). S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen. ACS Symp. Series, 218, 79 (1983). J. J . Pluth, J. V. Smith, J . M. Bennett and J. P. Cohen, Acta Crystallogr. , Sect. C , 40, 2008 (1984). J. B. Parise, Acta Crystallogr. , Sect. C , 40, 1641 (1984).
73
Molecular Design of Two-Dimensional Zirconium Phosphonate Catalysts
K. Segawa, A. Sugiyama, and Y. Kurusu Department of Chemistry, Faculty of Science and Technology, Sophia University, Tokyo 102, Japan
ABSTRACT Preparation and characterizationof two-dimensional zirconium phosphonate derivatives in either crystalline or amorphous forms have been investigated. Two composite zirconium phosphonates in single crystal phase have also been investigated and characterized by XRD, and 31PMASNMR. The catalytic performance over zirconium phosphonates are evaluated by hydrolysis of ethylacetate in aqueous solution. When the composite zirconium phosphonate is composed with an acidic function and with a hydrophobic function in single crystal phase, the catalytic activity in aqueous medium showed higher activity than that of single acidic zirconium phosphonate. The composite materials become accessible to any reactant molecule and improve hydrophobicity. INTRODUCTION Recently, several kinds of layered compounds have been proposed for use as catalysts; these include silicates, graphite, and acid salts of tetravalent metals. These materials can be expected to provide new applications such as shape selective ion-exchangers, adsorbents, and catalysts [ 11. Zirconium phosphates or phosphonates, whose compounds have the general formula Zr(03PR)2, can be obtained in amorphous forms or in crystalline forms having various layered structures. These compounds are lamellar structures comprising zirconium phosphates or organophosphonates. In the case of zirconium phosphates, such as Zr(03POH)2, the most extensively investigated crystal is the a layered acid salt, zirconium bis(monohydrogen orthophosphate), which is usually found as the mono-hydrated form. a-Zr(03POH)2-H20 crystallizes in the monoclinic system, as assigned by Troup and Clearfield from their single crystal work [2]. Each layer consists of planes of zirconium bridged through phosphate groups which alternate above and below the Zr atom planes 111. The proton of the phosphate group can be replaced by another cation without any alteration in the structure of the layer itself [3]. For the zirconium phosphonates, however, the two-dimensional tetravalent metal plane has a structure essentially similar to the zirconium phosphate structure; substituted for hydroxyl groups are the desired organic functional groups, oriented away from the basal surfaces in a bilayered fashion in the interlayer region [4]. Therefore, zirconium phosphonate materials can act as a series of modified surfaces, and become accessible for reactant molecules of catalysis.
74 K. Segawa, A. Sugiyama and Y. Kurusu
This work is aimed at establishing a preparation procedure for zirconium phosphonates not only in the single component phosphonate but also in the composite zirconium phosphonates in single crystal phase, and to establish a procedure to characterize these compounds. EXPERIMENTAL PreDaration of zirconium DhosDhonates Addition of a soluble Zr(1V)salt to phosphoric acid results in the precipitation of a gelatinous amorphous solid. The stoichiometric crystalline zirconium phosphate can be prepared by refluxing zirconium phosphate-gel in concentrated phosphoric acid [5]. The procedures for synthesis of zirconium phosphate have been described in detail elsewhere [6]. Zirconium phosphonates are solid materials typically synthesized in amorphous forms under aqueous conditions by the reaction of a soluble salt of a tetravalent metal and a phosphonic acid or an organophosphoric acid: 0
Zr4'
+
II
2(0H)2PR
-
Zr(O3PR)2
(1)
Several types of zirconium phosphonates, Zr(QPR)2, either in single component, such as one kind of R, or in composite zirconium phosphonates, such as two different Rs, are prepared: here R could be -H, -OH, -CH3, -C&, -C12H25, -C12H25, -C22H45, -CH2COOH, -C2H4COOH and -CH2S03H. Most of the zirconium phosphonate derivatives are obtained by the addition of aqueous solution of ZrOClz 8HzO (1.17 M) to phosphonate precursor, (OH)2P(O)R, (0.75 M, [RY[Zr]=2-10). For the crystallization,amorphous zirconium phosphonates were treated with HF solution (ca. 46 %) at 333K [7]. All crystals obtained by this procedure are washed with distilled water, acetone, and dimethylether; these washings are followed by drying at room temperatures. Characterizationof zirconium phosphonates The experimental procedures of BET, TGA, and XRD have been described in detail elsewhere [8]. The interlayer d-spacing from XRD pattern is determined by the angle of (001)reflection. For the measurement of ion-exchange capacity, 50 mg of sample was suspended with 10 cm3 of NaCl solution (0.1 N), and the pH values were measured with addition of NaOH solution (0.1 N) to obtain the potentiomeaic titration curve at room temperature. High-resolution solid-state MASNMR spectra are obtained on a Fourier Transform pulsed NMR spectrometer (JEOL, JNM-(3x270) equipped with a CP/MAS unit (JEOL,NM-GSH27MU). All 31P-NMR spectra combined with cross polarization (CP) and or with magic angle spinning (MAS) at 109.38 MHz are measured with high-power proton decoupling during data acquisition [6]. The 13C-MASNMR spectra are obtained at 67.94 MHz with CP and proton decoupling. Sample spinning speeds, determined from the side band spacing in spinning spectra, were 3.6 to 4.0 kHz. The catalytic activities of hydrolysis of ethylacetate at 341 K were measured in aqueous phase (0.68 M). 250 mg of catalyst was suspended in aqueous solution of ethylacetate, and the reaction rates were measured by GC (PORAPAK Q, 2-m).
Two-Dimensional Zirconium Phosphonate Catalysts 75
RESULTS AND DISCUSSION The single comwnent zirconium Dhosphonates The structure of zirconium phosphonates is that of a zirconium phosphate core layer, with pendant organic groups attached to this core and extending perpendicular to the plane of the layer. TABLE 1 shows physico-chemicalproperties of single crystalline zirconium phosphonates. The data of elementary analysis are in fairly good agreement with values calculated from each chemical formula. The XRD patterns of amorphous zirconium phosphonates do not show any crystal phases. The layer separation (interlayer d-spacing) of each crystalline zirconium phosphonate is determined from the first reflection line of the XRD pattern. The XRD data c o n f h the expected correlation of interlayer spacing with the size of the pendant organic group. Solid products consisting of layered sheets with ordered arrays of the pendant organic groups on both sides of the layer are obtained with most phosphonic acids. The TGA curves of zirconium phosphonates show the decomposition of pendant groups in several steps, due to the elimination of alkyl or hydroxy groups. The thermal stability values of zirconium phosphonates, which are determined by the first break-point of the TGA curve, are shifted towards lower temperatures as the number of carbons of alkyl groups increases. When the zirconium phosphonate is heated above 1300 K, regardless of the original structure, each XRD pattern is also changed to cubic zirconium diphosphate. No ion-exchange capacity was found for pure alkyl zirconium phosphonate; however, when R is replaced with -OH, -CH2COOH, -C2&COOH, or -CH2SO3H, the ion-exchange capacity showed numerical results similar to values calculated from each chemical formula. For the catalytic test reaction, hydrolysis of ethylacetate at 341 K in aqueous solution (0.67 M), showed relatively lower activity except over Zr(03CH2SOsH)2. In this case, the activity value is higher than that for the hydrogen type of Nafion. However, the reaction proceeds under the homogeneous reaction, since Zr(@CH2S03H)2 was a very water-soluble material. The results suggest that, even when the acidic functions of zirconium phosphonate are present (R; -OH, -CH2COOH, -C2&COOH) between each pair of layers, the reactant molecules are not accessible, because the interlayer d-spacings are narrower than those of alkyl zirconium phosphonates. As was stated previously, the Zr atoms lie very nearly in a plane and are bridged by phosphorous tetrahedra. These are situated alternately above and below the Zr atom plane. Three oxygen atoms of each phosphorous atom are bonded to three different Zr atoms, which form a distorted equilateral triangle. Thus each phosphonate, such as R of Zr(O3PR)2, is directed toward the interlayer space between each pair of layers. 31P-MASNMR and 13C-CP/MASNMR have been employed to study the phosphorous and carbon micro-environment of layered zirconium phosphonates. 31P-MASNMR spectra of crystalline zirconium phosphonates are shown in Fig. 1. Each chemical shift of crystalline zirconium phosphonate from phosphoric acid represents the single identical resonance line, which shows one kind of phosphorous environment between each pair of layers. The chemical shifts of 13C-CPMASNMRof zirconium phosphonates are in good agreement with that of each carbon of alkyl functional groups.
76 K. Segawa, A. Sugiyama and Y . Kurusu
TABLE 1 Physico-ChemicalProperties of Various Crystalline Zirconium Phosphonates Compound Zr(03PH12 2(03PcH3)2 zr(@pC6H5)2 2(03PC12H25)2 zr(o3pc22&5)2 ~(03~H)2'H20 Zr(WCH2COOW2 Zr(QPCz&COOH)z Zr(03PCH2S03H)2 H
LD. /nm
Thermal stabilitya /K
0.57 0.78 1.55 3.24 5.62 0.76 1.13 1.34 1.54
1238 873 733 754 505 823 632 633 533
Ion-exchange Catalytic activityb capacity Imeq g-1 /lO-7molg-1s-1 0.00 0.00 0.00 0.00 0.00 6.50(6.67)C 6.30 (5.45) 5.40 (5.06) 5.79 (4.55) 3.61
0.26 0.10 0.35 0.20 0.91 0.59 0.61 1.12 55.3 48.7
a Thermal stability is determined by the fist break-point of TGA. b Catalytic activity for hydrolysis of c Numbers in parentheses are calculatedfrom chemical formulae. ethylacetate at 341 K
L _k -- - B
C
E
21.0
*
*
60
h
0
-6060
*
*
*
h. 4*7
-6060
0
6.6
0
-6060
0
*
*
J d sh, I
-6060
I
I
0
-60
Chemical shift / ppm
G
H
I
I
_t L A LL ----9.0
9
2A
60
0
-6060
0
*
A -2.6 ,
-6060
0
Chemical shift / ppm
-6060
0
-60
Two-Dimensional Zirconium Phosphonate Catalysts 77
The composite zirconium phosDhonates in sinde crvstal -Dhase The acidic function of single zirconium phosphonate showed rather poor catalytic activities for hydrolysis of ethylacetate in aqueous solutions. In addition, over Zr(03PCH2S03H)2 catalyst, the reaction proceeds as a homogeneous reaction, even though the catalytic activity is higher than other acidic zirconium phosphonates. The objective of this study is to explore the role of a second phosphonate function in single crystal phase on the catalytic performance of acidic function and hydrophobic function of zirconium phosphonates and to leam how to exploit this second function to achieve a catalytic advantage in certain applications. For the preparation of composite zirconium phosphonates, we preferred two different functions: -OH, -CH2COOH, -C2H4COOH, and -CH2SO3H groups for acidic function, and -CH3, -C6H5, -C12H25, -C12H25, and -C22H45 groups for hydrophobic function, to give for Here x is the initial mole fraction of the acidic group for example Zr[(O3pOH)~(O3pC12H~)1-*]2. preparation. This x is not changed even after the crystallizationprocess; the values are confirmed by elemental analysis and by potentiomemc titration. For the crystallization process of amorphous composite phosphonates, Fig. 2 shows the BET surface area of Z~[(O~~OH)O.~(O~PC~H~)O.~ as a function of HF concentrations, [r;l/ [Zr], for crystallization. A much higher surface area of amorphous material is obtained; the BET area is about 300 m2g-l. When HF concentrations are increased for the crystallization, the BET areas are decreased. The crystallinity of composite zirconium phosphonates increased with increasing concentrations of HF. However, some segregation of each function is observed when HF concentrations, CF] / [Zr], exceed above 9. In this case, the XRD patterns show two different crystal phases; one for Zr(O3POH)2, which is 0.76 nm, the other one for Zr(@PC6Hs)2, which is 1.55 nm. For further preparation of crystalline composite zirconium phosphonate. we preferred n/ [Zr] = 6 or 8 in order to avoid segregation.
t
"
0
2
4
6
8
1
0
1
2
19 1 [Zrl Fig. 2 Surface area of composite Z~[(~~POH)O.~(O~~C&H~)O.~I~ as a function of [Fl / [Zr]
78 K. Segawa, A. Sugiyama and Y. Kurusu
I
I
2
I
I
30
40
I
20
10
2 8 I degree
Fig. 3 XRD patterns of composite zr[(e3PoH),(O3PClzH2)~-,]~as a function of x. Interlayer d-spacing of composite material depended on the bulkier functional groups. Figure 3 shows the XRD pattern of composite Zr[(OsPOH),(O3PCl2H25)1.,12 as the function of x. The patterns also do not show any segregation regardless of concentration of each component. In addition to the interlayer d-spacing, values determined from (01) reflection show constant (3.24 nm) values similar to that of single zirconium dodecyl phosphonate, Zr(03PC12H25)2. Ionexchange capacities are increased by increasing the concentration of P-OH groups. The results suggest that the functional groups are composed in single crystal phase. 110
100
-
-0 80 $2
90
A.
\
v) v)
.-
2
a
A *********,
.
a .
B ICIIC.mmmmmm
B
70-
a
mm m
a
601 50 40
a a
-
C
41+CYI......aa
I
.
.
I
.
.
I
.
.
a
l
.
8
.
Fig. 4 TGA curves of crystalline zirconium phosphonates: (A) 01-Zr(03PoH)~H~O, (B) Zrl(O3POH)o.~(O3PCl~H~5)0.5]2, and (C) Zr(QPC12H25)2. heating rate; 10 K-min-1
Two-Dimensional Zirconium Phosphonate Catalysts 79
The TGA curve of a-Zr(03POH)rH20 (A in Fig. 4) shows a two-stage decomposition curve due to rhe elimination of 1 mol of water of crystallization by lower temperature regions and to the condensation of phosphate groups with consequent loss of 1 mol of water at higher temperature regions [8]. For the alkyl zirconium phosphonate, such as Zr(O$C12H=)2, the TGA curve shows one-stage weight loss curve due to the elimination of dodecyl groups. And it has no water of crystallization between each pair of Zr atom planes. The TGA curve (B in Fig. 4) of crystalline Z~[(O~POH)O.~(O~PC~~H~~)O.~]~ shows a TGA curve similar to that of Zr(03PC12H25)~(C in Fig. 4) but not to that of a-Zr(03POH)~H20.The results suggest that the composite zirconium
phosphonate has no water of crystallization and that condensation of phosphate groups and elimination of dodecyl groups have occurred in similar temperature regions. Finally, regardless of the original structure of zirconium phosphonate, when those samples are heated above 1100 K, the crystals changed to complete cubic zirconium diphosphate, which has no functional groups. Fig. 5 shows 31P-and 13C-MASNMR spectra of crystalline Z ~ [ ( O S P O H ) ~ ( ~ P Cas~ ~ H ~ ~ ) ~ a function of x. For 31P-MASNMR spectra, two identical resonance lines are observed: -18.9 ppm for phosphorus of P-OH, and 7.7 ppm for phosphorus of P - C ~ ~ H ~The S . contribution of each resonance line varies as a function of x . The interlayer d-spacing of these compounds showed 3.24 nm without segregation regardless of x values, and the compounds have reasonable amounts of ion-exchange capacities in reference to the x values. 13C-CP/MASNMR spectra of composite materials show the same resonance lines as those of single zirconium dodecyl phosphonate, [Zr(03PC12H25)2]. The major resonance line, which is attributed to the carbon of -CH2-, is observed at 33 ppm from TMS. The other two identical lines in 13C-CP/MASNMRspectra are attributed to carbon of P-CH2- at 25.1 ppm and to carbon of -CH3 at 15.0 ppm.
A
I40
m 20
0 -20 -40
-
50403020100
Chemical shift / ppm Fig. 5 31P-MASNMR (A) and 13C-CP/MASNMR (B) spectra of composite Zr[(03POH),(03PC12H25)1,12 as a function of x.
A. Sugiyama and Y.Kurusu
10
I
8 -
6 -
0.0
0.2
0.4
0.6
0.8
1 .o
X
Fig. 6 Hydrolysis of ethyl acetate over Z ~ [ ( O ~ P O H ) ~ O ~ ~asCa ~function ~ H ~ of S )x.~ ~ ~ ] ~ Physico-chemicalproperties of composite Zr[(~POH)x(03pCl~H2s)~;rl~ either in amorphous or crystalline forms as a function of x have been studied. Ion-exchange capacities are almost consistent with their calculated values, and their values increased with increasing concentration of P-OH groups. The results suggest that functional groups are distributed uniformly between each pair of Zr atom planes and that the interlayer d-spacing obeyed the interlayer d-spacing of zirconium dodecyl phosphonate, Zr(QPC12Hz)z. For catalytic hydrolysis reactions in aqueous medium on zirconium phosphonates, the composite materials show higher catalytic activities than single zirconium phosphonate (Fig. 6). After introduction of hydrophobic functions to zirconium phosphate, the composite materials became accessible to any reactant molecule and improved in hydrophobicity. REFERENCES 1 M. S. Whittingham and A. J. Jacobson, Intercalation Chemistry (Materials Science Series) Academic Press, New York, 1982. 2 J.M. Troup and A. Clearfield, Znorg. Chem., 16 (1977)331 1. 3 G.Alberti, Acc. Chem. Res., 11 (1978)163. 4 G. Alberti, U.Costantino, S. Allulli and N. Tomassini, J . Inorg. Nucl. Chem., 40 (1978) 1 1 13. 5 A,Clearfield and J. A. Stynes, J . Inorg. Nucl. Chem., 26 (1964)117. 6 K. Segawa, Y.Nakajima, S , Nakata and S, Asaoka, J. Curd.. 101 (1986)81. 7 M. B. Dines and P. M. DiGiacomo, Inorg. Chem., 20 (1981)92. 8 K. Segawa, Y.Kurusu, Nakajima, Y.and M. Kinoshita, J. Curd., 94 (1985)491.
81
Mesoporous Materials Produced from Hydrothermally Synthesized Hectorites
Torii, T. Iwasaki, Y. Onodera and K. Hatakeda Government Industrial Research Institute, Tohoku, Nigatake 4-2-1, Sendai 983, Japan K.
Miyagino-ku,
ABSTRACT Novel mesoporous materials having exceptionally large pores were produced from hydrothermally synthesized silicate-bearing hectorites. Experiments suggest that interlayer anisotropic silicates act as larger pillars. Method o f preparing such materials is described, together with their porous properties. The mesoporous material from a precursory hectorite synthesized at 15OoC has a total specific surface area of 848 m2g-’, a pore volume of 0.98 cm3g-l and an pore average diameter of 46 1, values which are significantly higher than those of conventional pillared clays. INTRODUCTION Smectite minerals, which consist of two-dimensional silicate layers separated by hydrated exchangeable cations, swell with a variety of molecules and form intercalated complexes. Metal-oxide pillared clays, prepared from smectites and polynuclear metal complex cations, have attracted considerable attention as new types o f molecular sieves, which are structurally different from zeolites. These materials offer new possibilities as catalysts and adsorbents [ I ] . Recently a new hydrothermal method of producing silicate-bearing hectorites was proposed [2.3]. On dehydration of interlayer water these synthetic hectorites converted to porous materials with mesopores and micropores [4].Organophil ic hectorites prepared from silicate-bearing hectorites and a dialkyl dimethyl quaternary ammonium chloride showed attractive rheological properties in organic solvents [5]. Upon removal of organic materials by heating, they transformed into novel mesoporous materials characterized by extremely large specific surface areas and high thermal stability. The present paper is concerned with these mesoporous materials produced from hydrothermally synthesized silicate-bearing hectorites. EXPERIMENTAL
82 K. Torii. T. Iwasaki, Y. Onodera and K. Hatakeda
Preparation
of
mesoporous m a t e r i a l s
S i l i c a t e - b e a r i n g h e c t o r i t e s having d i f f e r e n t l a y e r charge and i n t e r l a y e r s i 1 i c a t e content were synthesized hydrothermal l y a t 1 25-300°C under autogeneous water vapor pressure f o r 2 h from a s l u r r y o f Si:Mg:Li:Na=4.00:2.70:0.30:0.35. O r g a n o p h i l i c h e c t o r i t e s were prepared from t h e s y n t h e t i c s i l i c a t e - b e a r i n g h e c t o r i t e s and a d i a l k y l dimethyl quaternary amnonium (ANK1A) c h l o r i d e c o n t a i n i n g 75% octadecyl,
24% hexadecyl, and 1 % octadecenyl groups as a l k y l groups ( t r a d e
name: Arquad 2HT-75,
L i o n Akzo C o . , L t d . ) .
Both h e c t o r i t e s and quaternary ammonium
were d i s s o l v e d i n hot water (8OoC) s e p a r a t e l y t o g i v e a c o n c e n t r a t i o n o f 2%. and then mixed, s t i r r e d , and b o i l e d f o r 30-60 min. A f t e r f i l t r a t i o n and washing w i t h warm water,
the o r g a n o p h i l i c h e c t o r i t e s were d r i e d and powdered. The amounts o f
quaternary ammonium e q u i v a l e n t t o t h e amount o f methylene b l u e adsorbed were used f o r r e s p e c t i v e h e c t o r i t e s except the value o f 0.96 meq g-l f o r samples H-01 and H-02. Mesoporous m a t e r i a l s were prepared from o r g a n o p h i l i c h e c t o r i t e s by h e a t i n g a t 300-900°C
i n the atmosphere f o r 1 h.
A n a l y t i c a l procedures S p e c i f i c surface areas, pore volumes, pore diameters and pore s i z e d i s t r i b u t i o n s were c a l c u l a t e d from the n i t r o g e n adsorption-desorption isotherms f o r 1 h using Micromeritics
a t -196OC on t h e samples heated a t 300'-900°C
a c c e l e r a t e d s u r f a c e area and porosimetry ASAP 2400. The micropore volume and mesopore surface area o f samples were obtained by T-plot method [6]. The diameter o f mesopores i s designated more than -20
8.
The amounts o f organic m a t e r i a l
present i n the h e c t o r i t e s were measured by thermal g r a v i m e t r i c a n a l y s i s u s i n g a Rigaku Thermof l e x thermal balance. X-ray powder d i f f r a c t i o n (XRD) analyses were c a r r i e d o u t w i t h a Rigaku d i f f r a c t o m e t e r
(RAD-I1 B) u s i n g monochromatized CuKa
r a d i a t i o n . Methylene b l u e (ME) a d s o r p t i o n c a p a c i t i e s were measured t o e v a l u a t e c a t i o n exchange c a p a c i t i e s o f h e c t o r i t e s .
RESULTS AND DISCUSSION Properties
silicate-bearing hectorites
and
organophilic hectorites
The e f f e c t o f hydrothermal s y n t h e s i s temperature on MB adsorbed,
interlayer
s i l i c a t e content and 001 spacing o f s i l i c a t e - b e a r i n g h e c t o r i t e i s shown i n Table -1 . 1. The MB a d s o r p t i o n increased from 0.28 t o 1.16 meq g w i t h increasing temperature i n the range 125°-3000C. whereas the i n t e r l a y e r s i l i c a t e content decreased from 64 t o 16 wt%. The change i n i n t e r l a y e r s i l i c a t e content
appeared
t o harmonize w i t h t h a t o f Langmuir s p e c i f i c surface area o f the s y n t h e t i c h e c t o r i t e s shown i n Table 3. As expected f o r a smectite, samples H-01-H-15
the layers o f s i x
expanded r e a d i l y on ethylene g l y c o l a t i o n ; however, abnormally
l a r g e 001 spacings f o r f o u r samples H-01-H-10
were observed i n c o n t r a s t t o
Mesoporous Materials Produced from Hectorites 83
Table 1. Effect of hydrothermal temperature on the methylene blue (MB) adsorbed, interlayer silicate content and 001 spacing of silicate-bearing hectorites Synthesis temp. OC
Samp I e H-01 H-02 H-05 H-10 H-12 H-15
MB adsorbed meq g-1 0.28 0.70 0.84 0.96 1. 08 1. 16
125 150 180 200 225 300
d (001)
Interlayer si I icatea wt. %
/a EG~
Air dried
64 48 34 29 18 16
18.8 17.4 17.3 14.3 13.2 13.6
23.9 20.0 18.5 17.4 17. 1 17. 1
acalculated from the dehydration amount of low temperature structural water between 300-650°C, bEthylene glycolated.
the spacing of 17 for the Na-smectites [71. Both 001 spacings of air dried and of ethylene glycolated hectorites decreased with increasing synthesis temperature, fitting well with the change in interlayer silicate content. These results suggest that hydrothermal products revealed smectite-like properties gradually and lost porous property as the synthesis temperature increased. The silicate-bearing hectorite is probably a kind of unstable smectite mineral. Sample yield, 001 spacings and content of intercalated AMQA cation of organophilic hectorites are shown in Table 2. The sample yield based on the precursory synthetic hectorite was 86-93%; thus the amount of AMQA cation in the organophilic hectorites showed slightly larger values compared with the expected amount. I n the samples OH-05-OH-15, the amount o f intercalated AMQA cation corresponded to the layer charge. Basal spacings of organophilic hectorites expanded by intercalation of AMQA cations, although those of three samples OH01-OH-05 were obscure. Table 2. Yield, 001 spacings, layer charge and dialkyl dimethyl quaternary ammonium (AMQA) cation content for the organophilic hectorites ~~
d (001)
Sample
Yielda 96
R Air dried
Layer chargeb eq/Olo (OH) 2 A
AMQA cat ion eq/Ol (OH) Used Contentc B C
Rate
C/B
C/A
0.45 0.44 0.38 0.43 0.46 0.49
118 118 I11 107 104 106
408 163
%
~~
OH-01 OH-02 OH-05 OH-I0
OH-I2 OH-I5
91
uc uc uc
90 93 93
38 27.8 31.3
86 92
0.13 0.32 0.38 0.43 0.46 0.49
0.53 0.52 0.42 0.46 0.48 0.52
111
107 104 106
aCalculate$ based on the precursory synthetic hectorite, bCalculated from MB adsorbed, Calculated from thermogravimetric data.
84 K. Torii.
T.Iwasaki. Y . Onodera and K. Hatakeda
Porous characteristics of the mesoporous material OH-02-600 Typical nitrogen adsorption-desorption isotherms at liquid nitrogen temperature for the mesoporous material OH-02-600 and its precursory hectorite H-02-300 are shown in Fig. 1. The isotherm of H-02-300 is of type I in the classification of Brunauer, Deming and Teller [8]and possesses a small hysteresis loop indicating that H-02-300 has both micropores and mesopores. Meanwhile the isotherm of OH-02-600 is of type IV and the hysteresis loop is of type H2 according to the manual of International Union of Pure and Applied Chemistry [9]. Some corpuscular systems tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined [lo]. The difference between the type IV for OH-02-600 and type I for H-02-300 reflects the larger interlayer spacing in the former materials. Type I and type I V isotherms were observed respectively for the AI2O3-pillared clay [l I] and Ti02-pillared clay [12.131. The most important difference between the mesoporous material OH-02-600 and the Ti0 -pillared clay is the considerably larger nitrogen amount adsorbed on 2 the former material. As shown in Table 3, the pore volume increased from 0.228 to 0.984 cm3g-' and the average pore diameter extended from 17. 1 to 46.4 8, by the transformation from H-02-300 to OH-02-600. The pore volume of OH-02-600 is fouror five-fold in contrast with Ti02-pillared clay (0.190-0.270 cm3g-') [12,131. These results ndicate that the mesoporous material OH-02-600 having exceptionally large pores was produced from the synthetic silicate-bearing hectorite H-02 by the intercalat on of AMQA cation and the removal of organic materials.
'1,
012
0:4
016
0:8
Relative pressure / P/PO
Fig. 1. Nitrogen adsorption-desorption isotherms at -196OC for the mesoporous material OH-02-600 and its precursory synthetic hectorite H-02-300. Open symbols: adsorption, Solid symbols: desorption.
Mesoporous Materials Produced from Hectorites 85
0
0 c X
?
2
5 . Q
ii
D
Pore diameter I Fig. 2.
i
Pore s i z e d i s t r i b u t i o n f o r t h e mesoporous m a t e r i a l OH-02-600.
F i g u r e 2 shows t h e pore s i z e d i s t r i b u t i o n s d e r i v e d from t h e d e s o r p t i o n branch o f the isotherm f o r t h e mesoporous m a t e r i a l OH-02-600.
appears t o be i n pores o f about 37
8.
Most o f t h e pore volume
The pore s i z e o f the mesoporous m a t e r i a l
OH-02-600 i s about t w i c e t h a t o f T i 0 2 - p i l l a r e d c l a y [ I l l . Pore volumes o f h e a t - t r e a t e d mesoporous m a t e r i a l OH-02 and i t s p r e c u r s o r y s y n t h e t i c h e c t o r i t e H-02 as a f u n c t i o n o f temperature a r e shown i n Fig. 3. The m a t e r i a l OH-02 was s t a b l e a f t e r being h e a t - t r e a t e d t o 6OO0C, a t which temperat u r e the volume s t a r t e d t o decrease. On t h e o t h e r hand, the pore volume o f t h e precursory h e c t o r i t e H-02 s t a r t e d t o decrease g r a d u a l l y a t 40OoC. As shown i n Fig. 3, t h e pore volume increased w i t h i n c r e a s i n g temperature i n the range of
1.0
-3
0.5
-
4
t 0
P
U
H-02
Heat-treatment temperature I @C Fig. 3. Pore volumes o f the mesoporous m a t e r i a l OH-02 and i t s precursory s y n t h e t i c h e c t o r i t e H-02 as a f u n c t i o n o f heat-treatment temperature.
86 K. Torii, T. Iwasaki, Y. Onodera and K. Hatakeda
300'-600'C.
T h i s f i n d i n g can be explained by the removal o f i n t e r l a y e r o r g a n i c
m a t e r i a l s by heat-treatment.
The sample c o l o r change from b l a c k t o w h i t e supports
t h i s explanation. The pore volume and s p e c i f i c surface area reached maximum (pore 2 -1 volume of 0.984 cm3g-l and s p e c i f i c surface area o f 848111g ) a t 600'C. The m a t e r i a l OH-02 showed decreases i n pore volume w i t h i n c r e a s i n g temperature from 600'
C t o 8OO0C, and r e t a i n e d a pore volume o f 0.345 cm3g-'
o f 255 m2g-l
-Effect
and a s u r f a c e area
a t 80OoC.
o f s y n t h e s i s temperature o f the p r e c u r s o r y h e c t o r i t e s on t h e porous
p r o p e r t i e s o f t h e mesoporous m a t e r i a l s Table 3 shows the s p e c i f i c surface areas, pore volumes and average pore diameter f o r several mesoporous m a t e r i a l s and t h e i r precursory s y n t h e t i c h e c t o r i t e s . The BET s p e c i f i c surface areas o f t h e mesoporous m a t e r i a l s d e r i v e d from t h e s i l i c a t e - b e a r i n g h e c t o r i t e s synthesized a t 150'-300°C t o 229 m2g-l
decreased from 848
w i t h i n c r e a s i n g synthesis temperature. The amount o f i n t e r l a y e r
s i l i c a t e s may r e f l e c t upon these s p e c i f i c surface areas (Fig. 4). The pore volumes a l s o decreased i n the same manner as the s p e c i f i c surface areas. The mesoporous m a t e r i a l s produced from the s i l i c a t e - b e a r i n g h e c t o r i t e s synthesized above 2OO0C possessed mesopores and micropores. This may be due t o t h e small content o f i n t e r l a y e r s i l i c a t e s . The s p e c i f i c surface area and pore volume o f t h e m a t e r i a l OH-01-600 whose precursory h e c t o r i t e was synthesized a t 125OC showed
Table 3. S p e c i f i c surface areas (SSA), pore volumes (PV) and average pore diameter (APD) f o r several mesoporous m a t e r i a l s and t h e i r precursory s y n t h e t i c hectorites
Samplea
H-01-300* OH-01 -600 H-02-300* OH-02-600 H-05-300* OH-05-600 H-lO-300* OH- 10-600 H- 1 2-300* OH-1 2-600 H-15-300* OH- 15-600
SSAb 2 -1 mg
618 738 532 848 534 560 488 410 318 261 269 229
Whole pore PV APD 3 -1 51 cm g A
0.243 0. 779 0. 228 0.984 0. 230 0. 622 0. 249 0. 598 0. 162 0. 243 0. 154 0. 255
15.7 42. 2 17. 1 46.4 17. 2 44. 4 20. 4 58. 3 20.4 37. 2 22.9 44. 5
Mesopore PV 3 -1 cm g B
0. 142 0. 776 0. 147 0.984 0..151 0. 622 0. 168 0. 584 0. 115 0. 194 0. 113 0. 231
Micropore PV cm g
-'
0.101 0. 003 0. 082 0.000 0. 078 0.000 0. 081 0. 014 0.048 0.049 0.040 0.024
Mesopore ratio B/A
0. 58 1. 00 0. 67 1.00 0. 66 1 . 00 0. 68 0. 98 0. 71 0. 80 0. 73 0. 91
Increment Pore vo 1ume ratio'
3. 21 4. 32 2. 70 2. 40
1. 50 1. 66
aLast t h r e e f i g u r e s designate the heat-treatment temperature f o r 1 h, bCalculated by BET equation f o r the samples unless o t h e y i s e s p e c i f i e d , and by Langmuir Pore volume r a t i o o f the mesoporous equation f o r the samples designated w i t h m a t e r i a l t o i t s precursory s y n t h e t i c h e c t o r i t e s .
*,
Mesoporous Materials Produced from Hectorites 87
lnteriayer silicate content / wt.%
Fig. 4. Specific surface areas for several mesoporous materials as a function of interlayer silicate content. slightly smaller values compared with the material OH-02-600. This may reflect a combination of low layer charge and high interlayer silicate content. The average pore diameter of the mesoporous materials changed from 42. 2 to 58. 3 & between the synthetic temperature range of 125°-3000C. The pore diameter of 46. 4 & for the material OH-02-600 calculated from (4 pore volume / specific surface area) is slightly greater than the 37 8 obtained from the pore-size distribution as indicated in F i g . 2. The expected layer structure change from the precursory silicate-bearing hectorites to the mesoporous materials is represented schematically in Fig. 5. In the silicate-bearing hectorites, anisotropic platy silicates exist lying flat between the silicate layers: they therefore give smectite-like basal spacings (Table 1 ) . Layers of silicate-bearing hectorites expand by the intercalation of the lengthwise AMQA cation and simultaneously the anisotropic silicates stand normally to the layer. Ultimately the mesoporous materials can be formed by the removal of organic materials. Rearranged interlayer anisotropic silicates act as
I
11
TI---
ir
I
intercalation of AMQA cation
Heat-treatme
a
0
Siilcate-bearing hectorlte
Organophilic hectorlte
Mesoporous materlai
Fig. 5. Schematic of the proposed formation of mesoporous materials from silicate-bearing hectorites.
88 K. Torii, T. Iwasaki, Y. Onodera and K. Hatakeda
long p i l l a r s i n the mesoporous m a t e r i a l s which u n t i l now had never been reported.
CONCLUSION 1.
H e c t o r i t e s which include a n i s o t r o p i c p l a t y s i l i c a t e s i n the i n t e r l a y e r s c o u l d be hydrothermally synthesized. Layer charge, s i l i c a t e content, etc. c o u l d be c o n t r o l l e d by the synthesis conditions.
2. Layers o f s i l i c a t e - b e a r i n g h e c t o r i t e s were expanded by the i n t e r c a l a t i o n o f the lengthwise a l k y l quaternary ammonium c a t i o n and simultaneously t h e i n t e r l a y e r a n i s o t r o p i c s i l i c a t e s stood normally t o t h e layer. 3. Mesoporous m a t e r i a l s i n which rearranged i n t e r l a y e r a n i s o t r o p i c s i l i c a t e s a c t as long p i l l a r s were produced as the heat- t r e a t e d products o f o r g a n o p h i l i c h e c t o r i tes. 4. Porous p r o p e r t i e s o f the mesoporous m a t e r i a l s w e r e mainly c o n t r o l l e d by t h e s i l i c a t e content and h e a t - t r e a t i n g condition s . 5. The mesoporous m a t e r i a l s produced from a precursory h e c t o r i t e synthesized a t 15OoC had a t o t a l s p e c i f i c surface area o f 848 m2g-l, cm3g-l and an average pore diameter o f 46
8,
a pore volume o f 0.98
values which a r e s i g n i f i c a n t l y
h i g her than those o f conventional p i l l a r e d clays.
ACKNOWLEDGMENT The authors wish t o express t h e i r sincere thanks t o Prof. M. Shimada o f t h e F a c u l t y o f Engineering, Tohoku U n i v e r s i t y , f o r h i s h e l p f u l suggestions. REFERENCES
1 2 3 4 5 6 7 8 9 10 11
12 13
J. Shabtai, M. Rose11 and M. Tokarz, Clays Clay Miner., 32(1984)99. T. Iwasaki and K. T o r i i , Ganko, 83(1988) 160. K. T o r i i and T. Iwasaki, Chem. L e t t . , (1988) 2045. K. T o r i i , T. Iwasaki, Y. Onodera and M. Shimada, Nippon Kagakukaishi, (1989), 345. T. Iwasaki, Y. Onodera and K. T o r i i . Clays Clay Miner., 37 (1989) 248. 6.C. Lippens and J. H. de Boer, J.Catal., 4(1965)319. G. Lagaly, Clay Miner., 16(1981) 1. S. Brunauer, L. S. Deming, W. Deming and E. T e l l e r , J. Amer.Chem. Soc., 62 ( 1 940) 1723. K. S. W. Sing e t a l . , Pure Appl. Chem., 57(1985)603. S. J. G r e w and K. S. W Sing, Adsorption, surface area and p o r o s i t y (Second E d i t i o n ) , Academic Press, London, 1982, p. 287. J. Shabtai, F. E. Massoth, M. Tokarz, G.M. Tsai and J. McCauley, i n G. E r t l (Ed), Proc. 8 t h I n t e r n a t . Congress C a t a l y s i s Vol. 4 (1984), Verlag Chemie. B e r l i n , P735. J. Sterte, Clays Clay Miner., 34 (1986) 658. S. Yamanaka, T. Nishihara and M. H a t t o r i , Mat. Chem. Phy., 1 7 (1987187.
89
Clays Pillared with Ceramic Oxides
Shoji Yamanaka and Makoto Hattori Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan
ABSTRACT Interlayers of montmorillonite can be pillared with various ceramic oxides by exchanging the interlayer cations with the precursory metal-hydroxy oligomeric cations, followed by dehydration. Metal oxide sol particles such as Ti02, SiOZ-TiOZ and Si02-FeZO can also be directly intercalated by ion-exchange. The silica mixed sol particfes are closely packed in the interlayer spaces and micropores are formed in the spaces surrounded by the sol particles and the silicate layers. The way of packing can be modified by using organic templates, which occupy a part of the interlayer spaces together with the sol particles. Burning off the organic part leaves behind mesopores in the structure. Clays pillared with ceramic oxides are characterized as microporous crystals having hydrophobicity and large acidity. INTRODUCTION Montmorillonite is a clay mineral, the structure of which is composed of stacked two-dimensional aluminosilicate layers with a thickness of molecular level. In the structure, A13+ cations of the octahedral sites are partially substituted by lower valence cations such as Mg2+. and the resulting net negative charges are balanced by hydrated exchangeable cations occupying the interlayer spaces, Various kinds of guest molecules intercalate into the interlayer spaces, replacing the interlayer water molecules around the cations [ l ] . From the viewpoint of having intracrystalline spaces in which guest molecules are accommodated, montmorillonite can be regarded as a two-dimensional zeolite. However, unlike zeolites, montmorillonite selects the guest molecules by affinity, i. e. by the strength of cation-dipole interactions rather than the molecular size. Non-polar molecules such as nitrogen, oxygen, and alkanes are not allowed to intercalate into montmorillonite, even if they are very small in size. In addition, the intercalated layers readily collapse if they are heated or degassed. In an effort to make the interlayer spaces more open and more easily accessible to non-polar as well as polar molecules, the silicate layers
90 S. Yamanaka and M. Hattori
have been pillared with thermally stable ceramic oxides [2]. PREPARATION OF PILLARED CLAYS General principles General procedures for the preparation of pillared clays are schematically The first and most important reaction for the introducillustrated in Fig. 1. tion of pillars is ion-exchange: the hydrated interlayer cations of montmorillonite are exchanged with precursory polynuclear metal hydroxy cations. After the ion-exchange, the montmorillonite is separated by centrifugation and washed with water several times to remove excess hydroxy ions. The interlayered hydroxy cations are then converted into the respective oxide pillars by calcination. The precursors developed so far and the interlayer spacings of their The pillar heights can be estimated by pillared clays are listed in Table 1. subtracting the thickness of the silicate layer (9.6 i)from the basal spacings.
ydra~ x ycation
:ide pillar
__)_
(ii)
Fig. 1. Schematic illustrations of the procedures for the preparation o f ceramic oxide pillared clays. (i) ion-exchange and (ii) dehydration by calcination
Table 1. Precursor cations developed for the introduction of oxide pillars and the basal spacings of the pillared clays. Pillar oxide A1203 Zr02 Fe203 Cr203 Biz03 A1 203-Si 02
Ti02 Si02-Ti02 Si02-Fe203
Precursor
Basal spacing,
17-19 17-20 17 21-17 16 17-1 9 24-27 40-50 40-100
Ref.
Clays Pillared with Ceramic Oxides 91
Most of the pillared structures are thermally stable up to about 500°C. and keep the specific surface area as large as 300-500 m2/g. The bismuth [ll] and the chromium oxides pillared clays collapse on heating to 300°C, the pillars being removed out of the interlayer spaces, although the chromium oxide with a larger basal spacing of 21 A is more thermally stable in a nitrogen atmosphere [lo]. New techniques Pillaring with oxide sol particles. As shown in Table 1, large oxide sol particles can be similarly exchanged with the interlayer cations of montmorillonite, as long as the particles are positively charged. Titanium oxide sols were prepared by the hydrolysis of titanium tetraisoproxide Ti(OC3H7)4, followed X-ray powder diffraction (XRD) by peptization with hydrochloric acid [13]. studies revealed that the Ti02 sol pillared structures with a basal spacing of about 27 A was stable, and no crystalline phases of TiOp were observed at least up to 500°C. though the samples contained about 50 wt% of Ti02 on 800°C dry basis. The nitrogen adsorption isotherm fits the BET equation, suggesting that the pore size of the titania pillared clay is large enough that multilayer adsorption of nitrogen molecules is possible. The pore size distributions derived from the nitrogen adsorption isotherms exhibit sharp distribution at 18-20 with a broad tail toward the larger pore sizes, if well-peptized titania sol solutions are used. The distributions are shifted toward larger pore size and become broader, if the peptization of the sol is not sufficient. This finding indicates that the sizes and the distributions of the pores are almost in accordance with those of the sol particles. A possible structural model for the arrangement of the titania sol particles The basal in the interlayer spaces is schematically shown in Fig. 2 [13]. spacing measured by the XRD analysis is the spacing of the parts well-ordered The sharp pore size disalong the direction normal to the silicate layers. tributions may come from the pores formed i n such parts. The broad pore size distributions accompanied by the sharp peak can be ascribed to the pores formed in the remaining disordered interlayer spaces. It should be noted that the structure shown in Fig. 2 is interesting not only for a porous structure, but also for microcrystalline Ti02 stabilized by incorporation in silicate layers 1181. Silica sol is also one of the oxide sols of interest as pillars. Although silica sol alone is negatively charged and can not be incorporated with negatively charged montmorillonite silicate layers, it can be intercalated if the surfaces are modified with positive ions. The addition of small amounts of Ti4' [15.16] and Fe3' [16,17] is sufficient to change the charge of the surface. In most cases, about 10 mol% of Ti4+ or Fe3' was added to SiO2 sols obtained by
92 S. Yamanaka and M. Hattori
Fig. 2. Schematic structural model of the interlayer arrangement of the titania sol particles 1131.
Ti4+ can be added in the form of a Ti02 sol the hydrolysis of Si(OC2Hg)4. In the case of the addition of Fe3+. it is necessary solution or Ti(OC3H714. to titrate the mixed solution with alkali up to about pH = 2.1 to deposit Fe3+ in the form of Fe-hydroxy cations. In both cases the basal spacings expand to more than 40 i. Contrary to the large basal spacings, however, the nitrogen adsorption isotherms are of the Langmuir type, indicating that the pore size is much smaller than the pillar height. Fig. 3 shows a structural model of the arrangements of Si02-Ti02 mixed sols particles. It appears that the sol particles are packed in such a way that micropores are formed in the spaces surrounded by the sol particles and the silicate layers of clay. Silica sols with much larger particle sizes such as commercial silica sols with average particle sizes of 100-200 can also be used for pillars, if small amounts of iron-hydroxy cations are deposited onto the surfaces. The larger the We can design the pore sol particle used, the larger the pores formed [17]. size and the distributions of the sol pillared clays by controlling the particle sizes of the sols intercalated. Use of organic templates In the current preparation of a new series of microporous crystals, organic templates are often used. Organic chemicals are also used in the preparation of pillared clays to control the pore sizes and the porosity as follows: The interlayer cations of silicate layer are exchanged with positively charged sol particles, and then a part o f the sols are exchanged with organic template cations such as octadecyl trimethyl ammonium (OTMA). During the exchange with the organic template, part of the sol particles are replaced with OTMA, changing the arrangement of the sol particles.
Clays Pillared with Ceramic Oxides 93
Fig. 3. Schematic structural model of Si02-Ti02 sol pillared clay [16].
In the Burning off the template leaves behind mesopores in the structure [19]. use of OTMA, the order of addition is very important. If OTMA is added first before adding sol oxides, OTMA occupies the interlayer spaces and prohibits the intercalation o f sol particles, since the organic cations are exchanged more selectively than sol oxides. This does not lead to increase in porosity. By using the s o l oxides first, a porous structure with a BET surface area exceeding 500 m2/g and a porosity of about 1.0 ml/g was obtained. Suzuki et al. [20] used polyvinyl alcohol (PVA) in the exchange reactions with aluminum hydroxy cations. They showed that neutral PVA molecules were intercalated into the interlayer spaces of montmorillonite and that even in the presence of PVA in the interlayer spaces, all of the interlayer cations were completely exchanged with aluminum hydroxy cations. The calcined sample showed The above two examples suggest that very sharp pore size distribution at 25 A. i n combination with suitable organic molecules or cations it would be possible to obtain different pore structures even from the same kind of metal-hydroxy cations and sol s.
ACIDIC PROPERTIES The acidic properties of alumina pillared clays have been extensively studied from the interest in using the pillared clays as cracking catalysts [21-241. Sakurai et al. [25] studied the acidic properties of the alumina pillared clays with different kinds of silicate layers and concluded that the alumina pillars between the silicate layers did not have any acidity and that the role played by the pillars was only to make the original acidity o f the silicate interlayers more easily accessible through opening the interlayer spaces. The acidity o f the sol oxide pillared clays was also studied by a titration method with Hammett indicators [16]. The acidity distributions of the three kinds of sol oxide pillared clays are shown in Fig. 4. The TiOE pillared clay
94 S. Yamanaka and M. Hattori
1.5-b
Ha
Fig. 4. A c i d s t r e n g t h d i s t r i b u t i o n f o r t h e c l a y s p i l l a r e d w i t h o x i d e sols heated a t 500°C: 0 , T i 0 : 0 , Si02Ti02; A , Si02-Fe O3 p i l ? a r e d c l a y s . The dashed l i n e siows t h e d i s t r i b u t i o n f o r a b i n a r y o x i d e Ti02-Si02 [16].
i s c h a r a c t e r i z e d by a v e r y s t r o n g a c i d i c s o l i d and furthermore, a l a r g e p o r t i o n o f i t s a c i d i t y comes from s t r o n g a c i d i c s i t e s : t h e p o r t i o n o f t h e weak a c i d i c s i t e s i s v e r y small.
The Si02-Ti02 p i l l a r e d c l a y i s a l s o a s t r o n g a c i d i c
s o l i d , b u t i t c o n t a i n s s i m i l a r amounts o f weak a c i d i c s i t e s .
The Si02-Fe203
p i l l a r e d c l a y a l s o has l a r g e a c i d i c amount, b u t most o f t h e a c i d s i t e s a r e v e r y weak.
The d o t t e d l i n e shows t h e a c i d i c d i s t r i b u t i o n o f t h e b i n a r y s o l i d o f
Si02-TiOz r e p o r t e d b y Shibata e t a l . [26].
T h i s c u r v e resembles t h o s e o f t h e
SiO2-TiO2 as w e l l as t h e T i 0 2 p i l l a r e d c l a y s i n t h e s t r o n g a c i d i c r e g i o n . Although i t i s n o t s u r p r i s i n g t h a t t h e Si02-Ti02 p i l l a r e d c l a y has v e r y s t r o n g acid sites, i t i s strange t h a t the Ti02 p i l l a r e d c l a y i s a very strong a c i d s o l i d , because T i 0 2 alone has l i t t l e a c i d i t y .
The s t r o n g a c i d i t y o f t h e T i 0 2
p i l l a r e d c l a y p r o b a b l y due t o s t r o n g i n t e r a c t i o n s between t h e s i l i c a t e l a y e r s and f i n e T i 0 2 p a r t i c l e s . WATER ADSORPTION Fig. 5 (a) shows t h e n i t r o g e n a d s o r p t i o n isotherms o f aluminum hydroxy p i l l a r e d c l a y s a f t e r heat-treatment a t 300~500°C. m u i r t y p e i s o t h e r m f o r microporous c r y s t a l s .
These a r e o f t h e t y p i c a l LangFig. 5 (b) shows t h e water ad-
s o r p t i o n isotherms on t h e same Al-hydroxy p i l l a r e d c l a y s
[27].
Unlike the
water a d s o r p t i o n isotherms f o r h y d r o p h i l i c z e o l i t e s , such as z e o l i t e s X and A , a p p a r e n t l y these isotherms cannot be e x p l a i n e d by Langmuir n o r BET a d s o r p t i o n equations: t h e water a d s o r p t i o n i n t h e e a r l y stages i s g r e a t l y suppressed, and shows h y d r o p h o b i c i t y .
Water a d s o r p t i o n isotherms f o r s e v e r a l microporous c r y s -
t a l s [ Z O ] a r e compared w i t h t h a t o f t h e alumina p i l l a r e d c l a y i n Fig. 6.
Zeo-
l i t e s NaX and 4A have v e r y steep Langmuir t y p e a d s o r p t i o n isotherms, w h i l e new microporous c r y s t a l s such as s i l i c a l i t e and A1P04-5 h a v i n g no c a t i o n s i n t h e
Clays Pillared with Ceramic Oxides 95
02
P/P,
0.6
04
0.8
1.0
P/P.
(b)
(a)
Fig. 5. (a) Nitrogen gas adsorption isotherms of Al-hydroxy pillared clays calcined at ( 0 ) 3OO0C, ( A ) 400°C and ( 0 ) 500°C [27]. (b) Water adsorption isotherms o f Al-hydroxy pillared clays calcined at ( 0 ) 200°C. ( 0 ) 300°C. ( A ) 400°C. ( A ) 500°C and ( 0 ) 600°C [27].
NaX
Al PO4 - 5
0.30
,
d *
n
0 L
In Y
C
=
8 6
0.10
Si licalite a
0
0.2
0.4
0.6 P / P.
0.8
1.0
Fig. 6. Comparison of the water adsorption isotherms of the alumina pillared clay and various microporous crystals.
pores show hydrophobicity. It is interesting to note that the shape of the isotherm for A1203 pillared clays is very similar to that of A1P04-5, though the total amount of adsorption of water is different. It i s also pointed out that in the case o f alumina pillared clay, the Al-hydroxy pillar o f the sample heated only at 2OO'C is not yet converted into the aluminum oxide pillar. It is still in the form of the hydroxide. However, the sample heated at 200°C shows hydrophobicity like those heated at higher temperatures. Such hydrophobicity is not limited to alumina-pillared clays, but all other pillared clays show similar
96 S. Yamanaka and M. Hattori
behavior [28]. The hydrophobicity seems to be a general characteristic in pillared clays regardless of the kind of pillar oxides introduced. ACKNOWLEDGMENT We gratefully acknowledge the support provided by the Grant-in-aid for Developmental Scientific Research (No. 01850179) of the Ministry o f Education, Science and Culture. REFERENCES 1.
B. K. G. Theng, "The Chemistry of Clay-Organic Reactions," Adam Hilger
(1974). 2. S. Yamanaka and M. Hattori, Hyomen. 27 (1989) 290. 3. G. W. Brindley and R. E. Sempels, Clays Clay Miner., 12 (1977) 229. 4. A. Schutz, W. E. E. Stone, G. Poncelet and J. J. Fripiat, Clays Clay Miner., 35 (1987) 251. 5. S. Yamanaka and G. W. Brindley, Clays Clay Miner., 27 (1979) 119. 6. G. J. J. Bartley, Catal. Today, 2 (1988) 233. 7. S. Yamanaka, T. Doi. S. Sako and M. Hattori, Mat. Res. Bull., 19 (1984) 161. 8. S. Yamanaka and M. Hattori, Catal. Today, 2 (1988) 261. 9. G. W. Brindley and S. Yamanaka. Amer. Miner., 64 (1979) 830. 10. T. J. Pinnavaia, M. S. Tzou and S. 0. Landau, J. Am. Chem. SOC.. 107 (1985) 4783. 11. S. Yamanaka, G. Yamashita, M. Hattori, Clays Clay Miner., 28 (1980) 281. 12. J. Sterte and J. Shabtai, Clays Clay Miner., 35 (1987) 429. 13. S. Yamanaka, T. Nishihara, M. Hattori and Y. Suzuki, Mat. Chem. Phys., 17 (1987) 87. 14. S. Sterte, Clays Clay Miner., 34 (1986) 658. 15. S. Yamanaka, F. Okumura and M. Hattori, Extended Abstract of '86 Annual Meeting of the Ceramic SOC. Jpn. (1986) 133. 16. S. Yamanaka, T. Nishihara and M. Hattori, Mat. Res. SOC. Symp. Proc., 1 1 1 (1988) 283. 17. S. Yamanaka, H. Matsumoto, F. Okumura, M. Yoshikawa and M. Hattori, Extended Abstract of the 28th Annual Meeting of the Basic Science Division o f the Ceramic SOC. Jpn. (1990) 43. 18. H. Yoneyama, S. Haga and S. Yamanaka, J. Phys. Chem., 93 (1989) 4833. 19. K. Takahama, M. Yokoyama, S. Hirao, S. Yamanaka and M. Hattori, J. Ceramic
SOC. Jpn. in press. 20. K. Suzuki. T. Mori, K. Kawase, H. Sakami and S. Iida, Clays Clay Miner., 36 (1988) 147. 21. E. Kikuchi and T. Matsuda, Catal. Today 2 (1988) 297. 22. M. L. Occelli and R. J. Rennard, Catal. Today, 2 (1988) 309. 23. H. Ming-Yuan, L. Zhonghui and M. Enze, Catal. Today, 2 (1988) 321. 24. M. L. Occelli, Catal. Today, 2 (1988) 339. 25. H. Sakurai, K. Urabe and Y. Izumi. Shokubai, 28 (1986) 397. 26. K. Shibata, T. Kiyoura, J. Kitagawa, T. Sumiyoshi and K. Tanabe, Bull. Chem. SOC. Jpn., 46 (1973) 2985. 27. S. Yamanaka, P. B. Malla and S. Komarneni, J. Colloid Interface Sci., 134 (1990) 51. 28. P. B. Malla, S. Yamanaka and S. Komarneni, Solid State Ionics, 32/33 (1989) 354. 29. S. T. Wilson, B. M. Lok, C. A. Messina. T. R. Cannan and E. M. Flanigen, ACS Symp. Ser. No.218, "Intrazeolite Chemistry," G. D. Stucky, F. G. Dwyer Eds. (1983) 79.
97
Zirconium Pillared Montmorillonite : Influence of Reduced Charge of the Clay
E.M. Farfan-Toms and P. Grange Unit6 de Catalyse et Chimie de MatCriaux DivisCs, UniversitC Catholique de Louvain, Place Croix du Sud 2, boite 17, 1348 Louvain-la-Neuve (Belgium) ABSTRAa Lithium introduced in the structure of the clay allows to control the density of the pillars and the strength of interaction between the pillar and the clay layer. At low calcination temperature, the interlayer distances and the surface area increased. The thermal stability of the clay, calcined at temperature higher than 4Oo0C, drastically decreases.
INTRODUCTION The intercalation of large size inorganic complexes between the layers of montmorillonite allows to prepare thermally stable microporous solids. Brindley and Sempels (l), Vaughan et al. (2) and Shabtai (3) have shown that the experimental conditions of A1 intercalation influences the physicochemicalproperties of the clay. The nature, amount and spacial distribution of the pillars change the thermal stability, texture and acidity of the pillared clays. For example, Rausch and Bale (4) have reported that the OWAl ratio modifies the structure of the A1 complex and that monomeric [Al(OH)x(H20)6-x]3-x' or polymeric [Al1304(OH)a(H20)12]species can be obtained. Clearfield ( 5 ) demonstrated that the polymerisation state of Zr species depends on the temperature, concentration and pH of the solutions. In any case, the height of pillars is largely controlled by the polymerisation state of the intercalated complexes. However, in order to maintain the accessibility of the inner surface, the density or spacial distribution of the pillars has to be controlled. This parameter has been studied by Plee et a1 (5), and Shabtai et a1 (7) for A1 pillared clays and Farfan-Toms et al(8) for zirconium. The distribution of the pillars may be controlled by the polymerization degree of the complex and by a precise adjustment of the charge density of the clay through a partial blocking of the exchangeable sites. The introduction of small cations (Li+, Mg2+ or Al3+) able to migrate at low temperatures to vacant sites in the octahedral layer and to partially neutralize the net octahedral charge due to isomorphic substitution represents the simplest way to prepare clays with reduced charge. This phenomenon is known as the Hoffman-Klemeneffet (9). Later Calvet et a1 (10) and Clementz et al(l1,12) demontrated that the charge decrease directly depends on the amount of Li ions introduced into the exchangeable sites. It is expected that the application of the Hoffman-Klemen effect will allow to modify the density of the pillars in Zr pillared montmorillonite. This paper reports the influence of Li concentration on the physico-chemicalproperties of Zr pillared montmorillonite.
98 E. M. Farfan-Torres and P. Grange
EXPERIMENTAL METHODS Preparation of the S ~les D The Li ions were introduced in two different ways: either before or after Zr intercalation. The montmorillonite (Weston L-Eccagun) was first exchanged with NaCl (1N) and washed. Two montmorillonites with reduced charge were prepared following the Brindley and Ertem method (13). Part of the Na+ montmorillonitewas first saturated with LiCl (1N) and washed. The Li+ clay thus obtained and Na+ clay suspension were stirred for 24 hours at 25OC and dried on glass plate. The films were then heated at 22OOC for 24 h in order to allow Li diffusion in the clay structure. Two different Li concentrations (F4.4 and F=0.6) were used. The Na+Li+ modified montmorillonite were dispersed in water acetone solution (l/l). The ZrOC12,8H20 solution was added to the Na+Li+ montmorillonite (0.02g.l-1;Zr/Clay=S.CEC). The suspension was stirred with NaOH solution (0.1 N) up to a OHEr ratio of 0.5. The final pH of the suspension was 1.85. After two hours of reaction at 4OoC the Zr pillared clay was washed up to constant conductivity of the solution, freeze-dried and calcined at different temperaturesup to 700°C (Em-02 and EIII-03). For comparison, Zr montmorillonite have been prepared with non modified Na+ montmorillonite (EIII-01). In addition, after calcination at 4OO0C, Li was also introduced in this sample (EIII-04). PHYSICO-CHEMICAL CHARACTERIZATION The Si-Al-Zr content of the clays, before and after pillaring, was determined by X-ray fluorescence (XRF-Philips PW 1450).The other elements were analyzed by Atomic Absorption (AAVarian techtron AA-5) after sulfofluorhydric leaching. The dichroic properties of the Li modified montmorillonite were followed by orientation of thin films (0-45') in the IR beam (FTIR Brucker IFS 88). The cationic exchange capacity (CEC) of the samples calcined at 400OC was evaluated. The basal spacing (d 001) (DRX-Kristalloflex-805 Siemens) and the surface area (MicromeriticsASAP 2400) was obtained on the solids calcined at different temperatures. X-Ray diffraction patterns have also been obtained after ethylenglycol saturation of selected samples. High resolution transmission electron microscopy (HREM) was performed (Jeol 100 CX Temscan) on ultrathin preparations (LKB Ultratome type 8802A). TPD (NH3) and infrared spectroscopy (pyridine)allowed to evaluate the acid properties of the solid calcined at 400 and 600OC. EXPERIMENTAL RESULTS The chemical composition and the cationic exchange capacity of the solids are reported in table 1. The relatively high Na content of the Zr modified pillared montmorilloniteshas to be noted (EIII02, EIII-03). Figures 1 and 2 report the IR spectra of the Na+ montmorillonite (A) and the Li+ modified montmorillonite [Na+Li+0.4 (B); Na+Li+0.6 (C)] before calcination (l), after calcination at 22OOC (2) and orientation of the sample in the IR beam (3). These figures illustrate the OH stretchnig vibration (fig. 1) and the bending (fig. 2) vibrations of the non pillared samples.
Zirconium Pillared Montmorillonite 99
Table 1. Chemical composition and CEC of the samples. Na+ mont Na++-Li+mont. Na+-Li+mont EIII-01 Em-02 ZIII-03 EIII-04 F = 0.4 F = (0.6)
E:3.66 : 0.83 0.1 1 0.05 2.74 10.31 86
63.98 22.35 3.17 0.86 0.09 0.11 1.63 0.55
64.41 22.72 3.08 0.86 0.12 0.09 1.20 0.70
7.26
6.78
50
37
49.20 51.73 52.69
20.35 11.06
48.26 18.40 18.18 16.37 2.39 2.63 2.53 0.72 0.66 0.75 0.05 0.06 0.05 0.03 0.03 0.03 0.12 0.20 0.18 0.38 0.47 0.39 17.35 17.35 20.30 8.84 7.73 11.14
48*
40*
16.40 2.24 0.59 0.08 0.00 0.05
37*
47*
Calcined at 4OOOC
g
I
a c
i /mi't
Fig. 2 Fig. I A : Na+ montmorillonite; B :Na+Li+(0.4) montrnorillonite;C : Na+Li+(0.6) montmorillonite. 1 :Fresh sample; 2 : montmorillonite treated at 220°C; 3 : wafer rotated at 45°C. The DRX spectra of the solids calcined up to 500°C are illustrated on fig. 3. The position of the d 001 diffraction line versus thecalcination temperature for the Na+ montmorillonite (Em-01) and the Zr pillared modified clays (Em-02, EIII-03) is reported on fig. 4. The same evolution of the basal spacing (d 001) for the pillared montmorillonite in which the Li has been introduced after the Z r is illustrated in fig. 5. It has to be mentioed that, after saturation of the solids by ethylene glycol, the interlayer distance of the samples calcined at 400°C is always slightly higher than before saturation. It has to be noted that the introduction of Li into the structure of the clay before pillaring and a calcination temperature lower than 300°C increase the surface area of the solids. A calcination temperature higher than 500°C gives amorphous solids. The Li clay structure collapses. In addition, these solids treated at 700°C present the same surface area as the Na montmorillonite.
100 E. M. Farfan-Torres and P. Grange
&
Q
20
18
10
L l 2
10
2 10
2 10
I
16 I
200
0
2
LOO
600
BM)
T IW
28
Fig. 4 : (d.OO1) Fig. 3 : DRX The specific surface area of the solids calcined at different temperatures, up to 7OO0C, is
16
0
The amount of NH3, desorbed up to 40O0C, for the solids calcined at 400 and 600OC is reported in table 2. For the samples EIII-02 and EIII-03, a large amount of adsorbed N H 3 still remains strongly
Table 2 :TPD :NH3 desorbed up to 400OC. samples
EIII-01 EIII-02 ~m-03 Em-04 Na+Mont. (Fa) Na+Li+Mont (F4.4) Na+Mont. (F=0.6)
NH3 (pmo1.g-1 at 400OC) 4oooC 6OOOC 329.0 272.0 249.0 165.0 390.0 200.0 49.0 29.0 75.0 31.5 54.0 25.5
E:!
Zirconium Pillared Montmorillonite
101
adsorbed at 400°C. In addition, it is observed that the Li modification does not drastically change the total acidity of the pillared clays. The FITR analysis of adsorbed pyridine evidences the same evoltuion for all the samples. The normalized intensities of the Br6nsted sites (1540 cm-I), Lewis sites (1448 cm-l) as well as the Lewis-Bronsted ratio with the outgazing temperature is reported in kble 3.
Table 3 :FTIR (pyridine) : normalized intensities (x 10-3) B. (154Ocm-1)
Samples
150
300
400
EIII-01 EIII-02 EIII-03 EIII-04
5.9 7.7 9.6 7.9
4. 6.: 6.6 6.9
.7 3.1 4.0 4.0
UB
L(148~m-~) 150 300 400
100
300
400
14.0 9.4 16.9 11.1 13.3 8.8 16.6 11.2
3.44 2.75 2.05 2.80
3. 2:. 2.02 2.32
% 2.17
20.3 21.2 19.6 22.3
2.80
DISCUSSION Structure and Droperties of the modified montmorillonitg The IR spectra of the reduced charge montmorillonite (fig.1) indicates that, after heating at 22OoC, the OH stretching vibration (3630 cm-1) is shifted to 3636 cm-l for the Na+Li+ montmorilloniteF S . 4 and 3639 cm-1 for the F S . 6 samples. This shift is more pronounced when the film is oriented at 45" in the IR beam. This suggests the dichroic character of this band. In addition, a shoulder at 3670 and 3700 cm-l appears. Prost and Calvet (10,14) attributed this dichroic band to OH groups perpendicular to the plan. The orientation change of the OH groups has been correlated to the interlayer cation migration in the octahedral cavities of the clay structure. The Li migration into a vacant site close to isomorphic substitution has to be linked to the inversion of the OH groups (10). The 3670 cm-l band is due to Al-Li-Mg configuration and the 3640 cm-l one associated with Al-Li-Al. Vedder (15) attributes the shoulder at 3700 cm-* to the Mg-Li-Mg structure. It has also been observed that Li introduction in octahedral sites induces a shift in the bending zone (10). The linear variation of the CEC with the Li concentration also supports the incorporation and migration of the Li cations in the octahedral vacant sites. Influence of Li on the structure and thermal stabilitv of Zr montmorilloni& A small increase of the (d 001) basal spacing is observed for the Li containing Zr pillared clays. However, the thermal stability of these solids drastically decrease. At high temperature, the collapse of the strucutre is also supported by the decrease of the surface area which is, at 700OC, almost identical to those measured for the montmorillonite. Different hypothesis may be proposed to explain the increase of the interlayer distance at low temperature: (i) a better polymerization of the intercalated complex; (ii) a modification of the dismbution of the pillars; (iii) a lower interaction between the pillar and the silica layer. The first hypothesis may easily be eliminated since the small variation of the height of the pillars (less than 1 A) cannot be explained by structural changes of the
102 E. M. Farfan-Torres and P. Grange
polymeric species introduced between the layers. In addition, the experimental conditions of the synthesis (pH, temperature, time) are always identical and the hydrolysis-polymerizationprocess of the zirconium salt should be identical. On the contrary, the important decrease of the charge of the clay may change the interaction strength between the polymer and the clay layer. This last assumption is strongly supported by the variation of interlayer distance of the Zr pillared after ethylene glycol saturation. Two hypothesis may explain the poor thermal stability evidenced by the collapse of the structure at 500°C : (i) the decrease of the CEC could induce the intercalation of smaller amounts of pillars regularly distributed; (ii) the distribution of the pillars could be less homogeneous. Based on the chemical analysis, it has been shown that the content of the different solids change by 3%. Assuming that the structure of the zirconium complex is represented by [(Zr40H)i4(H20)10]2+,a minimum of 4 moles of z r O 2 will be produced upon calcination. Four moles give 2000 meq charge, and 1000 meq for dimer species. The values of the observed basal distances seem to indicate that a dimer complex is intercalated. In addition, results of table 1 show that 45%, 60% and 50% of the sites are neutralized for the samples EIII-01, EIII-02 and EIII-03 respectively. The large number of exchanged sites could be correlated with the large distance between the sites, the loss of thermal stability being due to the low number of pillars and not to the poor spatial distribution of the complexes. At low calcination temperature, the decrease of the charge density which induces a larger distance between the layers and an enhancement of the distances between the pillars brings a better accessibility to the inner surfaces and this explains the high surface area of these pillared montmorillonites below 300OC. However, the surface area is drastically influenced at higher thermal treatment. The introduction of Li after pillaring (EIII-04) allows to explain part of this behaviour. For this solid, the basal spacing is not changed at low temperature, but the calcination decreases the thermal stability and the porosity. This is exclusively due to the lithium as the zirconium oxide pillars were present inside the layer before the modification of the clay. Such behaviour was observed for montmorillonite saturated with ammonium (16). For smectites, in which the charge is produced by isomorphic substitution in the tetrahedral layer, N a + are adsorbed on Si-0-A1 groups (beidellite for example). For montmorillonite, in which the charge is mainly due to octahedral substitution, migrates to octahedral layers (in the same way as Li) and induces dehydroxylation at lower temperature. In addition, Li acts as flux and improves the sintering of the clays. ACidiQ The Li diffusion in the clay structure slightly enhances the acidity of the Zr pillared montomorillonite as shown by the variation of the amount of desorbed NH3 We also observed a parallel decrease of the Lewis and increase of the Bransted sites. The total acidity of the EIII-02 and EIII-03 samples is reduced as compared with the pure Zrmontmorillonite. However, the acid strength is enhanced. The lowest charge on the surface layer could explain this behaviour.
Zirconium Pillared Montmorillonite 103
CONCLUSIONS The diffusion of Li+ in the octahedral cavities of the Na+montmorilloniteallows to control the density of the pillars of the Zr pillared montmorillonite. The solids, stable up to 30O0C, have larger surface area basal distancy than the pure Zr montmorillonite. The distance between the pillars increases while the interaction strength between the pillars and the clay layer decreases. However, the thermal stability of the Li-Zr pillared clays is drastically influenced after calcination at temperatures higher than 400OC. This is mainly due to Li acting as flux. AKNOWLEDGMENTS The financial support of the SPPS (Service de la Programmation de la Politique Scientifique), Belgium, is gratefully acknowledged. E.M. Farfan-Torres thanks the CGRI (Commissariat GCnCral de la CommunautC FranGaise de Belgique) for her grant. REFERENCES 1 G.W. Brindley and R.E. Sempels, Clay Miner., 12 (1977), 229-236. 2 D.E.W. Vaughan, R.Y. Lussier and J.S. Magee, U.S. Patent 4;176,090 (1979), 7 pp. 3 J. Shabtai, Chim. Ind., 61 (1979), 734-741. 4 W. Rausch and H.D. Bale, J. Chem. Phys., 40 (1964), 3891. 5 A. Clearfield, Inorg. Chem., 3 (1964), 146-148. 6 D. Plee, F. Borg, L. Gatineau and J.J. Fripiat, J. Am. Chem. Soc., 107 (1985), 2362-2369. 7 J. Shabtai, M. Rose11 and M. Tokarz, Clays Clay Miner., 32 (1984), 99-107. 8 E.M. Farfan-Torres and P. Grange, Preparation of Catalysts V; Elsevier, in press. 9 V. Hoffmann and R. Klemen, Z. Anorg. Allg. Agron., 13 (1950), 269-327. 10 R. Calvet and R. Prost, Clays Clay Miner., 19 (1971), 175-186. 1 1 D.M. Clementz, M.M. Mortland and T.J. Pinnavaia, Clays Clay Miner., 22 (1974), 49-57. 12 D.M. Clementz and M.M. Mortland, Clays Clay Miner. 22 (1974), 223-229. 13 G.W. Brindley and G. Ertem, Clays Clay Miner., 19 (1971), 399-404. 14 R. Prost and R. Calvet, C.R. Hebd. SCanc. Acad. Sci. Pans, 269 (1969), 539-541. 15 W. Vedder, Amer. Mineralogist, 49 (1964), 736-768. 16 B. Chourabi and J.J. Fripiat, Clays Clay Miner., 29 (1981), 260-268.
This Page Intentionally Left Blank
11 Structure m
This Page Intentionally Left Blank
107
Crystal Chemistry of Si-A1 Distribution in Natural Zeolites
Albert0 Alberti Instituto di Mineralogia, Universitl di Ferrara, Italy
ABSTRACT The Al-content in the tetrahedral sites of zeolites has been calculated according to Alberti and Gottardi's method for all ordered and some disordered zeolites. The results have been compared and integrated with the information given by MAS 27Al and "Si-NMR spectra. The most remarkable results are: 1. perfect order, as well as perfect disorder, are practically absent in zeolites 2. the validity of Loewenstein's rule is invariably stated 3. the "extended Loewenstein's rule" seems to be valid not only in zeolite Y,but also in all zeolites 4. decrease of the calculated Al-content by thermal treatment seems to indicate that dealumination of the framework occurs during dehydration. INTRODUCTION The properties of zeolites, most notably their stability, adsorptive capacity and catalytic activity, are strongly dependent on the precise location of Si and A1 in the anionic framework. This is one of the most challenging and debated problem in silicate crystal chemistry. The Al-content in a tetrahedron can be determined by: 1. 27Al and 29Si nuclear magnetic resonance spectra 2. neutron and X-ray diffraction data High resolution solid state magic-angle-spinning (HRMAS) ?'A1 and 2QSi-NMRspectroscopy has become a powerful tool for investigating the structural properties of zeolites. The 2QSispectrum provides a series of peaks corresponding to silicon atoms with 0 to 4 A1 nearest neighbors. From this NMR data the Si/AI framework ratio may be calculated. ?'Al-NMR has been applied primarily to observe the tetrahedral (framework) or octahedral (non-framework) form of A1 in zeolites. In some cases 27Al and 29Si MAS-NMR spectra have been used to determine Si-A1 distribution over two or more non equivalent crystallographic sites[1-41. It must be emphasized that the NMR technique reflects the local magnetic environment and ordering ofnuclei, while neutron and X-ray diffraction reflects long-range ordering. Therefore NMR and diffraction are complementary techniques, and their combined use often provides a much more complete description of Si-A1 distribution in zeolites.
108 A. Alberti
X-ray and neutron diffraction data supply good information on the Si-A1 distribution. The Si/Al ratio in a tetrahedral site, via X-ray or neutron diffraction, can be obtained: a. by the least squares refinement of the Si/Al scattering curves ratio b. from the dimension of the tetrahedron The former way is not useful when X-ray diffraction is used, because the difference between the scattering curves of Si and A1 is too small. If neutron diffraction is used, the neutron diffraction curves differ by as much as 25 %, so that the Si/Al ratio can be satisfactorily refined. Unfortunately, large crystals are needed at present (nearly lmm3 in volume), therefore this method can only be applied to a restricted number of zeolites. At present the Al-fraction in the tetrahedra of zeolites is normally deduced from the dimensions of the tetrahedra. Determinative curves are widely used, relating linearly the average T-0 distance of the tetrahedron, < T - 0 >, with the Al-content [5-61. A limit to these relationships is that the Si-0 and A1-0 distances are assumed to be a constant. On the contrary, the effects of local environment can change these distances remarkably. TO-T angles, tetrahedral O-T-O angles, coordination number and the sum of Pauling bond strengths on the bridging oxygens are some of the most important factors influencing T-0 distances [7-91. Alberti and Gottardi [lo] have shown that the variation of T-0 distances in relation to the bridging T-0-T angle can dramatically change the Al-content given by the linear relationship. Many studies have shown that the average Al-content in a zeolite deduced via linear relationships agrees quite satisfactorily with the Al-content given by the chemical analysis, if the (Si,Al) distribution is ordered; but it is systematically lower (- 0.05 of the Al/(Si+AI) ratio) in the case of a disordered distribution. Alberti and Gottardi [ll] have shown that this discrepancy occurs because the positions of the atoms given by the X-ray structure refinement, when disorder is present, do not correspond to the true position of the atoms, but t o the centre of gravity of the sites occupied in the different unit cells, each site having a weight depending on its occupancy and its scattering power. To summarize, the assessment of Al-content in the tetrahedral sites of zeolites via diffraction data is strongly influenced by: 1. the variation of the Si-0 and A1-0 distances depending on bonding forces on T and 0 atoms 2. the variation of the measured T-0 distances depending on disorder in the Si-A1 distribution
Recently, Alberti et al. [12]have proposed a method, which calculates the Al-content taking into account these two effects. In this method: a) the distances Si-0 and Al-0 are not constant, but depend on the local environment according to linear relationships b) distance vectors are used instead of their absolute values. This means that the true T-0 distance, on the left of the equation n
n
j=1
j=1
where n is the number of all the dj(T-0) distancea joining the true T and 0 positions, is used instead of the shorter observed T-0 distance on the right of the equation (1).This last T-0 distance is used to calculate the Al-content via linear relationship. In this way, the T-0 distance is calculated taking into account the static disorder in the Si-A1 distribution.
Si-A1 Distribution in Natural Zeolites 109
The average Al-content calculated using this method (from now on indicated by the symbol AG) usually agrees satisfactorily with the Al-content given by the chemical analysis. The average Al-content calculated via linear determinative curves (like Jones’ curve [5])agrees much less with this Al-content, especially in the case of a disordered Si-A1 distribution [10,12]. Therefore the Al-contents in the tetrahedral sites of zeolites reported in this work have always been calculated according to the AG method. To have more reliable results, only data from single crystal structure refinements have been used. It is commonly accepted that there are natural zeolites with an ordered Si-A1 distribution, and natural zeolites with a disordered Si-A1 distribution. It is less well-known that some natural zeolitic species show samples with an ordered Si-A1 distribution, while others are completely disordered. An attempt to rationalize this subject has been carried out by Gottardi and Alberti in previous works [13-141. The aim of this paper is: 1. to show that both complete order and complete disorder are practically absent in natural zeolites; 2. to show that the degree of order frequently varies strongly in different samples of the same zeolitic species; 3. to interpret this disorder from a crystallochemical point of view. In the discussion, synthetic zeolites like zeolite A, X and Iz have also been considered every time they give information useful for the comprehension of the order/disorder mechanism. MATHEMATICAL BASES OF ORDER-DISORDER According to the concepts reported by Smith and Brown [15], the Si-A1 order/disorder is a typical example of substitutional disorder. In substitutional disorder, two or more types of atoms randomly occupy one set of lattice nodes. Substitutional disorder of different atoms, in size and/or charge, would cause displacements not only in their lattice node, but also in the neighbor sites. The most striking effect of substitutional disorder is normally a thermal motion of the framework atoms which is apparently anomalously high. Using the ideas of Smith and Brown [15] as a starting point, we can treat substitutional Si-A1 order/disorder algebraically. Let us take Psi and PAIas the fractional frequency of atoms Si and A1 in a crystal and psij aa the fractional frequency of Si in the j-th lattice node, i.e. the probability of finding an Si atom in the node j. = 1- psi, will be the fractional frequency of A1 in the same lattice node. We can now introduce an order coefficient for the j-th lattice node which is defined as
if PA11
2 PAI.
For complete disorder, psil = Psi, PAI] = PAIand Ssi, = SAlj = 0. For complete Si order in the site j, Ssi, = 1, and for complete A1 order in the site j , s A l j = -1. In general for site j, if psij 2 Psi we have 0 5 Ssi, 5 1; if psij 5 Psi we have -1 5 Sail 5 0.
110 A. Alberti
Therefore, we introduce an order coefficient for the site j, Sj, which, according to Smith and Brown [15], will be called ”long-range order coefficient”,with the deliberate assumption that the probability pj is the same for all the symmetrically equivalent sites j. This coefficient is negative and obtained from equation (3) when p ~ l ,2 PA!; positive and obtained from equation (2) when psi, 2 Psi. We now introduce an ”average long-range order coefficient” S defined as:
j=1
j=1
where N is the number of independent T sites in the unit cell and wj is a weight coefficient depending on the multiplicity of the T site, i.e. 1.0 for a site in general position, 0.5 for a site on a symmetry element like 2, m, i and so on. S therefore represents the average of the Sj’s coefficients over the whole structure. DISCUSSION
In order to interpret the results of the following correctly, we must consider that they obey Loewenstein’s rule [M] which forbids the occurrence of Al-0-A1 linkages in tetrahedrally bonded aluminosilicates. As a result: 1. the %/A1 ratio is always 2 1.0 2. a ratio %/A1 = 1.0 necessarily gives an ordered Si-A1 distribution with a strict alternation Si-Al-Si in the tetrahedra. An obvious consequence is that odd-membered rings of tetrahedra are prohibited in frameworks with Si/AI = 1.0 3. in an even-membered ring of tetrahedra, and in particular in a 4-ring1a ratio Si/Al= 1.0 necessarily gives an ordered alternation Si-Al-Si. Obviously, in an odd-membered ring the Si/Al ratio must be > 1.0 4. an Si -+ A1 substitution can only occur in an Si(0AI) tetrahedron, i.e. only if the tetrahedron is surrounded by four Si-centered tetrahedra 5. if an oxygen atom in a structure is on a symmetry element 2,m, or T, the two bonded T sites cannot have a fractional frequency pal > 0.5. Therefore, for these T sites, PA1 = 0.5 corresponds to a perfect “short-range” order. 6. an Si + A1 substitution can be uninfluential on the Si-A1 distribution in the neighbor T sites, but may cause rearrangement of Si and A1 st least in the first and second neighbor T sitee, according to the so called “extended Loewenstein’a rule” (171. According to this rule, Si and A! are distributed so as to minimize the number of Al-A! pairs in second neighbor tetrahedra. Consequently, according to this rule, an Si -+ A1 substitution in a chain A1 - Si - A1 - Si - A1 - Si- A1
which gives a chain A1 - Si - A1 - Si - Si - Si - Al shoqld be unfavoured with respect to a chain A1 - Si - Si + A1 - Si - Si - A1 where A! - Si
- A1 alternations are not present.
Si-A1 Distribution in Natural Zeolites 111
Keeping these points in mind, it is now possible to analize Si-A1 order/disorder in zeolites, pointing out some particularly interesting features of Si-A1 crystal chemistry rather than drawing up a list of the Si-A1 distribution in these minerals. A catalogue of zeolites commonly considered to be ordered is shown in Table 1. It is to be noted that the real symmetry is always lower than the topological symmetry, with the exception of laumontite. A low Si/Al ratio seems to favour order, but in goosecreekite and in yugawaralite, two of the most ordered zeolites, the Si/Al ratio is as high as 3. In the case of high %/A1 ratio, order is favoured by the presence of divalent medium size cations (e.g. Ca). Wairakite and laumontite, with Si/A1=2, and goosecreekite and yugawaralite, with Si/AI=3, are Ca-rich zeolites. Table 1 - Zeolites with an ordered Si-A1 distribution. IUPAC code
Zeolite species
Topological symmetry
N:
Real symmetry
N:
Ideal Si/Al ratio
Extrafr. cations
G IS
Gismondine
I4lfamd
1
P21/c
4
1.o
Cii
GIS
Arnicite
I4 1/amd
1
I2
4
1.o
Na,K
CIIA
Willhenders.
Rsm
1
Pi
6
1.o
Ca,K
TI10
Thomsonite
Pmma
3
Pcnn c' = 2ca
6
1.o
Ca,Na~s
FAU
Zeolite X
Fd3m
1
Fd3
2
1.o
Na
LTA
Zeolite A
P m h
1
Fm3c a' = 2a
2
1.o
Na
NAT
Natrolite
I4 1/amd
2
Fdd2 3 1.5 a' = a f i , b' = b fi
Na
NAT
Mesolite
IQllamd
2
Fdd2 9 1.5 a' = 3afi, b' -- bfi
Na,Ca
NAT
Scolecite
I4 I /amd
2
Fd 5 a' = aJZ,V = b f i
1.5
Ca
ED1
Edingtonite
P421m
2
P21212
3
1.5
Da
ANA
Wairakite
Ia3d
1
I2fa
6
2.0
Ca
LAU
Laumontite
C2/m
3
C2/m
3
2 .o
Ca
BIK
Bikitaite
Cmcm
2
Pi b' = b/2
6
2.0
Li
YUG
Yugawaralite
C2f m
2
Pc a 3.0 a' = a/fi,c' = c / f i
GOO
Goosecreekite
P21Im
5
p2 1
a
3.0
Ca Ca
~~
+
number of topologically independent tetrahedra number of symmetrically independent tetrahedra in the real symmetry a, b, c, are the cell parameters in the topological syninietry a' ,b' ,c' are the cell parameters in the real symmetry . .
__
--
- ___-
112 A. Alberti
-
Table 2 Percent of Al in tetrahedral aitea , average "long-range order coefficient" Sj, average S and Si/AIA ratio for zeolitea with an ideal Si/AI=l.O. ~~
Tetrah. GL.MC.18 T1
1.1
.85
Gh.Sc.18
9.1
.81 .87
Gu.Ho.lg
9.1 8.1
A I D I C . ~ ~ Will.21
Thorn.=
8.7
.83
91.6 -.84
9.1
.81
8.8
.83
5.9
.88
4.9
8.0
.83
83.5 -.69
A9
T2
6.2
.88
6.Q
T3
96.8
-.93
95.5 -.91
92.8 -.85
91.9 -A4
91.6 -34
5.1
T4 T5
98.1 -.92
95.0 -.90
92.6 4 5
93.8 - 3 8
6.1 .86 92.1 -.86
93.1 4 7 86.8 -.I5
2.8
86.8 -.I5
T6
S Si/Al
0.86
0.90 1.06
1.15
A from chemical analyak
Zeolites w i t h Si/Al
2cO.Xz3
.El
1.05
.94 0.87
0.86
0.84 n.g.+
.90
1.05
0.81 1.08
31
Zc0.Xz4 8.6
0.75
1.18
.81
79.1 -34
0.12 1.18
+ not given
= 1.0
For this group of zeolites, according to Loewenstein's rule, only an enrichment in Si can occur ,with respect to the ideal ratio; i.e. the Si/A1 ratio cannot be less than 1.0. Table 2 reports the Al-content and the S - j coefficients for the different sites, and the "average long-range order coefficients" for this group of zeolites. Some conclusions can be drawn: 1. Al-content in the Si tetrahedra is always non-negligible; 2. with the increase of the Si/AI ratio, there is an increase not only in Si-content in the A1 tetrahedra, but also in Al-content in the Si tetrahedra. This can be explained if we assume that the "extended Loewenstein's rule", formulated to interpret the behavior of zeolite Y, is valid for all zeolites. In fact, from Table 2 it is evident that zeolites with a low Si/Al ratio, like gismondine from Montalto di Castro 1181, amicite 1201, or willendersonite (211, also have an high "average long-range order coefficient" S, whereas Zeolites X 123-241, which have a low S value, also have an high Si/Al ratio. As far as zeolite A is concerned, after a long and vehement debate, space group Fm& is now accepted as the correct one. In this space group there are two independent tetrahedral sites, T1 and T2, the former occupied by Si, the second by Al. The first refinement of hydrated Na-A 1251 in the Fmsc space group gives 6 % of A1 in site T1,13 % of Si in site T2, and Si/Al=l.l0 (see Table 3). Unfortunately, the chemical analysis has not been given. Pluth and Smith 126-301 refined a series of dehydrated zeolite A, exchanged with different cations. In all these refinements, the AG method gave an Al-fraction in T l always strictly equal to zero, an Si-fraction in T2 in the range of 12.9-8.6 %, and an Si/Al ratio in the range of 1.191.29. Moreover, the calculated Si/A1 ratio is noticeably different from the one determined by electron microprobe analyses (1.05-1.12) [26-30),by proton probe analysis (1.04) and by MAS *OSi-NMR spectra (1.03) 1311. The discrepancy between calculated and measured Si/Al ratio is anomalously high (three or more times the discrepancy normally found in the other zeolites), too high to be accepted aa a random error. According to Pluth and Smith [28],a peak of electron density at the center of the sodalite cage has been observed in numerous dehydrated divalent varieties of zeolite A, and it can be assumed that this peak belongs to an A104 extraframework complex.
Si-A1 Distribution in Natural Zeolites 113
Table 3 - Percent of Al in tetrahedral sites, "average long-range order coeficient" S, and Si/AI+ ratio for zeolite A.
-
Tetrah.
h Na-A26 d Na-Aae
T1
5.6
T2
S Si/Alp.'* Si/AI'.'.
d K-A2'
d Sr-A2'
d Rb-A3'
d Ag-A''
0.0
0.0
0.0
0.0
0.0
87.0
87.1
90.6
88.6
91.4
87.6
0.82
0.89
0.91
0.90
0.92
0.89
n.g.+
1.07'
1.05-1.12
1.09
1.12
1.05
1.16
1.29
1.21
1.26
1.19
1.28
P.'. electron microprobe analysis +
not given
crystal structure refinement
proton probe analysis 1.04;
NMR 1.03
There is no evidence of this peak in structures involving monovalent ions. However, in dehydratd Ag-A, according to Pluth and Smith 1291 "it is not unlikely that occluded AlO4-bearing species would be spatially disordered over the (sodalite) unit". Therefore we cannot exclude that, in other monovalent forms like Na-A and K-A, extraframework A1 is, in a lesser amount, inside the sodalite cage of dehydrated zeolite A. In Sr-exchanged A, a weak extraframework A1 signal in 27Al-NMR spectra has been detected. The intensity of the signal increases dramatically after calcination at 550°C [32]. As a result, occluded A104 species should develop during dehydration, and from X-ray data we can argue that dealumination of the framework involves the T1 tetrahedral site in particular. In fact, whereas the Al-content in the T2 site is the same both in hydrated and dehydrated forms, the Al-content in T1 decreases from 6 % in hydrated form to 0 % in the dehydrated ones. Unfortunately this hypothesis cannot be strongly supported owing t o the scarcity of crystallographic data, in particular for the hydrated form, in the true space group Fm%. MAS 2QSi-NMRspectra [31] are consistent with an occupancy by Si 1.5 % of the T2 site, and 100 % Si in the T1 site. This result agrees with our interpretation of the dehydrated structure, but not with that of the hydrated one.
-
a
b
Fig. 1. Clinographic projection of the natrolite tetrahedral chain; a) with typical natrolite Si/A1 order; b) with Al+Si substitution in T1 site.
114 A. Alberti
Zeblites w i t h Si/Al=l.S Only fibrous zeolites (natrolite, mesolite, scolecite, edingtonite) are in this group. In their topological symmetry there are two T sites, one having a multiplicity 1/4 of the other (from now on indicated as T 1 and T2 respectively). These tetrahedra are connected to construct a building block formed by five 4-membered rings (see Fig. la). In the case of order, T 1 is occupied by Si (and will be called Sil from now on), whereas T2 splits into two symmetrically independent tetrahedra, one occupied by Si, the other by A1 (called Si2 and A1 respectively). These minerals are an example of order-disorder transformation. Long-range order coefficients Sj and S in natrolite consistently vary from strong order to complete disorder (see Table 4). Edingtonite can also be strongly ordered or fully disordered. In these zeolites the %/A1 ratio is normally very near 1.5 [33]. Therefore, disorder is due to an Al-rSi substitution between A1 and Sil. In this case, according to Loewenstein's rule, Si must occupy both A1 sites, and the Si atom substituted by A1 in Sil will occupy one Si2 site, the other Si2 site being occupied by the residual A1 (see Fig. lb). Therefore the crystal structure refinement must give an Al-fraction in site Sil, equal to the Si-fraction in site A1 and two times the Al-fraction in site Si2. These ratios have been found when disorder is low, as in some natrolites, in mesolite and scolecite; but when the Al-fraction in Sil reaches values about 10 %, the disorder in the 4-ring formed by the T2 sites increases till it reaches complete disorder, with the same Al-fraction both in T1 and T2 (see Table 4 and Fig. 2). Curiously, complete disorder, which is not present in the majority of so-called disordered zeolites, has sometimes been found in these zeolites which are often taken as an example of order. For these phases, MAS 29Si and 27A1-NMR data could be very useful to obtain a better understanding of this order/disorder problem. Zeolites with Si/Al=2.O
In analcime-wairakite series (an interesting example of order/disorder in zeolites), wairakite, the calcium analogous to the sodic analcime has a remarkable order. Topology is cubic, with one independent T site. The structure of analcime has been determined and refined for a long time in the cubic space group [SO],even if there is evidence of its non-cubic symmetry. Mazzi and Galli [51] showed that analcime can have tetragonal or orthorhombic symmetry, whereas wairakite [52] has monoclinic symmetry. Table 4 - Percent of Al in tetrahedral sites, and "average long-range order coefficient" S, for zeolite with an ideal Si/Al=1.5. Tetrah.
Natrs4 Natras Natrs6 Natrs7 Natrs8 Natrsg Natr'O
Natrs7 Natr"
Tetran4'
Tl(Si1)
11.4
8.7
10.8
10.0
13.0
17.0
17.7
23.0
23.5
39.9
T2(Si2)
6.6
6.3
5.8
7.8
9.6
13.5
13.7
18.9
20.7
43.3
T2(AI)
91.3
88.1
87.4
86.2
85.4
79.9
75.8
70.9
69.2
S
0.82
0.81
0.80
0.78
0.74
0.65
0.61
0.50
0.47
Tetr&.
G ~ n n 'M ~ e ~ o l 'Sco14' ~
Edinfs Edinf6 Edinf7 Edinf'
Tl(Si1)
44.7
9.4
7.2
3.4
10.3
8.4
11.1
38.0
39.2
46.4
T2(Si2)
46.6
5.1
4.3
8.0
8.6
7.6
9.1
40.4
39.4
43.0
87.6
91.1
96.0
94.0
90.0
93.1
0.82
0.86
0.88
0.83
0.82
0.82
0.02
0.00
0.02
T2(AI)
S
0.01
Edinfs Edin;'
0.02 Edinf'
Si-AI Distribution in Natural Zeolites 115
0 0
-
F
-
P
3
-
-.5
-
0
0 0 -0
O
- 75
0
0
0 0
O
4
-1
0
0 b
o
0
o
Fig. 2. "Long-range order coefficients" Sj, for fibrous zeolites with Si/Al=1.5. The different symmetries follow from the different ordering of A1 in the tetrahedra, and from the related different charges in extraframework sites. Table 5 emphasises this relationship, showing the variation in the Al-content in T sites in the different samples of the analcime-wairakite series, as a function of the charge in cation sites. Fig. 3 schematizes this relation. The real symmetry of laumontite waa debated for a long time. Its structure, in fact, has been refined in the space groups Cm [53], C2 [54], and C2/m [55],the latter now considered aa the most probable space group. Among the phases with an ordered Si-A1 distribution, laumontite is the only one where the topological symmetry is the same as the real symmetry. Bikitaite shows a peculiar behaviour. The crystal structure proposed by Appleman et al. in 1960 1561 wits confirmed by Kocman et al. [57],assuming the space group P21. Afterwards, Table 5 - Al-fraction in tetrahedral sites, charge in cation sites, and "average long-range order coefficient" S, for analcime-wairakite series. Cation
ANAl"
ANA2"
T1 T2
0.41 0.05
M1
ANA361
ANA461
ANA551
ANA661
ANA'IK1 Wair52
0.38
0.36
0.36
0.32
0.25
0.24
0.04
0.08
0.18
0.19
0.28
0.41
0.44
0.84
0.82
0.84
0.79
0.76
0.73
0.61
0.60
0.04
M2
0.23
0.17
0.48
0.52
0.58
0.77
0.80
1.85
S
0.38
0.33
0.19
0.18
0.04
0.16
0.21
0.83
Tl/T2+
7.6
4.6
2.0
1.9
1.14
0.61
0.55
0.048
Ml/M2'
3.6
4.9
1.6
1.5
1.26
0.79
0.75
0.022
+
AI-fraction's ratio
* Cation charge's ratio
116 A. Alberti
Fig. 3. Schematized relation between the occupancy of M2 site and the Al-content in T1 site, in analcime-wairakite series. the crystal structure of bikitaite was refined in the lower space group PT by Bissert and Liebau [58] by X-ray diffraction, and by Stahl et al. 1591 by neutron diffraction. All these structure refinements were carried out on crystals from the same locality (Bikita, Zimbabwe). ' According to Bissert and Liebau [58] the structure is strongly ordered (S=O.91), (see Table 6) whereas according to Stahl et al. [59], a remarkable Al-fraction is present in a couple of Si sites (13 % and 17 % respectively). Consequently the order coefficient S decreases from 0.91 to 0.69. In the P21 structure refinement of Kocman et al. [57], there are three T sites, one almost completely occupied by Si (Al-content of 5 %), the other two in equal amounts by Si and Al. The most immediate hypothesis is that the refinement of Kocman et al. (571 in the space group P21 is incorrect, the true space group being P i . However, the low value of the coefficient of error R (0.037)and the temperature factors of framework atoms (Beq 0.6A' for the T sites, Be, 1.2A2 for the 0 sites), which are comparable with the corresponding values in P i refinements, make this hypothesis more difficult to support. Therefore, if we do not accept the hypothesis of a wrong P21 refinement, there are three crystals from the same locality (which have exactly the same chemical composition) with a completely different Si-A1 distribution.
-
-
Zeolites w i t h Si/Al=S.O Two zeolites with an almost ordered Si-A1 distribution are known in this group: yugawaralite [SO] (S=0.83), and gooscreekite [61] (S=0.89). Goosecreekite shows an interesting feature, unique to ordered zeolites. One of the Si-rich tetrahedra is surrounded by four other Si tetrahedra. Moreover, only one of the six independent Si sites has an Al-content remarkably different from zero (- 10 %). This tetrahedron is bonded with two Si and both the A1 tetrahedra, where (according to Loewenstein's rule) there is an Si-content near 10 %. Table 6 - Percent of A1 in tetrahedral sites, "long-range order coefficients" Sj, and average S, for bikitaite from Bikita. Tetrah.
T1 T2 T3
T4 T6 T6 S
BL6* P I
SKG6DP i
K G R ~ '~2~ 5.4
.a4
45.5
-.19
48.1
-.23
1.4
.96
2.9
.91
3.0
.91
4.0
.a7
0.0 90.0
1.0 -.86
13.1
.59
75.8
-.65
4.4
.86
16.7
.47
89.7
-35
77.8
a.68
0.91
0.69
0.42
Si-AI Distribution in Natural Zeolites 117
Table 7 - Zeolites with a disordered Si-AI distribution. IUPAC code ~~~
Zeolite species
Topological symmetry
Nt
Real symmetry
N:
Ideal Si/AI ratio
Extrafr. cations
3.0
Sr
1 or 6
1.5-5.5
Ca,Na,K
~~
BRE
Brewsterite
P21/m
4
P21/m0rP2~ 4 o r 8
CHA
Chabazite
Rzm
1
Rsmor
MAZ
Mazzite
P63/mmc
2
P63/mmc
2
2.5
Ca,Mg,I<
MOR
Mordenite
Cmcm
4
Cmc21
6
4.0-6.0
Na,Ca
HEU
Heulandite
C2/m
5
C2/m
5
3.0-5.0
Na,Ca,K
+
Pi
number of topologically independent tetrahedra
* number of symmetrically independent tetrahedra in the real symmetry
Zeolites with a disordered Si-A1 distribution
In this section only few zeolites, showing some interesting features, will be considered. Table 7 reports their crystallographic data. Brewsterite The Si-A1 order/disorder in this zeolite closely resembles that found in bikitaite. Its Si/Al ratio, given by the chemical analysis, is very near 3. In the topological symmetry there are 4 T sites; one is occupied by Si (Al-content 5 %), the others by Si 2/3 and A1 N 1/3 [62]. Alberti and Vezzalini [63] pointed out that the real symmetry of brewsterite is lower than P21/m, (probably P21). The order in brewsterite could therefore be higher than that found in the structure refinements, with 6 Si-rich and 2 Al-rich sites in the structure.
-
Chabazite Chabazite has only one topologically independent T site; therefore its structure should be perfectly disordered. However, Mazzi and Galli [64] have shown that chabazite contains partially ordered domains, which have not been clearly explained so far, but which indicate structural deviation from the trigonal symmetry. Some ordering of A1 and Si in the tetrahedra is undoubtedly present in chabasite, which could be related (as in analcime-wairakite series) to the arrangement of the extraframework atoms. The occurrence of willhendersonite, an ordered zeolite with chabasite topology [21], supports this statement. It should be pointed out that chabarite has an extremely wide range in the %/A1 ratio (from 1.4 up to 4.2) [65], which could generate a number of different ordered Si-A1 distributions and explain the apparent trigonal symmetry of chabazite as being the random aggregation of these domains, reciprocally oriented according to the trigonal symmetry elements. This hypothesis,does not agree with the HRMAS 2gSi-NMR results of Bodart et al. 131, who found that the A1 atoms in chabazite are distributed statistically. However, the low resolution of the 2gSi-NMR spectra and the superimposition of different Si(nAl) lines for zeolites of the chabasite group with two independent T sites make these results uncertain. Mazzite Mazzite has two topologically independent tetrahedra: T 1 with multiplicity 12, and T2 with multiplicity 24, located in the &membered rings and 4-membered rings of the gmelinite cage respectively.
118 A. Alberti
From the refinement of hydrated mazzite [66), the AG method gives an Al-content of 39 % in T1 and 24 % in T2, with an average value of 29 %, in satisfactory agreement with 28 9% given by the chemical analysis. Considering the different multiplicities of the T sites, for a random Si-A1 distribution the A l ~ z / A 1 ~ratio 1 is 2.0. In this case we have AITP/AIT~= 1.22. The structure refinement of mazzite dehydrated at 600°C [67] gives a ratio AITz/AITI = 1.10. Therefore, mazzite, either in hydrated or dehydrated form, shows a partial order with A1 preferentially in the T1 site. In dehydrated mazzite, 14 % and 8 % of A1 are in T1 and in T2 respectively, with an average value of 10 %, a value which is very low when compared with the Al-content of 28 % given by the chemical analysis. Once more, a dehydrated zeolite shows an Al-content lower than that of the hydrated phase. As in zeolite A, this discrepancy could be interpreted as due to dealumination of the framework during dehydration. A lot of work [4,68,69] has been done using “Al-NMR and 29Si-NMR spectra on the isostructural synthetic zeolite fl to determine Si/Al distribution in the two tetrahedral sites, These data clearly indicate that Si and Al distribution in zeolite $2is not random in nature, with an AlTZ/AlTI ratio in the range 0.9-1.6, depending on the different Si/Al ratios and different synthesis conditions, and with a preferential location in the &membered ring, as in natural mazzite. The same result was obtained by Alberti and Vezzalini (701using lattice energy calculations. Mordenite The topological symmetry of mordenite is Cmcm, but its real symmetry is lower, with a more probable space group Cmc21 [71]. The Al-content found in structure refinements of natural and exchanged mordenites, refined in the space group Cmcm, is normally low when compared with the Al-content given by chemical analysis. Alberti et al. [12]have shown that this result can be interpreted as the shortening of the measured T-0distances, as a result of averaging, when apparent symmetry (in this case Cmcm) is higher than the true symmetry (in this case Cmc21). The differences in Al-content in the four topologically independent tetrahedra are quite remarkable, as is shown in Table 8. Moreover, they recur in all the hydrated structures, thus indicating a significant partial Si,Al order. The two sites T3 and T4, which constitute the 4-membered rings, are the richest in Al, with, as concerns Al-content, T3 > T4 > Ti
> T2
Table 8 - Percent of A1 in tetrahedral sites, “long-rangeorder coefficients”Sj, and average S, for mordenites. T1
10.7’’
.08
12.5”
.09
7.TT3
.24
12.074 .20
T2
3.7
.69
6.1
.66
4.7
.53
8.9
T3
25.5
-.16
27.7
-.16
21.4
-.13
T4
15.5
-.04
18.2
-.05
14.7
-.05
S
0.29
0.25
0.29
6.875
.32
.41
0.0
1.0
20.8
-.14
30.2
-.33
21.5
-.08
7.2
.28
0.24
0.54
-
Si-A1 Distribution in Natural Zeolites 119
The tetrahedra of the 4-ring are the richest in A1 also in the related structures of dachiardite [76,77] and epistilbite [78]. The preference of A1 for the sites of the 4-membered ring in mordenite was also shown by Derouane and Fripiat [79] by means of a6 initio molecular orbital calculations. MAS "Si-NMR [2] was used to determine the Si-A1 distribution in mordenites. Unfortunately MAS "Si-NMR spectrum of low Si/A1 mordenite is broad and essentially featureless. Heulandite - clinoptilolite These two minerals have the same topology. There are 5 topologically independent T sites in the structure. In heulandite-clinoptilolite series there is an evident partial ordering of Si and A1 (see Fig. 4). The tetrahedron T2 is always the richest in Al, both in clinoptilolite and heulandite. In some clinoptilolites this site allocates more than 60 % of all aluminium present in the structure, reaching a fractional frequency of 0.45. This tetrahedron is connected to a symmetrically equivalent tetrahedron through an oxygen atom on a mirror plane. Therefore, taking into account Loewenstein's rule, this frequency is near its maximum value (0.50), (see point 5. of the Discussion), which corresponds to a perfect "short-range" order. In the four other T sites, there is no well-defined partial ordering. As T2 site allocates a large part of the Al-content, these last sites normally have an Si-content higher than the mean value. However, in a sample, a T site can have a remarkably high Si-content, but in another sample it can be near the mean value. MAS 27Al and 2QSi-NMRstudies [SS]have been carried out on natural clinoptilolites, but the spectra are, for our purposes, essentially featureless. This result is not surprising if we consider that, in the structure, there are 5 symmetrically independent T sites, (both containing Si and A1 with different ratios), so that 25 Si(nA1) peaks, with intensities varying from sample to sample, contribute to the observed spectrum. The limits of NMR spectra in such situations of complexity are evident.
1
I
I D
t -1
II i
n8
I Y
I
I
I
I
0
I
Fig. 4. "Long-range order coefficients" SJ,for heulandite-clinoptiloliteseries. Reference's number is shown. Empty squares = heulandites; full squares = clinoptilolites.
120 A. Alberti
CONCLUSIONS An accurate determination of the Si-A1 distribution in zeolites, using the Alberti and Gottardi method, has enabled us to show that: 1. the Al-content in the tetrahedral sites is always non-negligible, at least in hydrated phases, so that perfect, or almost perfect, order is practically absent in zeolites; 2. on the contrary, a partial ordering of Si and A1 exists in almost all zeolites, even when the topology suggests that perfect disorder should be the most likely situation to occur; 3. situations in disagreement with Loewenstein's rule have never been found in this work; 4. the "extended Loewenstein's rule" seems to be valid not only in zeolite Y, but also in all zeolites; 5. in dehydrated zeolites the calculated Al-content is normally lower than in the corresponding hydrated phases. Therefore, the hypothesis that dealuminaton is a normal process during dehydration should be carefully considered; 6. HRMAS 17Al and 2QSi-NMRspectra frequently give useful information for the determination of Si-A1 distribution in zeolites, but this technique should be considered as complementary to x-ray or neutron diffraction. Their combined use often provides a most complete and reliable description of the Si-A1 distribution in zeolites. Thb limits of NMR spectra in zeolites with disordered Si-A1 distributions and three or more symmetrically independent T sites must also be considered. ACKNOWLEDGEMENTS The authors thank the C.I.C.A.I.A. of the University of Modena for computing facilities. The Consiglio Nazionale delle Ricerche and the Minister0 della Pubblica Istruzione are also acknowledged for financial support. REFERENCES 1. G.T.Kokotailo,C.A.Fyfe,G.J.Kennedy,G.C.Gobbi,H.Strobl,C.T.Pasztor,
G.E.Barlow and S.Bradley in Y.Muramaki, A.Iijima, J.W.Wards (Eds.), New developments in zeolite science and technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22, 1986), Kodansha/Elsevier,Tokyo/Amsterdam,1986, p.361. 2. K.Itabashi,T.Okada,K.Igawa in Y.Muramaki, A.Iijima, J.W.Wards (Eds.), New developments in zeolite science and technology (Proc. 7th Int. Zeolite Conf., Tokyo,August 17-22, 1986), Kodansha/Elsevier, Tokyo/Amsterdam, 1986, p.369. 3. P.Bodart,I.B.Nagy,Z.Gabelica and E.G.Derouane, in D.Kallo and H.S.Sherry (Eds.), Occurrence, Properties and Utilization of Natural Zeolites, Akademiai Kiado, Budapest, 1988, p.245. 4. P.Massiani,F.Fajula,F.Figuerasand J.Sanz, Zeolites, 8 (1988) 332. 5. J.B.Jones, Act& Cryst., B24 (1968) 355. 6. P.H.Ribbe and G.V.Gibbs, Am.Mineral., 54 (1969) 85. 7. W.H.Baur, in M.O'Keeffe and A.Navrotsky (Eds.), Structure and Bonding in Crystals. Vol.11, Academic Press Inc., New York, 1981, p.31. 8. K.L.Geisinger,G.V.Gibbs and A.Navrotsky, Phys. Chem. Minerals, 11 (1985) 266. 9. A.Alberti (in preparation) 10. A.Alberti and G.Gottardi, Z.Kristallogr., 184 (1988) 49. 11. A.Alberti and G.Gottardi, in B.Drzaj, S.Hocevar, S.Pejovnik, (Eds.),Zeolites - Synthesis, Structure, Technology and Application, Elsevier, Amsterdam, 1985, p.255.
Si-A1 Distribution in Natural Zeolites 121
12. A.Alberti,G.Gottardi and T.Lai, (Proc.Nato Workshop, Dourdan, France, April 24-28, 1989) Plenum Publishing Company (in press). 13. G.Gottardi and A.Alberti, Bull.Soc.Geo1. Finland, 57 (1985)194. 14. G.Gottardi and A.Alberti, in D.Kallo and H.S.Sherry (Eds.), Occurrence, Properties and Utilization of Natural Zeolites, Akademiai Kiado, Budapest, 1988,p.223. 15. J.V.Smith and W.L.Brown, Feldspar Minerals Vol.1, Springer-Verlag, Berlin, 1988. 16. W.Loewenstein, Am.Mineral., 39 (1954)92. 17. S.Merlino, in D.Olson and A.Bosio (Eds.), Proc. 6th Int. Zeolite Conf., (Reno, USA, July 10-15,1983), Butterworths, Guildford, U.K., 1984,p.747. 18. R.Rinaldi and G.Vezzalini, in B.Drzaj, S.Hocevar, S.Pejovnik, (Eds.), Zeolites - Synthesis, Structure, Technology and Application, Elsevier, Amsterdam, 1985, p.481. 19. K.F.Fischer and VSchramm, Molecular Sieve Zeolites. Vol. I., Adv.Chem.Ser., 101 (1971) 250. 20. A.Alberti and G.Vezzalini, Acta Cryst., B35 (1979) 2866. 21. E.Tillmanns, R.X.Fischer and W.H.Baur, N. Jb. Miner. Mh., 1984 (1984) 547. 22. A.Alberti, G.Vezzalini and V.Tazzoli, Zeolites, 1 (1981)91. 23. D.H.Olson, J. Phye. Chem., 74 (1970) 2758. 24. G.Calestani, G.Bacca and G.D.Andreetti, Zeolites, 7 (1987)54. 25. V.Gramlich and W.M.Meier, Z.Kristallogr., 133 (1971) 134. 26. J.J.Pluth and J.V.Smith, J. Phys. Chem., 83 (1979)741. 27. J.J.Pluth and J.V.Smith, J. Am. Chem. SOC.,102 (1980)4704. 28. J.J.Pluth and J.V.Smith, J. Am. Cham. Sac., 104 (1982)6977. 29. 'L.R. Gellens,J.J.Pluth and J.V.Smith, J. Am. Chem. Soc., 105 (1983)51. 30. J.J.Pluth and J.V.Smith, J. Am. Chem. SOC.,105 (1983)2621. 31. C.Scott Blackwell, J.J.Pluth and J.V.Smith, J. Phys. Chem., 89 (1985)4420. Inorg. Chem., 23 (1984)2920. 32. D.R.Corbin,R.D.Farlee,G.D.Stucky, 33. A.Alberti,D.Pongiluppi and G.Vezzalini, N.Jb.Miner., Abh., 143 (1982)231. 34. G.Artioli, J.V.Smith and A.Kvick, Acta Cryst., C40 (1984)1658. 35. D.R.Peacor, Am. Mineral., 58 (1973)676. 36. A.Kirfe1, M. Ortnen and G.Wil1, Zeolites, 4 (1984) 140. 37. E.Krogh Andersen, I.G.Krog Andersen and G.Ploug-Sorensen Eur. J. Mineral. (in press) 38. F.Pechar, W.Schafer and G.Wil1, 2. Kristallogr., 164 (1983) 19. 39. A.Alberti and G.Vezzalini, Acta Cryst., B37 (1981) 781. 40. K.F.Hesse, Z. Kristallogr., 163 (1983) 69. 41. M.G.Mikheeva,D.Yu.Pushcharovskii,A.P.Khomyakov and N.A.Yamnova, Sov.Phys. Crystallogr. 31 (1986) 254. 42. F.Mazzi, A.O.Larsen, G.Gottardi and E.Galli, N. Jb. Miner. Mh., 1986 (1986) 219. 43. G.Artioli, J.V.Smith and J.J.Pluth, Acta Cryst., C42 (1986)937. 44. W.Joswig, H.Bart1 and K.Fuess, Z. Kristallogr., 166 (1984)219. 45. E.Galli, Acta Cryst., B32 (1976) 1623. 46. I.A.Belitsky,S.P.Gabuda,W.Joswig and H.Fuess, N.Jb.Miner.Mh., 1986 (1986) 541. 47. A.Kvick and J.V.Smith, J.Chem.Phys., 79 (1983) 2356. 48. F.Mazzi,E.Galli and G.Gottardi, N.Jb.Miner.Mh., 1984 (1984)373. 49. T.N.Madezhina,E.A.Pobedimskajaand A P.Khomjakov, Mineral.Zh., 6 (1984) 56. 50. G.??erraris.D.W.Jone6 and J.Yerkess, Z.Kristallogr., 135 (1972)240. 51. F.Mazzi and E.Galli, Am.Mineral., 63 (1978)448. 52. Y.Takeuchi, F.Mazzi, N.Haga and E.Galli, Am. Mineral., 64 (1979)993. 53. V.Schramm and K.F.Fischer ACS Adv.Chem.Ser., 101 (1971)259. 54. S.T.Amirov, V.V.Ilyukhin and N.V.Belov, Zap.Vses.Miner.Obsh., 100 (1971)20. 55. G.Artioli,J.V.Smith and A.Kvick, Zeolites, 9 (1989) 377.
122 A. Alberti
56. D.E.Appleman, Acta Cryst., 13 (1960) 1002 (abstr.). 57. V.Kocman,R.I.Gait and J.Rucklidge, Am.Mineral., 59 (1974) 71. 58. G.Bissert and F.Liebau, N.Jb.Miner.Mh., 1986 (1986)241. 59. K.Stah1, A.Kvick and S.Ghose, Zeolites, 9 (1989) 303. 60. A.Kvick,G.Artioli and J.V.Smith, Z.Kristallogr., 174 (1986)265. 61. R.C.Rouse and D.R.Peacor, Am. Mineral., 71 (1986) 1494. 62. G.Artioli,G.V.Smith and Ake Kvick, Acta Cryst., C41 (1985)492. 63. A.Alberti and G.Vezzalini, in Y.Muramaki, A. Iijima, J.W.Wards (Eds.), New developments in zeolite science and technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22,1986), Kodansha/Elsevier, Tokyo/Amsterdam, 1986,p.437. 64. F.Mazzi and E.Galli, N.Jb.Miner.Mh., 1983 (1983)461. 65. A.Alberti and M.F.Brigatti, Am.Mineral., 70 (1985)805. 66. E.Galli, Rend.Soc.It.Min.Petr., 31 (1975)599. 67. R.Rinaldi,J.J.Pluth and J.V.Smith, B31 (1975) 1603. 68. 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.Chesnick, Zeolites, 5 (1985) 179. 69. J.Klinowski and M.W.Anderson, J.Chem.Soc.Faraday,Trans. I, 82 (1986)569. 70. A.Alberti and G.Vezzalini, Bull.Mineral., 104 (1981) 5. 71. A.Alberti, P.Davoli and G.Vezzalini, 2. Kristallogr., 175 (1986)249. 72. V.Gramlich, DISS. No. 4633,ETH, Zurich (1971) 73. W.J.Mortier, J.J.Pluth and J.V.Smith, Mat. Res. Bull., 11 (1976)15. 74. W.J.Mortier, J.J.Pluth and J.V.Smith, in L.B.Sand and F.A.Mumpton (Eds.), Natural Zeolites, Pergamon, Oxford ,1978,p.53. 75. M.Ito and Y.Saito, Bull. Chem. SOC.Jpn., 58 (1985)3035. 76. G.Vezzalini, Z.Kristallogr., 166 (1984)63. 77. S.Quartieri, G.Vezzalini and A.Alberti, Eur. J. Mineralogy 2 (1990) 187. 78. A.Alberti,E.Galli and G.Vezzalini, Z.Kristallogr., 173 (1985) 257. 79. E.G.Derouane and J.G.Fripiat, in D.Olson and A.Bosio (eds.), Proc. 6th Int. Zeolite Conf., (Reno, USA, July 10-15,1983))Butterworths, Guildford, U.K., 1984, p.717. 80. A.Alberti, TMPM Tschermaks Min. Petr. Mitt., 18 (1972) 129. 81. A.Alberti and G.Vezzalini, TMPM Tschermaks Min. Petr. Mitt., 31 (1983) 259. 82. A.Alberti, TMPM Tschermaks Min. Petr. Mitt., 22 (1975)25. 83. N.Bresciani-Pahor, M.Calligaris, G.Nardin, L.Randaccio, E.Russo and P.CominChiaramonti, J. Chem. SOC.Dalton Trans., 1511 (1980) 84. K.Koyamaand Y.Takeuchi, 2. Kristallogr., 145 (1977)216. 85. T.W.Hambley and J.C.Taylor, J. Solid State Chem., 53 (1984) 86. N.Bresciani-Pahor, M.Calligaris, G.Nardin, L.Randaccio, J. Chem. SOC.Dalton Trans., 2288 (1981) 87. E.Galli, G.Gottardi, H.Mayer, A . P r e i s i and E.Passaglia, Acta Cryst., B39 (1983) 189. 88. S.Nakata,S.Asaoka,T.Kondoh and H.Takahashi, in Y.Muramaki, A.Iijima, J.W.Wards (Eds.), New developments in zeolite science and technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22,1986), Kodansha/Elsevier, Tokyo/Amsterdam, 1986, p.71.
123
NMR Studies of Cation Location in Zeolites
K.J.Chao', S.H. Chen' and S.B. Liu2 'Department of Chemistry, National Tsinghua University, Hsinchu 30043 ,Taiwan, R.O.C. 'Institute of Atomic and Molecular Sciences, Academia Sinica, P. 0. Box 23-166, Taipei 10764, Taiwan, R.O.C.
ABSTRACT After dehydration at 35OoC, most of the La3+ions in La, Na-Y tend to irreversibly migrate into sodalite or D6R cages while the Na+ions prefer to stay in supercages. In Cs, Na-Y, the large Cs+ions can not diffuse through the hexagonal prism into the D6R or sodalite cages and occupy the sites in supercages, irrespective of their hydration state. The IaQXechemical shift and the adsorption isotherm of xenon on Cs,Na-Y, Na-Y and La,Na-Y zeolites were used to probe the variation of cations location and occupation at different sites as a function of their hydration state. The results agree with 23Na 1D and 2D nutation NMR and the known structure data.
INTRODUCTION The distribution of cations in Na-Y, La,Na-Y and Cs,Na-Y zeolites were studied using xenon adsorption isotherms and Xenon- 129 NMR spectroscopy. The distribution of cations i n a hydrated zeolite i s mainly controlled by t h e i r sizes and can be described by a statistical model. In the dehydrated s t a t e , most of the cations are located on the intraframework s i t e s ; t h e i r occupancies are governed by mutual repulsions and cation-framework interactions [l]. By which, the environments of the framework silicon atoms and their corresponding 2% NMR spectra a r e affected [2,3]. The chemical s h i f t and lineshape of 2% NMR have been found t o depend on the nature and the distribution of cations i n the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3+ions from the supercages t o the small sodalite and/or D6B cavities by
124 K. J. Chao, S. H. Chen and S. B. Liu
dehydration has been followed by 2 9 S i NHR [2]. Xenon atom, with a diameter of 4.4 A, can only migrate into the supercages but not the sodalite or D6R cavities. Various investigations [4-61 showed t h a t 129Xe NYIR i s a sensitive probe f o r the geometric and electronic enviroments inside supercages of Y zeolites. A t room temperature, 70%of Na' ions of the hydrated Na-Y are located i n supercages [7]and these ions can be replaced by Cs+or La3+ions through conventional ion- exchange method. Since the size of the bare La3+ion (diameter=2.3 A) i s smaller than the aperture of a s i x member ring (diameter=2.6 A), Laj* ions can diffuse from the supercages i n t o the sodalite or D6R cages and replace the existing Na' ions by stripping off t h e i r hydration shells during heat treatment. After dehydration a t 350"C, X-ray and neutron diffraction [8,9] showed that most of the Na' ions i n La,Na-Y tend to migrate into supercages while the La3+ ions prefer to stay i n sodalite or D6R cages. Further calculation studies [lo], by considering the electrostatic field and the short range repulsion energies in Y zeolite, supported the above conclusion. In short, t h e preferred cation coordination s i t e s are i n sodalite or D6R cavities f o r dehydrated Y zeolites but favor to locate in supercages in the hydrated state Y zeolites. Consequently, some of the Na' were found t o move from supercages t o D6R or sodalite cages through dehydration in Na-Y. In the case of Cs,Na-Y, however, the larger Cs+ ions (diameter=3.3 A) can not enter the D6R or sodalite cages and therefore can only occupy t h e s i t e s i n supercages, irrespective of t h e i r hydration s t a t e . I n t h i s study, 1loXe NYR was employed t o study the cation effects i n Na-Y, Cs,Na-Y and La,Na-Y zeolites. The results are comparable with that of the 2 g S i and a3Na NMR investigations and the known structure data.
EXPERIIENTAL A binder-free Na-Y zeolite with Si/Al r a t i o of 2.29 was obtained from Strem Chemical Co., La,Na-Y and Cs,Na-Y zeolites were prepared by exchanging Na-Y z e o l i t e with LaClS and CsCl solution a t room temperature. The percentage of metal ion exchanged i n a zeolite has been determinated by Inductively-Coupled-Plasma Atomic Emission Spectroscopy and the number is used as prefix f o r the samples, e.g., Cs exchanged level of 667, i s represented as 66Cs ,Na-Y sample. A known amount of zeolite was loaded into a 10 mm NllR tube with an attached vacuum valve. The sanple was evacuated t o about 2x10-4 t o r r f o r 3 days a t room temperature, then it was heated to 350'C with a heating r a t e of 0.2*C/min, the sample ww allowed to maintain at this temperature f o r about 30 hours (2x10-6 t o r r ) After cooling t o room temperature, a known amount of xenon gas was introduced into the sample tube and was sealed by the vacuum valve. A l l the xenon adsorption isotherms were measured by volumetric method a t room temperature.
.
NMR Studies of Cation Location in Zeolites 125
The 1loXe "llspectra of adsorbed xenon were obtained on a Bruker YSL-300 spectrometer operating a t 83.0 MHZ and 295K. Typically, 200040000 signal acquisitions were accumulated f o r each spectrum with a recycle delay of 0.3s between 90' pulses. The 1aoXe NMR chemical s h i f t s were referenced t o t h a t of external xenon gas extrapolated t o zero pressure using Jameson's equation [ll]. A l l resonance signals of xenon adsorbed i n zeolites were shifted downfield from t h e reference but were taken to be positive in this report. The a3Na NHR experiments were performed on static samples and on a Bruker MSL-200 spectrometer. Dehydrated samples were evacuated at 350' C under shallow bed condition, cooled under vacuum (10-5 torr), stored in helium atmosphere and introduced into a sample cell in a glove-bag under helium atmosphere. The rehydrated samples were obtained by exposing dehydrated samples to water vapor at least three days over saturated NH4Cl solution at room temperature. A duraction of 0.5 s between scans were allowed for nuclear spin to recover to their equilibrium magnetization. The one-dimensional a3Na NMR spectra were recorded by using the spin-echo technique. The strength of the radio-frequency field for the two dimensional nutation experiments was 80 kHz and 128 t l values were used (0.1250pa). Each two- dimensional experiment took about 12 hours of spectrometer time. RESULTS AND DISCUSSION Xenon-129 NMR The dependence of 1aQXechemical s h i f t s on the concentrationof adsorbed tenon i n n
E
P
& 150 + 2
r vl
0
I
I
1
2
1
I
I
3
4
5
6
Xe atoms/cage Fig. 1. Dependence of the 1aOXe NMR chemical shift on the number of xenon atoms adsorbed on 66CsNaY(e), 69LaNaY(+) and NaY(A).
126 K. J. Chao, S. H. Chen and S. B. Liu
t h e supercage of Y z e o l i t e s i s shown i n Fig.1. The chemical s h i f t increased from 58 t o 110 ppn as t h e number of xenon atoms per supercage changes from 0.1 t o 3.6 on Na-Y, from 66 t o 97 ppm as t h e number of Xe/cage changed from 0.1 t o 2.6 on 69La,Na-Y and from 122 t o 180 ppm as t h e number of Xe/cage changed from 0.1 t o 5.3 on 66C8,Na-Y. Atomic xenon, with its large polarizability, has chemical s h i f t s extremely sensitive t o i t s physical surroundings. Using isotope Xenon-129 as a probe, I t o and Fraissard proposed [4] t h a t the ia9Xe chemical s h i f t s of xenon adsorbed on a z e o l i t e can be written as
where 8, i s chemical s h i f t of the reference and i s taken a s zero i n t h i s report; 6, i s the term from electronic f i e l d created by t h e cations; the term flxe-zeolite) ps i s related t o t h e interaction between xenon and z e o l i t e wall; ps corresponds t o the density which depends only on the z e o l i t e cage structure and it should be constant for a given type of zeolite; t h e last term of the equation arises from Xe-Xe interaction. At high Xe loading (>1 xenon atom/cage), ia9Xe chemical s h i f t increased l i n e a r l y with Xe density aa previously proposed by Jameson et al. (111. For Na-Y sample, the observed 1nBXe chemical shift data agree well with that of Ito and Fraissard [4]. The slope of ilDXe resonance on 69La,Na-Y at high xenon density i s identical t o t h a t of Na-Y and i s larger than t h a t on 66Cs,Na-Y. This indicates t h a t t h e environment of supercage f o r Xe-Xe collisions should be similar f o r Na-Y and 69La,Na-Y. A t lower xenon loading (< 0.8 Xe atom/cage), 69La,Na-Y shows the chemical s h i f t with a parabolic-type dependence on xenon loading. Similar behavior was observed on Yg-Y by Cheung e t . a l . [6] and it was explained as due t o t h e presence of strong adsorption s i t e s . The intercept of the curves, S, , i n Figure 1 represents the 12DXe chemical s h i f t a t the l i m i t of zero xenon loading, the values of 6, are 58, 66 and 122 ppm f o r Na-Y, 69La,Na-Y and 66Cs,Na-Y respectively. The intercept and slope of Na-Y a r e very coincide with those observed by I t o and Fraissard [4]. In their study, they also observed the similar JSvalues for NaY and de-A1Y and suggested that the Se from Na' ions is negligible even there are 4 Na' ions per supercage in Na-Y. Fig.2 shows the dependence of IaOXe chemical shifts on the concentration of xenon adsorbed on a series of Cs+ion-exchanged Na-Y. The differences (A&) between correspond t o the effect of t h e Cs' ions i n t h e supercages. 's,Cs,Na-Y and $Na-Y Fig.3 shows t h a t A & value of Cs,Na-Y increases with increasing Cs content. The electronic f i e l d caused by Cs* ions i s larger than t h a t caused by Na* ions and t h i s i s
NMR Studies of Cation Location in Zeolites 127
200
0
I
I
1
2
I
3 4 Xe atoms/cage
I
5
6
Fig. 2. Dependence of the iagXe NMR chemical shift on the number of xenon atoms adsorbed on NaY(A) and CsNaY with Cs exchanged percentage of 66(*), 56(r), 41(+) and
ao
60
40
20
0 0
1
2
3 CS+
4
5
ions/cage
Fig. 3. Variation of A & relative to NaY with the number of CS+ions per supercage.
6
128 K. J. Chao, S.
H. Chen and S. B. Liu
not consistent with the values predicted from the e / r values with Na*>>Cs*. Such discrepancy probably can be explained by the more direct contact between the Xe atom and the Cs+ion which has a larger diameter relative t o the Na+ion (diameter=1.9 A). Another possible explanation for the high *noXe chemical shifts observed on Cs,Na-Y compared to that of Na-Y and La,Na-Y may be attributed to the formation of Cs+-Xe complexes. This is due to the fact that Xe atom and Cs+ion have similar electronic configuration. Therefore, we conclude that the cationic f i e l d to the Xe atom depends on the size, location and nature of cations and it has d i s t i n c t effect by the direct contact of the Xe atom and the cation. After dehydration, most of La3* ions migrated from supercages t o sodalite or D6R cages. The effect of the La3+ions on 12QXechemical s h i f t is thus significantly smaller than that of the Cs+ions i n supercages. Comparing with Na-Y, 69La,Na-Y has the A & value of 8 ppm and a parabolicl-like curve at very low xenon concentration (Fig.1). This may be derived from the few stronger adsorption s i t e s i n the supercages. 6
0
200
400
600
800
1000
Pressure (torr) Fig. 4. Isotherms for xenon on 69LaNaY(o), NaY(n) and CsNaY with CS exchanged percentage of 23(r), 41(4) 56(r) and 66(.) at 295 K.
Xenon adsomtion isotherm The strong interaction between the Cs' ions and Xe atoms can also be confirmed by
NMR Studies of Cation Location in Zeolites 129
the adsorption isotherms of xenon adsorbed on a series of Cs' ion-exchanged Na-Y. The isotherm i s assumed t o follow Henry% law at low xenon pressure. The slope of the isotherms a t low xenon prssure and the corresponding Henry's equilibrium constant increase with increasing Cs content i n Y zeolites as shown i n Fig.4. The amount of xenon adsorbed at the sane xenon pressure i s also found t o increase with increasing Cs content i n t h e sample and is much higher than that of Na-Y or La,Na-Y. This provides an additional support to the 12QXeNMR results described in the previous section. Therefore the electronic f i e l d created by cations f o r the xenon atoms should increase with the Cs content i n Cs ,Na-Y. On the other hand, the presence of a few stronger adsorption sites for xenon atoms on 69La,Na-Y, concluded from i2gXe NME study, does not give the high amount of Xe adsorption relative t o Na-Y (Fig.4). Sodium- 23 NMR The occupations of the Na+ ions i n D6R, sodalite cages and supercages of Na-Y, Cs,Na-Y and La,Na-Y in the function of their hydration states were also monitored by 23Na NMR. The one- dimensional static 23Na NMR spectra are shown in Fig.5. After evacuation at 35OoC, the linewidth of the 23Na peak are broaden and the center of the 23Na profiles are shifted to high field. This is probably caused by the interaction between the localized Na+ ions and aluminosilicate framework. In the presence of water molecules, all sodium ions become mobile and water surrounded, resulting in a narrow resonance line on Na-Y and 66Cs,Na-Y. The different distributions of Na+ions in hydrated and rehydrated 69La,Na-Y derived from the irreversible migration of La3+ions account for the line broadening in the hydrated not in the rehydrated state. The 23Na nucleus has spin I=3/2 and the second-order quadrupole interaction that broadens its NMR spectrum [12],so discrimination of Na+ions in different sites via chemical shift becomes difficult and complicated. This problem can be overcome by using two- dimensional nutation technique.
200
PPmO
-200
'200
1
ppmO
1
-2001
'200
ppmO
- 2001
Fig. 5. 23Na NMR spectra of Nay, 66CsNaY and 69LaNaY in hydrated (a), dehydrated (b) and rehydrated (c) states.
130 K. J. Chao, S. H. Chen and S. B. Liu
The a3Na nutation spectra shown in Fig.6 indicate the distribution of Na' ions at more than one kind of sites in the hydrated 6gLa,NaY being different from that in the rehydrated 69La,Na-Y.
0
200
0
-200
0
a
I
200
I
I
0
I
I
-200
Fig. 6. 23Na 2D nutation NMR spectra of 69LaNaY in hydrated (a) and rehydrated (b) states.
NMR Studies of Cation Location in Zeolites 131
CONCLUSIONS The 129Xe chemical s h i f t and t h e adsorption isotherm of xenon adsorbed on Y z e o l i t e s a r e dependent on t h e size, location and nature of cations i n t h e z e o l i t e intraframwork space. The v a r i a t i o n of cation location i n a p a r t i a l l y cation-xchanged Na-Y can also be monitored by 23Na NMR. REFERENCES 1 R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves , Academic Press, London, 1978, p.32. 2 K.J. Chao and J.Y. Chern, J . Phy. Chem. 93 (1989) 1401. 3 J.Y. Chern and K.J. Chao, t o be published. 4 J. Fraissard and T. I t o , Zeolites, 8 (1988)350;J.Chem.Phys. 83 (1986)441; J.Chem.Soc., Faraday Trans. I. 83 (1987)451. 5 J.F. Wu,L.J. Ma, M.W. Lin, T.L. Chen and S.B. Liu, to be published. 6 T.T.P. Cheung, C.M. Fu and S.Wharry, J. Phys. Chem. 92 (1988) 5170. 7 W.J. Mortier, E.V.D. Bossche and J.B. Uytterhoeven, Zeolites, 4 (1984), 684. 8 M.L. Costenoble, W.J. Mortier and J.B. Uytterhoeven, J . Chem. SOC., Faraday I , 74 (1978)466. 9 A.K. Cheetham, M.M. Eddy and J.M. Thomas, J. Chem. SOC., Chem. Comu. 1984, 1337. 10 M.J. Sanders and C.R. A. Catlaw, Proc. 6th I n t . Zeolite Conf. , Reno, 1984, p.131 11 C.T. Jameson, J. Chem. Phys. 63 (1975) 5296. 12 G.A.H. Tijink, R.Janssen and W.S. Veeman, J. Am. Chem. SOC.109 (1987)7301.
This Page Intentionally Left Blank
133
Developments in X-ray and Neutron Diffraction Methods for Zeolites
J. M. Newsam# Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801, USA
ABSTRACT Some recent developments in the application of X-ray and neutron diffraction techniques to zeolite structuralproblems are summarized. Powder neutron diffraction (PND) has been applied further to studying hydrated materials, hydroxylic proton environments and zeolitehydrocarbon complexes, as well as to refinements of the framework structures of a range of zeolite and aluminophosphatecompositions. High resolution powder diffraction data collected using synchrotron X-radiation has been used for structure solutions and refinements, detection of subtle orderings or symmetry distortions, and quantitative studies of peak broadening phenomena, most notably arising from planar faulting. Techniques necessary for performing diffraction measurements on individual microcrystals > -5pm have been improved and the potential for using Laue geometry (with somewhat larger crystals) to accumulate diffraction data in time scales short enough to permit time-resolved structural studies has been demonstrated. In parallel, there have been continuous improvements in data analysis methods, notably in programs for Rietveld analyses of powder diffraction data, and the development of a new approach to framework structure solution using simulated annealing. BACKGROUND The five years since last considering specificallyrecent developments in X-ray and neutron diffraction methods for zeolites [ 11 have witnessed substantial progress. Some techniques, such as high resolution powder X-ray diffraction using synchrotron X-rays, have blossomed from earliest demonstrations of feasibility to widespread and productive application. Others, such as neutron powder diffraction, have shown steady progress. For still others, notably microcrystal diffraction, a variety of circumstances have contributed to extended gestation periods. Additionally, opportunities scarcely considered earlier (such as single crystal h u e diffraction, and certain developments in computer simulations that complement diffraction work) now command broad attention and warrant the commitment of substantial further investment. From a structural characterizationperspective synthetic zeolites pose a number of difficulties. Particle sizes are almost invariably small (typicallyless than 5pm) well below the minimum size limit usually required for study by conventional single crystal X-ray diffraction (SXD).Zeolites have relatively unfavorable scattering characteristics, being composed of lighter elements (Si, AI, 0,Na etc) generally close in atomic number (and therefore in X-ray scattering power). Zeolite structures have unit cell volumes in the 4x102 - 4x104 A3 range and are often complicated. Disorder on the length and time scales sampled by X-ray and neutron diffractionis common, the manifestation of #Present address: BIOSYM Technologies Inc, 10065 Barnes Canyon Rd. San Diego, CA 92121
134 J. M. Newsam
variations in framework atom positions (arising, for example, from local framework or nonframework cation ordering phenomena), in the locations of non-framework cations and, almost invariably, in the configurations and locations of non-framework sorbates or organic templating species. Inhomogeneity over various length scales, and impurity phases are frequently present. Additionally, synthesis conditions and the intrinsic similarities between distinct but related zeolite structure types combine to make local and extended defects (such as planar faults) common. Several of these materials issues are encountered in other crystalline systems. Their collective occurrence in the zeolite family represents a significant challenge. Partly because of these various difficulties, zeolite structural science is well poised to exploit developments in diffraction techniques. Even a relatively cursory inspection of the recent literature demonstrates that, indeed, zeolites and related crystalline microporous solids rank high amongst those classes of materials which are driving such developments. NEUTRON POWDER DIFFRACTION With many of the interesting natural zeolites for which larger single crystals are available already having been studied, recent single crystal neutron diffraction (SND)studies have been few in number. A study of bikitaite at 13K and 295K revealed an interesting one-dimensional hydrogenbonded water molecule chain [2] analogous to that observed earlier by PND in zeolite Li-A(BW) [3, 41. Powder neutron diffraction (PND) has been applied recently to a number of structural problems [5]. Our appreciation for the structural effects of gallium substitution for aluminum has been extended by PND studies of SOD [6], NAT [71, ANA [8] and RHO [9] framework gallosilicates. Possible A1 - Ga competition in a mixed alumino-gallosilicate ABW-framework material was also probed by PND in conjunction with solid state 2% nmr [ 101. Interestingly, this hydrothermal synthesis product showed a product A1:Ga ratio equal to that of the starting synthesis mixture. The structure refinement revealed that A1 and Ga were distributed, without segregation, randomly over the All site in the structure, despite the markedly differing expectation Ga-O and A 1 4 bond lengths. The significant neutron scattering length difference between Si and A1 has been exploited in measuring the partitioning of aluminum between the two inequivalent T-sites (T= Tetrahedral species, Si or Al) in zeolite L [ 111. This direct measure is complemented by (and in agreement with) the measurement in the same refinements based on bond length arguments. Powder neutron diffraction has also been a refinement technique of choice for some interesting new framework structures, notably the 18-ring aluminophosphateAlP04-54 [12], AlP04-11 [ 13, 141 and AlP04-25 (that is closely related to AlPO4-21, but proves to have a distinct topology) [15]. Both the aluminophosphateAlP04-5 [16] and its all silica analog, SSZ-24 [17], have also been studied. A key benefit of using PND is the sensitivity it offers to lighter elements, particularly protons. Following a first complete structural study of a hydrated polycrystalline zeolite in 1986 [3], protons associated with water molecules have been located in a number of systems including sodalite [6, 181, and NAT [7], ABW [4, 101 and ANA framework [8] gallosilicates. Hydroxylic proton positions have been determined in sodalite [6, 191 and, perhaps more interestingly, a bridging hydroxyl group that is apparently only a mildly acidic center has been observed in a steam-mated zeolite rho [20,21]. The flexibility of the RHO-framework has also been further probed by PND studies on cation-exchanged materials [22,23] and on ammonium-ionexchanged and heat-treated zeolite rho [24]. The conformation of ethylene glycol occluded in high silica sodalite has been determined [25]. In the category of zeolite-hydrocarboncomplexes, we have seen completion of studies at relatively low loading levels of benzene in zeolites L [26,27] and Na-Y [28]. Benzene has also been studied in
X-ray and Neutron Diffraction Methods for Zeolites 135
ZSM-5 [29] although the complexity of the framework structure makes full refinement of even the naked zeolite difficult. Although we await definitive PND work on higher unsaturated hydrocarbons, p-xylene (1,4-dimethylbenzene)has been successfully located (in the now familiar capping geometry above a site II cation) in a partially Yb-exchanged zeolite Y [30]. It is worth reiterating here that PND studies of zeolites containing sorbed hydrocarbons, although usually taxing experimentally, provide information that is invaluable to computer simulations and to studies of hydrocarbon dynamics. Recent single crystal X-ray diffraction studies (although currently limited to those materials for which single crystal specimens 2 -50pm are available) have determined benzene locations in Na-X [31], and, in ZSM-5, the geometry of the tetrapropylammoniumcation used as a template in synthesis [32] and the location of sorbed p-xylene [33]. Some success using powder X-ray diffraction (PXD) has also been noted [34,35]. SYNCHROTRON X-RAY DIFFXACTION Zeolitic materials have been prominent amongst those so far studied by high resolution powder diffraction using synchrotron X-rays [36]. High definition synchrotron PXD data has been helpful in a number of framework structure determinations and has facilitated studies of planar faulting (see below). Successful Rietveld refinementsof the framework structures of zeolite ZSM-11 137,381 and silica-ZSM-12 [39], and of the complete structures of zeolite Y containing cadmium sulfide [40] and cadmium selenide [41] clusters have been described. Critical in exploiting the excellent instrumental resolution that can usefully be configured at the synchrotron is the quality of the zeolite material. Generally, sample contributions to the measured peak widths are sufficient to prevent resolution much better than Ad/d = 2x10-3 being achieved. Even in cases where peaks are narrow, the appearance of previously unsuspected peak splittings (indicating subtle symmetry distortions)is not uncommon. The sample contributionsarise from inhomogeneity, defects or planar faulting, strain or particle size effects. The resolution accessible with a Ge (111) crystal defining the diffracted beam acceptance angle (and hence the resolution),Ad/d 3x104, determines that particles less than some lpm (1O"A) along the scattering vector direction will give rise to noticeable peak broadening. For particles much larger than lpm it can be difficult to obtain an uncomplicated powder average. It proves that today's synchrotron X-ray sources provide sufficent brightness for individual crystallites of this sort of size to be measurable individually using single crystal diffraction techniques. The feasibility of such measurements was demonstrated in late 1982 [l, 42-45], but advances have been hampered by facility problems, a paucity of allocated beam time and, perhaps most critically, by insufficient stability in the source and beam line X-ray optics. These problems are common to all SXD measurements at the synchrotron, and the expanding number of successful smcture refinements based on single crystal synchrotron X-ray data demonstrates that they have now largely been overcome. We have, in parallel, steadily improved our techniques for manipulating and mounting tiny particles, > -5pm, and for defining their orientations on the diffractometer [36]. Single crystal Laue diffraction simultaneouslyexploits both the brightness and the white character of the synchrotron X-ray spectral distribution, allowing intensity data to be acquired on an extremely short time scale. A substantial fraction of reciprocal space is accumulated at a single crystal orientation setting (currently by recording data on a film pack - Imaging Plates and Charge-Coupled Devices are both being developed as replacements). In some high symmetry cases one setting yields a unique segment of data, although in lower symmetry systems a small number of settings are necessary. Structure refinements [46] and a number of structure solutions [47-491 using h u e
-
136 J. M. Newsam
Figure 1. Stereoview representation of a hypothetical structure (hexagonal, Pg2m (No. 189), a = 12.38A and c = 17.40A) constructed from T&J cubes and derived from the A M framework (see text) diffraction patterns accumulated with synchrotron X-radiation have been demonstrated. Exposure times are short, with full data acquistions in a non-optimized mode requiring only a few minutes. Single Laue exposures from the enzyme lysozyme have been obtained in only two bunches from the storage ring at CHESS PO]. This demonstrated feasibility of recording diffraction patterns (and hence structural insight) on a nanosecond time scale permits us the vision of following at an atomic level and in a time-resolved manner sorptive, ion-exchange and perhaps ultimately catalytic processes within zeolites.
FRAMEWORK STRUCTURE SOLUTIONS Specimens suitable for conventional single crystal X-ray diffraction analyses have proven accessible for the mineral boggsite [51], and a number of microporous aluminophosphates and related substituted materials. The AFS (type species h4APSO-46) and A M (of CoAPO-50) frameworks [52] are particularly notable (see "The Atlas of Zeolite Structure Types" [53] or ZeoFile [54], a computerized data base of information on the known zeolite structure types derived from the Atlas). Both can be described as derived from Tg@o cubes. In AEY each cube is linked to an adjacent cube through one of the apical pendant linkages. In AFS, each cube has one vertex absent, with the three associated linkages joined with an identical, but inverted fragment to form a 3663 unit. In a plane, these units can interconnect to form a flat, hexagonaVtrigonal m y , although successive sheets along the unique (trigonal) axis can then be related by a strict translation or by a screw operation. The latter yields the A F S framework. The former was recognized as a likely (and since proven) model for beryllophosphate-H [55] (framework code BPH) and the aluminosilicate zeolite Linde Type Q [56]. Similarly, the sheets of linked complete cubes found related by simple translation along [OOI] in AFY can also be interlinked with successive layers related by mirror operations. The resulting structure (Figure 1) that has a c = 17.4A repeat remains currently merely an interesting hypothetical one (atomic coordinates optimized by distance least-squares are available from the author on request), but given the observation of the related AFS - BPH, and AFY frameworks, its occurrence might be considered likely. Conventional model building methods have enabled elucidation of the framework structures of Montesommaite [57], ZSM-18 [58], ZSM-57 [59],AlP04-52 [601 and AlP04-54 [61]. The structure
X-ray and Neutron Diffraction Methods for Zeolites 137
Figure 2. Representations of the three distinct topologies derived by simulated annealing based on data for a lithium gallosilicate (orthorhombic,Pna21 a = 18.5A, c = 7.5 A, 2 unique Tsites, 8 T-atoms in the unit cell) [66]. The correct model (which proves isotopological with the parent zeolite Li-A(BW)) is (c). of ZSM-18 had been a long-standing puzzle, and its solution by the model building approach, like the related structures of beryllophosphate-H (BPH) and Linde type Q (BPH), was made possible by the determination of the related AFS and AFY framework structures by conventional diffraction techniques. Our ability to exploit powder X-ray and neutron diffraction data in h e w o r k structure determinations continues to improve. Ab inifio structure solutions (by Direct Methods based on reflection intensities decomposed from powder diffraction profiles) were successful for the aluminophosphate AlPO4-TAMU ('in-house'PXD data) [62], the clathrasil Sigma-2 (synchrotron PXD data) [63], Li-A(BW) [ a ] , natrolite and erionite [65] (the last three being PND feasibility studies). It is often straightforward to obtain a unit cell size and likely space group symmetry (or symmetries) for a new zeolite material by indexing the PXD pattern. Sorption experiments yield the void volume and, conversely, the framework density and hence the (approximate) number of T-atoms contained within that unit cell. As all zeolites (and related Cconnected framework structures) obey well-defined geomemcal constraints, the number of possible topologies that can satisfy these data is limited. Further, the degree to which the constraints are satisfied by a given arrangement of T-atoms within the unit cell provides a measure of the reasonableness of that arrangement. Simulated annealing can then be used to adjust the T-atom coordinates so as to best satisfy the specified constraints (i.e. to generate likely structural models for the material in question). This new approach to framework structure solution [66] has already been applied successfully to simpler zeolite structural problems (Figure 2) and provides a rapid means of generating hypothetical frameworks. PLANAR FAULTING Planar faults are observed in a number of zeolite systems, at concentrations that are often approximately reproducible from one synthesis to the next. Zeolite beta, one of the best known examples, exhibits close to random disorder in the stacking of successive sheets. The structure is comprised of sheets that stack successively in a Right (41) or Left-handed (43) fashion [67,68,69]. In typical zeolite beta materials both modes of connection are near equally probable and the structure can then conveniently be viewed as a near-random intergrowth between the bea (with a pure LLLLL...or RRRRR... sequence) and beb (with recurrent alternation, RLRLRL...)frameworks illustrated in projection in Figure 3. In both of these structuresthere is a similar set of perpendicular 12-ring channels running horizontally, in the plane of the page. The intersections between the
138 J. M. Newsam
Figure 3. Representations of the two framework structures (drawn in projection as straight lines connecting adjacent T-sites; T = tetrahedral species, Si or Al) of which zeolite beta can be regarded as a disordered intergrowth [67,68,69]. orthogonal sets of channels are 12-ring apertures that define a pore path along the third, vertical direction. The planar faulting endemic to zeolite beta materials precluded the possibility of structure solution by direct diffraction methods. A variety of intergrowth structures in the phase field between the FAU framework and its hexagonal variant bss have recently been characterized. In this family both of the end-member structure types, FAU and bss, are well documented in the literature. The hexagonal bss framework had, however, remained only one of a large number of interesting hypothetical structures until determination of the structure of ZSM-20 [70]. Materials with still larger proportions of the hexagonal mode of stacking of the faujasite sheets than is found in ZSM-20 have also been described recently [71, 721. The accessibility of high resolution powder diffraction instrumentation at both neutron and synchrotron X-ray scattering centers [36] now permits detailed quantitative studies of a number of factors that contribute to measured powder (and single crystal) diffraction peak widths, such as finite particle sizes (and shapes), strain or inhomogeneity, or stacking disorder (as above). Coupled with improvements in experimentation (and steady progress in zeolite syntheses that afford quality materials for detailed studies) has been the development of an improved means of simulating the effects of stacking disorder on the associated diffraction patterns [73,74]. The powder diffraction profiles observed from even complicated systems such as zeolite beta and materials in the FAU-bss family can now be simulated with reasonable accuracy, and the character of the stacking arrangements therefore determined. ACKNOWLELXEMENTS I thank the various key contributors to our own zeolite structural characterization effort that are mentioned here, most notably M. W. Deem, M. T. Melchior, W. J. Mortier, S. B. Rice, B. G .
X-ray and Neutron Diffraction Methods for Zeolites 139
Silbernagel, K. G. Strohmaier, M. M. J. Treacy, D. E. W. Vaughan, J. P. Verduijn, D. Xie, C. Z. Yang, J. Yang and W. B. Yelon. REFERENCES 1. J. M. Newsam and D. E. W. Vaughan, in B. Drzaj, S . Hocevar and S . Pejovnik (Eds.), ZEOLITES: Synthesis, Structure, Technology and Application (Stud. Surf. Sci. Cat. No. 24), Elsevier, Amsterdam, 1985; pp. 239-248. 2. K. Stahl, A. Kvick and S. Ghose, Zeolites, 9 (1989) 303-311. 3. J. M. Newsam, J. Chem. Soc. Chem. Comm., (1986) 1295-1296. 4. J. M. Newsam, J. Phys. Chem., 92 (1987) 445-452. 5. J. M. Newsam, Materials Science Forum, 27/28 (1987) 385-396. 6. J. M. Newsam and J. D. Jorgensen, Zeolites, 7 (1987) 569-573. 7. D. Xie, J. M. Newsam, J. Yang and W. B. Yelon, in M. M. J. Treacy, J. M. White and J. M. Thomas (Eds.), Microstructure and Properties of Catalysts ( M R S Symp. Proc. Vol. 11 I), Materials Research Society, Pittsburgh, PA, 1988; pp. 147- 154. 8. W. B. Yelon, D. Xie, J. M. Newsam and I. Dunn, Zeolites, 10 (1990) 553-558. 9. J. M. Newsam, D. E. W. Vaughan and K. G. Strohmaier, (1990) in preparation. 10. J. Yang, D. Xie, W. B. Yelon and J. M. Newsam, J. Phys. Chem., 92 (1988) 3586-3588. 11. J. M. Newsam, J. Chem. SOC.Chem. Comm., (1987) 123-124. 12. J. W. Richardson, J. V. Smith and J. J. Pluth, J. Phys. Chem., 93 (1989) 8212-8219. 13. J. M. Bennett, J. W. Richardson, J. J. Pluth and J. V. Smith, Zeolites, 7 (1987) 160-162. 14. J. W. Richardson, J. V. Pluth and J. V. Smith, Acta Cryst., B44 (1988) 367-373. 15. J. W. Richardson, J. V. Smith and J. J. Pluth. J. Phys. Chem., (1990) in press. 16. J. W. Richardson, J. J. Pluth and J. V. Smith, Acta Cryst., C43 (1987) 1469-1472. 17, J. W. Richardson, J. V. Smith and S . Han, J. Chem. SOC.Chem. Commun., (1990) submitted. 18. J. Felsche, S. Luger and P. Fischer, Acta Cryst., C43 (1987) 809-811. 19. S. Luger, J. Felsche and P. Fischer, Acta Cryst.. C43 (1987) 1-3. 20. R. X. Fischer, W. H. Baur, R. D. Shannon and R. H. Staley, J. Phys. Chem., 91 (1987) 2227-2230. 21. R. X. Fischer, W. H. Baur, R. D. Shannon, R. H. Staley, L. Abrams, A. J. Vega and J. D. Jorgensen, Acta Cryst., B44 (1988) 321-324. 22. W. H. Baur, A. Bieniok, R. D. Shannon and E. Prince, Z. Kristallogr., 187 (1989) 253-266. 23. D. R. Corbin, L. Abrams, G . A. Jones, M. M. Eddy, G. D. Stucky and D. E. Cox, J. Chem. Soc. Chem. Commun., (1989) 42-43. 24. R. X. Fischer, W. H. Baur, R. D. Shannon, J. B. Parise, J. Faber and E. Prince, Acta Cryst., C45 (1989) 983-989. 25. J. W. Richardson, J. J. Pluth, J. V. Smith, W. J. Dytrych and D. M. Bibby, J. Phys. Chem., 92 (1988) 243-247. 26. J. M. Newsam, B. G . Silbernagel, A. R. Garcia and R. Hulme, J. Chem. SOC. Chem. Comm., (1987) 664-666. 27. J. M. Newsam, J. Phys. Chem., 93 (1989) 7689-7694. 28. A. N. Fitch, H. Jobic and A. Renouprez, J. Phys. Chem., 90 (1986) 1311-1318. 29. J. C. Taylor, Zeolites, 7 (1987) 311-318. 30. M. Czjzek, T. Vogt and H. Fuess, Angew. Chem., (1989) 786-787. 31. Y. F. Shepelev, A. A. Anderson and Y. I. Smolin, Kristallografiya, 33 (1988) 359-364. 32. H. van Koningsveld, H. van Bekkum and J. C. Jansen, Acta Crystallogr., B43 (1987) 127132. 33. H. van Koningsveld, F. Tuinstra, H. van Bekkum and J. C. Jansen, Acta Crystallogr., B45 (1989) 423-431. 34. B. F. Mentzen, F. Bosselet and J. Bouix, C. R. Acad. Sci., Ser., 305 (1987) 581-584. 35. B. F. Mentzen, Mater. Res. Bull., 22 (1987) 337-343. 36. J. M. Newsam and K. S . Liang, Int. Rev. Phys. Chem., 8 (1989) 289-338. 37. B. H. Toby, M. M. Eddy, C. A. Fyfe, G. T. Kokotailo, H. Strobl and D. E. Cox, J. Mater. Res., 3 (1988) 563-569. 38. C. A. Fyfe, H. Gies, G. T. Kokotailo, C. Pasztor, H. Strobl and D. E. Cox, J. Am. Chem. SOC., 1 1 1 (1989) 2470-2474. 39. H. Gies, B. Marler, C. A. Fyfe, G. T. Kokotailo and D. E. Cox, J. Phys. Chem., (1990) in press.
140 J. M. Newsam
40. N. Herron, Y.Wang, M. M. Eddy, G. D. Stucky, D. E. Cox, K. Moller and T. Bein, J. Amer. Chem. Soc., 111 (1989) 530-540. 41. K. Moller, M. M. Eddy, G. D. Stucky, N. Herron and T. Bein, J. Amer. Chem. Soc., 11 1 (1989) 2564-2571. 42. P. Eisenberger, J. M. Newsam, M. E. Leonowicz and D. E. W. Vaughan, Nature, 309 (1984) 45-47. 43. R. Bachmann, H. Kohler, H. Schulz, H.-P. Weber, V. Kupcik, M. Wendschuh-Josties, A. Wolf and R. Wulf, Angew. Chem. Int. Ed., 22 (1983) 1011-1012. 44. R. Bachmann, H. Kohler, H. Schulz and H.-P. Weber, Acta Cryst., A41 (1985) 35-40. 45. H. R. Hoeche, H. Schulz, H. P. Weber, A. Belzner, A. Wolf and R. Wulf, Acta Crystallogr., Sect. A, A42 (1986) 106-110. 46. I. G. Wood, P. Thompson and J. C. Matthewman, Acta Crystallogr., B39 (1983) 543-547. 47. J. A. Clucas, M. M. Harding and S. J. Maginn, J. Chem. Soc. Chem. Commun, (1988) 185187. 48. M. M. Harding, in M. A. Carrondo and G. A. Jeffrey (Eds.), Chemical Crystallography using Pulsed Neutrons and Synchrotron X-Rays (NATO AS1 Ser. C Vol. 221), D. Riedel, Dordrecht, Holland, 1988; pp. 537-561. 49. M. M. Harding, S. J. Maginn, J. W. Campbell, I. Clifton and P. Machin, Acta Crystallogr., B44 (1988) 142-146. 50. K. Moffat, W. Schildkamp, D. Bilderback and M. Szebenyi, (1988) unpublished. 51. J. J. Pluth and J. V. Smith, Amer. Mineral, 75 (1990) 501-507. 52. J. M. Bennett and B. K. Marcus, in P. J. Grobet, W. J. Mortier, E. F. Vansant and G. SchulzEkloff (Eds.), Innovations in Zeolite Material Science (Stud. Surf. Sci. Catal. No. 37), Elsevier, Amsterdam, 1987; pp. 269-279. 53. W. M. Meier and D. H. Olson, Atlas of Zeolite Structure Types; Butterworths, Guildford, UK, 1987. 54. J. M. Newsam and M. M. J. Treacy, (1990) in preparation. 55. G. Harvey, Zeit. Kristallogr., (1990) in press. 56. H. J. Bosmans and K. J. Andries, (1990) in preparation. 57. R. C. Rouse, P. J. Dunn, J. D. Grice, J. L. Schlenker and J. B. Higgins, Amer. Mineral., (1990) submitted. 58. S. L. Lawton and W. J. Rohrbaugh, Science, (1990) in press. 59. J. L. Schlenker, J. B. Higgins and E. W. Valyocsik, Zeolites, 10 (1990) 293-296. 60. J. M. Bennett, R. M. Kirchner and S. T. Wilson, in P. A. Jacobs and R. A. van Santen (Eds.), Zeolites: Facts, Figures, Future (Stud. Surf. Sci. Cat. No. 49), Elsevier, Amsterdam, 1989; pp. 731-739. 61. M. E. Davis, C. Saldarriaga, C. Montes, J. Garces and C. Crowder, Nature (London), 331 (1988) 698-699. 62. P. R. Rudolf, C. Saldarriaga-Molina and A. Clearfield, J. Phys. Chem., 90 (1986) 6122-6125. 63. L. McCusker, J. Appl. Crystallogr., 21 (1988) 305-310. 64. P. Norby, A. N~rlundChristensen and I. G. Krogh Andersen, Acta Chem. Scand., A40 (1986) 500-506. 65. M. Golab, &it. Kristallogr., 185 (1988) 695-695. 66. M. W. Deem and J. M. Newsam, Nature, 342 (1989) 260-262. 67. M. M. J. Treacy and J. M. Newsam, Nature, 332 (1988) 249-251. 68. J. M. Newsam, M. M. J. Treacy, W. T. Koetsier and C. B. deGruyter, Prw. Roy. Soc. (London), A420 (1988) 375-405. 69. J. B. Higgins, R. B. LaPierre. J. L. Schlenker, A. C. Rohrman, J. D. Wood, G. T. Kerr and W. J. Rohrbaugh, Zeolites, 8 (1988) 446-452. 70. J. M. Newsam, M. M. J. Treacy, D. E. W. Vaughan, K. G. Strohmaier and W. J. Mortier, J. Chem. Soc. Chem. Comm., (1989) 493-495. 71. D. E. W. Vaughan, US Patent Appl. (1990). 72. F. Delprato, L. Delmotte, J. L. Guth and L. Huve, Zeolites, 10 (1990) 546. 73. M. M. J. Treacy, J. M. Newsam and M. W. Deem, in Disorder in Crystalline Materials (MRS Symp. Proc.), Materials Research Society, Pittsburgh, PA, 1989; pp. 497-502. 74. M. M. J. Treacy, J. M. Newsam and M. W. Deem, (1990) submitted.
141
Effects of Structural Disorder on the Generation of Acidic Sites in Zeolite L
K. Tsutsumi, A. Shiraishi, K. Nishimiya, M. Kato, and T. Takaishi Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan
ABSTRACT Acidic properties of zeolite L were observed to correlate well with its structural disorders. The %i-MAS-NMR spectrum of zeolite L having a Si/AI ratio different from 3 revealed that Al distribution deviated from the ideal and suggested the presence of six different boat-shaped 8-ring patterns. Differential molar heats of adsorption of ammonia changed stepwise with the adsorbed amount, which reflects the difference in the acid strength o f protons located in structurally different 8-rings.
INTRODUCTION The typical unit cell content of zeolite
L
is
(K,Na)gAlgSi27072.nH20and i t s Si/AI ratio varies in the range of 2.6
- 3.5 [l-41. Takaishi recently
determined the distribution of Al atoms in the
L by analyzing 29Si-MASNMR spectra. He thereby deduced five kinds of
framework of zeolite
extra-framework cation sites as shown in Fig. l., and estimated the relative strengths of their cation affinities [5]. The acidic properties o f zeolite L should stem
D') in Fig. 1, since only site D is located on the wall of the
from protons on the site D (D' and
channel pore of zeolite L and only protons on the wall can participate in the reaction there occurring. In the ideal crystal with Si/AI ratio of 3 and no disorders, there are two kinds of site D, that is,
D' and D ' , shown in Fig. 1. A real crystal,
Fig. 1. Cation sites in zeolite L. A :Al atom, A :Al atom located on a hidden site,O :site A, :site B', @ :site B ' , @ :site C, Q :site D', (> :site D ' , ::I :site E
9
142 K. Tsutsumi, A. Shiraishi, K. Nishimiya, M. Kato and T. Takaishi
however, has a Si/Al ratio different from 3 and various disorders, both of which must cause the A1 distribution in the framework deviate from the ideal. The disordered Al distribution should generate new kinds of site D, modifying acidic properties of zeolite L. In this study, we analyze this situation using 2pSi-MAS-NMR spectroscopy and high-temperature ammonia-adsorption calorimetry. The acid strength will be determined from the heat of adsorption of ammonia. On adsorption of ammonia, the reaction,
takes place. The weaker the affinity of the proton to the site, the larger the adsorption heat (Q)
o f ammonia becomes. An experimental difficulty lies in the following point: the large heat of adsorption prevents adsorbates from diffusing freely on the adsorbent t o realize the equilibrium distribution on adsorption sites. This difficulty is mostly overcome by raising the adsorption temperature t o 473 K or so,and accelerating the diffusion rate [6,7].
EX PER IM ENTA L The starting zeolite L, Kp,5A1p.5Si26.5072,was supplied by Toso co. It was ion-exchanged in aqueous ammonium nitrate solution, dried and further heated in order to obtain (H,K)g.5A1p.5Si26,5072 samples. The Si/AI ratio was determined by 2pSi-MAS-NMR spectroscopy and the potassium content by activation analysis. Heats of adsorption of ammonia were measured with a twin-conduction-type microcalorimeter equipped with a volumetric vacuum line. The details and procedures have been described previously [6-81. Prior to calorimetric measurements, samples were activated by calcination under 1 mPa pressure on increasing the temperature a t a rate of 3 K min-' and a t the final temperature, in general 723 K, for 10 h. Adsorption of ammonia was carried out a t 473, 573 and 623 K. The 2pSi-MAS-NMR spectra were taken using a JEOL GX-270.
R ESULTS The molar differential heats of adsorption of ammonia a t 623 K are shown in Fig. 2 for the sample, H7,25K2.2sA1p.5Si3~.507~. The heat values clearly change stepwise with the adsorbed amount. Adsorptions a t lower temperatures gave also stepwise variation but less distinct, which indicates diffusion limitation in microporous adsorbents at lower temperatures. The step-wise variation of heat curves suggests the presence of several kinds of adsorption sites of ammonia, the energy level of which is different [9]. Since only site D is accessible t o adsorbates, the adsorption energy should be divided into three steps corresponding to
D', D" and the non-acidic
sites under the condition of the ideal framework retained in the sample. However, the observed
Structural Disorder and Acidic Sites in Zeolite L 143
NH3 Adsorbed I m mol g-1
0 H7.25K2.25-L
-,-
Evacuated at 723K-10H
2 150 -0
. 7
Y
.-5 c
g100 m
-
0 0 0 0
0 0
7J
4
r
+
I
50-
I
I
1
0
I
I
I
2 3 4 NH3 Adsorbed I Molecules U . C ’
I
I
5
6
Fig. 2. Calorimetrically determined molar heats of adorption of ammonia at 623 K on H7.25K2.25-L.
heat curves revealed at least six steps, which must arise from the deviation of Al distribution from the ideal in the framework.
D’slte
D”s1te
I
I
In an Ideal crystal
ANALYSIS AND DISCUSSION Site D is located near the centre of the boat-
by lntroductlon of a disorder
D’ is surrounded by three Al atoms while site D” has only one Al atom in its neighbor. The sample shaped 8-membered- oxygen-ring, and site
used here contained 9.5 A1 atoms per unit cell in contrast t o the stoichiometric value of 9.0 in an
D2
ideal crystal ; such a deviation should result in disordered Al distribution.
We considered three
kinds of disorders, D1, D2 and
D3
shown in Fig.3.
The disorder D1 involves a migration of Al atom in the 8-ring as well as A1 atom insertion. The D2 and D3 involue only Al migration. Then, the changes brought about by the disorders are expressed as,
D3
Fig. 3. Configurational pattern of Al atoms in the &ring. Filled circle: A1 atom Roman numeral: number of Al in the ring
144 K. Tsutsurni, A. Shiraishi, K. Nishimiya, M. Kato and T. Takaishi
+ IIJ = II’ + P + m’,
by D1,
I
+ m = II’ + rn’,
by D2t
I
+ m = II” + II”’,
by D3,
21
and (4)
in which the Roman numerals specify the number of Al atoms in the 8-ring, and the D ’ and D” sites are represented by III and I, respectively, for consistency. When one D1 occurs in the unit cell, three Si(3AI) atoms are newly produced, while two Si(2AI), one Si(1AI) and one Si(0AI) atoms disappear. Based on 29Si-MASNMR spectra which revealed the concentrations of Si(OAI),
Si( lAl), Si(2AI) and Si(3AI), the numerical possibility of the disorders can be calculated as shown in Table 1. The populations o f six different configurational patterns (I, II’, II”, It”’, III’ and IU) can thereby be easily calculated and the results are shown in the last column of Table 2.
Table 1. Analysis of 29Si-MAS-NMR spectrum of zeolite L used. Al
Observed Population/U.C. Calculated/U.C. ideal 0.50D1 0.14D2 1.OIDS Mixture
Si(3AI)
Si(2AI)
Si(1AI)
Si(0AI)
9.50
2.80
9.42
10.77
3.51
9.00 0.50 0.00 0.00 9.50
0.00 1.50 0.28 1.01 2.79
12.00 -1.00 -0.56 -1.01 9.73
12.00 -0.50 0.28 -1.01 10.77
3.00 -0.50 0.00 1.01 3.51
A reasonable distribution of cations on the six kinds of site D can be described as follows. A cation selectively occupies the site with the strongest cation affinity among the available sites. When there are several kinds of cations, there occurs competition among them to occupy the favorable site. In the case of (K,H)-L zeolite, K+ ions should eliminate protons in the competition.
The residual K+ ions in (H,K)-L must preferentially occupy sites B’ and B ’ . Since the cation affinity of site C is considered t o be similar to that of site D, site C is assumed to be occupied
Structural Disorder and Acidic Sites in Zeolite L 145
Table 2. Populations of the six patterns of the 8-ring of the sample.
Ideal Disorder D1 D2 D3
0.50D1+0.14D2+1.01D3 Total
I
II’
3.00
0.00
n’” Iu’
II”
0.00 0.00
0.00
m 3.00
2 1 1 0 1 1 1 1 0 0 1 1 -1 0 1 1 0 - 1 -2.15 0.64 1.51 1.01 0.64 -1.65
0.85 0.64 1.51 1.01 0.64
1.35
by protons as shown in Table 3. Next the relative strength of the cation affinity of the six kinds of site D which is accessible t o adsorbed molecules must be estimated. The AIOl in a zeolitic framework has an effectively negative charge, and the 8-ring attracts a cation more strongly with increasing number o f Al atoms contained. Protons located in the 8-ring with fewer Al atoms must become stronger acid sites. The six patterns of site D are thereby classified into three groups, I,
(II’, II”, II”’) and (JII, JII’) in ascending order of strength of cation
affinity or descending order of
ammonia adsorption heat. A t the present stage, one cannot anticipate detailed orders for each group, which are empirically determined in a course of analysis of experimental data.
Table 3. Distributions of cations in zeolite
L, (H ,K)9.5A19.5Si26.5072 Site
B’+B’
C
D’+D’
2
3
6
H7.25K2.25
2K
3H
0.25K 4.25H
H4.36 K5.14
2K
3H
3.14K 1.36H
H3.69K5.81
2K
3H
3.81K 0.69H
Composition number/U.C.
146 K. Tsutsumi, A. Shiraishi, K. Nishimiya. M. Kato and T. Takaishi
By combining this order with the numerical population of the six kinds of site D, a histogram of the ammonia adsorption heat can be obtained as shown in Fig. 4. It is noted that heats of adsorption on protons a t site I are expected to be very large but not known quantitatively a t
this stage, and schematically shown in the figure. Such a histogram is termed as a generating histogram for adsorption heat, since one can easily derive from it the adsorption heat curve for a sample having the same Si/AI ratio but a different [Ht]/[Kt]
ratio.
--
-
. I
J
Q .,Q., @,. ,.G T> /\
I
\/
/\
0.64
1.51
1.01
\ /
0.64
1.35
I
Fig. 4. Histogram of the energy level of six kinds of site D in H7.25K2.25-L.
We now derive the adsorption heat curve from the above generating histogram. The site with a weak adsorption heat for ammonia has a strong cation affinity, and 0.25 K+ ions in the present
sample selectively occupy sites located on the right side of the generating histogram. Next, 4.25 protons occupy sites in the region neighboring the above part, and there remain 1.5 vacant sites on the left side of the histogram as shown in Fig. 5. Comparing the observed heat with the generating histogram shown in Fig. 5, the fit is excellent. In order t o confirm the validity of the model, two different samples, the compositions of which are shown in Table 3, were examined. Since the starting zeolite was the same, they have the same Al distribution in the framework and hence should have the same generating histogram as those of the sample, H7.25K2.25A19.5Si28.5012.
The difference in their adsorption heat curves stems from the different positions of the border line between the regions occupied by protons and K+ ions in the histogram. The border lines are located a t 3.14 and 3.81 from the right side of the histogram in (H4.36K5.11)-Land (H3.6gK5.a1)-L,
Structural Disorder and Acidic Sites in Zeolite L 147
NH3 Adsorbed I m mol g-1
2 Evacuated at 723K-101
4.5
0
1
2
3
4
5
6
NH3 Adsorbed I Molecules U.C.?
Fig. 5. Comparison of calorimetrically determined heats of adsorption (circles) with the energy level histogram for H7.25K2.25-L.
NH3 Adsorbed I m mol g-1
2o
0
1
2. 3 4 NH3 Adsorbed I Molecules U.C.-’
5
6
Fig. 6. Comparison of calorimetrically determined heats of adsorption (circles) with the energy level histogram for H4.36K5.14-L.
respectively. The calculated ammonia-adsorption heat histograms in Figs. 6 and 7 thus obtained agree very well with the observed heats shown by circles in the figure.
148 K. Tsutsumi. A. Shiraishi, K. Nishimiya, M. Kato and T. Takaishi
NH3 Adsorbed / mmol g-1
Fig. 7. Comparison of calorimetrically determined heats of adsorption (circles) with the energy level histogram for H3.69K5.81- L.
It is concluded that the proposed model explains well a l l the observed results with the three
different samples and can be considered to be highly reliable. The strength of acid sites in zeolite L correlates well with the Al distribution in the framework, which can be modified by the deviation of Si/AI ratio from the ideal value of 3. The larger the extent of the deviation becomes, the lesser the number of site pattern I, or site D ’ , is. This gives a clue to the generation of strong acid sites in zeolite L.
REFERENCES
1. R. M. Barrer and H. Villiger, Z. Kristallgr., 128 (1969) 352. 2. Ch. Berlocher and R. M. Barrer, ibid., 136 (1972) 245. 3. J. M . Newsam, J. Chem. SOC.,Chem. Commun., (1987) 123. 4. J. M. Newsam, Mater. Res. Bull., 21 (1986) 661. 5. T. Takaishi, 1. Chem. SOC.,Faraday Trans.1, 84 (1988) 2967. 6. Y. Mitani, K. Tsutsumi and H. Takahashi, Bull. Chem. SOC. Japan, 56 (1983) 1921. 7. K. Tsutsumi and K. Nishimiya, Thermochimica Acta, 143 (1989) 299. 8. K. Tsutsumi, S. Hagiwara, Y. Mitani and H. Takahashi, Bull. Chem. SOC. Japan, 55 (1982) 2572. 9. K. Tsutsumi and Y. Mitani, Colloid & Polymer Sci., 263 (1985) 832.
111 Modification
This Page Intentionally Left Blank
151
Growth of Silica and its Controlling of Pore-opening Size on CVD Zeolites
Takashi Hibino, Miki Niwa, Yoshimi Kawashima, and Yuichi Murakmi Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01 Japan
ABSTRACT This paper describes how silica grows on the external surface of zeolite, and how it controls the pore-opening size. Silica growth and control of poreopening size is not affected by the kind of cation but only by the composition of zeolite. The larger the silica content, the more similar the silica layer to the basal plane grows. Due to the subtle difference between overlayer and zeolite, the pore-opening size can be narrowed. Because of the similarity, thicker layers are required for control in the highly siliceous zeolites. INTRODUCTION In previous investigations of chemical vapor deposition (CVD) of silicon alkoxide to control the pore-opening size of zeolites [l-51, the most interesting chemical aspect is the formation of a very thin layer of silica on the crystal plane and its function in pore-size reduction. Formation and function of such an ultra thin layer of metal oxides is not encountered very often. This study therefore presents problems different from either the monolayers of Langmuir-Blodgett film [6] or from the thicker oxide layers by usual CVD method [7]. Direct observation of silica deposited is very difficult because of its similarity to bulk zeolites as well as extremely small contents. Indirect approaches to reveal the structure of deposit silica and the function for pore-opening size controlling must be carried out. EXAFS study using germanium in the place of silicon showed that the formation of a thin layer was essential in controlling the pore-opening size [8,9]. As to the difference in deposit silica previously obtained, the difference in the number of required silica layers was interesting. In the case of H-mordenite (Norton, 100-H), mono to triple layers of silica were found to be required to control the poreopening size [2], while more (4 - 6) layers of silica were required for the ZSM-5 zeolite [4]. Therefore, the extent of its influence on the pore size
152 T. Hibino, M. Niwa, Y. Kawashima and Y. Murakami
seems to depend on the property and/or species of zeolites. In the present study, therefore, the silica-coated mordenites were prepared using decationized- and Na-mordenites with different silica to alumina ratios, and the growth of silica on zeolite and its function for pore-opening size reduction will be investigated. EXPERIMENTAL METHODS Zeolite and CVD --Six kinds of zeolites decationized-mordenites and Na-mordenites with different silica to alumina ratios were used in the present study (Table 1); decationized mordenites were prepared from corresponding Na forms. These zeolites were supplied by the Catalysis Society of Japan as reference catalysts. In addition, a dealuminated mordenite was prepared by leaching HM-10 with 0.4 mol dm-3 HC1 solution at 323 K for 30 hr. After washing with distilled water, this was treated in a sodium acetate solution at 353 K for 20 hr for the ion-exchange. The resulting composition of aluminum was measured by inductively coupled plasma (ICP) emission spectroscopy after digestion in HF. Deposition was performed using a vacuum system ill. Before deposition of the alkoxide, the sample was evacuated at 673 K for 2 hr, and the temperature was then lowered to 593 - 293 K for the deposition of silicon methoxide. Si(OCH3)4 vapor was then admitted t o the dried zeolite at a vapor pressure of 2.5 Torr. The resultant increase of weight was measured by the quartz microbalance. After the deposition, the decationized zeolite was calcined situ by oxygen at 673 K to remove the coke residue, while the Na-type mordenite was treated with water vapor at 593 K. The amount of Si deposited was Table 1. List of mordenites and saturated surface concentration of deposited silicon name
(Si02/A1203) surface area /m2 g-' ratio total externala
NaM-10 NaM-15 NaM-20 NaDM-10 SiO2
9.9 14.9 19.9 17.8
HM-10 HM-15 HM-20
9.9 14.9 19.9
-
187 309
299
-
-
187 308 299
9.1 11.5 14.9
aMeasured by benzene-filled pore method
593
10.3 8.8 5.4 6.2 1.3
291 284 282
14.0 13.6 9.9
593 593 593
9.8
13.4 16.9 13.3
353
-1
1st saturated deposit on temp./k conc. /Si nm
593
[lo].
Pore Size Control in CVD Zeolites 153
calculated from the weight gain after the calcination or the reaction with water, Si02 assumed t o have been formed. Catalytic Reaction and Infrared Study Catalytic cracking was performed by a conventional pulse technique. The catalyst was pretreated with helium at 723 K for 2 hr. 0.8 mm3 of octane isomers mixture was injected, and the products were analyzed using a liquid paraffin column operating at room temperature to 373 K. Na-mordenite was converted into the decationized form before the cracking. Profile of the deposition of silicon alkoxide was followed by an infrared spectroscopy, Jasco FTIR-3 in a transmission mode. RESULTS Accumulation of Silica by Successive Deposition As reported previously [ 4 ] , the deposition of silicon alkoxide on NaM ceased readily at 593 K, but it could be repeated again after the reaction with water vapor. Accumulation of silica by successive deposition was then measured on NaM, as shown in Fig. 1. The surface concentration on NaM-20 in the first deposition was smaller than on others, but increased gradually upon successively deposition. Totally, the accumulated surface concentration on NaM-20 obtained by 10 depositions was the highest among them. On the other hand, the value on NaM-10 in the first deposition was relatively high, but increased a little after that. The intermediate extent of silicon accumulation was observed on NaM-15 and NaDM-10, a dealuminated sample with 17.8 of silica to alumina ratio. After all, the larger the silica to alumina ratio of zeolite, the larger the degree of continuation of silicon accumulation. Furthermore, the accumulated silica increased on Si02 linearly with number of deposition. The surface silicon concentration at the first saturation was found to decrease with the silica to alumina ratio of zeolite, shown in Table 1. A relatively small concentration on silica was remarkable. Saturated silicon concentration therefore seemed to be correlated with the aluminum concentration of zeolites. Because of the consecutive reaction by produced water, the deposition on HM was hardly saturated at 593 K; however, it was saturated at room temperature. The saturated silicon concentration on the HM thus measured at room temperature (Table 1) decreased with the silica to alumina ratio of zeolite, similar to that on the NaM.
154 T. Hibino, M. Niwa, Y. Kawashima and Y. Murakami
5 10 Number o f successive depositions (-) Fig. 1. Increase of accumulated surface concentration of silica deposited NaM-20 (A),NaDM-10 by successive deposition on NaM-10 ( O ) , NaM-15 (01, ( 0 )and Si02 ( + I . 0
Infrared Study of the Deposition Deposition profile was then followed by an infrared study. Deposition of silicon alkoxide removed isolated silanol at 3745 cm-' selectively, while that of hydrogen-bonded at 3600 cm-' was kept unaltered, as shown in Fig. 2 a, b. Methyl group of surface residue of silicon compound was seen simultaneously with the disappearance of isolated silanol. Upon reaction of the surface deposited species with water, the stretch bands of the methyl group disappeared, and the absorption of isolated silanol was recovered completely or incompletely, depending upon the kind of zeolite and metal oxide. The behavior of intensity of isolated silanol was then measured quantitatlvely on M-10, M-20, and Si02 (Fig. 3 ) . On Si02. almost all the isolated silanol was consumed upon deposition but recovered completely by the reaction with water. The deposition was repeated on the Si02. In contrast, on M-10, about half the isolated silanol decreased in the first deposition, but the intensity was recovered almost completely by the reaction of water; however, the deposition gradually became difficult. On the other hand, the
Pore Size Control in CVD Zeolites 155
I
3900
3500 3000 Wovenumber (an-')
m
t
t
I
m
t
1
1
1
1
1
2700 Wovenumber
(cm-l)
Fig. 2-a (left). Infrared spectra on SiOz: (a), background: ( b ) , after deposition of Si(OCH3I4: (c), after hydration by HZO; further repetition of this procedure in (d) to (f): Fig. 2-b (right). those on NaM-10
,o
0
Fig. 3. Quantitative measurement of intensity of SiOH by repeating deposition and hydration cycle on SiOz ( + I , NaM-20 ( A ) , and NaM-10 (0).
156 T.Hibino, M. Niwa, Y. Kawashima and Y. Murakami
behavior on M-20 was analogous to that on Si02. The difference in behavior by the recycled deposition - activation was thereby in good agreement with that observed in the accumulation of silica shown above. On the other hand, broad bands of hydroxide at 3600 cm-I did not show significant change in intensity by the repetition of the deposition - reaction cycle. The intensity increased slightly after the third deposition only on M10.
Shape-selectivity in the Cracking of Octane Isomers Reduction of the pore-opening size of zeolites was tested by the cracking of 3-methylheptane and 2,2,4-trimethylpentane. In this case, the catalyst weight was chosen so that the conversion of these molecules was less than 30 %. The deposition of silica was done at 593 K for both NaM and HM. Both paraffins reacted in a similar degree on inherent species of mordenites, but, on NaM-10, NaDM-10, and HM-10, the conversion of 3-methylheptane was somewhat larger than that of 2,2,4-trimethylpentane. Conversion decreased upon deposition of silica. In particular, the extent of the decrease in the conversion of 2,2,4trimethylpentane was larger than that of 3-methylheptane. The conversion ratio of 2,2,4-trimethylpentane to 3-methylheptane decreased with increasing
1 a0
0,5
0
0
10
20
30
Surface si 1i c a concent r a t ion
40
(nm-*)
Fig. 4. Conversion ratio (2,2,4-trimethylpentaneto 3-methylheptane) vs. surface concentration of silica deposited on zeolites HI-10 (01, HM-15 ( B 1, HM-20 (A),NaM-10 (01, NaM-15 ( 0 1 , NaM-20 (A1 and NaDM-10 ( 0 ) .
Pore Size Control in CVD Zeolites 157
deposition of silica, except on SiNaM-10. The preferential reactivity of the smaller molecule, i.e., increase in shape-selectivity, indicated closure of the pore-opening by the deposition of silicon alkoxide. The relationship between the surface concentration of deposited silicon and the conversion ratio i s shown in Fig. 4. On the HM-10 and NaM-10, the cracking of 2,2.4-trimethylpentane was inhibited completely at the deposition of 14 nm-’ of silicon surface concentration. On the other hand, on HM-15, HM-20, NaM-15, and NaM-20. about 40 of Si was required for complete suppression of cracking of the larger molecule. The behavior on NaDM-10 was completely different from that on the native species HM-10, but similar to that on mordenites with 15 to 20 of silica to alumina ratio. Therefore, reduction of the pore-opening depended upon the silica to alumina ratio of zeolites. On the other hand, cations did not influence on this relationship. DISCUSSION
Infrared study shows clearly that the alkoxide reacted with isolated silanol in preference to hydrogen-bonded hydroxide [11,121. In other words, the deposition of the alkoxide i s difficult on the surface with a dense concentration of hydroxide which interacts with each other. Continual deposition of silica thereby shows that the distribution of surface hydroxide does not change after the deposition. This condition seems to be realized mostly on Si02, and among zeolites. on the most siliceous M-20, as found by ir and deposition studies. It can be postulated that the deposition of silicon alkoxide on these surfaces did not alter the surface structure. On the other hand, the conditions were not satisfied on M-10, since the deposition became difficult gradually, and the distribution of surface hydroxides changed a little. Because the surface cation density on mordenite i s 8 . 6 the foregoing discussion can ben applied to mono to triple layers of the deposited oxide. This difference in growth of silica due to the composition of zeolites may be explained simply by the similarity of reagent deposited and basal surface. Because silica grows on the surface through the siloxane bond (-0-Si-0-1, layers with the same properties and structure can be grown on the siliceous surface. Epitaxial growth of overlayer on the plane i s envisaged in the field of material physics. Epitaxy of a relatively thick layer measurable in microns i s explained primarily by the matching of crystal planes. We assume a similar epitaxy for the deposition of silica on the external surface of zeolites, although direct observation by electron microscopy i s extremely difficult.
158 T.Hibino, M. Niwa, Y. Kawashima and Y. Murakami
Difference in surface concentrations required for achieving the shapeselectivity indicates formation of silica with different surface conditions. The relationship between shape-selectivity and surface silicon concentration. however, does not largely depend on the included cation, proton or sodium, but rather on the composition of zeolites. Strong dependence on the composition was confirmed on the dealuminated mordenite. since the behavior was not in agreement with those on the native species but with those expected from the composition. Therefore, growth of silica and pore size enclosure can be summarized, silica to alumina ratio
low
high
thickness required for pore enclosure continuation of silica growth
thin low
thick high
On a highly siliceous external surface, the pore-opening is not readily narrowed because of the similarity between the zeolite and silica deposited. With increasing the aluminum content, silica grows in a somewhat different manner, with different bond length and bond angle from those of zeolites. A s a result, the protrudent siloxane bonds reduce the pore-opening size. REFERENCES 1 M. Niwa, S. Morimoto, M. Kato, T. Hattori, and Y. Murakami, Proc. 8th Inter. (1984) 701. Cong. Catal., 2 M. Niwa, S. Kato, T. Hattori, and Y. Murakami, J. Chem. SOC., Faraday I, 80 (1984) 3135. 3 M. Niwa, M. Kato, and Y. Murakami, J. Phys. Chem., 90 (1986) 6233. 4 M. Niwa, Y. Kawashima, T. Hibino, and Y. Murakami, J. Chem. SOC., Faraday I, 84 (1988) 4327. 5 M. Niwa and Y. Murakami, J. Phys. Chem. Solids, 50 (1989) 487. 6 For example, S. Palacin, A . Ruaudel-Teixier, A . Barraud, J. Phys. Chem., 90 (1986) 6237. 7 For example, S. Hayashi, T. Hirai, J. Crystal Growth, 41 (1977) 41. 8 T. Hibino, M. Niwa, Y. Murakami, M. Sano, J. Chem. SOC., Faraday Trans. I, 85 (1989) 2327. 9 T. Hibino, M. Niwa. Y. Murakami, M. Sano, S. Komai, T. Hanaichi, J. Phys. Chem., 93 (1989) 7847. 10 M. Inomata, M. Yamada, S. Okada, M. Niwa, and Y. Murakami, J. Catal., 100 (1986) 264. 11 W. Hertl, J. Phys. Chem., 72 (1968) 1248. 12 D. W. Sindorf. and G. E. Maciel, J. Phys. Chem., 86 (1982) 5208: D. W. Sindorf, and G. E. Maciel, J. Amer. Chem. SOC., 105 (1983) 3767.
159
New Method of Modifying Y-type Zeolite -Fe Supported Zeolite
S.HIDAKA,
R.IWAMOT0,
1.NAKAMURA and A.IIN0
C e n t r a l R e s e a r c h L a b o r a t o r i e s o f I d e m i t s u Kosan Co. Ltd.. 1 2 8 0 Kami i z u m i , Sodegaura, Kimitsu, Chiba, Japan 299-02 as a p a r t i c i p a n t o f Research A s s o c i a t i o n f o r Residual O i 1 Processing (RAROP).
ABSTRACT An i r o n supported Y-type z e o l i t e which was prepared from m o d i f y i n g NH4Y w i t h f e r r i c n i t r a t e s o l u t i o n showed h i g h 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 u n d e r t h e f l o w o f H2S/H2. D e t a i l e d i n v e s t i g t i o n s f o u n d t h a t p r e p a r a t i o n c o n d i t i o n s s u c h a s t e m p e r a t u r e a n d Feg+ c o n c e n t r a t i o n s i g n i f i c a n t l y a f f e c t c a t a l y t i c a c t i v i t y . The c a t a l y s t which was prepared by m o d i f y i n g NH4Y w i t h 0.25M Fe(N03)3 s o l u t i o n a t 323K showed t h e h i g h e s t a c t i v i t y among t h e t e s t e d samples. INTRODUCTION R e c e n t l y , we r e p o r t e d t h a t an Fe s u p p o r t e d z e o l i t e (FeHY-1) shows h i g h a c t i v i t y f o r a c i d i c r e a c t i o n s such as 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 and r e s i d h y d r o c r a c k i n g i n t h e p r e s e n c e o f H2S [ 1,2]. s p i n r e s o n a n c e (ESR),
Investigations using e l e c t r o n
F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o s c o p y (FT-IR),
M'dssbauer and t r a n s m i s s i o n e l e c t r o n microscopy (TEM) r e v e a l e d t h a t s u p e r f i n e f e r r i c o x i d e c l u s t e r i n t e r a c t s w i t h t h e z e o l i t e framework i n t h e super-cage o f Y-type z e o l i t e s [3,4].
Furthermore, we r e p o r t e d change i n physicochemical
p r o p e r t i e s and c a t a l y t i c a c t i v i t i e s f o r t o 1 uene d i s p r o p o r t i o n a t i o n d u r i n g t h e sample p r e p a r a t i o n p e r i o d [ 5 ] .
It was r e v e a l e d t h a t t h e a c t i v a t i o n o f t h e
c a t a l y s t was c l o s e l y r e l a t e d w i t h i n t e r a c t i o n between t h e i r o n c l u s t e r and t h e z e o l i t e framework. p r e p a r a t i o n c o n d i t i o n s on
I n t h i s work,
we w i l l
report the e f f e c t o f
t h e physicochemical p r o p e r t i e s and 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 i n t h e presence o f HzS.
EXPERIMENTAL Preparation
of c a t a l y s t
Various Fe-supported Y-type z e o l i t e s (FeHY-1) NH4Y (UCC:LZY-82)
were prepared b y s t i r r i n g
i n Fe(N03)3 s o l u t i o n a t v a r i o u s t e m p e r a t u r e s (293-37310
f o r 2 h. The samples o b t a i n e d were washed w i t h d i s t i l l e d water, d r i e d i n a i r a t 363K f o r 3 h and f i n a l l y c a l c i n e d i n a i r a t 773K f o r 3 h.
160 R. Iwamoto, S. Hidaka, I. Nakamura and A. Iino
of c a t a l y s t
Analyses
molar r a t i o
The amount o f Fez03 supported on z e o l i t e and t h e Si02/A1203
(S/A r a t i o ) o f t h e prepared c a t a l y s t s were o b t a i n e d b y X-ray f l u o r e s c e n c e s p e c t r o m e t r y (Rigaku Denki.
BET method (Yuasa,
3080E).
S p e c i f i c s u r f a c e areas were measured by
QUANTACHROME). U n i t c e l l dimension (U.D.)
was determined
f r o m t h e d i f f r a c t i o n a n g l e s o f (642) w i t h an X-ray powder d i f f r a c t o m e t e r
S i 1 i c o n was used as t h e reference.
(Rigaku Denki, RU-200). Measurement
of c a t a l y t i c
activity
To1 uene 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 high-pressure continuous f l o w m i c r o r e a c t o r . G r a n u l a r c a t a l y s t (32-64 mesh, 2.5 cm3) was loaded i n t o a s t a i n l e s s s t e e l t u b e r e a c t o r . T o l u e n e was f e d a t a r a t e o f 1 0 cm3h-l ( l i q u i d ) i n t h e f l o w o f H$(0.2vol.%)/H2
m i x t u r e gas (200 cm3min-')
a t 623K
and 6MPa. The e f f l u e n t was a n a l y z e d by gas chromatography (Shimadzu,
GC-9A)
by a f l a m e i o n i z a t i o n d e t e c t o r (FID). RESULTS AND
DISCUSSION
E f f e c t o f Fe3+ c o n c e n t r a t i o n Fe-supported
z e o l i t e s w e r e p r e p a r e d b y m o d i f y i n g NH4Y w i t h v a r i o u s
c o n c e n t r a t i o n s o f Fe(N03)3 s o l u t i o n f r o m 0.025 t o 0.5M a t 323K. F i g u r e 1 shows t h e solution
c o r r e l a t i o n between Fe3'
concentration i n the preparation
and 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 o f t h e o b t a i n e d c a t a l y s t . The
amount o f Fez03 l o a d i n g on t h e z e o l i t e i n c r e a s e d t o about 8 w t % i n p r o p o r t i o n t o Fe3'
c o n c e n t r a t i o n and l e v e l e d o f f a t 0.25M.
1 i n e a r l y w i t h Fe3' s h a r p l y above 0.1M.
S/A r a t i o a l s o i n c r e a s e d
c o n c e n t r a t i o n , w h i l e u n i t c e l 1 dimension (U.D.) These r e s u l t s i n d i c a t e t h a t non-framework
decreased
aluminum was
m a i n l y e x t r a c t e d below 0.1M and framework aluminum c o u l d be e x t r a c t e d above 0.1M.
S p e c i f i c s u r f a c e area f e l l d r a s t i c a l l y a t 0.5M,
s l i g h t l y below 0.1M.
a l t h o u g h i t changed
S u r p l u s d e a l u m i n a t i o n may cause p a r t i a l d e s t r u c t i o n of
t h e z e o l i t e f r a m e w o r k a t 0.5M.
I t i s suggested t h a t t h e
degree o f
d e a l u m i n a t i o n depends on t h e c o n c e n t r a t i o n o f p r o t o n s due t o t h e h y d r o l y s i s o f Fe3+[ 6,7]. F i g u r e 1 a l s o shows 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 , t o benzene and x y l e n e s
, and
selectivity
t h e amount o f c o k e d e p o s i t e d on t h e c a t a l y s t
a f t e r t h e reaction. The c o n v e r s i o n i n c r e a s e d i n p r o p o r t i o n t o t h e amount o f Fe203 f r o m 0.025 t o 0.25M,
t h e n decreased w i t h d e c l i n e i n c r y s t a l l i n i t y a t 0.5M.
The change i n s e l e c t i v i t y i s r a t h e r c o m p l i c a t e d . I t r e a c h e d a l o w a t
O.lM,
t h e n peaked a t 0.25M.
T h i s means t h a t i r o n c l u s t e r l o a d i n g a t l o w Fe3'
c o n c e n t r a t i o n shows l o w e r s e l e c t i v i t y t h a n a t 0.25M.
The l o w e s t s e l e c t i v i t y
New Method of Modifying Y-type Zeolite 161
a t 0.5M may be a t t r i b u t a b l e t o t h e change i n t h e s t a t e o f i r o n s p e c i e s due t o t h e framework d e s t r u c t i o n . The amount o f c o k e d e c r e a s e d d r a s t i c a l l y a b o v e 0.25M. c l o s e l y r e l a t e d t o t h e amount of framework aluminum. between t h e amount o f c o k e and U.D. acid sites i n zeolite.
S i n c e U.D.
is
t h e good c o r r e l a t i o n
suggests t h a t coke forms on B r d n s t e d
The removal o f framework aluminum corresponding t o
Brdnsted a c i d may be e f f e c t i v e f o r decreasing coke formation.
Furthermore,
t h i s r e s u l t a l s o i n d i c a t e s t h a t t h e a c t i v e i r o n c l u s t e r i s i n a c t i v e f o r coke formation i n s p i t e o f high a c t i v i t y f o r toluene disproportionation.
mdp
O W
10
(u3
al
R k
\m
c*H
0 0 E
-4
0)-
0
24.50
-
a 4
n
D\
24.40
40& 3 0 0
C E
U\
"
" V
0
0.1
0.2
0.3
0.4
0.5
Fe3+ concentration I M Fig. 1 C o r r e l a t i o n between Fe3+ c o n c e n t r a t i o n o f t h e p r e p a r a t i o n s o l u t i o n and physicochemical p r o p e r t i e s as we1 1 as c a t a l y t i c p r o p e r t i e s o f t h e Fe supported z e o l i t e . a) U n i t c e l l dimension b) Conversion o f t o l u e n e c) S e l e c t i v i t y t o benzene and x y l e n e s
162 R. Iwamoto, S. Hidaka, I. Nakamura and A. Iino
-Effect
o f p r e p a r a t i o n temperature
T h r e e Fe s u p p o r t e d z e o l i t e s w e r e p r e p a r e d b y m o d i f y i n g NH4Y w i t h 0.25M Fe(N03)3 a t v a r i o u s t e m p e r a t u r e s f r o m 293K t o 373K.
F i g u r e 2 shows t h e
i n f l u e n c e o f p r e p a r a t i o n t e m p e r a t u r e 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 and c a t a l y t i c a c t i v i t y o f t h e obtained catalysts. The amount o f Fez03 l o a d e d on t h e z e o l i t e i n c r e a s e d t o 15wt% a t 373K. H y d r o l y s i s o f Fe3+ and s u b s e q u e n t p o l y m e r i z i n g o f i r o n c l u s t e r t e n d t o a c c e l e r a t e a t a h i g h e r t e m p e r a t u r e s [ 5 ] . I n t h e t r e a t m e n t a t 373K, l a r g e r i r o n c l u s t e r may be s u p p o r t e d o n t h e s u r f a c e o f z e o l i t e because i t shows smal l e r quadrupole s p l i t t i n g from t h e i n v e s t i g a t i o n u s i n g M k s b a u e r spect r o s c o p y [ 81.
dp
04J
x 3 0
U\
dp
a,
.A
368 0
V\
273
P r e p a r a t i o n temperature Fig. 2
373
323
/
K
C o r r e l a t i o n b e t w e e n p r e p a r a t i o n t e m p e r a t u r e and 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 as w e l l as c a t a l y t i c p r o p e r t i e s o f t h e Fe supported z e o l i t e . (See f i g .
1 f o r abbreviations.)
New Method of Modifying Y-type Zeolite 163
Both t o l u e n e c o n v e r s i o n and s e l e c t i v i t y showed t h e h i g h e s t v a l u e o v e r t h e sample o b t a i n e d a t 323K. framework
We have r e p o r t e d t h a t d e a l u m i n a t i o n from z e o l i t e
and simultaneous i n t e r a c t i o n between i r o n c l u s t e r and z e o l i t e i s
n e c c e s a r y f o r t h e c a t a l y s t a c t i v a t i o n [ 5 ] . S i n c e t h e S / A r a t i o a t 293K i s l o w e r t h a n t h a t a t 323K, t h e i n t e r a t t i o n dose n o t appear t o be s u f f i c i e n t t o a c t i v a t e t h e c a t a l y s t i n t h e t r e a t m e n t a t 293K. c l u s t e r s u p p o r t e d a t 373K t r a n s f o r m s
On t h e o t h e r hand, i r o n
i n t o l a r g e r i r o n s u l f i d e by
p r e s u l f i d i n g b e f o r e t h e r e a c t i o n and decreased t h e a c t i v i t y [ 8 ] . Thus m o d e r a t e t e m p e r a t u r e as w e l l
a s Fe3+ c o n c e n t r a t i o n
m u s t be
c a r e f u l l y s e l e c t e d f o r t h e p r e p a r a t i o n o f t h e a c t i v e Fe-supported z e o l i t e . CONCLUSION As d e s c r i b e d above, iron cluster
i t was found t h a t
physicochemical p r o p e r t i e s o f t h e
supported on z e o l i t e and t h e
c a t a l y t i c a c t i v i t y for toluene
d i s p r o p o r t i o n a t i o n were 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 p r e p a r a t i o n condit i o n s . The c a t a l y s t which was prepared by m o d i f y i n g NH4Y w i t h 0.25M solution
Fe(N03)3
a t 323K showed t h e h i g h e s t a c t i v i t y among t h e samples obtained.
REFERENCES 1 S.Hidaka,A.Ii no, K.Ni ta,Y.Maeda,K.Morinaga and N.Yamazoe, i n Y.Murakarni, A . I i j i m a and J.W.Ward(Eds.), P r o c . 7 t h . I n t . Z e o l i t e Conf.,Tokyo, 1986, Kodansha, Tokyo, 1986, p.329. 2 S.Hi da ka, H. Sh ima kawa, A. I ino, T. M ibuchi, S.Nakay and K.Ni ta, K e t j e n Cata 1y s t Symposium 1986, Scheveningen, Akzo Chernie, 1986, p.H-9. 3 S. H id a ka, A. Iino, T. M ibuc h i , K. N it a, Y. Maeda and N.Y amazoe, Chem. Lett.( 1986) 1213. 4S.Hi da ka,A. Iino, T. M i b u c h i , K.Ni t a , Y. Maeda and N.Yamazoe, N i ppon Kagaku K a i s h i , 9 (1987) 1659. 5 S.Hidaka,A.Ii no, M.Gotoh, N . I s h i kawa,T.Mibuchi and K.Nita, Appl. C a t a l . 43 (1988) 57. 6 D.Nicho1 I s , i n J.C.Bai l a r and H.J.Emeleus(Eds.), Comprehensive I n o r g a n i c Chemistry, Pergamon, Oxford, 1973, p.1043. 7 L.Guczi, Catal.Rev.Sci.Eng., 23(3) (1981) 329. N.Yamazoe, B u l 1. Chem. SOC. Jpn. 61 8 S.Hidaka,A.Iino,K.Nita,K.Morinaga,and (1988) 3169
This Page Intentionally Left Blank
165
Modification of HZSM-5 by Diazomethane
Gen-min Lu, Song-Ying Chen and Shao-yi Peng Institute of Coal Chemistry, Academia Sinica, Taiwan, Shanxi, 03OOO1, People's Republic of China
ABSTRACT
The zeolite HZSM-5 was modified by the methylation of the surface protonic hydroxvl groups with diazonethane. After d i fication, i.r. peaks of surface methoxyl groups appeared at around 2970-2860 cm-', which corresponded to two decomposition peaks (295' and 535°C) in Ar and vere assigned to two states of surface methoxyl groups. The amount of irreversibly adsorbed pyridine and surface area dropped to one third of the original values after modification, while the change in catalytic activity for cumene cracking was not parallel to that of the acidity due to the decomposition of surface methoxyl groups under the reaction conditions. The surface Si :A1 ratio of nodif ied catalyst decreased to about one half its original value, which may be due t o the Migration of amorphous aluminum oxide in the pores t o the surface of the zeolite during the modification.
INTRODUCTION In recent Years, Plodification of zeolites, such as HZSM-5, by phosphoric compounds or metal oxides has been extensively studied, but little information is available on the modification of zeolites by diazomethane, which is an excellent methylating agent for protonic acidic sites. It is capable of entering into the small pores of zeolites because of its small molecular size and linear molecular structure. Yin and Pen8 (1,2) reported that the acidity and specific surface area of the inorganic oxide supports (A1,0,,SiOJ and zeolite catalysts changed significantly by diazomethane modification. In the present paper. the results of modification of HZSM-5 by diazomethane and the influence of modification on its properties of surface acidity, porosity and catalytic activity are reported.
EPER IMENTAL HZSM-5 vas prepared through ammonium exchange of the sodiur form (AF-5, Si :A1
=W).Four successive exchanges were carried out for lh with a 1M NH,CI solution at 96°C. After washing and drying, the NH,ZSM-5 was calcinated for 4h at 540°C in air.
166 G:m.
Lu, S.-y. Chen and S.-y. Peng
Diazomethane was synthesized from CH,NH,HCl in ethyl ether and the solution was used to react with HZSH-5. The modification reaction was carried out at 0-C until no further color change of the diazomethane solution was observed. The modified catalyst was dried first at aabient temperature in air and then at 120°C in Ar for 2h. The surface methoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabi 1 ities were studied by TemperatureProgrammed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed a m n i a was desorbed in Ar from R.T. to 550°C by 'PD at 1BoC/min. The same procedure was used for the TPDE of surface species on modified catalyst. The pretreated catalysts were contacted with pyridine gas at anbient temperature for one week, then the physically adsorbed pyridine was evacuated. The samples with and without pyridine adsorption were used for FSCA. The bonding energy of O,,, N,,, Si,,, Al,, and C,, and their relative amounts were measured. The specific surface area was measured by nitrogen adsorption at -195OC. The cumene cracking reaction was conducted by pulse technique under the following conditions: 0.10 catalyst, H, flow rate 75 mlhin, Pulse volume 1 ul.
RESULTS AND DISCUSSION
..
PorlaationddecolaDosltlonnfsurfacemethoxvlnrouDs Figure 1 shows the i.r. spectra of surface species on diazoaethane-modified HZSH-5. Three peaks appeared at around 2970-2860 CI-', indicating that surface methoxyl groups were formed during the modification reaction:
where Z-OH represented the protonic acidic site on the zeolite surface. Morrow (3) observed similar i.r. spectra for methanol adsorbed on SiO,, and the peaks were assigned to two states of the surface methoxvl groups. The 'PDE result in Figure 2 indicates two decomposition peaks at 295- and 535OC for the surface species, which were consistent with the above i.r. results. We could therefore assign the i.r. peaks at around 2970-2860 cm-' of modified HZSM-5 to two types of surface methoxyl groups which exibited different thermal stability. The TPDE product before 295'C was methane while the Products consisted of C,, C, and other compounds at higher temperatures, implying that at lower temperatures,
Modification of HZSM-5 by Diazomethane
167
the surface was demthylated via 2 (Z-O-CH,)
--------+
Z(Z-OH)
+
C
+
(2)
CH,
and the complex surface reactions took place at higher temperatures.
3200
3000
2800
cm-1
Figure 1. IR spectra of methoxyl groups on modified HZSW-5.
0
200
Figure 2. Temperature-Programmed Decomposition of surface methoxyl groups in Ar. Influenced' M the P r o wt h n f H Z S M - 5 As shown in Table 1, the aaount of irreversibly adsorbed pyridine dropped to one third its original value after aodification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of BPY peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. Ihe results in Table 1 aslo show that the drop in acidity paralleled that
168 G.-m. Lu, S.-y. Chen and S.-y. Peng
of the specific surface area. The XRD results indicate that the modification had little influence on the structure of HZSM-5. The drop in specific surface area after modification may be considered as the result of the formation of methoxyl groups in the pores of zeolite.
Original Modified Demethylated
0.530 0.170 0.718
444.7
100
117.7 420.5
70 >loo
a rmaol/g, 120°C; b 3OOOC; c 450"C, Ar, 2h.
Figure 3. TPD of irreversibly adsorbed ammonia on untreated (-1 and modified (-.-I HZSM-5 in Ar, The influence of the modification on the surface atomic ratios of Si :A1 as measured by ESCA is shown in Table 2. The enrichment of alumina was observed for modified HZSW-5. The surface Si:Al ratio was decreased t o about one half its original value. It seeled impossible for diazomethane modification to remove the framework alumina of the zeolite. We assumed that the enrichment was caused by the migration of the amorphous aluminum oxide to the surface of the zeolite. The mechanism should be studied further in detail.
Modification of HZSM-5 by Diazornethane 169
It is worth noting that the change in catalytic activity for cumene cracking after modification did not parallel the change of acidity in Table 1. This may be due to the difference in temperatures for adsorption and reaction or the partial decomposition of the surface methoxyl groups under reaction conditions (Figure 2 and equation (2)). It was also observed (Table 1) that the acidity and the catalytic reactivity for cuaene cracking increased when the aodif ied zeolite was treated at higher temperatures (450"C), indicating that the acidic property of the surface hydroxyl groups was enhanced by denethylation (2,4). Based on the above results, we conclude that the diazomethane modification of zeolites is an effective aethod to change selectively the anount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature (<3OO0C) catalytic reactions.
ACKNOWLEDGEWENTS We wish to thank WS. Ju-aing Hong of Beijing University for assistance of ESCA. This research was supported by the Chinese National Nature Scientific Foundation and partially by the Zhong Guancun Foundation of Instrumental Measurements. REFERENCES 1. Wen-juan Yin and Shao-yi Peng, in Y. Murakaai, A. Iijima and J. W. Ward ( E d s . ) , New Developaents in Zeolite Science and Technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22, 1986), Kodansha/Elsevier,Tokuo/ksterdam, 1986. 2. Wen-Juan Yin and Shao-yi Peng, J. Fuel Chem. Techn. (Chinese), 1509871, 205. 3. B. A. Horrow, J. Chem. SOC. Faraday Trans I, 70(1974), 1527. 4. E. Borello, A. Zecchina and C. Morterra, J. Phys. Cher., 71(1967), 2938.
This Page Intentionally Left Blank
171
\
Preparation of Metallosilicates with MFI Structure by Atom-Planting Method
T. Yashima, K. Yamagishi, and S. Namba Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
ABSTRACT The atom-planting method for the preparation of several metal losi 1 icates with MFI structure was studied. By the treatment of silicalite or ZSM-5 type zeolite with metal chloride vapor at elevated temperatures, metal atom could be introduced into the zeolite framework. From the results of alumination of silicalite it is estimated that the metal atoms are inserted into defect sites, such as hydroxyl nests in zeolite framework. The metallosilicate prepared had both Bronsted and Lewis acid sites with specific acid strength corresponding to the kind of metal element. INTRODUCTION Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [ 1 , 2 ] . It is expected that acid sites of different strength from those of alurninosi licate are generated when some trivalent elements other than aluminium are introduced into the framework o f silicalite. The Bronsted acid sites of metallosilicates must be Si(OH)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [ 31. It has been reported that aluminium can be introduced into the framework of silicalite with MFI structure by the treatment with AlCl3 vapor at elevated temperatures [4-81. By such treatment, not only Bronsted acid sites but Lewis acid sites are also generated, because aluminium atoms are introduced not only into the framework sites but alkso into the non-framework sites [6-81. It is expected that this method can be applied to prepare some metallosilicates with MFI structure. Namely, by treating silicalite with metal chloride vapor at
172 T. Yashima, K. Yamagishi and S. Namba
e l e v a t e d temperatures, t h e m e t a l l o s i l i c a t e s which i n c l u d e a metal element i n t h e zeolite
framework,
can
be prepared.
We named
this
method
"atom-planting".
Kraushaar and van H o o f f [ 9 ] have r e p o r t e d t h e s u c c e s s f u l i n t r o d u c t i o n o f T i i n t o dealuminated ZSM-5 z e o l i t e s by t r e a t m e n t w i t h T i c 1 4 vapor a t a h i g h temperature. T h i s method f o r t h e p r e p a r a t i o n o f t i t a n o s i l i c a t e may be c a l l e d a k i n d o f
atom-
p l a n t i n g method. In
t h i s paper, we p r e s e n t t h e "atom-planting"
metallosilicates. into
We
method f o r t h e p r e p a r a t i o n
d i s c u s s evidence o f t h e i n t r o d u c t i o n o f
metal
of
elements
t h e z e o l i t e framework as w e l l as t h e a c i d i t y o f t h e m e t a l l o s i l i c a t e s
pre-
pared b y t h i s new method. EXPERIMENTAL Preparation o f m e t a l l o s i l i c a t e s p a r e n t s i 1 i c a l it e s ( S i /A1 >800) and ZSM-5 t y p e zeol it e
The
synthesized
hydrothermally.
r e a g e n t were AlCl3,
planting vapor
(Si/A1=41), as
[lo],
and AsC13.
temperature,
SbC13
The temperature was t h e n brought t o
923 K ( o r 873 K f o r As),
after
which
as
dehydrated an
metal
in
atom-
choloride
(11 kPa) i n d r y He stream was added i n t h e r e a c t o r f o r 1 o r 2
t h i s treatment,
were
atom-planting
The procedure o f atom-planting i s
The p a r e n t z e o l i t e was placed i n a q u a r t z r e a c t o r and
He stream a t 773 K f o r 4 h.
dry
used
GaC13, InCl3, SbC15 which was p r e s e n t i n t h e form o f
a t e l e v a t e d temperatures follows.
T r i v a l e n t metal c h l o r i d e s
h.
After
t h e r e a c t i o n system was purged w i t h d r y He stream a t a t r e a t m e n t
temparature f o r 1 h t o remove r e s i d u a l m e t a l c h l o r i d e .
The t r e a t e d z e o l i t e
r i n s e d and cation-exchanged t o t h e NH4+ form w i t h 0.1 N NH4N03 s o l u t i o n .
was
washed
w i t h d e i o n i z e d water, t h e n d r i e d a t 383 K f o l l o w e d b y c a l c i n a t i o n a t 773 K. Characterization Bulk amounts o f elements were determined by atomic a b s o r p t i o n spectrophotomeamount o f framework A1 was determined b y * 7 A l MAS NMR.
The
try.
o f t h e m e t a l l o s i l i c a t e s were determined b y I R and
properties
IR
The
NH3-TPD
ments.
Before the
1.5 h.
I n t h e o b s e r v a t i o n o f p y r i d i n e adsorbed on m e t a l l o s i l i c a t e s ,
acidic
measure-
measurements, t h e sample wafer was evacuated a t 773 K
for
the
sample
wafer was exposed t o p y r i d i n e vapor (1.3 kPa) a t 423 K f o r 1 h, t h e n was
evacu-
ated
at
t h e same temperature f o r 1 h.
temperature. trometer
A l l I R s p e c t r a were
recorded
at
NH3-TPD experiments were performed u s i n g a quadrupole mass
as a d e t e c t o r f o r ammonia desorbed.
room spec-
The sample z e o l i t e dehydrated
at
773 K f o r 1 h was brought i n t o c o n t a c t w i t h a 21 kPa o f NH3 gas a t 423 K f o r 0.5 h,
t h e n evacuated a t t h e same temperature f o r 1 h.
The samples were c o o l e d
room temperature, and t h e s p e c t r a o b t a i n e d a t a h e a t i n g r a t e o f 10 K min-1 314 t o 848 K.
to from
Preparation of Metallosilicates by Atom-Planting Method 173
RESULTS AND DISCUSSION Mechanism o f atom-planting w i t h aluminium t r i c h l o r i d e First, zeolite
we should c l a r i f y whether m e t a l elements can be i n t r o d u c e d
into
framework, and where t h e s e elements occupy t h e framework i n
p l a n t i n g method.
the
I t i s observed by s o l i d s t a t e MAS NMR t h a t t h e s i g n a l
the atom-
attrib-
u t e d t o t e t r a g o n a l Z 7 A l i n c r e a s e d g r e a t l y a f t e r t r e a t i n g s i l i c a l i t e w i t h aluminium
t r i c h l o r i d e vapor a t e l e v a t e d temperatures.
From these r e s u l t s i t i s
con-
cluded t h a t aluminium atoms can be i n t r o d u c e d i n t o t h e z e o l i t e framework b y
the
atom-planting method [71. The
amount o f aluminium i n t r o d u c e d i n t o t h e framework reached c e i l i n g
with
i n c r e a s e o f r e a c t i o n t i m e and p a r t i a l pressure o f
and
aluminium
showed a g e n t l y - s l o p i n g peak a t around 940 K r e a c t i o n
level
trichloride.
temperature.
More-
over, t h e y d i d n o t correspond t o t h e amount o f s i l i c o n e removed from t h e s i l i c a lite
d u r i n g t h e r e a c t i o n [8,11].
the
atom-planting,
work,
From t h e s e r e s u l t s , i t i s suggested
aluminium atoms occupy s p e c i a l s i t e s i n t h e
that
zeolite
by
frame-
and do n o t s u b s t i t u t e s i l i c o n atoms i n t h e framework.
The oxygen atoms i n t h e s i l i c a l i t e framework can be c l a s s i f i e d i n t o mately
approxi-
t h r e e k i n d s by t h e l 8 0 i s o t o p e exchange method proposed by Endoh e t
al.
On t h e exchange r e a c t i o n o f 160 i n t h e z e o l i t e framework w i t h Cl8O2
in [12]. t h e gas phase a t 773 K, t h e most r e a c t i v e oxygen atoms were i n t h e t e r m i n a l S i O H groups on t h e e x t e r n a l s u r f a c e o f z e o l i t e c r y s t a l i t e , t h e medium r e a c t i v e gen
atoms were on t h e d e f e c t s i t e s and t h e less r e a c t i v e oxygen atoms
the
complete Si-0-Si
framework.
oxy-
were
The amounts o f these oxygen atoms i n t h e
l i t e framework can be c a l c u l a t e d from t h e r a t e o
in zeo-
t h i s i s o t o p e exchange r e a c t i o n
[ 11.121. We s y n t h e s i z e d n i n e s i l i c a l i t e s which had d i f f e r e n t c o n c e n t r a t i o n s o f
defect
s i t e s i n t h e z e o l i t e framework determined by l 8 0 i s o t o p e exchange method.
These
were t r e a t e d w i t h aluminium t r i c h l o r i d e vapor under t h e same
reac-
silicalites
t i o n c o n d i t i o n s ; 923 K temperature, 1 h time, 11 kPa aluminium t r i c h l o r i d e vapor F i g u r e 1 shows t h e p l o t s o f t h e amount o f aluminium atoms
pressure. into
introduced sites.
A
proposed
a
t h e framework a g a i n s t t h e amount o f oxygen atoms on t h e d e f e c t l i n e w i t h a s l o p e o f 4 was obtained.
straight
From t h i s r e s u l t we
r e a c t i o n scheme as f o l l o w s : I
I
-Si-
-Si
I
I
OH
-Si-OH
' - SPHi I
I
I
HO-SiI
+
AlCl3
-
&H+
,
-Si-0-Al-0-Si1
1
?
-Si I
1
-
+
3HC1
174 T. Yashima, K. Yamagishi and S. Namba
0 0,5 1,o Number of framework A 1 atoms /atom.u,c,-l Relationship between the number of oxygen atoms on defect sites and the number of A1 atoms introduced into zeolite framework. Fig. 1.
Namely, aluminium atoms can be inserted into the hydroxyl nests composed of four SiOH groups, and also into the lattice imperfections formed from the hydroxyl nests by the dehydration. The above results and discussion have been described in detail elsewhere [ll]. Atom-planting with various metal chlorides It is suggested, therefore, that other metal elements may be inserted Table 1. Atom-planting of silicalite into the hydroxyl nests in the zeolite with various metal chloridesa framework by the atom-planting method as well. Table 1 shows the results of atom-planting of silicalite with GaC13 [13]. InCl3 [13], SbC13 [14] at 923 K and with AsC13 at 873 K. Considerable amounts of these elements were introduced into silicalite. However, these values are not framework amounts but bulk amounts of the metal elements. If trivalent elements are introduced into the silicalite framework, the Bronsted acid sites will appear.
Amount of metal introducedb Sample
/mmol g-1
/atom u.c.-l
~
Parent(A1-)
A1 GaInSbAS-
0.02 0.74 0.30 0.24 0.12 0.17
0.11 4.3 1.7 1.4 0.7 1.0
a Reaction conditions: Temperature, 923 K (873 K for As); time, 2 h; partial pressure of metal chloride, 11 kPa b Determined by atomic absorption spectrophotometry.
Preparation of Metallosilicates by Atom-Planting Method 175
Therefore, these metallosilicates may have the Bronsted acid sites, because the Ga, In, Sb, and As introduced into the silicalite framework are trivalent. Figures 2 and 3 show the IR spectra of the atom-planted silicalites before and after adsorptionof pyridine. I n the region of wavenumber 3600 to 3800 cm-l, the parent silicalite showed only one absorption band at 3740 cm-l attributed to non-acidic SiOH group. After atom-planting, all samples showed a new absorption band at different wavenumbers, attributed to acidic SiOH groups which were confirmed by the disappearance with adsorption of pyridine. In the region 1400 to 1600 cm-1, after the adsorption of pyridine, all atom-planted silicalites showed three absorption bands at 1450, 1490 and 1540 cm-I attributed to pyridine interacted with Lewis acid sites, Lewis and Bronsted acid sites and Bronsted acid sites, respectively. Namely, all atom-planted silicalites with Al, Ga, In, Sb and As chlorides had both Bronsted and Lewis acid sites. From these results it is concluded that a part of the metal atoms introduced into the silicalite by the atom-planting method exists in the zeolite framework, resulting in metallosilicates. However, the absorbances of acidic OH groups in these metallosilicates were diferent from each other. These results suggest that the amount of each element introduced into the zeolite framework is diferent, probably because of the diference in thermal stability. In the case of Al-planted silicalite, it has been estimated that Lewis acid sites are generated by the aluminium introduced into the non-framework sites of silicalite [8]. For the other metalplanted silicalites, it is suggested that a apart of metal atoms is introduced into the non-framework sites, resulting in the formation of Lewis acid sites. Acidic properties of metallosilicates prepared by atom-planting method The solid acidity of these atom-planted silicalites was examined by ammonia TPD measurement. The ammonia TPD profiles of all these metallosilicates showed one peak at a higher temperature than 453 K. Figure 4 shows the relationship between peak temperatures of ammonia TPD and wavenumber of IR absorption bands due to the acidic SiOH groups of atom-planted silica1 tes. A straight line was obtained. From these results, the order of strength of acid sites would be as follows: Al-silicate > Ga-silicate > In-silicate
>
Sb-sil cate
>
As-silicate
Atom-planting of ZSM-5 type zeolite with antimony chloride If ZSM-5 type zeolite, which includes a considerable amount of a the zeolite framework, is treated with metal chloride vapor at eleva atures, the m e t a l l o - a l u m i n o - s i l i c a t e will be prepared. When ZSM-5 was treated with antimony trichloride vapor at 923 K for 2 h, two
uminium in ed temper(Si /A1 =41) absorption
176 T. Yashirna, K. Yamagishi and S. Narnba
Fig. 2. I R spectra for atom-planted zeolites (Al, Ga, In) and pyridine adsorbed on them. Solid line: atom-planted zeolites. Dotted line; pyridine adsorbed,
Wavenumber /cm-l Fig. 3. IR spectra for atom-planted zeolites (Sb, As) and pyridine adsorbed them. Solide line: atom-planted zeolites. Dotted line: pyridine adsorbed.
on
Preparation of Metallosilicates by Atom-Planting Method 177
3 7 I-& 3675
p\
Sb
Temperature /K Fig. 4. R e l a t i o n s h i p between peak temperature o f ammonia TPD p r o f i l e s and wavenumber o f I R a d s o r p t i o n band i n OH s t r e t c h i n g r e g i o n f o r atom-planted z e o l i t e s .
Wavenunber /cn-l Fig. 5.
I R s p e c t r a f o r Sb-planted z e o l i t e s i n OH s t r e t c h i n g region.
178 T. Yashirna. K. Yarnagishi and S. Narnba
a t t r i b u t e d t o a c i d i c S i O H groups were observed a t 3665 and 3610
bands
I R s p e c t r a f o r t h e Sb-planted ZSM-5,
the tion
band
as shown i n Fig. 5 [14].
a t 3665 cm-1 agrees w i t h t h a t f o r t h e
Sb-planted
cm-1
The
absorp-
silicalite.
a b s o r p t i o n band a t 3610 cm-l i s a t t r i b u t e d t o t h e a c i d i c SiOH group o f silicate. ZSM-5
As shown i n Fig. 5, t h e absorbance a t 3665 cm-l f o r
in The
alumino-
the
Sb-planted
was h i g h e r t h a n t h a t f o r t h e Sb-planted s i l i c a l i t e , w h i l e t h e
absorbance
a t 3610 cm-1 f o r t h e Sb-planted ZSM-5 was lower t h a n t h a t f o r t h e p a r e n t
ZSM-5.
These r e s u l t s show t h a t t h e amount of Sb i n t r o d u c e d i n t o t h e ZSM-5 framework
is
more t h a n t h a t i n t o s i l i c a l i t e framework and t h e amount o f A1 i n ZSM-5 decreased during
the reaction.
I t i s suggested, t h e r e f o r e , t h a t a p a r t o f t h e
aluminium
atoms i n t h e ZSM-5 framework a r e removed, f o r m i n g d e f e c t s i t e s i n t o which mony
atoms
sites
due
can be i n s e r t e d .
Therefore,
i n t h e Sb-planted ZSM-5,
anti-
strong
t o t h e r e s i d u a l aluminium and weak Bronsted a c i d s i t e s
due
acid
to
the
antimony i n t r o d u c e d a r e formed. I n conclusion, s e v e r a l m e t a l l o s i l i c a t e s w i t h M F I s t r u c t u r e can be prepared by the
atom-planting method, i.e.,
chloride
vapor
at
when s i l i c a l i t e o r ZSM-5 i s t r e a t e d w i t h
e l e v a t e d temperatures, metal atoms a r e
inserted
d e f e c t s i t e s , such as h y d r o x y l nests, i n t h e z e o l i t e framework. icates
metal
into
the
The m e t a l l o s i l -
prepared by t h e atom-planting method have b o t h Bronsted and
Lewis
acid
s i t e s o f s p e c i f i c strength.
REFERENCES 1
2 3 4 5
6 7 8 9 10 11 12 13 14
R.M. B a r r e r , Hydrothermal Chemistry o f Z e o l i t e s , Academic Press, London, 1982, p.251. M. T i e l e n , M. Geelen, P.A. Jacobs, Acta. Phys. Chem., 31, (1985) 1. B. N o t a r i , 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 . Sci. Catal., 37). E l s e v i e r , Amsterdam, 1988, p.413. C.D. Chang, C.T.-W. Chu, J.N. Miale, R.F. B r i d g e r , R.B. C a l v e r t , J. Amer. Chem. SOC., 106, (1984) 8143. R.M. Dessau, G.T. Kerr. Z e o l i t e s , 4, (1984) 315. M.W. Anderson, J. K l i n o w s k i , L. Xinsheng, J. Chem. Soc., Chem. Commun.. (1984) 1596. K. Yamagishi, S. Namba, S. Nakata, S. Asaoka, Innovation i n T. Yashima, Z e o l i t e M a t e r i a l Science(Stud. S u r f . Sci. Catal., 37). E l s e v i e r , Amsterdam, 1988, p.175. K. Yamagishi, S. Namba, T. Yashima, J. Catal., 121. (1990) 47. B. Kraushaar, J.H.C. van Hooff, Catal. L e t t . , 1, (1988) 81. T. M o e l l e r , i n I n o r g a n i c Chemistry, John W i l e y & Sons, New York, 1952, p. 627. K. Yamagishi, S. Namba, T. Yashima, J. Phys. Chem., submitted. A. Endoh, K. Nishiyama, K. Tsutsumi, T. T a k a i s h i , Z e o l i t e s as C a t a l y s t s , Sorbents and Detergent B u i l d e r s ( S t u d . S u r f . Sci. Catal., 46), Elsevier, Amsterdam, 1989, p. 779. K. Yamagishi, S. Namba, T. Yashima, submitted. K. Yamagishi, S. Namba. T. Yashima, Z e o l i t e s : Facts, Figures, F u t u r e (Stud. Surf. Sci. Catal., 49). E l s e v i e r , Amsterdam, 1989, p.459.
179
Chemical Interactions of Aluminophosphate Molecular Sieve with Vanadium Oxide
S. B. Hong, B. W. Hwang, Y. Yeom, S. J. Kim, and Y. S. UH* Division of Chemistry, Korea Institute of Science and Technology P.O. Box 131, Cheongryang, Seoul, Korea
ABSTRACT V,O, supported AIPO,-5 molecular sieve with different contents of V,O, have been characterized by X-RD, TPD, EPR, IR and diffused reflectance spectroscopy. There was strong interaction between the adsorbed vanadium species and the AIPO,-5 surface during the calcination step. The V=O stretching vibration band in supported V,O, appeared at 923 cm", indicatingthat surface vanadium species is located at the basic sites on the AIP0,-5 surface. According to the diffused reflectance spectra of the catalysts with low V,O, content, the vanadium species supported on AIPO,-5 surface was mainly in a distortedtetrahedral symmetry. However, upon increasing the V,O, contents, crystalline V,O, was also detected. Most of the properties including catalytic properties of the the vanadium-supportedAIP04-5were similar as to those of VAP0,-5. INTRODUCTION The aluminophosphate molecular sieve, AIPO,-5, itself has limited potentialas Catalyst, since its structure is neutral and has neither cation exchange capacities nor acidity [l-31. There are two possibilities for utilizing
the molecular sieves; one is modification of the framework by substiiution of metal atoms such as silicon [3-61and/or transition metals (5-111, and the other is introducingactive site by impregnation. The aluminophosphate molecular sieves have an interesting property for potential use as Catalyst supports, due to their excellent thermal stabilities and unique structures. AIPO,-5 is known to retain its structure after calcination at 1000°C and have uni-directionalchannels with pore size of 8 A bounded by 12-memberedrings [2]. To utilize molecular sieves as catalyst support, chemical interactions between the molecular sieve and active component, chemical stabilities, and surface structures must be determined. However, little attempt has been made to clarify the surface structures or properties of catalytically active components supported on the aluminophosphate molecular sieves. Liu et al. [12] reported that the crystallinity of AIPO,-5 decreased with impregnated MOO, content, in which MOO, was introduced either by impregnating (NH,),MoO,
or physically mixed with MOO,, followed by
calcination at 500°C. Winiecki et al. [13] reported that AIP0,-n molecular sieves were moderately stable in acid environments and initiated decomposition at pH value of 1.9 in HCI treatment. Vanadium pentoxide (V,O,)-based
catalysts, for example, are extensively used in industry for a number
of catalytic processes includingthe selective oxidation of aromatic hydrocatbons and transformation of SO, into SO, (14,151. The vanadium pentoxide catalysts are usually prepared in supported form on a proper
180
S.B. Hong, B. W. Hwang, Y. Yeom, S.J. Kim and Y. S.Uh
support. The interaction of V,O,
with various supports such as AI,O,
CeO,, SiO,, or TiO, has been
investigated [16-191. In this study, the structure of the vanadium species supported on AIPO,-5
molecular sieve has been
studied by X-ray diffraction, infrared, diffused reflectance,and EPR spectroscopy, temperature programmed desorption, and their properties comparedwith those of VAPO,-5. EXPERIMENTAL AIPO,-5 molecular sieve (BET surface area, 299 m'g-') used in this study was prepared by the method of Wilson et al. [l]. V,O~AIPO,-5
samples were prepared by impregnating with aqueous solutions of
ammonium vanadate after removing templating agent in the AIPO,-5 by calcining at 53OoC. The samples were dried at 110°C for 24 h then calcined in air at 550°C for 2 h. VAP0,d was preparedfrom gel mixture of phosphoric acid, pseudoboehmite,vanadium pentoxide, tri-propylamineand water (1.3 Pr,N :x V,05 :Al2O3
-
: P205: 40 H,O) by crystallization at 165°C for 3 7 days [A.
X-ray diffraction patterns of all the samples were taken on a Rigaku D/Max-IIA diffractometer using Nifiltered CuKa radiation. The contents of V,O,
supported on AIPO,-5 were determined by an atomic
absorption spectroscopy. EPR spectra were obtained on a Bruker ER-100. The g-values were calibrated with DPPH. Infrared spectra were recorded with an Analect6160 Fourier Transformed Spectrophotometer applying the KBr technique. UV-VIS diffused reflectance spectra were taken by a Shimazu 240 spectrophotometer equipped with a Type II diffused reflectance attachment. The samples (1.5 g) were prepared in one section of a cell and tapped into a fuzed-quartz cuvette. Magnesium oxide was used as the reflectance standard. Temperature programmed desorption experiment was made to analyze the acid and base properties of the molecular sieve with ammonia and carbon dioxide, respectively, on a heating rate of 1O"C/min. The sample was sufficiently flushed with He gas at room temperature to remove the physically
adsorbed ammonia or carbon dioxide. Dehydrogenationreaction of ethylbenzene was chosen as a test reaction for V20,/AIP04-5. The reaction was carried out on a flow reactor equipped syringe pump, and gas feeding system. The reactant was diluted with nitrogen. The products were analyzed by on-lined gaschromatograph(HP 5890) with 10% Carbowax
20M,31-13x 1.8" SS column. RESULTS AND DISCUSSION X-rav diff-r X-ray diffractionpattern of AIPO,-5 prepared in this study was the same as that prepared by Wilson et al. [2] X-ray diffraction patterns of VAPO,-5 and the V,O$AIPO,-5
samples showed the same XRD pattern as
AIPO,-5, although their relative intensities were slightly decreased for the impregnated sample due to dilution effects. The X-ray peaks due to V,O,
phase could be detected for the high vanadium oxide loaded
sample (Fig.lc).
The TPD pattern of NH, for AIPO,-5, VAP04d and V,O~AlPO,d showed that the peak positions and
Interaction of ALPO,-5 with Vanadium Oxide 181
their heights were nearly same, as shown in Fig. 2. These results indicate that there were no strong acid sites in the molecular sieves. The TPD pattern of CO, showed big differences depending on the modification as shown in Fig. 3. AIPO,-5 gave a major peak at 464K and a detectable
peak at 380K. These results show that AIPO,-5 has moderately strong basic sites. The basic sites may be
3
attributed to the polar nature of the frameworks. (rr V,O$AIPO,Q, in which vanadium is impregnated on
>
AIPO,-5, gave two well-defined peaks at 400K and Z
v)
440K and the peak heights were markedly increased
compared to AIPO,-5.
However, VAPO,-5,
5
2 -
substitutedvanadium in the framework, gave a single peak at 393K. These results suggested that VAP0,-
5 and V,O,/AIPO,-5
were different in vanadium state.
10 lntrared s U y Infraredspectra of the 7.9% V,O$AIPO,-5 samples before and after calcinationat 550°C are
20 30 2 8 value
40
Fig. 1. XRD patterns of V,O$AIPO,d
shown in Fig. 4. The sample before calcination,
a) 0 ?o'
b) 2%
C)
14%
which was impregnated and then dried at 110°C, showed characteristic bands of both V,O,
and AIPO,-5. Two bands at 1026 cm-' and 825 cm.' correspond to
the V=O stretching vibrations and the V-0-V deformations of crystalline V,O, were not observed in the samples with low V,O,
respectively. These bands
content, since they were screened by those of AIPO,-5.
However, they were readily detectable by increasing V,O,
content impregnated on AIPO,-5. Hence, the
state of vanadium species adsorbed on AIPO,-5 during impregnation step seems to be similar to that of crystalline V,O,
which will be evidenced further by diffused reflectance spectroscopy.
In the infrared spectrum of the sample after calcination at 55OoC,a new band was observed at 923 cm" (Fig. 4 (b)).AIPO,-5 and crystalline V,O,
did not show any band around 920 - 930 cm -'. The band at 923
cm-' was detectable for the sample of 0.7 % V,O,
content and its intensity increased with increasing V,O,
content supported on AIPO,-5 (Fig. 5). This indicates strong interaction between the adsorbed vanadium species and the AIPO,-5 surface during the calcination step. It has been reported that surface structures and properties of the V,O,
supported catalysts are
dependent on support material and the contents of loading [18,19]. In particular, the presence of basic sites on support surface was reported to cause a red shift of the V=O stretching vibration band of the supported V,O,
[20]. V,OdSiO,
catalysts with different V,O,
contents showed two V=O stretching vibration
bands at 1032-1035 and 927-954 cm", respectively, due to both acidic and basic sites on the surface. However, V,O,
supported on MgO showed only one band at 922 ern-', since MgO is a typical basic oxide. A
similar red shift was observed in the V,O,TTiO,
system. Hence, it can be concluded from the infrared
182 S.B. Hong, B. W. Hwang, Y. Yeom, S. J. Kim and Y . S.Uh
373 441
3
Q)
u)
C 0
C
u)
u) Q)
0 P
P
Q)
K
a
L
L Q)
Q,
z0
0 Q)
a
-
273 373 473 573 673 Temperature, K
773
Fig. 2. NH3 TPD patternsof modified AIPO,-5.
I
E0 0 0)
U
273
373 473 573 673 773 Temperature, K
Fig. 3. C02 TPD patterns of modified AIPO,-5.
1
I
1500 1000 500 wavenumber, cm-' Fig. 4. Infraredspectra of the 7.9% V,OJAIPO -5 a) before and b) after calcination at 55O0k
Interaction of ALPO,-5 with Vanadium Oxide 183
measurement in this study that AIPO,-5 has basic sites on the surface, where surface vanadium species is produced by the solid-solid reaction between the impregnated VO ,,
and AIPO,-5 during the calcination
step. In other words, the appearance of the band at 923 cm-1 was due to the V=O stretching vibration of surface vanadium species located at the basic sites of AIPO,-5.
1500
1000 500 wavenumber, cm-’
Fig. 5. Infraredspectra of V,O,/AIPO,-5 with different vanadium oxide content a)O%.. b)O.7%, c)1.6% d)2.6%, e)7.9%, 910% Diffused r e f l e m Diffused reflectance spectra in the UV-VIS region were examined to characterize the structure of vanadium species supported on the AIPO,-5 surface. The value of the electroncharge-transferenergy was reported to be strongly influenced by the number of ligands of the central vanadium ions and give informationon the symmetry of the vanadium ions in the clusters [21,22]. Diffused reflectance spectrum of the 7.9 YoV,O,/AIPO,d
sample after calcination at 550°Cwas drastically
different from that of the uncalcinedsample (Fig. 6). The sample before calcination showed a broad chargetransfer band at 400-550 nm. However, the sample after calcination showed a charge-transfer band at 270 nm. In a previous study on surface phases of V,O,
supported catalysts (23,241, V5+ions in a distorted
184 S. B. Hong, B. W. Hwang,
Y.Yeom, S. J. Kim and Y. S. Uh
octahedral symmetry showed the maximum absorption band at 400 nm, whereas the tetrahedrally coordinated
ions showed the maximum absorption band at a wavelength lower than 350 nm. Hence, it
was believed that the impregnated vanadium species on the AIPO,-5 surface has the V5+ ions in an octahedral symmetry similar to V,O,.
250
However, the symmetry of this vanadium species was changed to
45 0 650 wavelength, nm
850
Fig. 6. Diffused reflectance spectra Of 7.9% V,O,/AIPO,d a) before and b) after calcination at 55OOC.
250
450 650 wavelength, nm
Fig. 7. Diffused reflectance spectra Of V,0,/AIPO4-5 a) 0.7,b) 1.6, c) 2.6 d) 7.9,e) 10 Yo
Diffused reflectance spectra of the Vz0,/AIP04-5 samples of different V,O, By increasing V,O,
850
contents are shown in Fig. 7.
content, the absorption band appeared in the range between 400 and 550 nm, which
gave rise to the formation of crystalline V,O,. concluded that the crystalline V,O,
Therefore, as evidenced by infrared measurements, it is
phase is formed on the AIP04-5 surface after the formation of surface
vanadium species containing Vh in a tetrahedral symmetry. The diffused reflectance spectrum of VAP0,d [7,8] was similar to that of calcined Vz0,/AIP04-5.
!32mUiY Takahashi et al. [25] reported that the dispersed tetravalent vanadium (1=7/2) showed a hyperfine structure but broad band could be observed in the agglomeratedvanadium. Miyamoto et al. [8] and Jhung et al. [7]reported that EPR spectra of VAPO,-5 showed hyperfine structure. Miyamoto et al. [8] suggested that the hyperfine structure indicated atomically dispersion of vanadium in VAP0,d molecular sieve, in other words, vanadium was substitutedin the framework of AIP04d. The valence state of vanadium in as synthesized VAPO,-5 was mainly the tetravalent state, in which
Interaction of ALP0.-5 with Vanadium Oxide 185
VAPO,-5 was prepared either from vanadium(l1l) acetylacetonate [8]or vanadium pentoxide [7].If the sample was calcined under oxygen (air), the valence state was changed into pentavalent. The valence state was restored to tetravalent after treatment of the oxidized sample with hydrogen, toluene, xylene or thiophene even at room temperature; no change in XRD patterns was observed. These facts suggest that the valence state was reversibly changed from tetravalent to pentavalent and vice versa without changing crystal structure [7]. Vanadium loaded AIP04-5 showed broad bands. The broad features of EPR bands are attributed to the undispersed v4* species. However, the EPR bands of the sample gradually changed on Successive oxidationreduction treatment followed by vacuum treatment, resulting in a clear hyperfine structure. This indicates that vanadium ions are finely dispersed in AIPO,-5.
Fig. 8. EPR spectra of vanadium modifiedAIP0,- 5 (a) simply calcined at 500% for2 hr. (b) successive treatments. (c) VAPO,,d
The total conversion in the dehydrogenationof ethylbenzene was very low for vanadium free AIP0,-5 and increasedwith vanadium content. As shown in Fig. 9. the conversion increasedwith vanadium content in the low conversion region. The main product of the reaction was styrene. The other products were benzene, toluene and light
186 S. B. Hong, B. W. Hwang, Y. Yeom. S. J. Kim and Y. S. Uh
hydrocarbons, which may be produced due to the acid site of the catalyst. The selectivity toward styrene in the initial stage was approximately 56 %. However, the selectivity was gradually increasedwith reaction time resulting in 89 Yoat 100 min. of reaction time for VAPO,-5 (1.16 WE/,as VO , ), could be observed in the reaction with V,O$AIPO,d
catalyst. The same pattern
catalysts; 55% at initial stage and 97% at 100 min. This
indicatesthat acid sites exist in the catalyst and were poisoned during the reaction.
0.5
1.0
1.0
w t % of Vanadium
2.0
4.0
wt% of Vanadium
Fig. 9. Effect of vanadium content in VAP04-5 and V,O,/AIPO,-5 on reactivity at 45OoC, GHSV = 60,000 ml/(g-cat.hr) (a) VAP04-5and (b) Vz05/AIP0,-5 Typical values of the conversion and selectivities in dehydrogenation of ethylbenzene at 90 min of reaction time are shown in Table I. Table I. DehydrogenationReaction of Ethylbenzene Catalyst
Conv.(%)
Selectivity stvrene benzene toluene
~
V,O,/AIPO,d
(1.O wt 7’0)
29
90
6.3
3.3
V,0$AIP04-5 (4.0 wt Yo)
32
97
1.6
1.3
VAPO,-5 (0.3 wt Yo)
15
88
8.3
3.6
VAPO,-5 (1.2 wt %)
23
89
8.1
3.2
Time on stream: 90 min., GHSV: 1830 ml/(gcat.hr) EBIN2 = 1/35, Temperature: 525% In the lower range of vanadium content, there were little changes of conversion and selectivities between two modified moleclar sieves. However, the conversion and selectivity toward styrene was significatly imprved at higher vanadium content, which is easily obtained by impregnation method.
Interaction of ALPO,-5 with Vanadium Oxide 187
CONCLUSION Infrared spectra of VO ,,
supported catalysts after calcination at 550% showed the V=O stretching
vibration band at 923 cm.' . This indicates that surface vanadium species is located on the basic sites within the AIPO,-5 surface. According to the diffused reflectance spectra of the samples with low V,O,
contents,
the vanadium species supported on AIPO,-5 surface was mainly in a distorted tetrahedral environment. However, upon increasing the V,O,
content, crystallineVO ,,
was formed on the AIPO,-5 surface prior to the
formation of approximately a monolayer of the surface vanadium species. The vanadium species measured by EPR and DRS and catalytic properties of vanadium loaded V,O$AIPO,-5
were similar to those of
vanadium substituted VAPO,-5. ACKNOWLEDGMENT This research was supported financially by the Ministry of Science and Technology, Korea REFERENCES 1. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannanand E.M. Flanigen, US Pat. 4,310,440 (1982) 2. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 104 , 1146 (1 982) 3. E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson in "New Development in Zeolite Science and Technology" Y. Murakami et al. Ed., Elsevier, Amsterdam, p.103 (1986) 4. J.A. Martens, M. Mertens, P.J. Grobet and P. A. Jacobs, "Innovation in Zeolite Materials Science", P. J. Grobel et al. ed, Elsevier, 97 (1988) 5. C. Halik, S. N. Chaudhuri and J. A. Lercher, J. Chem. Soc., Faraday Trans I, 85 ,3879 (1989) 6. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E. M. Flanigen, J. Am. Chem. Soc., 106 ,6092 (1984) 7. S. W. Jhung, S.B.Hong, Y.S. Uh and H. Chon, MOST(Korea) Report No. 2N4431-3385-6, 1988 and Appl. Catal. submitted. 8. A. Miyamoto, Y. Iwamoto, H. Matusuda and T. Inui, "Zeolite: Facts, Figures, Future", Elsevier, Amsterdam. p 1233 (1989) 9. J. M. Bennett, B.K. Marcus, "Innovation in Zeolite Materials Science", P. J. Grobel et al. ed, Elsevier, 269 (1988) 10. A. F. Ojo, J. Dwyer and R. V. Parish, "Zeolite: Facts, Figures, Future", Elsevier, Amsterdam, 1989, p 1233 (1989) 11. S. Ernst, L, Puppe and J. Wwitkamp, "Zeolite: Facts, Figures, Future", Elsevier, Amsterdam, 1989, p447 (1989) 12. S.C. Wang and T.C. Liu, Proc. 5 t h ROWROC Joint Workshop on Catal., 90 (1988) 13. A. M. Winiechi and S. L. Suib, Langmuir, 5 ,333 (1989) 14. D .J. Hucknall, "Selective Oxidation of Hydrocarbons", Academic Press, New York, p.153 (1974) 15. J.C. Volta and J.L. Porlefaix, Appl. Catal., 18 , 1 (1985) 16. M.S. Wainwright and N.R. Forster, Catal. Rev. Sci. Eng., 1 9 , 211 (1979) 17. N.K. Nag, K.V.R. Chary, B.M. Reddy, R.R. Rao and S. Rahmanyam,App/. Catal., 9 , 225 (1984) 18. L.R. Le Coustumer, B. Taouk, L. Meur, E. Payen, M. Guelton, and J. Grimblot, J. Phys. Chem., 92 , 1230 (1 988) 19. F. Roozeboom, T. Fransen, P. Mars, and P.J. Gellings, 2.Anorg. A/@. Chem. 449, 25 (1979) 20. M. Iwamoto, H. Furukama, K. Matsukami, T. Takenaka, and S. Kagawa, J. Am. Chem. SOC. 105 ,3719 (1983) 21. M. Inomata, K. Mori, A. Miyamoto, T. Ui, and Y. Murakami,J. Phys. Chem., 87 ,754 (1983) 22. G. Rasch, H. Bogel and C. Rein, Z. Phys. Chem. (Leipzig) 259 ,955 (1978) 23. W. Hanke, R. Bienert and H.-G. Jerschkewitz,Z. Anorg. Allg. Chem., 414, 109 (9175) 24. G. Lischke, W. Hanke, H.-G. Jerschkewitz and G. Ohlmann, J. Catal., 91 ,54 (1985) 25. H. Takahasi, M. Shiotani, H. Kobayashi and J. Sohma,J. Catal., 14 , 134 (1969)
This Page Intentionally Left Blank
189
Optical Properties of Dyes Incorporated into Clay
T. Endo and M. Shimada Department o f M o l e c u l a r Chemistry and Engineering, F a c u l t y o f Engineering Tohoku U n i v e r s i t y , Aoba, Sendai, M i y a g i 980, JAPAN
ABSTRACT O p t i c a l p r o p e r t i e s o f xanthene and coumarine dyes i n c o r p o r a t e d i n t o a s w e l l i n g c l a y were studied. P r e p a r a t i o n o f t h e dye-clay composites were c a r r i e d o u t as f o l l o w s : a s w e l l i n g c l a y was added and t h o r o u g h l y mixed i n t o v a r i o u s s o l v e n t s c o n t a i n i n g t h e corresponding q u a n t i t y o f dye. A f t e r about one week, t h e r e s u l t i n g composites were q u i c k l y recovered by f i l t r a t i o n and washed s e v e r a l t i m e s w i t h s o l v e n t , t h e n d r i e d o v e r n i g h t a t 100°C i n a i r . The TG-DTA and t h e Xr a y d i f f r a c t i o n d a t a showed t h a t t h e i n t e r c a l a t e d dye molecules were i m m o b i l i z e d and t h e r m a l l y s t a b i l i z e d over t h e i r m e l t i n g temperatures. Also, t h e basal spacings o f t h e c l a y were expanded up t o t h e m o l e c u l a r s i z e o f dyes w i t h s l i g h t d i s t o r t i o n o f t h e l a y e r s t r u c t u r e on i n c r e a s i n g t h e dye content. A l l thft dominant fluorescences were observed, w h i c h were i d e n t i f i e d as t h e 71- 71 I t s quantum t r a n s i t i o n r e l a t e d t o t h e mesomeric s t r u c t u r e o f dye chromophore. e f f i c i e n c y i s b r i e f l y discussed i n c o n n e c t i o n w i t h c o n c e n t r a t i o n quenching phenomena. INTRODUCTION S t u d i e s concerning r e a c t i o n s i n microporous c r y s t a l s , e s p e c i a l l y i n t e r c a l a t i o n r e a c t i o n s , a r e a t t r a c t i v e f o r o b t a i n i n g i n f o r m a t i o n on t h e i n t e r f a c i a l b e h a v i o r between i n o r g a n i c and o r g a n i c phases.
S m e c t i t e and
compounds w i t h r e l a t e d l a y e r s t r u c t u r e s f r e q u e n t l y show s w e l l i n g a f t e r a b s o r b i n g o r g a n i c species o f many d i f f e r e n t k i n d s i n t h e f o r m o f n e u t r a l molecules and/or ionized entities.
Such b e h a v i o r was compared t o an a b s o r p t i o n i n w h i c h t h e
"host" m a t e r i a l t r e a t s t h e "guest" m a t e r i a l h o s p i t a b l y .
Previous works [ 1 ]
focused on t h e c o o r d i n a t i o n c h e m i s t r y o f molecules o r i o n s governed b y t h e charge d e n s i t y and i t s d i s t r i b u t i o n i n t h e i n t e r l a y e r space.
Frequently, u s e f u l
m a t e r i a l s were developed as c a t a l y s t s , f o r instance, p e t r o l e u m c r a c k i n g c a t a l y s t s w i t h a h i g h y i e l d of g a s o l i n e owing t o t h e g e o m e t r i c a l s e l e c t i v i t y and h i g h a c i d i t y o f t h e i n t e r l a y e r s u r f a c e [2].
Also, a p p l i c a t i o n s as i o n
exchangers, an e l e c t r o d e and e l e c t r o l y t e f o r b a t t e r i e s have been w i d e l y developed.
Most o f s t u d i e s on t h e i n t e r c a l a t i o n c h e m i s t r y seemed t o be
190 T. Endo and M. Shimada
m o t i v a t e d b y t h e p e c u l i a r i t y o f r e a c t i o n s p r o v i d e d b y micro-environments w i t h t h e two-dimensional space and surfaces. Here we r e v i e w o u r p r e v i o u s works on t h e s y n t h e s i s and o p t i c a l p r o p e r t i e s o f c l a y s i n t e r c a l a t e d w i t h dyes [3-51. rhodamine,
These dyes,
well-known examples b e i n g
p y r o n i n e and coumarine, a r e a v a i l a b l e f o r a l a s e r m a t e r i a l b e i n g
t u n a b l e o v e r a wide range o f frequencies.
When these dyes a r e i n t e r c a l a t e d i n t o
t h e c l a y , which i s t r a n s p a r e n t i n a U V - v i s i b l e region, i t i s l i k e l y t o f i n d novel f u n c t i o n i n o p t i c a l use o w i n g t o t h e r e s t r i c t i o n o f t h e m o t i o n o f dye molecules.
Furthermore, i t can be assumed t h a t a c h a r g e - t r a n s f e r i n t e r a c t i o n
between dyes i s p r e f e r a b l y enhanced, r e f l e c t i n g t h e h i g h e r f l u o r e s c e n c e e f f i c i e n c i e s o f dyes. EXPERIMENTAL Materials I n o r d e r t o p r e v e n t t h e e f f e c t o f Fe3+ i m p u r i t i e s on f l u o r e s c e n c e [6], an a r t i f i c i a l c l a y , s a p o n i t e ( r e f e r r e d t o as SA), material.
was used as t h e s t a r t i n g
Saponite w i t h h i g h transparency i n t h e v i s i b l e r e g i o n was o b t a i n e d
f r o m K U N I M I N E I n d u s t r y Co.
I t s chemical c o m p o s i t i o n determined b y a t o m i c
a b s o r p t i o n spectroscopy was represented as
( ~ a ~ . ~ ~ ~ s ~ . ~ ~ ~ 0 ' 7 7 + [ ( ~ ~ ~ . ~A l s~o , ~ t hi e~ . ~ ~ ) ( ~
c a t i o n exchange c a p a c i t y (CEC) was about 80.2 meq./lOOg-clay.
After dispersing
SA powder i n t o a 10 v o l % ethanol-90 v o l % w a t e r s o l u t i o n , t h e l a r g e s t f r a c t i o n o f s t a r t i n g c l a y was f r e e z e - d r i e d o r p i p e t t e d o n t o an aluminum p l a t e and d r i e d o v e r n i g h t i n a i r t o f o r m a f i l m y sample.
Xanthene dyes (rhodamine 590 and
p y r o n i n e Y) and coumarine 1 o f dye-laser grade were purchased f r o m t h e E x c i t o n d
Chemical Co. and used w i t h o u t f u r t h e r p u r i f i c a t i o n .
Ethanol, benzene, d i m e t h y l
formamide etc. o f s p e c i a l grade were used as solvents. Synthesis
of dye-clay
composites
The dye-clay composites were prepared b y d i s p e r s i n g t h e c l a y s i n each s o l v e n t c o n t a i n i n g t h e dye a t a q u a n t i t y o f 10-200% o f t h e CEC.
This experimental
procedure l e d t o a l m o s t complete i n t e r c a l a t i o n a t room t e m p e r a t u r e f o r 2-7 days. The composite was recovered by f i l t r a t i o n and washing s e v e r a l t i m e s w i t h each s o l v e n t f o r e l i m i n a t i n g an excess o f dye, and t h e n d r i e d i n a i r .
Assuming t h a t
t h e l o s s o f dye adsorbed on t h e s u r f a c e was f a i r l y s m a l l upon washing, t h e n e t w e i g h t of dye i n t e r c a l a t e d was e s t i m a t e d f r o m t h e r e s i d u a l dye c o n c e n t r a t i o n i n a s o l v e n t measured by a c o l o r i m e t r i c analysis. P h y s i c a l Measurements A l l experiments on t h e c h a r a c t e r i z a t i o n o f dye-clay composites were performed a f t e r d r y i n g o v e r n i g h t a t 100°C i n a i r , w h i c h should l e a d t o t h e e x p u l s i o n o f
Optical Properties of Dyes Incorporated into Clay 191
s o l v e n t s f r o m t h e i n t e r l a m e l l a r space o f t h e clays.
Before and a f t e r t h e
i n t e r c a l a t i o n , t h e p r o d u c t s were examined b y X-ray d i f f r a c t i o n , scanning e l e c t r o n microscope (SEM),
i n f r a r e d s p e c t r a and TG-DTA measurements.
The
e m i s s i o n and e x c i t a t i o n s p e c t r a o f t h e composites were measured i n t h e range o f 250 t o 800 nm a t room t e m p e r a t u r e u s i n g a s p e c t r o f l u o r o p h o t o m e t e r .
Also,
d i f f u s e r e f l e c t a n c e UV-Vis s p e c t r a were o b t a i n e d w i t h a spectrophotometer w i t h an i n t e g r a t i n g sphere attachment.
RESULTS AND
DISCUSSION
Xanthene dye-clay composites On a d d i t i o n o f Na+-saponite t o t h e rhodamine-ethanol s o l u t i o n and t h e pyronine-ethanol s o l u t i o n , t h e c o l o r o f t h e s o l u t i o n s g r a d u a l l y faded w i t h i n a
A l l t h e composites were i n t e n s e l y colored,
few hours, even a t room temperature.
namely b r i g h t r e d f o r Rhodamine 590 and c a r d i n a l f o r Pyronine Y. From t h e X-ray d i f f r a c t i o n d a t a and t h e c a l c u l a t e d s i z e s o f t h e dye molecules, t h e c o n f o r m a t i o n o f t h e dye molecules i n t h e i n t e r l a y e r was b r i e f l y estimated.
Fig. 1 shows t h e c o n f o r m a t i o n o f t h e dyes, i n which t h e xanthene
nucleus o f p y r o n i n e o r rhodamine was p o s i t i o n e d p a r a l l e l , and t h e phenyl group o f rhodamine perpendicular,
t o t h e s i l i c a t e l a y e r s o f t h e clay.
......... ... 9. 6A '16.OA
C la
SA
.... Rhodamine 590-Saponite
P y r o n i n e Y -Saponite
Fig. I. Schematic illustration on the conformations of rhodamine 590 and pyronine Y in the interlamellars of clay. T h e values indicated in the figure were estimated from X-ray diffraction patterns.
I t was e x p e r i m e n t a l l y shown i n r e f . [ 3 ]
t h a t t h e c o n f o r m a t i o n c o u l d be c o n f i r m -
ed by t h e p l e o c h r o m i c p r o f i l e i n t h e I R s p e c t r a o f t h e phenyl group.
Also, t h e
TG-DTA s t u d i e s showed t h a t t h e dyes were c o n f i n e d t o keeping such c o n f o r m a t i o n s between t h e i n t e r l a y e r s over t h e m e l t i n g t e m p e r a t u r e o f t h e dye (e.g.
300OC).
From t h e v a l u e o f t h e CEC, t h e i n t e r l a m e l l a r s u r f a c e area p e r c a t i o n e q u i v a l e n t c o u l d be c a l c u l a t e d t o be ca 91.7 A2.
W i t h t h e assumption t h a t t h e
c r o s s s e c t i o n o f t h e dye m o l e c u l e was a square, t h e approximate areas were e s t i m a t e d t o be ca 90 A* f o r p y r o n i n e and ca 180 A2 f o r rhodamine.
In this
192 T. Endo and
context,
M.Shimada
i t c o u l d be expected t h a t t h e rhodamine c a t i o n s were f o r c e d t o be
a l t e r n a t e l y and more t i g h t l y p o s i t i o n e d i n o r d e r t o m a i n t a i n t h e e l e c t r i c a l balance. As a f u n c t i o n o f t h e dye c o n t e n t i n t e r c a l a t e d , t h e r e l a t i v e i n t e n s i t y o f fluorescence, t h e maximum wavelength i n t h e f l u o r e s c e n c e spectrum o f
X
and
d-spacing a r e i l l u s t r a t e d i n Fig. 2(a) f o r t h e p y r o n i n e Y-saponite composite ( r e f e r r e d t o as PY-SA).
and Fig. 2(b) f o r t h e rhodamine 590-saponite composite
( r e f e r r e d t o as R590-SA).
Content of
rhodamine
J!
(mmolll 009-clay)
Fig. 2. Intensity,.,1
in fluorescence and d-spacing vs the contents of pyronine Y (a) and rhodamine 590 (b) incorporated into saponite.
The f l u o r e s c e n c e o f t h e composite showed a somewhat d i f f e r e n t p r o f i l e f r o m t h a t o f t h e dye s o l u t i o n .
The i n t e n s i t y o f fluorescence, e s p e c i a l l y ,
was
c o n s i d e r a b l y reduced, and t h e v a l u e o f A m a x a l s o s h i f t e d towards h i g h e r wavelengths b y i n c r e a s i n g t h e q u a n t i t y o f i n t e r c a l a t e d dye.
Representative data
o f f l u o r e s c e n c e e x c i t e d a t 387 nm f o r t h e PY-SA composite and a t 354 nm f o r t h e R590-SA composite a r e shown i n t h e f i g u r e s . spacing, r e l a t i v e i n t e n s i t y and ,,,,A, f u n c t i o n o f t h e dye content. p o l a r solvent.
It was noted t h a t t h e changes o f d-
i n v o l v e d a l m o s t t h e same tendencies as a
The r e d s h i f t o f X m a x i s o f t e n observed i n a
S i m i l a r l y , t h e p r e s e n t s h i f t was i n t e r p r e t e d as r e f l e c t i n g an
Optical Properties of Dyes Incorporated into Clay 193
electrostatic interaction between the aromatic rings of dyes and the oxygen planes of clay. In addition, the relative intensities were promptly decreased with smaller amounts of the dye. The concentration profile could be elucidated by conventional quenching phenomena. Coumarine dye-clay composite Fig. 3 shows the typical X-ray diffraction patterns of the saponites with coumarine concentrations of 65, 75, 85, 95 and 105 mmol/IOOg-clay. 18.5A I
5
10
15
20
25
30
35
Diffraction Angle ( d e g . )
Fig. 3. X-ray powder diffraction patterns for saponitecoumarine composites with coumarine at 65, 75, 8 5 , 9 5 and 105 mmo1/100 g-clay.
For the products obtained at both ends of this compositional region, sharp (001) reflections were observed at 13.0 A and 18.5 A. This implies that two kinds of galleries were fully constituted between silicate layers by the penetration of coumarine. However, in the intermediate region, the (001) reflections were often broadened and partially split. In Fig. 4, the relative intensity and the maximum wavelength of the fluorescence, A m a x and d-spacing o f the composites are summarized as a function o f coumarine content. The original d-spacing of 11.9 A in Na+-saponite was shifted to around 13.0 A on increasing the coumarine content. The value of 13.0 A was kept in the region of 20-70 mmol/lOOg-clay. The upper content of coumarine seemed to be independent of the CEC, because when montmorillonite or tetrasilicic mica with different CECs was used as a host material instead of saponite, the upper value was estimated to be 65-70 mmol/lOOg-clay.
194 T. Endo and M. Shimada
50
0
100
200
150
Content of Coumarine 1 (mmol I100g-clay)
Fig. 4. Intensity and maximum wabelength (,Imax) in emission, and d-spacing vs the coumarine content intercalated into saponite.
The d-spacings were s t e p w i s e changed from 13.0 A t o 18.5 A (see Fig. 3) a t higher adsorption levels.
The s c a t t e r i n d-spacings i s f r e q u e n t l y observed due
t o t h e e f f e c t o f adsorbed w a t e r i n t h e i n t e r l a y e r s . c o n c e n t r a t i o n o f 100 mmol/IOOg-clay,
However, above a coumarine
t h e v a l u e o f t h e basal spacing was a l m o s t
c o n s t a n t a t 18.5 A up t o t h e l i m i t o f coumarine adsorbed.
Hence, e v e r y
composite h a v i n g d-spacings o f 13.0 A and 18.5 A were denoted as d l - t y p e and dhtype, r e s p e c t i v e l y .
The p o s s i b l e g e o m e t r i c a l arrangements o f cournarine
molecules i n i n t e r l a y e r s a r e i l l u s t r a t e d i n Fig.
5.
The m o l e c u l a r s i z e and t h e c r o s s - s e c t i o n a l area o f coumarine 1 were c a l c u l a t e d t o be 3.2x10.4x7.5
(A) and 78 A2,
respectively.
From t h e d a t a o f observed d-
spacings and t h e c a l c u l a t e d m o l e c u l a r s i z e , t h r e e p o s s i b i l i t i e s f o r t h e c o n f o r m a t i o n o f coumarine molecules c o u l d be proposed. Fig. 5(a)],
I n t h e d l - t y p e [shown i n
s i n c e t h e t h i c k n e s s o f one a l u m i n o s i l i c a t e l a y e r was about 9.6 A,
t h e f u l l c l e a r a n c e space was e s t i m a t e d t o be about 3.6
A
equal t o t h e t h i c k n e s s o f t h e p l a n a r coumarine molecule.
T h i s v a l u e was a l m o s t Therefore, i t was
considered t h a t coumarine molecules were " f l a t " on t h e s i l i c a t e s u r f a c e s and covered each exchangeable c a t i o n s i t e w i t h o u t any overlap. [shown i n Fig. 5(b,c)],
t h e measured d-spacing was 18.5 A,
I n t h e dh-type so t h a t t h e
i n t e r l a m e l l a r spacing was e v a l u a t e d t o be about 8.9 A, i n which t h e coumarine
Optical Properties of Dyes Incorporated into Clay 195
( a ) dl-type
5A
ct3
/lI
............
(b) Model 1 (dh-type)
5A
( c) Model 2 (dh-type)
Fig. 5. Schematic illustration of the conformation of coumarine molecules intercalated into saponite; the d-type (a) and the dh-type of Model I (b) and Model 2 (c).
molecules were p o s i t i o n e d p a r a l l e l w i t h a double-layer s t r u c t u r e (Model 1) o r p e r p e n d i c u l a r t o t h e s i l i c a t e l a y e r s o f t h e c l a y (Model 2).
The p o s s i b i l i t y o f
t h e former c o n f o r m a t i o n r e l i e s on t h e f a c t t h a t t h e coulombic r e p u l s i v e f o r c e between p r o t o n a t e d coumarine molecules should be important,
As a r e s u l t , no
change i n d-spacing from 18.5 A was observed f o r samples heated t o 20OoC. Therefore,
t h e l a t t e r c o n f o r m a t i o n (Model 2 ) was considered t o be more
approximate f o r e l u c i d a t i n g a l l t h e e x p e r i m e n t a l data, e s p e c i a l l y t h e f o l l o w i n g d a t a w h i c h concern t h e f l u o r e s c e n c e p r o p e r t i e s o f t h e composites. The f l u o r e s c e n c e i n t e n s i t i e s showed t w o maximum values a t coumarine c o n c e n t r a t i o n s o f 20 and 80 mmol/IOOg-clay.
It i s p l a u s i b l e t o a t t r i b u t e t h i s
p r o f i l e t o c o n c e n t r a t i o n quenching as observed i n t h e xanthene dye-clay composites.
On t h e o t h e r hand, t h e second maximum was observed w i t h t h e
g e o m e t r i c a l t r a n s f o r m a t i o n f r o m t h e d l - t y p e t o t h e dh-type.
T h i s suggests t h a t
t h e i n t e n s i t y was a f f e c t e d b y t h e arrangement o f coumarine as r e p r e s e n t e d i n Fig. 5(c) were v e r i f i e d , t h e i n t e r v a l o f coumarine molecules would be e l o n g a t e d w i t h t h e change f r o m t h e d l - t y p e t o t h e dh-type.
A q u i t e reasonable e x p l a n a t i o n
i s t h a t t h e i n f l u e n c e of c o n c e n t r a t i o n quenching d i m i n i s h e d a t once as t h e
196 T. Endo and M. Shimada
change f r o m d l - t y p e t o dh-type occurred.
However, t h e observed e m i s s i o n was
decreased and quenched again b y t h e successive a d s o r p t i o n o f coumarine molecules, because t h e d i s t a n c e between coumarine molecules i n t h e i n t e r l a y e r s became shorter. CONCLUSION P r e v i o u s work [ 7 ] focused on t h e f a c t t h a t most i n t e r c a l a t e d species r e t a i n t h e i r s o l u t i o n - l i k e m o b i l i t y , even when t h e y a r e e l e c t r i c a l l y and g e o m e t r i c a l l y restricted i n the interlamellar.
The p r e s e n t r e s u l t s i n d i c a t e d t h a t xanthene
and coumarine dyes c o u l d be i m m o b i l i z e d and t h e r m a l l y s t a b i l i z e d i n t o t h e i n t e r l a y e r s o f s w e l l i n g clays.
On t h e b a s i s o f t h e f a c t t h a t t h e dye
chromophore was n o t o u t o f p l a n a r i t y , t h e mesomeric s t r u c t u r e r e a l i z e d t h e e l e c t r o n d i s t r i b u t i o n w i t h o u t any i n t e r r u p t i o n s .
TI
-
Moreover, t h e r e a c t i o n
c o n d i t i o n s were examined f o r c o n t r o l l i n g t h e dye c o n c e n t r a t i o n t o govern t h e e f f i c i e n c y o f fluorescence.
These t a i l o r - m a d e dye-clay composites appear t o be
u s e f u l i n fundamental s t u d i e s on t h e i n t e r c a l a t i o n s o f dye w i t h s p e c i f i c k i n d s o f m o l e c u l a r faces which can be v a r i e d w i t h r e s p e c t t o h y d r o p h o b i c i t y , a r o m a t i c i t y and p o l a r i t y .
As was p r e v i o u s l y suggested by A v n i r e t al.
[8,9],
t h e i n t e r a c t i o n s between t h e o r g a n i c dyes and t h e oxygen p l a n e o f t h e c l a y g i v e r i s e t o t h e s p e c t r o s c o p i c phenomenon o f metachromasy.
The f o r m a t i o n o f an
" e x c i p l e x " ( e x c i t e d s t a t e complex) due t o t h e a g g r e g a t i o n o f t h e p r o t o n a t e d o r g a n i c dyes i n t h e i n t e r l a y e r space a l s o appears probable.
Such o p t i c a l
s e n s i t i v i t y o f these composites f o r p a r t i c u l a r purposes a r e t h u s o f i n t e r e s t f o r f u t u r e development. REFERENCES 1. J.M.Thomas, i n M.S. Whittingham and A.J. Jacobson (Eds.), I n t e r c a l a t i o n Chemistry, Academic Press, NY, 1982. p.55. 2. J.P. R u p e r t , W.T. G r a n q u i s t and T.J. P i n n a v a i a , i n A.C.D. Newman (Ed.) Chemisrty o f Clays and C l a y M i n e r a l s , John W i l e y & Sons, NY. 1987, p.275. 3. T. Endo. T. Sat0 and M. Shimada, J. Phys. Chem. S o l i d s , 47(1986) 799. 4. T. Endo, N. Nakada, T. Sat0 and M. Shimada. i b i d . , 49 (1988) 1423. 5. T. Endo. N. Nakada, T. Sat0 and M. Shimada, i b i d . , 50 (1989) 133. 6. T. E k s t r o m , C. C h a t f i e l d , W. Wruss and M. M a l y - S c h r e i b e r . J. M a t e r . Sci., 20 (1985) 1266. 7. T.J. Pinnavaia and P.K. Welty. J. Am. Chem. SOC., 97 (1975) 3819. 8. D. A v n i r , Z. Grauer, S. Y a r i v , D. H u p p e r t and D. R o j a n s k i , New J. Chem., 10 (1986) 153. 9. Z. Grauer, A.B. M a l t e r . S. Y a r i v and D. A v n i r , C o l l o i d s and Surfaces, 25 (1987) 41.
IV. Diffusion
This Page Intentionally Left Blank
199
Molecular Mobility of Single Components and Mixtures on Zeolites
M. Billow
Central Institute of Physical Chemistry, Academy of Sciences of the G.D.R., Rudower Chaussee 5, Berlin - Adlershof, 1156 - German Democratic Republic. ABSTRACT The prerequisites of the evaluation of data characteristic of intracrystalline processes in the case of zeolite sorbents are discussed, along with the conditions under which diffusion can be compared to self-diffusion. Selected results of investigations carried out in the author's laboratory are given in order to demonstrate the consistency of sorption kinetic data with intracrystalline mobility data of single components on molecular sieves (MS). Various types of surface barrier which may influence the uptake rate are also described. The progress of research into multicomponent sorption kinetics is also reviewed, and further developments in this field are proposed, INTRODUCTION Since the time constants of catalytic reactions and the sorption uptake of molecules of various types on crystalline MS, e.g. zeolites, aluminophosphates and others, are within comparable ranges, the diffusion coefficient represents one of the important rate characteristics of both catalytic and sorptive sqparation processes. Therefore, numerous investigations have been dedicated to its determination and the interpretation of the underlying transport mechanisms [l-131. Although any sorption problem with practical relevance is a multicomponent system, only a relatively restricted number of papers deals with multicomponent molecular mobility, e.g. [6,141. In order to choose an appropriate approach to the use of diffusion coefficients as a reliable measure of intracrystalline mobility in MS, one has to .distinguish carefully various types of heat and mass transfer which may occur on different levels: (i) Sorption column (2,151. (ii) Sorbent particle (pellet) 12,13,16,171. (iii) MS crystal Il,4,8,181. In this paper only phenomena occurring on level (iii) will be discussed. In general, if sorption kinetics on porous solid particles are considered, one is dealing with complex physical phenomena which are often superimposed by external, e.g. apparatus effects. In order to obtain the correct diffusion data for the intracrystalline MS void volume, one should start therefore, from
200 M. BUlow
the following: Careful chemical, structural, physico-chemical identification and characterization of the MS crystals (sorption kinetics are very sensitive to the fine structure of the microporous solid, e.g. diffusion anisotropy, boundary effects (twinning), stacking faults, intergrowths between different lattices, presence of mesopores, amorphous material, templating phases and structural surface barriers). Ensuring the identity of the thermodynamic equilibrium properties of the sorption system, considered even in the case of sample change (concentration dependent diffusion coefficients). Excluding the influences of external effects on the measured sorption uptake rate. Carrying out experiments on a particle level nearest to the MS monocrystal (or monolayer of crystals). Consideration of the processes which possibly influence the uptake rate (for further analysis and interpretation, the number of processes superimposed upon each other should not exceed two). Separation of those processes by either appropriate variation of experimental conditions, or adequate theoretical modelling. Ensuring the consistency of data thus obtained via proofs of evidence from different experimental techniques applied to the system under consideration (proof of compatibility of microphysical processes, the possible model prerequisites and of the validity of the relationships between the rate constants). (viii) Comparison of data with results from mathematical experiments. If a MS monocrystal takes up a single component from a fluid phase and intercrystalline transport does not influence the uptake rate, one should be aware of the possibility that, besides intracrystalline diffusion, the following processes may either contribute or even govern the uptake rate: (i) Processes of sorption heat release [2,8,13,19-221. (ii) Surmounting various types of MS crystal interface barriers [7,23-271. (iii) Non-diffusional intracrystalline processes, e.g. molecular rearrangement [18,28,29]. In more film In
the case of multicomponent sorption kinetics the situation becomes even complex because replacement and counter-diffusion processes as well as resistances at interfaces could be involved [6,30,31]. the following we consider:
Relationship between diffusion and self-diffusion coefficients. (i) (ii) New experimental results for systems of actual interest. M i ) Types of surface barriers influencing the sorption uptake. (iv) Developments in the field of multicomponent molecular mobility. MOLECULAR MOBILITY OF SINGLE COMPONENTS This expos6 complements recently published comprehensive reviews [4,5,11] which relate to diffusion in zeolites.In comparison with the state of the art about ten years ago [l], significant progress has been achieved which was stimulated especially by discovering, and then covering the gap between the
Molecular Mobility on Zeolites 201
results of macroscopic (transient, esp. uptake rates) and microscopic (esp. n.m.r.1 methods [3,4,7,8,18,27,24,31-341. This progress was due to the development of both experimental and theoretical approaches to various aspects of molecular mobility phenomena. Experimental methods which contribute significantly to the actual knowledge [3,5] are as follows: piezonetric sorption uptake 1341, frequency response methods 135-381 , zero length column (flow GC) [391 , tracer exchange (self-diffusion) 146,411 , single crystal permeation (Wicke - Kallenbach's steady state type) 142,431, diffusioncontrolled reaction rate (effectiveness factor, steady state type) 1441, Fourier transform IR 131,451, X-ray techniques 1461, gas concentration jump method 1471 (macroscopic techniques) as well as n.m.r methods, e.g. pulsed field gradient [3,41 , tracer iesorptioF3 [3,4,251, relaxation time measurement [48,49] , line-shape analysis B 1501 , C [511, neutron spectroscopy, e.g. quasi-elastic neutron scattering 152,531 , broadening of elastic scattering peak 1543, (self-diffusion methods), e.s.r. pulsed field gradient 1551 (microscopic methods). Diffusion and self-diffusion From the theoretical point of view, it is necessary to show that no microphysical difference exists between the processes of diffusion, i.e. the transfer of molecules according to a gradient of their chemical potential or concentration, and self-diffusion, i.e. the re-distribution of molecules in space due to their random walk at equilibrium. The corresponding coefficients D and D can be defined according to Fick's 1st Law,
+J d =
-D grad c d
(1)
f
= D grad c, i as proportionality factors between the molecular flux densities J and the gradients of concentration c (for self-diffusion, a number of the molecules of the equilibrium system with one unique mobility parameter should be considered as tagged). An analogous consideration i s possible in the case of the 2nd Fick's Law. The general relationship between the coefficients DJ and D is: a dlnp D =D(l+(b/a)cl (2) d dlnc and was derived by means of the thermodynamics of irreversible processes [ 5 6 , 5 7 ] , where -for a sorption system- c and p represent the concentration and pressure in the sorption phase, and the equilibrated gas phase, respectively. The parameters a and b are defined via phenomenological coefficients [ 5 8 ] . Given the condition (b/a) c <<. 1, one obtains the Darken equation [ 5 9 ] , D = D - -dlnp (3) d dlnc which gives the possibility of comparing the quantities Ddand D, i.e. the justification of a united consideration of the results obtained by the methods listed in Table 1, for example. For application of eqn. (3) to MS, the following restrictions must be obeyed:
(i)
Requirement of local equilibrium involving the presumption of a quasihomogeneous sorbate, i.e. a sorbent system with the supposition of - a sufficient number of supercages for intracrystalline transport, - a sufficient number of crystals for intercrystalline transport,
202 11. Billow
each per unit volume. The length of molecular diffusion paths must be much longer than - the characteristic supercage diameter for intracrystalline transport, - the characteristic crystal size for intercrystalline transport. (iii) In the case of intercrystalline (long-range) diffusion - e.g. sorption uptake - and self-diffusion - e.g. n.m.r. - , data can be compared only in the Kundsen region, for avoiding deviations due to viscous flow etc. (iv) Any other disturbing kinetic effects, e.g. mass supply to the MS crystals, heat effects etc, must be excluded either experimentally, or by adequate theoretical treatment. (ii)
Table 1. Comparison of results of diffusion and self-diffusion measurements of benzene in MFI crystals under various experimental conditions. Sample type 3
SiIA1
Experimental
(size)/pm
Silicalite ( 30x25~15 1 (90x55~55) Silicalite (30x25x15 Na,H-ZSM-5 (55x12~10) H-ZSM-5 (8X5X3) Silicalite (105x45~45)
method
Concentration molec.1u.c.
N.m.r. tracer uptake
D x 10”
Ref.
2 -i
cm s
6.8 4.6
293 386
0.5
C - n.m.r.
1.5
303
2
51
H - n.m.r.
1.5
303
2
50
2.0
395
6
31
3.5 2.0
303 343
0.7 2
61
323
1
303
6
62
10
35
-
f3
50
TIK
I,
-
2
Fourier transform IR Sorption uptake ) l a 3 p=const, Cahn balance 34
60
7
Silicalite (190x56~35) Silicalite ( 30x25~15)
Sorption uptake 4.0 V=const, p=var. Sorption uptake 3 )10 p=const, spring balance 3.5
Silicalite (170~55x43)
)
lg3
1.0
388
)
Sorption uptake lQ3 p=const , microbalance 4.0
293
0.1
63
35 35
4.0
298
1
28
)lo3
Frequency response
18
Silicalite (6x3~2) H-ZSM-5 (6x3~2) (13x7~6) Na,H-ZSM-5 (35x15~10)
135
3.5
363
4
62
Na,H-ZSM-5 (35x15~10)
Sorption uptake 135 p=const, spring balance 3.5
363
4
62
Na,H-ZSM-5 (55x12~10)
50
Sorption uptake p=const, Cahn balance Sorption uptake V=constl p=var.
Sorption uptake V=const, p=var.
2.0 2.0
303 363
Molecular Mobility on Zeolites 203
Intracrystalline diffusion The validity of eqn. (3) for determining the intracrystalline self-diffusion coefficients from uptake data has been shown for the sorption of benzene by MFI structures. Table 1 illustrates the comparison of results of both diffusion and self-diffusion measurements carried out under various experimental conditions, described by different authors. It is remarkable that the thirteen different studies which have been conducted independently of each other and for which, in general, different MS samples have been used, led to nearly identical results. Careful investigations of benzene mobility in MFI structures gave evidence of its remarkable concentration dependence which has been interpreted on levels microphysical [181 and phenomenological [64] Other examples of independent and successful proof of both the validity of eqn. (31 and the occurrence of intracrystalline transport in MS have been reported 11-4,8,10,21,23,24,33,35,41,57,65]. Although for many of the results of sorption kinetic investigations in which the conditions (i - viii) have been met with sufficient care, there exists now a commonly accepted opinion about the underlying transport mechanism, the molecular mobility of hydrocarbons in NaX type zeolites, however, is still under controversial discussion [4,21,24,33,34,66-691. Independently of our arguments - to be given elsewhere - for the correctness of the piezometric data [21,24,33,34] being in agreement with those from n.rn.r. studies [3,701, various new results for benzene in NaX type zeolites shown in Figure 1 together with the n.m.r. self-diffusion coefficientsand the diffusion coefficients obtained by the Piezometric technique give evidence of both the correctness of, and in the case of the latter, the applicability of Fick's ditfusion model, even to thelverylrnobile hydrocarbon / faujasite systems. The consistent data are comprised of results frowfive independent methods: nym.r. self-diffusion 2 [70], H-n.m.r. [71], quasi-elastic neutron scattering [72], frequency response [73] and piezornetric sorption uptake [34]. On the other hand, the,self-consistency of data reported in [4,39,66,67]sugges tsl further analkf ysis ofiphenomena, probably interfered with both.thesamples andtheexperimen10-1 tal techniques used. n The intracrystalline mobility data 5 0 forthelbenzene / NaX type zeolite I I systemslare supported by our sorption 10" 0.5 1.0 1.5 2.0 uptake results for the systems n l m m o l g-'
.
I
(I
1
I
Figure 1: Self-diffusion coefficients of benzene in zeolite NaX at 458 K grom techniques: n.m.r. pulsed field gradient (0,U) 1701; n.m.r. H (S) [71] ; quasi-elastic neutron scattering (A) [72] ; sorption uptake (W [34]; single step frequency responsep) [73]; zero length column GC (+) [4]; data extrapolated from lower temperatures are astetisked; hatched area represents gravinetric data [21.
204 M. BUlow
p-diethylbenzene / NaKX (60pm) and NaX as well as benzene / NaBaX (the latter ones with a crystal size of 120pm and a 80% degree of cation exchange). The kinetics were investigated by means of the piezometric technique on crystal monolayers. These results demonstrate the influence of both the molecular structure of the sorbing species (diethylbenzene compared with benzene) and the peculiarities of the sorbate-cation interaction (benzene with Ba2+ instead of Na+ ions) on the diffusion coefficients and their concentration dependence. As can be seen from Figures 2 and 3, in both cases the diffusion coefficients are remarkably decreased compared to those for the benzene / NaX system. Due to its more complex molecular structure, with increased steric hindrance, I
NUX
0 A 0
01
1.0
n / m m i g-'
n/mmot
g-1
Figure 2: Diffusion coefficients of Figure 3: Diffusion coefficients of benzene on NaBaX zeolite. p-diethylbenzene on NaX and NaKX zeolites. p-diethylbenzene shows an intracrystalline mobility in both NaX and NaKX samples which is lower by about 1-2 orders of magnitude compared to the piezoeetric benzene data [341 with similar but more remarkably expressed concentration dependence [741. Note, however, that the benzene data of refs. [4,391 do not exceed the p-diethylbenzene data, even at higher temperatures. On the other hand, the mobility of benzene on NaBaX crystals is reduced by approximately three orders of magnitude with a changed concentration dependence. This finding could be explained by an interplay between the molecular interactions of the benzene molecules and the Ba2+ cations as well as steric hindrances [751. Intracrystalline molecular re-arrangement The increase in time resolution of advanced sorption uptake methods and the joint use of sorption and radio-spectroscopic techniques allow for a more detailed analysis of the so-called %on-Fickian" behaviour of sorbing species in the intracrystalline bulk phase [18,28,29,761. Correspondingly, information on molecular dynamics has been obtained for n-butane and 2-but ne %a in MFI zeolites by means of the single step frequency reeponse method and c n.m.r. line-shape analysis [291. As can be seen from Figures 4 and 5, the ad- / desorption for both sorbates proceeds very quickly, but with a
Molecular Mobility on Zeolites 205
significant difference: whereas in the case of n-butane the kinetic process is -6 described by one single diffusion coefficient (0.7....2.5 x 10 cmz s-iat 323 K) characterizing intracrystalline diffusion [77], for 2-butyne, after a fast 2 -i initial step 0 3 x 10-6 cm s at 323 K) the further sorption proceeds with a time constant being at least one order of magnitude larger, i.e. much more 1.5 2
n - butane lrlllcallte -I
1.51
olexponsion ldesorption k 1.50
-
2 n
lexponslon I desorption lk9
1co
o
a1 0.2 10
30 100
o M 02
10 30 100
o o j a2 to N loo o a1 a2 10 30 loo Figure 4: Sorption kinetics of Figure 5 : Sorption kinetics of 2-butyne on silicalite-I, n-butane on silicalite-I, single step frequency single step frequency response method, cf. 1291. response method, cf. [291. slowly. The analysis of uptake data by means of the computer program ZEUS [78] gave evidence of the absence of significant sorption heat effects on this I3 behaviour. The C-n.m.r. spectra for n-boutyne show the existence of one single sorbate phase. Furthermore, a 90 flip motion with a correlation time T << s could be derived which is probably related t o molecular jumps between the straight and sinusoidal micropore segments and vice versa. It can be proposed that superimposed upon the intrinsic random walk of molecules in the IWI channel network are processes of non-diffusional molecular re-orientation leading to an optimal sorbate arrangement. These processes are slow for the relatively "stiff" 2-butyne molecule (due to its triple bond) but fast for the "flexible" n-butane, i.e. the additional regime of sorption kinetics becomes observable if the time constant of diffusion 2 (R /D) is small compared to the time constant of re-orientation. Since the latter process should be independent of crystal size, size variation will give further evidence for appropriate systems. t Is
Surface barrier effects The concept of transport resistances localized in the outermost regions of crystals was introduced in order to explain the differences between intracrystalline self-diffusion coefficients obtained by n.m.r methods and diffusion coefficients derived from non-equilibrium experiments based on the assumption that intracrystalline transport is rate-limiting. This concept has been discussed during the past decade, cf. the pioneering work [79-811 and the reviews [2,7,8,23,32,82]. Nowadays, one can state that surface barriers do not occur necessarily in sorption uptake by MS crystals, but they may occur if the cross-sections of the sorbing molecular species and the micropore openings become comparable. For indication of their significance, careful analysis of MS
206 M. BUlow
uptake rate in dependence on the MS crystal dimensions is required. Various equilibrium and non-equilibrium effects have been derived to account for transport limitation at the interface of finite thickness: (i)
Equilibrium effects:
- Evaporation barrier [821. Different distance dependences of attraction and repulsion forces at a molecular level 1831. - Energetic heterogeneity of the crystal interface even in the case of an ideal MS crystal, more so for non-ideal ones [84]. - Real crystal structure after thermal and hydrothermal treatment of MS crystals [27,41]. - Real crystal structure after cation exchange 1271. - Deliberate modifications, e.g. surface silanation [7]. (ii) Non-equilibrium effects: - Changes of molecular conformations of the sorbate during the uptake process [23]. - Overloading of the outer crystal shell beyond the equilibrium value 1851 * - Presorption of other molecules, e.g. by-products, with specific interactions 171. - Temperature heterogeneities [21]. (iii) Complex phenomena: - Coking [861. - Micropore blocking by binding agents, amorphous compounds, reaction products, successive phase transition and chemical interaction between the zeolite and the binder [7,25,87,88]. -
I-
/‘ /‘
I
region of introcryslolllne diffusion at 593K
For the system n-decane / NaCaA type zeolite, the synergistic effect of both sample pretreatment and presorption, on the sorption uptake is shown in Figure 6. The region of intracrystalline diffusion was estimated by sorption uptake measurements on crystals with different sizes [411. Examples characterizing surface rate barriers in terms of constants are given in the literature cited.
-
Figure 6: Apparent diffusion coefficients of n-decane on NaCaA zeolites: A ,0 , m(5 3 K) A , 0 ,o (623 K) (zeolite from CK Bitterfeld);o, (573 K), (6 3 K) (zeolite from UCC) ; crystals: A , A ; pel1ets:O ,@ 0,. 0 a pellets pre-loaded with a mixture of n-alkylamines (chain length Clo-Cls), amount corresponding to an external crystal surface monolayer.
x,,n,m,x,A; I: ,H,A,
Molecular Mobility on Zeolites 207
MOLECULAR MOBILITY OF MIXTURES The development of mixture sorption kinetics becomes increasingly important since a number of puri ication and separation processes involves sorption at the condition of thermodynamic non-equilibrium. For their optimization, the kinetics of mu ticomponent sorption are to be modelled and the rate parameters have to be identified. Especially, in microporous sorbents, due to the high density of the sorption phase and, therefore, the mutual influences of sorbing species, a knowledge of the matrix of diffusion coefficients is needed [6]. The complexity of the phenomena demands combined experimental and theoretical research. Actual directions of the development in this field are as follows: (i)
Experimental methods,e.g. the extension of frequency response [89] and Fourier transform IR spectroscopy [31] methods to mixtures. (ii) Inverse methods for the determination of the matrix of diffusion coefficients (m.d.c.) in the case of multicomponent mixtures [6]. (iii) The problem of concentration dependent m.d.c. [90-921. (iv) Combined processes of diffusion and reaction. (v) The Volterra Integral Equations approach to multicomponent systems [94].
Since topic (v) represents - in the author's opinion - a far-reaching development with both theoretical and experimental significance it is described in more detail as follows. The approach is based on the universal transformation of solutions of rate equations for constant concentration conditions to those of variable concentration conditions as published earlier 193,941. The isothermic case of Fick's diffusion in a fluid mixture consisting of N components is considered for any geometry of the sorbina medium, e.g. MS crystals, at variable surface concentration. The model is described by the following equations and initial conditions [941:
(a 1
... local concentration of
the ith component in the crystal, p L
pressure of the ith component in the fluid mixture, R constant, T... absolute temperature, V V 5
...
a (t) 51
4
...
partial
... universal gas
... volume of the gaseous phase,
volume of the zeolite crystals), =
a (t,x) JxEr
= f
(pi(t) ,...,pN(t)),
i
=
l(1)N
(5)
V S
...concentration of the ith component at the crystal surface r v f ....sorption isotherm equations), and the initial conditions
(a
SL
,
S
L
PL(O 1 = PyL, P1(O+) = POL, $0
-
(a L
1 =
ii
(0+),
i = l(1)N
...concentration averaged over the crystal for the ith component);
(6)
the
indices (-1 and (+I at the time t = 0 denote the time regimes prior and after the beginning of the experiment, respectively.
208 M. BUlow
The solution of the problem can be obtained via the following system of non-linear Volterra Integral Equations [94]:
with the terms denoting
f ( p p
i
=
,...,PN(t))
- fl(PiOf...#PNO )
.
l(1)N
For fixed values of j = 1(1)N, the functions H
represent the
11
averaged
concentrations as obtained by solving the problem under the conditions of constant surface concentration. They satisfy Fick's diffusion equation N = DLLAHIL, 1=1
a-- H at Lj
C
i = l(1)N
(9)
with fixed boundary conditions H (0,~)= 0, X
E
Vgl
E
rV
LJ
H (t,x) L1
= 0,
x
i = 1(1)N,
,t
)
0,
rv , t
)
0.
i
=
1(1)N, i
f
(10)
j
S
H (t,x) = 1, x IJ
E
S
The solution of this problem can be given explicitly by (H LJ
,...,HNJ 1T
T
= (H
1.I ' * " 8 H N J )
- diag(h) B (h(Alt,x) l...,h(A v
N
t,x))
T
(11)
S
with A
=
l(1)N denoting the solutions of the eigenvalue problem
1
(D - AL I ) = 0,
i
=
l(1)N.
(12)
The matrix B relates to the corresponding eigenvectors. The vector is determined by a system of equations B
)i,
T ,..., A .) I NJ xEr LJ v
= (H
S
(13)
Molecular Mobility on Zeolites 209
The function h represents the normed solution of the single component problem at the condition of constant surface concentration. It satisfies the equation
a
xh
= A h
(14)
and the boundary conditions h(0,x) = 1, x E V P
h(t,x)
=
1, x E
rv ,
t)0.
s
This solution is assumed to be known. By means of the method described, the solution of the multicomponent sorption kinetics problem, at both constant and variable surface concentration, can be derived as far as if there is a known solution of the single component sorption kinetics problem at constant surface concentration. Without additional difficulties, the method allows for taking into account apparatus effects [95] which occur due to the influence of valves on the mass transport in the system or which take place in single step frequency response arrangement [961. These effects have to be considered in the case of sufficiently fast sorption uptake processes. Most probably, the approach described will promote the development of knowledge on multicomponent sorption kinetics and, thus, the practical use of this principle. CONCLUSIONS The rapid progress of the knowledge on intracrystalline rate processes reached especially by the development of various experimental techniques and their combined application to representative molecular sieve sorption systems, allows for a careful investigation of relationships between their structural, as well as physico-chemical properties and the non-equilibrium parameters. The research in multicomponent sorption kinetics which is of specific practical interest experiences acceleration since both experimental and theoretical studies are extended from single component towards multicomponent phenomena. In the latter field, the accumulation and analysis of a significantly large number of experimental sets of data will lead to new progress in the knowledge and application of sorption processes. ACKNOWLEDGMENTS The author is grateful to Drs. J.Caro, A.Micke, P.Struve, G.Tschirch (Berlin), M.KoCiiik (Prague) and Prof. L.V.C.Rees (London) for friendly co-operation. REFERENCES 1 R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London I New York I San Francisco, 1978. 2 D.M. Ruthven, Principles of Adsorption and Adsorption Processes, J.Wiley, New York, 1984. 3 J. KBrger, 8. Pfeifer and W. Heink, Adv. Magn. Res., 12 (1988) 1.
210 M. BUlow
J. KBrger and D.M. Ruthven, Zeolites, 9 (1989) 267. H.W. Haynes, J r . , Catal. Rev. - Sci. Eng., 30 (1988) 563. R.M. Marutovsky and M . Billow, Gas Sep. Purif., 1 (1987) 66. M. Bolow, Z. Chem., Leipzig, 25 (1985) 81. M. Billow, J. Karger, M. Kotit'ik,A.M. Vologtuk, Z. Chem., Leipzig, 21 (1981) 175. 9 P.A. Jacobs and R.A. van Santen (Eds.), Zeolites: Facts, Figures, Future (Proc. 8th Int. Zeolite Conf., Amsterdam, July 10-14, 19891, Part B, Elsevier, Amsterdam , 1989. 10 H.G. Karge and J. Weitkamp (Eds.), Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations (Proc. Int. Symp. Zeolites , Wnrzburg, September 4-8, 1989), Elsevier, Amsterdam 1989. 11 S.F. Garcia and P.B. Weisz, J. Catal., 121 (1990) 294. 1 2 L. Riekert, Adv. Catal., 21 (1970) 281. 13 M.M. Dubinin, V.V. Serpinsky and K.O. Murdmaa (Eds.), Adsorption and Adsorbents (Russ.) (Proc. 6th Allunion Conf. Adsorption Theory, MOSCOW, November 18-21, 19851, Nauka, MOSCOW, 1987. 14 E.F. Vansant and R. Dewolfs (Eds.), Gas Separation Technology (Proc. Int. Symp. Gas Separation Technology, Antwerp, September 10-15, 19891, Elsevier, Amsterdam, 1990. 15 S . Whitaker, in S . Whitaker and A.E. Cassano (Eds.), Chemical Reactor Analysis; Concepts and Designs, Gordon and Breach, New York 1986. 16 R. Aris, The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts, Vols. 1 and 2 , Clarendon, Oxford, 1975. 17 A.I. Liapis, (Ed.), Fundamentals of Adsorption - I1 (Proc.2d Int. Conf. Adsorption, Santa Barbara, May 4-9, 19861, Engineering Foundation, New York, 1987. 18 M . Billow, J. Caro, B. Rbhl-Kuhn and 8. Zibrowius, in ref. 10, p. 505. 19 K. Chihara, M . Suzuki and K. Kawazoe, Chem. Eng. Sci., 31 (1976) 505. 20 M . Kotiiik, M. Smutek, A.G. Bezus and A. Zikhov;, Collect. Czechoslov. Chem. Commun., 45 (1980) 3392. 21 M. Billow, W. Mietk, P. Struve and M. Kotiiik, J. Chem. SOC., Faraday Trans. I, 80 (1984) 813; 2167. 22 M.L. Sun, Ph D Thesis, L'Universite P. et M . Curie, Paris, 1988. 23 M . Bulow, P. Struve, C. Redszus and W. Schirmer in L.V.C. Rees (Ed.), Proc. 5th Int. Zeolite Conf., Napoli, June 2-6, 1980 , Heyden, London / Philadelphia I Rheine, 1980, p. 580. 24 M . BUlow, P. Struve and W. Mietk, Z. phys. Chemie, Leipzig, 267 (1986) 613. 25 J. KBrger, H. Pfeifer, R. Richter, H. Fnrtig, W. Roscher and R. Seidel, AICHE - J., 34 (1988) 1185. 26 M . Kocii'ik, P. Struve, K. Fiedler and M . Billow, J. Chem. Soc., Faraday Trans. I, 84 (1988) 3001. 27 M. Billow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt, W. Schirmer and J. Karger, J. Chem. SOC. Faraday Trans. I, 76 (1980) 597. 28 K. Beschmann, G.T. Kokotailo and L. Riekert, Chem. Eng. Process., 22 (1987) 223. 29 D. Shen, L.V.C. Rees, J. Caro, M. Billow, B. Zibrowius and H. Jobic, J. Chem. SOC., Faraday Transactions, in press. 30 Ch.N. Satterfield, J.R. Katzer and W.R.Vieth, Ind. Eng. Chem. Fundam., 4 5 6 7 8
Molecular Mobility on Zeolites 211
10 (1971) 478. 31 H.G. Karge and W. NieBen, Catalysis Today, in press. 32 M. Kol'it'ik, A. Zikhovi, P. Struve and M. Balow, in ref. 9, p. 925. 33 M. Billow, P. Lorenz, W. Mietk, P. Struve and N.N. Samulevit, J. Chem. SOC., Faraday Trans. I, 79 (1983) 1099. 34 M. Balow, W. Mietk, P. Struve and P. Lorenz, J. Chem. SOC., Faraday Trans. I, 79 (1983) 2457. 35 N. Van-Den-Begin, L.V.C. Rees, J. Caro and M. Billow, Zeolites, 9 (19891287. 36 Y. Yasuda, J . Phys. Chem., 80 (1976) 1867. 37 M. Betemps, in ref. 12, p. 526. 38 M. Balow, H. Schlodder, L.V.C. Rees and R.E. Richards, in Y. Murakami, A. Iijima and J.W. Ward (Eds.), Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22, 1986, Elsevier / Kodansha, Amsterdam / Tokyo, 1986, 579. 39 M. Eic, M.V. Goddard and D.M. Ruthven, Zeolites, 8 (1988) 40; 327. 40 R.M. Barrer and B.E.F Fender, J. Phys. Chem. Solids, 21 (1961) 12. 41 M. Billow, P. Struve and L.V.C. Rees, Zeolites, 5 (1985) 113. 42 E. Wicke and R. Kallenbach, Kolloid - Z., 97 (1941) 135. 43 D.T. Hayhurst and A.P.Paravar, Zeolites, 8 (1988) 27. 44 W.O. Haag, R.M. Lago and P.B. Weisz, Disc. Faraday SOC., 72 (1982) 317. 45 H.G. Karge and K. Klose, Ber. Bunsenges. Phys. Chem., 78 (1974) 1263; 79 (1975) 454. 46 B.F. Mentzen, in ref. 10, p. 477. 47 H. Nehishi, M. Sasaki, T. Iwaki, K. Hayes and T. Yasunaga, J. Phys. Chem., 88 (1984) 5564. 48 H.A. Resing and J.S. Murday, Adv. Chem., 121 (1973) 414. 49 H. Pfeifer, W. Schirmer and H. Winkler, Adv. Chem., 121 (1973) 430. 50 B. Zibrowius, J. Caro and H. Pfeifer, J. Chem. SOC., Faraday Trans. I, 84 (1988) 2347. 51 B. Zibrowius, M. Billow and H. Pfeifer, Chem. Phys. Letters, 120 (1985) 420. 52 R. Stockmeyer, Zeolites, 4 (1984) 81. 53 H. Jobic, M. Bge and G.J. Kearley, Zeolites, 9 (1989) 312. 54 P.A. Egelstaff, A.S. Domes and J.W. White (Eds.), in Molecular Sieves (Proc. 1st Int. zeolite Conf., London, July, 19671, SOC. Chem. Ind., London, 1968, p. 306. 55 K. Ulbricht, U. Ewert, T. Herrling, H.U. Thiessenhusen, G. Aebli, J. V6lter and W. Schneider, in G.R. Eaton, S . S . Eaton H.Ono (Eds.), E.P.R. Imaging and in vivo E.P.R., C.R.C. Press, New York, 1990. 56 J. Karger, Surface Sci., 36 (1973) 797. 57 M. Billow and J. KBrger, Thesis for Dr. sc. nat. Degree, Karl-Marx-Universitat, Leipzig, 1978. 58 R. Ash and R.M. Barrer, Surface Sci., 8 (1967) 461. 59 J. Crank, The Mathematics of Diffusion, Clarendon, Oxford, 1975, p . 213. 60 C. Fbrste, J. Karger, H. Pfeifer, L. Riekert, M. Billow, A. Zikinovi, J . Chem. SOC., Faraday Trans., 86 (1990) 881. 61 D.B. Shah, D.T. Hayhurst, G. Evanina and C.J. GUO, AIChE -J., 34 (1988) 1713. 62 A. Zikhovi, M. Billow and H. Schlodder, Zeolites, 7 (1987) 115. 63 P. Wu, A. Debebe and Y.H. Ma, Zeolites, 3 (1983) 118. 64 A. Zikknov6, M. Billow, M. KoEiiik and P. Struve, 2. Phys. Chem., Leipzig,
212 M. BUlow
270 (1989) 525. 65 H. Jobic, M. Bee, J. Caro, M. Billow and J. KBrger, J. Chem. Soc., Faraday Trans. I, 85 (1989) 4201. 66 M. Goddard and D.M. Ruthven, Zeolites, 6 (1986) 283; 445. 67 M. Goddard and D.M. Ruthven, in ref. 38, p. 467. 68 M. Blilow, in H. Tominaga (Ed.), New Developments in Zeolite Science and Technology (Discussion, 7th Int. Zeolite Conf., Tokyo, August 17-22, 19861, Japan Association of Zeolite, Tokyo, 1986, p. 67. 69 E. Aust, W. Hilgert and G. Emig, in ref. 10, p. 495. 70 P. Lorenz, M. Billow and J. Karger, Izv. Akad. Nauk U . S . S . R . , ser. chim., (19801, 1741. 71 B. Boddenberg and R . Burmeister, Zeolites, 8 (1988) 488. 72 H. Jobic, M. B&e, J. Karger, H. Pfeifer and J. Caro, J. Chem. SOC., Chem. Commun., (1990), 341. 73 L.V.C. Rees, Personal communication to M.B. 74 M. Billow, in preparation for Zeolites. 75 M. Billow and W. Mietk, in preparation for Zeolites. 76 E.R. Geus, J.C.Jansen and H.van Bekkum, in J.C. Jansen, L. Moscou and M.F.M Post (Eds.), Zeolites for the Nineties, Amsterdam, 1989, p. 293. 77 N. Van-Den-Begin and L.V.C. Rees, in ref. 9, p. 915. 78 M. Billow and A. Micke, in H . Kral (Ed.), Katalyse, Dechema-Wonographie No 118 (Vortrage der Dechema-Jahrestagung 1989, Frankfurt/M., June 1-2, 19891, p. 349. 79 R. Haul, D. Just and G. Dilmbgen, in J.H. de Boer (Ed.), Reactivity of Solids, Elsevier, Amsterdam, 1961, p. 65. 80 J. Karger, J. Caro and M. Billow, 2. Chem., (Leipzig), 16 (1976) 337. 81 J. Klrger and J. Caro, J. Chem. SOC., Faraday Trans., 73 (1977) 1363. 82 R . M . Barrer, Langmuir, 3 (1987) 309. 83 A.I. Vlasov, V.A. Bakaev, M.M. Dubinin and V.V. Serpinsky, Dokl. Akad. Nauk U.S.S.R., 251 (1980) 912. 84 M. Billow, Kolloidn. (Russ.1 40 (1978) 207. 85 J. Koresh and A . Soffer, J . Chem. SOC., Faraday Trans. I, 77 (1981) 3005. 86 M. Billow, J Caro, J. Vblter and J. KBrger, in 8. Delmon and G.F. Froment (Eds.), Catalyst Deactivation 1987, (Proc. 4th Int. Symp. Catalyst Deactivation, Antwerp, September 29-October 1, 19871, Elsevier, Amsterdam, 1987, p. 343. 87 M. Billow, P. Struve and S. Pikus, Zeolites, 2 (1982) 267. 88 W. Lutz, H. Fichtner-Schmittler, G. Becker and M. Blilow, Cryst. Res. Technol., 21 (1986) 1339. 89 Y. Yasuda, Y. Yamaha and T. Matsuura, in ref. 38, p. 587. 90 A.I. Fatahi, K.F. Loughlin and M.M.Hassan, in ref. 14, p. 203. 91 L.K.Filippov, M. Billow and I.V. Filippova, Gas Sep. Purif., 4 (1990) 41. 92 K. Dahlke and G. Emig, Catalysis Today, in press. 93 M. Koriiik, G. Tschirch, P. Struve and M. Billow, J. Chem. SOC., Faraday Trans. I, 84 (1988) 2247. 94 A. Micke and M. Billow, Gas Sep. Purif., in press. 95 A. Micke, M. Kofit’ik, M. BBlow, G. Tschirch and P. Struve, J. Chem. SOC.,
x.,
Faraday Trans., in press. 96 A . Micke, M. Kotiiik and J. Caro, J. Chem. SOC., Faraday Trans., in press.
213
Investigation of Diffusion and Counter-diffusion of Benzene and Ethylbenzene in ZSM-%type Zeolites by a Novel IR Technique W. NieRen and H.G. Karge Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, Faradayweg 4-6, 1000 Berlin 33 (West)
ABSTRACT A novel in-situ IR technique is applied to study diffusion and counter-diffusion of benzene and ethylbenzene in H-ZSM-5 zeolite. The transport of the adsorbate molecules from a diluted feed stream into the catalyst or vice versa is studied in a precatalytic regime but under conditions close to a catalytic run. Diffusivities are evaluated for the single component as well as for the binary case a t various temperatures and loadings. The diffusion coefficient determined for benzene a t 415 K and a coverage of about 0.11 mmol benzene per gram zeolite (0.6 molecules benzene per unit cell) amounts to D =9.7x 10-10 cm2 s-1 and is in excellent agreement with literature data obtained by different uptake techniques or NMR measurements. The activation energies for diffusion of benzene and ethylbenzene are 20 kJ mol-l and 25 kJ mol-1, respectively. Counter-diffusion, under similar conditions, reduces the diffusivities, which were observed in the single-component case, by about 50%. In a zeolite catalyst sample, which was coked via dealkylation of ethylbenzene at reaction temperatures somewhat higher than those of the sorption experiments, the diffusion coefficient of ethylbenzene remained essentially unchanged even though the sorption capacity significantly decreased due to deposition of carbonaceous material.
.
INTRODUCTION There exists a large body of data in the literature, concerning sorption kinetics and diffusivities for a great variety of sorbate/zeolite systems [l-41. Diffusion coefficients were almost exclusively obtained via measurements of sorption and/or desorption rates or NMR experiments. Sorption techniques frequently employ a balance determining the rate of mass transfer through the rate of weight gain (or weight loss) of a zeolite sample brought into contact with the sorbate phase of non-equilibrium partial pressure [5]. Another technique uses the pressure change a s a function of time when the sorbent, under constant volume and temperature, is exposed to the non-equilibrated fluid sorbate phase [61. NMR experiments, in contrast, are conducted under equilibrium conditions and provide self-diffusion coefficients [7-101. For a long time, significant discrepancies between results obtained from sorption kinetics and NMR investigations were generally observed [1,31. In 1977, H.-J. Doelle and L. Riekert [ l l l pointed out that sorption kinetics experiments may be, in many cases, severely disturbed by intercrystalline diffusion resistance and slow dissipation of the heat of adsorption or, in other words, actual non-isothermicity. They also suggested how to minimize these disturbances. Thus, in a number of subsequent investigations of several authors, agreement between the results of both
214
W. Niellen and H. G. Karge
of subsequent investigations of several authors, agreement between the results of both methods (uptake and NMR techniques) were achieved by improvement of the experimental design of the sorption studies [12-161. Uptake of sorbate molecules may be also measured through the increase of intensity of IR bands typical of the sorbate entering the zeolite sample [17-181.This technique might be particularly useful to investigate the socalled counter-diffusion, i.e., the diffusion of one component (A) versus another component (B) in the zeolite pore structure [19, 201. The IR method also renders possible the measurement of diffusivities when, under in-situ conditions, the properties of the sorbent changes, e.g., by coke deposition during a catalytic run. It is the aim of the present study to demonstrate the suitability of the IR technique for the purposes outlined and illustrate this by selected examples. EXPERIMENTAL SECTION Materials. An H-ZSM-5 type zeolite (overall Si/A1=33.5) was provided by DEGUSSA, Wolfgang, F.R.G. The well-shaped crystallites had an average size of 8.8 p x 5.2 p x 3.2 p. Pyridine, benzene and ethylbenzene were from MERCK, Darmstadt, F.R.G., spectroscopic grade, purified by distillation as well as repeated freeze-pump-thaw cycles and finally stored over highly activated 3A molecular sieve pellets. Apparatus. Preliminary experiments were carried out in a modified Kiselev-type cell [21] with a grating spectrometer, PERKIN ELMER model 325. Precise measurements of diffusivities were conducted by means of a fast Fourier Transform IR (FTIR) spectrometer, PERKIN ELMER model 1800 inserted in a complex set-up equipped with UHV, gas dosing and mass flow control systems. Details of the cell and experimental devices will be described elsewhere [221. Procedure. The preliminary key experiments were carried out in such a way t h a t a degassed zeolite wafer was loaded from the vapour phase of component A, the steady state IR spectrum of the sorbate/zeolite system measured, component B admitted, and the change in the intensities of typical bands of both compounds monitored at certain intervals. Systematic diffusion experiments were also conducted with self-supported zeolite wafers (7-14 mg cm-2) which were activated at Pa and 675 K for 1.5 h. Prior to contact with the sorbate, the IR cell was filled with dried helium a s a carrier gas. Subsequently, one or two components (benzene or ethylbenzene), carried by helium bubbling through thermostatted saturators, could be admitted. A system of mass-flow controllers allowed for an independent change of the partial pressures while the total pressure could be kept constant [221. The time required to pass the sorbate from the inlet valve to the place of the zeolite wafer was about 4 s. IR spectra were obtained in intervals as short as 0.37 s.
Diffusion of Benzene and Ethylbenzene in ZSM-5 215
RESULTS AND DISCUSSION The results of some prelimary key experiments [19,20] are shown in Fig, 1.
Y
v z 4
sample pre-loaded with pyridine and subsequently heated in 4 kPa
start
after 16h
afterLOh
sample pre-loaded with ethylbenzene (115 Pa) and subsequently contated with a mixture of 115 Pa EB and 1 kPa Bat 325 K
start
afterlh
after 3h
II-
5
ul
z U a
EB
EB
B
1495
1453
1478
B
W A V E N U M B E R [cm.']
Fig. 1. Counter-diffusion of (a) Benzene (B) vs. Pyridine (PY)and (b) ethylbenzene (EB) in Hydrogen Mordenite The sequence of IR spectra demonstrates that the molecules of the preloaded component A (pyridine, benzene) are displaced by the ingoing component B (benzene, ethylbenzene) when the preloaded sample is contacted with the vapour phase of the second compound. The process is slow because in both cases component A is more strongly held by the sorbent than component B (vide infra). But these experiments showed, that in principle, it should be possible to monitor counter-diffusion in zeolites via the IR method. In order to apply this technique for quantitative characterization of counter-transport phenomena in zeolites i t had to be checked first for the more simple case of single-component diffusion. As an example, benzene diffusion in H-ZSM-5 was chosen, because results for this case had been already reported by several authors. Their results, obtained from sorption kinetics as well a s from NMR experiments, were in very good agreement and, thus, provided a reliable basis for comparison [13,141. Fig. 2 displays a set of FTIR spectra obtained for the uptake of benzene into H-ZSM-5 a t 415 K employing the experimental device and procedure as described in the Experimental Section. One recognizes the increase in absorbance of the typical benzene band at 1478 cm-l as a function of time (spectra 1 to 4). The maximum absorbance, A,, of such bands can be used as a measure of the amount sorbed, M,, at time t into the porous structure of the zeolite crystallites. Therefore, evaluation of the sequence of these spectral uptake curves can provide data which may be used in the appropriate solution (equ. 1)of Fick's second law, and this generates the desired diffusivities 1221:
216 W. Nieoen and H. G. Karge
where M, D, t, 2a denote amount adsorbed, diffusion coefficient, time and the diameter of the crystallite, respectively. The parameter fi accounts for the "time-lag", i.e., the time required to establish a steady partial pressure a t the place of the sample 1221, corresponding to the valve effect 1231. I
Y
U
z
1
I
1
I
Lr
4-v-; 2
U c
-
I-
zm z
a
a
c
: He (800 mllmin) Carrier gas : 0 1.15 mbar Benzene Ethylbenzene : 0 mbar Temperature :415 K
-
1800
1
1
1700
1600
-
I
1500 W A V E N U M B E R [cm']
I
I
1400
1300
Fig. 2. Set of benzene spectra, Nos. 0 to 4, for successive states of sorption into H-ZSM-5, after 0,2.4,14.8,38.8 and 484 s. Figure 3 presents results obtained for the uptake of benzene into H-ZSM-5 for four temperatures. The fit of the experimental data according to equ. (1)yielded the diffusion coefficients indicated in the figure. For the same system, i.e., benzene/H-ZSM-5, a diffusion coefficient D,= 1.3 x cm2/s (at 415 K and a sorbed amount of n = 0.1 mmol/g) was reported [13,141 which is in excellent agreement with our result (D, = 9.7 x 10-lo f 0.5 x 1O-Io cm2/s under similar conditions) and provides confidence in the proposed IR technique. As one would expect, the diffusivities increase and the final uptakes (A, M,) decrease with increasing temperature, respectively. The parameter p exhibits only minor changes (see Table I). The results were well reproduced and identical for the case of desorption (compare Fig. 3) upon a decrease in the respective partial pressure (pressure jump down) as long as the loadings did not approach zero. Similar features were observed in the case of sorption of ethylbenzene. In fact, the final uptake was somewhat
-
Diffusion of Benzene and Ethylbenzene in ZSM-5 217
higher compared to the case of benzene, whereas fl appeared, in most experiments, reduced (compare Fig. 3 and Table 1).
0.5
-
- 0.4 0
E
6 0.3 w Y
2 0.2 P
3
0.1
0.0 SQUARE ROOT
OF T I M E
[s1/21
Fig. 3. Diffusion of benzene into H-ZSM-5a t various temperatures and for different pressure jumps.
Table 1 Diffusivities of benzene and ethylbenzene in fresh and coked H-ZSM-5 1 Benzene,B, Ap = 0 -+ 1.15mbar T[Kl DB[cm2/s3 l$j Is-'] n, ImmoYgl
355 375 395 415
4.6~ lo-'' 5.5~ 6 . 7 lo-'' ~ 9.7xlO-'O
0.3 0.2 0.2 0.4
-
0.35 0.24 0.16 0.11
395 415
2 Ethylbenzene, EB, Ap = 0 -+ 1.15 mbar DEB[cm2/sl PE13 [s11 n,,, [mmol/gl
TWI 355 375 395 415
3.2x10-" 4.3xlO-" 5.8xlO-'O 9 . 5 lo-'' ~
0.1 0.1 0.1 0.2
395 395 395
~~
13 30 70
5.8 x lo-'' 5.8 x 10 l o 5.8 x lo-''
3.1 x lo-'' 5.4 x lo-''
4 Counter-diffusion, B vs. EB T[Kl DB[cm2/sl DEB[cm2/sl
0.44
-
0.34 0.24
395 415
~
3.1 x lo-'' 5.1 x lo-''
-
0.55
5 Ethylbenzene, EB ( coking, 13,30,70 h) T[Kl Tcoking[hl DEB[cm2/sl DEB [ s ' l ~
3 Counter-diffusion, EB vs. B TIKI DB[cm2/sl DEB[cm2/sl
n, [mmoVgl coke [wt.-%I
~~
0.07 0.07 0.07
3.0 x lo-''
5.5 x lo-''
~~
0.29 0.28 0.24
~
-
3
2.9 x lo-'' 5.0 x lo-''
218 W. NieOen and H. G. Karge
Figure 4 demonstrates the counter-diffusion of ethylbenzene vs. benzene. In this experiment, benzene was first taken up at 415 K from a stream of benzene i n helium, with a benzene partial pressure of 1.15 mbar, until a steady state was reached (spectrum No. 0). Subsequently, ethylbenzene was admixed (1.15mbar) with the benzene partial pressure and the total rate of the gas flow remaining constant. Counter-diffusion i s indicated by the decrease of the typical benzene absorbance a t 1478 cm-l and the development of the typical ethylbenzene bands a t 1605,1496and 1453 cm-' (spectra Nos.1-5).
L
I
1
1800
1700
1600 1500 W A V E N U M B E R [cm '1
1400
1300
Fig. 4. Set of spectra (Nos. 1 to 5) for successive replacement of preadsorbed benzene (No. 0) by ethylbenzene, after 0,7.4,24.0,38.8,116.6and1842s.
1
0.3
-. zE
L
D
= 4 . 9 . 1 0 - l O ~ 2 I S fl
I
0.18s-1
0.2
w
Benzene : 1.15mbar Ethylbenzene : 0 1.15 mbar
-
Y
a +
-
0
3 0.1
o.a
1
-
I
5
1
1
I
I
25 15 20 10 S Q U A R E R O O T O F T I M E [sin]
Fig. 5. Counter-diffusion of ethylbenzene 415 K.
(
0400
)
1
30
vs. benzene ( x x x x ) in H-ZSM-5 at
Diffusion of Benzene and Ethylbenzene in ZSM-5 219
Experimental data and their evaluation according to equ. ( 1 ) for the ingoing component (here ethylbenzene),modified to l-M,/M, for the leaving component (here benzene), are represented in Fig. 5. From this figure it is evident that the diffusivities both of counter-diffusing benzene and ethylbenzene are reduced by about 50%, respectively, compared to the single-component diffusivities. The results of the inverse experiment, viz. counter-diffusion of benzene into H-ZSM-5, previously loaded with ethylbenzene from an ethylbenzenehelium stream at 415 K, are displayed in Fig. 6.
I
I
I
I
t
1
I
I
I : He (800 mllmin) Carrier gar Benzene : 0 - 1.15mbar Ethylbenzene : 1.15 mbar Temperature : 415 K
U c OV2
D
I
p
5.5~1010cm2ls
I
1.0s-1
*
0
.
0
Fig. 6. Counter-diffusion of benzene ( x x x x ) vs. ethylbenzene ( a t 415 K.
~
I
)
in H-ZSM-5
Interestingly, the changes in the absorbances of the respective IR bands were much smaller than in the experiment where ethylbenzene was the compound entering a zeolite sample already containing benzene (compare Fig. 5). This is obviously due to the fact that ethylbenzene is more strongly sorbed by the H-ZSM-5 sample than benzene (vide supra), which was confirmed by microcalorimetric measurements of the differential heat of adsorption of benzene (QB= 63.5 kJ mol-'1 and ethylbenzene (QEB=87.5kJ mol-') on the same zeolite samples [241 a t a coverages of 0.4 mmol g-l. Due to the smaller changes, the evaluation of the data is less precise than in the previously described experiments. However, the diffusivities for intracrystalline counter-transport are in the same range as in the reverse experiment (compare Fig. 6 and Table 1 ) . Application of the IR method proved to be also suitable for the measurement of diffusivities in coking porous catalysts. This was defionstrated by uptake experiments with ethylbenzene where the sorbent catalyst, H-ZSM-5, was intermittently coked in-situ via dealkylation of ethylbenzene a t temperatures (465K)somewhat higher than the sorption temperature (395 K). Coke deposition was monitored in-situ via the IR absorbance
220 W. NieBen and H. G. Karge
of the so-called coke band around 1600 cm-* and subsequently measured ex-situ in a TGA/GC device [25,26]. Interestingly, at coke loadings of about 3 wt.%, the diffusivities remained essentially unaffected. Similar conclusions were arrived at by Bulow et al. [27]. However, the final uptake of the sorbate, measured through A,, decreased with progressive coke formation (see Table 1). This observation suggests that a t least a significant fraction of the carbonaceous material is deposited inside the zeolite structure, reducing the sorption capacity. Most likely, parts of the zeolite crystallites are blocked. CONCLUSIONS Fast M'IR measurements of sorption and desorption of sorbate molecules into or from zeolite samples, employing an appropriate spectrometer system and sorption/desorption device, provide diffusivities for benzene/H-ZSM-5 and ethylbenzeneM-ZSM-5 which are of the expected order of magnitude. Even though the technique is open for further improvement, the results do not seem significantly disturbed by resistances of intercrystalline mass transport and/or heat transfer. This is supported by the excellent agreement between our results obtained for benzenem-ZSM-5 and reliable data reported in the literature. The same holds for the activation energies for diffusion, i.e. 20 k J mol-' and 25 k J mol-' for benzene [14] and ethylbenzene,respectively, evaluated from the data of Figure 3 and analogous measurements for ethylbenzene (not shown in the text) and Ap = 0.0 4 1.15 mbar. The novel IR spectroscopic technique opens a new route for counter-diffusion measurements and in-situ diffusion measurements of coking porous catalysts.
.
ACKNOWLEDGMENTS The authors are indebted to Professor Bulow for helpful discussions and Drs. P. Kleinschmit, A. Kiss and M. Siray (DEGUSSA) for providing the H-ZSM-5 sample. Financial support by the Bundesminister fur Forschung und Technologie (BMM'), Projects Nos. 03C 2311 and 03C 257 A7 and DEGUSSA, Wolfgang, F.R.G. is gratefully acknowledged. REFERENCES 1 R.M. Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular Sieves" Academic Press, London, (1978) pp. 256-338. 2 H. Pfeifer, "Nuclear Magnetic Resonance and Relaxation of Molecules Adsorbed on Solids" in: NMR - Basic Principles and Progress, Springer, Berlin, 7 (1972) pp. 105115. 3 D.M. Ruthven, "Principles of Adsorption and Adsorption Processes", John Wiley & Sons, New York, 1984, pp. 124-165. 4 D.M. Ruthven, in: "Molecular Sieves - II". Proc. 4th Int. Conf. Zeolites, Chicago, USA, April 18-22, 1977 (J.R. Katzer, Ed.) ACS Symp. Ser. 40, Am. Chem. SOC., Washington, D.C., (1977) pp. 320-334. 5 L. Riekert, Adv. Catalysis, 21 (1970) 281-322. 6 M. Bulow, P. Struve, G. Finger, Ch. Redzus, K. Ehrhardt, W. Schirmer a n d J . Karger, J. Chem. SOC.Faraday Trans. I, 76 (1980) 597-614.
Diffusion of Benzene and Ethylbenzene in ZSM-5 221
7 8
9 10 11
12 13 14
15
16
17 18 19 20 21 22 23 24 25 26 27
J . Karger, W. Seyd, H. Pfeifer and D. Geschke, Proc. 16th Colloque Am ere, Bucharest, Rumania, 1970 (I. Ursu, Ed.) Publishing House of the Acad. o the Socialist Republic of Rumania (1971)pp. 635-636. H. Pfeifer, W. Schirmer and H. Winkler, in: "Molecular Sieves", Proc. 3rd. Int. Conf. on Zeolites, Se t. 3-7,1973, Zurich, Switzerland (W.M. Meier and J.B. Uytterhoeven, Eds.); i d v . in Chemistry Series, 121,Am. Chem. SOC.,Washington, D.C., 1973,pp. 430-440. H. Resing and J.S. Murday, in: "Molecular Sieves", Proc. 3rd. Int. Conf. on Zeolites, Zurich, Switzerland (W.M. Meier and J.B. Uytterhoeven, Eds.); Adv. Sept. 3-7,1973, in Chemistry Series, 121,Am. Chem. Soc., Washington, D.C., 1973,pp. 414-429. M. Nagel, H. Pfeifer and H. Winkler, Z. Phys. Chem. (Leipzig) 255 (1974)283- . H.-J. Doelle and L. Riekert, in: "Molecular Sieves - II", Proc. 4th, Int. Conf. Zeolites, Chicago, USA, April 18-22,1977(J.R. Katzer ,Ed.) ACS Symp. Ser. 40,Am. Chem. SOC.,Washington, D.C., (1977)pp. 401-416. H.-J. Doelle and L. Riekert, Angew. Chem., 91 (1979)309-316. Ch. Forste, J. Karger, H. Pfeifer, L. Riekert, M. Bulow and A. ZikBnovB, J. Chem. SOC. Faraday Trans. I, 86 (1990)881-885. M. Bulow, J. Caro, B. Rohl-Kuhn and B. Zibrowius, in: "Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations"; Proc. Int. Symp. Zeolites, Wiirzburg, F.R.G., September 4-8,1988 (H.G. Karge and J. Weitkamp, Eds.) Elsevier, Amsterdam, 1989;Studies in Surf. Sci. and Catalysis, 46 (1989) pp. 505-507. N.G. Van-den-Begin and L.V.C. Rees, in: "Zeolites: Facts, Figures, Future", Proc. 8th Int. Zeolite Conf. Amsterdam, The Netherlands, July 10-14,1989 (P.A. Jacobs and R.A. van Santen, Eds.) Elsevier, Amsterdam, 1989; Studies Surf. Sci. and Catalysis, 49 B (1989)pp. 915-924. M. Eic and D.M. Ruthven, in: "Zeolites: Facts, Figures, Future", Proc. 8th Int. Zeolite Conf. Amsterdam, The Netherlands, July 10-14,1989(P.A. Jacobs and R.A. van Santen, Eds.) Elsevier, Amsterdam, 1989;Studies Surf. Sci. and Catalysis, 49 B (1989)pp. 897-905. H.G. Karge and K. Klose, Ber. Bunsenges. 78 (1974)1263. H.G. Karge and K. Klose, Ber. Bunsenges. 79 (1975)454-460. H.G. Karge, unpublished results. H.G. Karge and J. Weitkamp, Chemie-Ingenieur-Technik58 (1986)946-959. H.G. Karge, Z. hys. Chem. [NF], 122 (1980)103-116 H.G. Karge an8W. NieBen, Catalysis Today, accepted for publication. P. Struve, M. KoEifik, M. Bulow, A. Zikdnova and A.G. Bezus, Z. phys. Chemie, Leipzig 264 (1983)49-60. L.C. Jozefowicz, H.G. Karge and F. Asmussen, J. Chem. SOC.Faraday Trans. I, submitted for publication. H.G. Karge, Catalysis Today, submitted for publication. H.G. Karge, H. Darmstadt, M. Laniecki and R. Amberg, Applied Catalysis, submitted for publication. M. Bulow, unpublished results.
P
This Page Intentionally Left Blank
V . Catalysis
This Page Intentionally Left Blank
225
Zeolites as Partial Oxygenation Catalysts
D.R.C. Huybrechts, R.F. Parton and P.A. Jacobs K.U. Leuven, Dept. Biotechni sche Wetenschappen, Laboratorium voor Oppervlaktechemie, Kardinaal Mercierlaan 92, 8-3030 Heverlee (Leuven), Belgium
ABSTRACT The use of zeolite catalysts for partial oxygenations of organic substrates with dioxygen or with peroxidic oxygen sources is reviewed. Oxidative properties were introduced in zeolites by incorporation of transition metals, either as charge neutralizing or lattice cations, supported oxides, or encaged organometall ic complexes. Catalyzed reactions include the oxyfunctional ization of alkanes, the epoxidation or Wacker oxidation of alkenes, the oxidation of butadiene to furan or maleic anhydride and the hydroxylation of aromatics. INTRODUCTION For the past 25 years zeolite containing catalysts have been commercially used in a variety of petrochemical reactions. In the major part of these applications, such as cracking, hydrocracking, isomerisation and dewaxing, the zeolite acts either as an acid or as a bifunctional catalyst [ l ] . It is therefore understandable that most of the catalytic studies in the field of zeolites have concentrated on their acidity and properties as support for metal particles. On the contrary, relatively little information is available on he use of zeolites as redox catalysts, although in principle zeolites can be readily modified into oxidation catalysts by introduction of a transit on metal. Transition metal elements can be introduced in different zeol te structures in different oxidation states and coordinations, either as charge neutralizing cations, supported metal clusters, supported metal oxides, complexes or as lattice cations. Surveys of zeolitic oxidation catalysts were given by Ben-Taarit in a discussion o f transition metal induced catalytic properties of zeolites [ 2 ] , by Tagiev and Minachev [3], by Maxwell in his review on nonacid catalysis with zeolites [ 4 ] , and by Holderich in a review on zeolites as catalysts for organic syntheses [5]. In the present review, only reactions in which oxygen atoms from the oxidant are incorporated in the
226 D. R.C. Huybrechts. R. F. Parton and P. A. Jacobs
organic substrate w i l l be discussed. Consequently, o x i d a t i v e dehydrogenations [6] i s n o t included.
t h e important f i e l d o f
ALKANE OXIDATIONS WITH DIOXYGEN Oxidations of p a r a f f i n s on z e o l i t e s were already c a r r i e d o u t more than two decades ago by F r i p i a t and coworkers [7-81. n-Hexane was o x i d i z e d batchwise i n t h e l i q u i d phase by molecular oxygen a t 16OOC and under a t o t a l pressure o f 25 atm. Manganese, n i c k e l and c o b a l t i o n exchanged z e o l i t e s 13 X were compared w i t h homogeneous c a t a l y s t s , t r a n s i t i o n metals exchanged on alumina and nonc a t a l y t i c systems. I n a l l cases hexane i s probably slowly o x i d i z e d t o alcohols and ketones, which are then v i a a f a s t o x i d a t i v e cleavage converted i n t o two c a r b o x y l i c acids. A t t h e end o f t h e r e a c t i o n , independent o f t h e k i n d o f c a t a l y s t used, a 70 t o 80% s e l e c t i v i t y f o r a c e t i c a c i d i s reached. The r a t e o f formation o f a c e t i c a c i d i s i d e n t i c a l on t h e c o b a l t and manganese z e o l i t e and on t h e homogeneous manganese c a t a l y s t s . ALKENE OXIDATIONS WITH DIOXYGEN Ethylene oxide can be prepared by ethylene o x i d a t i o n a t 200-3OOOC i n the
presence o f a Ag c o n t a i n i n g A o r X z e o l i t e [ 9 - l o ] .
Ag i o n exchanged z e o l i t e s
have a low s e l e c t i v i t y f o r ethylene oxide, whereas c a t a l y s t s prepared by impregnation combine high a c t i v i t i e s w i t h h i g h s e l e c t i v i t i e s . Thus, on a 30% Ag-CaA, ethylene oxide i s formed w i t h 25% y i e l d and 70% s e l e c t i v i t y a t 250%. S i m i l a r r e s u l t s have been obtained w i t h Chabasite and Mordenite c o n t a i n i n g over 20 w t % o f s i l v e r [ll]. Moreover f o r such s i l v e r z e o l i t e s a c o r r e l a t i o n was found between t h e average s i l v e r p a r t i c l e s i z e and t h e ethylene oxide s e l e c t i v i t y . As t h e ethylene oxide s e l e c t i v i t y i s o n l y h i g h e r than 40% f o r p a r t i c l e s i z e s above 10 nm, i t i s c l e a r t h a t t h e e x t r a c r y s t a l l i n e s i l v e r phase contains t h e a c t i v e s i t e . Z e o l i t e s p l a y t h e r o l e o f low surface area supports. Mochida e t a l . s t u d i e d t h e o x i d a t i o n o f propylene by molecular oxygen over z e o l i t e Y exchanged w i t h Cu2+ [12-151. The o x i d a t i o n products obtained a t d i f f e r e n t feed gas compositions are l i s t e d i n Table 1. I n t h e absence o f steam i n the feed gas, Cu2+Y has a h i g h a c t i v i t y f o r deep o x i d a t i o n o f propylene t o C02 and H20 [12,14]. A t low propylene/02 r a t i o s , formaldehyde i s formed w i t h l e s s than 10% s e l e c t i v i t y . S e l e c t i v e o x i d a t i o n o f propylene i s o n l y p o s s i b l e a t high propylene/02 r a t i o s and low propylene conversions, a c r o l e i n being t h e main product formed [14]. A t low steam/propylene r a t i o s , a c r o l e i n i s formed w i t h 20-25% s e l e c t i v i t y a t propylene conversions o f about 20% [13]. It i s b e l i e v e d t h a t t h e presence o f steam suppresses deep o x i d a t i o n o f propylene and t h e r e f o r e allows h i g h e r
Zeolites as Partial Oxygenation Catalysts 227
s e l e c t i v i t i e s t o be obtained a t s i g n i f i c a n t propylene conversions. A t h i g h steam/propylene r a t i o s , t h e main products o t h e r than C02 are isopropanol below 200OC, acetaldehyde a t around 25OOC and acetone above 450OC, t h e s e l e c t i v i t y f o r a c r o l e i n reaching i t s maximum a t about 410OC [15]. The maximum y i e l d o f acetaldehyde and acetone i s 1 and 0.7% r e s p e c t i v e l y . Products o f propylene o x i d a t i o n on CuZtY.
Table 1.
Feed gas compos it i on (molar r a t i o s ) C3H6 : 02 : 1 1 1 1 1
25 0.14 1.6 1 0
Major products
Reference
H20 0 0 1.3 20
20
COP, formaldehyde C02, a c r o l e i n C02, a c r o l e i n COP, isopropanol , acetone, acetaldehyde, a c r o l e i n isopropanol , acetone, ethylene
12,14 14 13 15 15
Formation of isopropanol and acetone i s a l s o observed i n t h e absence o f oxygen, suggesting t h a t these products are formed from propylene by subsequent h y d r a t i o n and dehydrogenation. A p o s t e r i o r i i t f o l l o w s t h a t t h e CuZtY z e o l i t e i s h i g h l y a c i d i c i n r e a c t i o n conditions, and i s supposed t o p l a y a b i f u n c t i o n a l c a t a l y t i c r o l e i n these r e a c t i o n s [15]. Indeed, a c i d catalyzed h y d r a t i o n o f lower o l e f i n s w i t h formation o f alcohols has a l s o been observed over various proton-exchanged z e o l i t e s [16-191. Protons i n CuztY z e o l i t e can o n l y be generated upon r e d u c t i o n o f c u p r i c ions w i t h propylene o r d u r i n g t h e i o n exchange o f t h e parent NatY z e o l i t e w i t h copper, as a r e s u l t o f an astoechiometric exchange process. Some o f t h e r e s u l t s obtained by Ben-Taarit e t a l . f o r propylene o x i d a t i o n on Cu2+Y are s i m i l a r t o those reported by Mochida e t a l . when an excess o f propylene i s used i n t h e feed [2]. The former authors s t r e s s t h a t under these circumstances Cut2 i n CuztY i s transformed t o a Cuo/Cu20/Cu0 mixture. However, using optimized 02/propylene r a t i o s , f l o w r a t e s and temperatures, i t seems t h a t 70% s e l e c t i v i t y f o r a c r o l e i n a t 50% propylene conversion i s achievable. Under those c o n d i t i o n s t h e r e was no evidence f o r t h e formation o f e i t h e r a m e t a l l i c o r an oxide copper phase [2]. Although no d i r e c t evidence was a v a i l a b l e , Ben-Taarit e t a l . advanced a t e n t a t i v e mechanism f o r propylene o x i d a t i o n [ 2 ] . As o n l y heterogeneous o r mixed oxides c a t a l y z e t h e formation o f a c r o l e i n , t h e behavior o f C U + ~ Y z e o l i t e i s s i m i l a r t o t h e heterogeneous case. On the contrary, on Rh3+Y, acetone was s e l e c t i v e l y produced [2]. T h i s i s now i n l i n e w i t h t h e behavior o f Rh s a l t s i n
228 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
homogeneous conditions. It is difficult at this stage to rationalize the behavior of transition metal zeolite catalysts in this respect. Fripiat et al. studied the oxidation of propylene dissolved in benzene at 150OC under a total pressure of 45 atm [20]. On Mo-X, 7 M epoxide selectivity is obtained at propylene conversions of 7.5%. Side products are formed by further oxidation of the epoxide, by C-C cleavage in the olefin with formation of methanol, formic and acetic acid, and by fast esterification of the epoxide with these acids. Molybdenum containing zeolite catalysts have also been studied for the gas phase propylene oxidation [21]. On Mo-Y zeolites mainly acetone is formed, together with acetaldehyde, propanal, CO and C02. A maximum selectivity for acetone of ca. 12% has been reported. Benzene and other C6 hydrocarbons are present in the gaseous products, especially at oxygen/propylene ratios below 1. A large carbon imbalance is found, indicating the formation of strongly adsorbed species and/or propylene oligorners and coke. Mo impregnated Y zeolites, in which Mo is largely deposited on the external surface, are more active than Mo exchanged Y zeolites, but yield similar product distributions. Mo containing mordenite and ZSM-5 zeolites have low selectivities for partial oxidation of propylene. Formation of acetone is believed to proceed via protonation of propylene on the acid sites of the zeolite, and further reaction of the alkyl carbenium ion with Mo5+OH- to form isopropoxy species, which then yield acetone. Reaction of the carbenium ion with propylene or O2 is responsible for the oligomerization and total oxidation reactions respectively. Pd/Cu zeolite Y associations were found to be selective catalysts for oxidation of olefins in the presence of steam at temperatures ranging from 373 to 433K [22-301. Acetone and acetaldehyde were obtained by propylene and ethylene oxidation, with selectivities of at least 90%. Neither Pd/Y nor Cu/Y showed good activity in these reactions. The conversion of different olefins under the same experimental conditions decreases in the following order [23]: ethylene > propylene > 1-butene > cis-2-butene - trans-2-butene. These catalysts have been prepared according to two different approaches. Minachev et al. made impregnations of PdC12 and CuC12 onto zeolites and traditional amorphous catalyst supports [26-291. From this work two conclusions emerge: i/ a support effect seems to exist: when the chloride salts are impregnated on zeolites, the activity, stability and selectivity are by far superior to the case when salt impregnation is done on amorphous supports; i i / an optimum Cu/Pd ratio of 10/1 exists [27]. Tominaga et al. prepared a catalyst through an ion exchange method [22]. In the latter case the reaction scheme
Zeolites as Partial Oxygenation Catalysts 229
proposed is similar to that of the homogeneous Wacker reaction, where chloroanions are replaced by the anionic zeolite lattice Z- (Scheme 1) [22-23,301:
+ olefin + H20 Pd2+ ( 2 ' ) Pdo(H+Z')2 + 2 CU'+(Z')~ 2 (Cu+Z')(H+Z') + 0.5 O2 Scheme 1.
4
+
Pdo) ' Z ' H (
Pd2+(2')2 + 2 (Cu+Z') 2 C U ~ + ( Z - )+~H20
(H+Z')
carbonyl compound
Reaction scheme for Wacker type olefin oxidation on Pd2+Cu2+Y.
The active component for olefin oxidation i s Pd2+, while Cu2+ acts as a promoter for the reoxidation of PdO. The sequence of ion exchange of Pd2+ and Cu2+ on the faujasite zeolite influences the catalytic performance. Best results seem to be obtained when Pd2+ is introduced in the second step of the ion exchange as it will then be located mainly at the more easily accessible cation sites I 1 and/or I 1 1 [23]. The amount of exchanged Pdzt determines the catalytic activity of Pd2+Cu2+Y, provided that Cu2+ is present in sufficient amounts to assure fast regeneration of Pd2+. A Pd/Cu atomic ratio of four is required here. Increasing acidity in Pd2+Cu2+NaY results in a decrease of both the activity and selectivity in the olefin oxidation [26]. Apparently, the catalysts prepared by impregnation of the respective chlorides are more active than those prepared by ion exchange [3]. However, it is not clear whether the overall activity in the former case is determined by release of PdC12 and CuC12 into solution. When ion exchanged catalysts are used, the occurence of the latter phenomenon can definitely be excluded. In comparable reaction conditions as Pd2+Cu2+Y, Pd2+ and Cu2+ exchanged pentasil and ferrierite zeolites show a different type of activity [31]. The main products formed by propylene oxidation on these catalysts are acrolein and propionaldehyde below 120OC and 2-propanol above 120OC. Above 150°C consecutive oxidation of 2-propanol t o acetone is observed. The catalytic role of Pd and Cu in the 2-propanol synthesis is proposed to follow the Wacker concept. It i s striking that when Pdzt and Cu2+ are exchanged in 10-membered ring zeolites, oxidation of a primary carbon atoms seems possible, as acrolein and propionaldehyde are obtained from propylene. Pd/Cu-zeolites are also catalysts for the oxidative acetoxylation of propylene to allylacetate [32-391. The best results are obtained on a catalyst which is pretreated with an alkali solution to neutralize the acidic centres and containing Pd and Cu in an atomic ratio of 1.1 [37]. The alkali treatment suppresses the acid catalyzed addition of acetic acid to propylene, resulting in the formation of isopropylacetate, which is observed over non-neutral ized Na- and H-Y, as well as over unreduced and reduced Pd/Cu-Nay. Experiments with
230
I).
K. C. Huybrechts, R. F. Parton and P. A. Jacobs
180-labeled reactants indicate that an ally1 alcohol is the intermediate which is then esterified with acetic acid to form allylacetate [36,39]. The rate of formation of allylacetate is correlated with the zeolite structure and chemical composition (Figure 1) and with the nature of the exchanged alkali metal ion in zeolite Y. Surprisingly the traditional high alumina zeolites, containing a high number of Cu and Pd atoms, are this time much less active than some high silica zeolites (Figure 1). The latter zeolites are also very selective towards allylacetate. Unfortunately, the nature of such zeolites, denoted in the original publication as ZWK and ZWM, is not disclosed. In such zeolites the preferred Si02/A1203 ratio is between 20 and 50. The available data presently do not allow to rationalize these interesting findings.
P
0
0
20
40
60
80
100
Figure 1. Oxidative acetoxylation of propylene on Pd/Cu-zeolites; I = K,Naerionite, 2= ZWM-408, 3= ZWM-364, 4= ZWK-XI, 53 ZWK-I11 (derived from [37]). Butadiene oxidation on zeolites containing an active component of the type V2O5 and/or V2O5-P2O5 has been examined by Trifiro et al. [40-431. V205-P205 impregnated on HY, HZSM-5 and HZSM-11 zeolites show higher furan yields at lower reaction temperatures than the unsupported oxide (Figure 2). On HY and HZSM-5 zeolite based catalysts, butadiene is only oxidized to furan, whereas on HZSM-11 based and unsupported catalysts, further oxidation to maleic anhydride is observed (Figure 2).
Zeolites as Partial Oxygenation Catalysts 231
35
-El- V-P/HZSM- 1 1 F
- - t - - V - P ’ / H Z S M - l l MA
I
,,
II
I
.? ,
, .
,
.
F
+V-P/HY +V-P/HZSM-5
F
,,,”’
20 15 10
5
0 560
I
600
640
680
720
Temperature / K Butadiene o x i d a t i o n on z e o l i t e supported and unsupported V2O5-P2O5; Figure 2. F= f u r a n , MA= maleic anhydride, V-P= V2O5-P2O5 (derived from [40-421). 15
10
5
0 560
600
640
680
720
Temperature / K Figure 3.
Butadiene o x i d a t i o n on HY, and V2O5 and/or P2O5 loaded HY; V-P=
V2O5-P2O5 (derived from [40-411). A study o f t h e OH v i b r a t i o n s and o f the r a t e o f the a c i d c a t a l y z e d e t h y l e n e polymerization on t h e p a r e n t and V2O5-P2O5 loaded HY, HZSM-11 z e o l i t e s , showed t h a t f o r HY ans HZSM-5 both P and V i n the V2O5-P2O5 phase i n t e r a c t with t h e
232 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
hydroxyl groups of the structure [40-421. For HZSM-11 supported V2O5-P2O5 catalysts, such interaction was not observed. It is therefore proposed that the interaction of the hydroxyls in HY and HZSM-5 with the V205-P~O5 phase hinders the promoting effect of P on the catalytic behavior of V in the oxidation of furan. Indeed, the V205-P205/HY association shows no significantly different catalytic behavior than the V205/HY association (Figure 3). Butadiene oxidation to furan was also performed on ZSM zeolites prepared in the presence of V3+ [42-441. Non-oxidative activation prevents collapse of the zeolite structure to cristoballite and improves both the activity and selectivity in furan synthesis. When cyclohexene is oxidized with oxygen on a Co-zeolite, the major product is cyclohexenyl hydroperoxide together with minor amounts of 1,2epoxycyclohexane and 2,3-epoxy-l-cyclohexanol [45]. However a combination of Co-zeol ite with V O ( a ~ a c ) and ~ Mo(CO)~ (6/1/1) increases the conversion strongly and the epoxides dominate in the product mixture. PHTHALLOCYANINES AND PORPHYRINES IN ZEOLITES During the last decade a new generation of oxidation catalysts was built, which is able to oxidize a wide range of substrates and behaves similarly to the oxygenation enzymes, cytochrome P-450. These enzymes are composed of a protein part and a heme group with a transition metal. In the synthetic catalysts, which aim to mimic the enzyme activity, the protein is replaced by a zeolite framework, which is able to imitate its major functions, such as shape selectivity and protection of the heme group. The heme group is a porphyrine complex and is used as such as an homogeneous catalyst [46-481 for the partial oxidation of organic substrates. As the size of the synthetic tetraphenylporphyrine (TPP) or its derivatives exceeds that of the supercages of zeolite Y [49-501, synthetic analogues of porphyrines with smaller dimensions, phthallocyanines (Pc), or transition metal complexes such as those of the salen ligand (N,N’-ethylenebis(sa1 icy1 ideneaminato)) [51] are synthesized in zeolites. The structure of transition metal (TM) TPP, Pc and salen complexes is shown in Scheme 2. Nevertheless, there appeared one report in 1 iterature from authors who claimed the succesful synthesis of TPP complexes in zeolite Y [52]. Anyway, the former authors also report that TPP complexes are much more difficult to incorporate in zeolite Y than Pc chelates. The relative amounts of occluded TMPc and TMTPP are given in Table 2, indicating that approximately 10 times higher loadings can be obtained with TMPc complexes.
Zeolites as Partial Oxygenation Catalysts 233
NITROGEN 0
HYDROGEN
P
TMPc
P
TMTPP
A
Structure of TMTPP, TMPc and TM(sa1en) complexes.
Scheme 2.
Table 2. Ratio (R) o f TMPc to TMTPP occluded in zeolite Y after synthesis (after [52]).
R
Transition metal
12.0
co Fe Mn Ru
6.3
15.8 8.0
The synthesis of phthallocyanines in zeolite Y has been the subject o f several pub1 ications [52-581. The different procedures are represented in Scheme 3. In principle, dicyanobenzene is reacted under inert conditions with transition metal zeolites. The latter can be prepared either via ion exchange, or via carbonyl or metallocene impregnation.
B
C
Scheme 3.
+ TM-carbony1
+
4 DCB
+ mctilloccnc
Procedures for synthesis of TMPc in zeolites [52-581.
234 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
The advantages and disadvantages of these three syntesis procedures are summarized in Table 3. Table 3.
Synthesis procedures of zeolite-encaged metallo phthallocyanines.
Synthesis procedure Transition metal source Synthesis temperature (K) Creation of acid sites Homogeneous TM distribution Non-chelated residual TM TM at the outer surface Pc without TM Complexity of procedure Toxicity Procedure for sil iceous zeolites
A
B
C
cheap 473-573
expensive 393-423
expensive 393-453
t t t
t
t ttt ttt
t t
tt t t
In principle the advantages of catalysis with Y zeolite-encaged complexes are the following [59-621: a. a broad range o f solvents are applicable in contrast to homogeneous catalysts which have a limited solubility; b. dimerization and cluster formation is inhibited because no more than one complex occupies each supercage; C. oxidation of Pc complexes is suppressed; d. shape selectivity exerted by the zeolite framework is possible; e. a broad range o f temperatures and pressures are applicable; f. the complexes are mainly fixed by spatial restrictions rather than by ionic and covalent interactions; as a result the zeolite behaves as a solid sol vent; high dispersion and uniform distribution of the active sites is possible. 9. Possible disadvantages of such systems are a lower intrinsic reactivity and deactivation by blockage of the zeolite pores. Molecular graphics techniques indicate that the metallophthallocyanine plane is deformed into a saddle type complex upon encaging in the super cages of zeolite Y [57,63]. EXAFS analysis shows a longer interatomic distance Fe to N, suggesting that the iron atom is slightly out of the phthallocyanine ring 1301. These "ship-in-bottle" complexes have been used for the oxidation of several types of organic substrates. TMPc complexes encaged in supercages o f zeolite Y and X have an improved stability and activity compared to nonsupported complexes, as is shown in Table 4.
Zeolites as Partial Oxygenation Catalysts 235
Activity ratio of zeolite-encaged to homogeneous transition metallo Table 4. phthallocyanines. TMPcZa
Substrate
Source of oxygen
FePcY FePcY COPCX Fet .BuPcYb FePcY FePcY
methylcyclohexane cumene ethyl benzene cycl ohexane cycl ohexane n-octane
PhIO air air PhIO PhIO t - BUtOOH
Activity ratio TMPcZ/TMPc 5.1 22.2 100 6.9 0.9 200
Reference 49-50,63-64 65 66 67 67 57
a; Z = zeolite b; Fet.BuPcY = Fe-tertiary butylphthallocyanine Porphyrines and phthallocyanines suffer from oxidative degradation and oxidative dimerization [68]. The improved activity of the zeolitic systems is due to the effective site isolati.on within the pores, which prevents any bimolecular pathways to catalyst destruction [63]. Therefore, deactivation is more severe for the homogeneous catalysts than for the heterogenized TMPc. FePc itself is a poor catalyst for alkane oxidation with a high initial turn-over, but after less than 45 minutes it becomes completely inactive. On the contrary, FePc encaged in zeolite Y is stable for 24 hours [63]. The use of iodosobenzene (PhIO) as oxidant with zeolite-encaged phthallocynanines causes supplementary problems. Indeed, PhI02 is produced in the pores of the zeolite by an oxygen transfer reaction between two molecules of PhIO: PhIO
+
PhIO
PhI
+
PhIOZ
PhI02 is rather bulky and plugs the pores, thus preventing further access of reactants to the active sites [49-50,63-641. Therefore turn-overs are quite low when PhIO is used as oxidant. For the oxidation of methylcyclohexane on TMPcY [49-50,63-641 and of cyclohexane on Fet.BuPcY [67] turn-overs are 5.6 and 7.6 respectively. It should be noted that the reported turn-overs for oxidations with PhIO correspond to conversions of less than 1 substrate molecule per two supercages, or to total conversions of less than 0.1 X . Therefore the observed activities and selectivities may be influenced by sorption effects. Furthermore iodosobenzene is a rather expensive oxidant and not practical to use because of its low solubility in solvents. Therefore some researchers tend to use other oxidantia such as air [65,66] and tertiary butylhydroperoxide (t-ButOOH) [57]. In the oxidation of n-octane with t-ButOOH turn-overs as high as 6000 have been reported [57].
236 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
Following the normal reactivity order, tertiary carbon atoms are more reactive than secondary ones, which in turn are far more reactive than primary ones [63-64,671. Turn-over numbers in the oxidation of methylcyclohexane with PhIO on FePcY decrease with increasing loadings of the phthallocyanine on the zeolite, as shown in Figure 4 [49-50,63-64,691. This is due to pore blockage by the catalyst molecules themselves. 8
10
100
1000
+3
FePc * 1 0 /supercage Figure 4. Turn-over in methylcylclohexane oxidation by PhIO on FePcY as a function of FePc loading (turn-over for homogeneous FePc= 1.1) [63]. The turn-over number o f zeol ite-encaged phthallocyanines is dependent on the nature of the metal complex as well as on the organic substrate. This i s shown in Figure 5 for the oxidation of hexane and cyclohexane with iodosobenzene. The turn-over numbers obtained on Fet.BuPcY are about 4 times higher than those on FePcY, and those of cyclohexane are approximately 10 times higher than those of n-hexane [67]. The activity of phthallocyanines is also influenced by the structure of the zeolite in which they are encaged, as shown in Figure 6 for the oxidation of noctane with t-ButOOH. FePcY is more active than FePcVPI-5 but the latter catalyst deactivates much slower [57].
Zeolites as Partial Oxygenation Catalysts 237
10
8 cyclohexmo b
a
L 0
6
I
el
s
cc
4
2
0
FePcY
Fet.BuPcY
Catalyst F i g u r e 5.
n-Hexane and cyclohexane o x i d a t i o n by PhIO on Fet.BuPcY and FePcY
r671. 15 I
aR
A
\
=0
A
FePcY A
FePcVPI-5 A
el 0
A
.CI
ra
A
b 4)
P
el 0
A
u a
A A
el
d c)
BI
a
-
A
0
0 0
Figure 6. The ketone/alcohol r a t i o i n methylcyclohexane o x i d a t i o n w i t h PhIO i s s y s t e m a t i c a l l y h i g h e r on TMPc z e o l i t e s than on t h e homogeneous FePc [63], as shown i n Figure 7. This i s explained by t h e authors by a s o r p t i o n e f f e c t , since
238 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
more ketones are formed on X than on Y z e o l i t e . Indeed, alcohols w i l l be p r e f e r e n t i a l l y h o l d on t h e more h y d r o p h i l i c X z e o l i t e , and w i l l t h e r e f o r e become prime substrates f o r f u r t h e r o x i d a t i o n , assuming t h a t alcohols and ketones are formed consecutively. 0.60
c,
d
1
::::I
0.40
0.1 0
0.00
FePc
FePcY ( 2 . 3 6 )
FePcX ( 1 . 2 6 )
Catalyst Figure 7. Ketone/alcohol r a t i o i n methylcyclohexane o x i d a t i o n by PhIO on homogeneous FePc and on FePc f a u j a s i t e s [63]. The ketone/alcohol r a t i o i s a l s o dependent on phthallocyanine complex and o f t h e substrate. Indeed,
t h e nature o f t h e Fet.BuPc favors the
formation o f alcohols i n t h e o x i d a t i o n o f cyclohexane w i t h PhIO, as shown i n Figure 8 [67], whereas t h e use o f n-alkanes w i t h i n c r e a s i n g c h a i n l e n g t h enhances t h e formation o f ketones on FePcY w i t h t-ButOOH as oxidant, as i s seen i n F i g u r e 9 [57]. The major advantage t h a t z e o l i t e s o f f e r over amorphous supports 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 . Herron and Tolman demonstrated t h a t o x i d a t i o n o f smaller substrates i s favored, as shown i n Figure 10 f o r t h e competitive o x i d a t i o n o f cyclohexane (cC6) and cyclododecane (cC12) and o f n pentane (n-C5) and n-octane (n-C8) by PhIO [49-50,63-641. T h i s r e a c t a n t shape s e l e c t i v i t y can be improved by i n c r e a s i n g t h e s t e r i c c o n s t r a i n t s near the a c t i v e s i t e s i n t h e z e o l i t e . Therefore z e o l i t e s exchanged w i t h c a t i o n s o f i n c r e a s i n g s i z e f a v o r t h e o x i d a t i o n o f t h e smaller hydrocarbons [49-50,63-641. This p o i n t i s c l e a r l y shown i n Figure 11.
Zeolites as Partial Oxygenation Catalysts 239
0 a n
Y
0.60
b CI
0
a
2
0.40
CI
fl
\
Q)
a 0
0.20
M 0.00
FePcY
Fet.BuPcY
Catalyst Figure 8. Ketone/alcohol r a t i o i n cyclohexane oxidation by PhIO on Fet.BuPc and FePc zeol it e s [ 6 7 ] . 20
-
16 -
12
-
8 -
a 0
4 -
0 -
n-CS
n-C6
n-C7
n-C8
n-C9
n-C10
Substrate Figure 9.
[571*
Ketone/alcohol
r a t i o i n n-alkane oxidation by t-ButOOH on FePcY
240 D. R. C. Huybrechts, R. F. Parton and P. A. Jacobs
2.50 8 0
. I
Y
d
2.00
zx
*
R \ f Icl
1
F
1.50
I .oo
0
0
* d
a n
0.50
& 0.00
FePcY
FePcX
FePc
Catalyst Figure 10. Competitive oxidation of cC6 and cC12 and o f n-C5 and n-C8 by PhIO on FePcY [63].
6 -
4 I
0.06
.
0.10
0.1 4
0.1 8
Ionic radius I nm Figure 11. Reactant shape 'selectivity in the oxidation of cC6 and cC12 on FePcY as a function of the size of the exchanged cations [69]. Several authors [49-50,57,61,63-64,671 claim that the zeolite structure favors regioselective oxidation of the outer carbon atoms of n-alkanes by imposing steric constraints on the reaction. As shown in Figure 12 this shape
Zeolites as Partial Oxygenation Catalysts 241
selectivity increases wlth the chain length of the n-alkane [57]. The direct influence o f the zeolite structure on the shape selectivity in octane oxidation by t-ButOOH is shown in Figure 13 where the regio selectivity is higher for FePc encapsulated in zeolite Y than for VPI-5 molecular sieves [ 5 7 ] . 70
aR \
60
50
h
Y
.a P
40
e n
.y
V 0)
n
30
Q)
rn
0 .a
M
4
20
10 0
n-C6
n-C7
n-ca
n-C9
n-C10
Substrate Figure 12. Regioselectivity in the oxidation of n-alkanes by t-ButOOH on FePcY
WI. aR \
60
70 50
h
Y
.a
P
n
40
.LII
Y
0 Q) II
30
Q)
rn 0
.m
M
4
20
10 0
FsPcY
FcPcVPI- 5
Catalyst Figure 13. Regioselectivity in the oxidation o f n-octane by t-ButOOH on FePcY and FePcVPI-5 [57].
242 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
S t e r e o s e l e c t i v i t y i s a l s o induced by t h e z e o l i t e framework surrounding t h e a c t i v e s i t e i n t h e o x i d a t i o n o f norbornane and methylcyclohexane by PhIO [4950,63-641. The z e o l i t e o r i e n t a t e s t h e incoming substrate i n such a way t h a t one o f t h e two d i a s t e r e o t o p i c C-H bonds has a g r e a t e r chance t o be oxidized. The u l t i m a t e goal o f using TMPc complexes encaged i n Y z e o l i t e s could be t h e o x i d a t i o n o f methane t o methanol a t low temperature. Cytochrome P-450 i s unable t o o x i d i z e methane [70], because i t cannot coordinate t h e molecule i n t h e hydrophobic pocket o f t h e enzyme. However methane monooxygenases [71-721, which are non-heme c o n t a i n i n g enzymes w i t h i r o n ions as a c t i v e s i t e can o x i d i z e methane. The major d i f f e r e n c e between cytochrome P-450 and methane monooxygenases i s . t h a t t h e p r o t e i n p a r t o f t h e l a t t e r enzymes more r e a d i l y coordinates C 1 t o C5 substrates whereas t h e p r o t e i n p a r t o f t h e former i s more s u i t e d f o r c o o r d i n a t i o n o f C3 and l o n g e r alkanes [73-741. Attempts t o o x i d i z e methane t o methanol on homogeneous metal l o porphyrine complexes [ 74-77] and non-porphyrine complexes [78] w i t h reasonable y i e l d s have been unsuccesful t i l l now. To enhance t h e residence time o f methane a t t h e a c t i v e s i t e t h e use o f zeol ite-encaged complexes could be a p o t e n t i a l s o l u t i o n . Chan and Wilson [52] t r i e d t o o x i d i z e methane t o methanol by oxygen on TMPcY and TMTPPY (TM= Co, Fe, Ru, Mn) i n t h e temperature range o f 548 K t o 773 K. Only RuPcY, CoTPPY and MnTPPY are a c t i v e towards alcohol formation, y i e l d s up t o 0.5% being claimed (Table 5). A l l other c a t a l y s t s g i v e combustion o f methane t o carbon d i o x i d e and water. I n an attempt t o repeat these experiments, t h e present authors o n l y observed CO, COP and H20 formation, and r a p i d autoxidation o f the catalyst. Table 5. Catalyst
Methane o x i d a t i o n by oxygen on TMPcY and TMTPPY [52]a. Conversion o f CH4 (%) co2
RuPcY CoTPPY MnTPPY
4.8 1.9 1.8
87 94 95
S e l e c t i v i t y o f (%) H20 CH30H 1 120 126
11.3 5.8 3.5
a; Conditions T= 623 K, P= 50 psig, CH4/O2= 4, G H W 2600 h - 1 A l i p h a t i c s i d e chains o f aromatics, such as cumene [65] and ethylbenzene [66] are o x i d i z e d t o t h e corresponding alcohols and ketones by oxygen on FePcY and CoPcY r e s p e c t i v e l y (Scheme 4). Propylene i s o x i d i z e d on CoPcX t o small amounts o f carbon d i o x i d e and acetone and h i g h e r amounts o f formaldehyde and acetaldehyde [79].
Zeolites as Partial Oxygenation Catalysts 243
Scheme 4.
Side chain o x i d a t i o n o f aromatics on TMPc f a u j a s i t e s [65-661.
The synthesis o f t h e Y zeolite-encapsulated manganese complex o f t h e salen l i g a n d has been r e p o r t e d r e c e n t l y [ 5 1 ] . It was found t o have c a t a l y t i c a c t i v i t y i n t h e o x i d a t i o n o f cyclohexene, styrene, and s t i l b e n e w i t h PhIO. T y p i c a l l y , 1 Mn(sa1en) i s present per 15 supercages, r e s u l t i n g i n c a t a l y t i c turn-overs i n t h e o r d e r o f 60. The r e a c t i o n s i n v e s t i g a t e d w i t h t h e r e s p e c t i v e product y i e l d s are given i n Scheme 5. Typical o x i d a t i o n products are epoxides, alcohols and aldehydes. I n comparison t o t h e homogeneous case encapsulation seems t o lower t h e r e a c t i o n r a t e . From cyclohexene t h e expected o x i d a t i o n product cyclohexene oxide i s present i n excess and i s formed on t h e Mn(sa1en) s i t e . 2-cyclohexene1-01 i s probably formed on r e s i d u a l Mn c a t i o n s v i a a r a d i c a l mechanism.
0
PhIO
,
Mn (salen)Y
6
(7.7%)
(5.5%)
(5.4%)
0 H 2 - C H 0
Scheme 5.
Alkene o x i d a t i o n on a Mn(sa1en) complex [Sl].
(7.4%)
244 D. R. C. Huybrechts, R. F. Parton and P. A. Jacobs
A completely inorganic mimic o f cytochrome P450 has been developed by Herron and Tolman [80-831. The mimic provides remarkable r e g i o s e l e c t i v i t y i n t h e p a r t i a l o x i d a t i o n o f octane and substrate s e l e c t i v i t y 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 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 . The oxidant, hydrogen peroxide, i s generated i n s i t u from equimolar amounts o f H2 and 02 on Pd c l u s t e r s . However, t h e o x i d a t i o n products are s t r o n g l y adsorbed i n t h e i n t r a c r y s t a l l i n e volume o f t h e z e o l i t e . The r a t i o o f t h e o x i d a t i o n products from octane and cyclohexane increases from 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 increase o f t h e o x i d a t i o n o f t h e primary compared t o t h e secondary carbon atoms o f octane i s observed w i t h respect t o Fe-silica-alumina. R e g i o s e l e c t i v i t y i s enhanced compared t o t h e TMPcY systems, b u t again t h e o v e r a l l conversions are extremely low. OXIDATIONS WITH ORGANIC PEROXIDES OR HYDROGEN PEROXIDE Co2+ and Cu2+ exchanged X and Y z e o l i t e s c a t a l y z e t h e decomposition o f t b u t y l hydroperoxide w i t h generation o f t - b u t o x y and t - b u t y l p e r o x y r a d i c a l s . When t h i s decomposition i s performed i n t h e presence o f o l e f i n s , such as cyclohexene o r l-octene, t h e corresponding epoxides are formed w i t h s e l e c t i v i t i e s ranging from 10 t o 50% based on decomposed t - b u t y l hydroperoxide [84]. For cyclohexene o x i d a t i o n t r a c e s o f 3 - t - b u t y l -peroxy-cyclohexene, 2 cyclohexene-1-01 and 2-cyclohexene-l-one were detected as s i d e products. COX z e o l i t e s are a l s o used f o r t h e o x i d a t i o n o f o l e f i n s w i t h t-ButOOH [85]. Butene i s o x i d i z e d t o methyl e t h y l ketone, presumably a r i z i n g from t h e acid-catalyzed rearrangement o f t h e peroxide. A1 keno1 s and aldehydes are formed i n lower amounts. Cyclopentene i s more r e a c t i v e towards o x i d a t i o n than butenes. Epoxidation o f o l e f i n s over Mo c o n t a i n i n g Y z e o l i t e s was s t u d i e d by Lunsford e t a1 [86-901. Molybdenum introduced i n u l t r a s t a b l e Y z e o l i t e through r e a c t i o n w i t h M o ( C O ) ~ o r MoC15, shows a high i n i t i a l a c t i v i t y f o r epoxidation
.
o f propylene w i t h t - b u t y l hydroperoxide as oxidant and 1,2-dichloroethane as s o l v e n t [88]. The r e a c t i o n i s proposed t o proceed v i a t h e formation o f a Mo6+t - b u t y l hydroperoxide complex and subsequent oxygen t r a n s f e r from t h e complex t o propylene. The c a t a l y s t s u f f e r s however from f a s t d e a c t i v a t i o n caused by i n t r a z e o l i t i c polymerization o f propylene oxide and r e s u l t i n g b l o c k i n g o f t h e active sites. When cyclohexene i s used as substrate, cyclohexene oxide i s formed w i t h s e l e c t i v i t i e s exceeding 95% a t conversions up t o 89% [89]. The a c t i v i t y increases w i t h i n c r e a s i n g Mo content and i s poisoned by t r i b u t y l a m i n e , b u t n o t by triphenylamine, which i s t o o b u l k y t o e n t e r t h e z e o l i t e pores. These r e s u l t s
Zeolites as Partial Oxygenation Catalysts 245
demonstrate t h a t the epoxidation mainly occurs a t Mo6t ions l o c a t e d i n t h e c a v i t i e s o f t h e z e o l i t e and n o t on t h e external surface. The small amounts o f 2-cyclohexene-1-01
and 2-cyclohexene-1-one
formed are ascribed t o o x i d a t i o n
with O2 d i s s o l v e d i n t h e reactants. Mo c o n t a i n i n g Y z e o l i t e s were a l s o t e s t e d f o r cyclohexene o x i d a t i o n w i t h oxygen as oxidant and t - b u t y l hydroperoxide as i n i t i a t o r [86]. I n t h i s case t h e s e l e c t i v i t y f o r cyclohexene oxide was maximum 50% 2-cyclohexene-1-01 and 2cyclohexene-1-one being t h e main s i d e products. The proposed r e a c t i o n scheme involves a f r e e r a d i c a l chain mechanism w i t h intermediate formation o f cyclohexenyl hydroperoxide. Coordination o f t h e hydroperoxide t o Mo6t i n the z e o l i t e and oxygen t r a n s f e r from t h e r e s u l t i n g complex t o cyclohexene i s b e l i e v e d t o be t h e major step f o r formation o f cyclohexene oxide under these conditions. S e l e c t i v e h y d r o x y l a t i o n o f phenol w i t h hydrogen peroxide was r e p o r t e d on a c i d z e o l i t e c a t a l y s t s [91-921. Peroxonium ions, formed by H202 p r o t o n a t i o n , are t h e o x i d i z i n g species. 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 a f a u j a s i t e c a t a l y s t , a m i x t u r e o f hydroxybenzenes and t a r s i s obtained [91]. I n the presence o f H-ZSM-5 on the other hand, no t a r formation was mentioned (which does n o t n e c e s s a r i l y mean t h a t i t was absent) and p - s e l e c t i v i t i e s c l o s e t o 100% were r e p o r t e d f o r t h e h y d r o x y l a t i o n [92]. These s u p e r i o r s e l e c t i v i t i e s r e f l e c t t h e shape s e l e c t i v e p r o p e r t i e s o f ZSM type z e o l i t e s . Oxidations o f various organic substrates w i t h aqueous hydrogen peroxide have been r e p o r t e d on t i t a n i u m c o n t a i n i n g d e r i v a t i v e s o f s i l i c a l i t e - 1 , denoted as T i t a n i u m - S i l i c a l i t e - 1 o r TS-1 [93-971. Examples o f r e a c t i o n s which are catalyzed by TS-1 w i t h h i g h H202 y i e l d s and product s e l e c t i v i t i e s are l i s t e d i n Table 6. The o x i d a t i o n s are g e n e r a l l y c a r r i e d o u t a t atmospheric pressure and a t temperatures ranging from 273 t o 373 K. Table 6.
TS-1 catalyzed o x i d a t i o n r e a c t i o n s . ~
React ants
Products
o l e f i n t H202 d i o l e f i n t H 02 o l e f i n t CH 8H t H202 aromatic t primary a l c o o t H202 secondary alcohol t H 02 c-hexanone t NH t H262 vinylbenzene t i 2 O 2 alkane t H202
epoxide t H20 monoepoxide t H 0 glycolmonomethyfether t H20 hydroxyaromat ic t H20 aldehyde t H20 ketone t 2 H20 c-hexanone-oxime t H20 6-phenylaldehyde t H20 a1 kanol s and a1 kanones t H20
itof
Reference 98-102 103 104 105-107 108 108 109- 111 112 113-114
246 D. R. C. Huybrechts, R. F. Parton and P. A. Jacobs
O l e f i n s and d i o l e f i n s are s e l e c t i v e l y (mono)epoxidized on TS-1 by H202 [981031. The o n l y byproducts r e s u l t from consecutive opening o f t h e epoxide r i n g , and t h e i r formation can be reduced by treatment o f t h e c a t a l y s t w i t h a l k a l i n e o r s i l y l a t i n g compounds [99]. I n t h e o x i d a t i o n o f C2-C4 o l e f i n s , f u r t h e r r e a c t i o n o f t h e epoxide w i t h methanol i s observed a t lOOOC and y i e l d s s e l e c t i v e l y glycolmonomethylethers [104]. For 1 i n e a r o l e f i n s t h e epoxidation r a t e decreases w i t h increasing chain length, and 2 - o l e f i n s are more r e a c t i v e than 1 - o l e f i n s [97,102]. C i s - o l e f i n s are s e l e c t i v e l y epoxidized i n t h e presence o f t r a n s - o l e f i n s w i t h r e t e n t i o n o f t h e stereo c o n f i g u r a t i o n [loo-1021. The isomer d i s t r i b u t i o n i n t h e h y d r o x y l a t i o n o f phenol, a n i s o l e and toluene o r o t h e r aromatics on TS-1 i s influenced by t h e r e a c t i o n conditions, b u t i s characterized by a tendency towards p - s e l e c t i v i t y [ 105-1061. Furthermore secondary r e a c t i o n s leading t o polynuclear aromatic byproducts are minimized. Both phenomena are ascribed t o t h e pore s t r u c t u r e o f t h e c a t a l y s t , which i s i s o s t r u c t u r a l t o ZSM-5 [96]. The s e l e c t i v i t y f o r h y d r o x y l a t i o n as w e l l as t h e H202 e f f i c i e n c y decrease w i t h increasing conversions as i s shown i n Figure 14 f o r t h e h y d r o x y l a t i o n o f phenol [106]. 70
aR
100
ertrcbol
60
80
\
n
0 m
90
50
aR
.m
L.
40
Q)
b
a 0 0
30 -4
0
I
0
a
20
i
Q)
c
-4
5:
2o
10 0 0.22
0.3 3
0.44
0.5 5
0.66
Molar feed ratio H202 / phenol Figure 14. Hydroxylation o f phenol by H202 on TS-1 as a f u n c t i o n o f t h e feed r a t i o H202/phenol [ 1061. Hydroxylation o f phenol o r phenol ethers w i t h H202 on a so c a l l e d ‘ t i t a n o z e o s i l i t e ’ , has also been reported and i s very s i m i l a r t o t h e TS-1 catalyzed r e a c t i o n [115]. The e s s e n t i a l d i f f e r e n c e between TS-1 and
Zeolites as Partial Oxygenation Catalysts 247
t i t a n o z e o s i l i t e seems t o be t h e p a r t i c l e s i z e and t h e c r y s t a l symmetry. I t should be stressed t h a t t h e t i t a n o z e o s i l i t e has been synthesized according t o t h e so c a l l e d f l u o r i d e method. Benzene was hydroxylated w i t h H202 on various z e o l i t e s [107]. The e f f i c i e n c y o f H202 use f o r h y d r o x y l a t i o n decreases i n t h e order TS-1 > Fe-TS-1
> A1-TS-1 > Fe-ZSM-5 > A1-ZSM-5, w h i l e t h e s e l e c t i v i t y f o r phenol i n t h e o x i d a t i o n products f o l l o w s t h e opposite order. It i s proposed t h a t f u r t h e r o x i d a t i o n o f t h e primary product (phenol), i s suppressed by i t s p r o t o n a t i o n over a c i d z e o l i t e s . The o x y f u n c t i o n a l i z a t i o n o f alkanes w i t h H202 on TS-1 has o n l y been r e p o r t e d very r e c e n t l y [113-1141. Linear o r branched alkanes are o x i d i z e d t o secondary and/or t e r t i a r y alcohols and ketones, t h e l a t t e r ones being formed by consecutive o x i d a t i o n o f t h e secondary alcohols. Primary alcohols are n o t detected. A t 50OC maximum turn-overs o f n-hexane o f 35 mol/mol T i were reported r1131 whereas a t l O O O C , turn-overs o f 1000 are obtained f o r t h e same substrate [1141 Table 7 shows the H202 y i e l d and s e l e c t i v i t i e s o f the o x i d a t i o n products f o r d f f e r e n t a1 kanes [ 1141. Tab1e 7.
T S - 1 catalyzed alkane o x y f u n c t i o n a l i z a t i o n s a .
Substrate
H 0
ieldb
2(;) n-pentane n- hexane n-octane n-decane 2 -met hy 1pent ane 3-methyl pentane
68 70 65 56 59 58
P o s i t i o n o f oxygenation (%) T o t a l products Ketone f r a c t i o n c2 c3 c4tc5 c2 c3 c4tc5 68 53 35
32 47 35
2a
25
85 44
15 56
-
-
30 47
-
-
71 64 61 59
73 100
29 36
22 18 27
-
-
-
17 23
-
a; Reaction c o n d i t i o n s : 400 mg TS-1, 310 mmol alkane, 210 mmol H202 (35% i n H20), 0.81 mol acetone, 3 hours 1OO'C a t 1000 RPM; b; H202 y i e l d = f r a c t i o n H 0 used f o r alkane o x i d a t i o n on t o t a l H202 conversion, H202 conversions h f g b r than 9W i n a l l r e a c t i o n s . The data i n Table 7 show t h a t t h e s e l e c t i v i t y f o r 2-oxygenated products i n the o x i d a t i o n o f alkanes on TS-I i s somewhat h i g h e r than c o u l d be expected on s t a t i s t i c a l grounds. Only f o r 3-methylpentane, t h i s s e l e c t i v i t y becomes overcompensated by t h e higher r e a c t i v i t y o f t e r t i a r y C-H compared t o secondary C-H p o s i t i o n s . T h i s i n d i c a t e s t h a t the f i r s t step o f the oxidation, i.e. t h e formation o f alcohols from alkanes i s s l i g h t l y r e g i o s e l e c t i v e . W i t h i n t h e ketone f r a c t i o n , t h e s e l e c t i v i t y f o r 2-ketones i s even more pronounced, i n d i c a t i n g t h a t 2-alcohols are s e l e c t i v e l y o x i d i z e d t o 2-ketones i n t h e
248 D. R. C. Huybrechts, R. F. Parton and P. A. Jacobs
presence o f o t h e r alcohols. Both e f f e c t s are ascribed t o t h e geometric c o n s t r a i n t s imposed on t h e r e a c t i o n by t h e pore s t r u c t u r e o f TS-1. The decrease i n H202 y i e l d w i t h i n c r e a s i n g dimensions o f alkanes i s explained by t h e lower o x i d a t i o n r a t e o f more bulky substrates, which f a v o r s t h e main s i d e r e a c t i o n , i . e . decomposition o f H202 t o 02 [114]. Based on XRD and IR i n v e s t i g a t i o n s , t h e s i t e s o f t i t a n i u m s u b s t i t u t i o n i n TS-1 are represented as [96]
B 4t,."t, g\ /"to' do
O\ 0
It i s proposed t h a t hydrated o r dehydrated t i taniumperoxo compounds are formed i n TS-1 by H202 chemisorption on t h e t i t a n y l (Ti=O) group, and t h a t these complexes c o n s t i t u t e t h e actual oxidants [96]. I n t h e p a r t i c u l a r case o f alkane o x i d a t i o n , a homolytic r e a c t i o n mechanism i s proposed, as i s t e n t a t i v e l y represented i n scheme 6 [114]. 0 II
Ti
H202
Scheme 6. [114].
OOH
M \ + H20
\ /
-
M
'Ti
OH
Ti
Ti
/
-*
H-c- \
\
d
Ti
O---O-H*/
\ C-
Ti
\
0 11
Ti
HO-C-
/ \
Formation o f titaniumperoxocompounds and alkane o x i d a t i o n on TS-1
An a l t e r n a t i v e synthesis method for t i t a n i u m s i l i c a l i t e , c o n s i s t i n g o f t r e a t i n g dealuminated ZSM-5 w i t h TiC14 vapor a t 550OC, has been described by Kraushaar e t a l . and by Kouwenhoven [116-1181. Acid treatment o f v a r i o u s MFI type T i - S i z e o l i t e s , presumably r e s u l t i n g i n an e l i m i n a t i o n o f occluded a l k a l i c a t i o n s and r e s i d u a l amorphous Ti02, has been r e p o r t e d t o improve both t h e a c t i v i t y and s e l e c t i v i t y i n t h e phenol h y d r o x y l a t i o n [119]. Fe, Ga and A1 c o n t a i n i n g t i t a n i u m s i l i c a l i t e s combine t h e c a t a l y t i c a c t i v i t y o f T i i n r e a c t i o n s w i t h H202 w i t h those o f Fe, Ga o r A1 i n a c i d catalyzed r e a c t i o n s [107,120-1221. Recently, t h e synthesis o f TS-2, a t i t a n i u m c o n t a i n i n g s i l i c a l i t e - 2 z e o l i t e w i t h c a t a l y t i c p r o p e r t i e s comparable t o those o f TS-I has a l s o been r e p o r t e d [123]. 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 hydrogen peroxide, TS-2 shows a c a t a l y t i c a c t i v i t y comparable t o t h a t o f TS-1, w i t h a 1 a t t h e end o f t h e r e a c t i o n . The formation o f hydroquinone/catechol r a t i o o f
-
Zeolites as Partial Oxygenation Catalysts 249
parabenzoquinone i n t h e i n i t i a l stage o f t h e phenol h y d r o x y l a t i o n on TS-2 i s unexpected and c o u l d n o t be explained [123]. O x i d a t i o n o f c y c l i c ketones by H202 i n t h e presence o f an a c i d z e o l i t e was used f o r t h e p r e p a r a t i o n o f lactones and w-hydroxycarboxyl i c acids [124]. Thus, f o r c-pentanone o x i d a t i o n i n t h e presence o f H-ZSM-5, 6 - v a ~ e r o ~ a c t o n ei s obtained with 62.3% s e l e c t i v i t y a t 40% conversion, whereas 5-hydroxy-pentoic a c i d i s obtained w i t h 34% y i e l d i n t h e presence o f Zeolon H. MISCELLANEOUS REACT IONS Analogous t o t h e a c i d catalyzed h y d r o x y l a t i o n o f phenol on H-ZSM-5 w i t h hydrogen peroxide [92], benzene and chlorobenzene can be o x i d i z e d w i t h d i n i t r o g e n o x i d e on HZSM-5 [125-1261. I n t h e h y d r o x y l a t i o n o f benzene, the s e l e c t i v i t y f o r phenol i s h i g h a t conversions below lo%, w i t h s e l e c t i v i t i e s i n N20 o f about 30%. A small amount o f ortho-diphenol o r catechol was formed. The h y d r o x y l a t i o n o f chlorobenzene was also o r t h o - s e l e c t i v e (58%). The r e a c t i o n was proposed t o proceed v i a scheme 7.
O Scheme 7.
+
O
H+ A H-ZSM-5 O -
H
+
H+
!action scheme f o r benzene h y d r o x y l a t i o n by N20 on H- SM-5.
Benzene was a l s o o x i d i z e d by O2 t o dehydroxybenzenes and/or polyphenylene hydrocarbons on h i g h s i l i c a z e o l i t e s [127]. Fe3+ i m p u r i t i e s and Lewis-acid s i t e s i n t h e c a t a l y s t p a r t i c i p a t e i n t h e r e a c t i o n through c a t i o n - r a d i c a l formation from benzene. Van Bekkum e t a1 [128] used COX f o r t h e decomposition r e a c t i o n o f t-ButOOH and t h e o x i d a t i o n o f 2,6-dialkylphenols. Mainly t h r e e products are formed as shown i n scheme 8. Homogeneous c a t a l y s t s have a very low s e l e c t i v i t y f o r 2,6-dial k y l -pbenzoquinone ( I ) , whereas zeoi it e s combine h i g h a c t i v i t i e s w i t h high s e l e c t i v i t i e s towards formation o f ( I ) . The formation o f p o l y ( 2 , 6 - d i a l k y l - l , 4 phenylene e t h e r ) (111) i s suppressed on z e o l i t e s by s t e r i c c o n s t r a i n t s . The formation o f 3,31,5,5’-tetraalkyl-4,4’-diphenoquinone (11) i s suppressed i n t h e supercages, b u t promoted by h i g h concentrations o f phenol.
250 D.R. C. Huybrechts, R. F. Parton and P. A. Jacobs
$,
+
H
t.BOOH
\
\
R'
Scheme 8. A l k y l a t i o n o f 2,6-dialkylpheno isopropyl o r t - b u t y l ) [ 1281
.
by t-ButOOH on COX
(R= methy
Pd and Cu i o n c o n t a i n i n g z e o l i t e s c a t a l y z e t h e vapor phase o x i d a t i o n o f m e t h y l p y r i d i n e s [129]. Thus, on PdCuNa-mordenite, 2-methylpyridine was o x i d i z e d t o 2-pyridinecarbaldehyde w i t h 40% y i e l d . The reduced r e a c t i v i t y r a t i o o f 2,6d i m e t h y l p y r i d i n e s t o monomethylpyridines on z e o l i t e c a t a l y s t s compared t o oxide c a t a l y s t s , demonstrates t h e presence o f s t e r i c c o n t r o l i n these r e a c t i o n s . CONCLUSIONS From t h e previous paragraphes i t f o l l o w s t h a t a s u b s t a n t i a l amount o f experimental data e x i s t t h a t i l l u s t r a t e t h e oxygenation p r o p e r t i e s o f z e o l i t e c a t a l y s t s . I n very general terms z e o l i t e s are used t o heterogenize t r a n s i t i o n metal ions i n i o n exchange o r l a t t i c e p o s i t i o n s , t o s t a b i l i z e t r a n s i t i o n metal oxide d i s p e r s i o n s and t o prepare s h i p - i n - b o t t l e complexes. The behavior o f t r a n s i t i o n metal ions exchanged i n z e o l i t e s i s very s i m i l a r t o t h a t i n a homogeneous medium: CuPdY z e o l i t e s are e f f i c i e n t s u b s t i t u t e s f o r Wacker chemistry i n absence o f c h l o r i d e ions. Titanium i n t h e framework o f p e n t a s i l z e o l i t e s induces oxygenation a c t i v i t y w i t h d i l u t e d hydrogen peroxide as oxidant, thus c o n s t i t u t i n g a new c a t a l y t i c system. For V2O5-P2O5 oxide p a r t i c l e s s t r o n g l y adsorbed i n a z e o l i t e a change i n the usual s e l e c t i v i t y has been observed. Indeed, t h e o x i d a t i o n o f butadiene w i t h dioxygen stops a t t h e l e v e l o f f u r a n and n o t a t maleic anhydride on V2O5P205-HY.
Zeolites as Partial Oxygenation Catalysts 251
Metallophthallocyanines, porphyrines and salen complexes encaged i n mainly Y z e o l i t e s have been r e p o r t e d t o be a c t i v e and shape s e l e c t i v e i n t h e o x i d a t i o n o f a1 kanes and a1 kenes. ACKNOWLEDGEMENTS D.R.C.H. and R.F.P. acknowledge t h e support o f t h e National Fund f o r S c i e n t i f i c Research (Belgium).
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
J.W. Ward, Appl. Ind. Catal., 3 (1984). 271. Y. Ben T a a r i t and M. Che, Stud. Surf. Sci. Catal., 5 (1980) 167. D.B. Tagiev and Kh.M. Minachev, Russ. Chem. Rev., 50 (1981) 1009. I . E . Maxwell, Adv. Catal., 31 (1982) 1. W. Holderich, M. Hesse and F. Naumann, Angew. Chem., 100 (1988) 232. D.B. Tagiyev and K.M. Minachev, i n Y. Murakami, A. I i j i m a and J.W. Ward (Eds.), New Developments i n Z e o l i t e Science and Technology (Proc. 7 t h I n t . Zeolite Conf., Tokyo, August 17-22, 1986), Kodansha/Elsevier, Tokyo/Amsterdam, 1986, p. 981. J. Rouchaud, L. Sondengam and J.J. F r i p i a t , B u l l . SOC. Chim. France, 11 (1968) 4387. J. Rouchaud, L. Sondengam and J.J. F r i p i a t , B u l l . SOC. Chim. Belges, 77 (1968) 505. U.S. Pat., 3, 013, 985 (1961). N. Giordino, S. M o n t e l a t i c u and C. Zen, i n Proceedings o f t h e T h i r d 1973, Leuven I n t e r n a t i o n a l Conference on Molecular Sieves, Zurich, U n i v e r s i t y Press 1973, p. 449. G. Bagnasco, P. Ciambelli, E. Czaran, J. Papp and G . RUSSO, Stud. S u r f . Sci. Catal., 12 (1982) 275. I.Mochida, S. Hayata, A. Kato and T. Seiyama, J. Catal., 15 (1969) 314. I.Mochida, S. Hayata, A. Kato and T. Seiyama, J. Catal., 19 (1970) 405. I . Mochida, S. Hayata, A. Kato and T. Seiyama, J. Catal., 23 (1971) 31. I.Mochida, S. Hayata, A. Kato and T. Seiyama, B u l l . Chem. SOC. Japan, 44 (1971) 2282. F . F a j u l a , R. Ibarra, F. Figueras and C. Gueguen, J. Catal., 89 (1984) 60. M. Iwamoto, M. Tajima and S. Kagawa, J. Chem. SOC., Chem. Commun., (1985) 228. M. Iwamoto, M. Tajima and S. Kagawa, J. Catal., 101 (1986) 195. K. Eguchi, T. T o k i a i , Y. Kimura and H. Arai, Chem. L e t t e r s , (1986) 567. J. Rouchaud and J. F r i p i a t , B u l l . SOC. Chim. France, 1 (1969) 78. R.F. Howe and H. Minming, Proc. 9 t h I n t . Congr. Catal., 4 (1988) 1585. T. Kubota, F . Kumada, H. Tominaga and T. Kunugi, I n t . Chem. Eng., 13 (1973) 539. H. Arai, T. Yamashiro, T. Kubo and H. Tominaga, B u l l . Jap. P e t r . I n s t . , 18 (1976) 39. T.N. Shakhtakhtinskii, F. Beni Kharri, A.M. A l i e v , A.R. K u l i e v , V.P. Kasamanskii and I.M. Peresada, Azerb. Khim. Zhur., (1979) 41. T.N. Shakhtakhtinskii, F. Beni K h a r r i , A.M. A1 iev, A.R. Kul i e v , V.P. Kasamanskii and R.Z. Mikhailov, Azerb. Khim. Zhur., (1979) 51. Kh.M. Minachev, N.Ya. Usachev, Ya.1. Isakov, A.P. Rodin and V.P. Ka i n i n , I z v . Akad. Nauk SSSR, Ser. Khim., (1981) 724. Kh.M. Minachev, N.Ya. Usachev, A.P. Rodin and Ya.1. Isakov, I z v . Akad Nauk SSSR, Ser. Khim., (1982) 1975. Kh.M. Minachev, N.Ya. Usachev, A.P. Rodin and Ya.1. Isakov, I z v . Akad Nauk SSSR, Ser. Khim., (1982) 132.
252
D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
29
N. Ya. Usachev, A.P. Rodin, Yu.S. Khodakov and Kh.M. Minachev, I z v . Akad. Nauk SSSR, Ser. Khim., (1984) 747. B. Elleuch, C. Naccache and Y. Ben T a a r i t , Stud. Surf. Sci. Catal., 19 (1984) 139. K. Eguchi, 1. T o k i a i and H. Arai, J. Catal., 111 (1988) 457. V.V. Kharlamov, S.F. P o l i t a n s k i i , A.M. Shkitov, S.Yu. Panov, Kh.M. Minachev and O.M. Nefedov, I z v . Akad. Nauk SSSR, Ser. Khim., (1981) 1178. Kh.M. Minachev, O.M. Nefedov, V.V. Kharlamov, S.Yu. Panov and S.A. P o l i t a n s k i i , I z v . Akad. Nauk SSSR, Ser. Khim., (1981) 1490. Kh.M. Minachev, O.M. Nefedov, V.V. Kharlamov, S.Yu. Panov, A.M. Shkitov and M . I . Trofimov, I z v . Akad. Nauk SSSR, Ser. Khim., (1983) 2688. Kh.M. Minachev, O.M. Nefedov, V.V. Kharlamov and S.Yu. Panov, Proc.-Sov.I n d i a n Semin. Catal., Catal. Prog. Chem. Eng., l s t , (1984) 29. Kh.M. Minachev, O.M. Nefedov, V.V. Kharlamov, 0. Chizhov, A.P. Rodin and V . I . Kadentsev, I z v . Akad. Nauk SSSR, Ser. Khim., (1985) 1253. Kh.M. Minachev and V.V. Kharlamov, Proc. ZEOCAT Symp., Siofok, Hungary, 1985, Acta Phys. Chem., 31 (1985) 497. V.V. Kharlamov, A.G. Pogorelova, S. Yu. Panov, T.V. Yanchevskaya and Kh.M. Minachev, I z v . Akad. Nauk SSSR, Ser. Khim., (1988) 2694. Kh.M. Minachev and V.V. Kharlamov, Rev. Roum. Chim., 34 (1989) 527. G.C. Centi, Z. Tvaruzkova, F. T r i f i r o , P. J i r u and L. Kubelkova, Appl. Catal., 13 (1984) 69. Z. Tvaruzkova, G. Centi, P. J i r u and F. T r i f i r o , Appl. Catal., 19 (1985) 307. G. Centi, P. J i r u and F. T r i f i r o , Stud. Surf. Sci. Catal., 44 (1988) 247. F. T r i f i r o and P. J i r u , Catal. Today, 3 (1988) 519. K. Habersberger, P. J i r u , Z. Tvaruzkova, G. Centi and F. T r i f i r o , React. K i n e t . Catal. L e t t . , 39 (1989) 95. P.S. Belov, S.A. Nizova, R . I . Fedorova, V.V. M i t i n a and A.V. Pleshivtseva, Zh. P r i k l . Khim. (Leningrad), 62 (1989) 353. 8. Meunier, B u l l . SOC. Chim. France, (1986) 578. D. Mansuy, Pure & Appl. Chem., 59 (1987) 779. I.Tabushi, Coord. Chem. Rev., 86 (1988) 245. C.A. Tolman and N. Herron, ACS Prep. Div. Petr. Chem., 32(3) (1987) 798. C.A. Tolman and N. Herron, Catal. Today, 3 (1988) 235. C. Bowers and P.K. Dutta, J. Catal., 122 (1990) 271. Y.-W. Chan and R.B. Wilson, Jr., ACS Prep. D i v . P e t r . Chem., 33(3) (1988) 453. G. Meyer, D. Wohrle, M. Mohl and G. Schulz-Ekloff, Z e o l i t e s , 4 (1984) 30. A.N. Zakharov and B.V. Romanovsky, J. I n c l u s . Phenom., 3 (1985) 389. A.N. Zakharov, T.V. Korol 'kova and B.V. Romanovskii, Koord. Khim., 12 (1986) 94. A.N. Zakharov, B.V. Romanovskii, D. Luca and V . I . Sokolov, M e t a l l o r g . Khim., 1 (1988) 119. R.F. Parton, L. Uytterhoeven and P.A. Jacobs, Second I n t . Symp. Heterog. Catal. & Fine Chemicals, P o i t i e r s , (1990). 1. Kimura, A. Fukuoka and M. Ichikawa, Catal. L e t t . 4, (1990) 279. 8.V. Romanovsky, Proceed. 8 t h I n t . Congr. Catal., Verlag Chemie, Weinheim, 4 (1984) 657. B.V. Romanovsky, Acta Phys. Chem., 31 (1985) 215. 8.V. Romanovsky, i n Yu. Yermakov, V. Likholobov (Eds.), Proceed. 5 t h I n t . Symp. on Relations between Homog. and Heterog. Catal., VNU Science Press, 1986 p. 343. B.V. Romanovsky, A.N. Zakharov, T.V. Korolkova and A.G. Gabrielov, I n t . Heterog. Catal. Proc. 6 t h Confer., 2 (1987) 31. N. Herron, 3. Coord. Chem., 19 (1988) 25. N. Herron. A.D. Stuckv and C.A. Tolman. J. Chem. SOC., Chem. Commun.. (1986) 1521. T.V. Korol 'kova and 8.V. Romanovskii, Neftekhimiya, 26 (1986) 546.
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
55 56 57 58 59t 60 61 62 63 64 65
Zeolites as Partial Oxygenation Catalysts 253
66 G. Schulz-Ekloff, D. Wohrle, V. I l i e v , E. Ignatzek and A. Andreev, Stud. Surf. Sci. Catal . , 46 (1989) 315. 67 T. Kimura, A. Fukuoka and M. Ichikawa, Shokubai, 31 (1989) 357. 68 M.J. Nappa and C.A. Tolman, Inorg. Chem., 24 (1985) 4711. 69 R.F. Parton, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs, Stud. Surf. Sci. Catal , 46 (1989) 163. 70 P.R. O r t i z de Montellano, Cytochrome P-450: Structure, Mechanism and Biochemistry, Plenum Press, New York, (1986). 71 H. Dalton, Adv. Appl. Microbiol., 26 (1980) 71. 72 R.N. Patel, Chem. I n d u s t r . , Catal. Org. React., R.L. Augustine (Ed.), 22 (1985) 327. 73 A.M. Khenkin and A.A. Shteinman, K i n e t . Katal., 30 (1989) 7 ( t r a n s l a t i o n (1989) 1). 74 R.H. Fish, R.H. Fong, R.T. Price, J.B. Vincent and G. Christou, ACS Symp. Ser., B i o c a t a l . & Biomimetics, 392 (1989) 116. 75 V.S. Belova, A.M. Khenkin and A.E. Shilov, K i n e t . Katal., 28 (1987) 1016 ( t r a n s l a t i o n (1987) 897). 76 F.V. Stohl, J.A. Shelnutt, B. Granoff and D.E. T r u d e l i , Prep. Papers Am. Chem. SOC. Fuel Chem., 32 (3) (1987) 280. 77 F.V. Stohl, J.A. Shelnutt and B. Granoff, Proc. 9 t h I n t . Congr. Catal., Calgary, Vol. 3 (1988) 982. 78 V.S. Belova, A.M. Khenkin and A.EA. Shilov, Kinet. Katal., 29 (1988) 1279 ( t r a n s l a t i o n (1988) 1115). 79 H. Diegruber, P.J. Plath, G. S c h u l z - E k l o f f and M. Mohl, J. Mol. Catal., 24 (1984) 115. 80 N. Herron and C.A. Tolman, ACS Prep. Div. Petr. Chem., 32(1) (1987) 200. 81 N. Herron and C.A. Tolman, J. Am. Chem. SOC., 109 (1987) 2837. 82 N. Herron, Chemtech, September (1989) 542. 83 N. Herron, A.C.S. Symp. Ser., 392 (1989) 141. 84 K. -H. Bergk, F. Wolf and B. Walter, J. Prakt. Chem., 321 (1979) 529. 85 D.E. Van S i c k l e and M.L. Prest, J. Catal., 19 (1970) 209. 86 P.E. Dai and J.H. Lunsford, J. Catal., 64 (1980) 173. 87 P.E. Dai and J.H. Lunsford, J. Catal., 64 (1980) 184. 88 M.B. Ward,, K. Mizuno and J.H. Lunsford, J. Mol. Catal., 27 (1984) 1. 89 J.R. Sohn and J.H. Lunsford, J. Mol Catal , 32 (1985) 325. 90 J.H. Lunsford, Report 1988, from Gov. Rep. Announce. Index ( U . S . ) 88(22), a b s t r . No 885,205, (1988). 91 H.S. Bloch, U.S. Pat., 3, 580, 956 (1971). 92 C.D. Chang and S.D. H e l l r i n g , U.S. Pat., 4, 578, 521 (1986). 93 M. Taramasso, G. Perego and 8. N o t a r i , U.S. Pat., 4, 410, 501 (1983). 94 G. Belussi, M. C l e r i c i , F. Buonomo, U. Romano, A. Esposito and B. N o t a r i , Eur. Pat., 0 200 260 (1986). 95 G. Perego, G. B e l l u s s i , C. Corno, M. Taramasso, F. Buonomo and A. Esposito, i n Y. Murakami, A. I i u i m a and J.W. Ward (Eds.), New Developments i n Z e o l i t e Science and Technology (Proc. 7 t h Z e o l i t e Conf., Tokyo, August 17-22, 1986), Kodansha/El sevier, Tokyo/Amsterdam, 1986, p. 129. 96 B. N o t a r i , Stud. S u r f . Sci. Catal., 37 (1987) 413. 97 U. Romano, A. Esposito, F. Maspero, C. N e r i and M.G. C l e r i c i , Stud. Surf. Sci Catal , 55 (1990) 33. 98 C. N e r i , B. Anfossi, A. Esposito and F. Buonomo, Eur. Pat., 0 100 119 (1984). 99 M.G. C l e r i c i and U. Romano, Eur. Pat., 0 230 949 (1987). 100 M.G. C l e r i c i and G. B e l l u s s i , Eur. Pat., 0 315 247 (1989). 101 M.G. C l e r i c i and G. B e l l u s s i , Eur. Pat., 0 315 248 (1989). 102 T. Tatsumi, M. Nakamura, K. Yuasa and H. Tominaga, Chem. L e t t . , (1990) 297. 103 F. Maspero and U. Romano, Eur. Pat., 0 190 609 (1986). 104 C. Neri, F. Buonomo and B. Anfossi, Eur. Pat., 0 100 118 (1984). 105 A. Esposito, M. Taramasso and C. N e r i , Deutsches Pat., 31 35 559 (1982).
.
.
.
.
.
254 D. R. C. Huybrechts. R. F. Parton and P. A. Jacobs
106 A. Esposito, C. Neri, F. Buonomo and M. Taramasso, U.K. Pat., 2 116 974 (1983). 107 A. Thangaraj, R. Kumar and P. Ratnasamy, Appl. Catal., 57 (1990) L1. 108 A. Esposito, C. N e r i and F. Buonomo, Eur. Pat., 0 102 655 (1984). 109 P. R o f f i a , M. Padovan, E. M o r e t t i and 6. De A l b e r t i , Eur. Pat., 0 208 311 (1987). 110 P. R o f f i a , G. Paparatto, A. Cesana and 6. Tauszik, Eur. Pat., 0 301 486 (1989). 111 P. R o f f i a , G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, 6. P e t r i n i , S. T o n t i and P. Gervasutti, Stud. Surf. Sci. Catal., 55 (1990) 43. 112 C. N e r i and F. Buonomo, Eur. Pat., 0 102 097 (1986). 113 T. Tatsumi, M. Nakamura, S. Nagishi and H. Tominaga, J. Chem. SOC., Chem. Commun. (1990) 476. 114 D.R.C. Huybrechts, L. De Bruycker and P.A. Jacobs, Nature, 345 (1990) 240. 115 M. C o s t a n t i n i , J.-M. Popa and M. Gubelmann, Eur. Pat., 0 314 582 (1989). 116 B. Kraushaar and J.H.C. Van Hooff, Catal. Lett., 1 (1988) 81. 117 B. Kraushaar-Czarnetzki and J.H.C. Van Hooff, Catal. Lett., 2 (1989) 43. 118 C. F e r r i n i and H.W. Kouwenhoven, Stud. Surf. Sci. Catal., 55 (1990) 53. 119 J. E l Hage-A1 Asswad, J.B. Nagy, Z. Gabelica and E.G. Derouane, i n " Z e o l i t e s f o r t h e n i n e t i e s " (Recent Research Reports presented d u r i n g t h e 8 t h I n t . Z e o l i t e Conf., Amsterdam, J u l y 10-14, 1989), 1989 475. 120 G. B e l l u s s i , M.G. C l e r i c i , A. G i u s t i and F. Buonomo, Eur. Pat., 0 226 258 (1987). 121 G. B e l l u s s i , M.G. C l e r i c i , A. C a r a t i and A. Esposito, Eur. Pat., 0 266 825 (1988). 122 A. G i u s t i , S. Gusi, G. B e l l u s s i and V. Fattore, Eur. Pat., 0 293 950 (1988). 123 J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 58 (1990) L1. 124 C.D. Chang and S.D. H e l l r i n g , U.S. Pat., 4, 870, 192 (1989). 125 E. Suzuki, K. Nakashiro and Y. Ono, Chem. Lett., (1988) 953. 126 Y. Ono, K. Tohmori, S. Suzuki, K. Nakashiro and E. Suzuki, Stud. S u r f . Sci. Catal , 41 (1988) 75. 127 V.L. Zholobenko, L.M. Kustov and V.B. Kazanskii, K i n e t . Katal., 30 (1989) 901. 128 J.C. Oudejans and H. Van Bekkum, J. Mol. Catal., 12 (1981) 149. 129 L.Ya. L e i t i s , K.I. Rubina, M.V. Shimanskaya, V.V. Kharlamov and Kh.M. Minachev, React. K i n e t . Catal. L e t t . , 29 (1985) 175.
.
'55
Kinetics and Mechanism of Paraffin Cracking with Zeolite Cata1ysts
W.O.Haag, R. M. Dessau and R.M.Lago Mobil Research & Development Corp. Princeton, New Jersey, USA
ABSTRACT Acid catalyzed cracking of simple paraffins at high temperature and low partial pressure does not occur by the classical bimolecular carbenium chain reaction, but by protonation of the paraffins to produce penta-coordinatedcarbonium ion intermediates. Under limiting conditions, the kinetics of the two reaction paths both follow a first order rate law, although with very different activation energies. In the intermediate regime, the kinetics is quite complex. Hexane cracking offers several advantages as a means to characterize acid catalysts, and is best carried out at high temperature and low partial pressure. INTRODUCTION The catalytic cracking of hydrocarbons including paraffins is one of the most important commercial processes. Fundamental studies of the mechanism of paraffin cracking with solid catalysts in the last fifty years have been one of the cornerstones in creating our present knowledge of solid acid catalysis and carbenium ion chemistry. Furthermore, the cracking of representative light paraffins, especially of hexane [l-4, 121, has become a widely used characterization tool for acid catalysts including zeolites. On the basis of detailed mechanistic studies, it was previously shown [6] that under some reaction conditions the classical cracking mechanism is inappropriate; a novel mechanism involving pentavalent protonated paraffins has been proposed that has been widely accepted since. This paper presents further insight into details of this new mechanism and addresses key kinetic issues arising from the duality of cracking mechanisms.
256 W. 0. Haag, R. M. Dessau and R. M. Lago
THE TWO CRACKING MECHANISMS The previous detailed discussion [6] is briefly summarized here. The classical mechanism [7joccurs according to a carbenium ion chain reaction, illustrated with nhexane as the feed:
+ c-c-c-c-c-c + c-c-c + c-c-c-c-c-c+
HT
+
+ c-c-c-c-c
+ c-c-c-c-c-c+ c-c-c
p-Scission
+ c-c-c + c=c-c
(11
(2)
Since n-hexene, which involves the same hexyl carbenium ion intermediate, cracks about 260 times faster than hexane [8], step (2) is very rapid, and the rate determining step is
the hydrogen transfer reaction (1). For this reason this cracking pathway is referred to as bimolecular cracking. The origin of the first chain-initiating carbenium ion in step (1) has not been clearly identified so far. While olefinic products can undergo facile secondary reactions, the saturated products are relatively unreactive and are diagnostically significant; they consist predominantlyof propane and butanes rich in isobutane, independent of the structure of the feed paraffin. Cracking according to the second mechanism occurs by direct protonation of the paraffin with the solid catalyst acting as a superacid:
The protonated paraffin can be envisioned as a mixture of sigma-protonatedspecies containing three-center two-electron bonds that can be conveniently represented as a penta-coordinatedcarboniun ion [9]. Products are formed by collapse of the latter. Since
Kinetics and Mechanism of Paraffin Cracking 257
the rate determining step involves the interaction of a single paraffin molecule with the active site, cracking by this reaction path is called monomolecular cracking. The nature of the saturated products varies greatly with the feed, as shown for
cracking at 538°C with ZSMd (mol%):
from Hexane from3-Me-Pentane
29 56
11 21
30 21
22 1
7 1
0.5
-
Noteworthy are the formation of methane and ethane as well as of hydrogen. MONOMOLECUIAR CRACKING The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure andor low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5+ in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [lo]. The C6H15+ ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the
- z-
Fig. 1. Generation and collapse of C6H15+ ion formed by chemical ionization (CH5+) or by hydrogen zeolite (HZ). specie produced in the mass spectrometer is a free carbonium ion, whereas in the zeolite it has to be viewed as a more or less tightly bound ion pair. Quantitative comparisons of the product selectivities are not presently available but should prove instructive. Kinetics Over HZSM-5 catalyst at 538"C, n-hexane cracking follows a first order kinetic rate
258 W. 0. Haag, R. M. Dessau and R. M. Lago
law. This is apparent from Fig. 2a, which shows that the first order rate constant is independent of pressure from 10 to 250 torr, i.e., the rate is strictly propoltional to pressure. When at a constant pressure of 10 ton the conversion was varied by changing the contact time, again good first order behavior was observed (Fig. 2b). Similar
rI
(I)
4-
J 2I
0
I
I
Contact Time, W / F
Hexane Pressure, torr
Fig. 2. Hexane cracking with HZSM-5 (Si/AI = 35) at 538°C. a. First order rate constant determined at different hexane pressures. b. First order plot of conversion (E) at different contact times, hexane pressure = 10 torr. adherence to first order kinetics has been reported previously with hexane at 100 torr and 538°C [8],i.e., alpha test conditions [2]. These findings are at variance with recent literature suggestions that a simple first order kinetic model, as used in the alpha test, may be inadequate [4]. They also show that inhibition by olefins, observed under different conditions [4), is not important under alpha test conditions. The saturated reaction products methane to n-butane are formed in strictly parallel reactions. (Fig. 3) 7 r
6
s - 4
r"
2 0
0
10
20
30
Conversion, Yo Fig. 3. Yields of paraffin products from hexane cracking (HZSM-5, SVAI = 35, 538OC, 10 torr hexane).
Kinetics and Mechanism of Paraffin Cracking 259
BIMOLECUIAR CRACKING The conditions where the bimolecular reaction path predominates are low temperature and high olefin concentration. Although both mono- and bimolecular limiting conditions can be experimentally realized to a good approximation, experiments are often carried out under conditions were both mechanisms contribute to product formation and the kinetics is complex. For example, kinetic evaluation of hexane cracking at 37OOC and 150 torr hexane pressure shows thzt initially the reaction is slow and then accelerates (Fig. 4). 0 20
0.2
$
cr'
i 2
.-0
t E 0.4
f 10
I
c
8
0.6 0 0
0.5
1.o
Contact Time, s
0
1.O
2.0
Contact TIM, s
Fig. 4. Autocatalysis in hexane cracking (HZSM-5 Si/AI = 35, 370°C, 150 torr hexane). a. Conversion vs. contact time. b. First-order plot. We can identify this autocatalytic-typebehavior with a change in mechanism: initially cracking proceeds via the monomolecular path; once some olefinic products are made, they can initiate bimolecular cracking. A similar acceleration of cracking by olefins has been reported previously [5].Thus, direct paraffin protonation and cracking to olefins provides the long-sought initiators for the chain reaction of the classical cracking mechanism. Support for this change in mechanism with conversion has been provided previously [S] from the corresponding change in product distribution. It is also apparent from Fig. 4 that the bimolecular reaction occurs faster than the monomolecular initiation step. The duality of cracking mechanisms is summarized in Fig. 5, where RH = paraffin feed,
R1-C=C = olefinic product, KO= equilibrium constant of olefin chemisorption. Free Bronsted acid sites HZ interact directly with the paraffin feed by protonation, producing monomolecular cracking. When the acid sites are covered with adsorbed olefins to form
260 W. 0. Haag, R. M. Dessau and R. M. Lago
carbenium ion-like olefin complexes or alkoxy species, cracking proceeds via the classical bimolecular path involving hydrogen transfer. The fraction of acid sites (0) covered by
Monomolecular Mechanism Via Penta-Coordinated Carbonium Ion
Bimolecular Mechanism Via Tri-Coordinated Carbenium ion
Fig. 5. Dual mechanism of paraffin cracking. chemisobed olefins (R+) is assumed to be described by a Langmuir adsorption isotherm 0 = Ko[O] / (1 + Ko[O]. The associated rate equations are as follows (rl, r2 = rates of
mono- and bimolecular cracking, respectively):
r 2 = k2[RH][R+] = k RH K,[OI 2[ l+Ko[O]
K[ol r1 = kl [RH][H+] = k RH '[ ('- :K l o[O]
(3)
1 ) = k RH '[ l+Ko[O]
(4)
This kinetic expression has two limiting cases. At high temperature (KO small) and/or low paraffin pressure and conversion ([O] small):
Kinetics and Mechanism of Paraffin Cracking 261
At low temperature (large KO) and high paraffin pressure and conversion ( 101large):
r xr2 = k2[RH]
(7)
In the limiting cases (6) and (7),simple first order kinetics is expected. In the intermediate case (5), the kinetics is complex as seen in Fig. 4. At low contact time and conversion, r2 is negligible and the slope of the conversion curve represents kl ; at high conversion, the slope reflects k2. In this regime it is not admissible to do a single experiment and from the observed conversion calculate a rate constant by assuming first order kinetics. Apparent "rate constants" calculated in this way yield variable values that depend on the arbitrarily chosen conversion (slope of dashed lines, Fig. 4). The fraction of the total reaction following the bimolecular path is given by
At low conversion ([O] small) - % -
'2
k2
1 ' +'2
kl
KofO] -,c[RHJo E
where k2, kl, KOare constants at a given temperature, and where the olefin concentration [O] is approximately proportionalto the initial paraffin concentration [RH], and the conversion E. This change in mechanism with increasing conversion and increasing paraffin pressure has been verified previously [6] by the change in product distribution upon cracking of 3-methylpentane; the selectivity to propane and butane, products of bimolecular cracking, increases linearly with conversion and with initial paraffin pressure, in accordance with equation (9). Similar data were obtained from the cracking of hexane. Activation Enerqy While under the limiting conditions (6) and (7) simple first order kinetics applies, the
262 W. 0. Haag, R. M. Dessau and R. M. Lago
corresponding rate constants represent different reactions, i.e., paraffin protonation and hydrogen transfer, respectively, and hence can have different activation energies. Indeed, the Arrhenius plot for the cracking of hexane with ZSM-5, (Fig. 6), shows an Temperature, OC 300
540 482
240
Ea = 30 kcal/mole 1 .o
1.2
1.4
1.6
1.8
2.0
1o ~ / T ( K )
Fig. 6. Temperature dependence of first-order rate constant for hexane cracking (HZSMQ, 150 torr hexane). activation energy of 30 kcal/mol for the monomolecular path at high temperature and an approximate value of 6.5 kcal/mol for the bimolecularcracking at low temperature. This low value is reasonable in view of the concerted nature of the hydrogen transfer step. PARAFFIN CRACKING AS A CATALYST CHARACTERIZATIONREACTION Several different test reactions have been suggested to evaluate the catalytic activity of an acid catalyst as a measure of the number and strength of the active sites. The ideal
test reaction is experimentally easy, fast, reproducible, requires only a small amount of catalyst, has simple kinetics, and should show little deactivation. It should also not be diffusion limited and affected by the particle or crystal size. While no one reaction fits all
-
-
these criteria perfectly, we and apparently others find that hexane cracking comes closer to the ideal than most other reactions. The rate constant for hexane cracking is calculated from k = -F/W In(1-&) where F is the total gas flow rate in ml s-l, at the reaction temperature, W is the weight of catalyst in
Kinetics and Mechanism of Paraffin Cracking 263
grams and E is the fractional conversion. Thus, the unit of k is ml g-ls-l. The rate constant per unit volume of catalyst (units: s-l)can be obtained by multiplying k by the catalyst density. In principle, k can be determined at any suitable temperature. However this makes it difficult to compare results from different laboratories and/or for different catalysts if they were obtained at different temperatures. For this reason it has been suggested to determine the hexane cracking rate at standard temperature Tst of 538°C (811K, 1000°F). The value of that rate constant, relative to that of a standard SiO2/AI2O3
cracking catalyst with a rate constant k = 0.016 s-l,is called the a-value [l]. The rate constant at Tst can be obtained in one of two ways. It can be measured at any temperature and extrapolated to the standard temperature. However, this requires a knowledge of the appropriate activation energy. For the limited temperature range and catalyst types available earlier, an activation energy of 30 k c a h o l was found appropriate
[l]. As other catalysts became available and measurements are carried out at lower temperature, the applicable activation energy is no longer constant. As shown in Fig. 6, the assumption of a constant activation energy, for ZSM-5, is valid only between 538°C and about 4OOOC. It is obvious from Fig. 6, however, that measurements carried out at very low temperatures, such as 300°C or lower, can not be extrapolated to 538°C by using an activation energy of 30 kcal/mol. For example, Wang [1 11 extrapolated the cracking rate constant measured at 240°C to the reference temperature of 538"C, using an arbitrarily chosen activation energy of 30 kcal/mole; he calculated an alpha value of greater than 100 000, which differs from the actual value obtained at 538°C by over three orders of magnitude. Therefore, rather than varying the test temperature, a much simpler and more accurate procedure is to actually perform all measurements at the standard temperature of 538°C (1OOOOF), as has been proposed previously [2]. By an appropriate choice of
F,W and
conversion (e.g. 0.3-60%) rate constants differing by over four orders of magnitude can be readily measured. It has been found that even for the medium pore zeolite ZSM-5, of high activity, no diffusion limitations exist even at this relatively high temperature [8, 121 except for very large crystals exceeding 40 pm [l3]. At 538°C it is also easy to work
264 W. 0. Haag, R. M. Dessau and R. M. Lago
under limiting mono-molecular conditions, where the rate constant for medium pore zeolites such as ZSM-5 is independent of considerable variation in hydrocarbon partial pressure. While medium pore zeolites such as ZSM-5 do not deactivate significantly during hexane cracking at 538"C, large pore zeolites usually do. For maximum accuracy of results in these cases we found it advisable to use a low hexane partial pressure of about 10 torr. This not only completely eliminates catalyst deactivation during the test (Fig. 7),
0 0
10
20
30
40
50
60
70
Time on Stream, min. Fig. 7. Effect of hexane pressure on deactivation of US-Y catalyst at 538"C, -*- 100 torr, -0- 10 ton.
but also assures operation under limiting monomolecular test conditions that is more difficult to achieve with large pore zeolites. CONCLUSION
The cracking of simple paraffins is kinetically first order only under limiting reaction conditions, but very complex in the intermediate region. The two mechanisticallydifferent reaction paths that have been elucidated, mono- and bimolecular cracking, occur with greatly different activation energies, corresponding to different rate determining steps.
As a means to characterize the activity of acid catalysts, hexane cracking offers unique advantages and is best carried out at high temperature, such as 538OC, and low pressure.
Kinetics and Mechanism of Paraffin Cracking 265
REFERENCES 1 2 3
4 5. 6 7
8 9 10
11 12 13
P.B. Weisz and J.N. Miale, J. Catal., 4 (1965) 527; J.N. Miale, N.Y. Chen and P.B. Weist, J. Catal., 6 (1966) 278. D.H. Olson, W.O. Haag and R.M. Lago, J. Catal., 61 (1980) 390. L.P. Albridge, J.R. Laughlin and C.G. Pope, J. Catal., 30 (1973) 409; J.R. Anderson, K. Foger, T. Molo, R.A. Rajadhyaksha and J.V. Sander, J. Catal., 58 (1979) 114; Van den Berg, Thesis Univ. Eindhoven (1981) p. 22; M. Daage and F. Fajula, J. Catal., 81, (1983) 394; 405; U.S.4,392,003, July 1983; W.A. Wachter, Proc. 6th Int. Zeol. Conf., Reno, 1984 p. 141; A.G. Ashton, J. Dwyer, I.S. Elliott, F.R. Fitch, G. Qin, M. Greenwood and J. Speakman, ibid., p. 704; J. Heenng, L. Riekert and L. Marosi, ibid., p. 528; J.G. Post and J.H.C. Van Hoff, Zeol., 4 (1984) 9; J.R. Sohn, S.J. DeCanio, P.O. Fritz and J.H. Lunsford, J. Phys. Chem., 90 (1986) 4847. J. Abbot, Appl. Catal., 57 (1990) 105. P. Fejes and P.H. Emmett, J. Cat., 5 (1966) 193; P. Weisz, Chem. Tech., (1973) 498; D.M. Anufriev, P.N. Kuznetsov and K.G. lone, J. Catal., 65 (1980) 221; D.S. Santilli, Appl. Catal. 60 (1990) 137. W.O. Haag and R.M. Dessau, Proc. 8th Int. Congr. Catalysis, Verlag Chemie, Weinheim, Vol. II (1984) 305. C.T. Thomas, Ind. Eng. Chem., 41 (1949) 2564; B.S. Greensfelder, H.H. Voge and G.M. Good, Ind. Eng. Chem., 41 (1949) 2573; H.H. Voge in "Catalysis" (P.H. Emmett, Ed.), Vol. VI (1958) p. 407; B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw Hill Book Co., 1979, p. 29; H. Pines, The Chemistry of Catalytic Hydrocarbon Conversions, Academic Press, NY, 1981, p. 83. W.O. Haag, R.M. Lago and P.B. Weisz, Far. Disc. Chem. SOC.,72 (1982) 317. G. Olah, J. Am. Chem. Soc., 94 (1972) 808; G. Olah, Carbocations and Electrophilic Reactions, J. Wiley & Sons Inc., NY, 1974. R. Houriet, G. Parisod and T. Gaumann, J. Am. Chem. SOC.,99 (1977) 3599. 1. Wang, T. Chen, K. Chao and T. Tsai, J. Catal., 60 (1979) 140. M.F.M. Post, J. Van Amstel and H.W. Kouwenhoven in D.H. Olson and A. Bisio (Eds.) Proc. 6th Int. Zeolite Conf., Reno, Buttersworth, Guildford, 1983, p. 517. P. Voogd and H. van Bekkum, Appl. Catal., 59 (1990) 31 1.
This Page Intentionally Left Blank
267
Dual Function Mechanism of Alkane Aromatization over H-ZSM-5 Supported Ga, Zn, Pt Catalysts : Respective Role of Acidity and Additive P a u l Meriauaeau, Gilbert Sapaly and Claude Naccache.
Institut de Recherches sur la Catalyse, CNRS, 2, Einstein, 69626 Villeurbanne Cedex, France
avenue A .
ABSTRACT The propane reaction over H-ZSM-5, Ga-HZSM-5, Zn-HZSM-5 and Pt-HZSM-5 catalysts was studied. The activity of these catalysts increased with the acidity and with the addition of Ga, Zn or Pt. The highest selectivity towards aromatics was found over Ga-HZSM-5 (or Zn) catalysts. I t was demonstrated that the role of additives (Ga, Zn, Pt) was to increase the rate of propane dehydrogenation, thus favouring C3 oligomerization. Aromatization resulted in parallel path formation of dienes, trienes over the additive component and cycloalkanes followed by hydrogen transfer over acid sites. The intrinsic dehydrogenating properties of Ga (or Zn) supported on ZSM-5 was investigated by studying the reaction of propane at low conversion. The activity o f gallium (OX zinc) for dehydrogenation apparently increased with increasing zeolite acidity. The results are interpreted with a mechanism where C H7+ alkyl species formed on gallium ( o r zinc) exchanges with H3 , the simultaneous step intervenes in the propane dehydrogenation.
INTRODUCT ION One interesting and industrially important process developed recently,"the cyclar process" is the catalytic aromatization of light (C3-cg) hydrocarbons over pentasil based catalysts. These new classes of solids have been widely studied (1-9). These investigations led to the conclusion that the catalysts consisting of gallium, zinc, Pt, and modified H-ZSM-5 were more active and more selective towards aromatics than the parent H-ZSM-5 zeolite. The formation of aromatics from light alkanes comprised several main hydrocarbon reactions: alkane dehydrogenation catalyzed by €I+ and more efficiently by Ga, Zn or Pt, olefin oligomerization acid catalyzed reaction, dehydrocyclization to aromatics possibly catalyzed by H+ or/and by Ga, Zn, Pt.
268 P. Meriaudeau, G. Sapaly and C. Naccache
The present investigation was undertaken in order to determine the respective role of the acidity and of the additive (Ga, Zn, Pt) on the various steps in the formation of aromatics. In order to gain new evidence for the role of Ga or Zn in the alkane dehydrogenation, dehydrogenation of propane at low conversion was investigated. In this paper we also report the results of a study of propane dehydrogenation over gallium supported on non acidic Na-ZSM-5 and we compare the results with those obtained on Ga-HZSM-5. Zinc exchanged Na-ZSM-5 and H-ZSM-5 were also investigated. EXPERIMENTAL The ZSM-5 zeolites (Si/Al = 15 and 30) were synthesized using tetrapropylammonium bromide under hydrothermal conditions at 443 K according to the patent literature. The solids were calcined in an air stream at 773 K. Na-ZSM-5 was obtained by exchanging the calcined solids with NaCl solution, NH4C1 solution being used to obtain NHq-ZSM-5 samples. The crystallinity of ZSM-5 was checked by X-ray diffraction. H-ZSM-5 form was obtained by calcination at 723 K of the ammonium form. Ga-ZSM-5 samples were prepared by the incipient wetness impregnation technique, using Ga(N03)3 solution. The amount of dissolved Ga(N03)3 was adjusted in order to prepare Na-ZSM-5 and H-ZSM-5 containing 0.8 and 5 wt% Ga. Zn-ZSM-5 samples were prepared by exchanging Na- or NHd- forms with Zn(N0312 solution. Bulk Ga2O3 was prepared by precipitation of Ga(N03)3 solution with ammonia. Ga2O3 having 20 m2.g-l surface area is obtained after calcination at 773 K of the solid. The catalytic reactions were carried out in a continuous fixed bed microflow reactor at atmospheric pressure. The reaction temperature was fixed in the range 673-773 K and the catalyst weight in the 0.01-0.2 g range. The catalysts were in situ pretreated in a stream of air at 773 K before reaction, zeolitesupported Pt or Pt-Cu being further reduced at 773 K in H2. Metallic particle sizes determined by transmission electron microscopy were in the range 1-2 nm. The reactor effluents were analyzed on line by gas chromatography. High purity grade reactants supplied by commercial sources were used without further purification.
Dual Function Mechanism of Alkane Aromatization 269
RESULTS AND DISCUSSION Propane reaction. In a series of experiments propane (760 t o n ) reacted at 773 K over H-ZSM-5 (Si/A1 = 15) and H-ZSM-5 modified with Ga or Pt. The conversion of propane was maintained at around 30% by adjusting the flow rate between 1 and 10 l.h-l, higher flow rates being used for the most active catalysts. The catalytic activities for the different solids were normalized to that of H-ZSM-5. The data are summarized in Table 1. I t is apparent that the addition of Ga, Pt, Pt-Cu to the H-ZSM-5 zeolite increased its activity for the propane conversion. Table 1. Reaction of C3H8 over H-ZSM-5 based catalysts p(C3H8) toxr Reaction Temp: 773 K C3H8 conversion: 34%
-
Sample
Selectivity (carbon bases) Rate in a.u
E S M -5 1 Ga-HZSM-5 4.2 Pt-HZSM-5 10 Pt-CU-HZSM-5 17
C1
c2
C2
C3
C4
C5
Arom
19.2 10.5 15 2.5
4.7 2.3 3.4 0.1
20 7.1 33.4 26.6
5.4 3.2 7.9 9.5
18.8 6 16 13.7
3.9 0,5 1.7 1.3
28 70.4 22.6 48.3
Si/A1 = 15, 0.8 wt% Ga, 0.8 wt% Pt, 0.8 wt% Pt t 0.4 wt% Cu. I t is apparent from Table 1 that the rate of propane conversion increased substantially by the addition of Ga, Pt or Pt-Cu to H-ZSM-5 catalyst. Furthermore, in agreement with the literature data (1-6) the selectivity toward the formation of aromatics is also considerably improved over Ga-HZSM-5 catalyst, an increase of aromatic selectivity from 28 to 70% was observed at the same propane conversion. In contrast Pt-HZSM-5 appeared less selective for aromatic formation than H-ZSM-5. This was attributed to the high hydrogenolysis activity of Pt which led to the production of larger amounts of light alkanes particularly C2H6(8). To verify this, Pt was alloyed with Cu since it is well established that the hydrogenolysis activity of Pt is considerably decreased by alloying. The results presented in Table 1 confirm that the addition of Cu to Pt decreased the overall production of C 1 - c ~ from C3H8, and hence the selectivity towards aromatics was improved. Different reaction paths have been proposed to explain the conversion of propane into aromatics (2-6). Over H-ZSM-5, proton addition to C3Hg led to C3Hg+ carbonium ion intermediate, successive and parallel reactions
270 P. Meriaudeau, G. Sapaly and C. Naccache
occurred: C3H9+ eitner cracked into CH4 and ~ 2 ~or 4 dehydrogenated into C3H6, H+ being released in both cases. The consecutive reactions which occurred were propene oligomerisation to form Cn(nr6) alkenes, cyclisation through 1-5 or 1-6 ring closure, hydrid transfer reaction between alkylcyclohexanes and olefins forming aromatics. Over gallium modified H-ZSM-5 bifunctional mechanism was proposed, Ga (or Zn, Pt) promotes the dehydrogenation of propane into propene (2-8). The aromatization mechanism will be discussed. Hydrocarbon reactions over Ga2O3. While there is no doubt that in the aromatization of CjH8 over Pt-HZSM-5, the role of Pt is to dehydrogenate C3H8 into propene, and cycloparaffins into aromatics, the suggestion that Ga2O3 (and Zn+2 or ZnO) could play the same role was clearly demonstrated by studying the dehydrogenation of propane and cyclohexane over bulk Ga203(lO11). The results of the conversion of propane and cyclohexane are summarized in Table 2. Table 2. Activity reactions at 773 parentheses)
-
React ant
propane
and selectivity of Ga2O3 in hydrocarbon K (the pressure in torr indicated in
Conversion Product selectivity % carbon basis
%
Rate in mole l ' h
1.6
propene
91
0.12
<760) cyclohexane 147)
5.7
benzene
87
0.026
n-hexene" (53.5)
36
cracking cyclohexene cyclohexadiene benzene C7 aromatics
12.2 9.5 6.5 68 2.1
cyclohexadiene benzene crack ing
20.4 65.4 5
1.5-hexadiene* 90 (50)
* isomerisation reactions
m-2
0.19
'0- 44
not considered The interesting feature in the results is the existence of a catalytic activity for the dehydrogenation of propane and cyclohexane into propene and benzene respectively. Furthermore the high selectivity towards dehydrogenation shown by Ga2O3, the absence of significant cracking reactions, and the lack of propene oligomerization, indicate that Ga2O3 exhibits almost no acidic properties. Hence one may conclude that Ga2O3 phase in Ga-HZSM-5 catalyst would not assist the oligomerfzation and
Dual Function Mechanism of Alkane Aromatization 271
hydrid transfer reaction which participate in the formation of aromatics from propane. In the course of propane aromatization among other intermediates, 1-hexene, 1-5 hexadiene, cyclohexane were probably formed over Ga-HZSM-5. In order to better approach the understanding of the role of Ga2O3 in the propane aromatization, the reactions of the above cited molecules over bulk Ga2O3 were investigated. The results are given in Table 2. As expected, no acid catalyzed reaction such as like ring opening to n-hexene or isomerization to methylcyclopentane occurred for cyclohexane. Over Ga2O3 cyclohexane was dehydrogenated into benzene with a high selectivity which reinforces the conclusions about the dehydrogenating properties of Ga2O-j. As seen in Table 2, the rate of dehydrogenation of cyclohexane to benzene is low. In contrast n-hexene and 1-5 hexadiene over Ga2O3 formed benzene with a high rate relatively to cyclohexane reaction. The selectivity towards cyclic compounds approached 90%. The cyclic compounds, in addition to benzene, consisted of cyclohexene and cyclohexadiene starting with 1-hexene. and cyclohexadiene from 1-5 hexadiene. It follows from these results that n-hexene and 1-5 hexadiene are over Ga2O3 preferential dehydrogenated into hexatriene intermediates which at the high temperature reactions employed formed cyclodiolefins, the final aromatization reaction being dehydrogenation of the cyclohexadienes over Ga2O3. The essential role of Ga2O3 in the aromatization of alkanes and alkenes is to dehydrogenate the hydrocarbons, the aromatics were produced by cyclisation of the dienes and trienes. These conclusions are in agreement with those proposed in (12) for the mechanism of dehydrocyclization of paraffins over oxide catalysts. Dual site mechanism for the dehydrogenation of propane over Ga-HZSM-5, Zn-HZSM-5 Since i t is now well accepted that the role of the additive in the H-ZSM-5 based catalysts is to increase the rate of propane dehydrogenation it is clear that the additive should be sufficiently active to establish rapidly the thermodynamic equilibrium propane-propene. Ga2O3 or ZnO exhibits with respect to Pt catalysts lower dehydrogenating properties. Nevertheless, when supported on H-ZSM-5, they were found to promote more efficiently the aromatization of propane, and also the
272 P. Meriaudeau, G. Sapaly and C. Naccache
dehydrogenation of alkanes into alkenes. In the case of Zn-HZSM-5 it has been suggested (2) that propene is formed via hydride abstraction in C3H8 by Zn2+ cations which produces the transient species (Zn-H)+ and C3H7+, followed by deprotonation of the dimethyl carbenium ion. In a recent work (13) it has been shown that the dehydrogenation of propane into propene over ZSM-5 supported Ga2O3 is substantialy enhanced when Ga2O3 is supported on the H-form as compared with Ga2O3 supported on the Na-form. In order to confirm that the dehydrogenating properties of Ga or Zn increased when they are supported on H-ZSM-5 rather than on Na-ZSM-5 a comparative study of the reaction of propane over these catalysts was undertaken. C3H8 at 760 torr and 773 K was reacted over Ga-NaZSM-5, Ga-HZSM-5 (5 wt% Ga) and Zn-NaZSM-5, Zn-HZSM-5 (0.5 wt% Zn). The Flow rate and sample weight were adjusted in order to obtain low conversion of C3H8 (less than 4%) and hence as a consequence to minimize the subsequent conversion of the propene formed. In Table 3 are reported the observed selectivity into propene and the rates of formation of CH4 and C3H6. The rate of CH4 formation correlates with the catalyst acidity, while the rate of C3H6 formation correlates with the acidity and/or the dehydrogenating properties of the additives. Table 3. Reaction of C3Hg at 773 K, 760 torr over ZSM-5 based catalysts. Conversion range 1-3% Catalyst H- ZSM- 5 5 % Ga-HZSM-5 5 % Ga-NaZSM-5 0.5 % Zn-HZSM-5 0.5 % Zn-NaZSM-5
Selectivity % C3H6 21 46 63 57 62
rate in m o l e s h-l g cat’l CH4 C3H6 49 53 0.4 20.7 2.1
12 130 1 43 9.8
Table 3 indicates clearly that the rate of the cracking of propane, as evidenced by the production of methane, depends stxongly on the catalyst acidity, almost no cracking reaction of C3H8 being observed over the sodium-form. Table 3 also indicates that the rate of CH4 formation remained almost unchanged when 5 rt% Ga was added to H-ZSM-5, while the rate of CH4 production decreased by a factor of 2 by adding 0.5 wt% Zn. One can conclude in agreement with the literature (3,7) that the addition of Ga2O3 on H-ZSM-5 leaves the acidity of the zeolite almost unchanged
Dual Function Mechanism of Alkane Aromatization 273
while zinc decreases the acidity of the zeolite. Similar conclusions have been reached recently by investigating the zeolite acidity with infrared technique (14). From Table 3 it is clear that gallium enhanced substantially the formation of propene, with 5 wt% Ga the rate of C3H6 formation increased from 12 m o l e s l'h g-l up to 130 mmoles l'h g-l. Similarly Zn-HZSM-5, although less acidic than H-ZSM-5, was found to be 4 times more active for C3H6 production than H-ZSM-5. These results reinforce the conclusions already presented in the literature (2-8) suggesting that one of the major role of Ga and Zn is to promote the dehydrogenation of propane into propene. The striking feature shown in Table 3 is the important difference in the rate of propene formation as the zeolite was changed from its H-form into Na-form, Ga-HZSM-5 being found 100-fold more active than Ga-NaZSM-5, the effect of acidity being less pronounced when Ga is replaced by Zn. Thermodynamic calculation for the dehydrogenation of propane into propene at 773 K would give an equilibrium conversion of 16% at a total C3H8 pressure of 760 toxr. It is clear that in the reaction conditions used in this study, 4% maximal conversion of C3H8, the equilibrium of propane to propene was not reached. The higher activity of Ga (or Zn) when supported on H-ZSM-5 as compared with Na-ZSM-5 could not be explained only by a better dispersion of the gallium phase (or zinc phase). A dual-site mechanism for dehydrogenation of C3H8 on Ga(Zn)-HZSM-5 can be taken as an explanation for the above results, both Ga species (or Zn) and H+ intervening in the dehydrogenation mechanism of propane. One unpromoted H-ZSM-5 propene is formed by the following steps:
Protonation step ( 1 ) to give C3Hg+ carbonium ion followed by H2 elimination proceeds slowly and is probably the ratedetermining step in the dehydrogenation of propane over acid catalysts. Over Ga2O3, similarly to what has been proposed for zinc (2), it is likely that C3Hg is dissociatively adsorbed forming hydride and alkoxide surface species. Since it has been shawn (15) by infrared H2 is readily heterolytically dissociated on Ga2O3 at 293 K one can admit that the dissociative
274 P. Meriaudeau, G. Sapaly and C. Naccache
adsorption of C3Hg on Ga2O3 is a fast Process. Furthermore the surface alkoxide species may well be polar. Decomposition of the surface alkoxide through a concerted mechanism involving the attack of the alkoxide by the adjacent hydride generates C3H6 and H2, this step being the rate determining step. The two-step mechanism for dehydrogenation of C3H8 over Ga2O3 can be visualized:
When both Ga2O3 (or Zn) and H+ are present within the catalyst, one can assume that the polar propyl carbenium species produced by reaction (3) over Ga2O3 phase will possibly exchange with a zeolite proton through an alkyl surface migration
HI Ga
:3H7+
-d-
Ga + HZ
->
'H I Ga
-
y+ 0
-
Ga + C3H7+Z
(5)
This is a reasonable assumption in view of the high mobility of protons in zeol i tes
.
HH+ I 'I Ga-O-Ga+Hz + G a - O - G a C3H6 + HZ 3 C3H7+ Z Since equilibriums (6) are rapidly established, step (5) would be the rate determining step of the dehydrogenation of propane over Ga-HZSM-5 or Zn-HZSM-5 catalysts. It results that the dehydrogenating activity of the composite catalysts Ga-HZSM-5, Zn-HZSM-5, is considerably enhanced as compared with bulk Ga2O3 ( o r ZnO) or supported on non acidic carrier, because the slow rate-determining step (4) is by-passed by H+ and C3H7+ migration, step (5). According to the mechanism we propose for the dehydrogenation of propane over Ga (or Zn) doped HZSM-5 it is clear that in aromatization of C2-C5, the zeolite acidity is an important parameter not only for the necessary oligomerisation reaction of light alkenes but also for the production of olefins
Dual Function Mechanism of Alkane Aromatization 275
from alkanes. General considerations on the mechanism of C3H8 reaction over HZSM-5 and Ga- HZSM-5. The products obtained from the reaction of C2-C5 alkanes over H-ZSM-5 zeolites were nicely interpreted (3-8) according to the classical carbenium ion theory and the nonclassical theory developed for reactions occurring in superacid media where an alkane is protonated to form the carbocation species. The general scheme proposed for propane reaction over H-ZSM-5 is: a) cracking and dehydrogenation
b) Formation of higher alkenes by oligomerization. c) Cyclization through internal alkylation of the monoolefins to form alkylcyclohexanes or of diolefins, issued from hydride transfer reaction, to form alkylcyclohexenes. d ) Aromatization by hydride transfer between cyclic hydrocarbons and olefins. e) Additional cracking of the oligomers occurs during the process of aromatization. Over the bifunctional catalysts resulting from the addition of Ga2O3 (or ZnO) to H-ZSM-5, although the above reaction steps still occurred, the catalytic effect of the oxide should modify the course of the reactions in such a manner that steps a, c, d would not predominate in the aromatization of alkanes over these catalysts. (or Zn) increases the rate of propane Since Ga2O3 dehydrogenation, these results in a higher concentration of propene intermediates over Ga203-HZSM-5 as compared to that obtained on H-ZSM-5 unpromoted. Recently the oligomerization of C3Hg over H-ZSM-5 catalysts has been reported branched dimer, trimer, etc., formed by a carbenium ion mechanism, depend on propene concentration, temperature of reaction and acidity of the zeolite (16). High oligomer yield was obtained over the more acidic H-ZSM-5. It is clear that optimal oligomer yield, approaching the thermodynamic equilibrium will be reached with a good balance between the dehydrogenating function and the acid function of the composite catalyst.
276 P. Meriaudeau, G. Sapaly and C. Naccache
According to the results presented in Table 2 concerning the reactions of n-hexene 1.5 hexadiene and cyclohexane it was suggested that the predominant route to cyclic hydrocarbons in the reaction of propane over Ga203-HZSM-5 is the dehydrogenation of the olefin oligomers over Ga2O3 into dienes and possibly trienes. At high temperatures trienes are rapidly cyclized into cyclohexadienes. Alternatively protonation of the dienes results in the formation of olefinic carbenium ions which undergo intramolecular double bond alkylation and the subsequent 5 and 6ring cycloalkenes. The dehydrogenation of the olef inic oligomers into dienes as a possible route for aromatization over Ga o r Zn promoted H-ZSM-5 has also been suggested in (6, 17). Finally over Ga-HZSM-5 aromatics were formed by dehydrogenation of cyclohexenes and cyclohexadienes over Ga2O3 component.
Ga C3Hg -->
H+ C3H6 + H2 --> CnH2,
Ga olefins -->
dienes, trienes
5.6 ring olefins, dienes --> Aromatics + H2 dienes, trienes - - > In order to produce additional evidence for the above mechanism for aromatization over Ga203-HZSM-5 catalysts the reactions of n-hexene, 1,5 hexadiene, methylcyclopentane, methylcyclopentene, cyclohexene, cyclohexadiene at 773 K over H-XSM-5 and Ga-HZSM-5 were comparatively studied. In these experiments low pressure and low contact were employed to observe tho primary kinetic products uncomplicated by secondary reactions. The relative rates of the formation of benzene from the various hydrocarbons cited above are listed in Table 4. Table 4. Relative rate of benzene formed over H-ZSM-5 and GaHZSM-5 (Si/A1 = 30, Ga = 2 wt%) Reactant
n-H
MCP
MCPe
Hde
CHe
CHde
0.0
0.0
0.4
0.6
0.4
11.4
2% Ga-HZSM
50
43
100
95
83
100
Conversion
(100)
(35)
(83)
(100)
(59)
(83)
HZSM -
--------___----------------------------------------------n-hexene = n-H, methylcyclopentane = MCP, methylcyclopentadiene = MCPe, hexadiene = Hde, cyclohexene = CHe, cyclohexadiene = CHde. P reactant 0.35 torr. Catalyst weight: 10 mg. Flow rate 16 l/h, numbers in brackets are conversion. I
Dual Function Mechanism of Alkane Aromatization 277
The reaction of n-hexene at 773 K and high dilution over H-ZSM5 produced almost exclusively cracked products: propene. Under these conditions the forlaation of aromatics and paraffins wexe not observed. In contrast over Ga-HZSM-5 the main products were propene and benzene. The very rapid dehydrogenation of n-hexene over Ga-HZSM-5 into hexadiene and hexatriene which could easily form cyclic hydrocarbons by intramolecular alkylation catalyzed by H+ will explain the different behaviour of H-ZSM-5 and Ga-HZSM-5 in the reaction of highly diluted n-hexene. These suggestions are consistent in view of the finding that Ga-HZSM-5 shows dehydrogenating properties. Methylcyclopentane at low partial pressure and 773 K experienced over H-ZSM-5 almost exclusively ring opening reaction into n-hexene, which is consistent with the well-established mechanism over monofunctional acid catalyst: protonation of the 5-ring followed by carbon-carbon 5-ring bond 8-scission (18). Under identical experimental conditions MCP over Ga-HZSM-5 gave benzene as the major product. The interconversion of the 5-ring MCP into 6-ring benzene which operates over Ga-HZSM-5 must involve intermediates which are not formed on H-ZSM-5 under the experimental conditions mentioned in this paper. The present results indicate that over Ga-HZSM-5, Ga2O3 contributed dominantly to the conversion of methylcyclopentane into benzene. It is conceivable that over this catalyst MCP was deeply dehydrogenated into methylcyclopentadiene. Ring opening catalyzed by H+ followed by thermal cyclization of the hexatriene product, or ring-enlargement of MCPde would form cyclohexadiene and filially benzene upon dehydrogenation over Ga2O3. Conversion of MCP into benzene over Pt-Re-Al203 was also suggested to proceed also through dehydrogenation into MCPe, MCPde (19). Table 4 shows that benzene is formed at almost identical rates from cyclohexene, hexadiene, methylcyclohexene and cyclohexadiene react under low partial pressure over Ga-HZSM-5, which suggests that over these catalysts benzene is formed from the same intermediate. In contrast over H-ZSM-5, under identical experimental conditions, the rate of benzene formation from the hydrocarbons cited was one to two orders of magnitude lower. These results prove again that gallium plays a decisive role in aromatization. Over H-ZSM-5 the major hydrocarbon formed is methylcyclopentene from cycloherene (ring contraction)
278 P. Meriaudeau, C.Sapaly and C. Naccache
methylcyclopentene from hexadiene (C5 ring closure) cyclohexene and hexadiene from methylcyclopentene the classical ion carbenium mechanism being operative for these reactions. According to our results, Ga-HZSM-5 catalyzed c6 hydrocarbon aromatization via a coinmon intermediate which could be hexatriene and/or cyclohexadiene. Indeed regarding the dehydrogenating function of ga.11ium, cyclohexene and methylcyclopentene are rapidly dehydrogenated into cyclohexadiene and methylcyclopentadiene. The acid function of Ga-HZSM-5 allows MCPde ring opening to form heuatriene which immediately experienced thermal c6 ring closure into cyclohexadiene. In conclusion these results reinforce our previous finding on the role of gallium in aromatization of propane. Gallium is important in the dehydrogenation of propane into propene, and its dehydrogenation activity is promoted by the zeolite acidity. Proton acidity catalyzes propene oligomerization, the olefin dintribution approaching probably, at the high reaction temperature used, the thermodynamic equilibrium composition at high conversion. c6' olefins are on the dual-functional catalyst, Ga-HZSM-5, rapidly dehydrogenated into dienes and trienes, these reinctions predominating as compared with the also occurring Ht catalyzed intramolecular cyclization of the C6* olef ins. The common cyclohexadiene intermediate is dehydrogenated over gallium into benzene, or/and experienced hydride transfer with olefin over H+ to form benzene. REFERENCES 1 2
3 4
5
6 7
8
US Pat. 4.350 335 (1982) to A.W. Chester and Y.F. Chu (Mobil) : Eur. Pat. Appl. E.P. 50021 (1982) to D. Dave, A . Hall and P. Harold (B.P. Co). T. Mole, J.R. Anderson and C. Creer, Appl. Catal., 17 (1985) 141. H. Kitagawa, H. Sendola and Y. Ono, J. Catal., 101 (1986) 12. T. Inui, Y. Makino, F. Okazumi and M. Yamoto, J. Chem. Commun., (1986) 571. T. Inui, in "Successful Design of Catalyst (T. Inui ed.) Studies in Surface Science and Catalysis, Elsevier 44 (1989) p. 189. Y. Ono, H. Nakatami, H. Kitagawa and E. Suzuki in "Successful Design of Catalysts" (T. Inui ed.) Studies In Surface Science and Catalysis, Elsevier 44 (1989) p. 279. N.S. Gnep, J.Y. Doyemet, A.M. Seco, F. Ribeiro and M. Guisnet, Appl. Catal., 35 (1987) 108. P. Mariaudeau, G. Sapaly and C. Naccache in "Zeolites, Facts, Figures, Future" (P. Jacobs and R.A. Van Santen eds) Elsevier 49 (1989) p. 1423.
Dual Function Mechanism of Alkane Aromatization
9 10 11 12
13 14
15 16
17 18
19
279
L. Petit, J.P. Bournonville and F. Raatz in "Zeolites, Facts, Figures, Future" (P. Jacobs and R.A. Van Santen eds) Elsevier, 49 (1989) p. 1163. N.S. Gnep, J.Y. Doyernet and M. Guisnet, J. Mol. Catal., 45 (1988) 281. P. Meriaudeau and C. Naccache, J. Mol. Catal., 50 (1989) L7. B.A. Kazansky, G.V. Isagulyants, M.I. Rozengart, Yu. G. Dubinsky and L.I. Kovalenko in "Proc. Vth Intern. Congress Catal.", J.W. Hightower ed. (1973) p. 1277. P. Meriaudeau and C. Naccache, J. Mol. Catal., (1990) to be published. V.I. Yakerson, O.V. Bragin, Kh. M. Minachev, Catal. Letters 3 (1990) 339. P. Meriaudeau Results to be published. K.G. Wilshier, P. Smart, R. Western, T. Mole and T. Behrsing, Appl. Catal., 31 (1987) 339. J. Kanai in "Successful Design of Catalysts" (T. Inui ed.) Elsevier 44 (1989) p. 211. S.G. Brandenberger, W.L. Callender and W.K. Meerbott J. Catal. 42 (1976) 282. J.M. Parera, J.N. Beltramini, C.A. Querini, E.E. Martinelli, E.J. Churin, P.E. Aloe and N.S. Figoli J. Catal., 99 (1986) 39.
This Page Intentionally Left Blank
Autocatalysis, Retardation, Reanimation and Deactivation during Methanol Conversion on Zeolite HZSM5
H. Schulz, Zhao S i w e i and H. K u s t e r e r Engler-Bunte-Institute,
U n i v e r s i t y o f Karlsruhe.
K a i s e r s t r a O e 12, 7500 K a r l s r u h e , FRG
ABSTRACT T h i s i n v e s t i g a t i o n of t i m e r e s o l v e d s e l e c t i v i t y o f methanol c o n v e r s i o n on HZSM5 (and on HUSY f o r comparison) i n t h e t e m p e r a t u r e range 250 t o 500 OC r e v e a l s new s h a p e - s e l e c t i v i t y - c o n t r o l l e d regimes o f i n s t a t i o n a r i t y and changes i n a c t i v i t y w i t h r e a c t i o n t i m e and r e a c t i o n temperature: f i r s t l y a u t o c a t a l y s i s and s e c o n d l y c o n s e c u t i v e r e t a r d a t i o n w h i c h a r e dominant a t low t e m p e r a t u r e (250 - 290 OC); t h i r d l y r e a n i m a t i o n which i n c r e a s e s c a t a l y s t l i f e t i m e o d r a s t i c a l l y by s e v e r a l o r d e r s o f magnitude i n t h e t e m p e r a t u r e range 290 - 370 C and f o u r t h l y d e a c t i v a t i o n which d e f i n i t e l y reduces c a t a l y s t l i f e i n t h e t e m p e r a t u r e range 370 - 500 O C t h r o u g h "coking" i n i t s r e a l meaning. INTRODUCTION The approach
of
this
work
b a l a n c e s i n much d e t a i l
is
t o measure p r o d u c t c o m p o s i t i o n s
and mass
i n a t i m e r e s o l v e d manner and t o r e l a t e t h i s t o t h e
c o n t r o l l i n g k i n e t i c p r i n c i p l e s and e l e m e n t a l r e a c t i o n s o f p r o d u c t f o r m a t i o n and c a t a l y s t deactivation.
A d d i t i o n a l l y t h e o r g a n i c m a t t e r , which i s e n t r a p p e d i n
t h e z e o l i t e o r d e p o s i t e d on it, i s determined. The i n v e s t i g a t i o n c o v e r s a wide temperature
range
autocatalysis,
(250
-
500
retardation,
OC).
Four
reanimation
kinetic and
regimes
are discriminated:
deactivation.
A
comprehensive
p i c t u r e o f methanol c o n v e r s i o n on HZSM5 as a t i m e on stream and t e m p e r a t u r e function
i s developed.
This
r e p o r t e d i n l i t e r a t u r e [I
-
also
explains
consistently
individual
findings
43.
EXPERIMENTAL Methanol c o n v e r s i o n was performed i n a f i x e d bed r e a c t o r a t a t o t a l p r e s s u r e o f 5 b a r (pCH30H = 2.5 and pN2 = 2.5 b a r ) and a WHSV o f 1 h-I. The z e o l i t e c r y s t a l l i t e s were a p p l i e d as a ca. 10 p m l a y e r on t h e s u r f a c e o f fused s i l i c a p a r t i c l e s o f ca. 0.3 mm diameter, quartz-mass-ratio
of
1:14
(Fig.
1).
which corresponds t o a z e o l i t e -
No c a t a l y s t b i n d e r was used.
This ideal
282 H. Schulz, S. Zhao and H. Kusterer
REACTOR WALL QUARTZ FUSED S I L I C A PARJICLESm0.3 mm LAYER OF ZEOLITE CRYSTALLITES, h ~15Am MASS RATIO QUARTZ TO ZEOLITE 14:l F i g . 1: Schematic drawing o f zeolite/quartz-layered-catalyst
f i x e d bed.
f i x e d bed o f c a t a l y s t allowed an equal f l o w o f gases and a c l o s e - t o - i s o t h e r m a l mode o f o p e r a t i o n . W i t h t h i s v e r y i n e r t system o f h i g h c a t a l y s t d i l u t i o n no methanol decomposition t o CO and H2 occurred on HZSM5 up t o 475
OC.
Time r e s o l u t i o n o f s e l e c t i v i t y was achieved by r e c o v e r y o f samples from t h e p r o d u c t stream w i t h a d u r a t i o n o f sampling o f l e s s t h a n 0.1
h o t gas/vapour
D i f f e r e n t i a l y i e l d s o f i n d i v i d u a l compounds and o f coke,
second.
respectively
hydrocarbons which were adsorbed o r entrapped by t h e c a t a l y s t , were determined w i t h t h e h e l p o f r e f e r e n c e compounds product
stream.
Integral
coke
(neopentane,
yields
and
its
cyclopropane) H/C-ratio
added t o t h e
was
measured
temperature programmed combustion w i t h d i l u t e d oxygen and r e c o r d i n g o f c o n c e n t r a t i o n s of
CO,
C02, H2 and O2 i n t h e off-gas.
by the
"Soluble coke" from t h e
z e o l i t e l a t t i c e was recovered by means o f d i s s o l v i n g t h e z e o l i t e i n h y d r o f l u o r i c acid,
and e x t r a c t i n g t h e o r g a n i c m a t t e r w i t h methylene c h l o r i d e
[5].
This
s o l u t i o n was analysed by c a p i l l a r y GC and GC/MS. AUTOCATALYSIS AND RETARDATION Methanol
was
converted
temperature range 250
-
280
to OC
hydrocarbons
E.
HZSM5
and
HUSY
in
the
low
and t h e y i e l d o f hydrocarbons measured as a
f u n c t i o n o f d u r a t i o n o f experiment (Fig. autocatalytic.
on
2).
g. w i t h t h e HZSM5 and 260
The r e a c t i o n i s seen t o be h i g h l y OC
o n l y a f t e r 30 minutes a small
hydrocarbon y i e l d o f about 2 % i s observed. W i t h i n t h e n e x t 30 minutes t h e y i e l d increases
approximately
exponentially
and
after
60
minutes
duration
of
experiment a 100 % conversion of methanol t o hydrocarbons and coke i s obtained. Autocatalysis
i s v e r y s e n s i t i v e a g a i n s t temperature.
A t an o n l y 10
OC
lower
r e a c t i o n temperature t h e i n c r e a s e o f hydrocarbon y i e l d from about 5 t o 50 % occurs.
from 50 t o 100 minutes a f t e r s t a r t o f r e a c t i o n .
A t 280 OC i n c r e a s e o f
hydrocarbon y i e l d from z e r o t o 100 % occurs a l r e a d y i n t h e p e r i o d from 5 t o 20 minutes and a t 300 OC no a u t o c a t a l y s i s i s observable:
t h e degree o f c o n v e r s i o n
Methanol Conversion on HZSM-5 283
HUSY
0
50
50
150
DURATION
100
OF EXPERIMENT , min
F i g . 2: A u t o c a t a l y s i s and r e t a r d a t i o n d u r i n g methanol conversion on HZSM5 ( l e f t ) and HUSY ( r i g h t ) a t d i f f e r e n t temperatures. Y i e l d o f hydrocarbons ( y i e l d o f coke a f u n c t i o n o f d u r a t i o n o f t h e experiment ( i n l e t p = 2.5 bar, neglected) WHSV = 1 h ) C a t a l y s t s : HSZM5 Si/A1 = 26, o b t a i n e d from HUSY b a s i c c r a c k i n g c a t a l y s t Si/A1 = 4.5, obtained from Engelhard.
.,
DEGMR,
b e i n g 100 % from t h e beginning o f t h e experiment.
A u t o c a t a l y s i s o f methanol
conversion i s a l s o o b t a i n e d w i t h HUSY and t h e r e f o r e n o t c r e a t e d through shape s e l e c t i v i t y ( F i g . 3). A t 270
OC
w i t h t h e HZSM5, d i f f e r e n t i a l coke y i e l d has been
determined d u r i n g methanol conversion (Fig. and a t t a i n about 40 % a t 100 % conversion.
4).
The values a r e amazingly h i g h
T h i s ''coke''
i s o f course n o t a
substance o f c o k e - l i k e compostion b u t o n l y coke by d e f i n i t i o n as t h a t p a r t o f
F i g . 3: l e f t A u t o c t a l y s i s qnd r e t a r d a t i o n o f methanol conversion on HZSM5 and HUSY a t 260 OC (WHSV = 1 h- t PCH30H = 2.5 bar). F i g . 4: ( r i g h t ) A u t o c a t a l y s i s and r e t a r d a t i o n . Y i e l d s o f p a r a f f i n s , o l e f i n s , aromatics and "coke" as a f u n c t i o n o f d u r a t i o n o f t h e qxperiment. Methanol conversion on HZSM5, 270 OC, p = 2.5 bar, WHSV = 1 hD i f f e r e n t i a l coke y i e l d abtained through internalC@%dard).
.
0
284 H. Schulz. S. Zhao and H. Kusterer
TABLE 1 (LEFT) : YIELDS OF VOLATILE HYDROCARBONS OBTAINED FROM METHANOL
UTHTSFi5 AT INCREASING DURATION OF LXPERIMENT IN THE TIME RANGE OF AUTOCATALYSIS (pCH30H = 2 . 5 bar, 270 C, WHSV = 1 h-?; YIELD OF COKE NOT
TAKEN INTO ACCOUNT). TABLE 2 (RIGHT): COMPARISON OF COMPOSITION OF VOLATILE HYDROCARBONS IN TREXJTOCATALYSIS AND IN THE REJARDATION RANGE AT APPROXIMATELY THE SAME DEGREE OF CONVERSION, 250 C. FURTHER CONDITIONES IN TABLE 1. TABLE I 1
t h e hydrocarbon p r o d u c t which i s n o t v o l a t i l e and r e t a i n e d by t h e c a t a l y s t . i s seen i n T a b l e 1,
t h a t t h e i n i t i a l hydrocarbon p r o d u c t t o appear f i r s t i s
methane, a compound which commonly i s n o t formed v i a a c i d c a t a l y s i s . compounds t o appear a r e ethene,
ethane,
conform w i t h carbonium i o n c h e m i s t r y . compounds.
It
propane and n-butane
The n e x t
which a r e more
L a t e r i - b u t a n e and i s o p e n t a n e a r e m a j o r
These v o l a t i l e p r o d u c t s a r e c o m p a r a t i v e l y r i c h i n hydrogen and t h e
non v o l a t i l e p r o d u c t f r a c t i o n (coke) must t h e r e f o r e be c o r r e s p o n d i n g l y hydrogen d e f ic i e n t . Several c o n c l u s i o n s a r e drawn f r o m t h e s e f i n d i n g s :
1 ) The f r e s h v e r y a c t i v e a c i d i c c a t a l y s t i s n o t capable o f f o r m i n g hydrocarbons f r o m methanol (and d i m e t h y l e t h e r ) .
2 ) The b e g i n n i n g f o r m a t i o n o f s m a l l v o l a t i l e hydrocarbons i s 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 about t h e same amount o f non v o l a t i l e compounds r e t a i n e d on t h e catalyst.
These adsorbed hydrogen d e f i c i e n t hydrocarbons i n c r e a s e t h e r a t e o f
methanol consumption. They a r e p a r t o f t h e r e a c t i n g system.
3 ) I n i t i a l methane f o r m a t i o n f r o m methanol on t h e f r e s h c a t a l y s t i s proposed t o proceed on
Bronsted
acid
sites
as
a
reaction with
a
h y d r i d e donor
-
in
Methanol Conversion on HZSM-5
-
p a r t i c u l a r t h e a l l y l i c hydrogen o f propene
---- + -----
s t a b i l i z e d a l l y 1 c a t i o n CH2
6+ CH3
6-
- OH +
+
Ht
CH3
t o form
285
t h e s t r o n g l y adsorbed
CH - CH2.
- CH X CH2
+
+
---c CH4
CH2
- CH Z CH2 +
(1 1
H20
T h i s unusual r e a c t i o n needs b o t h t h e r e a c t i v e p r o t o n o f t h e f r e s h c a t a l y s t and t h e a c t i v a t e d hydrogen i n a l l y l i c p o s i t i o n .
A d d i t i o n a l l y no e a s i e r r e a c t i o n
pathway must be a v a i l a b l e . So t h i s r e a c t i o n produces hydrogen d e f i c i e n t s t r o n g l y chemi sorbed hydrocarbons i n t h e e a r l y s t a g e o f a u t o c a t a l y s i s . The r o l e o f t h e much d i s c u s s e d " p r i m a r y hydrocarbon f r o m methanol
reaction"
o f f o r m a t i o n o f a C2-
i s then l i m i t e d t o producing a very small
amount
chemisorbed ethene d u r i n g t h e i n c u b a t i o n p e r i o d . T h i s C2 w i l l r e a c t e a s i l y t o C3 v i a a l k y l a t i o n w i t h methanol.
A t t h e t i m e o f maximum r a t e o f hydrocarbon f o r m a t i o n t h e c o m p o s i t i o n o f r e a c t i o n p r o d u c t shows t h e p a t t e r n o f a carbonium i o n mechanism (much i-butane,
C6 and C,
i-pentane, (Table
1).
monomethyl
The z e o l i t e
hosts a
C r a c k i n g o f such s p e c i e s C6+
paraffins,
some a r o m a t i c s and o l e f i n s C3,
l a r g e amount o f
non v o l a t i l e
C4
hydrocarbons.
produces r e a c t i v e i n t e r m e d i a t e s f o r f a s t methanol
consumption. The
same
process
which
creates
autocatalysis,
v o l a t i l e hydrocarbons on t h e a c t i v e s u r f a c e ,
the
accumulation
of
non
causes r e t a r d a t i o n t h r o u g h p o o r e r
a c c e s s i b i l i t y o f a c t i v e s i t e s and s l o w e r mass t r a n s f e r . Autocatalysis
and r e t a r d a t i o n a r e moderated by t h e p o r e s t r u c t u r e o f t h e
1 and 2):
z e o l i t e (Figs.
(1) The i n c r e a s e o f r e a c t i o n r a t e w i t h t i m e i s s l o w e r
HUSY,
w i t h HZSM5 t h a n w i t h
because f o r m a t i o n o f
r e s t r i c t e d t h r o u g h shape s e l e c t i v i t y . attained earlier
(2)
HUSY t h a n w i t h
with
non v o l a t i l e
compounds
is
The maximum r e a c t i o n r a t e i s t h u s
HZSM5 and
has
a
smaller
value.
(3)
R e t a r d a t i o n i s f a s t e r w i t h HZSM5 because p r o d u c t shape s e l e c t i v i t y o f HZSM5 does n o t a l l o w b u l k y branched a l i p h a t i c and s u b s t i t u t e d a r o m a t i c compounds t o escape
( 4 ) R e t a r d a t i o n w i t h HUSY i s a t t r i b u t e d p r e d o m i n a n t l y t o
from channel c r o s s i n g s .
t h e coverage o f a c t i v e s i t e s w i t h h i g h e r m o l e c u l a r w e i g h t d e p o s i t e s . Thus shape selectivity
of
retardation.
Table 2 shows t h e c o m p o s i t i o n o f t h e v o l a t i l e hydrocarbons o b t a i n e d
from methanol
HZSM5
moderates
both
the
regimes
of
autocatalysis
and
on HZSM5 a t t h e same c o n v e r s i o n i n t h e a u t o c a t a l y s i s and t h e
r e t a r d a t i o n range.
The d i f f e r e n c e s a r e o n l y g r a d u a l however s i g n i f i c a n t .
I n the
r e t a r d a t i o n range s e l e c t i v i t y f o r methane and branched p a r a f f i n s i s l o w e r and i t i s h i g h e r f o r o l e f i n s and aromatics. Composition o f t h e v o l a t i l e a r o m a t i c hydrocarbons o b t a i n e d on HZSM5 shows t h e following minutes
changes w i t h t i m e on stream ( T a b l e 3). from
start)
m-xylene,
p-xylene,
I n t h e e a r l y p r o d u c t (38
pseudo-cumene
and
durene
are
the
286 H. Schulz, S. Zhao and H. Kusterer
w
COMPOSITION OF THE FRACAROMATICS OF THE VOLAT I L E PRODUCTS FROM METHANOL CONVERSION ON HZSM5 AT BIFFERENT REACTION TIMES: 270 C. DURATION OF 38 6o EXPERIMENT, mln/ 21
-
1.9 13.5
-
-
TABLE COMPOSITION (C-X) OF AROd R E T A I N E D I N HZSM5 DUEING METHANOL CONVERSION AT 270 C AS A FUNCTION OF DURATION OF EXPERIMENT. PRODUCT OBTAINED THROUGH DISSOLUTION OF THE ZEOLITE I N HF AND EXTRACTION OF THE ORGANIC MATTER. (PcH OH = 2.5 bar, 3 WHSV = 1 h-'). DURATION OF 36 60 EXPERIMENT, mln 22
-
1.9 2.7 4.6 10.4 0.8 21.6 4.9 36.1 2.3 15,8
6.4 11.7 17.6
14.8 19.3 3.4 4.5 1.8 4.4 2.0 23.5 32.3 14.9 28.2
6.1 1.9 0.7 16.4 3.4 9.4 13.7 26.4 9.3 12.2
dominant compounds. A t a l a t e r t i m e (60 minutes) t h e p-xylene/m-xylene increased,
5.1 1.6 1.3 3.0 10.0 13.3 7.9 27.5 23.4 6.7
r a t i o has
some ethylbenzene i s o b t a i n e d and durene and pseudo-cumene
become most
have
i m p o r t a n t as c o n s i s t a n t w i t h more e x t e n s i v e m e t h y l a t i o n o f t h e
aromatic r i n g and more s e r i o u s s p a c i a l c o n s t r a i n t s . Table 4 obtained
shows t h e
through
composition o f
dissolving
e x t r a c t i o n w i t h CH2C12.
used
aromatic
HZSM5
hydrocarbons which have been
samples
in
I n t h e e a r l y stage isodurene,
hydrofluoric
acid
trimethyl-ethyl-benzene
and a l k y l a t e d naphthalene a r e most important. A t a l a t e r t i m e 1-isopropyl-, dimethylbenzene a r e favoured.
and 2,4-
Several i n t e r e s t i n g c o n c l u s i o n s a r e p o s s i b l e :
- I n i t i a l l y when o n l y l i t t l e b u l k y m a t e r i a l i s present,
naphthalene can be
formed i n t h e crossings. - At
a
later
stage
alkylation with
propylene
to
form
1-isopropyl-2,4-
t r i m e t h y l b e n z e n e i s an i m p o r t a n t r e a c t i o n t o produce a b u l k y molecule. REAYIMATION W i t h i n c r e a s i n g r e a c t i o n temperature t h e HUSY z e o l i t e i s s e r i o u s l y coked. The HZSM5 however,
i s reanimated. Fig.
5 shows t h e c a t a l y s t l i f e t i m e and t h e y i e l d
o f coke as a f u n c t i o n o f r e a c t i o n temperature.
Catalyst l i f e time (time during
which 100 % methanol conversion i s o b t a i n e d ) i n c r e a s e s b y more t h a n 3 o r d e r s o f magnitude from about 5 minutes a t 270 OC t o approximately 2 0 0 hours a t 400 The average coke y i e l d , combustion,
as o b t a i n e d a t t h e end o f
d e c l i n e s from about 1 2 % a t 280
OC
OC.
l i f e t i m e by means of
t o about 0.1
% a t 315
OC.
Methanol Conversion on HZSM-5
287
CATALYST LIFE TIME
,q *
!a ,COKE
,{
200
YIELD ~!
300
500
400
REACTDN TEMPERATURE ,
OC
F i g . 5: Reanimation o f HZSM5 f o r methanol conversion. Coke y i e l d and c a t a l y f t l i f e t i m e as a f u n c t i o n o f r e a c t i o n temperature. p = 2.5 bar, WHSV = 1 hY i e l d o f coke determined by means o f combustion afE@O!reating the catalyst i n a Argon f l o w o f 100 ml/min a t t h e t e m p e r a t u r e o f r e a c t i o n : l i f e t i m e d e f i n e d as break-through-time o f methanol.
.
It i s concluded t h a t r e a n i m a t i o n o f t h e c a t a l y s t i s caused by i n t e r c o n v e r s i o n o f a r o m a t i c compounds i n t h e z e o l i t e . These r e a c t i o n s as i s o m e r i z a t i o n and a l k y l transfer
tend
to
establish
Dealkylation o f propyl-
equilibria
between
and e t h y l - s u b s t i t u e n t s
will
the
aromatic
a l s o occur.
compounds.
Now f r o m t h e
p o o l o f a r o m a t i c compounds t h e l e s s b u l k y ones can l e a v e t h e p o r e system. i n t e r c o n v e r s i o n r e a c t i o n s o f a r o m a t i c s s t a r t a t a p p r o x i m a t e l y 290 i n c r e a s i n g l y more e f f e c t i v e w i t h i n c r e a s i n g temperature.
OC
The
and a r e
Thus t h e i n t e r i o r o f
t h e HZSM5 channel system remains a c c e s s i b l e f o r t h e r e a c t a n t s and o n l y a c e r t a i n amount o f s t e a d y s t a t e m i x t u r e o f a r o m a t i c compounds occupies t h e pores. DEACTIVATION Above 400 O C coke y i e l d s t a r t s t o i n c r e a s e and c a t a l y s t l i f e t i m e decreases ( F i g . 5). i n detail.
C a t a l y s t d e a c t i v a t i o n t h r o u g h t h i s ' ' r e a l coking" has been i n v e s t i g a t e d As mentioned above an i n e r t r e a c t o r system was developed c o n s i s t i n g
o f an i n n e r fused s i l i c a r e a c t o r tube. T h i s was f i l l e d w i t h t h e c a t a l y s t : a t h i n
10
m l a y e r o f z e o l i t e c r y s t a l l i t e s on q u a r t z
particles.
When removing t h e
r e a c t i o n t u b e f r o m t h e r e a c t o r a f t e r about 40 % o f c a t a l y s t
l i f e time the
f o l l o w i n g p i c t u r e was observed ( F i g .
6). The q u a r t z p a r t i c l e s b e f o r e and b e h i n d
t h e c a t a l y s t bed had remained w h i t e .
The d e a c t i v a t e d c a t a l y s t b e h i n d t h e d a r k
narrow r e a c t i o n zone had t u r n e d t o b l a c k and t h e zone o f a c t i v e c a t a l y s t b e f o r e t h e r e a c t i o n zone had t u r n e d t o b l u e / g r e y .
This "blue
coke''
i s formed f r o m
o l e f i n s because methanol i s c o m p l e t e l y c o n v e r t e d t o hydrocarbons i n t h e r e a c t i o n zone and i s n o t p r e s e n t down stream o f t h e r e a c t i o n zone.
T h i s b l u e coke i s
s i t u a t e d on c r y s t a l l i t e s u r f a c e s because t h e ZSM5 p o r e system i s t o o narrow f o r h o s t i n g "coke-molecules".
I t has been observed p r e v i o u s l y [6,
71
t h a t such coke
288 H. Schulz, S. Zhao and H.Kusterer
CATALYST BED -QUARTZ+ LAYER OF HZSMS CRYSTALLITES ON QUARTZ PARTICLES ZONE + 0 - 1 ~ 1 1 - 1 1 1 - 0 4 FOUARTZPARTICLES
PAR TICLE S
Fig. 6: Schematic drawing o f ZSM5 c a t a l y s t bed d e a c t i v a t i o n . View o f t h e fused s i l i c a r e a c t i o n t u b e a t about 40 Z o f c a t a l y s t l i f e time. Black zone ( I ) o f d e a c t i v a t e d c a t a l y s t p a r t i c l e s covered w i t h coke ("methanol coke"). Small dark r e a c t i o n zone (11) i n which methanol conversion t o 100 % occurs. B l u e / g r e y zone (111) o f a c t i v e c a t a l y s t on which a small amount o f " o l e f i n coke" produced by t h e o l e f i n i c hydrocarbon p r o d u c t m i x t u r e has been deposited on t h e c r y s t a l 1 i t e surfaces. The q u a r t z p a r t i c l e s b e f o r e and behind t h e c a t a l y s t bed (zones 0) remain e s s e n t i a l l y white. seeds a r e capable f o r r e a c t i n g w i t h methanol d i r e c t l y t o form f u r t h e r coke t o g e t h e r w i t h methane as t h e coproduct. H-coke
+
2 ,CH3-OH
--c
H-C-Coke
+
CH4
(2)
t H20
T h i s t y p e o f coke f o r m a t i o n occurs i n t h e r e a c t i o n zone and leads t o coverage o f t h e z e o l i t e c r y s t a l l i t e s and c l o s u r e o f t h e pore entrances.
*'"
TABLE 5 COMPARISON OF HZSMS SAMPLES FOR HIGH TEMPERATURE DEACTIVATION WITH METHANOL AS REACTANT
0.5 9 ZEOLITE COATED ON 7 9 QUARrZ PARTICLES (0.2 ~ 2.5 ~bar, pN ~ = 2.5~ bar,~ WHSV == 1 h-'. 2
P
-
0.4 m), CATALYST BED VOLUHE 11
T = 475 ' C .
METHANOL CONVERSION
CATALYST
I DENT IF. No wwl
1
DURATION OF ZEOLITE SYNTHESIS
I I1 111 IV V
3 5 5
gc/
235
235
68
68
2.5
PROPANE/ PROPENERATIO AT
COKE FORM.
-
DAYS
cm3,
0.1 NTOS*l
9ZEOL
0.25 0.41 0.18 0.17 0.31
0.22 0.80 0.20 1.2
7.1 2.7 7.1 10.0
2.0 2.0 1.2 7.0 5.8
NTOS = NORMALIZED TIME ON STREAM; NTOS = 1 .o AT METHANOL BREAK THROUGH AT THE REACTOR OUTLET. **I CATALYST ORIGIN: I I 1 1 PROF. O'CONNOR. UNIV. OF CAPE TOWN, I V PROF. STEINBERG, UNIV. OF LEIPZIG, V DEGUSSA COMP. *)
-
Methanol Conversion on HZSM-5
289
C ATA LY ST IDENTI FICAT ION
AVERAGE CHqSELECTIVITY, C-'10
GRAMS METHANOL CONVERTED UNTIL METHANOL BREAK THROUGH PER GRAM OF H E M 5 (CATALYST LIFE TIME)
F i g . 7: H i g h t e m p e r a t u r e HZSM5 d e a c t i v a t i o n d u r i n g methanol conversion.
Amount
o f d e p o s i t e d coke, and average s e l e c t i v i t i e s o f coke and methane i n c o r r e l a t i o n t o c a t a l y s t l i f e time.
F i v e samples o f z e o l i t e HZSM5 have been c o m p a r a t i v e l y i n v e s t i g a t e d f o r t h e i r d e a c t i v a t i o n b e h a v i o u r a t h i g h t e m p e r a t u r e (475
C h a r a c t e r i z a t i o n d a t a and
OC).
r e s u l t s f r o m methanol c o n v e r s i o n a r e g i v e n i n Table 5. The i n i t i a l m o l a r p r o p a n e / p r o p e n e - r a t i o ( a t 10 % o f c a t a l y s t l i f e t i m e ) i s a measure o f c a t a l y s t a c t i v i t y .
I t grossly correlates w i t h the c a t a l y s t S i / A l -
r a t i o . F i g . 7 concerns f u r t h e r p r o o f s o f c a t a l y s t l i f e time.
No c o r r e l a t i o n i s
observed between t h e amount o f coke d e p o s i t e d and t h e t o t a l amount o f methanol which i s c o n v e r t e d d u r i n g t h e c a t a l y s t l i f e time. However, a c o r r e l a t i o n appears t o e x i s t between ( 1 ) coke s e l e c t i v i t y and c a t a l y s t l i f e t i m e and (2) methane s e l e c t i v i t y and coke s e l e c t i v i t y . These i n t e r r e l a t i o n s a r e c o n s i s t e n t w i t h t h e above model o f h i g h t e m p e r a t u r e d e a c t i v a t i o n b y coke f o r m a t i o n t h r o u g h a r e a c t i o n o f coke g r o w t h w i t h methanol. However,
t h i s mechanism needs coke seeds p r o v i d e d as " o l e f i n coke" on e x t e r n a l
a c i d i c centers.
Development o f ZSM5-catalysts
w i t h l o n g l i f e t i m e t h u s concerns surfaces.
f o r h i g h temperature a p p l i c a t i o n
minimizing o f
acid sites
on c r y s t a l l i t e
290 H. Schulz, S. Zhao and H. Kusterer
CONCLUSION Methanol conversion on ZSM5 and o t h e r c a t a l y s t s e l u c i d a t i n g probing reaction f o r c a t a l y s t a c t i v i t y ,
i s found t o be a v e r y
i n t e r n a l and e x t e r n a l a c t i v e
s i t e s and spacious c o n s t r a i n t s . D e a c t i v a t i o n mechanisms i n d i f f e r e n t temperature ranges a r e s p e c i f i e d . Basic r e a c t i o n steps and r e a c t i o n pathways a r e understood from t h e d e t a i l e d t i m e r e s o l v e d s e l e c t i v i t y data. LITERATURE 1 T. Mole, J. Catal., 84 (1983) 423 2 R. L. Espinoza, Ind. Eng. Chem.-Prod. Res. Dev., 23 (1984) 449 3 C. D. Chang, A. J. S i l v e s t r i , J. Catal., 47 (1977) 249 4 D. M. Bibby, Proc. 7 t h I n t e r n . Conf. on Z e o l i t e s . Tokyo, 1D-11 5 M. Guisnet, P. Magnoux and C. Canaff, i n R. S e t t o n ( E d i t o r ) , Chemical Reactions i n Organic Constrained Systems, Reidel, Dordrecht, Boston, 140 Lancaster, Tokyo, (1986) 131 6 H. Schulz, W. Bohringer, W. Baumgartner and Zhao Siwei, Proc. 7 t h I n t e r n . Conf. on Z e o l i t e s , Tokyo (1986) 915 - 922 7 H. Schulz, Zhao Siwei and W. Baumgartner, i n B. Delmon and C. Froment (Edts.), Stud. Surf. Sci. Catal., Vol. 34, (1987) 479 - 492
-
291
Shape Selective Reactions of Alkylnaphthalenes in Zeolite Catalysts
Jens Weitkampl and Manta Neuber2 1Institute of Chemical Technology I, University of Stuttgart, Pfaffenwaldring55, D-7000 Stuttgart 80,Federal Republic of Germany 2Engler-Bunte Institute, University of Karlsruhe, D-7500Karlsruhe. Present address: Hoechst AG, P.O.Box 80 03 20, D-6230 Frankfurt 80, Federal Republic of Germany
Abstract
The catalytic isomerization of 1-methylnaphthaleneand alkylation of 2-methylnaphthalene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into na hthalene and dimethyha hthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-#eta. Optimum catal sts for the shape selective methylation of 2-methylnaphthalene are HZSMJ and HZSM-1iy. All experimental findings concerning this reaction can be readily accounted for by conventionalproduct shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent publications from other groups.
Introduction Since its discovery about 30 years ago [l-31, shape selective catalysis in microporous crystalline materials has been the subject of countless investigations. Review articles are now available [4-101 in which the principles and classification of shape selectivity effects are discussed and numerous examples are given. The advantages of shape selective catalysis are already exploited in a number of industrial processes [ll-141. Astonishingly, virtually all these processes rely on a singre structural type of cafalyst, vu. zeolite ZSM-5 in various modifications, or its titanium containing analogue TS-1 [15]. It is, moreover, noteworthy that many of these processes convert and/or produce mononuclear aromatic compounds. It is not surprising, therefore, that a vast scientific literature exists on shape selective reactions of benzene derivatives in zeolite ZSM-5. In contrast, there are relatively few publications on the conversion of polynucleur aromutics in zeolite catalysts. L e e et al. [16] found unusually high selectivities for 44'diisopropylbiphenylwhen dealuminated mordenite was used as catalyst for the alkylation of biphenyl with propene. The reactions of 1- and 2-methylnaphthaleneon acid forms of zeolite X, Y,Omega, mordenite and ZSMd were studied by D i m i t r o v et al. [17], S o 1 i n a s et
292 J. Weitkamp and M. Neuber
al. [18-201 and M a t s u d a et al. [21]. Shape selectivity effects were reported by all these groups to be absent on the twelve-membered ring zeolites. In other respects, however, the results and conclusions of the three groups are contradictory. For instance, Solinas et al. report [191 that HZSMJ is practically inactive in the isomerization of l-methylnaphthalene at 210 to 330 "C, and this lack of activity is attributed to the narrow pores. By contrast, Matsuda et al. [21] observed considerable activity of HZSMd at 300 "C in the isomerization of both 1and 2-methylnaphthalene, and the external surface is claimed to be responsible for this catalytic activity. Dimitrov et al. [17], using MgY at 350 "C, found that isomerization of l-methylnaphthalene is accompanied by transalkylation (or disproportionation) into naphthalene and dimethylnaphthalenes. Solinas et al. [18, 191, using acid forms of zeolite Y and Omega, also detected naphthalene as a side product, but no dimethylnaphthalenes; from this they concluded that naphthalene forms via dealkylation rather than by transalkylation, even though this was not confirmed by the detection of methane in the gaseous product fraction. According to Matsuda et al. [21], disproportionation of both methylnaphthalenes takes even place on HZSM-5 at 300 "C and, starting from 2-methylnaphthalene, proceeds shape selectively with enhanced portions of the slim 2,6- and 2,7-isomers in the dimethylnaphthalenefraction. The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-251 on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 unlike on H-mordenite or HY the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 251. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel's experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. In this paper, we report on the shape selective isomerization of 1:methylnaphthalene in suitable zeolite catalysts in which the undesired transalkylation reaction is suppressed. Furthermore, new results concerning the alkylation of 2-methylnaphthalene with methanol are presented in an endeavor to contribute to a critical evaluation of Fraenkel's model. At the same time, the potential of shape selective catalysis for the manufacture of 2-methylnaphthaleneand 2,6-dimethylnaphthalenewill be shown which are raw materials for vitamin K and specialty polyesters or polyamides, respectively [29].
-
-
Experimental Materials The following zeolites were synthesized according to recipes derived from the literature: ZSM-5 [30], ZSM-11 [31], ZSM-12 [32], ZSM-20 [33], Beta [33] and EU-1 [34]. Zeolites L
Shape Selective Reactions of Alkylnaphthalenes 293
and Y were purchased from Union Carbide Corp., and mordenite from the Norton Co. Two samples of zeolite Y, designated YD1 and YD2 were made by dealumination with Sic& vapors after Beyer and Belenykaya [35]. All zeolites were characterized by X-ray powder diffraction and scanning electron microscopy. For chemical analysis, the zeolites were dissolved in aqueous HF (40%) whereupon aluminium and silicon were determined by atomic absorption spectroscopy. The Si/M ratio of ZSM-11 was 35, for the other zeolites it is given in Table 1. To remove the organic templates, the zeolites synthesized in our laboratory were calcined in air at 540 "C for 12 h, except for zeolite Beta which was calcined at 400 "C for 3 days. All zeolites were ion exchanged twice with a 0.5 n aqueous solution of NH,C1 for 2 h under reflux. After drying at 110 "C the powders were pressed binder-free, crushed and sieved. The particles with a size between 0.2 and 0.3 mm were used for the catalytic runs. Activation was done in-situ at 400 "C for 6 h in flowing nitrogen. 1-Methylnaphthalene,2-methylnaphthaleneand methanol were purchased from Merck or Fluka in the highest purity grade available and used without further purification. Catalytic experiments The catalytic experiments were carried out in a fixed bed flow-type apparatus in the gas phase at atmospheric pressure, ,@ = 100 kPa). The carrier gas (N, , nominal purity 99.99 v01.-%) was loaded with vapors of the reactant(s) in (a) thermostated saturator(s). The mass of dry catalyst was ca. 200 mg. In the isomerization experiments, the partial pressure in the feed and the modified residence time were pl-M-Np= 2.0 kPa and W/F,,-Np = 110 gh/mol, respectively. In the alkylation experiments, the partial pressures in the feed amounted to p>M-Np = 1.9 kPa and hoH = 0.95 kpa, and W/QM,, was 170 gh/mol. Product analysis was done by automatic on-line sampling and high resolution glc using a capillary column of 50 m length, OV-1 as stationary phase and H, as carrier gas. Various temperature programs were employed depending on the product mixture to be analyzed.
Results and Discussion Isomerization of 1-methylnaphthalene The typical catalytic performance of HY, a zeolite with a spacious, three-dimensional pore system is depicted in Fig. 1for two reaction temperatures. Both at 200 "C and at 300 "C, deactivation is severe. At low reaction temperatures (ca. 180 to 250 "C), isomerization into 2-methylnaphthalene is the main reaction. However, it is always accompanied by transalkylation into naphthalene and dimethylnaphthalenes.At 200 "C, for instance, the yield of transalkylationproducts ranges from ca. 2 to 5 %. Thermodynamic calculations based on compiled [36] Gibbs free enthalpies of formation result in an equilibrium composition of ca. 40 mole-% 1-M-Npand 60 mole-% 2-M-Np in the temperature range from 200 to 300 "C. Attempts to determine the position of equilibrium experimentally [37, 381 indicated that 30 mole-% 1-M-Np and 70 mole-% 2-M-Np is a more
294 J. Weitkamp and M. Neuber
TIME ON STREAM, h Fig. 1. Conversion of 1-methylna hthalene (Xl&Np) and product yields (Y) on zeolite HY at 200 "C and 300 "C (Tr = transJVlation products, 1. e., naphthalene and dimethylnaphthalenes; Cr = cracked products with ess than 11 carbon atoms; CI1 = hydrocarbons with 11 carbon atoms). realistic distribution and this is in agreement with our accumulated data. In any event, isomerization equilibrium is not attained at 200 "C on zeolite HY, even not when it is in its fresh state. Raising the temperature to 300 "C expectedly leads to increased conversion. However, undesired side reactions now become predominant, especially at the beginning of the run (cf. Fig. l), and as a consequence deactivation is even much faster. The same principal activity/selectivity dilemma occurred on certain other large pore zeolites, especially on HZSM-20 and, less pronounced on HL. It is noteworthy at this point that methane was not found as a product, although it was ascertained that it could have been detected if it had formed. This indicates unambiguously that naphthalene is formed by transalkylation rather than by dealkylation, at variance with the conclusions of Solinas et al. [18,19]. Indeed, dimethylnaphthaleneswere always present in the product, although the yield ratio YDM-Np/YNp never reached unity, even not at mild temperatures (e. g., 180 "C) where consecutive transalkylations into trimethylnaphthalenes were absent. This is an interesting finding which indicates that some dimethylnaphthalenes accumulate on the catalyst, in other words they are held strongly and not displaced efficiently from the zeolite surface by fresh methylnaphthalenes. Deactivation at mild temperatures,
Shape Selective Reactions of Alkylnaphthalenes 295
where cracking is absent, must then be due, at least in part, to self-poisoning by dimethylnaphthalenes. As already stated, isomerization on zeolite HY was always accompanied by disproportionation, even at 180 "C. With time on stream, Y,M-Np increases, because these heavy products are most efficiently held by the fresh catalyst. It is an interesting result that, at 180 "C, the yield of naphthalene passed through a maximum as well. Obviously, under appropriate reaction conditions, the disproportionation of methylnaphthalenes in zeolite HY exhibits an induction period, as does the disproportionation of ethylbenzene in large pore zeolites [39,40]. It was concluded at this point that zeolites with a very spacious pore system, such as faujasites or ZSM-20, are inappropriate catalysts for the isomerization of l-methylnaphthalene. Subsequently, a zeolite with much narrower pores was tested, viz. HZSM-5. Pertinent results are shown in Fig. 2. At 300 "C, the conversion is low and even a temperature increase of 100 "C does not bring about a considerable increase in conversion. We presume that the reaction of 1-methylnaphthalenein HZSM-5 is controlled by diffusion. There were practically no side reactions such as cracking, dealkylation or transallylation, in other words Xl-,-Np and YZ-M-Npare identical. This is at variance with the results of Matsuda et al. [21] who did observe some disproportionation on their HZSM-5 sample at 300 "C. More work is needed to elucidate the reasons for this different catalytic behavior of various samples of HZSMJ. As a whole, zeolite ZSMJ was discarded at this stage due to its too narrow pore system. In the endeavor to find good catalysts for the isomerization of 1-methylnaphthalene,the Spaciousness Index (SI) which was introduced earlier by our group [41,42] turned out to be a very useful guide. The SI values are slightly below 1for ZSMJ and other ten-membered ring zeolites and around 20 for very open and spacious structures such as Y or ZSM-20 [42]. In
TIME ON STRUM, h
Fig. 2. Isomerization of 1-methylnaphthalene on zeolite HZSMJ at 300 "C and 400 "C. There are no side reactions, hence X1-M-Np= YzM-Np.
296 J. Weitkamp and M. Neuber
100
I
I
HZSM-12
(SI = 3) Si/Al = 35
75-
: w >.
100
-
75
so:\
:
25-
I
I
I
I
H-Mordenitc 75
I
H-EU-1
(SI = 5) Si/AI = 20
-
-
- :05
I
- 1 00I
X
I
-
25I
I
1001
(SI = 7)
I
I
I
H-Beta
(SI = 16)
I
Si/A = 6.7
TIME ON STREAM, h
Fig. 3. Conversion of 1-methylna hthalene and product yields on zeolites with intermediate Spaciousness Indices (SI) at 300
"dl
subsequent runs, the catalytic performance of zeolites with intermediate Spaciousness Indices was investigated. Typical results are presented in Fig. 3. A comparison with the data for 300 "C in Fig. 1 clearly shows that in the zeolites with an intermediate Spaciousness Index (ca. 2 to 16), the undesired transalkylation is strongly suppressed, presumably due to transition state shape selectivity [7, 101, i. e., the bulky transition states or intermediates required for the bimolecular disproportionation cannot form inside these zeolites. In comparison with ZSMJ (cf. Fig. 2) the twelve-membered ring zeolites give much higher conversions,which are initid@ close to the equilibrium conversion of ca. 70 %. Inevitably, the zeolites with a Spaciousness Index between 2 and 16 have a higher Si/Al ratio than zeolites Y or ZSM-20. To separate the effects of the Si/Al ratio and the pore width, two dealuminated samples of zeolite Y, designated YD1 and YD2, were tested which resembled in their Si/AI ratios the Beta and EU-1 sample, respectively. Table 1 gives the isomerhation selectivitiesat 40 % conversion. While dealumination of zeolite Y brings about some improvement in selectivity, H-Beta is clearly superior to HYD1, and H-EU-1 is a much better catalyst than HYD2, which indicates that the pore width in the appropriate range is of prime importance. Fig. 3 shows that H-mordenite, while exhibiting an excellent selectivity, undergoes rapid deactivation which, without any doubt, is due to coking. The propensity of H-mordenite towards deactivation is well known from many other reactions and has generally been
Shape Selective Reactions of Alkylnaphthalenes 297
Table 1. Conversion of 1-methylnaphthaleneon various zeolite catalysts at 300 "C. Influence of the effective pore width, expressed b the Spaciousness Index (SI, [41, 42]), and the Si/AI ratio on the isomerization selectivity ( e time on stream was chosen in such a manner that XI-,-,, amounted to 40 %).
x
I
1
Zeolite Catalyst
SI
HY HYDl HYD2 HZSM-20 HL H-Beta H-Mordenite H-EU-1 HZSM-12 HZSM-52)
21 21 21 20 17 13-19l)
7 5 3
<1
Si/M 2.4 11 19 3.8 3.9 10 6.7 20 35 67
SZM-N~ in % 91 92 95 96 96 99.3 99.3 99.8 99.8 100 9
I
2,
For zeolite Beta, SI values between 13 and 19 have been reported in the literature. considerably below 40 %, cf. Fig. 2.
attributed [43-451 to the unidimensional pore system. The results presented in Fig. 3 suggest that other factors must, at least in part, play a role or contribute to the rapid coking of H-mordenite in the reaction studied here; otherwise one would expect the same rapid coking to occur on HZSM-12 since it possesses unidimensional, puckered twelve-membered ring pores as well [46]. Coke formation on zeolites is a very complex phenomenon [45, 471 which has probably often been oversimplified in the past and studied with a too small number of zeolites. The systematic use of more recent zeolites, such as ZSM-12 and others, in studies on coking might lead to a much better understanding of the factors which govern deactivation by coking. Evidence has been obtained by IR spectroscopy [48] that the build-up of coke during the conversion of methylnaphthalenes in HZSM-12 is hindered on account of the spatial constraints inside the pores. Alkylation of 2-methylnaphthalene with methanol In a preliminary screening, the alkylation of 2-methylnaphthalene was studied using a variety of acid zeolites with different pore widths. In principal agreement with the earlier work of Fraenkel et al. [22-251 it was found that the best selectivities for the slim alkylation products, i. e., 2,3-, 2,6- and 2,7-dimethylnaphthalene, are obtained on HZSM-5 and HZSM-11. On these catalysts it was observed that the alkylation is always accompanied by the undesired isomerization into 1-methylnaphthalene. Moreover, a peculiar deactivation behavior was encountered: With time on stream, the yield of 1-methylnaphthalene always dropped while the yield of alkylation products remained practically constant or even slightly increased. An example for the conversion and yield curves is given in Fig. 4. The distribution of the dimethylnaphthalene isomers is shown for the same experiment in Fig. 5. Bearing in mind that in equilibrium one would expect roughly 12 mole-% of each of the slim isomers, the
298 J. Weitkamp and M. Neuber
TIME ON STREAM, h
Fig. 4. Reaction of 2-methylnaphthalenewith methanol on HZSM-5 at 400 "C.
2.7-DU - N p
2,6-DU-Np [r:
z
8
HZSM-5 T = 400 F
20-
"
OO
'
4
8
'
I
12
"
16
'
I
'
20
TIME ON STREAM, h
Fig. 5. Alqlation of 2-methylnaphthalenewith methanol on HZSMJ at 400 "C. Composition of the dimethylnaphthalenefraction. shape selectivity effect is very pronounced: About 50 % of the dimethylnaphthalene fraction consist of the 2,64somer, and the three slim (2,3-, 2,6- and 2,7-) isomers together account for more than 90 % afrer c a 10 h on stream. It is, moreover, clearly seen that the shape selectivity becomes more pronounced as the zeolite is on stream. This is a well known effect in zeolite catalysis and has been referred to as coke selectivation [49]. We interprete the above effects as conventional product shape selectivity inside the pore system of zeolite ZSMJ or ZSM-11, and part of our arguments were presented earlier, in a preliminary note [a]. While the catalyst is on stream, coke is gradually formed and deposits, in part, inside the channel system. As a consequence, the diffusion pathways for product molecules increase. Slim molecules, such as 2,6-dimethylnaphthalene are less affected than
Shape Selective Reactions of Alkylnaphthalenes 299
bulkier ones, such as 1-methylnaphthalene.This way, both the changes in the composition of (Fig. 4) the dimethylnaphthalene fraction (Fig. 5 ) and the ever occurring decrease in Y1-M-Np can easily be rationalized. Our interpretation implies, of course, that the methylnaphthalenes do have access to the pentasil pores under reaction conditions. Indeed, it was demonstrated unambiguously by simple adsorption experiments [SO] that bmethylnaphthdene does enter the intracrystalline pore volume of ZSM-5 at temperatures as low us 100 "C. In other words, the assumptions on which Fraenkel's "half-cavity" model [22,24,25] is based, are highly questionable. We anticipate that at the much higher reaction temperature of 400 "C, even 1-methylnaphthalenedoes have "access to the pores". In additional experiments, HZSM-5 was precoked by converting methanol alone (into hydrocarbons) at 400 "C.Afterwards the zeolite was exposed to the 2-methylnaphthalene/methanol mixture, under the usual reaction conditions. The initial yield of l-methylnaphthalene was significantly reduced (1.5 % compared to 4 % for the fresh catalyst, cf. Fig. 4). Furthermore, the initial content of 2,6- + 2,7-dimethylnaphthalene in the dimethylnaphthalene fraction was 84 % instead of 70 % for the fresh catalyst. In another run, HZSM-5 was loaded with 0.5 wt.-% of Pt, and H, was used as carrier gas instead of N,. Under these conditions, the formation of coke was avoided or at least drastically diminished. In-line with our model, no changes in the product yields and in the distribution of the dimethylnaphthaleneisomers were observed with time on stream.
Conclusions Shape selective catalysis which has been applied to numerous reactions of mononuclear aromatics, can clearly be extended to the conversion of naphthalene derivatives. A striking example is the isomerization of 1-methylnaphthalene into the more valuable 2-methylnaphthalene: In acid zeolites with a Spaciousness Index [41,42] between ca. 2 and 16, the undesired disproportionation into naphthalene and dimethylnaphthaleneis almost completely suppressed. This is easily understood in terms of transition state shape selectivity. Particularly good catalysts for the selective isomerization of 1-methylnaphthalene are H-Beta, H-EU-1 and HZSM-12, which deactivate at a moderate rate. H-mordenite, while also selective for isomerization, undergoes rapid coking. The widely accepted explanation for the coking propensity of H-mordenite, viz. plugging of the unidimensional pores, has to be questioned: HZSM-12 possesses unidimensional pores as well, but it deactivates more slowly than mordenite. On zeolite HY, isomerization of 1-methylnaphthaleneis always accompanied by transalkylation. At mild reaction temperatures, e.g. 180 "C,this transalkylation seems to exhibit an induction period, an effect which is better known from the conversion of ethylbenzene in large pore zeolites [39,40]. Perhaps, this induction period is of more general importance in the disproportionation of alkyl aromatics in zeolites with sufficiently large pores [51]. 2-Methylnaphthalene can be methylated shape selectively using HZSM-5 or HZSM-11. The preferred products are 2,3-, 2,6- and 2,7-dimethylnaphthalene,i.e., the slim alkylation
300 J. Weitkamp and M. Neuber
products. On a fresh ZSM-5 catalyst, alkylation is accompanied by isomerization, it was discovered, however, that the yield of l-methylnaphthalene always declines with time on stream. In our opinion, classical product shape selectivity and coke selectivation readily account for the observed effects, so there is no need to invoke shape selective catalysis at the external surface, e.g., in "half-cavities",or a "nest effect".
Acknowledgements Financial support by Deutsche Forschungsgemeinschaft within the frame of Sonderforschungsbereich 250, "Selektive Reaktionsfiihrung an festen Katalysatoren" is gratefully acknowledged. We moreover acknowledge funding by Fonds der Chemischen Industrie and Max Buchner-Forschungsstiftung. References [l] P.B. Weisz and VJ. Frilette, J. Phys. Chem. 64 (1960) 382. [2] P.B. Weisz, V.J. Frilette, R.W. Maatman and E.B. Mower, J. Catal. 1(1962) 307. [3] N.Y. Chen and P.B. Weisz, in P.B. Weisz and W.K. Hall (Eds.), Kinetics and Catalysis, Chem. Eng. Progress Symposium Series, Vol. 63, AIChE, New York, 1967, p. 86. [4] S.M.Csicse , in J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis (ACS Monograph 171) Am. C em. SOC.,Washington,D.C., 1976, p. 680. [5] P.B. Weisz, Pure Ap 1. Chem. 52 (1980) 2091. [6] C.D. Chan , W.H. ! a n and W.K. Bell, in W.R. Moser (Ed.), Catalysis of Organic Reactions khemical Inckstries, Vol. 5), Marcel Dekker, New York, Basel, 1981, p. 73. [7] S.M. Csicsery, Zeolites 4 (1984) 202. [8] W.O. Haag, D.H. Olson and P.B. Weisz, in H. Griinewald (Ed.), IUPAC, Chemistry for the Future, Pergamon Press, Oxford, New York, 1984, p. 327. [9] S.M. Csicsery, Pure Ap 1. Chem. 58 (1986) 841. 10 J. Weitkamp, S. E r s t , .Dauns and E. Gallei, Chem.-In .-Tech. 58 (1986) 623. Ell] W. Holderich and E. Gallei, Chem.-In .-Tech. 56 (1984) 808 [12] N.Y. Chen and W.E. Garwood, Catal.kev. Sci. Eng. 28 (1986) 185. [13] N.Y. Chen, W.E. Garwood and F.G. Dwyer, Sha e Selective Catalysis in Industrial Ap lications (Chemical Industries, Vol. 36), Marcel ekker, New York, Basel, 1989. El41 B. otari, Stud. Surf. Sci. Catal. 37 (1988) 413. 151 G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, in Y. Murakami, A. Iijima and J.W. Ward (Eds. , New Developments in Zeolite Science and (Proc. 7th Int. Zeolite C o d , Tokyo, August 17-22, 1986), Kodansha/ % ~ ~ ~ ~ ~Amsterdam, ~ % k y 1986, o . 129. S.C. Rocke and f M . GarcCs, Catalysis Letters 2 (1989) 243. [16] G.S. Lee, J.J. [17] C. Dimitrov, Z. Popova and Mai Tuyen, React. Kinet. Catal. Lett. 8 (1978) 101. [18 V. Solinas, R. Monaci, B. Marongiu and L.Forni, Appl. Catal. 5 [19] V. Solinas, R.Monaci, B. Marongiu and L. Forni, Ap 1. Catal. 9 (20 V. Solinas, R. Monaci, E. Rombi and L. Forni, Stud. urf. Sci. Catal. 34 (1987) 493. [2l] T. Matsuda, K.Yogo, T. Nagaura and E. Kikuchi, in J.C. Jansen, L. Moscou and M.F.M Post (Eds.), Zeolites for the Nineties (Recent Research Reports, 8th Int. Zeolite Cod.; Amsterdam, July 10-14,1989), 1989, p. 431. [22] D. Fraenkel, M. Cherniavs and M. Levy, Proc. 8th Int. Congress on Catalysis, Vol. 4, Verlag Chemie, Weinheim, eerfield Beach, Basel, 1984, .545. [23] D. Fraenkel, B. Ittah and M. Le , 7th Int. Zeolite C o J , Preprints of Poster Papers, Japan Assoc. of Zeolites, Tokyo, 1 86, p. 271. [24] D. Fraenkel, M. Cherniavs ,B. Ittah and M. Levy,J. Catal. 101 (1986) 273. [25] D. Fraenkel, J. Molec. Cata .51(1989) L 1. E.G. Derouane, J. Catal. 100 (1986) 541. E.G. Derouane, J.M. Andre and A.A. Lucas, J. Catal. 110 (1988) 58.
x
R
5
fi
dj,
%
'r
T
Shape Selective Reactions of Alkylnaphthaleiies 301
[28] M. Neuber and J. Weitkamp, in J.C. Jansen, L. Moscou and M.F.M. Post (Eds.), [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]
Zeolites for the Nineties (Recent Research Reports, 8th Int. Zeolite Conf., Amsterdam, July 10-14, 1989), 1989, p. 425. H.G. Franck and J.W. Stadelhofer, Industrielle Aromatenchemie, Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, To 0,1987, p. 348-352. E.G. Derouane, S. Detremmerie, Z. Gabelica and7 4. Blom, Ap 1. Catal. 1(1981) 201. P. Chu, US Patent 3 709 979, assigned to Mobil Oil Corp., Jan. 1973. S. Ernst, P.A. Jacobs, J.A. Martens and J. Weitkamp, Zeolites 7 (1987) 458. S. Ernst, G.T. Kokotailo and J. Weitkamp, Stud. Surf. Sci. Catal. 37 (1988) 29. G.W. Dodwell, R.P. Denkewicz and L. B. Sand, Zeolites 5 (1985) 153. H.K. Beyer and I. Belenykaja, Stud. Surf. Sci. Catal. 5 (1980) 203. D.R. Stull, E.F. Westrum, Jr., and G.C. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley & Sons, New York, London, Sydney, Toronto, 1969,
8,
389-399. !?N. Vorozhtov and V.A. Koptyug, J. Gen. Chem. USSR 30 (1960) 1014. R.N. Bhattacha a, J. Indian Chem. SOC.58 (1981) 682. H.G. Karge, K. atada, Y. Zhang and R. Fiederow, Zeolites 3 (1983 13. J. Weitkam S. Ernst, P.A. Jacobs and H.G. Karge, Erdol, Kohle drdgas Petrochem. 39 (1986) 1f: J. Weitkamp, S. Ernst and R. Kumar, Appl. Catal. 27 (1986) 207. J. Weitkamp, S. Ernst and C.Y. Chen, Stud. Surf. Sci. Catal. 49,Part B (1989) 1115. E.G. Derouane, Stud. Surf. Sci. Catal. 20 (1985) 221. M. Guisnet, P. Magnow and C. Canaff, in R. Setton (Ed.), Chemical Reactions in Organic and Inor anic Constrained Systems, D. Reidel Publishing Co., Dordrecht, Boston, Lancaster, okyo, 1986, p. 131. M. Guisnet and P. Ma now, Appl. Catal. 54 (1989) 1. R.B. LaPierre, A.C. ohrman, Jr., J.L. Schlenker, J.D. Wood, M.K. Rubin and W.J. Rohrbaugh, Zeolites 5 (1985) 346. H.G. Kar e, in H. van Bekkum, E.M. Flanigen and J.C. Jansen (Eds.), Introduction into Zeolite $hence and Practice, Proc. Int. Summer School on Zeolites, Zeist, the Netherlands, July 1989, in preparation. M. Neuber, H.G. Karge and J. Weitkamp, Catal. Today 3 (1988) 11. R.M. Dessau, US Patent 4 444 986, assigned to Mobil Oil Corp., A ril24, 1984. J. Weitkamp, M. Schwark and S. Ernst, Chem.-1ng.-Tech. 61 (19893887. J. Weitkamp, S. Emst, H.G. Karge and R. Schneider, Proc. Int. Conf. on Catalysis and Adsorption by Zeolites, Leipzig, German Democratic Republic, August 20-23, 1990, accepted for publication.
3;
-
$
[45] [46] [47] [48] [49] [50] [51]
#
-
This Page Intentionally Left Blank
303
Alkylation of Biphenyl Catalyzed by Zeolites
Y. SUGIl, T. MATSUZAKIl, T. HANAOKAl, K. TAKEUCHIl, T.TOKORO1, and G. TAKEUCHI2 1National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan 2Research and Development Laboratories, Nippon Steel Chemical Co., Ltd., Kitakyushu, Fukuoka 804, Japan ABSTRACT Catalysis of 12-membered zeolites, H-mordenite (HM), HY, and HL was studied in the alkylation of biphenyl. The para-selectivities were up to 70% for isopropylbiphenyl (IPBP), and 80% for diisopropylbiphenyl (DIBP) in HM catalyzed isopropylation. Catalysis of HY and HL zeolites was nonselective. These differences depend on differences in pore structure of zeolites. Catalysis of HM to give the least bulky isomer i s controlled shape-selectively by steric restriction of the transition state and by the entrance of IPBP isomers. Alkylation with HY and HL i s controlled by the electron density of reactant molecule and by the stability of product molecules because these zeolites have enough space for the transition state to allow all IPBP and DIBP isomers. Dealumination of HM decreased coke deposition to enhance shape selective alkylation of biphenyl. INTRODUCTION Regiospecific functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [l-51. Catalysis using zeolites is the most promising way to prepare sterically small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Steric restrictions by zeolites increase the formation of preferred products and prevent the formation of undesirable products IS]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL in the alkylation of biphenyl. EXPERIMENTAL Ethene, propene, 1-butene, isobutene, and 1-hexene were used as alkylating agents. H-Zeolites, HY, HL, and HM (suppl'iedby TOSOH Corp.) were used as catalysts. Si/A12 ratios were 9.0-220 for HM, 5.8 for HY, and 6.1 for HL. High silica mordenites above 23 of Si/Alp were prepared by dealumination. Silicaalumina (SA, Si/A12=4.3) and HZSM-5 (Si/A12=50) were also used. Si/Al2
304
Y.Sugi et al.
Table 1. Catalytic Alkylation of Biphenyl with Propene React temp . ("C) SA( 4.3) HM(23) HY (5.8) HL( 6.1) HZSM-5(50)
180 250 200 180 300
Conv. (%)
67 4a 76 70 6
Selectivity(%) IPBP DIBP 62 73 60
38 27 40
70 100
30 0
Selectivity(%) in IPBP 23436 5 36 40 16
15 24 23 17 30
49 71 41 43 54
Selectivity(%) in DIBP 4,4'- 3,4'- 3,3'16 78 5 10
9 14 8 9
5 2 7 6
Reaction conditions: Biphenyl, 50 mmol; Propene, 100 mmol; Solvent, transDecalin 20 em3; Catalyst, 1 g; Reaction period, 4 h. ratios are shown in parentheses. All zeolites were calcined at 500°C for 5 h just before use. Typical reaction procedure is as follows: biphenyl (25 mmol), propene (50 mmol), HM (1 g), and trans-decalin (20 em3) were heated in an autoclave at Z O O o 30OOC for 4 h. Products were analyzed by a Hewlett-Packard Gas-chromatograph Model 5890 equipped with a 25 m capillary column of Ultra-1. Thermal gravimetric analysis (TG) of mordenites used for the reaction was conducted by Mettler Model TG-50. Amount of deposited coke was determined from weight loss between 400° and 7OO0C. RESULTS Isopropylation with Propene Catalyzed by HM Zeolite Typical results of catalysis of H-zeolites in isopropylation of biphenyl with propene are summarized in Table 1. Isopropylation with SA was nonselective to form a mixture of three isopropylbiphenyl isomers (IPBP). However, regioselective isopropylation to yield 4-IPBP occurred in the catalysis of HM(23) although the activity was much lower than that of S A . The selectivity of 4-IPBP for HM(23) was more than 70%, but the selectivity of 2IPBP was less than 5%. Higher para-selectivity was observed in the formation of diisopropylbiphenyl isomers (DIBP) as compared to the case of IPBP. The selectivity of 4,4'-DIBP in DIBP isomers was around 80%. In all cases, the formation of triisopropylbiphenyls was negligibly small at 250OC. Figure 1 shows change in the selectivity of products at 250°C in catalysis of HM(23). The selectivity of 4-IPBP gradually decreased with reaction period, and those of 2- and 3-IPBP increased. However, the selectivity of 4,4'-DIBP remains constant during the reaction except in early stages. Isomerization of 4-IPBP and 4,4'-DIBP was negligibly slow under our conditions. However, alkylated products were significantly isomerized during the reaction at 3OO0C to decrease the selectivity of 4,4'-DIBP in the level of 60%.
Alkylation of Biphenyl Catalyzed by Zeolites 305
0
n
h
n
I\
v
..
Q
120
240
360
480
600
72(
- 2-mP
Reaction perioqmin)
Fig. 1 Isopropylation of Biphenyl Catalyzed by HM Zeolite Reaction conditions: HM(23) 3 g, Biphenyl 150 mmol, Propene 750 mmol, 250'C. Effect of Alkenes on Catalysis of IIM Zeolites Effect of alkenes on catalysis of HM zeolites i s summarized in Table 2. Para-selectivity of ethylbiphenyls was 20% with significant formation of 2- and 3-isomers. The selectivity of 4,4'-diethylbiphenyl was as high as 7%. which i s very low compared to the case of 4,4'-DIBP. On the other hand, with alkenes higher than propene, the alkylation gave higher selectivities to form 4alkyl- and 4,4'-dialkylbiphenyls. In particular, isobutene gave regioselectively 4-t-butyl and 4,4'-di-t-butylbiphenyls. A mixture of 2- and 3-hexylbiphenyls Table 2 Alkylation with Various Alkenes Catalyzed by H-Mordenitea React. temp. Conv. Alkene
("C)
Ethene Propene 1-Butene Isobutene 1-Hexene
250 250 220 220 220
(%I >23 48 35 41 39
Selectivitv(%l MABPb DABPC
85 73 82 67 91j
15 27 17 33 9
Selectivity(%I i n IPBP 23436 5 3
-
{'$
45 24 13 4 14h 81
19 71 82(2f) 94(2g) 75h 90i
Selectivity( % ) in DIBP 4,4'-3,4'- 3,3'-
7d n.d.e n.d. 78 14 2 8 6 82 100 n.d n.d. n.d. n.d n.d. n.d.
a)Alkene 100 mmol. Catalyst HM(23) 1 g. Reaction period 4 h. b)Monoalkylbiphenyls. c)Dialkylbiphenyls. d)Determined based on the sum of the componets of m/e=210 by MS analysis. e)Not determined. f)4-t-Butylbiphenyl. g)4-Isobutylbiphenyl. h)2-Hexylbiphenyls (2-HBP). i)3-Hexylbiphenyls (3-HBP). j)2-HBP:3HBP=77:23.
306 Y. Sugi et al.
was obtained in a ratio of 4:l from 1-hexene. The latter were formed by alkylation after the isomerization of 1-hexene to 2-hexene. The paraselectivity of 3-hexylbiphenyl is higher than that of 2-hexylbiphenyl. Isopropylation Catalyzed by HY, HL and HZSM-5 Zeolites Features of the catalysis of HY and HL were quite different from that of HM(23). Product distribution resembled the case of SA. The selectivity of pIPBP at 200°C was as low as 45%, whereas selectivities of 3- and 4-IPBP were as high as 35% and 20%, respectively. The formation of DIBP was also nonselective. Figure 2 shows effect of reaction temperature on H Y catalyzed isopropylation. Product distributions varied with reaction temperature At low temperatures (e.g. 150°C), the alkylation occurs in the ortho- and paraposition to yield predominantly 2- and 4-IPBP. However, the thermodynamically more stable isomers, 3- and 4-IPBP, increased significantly with decrease of less stable 2-IPBP by raising the temperature. At high temperatures (e.g. 250°C), a nearly equimolar mixture of 3- and 4-IPBP was obtained with small amounts of 2-IPBP. A similar change in product distribution was observed in the formation of DIBP isomers. The products with 2-isopropyl groups were predominant at low temperatures, whereas the more stable isomers with 3- and 4-isopropyl groups increased with raising the temperature. Similar variations .""I
0
-c -->
%
40
30
c
c
5 20 fJY
10
n "
130
160
190
220
250
Reaction temperatwe(%)
Fig. 2 Effect of Reaction Temperature on Isopropylation Catalyzed by HY Zeolite Reaction conditions: HY(5.8) 1 g, Biphenyl 50 mmol, Propene 100 mmol, 4 h.
Alkylation of Biphenyl Catalyzed by Zeolites 307
variations were observed in the catalysis of HL, although decrease of 2-IPBP and increase of 3- and 4-IPBP were less extensive. Catalytic activity of HZSM-5 even at 300°C was extremely low under our conditions. The isopropylation was nonselective. Effect of Si/Al2 Ratios of HM on Catalysis Figure 3 shows effect of Si/A12 ratio of HM in the range of 10 and 220 on the isopropylation. Although some scatter was found, catalytic activity increased with increase of Si/A12. The para-selectivities of HM(10) were as low as 60% for IPBP and 50% for 4.4'-DIBP. However, catalytic feature changed significantly in the range of 10 and 30 Si/A12. Decrease of 2-IPBP and increase of 4-IPBP and 4,4'-DIBP were observed. The selectivities were gradually saturated as increasing Si/A12. The percentage of DIBP in products increased with increase of Si/A12. High silica mordenite such as HM(220) gave much DIBP even in the early stages of the reaction compared to HM(23). Amounts of coke deposition of HM determined by TG analysis in an air stream decreased with increase of Si/A12. Two kinds of peaks were found in the cases of high silica HM. The lower peak around 35OOC i s assigned to biphenyl derivatives desorbed from HM, and the higher peak i s due to deposited coke in HM pores. Isopropylation with isopropanol occurred by using high silica mordenite with
1
0
B
50
100
160
200
SI/A12 rat10
Fig. 3 Effect of Si/A12 Ratio of HM Zeolite on Isopropylation of Biphenyl Reaction conditions: HM lg, Biphenyl 50 mmol, Propene 100 mmol, 4 h.
308 Y.Sugi et al.
more than 20 Si/A12. Activity and selectivity resembled the case of propene. However, alkylation was inhibited in excess water in the case of HM(23). The reaction did not occur by HM with less than 15 of Si/A12. DISCUSSION Differences in Shape Selectivity of Zeolites Isopropylation of biphenyl catalyzed by solid acid catalysts gave a mixture of three isomers o f isopropylbiphenyl (IPBP) and many isomers o f diisopropylbiphenyl (DIBP). The 12-membered zeolites, HM, HY, and HL gave quite different features in selectivity. Isopropylation catalyzed by HM was highly selective to give 4-IPBP and 4,4'-DIBP. However, catalyses by HY and HL were nonselective similar to the case of amorphous silica-alumina. Three types of shape-selective catalysis are distinguished depending on whether pore size limits the entrance of reactant molecules, the departure of product molecules, or the formation of certain transition states [ 6 ] . The suitability of zeolite structure for the catalysis is essential for high shapeselectivity. Alkylation of biphenyl is also explained by steric control by pore size and shape of zeolite. HY, HL and HM have different pore structures although they have 12-membered ring pores with diameters of around 0 . 7 nm [ 7 ] . HY has three-dimensional channels and cavities of 1.3 nm in diameter, and HL and HM have straight channels. Pores of HY and HL are circular, the diameters of which are 0 . 7 4 nm, whereas HM has elliptical pores of 0.71x0.67 nm. Electron densities of ortho- and para-positions of biphenyl are higher than that of meta-positions. Sterically non-controlled isopropylation of biphenyl at low temperature occurred predominantly at ortho and para-positions to give 2and 4-IPBP because of the electrophilic nature of the alkylation. However, selective formation of 4-IPBP over HM is controlled by the steric restrictions depending on the elliptical pore of the zeolite and on the conformation of the transition state for the formation of products. The molecule 2-IPBP has approximately 0 . 7 5 nm of diameter i n a twisted bulky conformation with an angle of 64"[8]. The formation of 2-IPBP is prevented over HM because the corresponding transition state with bulky conformation requires more space than is available at the acidic sites of HM. On the other hand, the formation of 4IPBP proceeds unhindered because of its smaller transition state. The formation of 3-IPBP is also less hindered because of flexible conformations at transition states in HM pore. Shape selectivity was strongly influenced by the bulkiness of alkylating agents. The alkylation with ethene was nonselective. However, the alkylation with higher olefins occurred para-selectively as in the case of propene. These differences in para-selectivity reflect the steric circumstances of transition
Alkylation of Biphenyl Catalyzed by Zeolites 309
states in HM pore. In the case of ethylation, the space of pore will be enough to form transition state because of flexible conformation of 2-ethylbiphenyl.
However, the restriction at transition state should become severe to result in selective formation of para-alkylated products when higher olefins are used. 2- and 3-Hexylbiphenyls were obtained in a ratio of 4:l in the alkylation with 1-hexene. The para-selectivity of the latter isomers was higher than that of the former. These results are ascribed to steric restriction because of the differences in bulk of the 2- and 3-hexyl groups. Catalyses of HY and HL are not controlled by steric circumstances of pore and channel. HY and HL have enough spaces for transition state to allow the formation of all IPBP isomers. Product distribution changes markedly by increasing reaction temperature. Catalysis at low temperatures i s determined by the reactivity of each position of biphenyl molecule to yield 2- and 4-IPBP as principal isomers. However, the selectivity of 3-IPBP increases extensively with decrease of 2-IPBP with rising temperature, and an equimolar mixture of 3and 4-IPBP is produced at high temperatures. These changes in product distribution are ascribed to the isomerization of 2-IPBP to the more stable 3IPBP by a de-alkylation and alkylation mechanism. Catalysis of HZSM-5 at 3OOOC is nonselective with low activity. The reaction occurs at external surface because the pore i s too small to allow the entrance of biphenyl molecule. High selectivity of 4,4'-DIBP was observed in the catalysis of HM. The selectivity of 4,4'-DIBP was constant during the reaction with the accumulation of 2- and 3-IPBP and decrease of the selectivity of 4-IPBP. These results show that the alkylation proceeds by a consecutive reaction mechanism. The alkylation of 4-IPBP occurred regioselectively to give 4,4'-DIBP. Other isomers, 2- and 3-IPBP, do not participate in the reaction because these isomers are too sterically bulky to enter the pore of HM. On the other hand, catalyses of HY and HL were nonselective for the formation of 4,4'-DIBP. Three isomers of IPBP take part in the alkylation, which i s controlled by the electronic factor of reactant molecules at low temperatures and by the stability of product molecules at higher temperatures. Effects of Si/A12 Ratio of HM Zeolite on Catalysis Catalytic features of HM varied significantly with Si/A12 ratio, as shown in Fig. 3. Changes in the selectivity of IPBP and DIBP were observed especially by the catalysis of HM in the range of 10 and 30 Si/A12. The para-selectivities of HM(10) were as low as 60% for 4-IPBP and 52% for 4,4'-DIBP. However, the selectivities of 4-IPBP and 4,4'-DIBP gradually increased with increasing Si/A12 ratio. These results show that nonselective reactions occur at acidic sites on external surface. The percentage of DIBP in products increased with increase of
310 Y. Sugi et al.
Si/A12. High silica mordenite such as HM!220) gave much DIBP compared with HM(23) even in the early stages. Direct formation of DIBP from biphenyl proposed by Lee and his co-workers[51 should be involved in addition to consecutive formation from IPBP. In the case of high silica mordenite, coke deposition decreased with decreasing aluminum content. Coke deposition in 12-membered zeolites occurs in zeolite pore. The coke deposited in zeolite pores is usually an aromatic hydrocarbon with 2-4 nuclei [91. Biphenyl and its alkylated products easily produce coke by dehydrogenation. Deposited coke usually inhibits acid catalysis of HM because it chokes acidic sites. Coke deposition can be controlled, if acidic sites are reduced by dealumination. The dealumination changes acidic strength as well as the density of acidic sites [10,111. Acidic strength was not changed by dealumination below 100 of Si/A12. However, acid strength was weakened by further dealumination. These changes in acidic property may reflect coke deposition. High silica mordenite such as HM(220) effectively catalyzes alkylation with minimum coke deposition. Isopropylation by isopropanol occurred by using high silica mordenite with more than 20 Si/A12. Similar activity and selectivity were observed as in the case of propene. Although acid catalyzed reactions are usually inhibited by water, high silica mordenites produce hydrophobic circumstances at acidic sites to enhance the acid catalysis. ACKNOWLEDGMENT G.T. thanks Nippon Steel Chemicals Co. Ltd. and Nippon Steel Co. Ltd. for permission to publish of this article. REFERENCES 1 D.Fraenke1, M.Cherniavsky, B.Ittah and M.Levy, J. Catal., 110 (1986) 273. 2 T.Matsuzaki, Y.Sugi, T.Hanaoka, K-Takeuchi, H.Arakawa and G.Takeuchi, Chem. Express, 4 (1989) 413. 3 Y.Sugi, T.Matsuzaki, T.Hanaoka, K-Takeuchi, H.Arakawa and G.Takeuchi, Shokubai, 31 (1989) 373. 4 G.Takeuchi, H.Okazaki, T.Kito, Y.Sugi and T.Matsuzaki, Sekiyu Gakkaishi, in press. 5 G.S.Lee, J.J.Maj, S.C.Rocke and J.M.Garces, Catal. Lett., 2 (1989) 243. 6 S.M.Scicsery, Zeolites, 4 (1981) 202; Pure & Appl. Chem., 58 (1986) 841. 7 D.W.Breck, "Zeolite Molecular Sieves," John Wiley & Sons, New York(1974). 8 S.Tsuzuki, K.Tanabe, Y.Nagawa, H.Nakanishi and E.Oosawa, Nippon Kagaku Zasshi, (1986) 1607. 9 P.Dejaifve, A.Aurox, P.C.Gravelle, J.C.Vedrine, Z.Gabelica and E.G., Derouane, J. Catal., 70 (1981) 123. 10 H.G.Karge and J.Weitkamp, Chem.-1ng.-Tech., 58 (1986) 946. 11 M.Niwa, M.Sawa and Y.Murakami, Proc. 9th Intern. Congr. Catal., (1988) 380.
311
Correlation Between Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites and Selectivity of Conversion of Alkylbenzenes V.N.Romannikov, E.A.Paukshtis and K.G.Ione Institute of Catalysis, Novosibirsk 630090, USSR
ABSTRACT The oatalytio activi2;y o?+ Z 5 type zeoQtes+,+ modigied z+by Dy9: So , Ga , A1 , Be ), polyvalent oations (Ca , Mg , ?-Ins: were investigated in reaotions of toluene alkylation by ethylene and transalkylation of ethylbenzene. The presenoe in these samples of aprotic aoid oentres of different strength and absence of protio centres were established by IR speotrosoopy teohnique of adsorbed CO. The strength of aprotio oentres was oharaoterieed by the heat of CO adsorption and was shown to be a main faotor determining the seleotivity of oatalytio aotion of the systems studied. INTRODUCTION To develope the soientifio bases of zeolite oatalysts preparation for selective organio and petrochemical synthesis, the elaboration of methods of direoted modifioation of aoid properties of these systems seems to be very important, namely, the oreation in them of aoid sites of different nature and the determination of their role in multi-route catalytio transformations. To reveal faotors whioh influenoe aotivities of aoid-base oatalysts in alkylation and isomerization is the challenge to aotivity in this field. The greatest amount of work has been done in oonnection with the effeot of para-seleotivity, which is observed in alkylation of aromatic hydrooarbons on ZSM-5 type zeolites [I]. This effeot has been explained by a number of authors either by the influenoe of diffusion faotors [2,31 or by the isomerizing aotivity of the external surfaoe of zeolite orystals [41. In refs. [5,6] and espeoially in ref.[7] the para-seleotive effeot of ZSM-5 type zeolites is shown to be due to deoreasing their isomerizhg aotivity beoause of the deorease in the oonoentration of strong protio oentres as a result of modifiers intmduoed. Para-seleotive effect is related to the action of ohemical factors. However, in
312 V. N. Romannikov, E. A. Paukshtis and K. G. Ione
refs.[5-7] properties of aoid oentres were oharaoterized in terms of TPD of ammonia only; it does not allow workers to determine separately the role of protio and aprotio sites in effeots observed. In this work acidio and catalytio properties of ZSM-5 type zeolites oontaining polyvalent oations in exohange positions were Fnvestigated. Toluene alkylation by ethylene and ethylbenzene transalkylation were studied as model reaotions.
METHOD Catalysts Polyvalent oations were introduced into H-form of ZSM-5 type zeolite with Si02/A1203=95+5 by ion exchange method from water solutions. Cations were ohosen in suoh a way that their polarizing ability (e/r) ohanged in the widest interval. IR speotrosoopy investigations Sample pellets (8-10worn2) were oaloined in IR-oell at 450°C in air for 1 h and then in vaouum Pa) for 1 h. Speotra were recorded wing UR-20 speotrometer,speoially modified for operating in wide temperature range. Concentration and strength of aoid centres were oaloulated from spectral data of CO adsorbed at low temperatures [a], the former being oaloulated aooording to eq. ( 1 ) and the latter - aoooding to _.
eq. (2).
C ( p o l / g ) = (A0p)-’ Jlg(To/T)dv, (1 ) where A, is a ooeffioient of integral absorption, om/pol; p is surfaoe density of a pellet, g/om2 ; To and T a r e transmissions of IR-beam for individual line t h r o w the pellet before and after CO adsorption, respeotively, 46.
For more o a r e m identifioation of types of Lewis (aprotio) oentres, CO adsorption was oarried out in small doses (1-10 p o l per oell) up to saturation of aotive surfaoe aohieved. To reveal individual lines the separation of IR-speotra was oarried out ueing CK-2 o w e eynthesiser. Conoentration of aprotio oentres was determined wing ooeffioients of integral absorption f r o m ref.[91. The etrength of aprotio oentres was oharaoterized by the heat of CO adsorbtion aooording to eq. ( 2 ) . Conoentration of BGnsted (proQco (IcJ/mol) = 10,5
+ 0,5 (vco -
2143)
(2)
Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites 313
tio) centres was oaloulated aooording to eq. (1) from both the intensity of the OH-group band (VoH = 3610om-l, A0=70m/p01) and the intensity of oorresponding band of OH-groups in oomplexes with CO (’OH.. .CO = 3310om-’, A0=57 om/pmol). Coeffioients of integral adsorption were taken from ref. CIOI. Catalytio aotivity The investigation of aotivity and seleotivity of oatalysts was oarried out in a flow quartz reaotor at atmospherio pressure and following reaotion oonditions. Toluene alkylation Iry ethylene: temperature inside oatalyst bed -39Oz441O0C; aomposition of starting reaotion mixture - toluene and ethylene with molar ratio 2,50 f 0,05; WHSV to reaotion mixture 3,3-3,8 h-’ Transalkylation ethylbenzene: temperature inside oatalyst bed - 385~395°C;starting re8gent- ethylbenzene; WHSV to ethylbenzene - 3,4-3,6 h-’
.
Table 1.
Charaoteristios of zeolite samples studied.
Cation in zeolite
Conoentration Conoentration of aprotio of the most oentres type radius e/r strongproof tio oentres total inoluding 1 the strongest oation vOH=3610cm-, C K ~ + (rKn+), oonoent2 Cu”ol/g Pol/g vco ration. [Ill om-’ I,lmol/g ? 130 18 223018 H+ -2225 ...................................................................................................................................................................................................... Ca2+ 1,04 1992 < I 110 220058 -2190 ...................................................................................................................................................................................................... 0,74 2,70 3 120 221520 Mgz+ -2212 - 11 12 Y“:+ 0,97 3909 4 90 no data available 0992 3,26 < I 1% + - It 27 nz............... O P . 8 8 ...................3?................................................................................................................................................. 41 4 132 so3 + 0983 3,61 2 115 22308 -2225 - It 110 7 Gas+ 0,62 4984 3 II 8 All: 0,57 5,26 7 131 - I1 9 Be 0,34 59 8 8 7 85
-
FtESULTS AND DISCUSSION
Charaoteristios obtained by IR technique for zeolite samples in-
314 V. N. Romannikov, E. A. Paukshtis and K. G. Ione
vestigated a r e summarized in Table 1. It is shown,that both strong protio and strong aprotio aoid sites are present in the parent zeolite sample, the oonoentration of pmtio sites being equal to 130 pol/g. The introduotion of polyvalent oations into this zeolite leads to a sharp deorease of oonoentration of strong protic oentres: their residual oonoentration does not exoeed 7 pol/g, i.e. 556 of initial oonoentration. Simultaneously, the formation of aprotio aoid site8 of different strength was deteoted: weak with Qco Q 39 kJ/mol (Vco Q 2200 om'-' ,e/rG?); medium with QC0=45,0-46,5 kJ/mol (Vco = 2212-2215 om-', e/r=2,6-3,5); strong with a,,= 51,5-54,O kJ/mol (Vco = 2225-2230 om-', e/r 2 3,6). The total oonoentration of aprotio oentres waB found to be in the range of 100-130 pmol/g in all oases. Conoentrations of the strongest aprotic oentres and oorresponding values of Vco are presented in Table 1. Since aprotio sites in the zeolites under study were generated via ion exchange of protons inside orystal volume, the aprotio sites formed are also situated inside orystals. In connection with this, a position seleotivity of primary alkylation must be influenoed by structural restriotions whioh are put on the transition state by ZSM-5 type zeolite. Henoe, as follows from refs.l6,71, para-isomer must be a primary produot of alkylation. Taking into aooount these ideas,the sohemes of the main routes of investigated reaotions are aooepted (Figs. 1,2). As seen from the sohemes, the pathways of both reaotions are praotioally the same. The only differenoe is that in the oase of ethylbenzene alkylation prooeeds
aromattzatlon of ethylene
paraffins
+
aromatic hydmoarbons inoluding xylenes o+m (P j tsomert-, meta- 1somer 1-,or tho-
-m %at ton
I + C2H4 'fjHliCH3
(XpF)
I
aZkyZatlon of
*
tOZW7E
(-
o+m P 1 meta- tsomert-,ortho-
-m *zutton
-ET
1
Fig. 1 Soheme of main reaotion routes of koluene alkylation by : S '= seleotivity to the ethylene. (E?p= ethyltoluene; XYL=xylene;r produot on converted toluene)
Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites 315
paraffins
CHCH
'}-
+
C2H5C6H5
[
]
C2H4 i6H6
+
aromatio hydrooarbons
transalkplatton
b
4
C2H5C6H5
-
CgH6
Soheme of main reaotion routes of ethylbenzene transalky(DEB=diethylbenzene)
.
A
__
[ETI: (o+m)/p
-'
A
- 1.5
..................
30
I
a
I
-
1.0
- 0.5
- T -
(e/r ) of cation
I
I
6 39
4
1 45,0-+' -46,5
I
_-0
CH+)
1
5195
- 5490
Heat of CO adsorption( QCO,
kJ /mol )
Fig. 3 Dependenoes of the degree of toluene oonversion via alkylation by ethylene ( 1 ) and the de ee of isomerization of ethyltoluene formed (2) on the polarizf& ability of oations and on the heats of CO adsorption on oations in ZSM-5 zeolite. with partioipation of ethylene formed in the reaotion. The experimental results of investigation of oatalytio aotivity of samples (Table 1 ) in toluene alkylation by ethylene are presented in F i g . 3. T a k i n g into account the literature (1 I , one oould
316 V. N. Romannikov. E. A. Paukshtis and K. G. Ione
suggest that toluene alkylation by ethylene to para-position and suooessive isomerization of para-ethyltbluene in the oase of the samples studied prooeed with the partioipation of residual (non-ohanged) pmtio oentres (Table l),and the introduotion of oations influenoe the diff'wion oonditions in zeolite channels only. As a result oorresponding ohanges in both the activity of samples and the para-seleotivity of their aotion were observed. If this is valid, the correlations ( 3 ) must be fulfilled for all cations having Kn+ a raas'+- However, as one oan see from P i g . 4, these oorrelations are not fulfilled. Hence, if the introduotion of polyvalent cations is accompanied by a ohange in diffusion conditions in zeolite ohannel, this change is not the main faotor determining the level of para-seleotivity. The influenoe of interohannel diffusional limitation may be estimated by a oomparison of catalyst para-selectivities to separate product moleoules, the diameters of whioh are different. As is known, the kinetio diameter of moleoules inoreases in the sequence para-XYL <para-ET (para-DEB. Values of para-seleotivities to dial-
.o
1
0.9
30-
0.8
0.7 0.6
OS
10-
0.4
0
I
1
0.4
I
I
I
0.6
1
I
0.8
Radius of cation
Fig. 4
I
(A)
0.3
1.o
Dependences of toluene conversion via alkylation by ethylene ( 1 ) and para-seleotivity of reaotion ( 2 ) upon radius of cation introduoed.
Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites 317
Table 2. Values of para-selectivity (pS) to different dialkylbenzenes in reactions of toluene alkylation by ethylene and transalkylation of ethylbenzene.
09313 0,798 0,881 0,938 0,970
0,317 0,806 0 ,920 0,969 0,986
0,351 0,875 0,970 0 9 980
>0,995
1,121 1,096 1,101 1,045 1,026
1,107 1,086 1,054 1,011 1,009
Qlbenzenes formed via routes of reactions considered (Figs. 1,2) are presented in Table 2. A s one can see from Table 2, under the change of the value of para-selectivity to products under consideration in a wide range the ratios of these values decrease relatively, but not more than by 10-12%. Hence,para-selectivity in the reactions studied could be influenced by diffusional conditions in zeolite channel not more than by this magnitude. It is more correct to suggest that the selectivity of catalytio action of investigated zeolites is connected with the properties of acid-base centres varied by a change of the nature of the cation introduced. To elucidate this connection, it is necessary to discuss now some ideas on the subject. A s follows from Fig. 3, growth of catalyst activity in both alQlation and isomerization with increasing polarizing ability of cation in the range of e/r<4 is observed. Consequently, both these transfonnations proceed with participation of the one and the same catalytic site. However (Fig. 3 ) , if for the alkylation the presence of medium aprotic centres (Qc0= 45-46,5 kJ/mol, e/r=2,6-3,5) seems to be sufficient, the isomerization requires stronger ones. This may be explained under assumption that the aotivation of olefin on Lewis centres (which is necessary f o r alkylation) is easier than the activation of aromatic ring (which is necessary for isomerization). Hence, the alkylation by olefins requires centres of lower strength. In this connection three cases may be considered: 1 ) the strength of aprotio centres is suffioient f o r the activation of olefin and the following alkylation by interaction with aromatic hydrocarbon from gaseous phase, but insufficient for aromatic ring activation. Para-dialkylbenzene formed in this case does not undergo further isomerization on these centres. The group of centres
318 V. N. Romannikov,
E.A. Paukshtis and K. G. Ione
was determined here as being of medium strength with a heat of CO adeorption Qco = 45,O-46,5 kJ/mol. 2) The strength of weak aprotio oentres is insuffioient for the activation of either olefins or aromatio rings. 3) On strong aprotic centres,the aotivation of not only olefin moleoulee, but also aromatic rings is possible. For this reason, para-dialkylbenzene, formed on this type of oentre via alkylation, is oapable of further isomerizing with the partioipation of the same centre. As shown in Fig. 3 , under availability in zeolite of suoh oentres (in the range of e/r > 4 ) , oatalytic properties of a sample are close to that of the H-form of zeolite, and the oorrelation between meta- and para-ethyltoluenes is near thermodynamioal equilibrium magnitude. As a whole,the suppositions deeoribed above seem to be useful for an explanation of para-seleotive effeot in ZSN-5 zeolite catalysis. REFERENCES 1. N.Y.Chen and W.E.Gamood, Catal.Rev.-Soi.Eng., 28 (1986) 185. 2. W.Kaebing, C.Chu, L.B.Young, B.Weinstein and S.A.Butter, J.Catal., 67 (1981) 159. 3. Ya.I.Isakov, Kh.Y.Minaohev, T.A.IsakoVa, G.L.Bitman and S.P.Chernykh, Neftekhimia (RUB.),27 (1987)766. 4. G.Paparatto, LMoretti, G.Leofanti and F.Gatti, J.Catal., 105 (1987)2275. K.H.Chandavar, S.G.Hegde, S.B.Kulkami, P.Ratnasamy, G.Chitlangia, A.Singh and A.V.Deo, Proo.6th Int.Zeol.Conf., Reno, Butterworths (1983) 325. 6. J.-H.Kim, S.Namba and T.Yaehima, Bul1.Chem.Soc. Japan, 61 (1988) 1051. 7. J.-H.Klm, S.Namba and T.Yashima, YZeolites a6 Catalysts, Sorbents and Detergent Builder13,'~Amsterdam, Elsevier (1989) 71. 8. E.A.Paukehti6 and E.N.Yurohenko, Uspekhi Khimii (RUBS.), 52, N 3 (1983) 426. 9. R.I.Saltanov, E.A.Paukehti6 and E.N.Yurohenko, Kinetika i Kataliz (Ruse.), 23, N 1 (1982) 164. 10. L.V.Malysheva and E.A.Paukshti6, Kinetika i kataliz, in print. 11. G.B.Bokii, ~~Hristallokhimia"(Rs.~, Nauka, MOSOOW,l971 ,p.137.
319
N- and C-Methylanilines Formation on Zeolites with Different Structural and Acidic Proper ties
O.V. Kikhtyanin, K.G. Ione, L.V. Malysheva and A.V. Toktarev Institute of Catalysis, Novosibirsk 630090, USSR
ABSTRACT Zeolites of different structural types and chemical compositions have been investigated in aniline alkylation with methanol in a wide range of reaction temperatures. The results obtained have been oorrelated with IRS invest ations of zeolites studied. It is shown that wide-pore zeolites wi h low molar ratio Si02/A1203 are the most suitable mrkd.g&ic systems to obtain both N,N-dimethylaniline and C-alkylated anrlbes.
Y
INTRODUCTION In a number of recently published studies on alkylation of aniline with methanol [l-41 catalytic systems on the base of MFI zeolites.have been chosen as the main objects of investigation. In these publications it has been shown that the amount of N- and Cmethylanilines in reaction products depends on both reaction conditions (reaction temperature, composition and weight hourly space velocity of reaction mixture) and the way catalytic systems are modified. Selective catalysts f o r N-alkylation of aniline were the aim of investigation in [1,2l, but C-alkylation of aniline has not been oonsidered in detail. In the present study we attempted to investigate factors influencing C-alkylanilines formation from aniline amd methanol. Catalytical properties of MFI zeolites with different chemical compositions as well as of a number of wide-pore zeolites have been studied in this reaction. The results obtained have been correlated with those of IRS investigations of zeolite systems.
320 0. V. Kikhtyanin, K. G. Ione, L. V. Malysheva and A. V. Toktarev
Catalysts Zeolite powders were synthesized under hydrothermal oonditions aooording to well-known methods. XRD studies of the samples obtained have revealed that the oatalysts have the main phase in amounts exoeeqing 90%. Physioo-ohemioal properties of the samples are presented in Table 1. Table 1. Physioo-ohemioal properties of initial zeolite samples. Sample
Struotural SiO2 type mOmo1) 2 3
Kind of treatment
H-Na-ZSM-5 (27) MFI 27 Double ion exohange of Na-1190 form with 0.1M buffer sohH-Na-ZSM-5 ( 90) tion NH4C1+NH40H -I(300 H-Na-ZSM-5 (300) - II700 H-Na-ZSM-5 (700) -'I-11H-Na-Fe-ZSM-5 (300) -I*- Si02/Fe 0 =300 -11H-Na-Be-ZSM-5 (300) -(I- Si02/ Bh3=300 H-Na-B-ZSM-5 (300 ) -I1- Si02/B203 =300 -11H-Na-p H-Na-L H-Na-mordenite H-Na-L? H-Na-Y Mg-Na-Y Zn-Na-Y
BETA LTL MOR HA2 PAU -11-fl-
I9 7
11
7 4.5 4.5 4.5
-
ll-
-11-11-
-11, -11-
Double ion exohange of Na-Y form with O.5M solution of bfg aoetate or Z n nitrate
IRS studies Aoidio propertiee of the oatalysts have been determined from IR speotra of adsorbed CO aooording to 15-7l.CO moleoules were adsorbed at T=-llO°C by doses of 1-3 pmol up to a final pressure of 5 torr of CO in a oell. Strength of Bronsted aoid sites was oharaoterized by proton affinity soale (PA, kJ/mol), and strength of Lewis aoid sites by heat of CO adsorption. Conoentrations of Bronsted aoid sites were determined f r o m integral intensities of bonds of OH-groups oonneoted in H-oomplex with CO. Conoentrations of Lewis aoid sites were oaloulated from integral intensities of oorresponding bonds of adsorbed CO. The values of ooeffioients of integral absorption were taken from C5-81.
N- and C-Methylation of Anilines 321
Table 2. Aoidio properties of zeolite oatalysts. Sample
H-Na-ZSM-5 (27) H-Na-ZSM-5(90) H-Na-ZSM-5(300) H-Na-ZSM-5(700) H-Na-Fe-ZSM-5 (300) H-Na-Be-ZSM-5(300) H-Na-B-ZSM-5(300) H-Na-Y Mg-Na-Y Zn-Na-Y H-Na-p H-Na-LI H-Na-L H-Na-mordenite
Conoentration of Lewis Conoentration of Bronaoid sites, pnol/g sted aoid sites, pnol/g (PA, kJ/mol ) (Qco, kJ/mol) 37 41 47 54 internal external 100 125
85 20
80
5
54 64
0
45
87 72 50 47
0 0 10 0 0
80
157 25 46 99
0 0
0
47
5 0 0 0 0
16 36 0
22 21 13 13 0 0
0 0 0 0 0 0 0 10 0 11
1 50(1 1 80) 120(1 180) 30(1 180) 20(1 180) 28(1 225) 22( 1240) <5 (I 260) 800 (1 180) 0
29
1 1 0(1 180) 11 (1180) 56(1180) 12(1 180) 1 6(1 180) 140(1 180) 1 65(1 250)
0
4 0
0
300(1300) 46(1180) 26 (1 180)
1 6(1 180)
29(1 225 1 16(1 2 6 0 ) 0 0 0 0 100(1 180)
Catalytio testings. Catalyst samples were prepared by extrusion with 20% A1203. Flow isothermio reaotor with a fixed bed of a oatalyst was employed f o r oatalyst testings. Liquid reaotion produots were analyzed by GC on a oolumn contained Inerton N Super with 5% Carbowax 20M. RESULTS
Table 2 presents the results of investigation of aoidio properties of zeolite catalysts. It is ehown that inoreaslng the molar ratio of Si02/A1203 oauses deoreasing of oonoentration of Bronsted acid sites, but does not influenoe the strength of the sites (PA= 1180 kJ/mol). Inoreasing the A1 atoms oontent in zeolite oatalysts causes inorease in both oonoentration and strength of Lewis aoid sites. Substituted silioates with Fey B and Be possess aoid sites with deoreased strength. Wide-pore aluminosilioate zeolites in H-form have strong Bronsted (PA=1180 kJ/mol) and Lewis aoid sites, but there are Lewis aoid sites only in the oationio f o m of samples with FAU struoture - Hg-Na-Y and Zn-Na-Y. It oan be seen from Table6 3-6 that aniline oonversion and oontent of polyallrylated anilines in reaotion produots inorease when the molar ratio of SiO2/Al2O9 in ZSM-5 zeolites deoreaees. Using
322 0. V. Kikhtyanin, K. G. Ione, L. V. Malysheva and A. V. Toktarev
wide-pore zeolites of MAZ, MOR and LTL struotural types does not oawe inorease in aniline oonversion and polyalkylanilines oontent, but samples H-Na-p, H-Na-Y, I4g-Na-Y and Zn-Na-Y show inoreased aotivity in aniline oonversion and polyalkylanilines oontent. The main produots of C-alkylated anilines are paraTable 3. Catalytio prgperties of zeolites studied in anilinel methylation. Treao =250 C , methanol:aniline=3:1 (mol) , WHSV=l h- . Sample
Aniline oonversion,
Produots oomposition(*), %wt.
%
1
2
3
4
5
6
7
8
39.4 60.2 0 0.40 0 0 0 77.2 H-Na-ZSM-5 (27) 0 0 0 0 0 70.4 29.6 0 H-Na-ZSM-5(90) 45.7 72.0 28.0 0 0 0 0 0 0 H-Na-ZSM-5 ( 300) 19.6 0 0 0 0 0 H-Na-ZSM-5 (700) 18.6 79.4 20.6 0 0 0 0 0 H-Na-P 99.7 2.3 91-9 5 . 8 0 10.2 68.6 1.6 18.5 0 H-Na-Y 98.8 0 0 1.1 0.40 0 0 0 64.0 35.6 0 Mg-Na-Y 65.9 0 0 0.5 Zn-Na-Y 27.3 69.7 0.5 2.0 0 74.0 0.60 0 0 0 H-Na-L 55.9 53.9 45.5 0 66.7 28.6 4.2 0.5 0 0 0 0 H-Na-Sl 39.2 0 0 0 0 0 H-Na-mordenite 15.5 93.0 7.0 0 (*I- 1 - N-methylaniline, 2- N,N-dimethylaniline, 3- N-methyl-para-toluidine, 4- N,N-dimethyl-para-toluidine, 5- para-toluidine, 6- 2.4-xylidine, 7- mesidine, 8- others. Table 4. Catalytic progerties of zeolites studied in aniline m7thylation. Treao = 300 C, methanol:aniline=3:1 (mol), WHSV=l h- . Sample
Aniline oonversion, %
H-Na-ZSM-5 (27) H-Na-ZSM-5 (90) H-Na-ZSM-5(300) H-Na-ZSM-5(700) H-Na-P H-Na-Y Mg-Na-Y Zn-Na-Y H-Na-L H-Na-ti H-Na-mordenite
96.1
78.7 44.4
40.9 99.8 99.9 99.2 99.4 95.0 74.4 52.6
Produots oomposition, %wt. 1
2
3
17.1 74.6 0.6 45.8 49.4 1.1 51.2 45.0 1.2 59.4 38.7 0.6 3.9 20.3 11.9 0.8 6.6 19.5 8.4 27.1 15.2
2.7 22.2 40.0 60.2
13.0 29.1 73.7 0.3 36.5 10.4 22.1 11.7
4
5
6
7
8
0 0 0 0 0.4 0
0 0
0 0 0 0
7.7 0 3.7 2.6 1.3 62.6 46.1 45.3 32.4 3.8 12.6 4.4
0 0 0
2.2 1.1
5.1 0
0 0
0 0.6 2.4 2.2 20.2 0.1 0 2.8 5.1 2.9 13.2 0 0 3.8
0.4 0 1.6 0
0 0
0.1 0
substituted oompounds, namely, para-toluidine, N-methyl-paratoluidine and N,N-dimethyl-para-toluidine, but derivatives of ortho-toluidine oome to lesa than 2-3% in reaotion produots. Inoreasing the reaotion temperature oauses inorease the oontent
N- and C-Methylation of Anilines 323
of poly-C-alkylated anilines, the latter forming the next r o w :
4-methylaniline (para-toluidine), 2.4-dimethylaniline dine), 2.4.6-trimethylaniline (mesidine).
(2.4-xyli-
Table 5. Catalytio pqperties of zeolites studied in milinelmethylation. Treao =350 C, methanol:aniline=3:1 (mol), WHW= 1 h-
.
Sample
Aniline oonversion, % ~~~
H-Na-ZSM-5 (27) H-Na-ZSM-5 (90) H-Na-ZSM-5 (300) H-Na-ZSM-5 (700) H-Na-p H-Na-Y Mg-Na-Y Zn-Na-Y H-Na-L H-Na-CJ H-Na-mordenit e
Produote oomposition, %wt. 1
2
3
4
5
6
7
8
4.6 1.9 0.4 2.5 17.6 16.4 13.9 10.2 5.3
30.5
0 0 0 0
0 0 0 0
0 0 0 0
0.4 0.7
~
98.2 96.7 76.7 57 -7 99.0 97-7 99.5 1 00
95-0 95.5 82.8
9.6 53.6 66.2 34.3 63.0 60.2 33.8
20.1
0.1 0 0 0
0.5
11.1
2.3 3.5 41.2 16.0
1.7 11.0 0.4 0.7 3.3 27.4 15.5 51.5 23.0 41.7 12.8 21.1 42.0 30.6 15.1 9.0
1.9 6.4 13.9 11.3 0.1 0.2 3.0
0 0
4.4 2.5 32.3 14.9 18.2 26.4 4.3 16.3 25.0 21.4 24.4 22.3 0
0
0.7
0.5
o
o
0.2 0.1 0.2
Table 6. Catalytio progerties of zeolites studied in aniline me7 thylation. Ire,, = 400 C, methanol:aniline=3:l(mol), WHW= 1 hSample
Aniline oonversion, %
H-Na-ZSM-5 (27) 96.4 H-Na-ZSM-5 (90) 96.8 H-Na-ZSM-5 (300 H-Na-ZSM-5 (700 H-Na-p H-Na-Y 97.0 Mg-Na-Y 99.4 Zn-Na-Y I 00 H-Na-L 95.3 H-Na-LI 96.2 H-Na-mordenite 79.5
.
Produots oomposition, %wt. 1 9.7 11.2 26.8 49.8
2
3
4
19.6 19.0 33.4 31.5 9.2 40.3 30.4 7.1 29.5 26.5 9.9 7.6 0 0.3 14.2 7.4 0 0.1 10.1 1.7 0 0.1 9.2 0.7 0 0.9 6.5 0.2 9.4 12.6 21.9 19.2 13.2 28.5 14.5 32.5 33.2 18.8 24.1 9.1
5
6
7
2.5
1.6 0.7 0.2
0.4 13.8 0.2 6.8 0 5.6 0 4.7
0.1
0.4 1.4 10.2 12.5 9.9
0
25.8 33.5 29.5 13.1 33.7 5.2 8.6
8
13.8 28.3 31.0 11.2
39.3 11.4 33.7 11.5 5.2 17.9 1.0 1.0 0.5 11.7 11.4 0.8 0.3 2.3
DISCUSSION It is obvious that N-methylaniline is a primary pmduot of aniline methylation; in exoess of methanol this pmduot bealkylated again forming N,N-dhethylanilhe. It is seen (Tables 3-6) that deoreasing the molar ratio of Si02/A1203 in H-Na-ZSM-5 oatalyete oauses inoreaee in aniline oonversion and polyalkylanilines oontent.When oomparing these data with those of Table 2 it is seen
324 0. V. Kikhtyanin, K. G. Ione, L. V. Maly-sheva and A. V. Toktarev
that catalyst activity and fonnation of polyalkylated products are connected directly with the total conoentration of aoid sites in zeolite samples. However, w e of wide-pore catalysts yields nonequivalent results. The samples of BE2A and FAU structural types have higher activity and polymethylanilines seleotivity compared with those of H-Na-ZSM-5 zeolites. On the other hand, the other part of wide-pore samples has no advantage in catalytic behavior. Since the main difference between these two groups of wide-pore zeolites lies in the geometrioal properties of the frameworks, one may suppose that the internal geometry of the zeolite ohannels influences the catalytic properties of the samples. AS the higher activity in aniline alkylation is observed on zeolites with threedimensional frameworks (zeolites FAU, BETA), the formation of rather large molecules of methylanilines [91 could proceed in intersections of zeolite channels where geometrical dimensions are larger than those of pore openings. When reaction temperature increases, the formation of poly-C-alkylated anilines grows, the dependence between selectivity of such products and concentration of strong acid sites being observed. Para-toluidine is the main initial product of C-alkylation which then is alkylated by methanol again forming N,N-para-toluidine. A question arises concerning the manner of para-toluidine formation: either by direct alkylation of an aniline molecule into the aromatic ring, or by isomerisation of a methyl group from the N-atom to the para-position of the ring. To compare reaction abilities of N- and C-methylanilines, reactions of N-methylaniline and para-toluidine oonversions were studied on zeolite catalysts (Tables 7,8). It is seen that N-methylanilhe conversion is higher than that of para-toluidine, and the composition of reaction products depends on the concentration and strength of aoid sites in catalysts. !Pwo main directions of N-methylaniline Conversion are observed: disproportionation of N-methylaniline to aniline and N,N-dimethylaniline, and C-substituted anilines formation.Disproportionation of the methyl group proceeds on the catalysts with low strength of aoid sites. Another situation leads to C-methylanilines formation. On the other hand, study of para-toluidine conversion on zeolites ha6 shown (Table 8 ) that it is lower than that of N-methylaniline, and the reaotion ha6 a single route: disproportionation of paratoluidine to aniline, 2.4-xylidine and mesidine. On the basis of the results obtained the supposition may be made that para-tolui-
N- and C-Methylation of Anilines 325
dine is a more thermodynamioally stable produot of aniline alkylation, and isomerisation of N-methylaniline to para-toluidine oan take place with the partioipation of strong aoid sites of zeolites. In a reoent paper [31 we noted that some amounts of ortho-toluidine and its derivatives are also present but in very small quantities (2-3%) in reaotion produots. A s ortho-toluidine Table 7. Catalytio propertip of geolitey studied in N-methylaniline oonversion. Treao 400 C, WHSV=l h-
.
Sample
N-me thylaniline oonversion, %
H-Na-ZSM-5 (27) H-Na-ZSM-5 (90) H-Na-ZSM-5(300) H-Na-ZSM-5(700) H-Na-Fe-ZSM-5 (300) H-Na-Be-ZSM-5 (300) H-Na-B-ZSM-5 (300) H-Na-Y
aniline+N,N-dhethylaniline para-toluidine % mol. 2.o
86.1
2.8 14.3 14.3 14.3 12.5 25.0
80.0
49.0 46.3 46.1 38.1 27.8 97.1
1 .o
Table 8. Catalytio propertip of zeolites studied in para-toluidine oonv rsion. Ire,, =400 C, para-toluidine:benzene=l:4(mol), WHSV=l h-?
.
Sample H-Na-ZSM-5 (27) H-Na-ZSM-5 (90)
ParaProdUotB oomposition, %wt. toluidine 2.4-xylidine mesidine oonversion, 96 aniline 28.2
49.8
17.4
47.5
39.2 34.8
11 .o
17.7
is also formed in the reaotion of aniline methylation and is the first product of C-alkylation [31, its absenoe in large amounts in reaotion produots may be explained either by the oonversion of ortho-toluidine to para-toluidine, or by the transformation of ortho-toluidine aooording to the route: ortho-toluidine N- methyl-ortho-toluidine 2.4-xylidine. In the oase when the para-position in the aromatio ring is already oooupied, the next CH3-group oomes to the ortho-position by isomerisation also from N-atom into ammatio ring giving 2.4-xylidine and mesidine. The highest oontent o r the last two produots is observed in samples of FAU and BETA struotural types, moreover, the absenoe of strong Bronsted aoid sites does not influenoe markedly the formation of
-
-
326 0. V. Kikhtyanin, K. G. Ione, L. V. Malysheva and A. V. Toktarev
such produots (Mg-Na-Y and Zn-Na-Y samples). It may be said that cationio forms of wide-pore zeolites with three-dimensional frameworks are the most suitable for 2.4-xylidine and mesidine formation in reaotions of aniline methylation. formation from In broad outline the scheme of methylaniline aniline and methanol may be presented as follows: ANILINE
N-KETHYLANILINE
J ORTHO-TOLUIDINE
N,N-DIME"HYTANILINE
I PARA-TOLUIDINE
4
4 4 N,N-DIMETHYL-ORTHO-TOLUIDINE
I
N-m-PARA-TOLUIDINE
N-METHYL-ORTHO-TOLUIDINE
\
4
I
2.4-XYLIDINE
\
N,N-
4
N-MEFHYII-2.4-XYLIDINE -+
N ,N-
4 MESIDINE
+
N,N-
Thus, zeolites may be useful as catalysts for obtaining both N,N-dimethylaniline and C-alkylated products. Conoentration, strength of aoid sites and struotural properties of zeolites are important oharaoteristios for the elaboration of suoh oatalysts. REFERENCES 1. P.Y. Chen, S.J. Chu, N.S. Chang and T.K. Chuang, Stud. Surf. Sci. Catal., 49, Elsevier, Amsterdam, 1989, p.1105. 2. S.I. Woo, J.K. Lee, S.B. Hong et al, Stud. Surf. Soi. Catal., 49 , Elsevier, Amsterdam, 1989, p.1095. 3. K.G. Ione and O.V.Kikhtyanin, Stud. Surf. Soi. Catal., 49 , Elsevier, Amsterdam, 1989, p.1973. 4. P.Y. Chen, M.C. Chen, H.Y. Chu et al, in Y. Murakami, A. Iiji ma and J.W. Ward ( E d e . ) , New Developments in Zeolite Soienoe and Teohnology (Proo. 7th Int. Zeolite Conf., Tokyo, August, 1986), Kodansha/Elsevier, Tokyo/Amsterdam, 1986, p.739. 5. E.A. Paukshtis and E.N. Yurohenko, Uspekhi Khimii(Rw.), 52 (1983) 426. 6. R.I. Soltanov, E.A. Paukshtis and E.N. YurohenW, Kinet. and Catal.(Rw.). 23 (1982) 164. 7. E.A. Paukshtis and E.N. Yurohenko, Reaot. Kinet. Catal. Lett., 16 (1981)131. 8. L.V. Malysheva and E.A. Paukshtis, in print, 1989. 9. E.J. Weigert, J.Org.Chem., 52 (1987)3296.
327
Copper Ion-exchanged Zeolites as Active Catalysts for Direct Decomposition of Nitrogen Monoxide
Masakazu Iwamoto Catalysis Research Center,Hokkaido University,Sapporo 060, Japan
ABSTRACT The exhaust gases from vehicle engines and industrial boilers contain considerableamounts of harmful nitrogen monoxide (NO). To remove NO, catalytic reduction processes using NH3, CO or hydrocarbons have been applied, but several problems remain to be solved. It is suggested here that a catalyticdecomposition process be used for NO removal. The catalyticperformance of copper ionexchanged zeolites for NO decomposition is summarized. Maximum activity was observed around 823-376 K. From the correlation among the zeolite structure, the aluminum content,exchange level of copper ions, and the catalytic activity, it follows that the zeolite structure would be the factor determining the effectiveness of Cu2+ions, and the catalytic activity of the effective Cu2+ ion is probably controlled by the A1 content. A reaction mechanism is then suggested on the basis of IR, ESR, phosphorescence, temperature programmed desorption, and kinetic data. INTRODUCTION Acid rain and air pollution are very important problems that must be solved in the future because such pollution has major effects on terrestrial and aquatic ecosystems. At present, one of the most significant problems is removal of N G ,which are produced during high-temperaturecombustion and are an important group of air contaminants. In particular the decomposition or reduction of nitrogen monoxide (NO) is a major target to be achieved. Nitrogen monoxide is thermodynamically unstable relative to N2 and 02 at low temperatures; therefore its catalyticdecomposition is the simplest and cheapest method for the removal of NO from exhaust streams. To date, however, no suitable catalyst of consistently high activity has been found.192 Some noble metals3 and metal oxides495 are active in the reduced state, but oxygen contained in the feed gas or released by the decomposition of NO competes with NO for the adsorption sites and poisons the activity.* To remove surface oxygen and regenerate catalytic activity, high temperatures andlor gaseous reductants are required. Thus, catalytic reduction processes using NH3 or hydrocarbons have been applied as the second best method for removing NO$ although catalyk decomposition is essentially the best approach. PRESENT POSITION OF VARIOUS DECOMPOSITION CATALYSTS The catalytic propertiesof Cu-ZSM-5 zeolites, first found by Iwamoto and workers? will be outlined in the next section. Here I discuss how it occurred to us that Cu-zeolites are suitable as
328 M. Iwamoto
Table 1. Comparison of the activitiesof various catalystsfor NO decomposition.a),lo catalyst Weight pNo Flow rate Reaction temperature I K 43 1% /cm3.min-l 773 873 973 1073 3 3.13 30 6.2 26 53 (3304 30 30 41 1 3.13 &-a304 30 3 3.13 5 18 BaFeo3-X 20 5 7 18 40 / MgOll 0.5 3.00 YBa2C~307-~ 15 45 72 1.oo ~0.8srO.2c003l2 1 30 40 h i. ~ s ~ o . ~ 12 c u o ~1 3.13 2.4 3.13 30 12 33 56 WM203 CU-ZSM5 1 0.23 30 39 a! All values are conversions into N2. catalysts for NO decomposition. To answer this question is very easy. As mentioned in the introduction, in the 1970s it was widely known that catalytic decomposition is inhibited by the presence of oxygen.2 When we started the study of this 2 reaction about 10 years ago, the results of the temperature programmed dewrption of oxygen from copper ion-exchanged Y (Cu-Y) zeolites obtained earlier becamerelevant (Fig. 1).8 As can clearly be seen, Cu-Y adsorbs large amounts of oxygen and desorbs them at temperaturesas low as 573 K. We tried experiments of decomposition of NO over Cu-Y 600 Tinprmtm/.C zeolites. The experiments fortunately demonstrated weak but stable activities of the zeolites for NO decompo~ition.~These findings resulted in the Fig 1 TPD chromatograms metal ion-exchanged of oxygen
1
from several
development of Cu-ZSM-59 Y zeolites. A:Na+Y, B:Ni2+Y, C:Mn2+Y, Recently a few reports have claimed that direct D:C$+Y, and ECu2+Y. decomposition of NO is successfully catalyzed over several oxides, supported metals, or ion-exchanged zeolites. Hamada et al.10 compared the respectivecatalyticactivitiesby using their own experimental apparatus. The results are summarized in Table 1. Catalytic activities of Y-Ba-Cu oxides and La-Sr-Cu (Co) oxides 12 reported after Hamada et al.’s work are also given in Table 1. All of the data were obtained under steady-state conditions in flow systems. With oxide catalysts two kinds of catalysts, Cog04 or its activated type and oxygen-deficient perovskite-like compounds,are active for the decomposition. Research using perovskite-like oxides was intially performed by Uchijima,l3 who reported that a h0.85SQ.15c003 perovskite was the most active among the oxides examined for the NO decomposition. However, Shimada et al. 11 have clarified that YE@CU~@-~ oxides show higher activity than the La-Sr-Co oxide. In contrast, Tmoka et al. have recently claimed that La-Sr-Co-0 oxide8 with large surface areas can be p~pared by a new preparation method and show higher catalytic activity as shown in Table 1. Platinum
Cu(I1)-exchanged Zeolites for Direct Decomposition of Nitrogen Monoxide 329
supported on A1203 has been known to be an active catalyst for decomposition of NO at elevated temperatures. The catalytic propertiesof a copper ionexchanged ZSM-5 zeolite (Cu-ZSM-5) can be compared with others.
100
e3.0
80
\
E
60
* 20 0 213
313
413
513
613
113
813
Reaction temperature / K The extent of conversion Fig. 2. Temperature dependence of decomposition of NO over over Cu-ZSM5 is higher Cu-ZSM5-23.3-143 at 4.0 g.~.cm-~ and PNo = 1.0 %. than those over others at temperatures as low as 773 K. The results indicate that Cu-ZSMS is the most active catalyst at 773 K for the decomposition of dilute NO gas. The order of activity is CU-ZSM5> Ag-CogO4 > La-Sr-Co(Cu)-0> WAl2O3 > Y-Ba-Cu-O/MgO. Regarding transition metal ion-exchanged zeolites, catalytic activities in the reduction of NO with NH314915 or HC16 and the adsorption state of NO17 were extensively investigatedin the 1970s. Neverthless, surprisingly,little has been determined about their activity for the catalytic decomposition of NO. CATALYTIC DECOMPOSITION OF NITROGEN MONOXIDE OVER COPPER ION-EXCHANGFD ZEOLITES Catalytic Properties of Cd+-exchanged ZSM-5 Zeolites X-(X), Y-faujasite (Y),mordenite (M), femerite (F), L-type(L), and ZSM-5 (ZSMS) zeolites with various didalumina molar ratios were supplied by UCC, Norton, Tosoh, etc. Unless stated otherwise, the zeolite was ion exchanged in aqueous copper(I1) acetate solution of suitable concentration. These samples are called Cu-ZSM5-50-73 (cation - type of zeolite - silidalumina molar ratio - degree of exchange) etc., hereafter. The catalytic activity was measured using a fixedbed flow reactor made of 15 mm 0.d. quartz glass or stainless-steeltube. The gases before and after the reactor were analyzed by gas ~hromatography.~ The temperature dependence of the decomposition reaction is shown in Fig. 2. Maximum activity was observed around 823-873 K and the degree of decomposition decreased at higher temperatures (the best temperature for the reaction was dependent on the catalysts used and the partial pressure of NO in the feed). When the reaction temperature was again set at 773 K after the experiment at 973 K,the degrees of conversion of NO, and into N2 and 0 2 were the same as those of the original samples within experimentid emor. Hence the decrease in the catalytic activity at higher temperatures was not attributableto the deactivation of the catalyst. It is presumably due to changes in the reaction mechanism, the rate-limitings step, andor the state of active sites in the catalysts.
330 M. Iwamoto
No deterioration of the 100 effectiveness of the catalyst was 80 found at this temperature even after 30 h of continuous service; .I? 60 in this work experiments longer !$ than 30 h were not carried out. The degrees of conversion of NO 20 were not equal to those of N2 and 0 2 formation, as shown in Fig. n 2. The remaining nitrogen and 0 50 100 150 Exchange level / % oxygen balances can be attributed to the formation of NW. The Fig. 3. Correlation between exchange level of copper ions reaction, 2N0 = N2 + 0 2 , and the conversions of NO and into N2 and 0 2 . T = 723 K, W/F = 4.0 g.~.cm-~, PNo = 1.0 %, catalyst = ZSM5-23.3. proceeds first on the catalyst, then part of 0 2 produced reacts further with unreacted NO, 2N0 + 0 2 = 2NW. In the series of experiments of the Cu-zeolitedN0system, we found that several ion exchange treatments between Na' and Cu2+ brought about excess loading of copper ions on the ZSM-5 zeolite and the resultant zeolites are very active for the decomposition of NO.18 The correlation between the catalyticactivity and the exchange level of copper ion is depicted in Fig. 3. Clearly, two interesting observationscan be pointed put. First, the conversionsshow S-shaped dependences on the exchange level; the decomposition rates were very small on low-exchanged Cu-ZSM5 zeolites but increased rapidly above ca. 40% exchange. Two interpretations can be proposed. The first is the concept that there are two or more binds of cation sites in the ZSM-5 framework, one of which is the most easily exchangeable with a copper ion but is an inert site for the decomposition. This may be supported by the fact that at least two kinds of sites exist in ZSM-5-19 The second is the idea that the NO decomposition proceeds only with the cooperation of two adjacent active sites and therefore the catalytic activity emerges at high levels of ion exchange. At present, it remains unclear which explanation is correct. The second interesting point is the catalytic activity of the Cu-ZSM5 samples with 100%or more exchange; the decomposition rate to N2 and 02 increased monotonically with increasing exchange levels in this region. This indicates that the copper ions excessively loaded into the ZSM-5 zeolite are effective for the decomposition reaction, though their states are unknown. The best conversion into N2 in the present experiments was 80 - 85%, indicating that NO was nearly completely decomposed to N2 and 02 under these reaction conditions. The catalytic activities of H', Na', ,'K Mg2+, Caz', Cr3+, Fe3+, Cd', Ni2+, Cu2+,Zn2+ or Ag' exchanged zeolites for the decomposition were examined. Among these zeolites, only the Cu2+ or Cd' exchanged zeolites exhibited stable activities. However, the activity of Co-ZSMS50-80 was so low that the degree of NO removal was only 7.9% at 923 K and 4.0 g-scm-3. It is clear that the active catalysts are essentially Cu2+ exchanged zeolites alone.
e
B4
Cu( 11)-exchanged Zeolites for Direct Decomposition of Nitrogen Monoxide 331
Influence of Zeolite Structure and Silica1 Alumina Ratio on the Catalytic Activity It is very significant to investigate which of the zeolite structure and the aluminum content (or Si@/A1203 molar ratio) is the more important factor for controlling catalytic activity. Here several kinds of zeolites were used and typical results are quantitatively depicted in Fig. 4. Within the present experiments,ZSM-5 was the most active catalyst at around 773 K. In the
100 fp
80
%
60
.3
E
u8
20
0 0
20
40
60
80
100
Exchange level of Cu2+/ % Fig. 4. Influence of the exchange level of copper @) ions on the catalytic activity of zeolite. The reaction temperature was 823 or 873 and the contact time was 4.0 g.s. cm-3. nemother zeoliteswere zsM-523.3, M-10, y-5.2, and x-2.1
figure, with Y-type zeolite,the conversion of NO exhibited a characteristic correlation with the exchange level. On the basis of the location of C U ~ions + in the zeolite framework determined by X-ray diffraction 209 21 or other techniques>2 an hCreaSe in the catalyticactivity above 30% should be associated with the increase in the occupancy of supercage sites by the Cu2+ions. In IR experimentsit was confirmed that NO could adsorb as NO', NO- and (N0)2- species on the Cu-zeolite, and the anionic species decreased with adsorption time to yield N2 and N20 in the gas phase whereas NO+ increased.9 After adsorption of NO for about 1 h, anionic species had almost disappeared and the intensity of NO+ species became approximately constant. These results indicate that all the Cu+ ions generated through pretreatment at elevated temperature were oxidized to Cu2+ ions by oxygen produd in the NO decomposition at ambient t e m p t u r e and the resulting Cu2+ ions acted as adsorption sites for NO+ (Cu2' + NO = Cu+-NO+). This NO+ species could not be &sorbed by evacuation at room temperame. The IR spectra indicated the presence of a large amount of NO+ and small amounts of N@ and N@ after the evacuation, i.e., weakly adsorbed or physiwrbed NO molecules were absent from the zeolite under these conditions. These phenomena were further confirmed by ESR experiments; the adsorption-&sorptioncycles of NO resulted in a decrease-increase in the intensity of Cu2+ESR signals. The above IR and ESR experiments suggest that quantitative analysis of the adsorbed NO+ species makes it possible to measure the amount of Cu2+ ions9 active for NO adsorption. This amount should be a better guide than the amount of all loaded Cu2+ ions when we discuss the correlation among zeolite structure, A1 content and catalytic activity. The TPD technique 23 was employed to determine the amount. The thermal &sorption of NO consisted of two peaks at 373473 K and around 653 K. Oxygen desorption was also observed at around 653 K. Part of the NO peak at higher temperatureswould be due to the decomposition of N@ or NO3 adsorbates, since these IR bands disappeared at these tempexatms. The amounts of the lower NO peaks revealed that 85-95% of Cu2+ ions exchanged into the ferrierite and ZSM-5 were active for NO adsorption whereas only 40-45 % of Cu2* ions in Ltype and mordenite was useful.
332 M. Iwamoto
100
elt \
%;50
\
*
0.WL 0.0
'
'
0.1
0.2
*
03
A1 / (Si + Al) Fig. 5* Correlationbetweenthe A1 Content of each parent zeolite and the catalytic activity ion ) accessibleto NO. per copper (II (A) Cu-ZSM5-23.3-104; @) Cu-M-18.9-87; (C) CU-F-12.3-64,@) CU-M-10.5-72; (E) CU-L-6.0-39.
I
0
.1
1
u
10
Partial Pressure of NO / % Fig. 6. Dependence of the degree of conversion of NO on partial pressure of NO and contact time. (A) 753 K and 1.0 g.~.cm-~, (B) 753 and0.2, (C) 753 K and 0.025, @) 873 and 1.0. Catalyst = Cu-ZSM-5-23.3-122.
In the plots of the catalytic activity per Cu2+ion, to which an NO molecule can be accessible, against the A1 content, AU(Si+Al),we obtain a good correlation as depicted in Fig. 5. It is worth noting that not only the acid-base catalysis of proton-exchanged zeolites but also other kinds of catalyticreaction are controlled by the Al content. In the present NO-Cu zeolite system, the zeolite structurewould be the factor determining the effectiveness of Cu2+ions, and the catalyticactivity of the effectiveCu2+ion is probably contsolled by the Al content74 Effects of NO Pressure, Contact Time and Coexisting Gases on NO Decomposition The pressure or contact time dependence of the NO decomposition was examined over CuZSM5-23.3-122.24 The results are depicted in Fig. 6. Conversion of NO was 40-6056 at 0.2 gcat.s.cm-3 (GHSV = ca. 7500 h-1) and 13-25%even at 0.025 gat-scm-3 (ca. 60000 h-1). The results demonstrated that the present catalyst has excellent activity for the catalytic decomposition of NO even at such large SV, which is important for practical use. The effects of the addition of various gases on the catalytic activity of the Cu-zeolites were examined. Co;! showed no reductionin catalyticactivity. When H20 was added, a certain decrease in catalytic activity was observed. The influence of addition of oxygen was dependent on the zeolite structure, degree of exchange of Cu2' ions and the respective parhal pressures of NO and 02. For was added to 0.47% of NO, the conversion of NO decreased from example, when oxygen (8 ~01%) 55% (without oxygen) to 40% at 753 K on Cu-ZSM5-23.3-122. On the other hand, over CuZSM5-23.3-89 the conversion of NO decreased to 5% from 47% on addition of 3% oxygen to the N G H e (NO = 0.5 ~01%) stream. It should be noted that the catalyticactivity of Cu-ZSM5-23.3-122 is little influenced by the presence of oxygen in the feed. An excess loading of copper ions (exchange level above 100%)brings about an incrase not only in the catalytic activity but also in the tolerance to oxygen poisoning. So;! completely poisons the activity at 673-923 K. With ZSM-5,
Cu( 11)-exchanged Zeolites for Direct Decomposition of Nitrogen Monoxide 333
the desorption treatment of adsorbed So;! at higher temperatures resulted in regenelation of the decomposition activity. So;! would compete with NO for the adsorption sites and prevent the catalyticreaction. In contrast,the framework structures of Y and M zeolites were destroyed through the reaction with So.;! at elevated temperatures and the catalytic activity could not be regenerated.
Reaction Mechanism of NO Decomposition The reaction mechanism of and active sites for the reaction have been studied using various techniques combined with an isotopic tracer method. Infrared spectra were measured for detection of surface adsorbates on the Cu-ZSM5 zeolites; ESR, XPS,phosphorescence, diffuse reflectance UV, and Cu-MASNMR have been used to reveal the states of the copper ions in the catalysts. CO adsorption and TPD experiments have been employed to measure quantitatively the amounts of Cu' ions, NO adsorbed, and 0 2 remaining on the surface. Based on these investigations, we can propose a reasonable reaction mechanism which includes Cu' ions as active sites and NO- species as reaction intermediates. The reaction cycle is suggested to be as follows: + 2N0 elevated temp. 2cu2+ ;.2cu+ P 2Cu2+-NO- 02-N2, - 0 2 Finally, it will be discussed why the Cu-Zeolites exhibit such exceptionally high and stable activities. At present, there is no clear answer to this question but it appears that the decomposition activity is based on a combination of the following factors. (1) Copper ions are supported with atomic dispersion due to the ion-exchange properties of zeolites and are difficult to collect owing to the framework structure of zeolite. (2)Oxygen generated through the decomposition is not stabilized by the formation of oxide or strong adsorption,and &sorbs easily at relatively low temperatures22 (3)Thereduction of Cu2' to Cu' in the zeolite lattice is more difficultthan reduction of platinum and palladium ions but easier than that of other transition metal ions.25 The resulting Cu' ion in the zeolite is fairly stable both in a reductive atmosphere and under degassing treatment at elevated temperatures, whereas the precious metal ions are easily reduced to the respective metals and collect to yield metal particles. The easy reducibility of Cu2' and the stability of Cu' lead to a reversible redox behaivor between Cu2' and Cu' and rault in the appearance of the specific catalyticactivity. (4) The above redox behaivor of copper ions in zeolites is very distinct from those on other supports or in aqueous solution and is a specific phenomenon observed only on the zeolite. The copper (I) ion in zeolites is fairly stable,as mentioned above>5 whereas the copper(II) ion supported on silica gel is readily and directly reduced to copper(0). The difficulty of generating Cuo in zeolites may prevent the formation of copper particles. This may be the reason why we need a zeolite framework structureand why silica gel is a poor support for reaction.26
I
Acknowledgment This work was supported by Grants-in-Aid for Scientific Research from the Japan Ministry of Education, Science and Culture, and from the Tokuyama Science Foundation.
334 M. Iwamoto
REFERENCES 1. B. Harrison,M. Wyatt, K G. Gough, "Catalysis",Vol. 5 , Royal Society of Chemistry, London, 1982, pp. 127-171. 2. J. W. Hightower, D. A. Van Leirsburg, "The Catalytic Chemistryof Nitrogen Oxides",Ed. by R. L. Klimish and J. G. Larson, Plenum, London, 1975, p.63. 3. A. Amirnazmi, J. E Benson, M. Boudart, J. Cat& 30 (1973),55; A. Amirnazmi, M. Boudart, ibid., 39 (1975), 383. 4. E. R. S . Winter, J. Catd., 22 (1971), 158. 5. A. A. Chin, A. T. Bell, J. Phys. Chem., 87 (1983), 3700; Y.0. Park,R. I. Masel, and K. Stolt, Sur$ Sci., 131 (1983), L385. 6. K. C. Taylor, "Catalysis",Ed. by J. R. Anderson, M. Boudart, Springer-Verlag, Berlin, 1984, pp.119-170; A. Cruq and A. Frennet, "Catalysisand Autmotive Pollution Control", Elservier, Amsterdam, 1987; H. Bosch and F. J. J. G. Janssen, Cutd Today, 4 (1988), 1. 7. M. Iwamoto, S. Yokoo, K. Sakai, S. Kagawa, J Chem Soc., Faruday Trans. I, 77 (1981), 1692. 8. M. Iwamoto, K. Maruyama, N. Yamazoe, and T. Seiyama, J. Chem Soc., Chem. Commun., (1976), 615. 9. M. Iwamoto, H. Furukawa, Y.Mine, F. Uemura, S.Mikuriya, and S. Kagawa, J. Chem. SOC., Chem. C o m n . , (1986), 1272; M. Iwamoto, H. Furukawa, S . Kagawa, "New Developments in Zeolite Science and Technology",Ed. by Y, Murakami, A. Iijima, J. W. Ward, Elsevier, Amsterdam, 1986, p.943. 10. H. Hamada, Y.Kuwahara, Y. Kindaichi, and T. Itoh, Nut. Meeting Chem. Soc.Jpn., (1988),IVA39; H. Hamada, Y.Kintaichi, M. Sasaki, and T. Ito, Chem. Lett., (1990), 1069. 11. H. Shimada, S. Miyama, and H. Kuroda, Chem. Lett., (1988), 1797. 12. Y. Teraoka,H. Fukuda, S. Kagawa, Chem. Lett., (1990), 1; H. Yasuda, N. Mizuno, M. Misono, J. Chem. Soc. Chem. Commun., in press. 13. T. Uchijima, Hjomen, 18 (1987), 132. 14. T. Seiyama, T. Arakawa, T. Matsuda, Y. Takita, and N. Yamazoe, J. catal.,48 (1977), 1; M. Mizumoto, N. Yamazoe, and T. Seiyama, J. Catul., 55 (1978), 119. 15. W. B. Willamson and J. H. Lunsford, J. Phys. Chem., 80 (1976), 2664; T. Iizuka and J. H. Lunsford, J. Am. Chem. Soc., 100 (1978), 6106. 16. J. S.Ritscher and M. R. Sandner (UnionCarbide Co.),U.S. Parent, (1981),4297328. 17. C. C. Chao and J. H. Lunsford, J. Phys. Chem., 76 (1972), 1546; 78 (1974), 1174; E. F. Vansant and J. H. Lunsford, J. Phys. Chem., 77 (1973),2964; K. A. Windhorst and J. H. Lunsford, J. Am. Chem. Soc.,!I7 (1975), 1407. C. Naccache.and Y. Ben Taarit, Trans. Faruduy Soc., 2(1973), 1475; P. H. Kansai and R. J. Bishop Jr. J. Phys. Chem., 77(1973), 2308; M. Che, J. H. Dutel, P. Gallemt, and M. Primet, J. Phys. Chem., 80 (1976), 2371. 18. M. Iwamoto, H. Yahiro, Y.Mine, and S . Kagawa, Chem. Lett., (1989), 213. 19. W. J. Motier, "Compilationof Extra Framaoork Sites in Zeolite",Butterworth, London, 1982, p.53. 20. W. J. Motier, "Compilationof Extra Framework Sites in Zeolite",Butterworth, London, 1982, pp. 19-31. 21. P. Gallezot, Y. Ben Taarit, and B. Imelik, J. Catal, 26(1972), 295; I. E Maxwell and J. J. de Boer, J. Phys. Chem. 79 (1975), 1874. 22. M. Iwamoto, K. Maruyama, N. Yamazoe,T. Seiyama, J. Phys. Chem., 81 (1977), 622; M. Iwamoto, M. Nakamura, H. Nagano, S . Kagawa, J. Phys. Chem., 86 (1982), 153. 23. S. Kagawa, H. Furukawa, M. Iwamoto, Proc. 7th. Inter. Conger. Cat&., 1980, P. 1406. 24. M. Iwamoto, H. Yahiro, K. Tanda, "Successfil Design of Catalysts",Ed. by T. Inui, Elservier, Amsterdam, 1988, p.2 19. 25. M. Iwamoto, S. Ohura, and S . Kagawa, J. Chem Soc., Chem. Commun., (1981), 842; M. Iwamoto, H. Nagano, H. Furukawa, and S. Kagawa, Chem. Lett., (1983), 471. 26. W. Barbk, Esso Research and Engineering Co. Repon, GR-2-NOS-69, NAPCA Contact No. Ph 22-68-55, 1969 NOV.
335
Ship-in-Bottle Synthesis of Sterically Crowded FePhthalocyanines in NaY Zeolite Hosts and Their Catalytic Behavior in Regioselective Oxidation of Alkanes
Masaru ICHIKAWA:, Takuma KIMURA, and Atsushi FUKUOKA Catalysis Research Center, Hokkaido University, Sapporo 060, Japan
ABSTRACT Tetra-t-butyl Fe(1I) phthalocyanine(FePc(t-Bu)4: 19A diameter) was prepared inside NaY zeolite supercage by "ship-in-bottle'' technique from the presynthesized [HFe3(C0)ii3- in NaY and 4-t-butyl-dicyanobenzene. The structural properties of FePc and FePc(t-Bu)r in NaY were studied by EXAFS, MBssbauer, FT-IR, and diffuse reflectance UV-VIS. The spectroscopic results suggest that Nay-encapsulated FePc(t-Bu)4 is considerably distorted from the original Pc planar structure due to the intrazeolitic constraint inside NaY cages. Sterically crowded FePc(t-Bu)r/NaY exhibited higher activities and selectivities toward terminal hydroxylation of n-hexane and trans-epoxidation of stilbens with PhIO, compared with the external sample and FePc/NaY. Such a higher regioselectivity is likely associated with oxygen attack faborable for the ends of the long axis of substrate molecules around an oxidative Fe=O site of a sterically crowded FePr(t-Bu)s inside NaY cages. INTRODUCTION The use of organometallic compounds as catalyst precursors to be grafted on metal oxides and/or inside zeolite pores is an important aspect of both homogeneous and heterogeneous catalysis. The metal oxide-grafted organometallics including metal clusters offer the rational approaches for the preparation of tailored metal catalysts having a uniformal metal dispersion and well-managed metal compositions[ 13. We have previously reported[ 23 that RhFe and PdFe bimetallic cluster-derived catalysts exhibited the higher activities and improved selectivities for oxygenates in COtH2 reaction rather than the conventional metal catalysts. Zeolites are aluminosilicate crystallines consisting of pores of molecular dilaensions, interconnected by small windows(5-8A diameter). Strict regularity of the pore structure enables higher slectivities to be achieved in both catalysis and sorption processes. The intrazeolite circumstances alike a "solid-solvent" accomodate the selected reactant molecules and promote some inorganic and organic synthetic reactions, similarly in solution.
336 M. Ichikawa, T. Kimura and A. Fukuoka
"Ship-in-a-bottle" synthesis of metal complexes inside zeolite cages has gained growing attention for the purpose of obtaining the catalytically active precursors surrounded with configurationally constrained circumstances [3]. The zeolite framework may impose shape selectivity in metal-catalyzed reactions due to the limitation of size against in-coming reactant molecules [ 41. Metallophthalocyanine cornplexes [ 51 and their electron donor-acceptor complexes have extensively been studied as biological analogs such as chlorophylls and hemoglobins. They are applied to catalytic reactions such as sulfur oxidation, hydrocarbon oxidation, and hydrogenation of olefin, CO, and Nz [6]. Recently, Romanovsky [7] reported the intrazeolitic preparation of Co phthalocyanines, and Herron et al[8] have used Fe phthalocyanines in NaX as catalysts in the hydroxylation of alkanes. We have previously demonstrated [ 9 ] that Nay-encapsulated FePc is slightly distorted inside NaY cages possibly due to the intrazeolite constraint. Electron donor-acceptor(EDA) complex [Na+]r[FePc4-]/NaY gave a higher trans/cis ratio of 2-butenes in butadiene hydrogenation compared with the enternal sample. We extended to synthesize a tert-butyl-substituted FePc inside NaY zeolite, FePc(t-Bu)4 by the so-called "ship-in-bottle" technique, and characterized it by means of XANES, EXAFS, 57Fe Mdssbauer, FT-IR, and diffuse reflectance UVVIS. In this paper, the considerable distortion of an FePc ring plane in NaY cage is discussed in terms of the higher intrazeolitic constraint. The regioselectivities of product formation in the hydroxylation of n-hexane and epoxidation of stilbens were discussed in terms of the sterical crowdness around a central Fe in the phthalocyanine rings substituted a bulky alkyl groups inside NaY cages. EXPERIMENTAL [HFe3(CO)ii]- in NaY was presynthesized from Fez(C0)e (0.25 mmol) and powdered NaY (Linde LZ-Y52, 1 g, dehydrated under vacuum at 593 K for 2 h) [lo]. [HFe3(CO)ii]- formed on the external surface of NaY was removed by washing with a tetraglyme solution of bulky alkyl ammonium halides such as [N(n-CsHia)r]Br and [NMe3(CHzPh)]Cl. A trace amount of [HFe3(CO)ii]- salts was recovered in the extracted solution. [HFe3(C0)i 1 ]-/Nay was subsequently oxidized in air at room temperature for 50 h, followed by the reduction in Hz flow at 423 K for 12 h. The reduced sample was reacted with 4-tert-b~tyl-1~2dicyanobensene (2 amol) in a sealed glass tube under vacuum at 573-583 K for 50
h [ll], resulting in the formation of FePc(t-Bu)r in Nay. Unreacted t-butyl phthalonitrile was removed by sublimation at 573-583 K. FePc(t-Bu)r formed on the external surface of NaY was also removed by Soxhlet extraction with acetone for 24 h. The crude product was washed with hot ether and acetone, and the
Ship-in-Bottle Synthesis of Fe-Phthalocyanines in NaY and Their Catalytie Behavior 337
intrazeolitic FePc(t-Bu)r in NaY (FePc(t-Bu)r/NaY, green-blue powder) was obtained after drying in vacuo. FePc(t-Bu)a on NaY external surface (FePc(tBu)rtNaY) was independently prepared by the impregnation of FePc(t-Bu)r on NaY from the acetone solution (purchased from Tokyo Kasei Kogyo Co., Ltd., purified by sublimation at 673 K). Acetone and water were sufficiently eliminated by evacuation at 473 K for 2 h. The total loading of Fe in FePc(t-Bu)r/NaY and FePc(t-Bu)rtNaY was 1- 0.6 wt%. As a reference sample, the Nay-encapsulated FePc(FePc/NaY) was obtained similarly by using 1,2-dicyanobeneene instead of 4tert-butyl,l,2-dicyanobenzne and purified by the extraction with pyridine as previously reported [91. 4 t-8u-@fN
THF, MeOH
CN
XANES and EXAFS were conducted at BL-1OB n the Photon Factory of the 57Fe Mksbauer National Laboratory for High Energy Physics KEK-PF)[12]. Isomer shifts spectra were recorded with a Shimadzu MEG-2 spectrometer[l3]. were given relative to a-Fe. Infrared spectra were recorded by a Shimadzu Fourier-transform infrared spectrometer(FT1R-4100) with a resolution of 2 cm-1. Diffuse reflectance UV-VIS spectra were obtained on a Hitachi 330 spectrophotometer. The hydroxylation of n-hexane and epoxidation of stilbens were conducted at 300K in the suspended solutions with PhIO for each powdered sample such as FePc( t-Bu)r/NaY, FePc(t-Bu)4+NaY, FePc/NaY and FePctNaY. The products in solution were filtered and analyzed by FID gc using a capillary column(PEG25M). RESULTS AND DISCUSSION In Fig 1, IR spectra for FePc(t-Bu)4tNaY and FePc(t-Bu)4/NaY showed the bands characteristic of phthalocyanine ring( 1400-1320 cm-l; 1280-1250 cm- for C=N and C=C stretching vibrations of Pc ring, respectively) and peripheral tbutyl groups(1510-1470 cm-1). Those IR bands associated with the FePc ring are substantially shifted t o lower frequencies for the intrazeolite FePc(t-Bu)c relative to the external sample and a reference crystalline of FePc(t-Bu)rl,
338 M. Ichikawa, T. Kimura and A. Fukuoka
although not affected for those of a peripheral t-butyl group. The diffuse-reflectance UV-vis spectra of the powdered samples showed as in Fig 2 the dramatic changes of the relative peak intensities at 280 nm(B-band) and 550,580 nm(Q-band) between the NaY intrazeolite and external FePc(t-Bu)r , where the Q-band(r-x* transition of Pc ring) is markedly suppressed in the FePc(t-Bu)4 inside Nay, whereas those of B-band for peripheral benzene ring Fig 1. IR spectra of FePc (t-Bu)~t NaY and FePc (t-Bu)d/NaY in disc.
FeRb-BuLINaY
FePc (t-butyl)q
Fig 2. The diffuse-refrectance UV-VIS spectra of FePc ( t-Bu ) 4 /Nay and FePc(t-Bu)4 + NaY
1600 1500
1400
1300 km-' )
IMO
'I
?band
220
\ 400
P band
610
600
800
A (nm)
are highly pronounced, compared with the external sample. The Q- and B-bands of the intrazeolite sample are also relatively shifted to the longer wavelength region rather than those of the external one. These spectral difference in terms of band position and intensity in IR and UV-vis are explained by the structural deformation of FePc ring plane inside NaY cages due t o the intrazeolite constraint, possibly resulting in a considerable shrinkage of 7r-conjugated orbital systems of Pc ring plane. Furtherore, the curve-fitting analysis of Fe K-edge EXAFS indicated that the intrazeolite FePc(t-Bu)r gave a lesser coordination number of Fe-N bonding in Pc ring (C.N.=3.6) and a longer Fe-N interatomic distance (R=1.90A), rather than those of the external sample, FePc(t-Bu)dtNaY and in crystal of FePc(tBu)4(C.N.=3.8, 3.9; R=1.85, 1.86A, respectively). XANES data(Fig 3) for FePc(t-Bu)4/NaY also support the change of the coordination circumstance around a central Fe in Pc ring plane encapsulated inside NaY cages; a sharp peak appearing at ca 7110 ev due t o ls-4pd* transition characteristic of Fe phthalocyanine moiety relatively weakened in the intrazeolite sample of FePc( tBu)~, compared with those for the external and crystalline. According to these spectral date for EXAFS, XANES, UV-vis and IR it is suggested that the sterically crowded t-butyl substituted FePc (19x19 A in
Ship-in-Bottle Synthesis of Fe-Phthalocyanines in NaY and Their Catalytie Behavior 339
size) was formed and encapsulated inside NaY cages of 13 A diameter, where the phthalocyanine ring plane is significantly distorted due t o the intrazeolite constraint. It is also proposed that a central Fe atom is slightly out of the Pc ring plane inside NaY cage. Fig 3 . XANES Spectra of FePc(t-h)4crystal, FePc(t-Bu)4 t NaY and FePc(t-Bu)(/NaY samples at 300K
ioo.oo 99.80
Photon rmrgy
3 99.60 .-
(N)
c
3
x
3 -
FeR(I-Bu), t N a Y
99.40
99.20
99.00
0.10
P
U
7130 7150 7170
-o'02;0907110
7190
Velocity inm/s)
Photon rnrrgy taV1
0
.
1
4
r
M
Fig 4 . 57Fe Mbssbauer spectrum of FePc(t-Bu)r/NaY
at 78K
e s
0 -0.2-
m9o 7110 7130 7150 7170 7190 Pholon m r g y tN)
Mbssbauer spectra of Nay-encapsulated FePc( t-Bu)lr/NaY and external FePc( tThe Mbssbauer parameters after curveBu)4tNaY at 78 K are shown in Fig 4 . fitting analysis suggested that the Fe precursor in NaY reacted with 1,2dicyanobenzene to be converted into iron phthalocyanine derivatives without any other residual species of iron. The appreciable difference was observed for
I.S. and Q . S . values between the internal and external samples of FePc(t-Bu)4, suggesting that the electron density at the central Fe of FePc(t-Bu)r inside
-+
PhIO
-OH FePc/NaY FePc (t- BubINaY
340 M. Ichikawa. T.Kimura and A. Fukuoka
NaY pore is relatively reduced, compared with the external complex. Oxidation of n-hexane and stilbens was carried out with 30% by volume of substrate in CHzCl2: 1 ml of the stock solution, 20-100 mg(O.5-10x10-3 mmol FePc base) zeolite catalysts and 100 mg iodosobenzene(Ph0I) was sealed and stirred in air at 300K for 12 hrs. Products were identified by capillary g.c. and all yields and selectivities are compared against the intrinsic selectivity of the external samples(FePc(t-Bu)4tNaY and FePctNaY) under the same conditions. As shown in Table 1 where yields expressed as turnovers on the iron complex present, the intrazeolite effect in the oxidation of n-hexane regioselectivity is remarkable for FePc(t-Bu)4 rather than for FePc. The marked promotion in selectivity toward terminal hydroxylation to give 1-hexanol over 2- and 3-position products is observed in the intrazeolite sample of FePc(t-Bu)4, compared with those in the external one and non-substituted FePc samples. This selectivity controlling toward oxidation of the terminal carbon of the subtrate molecule is plausibly interpreted as an orientating influence of the zeolite channel and apertur window upon the substrate n-hexane as it approaches into the sterically crowded iron-oxidative site Fe=O in the intrazeolite FePc( t-Bu)4.
W
cis stilben
U
cis stilbenoxide
trans stilben
.trans stilbenoxide
Stereoselectivity is also observed in the oxidation of stilbens with the zeolite-entrapped t-butyl substituted FePc catalyst. With the cis-stilben as substrate the ratio of trans to cis product (trans-stilben is produced via the epoxidation of cis-sti1ben)is remarkably higher for FePc(t-Bu)s/NaY, compared with those for FePc(t-Bu)rtNaY and even for FePc/NaY and FePctNaY samples under the similar conditions, as shown in Table 2. It was found that the stereoselectivities toward trans-products (trans-stilben t trans-stilben oxide) kept remained(48-53% sel) for the different conversions of cis-stilben. The
Ship-in-Bottle Synthesis of Fe-Phthalocyanines in NaY and Their Catalytie Behavior 341
sterical crowdness around Fe=O site in FePc(t-Bu)r/NaY sample increased preference for oxidation at trans-configuration of the epoxi-intermediate having a relatively smaller cross-section rather than cis-form. Table 1. The oxidation of n-hexane with with PhOI in the presence of FePc (t-Bu)4/NaY, FePc(t-Bu)4tNaY,FePc/NaY and FePctNaY at 300K Catalyst
-
Y j e Id ( L O I / n o I FePc( t su),
I-hexanol
,F e P c )
2-hexanoi
3-hexanol
F e P c ( t - B u ) , /Nay
0.24
3.3
2.4
FePc(t-Bu)rSHaY
0.10
2.7
2.0
FePc/NaY
0.03
0.7
0.6
FePctNaY
0.02
0.6
0.5
n-hexane :28mmol PhIO:0.45 mmol for 12 hr FePc(t-Bu)4/NaY containing FePc(t-Bu)4tNaY
9.6XlO-3mmol FePc/NaY 5.5X10-3mmol ___________.___.____.______ FePc _ _t_ NaY ______ 5.5X10-3mmol _________.___
Table 2. The epoxidation of cis-stilben with PhOI in the presence of FePc (t-Bu)r/NaY, FePc(t-Bu)c+NaY and FePc/NaY at 300K. Catalyst
Reaction Temp.
FePc(t-Bu)./NaY
20
50 FePc(t-Bu),tNaY
FePc/NaY
20
(C)
T.S.:trans-stilben T.S.O.:trans-stilbenoxide C.S.O.:cis-stilbenoxide
Sel e c t i v i ty(:) T.S.
T.S.O.
14 20
34
33
C.S.O.
52 47
9
27
50
a
30
61
20
9
35
56
50
12
32
56
cis-stilben: 40 mg iodosobenzene lOOmg under Nz atmosphere FePc(t-Bu)r/NaY 3.OX10-3 FePc(t,-Bu)rtNaY 4.8X10-3 FePc/NaY 5.5XlO-3 mmol complex
In conclusion, the tert butyl substituted Fe-phthalocyanine was synthesized inside NaY pores by "ship-in-bottle'' technique. EXAFS, UV-vis and FTlR spectroscopic studies suggest that the FePc ring plane is substantially distorted due to the intrazeolite constraint, where a central Fe is slightly out of the Pc ring and its oxidation state is relatively changed, compared with The that of the external sample in terms of 5'Fe M8ssbauer parameters. sterically crowded FePc(t-Bu)r inside NaY leads to increased yields and selectivities toward the terminal oxidation of n-hexane and trans-product in the epoxidation of stilbens as the result of orientating the substrate molecules as the long-axis which approach to the oxidative Fe=O site in the encaged Pc ring.
342 M. Ichikawa. T. Kimura and A. Fukuoka
Acknowledgment We thank Dr. M. Tanaka, Dr. Y, Sakai, and Prof. T. Tominaga for 5IFe MBssbauer study, and Dr. N. Kosugi and Prof. H. Kuroda for the discussion on XANES and EXAFS results. X-ray absorption spectroscopy was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 87131). The part of this work is financially supported by the Grant-in-Aid for Scientific Research(No 6241009) from the Ministry of Science and Culture, Japan. REFERENCES 1 a. M. Ichikawa, "Tailored Metal Catalysts" Ed. Y. Iwasawa, p183263(D. Reidel Pub., Dordrecht, 1985); b Metal Clusters in Catalysis, Eds. B.C. Gates, L. Guczi, and H. Knhinger (Elsevier, Amsterdam, 1986) 2 M. Ichikawa, Polyhedron, 7, 2351(1988) 3 a) L.L. Sheu, H. Knhinger, and W.M.H. Sachtler, Catal. Lett., 2(1989)129. b) T.-N. Huang and J. Schwartz, J. Am, Chem. SOC., 104(1982)5245. 4 a) P.-L. Zhou and R.C. Gates, J . Chem. SOC., Chem. Commun., (1989)347. b) L. Rao, A. Fukuoka, and M. Ichikawa, J. Chen. SOC., Chem. Commun., (1988)458, c) A. Fukuoka, L. Rao, N. Kosugi, H. Kuroda, and M. Ichikawa, Appl. Catal., 50(1989)295. d) M. Ichikawa, L. Rao, T. Ito, and A. Fukuoka, Faraday Discuss. Chem. S O C . , ~ , 321(1989). 5 A.B.P. Lever, Adv. Inorg. Radiochem., 7(1965)27. 6 a)F.H. Moser and A . L . Thomas, The Pbthalocyanines (CRC Press. Boca Raton, 1983) b)K. Tamaru and M. Ichikawa, Catalysis by Electron Donor Acceptor Complexes (Kodansha-Halsted, Tokyo, 1975). 7 B.V. Romanovsky, in: Homogenous and Heterogenous Catalysis, Eds. Y.I. Yermakov and V.L. Kholobov (VNU Science; Utrecht, 1986) P.343. 8 N. Herron, G.D. Stucky, and C.A. Tolman, J. Chem. SOC., Chem. Commun.,(1986)1521. 9 T. Kimura, A. Fukuoka, and M. Ichikawa, Catal. Lett.,4, 279(1990) 10 M. Iwamoto, S. Nakanura, H. Kusano, and S. Kagawa, J. Phys. Chem., 90(1986)5244. 11 D. Dolphin, J.R. Sams, and T.B. Tsin, Inorg. Synth., 20(1980)160. 12 N. Kosugi and H. Kuroda, Program EXAFS2/V03, Res. Center Spectrochem, Univ. Tokyo (1987). 13 M. Tanaka, Y. Sakai, T. Tominaga, A. Fukuoka, T. Kimura, and M. Ichikawa, J . Radioanal. Nucl. Chem., Lett., 137(1989)287.
343
Titanium Silicalite: A New Selective Oxidation Catalyst
B. N o t a r i
ENI- R i c e r c a a S v i l u p p o 20097-San Donato Milanese, M i lano, I t a l y
ABSTRACT A new microporous c r y s t a l l i n e m a t e r i a l has been r e c e n t l y d i s c o v e r e d made o f T i 0 2 and SiO2 and having a S i l i c a l i t e - 1 s t r u c t u r e m o d i f i e d b y isomorphous s u b s t i t u t i o n o f S i ( 1 V ) w i t h T i ( I V ) , t h e r e f o r e named T i t a n i u m S i l i c a l i t e - I o r TS1. TS-1 has unique p r o p e r t i e s as heterogeneous c a t a l y s t f o r t h e o x i d a t i o n o f o r g a n i c compounds w i t h H202 as t h e o x i d a n t . H202 i s v e r y a t t r a c t i v e f o r i n d u s t r i a l a p p l i c a t i o n s because o f t h e low c o s t p e r oxygen atom and t h e absence o f by-products, even if i t r e q u i r e s g r e a t c a r e t o p r e v e n t u n c o n t r o l l e d decomposition. Many d i f f e r e n t r e a c t i o n s have been s t u d i e d u s i n g H202 and TS-1, and h i g h s e l e c t i v i t i e s have been o b t a i n e d i n many cases. F o r t h e p r o d u c t i o n o f c a t e c h o l and hydroquinone from phenol and H202, an i n d u s t r i a l process has been developed and a 10.000 t o n s / y p l a n t has been b u i l t which i s o p e r a t i n g s i n c e 1986 with excellent results. INTRODUCTION The
a c t i v i t y o f t i t a n i u m based c a t a l y s t s f o r t h e o x i d a t i o n o f
pounds i s w e l l known.
organic
com-
W u l f f e t a l . i n 1971 [I]patented f o r S h e l l O i l a process
f o r t h e s e l e c t i v e e p o x i d a t i o n o f p r o p y l e n e w i t h hydroperoxides l i k e ethylbenzene hydroperoxide catalyst
(EBH)
o r tertiary-butyl
hydroperoxide (TBH) w i t h t h e
use
of
A Shell
Oil
plant
made o f T i 0 2 d e p o s i t e d on h i g h s u r f a c e area Si02.
a
f o r t h e p r o d u c t i o n o f 130,000 t o n s / y o f propylene o x i d e a t M o e r d i j k , Holland, i s based on t h i s technology. Sheldon compounds
e t al. 1 ike
[ 2 ] have shown t h a t i n t h e e p o x i d a t i o n o f o l e f i n s w i t h
t i t a n i u m acetylacetonate o r tetra-n-butyl
titanate
TBH,
containing
Ti(1V) produce epoxides w i t h e x t r e m e l y h i g h s e l e c t i v i t i e s (98%). even though t h e r a t e o f r e a c t i o n i s g e n e r a l l y l o w e r w i t h r e s p e c t t o Mo(V1) o r V ( V ) c a t a l y s t s . Hydroperoxides H202
in
view
of
have been considered b e t t e r o x i d a t i o n agents w i t h r e s p e c t t h e i r e x c e l l e n t thermal and
chemical
stability,
s e l e c t i v i t i e s towards d e s i r e d p r o d u c t s and t h e i r s o l u b i l i t y i n o r g a n i c [3].
However
the
industrial
interest
in
t h e use
of
H202
for
the
to high
solvents selective
o x i d a t i o n s remains h i g h i n view o f t h e advantages o f f e r e d by t h i s o x i d a n t namely
344 B. Notari
The safe use o f H202 i n i n d u s t r i a l p l a n t s i s p o s s i b l e o n l y i f H202 decomposition c a t a l y s t s l i k e Fe sa t s low
c o s t p e r oxygen atom and absence o f by-products.
are
r i g o r o u s l y excluded.
in
des gned
Every s i n g l e p a r t of t h e p l a n t must be
o r d e r t o comply w i t h t h i s requirement. Shirmann As203
e t al.
and
i n 1977 [ 4 ] p a t e n t e d homogeneous c a t a l y s t s c o n t a i n i n g B2O3,
Moo3 which,
under p a r t i c u l a r anhydrous
conditions,
could
perform
e p o x i d a t i o n s o f o l e f i n s w i t h H202 i n o r g a n i c s o l v e n t s w i t h h i g h s e l e c t i v i t i e s . A r e s e a r c h program has been c a r r i e d o u t a t EN1 d i r e c t e d towards t h e s y n t h e s i s o f z e o l i t e - l i k e m a t e r i a l s : a new microporous m a t e r i a l made o f S i 0 2 and T i 0 2 been o b t a i n e d [5-71 which t u r n e d o u t t o be a v e r y e f f i c i e n t heterogeneous lyst of
f o r s e l e c t i v e o x i d a t i o n s w i t h H202. silicalite-I:
has cata-
I t s structure closely parallels
i t has been d e s c r i b e d as a s i l i c a l i t e
in
which
that
isomorphous
s u b s t i t u t i o n o f S i ( 1 V ) w i t h Ti(1V) has t a k e n place, and t h e r e f o r e named T i t a n i u m Silicalite-I
o r TS-1.
The changes i n u n i t c e l l dimensions brought about by
Ti
i n t h e s o l i d agree w e l l w i t h t h e values t h a t can be c a l c u l a t e d by t h e d i f f e r e n c e T i - 0 and Si-0 bond l e n g t h s .
between
The presence i n t h e I R spectrum o f an
ab-
s o r p t i o n band a t 960 cm-I which i s absent i n s i l i c a l i t e has been a s c r i b e d t o t h e presence ppm
o f T i i n the solid.
absent
in
The 29Si MAS NMR spectrum g i v e s a s i g n a l a t
s i l i c a l i t e , and t h e r e f o r e considered
characteristic
-115
of
TS-1.
F i n a l l y , t h e d i s t r i b u t i o n o f T i along t h e c r y s t a l has been shown t o be p e r f e c t l y homogeneous [ 81. The
c a t a l y t i c p r o p e r t i e s o f TS-1 a r e o f g r e a t s c i e n t i f i c
interest:
using
H202
i t i s possible t o perform
olefins
and
pounds
[12],
d i o l e f i n s t o epoxides [9-111,
ondary
a l c o h o l s t o ketones [13],
the
and
technological
selective
oxidation
the hydroxylation o f
aromatic
com-
t h e s e l e c t i v e o x i d a t i o n o f p r i m a r y a l c o h o l s t o aldehydes and
sec-
t h e ammoximation o f ketones [14,15].
Hetero-
geneous
c a t a l y t i c systems o f f e r t e c h n o l o g i c a l advantages i n
cations
w i t h r e s p e c t t o homogeneous systems: s i m p l e s e p a r a t i o n and r e c o v e r y
the
of
c a t a l y s t from t h e r e a c t i o n m i x t u r e , i t s r e c y c l e and
industrial
eventual
a p p l iof
regeneration
once d e a c t i v a t e d , e a s i e r r e c o v e r y o f r e a c t i o n products. In are
Fig. 1 c o n v e r s i o n and s e l e c t i v i t y values o b t a i n e d i n d i f f e r e n t
given.
hardly
be
obtained w i t h other catalysts: s i l i c a l i t e i t s e l f i s r a t h e r unreactive, w h i l e
on
other severe
Such
Ti(1V)
h i g h s e l e c t i v i t i e s a t h i g h H202 conversions
c o n t a i n i n g c a t a l y s t s H202 e i t h e r does n o t r e a c t
conditions,
is
decomposed i n t o H20 and 02.
In
the
can
reactions
or,
under
more
hydroxylation
of
benzene i n anhydrous s o l v e n t t h e s e l e c t i v i t y t o phenol i s reduced because o f t h e consecutive choice
of
reaction
of phenol t o p-benzoquinone,
but clearly a
operating conditions could e a s i l y increase the
phenol
more
yield.
ammoximation r e a c t i o n o f cyclohexanone t o cyclohexanone oxime, as we1 1 as reactions,
is
b e i n g c a r e f u l l y s t u d i e d and p o s s i b i l i t i e s o f
careful
industrial
The other appli-
Titanium Silicalite : A New Selective Oxidation Catalyst 345
Fig. 1.
TS-1 catalyzed oxidations H2 O2
R €A C TiON
c =c+ C/
H20,
-
/
con version
0
\
C - C
+HzO
Selectivity ReL
99
97,8
(9)
99,9
93,2 9a,2
(IL)
C/
OH
cations evaluated. The production o f hydroquinone and catechol by TS-1 catalyzed hydroxyl ation o f phenol with H202 appeared competitive with respect to existing industrial processes. A new industrial process has been developed based on TS-1 and a plant for the production of 10,000 tons/y o f diphenols has been built in Ravenna, Italy [7]. It operates since 1986 with excellent results. A plant for the industrial production o f TS-1 has also been built to provide the diphenols plant with the required amount of catalyst. SYNTHESIS OF TS-1
The study o f the catalytic properties o f any material requires that the product is obtained always with the same chemical composition, structure and catalytic activity, and this has been a serious problem in catalysis, But when the catalyst must be used in an industrial plant, this problem becomes vital. The whole plant is designed on the assumption that the desired reaction takes
346
B.Notari
place with the rate and selectivity defined in the project. Much care has therefore been given to the synthesis of this new catalyst, taking into account all variables that could influence the final result. The major variables are: -reagents used -purity, particularly effect of alkalies -Crystallite dimensions -non-framework Ti02 effects -crystallite agglomeration Reagents Reagents to be employed in the synthesis must be selected between a very large number of possible alternatives. With the use of tetraethylsilicate (TES) as the source of SiO2, tetraethyltitanate (TET) as the source of Ti02 and tetrapropylammonium hydroxide (TPAOH) as the base, indicated in the first patent [5], a high degree of reliability could be obtained, and these reagents have therefore been applied also for the industrial production in spite of their rather high prices. It soon appeared that one of the key features was the purity of reagents and in particular the effect of even minute traces of alkalies: this required the development of a process for the production of high purity TPAOH since commercial products then available proved unsatisfactory. The process [I71 is very efficient and has been developed to the industrial product ion. When TES and TET are brought in contact, mixed oligomers are formed: but when aqueous TPAOH is added, under certain conditions a precipitate can form. It has been reported [ 181 that hydrothermal treatment of clear solutions produces by crystallization orthorombic TS-1, while hydrothermal treatment of mixtures containing a precipitate produces monoclinic silicalite. A possib e explanation is that upon reaction with TPAOH, hydrolysis products containing T are formed: these products do not redissolve during the subsequent operations and therefore the Ti is not available for crystals formation. As a consequence o f this segregation the Ti containing compounds undergo independent transformations and appear in the final calcined product as Ti oxides. It is therefore of the utmost importance to prevent the formation of precipitates when the TES, TET and TPAOH mixture is prepared. A procedure which appears satisfactory involves mixing TES and TET and cooling to 273 K before the addition of TPAOH. Alternatively the mixture of TES and TET is added with vigorous stirring to an aqueous concentrated solution of TPAOH (15%). Both procedures have proved adequate.
Titanium Silicalite : A New Selective Oxidation Catalyst 347
Table 1. Effect of alkalies Rx 50 50 50 50 50 50
*
0
0 0 0 1765 3529
0 1060 3530 7060 0 0
H2%* Yield
orthorombic orthorombic orthorombic monoclinic orthorombic orthorombic
79.5 55.0 23.0 0.0 42.0 22.0
Moles of diphenols obtained per 100 moles H202 changed.
Effect of alkalies Synthesis o f TS-1 in the early experiments gave erratic results: the purity of the TPAOH base used in the different experiments was suspected to be responsible and it was hypothesized that alkalies could have an influence on the crystallization process. To clarify this point experiments were carried out with pure TPAOH and the same base to which controlled amounts of alkalies were added as indicated in columns 2 and 3 of Table 1. X ray diffraction and catalytic activity in the hydroxylation of phenol were used to measure the properties of the products obtained. The catalyst obtained with pure TPAOH has the orthorombic structure and gives a high yield of H202. When NaS or K+ is added the yield decreases and the magnitude of the effect is a function of the amount of added alkalies. Changes in the crystal structure follow a different trend: at low alkalies content the orthorombic structure is maintained; at high alkalies values it suddenly changes to monoclinic like silicalite, but the yield has dropped to zero. Hage-A1 Asswad et al. have independently reached similar conclusions [ 191, Crystallite dimensions Since the early catalytic experiments it appeared that the results were also influenced by the crystallite dimensions, with the best performances in the 0.2 - 0.3 range. Larger dimensions produced lower reaction rates and lower selectivities. With the reagents and method indicated, it was sufficient to regulate hydrothermal treatment temperature and time, 433 K and 3 h, to obtain the desired size. The separation of the crystals from mother liquors containing hydrolysis products of TES and TET must be carefully conducted: repeated washings are necessary to remove non-framework Ti02. Agglomeration In order to be used in an industrial plant the catalyst must be shaped in
348 B.Notari
particles of at least 20 - 30 of high mechanical resistance. Only when these requirements are satisfied it can be successfully used in the plant and survive the very severe regeneration procedures which must be periodically carried out to remove carbonaceous deposits and restore catalytic activity. The use of Ludox silica, silicates or other bonding agents has been unsuccessful. The solution to the problem has been found [20] with a procedure that brings about the formation of a thin layer of silica coating every single crystal and connecting all crystals of a particle together: this is obtained by dispersing the crystals into a TPA-silicate solution, transforming this suspension into particles of the desired dimensions through spray-drying and finally decomposing the organic silicate. The catalytic properties of the material so obtained are not significantly different from those of the 0 . 2 ~ mcrystallites, while the silica layer improves the mechanical properties of the particles making them satisfactory for industrial use. Kraushaar-van Hooff method Recently Kraushaar and van Hooff [21] have described a new method for the production of TS-1 based on the reaction of a Ti(1V) compound, typically TiC14, with a defective silicalite or a ZSM-5 from which Al(II1) has been removed by HC1 treatment. The Ti(1V) compound is contacted with the solid in the gas phase at 400-500 OC in a stream of nitrogen. The formation of TS-1 has been demonstrated by the changes occurring in the X-Ray diffraction pattern, the IR spectra and the 29Si MAS NMR spectra, all of which produce the patterns characteristic of classical TS-1. Also the catalytic properties are identical with those of TS-1, as shown by the results of the hydroxylation of phenol with H202. However, even small amounts of non-framework Ti02 dramatically change the catalytic performances: most of the H202 is decomposed to H20 and 02, the yield of diphenols drops to almost zero and tars are formed. The risk of non-framework Ti02 formation is definitely high in this method as a consequence of the hydrolysis of the Ti (IV) compound. The authors suggest that the new method could be of some value for titanium containing zeolites with structures different from silicalite, for instance large pore zeolites which could be useful in the oxidation of large molecules which cannot be oxidized with TS-1. STRUCTURE AND CATALYTIC ACTIVITY OF TS-1 The catalytic activity of TS-1 must no doubt be ascribed to the presence of Ti( IV): silicalite under the same experimental conditions is totally inactive. In order to explain the peculiar performances of TS-1 it has been proposed [7] that,these Ti(1V) are isolated from each other by long sequences of -0-Si-0-Si-
Titanium Silicalite : A New Selective Oxidation Catalyst 349
0-. D i f f e r e n t h y p o t h e s i s have been proposed concerning t h e c o o r d i n a t i o n o f T i ( 1 V ) in
the
solid.
Because
of
t h e analogy w i t h t h e
closely
related
Ti02/Si02
c a t a l y s t , t h e p o s s i b i l i t y t h a t T i ( 1 V ) a r e p r e s e n t as t i t a n y l groups >Ti=O o r t h e corresponding hydrated form w i t h contiguous
Si-OH groups has been considered:
Another p o s s i b i l i t y i s t h a t Ti(1V) a r e p r e s e n t i n t e t r a h e d r a l c o o r d i n a t i o n o f oxygens 1ike Si ( I V ) :
The very
960 cm-labsorption band i s i n f a v o u r o f t h e > T i = - group, s i n c e i t c l o s e t o t h e s t r e t c h i n g frequency o f t h e >Ti=O group (975
Boccuti e t al.
The same a u t h o r s f r o m
examination o f t h e UV-Vis s p e c t r a p o i n t e d o u t t h a t t h e >Ti=O
have
however
[22] p o i n t e d o u t r e c e n t l y t h a t t h i s a b s o r p t i o n i s b e t t e r e x p l a i n -
ed as t h e Si-0 s t r e t c h i n g m o d i f i e d b y t h e presence o f T i . the
cm-l):
comes
an e l e c t r o n i c t r a n s i t i o n a t 25,000
- 35,000
group
cm-1 which i s absent i n
should TS-1,
w h i l e t h e e l e c t r o n i c t r a n s i t i o n a t 48,000 cm-1 which i s p r e s e n t must be assigned to
Ti(1V) t e t r a h e d r a l l y c o o r d i n a t e d by -OH and
temperatures
-0-Si groups.
Upon
above 373 K a gradual l o s s o f water i s observed.
heating
at
On t h e b a s i s
of
t h e s e o b s e r v a t i o n s t h e y propose s t r u c t u r e s o f t h e type;
in
which
one o r two Ti-0-Si
bonds o f t h e c r y s t a l l i n e s t r u c t u r e
are
hydrated,
f o r m i n g s u r f a c e t i t a n o l s and s i l a n o l s groups which can r e v e r s i b l y dehydrate:
350 B. Notari
,OH
, Ti
HO,
si
-
- H20 , T 1
yo\
Si\
I t s h o u l d be noted t h a t t h e doubly h y d r a t e d form i s v e r y s i m i l a r t o t h e h y d r a t e d
t i t a n y l form: d i s t i n c t i o n between t h e two c o u l d t h e r e f o r e be o n l y apparent. By analogy w i t h t h e r e a c t i o n o f s o l u b l e Ti(1V) compounds w i t h H202 [22],
the
mechanism by which TS-1 a c t s as an o x i d a t i o n c a t a l y s t w i t h H202 c o u l d c o n s i s t i n the
i n t e r a c t i o n o f Ti(1V) o f t h e s o l i d w i t h H202 t o form a s u r f a c e
[7].
nate
peroxotita-
I n a second stage t h e s u r f a c e p e r o x o t i t a n a t e can p e r f o r m t h e
t i o n o f t h e o x i d i z a b l e o r g a n i c products:
oxida-
i f t h e s e a r e i n d i c a t e d by Red, we have:
Red. 0
According
t o t h i s proposal, t h e h i g h s e l e c t i v i t y o f TS-1 should be a s c r i b e d
the
that
fact
Ti(1V) This
are
H202 can be decomposed i n t o H20 and 02 o n l y when
two
i n near-neighbour p o s i t i o n s , a v e r y u n l i k e l y p o s s i b i l i t y
or in
to more
TS-1.
r e s u l t s i n a low decomposition r a t e o f H202 which f a v o u r s t h e t r a n s f e r
of
p e r o x i d i c oxygen t o t h e o r g a n i c compounds. The problem o f t h e r o l e o f a c i d i t y i n t h e o x i d a t i o n r e a c t i o n has been examinTo t h i s end s i l i c a l i t e s c o n t a i n i n g b o t h Ti(1V) and A l ( I I I ) , , o r
ed.
Fe(II1)
G a ( I I 1 ) have been synthesized [24-261 and used i n t h e e p o x i d a t i o n o f It
is
w e l l known t h a t t r i v a l e n t elements i n t r o d u c e d i n
definite
acidic
character t o the material.
The r e s u l t s
the
propylene.
framework
obtained
under
s i m i l a r experimental c o n d i t i o n s a r e g i v e n i n Table 2.
Table 2. E p o x i d a t i o n o f propylene Catalyst
T /K
Y H ?CH,CH,P
P\ '/c-c C/c-c
TS-1
313
97.7
Ti-Fe-Si
313
80
Ti- Oa- Si
293
6.5
/c-c C
?H
YH
/c-c C
1
0.2
1.1
11
5.5
3
56.1
37.3
or
yH
impart very
Titanium Silicalite : A New Selective Oxidation Catalyst 351
The
effect
substantial
amount o f t h e i n i t i a l l y formed epoxide undergoes t h e
catalyzed present
o f t h e a c i d i t y c r e a t e d by t h e t r i v a l e n t elements i s
a d d i t i o n o f water o r methanol t o t h e epoxide r i n g , only
to
evident: typical
this
considered an i n d i c a t i o n o f a v e r y weak a c i d i t y o f t h i s m a t e r i a l .
could
But t h e
t h a t e p o x i d a t i o n s e l e c t i v i t y can be increased b y t r e a t m e n t o f TS-1 w i t h agents l i k e Cl-Si-(CH3)3
ing
that trans-
or Si-ONa groups, w h i l e
formed b y t h e m o d i f y i n g agents i n t o i n a c t i v e Si-O-(CH3)3 TS-1
fact
o r CH3COONa [ I 6 1 can be regarded as evidence
c a t a l y t i c a c t i v i t y due t o T i i s n o t affected.
is be
modify-
t h i s weak a c i d i t y must be a t t r i b u t e d t o s u r f a c e s i l a n o l groups which a r e the
acid
This reaction
a v e r y l i m i t e d e x t e n t when TS-1 i s used, and
a
A l s o i n gas phase
reactions
does n o t show a c t i v i t y f o r t y p i c a l a c i d c a t a l y z e d r e a c t i o n s l i k e
methanol
t r a n s f o r m a t i o n i n t o hydrocarbons o r o l e f i n i s o m e r i z a t i o n . Assuming
Ti( IV)
that
is
distributed
statistically
in
all
tetrahedral
i t can be e a s i l y seen t h a t even f o r c r y s t a l l i t e s i z e s o f 0.2 pm
positions, great
m a j o r i t y o f Ti(1V) i s l o c a t e d i n s i d e t h e p o r e s t r u c t u r e .
every
T i ( 1 V ) i s a c a t a l y t i c c e n t r e w i t h equal a c t i v i t y ,
f o r molecules o f d i f f e r e n t s i z e s should be observed.
the
Assuming
diffusion
that
limitations
This i s i n f a c t the
case.
I t has been shown [ 2 7 ] t h a t t h e r a t e o f o x i d a t i o n o f p r i m a r y a l c o h o l s
decreases
regularly
a
drop
as t h e c h a i n l e n g t h increases, w h i l e f o r i s o - b u t y l a l c o h o l
i n t h e r a t e i s observed.
different
from
sudden
A l s o t h e r e a c t i v i t y o r d e r o f o l e f i n s on TS-1
t h e o r d e r observed w i t h
homogeneous
e l e c t r o p h i 1i c
is
catalysts,
w h i l e as a l r e a d y i n d i c a t e d v e r y b u l k y molecules a r e u n r e a c t i v e when TS-1 i s used as
the catalyst.
limitations
A l l these f a c t s can o n l y be i n t e r p r e t e d as due
to
o f t h e l a r g e r molecules, which means t h a t t h e c a t a l y t i c
diffusion sites
are
located i n s i d e the pore s t r u c t u r e o f the solid. CONCLUSIONS
A new microporous s o l i d m a t e r i a l has been o b t a i n e d made o f T i 0 2 and Si02 (TS1)
which
Si(1V)
has a s i l i c a l i t e - I s t r u c t u r e m o d i f i e d by isomorphous s u b s t i t u t i o n with
Ti(1V).
Its
synthesis
takes
place
in
the
presence
of of
t e t r a a l kylammonium bases under c a r e f u l l y c o n t r o l l e d c o n d i t i o n s .
TS-1
has
unique
p r o p e r t i e s as heterogeneous
o f o r g a n i c compounds w i t h H202:
oxidation
oxidation
catalyst
for
the
very high s e l e c t i v i t i e s are obtained
and t h i s para1 l e l s t h e behaviour o f T i ( I V ) based homogeneous c a t a l y s t s . I t i s proposed t h a t t h e o x i d a t i o n r e a c t i o n s proceed t h r o u g h t h e f o r m a t i o n
a s u r f a c e p e r o x o t i t a n a t e by subsequent organic
transfer
products.
catalysts each other,
of The
i n t e r a c t i o n o f framework Ti(1V) w i t h H202,
t h e oxygen from t h e p e r o x o t i t a n a t e t o difference
w i t h respect t o
other
of
and t h e
the
oxidizable
Ti( IV)
containing
i s a t t r i b u t e d t o t h e f a c t t h a t i n TS-1 a l l Ti(1V) a r e
isolated
from
w i t h t h e consequence t h a t t h e r a t e o f H202 decomposition i s reduced
352 B. Notari
t h u s f a v o u r i n g t h e s e l e c t i v e o x i d a t i o n o f t h e o r g a n i c products. The
production
competitive
with
of
diphenols
from
phenol and
H202
on
TS-1
o t h e r i n d u s t r i a l processes and a p l a n t has been
has
proved
built
which
operates s i n c e 1986 w i t h e x c e l l e n t r e s u l t s . The
discovery
of
TS-1 and i t s unique c a t a l y t i c
properties
constitutes
a
s i g n i f i c a n t c o n t r i b u t i o n t o t h e knowledge o f s i l i c a - b a s e d z e o l i t e - l i k e m a t e r i a l s containing
elements
different
from
A l ( 111)
and
opens
new
technological
p o s s i b i l i t i e s f o r o x i d a t i o n processes w i t h H202.
REFERENCES 1 H. W u l f f e t a l , USP 3,642,833; 3,923,843; 4,021,454; 4,367,342; B r i t . Pat. 1,249,079. 2 a) R.A. Sheldon and J.A. van Doorn, J. Catal., 31 (1973) 427. b ) R.A. Sheldon, J.A. van Doorn, W.A. Shram and A.J. De Jong, i b . 31 (1973) 438. 3 R.A. Sheldon i n "The Chemistry o f F u n c t i o n a l Groups, Peroxides", Ed. S. P a t a i 1983 J. W i l e y P.163. J.P. Shirmann e t a l . Ger. Pat. 2,752,626; 2,803,757; 2,803,791. M. Taramasso, G. Perego and B. N o t a r i , U.S.P. 4,410,501. M. Taramasso, G. Manara, V. F a t t o r e and B. N o t a r i , U.S.P. 4,666,692. B. N o t a r i , Stud. Surf. Sci. Catal., 37 (1987) 413. G. Perego, G. B e l l u s s i , C. Corno, M. Taramasso, A. Esposito, i n Y. Murakami, A. I i j i m a , J.W. Ward (Eds.), Proc. Seventh I n t . Conf. on Z e o l i t e s , Tokyo 1986, Tonk Kodansha p.129 9 C. N e r i , A. Esposito, B. A n f o s s i and F. Buonomo, Eur. Pat. 100,119. 10 C. Neri, B. A n f o s s i and F. Buonomo, Eur. Pat. 100,118. 11 F. Maspero and U. Romano, Eur. Pat. 190,609. 12 a) A. Esposito, M. Taramasso, C. N e r i and F. Buonomo, B r i t . Pat. 2,116,974. b ) A. Thangaray, R. Kumar and P. Ratnasamy, Appl. Catal., 57 (1990) L1. 4,480,135. 13 A. Esposito, C. N e r i and F. Buonomo, U.S.P. 14 P. R o f f i a , M. Padovan, E. M o r e t t i and G. De A l b e r t i , Eur. Pat, 208,311. 15 P. R o f f i a , M. Mantegazza, A. Cesana, M. Padovan and G. L e o f a n t i . X V I I t a l i a n N a t i o n a l Chemistry Congress, Oct. (1988) 259. 16 M.G. C l e r i c i and U. Romano, Eur. Pat. 230,949. 4,578,161. 17 F. Buonomo, G. B e l l u s s i and B. N o t a r i U.S.P. 18 B. Kraushaar-Czarnetzki and J.H.C. van Hooff, Catal. Lett., 2 (1989) 43. 19 J. E l Hage-A1 Asswad, J.B. Nagy, Z. Gabelica and E.G. Derouane, 8 t h I n t . Zeol. Conf. J u l y 1989. 20 G. B e l l u s s i , M. C l e r i c i , F. Buonomo, U. Romano, A. E s p o s i t o and B. N o t a r i , Eur. Pat. 200,260. van Hooff, Catal. L e t t . 1 (1988) 81. 21 B. Kraushaar and J.H.C. 22 M.R. Boccuti, K.M. Rao, A. Zecchina, G. L e o f a n t i and G. P e t r i n i , Stud. S u r f . Sci. Catal., 48 (1989) 133. 23 a) 0. B o r t o l i n i , F. D i F u r i a and G. Modena, J. Mol. Catal., 16 (1982) 69. b ) G. Amato, A. A r c o r i a , F.P. B a l l i s t r e r i . G.A. Tomaselli, 0. B o r t o l i n i , F. D i F u r i a , G. Modena and G. V a l l e . J. Mol. Catal., 37 (1986) 165. 24 G. B e l l u s s i , A. G i u s t i , A. E s p o s i t o and F. Buonomo, Eur. Pat. A.266,257. 25 G. B e l l u s s i , M.G. C l e r i c i , A. G i u s t i and F. Buonomo, Eur. Pat. A.266,258. 26 G. B e l l u s s i , M.G. C l e r i c i , A. C a r a t i and A. Esposito, Eur. Pat. A.266,825. 27 U. Romano, A. Esposito, F. Maspero, C. Neri, M.G. C l e r i c i i n "New Developments i n S e l e c t i v e O x i d a t i o n , Paper B-1, R i m i n i 1989.
353
The Effects of Iron Impurities on the Cracking Properties of Pillared Clays
M.
L. Occelli(l), J. M. Stencel(2) and S. L. Suib (3)
(1) Unocal, Brea, CA 92621 USA (2) Kentucky Center for Energy Research, Lexington, KY 40512 (3) University of Connecticut, Storrs, CT 06268 USA
USA
ABSTRACT Bentonites pillared with A1 O3 flusters can generate materials with BET surface area in the 290-316 m /g range having basal spacing near 19.4A (at 100°C). Naturally occurring iron impurities in the parent clay do not seem to affect physicochemical properties and a clay catalyst containing 3.8% Fe 0 generates as much.coke during gas oil conversion as a clay containing o h $ 0.3% Fe 0 . Thus the high coke make tendency of pillared clays cannot be a t t r k h e d to the presence of iron impurities in the parent material. INTRODUCTION Pi 1 1 ared cl ays are 1 ayered materi a1 s prepared by rep1 acing mono and divalent charge compensating cations in swelling smectites with large polyoxocations or charged cQ’Iloidal particles (1). On heating, the cationic pillars form oxide clusters that prop open the platelets of the clay thus exposing the silicate layers to sorption and catalysis. Bentonites and hectorites (2) pillared in this way have strong Lewis acidity and exhibit high cracking activity for gas oil cracking (1). The properties of these materials have been described in recent review articles (1,3). The limited hydrothermal stability (650-7OO’C at 100% steam, 1 atm) and the large amount of coke generated during gas oil cracking have prevented (to date) the commercial application of these new type of low cost catalysts in the fluidized cracking of oil fractions (1). Iron impurities in clays have been thought responsible for these type of catalysts’ low carbon selectivity (4,5). The purpose of this paper is to investigate and report the influence that the location, chemical state and environment of iron impurities have on the cracking properties of pillared clays prepared by reacting several smectites with aluminum chlorhydroxide solutions.
353 M. L. Occelli, J. M. Stencel and S. L. Suib
EXPERIMENTAL Pi 1 1 ared C1 ay PreDarat i on The low-iron (0.3% Fe203) containing bentonite sample was obtained from the Southern Clay Products Co, Gonzales, Texas (Bentolite L grade). The iron rich (3.8% Fe203) material used is a Wyoming bentonite obtained from the American Colloid Company. Chemical analysis is shown in Table 1. Bentonites pillared with alumina clusters were then prepared according to well known procedures (1) using, as before, an aluminum chlorhydroxide (ACH) solution (Reheis Chemical Company's Chlorhydrol(R)). Table 1. Oxide ComDosition* of Two Bentonites from: Wt%
Texas
Wvomi nq
A1 O3
15.7 71.7 3.60 1.70 0.30 0.30 0.20 0.16
20.2 63.5 2.4 1.1 3.8 0.17 2.23 0.35
si62
MgO C a0
*The difference from 100% is due to chemically bound water. In an effort to place iron in the pillars, Fe(N03).6H20 crystals were dissolved in Chlorhydrol(R). The resulting solution was then stirred at room temperature for 12 hours and added (in excess) to a vigorously stirred slurry containing 0.01 g clay/g DI water. The slurry was heated to 70°C and kept at this temperature for 2 hours. The pillared product, (Fe,ACH)-bentonite, was filtered, washed with DI-water at 70°C and oven dried at 12O0C/24h in flowing air. Iron was also introduced between the clay layers by ion exchanging at room temperature the bentonites with a 0.01M Fe(N0 )3 solution. The Fe-bentonite was then pillared using Chlorhydrol(') and the ACH-(Fe-bentonite) sample washed and dried as described above. A quartz-free nontronite sample (6) was expanded by reacting a slurry containing 0.0075 g claylg water with an excess of Chlorhydrol(R). A pillared product was obtained that after drying at -100°C had a d(001) spacing of 19.4A. Calcination in air at 400'C/10h reduced the d(001) value to 16.9A; the calcined ACH-Nontronite had BET surface area of 310 m 2/g and contained 31.9% Fe203, All powder diffraction measurements were obtained with a Siemens 0-500 diffractometer at a scan of l'/min using monochromatic CU-Ka radiation.
Iron Impurities and Cracking Properties of Pillared Clays 355
Pillared Clays Characterization Surface Acidity. Surface acidity was examined with a Nicolet 170 SX spectrometer. Spectra were acquired with 2 cm-' resolution (8192 data points) and apodised using the Happ-Genzel algorithm. Self supporting wafers (4-8mg/cm 2 in density) were prepared by pressing samples between 25 rnm diameter die for one minute at -6,000-7,000 lb pressure. Prior t o pyridine sorption, the wafers were mounted in an Abspec Inst. Corp. #200@ optical cell and degassed by heating at 200°C for 2h at torr. The pyridine-loaded wafers were then heated (in vacuo) in the 200-5OO'C temperature range. Spectra o f the 0-H stretching region were smoothed with a five point Savitzky-Golay algorithm and baseline slope corrected; peak intensities were normalized to the sample density. Crackinq ProDerties Catalytic evaluation of the different pillared clays was performed using a microactivity test (MAT) and conditions described in detail elsewhere (6). The weight hourly space velocity (WHSV) was 14-15; the reactor temperature was 51O'C. A catalyst-to-oil ratio of 3.5-3.8 was used. The chargestock's slurry oil (S.O., b.p. >354'C), light cycle oil (LCGO, 232'C < b.p. (354.C) and gasoline content were 62.7 vol%, 33.1 vol% and 4.2 vol% respectively. Conversions were on a vol% fresh feed (FF) basis and were defined as [V -V /V ] x 100, where Vf is the volume of feed f P f and V is the volume of product with b.p. > 204'C. Cracking activity is P defined as: % conversion/(100 - % conversion). RESULTS AND DISCUSSION Surface Properties ACH-bentoni tes prepared from Texas or Wyoming samples have similar surface properties. After drying in air, both materials have a d(001) of 19.4A; calcination in air at 400'/10 hours decreased the basal spacing to 18.1A and pillared products with 290-310 m 2/g surface area were obtained. Exchange reactions between Fe-bentonite and polycations of a1 uminum generated an ACH-(Fe-bentonite) sample containing 8.0% Fe203; after drying at 120'C/10h in air the clay's d(001) was 18.7A. The presence of residual Fet3 ions in exchange sites, decreases the Allj-pillar's density. Thus ACH-(Fe-bentonite) is somewhat less thermally stable than the other pillared clay catalysts prepared. After calcination in air at 40OoC/10h this catalyst lost 30% o f its surface area and its d(001) value decreased to 16.8A owing to dehydroxylation reactions of the [All,04(0H)24(H20)12]t7 cations.
356 M. L. Occelli, J. M. Stencel and S. L.Suib
The other iron-containing clay catalyst prepared by reacting a Texas bentonite with an ACH-Fe(N03)3 solution gave an (Fe,ACH)-bentonite sample containing 9.7% Fe203 that after drying in air at 12OoC/10h had d(001) = 17.8A and BET surface area of 296 m2/g. Calcination reduced pillar's height; after heating in air at 40OeC/10h, the d(001) value decreased to about 15.7A probably as a result of some iron removal from the pillars. Surface Acidity IR spectra for the pillared bentonites in the OH-stretching region show an intense and broad OH-band centered near 3640 cm-'; this band is shifted to near 3600 cm-l for the ACH-nontronite sample under study, Fig. 1. After pyridine sorption, only minor changes were observed in these spectra, indicating little reaction of the hydroxyl groups present with pyridine. As the degassing temperature is increased from 200°C to 500'C, OH bands decrease in intensity due to dehydroxylation reactions of the clay lattice, Fig. 1. Dehydroxylation is more facile in the iron-containing ACH-nontronite sample, Fig. 1F. Infrared (IR) spectra in the 1400-1600 cm-l region obtained by evacuating the pyridine loaded (calcined at 400'C) clays at different temperatures are shown in Figure 2. In analogy with the assignments made by Parry (8), bands near 1548 cm-l have been assigned to Nt-H groups in pyridium ions resulting from the presence of Bronsted (B) acid sites. These sites are thought to result from the partial substitution of A1 for Si in the clay tetrahedral layers and formation of -Si-OH--Al(IV) groups capable o f pyridinium ion formation. Bands near 1490 cm-l are attributed to pyridine sorbed on both Bronsted and Lewis (L) acid sites (8). The ACH-nontronite sample seem to have initially a greater density of Bronsted acid sites. The presence or absence of iron does not affect Bronsted acid site density or strength in the two ACH-bentonites (from Texas and Wyoming) tested. After degassing at 300'C/2h the intensity of the 1548 cm-l band decreased significantly in all the samples studied; at higher temperatures, evidence o f Bronsted acidity is lost, Fig. 2. The high acidity at 200°C of Fe-bentonite is attributed to the presence of OH groups associated with Fet3 ions in exchange sites, Fig. 2C. Above 200'C dehydroxylation occurs and a drastic reduction in intensity of the bands near 1548 cm-' and 1490 cm-l occur, Fig. 2C.
Iron Impurities and Cracking Properties of Pillared Clays 357
A
B
C
i
:
a
d
W
0 2
da $ m a
3100 36W 3500 3400 3300
3100 3000 3500 3400 3300
3100 3600 35W 3400 3300
WAVENUMBERSIcm-3
WAVENUMBERSlcm.3
WAVENUMEERS(cm')
3100 3600 3500 3400 3300
WAVENUMEERS(cm.3
3700 3000 3500 3400 3300
3700 3600 3500 3400 3300
WAVENUMBERS(cm-3
WAVENUMEERS(cm.I
'
Fig. 1. Hydroxyl absorption bands f o r several smectites p i l l a r e d w i t h aluminum chlorhydroxide (ACH) s o l u c t i o n s : A) Wyoming ACH-bentonite; B) Texas ACH-bentonite; C) Fe-bentonite; D) ACH-(Fe bentonite) ; E) (ACH, Fe)-bentonite and F) ACH-nontronite. Samples a) have been d r i e d a t 200'C and then loaded w i t h p y r i d i n e and degassed a t : b) 200'C, c) 300'C, d) 400'C and e) 500'C i n vacuo f o r 2 hours a t each temperature.
358 M. L. Occelli, J. M. Stencel and S. L. Suib
1kO 1550 1500 1450 1400 WAVENUMBERS(cm-9
1;OO
1550 1500 1450 1400 WAVENUMBERS(cm-1)
I
.
1600 1550 1500 1450 1400 WAVENUMBERS(crn-1)
R F
I,
1600 1550 1500 1450 1400 WAVENUMBERS(cm-1)
WAVENUMBERS(cm-1)
.
.
-
a
,
1600 1550 1500 1450 1400 WAVENUMBERS(cm-9
F i g . 2. IR spectra o f p y r i d i n e sorbed on: A) Wyoming ACH-bentonite; B) Texas ACH-bentoni t e ; C) Fe-bentoni t e ; D) ACH- (Fe-bentoni t e ) ; E) (ACH,Fe)-bentonite and F) ACH-nontroni t e . Samples have been degassed i n vacuo a t : a) 200'C, b) 300'C, c) 400'C and d) 500'C f o r two hours a t each temperature.
Iron Irnpuritics and Cracking Propertics of Pillared Clays 359
In pillared clays, acidity results mainly from dehydroxylation of the large interlayering cations which prop apart the silicate layers (9). On heating, these polyoxocations form oxide clusters (though containing Lewis acid sites) and protons which are retained as charge-compensating cations. Intensities of bands near 1450 cm-', assigned to pyridine coordinated onto Lewis acid sites, are less temperature sensitive and although band intensity monotonically decreases with temperature, some pyridine is present on these sites even after degassing at 500'C/2h, Fig. 2. In contrast to what seen in the spectra of the two pyridine-loaded ACH-bentonites (Figs. 2A, 2B), the band near 1490 cm-' in the spectrum of the ACH-(Fe-bentonite) sample, broadens and increases in intensity after degassing at 400'C or 500'C, Fig. 2D. This unexpected increase in band intensity with temperature, althcugh less pronounced, is present also in the spectra of (ACH,Fe)-bentonite and to an even lesser extent, in ACH-nontronite, Figs. 2E, 2F. It is believed that as pyridine desorbs (at T > 300'C) from the pillars L-acid centers, it reacts with Fet3 ions (in exchange sites) on the silicate layers. The presence of pyridine held to exposed Fe-ions (found between the A1203-pillars that prop apart the clay silicate layers) is believed responsible for the increased broadening and intensity of the absorption band near 1490 cm-'. Gas Oil Cracking Microactivity test results have been collected in Table 2. The reference comnercially available FCC contains an estimated 35% of a calcined, rare-earth exchanged zeolite Y (CREY). Prior to evaluation, this fresh catalyst was aged by steaming at 760'C/5h with -100% steam at 1 atm. Pillared bentonites, in general, collapse and lose their cracking activity after such a severe hydrothermal aging. Thus a milder thermal pretreatment (40OoC/10h in dry air) was used to age the clay catalysts in the hope of obtaining materials with comparable surface area and cracking properties. After pillaring and calcination at 400'C, the two ACH-bentonites are as active as the steam-aged zeolitic FCC but give much higher carbon and light gas yields, Table 2. As expected, the clay catalysts offer a LCGO advantage over the commercial FCC probably because the pillared clays' larger pore openings afford the cracking of high-molecular weight hydrocarbons in the slurry oil (SO) range (4,6). Most importantly, the results in Table 2 indicate that a pillared clay prepared using a Texas-bentonite containing only 0.35% Fe203, with a coke/conversion ratio o f 0.12, i s as carbon selective as a catalyst prepared using a Wyoming-bentonite containing ten times as much FeeOg (coke/conversion =
360 M.L. Occelli, J. M. Stencel and S. L. Suib
Table 2. Microactivity test results for several pillared clay catalysts after calcination in air at 400'C for 10h. The zeolitic cracking catalyst has been aqed for 5 hours at 760'C with 100% steam at 1 atm. ACH-Bentoni te from:
Wvomins Conversion (V% FF) Gasol ine ( V X FF) LCGO (V% FF) SO ( V X FF)
C (VX FF) ct = (VX FF) n - t 4 ( V X FF) i-C4 (VX FF) X4=(V% FF) CH4 (WtX FF)
H (SCF/BBL) Dgy Gas (WtX FF) Iron (Wt% Fe 03) Coke (Wt% FFf Coke/Conversion
BET S.A. (m2/g)
86.7 58.4 11.7 1.6
3.3 6.9 1.4 7.2 3.2 0.30 324 4.7 3.4 11.5 0.13 298
85.7 56.8 12.9 1.4 2.4 11.4 1.2 7.6 6.8 0.42 424 4.9
ACH(Fe,ACH)Nontroni te Bentonite
Zeol i tic Cracking Catal vst
78.3 32.6 17.3 4.4 1.o 5.6 0.4 2.1 4.3 1.35 1369 5.5
75.1 37.0 21.3 3.6 0.7 4.0 0.3 1.2 3.8 1.96 1817 5.8
10.0
31.9 28.2
9.7 20.6
6.5
0.12
0.36
0.27
0.076
314
310
0.30
296
85.4 59.1 9.8 4.8 4.7 7.1 2.0 8.5 2.4 0.26 356 5.6
---
161
When, instead of cracking a light gas oil (API gravity = 29.6, Aromatics = 23.1%, iron 0.9 ppm and sulfur 0.20), the two clay catalysts were used to crack a heavy gas oil rich in organic sulfur ( A P I gravity 23.7, aromatics = 34.1%, iron = 3.2 ppm and sulfur = 1.2%) a -10% decrease in cracking activity was observed probably due to the high aromatic content of this feed. However, the coke/conversion ratios of the two pillared bentonites remained essentially unchanged at 0.11 and 0.14, respectively. Thus, in agreement with previous results (2,10), the pillared clay's high coke make cannot be attributed to iron-catalyzed cracking reactions. Pillared clays containing 8-10% Fe203 such as Fe(ACH-bentonite) and (ACH, Fe) bentonite are somewhat less active than the ACH-bentonites under study owing to their lower thermal stability. Their lower gasol ine/conversion and high coke/conversion ratio indicate that when iron is placed either between the expanded silicate layers o f the clay catalysts [as in ACH-(Fe-bentonite)] or when it is part of the pillars [as in (Fe-ACH)-bentonite)] it can easily catalyze secondary cracking reactions forming large amounts o f coke and light gases (C2-C4, CH4, H2) by drastically reducing (cracking) hydrocarbons in the gasoline boiling range, see Table 2. 0.13).
-
-
-
Iron Impurities and Cracking Properties of Pillared Clays 361
The e f f e c t s o f i r o n become even more evident when ACH-nontronite ( c o n t a i n i n g 31.9% Fe203) i s used t o crack a l i g h t gas o i l a t MAT c o n d i t i o n s . It seems t h a t a t t h e temperatures used i n FCCU operations, Fet3 from t h e octahedral l a y e r migrates onto t h e s i l i c a t e l a y e r where i t can c a t a l y z e secondary cracking r e a c t i o n s g i v i n g low gasol ine/conversion r a t i o s t h a t can be explained by the h i g h l i g h t gas-make and by a near t h r e e - f o l d increase i n t h e coke/conversion r a t i o , Table 2. SUMMARY AND CONCLUSIONS
Bentonite from Texas o r Wyoming, when p i l l a r e d w i t h alumina c l u s t e r s , generate c l a y c a t a l y s t s having s i m i l a r surface area, basal spacing and thermal stability. I n f r a r e d spectra from p y r i d i n e chemisorption experiments have shown t h a t the two p i l l a r e d bentonites, i r r e s p e c t i v e o f t h e i r i r o n content, c o n t a i n both Bronsted (B) and Lewis (L) a c i d s i t e s and t h a t a t h i g h temperature a c i d i t y i s mainly o f t h e Lewis type. The presence o f i r o n (3.8% Fep03) i n the p i l l a r e d Wy-bentonite d i d n o t s i g n i f i c a n t l y a f f e c t a c i d s i t e s t r e n g t h o r density. I f l o c a t e d i n t h e p i l l a r s o r on the s i l i c a t e l a y e r s between p i l l a r s , i r o n appears t o enhance L-type a c i d i t y by i n t e r a c t i n g a t h i g h temperatures w i t h p y r i d i n e . When present i n exchange s i t e s , i r o n decreases p i l l a r d e n s i t y and w i t h i t t h e p i l l a r e d c l a y thermal stability. A t MAT conditions,
montmorillonites p i l l a r e d w i t h alumina c l u s t e r s , and having s i m i l a r surface area, generate ( a t a given conversion l e v e l ) s i m i l a r amounts o f coke (when used t o crack gas o i l ) i r r e s p e c t i v e o f t h e i r o n content o f t h e parent bentonite. Thus, t h e presence o f i r o n cannot be used t o e x p l a i n t h e h i g h tendency f o r coke (and l i g h t gas) make o f p i l l a r e d clay catalysts. ACKNOWLEDGMENT Special thanks are due t o M r . M. B e l l , Dr. P. R i t z , and D r . J. R. Glasmann (Unocal) f o r p r o v i d i n g x - r a y data, l a s e r Raman measurements, and procedures f o r n o n t r o n i t e p u r i f i c a t i o n . S. L. Suib and M. L. O c c e l l i acknowledge t h e K i n e t i c s and C a t a l y s i s D i v i s i o n o f t h e NSF f o r support of t h i s work under g r a n t CBT 8814974.
REFERENCES 1. 2. 3.
M. L. O c c e l l i i n "Keynotes i n Energy Related Catalysis," S. Kaliaguine Ed., Elsevier, p. 101 (1988). M. L. O c c e l l i , 0. H. Finseth; J. Catal. 99, 316, 1986. F. Figueras, Catal. Review 30, 3, 457, 1988.
362
51. L. Occelli. J. M. Stencel and S. L. Suib
R. J. Lussier, J. S. Magee, D. E. W. Vaughan; Preprints, 7th Canadian Symposium on Catalysis, p. 88, 1980. 5. D. Tichit, F. Fajula, F. Figueras, C. Guequen, and J. Bosquet, in "Fluid Catalytic Cracking: Role in Modern Refining," M. L. Occelli, Ed., ACS Symp. Series No. 375, p. 237, 1988. 6. M.L. Occelli, J. M. Stencel and S. L. Suib, submitted J. Mol. Catal. (1990). 7. M. L. Occelli; Ind. Eng. Chem. Prod. Res. Dev. 22, 553, 1983. 8. E. P. Parry, J. Catal. 2,371 (1963). 9. M. L. Occelli, R. M. Tindwa; Clays Clay Min. 31, 22, 1983. 10. M. L. Occelli, J. E. Lester; Ind. Eng. Chem. Prod. Res. Dev. 24, 27, 1985. 4.
363
Catalysis by Hydrotalcite in Liquid-phase Organic Reactions
Y. Ono. E. S u z u k i , and M. Okamoto Department o f Chemical Engineerinng, Meguro-ku, Tokyo 152, Japan
Tokyo I n s t i t u t e o f Technology,
Ookayama,
ABSTRACT The i n t e r l a y e r C1- a n i o n s i n a s y n t h e t i c h y d r o t a l c i t e - l i k e m a t e r i a l , Mg A12(OH)16C1~4H20, were found t o r e a c t w i t h o r g a n i c bromides i n a non-polar so v e n t o f toluene, a l m o s t a l l c h l o r i d e i o n s i n t h e i n t e r l a y e r space appearing i n t h e l i q u i d phase as t h e corresponding o r g a n i c c h l o r i d e s . The h y d r o t a l c i t e l i k e m a t e r i a l was found t o c a t a l y z e o r g a n i c r e a c t i o n s i n w h i c h t h e i n t e r l a y e r C1- anions p l a y t h e r o l e o f c a t a l y s t . Thus, t h e m a t e r i a l c a t a l y z e d t h e h a l i d e exchange r e a c t i o n s between a1 k y l h a l i d e s i n t o l u e n e 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 r i m e t h o x y s i l a n e t o g i v e s i l a n e and t e t r a m e t h o x y s i l a n e .
P
INTRODUCTION Hydrotalcite,
Mg6A12(oH)16c0~4H20. i s one o f t h e n a t u r a l l y o c c u r r i n g a n i o n i c
c l a y m i n e r a l s and can be s y n t h e s i z e d [1,2]. Mg6A12(OH)16C1~4H$,
A hydrotalcite-like material,
can a l s o be synthesized [2].
Here,
Mg*+ and A13+ c a t i o n s
c o n s t i t u t e p o s i t i v e l y charged h y d r o x i d e l a y e r s between which w a t e r molecules and C1- anions a r e i n t e r c a l a t e d ,
t h e l a t t e r b e i n g anion-exchangeable [3].
The e x c h a n b e a b l e a n i o n s a r e known t o r e a c t w i t h o r g a n i c h a l i d e s i n a nonp o l a r s o l v e n t such as toluene.
Thus,
b y the, a c t i o n o f i n t e r l a y e r I-anions i n
a h y d r o t a l c i t e - l i k e m a t e r i a l , Zn2Cr(OH)6I2-2H20, i n t o b u t y l i o d i d e [4].
b u t y l b r o m i d e was c o n v e r t e d
The f i r s t a i m o f t h i s work i s t o examine t h e r e a c t i o n s
o f i n t e r l a y e r C1- anions i n Mg6A12(OH)16C1~4H20 w i t h benzyl o r b u t y l bromide i n t o l u e n e t o expand t h e knowledge o f t h e r e a c t i v i t y o f t h e i n t e r l a y e r anions w i t h o r g a n i c h a l i d e s i n a non-polar solvent. The second a i m o f t h i s work i s t o demonstrate t h e c a t a l y s i s o f t h e i n t e r l a y e r a n i o n s i n o r g a n i c r e a c t i o n s such as h a l i d e - e x c h a n g e r e a c t i o n s b e t w e e n a l k y l h a l i d e s i n t o l u e n e (eq. 1).
364 Y. Ono, E. Suzuki and M. Okarnoto
Thus,
f o r example,
f o r a system o f benzyl chloride,
hydrotalcite-like material,
b u t y l b r o m i d e , and t h e
we expect t h e f o l l o w i n g r e a c t i o n s t o occur.
Butyl
b r o m i d e w o u l d u n d e r g o a h a l i d e s u b s t i t u t i o n b y t h e i n t e r l a y e r C1- a n i o n s , t h e i n t e r c a l a t e d B r - anions would,
l e a v i n g Br- anions i n t h e i n t e r l a y e r space;
i n t u r n , a t t a c k b e n z y l c h l o r i d e t o y i e l d b e n z y l b r o m i d e l e a v i n g C1- a n i o n s i n t h e i n t e r l a y e r space, t h e i n t e r l a y e r C1- anions b e i n g c y c l e d as f o l l o w s :
Here, H.T.-Cl-
and H.T.-Br-
d e n o t e h y d r o t a l c i t e - 1 i k e m a t e r i a l s c o n t a i n i n g C1-
and Br- i n t e r l a y e r anions,
respectively.
Since t h e h y d r o t a l c i t e - l i k e m a t e r i a l c o n t a i n s exchangeable i n t e r l a y e r anions, i t can be used as a c a t a l y s t f o r r e a c t i o n s f o r which anion-exchange
been used as c a t a l y s t . 4 (CH30)3SiH
__+
r e s i n s have
The d i s p r o p o r t i o n a t i o n o f t r i m e t h o x y s i l a n e (eq. SiH4
2) i s
(2)
3 (CH30)4Si
t
c a t a l y z e d b y an anion-exchange r e s i n such as D i a i o n PA-306 [5].
The c a t a l y t i c
d i s p r o p o r t i o n a t i o n o f t r i m e t h o x y s i l a n e i n t h e p r e s e n c e o f a h y d r o t a l c i t e - 1 ike m a t e r i a l c o n t a i n i n g i n t e r l a y e r C1- o r CH3O- anions w i l l a l s o be demonstrated. EXPERIMENTAL Synthesis
of
hydrotalcite-like materials
The h y d r o t a l c i t e - 1 i k e m a t e r i a l , aqueous s o l u t i o n s o f MgC12.6H20, a t 433 K [ 2 ] .
MggA12(OH)16C12.4H20,
AlCly6H20,
and NaOH (Mg2+/A13+
CH30Na (CH30-/N03-
i n a methanol s o l u t i o n o f
m o l a r r a t i o = 10) a t 338 K f o r 43 h.
and
disproportionation reactions
For t h e halide-exchange condenser,
m o l a r r a t i o = 3)
The m a t e r i a l c o n t a i n i n g CH3O- a n i o n s was o b t a i n e d b y a n i o n -
e x c h a n g i n g NO3- a n i o n s o f Mg6A12(oH)16(N03)2'4H20
Halide-exchange
was synthesized f r o m
reactions,
i n t o a 50-cm3 f l a s k e q u i p p e d w i t h a
30 cm3 o f t o l u e n e o r DMF as a s o l v e n t and 1.0 g p o r t i o n o f t h e
h y d r o t a l c i t e - 1 i k e m a t e r i a l (3.3
mmol o f i n t e r l a y e r C1- anions) were introduced.
A 33 mmol o f benzyl c h l o r i d e was added t o t h e m i x t u r e and t h e t e m p e r a t u r e was k e p t a t 343 o r 373 K w i t h s t i r r i n g ,
f o l l o w e d b y t h e a d d i t i o n o f a g i v e n amount
o f an a l k y l bromide ( o r i o d i d e ) (33-195 mmol). a n i t r o g e n atmosphere.
Reactions were conducted under
The l i q u i d p h a s e was w i t h d r a w n p e r i o d i c a l l y a n d
a n a l y z e d b y a gas c h r o m a t o g r a p h e q u i p p e d w i t h a 2-m l o n g SE-30 c o l u m n and a
Catalysis by Hydrotalcite for Organic Reactions 365
f l a m e i o n i z a t i o n detector. F o r 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 s , t h e h y d r o t a l c i t e - l i k e m a t e r i a l s were evacuated a t 453 K f o r 2 h b e f o r e use t o d r i v e o u t i n t e r l a y e r w a t e r molecules. A 41 mmol o f t r i m e t h o x y s i l a n e was added i n t o t h e f l a s k c o n t a i n i n g 0.62 g o f t h e m a t e r i a l (2.0 mmol o f i n t e r l a y e r a n i o n s ) u n d e r a n i t r o g e n a t m o s p h e r e . r e a c t i o n m i x t u r e was
analyzed f o r
chromatograph d e s c r i b e d above,
(CH30)3SiH
and (CH30)4Si
The
b y t h e gas
u s i n g heptane as a standard.
RESULTS AND DISCUSSION Reaction
of
i n t e r l a y e r anions w i t h o r g a n i c h a l i d e s
To know t h e r e a c t i v i t y o f i n t e r l a y e r C1- anions towards o r g a n i c h a l i d e s i n a non-polar solvent, o f H.T.-Cl-
t h e f o l l o w i n g e x p e r i m e n t s were c a r r i e d out.
A 1.0 g p o r t i o n
(C1-= 3.3 mmol) was added t o a t o l u e n e s o l u t i o n o f b u t y l o r b e n z y l
bromide (33 mmol) a t 373 K:
t h e f o r m a t i o n o f t h e corresponding a l k y l c h l o r i d e s ,
g e n e r a t e d by t h e h a l i d e exchange b e t w e e n H.T.-Clf o l l o w e d w i t h time.
and t h e a l k y l b r o m i d e s , was
As shown i n Fig. 1, t h e y i e l d o f b u t y l c h l o r i d e i n c r e a s e d
w i t h t i m e and a t t a i n e d a c e i l i n g value.
The exchange was a l m o s t complete i n 1
h, a b o u t 90% o f t h e c h l o r i d e i o n s i n t h e i n t e r l a y e r s a p p e a r i n g i n t h e l i q u i d phase as b u t y l c h l o r i d e .
- 3 0
E E
80
%01 C
a
-c
60
h)
ul t
40 $ 1
.oc a
20
& )r
rd
0 0
20
40 60 t I min
80
Fig. 1. Change i n b u t y l ( o r b e n t y l ) c h l o r i d e y i e l d w i t h r e a c t i o n t i m e i n a h a l i d e exchange between b u t y l ( o r b e n z y l ) bromide and t h e h y d r o t a l c i t e - l i k e m a t e r i a l c o n t a i n i n g i n t e r l a y e r C1- anions. Reaction c o n d i t i o n s : CqHgBr ( o r C H CH2Br)= 33 mmol, s o l v e n t ( t o l u e n e ) = 30 cm3, h y d r o t a l c i t e - l i k e m a t e r i a l (H.T.-C(i-?= 1.0 g ( i n t e r l a y e r C1- anions= 3.3 m o l ) , and r e a c t i o n temperature= 373 K.
366 Y. Ono. E. Suzuki and M. Okamoto
Compared t o t h e h a l i d e exchange b e t w e e n H.T.-Clh a l i d e exchange b e t w e e n H.T.-Cl-
and b u t y l b r o m i d e , t h e
and b e n z y l b r o m i d e p r o c e e d e d much f a s t e r .
Thus, as shown a l s o i n Fig. 1, t h e h a l i d e exchange was a l m o s t complete w i t h i n 10 min. Because o f t h e s i z e o f t h e a l k y l h a l i d e s , t h e h a l i d e exchange p r o b a b l y o c c u r s a t t h e e x t e r n a l edge s u r f a c e s o f t h e h y d r o t a l c i t e - l i k e m a t e r i a l , n o t i n t h e i n t e r l a y e r space,
T h i s i s supported b y t h e f a c t t h a t no expansion o f t h e
i n t e r l a y e r space was observed a f t e r t h e m a t e r i a l was used i n t h e halide-exchange reaction.
The d i f f u s i o n o f t h e h a l i d e i o n s i n t h e i n t e r l a y e r space i s n o t t h e
r a t e - d e t e r m i n i n g step, s i n c e t h e r a t e o f t h e r e a c t i o n g r e a t l y depends on t h e k i n d o f a l k y l bromide. H a l i d e exchanqe between a l k y l h a l i d e s i n t o l u e n e C a t a l y s i s o f t h e i n t e r l a y e r a n i o n s i n t h e h a l i d e exchange b e t w e e n a l k y l h a l i d e s (eq.
A r e a c t i o n between benzyl c h l o r i d e (33 mmol) and
1) was examined.
b u t y l b r o m i d e ( 3 3 mmol) i n t o l u e n e a t 373 K was c a r r i e d o u t u s i n g a 1.0 g The r e a c t i o n p r o c e e d e d and, a s l i s t e d i n T a b l e 1, t h e
p o r t i o n o f H.T.-Cl-.
y i e l d o f b e n z y l b r o m i d e was 31% a t 4 h, i n c r e a s i n g t o 37% a f t e r 20 h.
The
r e a c t i o n d i d n o t proceed i n t h e absence o f t h e H.T.-Cl-. The H.T.-Cl-
can a l s o be a c a t a l y s t f o r a l k y l i o d i d e p r o d u c t i o n i n t o l u e n e .
Thus, b e n z y l i o d i d e c o u l d be o b t a i n e d b y t h e r e a c t i o n o f b e n z y l c h l o r i d e ( 3 3
mmol) w i t h b u t y l i o d i d e (33 mmol) a t 373 K (Table 1).
T a b l e 1. H a l i d e exchange b e t w e e n b e n z y l c h l o r i d e and b u t y l bromide ( o r iodide1.a ~~
~
Benzyl bromide ( o r iodide) y i e l d Alkyl halideb
CqHgBr CqHgBr C4H9I C4H9I
Weight o f H.T.-ClC 4 1.0
0
1.0 0
/
Reaction t i m e 4
20
31
31
0
/
/ % h
0
26
-
0
0
aSolvent (toluene)= 30 cm3 and r e a c t i o n temp e r a t u r e 373 K. bBenzyl c h l o r i d e = b u t y l c h l o r i d e ( o r i o d i d e ) = 33 mmol. Cone g r a m o f H.T.-Clc o n t a i n s 3.3 mmol o f i n t e r l a y e r C1- anions.
Catalysis by Hydrotalcite for Organic Keactions 367
H a l i d e exchange u s i n g v a r i o u s bromoalkanes The h a l i d e exchange i n DMF b e t w e e n b e n z y l c h l o r i d e a n d b u t y l b r o m i d e g a v e F i g u r e 2 shows t h e t i m e
h i g h e r y i e l d o f benzyl bromide t h a n t h a t i n toluene.
course o f benzyl bromide y i e l d i n a r e a c t i o n between benzyl c h l o r i d e (33 mmol) and b u t y l b r o m i d e (33 mmol) i n DMF a t 343 K u s i n g a 1.0 g p o r t i o n o f H.T.-Cl-. The y i e l d i n c r e a s e d w i t h t i m e a n d was 51% a t 4 h. I n t h e h a l i d e exchange i n DMF,
t h e y i e l d was 53%
i n t h e a b s e n c e o f H.T.-Cl-,
I n a p r o l o n g e d r u n o f 20 h, t h e r e a c t i o n proceeded even
t h e y i e l d b e i n g 27% a t 4 h.
A l k y l b r o m i d e s can be o b t a i n e d f r o m t h e c o r r e s p o n d i n g c h l o r i d e s i n t h e presence o f NaBr o r t r i - n - b u t y l a m i n e ,
u s i n g bromoalkanes such as e t h y l bromide,
p r o p y l bromide,
as w e l l as b u t y l bromide as a b r o m i n a t i o n
a g e n t [6,7].
and 1,4-dibromobutane
These b r o m o a l kanes w e r e t e s t e d a s b r o m i n a t i o n a g e n t s t o w a r d s As l i s t e d i n T a b l e 2 ( e n t r i e s 1
b e n z y l c h l o r i d e i n t h e p r e s e n c e o f H.T.-Cl-. and 3-5),
t h e bromoalkanes gave benzyl bromide y i e l d s a t 4 h o f 51-652 under t h e
same r e a c t i o n c o n d i t i o n s as those i n Fig. 2.
Benzyl bromide y i e l d a t 20 h and
a t e q u i l i b r i u m a r e a l s o l i s t e d i n T a b l e 2.
It i s c l e a r t h a t t h e r e a c t i o n s
p r o c e e d c l o s e t o e q u i l i b r i u m e v e n a t t h e r e a c t i o n t i m e o f 4 h. T a b l e 2,
1,4-dibromobutane
i s t h e m o s t e f f i c i e n t b r o m i n a t i o n agent.
halide-exchange r e a c t i o n s proceed c l o s e t o e q u i l i b r i u m u s i n g H.T.-Cl-
60
I
As seen i n
50 -
as a s o l i d
I
I
I
The
o/o0 o 4 .
40
/
30
/*-
20
*/.-*-
10 0
I
0
1 2 3 Reaction time I h
I
4
Fig. 2. Change i n benzyl bromide y i e l d w i t h r e a c t i o n t i m e i n a h a l i d e exchange between benzyl c h l o r i d e and b u t y l bromide. Reaction conditions: CgH CH C1= CqHgBr= 33 mmol, s o l v e n t (DMF)= 30 cm3, hydrot a l c i t e - l i k e m a t e r i a l (H.?.-il-)= 1.0 g ( i n t e r l a y e r C1- anions= 3.3 mmol), and r e a c t i o n t e m p e r a t u r e = 3 4 3 K. I n t h e p r e s e n c e ( 0 ) o r absence ( 0 ) o f t h e h y d r o t a l c i t e - 1 ike m a t e r i a l .
368 Y. Ono. E. Suzuki and M. Okamoto
T a b l e 2. H a l i d e exchange between benzyl c h l o r i d e and v a r i o u s bromoal kanes.a Bromoalkaneb
Entry
Benzyl bromide y i e l d Reaction t i m e
/
/ %
h Equ 1ib r iurn
1
CqHgBr C4HgBrC C3H7Br C HgBr Br(ZH2)qBr
2 3 4 5
4
20
51
53 90 58 63 66
-
53 60 65
54 90 60 64 66 ~-
~
~
a S o l v e n t (DMF)= 30 cm3, H.T.-Cl-= 1.0 g, a n d r e a c t i o n temperature= 343 K. bBenzyl c h l o r i d e = bromoal kane= 33 mmol. CReaction c o n d i t i o n s as above e x e p t f o r CqHgBr= 195 mmol.
c a t a l y s t i n p l a c e o f a homogenenous c a t a l y s t of t r i - n - b u t y l a m i n e
o r NaBr [6,7].
A 90% y i e l d o f benzyl bromide was a t t a i n e d ( e n t r y 2 i n Table 2) when 195 mmol o f b u t y l bromide was used (C4HgBr/C6HtjCH2Cl m o l a r r a t i o = 6) and t h e r e a c t i o n was conducted f o r 20 h. Disproportionation
of t r i m e t h o x y s i l a n e
F i g u r e 3 shows t h e change i n t r i m e t h o x y s i l a n e c o n v e r s i o n w i t h r e a c t i o n t i m e f o r t h e r e a c t i o n s i n t h e presence o f t h e h y d r o t a l c i t e - l i k e i n t e r l a y e r C1- anions, unreacted (CH30)3SiH. a t 6 h.
Here,
material containing
t h e c o n v e r s i o n was c a l c u l a t e d f r o m t h e amount o f
The conversion i n c r e a s e d w i t h r e a c t i o n t i m e and was 70%
The s e l e c t i v i t y o f t h e r e a c t i o n i s d e f i n e d as f o l l o w s : [amount of (CH30)4Si produced]
Se 1e c t i v i ty=
ioa
(3)
[amount o f (CH30)3SiH consumed] x (3/4) The s e l e c t i v i t y was loo%, i n d i c a t i n g t h a t o n l y 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 (eq. 2 ) proceeds. T a b l e 3 l i s t s t h e t r i m e t h o x y s i l a n e c o n v e r s i o n a t 6 and 9 h u s i n g hydrotalcite-like
m a t e r i a l s c o n t a i n i n g i n t e r l a y e r c1- and CH3O- anions.
and 2 i n Table 3, 79% a t 9 h.
Entries 1
show t h a t t h e c o n v e r s i o n i n c r e a s e d w i t h r e a c t i o n t i m e and was
The s e l e c t i v i t y , however, decreased,
o t h e r t h a n eq. 2 occur.
indicating that reactions
The u s e o f t w i c e t h e w e i g h t o f t h e m a t e r i a l ( e n t r y 3
i n Table 3) i n c r e a s e d t r i m e t h o x y s i l a n e c o n v e r s i o n t o 92%. w h i l e t h e s e l e c t i v i t y was 95%
D e v i a t i o n o f t h e s e l e c t i v i t y from 100% would be due to,
f o r example,
Catalysis by Hydrotalcite for Organic Reactions 369 I
0
1
I
I
I
I
I
1
I
I
I
I
2
3
4
5
6
Reaction time I h Fig. 3. Change i n trimethoxysilane conversion with reaction time. Reaction conditions: (CH30)3SiH= 41 mmol, hydrotalcite-like material (H.T.C1-)= 0.62 g (interlayer C1- anion= 2.0 mmol), and reaction temperature= 323 K.
Table 3.
Disproportionation o f trimethoxysilane.a
-
Entry
H'T'-X
Weight o f H.T.-X- / g
Reaction time / h
trimethoxysilane conversion / %
Selectivity o f reactionb / %
-~
1 2
3 4 5
H.T.-ClH.T.-ClH.T.-Cl-
0.62 0.62
H.T.-CH30H.T.-CH30-
0.62 0.62
1.2
6 9 6 6 9
70 79 92 a3 95
100 92 95
100 95
~
a(CH30)3SiH= 41 mmol and reaction temperature= 323 K. bOef ined by eq. 3.
polymerization o f (CH30)qSi into si loxanes by the action o f undetectable amount o f water. The hydrotalcite-l ike material containing interlayer CH3O- anions catalyzed the reaction (entries 4 and 5 in Table 3). the trimethoxysilane conversion being 8 3 and 95x with 100 and 95% selectivity, respectively. CONCLUSION The interlayer Cl- anions i n a synthetic hydrotalcite-1 ike material, Mg6A12(OH)16C1~4H20, react with butyl bromide or benzyl bromide in a non-polar solvent o f toluene. Almost all the chloride ions in the interlayer space
370 Y. Ono. E. Suzuki and
M.Okamoto
appear i n t h e l i q u i d phase as t h e corresponding o r g a n i c c h l o r i d e s . t h e r e a c t i o n g r e a t l y depends o n t h e k i n d o f o r g a n i c b r o m i d e .
The r a t e o f I n t e r l a y e r C1-
anions m i g r a t e t o t h e e x t e r n a l edge s u r f a c e s where t h e r e a c t i o n s proceed. The h y d r o t a l c i t e - l i k e m a t e r i a l c a t a l y z e s o r g a n i c r e a c t i o n s i n w h i c h t h e i n t e r l a y e r C1- a n i o n s p l a y t h e r o l e o f c a t a l y s t , halide-exchange
The m a t e r i a l c a t a l y z e d t h e
r e a c t i o n s between benzyl c h l o r i d e w i t h b u t y l bromide o r b u t y l
i o d i d e i n toluene.
The h y d r o t a l c i t e - l i k e
material also catalyzes a
d i s p r o p o r t i o n a t i o n o f t r i methoxys i1ane t o g i v e s i 1ane and t e t r a m e t h o x y s i1ane. The h y d r o t a l c i t e - l i k e m a t e r i a l was f o u n d t o be a p o t e n t i a l c a t a l y s t f o r o r g a n ic r e a c t i o n s .
REFERENCES
C. Frondel, Am. Miner., 26 (1941) 295: R. Allmann, Chimia, 24 (1970) 99. S. M i y a t a and T. Kumura, Chem. Lett., (1973) 843; S. M i y a t a , C l a y s C l a y Miner., 23 (1975) 369; S. Miyata, ibid., 28 (1980) 50. 305. 3 S. Miyata, Clays C l a y Miner., 31 4 K. J. M a r t i n and T. J. Pinnavaia, J. Am. Chem. SOC., 108 (1986) 541. 5 Eur. P a t e n t Appl., (1986) 201919. 6 W. E. W i l l y , D. R. McKean, a n d B. A. G a r c i a , B u l l . Chem. SOC. Jpn., 49 (1976) 1989. 7 Y. Sasson and M. Y.- Weiss, J. Mol. Catal., 10 (1981) 357.
1 2
(1985J
371
Iron-exchanged Montmorillonite as an Efficient Acid Catalyst in Liquid-Phase Organic Synthesis
Y . I z u m i and M . Onaka
Department o f s y n t h e t i c Chemistry, School o f Engineering, Nagoya U n i v e r s i t y , F u r o - c h o , C h i k u s a - k u , Nagoya 4 6 4 , Japan
ABSTRACT Fe3'-exchanged carbonyl
montmorilloni t e
groups
much
more
a c t e d as a s o l i d
efficiently
than
a
acid
catalyst
homogeneous
superacid
t r i f l u o r o m e t h a n e s u l f o n i c a c i d and i t s s i l y l e s t e r i n l i q u i d - p h a s e to
be
added t o c a r b o n y l compounds. of
clay
The t r i a l k y l s i l y l c a t i o n s
s i l i c a t e l a y e r s a p p e a r t o be r e s p o n s i b l e
of
carbon-carbon
b o n d - f o r m i n g r e a c t i o n s u s i n g e n o l s i l a n e s and s i l y l c y a n i d e a s t h e
surface
activating
nucleophiles
formed
for
on
the
enhancing
the
r e a c t i o n s as h i g h l y a c t i v e Lewis a c i d s i t e s .
INTRODUCTION In
r e c e n t y e a r s , s y n t h e t i c o r g a n i c c h e m i s t s began t o show much
applying
inorganic
promoters,
s o l i d materials t o liquid-phase
o r reagent supports.
reactions
interest as
A microporous c r y s t a l o f c l a y
[1,2].
reactions
The p r e s e n t a u t h o r s f i r s t r e p o r t e d
c a t a l y s i s o f m e t a l c a t i o n - e x c h a n g e d m o n t m o r i l l o n i t e (M"'-Mont, Sn4')
in
reactions
carbon-carbon using
bond-forming
r e a c t i o n s (Eq.
1)
catalysts,
montmorillonite
has a l s o been a p p l i e d as a s o l i d a c i d t o c a t a l y z e v a r i o u s t y p e s o f organic
in
liquid-phase
the Mnt:
such
novel A13+,
as
acid Fe3',
aldol-type
e n o l s i l a n e s w h i c h a r e w i d e l y a p p l i e d as c a r b a n i o n
donors
in
o r g a n i c s y n t h e s i s [15-171. The c l a y c a t a l y s t a c t i v a t e s t h e c a r b o n y l compounds t o react
with
homogeneous
e n o l s i l a n e s much more e f f i c i e n t l y t h a n z e o l i t e s
and
a c i d s s u c h as BF3 e t h e r a t e , m e t h a n e s u l f o n i c a c i d , and
conventional even
better
t h a n a s u p e r a c i d o f trifluoromethanesulfonic a c i d o r i t s s i l y l e s t e r [3-51. We h e r e f u r t h e r d e m o n s t r a t e t h e m e r i t s o f Fe3'-exchanged the
conventional
homogeneous
montmorilloni t e over
acid catalysts, applying i t t o
other
types
of
l i q u i d - p h a s e c a r b o n - c a r b o n b o n d - f o r m i n g r e a c t i o n s between c a r b o n y l compounds and useful
nucleophilic
c a r b a n i o n r e a g e n t s s u c h as s i l y l k e t e n e a c e t a l
(an
ester
372 Y. Izurni and M. Onaka
e n o l a t e ) [ 6 ] and c y a n o s i l a n e [ 7 ] , b o t h o f wh ch a r e t h e s i l y l a t e d e q u i v a l e n s o f espectively.
c a r b o x y l i c a c i d e s t e r and hydrogen c y a n i d e ,
R4
R’
R4
wosi Me3 C‘’ II
Mnt-Mont
R5
t
-MegSiO
0
R2 q R 3
A5 0
-78
-
2O0C
R1: a l k y l , v i n y l ; R2: a l k y l ; R3: a l k y l , a l k o x y ; R4: H, a l k y l ; R 5 : a l k y l
EXPERIMENTAL Preparation o f c l a y c a t a l y s t s p u r i f i e d Nat-exchanged m o n t m o r i l l o n i t e ( ” K u n i p i a F ” , 1 . 1 9 meq/g)
A
exchanged
with
aqueous
as
Fe-Mont)
(abbreviated
Fe(NO3I3. was
The
washed and
Fe3+-exchanged dehydrated
under
was
montmoril l o n i t e relatively
of clay.
o f 12OoC/0.5 T o r r f o r 24 h t o a v o i d c o l l a p s e o f t h e l a y e r The BET s u r f a c e a r e a o f Fe-Mont t h u s o b t a i n e d was 37 m2 / g .
mixture
A
&
mild
structure
conditions
General p r o c e d u r a
ion
clav-catalyzed liquid-phase organic synthesis
o f a c a r b o n y l compound ( 1 mmol) and a n u c l e o p h i l i c
reagent
(1.1
mmol) was added t o a suspended m i x t u r e o f CH2C12 s o l v e n t (2-5 m l ) and a powdered Fe-Mont
catalyst
conditions
noted
(0.2-0.5
60 mesh pass) a t O°C,
g,
i n Tables.
s t i r ed
and
A f t e r t h e complete consumption
under
the
of
the
carbonyl
The f i l t r a t e
compound, t h e c l a y c a t a l y s t was f i l t e r e d o f f t h r o u g h a C e l i t e pad was d i s t i l l e d t o r e c o v e r t h e p r o d u c t s f o r a n a l y s i s . RESULTS AND DISCUSSION A d d i t i o n r e a c t i o n s o f s i l y l ketene a c e t a l s This
to
a ,B -acetvlenic ester$
t y p e o f a d d i t i o n r e a c t i o n shown i n Eq. 2 and 3 [ 8 , 9 ] i s expected t o
a c c e l e r a t e d e i t h e r t h r o u g h a c t i v a t i o n o f t h e c a r b o n y l group o f ester
(ynoates)
by a c i d , o r t h r o u g h enhancement o f
a ,
be
4 -acetylenic
nucleophilicity
of
ester
e n o l a t e w i t h a s t r o n g base, f o r example, by use o f a l i t h i u m e n o l a t e . Table
summarizes t h e r e s u l t s o f t h e r e a c t i o n s o f ynoates
1
(2a-c)
with
a
s i l i c o n e n o l a t e ( s i l y l ketene a c e t a l ) and a l i t h i u m e n o l a t e o f methyl p r o p i o n a t e (Eq.
1).
Except of
yields.
However,
generally
f o r the r e a c t i o n o f 2c,
Fe-Mont
catalyzed
exclusive
1,2-
3
high
s i l y l ketene a c e t a l t o 2a and 2b t o g i v e an adduct o f
addition
applied
even t r i m e t h y l s i l y l trifluoromethanesulfonate homogeneous
strong
acid, f a i l e d
to
effect
in
(TMSOTf), the
a
addition
reaction. I n t h e cases o f u n s u b s t i t u t e d ynoates 2c, Fe-Mont m a i n l y induced 1 , 4 - a d d i t i o n to
produce ( E ) - v i n y l s i l a n e 5c
[lo];
t h i s may have been caused by t h e
formation
Iron exchanged Montmorillonite as Catalyst for Organic Synthrsi< 373
"24[,,41 3
R
A+
Me0
1
2
4
"p\
R'
r'7
R
SiMeg
OSiMeg
R
5' 2 a : R=Ph, R'=Me;
Table 1.
2b: R=Me, R ' = E t ;
5 2 c : R=H, R'=Me
A d d i t i o n o f e s t e r e n o l a t e s 1 t o 2 on c l a y c a t a l y s t .
M i n enolate
Run
C02R'
Me02C
Acceptor
1 2 3
SiMe3 SiMe3 Li
2a 2a 2a
Fe-Mont TMSOT f
4
SiMe3 SiMeg Li
2b 2b 2b
SiMe3 SiMe3 SiMeg Li
2c 2c 2c 2c
5 6 7
8 9 10
Condi t i ons
Catalyst
CH2C12/-78OC/1.5 CH C1 / R T / l d HM6A-fHF/-7E0C/2
Products (% y i e l d ) h h
3a (86) N R ~ 4a ( 5 8 )
Fe-Mont TMSOT f
CH2C12/-78OC/1 h CH C1 / R T / l d h HM6A-?HF/-78'C/l
3b (89) N R ~ CMb
Fe-Mont Fe-Mont TMSOT f
CH C1 /-?E0C/5 h PhEH 70°C/5 h CH C? /RT/1 d HM6A-?HF/-78°C/1 h
3c ( l o ) , 5c ( 6 1 ) 3c ( 3 1 , 5 c ( 7 7 ) N R ~ CMb
-
-
A complex m i x t u r e o f p r o d u c t s was o b t a i n e d
a No r e a c t i o n o c c u r r e d .
o f a t r a n s i e n t intermediate 5 ' through s i l i c o n t r a n s f e r from the enolate
oxygen
to
to
toluene
rate
slowed
the
a c a r b o n atom.
improved down mixed
the
Changing t h e s o l v e n t f r o m d i c h l o r o m e t h a n e
s e l e c t i v i t y f o r 1,4-addi t i o n , a l t h o u g h t h e r e a c t i o n
8). The l i t h i u m e n o l a t e o f 1 was r e a c t i v e t o w a r d 2 i n
(run
a
HEMPA-THF
b u t a complex m i x t u r e o f p r o d u c t s was o b t a i n e d e x c e p t
solvent,
for
the
r e a c t i o n w i t h 2a. T a b l e 2 shows t h e r e s u l t s o f t h e a d d i t i o n o f s i l i c o n and l i t h i u m e n o l a t e s methyl
2b and 2c ( E q . 3 ) .
to
acetate
butyldimethylsilyl trimethylsilyl moreover, inactive
it to
ketene
ketene caused 2c.
acetal
acetal
o f 1,
of
Under t h e
6
is
far
requiring
exclusive 1,2-addition
Fe-Mont less
higher
t o 2b i n a
catalysis, reactive
reaction good
S a t i s f a c t o r y y i e l d s o f t h e expected p r o d u c t s
the
than
of tthe
temperature;
yield, could
but not
was be
Y. Izurni and M. Onaka
374
o b t a i n e d f r o m t h e TMSOTf-catalyzed r e a c t i o n and t h e l i t h i u m - e n o l a t e a d d i t i o n . p r o m o t i v e e f f e c t o f Fe-Mont on t h i s t y p e o f a d d i t i o n r e a c t i o n s
The
is
thus
s i g n i f i c a n t , compared w i t h t h e c o n v e n t i o n a l homogeneous s y n t h e t i c methods.
R
K
n1i
t
Me0
u
-
n
OR'
tBuMe2Si0
c, (3)
I
C02 R '
6
8
7
2
2 a : R=Ph, R'=Me;
2b: R=Me, R ' = E t ;
2 c : R=H, R'=Me
A d d i t i o n o f e s t e r e n o l a t e s 6 t o 2 on c l a y c a t a l y s t
T a b l e 2.
Condi t i ons
Run
M i n enolate
Acceptor
1 2 3
S i BuMe2 S i tBuMe2 Li
2b 2b 2b
Fe-Mont TMSOTf
CH2C12/RT/1.5 h CH C1 /RT/l d HdA-?HF/-78OC/2
4
S i tBuMe2 S i tBuMe2 Li
2c 2c 2c
Fa-Mont TMSOTf
CH2C12/RT/1 d CH C1 /RT/2 d HMBA-?HF/-78OC/2
5 6
Catalyst
-
-
Products (96 y i e l d )
h
7b ( 7 8 ) 8b ( 1 9 ) CMa
h
7c ( 4 ) 7c ( 3 ) CMa
a A complex m i x t u r e o f p r o d u c t s was o b t a i n e d .
Cvanosilvlation
c a r b o n v l compounds
Cyanotrimethylsilane
with
cvanotrimethvlsilane
i s v e r y s o l u b l e i n o r g a n i c s o l v e n t s and c a n
i t i s widely applied t o organic synthesis i n
handled,
so
hydrogen
c y a n i d e and i n s o l u b l e i n o r g a n i c c y a n i d e s .
reactions sources
place
be
of
safely
poisonous
For instance. i t s
addition
or
t o a l d e h y d e s and k e t o n e s s e r v e t o p r o t e c t c a r b o n y l f u n c t i o n s , o f c y a n o h y d r i n s and
0 -aminoalcohols.
The c y a n o s i l y l a t i o n o f
carbonyl
compounds (Eq. 4 ) p r o c e e d t h e r m a l l y o r w i t h t h e a i d o f L e w i s a c i d s s u c h as and A1C13, o r o f s o l u b l e c y a n i d e i o n s (KCN-18-Crown-6,
'Bu4NtCN-)
as Zn12
(11-141.
Mnt-Mont o=c
Me3SiCN
t
3
Table
in
(HoS-8.2) the
(4)
0 OC
k 2
MegSiCN
c
shows t h e r e s u l t s o f t h e r e a c t i o n between c a r b o n y l t h e presence o f s o l i d a c i d s .
which i s
compounds the
most
with acidic
among t h e s o l i d c a t a l y s t s t e s t e d , r e v e a l e d t h e h i g h e s t a c t i v i t y
a d d i t i o n r e a c t i o n o f 2-octanone.
Ca2'-exchanged
Fe-Mont,
montmorillonite
Compared w i t h Fe-Mont,
(Ca-Mont,
much
less
for
acidic
t 0 . 8 < H 0 5 ; t l . 5 ) and s i l i c a ( t 1 . 5 < t b S
Iron-exchanged Montmorillonite as Catalyst for Organic Synthesis 375
t3.3)
p o o r e r y i e l d s even when r e a c t i o n t i m e was p r o l o n g e d .
gave
imp1 ie s
tha
This
t h e s t r o n g l y a c i d i c s i t e s on Fe-Mont c a n e f f e c t i v e l y
result
promote
the
c y a n o s i 1y l a t o n , t h a t i s , a c a r b o n y l g r o u p c o o r d i n a t e s t o a s t r o n g a c i d s i t e
to
b e a c t i v a t e d , f o l l o w e d by t h e a d d i t i o n o f Me3SiCN. C y a n o s i l y l a t i o n o f c a r b o n y l compounds on s o l i d a c i d c a t a l y s t s a
Table 3.
Time/h
Y i e l d/%
Fe-Mont
0.2
96
2-octanone
None Fe-Mon t Ca-Mont Si02
4.5 0.2 4.5 4.5
0 96 42 0
Benzophenone
Fe-Mont
0.7
98
Substrate
Catalyst
Benzaldehyde
a Fe-Mont: 0 . 2 g , o t h e r s o l i d s : 0 . 5 g , a t O°C i n CH2C12 2 m l p-Benzoquinone,
a d i k e t o n e , was a l s o e f f i c i e n t l y c y a n o s i l y l a t e d
c a t a l y s t t o g i v e a bisadduct
by
Fe-Mont
( 1 , 4 - b i s ( t r i r n e t h y l s i l o x y ) - l ,4-dicyanocycohexa-2,5-
d i e n e ) e x c l u s i v e l y f o r 1 . 5 h under t h e same r e a c t i o n c o n d i t i o n s as d e s c r i b e d Table
3 (yield=91%).
R e p o r t e d l y L e w i s a c i d s s u c h as Zn12 and A l C l 3 were
in
quite
i n a c t i v e f o r t h i s r e a c t i o n 1141. CONCLUSION Fe3'-exchanged montmorillonite,
montmoril l o n i t e , not
only
as
we1 1
as
Al"
and
w o r k s as an e f f i c i e n t s o l i d a c i d
Sn4'-exchanged catalyst
in
a d d i t i o n r e a c t i o n s o f c a r b o n y l compounds u s i n g s i l y l a t e d n u c l e o p h i l e s , b u t enables
an e a s y work-up p r o c e d u r e w h i c h m e r e l y r e q u i r e s f i l t r a t i o n t o
the also
separate
the products from the c a t a l y s t . Such
u n i q u e a c i d c a t a l y s i s o f Fe-Mont,
w h i c h i s sometimes s u p e r i o r
to
the
a c i d c a t a l y s i s o f s u p e r a c i d as seen i n t h e p r e s e n t p a p e r , i s due poss b l y t o t h e Me3Sit
init ally
from
n u c l e o p h i l i c r e a g e n t s and t h e p r o t o n s o f Fe-Mont l a y e r s u r aces
(Eq.
c a t i o n s a s v e r y s t r o n g L e w i s a c i d s i t e s t h a t a r e formed
silylated
5 ) as r e c e n t l y s u g g e s t e d b y t h e p r e s e n t a u t h o r s [ 3 ] . Fe3'* (H20)*( S i 1 i
ate)^X
X=
R
R'.
o r CN
( Fe-OH)2t*Ht*
-
( S i 1i
ate)^(5)
( Fe-OH ) 2t* ( Me3Si ) '* ( S i 1 ica t e ) 3-
376
Y. Izumi and M.Onaka
REFERENCES
1
2 3 4 5 6 7 8 9 10 11
J . M. Thomas, " I n t e r c a l a t i o n C h e m i s t r y , " ed. by M. S . Whittingham and A . J . Jacobson, Academic Press, New York, 1982, p.55. J. A . B a l l a n t i n e , J . H. P u r n e l l , J. Mol. C a t a l . , 27 (1984) 157. M. Kawai, M. Onaka, Y . I z u m i , B u l l . Chem. SOC. J p n . . 61 (1988) 1237. M. Kawai, M. Onaka, Y . I r u m i , B u l l . Chem. SOC. J p n . , 61 (1988) 2157. M. Kawai, M. Onaka, Y . I z u m i , Chem. L e t t . , (1986) 381. M. Onaka, T. Mimure, R . Ohno, Y . I r u m i , Tetrahedron L e t t . , 30 (1989) 6341. M . Onaka, K . H i g u c h i , K . S u g i t a , Y . I z u m i , Chem. L e t t . , (1989) 1393. R . D . C l a r k , K . G. Untch, J . Org. Chem., 44 (1979) 248. G. H . Posner, Org. React., 19 (1972) 1 . 5c c o n s i s t e d o f a s i n g l e p r o d u c t as examined by c a p i l l a r y GC a n a l y s i s , and ( E ) - c o n f i g u r a t i o n was c o n f i r m e d by 'H NMR. D. A . Evans, L . K. T r u e s d a l e , G. L . C a r r o l l , J . Chem. S O C . , Chem. Commun.,
(1973) 55. 12 H. Neef, R . M u l l e r , J . P r a k t . Chem., 315 (1973) 367. 13 W. L i d y , W . Sundermeyer, Chem. B e r . , 106 (1973) 587. 14 D . A . Evans, L . K . T r u e s d a l e , T e t r a h e d r o n L e t t . , (1973) 4929. 15 A . T . N i e l s e n , W. J. H o u l i h a n , O r g . R e a c t . , 16 (1968) 1 . 16 2 . G. Hajos, "Carbon-Carbon Bond F o r m a t i o n , " ed. by R . L . A u g u s t i n e , Dekker, New Y o r k , 1 ,1979, p . 1 . 17 T . Mukaiyama, Ore. React., 28 (1982) 203.
Influence of Pore Structure on the Catalytic Behavior of Clay Compounds
E. Kikuchi and T. Matsuda
Department of Applied Chemistry. School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169. Japan
ABSTRACT
The effect of pore structure on the catalytic activity of pillared clays was investigated using alumina-pillared montmorillonite (Al-mont) with a microporous structure and alumina-pillared saponite (Al-sapo) having mesopores. The disproportionation and isomerization of trimethylbenzene, and the cracking of cumene were adopted as model reactions. The catalytic activity of pillared clays was affected by pore structure as well as by acidity: Al-mont having less acidity was more active for the disproportionation reaction than Al-sapo. although the effect of pore structure on the cracking and isomerization was small compared with the disproportionation. The microporosity is considered to enhance the concentration of reactant molecules at the acid sites, resulting in a high catalytic activity. It is suggested that the significant influence of pore structure on disproportionation is attributed to the second order kinetics of disproportionation which is a bimolecular reaction.
INTRODUCTION The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented. Weisz and his associates [ l ] reported that faujasite Y zeolite showed 103 t o 104 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity o f zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. Pillared clay is a new family o f molecular sieve materials obtained by
378 E. Kikuchi and T. Matsuda
exchanging charge compensating c a t i o n s between t h e s i l i c a t e l a y e r s o f c l a y w i t h large inorganic
polyoxycations
such
i n o r g a n i c o x i d e c l u s t e r s a r e formed:
as
[ A l l 304(OH)24(H20)1~]7t.
On h e a t i n g ,
t h e y p r o p open t h e c l a y l a y e r permanently
t o generate a microporous structure.
I t has r e c e n t l y been p o i n t e d o u t [ 3 , 4 ]
t h a t t h e a g g r e g a t i o n manner o f c l a y
l a y e r s d i f f e r e n t i a t e s t h e pore s t r u c t u r e o f p i l l a r e d c l a y s and t h a t t h i s h i g h l y dependent on t h e n a t u r e o f t h e c l a y .
Edge-to-face
o r edge-to-edge
is
layer
a g g r e g a t i o n g i v i n g mesopores competes f a v o r a b l y w i t h face t o f a c e a g g r e g a t i o n when t h e l a y e r s i z e i s s m a l l o r t h e l a y e r morphology i s l a t h - l i k e . w e l l ordered face t o face aggregation, zeolite-like
I n contrast,
which leads t o t h e formation o f a
r e g u l a r microporous structure,
o c c u r s i n t h e case o f c l a y s w i t h
l a r g e l a y e r s i z e o r p a n c a k e - l i k e morphology.
Small p a r t i c l e s a p o n i t e has a h i g h
tendency t o g i v e edge-to-edge
w i t h l a r g e p a r t i c l e s tends
o r edge-to-face to
aggregate
aggregation w h i l e montrnorillonite
i n the
face-to-face
manner.
These
a g g r e g a t i o n manners have been demonstrated b y e l e c t r o n microscopy and a d s o r p t i o n measurements [5,6].
I n t h i s study, we i n v e s t i g a t e t h e m i c r o p o r o u s e f f e c t u s i n g
p i l l a r e d m o n t m o r i l l o n i t e and s a p o n i t e .
EXPERIMENTAL Ca t a 1y s t The c l a y s used i n t h i s s t u d y were sodium-type
n a t u r a l m o n t m o r i l l o n i t e and
s y n t h e t i c s a p o n i t e o b t a i n e d f r o m Kunirnine I n d u s t r y Co. capacities
were
1.2
and
[Al,304(OH)24(H20)12]7t, addition o f
0.8 rneq./g,
respectively.
was p r e p a r e d f r o m
intercalating
since the addition of The
NaOH s o l u t i o n
was
previously
The h y d r o l y z e d
u n t i l t h e p r e c i p i t a t e vanished, t o AlC13 s o l u t i o n
i n t e r c a l a t i o n method o f
[Al,30,(OH)24(H20)12]7+
agent,
h y d r o l y z e d AlC13 s o l u t i o n by
NaOH s o l u t i o n t o make t h e OH/A1 m o l a r r a t i o 2.5.
AlC13 s o l u t i o n was aged f o r about 12 h a t 50°C precipitate.
An
T h e i r c a t i o n exchange
sodium-type
described
in
yielded
clay with
detail
[7].
a
The
i n t e r c a l a t e d p r o d u c t was c a l c i n e d a t a g i v e n t e m p e r a t u r e i n t h e range 400-600°C f o r 4 h.
P i l l a r e d m o n t m o r i l l o n i t e and s a p o n i t e t h u s o b t a i n e d a r e a b b r e v i a t e d as
Al-mont and Al-sapo,
respectively.
Apparatus and procedures The
disproportionation
and
isornerization
of
trimethylbenzene(TrM6)
s t u d i e d a t 2OO0C u s i n g a c o n t i n u o u s f i x e d bed r e a c t o r . d i l u t e d w i t h n i t r o g e n i n a m o l a r r a t i o of c a r r i e d o u t a t 4OO0C u s i n g a p u l s e r e a c t o r .
1:9.
were
The r e a c t a n t TrMB was
The c r a c k i n g o f curnene was
The c a t a l y s t was t r e a t e d i n a
stream o f n i t r o g e n f o r 1 h a t a d e s i r e d t e m p e r a t u r e i n t h e range 400-600°C
prior
Pore Structure and Catalysis of Clay Compounds 379
t o reaction. Temperature programmed d e s o r p t i o n measurements The number o f a c i d s i t e s o n p i l l a r e d c l a y s was d e t e r m i n e d b y means o f temperature programmed d e s o r p t i o n (TPD) o f ammonia.
I n each TPD experiment,
a
sample weighing about 0.5 g was t r e a t e d in vacuo f o r 1 h a t a g i v e n temperature i n t h e range 400
-
60OoC.
Amnonia was adsorbed a t a d e s i r e d temperature (100-
30OoC) f o r 30 min and evacuated f o r 30 min. a r a t e o f 10°C/min detector.
T h i s sample was heated t o 7OO0C a t
and desorbed ammonia was m o n i t o r e d by thermal c o n d u c t i v i t y
As water was desorbed s i m u l t a n e o u s l y w i t h ammonia,
spectrum was o b t a i n e d by p o i n t - b y - p o i n t
t h e ammonia TPD
s u b t r a c t i o n o f t h e water d e s o r p t i o n
spectrum o b t a i n e d w i t h t h e sample which had n o t adsorbed ammonia.
RESULTS AND DISCUSSION F i g u r e 1 shows t h e ammonia TPD s p e c t r a o b t a i n e d w i t h Al-mont c a l c i n e d a t 40OoC.
Al-sap0
was more a c i d i c
t h a n Al-mont.
and Al-sap0
It i s generally
assumed t h a t t h e a c i d s i t e s on p i l l a r e d c l a y s a r e a t t r i b u t a b l e e i t h e r t o t h e s i l i c a t e layer o f clays or t o the p i l l a r s .
I t was shown p r e v i o u s l y [8,9] t h a t
t h e a c i d i t y i n c r e a s e d w i t h i n c r e a s i n g number o f p i l l a r s .
The number o f p i l l a r s ,
however, cannot s e r v e t o e l u c i d a t e t h e d i f f e r e n c e i n a c i d i t y between Al-mont and Al-sap0 because more a c i d i c Al-sap0 has s m a l l e r number o f p i l l a r s than Al-mont. being
2.20
and
3.3 mnol/g, r e s p e c t i v e l y .
Many
investigators
have proposed
m
8
H L
v 0 l
W
u 4-
0
c
0 F
&I
I 9 U
c W c u
0
0
100 Desorptlon temperature/°C Fig. 1. Ammonia TPD s p e c t r a o f Al-mont(--.-) Fig. 2. Ammonia TPD s p e c t r a o f Al-mont. 2OOOC (---), 25OoC (---- ), and 3OO0C(---).
200 300 400 k s o r p t Ion temperature/%
and Al-sapo(-)
500
c a l c i n e d a t 40OoC.
Ammonia was adsorbed a t 100°C(-),
:M(I E. Kikuchi and T. hlatsuda
-
10.
,V
I
V v) W
% 12.5
c
c
7,
‘m
I
-8
01
7
P
\
U
c
a
U v)
c
c
3
5.
c
0
V
5.0
U W
U 01
4
m
rY
2
7,5
v)
0
g
10.0
\
U
rY
*
**
2,5
z
0 0.2 0,4 0.6 Concentration o f a c i d s i es adsorbing ammonia a t 2000C/nmol g-
F
Coricerilratioii of a c i d sites/imno1 (J-1
F i g . 3. R e l a t i o n between t h e c r a c k i n g a c t i v i t y o f Al-mont and t h e c o n c e n t r a t i o n o f a c i d s i t e s . Ammonia was adsorbed a t 100°C( 0 ) .2OO0C( A ) , and 25OoC( 0 ). Fig. 4. The c r a c k i n g a c t i v i t y o f Al-mont( 0 ) and Al-sapo( A t h e c o n c e n t r a t i o n o f a c i d s i t e s a d s o r b i n g ammonia a t 20OoC.
)
as a f u n c t i o n o f
[ l o - 1 2 1 t h a t a c i d i t y i s g e n e r a t e d by t h e decomposition o f p i l l a r s as f o l l o w s .
+
[A11304(OH)24(H20)12]7t
I f t h i s i s t h e case, capacity (CEC).
+
7 Ht
+
20.5 H20
p i l l a r e d c l a y would have c o r r e s p o n d i n g c a t i o n exchange
The C E C v a l u e s o f t h e s e c l a y s d e c r e a s e d t o z e r o on
i n t e r c a l a t i o n w i t h [Al,304(OH)24(H,0)12]7t a l t h o u g h a f t e r c a l c i n a t i o n a t 40OoC. 0.13 meq.g-l
6.5 A1203
o f CEC.
respectively.
cations. Al-sapo
C a l c i n a t i o n i n c r e a s e d CEC,
and A l - m o n t
showed 0.35
and
Thus, t h e d i f f e r e n c e i n t h e a c i d i t y observed
between Al-mont and Al-sapo seems t o be a t t r i b u t a b l e p r e d o m i n a n t l y t o t h e number o f c a t i o n exchangeable s i t e s g e n e r a t e d on decomposition o f p i l l a r s . As expected from t h e TPD r e s u l t s , Al-sapo was more a c t i v e f o r t h e c r a c k i n g o f cumene on a p e r w e i g h t o f c a t a l y s t b a s i s t h a n Al-mont. t h e c a t a l y t i c a c t i v i t y on a b a s i s o f a c t i v e s i t e s , a c t i v e s i t e s on these c a t a l y s t s . temperature F i g . 2.
o f ammonia a d s o r p t i o n .
By i n t e g r a t i n g
I n o r d e r t o compare
we e v a l u a t e d t h e number o f
TPD s p e c t r a were measured w i t h v a r y i n g t h e Typical
these spectra,
r e s u l t s on Al-mont the concentration o f
a r e shown
in
acid sites
c o r r e s p o n d i n g t o d i f f e r e n t s t r e n g t h o f a c i d i t y can be determined. The t y p i c a l c r a c k i n g a c t i v i t y o f Al-mont. constant,is
expressed by t h e f i r s t o r d e r r a t e
shown i n Fig. 3 as a f u n c t i o n o f t h e c o n c e n t r a t i o n of a c i d s i t e s
t h u s determined.
Here, t h e c o n c e n t r a t i o n o f a c i d s i t e s was changed by c a l c i n i n g
Pore Structure and Catalysis of Clay Compounds 381
t h e p i l l a r e d c l a y a t 400,
500,
and 600OC.
C a l c i n a t i o n a t h i g h e r temperature
It i s obvious f r o m these r e s u l t s t h a t t h e
c o n s i d e r a b l y decreased t h e a c i d i t y .
a c i d s i t e s a d s o r b i n g ammonia a t 2OO0C a r e r e s p o n s i b l e f o r t h e c r a c k i n g o f cumene. F i g u r e 4 compares t h e c r a c k i n g a c t i v i t i e s o f Al-mont function o f the concentration o f acid sites. concentration o f a c t i v e sites, t h a n Al-sapo.
Al-mont
and A l - g a p 0
as a
When compared a t a d e f i n i t e
exhibited o n l y s l i g h t l y higher a c t i v i t y
The a c t i v i t y o f an a c i d c a t a l y s t i s a f f e c t e d by t h e n a t u r e o f t h e
a c i d i t y as w e l l as b y t h e number o f a c i d s i t e s .
It has been shown [7,13]
Al-sapo i s f a r more Bronsted a c i d i c t h a n Al-mont,
due t o t h e presence o f Si-0-A1
linkages i n t h e tetrahedral layer.
that
Thus, a c i d i t y cannot e l u c i d a t e t h e a c t i v i t y
d i f f e r e n c e between A l - m n t and Al-sapo.
As proposed by Wang and L u n s f o r d [ 2 ] t o
e x p l a i n t h e s u p e r i o r i t y o f HY over s i l i c a - a l u m l n a ,
t h e a c t i v i t y o f an a c i d s i t e
seems t o be a f f e c t e d by t h e c o n c e n t r a t i o n o f r e a c t a n t around t h e s i t e .
I n that
case, t h e a c t i v i t y should depend on t h e pore s t r u c t u r e o f t h e c a t a l y s t s i n c e t h e c o n c e n t r a t i o n o f r e a c t a n t molecules i s considered t o be structure o f a solid.
s e n s i t i v e t o t h e pore
We deduce t h a t t h e h i g h c r a c k i n g a c t i v i t y o f Al-mont
is
due t o i t s r e g u l a r microporous s t r u c t u r e p e r m i t t i n g a h i g h c o n c e n t r a t i o n o f cumene a t t h e a c i d s i t e s . The same e f f e c t should be expected i n o t h e r c a t a l y t i c r e a c t i o n s . confirm
this,
disproportionation
0
0.1
and
O,2
isomerization
0,3
I n order t o
reactions
0,4
Concentration f acid s i es adsorbing anmonia a t 2508C/nmol g-
t
o)
and Al-sapo( A ) as Fig. 5 . The d 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 Al-mont( a f u n c t on o f t h e c o n c e n t r a t i o n o f a c i d s i t e s a d s o r b i n g ammonia a t 25OOC.
were
382 E. Kikuchi and T.Matsuda
I smer iza t ion
c I
u
0)
m c I
ml
5
4
\
c, K
m m c
c,
0
u
2
W c,
m
E
r-
5? 0
0,2 0,4 0,6 Concentration o f a c i d s i es adsorbing a n o n i a a t 200O~/mn01 g-
0
0-1
0,2
0,3 0,4 Concentratlon o f a c i d s i f e s adsorbing
f
a m n i a a t Z S O O C / ~ I g-
Fig. 6. The 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 a c t i v i t i e s o f Al-mont( Al-sapo( A ) as a f u n c t i o n o f t h e c o n c e n t r a t i o n o f a c i d s i t e s . investigated.
F i g u r e 5 shows t h e r e l a t i o n s h i p between t h e a 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 1.2.4-trimethylbenzene o f acid sites, 25OoC.
o ) and
(1,2,4-TrMB)
and t h e c o n c e n t r a t i o n
which was determined from t h e amounts o f ammonia adsorbed a t
F o r t h i s r e a c t i o n , a b e t t e r l i n e a r r e l a t i o n was o b t a i n e d w i t h t h e
r e s u l t s o f TPD o f ammonia adsorbed a t 25OoC.
Al-mont was f a r more a c t i v e f o r
t h i s r e a c t i o n t h a n Al-sapo. I t i s n o t e d t h a t t h e microporous e f f e c t was g r e a t e r i n t h e d i s p r o p o r t i o n a t i o n
o f 1.2.4-TrMB paper [14],
t h a n i n t h e c r a c k i n g o f cumene. t h e d i s p r o p o r t i o n a t i o n o f 1.2.4-TrMB
As shown i n t h e p r e v i o u s a t 2OO0C p r o c e e d s v i a a
b i m o l e c u l a r t r a n s i t i o n s t a t e and obeys t h e second o r d e r k i n e t i c s ,
I n contrast,
t h e c r a c k i n g o f cumene i s t h e f i r s t o r d e r k i n e t i c s w i t h r e s p e c t t o cumene concentration. significantly
Thus,
i t seems t h a t
t h e microporous e f f e c t
i s e x e r t e d more
i n t h e second o r d e r r e a c t i o n ( d i s p r o p o r t i o n a t i o n )
f i r s t order reaction (cracking)
than
i n the
i f pore s t r u c t u r e p l a y s an i m p o r t a n t r o l e i n
l o c a l i z i n g c o n c e n t r a t i o n o f r e a c t a n t molecules. F u r t h e r p r o o f was o b t a i n e d b y c o m p a r i n g t h e c a t a l y t i c a c t i v i t y f o r 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 z a t i o n o f t r i m e t h y l benzene. i s expected t o obey f i r s t o r d e r k i n e t i c s ,
As t h e i s o m e r i z a t i o n
t h e microporous e f f e c t would appear
more i n d i s p r o p o r t i o n a t i o n t h a n i n i s o m e r i z a t i o n . trimethylbenzene (1.2.3-TrMB) isomerization
Here,
1.2.3-
was used as r e a c t a n t i n s t e a d o f 1,2,4-TrMB, s i n c e
conversion o f 1,2,4-TrMB
was t o o small t o d i s c u s s
t h e change i n
Pore Structure and Catalysis of Clay Compounds 383
Table. 1. Rate c o n s t a n t s f o r 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 1.2.3-TrMB a t 2OO0C. Catalyst
Al-mont
Al-sapo
kdis/mOl g - l
St3C-l
5.0
10-7
2.1
10-7
kiso/mol
sec-’
3.7
10-7
3.5
10-7
g-l
0.60
1.35
kdis/kiso
F i g u r e 6 shows t h e r e l a t i o n s h i p b e t w e e n t h e a c t i v i t y f o r
the a 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 and i s o m e r i z a t i o n o f 1.2.3-TrMB sites.
and t h e c o n c e n t r a t i o n o f a c i d
The a c i d s i t e s adsorbing ammonia a t 2OO0C were r e s p o n s i b l e f o r t h e
i s a n e r i z a t i o n o f 1,2,3-TrMB.
When compared a t a g i v e n c o n c e n t r a t i o n o f a c i d
s i t e s , Al-mont
e x h i b i t e d about 5 t i m e s h i g h e r a 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 a n Al-sapo.
I n contrast,
times h i g h e r t h a n Al-sapo.
t h e i s o m e r i z a t i o n a c t i v i t y o f Al-mont was about 2 T h i s d i f f e r e n c e i s a t t r i b u t e d t o t h e second o r d e r
k i n e t i c s o f d i s p r o p o r t i o n a t i o n , which i s a b i m o l e c u l a r r e a c t i o n .
Table 1 shows
t h e r a t e c o n s t a n t a t 2OO0C a n d t h e r a t i o f o r d i s p r o p o r t i o n a t i o n t o isomerization,
namely,
isomerization.
the
selectivity
Microporous Al-mont
for
disproportionation
against
exhibited high selectivity f o r
d i s p r o p o r t i o n a t i o n compared w i t h Al-sapo.
coNcLusIoN Al-mont h a v i n g z e o l i t e - l i k e r e g u l a r micropores i s a c t i v e f o r t h e c r a c k i n g o f cumene, and 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 z a t i o n o f TrMB compared f a v o r a b l y w i t h Al-sapo w i t h mesopores, a l t h o u g h Al-mont
i s l e s s a c i d i c t h a n Al-sapo.
The
microporous s t r u c t u r e i s considered t o enhance t h e c o n c e n t r a t i o n o f r e a c t a n t molecules a t t h e a c i d s i t e s ,
resulting i n
high catalytic activity.
The
microporous e f f e c t i s e x e r t e d more s i g n i f i c a n t l y i n d i s p r o p o r t i o n a t i o n than i n cracking
and
isomerization.
Thus,
the
catalytic
selectivity
for
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 i s a l s o a f f e c t e d by t h e m i c r o p o r o s i t y .
Al-
mont e x h i b i t s h i g h e r 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 a g a i n s t i s o m e r i z a t i o n t h a n Al-sapo.
These p r o p e r t i e s a r e a t t r i b u t a b l e t o t h e second o r d e r k i n e t i c s o f
d i s p r o p o r t i o n a t i o n , which i s a b i m o l e c u l a r r e a c t i o n .
381
E. Kikuchi and T.Matsuda
REFERENCES 1 J.M. Miale, N.Y.Chen and P.B. Weisz, J. Catal., 6(1966)278. 2 K.M. Wang and J.H. Lunsford, J. Catal., 24(1972)262. 3 T.J. Pinnavaia, M. TZOU, S.D. Landau and R.H. Raythatha, J. Mol. Catal., 27( 1984)195. 4 M.L. O c c e l l i , S.D. Landau and T.J. Pinnavaia, J. Catal., 90(1984)256. 5 M.L. O c c e l l i , S.D. Landau and T.J. Pinnavaia, J. Catal.. 104(1987)331. Sanders, J. Catal., 107(1987)557. 6 M.L. O c c e l l i , J. Lynch and J.V. 7 T. Matsuda, H. Nagashima and E. Kikuchi, Appl. Catal., 45(1988)171. 8 T. Mori and K. Suzuki. Chem. Lett., (1989)2165. 9 J. Shabtai, F.E. Massoth, M. Takarz. G.M. T s a i and J. McCauley, Proc. I n t . Congr. Catal., B e r l i n , vo1.4, 1984, p.735. 10 D. Plee, A. Shutz, G. Poncelet and J.J. F r i p i a t , C a t a l y s i s by Acid and Bases (Stud. Surf. Sci. Catal., 20), E l s e v i e r , Amsterdam, 1985, p.343. 11 G. Poncelet and A. Shutz, Chemical Reaction i n Organic and I n o r g a n i c Constrained System (NATO AS1 S e r i e s Ser.C, 165). 1985, p.165. 12 D. T i c h i t . F. F a j u l a , F. Figueras, J. Bousquet and C. Gueguen, C a t a l y s i s by A c i d and Bases (Stud. Surf. Sci. Catal., 20). E l s e v i e r . Amsterdam, 1985, p. 351. 13 K. Urabe, H. Sakurai and Y. Izumi. Shokubai ( C a t a l y s t ) , 28(1987)422 ( i n Japanese). 14 E. Kikuchi, T. Matsuda, H. F u j i k i and Y. M o r i t a , Appl. Catal., 11(1984)331.
Studies in Surface Science and Catalysis Advisory Editors : 8. Delmon, Universite Catholique de Louvain, Louvain -la-Neuve, Belgium J. T. Yates, University of Pittsburgh, Pittsburgh, PA, U. S.A.
Volume
1
Preparatlon of Catalysts I . Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium held at the Solvay Research Centre, Brussels, October 1 4 - 17, 1975 edited by B. Delmon, P. A. Jacobs and 0. Poncelet
Volume
2
The Control of tho Reactlvlty of SolMs. 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 8. Delmon
Volume
3
Preparatlon of Catalysts II. Scientific Bases for the Preparation of HeterOgen80US 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 Ropertles of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourbon
Volume
5
Catalysis by Zeolites. Proceedings of an International Symposium organized by the lnstitut de Recherche sur la Catalyse -CNRS-Villeurbanne and sponsored by the Centre National de Recherche Scientifique, Ecully(Lyon), September 9 - 1 1, 1980 edited by B. Irnellk. C. Naccache. Y. Ben Taarlt. J. C. Vadrlne, G. Coudurler and H. Rallaud
Volume
6
Catalyst Deactlvatlon. Proceedings of the 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 edited by T. Selyama and K. Tansbe
Volume
8
Catalysis by Supported Complexes by Yu. I . Yermekov, B. N. Kuznetsov and V. A. Zakharov
Volume
9
Physics of Solid Surfaces. Proceedings of the Symposium held in Bechyne, Czechoslovakia, September 29-October 3, 1980 edited by M. LiznI6ka
Volume
10
Adsorption at tho Gas-SolM and Llquld-SoHd Intorfaso. Proceedings of an International Symposium held in Aix -en - Provence, September 2 1 - 23, 19 8 1 edited by J. Rouquerol 8nd K. S. W. Sing
Volume
11
Metal -Support and Metal-Addltlve Effects In Catalysis. Proceedings of an lnternational Symposium organized by the lnstitut de Recherches sur la Catalyse CNRS-Villeurbanne. and sponsored by the Centre National de la Recherche Scientifique, Ecully (Lyon), September 14 - 16, 1982 edited by B. Imeiik, C. Nsccache, G. Cwrdurler. H. Prdlaud, P. Merlaudeau, P. Gallezot, 0. A. Martln and J. C. Veddne
Volume
12
Metal Microstructures In Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Eremen, September 22-24, 1982 edited by P. A. Jacobs, N. 1. Jsegar, P. Jlru and 0. Schulz-Ekloff
Volume
13
Adsorptlon on Metal Surfaces. An Integrated Approach edited by J. &nard
Volume
14
Vibration at Surfaces. Proceedings of the Third International Conference Asilomar, California, U. S. A., September 1-4, 1982 edited by C. R. Brundk and H. Morawltz
Volume
15
Hetorogeneous Catalytlc Reactlons lnvohrlng Molecular Oxygen by G. 1. Golodets
Volume
16
Preparation of Catalysts Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain - la - Neuve, September 6 - 9, 1962 edited by 0. Poncelet, P. Grange and P. A. Jacobs
Volume
17
Spillover of Adsorbed Species. Proceedings of the International Symposium, Lyon -Villeurbanne, September 12 - 16, 1983 edited by G. M. Pajonk, S. J. Telchner and J.E. Qermaln.
Volume
18
Structure and Raacdvlty of Modifled Z . d t t u . Proceedings of an International Conference, Prague, July 9- 13, 1984 edited by P. A. Jacobs, N. 1. Jaoger, P.JIA, V. 6. Kazansky and 0. Schulz-Ekloff
Volume
19
Catalyris on tho Energy Scorn. Proceedings of the 9th Canadian Symposium, Ouebec, P. Q., September 30-October 3, 1984 edited by S. Kdlagulne and A. Mahay
Volume
20
Catalysls by Adds and B.000. Proceedings of an International Symposium organized by the lnstitut de Recherches sur le Catalyse-CNRS- Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Villeurbanne (Lyon) , September 25-27, 1984 edited by B. Imeilk, C. Naccache, 0. Coudurler, Y. Ben Taarlt and J. C. Veddne
Volume
21
Admrptlon and Catalysls on Oxlde Surfaces. Proceedings of a Symposium, 6runel University, Uxbridge, June 26- 29, 1984 edited by M. Che and 0. C. Bond
Volume
22
Unrtwdy Procosms In Catalytic Roactorr by Yu. Sh. Matro8
Volume
23
Physic. of solid surfaces 1984 edited by J. Koukd
Volume
24
Zoolhu, Synthulr, Structuro. Tachnology and Appkatlon. Proceedings of the International Symposium, Portoroz - Portorose. September 3 8, 1984 edited by B. Drzaj, S. Hocovar and S. Pejovnlk
m.
-
Volume
25
Catalytic Polymerizatbn of Oleflnr. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, Japan, July 4-6, 1985 edited by T. KeII and K. Soga
Lolume
26
Vlbratlonr at sUrfeCO8 1986. Proceedings of the Fourth International Conference, Bowness-on-Windermere, U. K., September 1 5 - 19, 1985 edited by D. A. Klng,.N. V. Rlchardron and S. Holioway
Volume
27
Catelytlc Hydrogenation by L. Cerveny
Volume
28
New Development8In Zeolite Sclence and Technology. Proceedines of the 7th International Zeolite Conference, Tokyo, August 17 -22, 1986 edited by Y. Murakami, A. Iljima and J. W. Ward
Volume
29
Metal ClUrter8 in COtOlyri8 edited by 8. C. Gatrs, L. Guczl and H. Knozinwr
Volume
30
Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium (CAPOC 11, Brussels, September 8 - 1 1, 1986 edited by A. Cnicq and A. Frennet
Volume
31
Preparation of Catalyrtr N . Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, louvain - la - Neuve, September 1 - 4, 1986 edited by B. h i m o n , P. Grange, P. Jacobs and G. Poncelet
Volume
32
Thin Metal Films and Gar Chemlrorptlon edited by P. Wlrrmann
Volume
33
Syntherir of Hlgh - Slllca Aluminorollcate Zaollter by P. A. Jacob8 and J. A. Martanr
Volume
34
Catalyrt Doadvatlon 1987. Proceedings of the 4th International Symposium, Antwerp, September 29 -October 1, 1987 edited by B. Delmon and 0 . F. Froment
Volume
35
Keynotea In Enrrgy Related Catalyslr edited by S. KaLguine
-
Volume 38
Methane Converdon. Proceedings of a Symposium on the Productions of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D. M. Blbby, C. D. Chang, R. F. Howe and S. Yurchak
Volume 3 7
Innovation In Zsollte Materlalr Sdence. Proceedings of an International Symposium, Nieuwpoort(Belgium), September 13- 17, 1987 edited by P. J. Grobet, W. J. MwHer, E. F. Vannunt and G. Schulz-Ekloff
Volume
38
Catalyrir 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, Sen Diego, CA, May 7 - 22, 1987 edited by J. W. Ward
Volume 3 9
Characterizetion of Porour Soldr. Proceedings of the IUPAC Symposium (COPS 1). Bad Soden a. Ts., F. R. G., April 26-29, 1987 edited by K. K. Unger, J. Rouquerd, K. S. Slng and H. Kral
Volume 40
PhydC8 O f 80iM SUrfOCOr 1987 edited by J. Koukal
Volume
41
Heterogeneous Catalysis of Fine Chemlcds I edited by M. Gulsnet, J. Banauit, C. Bouchouie, D. Dupez, C. Montassier and G. Pirot
Volume
42
Laboratory Studies of Heterogeneous Catalytic Proceases by E. G. Chrlstoffel and 2 . Pail
Volume
43
Catalytic Processes under Unsteady - State Conditions by Yu. Sh. Matros
Volume 44
Successful Design of Catalysts - Future Requirements and Development edited by T. lnul
Volume
Transition Metai Oxides. Surface Chemistry and Catalysis by H. H. Kung
45
Volume 46
Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, F. R. G., September 4-8, 1988 edited by H. G. Kerge and J. Weltkamp
Volume 47
Photochemistry on Solld Surfaces edited by M. Anpo and T. Matauura
Volume 48
Structure and Reacthrlty of Surfaces. Proceedings of a European Conference, Trieste, Italy, September 1 3 - 16, 1988 edited by C. Morterra, A. Zecchlna and G. Costa
Volume
49
Zeolites : Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, The Netherlands, July 1 0 - 14, 1989 edited by P. A. Jacobs and R. A. van Santen
Volume
50
Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M. L. Occelli and R. G. Anthony
Volume
51
New Solld Acids and Bows-their catalytic properties by K. Tanabe, M. Mlsono, Y. Om, and H. Hattori
Volume
52
Recent Advsncw In Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, 17 - 19 April, 1989 edited by J. KHnowskl and P. J. Barrio
Volume
53
Catalysta In Petroleum Refining 1989. Proceedings of the Conference on Catalysis in Petroleum Refining, Kuwalt, March 5-8, 1989 edited by D. L. Trlmm, S.Akaohah. M. Absl-Halab1 and A. Bishara
Volume
54
Future Opportunitks In Catalytic and Separation Technology edited by M. Mlwno, Y. Moro-oka and S. Klmura
Volume
55
Now Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centl and F. Trlflro
Volume
56
Catalytic Olefin Polymerlzatlon. Proceedings of the International Symposium on Recent Developments in Olefin PolymerizationCatalysts, Tokyo, October 23- 25, 1989 edited by T. Kell and K. Soga
Volume 57A
Spectroscopic Characterization of Heterogeneous Catalysts. Pert A: Methods of Surface Analysis edited by J. L. G . Fierro
Volume 578
Spectroscopic Characterization of Heterogeneous Catalysts. Part B :Chemisorption of Probe Molecules edited by J L. G . Fieno
.
Volume 58
Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanipan and J.C. Jansen
Volume 59
Heterogeneous Catalysis of Fine Chemicals II edited by M . Guisnet, C. Bouchoule, D. Duprez, R .Maurel, C. Montessier and G. P6mt
Volume 6 0
Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi
This Page Intentionally Left Blank
