Zeolites and Microporous Crystals: Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, August 22-25, 1993 (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 83 ZEOLITES AND MICROPOROUS CRYSTALS Proceedings of the International Symposiu...

Author: Tadashi Hattori

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Studies in Surface Science and Catalysis 83 ZEOLITES AND MICROPOROUS CRYSTALS Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, August 22-25, 1993

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Studies in Surface Science and Catalysis Advisory Editors : 6 . Delrnon and J. T. Yates Vol. 83

ZEOLITES AND MICROPOROUS CRYSTALS PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON ZEOLITES AND MICROPOROUS CRYSTALS, NAGOYA, AUGUST 22-25, 1993 Edited by Tadashi Hattori

Nugoyo Uniuersity

Tatsuaki Yashima

Tokyo Institute of Technology

KODANSHA Tokyo

1994

ELSEVIER Amsterdam - London -New York -Tokyo

Copublished by KODANSHA LTD., Tokyo and ELSEVIER SCIENCE B.V., Amsterdam exclusive sales rights Japan KODANSHA LTD. 12-21, Otowa 2-chome, Bunkyo-ku, Tokyo 1 12, Japan for the rest of lhe world

ELSEVIER SCIENCE B.V. 25 Sara Burgerhartstraat, P.O. Box 211, 1000 AE Amsterdam, The Netherlands

ISBN 0-444-98657-X ISBN 4-06-206909-1

Copyright

(Japan)

0 1994 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) PRINTEDIN

JAPAN

Organization

Organizing Committee CHAIRMAN: Murakami, Y.

Nagoya University

GENERAL SECRETARY Izumi. Y.

Nagoya University

COMMITTEE: Hattori, T. Inui, T. Kikuchi, E. Kinoshita, A. Nakajima, H. Namba, S. Niwa, M. Ono, Y. Sagara, H. Segawa, K. Tada, K. Tatsumi, T. Tsutsumi, K. Usui, K. Utada, M. Wada, K. Yamamoto, T. Yamanaka, S. Yanagawa, T. Yashima, T. Yoshiyagawa, M.

Nagoya University (Program) Kyoto University Waseda University Catalysts & Chemicals Industries Asahi Chemical Industry The Nishi Tokyo University Toltori University Tokyo Institute of Technology JGC Corporation Sophia University Toray Industries The University of Tokyo Toyohashi University of Technology (Finance) Mizusawa Industrial Chemicals The University of Tokyo Mitsubishi Kasei Industries Idemitsu Kosan Hiroshima University Lion Corporation Tokyo Institute of Technology (Publications) Tosoh Corporation

Local Arrangements COMMITTEE: Mori, T. Onaka, M. Satsuma, A. Urabe, K.

National Industrial Research Institute of Nagoya Nagoya University (Secretary) Nagoya University (Symposium Site Arrangements) Nagoya University (Program) v

vi

Organization

International Advisory Board Haag,W.0. Iijima, A. Jacobs, P.A. Koizumi, M. Naccache, C. Notari, B. Ratnasamy, P. Takaishi, T. Tominaga, H. Weitkamp, J. XU,R.-R.

Mobil Research and Development, U.S.A. The University of Tokyo, Japan Katholieke Universiteit Leuven, Belgium Ryukoku University, Japan Institut de Recherches sur la Catalyse, France ENI-Research and Development, Italy National Chemical Laboratory, India Toyohashi University of Technology, Japan Saitama Institute of Technology, Japan University of Stuttgart, Germany Jilin University, China

Supporting Organizations & Foundations The organizers are grateful to their Generous Support. Aichi Prefecture Nagoya City Nagoya Convention & Visitors Bureau Nagoya University Foundation Tokuyama Science Foundation The Daiko Foundation Research Foundation for the Electrotechnologyof Chubu

List of Contributors Numbers in parentheses refer to the pages on which a contributor's paper begins.

Alfredsson, V. (77) National Centre for HREM, Chemical Centre, Lund University, Lund, Sweden Aly, H.M. (433) Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A. Anderson, M.W. (77) Department of Chemistry, UMIST, Manchester M60 lQD, U.K. Arai, Y. (251) Kajima Technical Research Institute, Tobitakyu, Chofu, Tokyo 182, Japan Asano, K. (417) Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 1 13, Japan Benazzi, E. (3) Institut Francais du Petrole, 1-4 Avenue Bois Preau, BP 31 1,92506 Rueil Malmaison Cedex, France Bezoukhanova, C.P. (371) Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Department Interface Chemistry, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Bhat, Y.S. (163) Research Centre, Indian Petrochemicals Corporation Ltd., Baroda 391 346, India Birke, P. (425) Geschaftsbereich Katalysatoren, Leuna-Werke AG, D-06236 Leuna, Germany Bonneviot, L. (101) Departement de Chimie, CERPIC, Universite Laval, Ste Foy, Quebec, G1K 7P4, Canada Bovin, J-0. (77) National Centre for HREM, Chemical Centre, Lund University, Lund, Sweden Bulow, M. (209) The BOC Group Technical Center, 100 Mountain Avenue, Murray Hill, N.J. 07974-2064, U.S.A. Cahill, R.A. (433) Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A.

carr, S.W. (77) Unilever Research, Port Sunlight Lab., Wirral, Merseyside L63 3JW, U.K. vii

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List of Contributors

Cartier, C. (101) Departement de Chimie, CERPIC, Universite Laval, Ste Foy, Quebec, GlK 7P4, Canada Cativiela, C. (391) Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza - C.S.I.C., 50009 Zaragoza, Espana Cejka, J. (287) Institute for Physical Chemistry and CD Laboratory for Heterogeneous Catalysis, Technical University of Vienna, A- 1060 Vienna, Getreidemarkt 9, Austria Chen, J.D. (407) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands Chen, P.Y. (48 1) Union Chemical Laboratories, Industrial Technology Research Institute, 321 Kuang Fu Road, Section 2, Hsinchu, Taiwan Cheng, S.-F. (33) Department of Chemistry, National Taiwan University, Taipei, 107 Taiwan, China Chu, S.J. (481) Union Chemical Laboratories, Industrial Technology Research Institute, 32 1 Kuang Fu Road, Section 2, Hsinchu, Taiwan Clearfield, A. (433) Department of Chemistry, Texas A&M University, College Station, Texas 77843, U S A . Corma, A (461) Instituto de Tecnologia Quimica, UPV-CSIC, Universidad Politecnica de Valencia, Camino de Vera s/n, 4607 1 Valencia, Spain Dai, P.E. (489) Texaco Inc. Research and Development, P.O.Box 1608, Port Arthur, Texas 77641 U.S.A. Dakka, J. (407) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands DeGuzman, R.N. (19) U-60, Department of Chemistry, University of Connecticut, Storrs, CT 06269, U.S.A. de Menorval, L.C. (391) Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees (URA 4 18 CNRS), E.N.S.C.M., 8 rue Ecole Normale - 34053 Montpellier Cedex 1, France Denis, I. (101) Departement de Chimie, CERPIC, Universite Laval, Ste Foy, Quebec, GlK 7P4, Canada Derouane, E.G.(1 1) Laboratoire de Catalyse, Facultes Universitaires N.-D. de la Paix, Rue de Bruxelles, 61, B-5000 Namur, Belgium Di Renzo, F. (41) Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 418 du CNRS, ENSCM, 8 rue de 1’Ecola Normale, 34053 Montpellier, France


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Durante, V.A. (321) Sun Refining and Marketing Company, Research and Development, P.0.Box 1 1 35, Marcus Hook, PA 19061-0835, U.S.A. Fajula, F. (41) Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 41 8 du CNRS, ENSCM, 8 rue de 1’Ecola Normale, 34053 Montpellier, France Figueras, E (391) Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees (URA 4 18 CNRS), E.N.S.C.M., 8 rue Ecole Normale - 34053 Montpellier Cedex 1, France Fraile, J.M. (391) Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza - C.S.I.C., 50009 Zaragoza, Espana Fricke, R. (57) Center of Heterogeneous Catalysis, Rudower Chaussee 5, D- 12484 Berlin-Adlershof, Germany Fujimoto, Y. (273) Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1 Kioi-Cho, Chiyoda-ku, Tokyo 102, Japan Fukunaga, T. (331) Central Research Laboratories of Idemitsu Kosan Co., Ltd. 1280 Kamiizumi, Sodegaura, Chiba, 299-02, Japan Fukuoka, Y. (473) Chemical Development Laboratory, The Asahi Chemical Industry Co., LTD, Shionasu, Kojima, Kurashiki-shi, Okayama 7 1 1, Japan Fung, B.M. (233) Department of Chemistry, University of Oklahoma, Norman, OK 73019-0370, U.S.A. Garcia, J.I. (391) Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza - C.S.I.C., 50009 Zaragoza, Espana Giordano, G. (41) Dipartimento di Chimica, Universita della Calabria, 87030 Rende, Italy Goslar, J. (179) Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17,60-179 Poznan. Poland Grobet, J. (371) Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Department Interface Chemistry, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Grobet, P.J. (371) Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Department Interface Chemistry, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Grohmann, I. (57) WIP, KAI e.V., Rudower Chaussee 6, D- 12484 Berlin-Adlershof, Germany

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Guczi, L. (347) Department of Surface Chemistry and Catalysis, Institute of Isotopes of the Hungarian Academy of Sciences, P.O.Box 77, Budapest, Hungary, H-1525 Gunnewegh, E.A. (379) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands Hagen, A. (313) University of Leipzig, Department of Chemistry, Institute of Technical Chemistry, Linnestr.3, D04103 Leipzig, Germany Halgeri, A.B. (163) Research Centre, Indian Petrochemicals Corporation Ltd., Baroda 391 346,India Haller, G.L. (321) Department of Chemical Engineering, Yale University, P.O.Box 2159, New Haven, CT 06520, U.S.A. Hamdan, H. (125) Department of Chemistry, Fakulti Sains, Universiti Teknologi - Malaysia, KB 791, Skudai, 80990 Johor, Malaysia Hampson, J.A. (197) Physical Chemistry Laboratories, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K. Hashimoto, K. (225) Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Honmachi Yoshida, Sakyo-ku, Kyoto 606, Japan Herrero, C.P. (85) Instituto de Ciencia de Materiales, C.S.I.C., Serrano, 115 dpdo., 28006 Madrid, Spain Hibino. T. (155) Synthetic Crystal Research Laboratory, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Hoefnagel, A.J. (379) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands Hcelderich, W.F. (399) Institute for Chemical Technology and Heterogeneous Catalysis, University of Technology RWTH Aachen, Womngerweg 1,52074 Aachen, Germany Howe, R.F. (187) Department of Physical Chemistry, University of New South Wales, Box 1, Kensington NSW, 2033, Australia Hwang, I.C. (339) Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea Ihm, S.-K. (355) Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373- 1 Kusongdong, Yusonggu, Taejon, 305-701, Korea


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Iida, S. (453) Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Imai, H. (25) Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan h i , T. (263) Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Ishida, H. (473) Chemical Development Laboratory, The Asahi Chemical Industry Co., LTD, Shionasu, Kojima, Kurashiki-shi, Okayama 7 1 1, Japan Ishikawa, N. (331) Central Research Laboratories of Idemitsu Kosan Co., Ltd. 1280 Kamiizumi, Sodegaura, Chiba, 299-02, Japan Ishiyama, 0. (25) Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Iwayama, K. (243) Chemicals Research Laboratories, Toray Industries Inc., 9- 1 Oe-cho, Minato-ku, Nagoya 455, Japan Izumi, Y. (453) Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Jackson, K.T. (1 87) Department of Physical Chemistry, University of New South Wales, Box 1, Kensington NSW, 2033, Australia Jacobs, P.A. (371) Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Department Interface Chemistry, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Jong, S.-J. (33) Department of Chemistry, National Taiwan University, Taipei, 107 Taiwan, China Jullien-Lardot, V. (1 1) Laboratoire de Catalyse, Facultes Universitaires N.-D. de la Paix, Rue de Bruxelles, 61, B-5000 Namur, Belgium Karge, H.G. (135) Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany Kasai, H. (251) Kajima Technical Research Institute, Tobitakyu, Chofu, Tokyo 182, Japan Kato, C. (171) Department of Applied Chemistry, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan

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Kawai, T. (217) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Kessler, H. (3) Laboratoire de Materiaux Mineraux, URA CNRS 428, Ecole Nationale Superieure de Chimie, Universite de Haute Alsace, 3 Rue Alfred Werner, 68093 Mulhouse Cedex, France Kikuchi, E. (295) Department of Applied Chemistry, Schooi of Science & Engineering, Waseda University, 3-4- 1 Okubo, Shinjuku-ku, Tokyo 169, Japan Kim, J. (321) Department of Chemical Engineering, Yale University, P.O.Box 2159, New Haven, CT 06520, U.S.A. Kim, J.-H. (279) National Institute of Materials and Chemical Research, Higashi, Tsukuba, Ibaraki 305, Japan Kono, M. (473) Chemical Development Laboratory, The Asahi Chemical Industry Co., LTD, Shionasu, Kojima, Kurashiki-shi, Okayama 7 11, Japan Kosslick, H. (57) Center of Heterogeneous Catalysis, Rudower Chaussee 5 , D- 12484 Berlin-Adlershof, Germany Kouwenhoven, H.W. (379) Technical Chemical Laboratory, ETH-Zentrum, Universitatsstrasse 6,8092 Zurich, Switzerland Kowalak, S. (179) Faculty of Chemistry, A.Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland Kraak, P. (425) Geschaftsbereich Katalysatoren, Leuna-Werke AG, D-06236 Leuna, Germany Kubo, M. (1 17) Department of Molecular Chemistry & Engineering, Faculty of Engineering, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980, Japan Kuroda, K. (1 7 1) Department of Applied Chemistry, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan Laniecki, M. (363) Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6,60-780 Poznan, Poland Larsen, G. (321) Department of Chemical Engineering, Yale University, P.O.Box 2 159, New Haven, CT 06520, U.S.A. Lee, C.S. (233) Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, 10764 Taiwan, China Lee, D.-K. (355) Department of Chemical Engineering, Gyeongsang National University, 900, Kajwadong, Chinju, 660-701, Korea

List o f Contributors

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Lee, J.-H. (355) Department of Chemical Engineering, Chungbuk National University, Gaesindong, Cheongju, 360-763, Korea Lercher, J.A. (287) Institute for Physical Chemistry and CD Laboratory for Heterogeneous Catalysis, Technical University of Vienna, A-I060 Vienna, Getreidemarkt 9, Austria Li, C.-L. (497) Petroleum Processing Research Center, East China University of Chemical Technology, 200237 Shanghai, China Li, L.-T. (497) Petroleum Processing Research Center, East China University of Chemical Technology, 200237 Shanghai, China Lin, J.-T. (33) Department of Chemistry, National Taiwan University, Taipei, 107 Taiwan, China Lin, W.C. (481) Union Chemical Laboratories, Industrial Technology Research Institute, 321 Kuang Fu Road, Section 2, Hsinchu, Taiwan Liu, F. (497) Petroleum Processing Research Center, East China University of Chemical Technology, 200237 Shanghai, China Liu, L.-L. (497) Petroleum Processing Research Center, East China University of Chemical Technology, 200237 Shanghai, China Liu, S.B. (233) Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, 10764 Taiwan, China Lortie, C. (101) Departement de Chimie, CERPIC, Universite Laval, Ste Foy, Quebec, GlK 7P4, Canada Lu, G.-M. (347) Department of Surface Chemistry and Catalysis, Institute of Isotopes of the Hungarian Academy of Sciences, P.O.Box 77, Budapest, Hungary, H-1525 Martin, B.R. (489) Texaco Inc. Research and Development, l?O.Box 1608, Port Arthur, Texas 77641 U.S.A. Masuda, T. (225) Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Honmachi Yoshida, Sakyo-ku, Kyoto 606, Japan Matsuda, T. (295) Department of Applied Chemistry, School of Science & Engineering, Waseda University, 3-4- 1 Okubo, Shinjuku-ku, Tokyo 169, Japan Matsumoto, H. (251) Shin Tohoku Chemical Industry Ltd., Kamisugi, Aoba, Sendai, Miyagi 980, Japan

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Mayoral, J.A. (391) Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza - C.S.I.C., 50009 Zaragoza, Espana Min, E.-Z. (443) Research Institute of Petroleum Processing, China Petrochemical Corporation, P.O.Box 914, Beijing 100083, China Mirth, G. (287) Institute for Physical Chemistry and CD Laboratory for Heterogeneous Catalysis, Technical University of Vienna, A-1060 Vienna, Getreidemarkt 9, Austria Mitsui, 0. (473) Chemical Development Laboratory, The Asahi Chemical Industry Co., LTD, Shionasu, Kojima, Kurashiki-shi, Okayama 7 11, Japan Miyamoto, A. (1 17) Department of Molecular Chemistry & Engineering, Faculty of Engineering, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980, Japan Mizuno, S. (273) Department of Chemistry, Faculty of Science and Technology, Sophia University, 7- 1 Kioi-Cho, Chiyoda-ku, Tokyo 102, Japan Murakami, Y. (155) Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Murakami, Y. (25) Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Nakamura, M. (417) Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 1 13, Japan Nakashiro, K. (303) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Nakatsuka, Y. (155) Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Namba, S. (279) Department of Materials, The Nishi-Tokyo University, Uenohara-machi, Kitatsuru-gun, Yama nashi 409-01, Japan Neeleman, E. (407) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands Niwa, M. (1 55) Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680, Japan


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Nusterer, E. (287) Institute for Physical Chemistry and CD Laboratory for Heterogeneous Catalysis, Technical University of Vienna, A-1060 Vienna, Getreidemarkt 9, Austria O’Young, C.-L.(19) Texaco Inc., PO Box 509, Beacon, NY 12508, U.S.A. Ogawa, M. (171) Department of Applied Chemistry, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan Ohsuna, T. (77) College of Science & Engineering, Iwaki Meisei University, Iwaki, Japan

On,D.T. (101) Departement de Chimie, CERPIC, Universite Laval, Ste Fdy, Quebec, G1K 7P4, Canada Ono, Y.(303) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Osako, K. (303) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan

Otterstedt, J-E. (49) Department of Engineering Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden Paczkowski, M.E. (399) Institute for Chemical Technology and Heterogeneous Catalysis, University of Technology RWTH Aachen, Womngerweg 1,52074 Aachen, Germany Parton, R.F. (371) Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Department Interface Chemistry, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Pawlowska, M. (179) Faculty of Chemistry, A.Mickiewicz University, Grunwaldzka 6,60-780Poznan,Poland Pester, R. (425) Geschaftsbereich Katalysatoren, Leuna-Werke AG, D-06236 Leuna, Germany Pilz, w.(57) WIP, KAI e.V., Rudower Chaussee 6, D- 12484 Berlin-Adlershof, Germany

Pires, E. (391) Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza - C.S.I.C., 50009 Zaragoza, Espana Ramaswamy, A.V. (109) Catalysis Division, National Chemical Laboratory, P u n e l l 108, India Ramli, Z. (125) Department of Chemistry, Fakulti Sains, Universiti Teknologi Malaysia, KB 791, Skudai, 80990 Johor, Malaysia

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Rao, P.R.H.P. (109) Catalysis Division, National Chemical Laboratory, Pune-41108, India Rees, L.V.C. (197) Physical Chemistry Laboratories, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K. Resasco, D.E. (321) Sun Refining and Marketing Company, Research and Development, P.O.Box 1135, Marcus Hook, PA 19061-0835, U.S.A. Roessner, F. (3 13) University of Leipzig, Department of Chemistry, Institute of Technical Chemistry, Linnestr.3, D04103 Leipzig, Germany Sagae, A. (251) Kajima Technical Research Institute, Tobitakyu, Chofu, Tokyo 182, Japan Sato, M. (93) Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan Sato, T. (251) Shin Tohoku Chemical Industry Ltd., Kamisugi, Aoba, Sendai, Miyagi 980, Japan Schodel, R. (425) Geschaftsbereich Katalysatoren, Leuna-Werke AG, D-06236 Leuna, Germany Schoeman, B.J. (49) Department of Engineering Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden Schott-Darie, C. (3) Laboratoire de Materiaux Mineraux, URA CNRS 428, Ecole Nationale Superieure de Chimie, Universite de Haute Alsace, 3 Rue Alfred Werner, 68093 Mulhouse Cedex, France Segawa, K. (273) Department of Chemistry, Faculty of Science and Technology, Sophia University, 7- 1 Kioi-Cho, Chiyoda-ku, Tokyo 102, Japan Senoh, N. (155) Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680, Japan Serrette, G.P.D. (433) Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A. Shea, W.-L. (433) Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A. Sheldon, R.A. (407) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands Shen, Y.-F. ( I 9) U-60, Department of Chemistry, University of Connecticut, Storrs, CT 06269, U.S.A. Sherwood Jr., D.E. (489) Texaco Inc. Research and Development, P.O.Box 1608, Port Arthur, Texas 77641 U.S.A


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Shi, L. (497) Petroleum Processing Research Center, East China University of Chemical Technology, 200237 Shanghai, China Shiu, P.F. (233) Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, 10764 Taiwan, China Sterte, J. (49) Department of Engineering Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden Storek, W. (57) Federal Institute for Materials Research and Testing BAM, Rudower Chaussee 6, D-12484 Berlin-Adlershof, Germany Sugimoto, M. (331) Central Research Laboratories of Idemitsu Kosan Co., Ltd. 1280 Kamiizumi, Sodegaura, Chiba, 299-02, Japan Suib, S.L. (19) U-60, Department of Chemistry, University of Connecticut, Storrs, CT 06269, U.S.A. Suzuki, M. (243) Chemicals Research Laboratories, Toray Industries Inc., 9- 1 Oe-cho, Minato-ku, Nagoya 455, Japan Tatsumi, T. (417) Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan Terasaki, 0. (77) Department of Physics, Tohoku University, Aramaki, Aoba, Sendai 980, Japan Tominaga, H. (4 17) Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan T~ai,T.-Y. (433) Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A. Tsutsumi, K. (217) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Tuan, V.A. (57) Center of Heterogeneous Catalysis, Rudower Chaussee 5 , D-12484 Berlin-Adlershof,Germany Urabe, K. (453) Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan

van Bekkum, H. (379) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands van Koten, M.A. (379) Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136,2628 BL Delft, The Netherlands

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Vetrivel, R. (109) Catalysis Division, National Chemical Laboratory, Pune-41108, India Vogt, A.H.G. (379) Technical Chemical Laboratory, ETH-Zentrum, Universitatsstrasse 6, 8092 Zurich, Switzerland Vogt, F. (425) Fachbereich Chemie, Martin-Luther-Universitat Halle-Wittenberg, D-062 17 Merseburg, Germany Walther, G. (57) Center of Inorganic Polymers, Rudower Chaussee 5, D-12484 Berlin-Adlershof, Germany Wami, H. (251) Kajima Technical Research Institute, Tobitakyu, Chofu, Tokyo 182, Japan Wang, G.-R. (67) Institute of Industrial Catalysts, Dalian University of Technology, Dalian 116012, China Wang, X.-Q. (67) Institute of Industrial Catalysts, Dalian University of Technology, Dalian 116012, China Wang, X.-S. (67) Institute of Industrial Catalysts, Dalian University of Technology, Dalian 116012, China Watanabe, D. (77) College of Science & Engineering, Iwaki Meisei University, Iwaki, Japan Wieckowski, A.B. (179) Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60- 179 Poznan. Poland woo, S.I. (339) Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea Wu, K.C. (481) Union Chemical Laboratories, Industrial Technology Research Institute, 321 Kuang Fu Road, Section 2, Hsinchu, Taiwan Yamaguchi, F. (25) Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Yamamoto, H. (273) Department of Chemistry, Faculty of Science and Technology, Sophia University, 7- 1 Kioi-Cho, Chiyoda-ku, Tokyo 102, Japan Yamanaka, S. (147) Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, HigashiHiroshima 724, Japan Yamawaki, M. (303) Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Yanagihara, T. (217) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan


xix

Yanagisawa, K. (417) Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan Yang, C.L. (481) Union Chemical Laboratories, Industrial Technology Research Institute, 321 Kuang Fu Road, Section 2, Hsinchu, Taiwan Yashima, T.(279) Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Yu, S.-X. (67) Institute of Industrial Catalysts, Dalian University of Technology, Dalian 116012,China Zerger, R.P.( 19) U-60, Department of Chemistry, University of Connecticut, Storrs, CT 06269, U.S.A.


Preface

This volume comprises the proceedings of the International Symposium on Zeolites and Microporous Crystals (ZMPC '93) held at the Nagoya Congress Center in Nagoya, Japan, August 22-25,1993. This symposium was organized by the Japan Association of Zeolites in collaboration with the International Zeolites Association and 13 academic societies in Japan. The Japan Association of Zeolites held the International Symposium on Chemistry of Microporous Crystals (CMPC) in Tokyo in 1990. The success of CMPC and the continuing development in this field led the Japan Association of Zeolites to organize new members of a steering committee to hold ZMPC '93 as a continuation of CMPC. The aim of this symposium is to bring together experts in numerous areas of zeolite and zeolite family studies from various parts of the world to discuss problems of common interest and to exchange ideas and experiences. This will help open new horizons in the chemistry of zeolites and microporous crystals. Fortunately, our efforts attracted much attention and this symposium had 295 attendants from 29 nations with 157 oral and poster papers. At this meeting various trends in the following areas were noted: -crystal chemistry ( 13) -synthesis (21) -ion exchange and modification ( 13) -adsorption and diffusion (21) -intercalation and cross-linking ( 12) -host-guest interaction (9) -catalysis ( 5 8 ) -applications (10) This volume is a collection of 9 plenary lectures and 27 invited papers as well as 22 contributed papers out of 24 papers presented orally. All papers were subjected to scientific review by referees selected from among the participants. All the authors with few exceptions were asked to revise their papers and they responded with careful revisions. The editors sincerely thank the referees for their dedicated efforts in reviewing and the authors for their faithful response, both of which ensured the scientific quality of this volume. The editors express their thanks to Professors Makoto Onaka, Atsushi Satsuma and Kazuo Urabe of Nagoya University, and to Mr. Ippei Ohta of Kodansha Scientific for their invaluable assistance in the editing of this volume. Tadashi Hattori Tatsuaki Yashima

December 1993

xxi


Contents

Organization

v


Preface

vii

xxi

I . Synthesis Further Results in the Synthesis of Microporous Alumino-and Gallophosphates in the Presence of Fluoride (C. Schott-Darie, H. Kessler and E. Benazzi) ..................3 Preparation, Characterisation, and Catalytic Properties of Microporous Zirconophosphate Molecularly Engineered Layered Structures (MELSB) (E. G. Derouane and V. Jullien-Lardot) ..................................................................

11

Synthesis of Manganese Oxide Octahedral Molecular Sieves (OMS) (Y.-F. Shen, R. N. DeGuzman, R. P. Zerger, S. L. Suib and C.-L. O’Young) ...........................

19

Preparation and Properties of the Pyridine Intercalates of Bismuth Molybdic Acid (Y. Murakami, F. Yamaguchi, 0. Ishiyama and H. Imai) ....................................

25

Synthesis of Titanium Pillared Clay Using Organic Medium (S.-J. Jong, J.-T. Lin and S.-F. Cheng) .......................................................................................

33

Oxygenated Stabilizing Agents in the Synthesis of MFI Zeolites (G. Giordano, F. Di Renzo and F, Fajula) .................................................................................

41

The Synthesis of Discrete Colloidal Zeolite Particles (B. J. Schoeman, J . Sterte and J.-E, Otterstedt) ....................................................................................... 49 Study on the Isomorphous Substitution of Silicon by Tetravalent Elements (Zr, Ge, Ti) in the Framework of MFI Type Zeolites (R. Fricke, H. Kosslick, V.A. Tuan, I. Grohmann, W. Pilz, W. Storek, and G. Walther) ....................................

57

Synthesis and Catalytic Reaction of [Zr] ZSM-5 (G.-K. Wang, X.-Q Wang, X.-S. Wang and S.-X. y u ) ..........................................

67

xxi i i

xxiv

Contents

11. Structure Fine Structures of Zeolites: Defects, Interfaces and Surface Structures. An HREM Study (0. Terasaki, T. Ohsuna, V . Alfredsson, J.-0. Bovin, S. W. Carr, M. W. Anderson and D, Watanabe) ........................................................................

77

Statistical Mechanics of Si, A1 Ordering in A-type Zeolites ( C . P. Herrero)

85

*****.---*..

Topological and Stereochemical Characteristics of Zeolite Frameworks (M. Sato) ..a93 Symmetry and Location of Titanium Within Titanium Silicalite Framework of MFI Structure (D. T. On, I . Denis, C . Lortie, C. Cartier and L. Bonneviot) ............101 The Topography of Vanadium in Silicalite-2 Crystal Lattice and its Catalytic Role in the Oxyfunctionalization of Alkanes (R. Vetrivel, P. R. Hari Prasad Rao and A, V, Ramaswamy) .................................................................................

109

Structure and Dynamics of Ion-exchanged Zeolites as Investigated by Molecular Dynamics and Computer Graphics (A. Miyamoto and M. Kubo) ..................... 117 Structural Characterization of Rhenium Impregnated Zeolite Y and ZSM-5 by 2gSi and 27AlMAS NMR and IR Spectroscopy (H. Hamdan and'Z. Ramli) **..*el25

111. Modification Solid-state Reactions of Zeolites (H. G . Karge)

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

Anion Exchange Reactions in Layer Structured Crystals (S. Yamanaka)

**..*.*.****-..

135 147

Reactant Shape-selectivity for Cracking of Linear Paraffin on HZSM-5 Modified by CVD of Silicon Alkoxide: A Strong Dependence upon the Reaction Temperature (M. Niwa, N. Senoh, T. Hibino, Y. Nakatsuka and Y. Murakami) **.***155 New Approaches in Shape Selective Alkylation Reactions Using Pore Size Regulated MFI Zeolites (A. B. Halgeri and Y. S. Bhat) .................................

163

Layered Silicate-Organic Intercalation Compounds as Photofunctional Materials (M. Ogawa, K, Kuroda and C, Kate) ............................................................

171

Polymerization Inside the Molecular Sieves (S. Kowalak, M. Paw+owska, A.B. Wieckowsk and J. Goslar) ........................................................................

179

Studies of Zeolite Single Crystals: Ethene Oligomerization in HZSM-5 (K, T, Jackson and R. F, Howe) ..................................................................

187

Contents

xxv

IV. Adsorption Adsorption of Lower Hydrocarbons in Zeolite NaY and Theta-I. Comparison of -Low and High Pressure Isotherm Data (J. A. Hampson and L. V. C. Rees) * . * * . * - . . 197 Determination of Sorption Thermodynamic Functions for Multi-component Gas 209 Mixtures Sorbed by Molecular Sieves (M, Biilow) ....................................... Adsorption Characteristics of Hydrophobic Zeolites (K. Tsutsumi, T. Kawai and T, Yanagihara) .......................................................................................

217

Measurements of Adsorption on Outer Surface of Zeolite and Their Influence on Evaluation of Intracrystalline Diffusivity (T. Masuda and K. Hashimoto) ............225 Interpretation of Xenon Adsorption Isotherms and Xe- 129 NMR Chemical Shifts on Ion-exchanged NaY Zeolites (S. B. Liu, C. S. Lee, P. F. Shiu, and B. M. Fung) .......................................................................................... 233 Adsorption of C, Aromatic Isomers on Faujasite Zeolite (K. Iwayama and M. Suzuki).............................................................................................

243

Study on a New Humidity Controlling Material Using Zeolite for Building (A. Sagae, H. Wami, Y. Arai, H. Kasai, T. Sat0 and H. Matumoto) .....................

25 I

V. Catalysis Novel Catalytic Functions of Metallosilicates Exerted by Isomorphous Substitution (T, Inui) ..........................................................................................

263

Selective Synthesis of Ethylenediamine from Ethanolamine and Ammonia over Zeolite Catalysts (K. Segawa, S. Mizuno, Y. Fujimoto and H. Yamamoto) * * . * * * * * . . * * 213 Para-Selectivity of Zeolites and Metallosilicates with MFI Structure (S. Namba, J,-H. Kim and T, Yashima) ........................................................................

279

Transition State and Diffusion Controlled Shape Selectivity in the Formation and Reaction of Xylenes ( G . Mirth, J . Cejka, E. Nusterer and J. A. Lercher)............... 287 Selective Synthesis of 4,4’-DiisopropylbiphenylUsing Mordenite Catalysts (T, Matsu& and E. Kihchi) .....................................................................

295

Mechanism of the Activation of Butanes and Pentanes over ZSM-5 Zeolites ( Y . Ono, K. Osako, M. Yamawaki and K. Nakashiro) .......................................

303

Conversion of Ethane into Aromatic Hydrocarbons on Zinc Containing ZSM-5 Zeolites Prepared by Solid State Ion Exchange ( A . Hagen and F. Roessner) .**..-313

xxvi

Contents

Platinym-Nickel/L-zeolite Bimetallic Catalysts: Effect of Sulfur Exposure on Metal Particle Size and n-Hexane Aromatization Activity and Selectivity (G. Larsen, D. E. Resasco, V. A. Durante, J. Kim and G. L. Haller) .....................

32 1

Characterization and Catalytic Performance of the Platinum K L Zeolite Treated with Chlorotrifluoromethane (M. Sugimoto, T. Fukunaga and N. Ishikawa) . . * . . * * * . 33 1 Characterization and Catalytic Properties of Zeolite-Supported Platinum-Iridium Bimetallic Catalysts Prepared by Decoration of Iridium (1. C. Hwang and S. 1, Woo) ..................................................................... 339 Infrared Spectroscopic Study of CO Adsorption on Pt-Co Bimetallic Particles Entrapped in NaY-Zeolite (G.-M, Lu and L. Guczi) .......................................

347

Some Characteristics of Transition-metal Containing Y-Zeolite in CO Hydrogenation (S.-K. Ihm, D.-K. Lee and J.-H, Lee) ................................................

355

Ni-Mo-Y Zeolites as Catalysts for the Water-Gas Shift Reaction (M.kaniecki)..........................................................................................

363

Synthesis, Characterization and Catalytic Performance of Nitrosubstituted Fephthalocyanines on Zeolite Y (R. F. Parton, C. P. Bezoukhanova, J. Grobet, p. J . Grobet and p. A. Jacobs) ..................................................................... 37 1 Zeolite Catalyzed Aromatic Acylation and Related Reactions (H. van Bekkum, A. J. Hoefnagel, M. A. van Koten, E. A. Gunnewegh, A. H. G. Vogt and H. W. Kouwenhoven) ..............................................................................

379

Diels-Alder Condensation of Methyl and (-)-Menthy1 Acrylates with Cyclopentadiene over Zeolites and Cation Exchanged Clays (F. Figueras, C. Cativiela, J. M. Fraile, J. I. Garcia, J. A. Mayoral, L. C. de MEnorval and E. Pires) ...............39 1 Controlled Preparation of Neoalkanals and Novel Cyclic Dioxepenes, Depending upon the Use of Shape Selective and Non Shape Selective Catalysts (W. F. Hoelderich and M. E. Pac-kowski) ...................................................... 399 Redox Molecular Sieves: Recyclable Catalysts for Liquid Phase Oxidations (R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman) ....................................

407

Titanium Silicalites as Shape-Selective Oxidation Catalysts (T. Tatsumi, K. Yanagisaws, K. Asano, M. Nakamura and H. Tominaga) .......................................... 417 Carbon Supported TS-I Catalysts (P. Birke, P. Kraak, R. Pester, R. Schodel and F. Vogt) ................................................................................................

425

Alteration of Alumina Pillared Clays for Enhanced Catalytic Activity (A. Clear433 field, H. M. Aly, R. A. Cahill, G. P. D. Serrette, W.-L. Shea and T.-Y. Tsai) ..*-..*.. Development of Pillared Clays for Industrial Catalysis (E. Min)

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

443

Contents

xxvii

Ni-Exchanged Sepiolite as a Fibrous Clay Catalyst for Selective Dehydration of n-Butyl Alcohol to Dibutyl Ether (K. Urabe, S. lida and Y. Izumi) .................. 453 Role of the Zeolite Catalysts in the New Refining Strategies (A. Corma)

............46 1

Liquid-Phase Hydration of Cyclohexene with Highly Silicious Zeolites (H. Ishida, Y. Fukuoka, 0. Mitsui and The late M. Kono) .................................

473

The Synthesis of Methyl Isobutyl Ketone over Palladium Supported Zeolites (P. Y. Chen, S. J. Chu, W. C. Lin, K. C. Wu, and C. L. Yang) ...........................

48 1

Influence of Zeolite Secondary Porosity on Performance of Resid Hydrocracking 489 Catalysts (P. E. Dai, D. E. Sherwood Jr. and B. R. Martin) .............................. Regeneration Behaviors of Hydroisomerization Catalysts (L. Shi, F. Liu, L.-L. Liu, C.-L, Li and L.-T. Li)

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

497



Further Results in the Synthesis of Microprous Numino- and Gallophosphates in the Presence of Fluoride

C. Schott-Dariel, H. Kesslerl and E. Benazzi2 1Laboratohede Mat6riaux MinQaux, URA CNRS 428, Ecole Nationale Sup6rieurede Chimie, Universit6 de Haute Alsace, 3 Rue Alfred Werner, 68093 Mulhouse Cedex, France 2Institut Franqais du Wtrole, 1-4 Avenue Bois M a u , BP 311,92506 Rueil Malmaison Cedex, France

ABSTRACT The recent results in the synthesis of alumino- and gallophosphate molecular sieves in the presence of fluoride are described with emphasis on the location of fluorine in the structures. In the A1203-P205 system, with quinuclidine as a template, a tetragonal variant of AlP04-16 with F located in the D4R's was obtained. The triclinic CHA-like A W 4 precursor of AW4-34, with F bridging two A1 of a 4MR was produced from morpholine, 1-methylimidazole,piperidine or pyridine. In the Ga203-P205 system, the mclinic precursor of GaP04-34 was obtained with 1methylimidazole or pyridine. In non-aqueous medium (ethylene-glycol), pyridine led to GaP04 LTA. Cloverite whose usual template was quinuclidine. was further obtained with methylquinuclidine, 3-azabicyclo [3,2,2] nonane or piperidine. A novel structure with 16MR openings was formed on starting with hexamethylenediamine.In this structure three types of F atoms are present, one is occluded in D4Rs and two others are bridging Ga atoms. INTRODUCTION A strong structure-orienting effect of fluoride, added as HF to the starting mixture, was previously observed in the synthesis of the cubic LTA [l] and -CLO [2] type gallophosphates.The structure of both materials can be built up from double-4-ringscontaining a fluoride anion which is neutralized by the protonated organic molecule, i.e., di-n-propylamine and quinuclidine respectively. Without HF in the synthesis mixture, the hexagonal gallophosphate GaP04-a [3], similar to phase 6 [4] or GaP04-C3 [5] was obtained with both organics. A less strong but still an orienting effect of fluoride appeared in the synthesis of the triclinic CHA-like ALP04 with morpholine as a template [6]. In this material, F is bridging two A1 atoms of a 4-membered ring. On calcination, morpholine and fluorine are removed and the rhombohedra1 CHA-type AlP04-34 is produced. With the same starting composition, but in the absence of HF,no crystallization was observed. Other types of bridging with F atoms were reported recently by F6rey et al. for two gallophosphates synthesized in the presence of 1.4-diazabicyclo[2.2,2] Octane (DABCO). In these solids, fluorine is involved in the comer-sharing of a Ga03FOHH20 octahedron and a Ga03FOH 3

4

C. Schott-Darie, H. Kessler and E. Benazzi

trigonal bipyramid [7], and the edge-sharing of dimeric Ga2070F2 (0= 0 or OH) octahedral entities [81. In the present paper are described our further results in the synthesis of microporous aluminoand gallophosphates in the presence of HF and various organic templates. Some syntheses were performed in a "non-aqueous"ethylene-glycol medium. EXPERIMENTAL

Svnthesis The reactants were 85 % phosphoric acM (Prolabo, Normapur), 40% hydrofluoric acid (Prolabo, Normapur). The aluminium source was aluminium triisopropoxide (Aldrich, 98%). Two gallium sources were used, a gallium nitrate solution (Rhijne-Poulenc) and an amorphous source obtained after heating the nitrate source at 250OC for 24 hours. All the syntheses were carried out in the presence of a template. The templates used include the following : hexamethylenediamine, morpholine. l-methylimidazole, pyridine, piperidine, quinuclidine, methylquinuclidine, 3-azabicyclo[3,2,2]nonane, The order of addition of the reactants under stirring was the following : H3P04, water, aluminium or gallium source, hydrofluoric acid and finally the template. The obtained gel was mixed and transferred into a PTFE-lined stainless steel autoclave. After heating at 15O-17O0Cfor 24-96 hours, the solid was filtered, washed and dried at 7OOC. The "non-aqueous" medium (ethylene-glycol) syntheses were carried out with the same gel composition and procedure as reported by Huo and Xu [9]. Charac-

. .

r X-Rav diffraction. The powder patterns were obtained with CuKa radiation on a Philips PW 1800 diffractometer (variable slit). High-temperature X-ray diffraction was performed either using a high-temperature Guinier camera or on a diffractometer equipped with a hightemperature CGR chamber (CuKd. The temperature range was 25-600OC and the sample was kept in a He atmosphere.

Chemicalanalvws. Al, Ga and P were analyzed by inductively coupled plasma emission spectroscopy. F- was determined by using a fluoride ion selective electrode after mineralization. Carbon and nitrogen were analyzed by turning them into C@ and N2 respectively, by combustion of 1-2 mg of sample at lo00 OC in an excess of @ (C), and a He -3%02 mixture (N). The nitrogen oxides were reduced to N2 by metallic copper at 500 "C. C@ and N2 were titrated by a coulomemc and catharometric technique respectively. The amount of organic species was also obtained by thermogravimetry, as well as the hydration water. . TG analysis was performed on a Mettler 1 thermoanalyser by heating in air at 4'C.min-1. and DTA was carried out in air and in argon on a BDL-Setaram M2 apparatus with a heating rate of 10°C min-1. Solid State NMR spectroscopy. The NMR spectra were recorded on a Bruker MSL 300 spectrometer for 13C. 19F, 27Al and 31P. The NMR acquisiton conditions are given in Table 1.

Microporous Alumino- and Gallophosphates in the Presence of Fluoride

5

Table 1. Recording conditions of the M A S NMR spectra 13c

Standard

Frequency (MHz) Pulse width (10%) Recycle time (s) Spinning rate (kHz) No. of scans

(CH314Si 75.4 3.3 15 4.5 150

*9F CFC13 282 5.5 4 8 180

31P

27~1

Al(NO3)aq 78.2 10 2 8 150

85 % H3PO4 121.4 2.3 10 10 8

RESULTS

&Ol- P?Os svstem. On attempting the synthesis of cloverite with the ALP04 composition and the usual template for cloverite, i. e., quinuclidine. a tetragonal variant of AlP04-16 was obtained (a = 9.346A, c = 13.508A), whereas the material prepared in the absence of HF is cubic with a = 13.3832(6)A [lo]. Thus, like in octadecasil [ll]. the presence of F induces a tetragonal distortion of the structure. Preliminary results of a Rietveld refinement of the structure show that fluoride is located in the double-4-rings like in octadecasil. As previously reported, a triclinic CHA-like AlPO4, was produced in the presence of HF with morpholine as a template [6]. Two F atoms were found to bridge two A1 atoms of a 4-ring which connects two doubled-rings of the chabazite-like structure. The additional F atoms are neutralized by two protonated organic molecules located in each chabazite cage. The unit cell formula is ( A ~ P O ~ ) ~ ( C ~ H ~ This O O )triclinic ~ F ~ . material was further obtained with 1-methylimidazole, pyridine and piperidine whose geometry is similar to that of morpholine (Table 2). This triclinic phase is a precursor of AlP04-34. Indeed, by calcination at about 50O0C, the organic template and HF are removed and the rhombohedra1chabazite structure of ALP04-34 is formed. Table 2. Alumino- and gallophosphates obtained in aqueous medium with various templares and in the presence of HF Template Hexamethylenediamine Morpholine 1-Methylimidazole Pyridine Piperidine Quinuclidine Methylquinuclidine 3-azabicyclo[3,2,2]nonane * Non performed synthesis

AM4

*

0

Tricl. CHA Tricl. CHA Tricl. CHA Tricl. CHA Tetrag. A M 4 - 16

* *

4

Novel structure

**

Tricl.CHA Tricl. CHA Cloverite Cloverite Cloverite Cloverite

** Amorphous

In figure 1 is shown the X-ray diffraction spectrum of the calcined l-methylimidazoleAlPO4 recorded at 615°C on the high temperature diffractometer compared with the calculated spectrum for rhombohedfal AP04-34 taking into account the framework atoms only [12]. A good

6

C. Schott-Darie, H. Kessler and E. Benazzi

agreement can be observed. On rehydration at room temperature, there is a change in the spectrum owing to the interaction of the water molecules with the A1 atoms of the framework (Fig. lc). Two different synthesis routes of AlP04-34, using both tetraethylammonium hydroxide as the template, were previously reported [ 13, 141. In the first one [ 131, the P2O5 source used was a mixture of Al(H2PO4)3 and H3PO4, moreover calcined AlP04-5 was added. In the second one [141 the P2O5 source was H10Pg025 and a wet milling of the starting gel was performed at room temperature (4 h) before heating. The fluoride route described here, with four different templates, is a third one via a triclinic chabazite precursor.

10

20

30

29

Fig. 1. Powder X-Ray diffraction: (a) calculated spectrum for AlP04-34 (framework only [lo]), (b) high-temperature spectrum of AlPO4-34 (calcined 1-methylimidazole-Al, 615"C), (c) room temperature spectrum of rehydrated AlP04-34.

.--

As for the A1203-P205 system, the novel triclinic chabazite-like precursor of GaP04-34 was obtained with 1-methylimidazoleor pyridine (Table 2). However, with morpholine no crystallization was observed, and with piperidine as the template the gallophosphate

Microporous Alumino- and Gallophosphates in the Presence of Fluoride

7

cloverite was produced. The latter organic molecule was reported to direct towards cloverite in non-aqueous medium by Huo and Xu [9]. It is observed here that cloverite can be obtained even in aqueous medium with this template. As shown in table 2, cloverite was also produced from methylquinuclidine and 3-azabicyclo [3,2,2]nonane. Actually, both molecules are rather similar to the usual template quinuclidine. The latter directs towards cloverite in a large range of crystallization temperatures (80OC-220OC) and heating times (7 h - 90 h). A novel gallophosphate was obtained with hexamethylenediamine as a template (Table 2). A gel of composition 2.8 HMDA : lGa2O3 : lP2O5 : 1HF : 80H20 was heated at 17OOC for 24 hours [ 151. The powder X-ray diffraction spectrum of the obtained solid is given in figure 2.

,

0

10.0

30.0

20.0

40.0

50.0 20

Fig. 2. Powder XRD spectrum of hexamethylenediamine-GaPO4 Figure 3 shows the 19F MAS NMR spectrum. Three lines are observed at the chemical shift values of -67.8 ppm, -99.2 ppm and -1 13 ppm. The first signal can be assigned to fluorine occluded in a double-4-ring. As a matter of fact, the chemical shifts observed for F in cloverite and in LTAtype GaPO4 are -68.1 ppm and -72.0 ppm respectively [2,1]. In both structures, fluorine was located in double-4-rings [16, 171. The signals at -99.2 pprn and -1 13 ppm can be assigned to F atoms bridging Ga atoms, indeed, the chemical shift of fluorine bridging two gallium atoms of a 4membered ring in the triclinic precursor of GaP04-34 is -97.3 ppm. The assignments are in agreement with the crystal structure of the material which was independently synthesized and determined on a single crystal by Fkrey et al. [18]. The symmetry is orthorhombic with unit cell parameters a = 24.638 A, b=18.408A, c = 10.25A. space group P21212. The structure shows large openings made of 16-membered rings built up from PO4, GaO4, GaX5 and G& polyhedra (X = 0, OH, F). One type of fluorine is located in double-4-rings and two other types are bridging Ga atoms. The 31P M A S NMR spectrum is reported in figure 4. Two main lines are observed at -3.3 ppm and -9.1 ppm, the latter showing moreover two shoulders. A weak signal is also observed at about -15 ppm. As for cloverite, the resolution of the 31P MAS NMR spectrum of this novel gallophosphateis rather poor.

8

C. Schott-Darie. H. Kessler and E. Benazzi

I

-670

-113

1

Fig. 4.31P M A S NMR spectrum of hexamethylenediamine-GaP04

Fig. 3. l9F MAS NMR spectrum of hexamethylenediamine-GaPO4

The thermal stability of the material was determined by TG, DTA and high-temperature Xray diffraction. The total weight loss (up to 70O0C),corresponding to the removal of the hydration water, the organic molecule and HF was 21.5 wt.%. The DTA curve (air flow) is shown in figure 5. The endotherm observed at 100°C corresponds to the removal of water and the strong exotherm at 420°C to the oxidation of the organic species. The small endothermic peak observed at 38OOC is unexplained at this time. According to high temperature X-ray diffraction (Guinier camera), there is a slight structure change at 350°C when the organic template is removed, and a transformation into a mixture of quartz- and cristobalite- type GaP04 at 650°C.

-

420°C

I+ 1-

I

350°C

I

I

100°C

380°C

Fig. 5. DTA curve of hexamethylenediamine-GaP04,air flow,heating rate 1O0C.mh1

Microporous Alurnino- and Gallophosphates in the Presence of Fluoride

9

"Non-wous" synthesismediram MzQ3&Q5-(M - Al-GaL In "non-aqueous" medium only piperidine or pyridine were used as templates and ethylene-glycol was an additional organic species. The medium was not completely water-free because of the water contained in the phosphoric and hydrofluoric acids. The alumino- and gallophosphate phases obtained at 170°C, 11 days and with a starting composition similar to that used by Huo and Xu [9], i. e., 4.3 R: lM2O3 : lP2O5 : 1.7HF : 44EG (R = organic template, M = Al, Ga; EG = ethylene-glycol)are given in table 3. Table 3. Alumino- and gallophosphates obtained in "non-aqueous" medium in the presence of HF ~~

Template Piperidine Pyridine

ALP04 Tricl.CHA Tricl.CHA

Gap04 Cloverite GaP04LTA

In the system A1203-P205, only the triclinic chabazite fluoroaluminophosphate was produced. For the system Ga203-P205 the synthesis of cloverite with piperidine is confirmed. Surprisingly, with pyridine, LTA-type GaPO4 was obtained. 19F MAS NMR shows that for the three types of materials fluorine is in the same environment as in the corresponding ones synthesized in aqueous medium, indeed, the observed chemical shift values are very close for similar structures. In aqueous medium the LTA structure type was produced with di-n-propylamine and HF [l]. In this solid, the N atom of the di-n-propylammonium cation was found in the 8membered ring of the a cage, each propyl group pointing towards the center of two adjacent a cages. Seeing the differences between di-n-propylamine and pyridine, it will be of interest to determine the location of the latter and also whether ethylene-glycol is occluded in the structure. CONCLUSION It appears that a large variety of materials with different fluorine locations can be obtained by using the fluoride synthesis route in aqueous as well as in "non-aqueous" media. F is occluded in double-4-rings only in the LTA- and -CLO type gallophosphates, whereas in the HMDA-GaP04 it is moreover connecting two Ga atoms of GaX5 and GaX6 polyhedra (X = 0, OH, F). In the triclinic CHA-like materials two F atoms are bridging two out of six A1 or Ga atoms ; this results in a triclinic distortion of the chabazite structure which is removed on calcination. ACKNOWLEDGEMENTS The authors are grateful to Drs J. Baron and L. Delmotte for assistance in powder X-ray diffraction and NMR spectroscopy, J. Patarin for Rietveld refinement of AIP04-16 and M. Soulard for assistance in thermal analysis and for performing the high-temperature X-ray diffraction studies REFERENCES

1 A. Merrouche, J. Patarin, M. Soulard, H. Kessler, D. Anglerot, in M.L. Occelli and H. Robson (Eds.). Synthesis of Microporous Materials : Molecular Sieves, Van Nostrand-Reinhold,New York, 1992, p. 384.

10

C. Schott-Darie, H . Kessler and E. Benazzi

2 A. Merrouche, J. Patarin, H. Kessler, M. Soulard, L. Delmotte, J.L. Guth and J.F. Joly, Zeolites, 12 (1992) 226. 3 S.T. Wilson, N.A. Woodward, E.M. Flanigen and H.G. Eggen, Eur. Pat. Appl., (1987) 226 219. 4 J.B. Parise, J. Chem. Soc.,Chem. Comm. (1985) 606. 5 G. Yang, S. Feng and R. Xu, J. Chem. Soc.,Chem. Comm. (1987) 1254. 6 H. Kessler, in R.L. Bedard et al. ( a s . ) , Synthesis, Characterization and Novel Applications of Molecular Sieve Materials, Materials Research Society, Pittsburgh, 1991, p. 47. 7 T. Loiseau and G.FCrey, Eur, J. Solid. State Inorg. Chem., 30 (1993) 369. 8 T. Loiseau and G. FCrey, J. Chem. Soc.,Chem. Comm., (1992) 1197. 9 Q. Huo and R. Xu,J. Chem. SOC., Chem. Comm., (1992) 1391. 10 J.M. Bennett and R.M. Kirchner, Zeolites, 11 (1991) 502. 11 P. Caullet, J.L. Guth, J. H a m , J.M. Lamblin and H. Gies, Eur. 1. Solid. State Inorg. Chem., 28 (1991) 345. 12 E. Jahn, private communication. 13 D.A. Lesch, R.L. Patton and N.A. Woodward, Eur. Pat. Appl., (1988) 293939. 14 E. Jahn, P. Daniels and H. Gies, 5th German Zeolite Workshop, Leipzig, March 14-16, 1993. 15 E. Benazzi, C. Schott-DaAe, H. Kessler and J. F. Joly, French Patent Application 93/01386, to IFP 16 M. Estermann, L.B. McCusker, C. Baerlocher, A. Merrouche and H. Kessler, Nature, 352 (1991) 320. 17 A. Simmen, J. Patarin and C. Baerlocher in R. von Ballmoos et al. (Eds.), Proc. 9th Int. Zeolite Conf., Montreal, July 5-10, 1992, Butterworth-Heinemann,Stoneham, 1993, p.433. 18 G.FCrey, private communication.

Preparation, Characterisation, and Catalytic Properties of Microporous Zirconophosphate Molecularly Engineered Layered Structures (MEIS*)

E. G. Derouane and V. Jullien-Lardot Facultbs Universitaires N.-D. de la Paix, Laboratoire de Catalyse Rue de Bruxelles, 61,B-5000 Namur. Belgium

ABSTRACT Microporous and mesoporous MELS@were prepared by partial pillaring of aZr(HP04),with n-alkyl (n, = 6-12)diphosphonic acids. The MELS@porosity and acidic properties were evaluated by various techniques: X-ray diffraction, N, physisorption, 31P MAS NMR, alkylamine adsorption, etc.. . The use of methanol in the synthesis promotes the generation of porosity by partial esterification of POH groups which are not reacted with the diphosphonic acid. INTRODUCTION MELS@,Molecularly Engineered Layered Structures, are pillared layered materials [1,2]which are usually obtained by reaction of layered zirconium phosphates, particularly a - Z r (HP04)2. H2O (ZrP), with diphosphonic or diphosphoric acids. Preferred pillars are either alkyl or phenyl derivatives as the anchoring site has a cross-section of about 25 A2. Problems encountered when pillaring ZrP with diphosphonic acids are: 1. the identification of key parameters leading to crystalline materials; 2. the characterisation of structural modifications occuring when pillar density decreases, which is necessary t o obtain microporosity; 3. the evaluation of the MELS@products acidic and catalytic properties. These questions were addressed for MELS@materials pillared by n-alkyl (nc = 6-12)diphosphonic acids which allow a fine tuning of the interlayer spacing. As nalkyl pillars have a cross-section of 15 812, fully pillared n-alkyl MELSB have no useful microporosity. Partial pillaring is achieved by controlled pillaring of ZrP, either by direct synthesis of mixed diphosphonic and diphosphoric acid pillared MELSB followed by hydrolysis of the diphosphonate pillars, o r by direct synthesis in - diphosphonic acid mixtures. The last route was used in the present work.

I2

E. G . Derouane and V. Jullien-Lardot

EXPERIMENTAL MELS" were prepared from ZrOC12.8H20 and mixtures of n-alkyl diphosphonic acids (DPA; n,= 6-12) and phosphoric acid (PA) (PADPA molar ratio = 0-10) dissolved in methanol. Slight excess of phosphorus species (Ptot/Zr 22) facilitates crystallisation. The obtained suspension was autoclaved for 3-175 h. When used, HF as mineralising agent was present in a molar ratio HFIZr 5 6 to maintain product yield. After hydrothermal treatment, the mixture was quenched t o room temperature, filtered and the remaining solid washed with acetone and water before drying a t 373 K. Samples are identified as MELS-n, (n, = number of carbon atoms in the alkyl chain). X-ray diffractograms were obtained using a Philips PW 1349130 or a Scintag diffractometer, both with Ni-filtered CuK, radiation. 31P MAS NMR spectra were acquired on a Bruker MSL-400 spectrometer operating a t 161.977 MHz. Spinning at 10 kHz provided high resolution spectra with low intensity spinning side-bands. All chemical shift values were referenced against liquid phosphoric acid (85 %I, but using as external reference NH4H2P04 (-3.24 ppm). 13C MAS-CP NMR spectra were acquired a t 100.613 MHz with proton decoupling. A Stanton-Redcroft ST-780 thermoanalyser was used for TG/DTA analyses using 15 to 30 mg of solid in flowing dry air (10 mumin; 10 Wmin) from 293 t o 1073 K, t o enables the determination of the solid composition in term of Pt,t/Zr. nButylamine titration was adapted from Clearfield 131. Quantification of the amine desorption was achieved by titration with a solution of NH2S03H a t constant pH, of the basic molecules desorbed during thermal treatment of the samples (SETARAM TG-DSC 111, 10 Wmin, carrier gas = He). Nitrogen BET surface areas and porosity measurements were obtained on dehydrated samples (evacuated a t 413 K for 15 h) using the Harkins-Jura and Barret-Joyner-Halenda formalisms. RESULTS AND DISCUSSION Svnthesis Fully pillared MELS@were synthesized to optimise the synthesis parameters, with and without addition of HF as mineralising agent. The evolution of product crystallinity as a function of the synthesis conditions was followed by XRD and 31P MAS NMR. The latter technique is necessary to identify the MELSBphase in materials appearing amorphous to XRD. The MELSBphase is characterised by a narrow line @ lppm (PCH2 groups of the diphosphonate pillars bound to ZrP) and diphosphonic acid groups by a line @ 25-30 ppm. The change in chemical shift is due to the sharing of three oxygens with Zr in the diphosphonate compared to only two bound to H for the diacid.

Zirconophosphate Molecularly Engineered Layered Structures

1

I3

POH

FWHM(")

PCHz = 0.10

0.7

7

I 1 1.4

13.25 A

= 0.77

Fig. 1. Powder XRD patterns of partially pillared MELSB-8 (HF) materials as a function of the POWPCH2 ratio. Crystallisation is favoured when chain length, time, and temperature increase. The addition of HF also increases the rate of crystallisation, as expected. When diluting diphosphonate pillars by POH groups in mixed MELSB, phase segregation, i.e., clustering of the POH and diphosphonate entities, must be avoided. In the following discussion, mixed MELSB compositions are described in terms of POWPCH2 ratio (POH corresponds to the incorporation of PA whereas two PCH2 groups are obtained by reaction of one diphosphonic acid molecule). If crystallisation is achieved in the presence of both DPA and PA, preferential incorporation of DPA is always observed. In spite of the addition of HF, the crystallinity of partially pillared MELS@never reaches that of fully pillared MELS. The crystallinity of partially pillared MELSB decreases with pillaring density. Interlayer spacing also decreases. Figure 1 shows the XRD spectra of MELSB-8 materials synthesized in HF medium for various values of the POWPCH2 ratio. As the latter increases, the interlayer spacing corresponding t o the small angle reflection decreases and line broadening is also observed. These observations indicate increasing disorder (line broadening cannot be accounted for by a decrease in particle size) and relaxation of the layered structure. These results suggest that pillars may not be distributed homogeneously. Further reduction of the amount of pillars (POWPCH2 ratio > 1) alters the XRD pattern as shown in Fig. 2. An additional diffraction peak appears a t small angle value ( d u l = 20 A),a t values of POWPCH2 equal t o 1.6-2.7 and 3.4-4.6 for n-dodecyl and n-octyl pillars, respectively. This phenomenon is thus favored by longer n-alkyl chains leading to larger interlayer spacing.

14

E. G. Derouane and V . Jullien-Lardot

POH PCH2

7.7

4.6

3.4 1.7 1.4

15

10

5 f

20

Fig. 2. Evolution of the powder XRD pattern of MELS@-8(HF) as a function of the POWPCHz ratio. It has been suggested that increasing amount of POH groups favors segregation of the POH and pillaring species; this has been demonstrated for mixed non-pillared phosphonate MELS@ (31. The alternation of POH and pillared layers could result in a reflection at smaller angle, but the characteristic spacing of ZrP should be preserved, which is not the case (Fig. 3, Model A). Another possibility is the existence of microdomains containing exclusively phosphate groups and pillars linked by one end only. However, 31P NMR analysis does not reveal the presence of free phosphonic acid (Fig. 3, Model B). Consequently, we believe that Model C


15

MODEL A P OH

P OH

OH

P

OH OH

P

OH OH

P OH

OH

P OH OH

P

OH OH

OH

d3

MODEL B

MODEL C

Fig. 3. Possible structural models explaining larger interlayer spacing. (Fig. 3) is most probable. The new reflection would thus arise from the propagation of stacking defects, also evidenced by the broadening of the XRD peaks. When the

16

E. G . Derouane and V . Jullien-Lardot

POH/PCHz ratio exceeds 2.7 for MELSB-12 and 4.6 for MELSB-8, a narrow diffraction line appears a t a d-spacing of 8.6 h;. This peak evidences cocrystallization and segregation of a non-pillared and a partially pillared phases. The appearance of the non-pillared phase is preceded or accompanied by the observation of a 1% N M R resonance a t 54 ppm indicative of the presence of POCH3 groups. Considering that the spacing is intermediate between the interlayer distances of ZrP (7.56h;) and zirconium methylphosphate (9.9 A), the non-pillared phase is believed t o be ZrP partially esterified by the methanol solvent . N2 isothermal adsorption was used to investigate the porosity of the partially pillared MELSB as a function of the POWPCH2 ratio. Partially pillared MELSB are characterised by a type I1 isotherm when they are rich in pillars. As the amount of pillars is decreased (POWPCH2 2 0.3), a transition is observed to a type IV isotherm and an adsorption-desorption hysteresis also appears (Fig. 4). Although micropores are evidenced by the isotherm shape, t-plots are not suitable for their evaluation because of the low affinity of MELSB materials for N2. Table 1 summarises the important characteristics of MELS@-8materials prepared with and without HF.

30 -

20 -

J

MELS 8 (HF) POHPCH2 = 0.09 I'

a

10 -

--f >

-a,n;

I

I

I

I

-

I

I

I

I

I

I

I

I

MELS 8 (NM) POWPCH2 =1.1 . ; b d

100 -

a

-

0

a

n

0

n

50-a@a

b

-

-odd I

fin8

0

0

I

0

8

m I

150 -

0

8

0 0

I

MELS 8 (HF) POWPCH2 120 =0.75 0..

80 40 -

.pd a

-

,

I

MELS 8 (HF) POHPCHz = 0.29

80 40 -

a

fi

8

120

I

I

I

I

I

I

I

1

1

1

1

1

l

1

1

l


Sample

POWPCH2 ratio

V m a o (%)

SA (m2.g-1)

D (A)

MELS'W MELS@-8 MELS@-8(HF) MELSB-8 (HF)

0.45 1.1 0.29 0.75

82 86 69 90

314 213 236 132

37.1 37.1 38.5 38.8

17

The mean mesopore diameter stays constant a t about 38 A. The specific surface area increases up to a POHPCH2 ratio of 0.3, and then decreases as disorder increases, segregation of the pillars and of the phosphate groups occurs, and eventually, the structure progressively collapses. A less conventional way to evaluate the accessible interlayer free volume is the sorption of amines of various sizes (n-butyl, di-n-butyl, and tri-n-butyl). The sorption data are summarised in Table 2. The accessibility of the interlayer volume is proven by the increase in interlayer spacing, from @ 13.8 A for MELSB-8 up t o 15.5 A following sorption. Amine sorption is, however, not reliable to quantify the number of acidic sites. Indeed, the number of amine molecules adsorbed per acidic site (n amindn POH) decreases as the amine gets bulkier, because of the proximity of the POH groups. The number of amine molecules adsorbed per POH group is also observed to increase when the POHPCH2 ratio decreases, because of intermolecular interactions between amine molecules and the possibility for more than one n-butylamine molecule to neutralise one POH group. Table 2. Sorption of amines of various sizes in MELSB-8.

POWPCH~ratio 1.02

0.48 0.12

I

Amine n-butylamine di-n-butylamine tri-n-butyl-amine n-butylamine di-n-butylamine n-butylamine di-n-butylamine

I

n Amine/n POH 0.99 0.80 0.48

2.01 0.97 3.03 1.31

1

d

(A)

15.50 15.24 14.49 14.84 14.59

1

Additional evidence for the accessibility of the interlayer volume t o the amine molecules is also obtained from 31P MAS NMR measurements. Upon amine

18

E. G. Derouane and V. Jullien-Lard01

impregnation, the line @ lppm (PCH2) is not affected whereas the resonance @ -25 ppm (POH) is shifted to lower field as expected when deprotonation occurs [4]. The chemical shift variation is less important for bulkier amines (-1.8 ppm for tri-nbutyl vs. -3.3 ppm for n-butyl), proving that the deprotonation is less efficient. This result is consistent with the data shown in Table 2, i.e., that one tri-n-butylamine molecule only interacts with two POH sites compared to a 1:l amine/POH ratio for n-butylamine. Nevertheless, the resonance at 1 ppm (PCH2) becomes narrower upon amine sorption. Thus, all pillars are affected, which is a further proof of limited pillar segregation (evidenced as stacking defects by XRD). The acid catalytic activity of the partially pillared MELS@-8 samples was evaluated by measuring the rate of t-butylacetate decomposition a t 371 K. For various MELS@-8samples with different POWPCH2 ratio, turnover frequencies are nearly constant @ 10-2 sec-1 (k 20%), indicating that all POH groups are accessible. CONCLUSION Porous n-alkyl (nc = 6-12) diphosphonate MELS@ can be obtained by direct crystallisation in the presence of diphosphonic-phosphoric acid mixtures, using methanol as solvent. Preferential incorporation of DPA is always observed. Increasing the POWPCH2 ratio leads firstly to microporosity, and secondly to some additional mesoporosity as stacking defects appear, because of pillar segregation. At very high POWPCH2 ratio, cocrystallisation of a zirconium methylphosphate and of a partially pillared MELSB is observed. The partially pillared MELSB have acid POH sites which possess catalytic activity. ACKNOWLEDGEMENT The authors thank Catalytica Inc. for generous support of this research. They also acknowledge constructive discussions with Mrs. S. Bloch, Drs. R.L. Garten, D.L. King, C.S. Schramm, M.D. Cooper, W.A. Sanderson, S. Justi, and J.D. Fellmann.. REFERENCES 1 K. Segawa, A. Sugiyama, and Y . Kurusu, Stud. Surf. Sci. Catal., 60 (1991)73. 2 D.L. King, M.D. Cooper, W.A. Sanderson, C.M. Schramm, and J.D. Fellmann, Stud. Surf.Sci. Catal., 63 (19911247. 3 A. Clearfield and R.M. Twinda, J.Inorg.Nucl.Chem., 41 (1979)871. 4 D.J. Mac Lachlan, K.R. Morgan, J.Phys.Chem., 94 (1990)7656.

Synthesis of Manganese Oxide Octahedral Molecular Sieves (OMS)

Yan-Fei Shen’, Roberto N. DeGuzman’, Richard P. Zerger’, Steven L. Suib’, and ChiLin @Young2

’ U-60, Department of Chemistry, University of Connecticut, Storrs, CT 06269 USA * Texaco Inc., PO Box 509, Beacon, NY, 12508 USA ABSTRACT A series of manganese oxide octahedral molecular sieves (OMS) with different tunnel sizes, ranging from 2.3 to 6.9 A, have been synthesized. The materials were prepared by redox reactions between Mn2+and MnO, or other oxidants under different conditions. Two synthetic mechanisms are proposed based on the intermediates. The first mechanism involves amorphous materials and the second one involves layered materials as the intermediates. Like the synthesis of zeolites, template, temperature, and pH are important parameters that control the tunnel structures of OMS. INTRODUCTION Octahedral molecular sieves (OMS) represent another family of molecular sieves; they use octahedra as the basic structural unit to form three-dimensional framework structures. There are naturally occurring manganese oxides with mono-directional tunnel structures, e.g. todorokite (OMS-1) has (3x3) tunnels, hollandite (OMS-2) has (2x2) tunnels, and pyrolusite has (1x1) tunnels (Figure 1). The pore sizes of OMS-1 and OMS-2 are 6.9 and 4.6

respectively; most organic molecules can be adsorbed. The materials have

cation-exchange properties; most metal ions can exchange and occupy tunnel sites. Because of these unique properties, the materials could have various applications for adsorption, electrochemical sensors, and catalysis. Pyrolusite has a pore size of 2.3 8, and no ionexchange properties. The materials can be synthesized by redox reactions between Mn” and MnO,‘ or other oxidants under different conditions. Interestingly, there are some similarities between the syntheses of OMS and zeolites. Like the synthesis of zeolites, template, pH, and temperature are important parameters that determine the structures of OMS.

19

20

Y.-F. Shen, R. N. DeGuzman, R. P. Zerger, and S. L. Suib, and C.-L. 0' Young

Fig. 1. Tunnel structure of manganese oxide octahedral molecular sieves: (A) pyrolusite (lXl), (B) OMS-2 (2X2), (C) OMS-1 (3x3). EXPERIMENTAL A. Synthesis of OMS-2. pyrolusite. and nsutite

OMS-2 was prepared by two methods, referred to as Method 1 and Method 2. Method 1 involved the redox reactions between permanganate ion (MnO,) and manganous ion (Mn2') at low pH's 5 3. Hydrothermal conditions could be used to improve the product crystallinity. In the presence of counter cations, such as K', Cs', and BaZ+,OMS-2 was formed at temperatures between 80 to 140°C.Pyrolusite was formed at temperatures higher than 160°C. At lower temperatures < 80°C or in the presence of smaller counter cations, such as Na+, Ca2+,and Mg2+,nsutite was formed. The size and concentration of counter cation, pH, and temperature were important parameters [l]. Without hydrothermal conditions, OMS-2 also could be prepared by refluxing an acidic solution of KMnO, and Mn2+at 100°C for 24 hours. The same effects of pH, temperature, and counter cation were observed. Method 2 involved the formation of layered K-buserite at high pH's, followed by calcination at high temperatures [2,3]. A typical preparation is as follows: a solution of 35 g KOH in cold 200 mL water was added to a solution of 30 g MnSO,.H,O in 200 mL water. Oxygen was bubbled vigorously through the solution for 4 hours. The black K-buserite product was washed with water, and placed in a furnace at 600°C for 18-24 hours. Yield was about 17.0 g.

B. Svnthesis of OMS-1 OMS-1 was prepared by reaction of birnessite, a layered manganese oxide, in a Mg2' form that was autoclaved at 155-170°C for 10-40 hours. The synthetic birnessite was prepared by redox reactions of Mn2' and MnO, at high pH's. The nature and thermal

Manganese Oxide Octahedral Molecular Sieves

21

stability of OMS-1 products depend strongly on preparation parameters, such as the MnO, /Mn2' ratio, pH, the aging time of birnessite precursors at room temperature, and time and temperature of autoclave treatment of birnessite. Details of the synthesis have been published elsewhere [4,5]. C. Characterization Methods The XRD powder patterns of OMS-2, pyrolusite, and nsutite are distinct enough to identify their tunnel structures. The XRD powder patterns of OMS-1 are similar to those

of layered precursors, birnessite and buserite, they all have a d-spacing around 10 A. TEM and adsorption of OMS-1 were studied to further confirm the (3x3) structure. TGA, BET, and ESR were used to study the thermal stability of OMS. RESULTS AND DISCUSSION A. Classification of Molecular Sieves (MS)

Based on the basic structural unit, molecular sieves can be classified as tetrahedral molecular sieves (TMS), octahedral molecular sieves, and mixed molecular sieves (Figure 2). Most common molecular sieves, such as zeolites, AIPO,, SAPO, MeAPO, and MeAPSO are tetrahedron based MS. Todorokite, hollandite, and pyrolusite are octahedron based MS. Titanosilicates (ETS-4 and ETS-10) and phosphomolybdates are mixed MS, which have tetrahedra and octahedra to form the frameworks. The pore sizes of some representative TMS and OMS together with the kinetic diameters of some molecules are shown in Figure 3. OMS-2 has a pore size between NaA and ZSM-5 zeolites; OMS-1 has a pore size between ZSM-5 and NaX zeolites. The tunnels of OMS-1 are big enough to adsorb most hydrocarbon molecules; OMS-2 can only adsorb straight chain but not branched chain hydrocarbon molecules. A hypothetical OMS (4x4) structure has a pore size of 9.2

4

between A1P04-8 and VPI-5. The tunnels of pyrolusite are too small to adsorb any hydrocarbon molecules.

B. Synthesis of OMS OMS-1 and OMS-2 have similar structures; however, they are synthesized by different procedures and conditions. OMS-2 is prepared by autoclaving or refluxing a solution of

MnOd and Mn2+at pH's 5 3,80-140"C, and in the presence of enough counter cations with an ionic diameter between 2.3 and 4.6

A. Nsutite structure is formed with counter cations

smaller than 2.3 A. The counter cations act as templates. However, no OMS-1 or bigger

22

Y.-F. Shen, R. N. DeCuzrnan, R. P. Zerger, and S. L. Suib, and C.-L. 0' Young

LJ Molecular Sleves

Sa rd

t I-

A I S M , ZbolKM

Alumlnoclllcstw B, Ga-, n-,Fa, cr4w MetellaslllcstM

MS

t

AI-PO,AIPo4

Me-AI-P-O, Mew0

Me-AI-PSI-O, MeAPSO

Figure 2. Classification of molecular sieves.

Molecular Dlameter, A: CO 3.8, n-Paraffins 4.3, I-Butane 5.0, Neopentane 6.2, Benzene 5.8, o-Xylene 6.3

Figure 3. Pore size of molecular sieves. (n) and (nxn) represent the number of tetrahedron and octahedron in the pores, respectively.

Manganese Oxide Octahedral Molecular Sieves

tunncl

structures

are

formed

when

larger

organic

atnine

cations,

such

23

as

tetraalkylammonuiin cations, are used as templates. The presence of organic amine might disturb the redox reactions between MnZCand MnO,-. The synthetic O M S 2 is thermally stable up to 600°C based on the results of XRD,TGA, and BET. Pyrolusite is formed at higher temperatures > 120”C, and nsutite is produced at lower temperatures < 80°C. At high pN’s > 12, birnessite and buserite are produced from the reactions of MnZt

and MnO,. OMS-2 can be formed by calcining K-buserite at high temperatures > 600°C. The thermal stability of OMS-2 prepared by this method is higher, up to 800°C; the average oxidation state of Mn is lower. OMS-1 also uses the layered materials as the precursor. T h e precursor of OMS-1, Mg-birnessite, is best synthesized at pH 2 13, MnZt/MnO,‘ ratio of 0.3 to 0.4, and aging at room temperature for 1 week. The purest and the most thermally stable synthetic OMS-1 is then obtained by autoclaving such precursors at 1SO-180°C for more than two days. XRD, TGA, ?’EM, BET, and adsorption results show the synthetic OMS-1 is thermally stable up

to 500°C [4,5]. Two mechanisms are proposed for the synthesis of manganese oxide OMS (Scheme

I). Both start from the redox reactions between MnZ+and MnO, or other oxidants. The first mechanism involves amorphous manganese oxides at lower pH’s as the intermediates, Different manganese oxide OMS, such as OMS-2, pyrolusite, and nsutite, are then formed by controlling temperature and counter cation. The second mechanism involves layered manganese oxides, birnessite and buserite, at high pH’s as the intermediates. OMS-1 and OMS-2 are then formed by controlling temperature and counter ion. Scheme 1. Two Proposed Synthetic Mechanisms of OMS Mechanism I Mn(2+) Solution

+

1-

Mn04(-) Solution or Other Oxidsnti

1

Low pH

’

Amorphous Mmterlair

Mechanism I1 HipH

,

Layered MItedmls

T. T h e Template

’

OMS-2. Pyroluslte (1x1). & NIUtltO

T. Tlme

OMS-1 & OMS-2

Template

It is interesting that there are many similarities between the syntheses of OMS and

zeolites. The effects of template, pH, and temperature have been observed and are wellknown in zeolite synthesis. The conversion of layered silicates to mesoporous aluminosilicates also has been reported recently [6].

24

Y.-F. Shen, R. N. DeGuzman, R. P. Zerger, and S. L. Suib, and C.-L. 0 Young

ACKNOWLEDGEMENTS We acknowledge Texaco Inc. and Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences for support of this research. REFERENCES 1. C.-L. O’Young, in M. L. Occelli and H. E. Robson (Ed), Synthesis of Microporous Materials, Vol. 11, Van Nostrand Reinhold, NY, 1992, p.333. 2. R. N. DeGuzman, Y.-F. Shen, E. J. Neth, S . L. Suib, C.-L. O’Young, S . Levine, and J. M. Newsarn, to be published. 3. R. Giovanili and B. Balmer, Chimia, 35 (1981) 53. 4. Y.-F. Shen, R. P. Zeger, S . L. Suib, L. McCurdy, D. I. Potter, and C.-L. O’Young, J. Cliem. SOC. Cfiem. Comm.,(1992) 1213. 5. Y.-F. Shen, R. P. Zeger, R. N. DeGuzman, S. L. Suib, L. McCurdy, D. I. Potter, and C.L. OYoung, Science, 260 (1993) 5 11. 6. S . Inagaki, Y. Fukushima, A. Okada, T. Kurauchi, K. Kuroda, C. Kato, Proceedings from the 9th IZC, edited by R. von Ballrnoos, J. B. Higgins, and M. M.J. Treacy, Vol I, (1992) 305.

Preparation and Properties of the Pyridine Intercalates of Bismuth Molybdic Acid

Yasushi Murakamil, Fujito Yamaguchi, Osamu Ishiyama and Hisao Imai Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan ABSTRACT

The pyridine intercalate of bismuth molybdic acid was prepared from the reaction between dehydrated bismuth molybdic acid and gaseous pyridine. The extent of the pyridine intercalation was 2/3 and the bilayer of pyridinium ion was formed between the oxide sheets. The intercalation reaction with 3- or 4-methylpyridine proceeded similarly to that with pyridine. The pyridine intercalate had the space for the adsorbtion of water while no water was adsorbed on the 4-methylpyridine intercalate because of methyl group hinderance. The intercalate with a monolayer of 4-methylpyridinium ion was obtained by heating the bilayer intercalate at 250". INTRODUCTION

Layered transition metal oxides have been extensively studied due to their potential use in catalysis and superconductivity. We have developed a preparation method for a new type of heteropolyacid, bismuth molybdic acid (HBM) [ 11, and the intercalation reaction between bismuth molybdic acid and pyridine [2]. Recently we determined the layer structure of HBM represented by the formula BiMo,O$OH),, which is monoclinic, P2,/m, with a = 6.341 A, b = 11.596 c = 5.790 B= 113.31') z = 2 (31. A two-dimensional sheet consists of the bismuth-centered oxygen pentagon and the molybdenum-centered oxygen quadrangle. Bismuth and molybdenum in the two-dimensional oxide sheets are cross-linked by hydroxyl groups Fig. 1 Portion of the structure of with hydrogen bonds. The layer is parallel to the HBM perpendicular to (lor). (101) plane and the basal spacing d for HBM is 5.0 O:Bi, o:Mo, and o:O, OH

A,

A,

*To whom correspondence should be addressed. *Present address: Department of Fine Materials Engineering, Shinshu University, 3-15-1 Tokida, Ueda 386, Japan. 25

26

Y. Murakami, F. Yamaguchi. 0. Ishiyama and H . lmai

A (Fig. 1). In this study the pyridine intercalate (HBMPy), the 3-methylpyridine intercalate (HBM3MP) and the 4-methylpyridine intercalate (HBM4MP) of HBM are prepared, and their properties of reactivity and structures are discussed on the basis of chemical compositions, guest species and basal spacings.

i'I

to vacuum

EXPERIMENTAL

HBM was prepared from a mixture of the nitric acid solutions of bismuth nitrate and sodium molybdate [1,2]. The intercalation reaction was HBM pyridine or carried out in a closed H-shaped reactor (Fig. 2). methylpyridine HBM dried at 150°C was loaded in one side of the reactor bottom and pyridine or methylpyridine in the Fig. 2 Reactor for intercalation. other side. After degassing with freeze of pyridine, the reactor was sealed. HBM was treated only with gaseous pyridine or methylpyridine by heating the whole H-shaped reactor at 16O0C for a few days. The sample treated was airdried at room temperature. Transmission electron micrographs were obtained using a JEOL EM-200EX microscope. Elemental analyses were performed using a Yanako MT-2 organomicroanalyzer. Infrared spectra were recorded using KBr disks on a JASCO mAR-3 Fourier transform infrared spectrometer. Thermogravimetric analyses were performed using a Shinkuriko TGD7000 thermal analyzer at 1O0C/min. X-ray powder patterns were obtained with CuKa radiation using a Rigaku RU-200 diffractometer. RESULTS

Crystal morphology Figure 3(a) shows a transmission electron micrograph of a synthetic HBM. The single prismatic crystal observed is about 0.5-0.7 P m wide and 2 Pm long. On the other hand, the aggregation of needle-like crystallites was observed in HBMPy as shown in Fig. 3(b). The needle-like crystal is about 0.05 m thick and 0.5 P m long. These micrographs indicate that intercalation causes the crystal of HBM to disintegrate when HBM was treated with pyridine, with a resulting reduction in particle size. Chemical Analyses Chemical analyses of HBM and its intercalates are given in Table 1. The extent of the intercalation x was calculated from the percentage of C, N and H in the sample. The experimental data agreed with the calculated data. The extent of the pyridine intercalation of

Pyridine Intercalates of Bismuth Molybdic Acid

27

Fig. 3 Transmission electron micrographs of (a)HBM and (b)HBMPy. Table 1. Elemental analyses of intercalates.

*(%I

obs.

cal.

obs.

cal.

H(%) obs. cal.

C(%)

X

n

HBMPy

0.71

7.22

7.22

1.69

1.69

0.85

0.84

0.36

HBM3MP

0.62

7.38

7.51

1.46

1.46

0.97

0.98

0.24

HBM4MP

0.68

7.91

8.25

1.60

1.61

0.93

0.97

0.00

HBM4MPm

0.55

6.93

6.84

1.33

1.33

0.64

0.84

0.00

formula: (HGu)xH,-xBiMo20,

nH20

HBM was determined to be 0.71, approximately equal to 2/3. Similarly, the extent of the intercalation of HBM nearly equaled 213 for 3- and 4-methylpyridine whereas no reaction proceeded between HBM and 2-methylpyridine. The extent of the 4-methylpyridine intercalation of HBM decreases by heating HBM4MP at 250'c for 4 hours, to 0.55, approximately equal to 1/2 as the intermediary intercalate HBM4MPm. The extent of the water content n of HBMPy was determined by thermogravimetric analysis to be 0.36, approximately equal to 113. The water content corresponded to the number of protons uncoordinated by pyridine. The fact that HBMPy readsorbed water molecules after dried at 160'c and cooled in air suggests the reversible process of the adsorption-desorption of water. On the other hand, HBM4MP contained no water molecule. The extent of the water content of HBM3MP was less than that of HBMPy. Infrared spectra infrared spectra of intercalates are shown in Fig. 4. The pyridine bands observed in HBMPy were characteristic of a pyridinium ion. No band attributed to other pyridine species

28

Y . Murakami, F. Yamaguchi, 0. lshiyama and H . lmai

0

T nfr9-J lllLlLuW..

,,

.=n,=.rtra nf fa\URMP.r UyW'L'u "I \U,"Y'."

A: absorption

500

Fig. 5 Thermogravimetric analyses of (a)HBMPy, (b)HBM3MP and (c)HBM4MP.

1700 1400 Wave number / cm-' T;;" I'6. A 7

1 0 0 200 300 400 Temperature / "C

/h\URM?MP \","YL.'JL"'

, /r\URMAMP \',"U""""

9"A

. . 1 -

/A\URMAMPm 111. \..,'LY"'T'."

attributed to pyridinium ion or methylpyridinium ion.

was observed in the infrared spectra. For methylpyridine intercalates, the bands were attributed to a methylpyridinium ion. The bands due to 4-methylpyridinium ion were not shifted by the heat treatment at 250'c for 4 hours. These spectra suggest that both pyridine and methylpyridine coordinate to protons located between the bismuth molybdenum oxide sheets of the intercalates of HBM.

Thermogravimetric changes of intercalates are shown in Fig. 5. The decrease in weight attributed to dehydration was observed below 120'c for HBMPy. However, the system for the intercalation was excluded from water. Therefore, no water should be contained in HBMPy before and during the intercalation. In order to elucidate the water content, the thermogravimetric analysis was performed after HBMPy was heated at 120'c for the dehydration and kept in air at room temperature. In spite of the pretreatment of HBMPy at 120'c, weight decrease was observed up to 120°C. Therefore, the weight decrease below 120'c was attributed to dehydration of the water adsorbed in air following the intercalation reaction. Further weight decrease was observed above 18O0C, attributed to the liberation of pyridine. The amount of the pyridine decrease agreed with that estimated by chemical


29

analysis. The weight was constant above 330"c, and a-Bi2Mo3OI2 and MOO:, phases were detected by X-ray diffraction analysis after a heat treatment at 500°C. The layer structure was broken with the liberation of pyridine. In contrast with HBMPy, no weight decrease due to the dehydration was observed below 120°C for HBM4MP. Thus no water was contained in HBM4MP. In addition, the weight of HBM4MP decreased by two steps. The amount of the total decrease corresponded to the content of the 4-methylpyridine determined by chemical analysis. Therefore, the liberation of 4-methylpyridine took place by two steps. The first liberation of 4-methylpyridine occurred at lower temperatures of 150-250°C than the liberation of pyridine from HBMPy, whereas the second liberation proceeded at higher temperatures of 330-400°C. Thus the intermediary intercalate HBM4MPm was more stable than HBMPy while the synthetic intercalate HBM4MP was less stable. After the decomposition of HBM4MPm, the layer structure was broken and a-Bi2Mo3OI2 and Moo3 phases were formed. The weight decreases were observed by three steps for HBM3MP. The distinction among these steps was ambiguous. The decrease below 120°C was attributed to the dehydration in a similar manner as for HBMPy. Weight decreases due to the liberation of 3-methylpyridine were observed both at 150-25O0C and at 250-330°C. However, an intermediary intercalate like HBM4MPm cannot be obtained by the heat treatment of HBM3MP at an appropriate temperature. Powder X-ray diffraction A number of diffraction peaks were observed in powder X-ray diffraction patterns for all intercalates. However, the diffraction peaks of the intercalates were considerably broad. Basal spacing, d, of each intercalate was determined from the diffraction peak at lowest angle and summarized in Table 2. Guest molecules expand the layer structure to increase the basal spacing. Significant expansion was observed by the intercalation of pyridine. The extent of the expansion is comparable to double the size of pyridinium ion. The presence of the methyl group at the 4-position of the pyridine ring increases the basal spacing. The basal spacing decreases by heating HBM4MP at 250°C.

Table 2. Basal spacing of intercalates. dfA ~

HBM

5.o

HBMPy

16.3

HBM3MP

16.4

HBM4MP

17.1

HBM4MPm

12.2

DISCUSSION Reactivity An intercalation compound of HBM with pyridine was previously obtained from the reaction of HBM in liquid pyridine [2]. Since HBM has hydroxy groups, water contaminated

30

Y. Murakami, F. Yamaguchi, 0. Ishiyama and H. Imai

the liquid pyridine during heating for the intercalation. Therefore, molybdate ion in HBM is liable to be dissolved in the basic aqueous solution of pyridine. In this experiment, HBM was pretreated at 150°C to be dehydrated, and reacted for intercalation in gaseous pyridine at 160'c with exclusion of moisture so that the effect of water on the reaction was negligible. Thus the intercalation is not explained by the replacement of proton located between the oxide sheets of HBM by pyridinium ion in liquid pyridine although IR results suggest the presence of the pyridinium ion. The lattice was expanded by the coordination of pyridine to proton between the oxide sheets of HBM. The reactivity of the intercalation depends on the coordination ability of a guest molecule. Both pyridine and methylpyridine sufficiently donate electrons to the bismuth molybdenum oxide layer. The reason for no intercalation of 2-methylpyridine is that methyl group at 2-position prevents the coordination of the nitrogen atom in the pyridine ring to the proton between the oxide sheets of HBM. Structure e Bilayer intercalates. The increase in basal spacing is 11.3 A with the pyridine intercalation of HBM. Since the size of pyridine is about 5.0 A long in the CN direction, the presence of a bilayer of pyridinium ions between oxide sheets is proposed for HBMPy. The specific layer area per BiMo207(OH) unit is calculated from the lattice parameters of HBM to be 39.1 i2. When the extent of the pyridine intercalation is 2/3, the specific layer The structure . for the bilayer of pyridine has been area per pyridine molecule is 58.7 i2 proposed in the system of the intercalates of MOO 4,5]. The specific layer area per MOO, 30 [ unit or pyridine molecule is estimated to be 14.66 A2 [4]. If a structural model for retaining the original layer of bismuth molybdenum oxide of HBM were correct, the specific layer area per pyridine molecule would be too large for HBMPy. We therefore conclude that the structure for the host layers has been changed from the single oxide sheet. We proposed a new host layer of HBMPy in which the oxide sheet of HBM is doubled. The specific area of the oxide double sheet is 29.4 i2 per pyridine molecule. The thickness of the oxide double sheet can be estimated to be about 7.0 A from the HBM structure and the thickness of the MOO, double layer (6.9 A). 0 The basal spacings of HBMPy and HBM4MP give a relative expansion of 0.8 A for replacement of -H with -CH,. This expansion is shorter than that with the intercalation of Moo3 (1.74 A) [5]. The difference in both the specific layer area and the expansion can be explained by the tilt of guest molecules. The CN axis of pyridine is perpendicular to the MOO, because of the direct coordination of pyridine to the molybdenum atom [4,5]. On the other hand, the pyridine species in HBMPy are pyridinium ions according to infrared spectra. When the N atom in pyridinium ion is positively charged with the coordination to the oxide sheet, the proton position deviates from the pyridine plane and the CN axis is tilted from the direction perpendicular to the oxide sheet. The structural relation between HBMPy and HBM4MP is displayed in Figs. 6(a) and (b). The difference of the water content between 0


31

Fig. 6 Proposed structures for (a)HBMPy, (b)HBM4MP and (c)HBM4MPm. HBMPy and HBM4MP is explained by these models. The methyl group at the 4-position of the pyridine ring fills the space between the pyridine rings for the adsorption site. The structure of HBM3MP is similar to that of HBMPy from the thermogravimetric data. The complexity of the thermal behavior of HBM3MP depends on the geometrical antisymmetry of 3-methylpyridine. The methyl group at the 3-position of the pyridine ring has little effect on the structure of intercalate of HBM. Monolayer intercalates. The liberation of methylpyridine from HBM4MP and HBM3MP took place by two steps. Especially for the 4-methylpyridine intercalates, thermogravimetric intermediates were stable and isolated as HBM4MPm. The increase in basal spacing from HBM to HBM4MPm is 5.2 A, which is smaller than the size of 4-methylpyridine (5.9 i). The CN axis of 4-methylpyridine is tilted in HBM4MPm as shown in Fig. 6(c). The bilayer of 4-methylpyridine between the oxide layer was rearranged into the monolayer of 4-methylpyridine. Stability of the monolayer intercalates depends on the pyridine-ring orientation and the interaction between guest molecules and the host layer. Structural investigations of the pyridine intercalates of TaS, and NbS, have presented the model of the standing pyridine-ring monolayer in which the CN axis is parallel to the dichalcogenide layers [6,7]. The monolayer model explains that the extent of the pyridine intercalation is 1/2 for the dichalcogenides. The extent of the intercalation for the monolayer intercalate HBM4MPm was in good agreement. For HBM4MPm, however, the pyridine-ring was protonated and the CN axis of the pyridinering was nearly perpendicular to the oxide layer. These differences are accounted for by the

32

Y . Murakami, F. Yamaguchi, 0. Ishiyama and H . lmai

electric charge density of the host layer. The transition metal disulfide layers which consist of two sulfide anion sheets with the metal atoms in the central plane are electrically neutral and stacked by van der Waals interactions. Since the sulfide anion was less ionic than the oxide anion, the N atom with a lone pair of electrons is apart from the layer and thus the CN axis is parallel to the layer. On the other hand, the bismuth molybdenum oxide layer is negatively charged and cations are located between the layers. The N atom in the pyridinering is positively charged with the coordination to proton. The CN axis is directed alternatively to either side of the oxide layers by the attractive forces between the N atom and the oxide layers. The stability of the monolayer intercalates depends on the arrangements of guest molecules. When a pyridinium ion is close to the neighboring ion with the alternative direction of the CN axis, the positively charged N atom of one is adjacent to the nonpolar pyridine-ring of the other. The monolayer intercalates of pyridine are unstable because of a mutual repulsion between the N atom and the pyridine-ring. For the monolayer intercalate of 4-methylpyridine, the neighborhood of the N atom of one is the methyl group of the other. The weak repulsion between the N atom and the methyl group results in the remarkable stability of HBM4MPm. References 1 Y. Murakami, N. Ishizawa and H. Imai, Powder Diffraction, 5 (1990) 227. 2 Y. Murakami and H. Imai, J. Mat. Res. Lett., 10 (1991) 107. 3 Y. Murakami, F. Yamaguchi, 0. Ishiyama, H. Imai, M. Yashima, M. Kakihana and M. Yoshimura, submitted for publication. 4 J. W. Johnson, A. J. Jacobson, S. M. Rich and J. F. Brody, J. Am. Chem. SOC.,103 (1981) 5246. 5 J. W. Johnson, A. J. Jacobson, S. M. Rich and J. F. Brody, Revue de Chimie minkrale, 19 (1982) 420. 6 A. J. Jacobson, in E. S. Whittingham and A. J. Jacobson (Eds.), Intercalation Chemistry, Academic Press, New York, 1982, p.229. 7 C. Riekel and C. 0. Fischer, J. Solid State Chem., 29 (1979) 181.

Synthesis of Titanium Pillared Clay Using Organic Medium

Sung-Jeng Jong, Jenn-Tsuen Lin and Soofin Cheng* Department of Chemistry, National Taiwan University, Taipei, Taiwan 107, R.O.C.

ABSTRACT A method for preparing Ti-pillared montmorillonite with narrow distributed micropore size and high surface area was developed. The pillaring agent was prepared by mixing a Tic&/ethanol solution with a solution of glycerin and water. Basal s acings of 21.3 A, 17.7 A and 17.4 A, corresponding to interlayer spaces of 10.8, 8.2 and 7.9 , were observed for the as-synthesized, 773 and 973 K calcined samples, respectively. These distances implied that Ti was incorporated into the interlayers in some form of polynuclear clusters. The 573-773 K calcined samples contained both micro- and meso-porous structures, with the former contributed ca. 88% of its total pore volume. The presence of glycerin in the preparation procedure was found to be essential in order to prepare Ti-pillared clay of high crystallinity and narrow distributed micropore size. Its role was proposed to slow down the hydrolysis of titanium ethoxide species so that titanium polyoxo clusters of more homogeneously distributed sizes could be formed during the pillaring process. The acidity and shape-selectivity of the resultant Ti-PILC were also examined.

1

INTRODUCTION Pillaring the layered compounds with bulky inorganic species is a well-known route to prepare microporous materials. The increasing number of studies in this field is stimulated by their potential applications in numerous fields, such as absorption, separation, conductivity and catalysis. Among the available layered compounds, smectite clays have received the most attention and different applications have been demonstrated by incorporating different pillar species. For instance, alumina- and zirconia-pillared clays have great potential in the area of acid catalytic reactions, such as cracking and alkylation [l-31, chromia-pillared clays have been applied as dehydrogenation catalysts [4], and iron-pillared clays have been used as a catalyst in FischerTropsch reaction [5]. Titania-pillared clay has received relatively less attention, although TiOz has shown several significant and distinctive properties as a catalyst or catalyst support 16-91. Moreover, titanium oxide is a typical photocatalyst and is responsible for a variety of organic reactions [lo]. It is noteworthy that the photocatalyticactivity of microcrystallineT i 4 pillared in montmorillonite was reported to be more active than the T i 4 powder in decomposition of 2-propanol and carboxylic acids (2-9 carbon chains) [ll]. In spite of its promising applications, only a few preparation methods for titanium pillared 33

34

S.-J. Jong, J.-T. Lin and S. Cheng

clays has been published so far. From these publications, it is known that the preparation conditions are critical, primarily due to the diversity of titanium species formed in aqueous solutions. In 1986, Sterte [12] reported for the first time on the preparation of Ti-pillared clay. A TiClJHCl solution was used as the pillaring agent. The basal spacing of the products heated at temperatures above 473 K determined to be about 28 A. Later, Bernier et al. [13] performed a detailed study on the experimental conditions with the same pillaring reagent and found that the synthesis conditions were critical with respect to the morphology and texture of the final product. The basal spacing of the uncalcined samples could be varied from 24.9 to 13.8 A. A different method was reported by Yamanaka et al. [14, 151, who used titanium oxide sol as the pillaring agent by hydrolysis of titanium tetraisopropyloxide and peptization with HC1. The resultant products had a basal spacing of ca. 27 A. The pore size was found to correlate with the size of the sol particles, which was in turn dependent on the peptization condition. The third method reported in the literature uses a trinuclear acetatcchlorohydroxo titanium(II1) complex as the titanium source [16]. The resultant compounds have a basal spacing around 22 A, but the specific surface area is only ca. 120 m2/g. Recently, we have developed a new method for preparing Tipillared montmorillonite with narrow micropore-sizedistribution by adding glycerin in the pillaring solution [17]. The effect of glycerin and variables in the preparation condition were discussed in this paper. EXPERIMENTAL METHODS A Wyoming Na+-Ca’+-montmorillonite(commercial designation, Volclay SPV 200) was obtained from the American Colloid Company. Impurity quartz was removed by conventional sedimentation techniques. The < 2pm fraction was used as starting material. The cation-exchange capacity of the montmorillonite was determined to be 83 meq/100g. The pillaring agent was prepared by first mixing TiCI, with twice amount of ethanol and stirring until homogeneous. Among various reaction conditions examined, four pillaring processes were found to result in pillared clays of higher thermal stability. Three of them (samples termed as Gl-G3) had glycerin added in the pillaring solutions, and the other (sample termed as B) without glycerin was served as blank for comparison. Their preparation procedures were described below. 2 mL of the partially hydrolyzed Ti-ethoxide solution was added to a 20 mL glyceridwater (1:l volumetric ratio) solution and stirred for 4 h. The clear mixture was then added dropwise to one gram of clay dispersed in 100 mL of deionized water. The pH of the solution was measured to be 0.44. After stirring for 3 h, the clay was filtered and washed thoroughly with deionized water. The pH of the filtrate was ca. 1.5. 2 mL of the partially hydrolyzed Ti-ethoxide solution was added dropwise to one gram of clay dispersed in 20 mL glyceridwater (1:l ratio) solution and stirred for 24 h, followed by filtering, washing and drying. G3- 2 mL of the partially hydrolyzed Ti-ethoxide solution was added dropwise to one gram of

a-

m-

Titanium Pillared Clay Using Organic Medium

35

clay dispersed in 20 mL glycerinlwater (1:l ratio) solution. After stirred overnight, the mixture was diluted with 200 mL of water and stirred for another 5 h, followed by filtering, washing and drying. B- 2 mL of the partially hydrolyzed Ti-ethoxide solution was added dropwise to one gram of clay dispersed in 100 mL of water and stirred for 24 h, followed by filtering, washing and drying. XRD analyses of the pillared clays were performed on the oriented samples prepared by spreading ca. 0.5 mL of water suspension of the sample on a quartz slide. The XRD patterns were obtained on a Philips PW 1840 automated powder diffractometer, using Ni-filtered CuKa radiation. Temperature programmed desorption of ammonium were carried out with a Du-Pont 9900 thermogravimetric analysis system. N2adsorption-desorption isotherms were measured with a Cahn TG-121 microbalance. I3C NMR spectra were obtained from a Bruker AC 300F NMR spectrometer. Carbon content was analyzed with a Perkin-Elmer 2400 EA instrument. For CO hydrogenation reaction, 10 wt. % of FqO, powders physically mixed with the pillared clays were served as the catalysts. The reaction was carried out in a pressured plug-flow type reactor under 50 atm CO/H2(1:1 ratio) pressure. The catalyst was pre-reduced with a gas stream of H2/N2(1:9 ratio) at one atmosphere, 673 K for 16-20 h. The catalytic reaction was carried out at 573 K with a flow rate of the reactants of 20 mL/min. The liquid products were collected with a trap kept at ambient temperature for a two-day period, while the gaseous products were analyzed with a on-line HP 5890A gas chromatograph. A molecular sieve 5A column was used for the separation of inorganic gases, and a porapak S column for the separation of organic products. Both TCD and FID detectors were used.

RESULTS AND DISCUSSION Crystallinitv

.. .

21.311

(4 10,9A (5.3A

(0

20

30 2e(')

40

50

36

S . 4 . Jong, J.-T. Lin and S. Cheng

determined from the 001 reflection, which appeared as the strongest peaks in the XRD patterns. Samples Gl-G3 have very similar basal spacings, while sample G3 has the highest crystallinity. The as-synthesized samples have basal spacings around 21.3 A, that shrinks to 17.7 A after calcination at 773 K and to 17.4 A after calcination at 973 K. By subtracting the basal thickness of clay of 9.5 A, the interlayer space is 10.8, 8.2 and 7.9 A, respectively. These distances imply that Ti is incorporated into the interlayers in some form of polynuclear clusters with the diameter around 8 A. For comparison, Fig. 1(B) shows the XRD patterns of sample B, which was prepared without glycerin. The resultant pillared clay is poorly crystalline. Only one broad peak appeared at ca. 15.3 A for the as-synthesized sample. This peak shrunk remarkably in intensity and three weak peaks around 25.7, 12.9 and 10.1 A were resolved after the sample was calcined at 773 K. These peaks, however, disappeared again after the sample was calcined at 973 K. Surface area and porositv The surface areas of the samples calcined at 573, 773 and 973 K are schematically shown in Fig. 2. All have relatively high surface areas. The values derived from B.E.T. are lower than those from Langmuir isotherms. Among them, sample B seems to be most thermally stable, and more than 90% of its surface area is retained after 973 K heat treatment. On the other hand, samples Gl-G3 have their surface areas retained up to 773 K, but loose ca. 30% of them after calcination at 973 K. Moreover, different preparation procedures in adding the glyceridwater solution also have influence on the thermal stability of the resultant Ti-PILCs. Among Gl-G3 samples, G3 is most thermally stable and G2 the least sable. Therefore, it is concluded that large amount of water in the pillaring reaction is necessary in order to prepare Ti-PILC of high thermal stability.

loo J

473

573

873 773 cdc. temp. (K)

873

873

1

Fig. 2. Surface areas of Ti-PlLCs after calcination at various temperatures; sample Gl(O), G2 (A), G3 (H) and B (V); BET- solid lines, Langmuir- dashed lines. Fig. 3 shows the typical N2adsorption-desorption isotherms of Ti-pillared samples prepared with glycerin (sample G3 as representation) and without glycerin (sample B). The abrupt increase in adsorption volume at low partial pressure is attributed to micro-pore @ore diameter < 20 A)

Titanium Pillared Clay Using Organic Medium

37

1

140

I 20t 0.0

0.1 0.2

0.3

0.4

0.5

0.6 0.7

0.8

0.9

1.G

0.0

P/Po

0.1

0.2

0.3 0.4 0.5

0.6

0.7 0.8

0.9

1.0

P/Po

Fig. 3. N2adsorption-desorption isotherms of Ti-PILCs prepared with glycerin (A) and w i t h 1 glycerin (B); after calcination at (0)573 K, (A) 773 K and (W)973 K. condensation, while the hysteresis observed at higher partial pressure is contributed from the meso-porous structures @ore diameter of 20-500 A) [ 181. The diameters of the micropores are consistent with the interlayer space obtained from XRD patterns. Therefore, the micropores should be the free space in between the adjacent layers, which were propped open by the pillars. It is noteworthy that the micropore condensation is more obvious on sample G3 than on sample B. Their differences are further demonstrated in their profiles of pore size distribution in mesopore region (Fig. 4). The latter was calculated on the basis of the desorption branch of the isotherms. A maximum around 26 A, corresponding to the hysteresis in the adsorption-desorption isotherms, was observed on both samples, while sample B has another maximum at ca. 12 A, which actually spreads over 10-23 A. In other words, the pore size distributes over a wider range when glycerin is not present in the pillaring solution.

-5

:6G1 140t 120..

..

TI

[A) I

I

(8)

140 .120 -.

100..

' B

80.60.40.-

40

20 -0 --

20

-20

J

3

10

20

30

dUl

40

50

6 ~

"I

Pore-size distribution of Ti-PILCs prepared (A) with glycerin (sample G3) and (B) without glycerin (sample B); after calcination at 573 K. Fig.

t.

For sample 6 3 calcined at 573 K, the micropores contribute ca. 88% of its total pore volume. After calcination at 773 K, the volume of micropores was retained while that of mesopores increased. The latter was probably contributed from the void space remained after the organic species being completely burned off. This proposal was confirmed by the carbon analysis results, which showed that ca. 30% of carbon still remained in the sample after 573 K calcination but

38


nearly completely removed after 773 K calcination. When the calcination temperature was raised to 973 K, the volume of micropores in sample G3 was reduced to ca. 65 % of its origin while that of mesopores was almost unchanged. Judging from the results of XRD analyses, the loss of part of microporous structures is probably due to the severe distortion of the lamellar structure so that those pores in the interior portion of the particles were not accessible by N, molecules. Nevertheless, the pillared structure was retained after calcination at 973 K and the micropores still contribute ca. 70% of its total pore volume. On the other hand, for sample B, the microporous structures contribute only ca. 50% of its total pore volume. Effect of elvcerin *3CN M R spectra were used to determine the roles of ethanol and glycerin in the formation of titanium polvnuclear species. Fig. 5(a) shows that two peaks appear at 16 and 68 ppm on the

111

(a i

I

1

h

I

I

80

I

I

70

I

I

60

I

I

50

I

I

40

I

I

30

I

I

20

I

I

10

PPM

Fig. 5. I3C NM, Zr-PILC.

0 373

l

l

473

l

1

I

573

I

673

t

\,

173

Temperature (lC)

Fig. 6. TPD of NH, over (a) Al-PILC, (b) Ti-PILC, and (c) Zr-PILC. Table 1 tabulates the catalytic activities and product distribution of physically mixed FqO, with the above mentioned three pillared clays in comparison with that of plain Fe catalyst and FqO, physically mixed with ZSM-5 zeolite. The Ti-PILC catalyst seems to give the highest conversion among the five catalysts. Nevertheless, the product distributions over the three pillared clays are similar. Shape-selectivity is demonstrated on the pillared clays as well as on ZSM-5 zeolite by the increase in gasoline range products and the suppression of high molecular weight waxy products, The mechanism should be similar to that proposed by Caesar et al. [19] that ethene and other light olefins formed over iron catalyst were transferred to the acid sites on PILCs

40


or zeolites, where polymerization occurred and heavier hydrocarbons and aromatics shape-selective by the pore sizes were formed. The results that only little amount of aromatics was obtained over the PILC catalysts were accounted for by the weaker acidity of PILCs as comparing with that of ZSM-5zeolite. The selectivities of aromatic products over PILC catalysts were varied in proportion to the quantities of strong acid sites measured from NH3 TPD experiment. Table 1. CO hydrogenation over Fe/PILCs Catalysts

Fe

FelZSM-5

Fe/AI-PILC

Fe/Zr-PILC

Fe/Ti-PILC

CO conv. (mol 96) CHs selec. (mo1W) CHs composition (wt. W ) Cl c2

68.1 57.4

80.1 51.3

61.7 57.4

52.8 56.8

84.9 61.5

13.6 17.7 20.4 13.4 13.9

29.3 7.3 12.2 19.4 16.1 15.7

17.5 14.6 19.2 14.2 33.3 1.2

21.0 15.6 21.2 13.2 28.8 0.2

19.0 16.8 20.0 13.3 30.1

~

c3 c4

c 5+ aromatics wax

-

16.8

-

0.8

573 K,50 atm

Acknowledgement: Financial support from the National Science Council of the Republic of China is gratefully acknowledged.

REFERENCES 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18. 19.

F. Figueras, Cutul. Rev.- Sci. Eng., 30 (1988) 457. T. Matsuda, M. Matsukata, E. Kikuchi and Y. Morita, Appl. Cutal., 21 (1986) 297. M. L. Occelli: "Physicochemical Properties of Piilared Clay Catalysts" in "Keynotes in EnergyRelated Cufulysis",S . Kaliaguine ed. p. 101-137, Studies in Surface Science and Catalysis, Vol. 35, Elsevier Science Publishers, Amsterdam, Oxford, New York, Tokyo (1988). T.J. Pinnavia, M.S. Tzou and S.D. Landau, J . Amer. Chem. SOC.107 (1985) 2783. Y. Kiyozumi, K. Suzuki, S. Shin, K. Owaga, K. Saito and S. Yamanaka, Jpn. Kokui Tokyo Koho, 59-216631 (1984). S . Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. SOC., 110 (1978) 170. S. Matsuda, M. Takeuchi, T. F. Hishinuma, Nakajima, T. Narita, Y. Watanabe and M. J. Imanati, Air Pollut. Control Assoc., 28 (1978) 350. A. Vejux and P. Courtine, J. Solid Stute Chem., 23 (1978) 93. M. Gasior and T. Machey, J . Curd., 83 (1983) 472. I. Izumi, W. W. Dunn, K. 0. Wilboun, F. F. Fan, and A. J. Bard, J. Phys. Chem., 84 (1980) 3207. H. Yoneyama, S. Haga and S. Yamanaka, J. Phys. Chem., 93 (1989) 4833. 1. Sterte, Clays & Cluy Miner., 34 (1986) 658. A. Bernier, L. F. Admaiai and P. Grange, Appl. Cutul., 77 (1991) 269. S. Yamanaka, T. Nishihara, M. Hattori and Y. Suzuki, Mut. Chem. Phys., 17 (1987) 87. S. Yamanaka and M. Hattori in Chemistry of Microporous Crystals, T. Inui, S . Namba and T. Tatsumi eds. Elsevier, Amsterdam-Oxford-New York, 1991, p. 89. T. Kijima, H. Nakazawa and S. Takenouchi, Bull. Chem. SOC.Jpn., 64 (1991) 1395. J.-T. Lin, S.-J. Jong and S. Cheng, Microporus Muter. 1 (1993) 287. C. N. Satterfield, "Heterogeneous Catalysis in Industrial Practice", McGraw-Hill, N.Y., 1991, p. 39. P. D. Caesar, J. A. Brennan, W. E. Garwood and J. Coric, J. Card. 56 (1979) 274.

Oxygenated Stabilizing Agents in the Synthesis of IWFI Zeolites

G. Giordano', F. Di Renzo' and F. Fajula2 Dipartimento di Chimica, UniversitA della Calabria, 87030 Rende, Italy Laboratoire de Chimie Organique Physique et Cinktique Chimique Appliqukes, URA 418 du CNRS, ENSCM, 8 rue de 1'Ecola Normale, 34053 Montpellier, France ABSTRACT This communication examines the role played by the guest species in stabilizing the MFI framework and widening its crystallization field. A series of syntheses in presence or in absence of organic molecules (TPA, ethylene glycol) has been carried out in order to compare the different phases formed. Crystallization selectivity, crystal habit and size, elemental composition, adsorption properties and catalytic behaviour have been examined for the different MFI zeolites synthesized. INTRODUCTION The use of organic tetraalkylammoniurn cations as a tool for the synthesis of zeolites [l] has allowed the discovery of a large number of new structures. One of the most important zeolite phases synthesized in the last twenty years is the MFI structure, which presents unique peculiarity due to its particular channel system, adsorption and catalytic properties. The first synthesis of MFI zeolite has been carried out from an initial hydrogel which contained tetrapropylammonium (TPA) cations [2]. Further patents and open literature reports describe the preparation of MFI zeolite in a very large Si/AI and OH-/Si@ range, in the presence or in the absence of alkali cations [3-51,with other nitrogen-containing organic molecules [4-61, and also in absence of organic compounds [7-lo]. The patent literature shows that MFI zeolites can be also obtained from starting hydrogels containing alcohols, ethers, glycols, diols, and other oxygen-containing molecules [11-151. In the present study, the formation of MFI zeolite in the absence of any organic molecule or in the presence of ethylene glycol (EG) is investigated, and the resulting solids are compared with the same zeolite synthesized in the presence of TPA cations. The Si/A1 range in which pure MFI zeolite can be obtained from the EG systems has been investigated, and the preparation of the pure-silica zeolite has been attempted. The influences of the ratios OH/Si@ and organics/Si@ have been investigated. These ratios and the synthesis time control the nature of the phases formed. The influence of EG in stabilizing the MFI structure and widening the crystallization field is pointed out. Competition with other phases, thermal 41

42

G . Giordano, F. Di Renzo and F. Fajula

stability, elemental composition, texture, adsorption and catalytic properties of the MFI zeolites formed have been studied and discussed. EXPERIMENTAL Systems having

the

following

molar

composition

were

studied:

xNa20.yEG-zA1203-Si~*9.4H20, where EG stands for ethylene glycol, x ranges from 0.04 to 0.30, y from 0 to 7, and z from 0 to 0.05. A reference MFI zeolite was obtained in the presence of TPA'

ions from an initial hydrogel 0.05Na~0~0.13TPABr~0.015A1~0~*SiO~10

H20 held 4 days at 150°C. The crystallization experiments were canied out at 170°C under autogenous pressure in stainless steel reactors. The as-synthesized samples were characterized by the following techniques: powder X-ray diffraction, thermo-gravimetry, elemental analysis, scanning electron microscopy, and energy-dispersive electron probe microanalysis. Thermal stability has been evaluated by in situ powder X-ray diffraction in the range between room temperature and 900°C and by n-hexane adsorption after calcination under air flow up to 850°C at 5 "C/min. Finally, N2 adsorptioddesorption was used to determine the microporous volumes, and nhexane cracking (350"C, 1 Atm, WHSV 2.7 h-l ) was used as a model reaction for catalytic tests. The samples used for N2 adsorption measurements and catalytic tests have been calcined at 500"C, exchanged by W C l solution and calcined again at 500°C. RESULTS AND DISCUSSION Influence of NaOH The influence of the sodium hydroxide content in the starting hydrogel is summarized in table 1. MFI zeolite is formed in competition with sodalite, whose presence has been systematically reported in the ethylene glycol synthesis systems [ 16-18]. The crystallinity values reported in table 1 have been evaluated by comparison with the XRD intensities of well-crystallized standard samples. The evaluation of the crystallinity at intermediate stages of the syntheses indicate that the crystallization rate is highest at intermediate alkalinity levels. The crystallization yield, calculated as (mass of solid product)/(mass of Si*+NaAl* engaged in the synthesis), systematically decreases with increasing alkalinity, in agreement with the increasing silica solubility. T a b l a 1. P b s s a s f o r n e d a s a f u n c t i o n o f N a 2 O c o n t e n t from h y d r o g e l s r N a z O ~ 7 E G ~ 0 . 0 2 5 8 1 z O ~ ~ S .4H20 i O z ~ Sat 170°C. Sample

s

1

2 3 4

5

s102

S rOa

0.04 0.08 0.16 0.24 0.30

0.03 0.11 0.27 0.43 0.55

phase X at 5 d SOD MFI amorphous 3 21 10 40

-

12

40 3

p h a s e Y. a t 11 d SOD HFI

solid yield

amor2hous 22 10 21 59

7a a4 53 41

0.96 0.79 0.71 0.58

Oxygenated Stabilizing Agents in Zeolite Synthesis

0

0 H -1 Si 0 2

43

0.5

Fig. 1. Yields of MFI (n)and sodalite ( 0 ) from hydrogels xNa20.7EG.0.025A1203.SiO2.9.4 H30 " at 17OOC as functions of alkalinity.

The yields of each phase, evaluated as (solid yield)x(fraction of the given phase as evaluated by XRD)x(mass fraction of Si02-tNaAIQ in the given phase), are reported in figure 1 as a function of alkalinity. The MFI yield decreases with alkalinity, whereas the sodalite yield features a minimum at the intermediate levels of alkalinity. No data indicate true metastability of a phase towards another, viz. dissolution of the first phase formed to feed the growth of a more stable phase. The competition between MFI and sodalite is probably controlled by their relative rates of crystallization. Crystallization of sodalite is favoured at the highest alkalinity levels. Influence of A1 Data about the influence of the aluminium content in the starting hydrogel on the crystallization selectivity are reported in table 2. At very low aluminium levels no MFI is formed, and the crystallization of ZSM-48 [19] or cristobalite is instead observed. MFI is the main product at intermediate aluminium levels, whereas sodalite and, to a lesser extent,

T a b l e 2. P h a s e s f o r s e d as a f u n c t i o n o f 81203 c m t e n t ? r o m h y d i o g e l s 0 . 1 6 N a z 0 ~ 7 E G ~ z 9 1 z G ~ S i 0 2 . 9 .Hz0 4 a t 170°C.

A1203 SiOz

OH-

6

0.005

0.31

7 3 8

0.010 0.025

0.30 0.27 0.27

Sample d

p h a s e % at 5 d

p h a s e % at 11 d

~ ~ H F I

7 7 c r i+ 21M48 62YFI-38crl 8 4 X F I c lOSGD

SiUz

solid yield

~~

0.050

48HFI+lOSGD 60SODc8iIOR-7HfI

~

H48

= ZSH-48; c r : = c r i s t o b a l i t e

74SODc13HORLi3HFI

0.90 0.99 0.79 0.96

44

G. Giordano, F. Di Renzo and F. Fajula

mordenite are the main phases formed at higher aluminium levels. The insertion of aluminium seems to be easier in the sodalite framework than in a pentasil network, also when ethylene glycol is present. Stabilizing effect of the ethylene ~lYcol Table 3 represents the influence that the EG content of the synthesis batch exerts on the crystallization selectivity. At all alkalinity levels, the MFI selectivity is higher when ethylene glycol is present. The stabilizing effect of EG is striking at low alkalinity: MFI can be formed in the absence of any organic agent, but it is metastable towards the formation of mordenite and quartz. In the presence of ethylene glycol (EG/SiO;!= l), MFI is more rapidly formed and it remains stable on long staying in synthesis conditions (11 days). The XRD crystallinity of MFI obtained in the presence of EG is only slightly lower (92%) than the crystallinity of a reference MFI formed in TPA medium. T a b l e 3 . P h a s e s l o r n e d a s a f u n c t i o n or’ N a z 0 and e t h y l s n e g l y c o l 4 a t 170°C. c o n t e n t from h y d r a g e l s x N a z O . y E G . 0 . 0 2 5 ~ 1 ~ 0 3 . S i 0 2 9 . Hz0

EG

s

NaaO Si02

9 10

0.12 0.12

0 i

11 12 3 13

0.16 0.16

0

0.16

7

0.30 0.30 0.30

0

Sample

SiOz

phase X a t 5 d

phase X a t l i d

0.19 0.19

32MFI+8qua 92MFI

50quat50tiOR 92XF I

0.27 0.27 0.27

42quaA37HORsi2ANA 63MFIclOHCR 75HFI+7HOR+5qua 48HlI+lOSOC 84HFI+IOSOD

0.55 0.55 0.55

60ANA+13HCR 12AN.4+5HGR 12SOC73HTI

OH-

solid y i e Id

SiOz ~

14 5

1

1

7

83ANA+ 17HOR 76ANA-24HOR 59SOD+14HFI

~~

0.80 0.82

0.54 0 . a2

0.79 0.2;

0.25

0.5a

qua = q u a r t z

The selectivity data for final products reported in table 3 are represented in figure 2 as a function of OH-/Si02 and EG/(Si+Al) of the initial hydrogel. The presence of ethylene glycol strongly favours the formation of MFI at the lowest alkalinity levels. At higher alkalinity levels, zeolite frameworks which do not feature five-member rings are preferentially formed, and the presence of ethylene glycol favours the formation of sodalite instead of analcime. The crystallization yield reported in table 3 steadily decreases at increasing alkalinity, following the increased solubility of the silicate species. The decrease in crystallization yield is markedly less intense in the presence of ethylene glycol, suggesting that the adsorption of the organic molecule prevents the hydrolysis of the T-0-T bridges. This effect can be considered as a model for the action of ethylene glycol, and possibly of other organic molecules, in enlarging the field of formation of MFI. The incorporation of ethylene glycol in the porosity of MFI stabilizes this zeolite against hydrolysis. Solids unable to incorporate EG, like quartz or analcime, are less stabilized by its presence in the synthesis medium, and the formation field of MFI is accordingly widened.


45

+ \

(3

w

1 \QUARTZ \ 0

I

0

1I

\

OH'/ S i 0 2

0,5

Fig. 2. Crystallization fields of the main phases formed from hydrogels at 170°C, as functions of ethylene glycol content and xNa 0~yEG.0.025A1~0~.Si0~.9.4H70 a l d n i t y . Mordenite always contami-ates quartz and analcime. On the other hand, it must be observed that no MFI is formed when hydrolysis is nearly completely prevented. Table 4 reports the results of this work on ethylene glycol-water systems in comparison with literature data about anhydrous ethylene glycol systems. In the absence of water, sodalite is the only zeolitic phase formed, as the consequence of a true template effect of ethylene glycol [20]. In the presence of water, EG still exerts some directing effect towards the formation of sodalite, but at low allralinity and low aluminium concentration its main effect is the pore-filling and stabilization of the MFI zeolite, probably formed by the same mechanism which operates in the absence of any organic agent. T a b l e 4 . Comparison o f e t h y l e n e g l y c o l s y s c e m s w i t h e c h y l e n e glyc?lw a t e r systems. Reference d

CIS1 c171 t h i s work t h i s work

OH-

sioz

0.05-0.7 0.4-4 0.2-0.6 0.1-0.4

Si A1 m

0.5-50 10-20 20-SO

EG

% 0.3-20

5-30 1-7 1-7

EG NazO

320 nain phase SiOz forned

a.

2.5-143 6.2-30

0.

23-44 6.3-90

9.4

SOD SOD SOD

S.4

HFI

MFI characterization Table 5 shows the morphology and the chemical characterization of MFI zeolite products. The amount of water detected in MFI is connected with the content of other guest species present in the zeolite. In fact, the largest water content is detected for the organic-free MFI (sample 15), evidencing that in this case the water exerts a pore-filling action. The sodium content in the zeolite indicates that Na' ions are incorporated to neutralize the

46

G . Giordano, F. Di Renzo and F. Fajula

T a b l e 5 . Main c h a r a c t e r i s t i c s o f HFI s a m p l e s . Sample

habit

size

(urn)

#

4 7 12

15= 16d

prisms prisms prisms prisms spheres

14~14x10 12~6x2 40x40~20 1.3xlx0.7 0.3SDS-I

composition per U.C. Na A1 o r g . HzO 6.4 2.4

5.8 6.2 2.4

6.9 2.6 8.2 6.8 3.2

7.8 9.7 6.6 0.0 3.1

9.0 6.1 5.8 48.6 2.2

Thernal s t a b i l i t y T d ( " C ) a n-CS(nl/g)b

650

0.043

650 900

0.070 0.126

900

0.100

a : Temperature a t which c r y s t a l c o l i s p s e b e g i n s . b : S o r p t i o n c a p a c i t y a f t e r c a l c i n a t i o n a t 85O'C. C : 7 6 % - c r y s t a l l i n e n F I formed w i t h o u t o r g a n i c a g e n t , from a h y d r o g e l o f t h e same c o m p o s i t i o n a s e x p e r i n e n t 9 , h o l d 48 h o u r s a t 1 5 0 ° C . d : TPA-containing r e f e r e n c e sample.

negative charges linked to framework aluminium. The amount of TPA occluded in the structure is in agreement with literature results for MFI with similar aluminium content [21]. The ethylene glycol content reflects the competition of EG and water for pore-filling, depending on the hydrophilic-hydrophobic character of the zeolite. The largest EG content is measured for the zeolite with the lowest aluminium content (sample 7). The Al/(Si+Al) ratio of MFI is always higher than the Al/(Si+Al) ratio of the synthesis hydrogel, indicating that aluminate species are incorporated more effectively than silica. The difference between the incorporation yields of silica and aluminate fades out as the aluminium content decreases. Aluminium-containing species appear to be the limiting parameter of the MFI crystallization, in agreement with the absence of MFI in the crystallization products of hydrogels with Si/Al ratio higher than 50. This limit suggests that it is not possible to obtain the MFI silica end term from ethylene glycol systems, like from organic-free systems. Thermal stability of the assynthesized materials has been evaluated by the temperature at which a loss of crystallinity is detected in the powder diffractograms recorded at increasing temperatures, and by the nhexane sorption capacities, measured at a relative pressure P/P"=O.17 after calcination at 850°C. The data presented in the last columns of table 5 show that MFI synthesized without organics (sample 15) or in the presence of TPA (sample 16) exhibit a greater stability than solids obtained in the presence of ethylene glycol. Catalvtic behaviour Table 6 summarizes the nitrogen micropore volume measured on the activated catalysts (calcined-exchanged-calcined) and the n-hexane conversion after two hours on stream at 350°C. The aluminium content of the materials is also recalled. It is apparent that the cracking activity is independent on the origin of the MFI zeolite and correlates with the aluminium content, and thereby with the number of active centres. The low activity of sample 4 in comparison to sample 15, which contains an equivalent amount of aluminium, stems from the lower micropore volume of the former.


47

Table 6 . H i c r o p o r e volume and c a t s l y t i c a c t i v i t y o f XFI c s t s l y s t s Sample m i c r o p o r e volume n - ‘nexane d ( m l / g Nz) U.C. conversion ( 2 )

15

0.084 0.121 0.125

16

0.095

4 1 _2 _

~~

6.9 8.2 6.9 3.2

16.5 30.0 29.1

11.0

Conclusions The influence of ethylene glycol in promoting the crystallization of MFI-type zeolites has been investigated and the results compared to solids prepared in the absence of any organic agent and in the presence of TPA cations. Compared to the organic-free syntheses, ethylene glycol enlarges the crystallization domain of MFI and stabilizes the structure in its formation medium through a pore fdling action. The composition of the EG-water-sodiumaluminosilicate gels effective in MFI crystallization is limited to a lowest aluminium content, preventing the formation of an Al-free MFI end member. Catalysts active for n-hexane cracking can be readily prepared by standard calcination and ion-exchange procedures. At 350°C and atmospheric pressure, the catalytic activity is found to be related to the aluminium content, regardless of the origin of the zeolite. REFERENCES 1 R.M. Barrer and P.J. Denny, J. Chem. Soc., (1961) 971. 2 R.J. Argauer and G.R. Landolt, US Pat. 3 702 886 (1972). 3 R.W. Grose and E.M. Flanigen, US Pat. 4 061 724 (1977). 4 P.A. Jacobs and J.A. Martens, Synthesis of High-Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, 1987. 5 D.T. Hayhurst. A. Nastro, R. Aiello, F. Crea and G. Giordano, Zeolites, 8 (1988) 416. 6 J.P. Gilson, in E.G. Derouane, F. Lemos, C. Naccache and F.R. Ribeiro (Eds.), Zeolite Microporous Solids: Synthesis, Structure, and Reactivity (NATO AS1 Series C 352), Kluwer Academics, Dordrecht/Boston/London, 1992, p. 19. 7 B. N o h , G. Manara, G. Bellussi and M. Taramasso, European Pat. Appl. 98 641 (1984). 8 E. Narita, K. Sato, N. Yatabe and T. Okabe, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 507. 9 V.P. Shiralkar and A. Clearfield, Zeolites, 9 (1989) 363. 10 W. Inaoka, S . Kasahara, T. Fukushima and K. Igawa, Stud. Surf. Sci. Catal., 60 (1991) 37. 11 Mobil Oil Co., Dutch Pat. 7 906 451 (1981). 12 Snam Progetti, Dutch Pat. 8 101 216 (1981). 13 J.L. Casci, B.M. Lowe and T.V. Whittam, European Pat. Appl. 42 225 (1981). 14 M.F.M. Post and J.M. Nanne, Canadian Pat. 1 135 679 (1982). 15 W. Hoelderich, L. Marosi, W.D. Mross and M. Schwarzmann, European Pat. Appl. 51 741 (1982). 16 D.M. Bibby, N.I. Bawter, D. Grant-Taylor and L.M. Parker, in M.L. Occelli and H.E. Robson (Eds.), Zeolite Synthesis (ACS Symposium Series 398), Am. Chem. Soc., Washington D.C., 1989, p.209. 17 W.A. van Herp, H.W. Kouwenhoven and J.M. Nanne, Zeolites, 7 (1987) 286. 18 D.M. Bibby and M.P. Pale, Nature, 317 (1985) 157. 19 F. Di Renzo, G. Giordano, F. Fajula, P. Schulz and D. Anglerot, French Pat. Appl. 92 14776 (1992). 20 J.W. Richardson, J.J. Pluth, J.V. Smith, W.J. Dytrych and D.M. Bibby, J. Phys. Chem., 92 (1988) 243. 21 G. Debras, A. Gourgue, J.B. Nagy and G. De Clippeleir, Zeolites, 5 (1985) 377.


The Synthesis of Discrete Colloidal Zeolite Particles

B. J. Schoeman, J. Sterte and J-E. Otterstedt Department of Engineering Chemistry, Chalmers University of Technology, S-412 96 Goteborg. Sweden

ABSTRACT Highly crystalline discrete colloidal particles of hydroxysodalite with an average particle size of 37 nanometers and a narrow particle size distribution have been synthesized at 100°C in clear homogeneous solutions. A method of particle size tailoring is illustrated by means of the mid-synthesis addition of the growth-limiting nutrient, alumina, as well as by the seeding technique employing seeds of hydroxysodalite with a particle size of 37 nm. The sols may be stored as free flowing powders and redispersed to their original form. INTRODUCTION The synthesis of zeolites in alkaline aluminosilicate solutions in the presence of organic cations has been widely studied since the use of these cations result in siliceous zeolites, notably zeolite N-A and TMA-sodalite, as compared to the corresponding zeolites in the wholly inorganic aluminosilicate system [ I ] . Extremely silica-rich zeolites, such as ZSM-5, ZSM-I 1 , Phi and Beta, have been synthesized in the presence of organic cations 121. As a result, a wide variety of zeolitic structures are available for potential use within such diverse fields as glass ceramics 13 I, insulators and semiconductors 141and, of course, catalysis to mention a few. One material property which one would expect to be of importance is particle size. Within the field of catalysis, particle size effects have been investigated and in certain cases noted as strongly influencing catalytic activity 15-71.We have reported on the synthesis of colloidal zeolite from clear homogeneous TMA,Na-aluminosilicate solutions and showed that a carefully controlled sodium content allows for the synthesis of colloidal zeolite N-Y and N-A 181 whereas the synthesis of colloidal hydroxysodalite is insensitive to the sodium content in the solution (at least within the range studied) 19,101. In the case of zeolite N-Y and N-A, the sodium content at the start of crystallization determines the zeolite phase produced and the role of sodium would appear to be the growth-limiting nutrient [ 1 1 I. The growth-limiting nutrient in the the synthesis of hydroxysodalite was shown to be alumina I lo].The mid-synthesis addition of alumina resulted in further growth of the heel particles without a secondary nucleation event. In this presentation, particle size tailoring will be discussed. 49

50

B. J. Schoernan, J . Sterte and J-E. Otterstedt

EXPERIMENTAL Materials and preparation of the synthesis mixtures Ludox SM (30.2 wt% SiO,, 0.66 wt% Na20, particle size 7 - 8 nm, DuPont) was used as the silica source. The aluminate used i n all runs except run HS4 and HS6 was prepared from AI,(SO,),.

18H,O (puriss, Kebo Lab, Sweden) whereas a sodium aluminate powder (55 wt%

A120.,, 40 wt% Na20, Kebo Lab. Sweden) was used in runs HS4 and HS6. The source of alkali was TMAOH.SH,O (Sigma) and NaOH pellets (p.a., Eka Nobel AB. Sweden). Double distilled water was used throughout this work. Clear TMA,Na-aluminosilicate solutions were prepared by adding a clear TMAaluminate solution to the silica source with strong mixing to avoid the formation of a solid phase. The preparation of the TMA-aluminate solution where AI,(SO,),. 1 8 H 2 0 was used has been described in detail in our earlier work 181. In those runs where the sodium aluminate powder was used, the aluminate was simply dissolved in the TMAOH solution with mixing until complete dissolution. The synthesis mixtures were heat treated with reflux at I00"C without stirring in polypropylene bottles submerged in a polyethylene glycol oil bath placed in a well ventilated area. Analysis Purified sols were obtained by separating the mother liquid from the solid phase by a series of centrifugations, (2 h, relative centrifugal force, RCF, of 49 OOOg), decantering. addition of distilled water and dispersion (in an ultrasonic bath) steps. Four such steps normally sufficed to remove most of the soluble amorphous phase. I t can be noted that the particles can be dispersed to their original state by simply letting the sample stand for several hours. Mass growth curves and zeolite yields were obtained upon purifying samples by weighing ca. 30 g sol in pre-weighed polypropylene centrifuge tubes and centrifuging as described above. The solid samples were dried

(2 h. 105°C)while in the centrifuge tubes and thereafter equilibriated over a saturated CaCl, solution for at least 16 h. The centrifuge tubes were then weighed in order to determine the weight of solids. Particle size and particle size distribution (PSD) analyses were performed by means of dynamic light scattering with a Brookhaven Particle Sizer, BI-90, on dilute as-synthesized samples as well as on purified aqueous sols. Particle size, PSD and crystal morphology were determined by transmission electron microscopy (TEM), model JEOL 2000 FX.In those cases where the particles could be observed with scanning electron microscopy ( z ca. 80 nm), a JEOL model JSM5200 electron microscope was used, X-ray diffraction (XRD) analysis for phase identification was performed on freeze dried purified sols using a Siemens powder X-ray diffractometer ( D - m , Position sensitive detector). N, adsorption was measured at liquid nitrogen temperature with a Digisorb 2600 surface area analyzer, Micrometrics Instrument Corporation. The freeze dried purified sols were outgassed at 2500C for 3 h prior to measurement. The surface areas were calculated with the BEiT equation.

Discrete Colloidal Zeolite Particles

51

RESULTS AND DISCUSSION General Addition of a clear TMA-aluminate solution to the colloidal silica sol to obtain a synthesis mixture with the molar composition 14(TMA)?O 0.85Na20 I .OAI2O34OSiO2 805H20 , denoted run HS I , results in a clear synthesis mixture free from a solid amorphous gel. The amorphous silica particles present depolymerize upon heat treatment as seen by the disappearance of the bluish haze initially present. No particles can be detected by dynamic light scattering during the apparent induction period. After a synthesis time of ca. 40 hours, the bluish haze appears once again, only this time indicating the advent of particle growth. As seen in Figure 1, the zeolite content increases to a final value of 0.06 g zeolite/g sol after a synthesis time of 50 hours, and the corresponding particle size, determined by dynamic light scattering, is 37 nm. The particle concentration is

; - 10.w] 12

{

-

a

.

-

m

m

w

8-

5 *-

E

4-

w

2'

8 t

12

Fig 1. The increase in zeolite content as a function of synthesis time in run HS 1 and run HS5 (mid-synthesis addition of alumina after a synthesis time of 55 hours).

2 THETA

36

Fig 2. XRD diffractograms for (a) the purified product of run HS I and (b) the pH adjusted sol of run HS 1.

-

therefore 1. I lo15 per g sol. Prolonged hydrothermal treatment of the sol does not result in an increase in the average particle size. XRD analysis, Figure 2, shows that the purified product consists of highly crystalline hydroxysodalite with a unit cell constant of 8.921A. An interesting point to note in this respect is that the XRD peaks are not as broad as one might expect for such small particles. Shown in Figure 2. as a comparison, is the XRD diffractogram for the as-synthesized product that has been pH adjusted using cationic resin (Dowex HCRS-(E)) in the H+ form. The pH adjustment essentially removes the free alkali present in solution but not the aluminosilicate present in solution, which therefore remains in the sample upon freeze drying. This form of the product displays a more pronounced peak broadening, thus indicating that the presence of amorphous material contributes to a false peak width at half peak height. It is, however, possible to determine the peak width at half peak height in the purified sols diffractogram (Figure 2a) in order to use Scherrer's equation 181 to estimate crystal size. The resulting particle size according to this method is SO nm. The particle size distribution, PSD, determined from light scattering results and expressed as the coefficient of variation 1101, is only 7%, or in other words, the colloidal particles

52

B. J. Schoeman, J. Sterte and J-E. Otterstedt

form a 'monodisperse' particle population. The uniformity in the particle size is confirmed by the TEM micrograph shown in Figure 3. There is a slight discrepancy between the DLS average particle size, 37 nm and the particle size evident in the TEM micrograph where the particles appear to be of the order of 20 - 25 nm. The hydroxysodalite particles (which are non-porous to N?) have a specific surface area of 18s mzlg - comparable to commercial silica sols such as Ludox HS (particle size ca. 14 nm and a specified specific area of 210 - 230 mzlg).

Fig 3. TEM micrograph of hydroxysodalite obtained in run HS I.

Fig 4. SEM micrograph of large particulate zeolite N-A obtained in run HS2.

Influence of the alumina content In run HS2, the alumina content was increased by a factor of two, as compared to that in run HSI, to give a synthesis mixture with a molar composition I4(TMA)z0 0.8SNa20 2.OAl2O3 4OSiOz 805HzO. The apparent induction time in this run was considerably longer - ca. 62 hours. Furthermore, the solution contained visible signs of a raft-like material together with colloidal material after a synthesis time of 84 hours. The colloidal material was separated and identified as hydroxysodalite by XRD while the DLS average particle size was 25 - 30 nm. A large particle fraction, ca. -500nm, Figure 4, could be identified as zeolite N-A. It is possible that the raft material present in the synthesis mixture was associated with the formation of the A-type material - a phenomenon reported in the literature 1121, but not with the formation of hydroxysodalite since HS could he crystallized in run HS I without the presence of such rafts. The alumina content in run HS3 was increased once again to give a molar cornposition in the synthesis mixture of I4(TMA)20 0.98Naz0 3.OA1203 4OSiOz 860Hz0. The product after a crystallization time of 44 hours was exclusively zeolite N-A with a particle size of ca. 200 nm. Two aspects should be noted with regard to the above results, in particular, run HSI. Firstly, the synthesis of colloidal hydroxysodalite is accomplished at a temperature of 100°C. I t is well known that relatively smaller crystals can be obtained by reducing the temperature of crystallization as shown by Zhdanov [ 131; however, in our work, this is not necessary. Secondly, it is not necessary to employ exceedingly high alkali contents to either achieve clear homogeneous


53

solutions or to synthesize colloidal zeolite. It is interesting to compare the synthesis composition of run HS2, which yields a particle size of ca. 30 nm, with that of synthesis mixtures reported by Hopkins 141and Kostinko I151 that yield hydroxysodalite and shown below in Table 1: Table 1. Molar compositions from the literature and this work used to synthesize hydroxysodalite.

(TMA)?O HS2 Hopkins 1141 Kostinko 115) a

7 4.3

Na?O

A124

0.42 3.1 3.2

1 .o 1 .o I .o

SiO?

HzO

Alkalinity a

20 20 2

400 280

1 .OM R2W 1 .SM R2W 3.7M R2W

48

R2O is both the TMAOH and NaOH content. No particle size in the above reference works is specified but, performing the preparation

according to the method given in the work of Hopkins, a gel is formed which upon hydrothermal treatment yields a product with a relatively broad particle size distribution, 150 - 300 nm. Noteworthy is the fact that the ratio of the alkalinity, expressed as the sum of Na2O and (TMA)20, to both A120, and Si02 is similar in run HS2 and in the mixture of Hopkins whereas the mixture in run HS2 is more dilute. Expressing the alkalinity in terms of molarity. Table 1, i t is clear that high alkalinities are not a criterion to be fullfilled in order to synthesize colloidal zeolite HS. Two differences in the compositions of run HS2 and that of Hopkins are apparent - the Na,O/(TMA),O ratio and, secondly the alumina source. In the run of Hopkins. sodium aluminate trihydrate supplies the alumina whereas the alumina source in this work is a freshly precipitated alumina. It is well known that the reagents influence the crystallization products and therefore run HS4 was performed wherein the molar composition was the same as in run HSI but where the alumina source was a commercial sodium aluminate. The sodium content in run HS4 is somewhat higher than in HSI (Na20/AI103 = 2.2) but a s reported previously, the sodium content does not appear to affect the crystallization significantly, at least not the ultimate size of the crystals. This is confirmed by the fact that the particle size obtained in run HS4 is 36 nm, Figure 5.The lower sodium content does allow one to obtain a synthesis solution free of a solid amorphous material. I t appears therefore that such a clear solution aids in the successful synthesis of colloidal hydroxysodalite. Particle size tailoring An analysis of the alumina and sodium content in the hydroxysodalite particles in the ultimate product of run HSI shows that only ca. 18% of the sodium present in the synthesis mixture can be accounted for in the crystal fraction while 90% of the alumina has been consumed by the crystals. This indicates that alumina is the growth-limiting nutrient. Hence an amount of alumina equal to the amount present at the start of crystallization in run HSI was added to the synthesis mixture of run HSS after a crystallization time of SS hours. Up to this point in run HSS, the crystallization was

54

B. J . Schoeman. J . Sterte and J-E. Otterstedt

merely a repeat of run HSI. As a result, the particle size after 55h was measured as 37 nm, the zeolite content, 5.98%. and the particle concentration, 1.1 101s per gram sol. As seen in Figure 1,

Fig 5. TEM micrograph of hydroxysodalite synthesized in run HS4 with sodium aluminate as the alumina source the mid-synthesis addition of alumina results in the increase in the zeolite content to 1 1 .S% (with a correction for dilution by the TMA-aluminate solution) and the corresponding particle size was measured as 48 nm. Once again, the particle size reaches this ultimate size and remains constant over a period of at least 20 hours. The specific surface area of the purified product decreased to 143 m,/g while the particle concentration remained essentially constant, 0.98.lOl5 per g sol (also corrected for dilution). An increase in the particle size by a factor of 1.3 is equivalent to an increase

in the zeolite content by a factor of approximately 2.2. Together with the fact that the particle concentration remains constant, one can conclude that the heel particles initially present with a particle size of 37 nm have continued to crystallize upon the mid-synthesis addition of alumina, the growth-limiting nutrient, without a secondary nucleation event taking place. Finally, the TEM micrographs, Figures 6a and b, show that there is a distinct difference in the particle size before and after the mid-synthesis addition of alumina, thus confirming the above conclusion. An alternative particle size tailoring method is via the addition of a purified hydroxysodalite heel sol to a synthesis mixture which otherwise would yield hydroxysodalite with a particle size of 37 nm as in run H S l . This is illustrated by taking into account two results presented above. A synthesis mixture with a molar composition as in run HSI yields a colloidal suspension with a zeolite content of 6wt%. The result of run HS5 in which growth of hydroxysodalite occurred upon the existing heel particles should allow one to tailor the size of zeolite particles by varying the amount of seed material. The effect should be that the soluble aluminosilicate material in solution should distribute itself among the available seed, hence the final size of the particles could be calculated beforehand. This has been done in run HS6 using 2.3wt% seeds. Since


55

d, , d, = initial and final particle size respectively, m, = mass seed material and m 1 = mass material able to be deposited, the seed particles with a particle size of 37 nm should increase to 58 nm. In actual fact, this is indeed the case as shown by dynamic light scattering. Furthermore, an increase in the zeolite content is measured from 2.3 wt% (due to the seeds) to 8.2 wt% corresponding to the increase in particle size to 58 nm. The product of run HS6 is depicted in Figure 7b and compared

Fig. 6 (a) TEM micrograph of zeolite HS before alumina addition and (b) after mid-synthesis addition of alumina.

Fig 7 (a) TEM micrograph of the seed particles used in run HS6 and (b) the resulting product in run HS6. with that of the seed material, Figure 7a. From Figure 7b, it is apparent that particles with an average size of about 10 nm are present. These particles are strongly associated with the heel particles since the centrifugation conditions were such that 10 nm particles would not be removed from the solution. As a result, a surface nucleation mechanism appears to be operating in this case. The reason why this phenomenon is not observed in run HSS,Figure 6b, is not known but work is under way in this regard since interesting growth mechanism information can be obtained.

56

B. J . Schoeman. J. Sterte and J-E. Otterstedt

Colloidal zeolite powder The purified colloidal suspension in run HSl was adjusted to pH 11.5 with TMAOH and freeze dried, The powder could be redispersed in water whereafter the measured particle size was 38 nm thus showing that colloidal zeolite HS sols can be stored as powders and redispersed to their original form without loss of their colloidal properties.

CONCLUSIONS Discrete and rather monodisperse colloidal hydroxysodalite particles with an average particle size of 37 nm can be synthesized in clear homogeneous solutions at the relatively high temperature of 100°C and without the presence of what one might term exceedingly high alkali contents. The particle size in the colloidal suspension can be size tailored by the addition of a growth-limiting nutrient, in the case of hydroxysodalite, alumina, or by seeding a synthesis mixture with discrete colloidal particles of a well-defined size. Acknowledgments This work has been financed by the Swedish Research Council for Engineering Sciences (TFR) whom the authors would like to thank. The authors would also like to thank B. Stenbom for providing the TEM micrographs. References 1 R. M. Barrer, 'Hydrothermal Chemistry of Zeolites', Academic Press, London, 1982, p. 160. 2 P. A. Jacobs and J. A. Martens, Stud. Surf. Sci. Catal., 33 (1987) 1. 3 R. L. Bedard and E. M. Flanigen, in K. von Ballmoos, J. B. Higgins and M. M. J. Treacy 9th Int. Zeolite Conf., Montreal, July 5-10, 1992, Butterworth-Heinemann, (Eds.), ROC. Boston, 1993, p. 667. 4 A. Stein and G. A. Ozin, in R. von Ballmoos, J. B. Higgins and M. M. J. Treacy (Eds.), Roc. 9th Int. Zeolite Conf., Montreal, July 5-10, 1992, Butterworth-Heinemann, Boston, 1993, p. 93. 5 K. Rajagopalan, A. W. Peters and C. C. Edwards, Applied Catalysis, 23 (1986) 69. 6 V. P. Shiralker, P. N. Joshi, M. J. Eapen and B. S. Rao, Zeolites, 11 (1991) 51 I. 7 A. J. H. P. van der Pol, A. J. Verduyn and J. H. C. van Hooff, in R. von Ballmoos, J. B. Higgins and M. M. J. Treacy (Eds.), Proc. 9th Int. Zeolite Conf., Montreal, July 5-10, 1992, Butterworth-Heinemann, Boston, 1993, p. 607. 8 Schoeman, B. J., Sterte, J., Otterstedt, J-E., 'Colloidal zeolite suspensions', Accepted for publication in Zeolites. 9 Schoeman, B. J., Sterte, J., Otterstedt, J-E., J. Chem. SOC.,Chem. Comm., (1993)994. 10 Schoeman, B. J., Sterte, J., Otterstedt, J-E., 'The synthesis of colloidal zeolite hydroxysodalite by homogeneous nucleation', Accepted for publication in Zeolites. 11 Schoeman, B. J., Sterte, J., Otterstedt, J-E., 'The synthesis of colloidal zeolite N-A', To be submitted for publication in Zeolites. 12 R. Aiello, R. M. Barrer, and 1. S. Kerr, in R. F. Could (Ed.), Molecular Sieve Zeolites-1 (ACS Monograph 101), Am. Chem. SOC.,Washington D.C., 1971, p. 44. 13 S. P. Zhdanov, in R. F. Could (Ed.), Molecular Sieve Zeolites-I (ACS Monograph 101 ). Am. Chem. SOC.,Washington D.C., 1971, p. 20. 14 P. D. Hopkins, i n M. L. Occelli and H. E. Robson (Eds.), Zeolite Synthesis (ACS Monograph 398),Am. Chem. SOC.,Washington D. C., 1989, p. 152. 15 J. A. Kostinko, in C. D. Stucky and F. G. Dwyer (Eds.), lntrazeolite Chemistry (ACS Monograph 218), Am. Chem. SOC.,Washington D. C., 1983, p. 3.

Study on the Isomorphous Substitution of Silicon by Tetravalent Elements (Zr, Ge, Ti) in the Framework of MFI Type Zeolites

R. Fricke', H. Kosslick', V.A. Tuan', I. Grohmann2,W. Pilz2, W. Storek3. G. WaltheP 1 Center of Heterogeneous Catalysis, Rudower Chaussee 5, D-12484 Berlin-Adlershof, Germany 2 WIP, KAI e.V., Rudower Chaussee 6, D-12484 Berlin-Adlershof, Germany 3 Federal

Institute for Materials Research and Testing BAM, Rudower Chaussee 6, D-12484 BerlinAdlershof, Germany 4 Center of Inorganic Polymers, Rudower Chaussee 5, D-12484 Berlin-Adlershof, Germany ABSTRACT Silicalite samples containing tetravalent metals (Ge, Ti, Zr) have been hydrothermally synthesized and characterized by various spectroscopic and thermoanalytic methods. Zr-Sil. shows nearly no increase of the unit cell volume. Strong indications for an incorporation of Zr in the framework arise from a Raman band at 685 cm-1 and a DTA peak that is about 30 K higher than for silicalite. In contrast to silicalite a symmetry change from orthorhombic to monoclinic is not observed. In Ge-Sil. a 29Si Nh4R signal at -1 10 ppm of Si-0-Ge groups can be resolved under optimal conditions only. Combined Raman (band at 960 cm-l), X P S and ESR measurements of Ti-Sil. allow to distinguish between Ti isomorphously substituting Si in the framework and between extra-framework Ti. INTRODUCTION The isomorphous substitution of silicon in zeolites of MFI structure by other tetravalent metals would not be promising if only acid catalyzed reactions are concerned. Due to identical charges no charge compensation which leads to the generation of acid Bronsted centers (Si-OH-Me) is necessary. There are, however, important reasons which justif) als the investigation of this type of Me-silicate: i. the kind of metal introduced into silicalite can be a well-known component of catalysts for other types of reaction (for instance, vanadium or titanium as the most famous catalyst components for oxidation reactions), ii. in the absence of any 'electronic distortion' it might be advantegeous to study the degree of structural distortion caused by the incorporation of tetravalent metals having different ionic radii and electronegativity, and its influence on the characterizing parameters. In summary, despite the catalytic reasons, general phenomena of the isomorphous substitution of silicon by other metals in zeolites can be helpfilly investigated .

57

58

R. Fricke, H. Kosslick, V. A. Tuan. I . Grohrnann, W. Pilz, W. Storek and G. Walther

In the present paper the synthesis and physico-chemical characterization of Ge-, Ti- and Zr-Silicalite zeolites (always in comparison to a pure silicalite I sample) will be discussed. New and complementary results will be presented and an attempt is made to generalize some of them within this class of zeolites. EXPERIMENTAL Svnthesis and Samples The samples were prepared under hydrothermal conditions in Teflon lined autoclaves. The Si/Me ratio of the starting gel, the chemical compounds used for the preparation of the gel as well as the conditions of the hydrothermal synthesis are listed in Tab. 1.

1 Effective ionic radii for Me4+ in tetrahedral coordination (from Shannon and Prewitt, 1969, 1970) 2 from Pauling 1967, 3 TPAl3r : Tetrapropylammoniumbromide,+additionally methylamine and HF were added, 4 TEOS: Tetraethylorthosilicate, TEOT: Tetraethylorthotitanate, 6 TPAOH: Tetrapropylammoniumhydroxide, Zr-iPr: Zirconiumisopropoxide The recipes for the synthesis of Ge- and Ti-Silicalite have been already published [1,9]. The ZrSilicalite samples were prepared under hydrothermal conditions by heating a starting gel having the molar composition: f i r 0 2 * Si02 * 0.5 TPAOH * 36.1 H20, where x = 0.01, 0.02 and 0.04. TPAOH was used as structure directing agent (template). The gel was prepared as follows: A solution containing the required amounts of TEOS (Merck), TPAOH (Merck) and distilled water was mixed under vigorous stirring until homogeneity was achieved (about 30 min.). To this solution the necessary amount of zirconiumisopopoxide (Aldrich) solved in 50 - 80 ml isopropylalcohol was added dropwise under continued stirring; the product was hydrolyzied and the alcohol evaporated under manyfold dilution. The homogeneous gel was then heated in Teflon-lined autoclaves for 4 days at 443 K under static conditions and autogeneous pressure. Thereafter, the autoclaves were quenched in cold water and the synthesis products were withdrawn immediately by filtration. The products were repeatedly washed with distilled water, dried and calcined for 4 hours at 823 K in air to remove the template.

Methods XRD patterns were taken with an HZG 4 difiactometer using Ni-filtered Cu KO radiation. SEM pictures were obtained on a TESLA B300 electron microscope. A MOM device (Hungary) was used

Isomorphous Substitution by Tetravalent Elements

59

for the thermoanalytic analysis. The 29Si MAS Nh4R spectra were obtained on a Bruker MSL 400 instrument at 79.3 MHz under conditions already desribed [l]. IR spectra were taken on a Bruker IFS 66 FT-spectrometer, diffise reflectance IR spectra (DRIFT) on an IRF-180 ZWG spectrometer. A Dilor X Y spectrometer equipped with an Ar+ laser of 50 mW was used for measuring the Raman spectra . ESR measurements at 77 and 293 K were carried out on a ZWG-ERS-200 spectrometer working in x-band. The X P S spectra were recorded employing an ESCALAB 200X photoelectron spectrometer. RESULTS Zirconium-Silicalite Although Zr02 attracts increasing attention as a source for the preparation of superacid catalysts [2] there is nearly no attempt to isomorphously substitute Zr for Si into the framework of MFI type zeolites. One important reason migih be that the ionic radius of Z 8 + in a four-fold coordination (0.59 A) is too large in comparison to that of Si4+ (0.26 A) making a stabilisation of the silicalite lattic after the incorporation of zirconium in the framework rather improbable. Despite same information from the patent literature Dongare et al. [3] were the first, to our knowledge ,who tried to synthesize and characterize Zr-Silicalite. In particular, from their results of IR spectroscopy, the determination of the lattice constants and from catalytic results in the hydroxylation of benzene to phenol and phenol to dihydroxybenzenes the authors conclude that Z P + has isomorphously substituted Si4+ in the framework of silicalite. Our own results on Zr-silicalite presented here allow not only a comparison with those of Dongare et al. but accomplish their studies by important measurements with DTA/TG, 2% MAS NMEt and Raman spectroscopy. As concluded from the SEM picture (Fig. 1) the sxynthesis products show parts of different morphologies: The main part consists of small 0.5 pm crystals, additionally, there exists a small quantitiy of twinned crystals typical also for ZSM-5 products.

Fig. 1. SEM picture of Zr-Silicalite

R. Fricke, H . Kosslick, V . A. Tuan. 1. Grohmann, W . Pilz, W . Storek and G. Walther

60

XRD pattern show mainly two results: i. There is no influence of the Zr contents on the appearance of the diffraction pattern ii. The symmetry of the samples remains orthorhombic after activation which is in contrast to pure silicalite where monoclinic symmetry is observed. This indicates incorporation of Zr. The lattice parameters summarized for silicalite and the Me-silicates in Tab. 2 show that the unit cell volume Vu,c, of Zr-silicalite is only slightly increased when compared with that of pure silicalite. This suggests that only small amounts of Zr are incorporated into the framework. The increase of Zr in the gel (SVZr-23) does not lead to a change of the lattice parameters which allows to conclude that at maximum 1 Zr/unit cell can be expected under the present conditions.

47 23

20.0409 20.0448

19.8695 19.8803

13.3723 13.3755

5324.89 5330.10

orthorhombic orthorhombic

Infrared measurements carried out in the lattice vibration region do not show any band between 900 and 1000 cm-l. This observation does not agree with the results of Dongare et al. [3] who claimed a band at 963 cm-l. No shift of IR bands is observed which supports the conclusion that only small amounts of Zr are incorporated. Thermoanalytic measurements which are sensitive to structural changes already for a low degree of metal incorporation show a strong exothermic peak at about 683 K that is due to the decomposition of the template. The distinct increase of the decomposition temperature for about 30 K in comparison to silicalite already for the Si/Zr=95 sample can be taken as strong evidence for the incorporation of Zr. In contrast to the expected low degree of substitution the thermal effect is rather large suggesting a high degree of structural distortion due to the incorporation of the large Zr ions. The value of the decomposition temperature (using TPA+ as template in all cases) is 653 K for silicalite I and 688 and 713 K for Ti-silicalite and Ge-silicalite, respectively. It shows, therefore, that it is strongly dependend on the kind of metal. 29Si MAS NMR spectra of Zr-silicalite samples are very similar to those of Ti-silicalite (TS-I) or of ZSM-5 [4,5]. They consist of a main signal at -1 13 ppm and a shoulder at about -1 16 ppm. A separate small peak at -103 ppm is additionally observed. Following the discussion in literature the main peak at -113 ppm can be assigned to Si(4Si) coordination. In a detailed 29Si NMR study of various ZSM-5 samples Axon and Klinowski [5] firther came to the conclusion that the signal at 103 ppm is indicative of Si(OSi)3O- framework defects. This seems reasonable in particular in the case of Zr-silicalite where at least a partial incorporation of the large Z d + ions should lead to structural distortions and defects of the silicalite structure.


61

A Raman spectrum of the sample with Si/Zr=95 is shown in Fig. 2. The main signals at 383 and 804 cm-1 coincide with those of silicalite I and can therefore be assigned to the oxygen motion along the T-0-Tline and to the Si-0-Si stretching vibration, resp.. A broad line at about 657 cm-* is absent in the spectrum of silicalite and is assumed to be caused by the incorporation of small amounts of Zr into the solid zeolite. _ _ ~

c

4

Fig. 2. Raman spectra of Silicalite

h and Zr-Silicalite nm

i y ~

m

ZY)

-loll-%

-

Germanium Silicalite There are only few attempts to synthesize Ge-silicalite. In some papers Gabelica et al. [6-81 could succesfilly show that Ge4+ can be isomorphously substitute silicon in the framework of silicalite. By means of various chemical and spectroscopic methods the authors claimed the incorporation of about 32 Ge/u.c. as a maximum leading to a Si/Ge ratio of nearly 2. The unit cell volume increased from 5345 A3 (silicalite) to 5428 A3 for Ge-Sil with the highest degree of substitution. The presence of structure defects caused by Ge is documented though no Ge-0-Ge bonds could be observed at that high degree of Ge incorporation. Very recently, Kosslick et al. [l] published an extensive study on Ge-silicalite. They found a Ge incorporation up to 12 Ge/u.c., leading to a unit cell expansion of about 52 A3 .On the basis of these values it is concluded that Ge does not occupy silicon sites of large T-0-T angles, i.e. that there exists a site preference of Ge atoms in the silica framework. that causes an easy incorporation of Ge up to 12 Ge/u.c.. Additional Ge may be incoporated only with firther distortions of the framework.Themain quantitative results that are related to the question of the isomorphous substitution of Si by Ge were as follows [ 11: i. XRD: identical pattern with silicalite or ZSM-5 (MFI-structure),VU,,=5389 A3 (Sil.: 5341 A3) ii. SEM: crystals up to 16 pm iii. n-hexane adsorption:ca. 1.2 mmoYg iv. DTA: exothermic peak at 718 K v. 29Si MAS NMR: -1 13 and -116 ppm: Si(4Si); -1 10 ppm : Si(lGe3Si) vi. IR: 3670 cm- : Ge-OH band 670 cm-l : Si-0-Ge (symmetric) 1030 cm-1: Si-0-Ge (asymmetric) 685 cm-l: Si-0-Ge (symmetric). vii. Raman: It should be mentioned that these parameters become evident mainly for samples with a high degree of substitution (SUGe-41).

62

R. Fricke, H . Kosslick, V. A. T u a n , I . G r o h m a n n , W. Pilz, W . Storek a n d G. Walther

Titanium-Silicalite (TS- 1) The TS-1 zeolite, first synthesized by Taramasso et al. [9,10] is without any doubt the most spectacular sample of a zeolite where a metal has substituted for silicon within the MFI structure. This attention is mainly caused by its remarkable properties in the oxidation I epoxidation of a great variety of organic compounds using hydrogen peroxide as oxygen source [11,12]. Titanium atoms in tetrahedral framework positions were assumed by these authors to be the active center for the catalytic reaction. This interpretation is, however, not accepted in full detail by other authors and a lot of investigation and speculation on the nature of the active center has been published. We have synthesized a series of three TS-1 samles having different Ti contents of Si/Ti=23-100 according to the published synthesis method and the conditions given in Tab. 1. The characterization has been carried out using XRD, SEM, EDX, 29Si MAS NMR, X P S , thermoanalysis, Raman and ESR spectroscopy. XRD pattern reveal that the synthesis products were highly crystalline and show MFI structure without any admixtures. In comparison with silicalite the lattice parameters show enhanced values leading to an increase of the unit cell volume (Tab. 2) by about 35-40 A3. Though the XRD pattern gave no indication to inhomogeniety the SEM pictures show a mixture of hexagonal and cubic crystals the portion of each depending on the Si/Ti ratio and modifications of the synthesis procedure. An estimation of the Si/Ti ratio by EDX gave a value of about 100. 2% MAS N M R measurements show a main peak at -1 13 ppm with a shoulder at about -1 16 ppm and confirm therefore the results already published in literature [4,5]. Like in the case of Zrsilicalite a small but well separated peak at about -103 ppm is additionally observed. DTA results show that the template is decomposed at a characteristic temperature of 683 K which 40 K higher than for silicalite. In contrast to Ge- and Zr-silicalite IR spectra show a characteristic band at about 960 cm-1 which is often taken as evidence for the incorporation of Ti into the framework of silicalite [ 13,141. Further evidence for the incorporation of Ti has been given by recent X P S investigations [15]. A binding energy of 460.3 eV is found for the Ti 2~312photoelectrons. This value is about 1.2 eV higher than that of octahedrally coordinated Ti in anatase and is attributed to tetrahedrally coordinated titanium in silicalite. This suggestion is supported by X P S studies of T i 0 2 4 0 2 glasses [ 161 and mixed oxides [ 171 where a similar shift has been observed k

1

I

325 C 0

315

U

n 305

t s 295

472

468

464

460

456

452

Binding Energy / eV Fig. 3. X P S spectrum of TS-1 Raman spectroscopy is used to obtain information on the state of Ti in TS-1 compared to anatase and some other titanium containing compounds or solids (Fig. 4). The results clearly show that the


63

Raman spectra of the TS-1 samples all contain the characteristic bands of silicalite. In addition, however, a band at about 960 cm-1 appeared in the spectrum [18] that is absent in those of anatase or any other modification of Ti02 (mtile, brookit) of Ba2TiOq or in other metal substituted h4FI zeolites. Carefilly concluded it seems reasonable to connect the appearance of this 960 cm-l band with the presence of Ti in the solid silicalite. No indications were found for bands representing

Fig. 4. Raman spectra of Silicalite (1) TS-1 (2), ('anatasel-containing) TS- 1 (3) and anatase (4) octahedrally coordinated Ti so that the presence of extra-framework Ti is excluded. This conclusion is also supported by the Raman spectrum of a sample with Si/Ti=23 where bands characteristic for anatase (144, 517, 640 cm-l) could be observed. It should, however, be mentioned that a quantitative estimation of each of the components is not possible because Raman spectroscopy is more sensitive against the presence of Ti in anatase than in the MFI structure. In an early study Varshal et al. [ 191 investigating Ti in oxygen coordination of various solids have observed Raman bands at 950 and 930 cm-1 in titanium-containing cristobalite and silica-glasses, respectively. Comparison with other possible coordinations of titanium and extended vibrational spectroscopic studies on titanium-containing vitrous silica and silicates confirmed applying also arguments from structural symmetry and selection rules [20] suggests this new band of TS-1 to be assigned to a stretching mode of tetrahedrally coordinated Ti-(0Si)q [18]. There are only few ESR spectroscopic studies of TS-1 [21,22]. The main reason for this is probably that Ti is in the four-valent state which is not paramagnetic, i.e. cannot be detected by ESR. Therefore, reduction with hydrogen or carbon monoxide is necessary to obtain Ti3' species. Although it is not obvious from the beginning on what the result of the reduction treatment migth be we have carried out such measurements in combination with adsorption studies with molecular oxygen, water and 1,3,5 triisopropylbenzene. The latter compound has been used because the

64

R. Fricke, H. Kosslick, V. A. Tuan, 1. Grohmann, W. Pilz, W. Storek and G. Walther

kinetic diameter of that molecule is about 8 A which does not allow it to enter the pores of the MFI structure (-5.7 A). The aim of this part of present studies (the details of which will be published elsewhere [23]) is to get information on the location, the reduction behaviour and a possible migration of Ti in TS-I . TS-1 samples in the as-synthesized or calcined form show no ESR signal. Reduction treatment of sample Si/Ti=23 ('anatasel-containing according to Raman spectrum) which includes a stepwise increase (100 K, 2 h, starting at room temperature) of the applied temperature show that reduction of the present Ti takes place starting at about 673 K. Further increase of the reduction temperature up to 873 K enhanced the intensity of the temperature dependent Ti3+signal (visible at 77 K only)

E l a w.1

1'"lQ.2.001

Fig. 5. ESR spectra of TS-I samples after reduction with hydrogen at 823 K ('anatasel-containing) TS- 1 (above), signal after leaching (below)

which is slightly axial and has an average g-value of g=1.938 (Fig. 5a). Leaching of the sample followed by the reduction treatment leads to a dramatic decrease of the signal intensity (Fig. 5b). If repeating the reduction treatment with sample Si/Ti=50 the corresponding Ti3+ signal is by a factor of more than 5 lower, i.e. the decrease in Ti3+ intensity is not proportional to the Ti content of the sample. Adsorption of molecular oxygen on these reduced samples leads to a disappearance of the Ti3+ signal and the appearance of 02- signals. Adsorption of water vapor as well as of 1,3,5 triisopropylbenzene has the same effect on Ti3+ [23]. DISCUSSION One of the most important tasks during the investigation of isomorphous substitution is to show evidently that the metal atoms have been incorporated into the framework of the zeolite. In the case of three-valent metals this can be proven by TPD of ammonia and by IR measurements where the

Isornorphous Substitution by Tetravalent Elements

65

observation of the acid bridging Si-OH-Me group unambigously shows that the metal is located within the zeolite framework. In the case of tetravalent metals these bridging OH groups are not formed, i.e. other methods or indications are necessary to show the incorporation and the tetrahedral coordination of the metal ion. Most of these studies have been carried out with TS-1. Several authors took the 960 cm-l I R band as an indication for the presence of Si-0-Me vibrations. The increase of the unit cell volume compared to that of pure silicalite or a shoulder at about -116 ppm in the 29Si MAS NMR were taken as fbrther indications. There are, however, some doubts on the absolute validity of these indications especially because some authors could show by means of various modifications of the synthesis procedure that it is possible to synthesize samples having all these spectroscopic properties but showing dramatic differences in their catalytic behaviour [24-261. This result and some of those described above indicate that it is highly recommended to search for each tetravalent metal the appropriate method for the indication of the isomorphous incorporation into the framework. This should be discussed for several results obtained in our laboratory. i. 2% MAS NMR spectra of MeIV-ZSM-5 samples usually show broadening of the signals with increasing Me content which is connected with the increasing degree of distortion of the silicalite lattice. Therefore, they do not allow to separate a signal of the Si(lMe3Si) chemical shift. In the case of Ge-ZSM-5 samples, however, a narrowing of the signal is observed at Si/Ge=l 1 allowing a deconvolution and the identification of a signal at -1 10 ppm as the Si(lGe3Si) chemical shift [l]. The broadening of the signal especially at the left flank of the main signal is taken as qualitatively indication for the Ge incorporation. ii. In the case of Zr-ZSM-5 where obviously the probability of the incorporation of Zr atoms is lower the combination of at least two results can be taken as evidence that a part of Zr is incorporated into the framework: Compared with pure silicalite the decompositon temperature of template is distinctly (about 30-40 K) higher. In the same way Zr-ZSM-5 does not show symmetry change to monoclinic as silicalite does but remain the orthorhombic one already observed for the as-synthesized form. iii. Raman spectroscopy is especially suitable for the investigation of TS-I . Extreme differences in the sensitivity of detecting Ti in various compounds (anatase etc.) or matrices allow to assign a band at about 960 cm-l to Si-0-Ti vibrations with Ti having tetrahedral coordination [18]. ESR spectra recorded aRer reduction of TS-1 shows that a part of Ti3+ is located on tetrahedral extra-framework positions. Additional evidence is obtained also by XPS measurements studying the activated as well as the leached form of an TS-1 sample with high Ti content. The adsorption of triisopropylbenzene the kinetic diameter of which (8.5. A) does not allow penetration into the zeolite pores gives fbrther evidence from ESR that the main part of these reduced extraframework Ti3+ species are located on the outer surface. TS-1 samples which are 'pure' according to Raman measurements do not show this behaviour. The authors gratefblly acknowledge the technical assistance of B. Kurrat and U. M a . R.F. thanks the 'Fond der Chemischen Industrie (VCI)' for financial support.

66

R. Fricke, H. Kosslick, V. A . Tuan, 1. Grohmann, W. Pilz, W . Storek and G . Walther

REFERENCES 1 H. Kosslick, V. A. Tuan, R. Fricke, Ch. Peuker, W. Pilz and W. Storek, J.Phys.Chem., 97( 1993)5678 2 J. M. Parera, Catal.Today, 15( 1992)481 3 M.K. Dongare, P. Singh, P. P. Moghe and P. Ratnasamy, Zeolites, 11(1991)690 4 A. Tuel and B. Ben Taarit, JCS,Chem.Com.,(l992)1578 5 S. A. Axon and J. Klinowski, Appl.Catal.A,81(1992)27 6 Z. Gabelica and J. L. Guth, in P. A. Jacobs and R. A. van Santen (Eds.), Zeolites: Facts, Figures, Future, Elsevier, Amsterdam, 1989, p.42 1 7 A. Lopez, M. Soulard and J. L. Guth, Zeolites, 10(1990)134 8 M. H.Tulier, A. Lopez, J. L. Guth and H. Kessler, Zeolites, 11(1991)662 9 M. Taramasso, G. Perego and B. Notari, US Patant 4 410 501 (1983) 10 G. Perego, G. Belussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, in Y. Murakami, A. Lijima and J. W. Ward (Eds.), New Developments in Zeolite Science and Technology (Proc. 7th 1nt.Zeolite C o d , Tokyo, August 17-22, 1986), KodanshaElsevier, Tokyo/Amsterdam, 1986, p. 129 11 B. Notari, in P. J. Grobet et al.(Eds.), Innovation in Zeolite Materials Science, Elsevier, Amsterdam, 1988, p.4 13 12 M. G. Clerici and P. Ingallina, J.Catal. 140(1993)71 13 M. R.Boccuti, K. M. Rao, A. Zecchina and G. Leofanti, in C. Morterra, A. Zecchina, C. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, p. 133 14 A. Thangaraj, R.Kumar, S. P. Mirajkar and P. Ratnasamy, J.Catal. 130(1991)1 15 I. Grohmann, W. Pilz, H. Kosslick and V. A . Tuan, 5th Confon Applic.of Surface and Interface Analysis (ESCASIA '93), Oct. 4-8, 1993, Catania, Italy, presentation accepted 16 Sh. Mukhopadhyay and S. H. Garofalini, J.Non-Cryst.Sol., 126(1990)202 17 A. Yu.Stakheev, E. S. Shpiro and J. Apijok, J.Phys.Chem., 97(1993)5668 18 W. Pilz, Ch. Peuker, V. A. Tuan, R. Fricke and H. Kosslick, Ber.Bunsenges.Phys.Chem., 97(1993)1037 19 B.G. Varshal, A. V. Bobrov, B. N. Marvin, V. V. Iljuchin and N. V. Belov, Dokl.Akad.Nauk SSSR (Russ), 216(1974)374 20 A. Chmel, G.M. Eranosyan and A. A. Karshak, J.Non-Chryst.Sol., 146(1992)213 21 A. Tuel, J. Diab, P. Gelin, M. Dufaux, J.-F. Dutel and Y. Ben Taarit, J.Mol.Catal., 63(1990)95 22 A. Zecchina, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti and M. Padovan, in P. A. Jacobs et al. (Eds.), Zeolite Chemistry and Catalysis, Elsevier, Amsterdam, 1991, p. 251 23 R.Fricke, H.Kosslick and V. A. Tuan, to be published 24 B. Kraushaar-Czarnetzki and J. H. C. van Hooff, Catal.Lett., 2(1989)43 25 D. R.C. Huybrechts, I. Vaesen, H. X. Li and P. A. Jacobs, Catal.Lett., 8(1991)237 26 A. J. H.P. van der Pol, A. J. Verduyn and J. H. C. van HOOKAppl.Cata1.q 92(1992)113

Synthesis and Catalytic Reaction of [Zr] ZSM-5

Gui-Ru Wang*, Xue-Qin Wang, Xiang-Sheng Wang and Shun-Xiang Yu Institute of Industrial Catalysts, Dalian University of Technology Dalian 116012,China.

ABSTRACT The effect of NazO/SiOz and SiOz/ZrOzmolar ratios on the synthesis of [Zr] ZSM-5 type zeolites and their catalytic activities in phenol hydroxylation were investigated. The effect of variation of NazO/SiOz and SiO,/ZrO, molar ratios in the starting reaction mixture on the synthesis of [ZrlZSM-5 was observed. At higher NazO/SiOz ratio, a large amount of a-SiO, was formed. In contrasttat lower NazO/SiOz ratio,some amorphous material was formed. [ZrlZSM-5 zeolites are able to be synthesized within Na,O/ SiO, ratios ranging from 0.033 to 0. 167. But the [ZrlZSM-5 zeolites formed from various NazO/SiOz ratios have different catalytic activities. At SiOz/ZrOzratios in the range 15- 95, [ZrlZSM-5 zeolite can also be synthesized. But the catalytic activities obviously depend on the SiOz/ZrOzratio. I t was also confirmed that Zr is incorporated into the zeolite framework and that Zr in the lattice plays an important role as an active site in the hydroxylation.

1. INTRODUCTION Zirconium silicalite ([ZrlZSM-5) ,like titanium silicalite ([TiIZSM-5 ,has very interesting properties toward catalytic oxidation. These isomorphous substituted zeolites are very promising in the catalytic oxidation of manufacturing fine chemicals[1]. A large number of patents and papers about the synthesis and characterization of [TilZSM-5 and its application in catalytic oxidation have been reportedC2-51. A few papers about t h e synthesis and characterization of [ZrIZSM-S have been reportedc6 81. However, articles concerning the effects of NazO/SiOz and SiOz/ZrOp ratios on the synthesis of [ZrIZSM-5 zeolite and its catalytic activity in phenol hydroxylation are not available in the literature. T h i s paper reports the influence of NazO/SiOz and SiO,/ZrO, ratios on t h e synthesis of [Zr]ZSM-5 zeolites and their catalytic activity.

-

2. EXPERIMENTAL 2. 1 Synthesis All the zeolite syntheses were carried out in stainless steel autoclaves (volume 100 0

To whom correspondence should be addressed.

67

68

G.-R. Wang, X.-Q. Wang, X.-S. Wang and S.-X. Yu

to 500 ml) at 443K,under autogenous pressure. The silicon source used is commercial sodium silicate ( SiOZ/NaZO = 3. 51) ,zirconium source is zirconium oxychloride (ZrOCl,) (AR) and the template is hexandiamine (HDA) (AR). First, two aqueous solutions were prepared according to different ratios :solution A containing sodium silicate and HDA ,solution B containing Zr0ClZand H,SO,. Then solution B was slowly added into solution A under continual stirring. A white gelatinous precipitate was formed and was stirred subsequently at ambient temperature. After 2h the mixture was transferred to an autoclave and heated to 363K. The mixture was kept to age for 8h at this temperature. Then the autoclave was heated to 443K and kept for 48h to 72h. After crystallization the product was washed with ion-free water to pH 7-8

and

dried at 373K for 6h in air. Before use the synthesized samples were further calcined in steps at 628K, 678K, 728K and 778K. The temperature was maintained for 2h at each step. 2. 2 Characterization The XRD spectra of the samples were recorded on a D/max-rb 12KW X-ray diffractometer with nickel-filtered Cu Ka radiation. The XRD powder patterns were recorded at a scanning rate of 28=0. 5"/min with silicon as an internal standard. The IR framework spectra (200-1300

cm-') of the samples were recorded with the 260-50 Hitachi spectra

photometer. The scanning electron micrographs G E M ) of the samples were obtained using the JEM-100 CEX scanning microscope. The elemental analyses of the samples were performed by means of ICP spectrometer. The adsorption capacity of water and n-hexane was carried out at a relative pressure of P/P, = 0. 5 and adsorption temperature 298K. The UV-VIS diffuse reflectance spectrum of the samples was recorded on a UV-240 UV-

VIS autographic (recording) apparatus. The surface acidic properties of the samples were examined by NH,-TPD. 2.3 Catalytic reaction The catalytic reactions were carried out in a batch reactor (100 ml capacity) under the following reaction conditions: Temp. = 343K, wt. of catalyst = lg, phenol/HzO\- 2 (30% aq. sol. ) molar ratio= 1,reaction time= 3h. The products were analyzed with G. C. The catalytic activities were characterized by the yield of catechol and hydroquinone in the hydroxylation of phenol.

3. RESULTS AND DISCUSSION 3. 1 Effect of NazO/SiOz ratio The effects of NazO/Si02 ratio on the formation and catalytic activity of [ZrlZSM-5 are listed in Table 1.

68

Synthesis and Catalytic Reaction of [ZrIZSM-S

Tabie 1 XRD pattern and catalytic activity of zeolites formed at various Na20/Si02 ratios Sample '

2

3

4

5

6

7

8

0.008

0.033

0.067

0.100

0.133

0.167

0.200

0.233

ZSM-5f

ZSM-5

ZSM-5

ZSM-5

ZSM-5

ZSM-5

ZSM-5f a-SiO2

a-SiO,+

1. 37

2. 35

11. 86

7. 80

0. 63

1 ~

~~

Na20/Si02

XRD

pattern

amorphous

ZSM-5

Catalytic activity

1. 05

No reaction No reaction

mol %

* Synthesis conditions (molar ratio) SiOz/ZrOz= 94,HZO/Si02=40,HDA/Si0z= 1.67 The results show that NazO/SiOz ratio has a clear effect not only on the phase purity of CZrIZSM-5,but also on its catalytic activity. At higher NazO/SiOz ratios a large amount of a-SiOz was formed besides [Zr]ZSM-5

. In contrast, some amorphous

matter was

formed along with [Zr]ZSM-5 at lower NazO/SiOz ratio. With NazO/SiOz ratio ranging from 0.033 to 0. 167,[Zr]ZSM-5 zeolites formed are much purer,but they have different catalytic activities. The difference is probably due to the difference in the amounts of zirconium introduced into lattices of zeolite synthesized as different NazO/SiOz ratios. NotariCS] proved that the presence of sodium or potassium can prevent the insertion of titanium into the silicate framework. T h u s it is reasonable to assume that the presence of sodium would also prevent the introduction of zirconium into lattices of zeolite synthesized at higher NazO/SiOz. As for the decreased catalytic activities of zeolite synthesized at lower NazO/SiOz there is still no explanation and the problem requires further investigation. 3. 2 Effect of SiOz/ZrOz ratio.

The effects of SiO,/ZrOz ratio on the formation and catalytic activity of [ZrlZSM-5 are listed in Table 2. The results show that [Zr] ZSM-5 zeolites with different catalytic activities were formed at SiOz/Zr02ratios between 15 and 94 and that the catalytic activity of zeolites decreased significantly with decrease in SiOz/ZrOzratio. The decrease of catalytic activity is probably because of the decrease in the crystallinity of [ZrlSM-5 zeolite synthesized. Our experiments show that the time of crystallization increases with decrease in S O z / ZrOz ratio. The formation of pure [ZrlZSM-5 zeolite is difficult at lower SiOz/ZrOz. From Table 2 it can be seen that [AIIZSM-5 and ZrOz are catalytically inactive. But

[Ti] ZSM-5 and [Zr]ZSM-5 possess catalytic activity in the hydroxylation of phenol. However, the catalytic activity of [ZrlZSM-5 is lower than that of [TiIZSM-5.

69

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G.-R. Wang, X.-Q. Wang, X . 4 . Wang and S.-X. Y u

Table 2 Unit cell parameters and catalytic activity of zeolites formed at various SiOa/Zr02 ratios Sample'

SiOJZrOl

XRD

(mixture)

pattern

[AIIZSM-5"

-

[TiIZSM-5 ' '

Unit cell parameters(A) a

b

Unit cell

volume C

C( h )'I

ZSM-5

20.067

19. 914

13.391

5351. 168

ZSM-5

20.104

19.910

13.444

5381.238

SiOt/ZrO: (product)

m

Catalytic activity mol %

No reaction 14.54

[ZrlZSM-5(1)

94

ZSM-5

20.087

19. 975

13.447

5395.296

109.48

11.86

CZrlZSM-(2)

30

ZSM-5

20.089

19.983

13.474

5409.082

44.83

a. 26

[Zr]ZSM-5(3)

20

ZSM-5

[ZrIZSM-5(4)

15

EM-5

ZrO t

0

* Synthesis conditions:NazO/SiOz=O. 1 HzO/Si02=40 HDA/SiO2=1.

3. 51 2. 64

No reaction

67

* * [TiIZSM-5 (SiOz/TiOZ= 73)zeolite and [AlIZSM-5 (SiOZ/AlzO3=54. 7) zeolite were synthesized in our laboratory. 3.3 Three-element phase diagram A three-element phase diagram of formation of [Zr]ZSM-5 with various SiOz/ZrOz and NazO/Si02ratios is shown in Figure 1. It can be seen from Fig. 1 that the crystallization formulae can be changed only within a small region. The reason for this narrow region is probably because the radius of zirconium atom is larger than that of titanium atom, so it is more difficult for zirconium to enter the framework.

3. 4 Characterization of [ZrlZSM-5 zeolites [ZrlZSM-5 zeolites have been successfully prepared using the hydrothermal synthesis procedure at the optimum conditions of NazO/SiOz ratio=O. 1 and SiOz/ZrOz ratio= 94. The results of SEM XRD, I R , UV-VIS, NH3-TPD, adsorption and activity-test of [ZrlZSM-5 are shown in Figs. 2-6 and in Tables. 2 and 3 respectively. T h e results of characterizations confirm that Zr is incorporated into the zeolite framework and that Zr in the lattice plays an important role as an active site in the hydroxylation of phenol. Inspection of XRD pattern clearly indicates that synthesized [ZrlZSM-5 possesses the pentasil - type framework structure and orthorhombic symmetry. T h e parameters and volume of unit cell of [ZrlZSM-5 are larger than those of [AIIZSM-5 and [TilZSM5. An increase in parameter and volume of unit cell results by decreasing SiOz/ZrOz ratio for [ZrlZSM-5. T h e increase in unit cell volume is due to the introduction of larger ion Zr'+ ( 0 . 73A ) in the framework lattice. T h e electron micrograph confirms the absence of amorphous matter outside the crystals of [ZrlZSM-5. The average size of the crystals is around 2 pm.

Synthesis and Catalytic Reaction of [Zr]ZSM-5

ZrOZ

A

SiO,

Ya20 Fig. 1. Three-element phase diagram of [ZrlZSM-5

T h e framework IR spectrum of [Zr]ZSM-5 recorded in the range of 200-1300

cm-'

shows that there are absorption bands at 321,389 and 746 cm-' in addition t o other bands characteristic of the MFI structure. F. Gonzlez-Vichez et al[10]. found that the absorption bands of IR spectrum at 320,392,740 ad 846 cm-' are brought about by the vibrations of the Z r - 0 bond in the transition metal tetraoxide tetrahedron. T h u s we suggest the existence of [ZrO,] tetrahedron in the framework of [Zr]ZSM-5 zeolite. This suggestion conforms with that of Pang[G]. Just like a strong transition band of [TilZSM-5 at 212nm[5] ,the UV-VIS spectrum of [ZrlZSM-5 exhibits a strong transition band around 472OOcm-' (212nm). T h e strong transition band of [ZrlZSM-5 was attributed to a transition having charge transfer character involving the Zr (IV) sitestwhereas pure silicalite does not give such a signal. T h e adsorption capacity of ZSM-5 zeolites reveals that [Zr]ZSM-5 possesses hydrophobic characteristic. The adsorption amount of n-hexane on [Zr] ZSM-5 is larger than that of H,O on [ZrlZSM-5. In the NHt3-TPD spectrum of [AlIZSM-5 there are two desorption peaks at about 573K (weak acidic site) and 770K (strong acidic site) while in that of [ZrIZSM-5 there is only one desorption peak at about 573K. So we can conclude that [Zr]ZSM-5 zeolite possesses only weak acidic sites. The weak acidity of [ZrlZSM-5 zeolite corresponds the electroneutrality of the [ZrO,] composition of the framework of zeolite.

71

72

G.-R. Wang, X.-Q. Wang, X.-S. Wang and S.-X. Y u

I

I

20

10

Fig. 2. SEM of [ZrlZSM-5

1400

1000

600

Fig. 3. XRD pattern of [ZrlZSM-5

200

W

P

wavenumber (cm.' )

Fig. 4. Framework IR spectrum

of [ZrIZSM-5

40

30

u

wavelength

0

W

p 0

(nm)

Fig. 5 UV-VIS diffuse reflectance spectra of samples

Synthesis and Catalytic Reaction of [Zr] ZSM-5

Table 3 Adsorption capacity of n-hexane and HzO Sample

Adsorption Relative temp

pressure

Adsorption capacity ml/g

n - hexane

K

P /P

n-hexane

H,O

H,O

[ZrIZSM-5

298

0. 5

0. 150

0.040

3. 75

[AIIZSM-5

298

0. 5

0.165

0.084

1. 96

373 473 573 673

713 873 973

Desorpt ion temperature (K 1 Fig. G. TPP spectra of samples

The [Zr]ZSM-5 zeolite is found to be active in the hydroxylation of phenol with 30% HzOzto catechol and hydroquinone. But ZrOz and [AIIZSM-5 zeolite are catalytical-

ly inactive. This indicates that Si'+ ions in the lattice framework are replaced by Zr'+ ionsc1' and that Zr in the lattice plays an important role as an active site in the hydroxylation.

4. CONCLUSION NazO/SiOz and SiO,/ZrO, ratios in the starting reaction mixture have obvious effects not only on the crystallinity of [Zr]ZSM-5 formed,but also on the amount of zirconium introduced into the lattice of [Zr]ZSM-5 zeolite formed. The catalytic activities of [ZrlZSM-5 zeolites formed with various NazO/SiOz and SiOz/ZrOz ratios are quite different. A three-element phase diagram of formation of [ZrlZSM-5 shows that the crystal-

73

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G.-R. Wang, X.-Q. Wang, X . 3 . Wang and S.-X. Y u

lization formulae can be changed only within a small region. The results of characterization and catalytic activity determination confirm that Zr is incorporated into the zeolite framework and that Zr in the lattice plays an important rote as an active site in the hydroxylation.

ACKNOWLEDGMENT The authors express their sincere thanks to Professor Z. H. Zou (Department of applied chemistry ,Dalian University of Technology) for his help in registering the UV-VIS diffuse reflectance spectra.

REFERENCES [ 11

M. K. Dongare, P. Singh, P. P. Moghe, and P. Ratnasamy, Zeolites, vol 11 (1991) 690 R. A. Sheldon, in G. Centi and F. Trifiro (Editors) ,New Developments in se[Z] lective Oxidation (Studies in Surface Science and Catalysis ,Vol. 55) ,Elsevier,Amsterdam, (1990) 1-32. [3] A. J. H. P. van der Pol and J. H. C. van Hooff,Appl. Catal. ,92(1992) 93-111. A. Thangaraj,R. Kumar and P. Ratnasany,Appl. Catal. ,57(199O)Ll-L3 [4] [5] G. Bellussi and V. Fattoretin P. A. Jacobs et al. (Editors) ,Zeolite chemistry and Catalysis, (1991) Elsevier Science Publishers B. V. ,Amsterdam,79. Pang Wenqin,Yu Long, Wu Yaping,Chemical Journal of Chinese Universities. [G] vol 7 No 1 (1986) 63. Costantin,Michel. ,Guth,Jean Louis;Lopez,Annie;Popa ,Jean Michel. E P 466 [7] 545 (1990). Grace,W. R. and Co. Jpn. Kokai Tokkyo Koho Jp 02 296 715 11989). [8] - (1987) 413. [9] B. Notari,Stud. in Surf. Sci. and Catal. ,37 F. andGriffith, W. P . . J. C. S.Dalton, 1 3 , 1416(1972). Gonzlez-Vichez, [lo]

11. structure


Fine Structures of Zeolites: Defects, Interfaces and Surface Structures. An HREM Study

O.Terasakil, T.0hsuna2, V.Alfredsson3, J-0. Bovin3, S.W.Cad, M.W.Anderson5 and D.Watanabe2 1Dept. of Physics, Tohoku Univ., Aramaki Aoba, Sendai 980, JAPAN Vollege of Science & Engineering, Iwaki Meisei Univ., Iwaki, JAPAN 3National Centre for H E M , Chemical Centre, Lund Univ., Lund, SWEDEN 4Unilever Research, Port Sunlight Lab., Wirral, Merseyside L63 3JW, UK 5Dept. of Chemistry, UMIST, Manchester M 60 lQD, UK

ABSTRACT HREM study on fine structures of zeolites, especially defects, interfaces and surface structures is reported by taking examples of LTL, FAU and EMT. Four different boundaries in LTL are shown and an easy way to distinguish them is suggested. It is concluded from observations of atomic resolution surface-profile imaging that double-hexagonal rings act in crystal-growth process as growth-units in FAU and probably in EMT. The first observation of the effect of dealumination on FAU and EMT is reported in atomic scale. Application of HREM to determine the complicated structure, which is impossible to be solved without knowledge of the nature of heavy defects, is also shown for the case of ETS-10 in order to show the advantage of the HREM. I. INTRODUCTION Zeolites have attracted a lot of attention both as containers for making quantum confined materials and as catalysts. In order to determine the structures X-ray single crystal diffraction is the best method of choice if a large and near perfect crystal is available for the experiment. However, synthetic crystals contain many different kinds of defect and are usually too small in size. Furthermore some crystals, i.e. BETA, ETS-10, etc., contain so many faults that their structures are very hard to be solved. High resolution electron microscopy (HREM) is the ideal technique for studying the fine structures of zeolites such as interfaces, boundaries and surface steps, and scanning electron microscopy (SEM) for external crystal morphology. However, zeolites are very sensitive to electron beam irradiation and become amorphous quickly under the beam. Here we will report our recent results on these topics in order to show the great advantages of using the techniques for the crystallographic characterization of zeolites.

78

0.Terasaki, T. Ohsuna. V. Alfredsson, J - 0 . Bovin, S. W. Carr, M . W. Anderson and D. Watanabe

11. LTL

LTL has a one-dimensional channel with 12 membered ring aperture(Fig. 1) and belongs to the space group P6/mmm. The channels are well separated by the framework atoms and therefore it is a very attractive container for making one-dimensional materials. Recently the Pt-LTL system has attracted a lot of attention from a catalytic point of view especially for converting n-hexane into aromatics. Blocking of the one-dimensional channels by crystal defects induces serious problems for both applications. There are several different kinds of boundaries which are produced during the process of crystal growth. It is quite common in LTL to find the growth of one crystal onto another or into another in a wedge shape. The orientational relation between the crystallites is easily determined by an electron diffraction(ED) pattern. In order to avoid sample preparation artifacts for EM experiments, crystals were suspended without crushing after dispersion by ultrasonic wave. Figs. 2 (a), (b) & (c) are ED patterns from a single crystallite, composite crystals with rotation(ti1ting) angles about the 6-fold c-axis between the crystallites of ca. 30' and a few degrees, respectively. In fig. 2(b), the spot indicated by an arrow corresponds to the 700 and 440 reflections, which are very close in lattice spacing, from different crystallites. Two examples of crystal overgrowth are shown in Figs. 3 (a) & (b). The only difference between the two is in the magnitude of the rotation angle. We can observe different Moire patterns, i.e. overlapping lattice, in HREM images depending on the angle, and the lattice is basically incommensurate. We have reported the coincidence boundary of this type, 413 x d l 3 R32.2' which blocks more than 90 % of the onedimensional channels of LTL if a crystal grows on the other with rotation by ca. 30' along the caxis[l]. ED pattern and HREM image of this case correspond to Fig. 2(b) and Fig.3(a), respectively. In Fig. 3(a), the upper right(A) and lower left(B) crystallites overlap at right side(C). The occurrence of this boundary is quite common. The type of a coincidence boundary is determined exactly only by HREM images, because the structure is incommensurate and takes "domain structure", and an ED pattern gives only average information. The case of small angle tilt corresponds to ED pattern of Fig.2(c) and HREM image of Fig. 3(b). In Fig. 3(b), the left part(A) The next two examples are corresponds to ca. 10' tilt and the right part(B) to ca 4' tilt. unoverlapped crystals and therefore the effect on both containers and catalysts is expected to be small. Fig.3(c) shows an HREM image of the case where a crystallite intergrows into another in a wedge shape. Boundaries tilted by ca.30' are indicated by white arrows, and the corresponding ED pattern is also of Fig. 2(b) type. The last example is a "dislocation" and the HREM image is shown in Fig. 3d. The diffuse intensity produced from the defect shown in Fig.3d is too small to be detected in ED pattern. It is not easy to take such HREM images, but by ordinary EM it is very easy to observe images at low magnification in order to check whether a crystal overgrows on another and easy to observe ED patterns, because both can be done under much less electron irradiation density in comparison with taking HREM images. From the combination of these experiments, it is relatively easy to characterize LTL crystal. Synthetic techniques have shown a big improvement in reducing the density of the crystal overgrowths, as shown in Fig. 4(a),(b) and (c).

Fine Structures of Zeolites: HREM Study

79

In. FAUEMT a). Intergrowth Delprato et al. succeeded in synthesizing EMT, a hexagonal variation of FAU, by using 18crown-6( 18-c-6) as a structure directing agent[2]. They also showed that FAU could be synthesized by 15-crown-5(15-c-5). We recently reported on the first synthesis aimed at preparing controlled intergrowths of the two phases by using mixtures of the crown ethers while keeping everything else constant, gel composition A1203: 1OSiOz: 2.4Na20: 140H20: 1 crown ether[3]. Figs. 4(a) and (b) are SEM images for the crystals synthesized using 100 % 15-c-5 and 100 % 18-c-6, respectively. The shapes are octahedra and hexagonal plates, which are compatible with their symmetries. From the crystal synthesized with 67 mole % of 18-c-6 , it was confirmed by HREM images( e.g. Fig. 5 shows an HREM image taken at the [ 1101 direction of FAU) and ED patterns that there was a spatial correlation between the blocks of EMT and FAU structures. While, it is concluded from X-ray powder diffraction patterns that the critical concentration of 18-c-6 required to generate the EMT is ca. 50 mole%. The variation in crystal structure of EMT and FAU might correlate with the variation in surface solution concentration of 18-c-6, and an oscillatory crystal growth between EMT and FAU is observed in the system[3,4]. In order to understand the role of 18-c-6 for directing the structure to EMT, it is important to determine how the Na+/18-c-6 complex fits into growing surface. As the content of Na20 in gel increases from 2.4 to 2.6, 2.75 and 2.9 in 100 % 18-c-6, we can observe an increase of cubic phase in both HREM and SEM images. Fig. 4(c) is a SEM image obtained from crystals synthesized with 2.6 Na20. A common feature of the external morphology obtained by SEM is that part of octahedra are attached to the surfaces of the hexagonal plates. This suggests that rates of nucleation and growth are larger for EMT than FAU phase at these conditions[4]. b). Surface It is vitally important to observe growth surfaces for understanding crystal growth processes in hydrothermal synthesis. The growth form is governed by the anisotropy of the growth rate and is thus sensitive to the growth conditions. Using HREM, structures of clean surface-steps of FAU have been studied by atomic resolution surface-profile imaging. Two FAU crystals synthesized by different methods (i) without template(A) and (ii) by using the crown-ether as template(B), were observed. The height of the steps at the crystal surface corresponds to one faujasite sheet for both cases. The simulated image of surface model with an incomplete sodalite cage gives the best fit for the crystal A( see Fig.6(a,b)). For crystal B, on the other hand, a simulated image with a complete double-hexagonal ring fits best(Fig. 6(c,d)). Therefore it is concluded that double-hexagonal rings play an important role or step in the crystal growth process[5]. In other words, once the doublehexagonal rings are formed from either monomers or polymers, they are stabilized to advance towards next growth process and consequently the double-hexagonal ring acts as a growth unit.

80

0. Terasaki. T. Ohsuna, V. Alfredsson, J - 0 . Bovin, S. W . Carr, M . W. Anderson and D. Watanabe

IV.Dealumination Dealumination is a well-established technique for improving thermal stability and resistance to acid of zeolites. We have succeeded in dealumination from ordinary FAU with Si/Al=2.8 by HCl and calcination to FAU with Si/Al=340. The crystallinity is not lost although the treatment is very severe. We can observe from HREM images that very large mesopores are produced during the process[6] and there are amorphous layers at the crystal surface[5]. The size of crystal synthesized without crown ether is normally very small and the shape does not show a regular habit; it was difficult to observe the initial stage of dealumination. On the other hand, we have confirmed that EMT and FAU are made with very regular external shapes, i.e. hexagonal plates and octahedra, respectively, by using the two different crown-ethers mentioned above. The crystals were dealuminated mildly by using ammonium hexafluorosilicate from a WAl ratio of 3.5 to ca. 5.5. A few distinct features were found from the acid treatment; the most prominent are (i) amorphous layers are formed which follow the original crystal shape, (ii) many mesopores are formed between the amorphous layers and crystals, and (iii) boundaries or defects are preferentially attacked. Typical example is shown in Fig. 7 and the details will be published in elsewhere[7].

V. ETS-10 In 1989 a new family of microporous titanosilicates was discovered. They are ETS-4 and ETS-10 and both show adsorption characteristics of microporous materials. Their structures are suggested to be constructed from non-traditional primary building units. In the case of ETS-10, it displayed characteristics indicating an effective pore diameter of approximately 8 A@]. The difficulty in solving the structure resides in the fact that (i) it can only be synthesized as powder( particle size is ca. 5pm) and (ii) it contains disorder-exemplified by broad powder X-ray diffraction pattern. But by ED patterns we can observe from a crystal intensity distribution of diffuse scattering which is dependent on the manner of disorder as well as reflection indices. Fig. 8 shows an HREM image of ETS-10 taken along the channels and clearly suggests the manner of defects. From these observations we can derive a basic unit, i.e., a rod and basic structure of layer whch is composed of the rods. The rod has the composition of TiSi5013 and the rods are running parallel to the two principal axes. We can derive many different stacking of layered structures, i.e. polymorphs. Two end members have C2/c symmetry and P41 symmetry showing chirality, respectively. Both contain 12-ring pores of three-dimension and Ti takes octahedral sites. The details will be published soon in elsewhere[9,10]

Acknowledgements A part of this study is supported by Tosoh(0T & TO) and Sumitomo Chemical(0T). Support from British Council to MWA for travel is also acknowledged.


References [l]O.Terasaki, J.M.Thomas & G.R.Millward: Proc. R. SOC.(Lond.) A395 (1984), 153. [2]F.Delprato, L.Delmotte, J.L.Guth & L.Huve: Zeolites 10 (1990), 546. [3]0.Terasaki, T.Ohsuna, VAlfredsson, J-0 Bovin, D.Watanabe, S.W Carr & M.W.Anderson: Chem. Mater. 5 (1993), 452. [4]T.Ohsuna, O.Terasaki, V.Alfredsson, J-0 Bovin, D.Watanabe, S.W.Carr & M.W.Anderson: To be submitted( 1993). [S]V.Alfredsson, T.Ohsuna, 0.Terasaki & J - 0 Bovin: Angew. Chem. Int. Ed. Engl. 32(1993), 1210. [6]H.Horikoshi. S.Kasahara, T.Fukushima, KAabashi, T.Okada, 0.Terasaki & D.Watanabe: J. Chem. SOC.Jpn. (1989), 398. in Japanese. [7]T.Ohsuna, O.Terasaki, D.Watanabe, M.W.Anderson & S.W.Carr: To be submitted(l993). [8]S.M.Kuznicki, K.A.Thrush, F.M.Allen, S.M.Levine, M.M.Hami1, D.T.Hayhurst & M.Mansou: Molecular Sieves, Synthesis of Microporous Materials. vol. 1, ed. M.L.Occelli & H.Robsm, pp. 427-453, 1992. [9]M.W.Anderson, O.Terasaki, T.Ohsuna, A.Philippou, S.P.MacKay, A.Ferreira, J.Rocha & S.Lidm: Submitted( 1993). [lO]O.Terasaki, T.Ohsuna, M.W.Anderson, S.Lidin & D.Watanabe: in preparation( 1993).

Fig. 1. Schematic drawing of projection of LTL framework along the c-axis.

Fig. 2. [00.1] electron diffraction patterns obtained from LTL. From a perfect crystallite(a), composites of crystallites with ca.30' rotation or tilt(b) and with small angles(c).

81

82

0. Terasaki, T. Ohsuna, V. Alfredsson, J - 0 . Bovin, S. W. Carr, M. W. Anderson and D. Watanabe

Fig. 3. HREM images of LTL taken with [OO.11 showing structural details. Moire effects are due to overlapping crystals with rotaion angle of ca. 30"(a), of ca. 4' and 10" (b). (c) shows ca. 30" tilt boundaries and (d) "dislocation" type.


Fig. 4. SEM images from FAU(a), EMT(b) and from crystal synthesized with an excess of Na20(c).

Fig. 6. HREM images of surface structures of FAU, synthesized without template (a,b) and with crown-ether(c,d).

83

84

0. Terasaki, T. Ohsuna, V . Alfredsson, J - 0 . Bovin, S. W. Carr, M. W. Anderson and D. Watanabe

Fig. 8. HREM image of ETS-10. Fig. 5. HREM image of intergrowth of EMT and FAU.

Fig. 7. H E M image of dealuminated FAU by ammonium hexafluorosilicate.

Statistical Mechanics of Si, Al Ordering in A-type Zeolites

Carlos P. Herrero Instituto de Ciencia de Materiales, C.S.I.C., Serrano, 115 dpdo., 28006 Madrid, Spain

ABSTRACT The distribution of Si and A1 atoms on the framework of zeolite A is analyzed by the Monte Carlo method for Si/AI ratio from 1 to 3. Atom configurations are generated at 400 K by using an interatomic potential which includes long-range electrostatic interactions. Special emphasis is laid upon the study of thermodynamical variables related with this atom distribution (internal energy, configurational entropy, free energy). Our results indicate that the Si,A1 ordering contributes appreciably to stabilize the framework, especially for Si/Al ratio near 1.

INTRODUCTION Computer simulation of atom distributions on underlying lattices by the Monte Carlo (MC) method is nowadays widely employed to study different structural and thermodynamical properties of solid compounds. In particular, this computational technique has been extensively employed to analize atom ordering in metal alloys, as well as magnetic properties of solids [l]. In recent years, this method has been applied to study the arrangement of silicon and aluminum atoms in tetrahedral (T) networks of alurninosilicates [Z-41. Several works have been

devoted to analyze this atom distribution in the faujasite framework. In this case, the structural features of the atom distribution obtained from MC simulations were compared with 29Si Nuclear Magnetic Resonance (NMR) data, and the results found by both techniques showed good agreement [4]. Moreover, several thermodynamical quantities characterizing the Si,Al arrangement in faujasite-like zeolites have been derived from MC simulations (51. In this paper, we report on the structural and mainly the thermodynamical properties (internal energy, configurational entropy, free energy) of the Si,A1 distribution on the zeolite-A framework. Previous work indicated that, as in the case of faujasite-like zeolites, "Si NMR data of ZK4 and N-A zeolites [6,7] can be interpreted from results of MC simulations performed by using adequate interatomic potentials [8]. METHOD OF CALCULATION Our simulation cell was generated as a 2 x 2 x 2 supercell from a unit cell with space group 85

86

C. P. Herrero

PmSm. The atom coordinates were taken from Pluth and Smith [9] for dehydrated zeolite A. The composition of the simulation cell is N~(AlnSilg2-,,03~4), where the number of A1 atoms, n, is in the range 48-96. In the following, we will call 21 and 21 the atomic fraction of A1 and Si, respectively, in the tetrahedral sites (21 = n/192, 2 2 = 1 - 21). The exchangeable Nat cations are assumed to be randomly distributed over the available extraframework sites. In our calculations, the framework geometry is assumed to be fixed, irrespective of the Si,Al ordering. In the course of the Monte Carlo simulations, silicon and aluminum atoms are free to distribute over the tetrahedral sites of the framework according to the interatomic potential, without any symmetry constraint. The lattice energy for a given T-atom distribution is calculated as a sum of three contributions: Coulomb interactions, short-range dispersion-repulsion terms, and oxygen polarization. The energy contribution of the long-range Coulomb interactions is calculated by the Ewald method, using a point charge model, and for the short-range dispersion-repulsion energy, we employ a Buckingham potential. The electric field at each oxygen center depends on the actual Si,Al distribution, and consequently the oxygen polarization energy cannot be neglected. An important parameter in the calculation of the electrostatic energy associated to a given atom distribution is the charge difference, Sq, between silicon and aluminum atoms in this structure. For this charge difference we have taken the value Sq = 0.26e (e, elementary charge), which gives the best agreement between the atom distribution obtained from MC simulations and that derived from "Si NMR data [6,7]. More details on the interatomic potential were given elsewhere [4],and will not be repeated here. We have simulated the atom distribution on the zeolite-A framework for 25 compositions in the 21-range from 0.25 to 0.5 (Si/Al ratio from 1 to 3). For each framework composition, we sample the canonical ensemble ( N , V , T fixed) by the Metropolis procedure [I] to obtain information upon the atom ordering at the temperature of hydrothermal synthesis ( T H 400 K). For a given 21, the sampling consists of 5 x lo5 Monte Carlo steps, each one including an attempt to interchange each A1 atom in the simulation cell with a nearby Si atom.

-

The configurational entropy of the atom distribution at temperature TH is calculated by thermodynamic integration along a reversible path by means of the equation

where SC(m)is the configurational entropy of a random T-atom distribution, which has been taken as reference state at T = 00. The heat capacity, c,, is obtained from our MC simulations by the formula [ l o ]

where Icg is the Boltzmann constant and the average square fluctuations of the configurational energy, U , are given by

( b u y = < u2> - < u

>2

(3)

Statistical Mechanics of Si, A l Ordering

87

and the angle brackets mean averages over a MC trajectory. More details on the calculation of the configurational entropy from MC simulations can be found in ref. [5]. Changes of the Helmholtz free energy, A F , with respect to a random atom distribution have been calculated as

where E denotes the average configurational energy at TH. RESULTS The results of our simulations are in line with the avoidance of A1 atoms in neighboring tetrahedra (Loewenstein's rule). There appears also a tendency for the A1 atoms to be dispersed more than required by this rule, with an effective repulsion of aluminum atoms in next-nearest tetrahedra. The short-range order present in the atom distribution was quantified by means of the pair correlations between nearest and next-nearest T atoms, and the results obtained were in good agreement with those derived from "Si NMR spectra, as shown elsewhere [ll]. We find that long-range ordering occurs for Si/Al lower than 1.3, where two different subsets of T atoms (say T I and Tz), which alternate in the structure, with different atom occupancy can be distinguished. This can be seen in Fig. l a , where we show a sketch of the zeolite-A framework with the resulting atom distribution for Si/AI = 1, which agrees with the previously known sub-lattice ordering in the space group Fm3c [9,11]. For Si/Al > 1.3, only short-range order is found. In F i g . l b we display a microstate of the atom distribution corresponding to Si/A1 = 3, which shows an apparent disorder in the atom distribution, and suggests that the corresponding configurational entropy will be appreciable, and will have to be taken into account in the calculation of the framework stability.

(a) Si/Al= 1

(b) Si/AI = 3

Fig. 1. Sketch of the zeolite-A framework showing Si,Al configurations obtained from MC simulations at 400 K: a) Si/A1 = 1; (b) Si/Al = 3. Only tetrahedral sites of the framework are shown. Open and fi(led l circles represent Si and A1 atoms, respectively. The average values of the internal energy, obtained from our Monte Carlo runs for different A1 loadings, are presented in Fig. 2. In this figure, we give for each framework composition the

88

C. P. Herrero

energy difference, AE, with respect to a random atom distribution. This difference is a measure of the change in the internal energy of the material due to Si,A1 ordering in the tetrahedral network. The lattice stabilization 1 AEl increases with increasing A1 content, and reaches a value of about 1100 kJ/mol for z1= 0.5. In spite of the continuous decrease of AE as a function of zl,this energy difference does not follow a linear dependence in the whole composition range. In fact, one can distinguish three different regions with nearly linear dependence on z1, but with different slopes. This behavior is related to the appearance of different ordering schemes in different composition regions, as observed previously for faujasite-like zeolites. This point will be discussed below in connection with the composition dependence of the free energy.

0.25 0.30 0.35 0.40 0.45 0.50

Al fraction Fig.2. Average configurational energy obtained from MC simulations at TH = 400 K versus A1 atomic fraction in the tetrahedral sites. For each framework composition, the zero energy corresponds to a random T-atom distribution. As indicated above, the configurational entropy associated to the atom distribution is not negligible for framework compositions in which there appear an appreciable degree of disorder. This makes that the only calculation of the internal energy is not enough to discriminate between different ordering patterns, and to analyze the stability of the framework as a function of the A1 loading. Moreover, the entropy is a thermodynamical variable specially adequate to quantify the degree of disorder present in the atom distribution. In Fig. 3 we present the dependence of the configurational entropy on the A1 content. As expected, the difference between the entropy of the simulated atom distribution and that corresponding to a random atom arrangement increases for increasing A1 loading. This is due to the fact that the atom distribution is more ordered for higher A1 fractions. For Si/AI= 1 (z1 = 0.51, one has essentially an ordered atom distribution with A1 on sites TI and Si on sites Ta, and consequently, S, reaches its minimum value. We note in passing that for low A1 contents, the calculated entropy decreases for decreasing synthesis temperature, in agreement with the expectation that the atom distribution will be more ordered for lower synthesis temperatures.

88

Statistical Mechanics of Si, A1 Ordering

8 M 3

\

-0.4 -0.6

89

\-..

- .,\

I

-

h...

.' ...-.... .... 2.

B

3

-0.8

-

-1.0 -

8

-1.2

Once calculated AE and AS for each composition, we obtain the free energy A F by means of eq. 4. Since MC simulations yield atom distributions in thermodynamic equilibrium at the selected temperature, the free energy will reach for each A1 loading the minimum value attainable in the context of the interatomic potential employed here. As found for the energy and the entropy, changes of free energy with respect to a random T-atom distribution grow up for increasing sl.The maximum lattice stabilization is found for s1 = 0.5, where A F = 693 kJ/mol. On the other side, for z1 = 0.25 we obtain A F = 253 kJ/mol. At this point, it is more interesting to calculate the change of free energy per A1 atom, AF/n, which is shown in Fig. 4 versus the number of A1 atoms per simulation cell. This function presents approximately a linear dependence as a function of n in each one of the composition regions: A, n = 48 - 66;

= 8

-5.0

\

8 3t;l!

-5.5

-6.0

2

@8

g

- *\\

-6.5

-

-7.0 1

-

.,'

B -... a.....w.,

. I . . . _ . . .

'4,

c\' t

\-

-7.5

Al atoms per cell Fig.4. Free energy per A1 atom, AFln, for several A1 contents in the range n = 48 - 96. The stability of the framework increases with decreasing free energy.

C. P. Herrero

90

B, n = 66 - 82; C, n = 82 - 96. The dotted lines in Fig. 4 are fits to the calculated points, with slopes of -57.5, -10.4, and -52.1 J/mol in regions A, B, and C, respectively. DISCUSSION The behavior of the function A F / n shown in Fig. 4 is similar to that found for faujasitetype zeolites 151, where we found also three different composition regions with linear behavior. In that case, the breaks of the function A F / n appeared at the same framework compositions where x-ray diffraction data detected discontinuities in the lattice parameter a0 [12]. A detailed analysis of the atom distributions simulated for faujasites confirmed the suggestion of Dempsey [12] that those breaks in the composition dependence of a0 are associated to changes in the relative disposition of the A1 atoms in the hexamer rings of the framework. For zeolite A, we find a similar situation, and the atom distribution can be qualitatively described by the ratio between the number of A1 pairs in meta, N,, and in para, N,,, positions in the hexamer rings of the zeolite-A network. Thus, for region A ( n = 48 - 6 6 ) , we have N,,,/Np 1. In region B, N ,

-

increases fast as a function of zl,whereas Np decreases continuously. In region C, where the atom distribution displays sub-lattice ordering, Np 0 and most of the A1 atoms are located in meta positions. Fig. 4 indicates also that in region A the framework is less stable than in region

-

C, in agreement with the observation that pure A-type zeolites are difficult to be synthesized with z1 < 0.35 [7].

0.38

W

0.30 0.25 0.30 0.35 0.40 0.45 0.50

Al fraction E the entropy and energy conFi 5. Composition dependence of the ratio T H A S ~ A between tri utions to the free energy.

t

Finally, we would like to emphasize the importance of MC simulations to study the ordering of Si and A1 in aluminosilicates. A clear advantage of MC simulations over energy minimization is the fact that the former are carried out at finite temperatures (7' > 0; in our case, the synthesis temperature), and thus one takes into account thermal effects, which are in principle not negligible. This kind of simulations can also give a quantitative measure of the configurational entropy of the atom distribution, as shown above. The entropy contribution to the free energy is

Statistical Mechanics of Si, A1 Ordering

91

not negligible versus the corresponding stabilization due to the change of internal energy in the lattice. This can be seen in Fig. 5 , where we display the XI-dependence of the ratio T H A S ~ A E for A-type zeolites. This ratio lies between 0.3 and 0.4 in the range 21 = 0.25 - 0.5, indicating that the entropy contribution to the free energy at TH is more than 30% of the energy gained by the atom ordering. The ratio between the entropy and the internal energy terms tends to go up with increasing A1 content, but there appears a discontinuity at 3c1 0.38. This fact will be related with the characteristics of the atom distribution, but its interpretation is not clear at present. Further investigation is necessary to clarify this point. N

ACKNOWLEDGMENTS R. Ramirez and L. Utrera are thanked for assistance with the computer facilities. This work was supported by CICYT (Spain) under contract number MAT91-0394. REFERENCES 1 K. Binder and D.W. Heermann, Monte Carlo Simulation in Statistical Physics, Springer, Berlin, 1988. 2 A.J. Vega, Am. Chem. SOC.Symp. Ser., 218 (1983) 217. 3 C.M. Soukoulis, J. Phys. Chem., 88 (1984) 4898. 4 C.P. Herrero and R. Ramirez, J . Phys. Chem., 96 (1992) 2246. 5 C.P. Herrero, L. Utrera and R. Ramirez, Phys. Rev. B, 46 (1992 787. 6 J.M. Bennett, C.S. Blackwell and D.E. Cox, J . Phys. Chem., 87 1983) 3783. 7 R.H. Jarman, M.T. Melchior and D.E.W. Vaughan, Am. Chem. SOC.Symp. Ser., 218 (1983) 267. 8 C.P. Herrero, J . Phys. Chem., 97 (1993) 3338. 9 J.J. Pluth and J.V. Smith, J. Am. Chem SOC.,102 (1980) 4704. 10 D. Chandler, Introduction to Modern Statistical Mechanics, Oxford University Press, New York, 1987. 11 C.P. Herrero, J. Phys.: Condensed Matter, 5 (1993) 4125. 12 E. Dempsey, G.H. Kiihl and D.H. Olson, J . Phys. Chem., 73 (1969) 387.

1


Topological and Stereochemical Characteristics of Zeolite Frameworks

M. Sat0 Department of Chemistry, Gunma University Kiryu, Gunma 376, Japan ABSTRACT A new approach for the characterization of zeolite frameworks has been tried by applying the concept of topological and stereochemical compatibility to consecutive concentric clusters (CCL). Topological compatibility means the topological consistency between a kernel and peripheral clusters, while the stereochemical one imposes chirality and steric compatibility in threedimensional space. This method is successfully applied to the characterization of 12 distinct zeolite frameworks: AFT, AEI, CHA, EMT, FAU, GIs, GME, KFI, MER, PAU,PHI, and RHO. I NTRODUCT ION Zeolite frameworks can be topologically characterized in terms of the secondary building units (SBU) criterion [l] or the CCL one [ 2 ] . The SBU is a simple and effective geometrical means of characterizing the zeolite frameworks, but inferior because it lacks a mathematical foundation. In contrast, the CCL is based on the graph theory and can be applied widely to existing and non-existing frameworks. Any kind of zeolite framework can be completely covered with CCLs by extending the topological distance from 0 to infinity, and the characteristics of zeolite frameworks are realized on those of the CCLs. However, it is also true that the CCL representation becomes very complicated with increase of topological distance and it is not always realistic as a framework characterization. In this paper, a new topological and stereochemical approach to characterize zeolite frameworks has been tried on the complicated CCLs.

93

94

M. Sato

TOPOLOGICAL COMPATIBILITY An nth CCL is defined as a set of all the points ranging from topological distance 0 to n , and all the lines responsible for the connection between them. Fig.1 shows some examples of consecutive CCLs up to the 3rd step. Both 0th and 1st CCLs are common to any kind of frameworks, but 2nd and 3rd CCLs are not. As can be seen, FAU (faujasite) can be topologically differentiated from both ANA (analcime) and LAU (laumontite) in the 2nd step, because they contain different kinds of 2nd CCLs. Also LEV (levynite) containing two kinds of CCLs is obviously differentiated from all the others in the 2nd step. ANA is differentiated from LAU in the 3rd step. These are topological criteria for characterizing zeolite frameworks. In these consecutive CCLs, it must be noted that an (n+l)th CCL is formed on the basis of nth CCLs, all of the same kind or of several kinds. Strictly speaking, an (n+l)th CCL is constructed on the basis of five nth CCLs, one central CCLs (kernel CCL) and four surrounding ones (peripheral CCLs). This can be mathematically expressed as, C(n+l,O)= C(n,O) + C(n,l) + C(n,2) + C(n.3)

+ C(n,4)

in which C(n+l,O) and C(n.0) denote the (n+l)th and nth kernel CCLs having its origin at site 0, while C(n,p) (p=1,2,3,4) denote the nth peripheral CCLs having their origins at the sites 1,2,3, and 4. Sites 1,2,3,4 are those next to site 0. The symbol + indicates coupling the clusters. Fig. 2 shows their connective FAU in Fig. 1. In this construcrelations which is realized on tion, a topological compatibility between the kernel and the peripheral clusters is required to form a new large CCL. Two CCLs are completely compatible when they overlap each other perfectly. This is the case when the same kind of CCLs have a common origin. However, if their origins are in different sites, they can overlap partially. Two different kinds of CCLs can overlap partially or hardly. Fig. 1 shows that the 2nd CCLs of FAU and ANA are partially compatible to form the 3rd CCL of LEV, but those of LTA and FAU are not.

Topological and Stereochem~calCharacteristics

0th

3rd

2nd

1st

ANA

LAU i

a-

+

f

/

\ LEV

&

FAU

LTA

Fig. 1 Consecutive CCLs (concentric clusters) and corresponding zeolite species. ANA: analcime, LAU: laumontite, LEV: levynite, FAU: faujasite, LTA: Linde type A.

95

96

M. Sat0

C(n+l,O)

Fig. 2

Topological compatibility between a kernel and four peripheral CCLs to form a new large CCL

STEREOCHEMICAL COMPATIBILITY In addition to topological compatibility, stereochemical compatibility must be taken into consideration in the formation of real zeolite frameworks. For example, the 2nd CCL of FAU cannot be in a plane configuration, but is in two kinds of stereochemical configuration, i.e. left and right-handed. In this case, a given kernel CCL can be combined with a peripheral one ta satisfy its chirality consistency. Steric hindrance is a more important factor for the combination of CCLs, not only between kernel and peripheral CCLs, but also between two or more peripherals CCLs. Stereochemical compatibility is essential for the characterization of real frameworks. One example is shown on frameworks such as AEI (ALP04-18). AFT (ALP04-52). CHA (chabazite), EMT (hexagonal faujasite), FAU (faujasite), GIS (gismondine), GME (gmelinite), KFI (ZK-5), MER (merlinoite), PAU (paulingite), PHI (phillipsite) and RHO (rho).

Topological and Stereochemical Characteristics

97

All of these differ in framework topology [ 3 ] . However, it is 2nd CCL of noteworthy that they constitute only one kind of FAU, in which three four-membered rings are arranged to share their edges. Stereochemically, they can be characterized in two forms, L (left-handed) and R (right-handed), as shown in Fig. 3.

Fig. 3 Left- and right-handed CCLs allowed in the 2nd CCL of FAU The compatibility between a central cluster and the neighboring clusters can be realized by two symmetry operations, i.e., reflection and rotation. A rotation operation relates a central left-handed cluster with a left-handed one in the first neighbor, while a reflection operation relates a left-handed one with a right-handed one. Fig 4 shows their compatible relation at site 1. As already shown, a given kernel cluster has 4 distinct sites to combined with neighboring clusters. Thus, an L-handed kernel as well as an R-handed one has total 16 stereochemically distinct combinations of clusters. However, the steric hindrance between a kernel and peripheral clusters reduces the number to 9 (Table 1). In them, the arrangements LRLR, LRRR, RRLR are converted to those RLRL, RLRR, RRRL respectively by a symmetry

98

M . Sato

operation of rotation. Thus, only 6 combinations are allowed to form 3rd CCLs. All of them based on the L handed kernel are shown in Fig. 5. 3

I--

2

4

3

1 3

1

2

4

4

2

.......... .

T I

Fig.4

Chirality compatibility between a kernel CCL (solid) and a peripheral one (dotted) in terms of rotation (L) and reflection (R)

Table 1

Stereochemically distinct combinations for both L and R kernels. The symbol + means the combinations allowed to appear as the 3rd CCL.

Topological and Stereochemical Characteristics

Fig.5

LLRR

LRLR

LRRR

RRLL

RRLR

RRRR

Six types of 3rd clusters allowed for L-handed kernel

CHARACTERIZATION OF FAUJASITE SERIES FRAMEWORKS Now, it is possible to characterize the above faujasite series frameworks in terms of the 3rd cluster types. Visual examination is very difficult to perform, because all the framework nodes should be examined as centers of the CCLs. A computer program which serves to identify cluster type has been developed for this. The result is shown in Table 2. GIS and MOR as well as FAU and AEI cannot be differentiated in the 3rd CCL, wh.ile others can be clearly differentiated. The absence of the arrangement LLRR in this table suggests that the configuration of the cluster LLRR is closed and cannot be developed further.

99

CONCLUSION The concept presented here can be widely utilized not only for the characterization of various kinds of zeolite frameworks, but also for the generation of existing and non existing zeolite may be topologically frameworks. A large number of clusters compatible, but the numbers is reduced by stereochemical compatibility. This may be the main reason why the number of real A computer program for framework frameworks is restricted. generation based on this concept is now in progress.

REFERENCES 1

2 3

W.M.Meier, Molecular Sieves, Soc.Chem.Ind.London UK, (1968), p.10. M.Sato, J.Phys.Chem.91,(1987),4675. W.M.Meier and D.H.Olson ed. Atlas of zeolite structure types, Zeolites 12,(1992),No.5.

Symmetry and Location of Titanium Within Titanium Silicalite Framework of M[FI Structure

D. Trong On, I. Denis, C. Lortie, C. Cartier and L. Bonneviot Departement de Chimiet, CERPIC, Universith Laval, G1K 7P4, Ste Foy, Qc, Canada. ABSTRACT A series of titanium silicalites of MFI structure, active in n-hexane oxyfunctionalization, were investigated by FT-IR, XPS, XANES and EXAFS spectroscopies to characterize the titanium sites. Most of the titanium ions are sited in a non-substitutional framework sites of C4v symmetry rather than Td. The framework IR bands reveal that the [SiO4] units, linked to titanium via double Ti-0-Si bridges, have a symmetry lowered from Td to at least CzV. The decrease of the 960 cm-1 IR band upon the effect of adsorption of water or H202 is attributed to the partial hydrolysis of the double bridges leading to a linkage by single bridges. A molecular simulation investigation shows that such sites can be accommodated in the structure by disruption of the four (Si) membered rings. INTRODUflION Titanium silicalites (TS)were recently found selective for various reactions. Among them, oxyfunctionalization of alkanes with H202 is probably one of the most interesting [l-31. The titanium ions responsible for the catalytic activity are believed, on the basis of unit cell expansion with increasing Ti content, to occupy substitutional T sites. Such lattice expansion has been confirmed to occur up to 4% molar fraction of titanium in TS-1 [4,5]. Titanium can also be incorporated in the framework of other silicalites of MEL (TS2),p, or ZSM-48 (TS-48) structure 16-10]. Despite this success, the incorporation of transition metal ions into a zeolite framework and its characterization are still a challenge since these ions preferentially occupy octahedral sites that a zeolite framework can not provide. The location of the titanium site is a puzzling case not yet fully understood. Though the lattice expansion is a good criteria for framework incorporation, it does not necessitate the occupancy of substitutional site to take place. Our recent EXAFS investigation indeed proved that the framework TiOx species are non-substitutional species in TS2 [7,81. They are linked to framework SiO4 tetrahedra via an edge-sharing type of binding. This was later confirmed for TS-1 whose dehydration effect was 101

102

D. T. On, 1. Denis, C. Lortie, C. Cartier and L. Bonneviot

investigated by EXAFS [12]. It was found that the double Ti-O-Si bridge that connects a [Ti041 unit to a [SiO4] unit is partially hydrolyzed leaving those two units linked via a corner, i.e., through a single Ti-0-Si bridge. This study deals with the problem of rationalization concerning the 960 cm-I IR band evolution in connection with the titanium symmetry upon adsorption of water or hydrogen peroxide and, with the titanium location in the framework. EXPERIMENTALS Titanium silicalites (TS-I) were prepared from the addition of water to a mixture of tetra(ethoxy)silicon(IV) and tetra(iso-propoxy)titanium(IV)compounds in presence of a solution of tetrapropylammonium hydroxide in propanol. The hydrothermal treatment was carried out at 175OC for 4 days in a Teflon coated stainless steel autoclave. The solid materials was filtered, washed and calcined at 500 "C. Four samples were synthesized with a Ti/Ti+Si ratio of 1.2, 2.1, 2.6 and 3.4% obtained from chemical analysis. Dehydrated samples were obtained by evacuation at 45OOC in N2 a n d transferred under dry N2 in the appropriate cell for measurements. The Ba2Ti04 and hexadecaphenyloctaeiloxyspiro(9,9)titanate(WtHDPOSST, were prepared as indicated in the literature [ll]. The XRD were recorded on a Rigaku D-Max IIIVC X-ray spectrometer. IR spectra were recorded on self supporting pellets of samples diluted in KBr using a Bomem 102 FTIR spectrometer. The XPS data were performed on a V.G. Scientific Escalab Mark I1 [5]. The X-ray Absorption spectra at the titanium edge were collected at the radiation synchrotron facility of the LURE (France) and treated as previously [7,8,12]. The white radiation was monochromatized by a Si (311) two-crystal monochromator. The Fourier transform were obtained on filtered and k3 weighted EXAFS signals (Kaiser window [z =3.7], kl = 2.50 A-1 to k2 = 12.30 A-1) and the simulation were performed as previously [7,8,12,131. The reaction of n-hexane with hydrogen peroxide in methanol was performed at 55OC in a pyrex flask with a reflux condenser. The catalysts/hexane, hexane/H202, hexane/MeOH ratios were kept constant at 43.5 g/mol, 1.15 mol/mol and 34.3 mol/mol while, in comparison, they were held at 42.9,1.17 and 34.9 (same units), respectively, in the work by Clerici et al. [31. The product analysis was performed on a GC equipped with a capillary column. H202 was titrated at the end of the reaction. RESULTS AND DISCUSSION The XRD confirmed that the synthesized materials had the MFI structure with a crystallinity of 90-95% within this series. A linear dependence of the 960 cm-I IR band and lattice expansion with the Ti loading was taken as a fingerprint of the titanium incorporation in the framework 141.

Symmetry a n d Location of Titanium i n MFI Structure

103

The reaction products of n-hexane with hydrogen peroxide were very similar within this series of samples. After 2 hours the H202 consumption was almost completed (93-98%) with an hexane conversion of 42-45%. The yield of H202 toward oxyfunctionalization was in the range of 70-83%. The functionalization in position 2 which produced 2-hexanol and 2-hexanone was favored in comparison to functionalization in position 3. The C2/C3 ratio was varying in the range of 2.8-3.5. This results were quite similar to those reported by Clerici et al. (see table 1). Table 1. Reactivity of the catalysts in n-hexane functionalization sample Ti/Ti+Si Hexane conv./% H202 conv./% H202 yield/% 0.026 43.5 97.5 74.4 TSI a TS-1 b 0.028 90 Ti02 C 0 4 0 SiO2 d 0 0 0

C2/W ol/one 3.1 0.47 2.3 0.67

a) this work, b) Clerici et al. c) anatase, Degusssa P 25 and fumed silica Cab-0-Sil M5. Frame work IR characterization The characteristic IR band at 965 cm-1 was found to decrease in intensity by about 30% and 60% after adsorption of H20 and H202, respectively, in comparison with its original intensity in the calcined material. In the same time, its position was shifted from 965 to 970 cm-1. A complete restoration of this band was obtained after H20 adsorption followed by a subsequent calcination. Only a partial recovery was found to occur after a first H202 adsorption-calcination cycle. After subsequent cycles no more loss of intensity was observed. To investigate the IR 1000-11200 cm-1 range, the pellets were prepared with a higher dilution of the sample in KBr to avoid the saturation of the transmitted signal. Figure 1 depicts the absorbance of a pure silicalite superimposed with the absorbance of a TS-1. This clearly gives evidence that the incorporation of titanium is not only associated with the new peak at 965 cm-1 but also with a shift toward lower energies of the main peak position as well as a modification of the peak shape on the high energy range. These changes were further examined on difference IR spectra of TS-1 of various loadings with pure silicalite as a reference. The results shown on figure 2, were found systematically reproducible. The signal to noise ratio was still very good to trust the signal shape obtained by difference. In the central part of the figure, a very sharp peak looks like a differential spectrum. This is not due to new oscillators, this is rather the result of the difference between two strong bands slightly shifted one with another by about few cm-1. Such an effect is probably due to a slight shift toward

104

D. T. On, 1. Denis. C. Lortie, C. Cartier and L. Bonneviot

0.4

8

8

5

5

e

e

P

9

8

2

4

4

900

1000

1100

1200

1300

cm-' Fig. 1. FT-IR spectra of (- - -) pure silicalite and (---) 2,6% TSI.

-0.3

I0

1000

..1200

1400

cm-1

Fig. 2. Difference IR spectra of dehydrated a) 1.2, b) 2.1 and c) 2.6%TS-I.

lower energies of the overall framework vibrational frequencies in presence of titanium. There is also a negative peak whose linewidth is large enough to discard any artefact previously described. This negative peak at about 1130 cm-1 is the trace of the suppression of one type of oscillators when the titanium is incorporated in the framework. Finally, two other peaks are revealed by the difference IR spectra at about 1080 and 1200 cm-1. These peaks and the 965 cm-1 peak could account for the splitting of the asymmetric stretching mode of the [SiO4] units linked to titanium by three new vibrational modes. A splitting by three of such magnitude occurs for the sulfato, anion whose symmetry is lowered for Td to CzV in complexes where it binds two cobalt cations [14]. For the reasoning part, this interpretation applied to [SiO41 is in agreement with previous authors [15,161 with, nevertheless, the difference that instead of two bands at 967 and 1083 cm-1, there is three bands to take into account at 965, -1080 and -1200 cm-1. Along this line, the IR results and the EXAFS data will be consistent with the double bridge formation between Ti and Si.

- x

. .

The XPS spectra of the dehydrated and the hydrated materials were found identical and exhibits a single doublet characteristic of tetravalent titanium. The 2P3/2 line rise at 459.7 eV at the same position found for tetrahedral titanium species in the titanium glasses or TS-2 materials [71. After adsorption of H202, a second doublet appears at the expense of the other, the new 2P3/2 line rising at 458.3 eV like for octahedral titanium in Ti02 anatase.

Symmetry and Location of Titanium in MFI Structure

105

8

lal5.iA c/-I

r

0 0

2 2

anatase

I

u 0 4

0 4

6 6

4 2

* 1

Fig. 4. EXAFS FT transforms of (---) dehydrated 2.6%TS-1 and (- - -) BCTST compound

. .

ed X-rav A

3

R / i

RIA Fig. 3. EXAFS FI' transforms of 2.6%TSand anatase.

2

b b The full analysis and simulation of the EXAFS signal of TS-1 and TS-2 samples was previously performed [7,8, 121. For the sake of brevity, the focus will be restricted on a qualitative comparison of the Fourier Transformed (FT) of the samples in various states. The first peak of the l T profiles arising in the 0.5-1 A range is mainly due to the mathematical residue of the baseline extraction, no further comments will be made about it (see figure 3). The second and more intense peak is attributed to the first shell of oxygen neighbors. Since the lT transform is not phase corrected, this peak is at 0.5 8, lower than the real Ti-0 distance. This first peak at 1.3 A is simulated at 1.79 8, for tetrahedral siloxytitanium compound, HDOSST, consistently with its XRD structure (1,78-1,79&. By comparison, the dehydrated TS-1 has a much lower first shell. This decrease has been described previously as the effect of the presence of a very close Si neighbor at about 2.2-2.3 A that negatively interferes with the oxygen EXAFS oscillations [81. The strong second peak in the HDOSST is due to 4 Si atoms at about 3.5 A, i. e, exactly where should lie the silicon neighbors in a regular T site. This comparison with the model compound and TS-1 clearly supports the first EXAFS simulations made previously [8,12]. The comparison between anatase and TS-1 FT's demonstrates that after water or H202 adsorption, the Ti-0 distance never reached the values expected for an octahedral environment (dTi-0 1.95 A). A slight increase of the peak at about 2.9 A in agreement with previous results 1121. This effect is more pronounced for H202 than H20. This can be related to the framework IR data assuming that the opening of the double bridge via the hydrolysis of one of the Ti-0-Si bridge is more efficient with H202. Finally, the Ti-0

-

106

D. T. On. 1. Denis, C. Lortie. C . Cartier and L. Bonneviot

0

15 30 45 Energy /eV

60

0

Fig. 5. XANES at Ti K-edge of dehydrated 2.6%TS-I, and reference compounds

15

30 45 Energy /eV

60

Figure 6 : XANES at Ti K-ed es of 2.6% TS-1 after various treatments an anatase

B

distances in the H202 adduct seems to be distributed in two groups of distances, short and long distances (-1.8 8, and 2.1 A), consistent with the formation of peroxotitanium species [171.

. .

X-rav AbsorDtion Near Edge characThough the titanium near edges of dehydrated TS-1 are similar to those of tetrahedral titanium in Ba2Ti04 and HDOSST compounds, there is some striking differences (see figure 5 and 6). Along the series, Ba2TiO4, BCTST and TS-I, the pre-edge position shifts from 2.6, 2.9 to 3.2 eV, a shoulder at about 13 eV increases progressively and the post-edge evolves toward longer distances. These evolution are consistent with a distortion leading to a stronger crystal field in the xy plane and a weaker field effect in the z direction. This would be drastically produced in a square planar symmetry 1181. A square planar titanium phtalocyanin indeed exhibits a preedge at 3.5 eV, close to the TS-1 pre-edge position [19]. Nevertheless, the shoulder at 13 eV is not strong enough for TS-1to account to for a pure D4h symmetry. Titanium is more likely to occupy a C4" site. The hydration might be understood as an addition of a water molecule that increases the coordination number from 4 to 5 accompanied with an equilibrated reaction of hydrolysis of one of the two Ti-0-Si bridges that links Ti to Si. By contrast, hydrogen peroxide leads to a substantial displacement toward an hexacoordinated state in a strongly distorted octahedral symmetry (intense pre-edge, mixture of short and long Ti-0 distances and XPS 2P3/2 shift). In the same time, most of the rings formed by

Symmetry and Locarion of Titanium i n MFI Structure

107

the double bridge are open with respect to the strong intensity decrease of the 965 cm-* band.

e work

The lattice expansion, the symmetry, the binding mode of titanium and the loss of crystallinity clearly characterize a disruptive framework site. The search for the site using Polygen Quanta from Molecular Simulations was dictated by the separation by about 4.3-4.6%i (about twice the Ti--Si EXAFS distance) between two framework silicon ions. The result shows that the four silicon membered rings fulfill the conditions.

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D. T. On, 1. Denis. C. Lortie, C. Cartier and L. Bonneviot

According to this ,one can envisage the monomeric site as shown on the scheme. Such sites are most probably those active in alkane oxyfunctionalization owing to its highly strained environment, its open coordination shell and its capacity to form the peroxotitanium species. CONCLUSION The titanium environment in TS-1 materials has been investigated with a combination of techniques designed to probe long and short range structure as well as local symmetry. A consistent picture of the monomeric site has been proposed where the titanium is tetracoordinated in a C4" symmetry. Its linkage to the framework occurs via two double bridges. The disrupted four silicon membered ring of the framework is the more reasonable location to accommodate such an odd site. REFERENCES 1 T. Tatsumi, M. Nakamura, S. Nagashi & H. Tominaga, J.Chem.Soc.,Chem. Comm., (1990)476 2 D.R.C. Huybrechts, L. De Bruycker, & P. A. Jacobs, Nature, 345 (1990) 240 3 M. G . Clerici, Appl.Catal., 68 (1991) 249. 4 M. Tamarasso, G. Perego, and B. Notari, U. S. Pat.4410501 (1983). 5 A.J.H.P. van der Pol and J.H.C. van Hoof, Appl. Catal., 92 (1992) 93. 6 J. R. Reddy and R. Kumar, J. Catal., 130 (1991) 440. 7 D. Trong On, L. Bonneviot, A. Bittar, A. Sayari and S. Kaliaguine, J. Mol. Catal., 74 (1992) 233; A. Bittar, D. Trong On, L. Bonneviot, S. Kaliaguine and A. Sayari, in R. von Ballmoos, J. B. Higgins and M. M. Treacy (Eds.), Proc. 9th Int. Zeolite Conf., Montreal, July 1992, Butterworth-Heinemann, Boston, 1993, p. 453 8 D. Trong On, A. Bittar, A. Sayari, S. Kaliaguine and L. Bonneviot, Catal.Letters, 16 (1992) 95. 9 M. A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. Soc., Chem. Comm., (1992) 589. 10 D. P. Serrano, H.-X. Li and M. E. Davis, J. Chem. SOC.,Chem. Comm., (1992) 745. 11 M. B. Hursthouse and Md. A. Hossain, Polyhedron, 3 (1984) 95. 12 L. Bonneviot, D. Trong On and A. Lopez, J. Chem. Soc., Chem. Comm., (1993) 685. 13 A. Michalowicz in Logiciel pour la Chirnie, SOC.Fr. Chimie, Paris, 1991, p. 102; A. Michalowicz and V. Voinville, ibid,p.l16. 14 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th Edition, Wiley, New York, 1986, p. 249. 15 M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal., 48 (1990) 133. 16 G. Deo, A. M. Turek, I. E. Wachs, D. R. C. Huybrechts and P. A. Jacobs, Zeolites, 13 (1993)365. 17 F. A. Cotton and G. Wilkinson, Advance in Inorganic Chemistry, 5th edition, Wiley, New York, 1988, p.659. 18 C. Cartier, M. Momenteau, E. Dartyge, A. Fontaine, G. Tourillon, A. Michalowicz and M. Verdaguer, J. Chem. SOC.Dalton Trans., 19 C. A. Yarker, P. A. Johnson, A. C. Wright,J. Wong, B. Greegor, F. W. Lytle and R. N. Sinclair, J. Non-Crystalline Solids, 791(986) 117.

The Topography of Vanadium in Silicalite-2 Crystal Lattice and its Catalytic Role in the oxyfunctionalization of Alkanes

R. Vetrivel, P.R. Hari Prasad Rao and A.V. Ramaswamy Catalysis Division, National Chemical Laboratory, Pune - 41 108, India

ABSTRACT The location of vanadium in the crystal structure of silicalite-2 and its interaction with alkanes are studied by computer modeling and semi-empirical quantum chemical calculations. We report here the relative substitution energy of vanadium at different crystallographic sites and discuss the topography of vanadium in silicalite-2 lattice. The product distribution for the oxyfunctionalization of n-alkanes is rationalized based on the cluster model calculations.

INTRODUCTION The presence of transition metals in the framework of a zeolite lattice can impart oxidation activity, which can be combined with the shape selective properties of the molecular sieve. Vanadium incorporated molecular sieves are a new class of materials which catalyze various useful oxidation reactions [1-41. Oxyfunctionalization of alkanes with high selectivities on natural and synthetic metalloporphyrin systems and on vanadium(V) 0x0 peroxo complexes is known [5-71. Similar oxidative catalytic properties have been recently reported for vanadium containing silicalite-2 (VS-2) [8- 111. A thermally stable, vanadium containing silicalite-2 with MEL structure has been synthesized [9] and characterized using various spectroscopic techniques [9-111. There is strong evidence supporting the presence of vanadium in the framework lattice and this vanadium catalyzes the oxyfunctionalization of alkanes to corresponding alcohols, aldehydes and ketones at various carbons including the primary carbon. This unique catalytic behavior and the nature of vanadium as well as its environment in the molecular sieve are probed in the present study using computer modeling and quantum chemical cluster calculations. METHODOLOGY AND CLUSTER MODELS Method We have used the standard Extended Hucke Molecular Orbital(EHF 0 ) methot [12] for the electronic structure calculations. Among the various semi-empirical quantum chemical methods, reliable calculation parameters for vanadium atom are available only in EHMO method. Although 109

110

R. Vetrivel. P. R. Hari Prasad Rao and A . V . Ramaswamy

more accurate ab initio calculations are desired, EHMO calculations are adopted in the present study considering the following facts: i) EHMO calculations are computationally efficient and are feasible for performing a multitude of calculations on related clusters. ii) The absolute values of the energy calculated are not used but their relative difference is compared for different geometries of similar clusters and iii) The method has been extensively used successfully in the past. It was found to be a valid method for comparison of energy values of chemical systems, where the number of atoms and each kind of formal chemical bond is conserved, as is the case in the present study [ 13,141. The ionization potential and exponent used for 4s and 4p orbitals of V are -6.303eV; 1.3763 and -3.461eV; 1.1338 respectively. The ionization potential, double-zeta exponent values and the contraction coefficients for 3d orbitals of vanadium are -6.303eV; 4.75; 1.50; 0.456 and 0.752 respectively.

Model The cluster model chosen for the present study is (OH)3-T-O-T(OH)3, (where T=Si or V). The T atoms and the oxygen atoms were located as the framework positions of MEL crystal structure report [15]. The charge on vanadium was 5+ corresponding to the calcined form. The details of the cluster generation and termination are reported elsewhere [ 161. The interaction energy values between the zeolite framework cluster and alkane are calculated as the difference between the total energy of adsorption complex and the sum of the total energy values of framework cluster and the alkane. The Quanta and CharmM software packages supplied by Polygen Corporation, U.S.A., were used in an IRIS workstation for the calculations. RESULTS AND DISCUSSION Probable location of vanadium in MEL lattice It is a hard task to rationalize the quantity of vanadium ions entering the framework and the crystallographic sites where these cations are located. The quantity and location of vanadium in VS-2 will depend on the local geometry of different crystallographic sites and the extent of relaxation possible in the lattice due to substitution of larger vanadium ions in place of silicon. The geometry of the seven crystallographic ally distinct T-sites in MEL lattice are described in Table 1. The ease of substitution of vanadium at a given crystallographic site and the extent of relaxation occumng in the lattice are reflected by the 'substitution energy'. The substitution of vanadium in the place of silicon in MEL lattice [15] is considered to happen as follows: [(OH)3Si-O-Si(OH)3] + [V(OH)4]+

+

[(OH)-jSi-O-V(OH)3]+

+ [SiO4]

The substitution energy is calculated as the difference between the sum of total energy of the clusters in the products and reactants of the above equation. Since there are four neighboring T-sites to every T-site as shown in Table 1, the substitution energy for a given site is calculated as an average value derived from four dimer clusters [16] and they are also listed in Table 1.

Topography of Vanadium in Silicalite-2

I 11

Table 1. The neighboring T-sites as well as their geometry in MEL lattice and vanadium substitution energy. Site No.

Neighboring T-sites

Average T-0 Average T-0-T bond length(W) bond angle (degrees)

Average 0 - T - 0 bond angle (degrees)

Substitution energy for v5+ (kcal/mol)

T1 l-2 T3 T4 T5 T6 T7

1,1,2,2 1,3,4,5 2,3,6,7 2,4,5,6 2,4,5,7 3,3,4,4 3,5,7,7

1.625 1.563 1.605 1.598 1.598 1.580 1.605

109.68 109.45 109.43 109.47 109.49 109.27 109.42

-7.33 -5.80 -6.20 -6.70 -6.73 -6.99 -5.60

148.20 155.98 149.38 154.73 156.33 151.30 157.00

The substitution of vanadium in place of silicon at all seven crystallographic sites of the MEL lattice was considered and the calculated substitution energy values are also listed in Table 1. The substitution energy for vanadium is not very different for distinct sites, hence a random distribution of vanadium is possible. The product distribution obtained in the oxidation of alkanes over VS-2 showed that oxyfunctionalization occurs at different carbon atoms [8]. Oxyfunctionalization of different carbon atoms of n-alkanes are either due to different modes of activation of n-alkanes or due to vanadium atoms present in different sites. Modeling studies carried out to gain better understanding of the interaction between n-alkanes and vanadium are discussed in the following section. The results in Table 1 show that the substitution of vanadium at sites 1.43 and 6 are relatively more favorable than at sites 2,3 and 7. In general, it was observed that the T-sites with longer T - 0 distances and smaller T-0-T angles are preferred for V substitution. Additionally, the orientation of adjacent tetrahedra decided by the dihedral angle 0 - V - 0 3 , also influences the vanadium substitution site. The topography of site I , which is the most favored site for vanadium substitution is common to the 10-member and 6-member channels along both the 'a' and ' b axes, as shown in Fig. 1. The location of other T-sites is also shown in Fig. 1. For the alkane molecules diffusing in the channels, sites 1 and 5 are more accessible since they are at the channel intersection while the sites 4 and 6 are on the walls of straight 10-member channel. For the same reason, V at T4 and T6 sites are expected to oxyfunctionalize the secondary carbon atoms, while V at T1 and T5 are expected to oxyfunctionalize the primary or secondary carbon atoms depending on the mode of activation of alkanes. The mode of activation of n-alkanes The n-alkanes were found to have conformational freedom inside the two-dimensional channels of the MEL framework. The channel intersection in the MEL framework is 5.6 X 5.6A and the channels are perpendicular to each other. Modeling studies showed that the n-alkanes in one of the straight channels can project its terminal methyl group into either the 6-member channel or 10-member channel in the perpendicular direction leading to the activation of C-H bond of primary carbon atom. The diffusion in the 6-member channel is restricted due to unfavorable van der Waals interaction, while the free diffusion into 10-member channel is possible. As discussed

112

R. Vetrivel, P. R. Hari Prasad Rao and A . V . Ramaswamy

Fig.1 The topography of various T-sites in MEL lattice. There is a reflection plane ( 0 )cutting across the channel. The A and B layers along b-axis is shown and the stacking order is AABBAA ...

earlier, the most preferred site of vanadium substitution in MEL lattice is common to 6- and 10member channels. Since, the n-alkanes cannot diffuse any further into a 6-member channel, the primary carbon alone is in contact with the vanadium site. Of course, when the molecule is approaching vanadium from the 10-member channel, due to diffusional freedom, there is more probability for the second and consequent carbon atoms to come in contact with the active site. Thus, when vanadium is present in the junction of 10- and 6-member rings, the activation of C-H bond of primary and secondary carbon atoms are possible. Our studies indicate that the 6-member channels, which are generally considered as too small for any reactants of catalytic interest, can play a role in the oxyfunctionalization of primary carbons. The primary carbon oxidation phenomenon has been observed in VS-1 and VNCL-1 [ l I] zeolites also, where 6-member channels intersect larger 10-member or 12-member channels. It is probable that the vanadium prefers to get substituted at the junction of 6-member and larger channels in VS-1 and VNCL-1 also.


113

Fig. 2 Methane molecule approaching vanadium at T I site from the 10-member channel. Further, cluster model calculations were carried out to understand the electronic interaction between n-alkanes and vanadium ion present at the junction of 10- and 6-member channels. The activation of methane and ethane over a model cluster containing four tetrahedral sites, terminated by

Fig. 3. Methane molecule approaching vanadium at T i site from the 6-member channel hydrogen atoms, namely T4O 13H 10 was studied. The cluster model represents one vanadium substituted for Si at T i site of the cluster containing T2-Ti-Ti-T2 sites. The methane and ethane molecules approaching the vanadium from the 6-member channel and from the 10-member channel

114

R. Vetrivel, P. R. Hari Prasad Rao and A. V . Ramaswarny

were considered and they are represented in Figs. 2 and 3, respectively. The results of EHMO calculations on these cluster models are given in Table 2. From the adsorption energy values given in Table 2, it can be observed that the approach from either of the channels does not make much difference for the adsorption energy of methane molecule. However, for the ethane molecule the Table 2. Activation of methane and ethane over VS-2 clusters Cluster Model

Total energy(eV)

Adsorption energy(eV)

Tetrameric Cluster alone

-2058.98

-----

Methane from 10-M.C. Methane from 6-M.C.

-2208.73 -2208.26

-4.97 -4.50

Ethane from 10-M.C. Ethane from 6-M.C.

-2349.04 -2346.92

-2.98 -0.86

adsorption energy is less exothermic than methane when it approaches from both channels. This result indicates that there is steric hindrance for the larger n-alkanes. Additionally, it was found that when the ethane molecule approaches from the 6-member channel, the steric hindrance is maximum. However, these EHMO results should be taken as a prediction of qualitative trend only, due to the inherent approximation in the method and the restricted cluster model. In Table 3, the oxyfunctionalization occurring at primary and secondary carbon atoms of nalkanes over VS-2 catalysts [I I ] are listed. The primary carbon oxyfunctionalized products Table 3. Oxyfunctionalization of alkanes over VS-2 (ref. 11) Reactant

n-hexane n-heptane n-octane

Product distribution (wt. %)

Primary / secondary carbon oxyfunctionalization selectivity

Primary carbon oxyfunctionalized product

Secondary carbon oxyfunctionalized product

Observed value

Expected value

10.9 07.3 07.8

68.7 68.5 67.8

0.16

0.50 0.40 0.33

0.11 0.12

included both alcoholic and aldehydic compounds, while the secondary carbon oxyfunctionalized products included alcoholic and ketonic compounds. The kinetic study of formation of alcohol, aldehyde and ketone indicated that the alcohol formation is the first step, while the aldehyde and ketone were formed in the subsequent steps. It is believed that the oxyfunctionalization of alkanes


I I5

is selectively effected only by those vanadium species which are in the framework positions. The results in Table 3 also brings out the regioselectivity in the oxyfunctiondization of n-alkanes. Our calculations indicate that the vanadium in a specific topography at the pore intersection, namely at the junction of 10-member channel and 6-member channel, can only oxyfunetionalize the primary carbon, while the vanadium at the pore walls oxyfunctionalize secondary carbon atoms. The observed selectivity for primary carbon oxyfunctionalization is much smaller than the theoretical expected value, as shown in Table 3. The cause of this low selectivity is the competitive secondary carbon oxyfunctionalization occurring due to different modes of activation. When the n-alkane approaches the vanadium from the 6-member channel, primary carbon is oxyfunctionalized and when the n-alkane approaches the vanadium from the 10-member channel, either primary or secondary carbon is oxyfunctionalized. A useful suggestion arising from the present investigation is that the primary carbon oxyfunctionalization selectivity could be improved by partially blocking the free diffusion of n-alkanes in the 10-member channel. CONCLUSIONS In this study, the probable positions for substitution of vanadium in the MEL lattice have been derived. A random distribution of vanadium in silicalite-2 lattice is predicted, while site 1 is energetically the most preferred one for V substitution. The oxyfunctionalization of n-alkanes at primary and secondary carbon atoms is due to vanadium present in different topography. The activation of alkane molecules on the most preferred vanadium substitution site, namely at T I , is studied using methane and ethane as representative molecules. It is observed that different modes of activation of n-alkanes over the same vanadium site is also a reason for the oxyfunctionalization occurring at different carbon atoms. These results rationalize the product distribution obtained for the oxidation of long chain alkanes over VS-2 catalysts. REFERENCES 1 A. Miyamoto, Y. Iwamoto, H. Matsuda and T. Inui, Stud. Surf. Sci. Catal., 49 (1989) 1233. 2 F. Cavani, F. Trifiro, K. Habersberger and Z. Tvaruzkova, Zeolites, 8 (1988) 12. 3 Z. Tvaruzkova, G. Centi, P. Jiru and F.Trifiro, Appl. Catal., 19 (1985) 307. 4 L.W. Zatorski, G. Centi, J.L. Neito, F. Trifiro, G. Bellussi and V. Fattore, Stud. Surf. Sci. Catal., 49 (1989) 1243. 5 B. Meunier, Bull. SOC.Chim. Fr., (1986) 578. 6 N. Herron and C.A. Tolman, J. Am. Chem. SOL, 109 (1987) 2837. 7 H. Mimoun, L. Saussine, E. Daire, M. Postel, J. Fischer and R. Weiss, J. Am. Chem. SOC., 105 (1983) 3101. 8 P.R. Hari Prasad Rao and A.V. Ramaswamy, J. Chem. SOC.,Chem. Comm., (1992) 1245. 9 P.R. Hari Prasad Rao, A. V. Ramaswamy and P. Ratnasamy, J. Catal., 137 (1992) 225. 10 P.R. Hari Prasad Rao, A.A. Belhekar, S.G. Hegde, A.V. Ramaswamy and P. Ratnasamy, J. Catal., 141 (1993) 595. 11 P.R. Hari Prasad Rao, A. V. Ramaswamy and P. Ratnasamy, J. Catal., 141 (1993) 604. 12 R. Hoffrnann, J. Chem. Phys., 39 (1963) 1397. 13 R. Hoffrnann, Science, 21 1 (1988) 995. 14 W.J. Hehre, Acc. Chem. Res., 3 (1976) 399. 15 C.A. Fyfe, H. Gies, G.T. Kokotailo, C. Pasztor, H. Strobl and D.E. Cox, J. Am. Chem. SOC., 11 1 (1 989) 2470. 16 R. Vetrivel, S. Pal and S. Krishnan, J. Mol. Catal., 66 (1991) 385.


Structure and Dynamics of Ion-exchanged Zeolites as Investigated by Molecular Dynamics and Computer Graphics

A. Miyamoto and M. Kubo Department of Molecular Chemistry & Engineering, Faculty of Engineering, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980, Japan ABSTRACT The applicability of molecular dynamics (MD) and computer graphics (CG) t o investigating the structure and function of ion-exchanged zeolites was demonstrated for (i) reproducing the known structures of various zeolites, (ii) determining the unknown positions of Al in T-site and exchanged cations, ( i i i ) understanding the role of zeolite framework in CuZSM-5 for the direct decomposition of NO to N2 and 0 2 . and (iv) understanding the dynamic mechanism in the molecular sieving process of 0 2 and N 2 in A-type zeolites.

INTRODUCTION Much attention has been given to

ion-exchanged zeolites

in relation

applications to catalysts, adsorbents, and various functional materials.

t o their

I t has also been

found that exchanged cations play important roles as the center of adsorption and It is therefore highly important

catalytic reactions. exchanged

cations

materials.

to

understand

adsorption

and

to determine the position of

catalytic

reactions

in

zeolitic

In addition to a number of experimental methods, it would be desirable to

establish theoretical methods for the purpose.

Molecular dynamics (MD) has been

widely

and

applied

to

understanding

the

structure

physical

properties

of

various

substances including simple liquids, water, molten salts, liquid metals, glasses, proteins, polymers, and zeolites [I-31. dynamics

of

various

On the basis of our previous studies on the structure and

materials

[2,4-111, the

objective

of

the

present

study

is t o

summarize various applications of MD and computer graphics ( C G ) to the structure and function of ion-exchanged zeolites. MEI'HOD

MD calculations were made with the XDORTO program developed by Kawamura 1121. The Verlet algorithm was used for the calculation of atomic motions, while the Ewald method was applied to the calculation of electrostatic interactions.

Temperature and

pressure were controlled by means of scaling of atom velocities and unit cell

I I7

118

A. Miyamoto and M . Kubo

Fig. 1 Trajectories of atoms in ZSM-5 without Al(a) and CaNaA(b) calculated by the MD method at 600 K(so1id line) and average positions determined experimentally(+) parameters under the three-dimensional periodic boundary condition.

The two body

central force interaction potential, Eq. 1 , was used for all calculations; the first, second, and third terms refer to the Coulomb, exchange repulsion. and Morse interactions, respectively.

where

Zi is an atomic charge, e an elementary electric charge, rij an

distance, and fo a constant.

interatomic

Parameters a and b in Eq. 1 represent the size and stiffness

of an atom, respectively, in the exchange repulsion interaction, while Dij, rij*, and P i j represent the bond energy, equilibrium bond distance and stiffness, respectively, the Morse function.

The MD calculations were made with

OMRON LUNA-BSK,

Apollo 9000 Model 7 10, and Silicon Graphics IRIS-4D/25TG workstations. visualization

was

made

with

BIOSYM

Insight-I1 software

workstation, while a dynamic visualization

on

SG

in HP

A static IRIS-4D/25TG

was done with the MOMOVIE program

developed in our laboratory on the OMRON LUNA-88K workstation [7,8].

RESULTS AND DISCUSSION Availabilitv of the s i m d e diatomic Dotential for reuroduc ine the structure of various zeolites 141. A simple diatomic potential such as Eq. 1 has been shown to be effective for reproducing the bulk and surface structures of various metal oxides [9-111. potential was zeolites.

The

demonstrated to be effective for reproducing the known structures of

As two examples, Fig. 1 shows results of MD calculations for ZSM-5 without A1

incorporation and CaNaA.

The trajectories of Si and 0 atoms for ZSM-5 (Fig. la) are

close to the average positions of the ions determined by X-ray crystal structure analysis [13].

Under the condition, the mean

square displacement of ions from the

Molecular Dynamics and Computer Graphics

Fig. 2

CG pictures of CaNaA(a) and NaA(b) zeolites calculated by the MD method.

positions of the X-ray analysis were 0.103 ion.

119

A2

for the 02- ion and 0.039

A2

for the Si4+

These values are reasonable in comparison with the temperature factor in the X-

ray crystal structure analysis.

Similarly, the trajectories of Ca, Na, Al, Si, and 0 atoms

calculated by the MD method are close to those determined experimentally (Fig. Ib), indicating the availability of the present MD method for various zeolites.

reproducing the structure of

CG pictures of CaNaA and NaA zeolites calculated by the MD method are

shown in Fig. 2. Availabilitv

of the Dresent MD method for dete rmininp the _osition D of

A1 and

uchaneed cation in zeolites 14,5L In ion-exchanged ZSM-5, the position of exchanged cation has not been determined exactly, although a quantum chemical calculation suggests that an A1 ion is located at the T12 site among the 12 possible sites of T atoms [14].

Thus, one of the 8 Si atoms in

the T12 site was replaced with an A1 atom, and MD calculations were made for different initial position of exchanged cation to investigate the trajectory of the cation.

When

the initial position was close to the possible position of the ion-exchange site in the neighbor of A13?

the ion migrated readily to the position.

When the initial position

was far from the ion-exchange site, high temperature was necessary for the ion to Fig. 3 shows examples for NaZSM-5.

migrate to the position.

Although Na ion cannot

reach the vicinity of A1 cation at 300 K, the Na ion can migrate easily at 600 K to reach the ion-exchange site.

This indicates that the MD method is effective for estimating

the position of Na cation in Na-ZSM-5. and Cs-exchanged Z S M J .

Similar results were also obtained for H-, Li-, K-,

Although H+ or Li+ ion with a small ionic radius is located in

the vicinity of oxygen anions, Na+ , K+ , and Cs+ ions with a larger ionic radius spread out of the micro-pore of ZSM-5.

The adsorption data on alkali-exchanged ZSM-5 are

consistent with the pictures [ 151. According to X-ray crystal structure analysis [16], the Na ions in Na-exchanged mordenite (NaM) are

classified into two types; the first one denoted by Na(1) is located

A . Miyamoto and M. Kubo

120

Fig. 3 Trajectories of NaZSM-5 calculated by the MD method at 300 K(a) and 600 K(b)

Fig. 4 Calculated trajectories of atoms in NaM with inadequate AI distribution(a) and adequate Al distribution(b) in the 8-membered ring. at the 8-membered ring of the MOR structure, while the other one denoted by Na(2) not localized in the NaM crystal.

is

The distribution of Al atoms in the 8-membered ring

was determined by M D calculation to reproduce the known position of Na(1). shows an example of MD calculations for the NaM at 300 K.

Fig. 4a

Here, the solid lines refer to

the calculated trajectories of the atoms in NaM, while the crosses (+) show the average positions of the atoms determined with X-ray crystal structure analysis [16].

The

assumed position of Al is not adequate, because the calculated position of Na(1) does not agree

with

the

experimental one.

Similar calculations were

distributions of Al in the 8-membered ring.

made

for

various

Fig. 4b shows the case of the best fit;

namely, the calculated trajectory of Na( 1) is located near the experimental position. These results indicate that the position of Na ion is a sensitive reflection of the position

of Al in the framework and the distribution of Al can be determined from the known position of Na. On the basis of the A1 distribution in the framework of the 8-membered ring shown in Fig. 4b, MD calculations were made for different distribution of A1 in the framework

of a 12-membered ring to determine the most favorable Al site in the 12-membered ring,

The calculated distribution of A1 was confirmed to be consistent with the


121

Fig. 5 Trajectories of atoms in CuZSM-5 with two Cuf ions in the unit cell at 600 K(a) and CG picture of CuZSM-5 with a Cu+ in the unit cell at 600 K(b).

It was also found that the electrostatic interaction plays an

Lowenstein rule.

important role as an origin of the Loewenstein rule for the Al-distribution. Role of zeolite framework in Cu-ion-exchanged zeolites for the decomuosition of NO 161 Much attention has been given to the direct decomposition of NO to N2 and 0 2 in relation to the catalytic process for environmental protection.

It has been found that

among various zeolites examined CuZSM-5 exhibits the best performance for the reaction. The decomposition of NO to N2 and 0 2 on CuZSM-5 is considered to proceed by the redox cycle between Cu+ and Cu2+ [17].

Thus, the structure and dynamics of Cu+

and Cu2+ ions in ZSM-5 were calculated using the MD method.

Similar to the method

for the NaZSM-5, MD calculations were made for the different initial position of Cu+ in CuZSM-5.

Fig. 5a shows

an example for two AI3+, namely two Cu+ ions, in a unit cell of

ZSM-5.

Both Cu+ ions migrate easily at 600 K to the ion-exchange site in the vicinity of

A13+.

Similar results were also obtained for different numbers of A13+ in the unit cell.

As an example, Fig. 5b is a CG picture of the final structure of Cu+

in CuZSM-5 with a

C u + ion in the unit cell. At a lower temperature, Cu2+ ion in CuZSM-5 is considered to be in the C u 2 + 0 H - state. However, at

an

elevated temperature, the

C u 2 + 0 H'

species is considered to be

dehydrated: 2Cu2+OH-+ Cu2+ + Cu2+02- + H20. Thus, the structure and dynamics of Cu2+ and C u 2 + 0 2 - species were simulated and an example of the calculated trajectories is shown in Fig. 6a.

Both Cu2+ and C u 2 + 0 2 -

species are located at the ion-exchange site in the vicinity of A13+, and the migration of 0 2 - in C u 2 + 0 2 - to another Cu2+ species does not occur.

Similar results were also

obtained for different distributions of A1 in T12 site. As shown in Fig. 5, the mobility of each Cu+ ion is limited near the A13+ ion. This means that the negative charge around A102- is locally neutralized by the positive

122

A. Miyamoto and M. Kubo

a

C

T

2

6

4

8

1

0

1

2

distance/A Fig. 6 Trajectories of atoms in CuZSM-5 with two Cu2+ ions(Cu2+ and Cu2+02-) in the unit cell at 600 K(a) and the Coulomb energy against the distance between positive and negative charges(b). charge of Cu+ for the Cu+

state in CuZSM-5.

As shown in Fig. 6, on the other hand,

C u 2 + species at the ion-exchange site forms a net positive charge, while C u 2 + 0 2 - species cannot neutralize the negative charge around A102'. This results in separated positive and negative charges in the ZSM-5 crystal for the Cu2+ state.

As shown in the

relationship of the Coulomb energy with the distance between positive and negative charges

(Fig.

6b), the

separation

of

positive

and

negative

charges

significantly

increases the electrostatic energy, thus decreasing the stability of the Cu2+ state. The Si/AI ratio of ZSM-5 is usually higher than that of other zeolites, indicating that the average distance between ion-exchange sites for ZSM-5 is longer than that for the other zeolites.

As shown in Fig. 6b, the electrostatic instability of the Cu2+ state

increases with increasing distance, suggesting that the stability of Cu+ relative to Cu2 i. for ZSM-5 is higher than that for the other zeolite. Since the desorption of 0 2 from the oxidized state(Cu2+) is considered to be the key step in the decomposition of NO to N2 and 0 2 , the relative stability of Cu+ is consistent with experimental results on the higher activity of CuZSM-5 for the decomposition of NO and the increased specific activity of Cu ion with Si/Al ratio in CuZSM-5 [17].

. .

ic behavior of alka li-cations in the molecular sievine effect of 0 2 a n d J ~ L 4 k

IYE zeolites 171 Much attention has been given to the molecular sieving function of zeolites for the In addition to a number of interesting separation of 0 2 and N2 in air [l8]. experimental approaches, it would be interesting to apply theoretical methods to the subject for atomistic understanding of the separation mechanism and for the scientific design of separation materials. Dynamic behaviors of 0 2 and N2 molecules in the micropore of CaNaA were calculated


123

Fig. 7 Trajectories of 02(a) and N2(b) molecule in NaA zeolite at 262 K by the M D method for different initial positions, initial velocities, and initial directions It was found that both 0 2 and of 0 2 or N2 molecules at various temperatures (50-600 K). N 2 can easily move to the next supercage through the window at any temperature.

In

other words, molecular sieving effect was not observed for CaNaA zeolite, in agreement with the open 8-membered ring (Fig. 2a), which is much larger than the radius of 0 2 or N 2 molecule.

M D calculations were also made for 0 2 and N2 molecules in the micropore of NaA (Fig. 2b) for different initial positions, initial velocities, and initial directions of 0 2 or N2 molecule at various temperatures (50-600 K). It was found that the dynamic behaviors At a higher of 02 and N2 change greatly with temperature and kind of molecule. temperature such as 300 K, both 0 2 and N2 can migrate through the space near the Na+ cation at the window.

At lower temperatures such as 262 K, the diffusion process of 0 2

at the window of NaA was very different from that of N2.

Fig. 7 shows examples of

diffusion processes of 0 2 and N2 in NaA zeolite at lower temperatures. velocity, position, and direction was common to both molecules.

The initial

Although the 0 2

molecule can migrate through the window to the next cage, the N2 molecule is repelled by the Na ion at the window and cannot diffuse to another cage.

These results at

various temperatures are consistent with experimental data obtained by Breck et al.

[181. The dynamic visualization of the diffusion process of 0 2 and N2

at different

temperatures suggested that the mobility of Na+ located at the window is important for the significant effect of temperature on the molecular sieving function. behavior of Na+

The dynamic

ions can be seen in the changes of mean square displacement (MSD)at

various times of MD calculation.

As shown in Fig. 8, MSD of Na ions located at the 8-

membered ring (Na2) is much larger than that of 0 2 - , Si4+, A13+, or the other Na ions (Nal).

In other words, the Na ion at the window plays the most important role in

124

A . Miyamoto and M . K u b o

-

MSD(0)

^

^

f

0.050

-

MSD(Si)

0.050

MSD(A1)

1I

0.050

MSD(Na2)

A

-

Mj,

,

0

Fig. 8

1

2

3 Timelpicosecond

4

0.050

5

Changes in mean square displacement (MSD) of various atoms in NaA at 300 K.

determining the effective pore radius of NaA ion. increases with temperature.

It was also found that MSD of Na2

Consequently, the significant effect of temperature on

the molecular sieving function of NaA can be understood in terms of the increased mobility of Na+ ions at the window at

higher temperatures.

REFERENCES 1. e.g. M. Doyama, 1. Kihara, M. Tanaka, and R. Yamamoto eds., Computer Aided Innovation of New Materials II(Proc. CAMSE'92, Yokohama, Sept. 22-25, 1992) North-Holland, Amsterdam, 1993, pp. 985-1 114, and references therein. 2. e.g. H. Niiyama, T. Hattori, and A. Miyamoto eds., Computer Assisted Research for Catalyst Design(Cata1. Today 10( 1991) 119-232), Elsevier, Amsterdam, 1991, and references therein. 3. e.g. C.R.A. Catlow, P.A. Cox, R.A. Jackson, S.C. Parker, G.D. Proce, S.M. Tomlinson and R.A. Vetrivel, Mol. Simul. 3 (1989) 4 9 and references therein. 4 . A. Miyamoto, K. Matsuba, M. Kubo, K. Kawamura and T. Inui, Chem. Lett. (1991) 2055. 5 . A. Miyamoto, K. Kagawa, M. Kubo. K. Matsuba and T. Inui, in M.Doyama et al. (eds.) Computer Aided Innovation of New Materials 11, North-Holland, Amsterdam, 1993, p.1025. 6. A. Miyamoto, M. Kubo, K. Matsuba and T . h i , in M.Doyama et al. (eds.), Computer Aided Innovation of New Materials 11, North-Holland, Amsterdam, 1993, p. 1013. 7. M. Kubo and A. Miyamoto, in M.Doyama et al. (eds.) Computer Aided Innovation of New Materials 11, North-Holland, Amsterdam, 1993, p.295. 8 . M. Kubo, T. Inui, and A. Miyamoto, Proc. 4th Intern. Conf. Fund. Ads. in press. 9 . A. Miyamoto, T. Hattori and T. Inui, Physica C, 190 (1991) 93. 10. A. Miyamoto, T . Hattori and T. Inui, Appl. Surf. Sci. 60 (1992) 660. 11. A. Miyamoto, K. Takeichi, T. Hattori, M. Kubo and T . Inui, Jpn. 3. Appl. Phys. 31 (1992) 4463. 12. K. Kawamura, in F. Yonezawa (ed.), Molecular Dynamics Simulations, Springer Verlag, Berlin, 1992, p.88. 13. D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J. Phys. Chem. 85 (1981) 2238. 14. J.G. Fripat, F. Berger-Andre and E.G. Derouane, Zeolite, 3 (1983) 306. 15. T. Yamazaki, I. Watanuki, S . Ozawa and Y. Ogino, Langmuir, 4 (1988) 433. 16. W.M. Meier, Zeit. Krist., 115 (1961) 439, 17. M. Iwamoto, in T. Inui (ed.), Chemistry of Microporous Crystals, Kodansha, Tokyo, 1991, p.327. 18. D.W. Breck and J.V. Smith, Sci. Am., 200 (1959) 85.

Structural Characterization of Rhenium Impregnated Zeolite Y and ZSM-5 by 9 i and nAl MAS NMR and IR Spectroscopy

H. Hamdan and Z. Ramli Department of Chemistry, Fakulti Sains, Universiti Teknologi Malaysia, KB 791, Skudai, 80990 Johor, Malaysia ABSTRACT Rhenium impregnated on zeolite catalysts is expected to provide variable acidity and selectivity towards metathesis of olefin in palm oil. Since the reactivity depends on the bonding and surface area of the catalytic system, the structural characteristics of Re impregnated zeolite Y and ZSM-5 were established by IR and MAS NMR. Three new peaks are observed at 912,924 and 934 cm-l by IR. The first two peaks are assigned to tetrahedral and octahedral centres of the dimeric Re207 respectively. The peak at 934 cm-* is assigned to distorted octahedral Re207 formed when two oxygens of Re are bonded to the zeolite support. The impregnation of Re on the zeolite surface is further supported by 2% and 27A1 MAS NMR. The number of Re207 (moVuc) attached on zeolite Y and ZSM-5 is 2.2 and 1.O respectively. A possible mechanism of impregnation is proposed. INTRODUCTION The importance of zeolite Y and ZSM-5 as heterogeneous and shape selective catalysts in a variety of cracking and hydrocracking reactions of petroleum have been extensively studied and is well established [l-21. The potential of these zeolites as support and catalysts in other organic reactions particularly the metathesis of olefins in palm oil is of particular interest. One of the most exciting applications of the reaction is the catalytic metathesis of alkenes containing functional groups. Although the metathesis reaction is not an acid-catalysed reaction in nature, the activity of the reaction depends very much on the acidity and surface area of the catalyst. Supported rhenium oxide has been found to be a highly active and selective heterogeneous catalyst for metathesis at room temperature and atmospheric pressure [3-51. When supported on silica-alumina, the activity of the catalyst is further enhanced compared to rhenium oxide supported on alumina. The increase in activity is due to the increase in Brbnsted acidity which reacts with Re04- to produce active sites which are conducive to metathesis reaction [4]. Following the encouraging results reported by Komatsu et a1 [6] on his study of metathesis reaction of propene catalysed by molibdenum on zeolite Y, we chose to study the possibility of impregnating rhenium oxide on zeolites Y and ZSM-5. Such catalytic systems are highly desirable not only because they are expected to provide variable acidity but also selectivity towards certain products of the metathesis of olefins in palm oil. 125

126

H. Hamdan and 2. Ramli

The reactivity of the sites which are responsible for metathesis reaction depends on the bonding sites, surface area and structure of rhenium oxide and the zeolite support. The objective of our work was to establish the structural characteristics of the rhenium oxide on zeolite Y and ZSM-5 supports, mainly by using IR and MAS NMR spectroscopies. The results obtained from this study were compared with those previously reported by other researchers on the impregnation of rhenium oxide on other types of amorphous supports [3-81.

EXPERIMENTAL (0 SamDlearation The zeolites NaY and ZSM-5 used as the starting materials were prepared in our laboratory following established procedures [9,11]. Na,NH4Y (Sample 1) and NH4ZSM-5 (Sample 6) were prepared by one-fold exchange of NaY and ZSM-5 with aqueous NH4N03 solution. Dealuminated zeolite Y (Sample 2) was prepared by taking 20 g portions of Sample 1 and heating it in a tubular quartz furnace, with water being injected at a rate of 7 mVh into the tube by a peristaltic pump so that the partial pressure of H20 above the sample was 1 atm. The HZSM-5 (Sample 7) was prepared by calcination of Sample 6. Times and temperature of the treatment are given in Table 1.

Samples Prepared from sample no.

7 8

I

6 7

*treatment

I (C)

I (I)

460 OC,2h 6 w% Re on 7, (C) 500 OC, 2h

* - (S)denotes hydrothermal treatment, (I)

Surface Area (m2/g)

I I

608.6 532.1

Pore Volume (cc/g)

I

0.267 0.230

impregnation of Re, (C) calcination

(ii) Preparation of the catalyst The catalysts were prepared by impregnating samples 2 and 7 with a calculated amount of aqueous solution of ammonium perrhenate (NHqReO4) followed by drying at 110 OC and calcination at 500 O C in air. In this study, a number of Re207/zeolite catalyst systems were prepared with Re loadings in the range of 1-6 w% of Re. The impregnated samples 2 and 7 are denoted as samples 3-5 and 8 respectively. Details on the treatment of each sample are listed in Table 1. (v) Adsorption and XRD measurements The specific surface areas and the pore volumes of the zeolites catalysts (see Table 1) were determined by the BET method at 77 K with nitrogen as the adsorbate. Crystallinity of zeolite samples was determined by comparing the intensities of XRD peaks with the starting materials which were supposed to be 100% crystalline.

Re Impregnated Zeolite Y and ZSM-5

127

(iii) Mid-IR spectra were measured at room temperature using a Mattson FTIR spectrometer Galaxy 6020 and the KBr wafer technique. The KBr wafer is made by mixing about 0.25 mg of zeolite with 300 mg KBr powder and pressing under vacuum. All measurements were performed at room temperature to keep the hydration state of the zeolites constant and minimize spectral changes. v(i)-

. . NMR [MAS N M R l

29Si MAS N M R spectra of the samples were measured at 79.5 MHz using a Varian 400 spectrometer. Samples was spin at 5 kHz using air as the spinning gas. Radiofrequency pulses of 4 ps were applied with 40 s recycle delay. 1400 transients were acquired for each spectrum. 29Si chemical shifts were quoted in ppm from external TMS, used as reference. 27Al MAS NMR spectra were measured at 104.21 MHz using a Bruker MSL 400 spectrometer. Acquisition was camed out using 0.6 ps radiofrequency pulses with 0.2 s recycle delay. Samples were spun at 6.8 kHz. Chemical shifts are quoted in ppm from external Al(H20)63+. 5000 transients were acquired for each spectrum.

RESULTS The XRD patterns of all samples before and after impregnation indicate very good retention of crystallinity and structure. The diffraction patterns of NHqReOq is apparent only with 6% Re loadings. After calcination of the samples, the reflections at 28 = 16.48O, 25.37O, and 34.730 characteristic of Re species in the zeolite were not observed. This suggests that before calcination, the impregnated Re species of higher percent loading exist as isolated NH4Re04 species on the zeolite surface, which are detectable by XRD. The disappearance of these reflections after calcination suggests that the Re salt decomposes to form the metal oxide bonded to the surface of the zeolite framework. There is no loss of Re during the process as proven by the elemental analysis. The decrease in the unit cell sizes calculated from XRD data as listed in Table 4, observed for the calcined Samples 3, 4 and 5 is due to further dealumination of the zeolite framework upon calcination of the ammonium containing Re impregnated zeolites. The retention of crystallinity and adsorption of Re onto the zeolite surfaces are further supported by the IR spectra as shown in Figure 1. The absorption frequencies of samples 2-8 are listed in Table 2. The IR spectra reveal three additional peaks for all Re impregnated zeolites samples with the wavenumbers of ca. 912,922 and 934 cm-l. The peaks at 912 and 922 cm-1 correspond to the tetrahedral ReO4- and octahedral ReO3+ species respectively as assigned in previous works [8]. These two absorptions have been observed for dimeric Re207 species. However, the peak at 934 cm-1 has never been observed nor reported for any Re species, impregnated on other type of supports. The intensity of the Re-0 IR absorption bands in Sample 5 increases with increasing amount of Re loadings as shown in Figure 2. Our calculations for 6% Re loading in sample 5 and 8 show that the average number of impregnated Re207 molecules is 2.2 and 1.O molecules per unit cell

128

H. Harndan and 2. Rarnli

of zeolite respectively. Adsorption studies also indicate a decrease in the surface area and pore volume of the zeolites (seeTable 1). The decrease in the pore volume and surface area of the zeolites indicates that the much larger Re207 molecules must have been, to some extent entered the zeolite pores and anchored onto the surface of the pores. Nevertheless, the surface area of the zeolite support remains much larger than that observed on a silica-alumina support [4].

kfiE4 1200

I000

BOO

600

400

wavenumbers

Figure 1 IR spectra of (a) Sample 2, (b) Sample 5, (c) Sample 7 and (d) Sample 8

Figure 2 IR vibrational frequencies of R e 0 in zeolite Y with (a) 1%, (b) 3% and (c) 6% Re loadings :r Re

Table 2

2 3 4 5 7 8

int. asym ext. sym int sym 1040 783 574 581 1040 804 581 1042 805 1044 807 583 1098 794 544 546 798 1100

bend 459 457 458 455 452 450

tetra oct

dis. oct

912 912 912

924 924 922

934 934 932

910

922

934

29SiMAS NMR spectra of sample 2 (see Fig. 3a) before impregnation of Re is in complete agreement with earlier works[9-12] and consist of up to five signals in the chemical shift range of -85 to -105 ppm corresponding to Si(nA1) building blocks. The spectra were deconvoluted using Gaussian peak shapes and the so-obtained relative intensities of the individual signals are given in Table 3. The framework Si/AI ratios of the samples calculated from the spectra are as listed in Table 4. The 2% MAS NMR spectra of Re impregnated Samples 5 and 8 (see Figure 3b and 4b respectively) are virtually unchanged. Besides a slight decrease in intensities expected from hrther dealumination upon calcination of Sample 5 as described earlier, the intensities of the signals Si(OA1) and Si( 1Al) remain relatively unchanged whereas those of Si(2A1) and Si(3A1)


129

are significantly reduced and broadened. The spectra in Figure 3 clearly indicate broadening of these signals which are split into at least two distinct overlapping peaks in which one of the peaks is shifted towards more negative frequencies. The effect is not significant for Sample 8. There is only a slight decrease of intensity and broadening of the signal corresponding to Si( 1Al). Assuming that the percentage of the Si(nA1) sites impregnated with Re is proportional to the distribution of the shifted peaks since the splitting is due to the influence of Re207 impregnated on the zeolite surface, the amount of Re207 adsorbed onto the surface of each Si(nA1) sites in samples 5 and 8 were calculated from the distributions of the Si(nA1) signals in the 29Si MAS NMR spectra (see Fig. 3b) as listed in Table 5. The data obtained agree well with the values calculated from wet analysis. 27Al MAS NMR spectra of Sample 5 before and after Re impregnation (see Figure 3c and d) indicate a small loss of aluminium from the framework due to dealumination. Most of the aluminium is in the tetrahedral form.

Table 3

Relative distribution of Si(nA1) configurations in samples Y and ZSM-5 before and after Re207 impregnation, as calculated by Gaussian deconvolution of 29Si MAS N

Table

Note: n.m; not measured

Table 5

I

Si(nA1)

n= 0 1

2 3 4

Quantitative determination of the distribution of Re207 adsorbed per unit celI1 of F 40) Sample 5 (Si/Al = 3.8; A ~ =

I

%Si sites

26.45 47.85 20.62

1 % Si sites

1 % Sisites 1

Dist. of

with A1 0.00

with Re

Al

0 0 17 21 52

0 26 11

65.06 28.08 5.44 1.46

I

No. Si site with Al

0 26 6 1 0

I

No. Re-0-AI sites 0 0 1 1 0

1

130

H. Hamdan and 2. Ramli

1

I

- 80

I

- 90

I

1

I

- 100-

I

I

- 110

60

ppm from TMS

I

30

I

0

I

-30

ppm from AI(H~o)$+

Figure 3 29% and 27Al MAS NMR spectra of Sample 2 (a) and (b) and Sample 5 (c) and (d) respectively

-100

-110

.120

ppm from TMS

Figure 4 29Si MAS NMR spectra of (a) Sample 7 and (b) Sample 8 DISCUSSION The calcination of the Re impregnated zeolites does not change the framework structure of the zeolites. A number of important features are observed upon fbrther examination of the relative intensities of Si(nA1) sites in the 2% MAS N M R spectra of Sample 5 (see Figure 3b), which contribute to the mechanism of impregnation and structural characteristics of the Re2O7. There is a significant change to the lineshape of the signals corresponding to Si(3A1) and Si(2Al) after Re impregnation. The broadening and splitting into at least two distinct overlapping peaks in which one of the peaks is shifted towards more negative frequencies is due to the influence of Re207 impregnated on the zeolite surface. Being more electronegative than H+ or Na+, Re’+ decreases the shielding on the silicon and causes the signal to shift to a smaller


131

frequency than the precursors. This splitting is not observed in the Si(lA1) site or Si(OA1) site. Our observation is in complete agreement with those suggested by Mol [4] and Ellison [ 131 that impregnation of Re favours the more acidic site. In zeolites, these sites correspond to the bridging OH groups attached to the framework aluminium. Since the number of Si with A1 sites available are far greater than the number of Re molecules, the Re molecules would naturally firstly attack the Si sites with more A1 attached to it. Consequently, the signals corresponding to those sites attached to Re will be shifted with respect to those sites originally present and are indeed observed as shoulders in Figure 3b. For sample 8, the Re207 will tend to attach to the Si(lA1) sites. The shift in the Si( 1Al) peak is less obvious and causes the signal to be broadened rather than split followed by a decrease in the intensity. Since Re207 molecules are too large to enter into the small cages, only those distributed in the large cage of zeolite Y and ZSM-5 will be accessible to impregnation. During calcination, the ReO4- ions are preferably bonded to sites which were previously occupied by the bridging hydroxyl groups. The reaction of ReO4- ions with the bridging hydroxyl groups results in electropositive rhenium centres which will easily accept the complexation of the electron rich carbon-carbon double bond of alkenes. THE Re207/ZEOLITE STRUCTURE Extensive studies have been carried out in order to establish the possible structure of Re207 on various types of amorphous support. Regardless of the chemical nature of the support being studied, it has been proven that in the hydrated form, Re exists as dimeric tetrahedral and octahedral Re207 with the octahedral end attached to the support, as depicted by cornformation I in Figure 5. The same is observed for the zeolite impregnated Re catalysts. This is proven by the presence of two absorption peaks observed in the IR spectra. However the IR spectra also indicate an additional absorption at ca. 934 cm-l. The appearance of the peak at a higher wavenumber than 924 cm-1; that is assigned to octahedral Re207 suggests that when

impregnated on the zeolites there exist another Re207 conformation of a higher symmetry. Realizing that zeolite is structurally different from the amorphous Si02-Al203 precursors in that it consists of a crystalline framework of SiO4 and A104 tetrahedra, it is therefore possible for two of the octahedral Re centres to be bonded to two adjacent framework aluminiums as shown by conformation I1 in Figure 5. Such conformation will result in a distorted octahedral Re centre and causes a shift of the octahedral IR absorption band from 924 to 934 cm-l. Our studies on Re impregnated zeolite Y with different framework Si/Al composition indicate that the probabilities of existence of such conformation I1 depends on the distribution of the SGO-A1 sites in the zeolite framework. It strongly indicates that conformation I1 predominates in the presence of silicon sites having a larger number of aluminium neighbours in the first coordination sphere. In zeolite Y of higher Si/Al ratios, the Re207 of conformation I1 is not observed. However, further study by MAS NMR of Re207 impregnated on zeolites of various framework compositions are necessary to prove these conclusions.

132

H. Hamdan and Z. Ramli

Comformation I

Conformation II

Figure 5 Structural representation of Re207 impregnated on zeolites

CONCLUSIONS The results of this study have shown that it is possible to prepare Re impregnated zeolite catalysts. The quantity of Re207 adsorbed on the surface of the zeolite support depends on the percentage of Re loadings and the size of pores in the zeolite. The IR spectra reveal that upon impregnation and calcination of Re on zeolites, the Re207 exists as dimeric octahedral and tetrahedral entities of conformation I and as distorted Re207 dimer of conformation 11. 29Si MAS NMR spectra of Re impregnated zeolite Y indicate that Re207 favours the more acidic Si-0-AI sites of the zeolite framework. The acidity and activity of Re impregnated zeolite Y and ZSM-5 catalysts on the metathesis of olefins are currently being investigated. ACKNOWLEDGEMENT We wish to express our appreciation to Prof. A. Corma from CSIC-UPV, Valencia, Spain and Prof M.E.G. Derouane from Univ. of Namur, Belgium for the NMR spectra, and UTM for financial support. REFERENCES 1 . I.E. Maxwell and W.H.J. Stork in Zeolite Science and Practice, Stud. Surf. Sci. Catal., 58 (1991) 571 2. J.Weitkamp, Hydrocracking and Hydrotreating Process, ACS Symp. Series 20, American Chemical Society, 1975, p. 1 . 3. Xu, Xiaoding, C. Boelhoumer, D. Vonk, J.I. Benecker and J.C. Mol, J. Mol. Catal., 36 (1986) 3 1 . 4. A.Andreini, X. Xu and J.C. Mol, Appl. Catal., 27 (1986) 31. 5. R. Amigues, Y . Chauvin, D. Commereue, C.T. Hong, C.C. Lai and Y.H. Liu, J Mol. Catal., 65 (1991) 39. 6. T.Komatsu, S. Namba and T. Yashima, Acta Phys. Chem. 31 (1985) 251. 7. J. Ardreini, J. Mol. Catal., 65 (1991) 359. 8. R. Nakamura, F. Abe and E. Echigoya, Chem. Lett., (1981) 5 1 . 9. H. Hamdan, B. Sulikowski and J. Klinowski, J. Phys. Chem., 93 (1987) 350. 10. E.R. Andrew, Int. Rev. Phys. Chem., l(1981) 195. 1 1 . H. Hamdan and J. Klinowski, Chem. Phys. Lett., 139 (1987) 576. 12. D. Freude in Recent Advances in Zeolite Science, Stud. Surf. Sci. Catal., 52 (1989) 169. 13. A. Ellison in Olefin Metathesis and Polymerization Catalysts, Synthesis, Mechanism and Utilization, Kluwer Acad. Pub., 1989, p. 335.

111. Mdbation


Solid-state Reactions of Zeolites

Hellmut G. Karge Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany ABSTRACT The principle of preparation of modified zeolites by solid-state reaction is illustrated using simple systems such as mixtures of alkaline, alkaline earth or lanthanum chlorides and ammonium or hydrogen forms of zeolites. The advantages of this approach are discussed. Solid-state reaction is an attractive route to preparation of acidic or bfinctional catalysts. Finally, it is shown that solid-state modification is also possible using the as-synthesized sodium forms for instance for an ion exchange with mono-, bi- and trivalent cations, which is investigated by IR,2sNa MAS NMR, TPE and, in the case of FeC12, additionally with Mbsbauer spectroscopy. INTRODUCTION Zeolites may be modified either by changes in the chemistry of their anion framework or by exchange of the charge-balancing cations. Post-synthesisinsertion of cations into the anionic framework of a zeolite is a relatively new field of research activity. However, very early on their ion exchange properties attracted the attention of scientists both for basic and applied research. A large body of pertinent literature on ion exchange in zeolites exists, and the field has been reviewed several times [l-21. The studies on ion exchange in zeolites almost exclusively concerned the conventional ion exchange. Briefly, this is carried out by suspending crystallites of a zeolite with a cation A in a salt solution of the cation B which is supposed to go into the zeolite structure and there replace cation A. However, there were some early observationsthat ion exchange with zeolites might occur in solids as well. Thus, solid-state ion exchange in zeolites has been observed and reported in some early studies by Rabo et al. [3] and CIearfieId et ul. [4]. After this early work on solid-state reaction of zeolites, there has been essentially no activity in this field for a rather long period. Only during more recent years, we have seen a growing interest in this route to modification of zeolites and zeolite-like materials which indeed is attractive in many aspects. One advantage of solid-state ion exchange results from the fact that it does not require handling of large volumes of solutions containing salts of the exchange cations as is the case with the conventional ion exchange. Thus, the necessity of separating the solid materials (zeolite powder) and aqueI35

136

H. G. Karge

ous solution after the ion exchange is avoided as well as problems of environmentally protective discharge of large amounts of salt solutions. This may be of particular importance in view of both economical and ecological aspects. Furthermore, with solid-state water-free ion exchange, introduction of cations into zeolite structures is facilitated in those cases where a hydration shell prevents the cation fiom penetrating into narrow pores or small cavities. Frequently, solid-state reaction enables one to obtain a high degree of exchange (up to 100%) in a one-step process. If one of the products of the solid-statereaction between the compound of the cation to be incorporated and the zeolite is volatile (HCI, H20), this product may be easily removed and the equilibrium shifted to the side of the highly exchanged zeolite. Systematic work on this phenomenon is now carried out in several research groups, Kucherm and Slinkin [5-71resumed the ESR approach used by Clearfield et al. [4] to follow solid-state ion exchange of transition metal cations into zeolites. Karge et al. [8-lo] investigated systematicallythe ion exchange of alkaline, alkaline earth and rare earth cations into zeolites via solid-state reaction. A great variety of methods have been employed for investigationof solid-state reactions of zeolites including infrared spectroscopy (IR), electron spin resonance spectroscopy (ESR), X-ray diffraction (XRD), magic angle spinning nuclear magnetic resonance ( M A S NMR), Mossbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and temperature-programmed evolution (TPE) of volatile reaction products (hydrogen halides, water), monitored by titration, gas chromatography (W) or mass spectrometry ( M S ) . In the present study, a brief review on results obtained by solid-state reaction of chlorides of alkaline, alkaline earth and rare earth metals with hydrogen and ammonium forms of faujasite-type zeolites and ZSM-5 should first illustrate the principle and potential of solid-state ion exchange in zeolites. This contribution will then focus on very recent results of so-called contact-induced ion exchange. In that context it will be shown that solid-state ion exchange also provides an attractive route to obtain active catalysts directly from the sodium form of as-synthesized zeolites. Finally, examples will be discussed where obviously the presence of a particular gas phase facilitates the transport of the ingoing cation from its solid compound into the pore structure of the zeolite. EXPERIMENT The experimental procedure for solid-state ion exchange in zeolites is rather simple. The only prerequisite is to generate an intimate mixture of the zeolite powder and the compound, e.g., halide or oxide of that cation which is desired to go into the zeolite structure. Such a mixture may be obtained by thorough grinding or milling of the components or via suspendingthe finely dispersed powders in an inert solvent (e.g., heptane) and subsequent evaporation [8]. The thus obtained mixture is then heated in high vacuum or in a stream of an inert gas to remove volatile products such as HCI, H20,N H 3 etc. It has been demonstrated that the presence of water (e.g. moisture of the ambient or crystal water) is neither detrimental to solid-state ion exchange nor a prerequisite, i.e. solid-state ion exchange can be conducted under completely water-free conditions. Application of the various techniques employed to confirm and study qualitatively andor quantitatively the solid-state reactions of zeolites as well as the particular procedures developed have been


137

described in detail elsewhere [8-121or are indicated in the text. Also, the materials used have been characterized in the respective references or will be specified in the appropriate paragraphs of this paper. RESULTS AND DISCUSSION Alkaline metal chlorides/ NHq-ZSM-5. H-ZSM-5 or -3 Mixtures of solid alkaline chlorides MeCl (Me=Li, Na, K, Rb, Cs) and NHq-ZSM-5, H-ZSM-5 [8] or NHq-Y-zeolite react at elevated temperatures to evolve gaseous hydrogen chloride and form the respective cationic form of ZSM-5 or Y-type zeolite. The acidic OH groups are completely, the non-acidic ones are partially removed, as evidenced by IR.Thermogravimetric analysis, titration of the evolved gases, and TPE-MSrender the discrimination between a low-temperature and a hightemperature exchange reaction possible. The system M e C m - Y has been studied more recently. Application of TPE-MS in this case is illustrated in Figure 1 where the evolution of HCI and N H 3 as the products of solid-state reactions in the systems M a w - Y (with Me=Li, Na, K, Rb, Cs) is monitored. Two reaction regimes are clearly discriminated, viz. a low-temperature reaction and a high-temperature reaction range. The low-temperaturereaction is particularly pronounced in the case of LiCI; its contribution decreases in the sequence Li>Na>K>Rb>Cs. Simultaneously, the peak temperature of the high-temperature reaction decreases in the same sequence. It is worth mentioning that with the exception of L i C w - Y (where the high-temperature reaction is essentially lacking) the decrease of the high-temperature peak in the sequence Na>K>Rb>Cs parallels the decrease in the lattice energies of the chlorides [8]. Comparative measurements of mixtures of inorganic chlorides and zeolites prepared via (i) grinding and milling or (i) precipitation of suspensions in volatile solvents have shown that obviously solid-state reaction of zeolites takes place to some extent even during mechanical mixing prior to heating. Evaluation of the analytical data shows good agreement between the 400 600 800 1000 aluminium content of the framework and the TEMPERATURE I K amount of neutralizingcations. From comparison of the TPE-MS curves Fig. 1. Temperatue-programmed evolution (TPE-MS) of gases (m/e = 36, HCl, -; m/e = for pure w - Y and the MeCViNH4-Y mix18,H20,o 0 0 ; m/e = 17,NH3, o o o for --Y tures it is evident that the dehydroxylation peak in MeCvNHq-Y mixtures) upon solid-state rearound 900 K is missing in the case of the mixaction.

'

138

H. G . Karge

tures. This is a clear indication of the fact that all the OH groups which may form upon heating of the mixtures have been eliminated by solid-state reaction with the chlorides. Consequently, no hydroxyls were left to react under formation of water at higher temperatures as is the case with pure NHq-Y. Alkaline earth and rare earth cations In recent systematic studies Kurge et ul. [9-101 have shown that acid alkaline earth- or rare earth-containingzeolite catalysts can be prepared via solid-state reaction between salts of the ingoing cation and zeolites. Table 1. Mass balance* for solid-state ion exchange between CaC12 and m - M O R (4) HCI, NH&I evolved

CIextracted

(6) Ca2+ extracted

2.54

1.88

0.95

(9) CaC12 reacted

(10) Ca2+ irrev. held

(11) CaC12 occluded

(6)-(7)

1/2x(4)

(9)-(8)

(3)-(6)-( 10)

0.01

1.27

1.26

0.27

(1)

(2)

Al

Al

total

tetrah.

(3) CaC12 employed

2.50

2.50

2.48

(7) CaC12 extracted

(8) Ca(0H)z extracted

1/2x(5) 0.94

(5)

* All data in millimoles per gram water-free zeolite The stoichiometry of the solid-state exchange between, e.g. m - M O R and CaC12 is perfect in that the amount of Ca2+ reacted (in meq. per gram) exactly corresponds to the content of framework Al (AF) per gram as is demonstrated by the results collected in Table 1. Solid-state ion exchange of protons of deammoniated NHq,Na-Y for lanthanum cations of lanthanum chloride results in a complete replacement of the OH groups of (deammoniated) NHq,Na-Y under evolution of HCI, whereas solid-state reaction in H-ZSM-SLaC13 is incomplete under the same conditions. This is demonstrated by Figure 2 which displays TPE-MS curves for the latter system. Only about 60% of the protons are replaced by La3+ even with an excess of Lac13 (La/Al=0.67, curve b) after heating the LaCI3h-I-ZSM-5 mixtures up to 950 K [lo]. This is probably due to the difficulty in counter-balancinga total of three negative charges by one La3+ in the case of the aluminium-poor H-ZSM-5 samples (Si/Al=23) where the centers of the negative charges are largely separated, by contrast to w - Y (Si/Al=2.7) After heat-treatment at 675 K, stoichiometric mixtures of NH&Na-Y/LaC13 yields a material inactive for catalyzing hydrocarbon reactions such as disproportionation of ethylbenzene or cracking of n-decane. When heat-treatment of N€Q,Na-Y/LaC13 mixtures at 675 K is followed by admission of small amounts of water at the same temperature, OH groups with IR bands form which are typical of LqNa-Y, and an active catalyst is obtained. Its performance in ethylbenzenedisproportionationor decane cracking is superior or at least equal to that of La,Na-Y with a similar composition obtained by conventional ion exchange [lo, 131.


139

Conventionally exchanged La-Y samples with a degree of exchange close to 100% are obtainable only by intermittent (b) La I Al I0.67 calcination and re-exchange, whereas a 0.8 (c) LnCI3 x H20 solid-state reaction between Lac13 and al? i n most 100% exchanged q - Y easily 10.6 results in highly (99%) exchanged La-Y. Even though the presence of traces of 9 water is not a prerequisite for conduction 3 90.4 of solid-state ion exchange (vide infa), w G the thus obtained materials were rendered x active catalysts in acid catalytical hydro0.2 carbon reactions such as n-decane cracking or ethylbenzene disproportionation only after brief contact with small amounts of 300 500 700 900 water vapour [9-10, 131. TEMPERATURE I K However, solid-state reaction does Fig. 2. Temperature-programmed evolution (TPEnot only occur between the hydrogen form MS) of HCl (m/e = 36) upon solid-state reaction beof zeolites (H-ZSM-5, H-Y, H-MOR) and tween Lac13 and H-ZSM-5; (a) LdAl = 0.33, @) LdAI = 0.67 the salts or oxides of the cation to be introduced. Results obtained by 0 -6.0-9.0 solid-state 23Na MAS NMR spectroscopy, X-ray difliactometry and IR spectroscopy show that contact-induced ion exchange occurs at ambient temperatures in a mechanical mixture of, for example, LiCl, KCl, BeC12, or CaCI2, and hydrated Nay zeolite

-

.

khLi

~41.

The as-synthesized sodium form of faujasite type Na-Y zeolite has also been successfully reFig. 3. 23Na MAS NMR spectra (referenced to crystalline acted with LaC13. XRD patterns NaCl of (a): parent Na-Y; (b): ground mixture of (a) and crystalline NaCI; (c): sample (b) heat-treated at 850 K; (d): sample exhibited the appearance of re(c) washed with water flections typical of La-Y and NaCl, the latter forming outside the zeolite grains as tiny NaCl crystallites. The contact-induced ion exchange between Lac13 and Na-Y is clearly demonstrated by 23Na MAS NMR as shown in Figure 3. I

I

0

I

I

1

-

1

I

I

-20

1 - 1

I

I

I

1

-

1

I

-20 0 CHEMICAL SHIFTS, GN~CI, cryst. [ppml

-20

0

I

0

I

I

-20

I

140

H . G. Karge

The assignments of the signals (referenced to NaCl) are as follows: crystalline NaCI, 6 = 0; Na+ at SIII sites in front of 4-rings in the a-cage, 6 = -6.0; Na+ at SII sites in the a-cage, 6 = -8.2 to -9.1; Na+ in P-cages, 6 = -13 to -13.5 [ 151. From Figure 3 it can be deduced that grinding of a mixture of Lac13 and Na-Y at ambient results in a preferential exchange of Na+ by La3+ in the large cavities (acages). The signal around -9 ppm is significantly decreased and visible only as a shoulder of the intense signal at -13.1, indicating Na+ in the P-cages (compare spectra a and b). Upon heattreatment, La3+ enters the small cavities (P-cages) and expels the Na+-ions from there into the acages which results in an increase of the signal at 6 = -8.8 ppm on the expense of the -13.1 signal (compare spectra b and c). Finally, after washing the heat-treated sample, small signals at -6.0 and -9.0 are left due to residual Na+ at SIII and SII sites in a-cages (note that the scale has been extended by a factor of 5). The La, Na-Y samples obtained via contact-induced solid-state exchange between Lac13 and Na-Y are, after contact with traces of water, active catalysts in acid-catalyzed hydrocarbon reactions [161. This is a novel technique to obtain lanthanum-containing zeolite-based catalysts. Transition metal cations Modification of zeolites by solid-state reaction is also an attractive route to obtain transition metal-containing materials. The high-temperature interaction between H-ZSM-5 zeolite and solid MnC12, MnSO4, Mn(CH3C00)2 and Mn304 was studied by temperature-programmed desorption of ammonia, IR,ESR and mass spectrometry [ 171. It has been shown that the degree of solid-state ion exchange for MnC12 is strongly affected by the temperature of the heat treatment. Depending on the amount of manganese cations present in the zeolitefsalt mixture, at 770 K an exchange degree of 60 to 80% can be obtained. For all the Mn compounds studied, the solid-state reaction, resulting in

Table 2. Changes in the content of acidic OH in mixtures of manganese compounds with H-ZSM-5 after 6 h heat-treatment in vacuum or a flow of nitrogen compound Mn2+ temperature OH groups consumed* (mmOVf3) (mm0Yg) CK) vacuuma flowb 0.21 Mnc12 0.33 570 0.33 670 0.38 0.41 0.33 770 0.56 0.57 0.57 0.45 770 MnSO4 0.47 770 0.16 0.18 0.60 770 0.46 Mn304 adetermined from IR data (band at 3610 cm-l after 6 h heat treatment in high vacuum) bdetermined from TPDA; high temperature peak approximately at 700K *initial OH content (pure H-ZSM-5) 0.91 mmoVg


141

replacement of protons of acidic skeletal OH groups by Mn2+ ions, appears to take the same course: most of the Mn2+ ions are exchanged in the initial stage of the reaction and then the reaction rate considerably decreases, levelling off to zero. However, a distinct effect of the anion of the admixed compound is observed. This is shown in Table 2 where the percentage of the OH groups consumed upon reaction of manganese compounds with the parent H-ZSM-5 (0.91 mmol g-l) is indicated. The solid-state reaction proceeds most easily with MnC12. The reaction rate increases with the reaction temperature and is highest at 770 K. Under similar conditions, the degree of exchange is lower with Mn304. and MnSO4, viz. 51% and 18%, respectively [ 171. Cu / O H molar ratio While in the L a C 1 3 W - Y system already a stoichiometric mixture led to a 100% exchange [lo], in cases of transition metal compounds such as, for instance, CuCV H-ZSM-5 a strong effect of the Cu+/OH ratio and reaction temperature was observed. As can be recognized from Figure 4, the 550 600 650 700 Temperature I K solid-state reaction was considerFig. 4. Temperature-programmed evolution of HCI (m/e=36) ably enhanced by an excess of from CuCVH-ZSM-5 mixtures with Cu+/OH equal to (a) 0.32; CuCI. This was measured through (b) 1.00; (c) 1.60; (d) 2.25 the MS signal intensity of the HCI evolved ( d e = 36) during ternperature-programmed heating of CuCVH-ZSM-5 mixtures. Concomitant IR measurements for Cu+/OH ratios of 0.32 and 1.00 revealed an increase of the exchange degree (consumption of acidic OH groups) from 30 to 53%. Similarly, rising the temperature from 670 K to 770 K for a CuCVH-ZSM-5 mixture with Cu+/OH=l .O resulted in an increase of the degree of exchange from 38% to 53% [18]. The ESR spectra of a CuCLfH-ZSM-5 mixture exhibited, after treatment at elevated temperatures in vacuum and subsequent oxidation, signals which are typical of Cu2+-ZSM-5, i.e. they showed the characteristic g values, viz. g 1 = 2.073 and glI = 2.33 and hyperfine splitting constant of all = 1.25 mT (Figure 5). As can be seen fbrther from Figure 5, the intensity of the signal and the hyperfine splitting was significantly enhanced when the temperature of solid-state reaction was increased from 570 K to 770 K or the duration of thermal treatment fiom 12 h to 24 h, in agreement with IR measurements (vide supra). XRD patterns prior and subsequent to heat treatment indicated the disappearance of CuCl reflections due to the solidstate reaction of CuCl with H-ZSM-5 [ 181. Solid-state reaction was also observed between ZnO and H-ZSM-5 [ 191 yielding Zn-ZSM-5 according to

142

H. G . Karge

ZnO + 2 H-OZ + Zn(0Z)z

+ H20

Where OZ- designates the anion zeolite framework. This reaction occurred after intimate I mixing of ZnO with H-ZSM-5 and activation at 723 K (2 h). It was confirmed by IR with and without utilization of pyridine as a probe molecule. The reaction yielded Zn-ZSM-5 catalysts which contained about 2.0 wt.% Zn and exhibited similar activity and selectivity as Zn-ZSM5 catalysts prepared by conventional ion exchange or impregnation of H-ZSM-5 with Zn(NO3)2 followed by thermal treatment CUCI / H-ZSM-5 (compare Figure 6). Calcination in air at 823 K prior to the catalytic reaction decreased the activity which was ascribed to a loss of zinc during thermal treatment. Contact-induced incorporation of iron CUCI / H-ZSM-5 into a zeolite was studied with F e C 1 2 m - Y [20]. XRD, TPE-MS and Mossbauer spectroscopy were employed. XRD patterns of the Fig. 5 . X-Band ESR spectra of stoichiometric FeC12/ m - Y mixtures prior to and after conCuC12/H-ZSM-5 (a) and CuCVH-ZSM-5 (b-d) tact-induced and solid-state ion exchange (at mixtures treated in vacuum at elevated temperaelevated temperatures) revealed the disappeartures followed by oxidation in 1.3 @a oxygen at 570 K for lh and evacuation at 420 K; (a) 770 ance of the FeCl2 reflections and appearance of K, 12h (b-d) 570 K, 12h; 770K, 12 h; 770K, 24h. the W C l reflections, thus confirming the phenomenon of solid-state reaction (Figure 7). The TPE pattern did not only 80 ........................................................................................................................................... show the d e = 18 peak around 400 K, which is usually encountered ............................................................... during solid-state reaction being due to the desorption of coordinately ............................................................... bound H20, but an additional water - Zn-ZSM-5 ZnOIH-ZSM-5 ZnO+H-ZSM-5 H-ZSM-5 ZnO desorption peak at 510 K emerged. thermal treatment The latter peak coincided with the release of ammonia ( d e = 17). Furthermore, the F e C 1 2 m - Y system Fig. 6. Comparison of ethane conversion over differently exhibited a small but well-reproprepared Zn-containing ZSM-5: Zn-ZSM-5: 3 times exduced delay of HCI evolution, comchanged, Zn(NO3)2 soln., 353 K; ZnO/H-ZSM-5, incipient wetness method; ZnO+H-ZSM-5, sotid-state reaction

T G I 7F

I

I


143

pared to NH3 which evolved earlier. Both observations suggest that during contact-induced ion exchange hydroxy-iron ions intermittently form, such as Fe(OH)+, Fe(OH)2+ or Fe(OH)2+: as proven by Mossbauer spectroscopy, the larger portion of is initially oxidized to Fe3+ (vide infu). The water release at 520 K is then due to the subsequent reaction with HCI, originating from NHqCl decomposition, according to

Fs+

Fe(OH)(n-*)+ + HCI + FeCl(n-l)+ + H20

(2)

At somewhat higher temperatures Fe(n-l)+ undergoes solid-state ion exchange: FeCI("-*)+

+ H+ +Fen+ + HCI

(3)

This sequence of reactions would explain the delayed release of HCI compared to NH3 as found with TPE-MS. Mossbauer spectra of the system FeC12W-Y showed that after contact-induced ion exchange at ambient most of the Fe2+ was oxidized to Fe3+ (720 K) W species. However, at elevated tempera4 LL tures changes in the relative concentraW tions of the various FdII and Ferr [r species occured, most dramatically in W I the temperature range around 520 K to I620 K where reactions (2) and (3) proceeded. These changes were due to auto-reduction [21] of the F&I-containing zeolite involving oxidation of framework oxygen atoms to molecular W oxygen: a a = 24.785 A I - 3 2 { Al04/2}3- Fe3+ + 2 { AlO4/2}22 F$+ + ~ 2 0 3+ 112 0 2

&

ccn

z

z -

--

L 11UIL

1 I

30

BRAGG ANGLE, 28 [DEGREES] Fig. 7. Schematic presentation of XRD patterns of XRD patterns of (a) the parent W - Y zeolite; (b) the mixture of m-Y/FeC12.4 H20 ground in air at ambient temperature; (c) material (b) heated in air up to 730 K (heating rate: lOWmin). *) FeC12.4 H20; **) W C l

Here {AlO4/2}- denotes a part of the zeolite structure (a TO412 tetrahedron with T = Al). By carehl analysis of the Miissbauer spectra (Figure 8) obtained prior to and after heat-treatment at increasing temperatures a total of two Fe(II1) and seven Fe(II) species, differing in their coordination, were identified, viz. tetrahedrally and octahedrally coordinated Fe(III), FeC12 . x H20, two types of

144

H. G. Karge

Fe(I1) ions in tetrahedral and 4 types of Fe(II) species in slightly varied octahedral environment. Bihnctional catalysts Bihnctional catalysts possessing both, an acidic and a hydrogenation I dehydrogenation finction can be obtained via solid-state ion exchange as well. As an example, introduction of Pd2+ into H-ZSM-5 was studied [22]. It turned out that simultaneous or, even better, preceding solid-state ion exchange with a polyvalent, non-reducible cation such as Ca2+ via reaction of CaC12/PdC12 with HZSM-5 yielded an appropriate catalyst precursor. After reduction of the resulting Pd,Ca,H-ZSM-5, finely dispersed Pdo particles were formed. They resided in the interior of the zeolite structure as was proven by reactant shape-selective hydrogenation of olefins. The unreduced Ca2+ cations probably hnction as anchors for the Pdo particles formed [23]. The reduced Pd,Ca,H-ZSM-5 was active in hydrogenation of ethylbenzene to ethylcyclohexane or dehydrogenation of ethylcyclohexane to ethylbenzene. The activity, selectivity and time-on-stream behaviour of this catalyst was equivalent or even superior to that of a conventionally prepared PdO-containing H-ZSM-5 with the same Pd-loading (1.O wt.%).

Mechanism of solid-stateion exchange in zeolites

7

p m b i e n t

b - e : treatment in high vacuum -5 -3

-1 1 3 5 VELOCITY [mm/s]

Fig. 8. Massbauer spectra of (a) W-YFeC12. 4 H 2 0 ground at ambient temperature in air and material (a) after heat treatment in vacuum at (b) 420 K, (c) 520 K, (d) 620 K and (e) 720 K.

The mechanism of solid-state reaction between compounds (e.g. halides, oxides) of cations to be incorporated and zeolites is not yet clarified. In particular, it is still unclear, whether molecules (e.g. NaCI) of the compound of the cation to be introduced (e.g. Na+) migrate as such (mechanism I) or if cations and anions move and react separately (mechanism 11). An attempt was undertaken to answer this question by comparison of the solid-state exchange behaviour of, for instance, CsCl and a caesium salt of a bulky anion (which cannot enter the zeolite pores) such as Pw120403-. Indeed, in the former case reaction with the H-ZSM-5 led to an 82% exchange, whereas in the second experiment the reaction resulted only in a 27% consumption of the acidic OH groups of the parent zeolite. The fact that the exchange with the caesium phosphorous tungstenate was not zero, as one might have expected if mechanism I would be operative, was probably due to partial thermal decomposition of Cs3PW12040 at the (mini-


145

mum) reaction temperature of 475 K. Decomposition was indicated by the blue color into which the originally white Cs3PW12040 / H-ZSM-5 mixture turned upon heat-treatment. In fact, most likely, tungsten oxides formed and behaved similarly as molybdenium or chromium oxides do, e.g. in the state of Mo(V) or Cr(V) where they were successfblly introduced by solid-statereaction [6]. The presence of water, even in traces, is not a prerequisite for solid-state ion exchange in zeolites to occur. This was shown by experiments carried out in a glovebox where any traces of water were excluded. Also, the reaction proceeded well with water-insoluble salts such as AgCl or Hg2C12. However, the presence of small amounts of H20 may facilitate the solid-state exchange. This is suggested by the observation of easy contact-induced exchange under ambient conditions or in the case of salts with crystal water (vide supra). In some cases the presence of a particular vapor phase affects the introduction of cations from a solid into the zeolite structure. Sachtler et al. [24], for instance, reported introduction of Pd2+ into H-ZSM-5 in the presence of C12. In this case, cation incorporation probably occurs via sublimation. Miessner et al. [25] have found that the introduction of rhodium cations via solid-state reaction is significantly accelerated in the presence of carbon monoxide. CONCLUSIONS Preparation of modified zeolites by solid-state reaction is possible with a great variety of systems containing a solid compound (e.g. chloride, oxide) of the cation desired to enter the zeolite structure and ammonium, hydrogen or sodium forms of zeolites. Solid-state reaction is confirmed, for instance, by TGA, TPE-MS, XRD, IR,ESR, M A S NMR and Masssbauer spectroscopy. Solid-state modification of zeolites offers an attractive route to obtain active acidic or bifbnctional zeolite catalysts. Generally, the procedure is easy, successfbl in cases where conventional exchange is difficult or impossible due to steric reasons and environmentally favorable, because handling and discharging of large volumes of salt solutions can be avoided. Even though some interesting observations were made concerningthe mechanism of solid-state preparation of modified zeolites, this particular problem requires fbrther investigation. REFERENCES R. M. Barrer, Proc. 5th Int. Zeolite Cod., Naples, Italy, June 2-6, 1980 (L. V. C. Rees, Ed.), Heyden, London, 1980, pp. 273-290 R. P. Townsend, in "Introduction to Zeolite Science and Practice" (H. van Bekkum, E. M. Flanigen and J. C. Jansen, Editors), Elsevier, Amsterdam, 1991; Studies Surface Science 58 (1991) 359-388 [31 J. A. Rabo, "Salt Occlusion in Zeolite Crystals", in "Zeolite Chemistry and Catalysis", (J. A. Rabo, Ed.) ACS Mongraph 171, Am. Chem. SOC.,Washington, D.C., USA, 1976, pp. 332-349 [41 A. Cledeld, C. H. Saldarriaga and R. C. Buckley, Proc. 3rd Int. Cod. Molecular Sieves; Recent Progress Reports; Zurich, Switzerland, Sept. 7-13, 1973 (J. B. Uytterhoeven,Ed.)University ofLeuven Press, Leuven, Belgium, 1973; paper No 130, pp. 241-245 A. V. Kucherov and A. A. Slinkin, Zeolites 6 (1986) 175-180 A. V. Kucherov and A. A. Slinkin, Zeolites 7 (1987) 38-42 A. V. Kucherov and A. A. Slinkin, Zeolites 8 (1988), 110-116 H. K. Beyer, H. G. Karge and G. Borbely, Zeolites 8 (1988) 79-82 H. G. Karge, H. K. Beyer and G. Borbely, Catalysis Today 3 (1988) 41-52

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[lo] H. G. Karge, G. Borbely, H. K. Beyer and G. Onyesty& Proc. 9th Int. Congress on Catalysis, Calgary, Canada, June 26-July 1, 1988, (M. J. Philips and M. Ternan, Eds.), Chemical Institute of Canada, Ottawa, 1988, pp. 396-403 [ 1 1 H. G. Karge, "Modificationof Zeolites and New Routes to Ion Exchange", in: "Zeolite Microporous Solids: Synthesis, Structure and Reactivity", Proc. NATO Adv. Inst., Sintra-Estoril, Portugal, May 13-25, 1991 (E.G. Derouane, F. Lemos, C. Naccache and F. Ribeiro, Eds.) Kluwer Acad. Publ., Dordrecht, The Netherlands, 1992; NATO AS1 Series 352 (1992) pp. 273290 [12] H. G. Karge and H. K. Beyer, in "Zeolite Chemistry and Catalysis", Proc. Int. Symp., Prague, CSFR, Sept. 8-13 (P. A. Jacobs, N. I. Jaeger, L. Kubelkova and B. Wichterlovi, Eds.) Elsevier, Amsterdam, 1991; pp. 43-64; Stud. Surf. Sci. Catalysis [13] H. G. Karge, V.Mavrodinova, 2. Zheng and H. K. Beyer, Appl. Catalysis 75 (1991), 343-358 [14] G. Borbely, H. K. Beyer, L. Radics, P. Shdor and H. G. Karge, Zeolites 9 (1989) 428-43 1 [ 151 H. G. Karge, H. K. Beyer and G. Pa-Borbely , submitted to Zeolites [16] H. G. Karge and H. K. Beyer, DGMK-Berichte, Tagungsbericht 9101, DGMK-Fachbereichstagung "C1 - Chemie - Angewandte Heterogene Katalyse C4 Chemie", Leipzig, Germany, Feb. 20-22, 1991, ISBN No. 3-928164-07-4, ISSN No. 0988-068X, pp. 191-206 [17] S. Beran, B. Wichterlova and H. G. Karge, J. Chem. SOC.Faraday Trans. 86 (1990) 3033-3037 [18] H. G. Karge, B. Wichterlova and H. K. Beyer, J. Chem. SOC.Faraday Trans. 88 (1992) 13451351 [ 191 F. RoBner, A. Haglu, U. Mroczek, H. G. Karge and K.-H. Steinberg, in "New Frontiers in Catalysis", Proc. 10th Int. Congress on Catalysis, Budapest, Hungary, July 19-24, 1992, L. Guczi, F. Solymosi and P. Tetenyi, Eds.) Akaddmiai Kiado, Budapest, 1993, pp. 1707-1710 [19] K. Lazar, submitted for publication in Zeolites [20] K. Lazar, G. Pa-Borbdly, H. K. Beyer and H. G. Karge, submitted to J. Chem. SOC.Faraday Trans. [21] P. A. Jacobs, W. de Wilde, R. A. Schoonheydt, J. B. Uytterhoeven and H. Beyer, J. Chem. SOC. Faraday Trans. I, 72 (1976) 1221-1230 [22] H. G. Karge, Y.Zhang and H. K. Beyer, in "New Frontiers in Catalysis", Proc. 10th Int. Congress Catalysis, Budapest, Hungary, July 19-24, 1992 &. Guczi, F. Solymosi and P. Tetenyi, Eds.) Akademiai Kiado, Budapest, 1993, pp. 257-270 [23] M. S. Tsou, H. J. Jsiang and W. M. H. Sachtler, Appl. Catalysis 20 (1986) 231-238 [24] 0. C. Feeley and W.M. H. Sachtler, Appl. Catalysis 75 (1991) 93-103 [25] L. Schlegel, H. Miessner and D. Gutschik, submitted to "CatalysisLetters" (by courtesy of the authors)

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-

Anion Exchange Reactions in Layer Structured Crystals

Shoji Yamanaka Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan ABSTRACT Two types of new anion-exchangeable layer structured crystals have been developed: basic copper acetate, and y-zirconium phosphate. Basic copper acetate, C U ~ ( O H ) ~ ( O C O C H ~ )adopts ~H~O, the botallackite type layer structure with the acetate ions directly coordinated to the copper ions, which are exchangeable with various anions such as NO;, CIO,-, CI', Br-, I', SO,*-, MnO, , and carboxylate ions. The MnO, exchanged product thermally decomposes to an amorphous mixture of CuO and CuMn20,, which shows a high catalytic activity toward the oxidation of carbon monoxide. y-Zirconium phosphate is formulated as Zr(PO,)( H2P04).2H20. The dihydrogenphosphate groups located on the interlayer surface are exchangeable with various phosphoric ester ions, forming organic derivatives of inorganic layers. INTRODUCTION Ion exchange is an important route in the preparation of intercalation compounds [1,2]; the interlayer cations of most of layer structured crystals are easily exchanged with various organic as well as inorganic cations. Since the layers are only weakly bound with each other, the interlayer space can expand to such an extent that guest cations even much larger than the thickness of host layers can be accommodated between the layers. A large number of such cation-exchangeable layer structured crystals are known [I]; swellable clay minerals, zirconium and titanium phosphates, and transition metal oxysalts such as titanates, uranates, vanadates, molybdates. Exchangeable cations are located between the layers to balance the negative charge created within the layers by substitution of cations with lower valent cations, or by the presence of excess anions in the framework. Microporous pillared clays are prepared by exchanging the interlayer cations of the silicate layers of clay with voluminous hydroxy metal cations such as [Al 1304(OH),4]7+and [Zr4(OH),4]2+,which are converted into oxide pillars between the layers [3]. Oxide sol particles can also be introduced into the interlayer space by an ion exchange. so long as the particles are positively charged [4,5]. In contrast to a variety of cation-exchangeable layered crystals, anion-exchangeable ones are very rare. This is probably because anions are used as building units constructing the framework of crystal structures, and strongly bound to cations. It is very difficult for large anions to move under an ambient temperature. Hydrotalcite-type layered double hydroxides (LDHs) with a general formula I47

148

S . Yamanaka

0

A7

2

(a) (b) Fig. 1 . Comparison of the botallackite (a) and the LDH (b) type layer structures. of [M", -xM"',(OH)2]X+ Yz-x,z nH20 are exceptionally rare examples of anion-exchangeable crystals

[6]. As often referred to as anionic clays, LDHs are structural complements of smectite cationic clays; the positive charge of the layers are created by the substitution of divalent cations with trivalent ones in the octahedral layers. The charge-balancing anions are located between the hydroxy layers, and easily exchanged with various anions. An attempt to introduce anionic pillars into LDHs have been made by Pinnavaia [7]. In this study, new types of anion-exchangeable layered crystals have been developed, in which the exchangeable anions themselves are used to construct the layered framework of the crystals; basic copper acetate, and y-zirconium phosphate. BASIC COPPER ACETATE Svnthesis and structure Basic copper acetate was prepared by titrating a 0.1 M copper acetate solution with a 0.1 M NaOH solution up to OHKu = 1 . Green-colored platelet crystals with a composition of Cu2(OH)?(OCOCH3).H20were obtained [8]. The X-ray powder diffraction (XRD) pattern of the basic salt can be indexed on the basis of a monoclinic cell; the lattice dimensions of the a-b plane ate comparable with those of the basic copper salts of the botallackite type such as CU,(OH)~X(X = C1, Br, and NO,), only the basal spacing being changed in accordance with the size of the acetate ion. On heating to 100°C or by evacuation, the interlayer water is reversibly removed with a decrease of the basal spacing from 9.30 to 7.20 A. The structures of the botallackite and LDH are schematically compared in Fig. 1. Both structures can be derived from the Cdl, layer structure. In the LDH layer, the hydroxide layers are completed by hydroxy groups; the divalent metal ions are partially substituted with trivalent metal ions. The layers im positively charged. This excess charge is balanced by anions located between the layers. On the other hand, the botallackite has neutral hydroxy layers. A quarter of the hydroxy groups are substituted with acetate ions. It is interesting to note that though the acetate framework ions are directly bound to Cu2+ ions, these are easily exchangeable with various anions as described below.

A n i o n Exchange in Layer Structured Crystals

Anion exchange The acetate ions of basic copper acetate are easily exchanged with various anions merely by dispersing in aqueous solutions of NaX ( X = CI, Br. I. NO,, CIO,), and Na2S04 at room temperature. the basal spacing being changed to those of' the corresponding basic salts already reported; Cu2(0H),(CH,COO) + X- -F C U ~ ( O H ) ~+ X CH3COO-

149

CU2(0H)3N0,

/ \

Cu,(OH),(OCOCH,).H,O

Cu,(OH),,(CIO,),

1 A/

Cu,(OH),Br

Cu,(OH),CI

Fig. 2 . Reversibility in the anion exchange.

The ions CH,COO- NO3-, and CIO; are reversibly exchanged with each other, as shown in Fig. 2 . The exchange with small size ions such as chloride and bromide ions are irreversible. The competitive ion exchange reactions studied on several pairs of anions showed that the selectivity of the anions by the basic copper layers were in the following order; CI- > Br- > NO,- > CH3COO' , CIO, . The study by scanning electron microscopy clearly indicated that the shape of the crystals were retained before and after the ion exchange reactions, suggesting that the reactions occur topotactically. The exchange with carboxylate anions were performed by using various sodium salts (nCnH2,+,COONa, n = 0-1I ) [9]. The basal spacings of the exchanged crystals iue shown in Fig. 3

0

10

5

n

Fig. 3. Basal spacings of' basic copper acetate after reaction with carboxylate ions with different number of carbon atoms (n) in the alkyl chains.

Fig. 4. Schematic structural models of the onetntation of interlayer alkyl chains of the exchanged products I, and 11.

150

S. Yamanaka

as a function of the number of carbon atoms (n) in the alkyl chains. Some products have more than one kinds of basal spacings depending on the preparation conditions. Two linear relationships are observed with slopes corresponding to 2.55 (1) and 2.0 (11) &carbon atom. These slopes suggest that the alkyl chains are oriented in bimolecular layers almost perpendicular and inclined at an angle of about 52" to the layers, respectively as shown in Fig. 4. Chemical and thermogravimetric analyses showed that the samples with the higher slope (1) have a composition of x = about I , and the ones with the lower slope (11) have a composition of x = about 0.85 in Cu2(0H),~,(CnH,,+,COO),.

Cu2(OH)3Mn04 Decomposition 200°C Crystallization 440°C

I

W, 18.8% (Calcd. 20.0%)

1/2 CuMn204 + 3/2 CuO 940°C

1

W, 2.6% (Calcd. 2.7%)

CuMn02 + CuO 1005°C

I

W32.6% (Calcd. 2.7%)

CuMn02 + U2 Cu20

Fig. 5. Thermal decomposition of Cu,(OH),MnO,

MnO,exchanPed product The acetate ions can be ion exchanged with MnO, anions, though the XRD pattern of this product cannot be indexed on the basis of the usual unit cell derived from a simple botallackite type structure. The exchanged product Cu2(OH),Mn04 thermally decomposes as shown in Fig. 5 [8]. In relation with a well known Hopcalite catalyst (amorphous CuMn204) for oxidation of carbon monoxide to carbon dioxide, a similar catalytic activity was expected to the thermally decomposed product. The conversion of CO to CO, was tested at 30°C for the samples treated at temperatures ranging from 250 to 50OoC, and the results are shown in Fig. 6. The conversion was found to be almost 100% in the beginning of the reaction. The activity for the conversion tends to last longer for the samples treated at a higher temperature in the above temperature range. However, above 500"C, the sample showed a lower level of activity from the beginning, and a substantial deactivation 100

100

W'C

...o...

k? 50

8 0

1

Time. h

Fig. 6. Percent CO conversion versus time for Cu (OH) Mn04 decomposed at different temperatures; (A) 250", ( A )300", ( 0 ) 350°, 40&, and ( 0 ) 500OC.

6)

Anion Exchange in Layer Structured Crystals

151

I

occurred only after one hour. The sample calcined at 400°C showed the highest activity and the initial activity lasted longer than one week. The high level of the activity was completely recovered by the reactivation by heating at 400°C in an oxygen atmosphere. According to Puckhaber et al. [lo], the key to prepare active Hopcalite catalysts is in how to obtain pure amorphous CuMn,O,, because the formation and segregation of the crystalline spinel phase lead to deactivation of the catalyst. The activity was found to vary with the conditions of preparation. By using the crystalline sample of Cu,(OH),MnO,, amorphous phase was easily obtained by the thermal decomposition. The activity is probably due to the formation of an amorphous mixture of CuO and CuMn,O,. The advantage of using the basic copper permanganate would be found in obtaining homogeneous mixture of Cu and Mn ions in an atomic level, and the reproducibility of the thermal decomposition. ZIRCONIUM PHOSPHATE Structure Zirconium phosphates, Zr(HP04), n H 2 0 are well known cation exchangeable layer structured crystals. The phosphate groups are bonded to zirconium atoms, and situated alternatively above and below the zirconium atom planes [ 11,121. The tips of phosphate groups, which are not bonded to zirconium atoms, bear protons and are directing toward the adjacent layers. These protons are responsible for the cation exchange capacities of zirconium phosphates. There are two polymorphs in the layer structured zirconium phosphates; a monohydrate, a and a dihydrate, y. Although the two layer structures had been considered to be very similar [ 131, a recent study by solid state NMR has revealed that the y phase has two different types of phosphate groups, and should be formulated as Zr(P0,)(H2P0,).2H,0 rather than Zr(HPO,),.H,O of the o! phase [14,15]. The two types of structures are compared in Fig. 7. The phosphate groups in the y phase are all located on the interlayer surface, each phosphate groups being bonded with three different zirconium atoms. In the y phase, PO, groups are within the structure as framework anions, and only H2P04 groups are on the interlayer surface.

(a) (b) Fig. 7. Comparison of the two layer structures of zirconium phosphates; (a) a,and (h) y. The interlayer water molecules are not shown.

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S. Yamanaka

Table 1 . Phosphoric esters and the basal spacings of the exchanged products. Basal spacing, A

Phosphoric esters C6HSOPO3H2 HOCH2CH(OH)CH,OPO,H, (HOCH,),CHOPO,H, n-CnH2,+,0P0,H2 (n = 1-18) CH,-(OCH2CH;),-OP0,H, (n = 1-3)

16.38 15.36 15.09 1 1.8-37.2 14.8-19.9

Ref. 17 18 18 19 20

Anion exchang Rahman and Barrett [ 161 first investigated the exchange of phosphate ions of a-zirconium phosphate with phosphate ions in solutions by using 32P isotope. They found that the phosphate groups internal as well as outer surfaces of a-zirconium phosphate were exchanged with the labeled phosphate groups in the contacting solution. We have found that similar ion exchanges occur more easily in y-zirconium phosphate [ 171; the H2P04 groups are exchanged with various phosphate ester groups, where the phosphate groups are labeled in the form of ester groups (ROP0,H) in stead of by j2p isotope; Zr(PO,)(H,PO,)

+

ROPO,H-

-

Zr(PO,)(ROPO,H)

+

H2P04-.

The reactions are carried out at 70°C in aqueous solutions or mixed solutions of acetone + water containing phosphoric acid esters. The esters should be hydrogen form. If salt forms are used, yzirconium phosphate is changed into a stable Na ion-exchanged form, Zr(PO,)(NaHPO,). which is inert against the exchange reaction [ 181. Table1 shows a list of phosphoric esters exchanged so far, together with the basal spacings of the resulting organic derivatives. The reactions are not confined to these phosphoric esters. Much larger number of phosphoric esters can be used for the exchange, as long as the esters are stable in aqueous solutions at about 70OC. Exchange with phosphonate ions are also possible [21]. In our previous studies, we assumed that y zirconium phosphate had a structure similar to that of the a phase. All the formulae reported should be revised. It is interesting to note that the resulting exchanged products are organic derivatives of inorganic

Fig. 8. A schematic illustration of the arrangement of oxyethylene chains grafted onto the interlay surface of zirconium phosphate.

Anion Exchange in Layer Structured Crystals

153

layers. The organic functional groups are grafted onto the interlayer surface, and arranged in a regular manner. The derivatives obtained by the ion exchange with phosphoric esters having oxyethylene chains exhibit properties characteristic for crown ethers (Fig. 8) [20]; the interlayer oxyethylene chains can take up alkali metal salts such as LiClO, [22], iodides, and thiocyanides [20]. Similar organic derivatives can also be prepared by a direct reaction of epoxide compounds with the interlayer HZPO, groups [23,24]. The H2P04 groups are exchangeable with pyrophosphoric acid groups [25]. However, the pyrophosphate ions introduced between the layers are hydrolyzed almost simultaneusly, giving rise to a structural transformation of the y to the a phase. ACKNOWLEDGMENT This study was partly defrayed by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture. REFERENCES 1. S. Yamanaka and F. Kanamaru, Kagaku-sosetu, 40 (1983) 65. 2. S. Yamanaka, Ceram. Bull. 70 (1991) 1056. 3. S. Yamanaka and M. Hattori, in T. Inui, S. Namba, and T. Tatsumi (Eds.) Chemistry of Microporous Crystals, KodanshdElsevier, Tokyo/Amsterdam, 1991, p.89. 4.S . Yamanaka, Y. Inoue, M. Hattori, F. Okumura, and M. Yoshikawa, Bull. Chem. SOC.Jpn., 65 (1992) 2494. 5. S . Yamanaka and K. Takahama, in C. A. C. Sequeira, M. J . Hudson (Eds.) Multifunctional Mesoporous Inorganic Solids, Kluwer Academic Publishers, The Netherlands, 1993, p.237. 6 . A. D. Roy, C. Forano, K. E. Malki, and J.-P. Besse, in M. L. Occelli and H. Robson (Eds.) Expanded Clays and Other Microporous Solids, Van Nostrand Reinhold, New York, 1992, p. 108. 7. T. J. Pinnavaia, in M. L. Occelli and H. Robson (Eds.) Expanded Clays and Other Microporous Solids, Van Nostrand Reinhold, New York, 1992, p. 1. 8. S. Yamanaka, T. Sako, K. Seki, and M. Hattori, Solid State Ionics, 53-56 (1992) 527. 9. S . Yamanaka, T. Sako, and M. Hattori, Chem. Lett., ( 1989) 1869. 10. L. S. Puckhaber, H. Cheung, D. L. Cocke, and A. Clearfield, Solid State Ionics, 32/33 (1 989) 206. 11. S . Yamanaka and M. Hattori, in T. Kanazawa (Ed.) Inorganic Phosphate Materials, KodanshdElsevier, Tokyo/Amsterdam, 1989, p. 131. 12. A. Clearfield (Ed.), Inorganic Ion Exchange Materials, CRC Press, Boca Raton, Fl, 1982. 13. S. Yamanaka and M. Tanaka, J. Inorg. Nucl. Chem., 41 (1979) 45. 14. N. J. Clayden, J. Chem. SOC.Dalton Trans., (1987) 1877. 15. A. N. Christensen, E. K. Andersen, I. G. K. Andersen, G. A. Alberti, M. Nielsen, and M. S . Lehmann, Acta Chem. Scand., 44 (1990) 865. 16. M. K. Rahman and J. Barrett, J. Chromatogr., 69 (1972) 261. 17. S. Yamanaka and M. Hattori, Inorg. Chem., 20 (1981) 1929. 18. S . Yamanaka, K. Yamasaka, and M. Hattori, J . Inorg. Nucl. Chem., 43 (1981) 1659. 19. S . Yamanaka, M. Matsunaga, and M. Hattori, J. Inorg. Nucl. Chem., 43 (1981) 1343. 20. S. Yamanaka, K. Yamasaka, and M. Hattori, J. Inclusion Phenomena, 2 (1984) 297. 21. S. Yamanaka and M. Hattori, Inorg. Chem., 20 (1981) 1929. 22. S. Yamanaka, M. Sarubo, K. Tadanobu, and M. Hattori, Solid State lonics, 57 (1992) 271. 23. S. Yamanaka, Inorg. Chem., 15 (1976) 281 1 . 24. S. Yamanaka, T. Ohno, and H. Nakano, Chem. Lett. to be submitted. 25. S. Yamanaka, K. Asano, and M. Hattori, Phosphorous Res. Bull., 1 (1991) 51.


Reactant Shape-selectivity for Cracking of Linear Paraffi on HZSM-5 Modified by CVD of Silicon Alkoxide :A Strong Dependence upon the Reaction Temperature

Miki Niwal, Norihisa Senoh', Tgkashi Hibinoz, Yasuo Nakatsuka3, and Yuichi Murakami3 'Department of Materials Science, Faculty of Engineering, Tottori University, Koyama -cho, Tottori 680 Japan 2Synthetic Crystal Research Laboratory, School of Engineering, Nagoya University 3Department ofApplied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-h, Nagoya 464-01 Japan

ABSTRACT Chemical vapor deposition (CVD) method of silicon alkoxide was applied to HZSM-5 zeolite in order to enhance the shape-selectivity in cracking of octane isomers. Adsorption experiments using octane and 3-methylheptane showed that silica deposited on the external surface controlled the pore-opening size finely to retard the diffusion of 3-methylheptane only. Shapeselectivity for cracking of linear paraffin was observed at 773 K, but not at lower temperatures; a strong temperature dependence was observed. Mechanism of cracking on HZSM-5 was so complex, and strongly held residues such as large olefinic compounds seemed to retard the reaction at lower temperatures. This was, however, substantial, because cracking of paraffin is usually performed at higher temperatures.

INTRODUCTION Although the zeolite ZSM-5 is used in the dewaxing process [l]to cut molecules with lowboiling points, the selective cracking of linear paraffins cannot always be achieved; the selectivity depends upon reaction conditions such as temperature, partial pressure of reactant, and level of conversion. At temperatures above 773 K, where the FCC (fluidized catalytic cracking) process is usually operated, the selectivity is not high, as previously reported [2]. The improvement of the selectivity of ZSM-5 zeolite is thereby required, when this zeolite must be mixed with the FCC catalyst in order to increase the octane number of gasoline [3,4]. We have already proposed the CVD of silicon methoxide on the external surface of zeolite to finely control the pore-opening size [5,6]. With this method, silica is deposited only on the external surface without any change of interior of zeolite, and shape-selective catalytic reactions and sorption can be achieved on the modified zeolites. The purpose of this investigation is thereby to discover whether the reactant shape-selectivity of cracking of paraffins can be achieved on the CVD modified HZSM-5.

I55

156

M. Niwa, N . Senoh, T. Hibino, Y. Nakatsuka and Y. Murakami

EXPERIMENTAL METHODS HZSM-5 was supplied by Mobil Catalyst of Japan; the silica to alumina ratio was 76.4, and the external surface area measured by the benzene-filled pore method was 10.8 m2g-l. The homogeneously distributed round sphere of the crystal did not contain the impurity of aluminum, as found from SEM and NMR studies. CVD of Si(OCHJ, was performed in a vacuum system. Zeolite was evacuated at 673 K, to which the vapor of alkoxide was admitted at 593 K, and the resultant increase in weight was followed by expansion of quartz micro-balance. The obtained material was calcined in oxygen to remove the organic residue, and the thus obtained weight increase was used to show the extent of modification. Rate of adsorption of octane isomers was measured gravimetrically at 273 K. Test of catalytic activity for the cracking was made by the usual pulse technique and by the continuous flow method. Either octane or 3-methylheptane was fed into the reactor made of Pyrex glass, individually. Distribution of products was measured only in the continuous flow method using an SE-30 separation column. Rate constant of cracking was measured based upon the firstorder rate equation.

RESULTS Adsorption Experiments To discover the extent of control of pore-opening size, adsorption experiments were performed at 273 K. Fig. 1-(a) and (b) shows the rate of adsorption of octane and 3-methylheptane, respectively. Adsorbed amount divided by equilibrium

1

8.9

a ~

0.6

0.7 8.6

\

a

8.5 8.4

8.3 8.2 8.1 8

1

adsorbed amount (Q/Qe)was plotted against the root t (elapsed time of adsorption). Rate of adsorption of octane was almost constant throughout inherent HZSM-5 and

8.9

a

8.6 0.7

\

0.5

~

8.6

-a

8.4

8.3 8.2 8.1

8

J-

t

(m i n

1/21

Fig. 1-(a), (b). Adsorption measurements of octane (a, upper) and 3-methylheptane (b, lower) at 273 K on the HZSM-5 (a) and SiHZSM-5 with 7.8 (t),11.2 (O), and 14.4 (A) wt% of silica.

HZSM-5 Modified by CVD

157

some kinds of SiHZSM-5, as shown in Fig 1-(a). On the other hand, the rate of adsorption of 3-methylheptane was suppressed significantly by increasing the amount of silica deposited. This finding of adsorption shows that the pore-opening size of HZSM-5 was finely controlled so that the adsorption of 3-methylheptane was suppressed while that of octane remained unchanged. Negative values of adsorption as extrapolated into zero second seem to be caused by experimental errors because of the rapid rate of adsorption.

Test of Catalvtic Cracking Activitv bv the Pulse Method. In the previous investigation of cracking of paraffins on H-mordenite IS], test of cracking was performed at 573 K, and high shape-selectivity was observed. At first, we selected this temperature 573 K also for this investigation on HZSM-5. Fig. 2-(a) shows the change in rate constant for cracking of octane isomers by increasing the amount of deposited silica, measured by the pulse method. However, loss of activity in the cracking of both paraffins by the deposition of silica was remarkable. No selectivity was observed on the SiHZSM-5 at all. Because the drastic loss of activity was unexpected, the experimental conditions were varied, and shape-selectivity was obtained on the SiHZSM-5 at 773 K, as shown in Fig. 2-(b); the rate constant of 3-methylheptane decreased by increasing the amount of silica, but that of octane decreased only slightly.

l ” ” ” ” ” ” ” ” ” I

OFT---

Weight increase by CVD (wt%)

+

Weight Increase by CVD (wt%)

Fig. 2-(a), (b). Change in rate constant by increasing the deposition amount of silica for cracking of octane (o),3-methylheptane(A), and 2,2,4-trimethylpentane (0)on SiHZSM-5 at 573 K (a, left) and 773 K (b, right).

I57

158

M. Niwa, N. Senoh, T. Hibino, Y. Nakatsuka and Y . Murakami

It is well known that the cracking of olefins proceeds rapidly, because the carbenium ion is easily formed through the protonation. Fig. 3 shows that hexene isomers rapidly reacted at 573 K, and outstanding shape-selectivity was observed, unlike the paraffins shown above; the rate constant of 1-hexene remained almost constant on the SiHZSM-5, while that of 2,3-dimethyl-1-butene decreased by increasing the amount of silica, and that of 2-methyl-1-pentene also gradually decreased. The enhancement of shape-selectivity of HZSM-5 by CVD of Si(OCH,), thereby depends on the reactant molecule as well as on the reaction temperature.

at 573 K

Fig. 3. Change of rate constant with increasing the deposition amount of silica for cracking of 1-hexene (o), 2-methyl-l-pentene(A), and 2,3dimethyl-1-butene (n) on SiHZSM5 at 573 K.

-

i

0 0

10 Weight Increase by CVD (wt%)

Catalvtic Cracking of Octane Isomers by the Continuous-Flow Method Because no information about the mechanism on the CVD zeolite was available, we studied the kinetics of cracking of octane isomers by the continuous-flow method. Simultaneously, the selectivity observed by the pulse method was confirmed. In the continuous flow method, the activity of catalyst declined slightly at the early stage of the reaction, but it was readily stabilized; the catalyst activity was easily reproducible by burning the coke residue in oxygen, so the kinetic parameter was measured continuously by varying the total flow rate. A first-order equation can be given for the conversion of x : -ln(l-X/lOO) = k (V/F)

(1)

where k, V, and F denote the rate constant, volume of catalyst, and total flow rate, respectively. Qpical first-order plots were observed as for the cracking of octane at 773 K, as shown in Fig. 4-(a). Plot of -In (1-~/100) against the V/Fshowed straight lines with an intercept of zero on HZSM-5 and SiHZSM-5 with partial pressures of 2.8 to 10 Torr; the first-order rate constant k was obtained from the slope. The plot for the cracking of 3-methylheptane at 773 K, however, did not show a linear relationship between them, but deviated remarkably with longer contact time, as shown in Fig. 4-@). First-order kinetics with respect to 3-methylheptane was


thereby observed only at short contact time, and the first-order reaction seemed to be suppressed by increasing the contact time. On the other hand, plots for octane cracking at 673 K showed stimulation of reaction rate at the longer contact time, and that for 3-methlheptane showed almost first-order kinetics. The kinetics of cracking on the HZSM-5 was therefore complex, and the behavior depended on not only the temperature but also on the reactant. However, the kinetics of the reaction was not influenced by the deposition of silica. The complex behavior is therefore based upon the inherent property of the zeolite and the paraffin compounds.

VF ( ~ o - ~ r n i n ) Fig. 4-(a), (b). Kinetic plot of cracking of octane (a, left) and 3-methylheptane (b, right) at 773 K on the HZSM-5 (0,o) and SiHZSM-5 with 5.69 (o),11.3 (a), 16.0 (O), and 21.0 (A) wt% SO,. Partial pressures of octane and 3-methylheptane were those of partial pressure chilled at 273 K (2.8, and 4.3 Torr), respectively. Only in the experiment shown by (o), 10 Torr of octane was used.

The rate constant k was thus obtained from these relationships, and it was obtained under the condition of short contact time when the plots did not deviate from the linear relationship. Fig. 5 shows the dependence of rate constants thus obtained against the amount of silica deposited. We can confirm that shape-selectivity was realized at 773 K. However, at 673 K, no selectivity was obtained, but rather, loss of catalyst activity on the SiHZSM-5 was noteworthy. The dependence of shape-selectivity upon reaction temperature therefore agreed with those found by the pulse method.

159

160

M. Niwa, N . Senoh, T. Hibino, Y . Nakatsuka a n d Y. Murakarni

Fig. 5 Change in rate constant of cracking of octane (0 ,a) and 3methylheptane (A, A) at 773 K (closed) and 673 K (open) by varying the amount of silica, measured by the continuous-flow method.

6 0 0 ~ .

1

I

8

1

I

--8 rjJ 400

3

8 20 3

2

0

10

20

Deposition amount/wt%

Fig. 6 shows the distribution product which was obtained from cracking of octane at 773 K. Products included C,,C4and C1tC2,and the distribution was not changed by varying the amount of silica deposited.

Fig. 6 Distribution of products CltC2 (o), C, (o),C4@), isopentane (+), C, (A) and C, (A) obtained from cracking of octane at 773 K.

$a

d

0 Weight Increase by CVD (wt%)

Discussion Adsorption experiments using octane isomers show the fine control of pore-opening size by CVD of Si(OCH,),; the rate of adsorption of branched paraffin was suppressed significantly while that of linear paraffin remained unchanged. Because the rate of adsorption of either octane or 3-methylheptane was fast, it was difficult to determine the diffusion constant directly from the


161

adsorption measurements. In this investigation, therefore, we indicate only the change in diffusion rates which depends upon the kind of molecule. This is essential for obtaining reactant shape-selectivity . It has been reported that the reaction mechanism of the cracking of simple paraffin compounds is very complex [7]. There are at least two types of mechanism, i.e., bi-molecular and mono-molecular mechanism of cracking, and the selection of the proper mechanism depends upon the reaction conditions. Under the conditions of high temperature and low partial pressure, the mono-molecular mechanism via the formation of penta-coordinated carbonium cation is postulated. On the other hand, bi-molecular mechanism of cracking through the formation of olefin intermediates becomes abundant with decreasing the temperature or with increasing partial pressure of paraffin compound. Complex behavior is noted in the intermediate region. Abnormal temperature dependence of the cracking of n-decane on HZSM-5 has been reported and explained based on the occurrence of two kinds of mechanism [8]. They explained the complex behavior of cracking based upon the change in the concentration of Bronsted acid occupied by the adsorbed olefin by varying the reaction temperature. In addition, in this investigation on SiHZSM-5, the influence of the enclosure of pore-opening size on the mechanism must be considered.

high Temp

/ low Temp

(penta-coordinated carbonium cation) ~

Cracking products

(carbenium cation)

Scheme for the cracking on HZSM-5 at high and low temperatures. The cracking of octane isomers at 773 K clearly shows the reactant shape-selectivity for the linear paraffin. This selectivity is different from the transition-state selectivity, which was claimed for the cracking on HZSM-5 due to spatial allowance [9,10]. Because the selectivity is obtained at high temperatures such as 773 K, this is substantial as a catalyst component which will be mixed with the FCC catalyst, because the process is operated at temperatures higher than 773 K where the selectivity is apt to deteriorate. Unexpected loss of catalyst activity on the SiHZSM-5 at low temperatures such as 673 or 573 K gives rise to a problem which may be solved from the viewpoint of reaction mechanism. As mentioned above, the bi-molecular reaction mechanism is abundant at this temperature region, and the olefinic intermediate stimulates or retards the reaction, depending on the conditions. However, small olefins such as 1-hexene reacted rapidly, and typical shape-selectivity was realized. To understand this phenomenon, we must consider the influence of the unique

162

M. Niwa, N. Senoh, T. Hibino, Y. Nakatsuka and Y . Murakami

structure of CVD zeolites. Large (or polymerized) olefinic compounds which could not be desorbed from the pore may be assumed. The formation of the strongly held residue markedly retards the cracking on the acid sites. Under these conditions, we do not observe shapeselectivity, only the loss of activity.

References 1. N. Y. Chen, R. L. Goring, H. R. Ireland, T. R. Stein, Oil GasJ., 75(1977)165. 2. V. J. Frillette, W. 0. Haag, R. M. Lago, J. Catal., 67(1981)218. 3. R. J. Madon,J. Catal., 129(1991)275. 4. F. N. Guerzoni, J. Abbot, J. Catal., 139(1993)289. 5 . M. Niwa, S. Morimoto, M. Kato, T. Hattori, and Y.Murakami, Proc. 8th Inter. Congr. Catal., 1984 701. 6. M. Niwa, S. Kato, T. Hattori, and Y. Murakami, J. Chem. SOC., Furaduy I, 80(1984)3135. 7. W. 0. Haag, R. M. Dessau, R. M. Lago, in T. Inui, S . Namba, and T. Tatsumi (Eds.), Chemistry of Microporous Crystals (Proc. of the Int. Symp. on Chem. of Microporous Crystals, Tokyo, June 26-29, 1990), KodanshaElsevier, Tokyo/Amsterdam, 1991, p.255. 8. L. Riekert, J.-Q. Zhou, J. Catal., 137(1992)437. 9. W. 0. Haag, R. M. Lago, P.B. Weisz, Faraday Disc., 72(1982)317. 10. S. Namba, K. Sato, K. Fujita, J. H. Kim and T. Yashima, in Y.Murakami, A. Iijima, and J. W. Ward (Eds.), New Developments in Zeolite Science and Technology (Proc. of the 7th Int. Zeolite Conf, Tokyo, June August 17-22, 1986), KodanshaElsevier, Tokyo/Amsterdam, 1986, p.661.

New Approaches in Shape Selective Alkylation Reactions Using Pore Size Regulated MFI Zeolites

A.B. Halgeri and Y.S. Bhat Research Centre, Indian Petrochemicals Corporation Ltd., Baroda 391 346, India ABSTRACT Pore size regulated MFI metallosilicates exhibited a v e r y high para selectivity during monoalkylbenzene alkylation. A t nearly s a m e para-dialkylbenzene selectivity (98%) t h e mono alkylbenzene conversion decreased in t h e o r d e r AlMFI > Ga-MFI > FeMFI, which i s t h e s a m e order of their acidity. A test reaction w a s used to follow t h e extent of pore size reduction during silylation. The changes in reaction condition do not influence high para selectivity feat u r e of MFI metallosilicates. The para product selectivity in case of C1-C3 alkylations of mono alkylbenzenes decreased in t h e order toluene isopropylation > ethylbenzene ethylation > toluene ethylation > toluene methylation. INTRODUCTION

are t h e starting materials for various chemical The raw material for polyester fibre, terephthalic acid is obtained

Para-dialkylbenzenes processes. from

t h e oxidation of para-xylene [ 1I.

Para-ethyltoluene

on dehydrogenation

forms para-methylstyrene, t h e polymer of this monomer has got certain advantageous properties over t h e conventional polystyrene [ 11.

Para-diethylbenzene

(P-DEB) is used as a desorbent in t h e separation of para-xylene from isomeric C8 aromatics [21.

Perfumes are made from para-cymene [31.

The formation of

para isomers is accompanied by other isomers of dialkylbenzenes.

This re-

duces t h e purity of para dialkylbenzenes. On Al-MFI zeoIite of smaller crystal size near thermodynamio equilibrium

ia formed during alkylation of mono alkylIt is widely known that Al-MFI zeolites modified with oxides of

composition of para-dialkylbenzene benzene [41.

magnesium [5,71, phosphorus [61 or boron [5-71 exhibit a high para selectivity

for alkylation of alkylbenzenes.

Higher para-dialkylbenzene

selectivities a r e

reported by coking t h e zeolite, adsorbing bulkier nitrogen compounds, using large crystal or chemical vapour deposition (CVD) i8.91.

163

The

CVD technique

164

A . B. Halgeri and Y. S . Bhat

has opened up a new domain, precise pore size control which can be used to design the zeolite for a specific application. The isomorphous substitution of Fe and G a in place of A1 in MFI zeolite has also resulted in dialkylbenzenes composition different from the thermodynamic equilibrium [ 101. As the acid property of MFI metallosilicates is different from one another it is interesting to look into the para selectivity aspect of silylated zeolites during alkylbenzene alkylation. In the present work the para selectivity enhancement feature of pore opening size regulated, silylated metallosilicates during mono alkylbenzene alkylation with C1-C3 alcohols is reported. The pore opening size regulation by chemical vapour deposition of silica involves blocking of non-selective external surface and pore mouth sites without altering internal zeolite structure. The para selectivity increase is illustrated for various alkylation reactions viz. toluene methylation, ethylation, isopropylation and ethylbenzene ethylation. The aspect of acidity of the metallosilicates and mono alkylbenzene conversion has been studied. EXPERMENTAL

The isomorphous substituted MFI metallosilicates were synthesized according to the published information[ 101. The zeolites were characterized by XRD for phase purity, SEM for crystal size, I R for pentasil structure, ESCA

for elemental detection and TPD of ammonia for acidity. The zeolites were converted to the proton form before they were chemically modified by silica deposition. The pore opening size regulation was achieved by depositing tetraethyl orthosilicate at 503 K in-situ

followed by calcination at 813 K for 8

hours[S,lll. This step w a s monitored by a test reaction. The catalytic reaction runs were carried out in a fixed bed, continuous, down flow integral reactor

at atmospheric pressure. The mixture of reactants was introduced by a Sage syringe pump and evaporated in a preheater. From the preheater vapour w a s carried by hydrogen gas to the catalyst bed maintained at the desired reaction temperature. The products of the reaction were analysed in a Varian V i s t a 6000 gas chromatograph using a 50 meter length LB-550 capillary column. RESULTS AND DISCUSSION

Alkvlation activity of MFI metallosilicates The crystal size, morphology and Si02/M203 ratio of metallosilicates used in this

study are summarized in Table I. The activity

and

selectivities

of

Pore Size Regulated MFI Zeolites

165

metallosilicates for ethylbenzene ethylation are presented in Table 2. The catalyst activity expressed in t e r m s of ethylbenzene conversion decreased in the order for the three zeolites Al-MFI > Ga-MFI > Fe-MFI. This is in t h e 8 a m e order of acidity of the Table

metallosilicates

Crystal size, morphology metallosilicates

1.

Metallosilicte

Crystal size

A1-MFI Ga-MFI Fe-MFI

0.5 1.0 1.0

-

as measured by TPD of ammonia. and

Si02 / M203

ratio of MFI

morphology

__-

Spheroidal Spheroidal Spheroidal

1.0 2.0 1.5

The

Si02/M203

-

90 85 93

---

----_-__________________

Table 2.

Performance comparison of metallosilicates for ethylbenzene ethylation Al-MFI

Ethylbenzene conversion,(wt%) Selectivity to products,( w t % ) Benzene Diethyl benzene

meta Para ortho D i e t hyl benzene isomer s, (%> meta para ortho

Metallosilicate Ga-MFI

Fe-MFI

34.28

26.41

22.77

19.98

15.05

00.88

47.60 24.25 2.00

50.07 26.45 0.84

59.11 38.12 0.62

64.36 32.79 2.85

64.72 34.19 1.09

60.41 38.96 0.63

conditions : Temperature = 623 K, WHSV = 5.2 h-',

H2/HC = 3

TPD profiles for all three metallosilicates are reported elsewhere 1121.

The

profiles consisted of a high and a low temperature peak corresponding to strong and weak acid sites. The substitution of G a and Fe for A1 in MFI struct u r e resulted in the decreased strength of both type of acid sites, weak as well as strong. Hence t h e total acidity of the metallosilicates decreased in t h e order Al-MFI > Ga-MFI > Fe-MFT. Another important observation is that benzene formation due t~ dealkylation of ethylbenzene w a s lowest with metallosilicate of lower acidity, Fe-MFI. Pore o w n i n g & regulation and selectivity enhancement The technique chosen for t h e pore size regulation of MFI metallosilicates w a s vapour deposition of bulky molecule, tetraethyl orthosilicate at 503 K

166

A. 8. Halgeri and

followed

by

Y. S.

Bhat

calcination

at 813 K t o decompose t h e alkoxy compound. As t h e

molecular size of tetraethyl orthosilicate is larger than t h e zeolite pore opening, on its decomposition t h e deposition of silica t a k e s place on t h e external

surface and pore mouth entrance. The initial

deposition

reaction involves

hydroxy groups located on t h e zeolite external surface, and of subsequent reaction between gaseous alkoxide and surface residue or between deposite molecules. The internal s t r u c t u r e remains unaffected only t h e pore opening size is reduced [91. The effects of pore opening size reduction on t h e diffusivity of aromatic molecules inside t h e zeolite w a s monitored by a test reaction. A mixture of t w o reactant probe molecules of different kinetic diameter w a s employed. The reaction mixture consisted of 80% meta-xylene and 20% ethylbenzene. Essentially two reactions occur on metallosilicates with t h e probe molecules : (i) m e t a xylene conversion to para and ortho-xylenes, and (ii) ethylbenzene dealkylaion to benzene and ethylene. The amount of silica deposited or t h e extent of pore opening size reduced is proportional to t h e period for which silylation was carried out. There was not any conversion of meta-xylene a f t e r 210 minutes of silylation period where as ethylbenzene conversion w a s still at appreciable level (Fig. 1).

This can be ascribed t o smaller kinetic diameter of ethylbenzene

compared to meta-xylene and the latter is very close to pore opening size. A s the silylation period progressed t h e pore opening size w a s getting reduced and became less than that of meta-xylene. With the result, meta-xylene could not enter inside the zeolite channel, b u t t h e size of ethylbenzene being still smaller than t h e pore opening, it diffused inside. This test, with probe mole-

cules clearly illustrated t h e effect of pore opening size regulation on t h e diffusivity of reactant molecules inside t h e channel.

MFI metallosilicates for a typical mono alkylbenzene reaction is given in Table 3. A s compared to metallosilicate the pore regulated metallosilicates showed a lower ethylbenzene conversion and The performance of pore regulated

higher para-diethylbenzene selectivity. A t around 98% para selectivity, t h e alkylbenzene conversion on pore regulated metallosilicates increased in t h e order Fe-MFI < Ga-MFI < Al-MFI.

This is in t h e same o r d e r of their acidity.

There is a good correlation between acidity and mono alkylbenzene conversion. Like metallosilicates, on pore regulated metallosilicates also dealkylation w a s least

on t h e molecular sieve of lower acidity. Table 4 compares t h e para

selectivity at nearly t h e s a m e mono alkylbenzene conversion level. I n case of mono alkylbenzene alkylation, t h e alkyl group already present


167

in t h e benzene ring activates t h e ortho and para positions for alkylation. Due to space constraint inside MFI metallosilicate, alkylation t a k e s place only at para position, while ortho alkylated product forms on t h e external surface sites.

M e t a isomer is

formed Prom isornerization of

ortho and

para isomers

on these sites. The pore size regulation by chemical vapour deposition of silica Table 3.

Ethylbenzene ethylation activity of metallosilicates

pore

size regulated ~

AI-MFI Ethyl benzene conversion,(wt%) Selectivity to products,( w t % ) Benzene Diethyl benzene meta para ortho Diethylbenzene isomers

meta para ortho

~~~~~

Fe-MFI

14.83

12.09

10.40

21.93

16.21

2.21

1.42 70.93 0.00

1.52 77.48 0.00

1 .80 94.24

1.95 98.05

1.92 98.08

1.88 98.12

0.00

0.00

0.00 ~

~~

E8 Conversion

0.00

~~~

conditions : Temperature = 623 K, WHSV = 5.2 h-’,

w

~~

Silylated metallosilicate Ga-MFI

MFI

HZ/HC = 3

I

0

2 z

w U w a

u

I

’

Conversi01 I

SILYLATION TIME (MIN.) Fig. 1

Test reaction t o differentiate t h e change in pore opening size after silylation. Reaction conditions: T=678 K, WHSV=8/h, H2/HC=3

168

A. B. Halgeri and

Table 4.

Y. S . Bhat

Performance comparison of alkylation activity at nearly = m e ethylbenzene conversion kwel

_ _ _ I _ _ -

A1-MFI (a) Ethylbenzene conversion,(wt%) Selectivity to products,( w t % ) Benzene Diethyl benzene

meta para ortho Diethylbenzene isomers meta Para ortho

Silylated metallosilicate Ga-MFI (b)

Fe-MFI (C)

11.01

10.79

18.05

15.78

2.21

1.30 72.71

1.35 78.65

0.00

0.00

1.80 94.24 0.00

1.75 98.25

1.69 98.31 0.00

1.88 98.12 0.00

0.00

10.40

conditions : Temperature = 623 I(, €i2/IIC = 3 WRSV : a = 9.5 h-l, b = 7.9 h-l, c = 5.2 h-'

Fig. 2.

Performance of gallosilicate for toluene ethylation ae a function of silylation

Reaction conditions: T=623 K, WHSV = 5.2 h-l

period.

Toluene/Ethanol = 5


Table 5.

169

Catalytic activity of Al-MFI metallosilicate for various alkylation reactions

________-________-______I___________----

Reactions 1

Mono alkylbenzene conversion,(wt%) Selectivity to products,(wt%) Cymene

6.12

meta

0.00

Para ortho Diet h yl benzene

28.98 0.00

meta

3

2

19.51

19.27

4

8.03

3.28 70.72

para ortho Ethyltoluene

0.00

meta

5.45 83.55

para ortho

0.00

Xylene

meta

5.16 83.88

para ortho

0.00

Others Dialk yl benzene isomer

meta para ortho

7 1.02

26.00

10.04

27.42

0.00 100.00 0.00

4.43 95.57

6.12 93.88

0.00

0.00

7.11 88.02 4.87

conditions : Temperature = 623 K, WHSV = 5.2 h-l, H2/HC = 3 (*)Temperature = 573 K, silylation period = 180 min 1 = Toluene isopropylation, 2 = Ethylbenzene ethylation, 3 = Toluene ethylation, 4 = Toluene methylation covers t h e pore mouth and external surface eites, t h e extent of formation of ortho and m e t a decreases with increase in silylation period. I n other words, para-dialkylbenzene

is formed as t h e primary product of alkylation and its

f u r t h e r isomerization is suppressed by silica deposition. enhancement

in

para-diethylbenzene

selectivity

Fig. 2 shows the

with progress in silylation

period.

-Effect

of reaction conditions The variation in reaction conditions do not influence high para selectivi-

t y feature of t h e silylated metallosilicate. The details of the experimental results with modified Ga-MFI metallosilicate was reported elsewhere [ l l I . Similar observations were made in case of modified Al-MFI and FeMFI metallosilicates.

A.

170

B. Halgeri and Y.S. Bhat

Comparison of para selectivity in C1-C3 alkvlation Table 5 compares t h e performance of pore size regulated f o r various alkylation reactions o n silylated AI-MFI metallosilicate. The alkylation reactions studied were toluene methylation, ethylation, isopropylation a n d ethylbenzene ethylat,ion. A t 623K t h e alkylbenzene conversion was higher in ethylation t h a n methylation or isopropylation. Para-dialkylbenzene selectivities were 100, 95.95 93.5 and 88% respectively for para-cymene, para-diethylbenzene, para-ethyltol-

uene and para-xylene. This is in t h e o r d e r of number of carbon atoms p r e s e n t in t h e side chains of dialkylhenzenes which in t u r n is related to diffusivity difference between p a r a a n d o t h e r isomers. The dialkylbenzenes selectivity was in t h e o r d e r cymenes < xylenes < diethylbenzenes < ethyltoluenes. The lowest selectivity w a s for cynienes as it got isomerized t o n-propyltoluenes of smaller kinetic diameter. The silylation was not sufficient in case of xylenes to pre-

vent trimethylbenzenes formation from secondary reactions.

Diethylbenzenes

and ethyltoluenes are of higher kinetic diameter than xylenes. The selectivity

was lower for t h e former

a s t h e extent of dealkylation is

more d u e to t h e

ethyl g r o u p s p r e s e n t in t h e benzene ring. A similar behaviour w a s observed with

silylated Ga-MFI a n d Fe-MFI nietallosilicates.

ACKNOWLEDGEMENT The a u t h o r s are grateful t o

nr.

I. S. Bhardwaj, director (R & D) for his

interest in t h e work a n d permission to publish t h e paper. REFERENCES 1

2 3 4 5 6 7 8 9

10 11

12

N. Y. Chen, W. E. Garwood a n d F. G. Dwyer, Shape Selective Catalysts in Industrial Applications, Marcel Decker Inc, N e w York, 1989 R.V. Jasra and S. G.T. Bhat, Sep. Sci. a n d Tech., 23 (10 & 1 1 ) (1988) 945. D. Fraenkel and M. Levy, J. Catal., 118 (1989) 10. L. B. Young, S. A. Butter a n d W. W. Kaeding, J. Catal., 76 (1982) 418. W. W. Kaeding, C. Chu, L. B. Young, B. Weinstein a n d S. B. Butter, J. Catal., 67 (1981) 159. J. H. K i m , S . Namba a n d T. Yashima. Bull. Chem. Soc. Jpn., 61 (1988) 1051. J. H. Kim, S. Namba and T. Yashima, Stud. Surf. Sci. Catal., 46 (1989) 71. M. Niwa, S. Kato, T. Hattori a n d Y. Murakami, J. Chem. Soc. Faraday I, 80 (1984) 3135. M. N i w a , M. Kato, T. Hattori and Y. Murakami, J.Phys. Chem., 90 (1986) 6233. J. H. Kim, S. Namba a n d T. Yashima, Zeolites, 11 (1991) 59 A. 8. Halgeri, Y. S. Bhat, S. Unnikrishnan a n d T. S. R. Prasada Rao, P r e p r i n t s of ACS Symp. on Alk., Arom., Oligo. a n d Isom. of s h o r t chain hydrocarbons over Aetr. Cat., N e w York, 36(4) (1991) 792. P.A. Parikh, N. Subramanyam, Y.S. Bhat a n d A.B. Halgeri, Catalysis Letters, 14 (1992) 107.

Layered Silicate-Organic Intercalation Compounds as Photofunctional Materials

Makoto OGAWA,I Kazuyuki KURODA and Chuzo KATO Department of Applied Chemistry, Waseda University Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan. 1 Present Address: Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama 351-01, Japan

ABSTRACT The alkylammonium-exchanged swellable layered clay minerals have been prepared and used as immobilizing media for photoactive organic compounds. Anthracene, pyrene and p-aminoazobenzene were incorporated into the interlayer spaces of the long-chain alkylammonium exchanged-smectites. It was revealed from the photoprocesses of the intercalated species that the adsorptive properties of the host materials varied depending on the arrangements of the interlayer alkylammonium ions. Besides the hydrophobic modification by long chain alkylammonium ions, 2dimensional microporous structure was obtained by pillaring with tetramethylammonium ion. INTRODUCTION The study of photoprocesses on solid surfaces is a growing new field which yields a wide variety of useful applications such as sensitive optical media, reaction paths for controlled photochemical reactions, molecular devices for optics, etc.[ 11 In this context various host-guest systems where organic polymer, porous materials with 2- and 3-dimensional pore structures, various states of surfactant assembly in solutions etc. have been used have been investigated.[2] The nanomaterials with ordered structure have an advantage so that the properties of the immobilized species can be discussed on the basis of their defined nanoscopic structures. Their structure-property relationships will give the fruitful information on designing materials with novel chemical, physical and mechanical properties. Among possible ordered media, layered materials such as smectites provide unique two dimensional immobilizing media for photoactive species. Smectites are 2:l type layered clay minerals consisting of negatively charged silicate 171

172

M. Ogawa, K . Kuroda and C. Kato

layers and readily exchangeable interlayer cations. [3] They possess various attractive features such as the swelling behavior, ion exchange properties, adsorptive properties, large surface area, and so on. Accordingly, the photoprocesses of photoactive species adsorbed on smectites have been reported and the structure-property relationships in the unique host-guest systems have been discussed.[2,4-71 If smectites have metal cations in the cation exchange sites, their surfaces are hydrophilic and are often not a good adsorbent for poorly water-soluble species which cannot compete with water for adsorption. However, when the interlayer cations are replaced by organoammonium ions, the surfaces become organophilic and the organophilic-clays have been used as adsorbents for poorly water-soluble species.[8,9] Recently, the immobilization of photoactive species in the interlayers of the alkylammonium-smectites have been reported.[l0-181 In this paper, we summarize our recent results on unique photoprocesses of the organic compounds intercalated in the alkylammonium exchanged layered silicates. Our attention has been focused on the role of alkylammonium ions on the photoprocesses, in order to show the possible surface modification at a molecular size level by using layered structures of swelling clay minerals. METHOD Materials Na-montmorillonite (Kunipia F, Kunimine Industries Co., the cation exchange capacity (C.E.C.) was 119 meq./ 100 g clay.) obtained from Aterazawa mine (Yamagata, Japan) and synthetic Na-saponite (Sumecton-SA, Kunimine Industries Co., the C.E.C. was 71 meq./ 100 g clay.) were used as starting materials. Tetramethylammonium ((CH3)4N+;abbreviated as TMA-), dodecylammonium (C12H25NH3+;DA-), octadecyltrimethylammonium ( C I ~ H ~ ~ ( C H ~ODTMA-) ) ~ N + ; and dimethyldioctadecylammonium((C~~H~~)~(CH~)~N+; DMDODA-) chlorides were used as received. Anthracene (WAKO Pure Chemical Ind. Co.), pyrene (Tokyo Kasei Ind. Co.) and p-aminoazobenzene (abbreviated as p-AZ, Tokyo Kasei Ind. Co.) were used after recrystallization from appropriate solvents.

Pyrene

p -Aminoazobenzene

Scheme I. Guest species used in this study

Intercalation Compounds as Photofunctional Materials

173

Sample Preparation Preparation of Alkvlammonium-Smectites Organoammonium-exchanged-smectiteswere prepared by a conventional ion exchange in aqueous solutions of appropriate organoammonium salts. The amounts of added organoammonium salts were just adjusted at the cation exchange capacity of the host materials, because excess organic salts may be adsorbed by the clays in excess of the cation exchange capacity. After the ion exchange, the products were washed with deionized water repeatedly until the negative AgN03 test was obtained. Intercalation of Guest Species into Alkvlammonium-Smectites Intercalation of organic species into long chain alkylammonium-smectites was carried out according to the method described in our previous reports.[16,19,20] The mixture of a host material and a guest species was ground with a mortar and a pestle at the room temperature. The weight ratios of the mixture for host : guest were varied in order to prepare intercalation compounds with different amounts of adsorbed guest species. Characterization X-Ray powder diffraction was performed on a Rigaku RADII-A diffractometer using Mn filtered Fe Ka radiation. Diffuse reflectance UV-vis absorption spectra were recorded on a Shimadzu UV-21OA spectrophotometer. Emission spectra were recorded on a Shimadzu RF-5000 spectrofluorophotometer. RESULTS AND DISCUSSION Intercalation of Anthracene and Pyrene into OrPanoammonium-Clays In our preliminary study on the reactivity of alkylammoniummontmorillonites, it was shown that the alkyl chains longer than C12 are required for the intercalation of naphthalene and anthracene.fl91 The hydrophobic interactions between the guest species and alkylammonium ions are thought to be the driving force for the intercalation. Additionally, the spectroscopic properties of the intercalated arenes are affected by the kind of the alkylammonium ions. Since the aromatic hydrocarbons have been utilized to probe into various surfaces, we used anthracene and pyrene as a probe to investigate the adsorptive properties of alkylammonium-smecti tes. The DMDODA- and the ODTMA-montmorillonites are used as the host materials. Since the basal spacing of the ODTMA-montmorillonite was 2.2 nm, alkylammonium ions are arranged as pseudo-trimolecular layers with their alkylchains parallel to the silicate sheet.[21] For the DMDODA-montmorillonite, two types of arrangements are expected from the basal spacing of 3.0 nm; one is monomolecular coverage with their alkyl chains inclined to the silicate sheets at ca. 53 deg and the other is bimolecular coverage with their alkyl chains inclined to the silicate sheet at ca. 23 deg. The schematic structures of the products are shown in Fig.1.

174

M. Ogawa, K. Kuroda and C. Kato

Fig.1. The schematic structures of (a) the DA-, (b) the ODTMA- and (c,d) the DMDODA-montmorilloni tes. By the reaction between the ODTMA-montmorillonite and anthracene, a new d(001) diffraction peak with the basal spacing of ca. 3.7 nm appeared and the intensity of the d(001) diffraction peak d u e to the unreacted ODTMA-montmorillonite decreased. The change in the XRD pattern of the DMDODA-montmorillonite by the reaction with anthracene is different. The basal spacings increased gradually u p to 3.8 nm depending on the relative amount of the added anthracene. When pyrene was used as the guest species, similar difference in the change in the XRD patterns was observed. While the basal spacing of the ODTMA-montmorillonite-pyrene intercalation compound was 3.6 nm, the basal spacings of the DMDODAmontmorillonite-pyrene intercalation compounds varied gradually from 3.0 to 3.9 nm depending on the amounts of added pyrene. The absorption and emission spectra (the excitation wavelength is 330 nm) of the intercalated anthracene are in mirror symmetry similar to those in solution while the bands appeared in different wavelength regions from those observed for an ethanolic solution and those for anthracene crystal. The wavelengths of the absorption and fluorescence maxima due to 0-0 transition are listed in Table 1. Very small stokes shifts being similar to that observed for anthracene in ethanol were observed for the DMDODA-montmorillonite-anthracene intercalation compounds. The amounts of the intercalated anthracene did not affect the absorption and


175

fluorescence wavelengths, meaning that the intercalated anthracene molecules were located in a similar environment. Therefore, anthracene molecules were thought to be solubilized in the alkyl-chains of DMDODA. On the other hand, the stokes shift observed for the anthracene intercalated in the ODTMA-montmorillonite is much larger and the value is close to that observed for anthracene crystal, suggesting that the intercalated anthracene molecules are probably surrounded by neighboring anthracene molecules in the interlayer space. Table 1. The absorption and fluorescence maxima of 0-0 transition of anthracene intercalated in the ODTMA- and DMDODA-montmorillonites. Absorption Emission Stokes Max/nm Max /nm shift /cm-' Anthracene Crystal 395 421 1.5~103 Anthracene in Ethanol 375 377 2x102 DMDODA-Mont-Anthracene (100:19) 381 388 4x102 ODTMA-Mont-Anthracene (100:43) 393 417 1.4~103 The above idea on the different adsorption states of the two types of compounds is supported by the pyrene fluorescence. When pyrene is forced into close proximity or in high concentration solution, excited state dimers (excimers) are observed. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. In the fluorescence spectra of the pyrene intercalated compounds, monomer fluorescence with vibrational structure was observed around 400 nm together with the broad peak due to excimer emission (500 nm). Table 2 summarizes the results of the pyrene intercalated compounds. The ratio of monomer to excimer for the DMDODA-montmorillonite system is three times higher than that for the ODTMA-system, suggesting that the adsorbed pyrene molecules are isolated in the interlayer space of the DMDODAmontmorillonite compared with those doped in the ODTMA-montmorillonite. Table 2. The basal spacings, the amounts of adsorbed pyrene, the concentration of pyrene, the ratio of monomer to excimer. P Host Basal Amount of the Conc. of Ratio of spacings adsorbed pyrene monomer to / nm pyrene (g/ /(mol/l)*l excimer*2 100 g clay) DMDODA-Mont 3.8 22 1.o 0.58 ODTMA-Mont 3.6 22 1.1 0.19 *1 The values were determined on the basis of the clearance spaces and the adsorbed amounts of pyrene of the intercalation compounds. *2 The values were determined from the luminescence spectra.

176

M.Ogawa, K . Kuroda

and C. Kato

In order to elucidate the difference in the adsorption states, saponite with the C.E.C. of 71 meq./ 100 g clay was used as host material. Because of the lower layer charge density of the saponite compared with that of the montmorillonite, the DMDODA-saponite showed the smaller basal spacing of 2.2 nm. Judging from the value, the intercalated DMDODA ions arranged as a pseudo-trimolecular layer in the interlayer space of the saponite similar to that for the ODTMA-montmorillonite. Pyrene was intercalated into the interlayer space of the DMDODA-saponite and the change in the fluorescence spectra as a function of the loaded amount was similar to that observed for the ODTMA-montmorillonite system. This indicates that the arrangements of the intercalated alkylammonium ions is the important factor for the difference in the adsorption states of the guest species. In other words, we can create various reaction environment by selecting the hosts with various layer charge densities and guests with appropriate size. The results of the photophysical and chemical studies on the extended host-guest systems will be reported subsequently. It should be noted that the very high concentrations of the guest species were achieved in the present systems. For example, pyrene monomer fluorescence, which is not observed for an 1x10-2 mol/l of pyrene ethanolic solution, is observed at the concentrations of the 1.1 and 1.0 mol/l for the DMDODA- and the ODTMAmontmorillonites, respectively. To our knowledge, immobilization of guest species in detergent molecules a t such high concentrations with retaining an ordered structure is difficult. In the previous studies on the photoprocesses of dye molecules on clay minerals and other microporous materials, the amounts of adsorbed species are very low, if compared with those achieved in the present system. Since the high concentration of photoactive centers is a merit for the application of such types of composite materials as well as their structural regularity, the stability and so on, the assembly of detergent molecules formed in layered materials is a medium of importance from both practical and scientific viewpoints. Intercalation of v-AZ into an oraanophilic montmorillonite Photochromism of azobenzene and its derivatives due to cis-t r u n s isomerization has widely been investigated. Photocontrol of chemical and physical functions has vigorously been studied by using photochemical configurational change of azobenzene derivatives. Additionally, the attractive cis-trans isomerization of azobenzenes are largely affected by the surroundings. Therefore, intercalation and photochemical isomerization of p-AZ was investigated by using alkylammoniumexchanged swelling layered materials. As an example, DA-montmorillonite was used as the host material. When the mixture of the DA-montmorillonite (the basal spacing is 1.8 nm) and p-AZ was ground, an intercalation compound with the basal spacing of ca. 3.0 nm formed. The absorption spectrum of the intercalation compound showed an absorption band at 395


177

nm. Although the absorption maximum was slightly red shifted from that observed for pAZ dissolved in benzene (377 nm), it can be assigned to trans-p-AZ. Since p-AZ was not intercalated into the alkylammoniummontmorillonite with shorter alkylchain, the hydrophobic interactions are thought to be the driving force for the intercalation. Fig. 2 shows the change in the of the DAabsorption spectra m o n t m o r i l l o n i t e - p - A Z intercalation The compound upon UV irradiation. I spectrum (a) was recorded after the sample was 3 00 500 71 0 Wavelength / nm stored in the dark for 1 day and corresponds to Fig. 2. Change in the absorption the trans-isomer of p-AZ. By UV irradiation spectra of the DA-montmorillonitefor 5 min, the band intensity decreased p-AZ intercalation compound (spectrum (b) in Fig. 2). When the sample was prepared on an acrylate plate: spectrum a, stored in dark; stored in dark after the irradiation, the spectrum b, after UV light intensity of the band at 395 nm due to transirradiation for 5 min; spectrum c, isomer increased gradually. Fig. 2 (c) shows after placed in dark for 5 inin after the absorption spectrum after storing the 5 min UV irradiation. sample in the dark for 5 min. After one hour, the spectrum became identical to the spectrum (a). At 60 OC, the spectral recovery completed within a few minutes, whereas it took an hour at room temperature. The reversible spectral change was observed repeatedly. Therefore, it can be ascribed to the photoisomerization and the thermal back reaction of the intercalated p - AZ. Additionally, the thermal cis-trans backward reaction took a longer period than that observed in solution. This novel photoresponsive system suggested the possibility of controlling attractive properties of intercalation compounds by light. In order to obtain further information on the unique photochemical behavior, studies on the intercalation and the photochemical behavior of p-AZ in the alkylammonium type host materials with different arrangements of the alkyl-chains are now underway and will be reported subsequently. The tetramethylammonium-pillared-saponite Besides the organophilic assembly of detergent in the interlayer space, we can create microporous structure by pillaring the layered structure with TMA ions. The novel oriented transparent film of the TMA-saponite has been prepared by casting the aqueous suspension of the TMA-saponite. The interlayer TMA ion provides the

178

M . Ogawa, K . Kuroda and C. Kato

interconnected micropore in the interlayer space. (Fig.3) The film is also .regarded as a unique anisotropic medium for immobilizing photoactive species because the direction of the micropore is parallel to the substrate and the film is transparent in wavelength region from 250 to 2000 nm.[221

+I

Silicate sheet

I /

CONCLUSION Various types of organoammoniumFig.3. Schematic structure of smectites have been prepared and have been the TMA-saponite. applied as immobilizing media for photoactive organic molecules. By using the negatively charged silicate layers and the appropriate alkylammonium ions, novel molecular assembly based on the inorganic-organic nano-composites can be prepared. REFERENCES 1 M. Anpo and T. Matsuura (Eds.) Photochemistry on Solid Surfaces, (Studies in surface science and catalysis 471, Elsevier, Amsterdam, 1989. 2 V. Ramamurthy (Ed.) Photochemistry in Organized b Constrained Media, VCH Publishers Inc., New York, 1991. 3 8.K.G.Theng (Ed.) The Chemistry of Clay-Organic Reactions, Adam Hilger, London. 1974. 4 J.K.Thomas, Chem.Rev., 93, (1993) 301.; J.K.Thomas, Acc.Chem.Res. 21 (1988) 275. 5 H.Usami, K.Takagi, and YSawaki, J.Chem.Soc. Perkin Trans. 2, (1990) 1723. 6 H.Miyata,H. YSugahara, K.Kuroda and C.Kato, J.Chem.Soc., Faraday Trans. 1. 83 (1987) 1851. 7 M.Ogawa, M.Inagaki, N.Kodama, K.Kuroda and C.Kato, J.Phys.Chem. 970993) 3819. 8 R.M.Barrer, Zeolites and Clay Minerals US Sorbents and Molecular Sieves, Academic Press, London, 1978. 9 S.A.Boyd, J.F.Lee and M.M. Mortland, Nature, 333 (1988) 345.; J.F.Lee, M.M.Mortland and S.A.Boyd, J.Chem.Soc. Faraday Trans. I, 85 (1989) 2953. 10 Y.Okahata and A.Shimizu, Langmuir, 5 (1989) 954. 11 T.Nakamura and J.K.Thomas, Langmuir, 3 (1987) 234. 12 V.Kuykendal1 and J.K.Thomas, Langrnuir, 6 (1990) 1346.; ibid., 1350. 13 T.Seki and K.Ichimura, Macromolecules, 23 (1990) 31. 14 H.Tomioka and T.Itoh, J.Chem.Soc., Chem.Cornmun., (1991) 532. 15 K.Takagi, T.Kurematsu and Y.Sawaki, J.Chem.Soc. Perkin Trans. 2, (1991) 1517. 16 M.Ogawa, K.Fujii, K.Kuroda and C.Kato, Mater.Res.Soc.Symp.Proc.,233 (1991) 89. 17 M.Ogawa, T.Aono, K.Kuroda and C.Kato, Langmuir, 9 (1993) 1529. 18 M.Ogawa, T.Handa, K.Kuroda, C.Kato and T.Tani, J.Phys.Chem., 96 (1992) 8116. 19 M.Ogawa, H.Shirai, K.Kuroda and C.Kato, Clays Clay Miner.., 40 (1992) 485. 20 M.Ogawa, K.Kuroda, and C.Kato, Chem. Lett., (1989) 1659.; M.Ogawa, T.Handa, K.Kuroda, and C.Kato, Chem. Lett., (1990) 71.;M.Ogawa, T.Hashizume, K.Kuroda, and C.Kato, Inorg.Chem., 30 (1991) 584. 21 GLagaly, Clay Miner., 16 (1981) 1. 22 M.Ogawa, M.Takahashi, C.Kato and KKuroda, submitted.

Polymerization Inside the Molecular Sieves

S. Kowalakl, M. Pawtowskal, A.B. Wiqckowski2, J. Goslar2

lFaculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland, 2Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-1 79 Poznan, Poland

ABSTRACT Attempts have been made to use the molecular sieves as a matrix for intracrystalline polymerization. Although the polycondensation of resols is significantly affected by the presence of certain molecular sieves, the latter do not facilitate the polymerization of styrene. Styrene, however, reacts with zeolites H-ZSM-5, H-mordenite and also with silicalite-1 and AIPO4-11 modified with fluorine to form the colored species that cannot be removed from the molecular sieves. ESR measurement indicates a radical nature of these species. The protonic acid sites and specific geometry of the channels (0.5 - 0.7nm) seem to be indispensable for their generation. Strong chemical interaction between styrene and the molecular sieves decreases the rate of styrene polymerization. INTRODUCTION Polymerization and oligomerization often accompany the organic reactions catalyzed by zeolites and other crystalline molecular sieves. These undesired processes result in the blockage of active sites by the macromolecular deposit and subsequently in reducing catalytic activity [ 13. There are, on the other hand, attempts reported to use the molecular sieves as polymerization catalysts. Pichat [2] oligomerized acetylene over Ni - modified zeolites. Dutta [3] found catalytic activity of zeolites modified with Co and Ni for the above reaction. Catalytic activity of zeolites for oligomerization of olefins has been studied by several authors [4 - 91. Oligomerization of cyanobenzene inside the faujasite supercage was employed for the encapsulation of metallophthalocyanines into zeolite [lo]. Bein [11 - 131 polimerized thiophene, pyrrole and aniline inside the zeolite crystalline structure. The resulting polymers showed an electric conductivity and could be applied as molecular wires. A substantial part of the recent American Chemical Society Symposium on Supramolecular Architecture [14] was devoted to polymers inserted into zeolite hosts. It is conceivable that the polymer chain formed inside the molecular sieve channel can be strung throughout several crystallites as illustrated in Figure 1. Such an interaction between the polymer chain and molecular sievepller should be much more effective than that between polymer and a I79

180

S. Kowalak, M. Pawtowska, A. B. Wipkowski and J. Goslar

MOLECULAR

SIEVE

POLYMER

CHANNELS

CHAIN

Fig. 1. Model of stringing of the polymer chain throughout the molecular sieve channels. conventional filler. We have already found [ 15 ] that polycondensation of resols is significantly accelerated by catalytic action of the molecular sieves. The rate of polycondensation of phenolformaldehyde mixture changed in the following order: zeolite 4A > zeolite L > mordenite > 13X > silica gel >> AlPO4-5. The presence of the molecular sieves also affected physical properties of the resulting products. The flexural strength of the modified resols increased after adding the fillers in the following order: no filler (0.96J/cm2) < Si02 (1.19J/cm2) < 4A (1 .62J/cm2) < 13X (2.09 J/cm2) < L (2.28 J/cm2) < mordenite (2.67 J/cm2). The resistance to bending of the samples containing zeolites 13X, L, and mordenite is distinctively higher than for the samples containing other fillers. It is very likely that it results from the bonding of the resin to the intracrystalline pore system of these zeolites. The pore diameter of the above three zeolites is sufficient to accommodate both polycondensation substrates as well as the resulting polymer chain. Although zeolites 4A increase the polycondensation rate, the properties of the resulting resol are similar to those of sample containing silica gel, because only the outer surface of zeolite can be involved in an interaction with the resin. In the following study we have chosen styrene as a substrate for polymerization in the presence of various molecular sieves. The aim was to check, whether the interaction between the zeolite channels and polymer formed can be noticed similarly as for resols [15] and subsequently, whether the properties of the polymer are affected by the presence of the active molecular sieve filler. For comparison, some other chemicals were used as fillers. Surprisingly, we noticed in our preliminary experiments that some of zeolites under study turned pink or purple after contact with styrene. The colored product could not be removed from zeolite even after several days of solvent extraction, which suggested chemical bonding of the species to the molecular sieve structure. The color already appeared at room temperature and it became more intense after heating at elevated temperatures. A similar observation has been published recently by Pollack [ 161. We have tried to find the factors (geometry of zeolite, chemical nature of the surface) responsible for generation of the colored species. Some speculation on their structure is based on spectral measurements.

Polymerization Inside Molecular Sieves

181

EXPERIMENTAL The following molecular sieves have been used for the experiments: Na-X, H-X, Na-Y, H-Y (manufactured in Institute of Industrial Chemistry, Warsaw, Poland ), Al-Y-F ( aluminum form of zeolite Y was modified with fluorine [17]), Na-mordenite (Leuna, Germany), H-mordenite (Norton lot2), Na-ZSM-5 (Institute of Industrial Chemistry, Warsaw ), H-ZSM-5 (BASF), Silicalite-1 (Union Carbide), dPo4-5, APO4-11, AlPO4-11 modified with fluorine (prepared in our laboratory [18]).Some other reagents were used for comparison: y-Al2O3, fluorinated A1203 [ 191, P2O5, H2SO4, HC104, HCl (POCh, Poland). The molecular sieves samples (0.5g) were always activated at 450oC before the reaction with styrene, unless other temperature is indicated. Styrene (POCh, Poland) was purified before the reaction in order to remove the inhibitor. A column filled with inhibitor remover (Aldrich) was employed to exclude a potential influence of inhibitor on color changes. About 1 cm3 of styrene was inserted into the vials containing the activated molecular sieves or other reagents applied . One series of the samples was left with styrene at room temperature, and the other one was heated at 8OoC for 24 hours. Some of the molecular samples turned pink or purple right after adding styrene. The color developed more intensively at an elevated temperature. We had hoped the colored species were able to be removed from the molecular sieves by solvent extraction. However, even several days of Soxhlett extraction with benzene did not result in the separation of the colored product. The benzene extract contained only polystyrene, which was confirmed by IR spectroscopy.The colored species could not be removed from the molecular sieves by means of thermal evacuation in a sublimation apparatus either. It is worthwhile to notice that the thermal decomposition of the pink color product started only at a temperature above 400OC. Another way to separate colored species from zeolite was to dissolve zeolite with HF. The most intense purple color was noticed for zeolite H-ZSM-5. The color disappeared after dissolving zeolite and the IR spectrum of organic remnant was identical to that of polystyrene. After reacting with styrene the sample were always washed with benzene . The color of the samples contacted with styrene is listed in Table 1. It has become clear that only some of the molecular sieves containing acid sites indicate the development of pink color. In order to confirm that the acid sites are indispensable for development of pink color, the samples, after the reaction with styrene and washing with benzene were treated with aqueous ammonia solution. As indicated in Table 1 such a treatment resulted in the vanishing of the color or at least the diminishing of intensity. The IR spectra of the samples were recorded using KBr pellets or self supported wafers and a vacuum cell. The Bruker IFS 113v spectrometer was employed for recording the spectra. Electronic spectra of selected samples were recorded as nujol film by means of Shimadzu UV-160 spectrometer. ESR spectra of selected samples were measured by means of SE/X-2547 spectrometer produced by Radiopan, Poznari. The field frequency was 9.4GHz and the magnetic field modulation was 100kI-E. Spectra were recorded at room temperature without former evacuation.

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Elimination of protonic sites in H-mordenite and in H-ZSM-5 by high temperature treatment results in the lack of color development. The strong acid sites , however, are also present on the surface of H-faujasites, fluorinated AI-Y, fluorinated alumina, not to mention the mineral acids employed in these experiments. None of the latter forms the colored species with styrene. Therefore we cannot agree with Pollack's suggestion that styrene might be used as an acid sites indicator. The geometry of the inner voids in the molecular sieves seems to play a very important role in the generation of colored species and for their high stability. Zeolites ZSM-5 and silicalite-1 belong to MFI structure and their ten-membered channel size is 0.53 x 0.56nm. The AEL structure of AIPO4-11 also shows the ten-membered ring system ( 0.39 x 0.63nm ). The channel pore system in mordenite comprises 12 membered rings (0.65 x 0.70nm) and 8 membered ring channels (0.26 x 0.57nm). The other molecular sieves used for the study show apertures larger then 0.7nm. Diameter of the supercage in faujasites is almost 1.3nm large. It is likely that the larger intracrystalline voids enable the formation of longer polystyrene chains, whereas in the case of relatively narrow channels, only small oligomers can be formed due to geometric constraints. The complex of the oligomer with molecular sieve structure is probably responsible for the pink color. Pollack [16] suggested that the color is attributed to styrene cation radicals generated by protonation of the vinyl group. Although our electronic spectra (Fig. 2) and the ESR data (Table 2 ) are very similar to those presented in work [16], we believe that the colored species cannot be a result of simple styrene protonation. If it were the case , the color development should also be seen for other molecular sieves and mineral acids. Absorption maximum at about 560nm noticed both in ours and Pollack measurements differs from the values attributed to styrene radical ions. The spectra published by Keene [20] show the maximum at 410nm and those presented by Shida [21] at about 600nm.

I

558

I

300

I

I

500

I

100

nm

Fig. 2. Typical electronic spectrum of zeolite H-ZSM-5 contacted with styrene at S O W . The recorded maximum is close to the values attributed to diphenylethyl cation [22]. A small band at about 770nm is in the range similar to paracyclophane or 1,2,5,6, dibenzocyclooctatetraene. The ESR data indicate a presence of radicals in the molecular sieves treated with styrene (Table 2). However , the radicals have been detected not only for the colored samples, and therefore it is not clear whether all free radicals generated in the molecular sieves are involved in a color development .


183

Since the first aim of the study was to apply the molecular sieves as potential active filler and catalyst for styrene polymerization, the polymerization rate in the presence of some molecular sieves was preliminary estimated by means of measurement of viscosity of polymer formed.

RESULTS AND DISCUSSION

Table 1 shows the color of the indicated samples of the molecular sieves and other reagents after reaction with styrene at room temperature and at 8OOC. It also shows the color changes after treatment with ammonia. Table 1. Color of the samples after the contact with styrene. Sample 1. Na-X 2. H-X 3. Na-Y 4. H-Y 5 . AI-Y-F

6. Na-mordenite 7. H-mordenite 8. Na-ZSM-5 9. H-ZSM-5 10. silicalite-1 1 1. AIPO4-5 12. AIPO4-11 13. AlPO4-11-F 14. A1703 15. Al203-F 16. P205 17. H2SO4 18. HClO4 19. HC1 20. H-mordenite calc. at 700OC

Color after contact +NH7 at 20OC white white white white white white light yellow tan tan light tan white white pink white white white purple beige purple beige white white light grey white white light pink white white light yellow white khaki yellow black colorless colorless white white

with at 8OOC light yellow light yellow white yellow yellow white grey pink light yellow purple purple white light grey pink pink white yellow brown green

styrene

+ NH3 light yellow white white light yellow light yellow white tan white beige beige white white white light yellow white tan

As indicated in the above table the pink or purple color develops only after the reaction of styrene with H-ZSM-5, H-mordenite, silicalite-1 and also, but less intense with fluorinated WO4-11. The sodium forms of mordenite and zeolite ZSM-5 do not show the pink color with styrene. To some extent we have to accept the opinion of Pollack [ 161 that protonic acid sites are indispensable for the pink color product generation. In the case of silicalite and fluorinated AIPO4-11 the number of acid sites is certainly very limited, yet still sufficient to form the conspicuous complex with styrene.

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The LR spectra of the pink samples (Fig. 3) show the distinctive bands at 1541 and 696 cm-l, which are in the same range as the most intense bands of 1,1, diphenylethylene [24]. The intensity of hydroxyl groups in H- forms of mordenite (Fig. 4) and zeolite ZSM-5 are noticeably reduced after admittance of styrene, which confirms that the protonic sites are involved in the reaction with styrene. Table 2. Results of the ESR measurement for the selected samples contacted with styrene at 80OC Sample 1. H-ZSM-5 2. silicalite-1

g- value

Line width (Gauss)

2.0023

10.7

2.0026

11.6

3. H-mordenite

2.0023

10.1

4. H-Y

2.0034

9.7

5. mod-11

2.0026

10.0 (narrow), 26 (broad)

6. AIPO4-11-F

2.0026

10.0 (narrow), 26 (broad)

a

-

2000

I

1600

I

I

1200

800

L

CM-~

Fig. 3. IR spectra (KBr) of H-ZSM-5 (a) and the same sample treated with styrene at 8OoC (b).

Fig. 4. IR spectra (self supported) of Hmordenite (a) and the same sample with styrene (b).

Table 3. Viscosity of the polystyrene formed in a presence of the indicated samples. ~~~~~

Sample

none

Viscosity 1.254 (q reduced dcVg )

silica gel

Na-Y

H-mordenite

H-2 SM-5

0.482

0.493

0.192

0.08 1

The preliminary styrene polymerizarion experiments carried out in the presence of some moleculai sieves at 8OoC for 24 hours (Table 3) show that the polymerization rate (estimated as viscosity 01 the product) is lowest for the samples containing H-ZSM-5 and also low for H-mordenite. H-ZSM-!


I85

shows the deepest purple color with styrene. It suggest that strong chemical interaction between this zeolite and radicals having been formed during the process retards their propagation and subsequently diminishes the rate of polymerization. Although the chemical structure of colored species formed from styrene on certain molecular sieves containing protonic acid sites is still not clear to us, and we can only speculate a presence of small oligomeric cation radical, we would like to emphasize the importance of the molecular sieve pore geometry for generation of this species. The geometry is probably also decisive for its high thermal stability. Perhaps, it is the next and a very spectacular example of the shape selectivity of the molecular sieves. ACKNOWLEDGEMENTS This work was supported by the grant (2 0765 91 01) fiom the Polish Committee for Science Research (KBN). The grant from The Batory Foundation is greatly appreciated. We thank Dr. W. Augustyniak and Dr. R. Fiedorow for the helphl discussion. REFERENCES 1 H.G. Karge, in H. van Bekkum, E.M. Flanigen , J.C. Jansen (Ed.) Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis 58, Elsevier, Amsterdam, Oxford, New York, Tokyo, 1991,p.531. 2 P. Pichat, J.C. Vedrine, P. Gallezot, and B. Imelik, J. Catal., 32 (1974) 190. 3 P.K. Dutta, M. Puri, J. Catal., 111 (1988)453. 4 A.K. Gosh, R.A. Kydd, J. Catal., 100 (1986)185. 5 M. Zerdkoohi, J.F. Haw, J.H. Lundsford, J. Am. Chem. SOC.,109 (1987) 5278. 6 J.F. Haw, B.R. Richardson, J.S. Oshiro, N.D. Lazo and J.A. Speed, J. Am. Chem. SOC., 111 (1 989) 2052. 7 I. Kiricsi, H. Foerster, J. Chem. SOC.,Faraday Trans. 1,84 (1988)491. 8 H. Foerster, 0. Zakharieva-Pencheva, J. Mol. Struct., 175 (1988) 189. 9 R. Piffer, H.Foerster, W Niemann, Catal. Today, 8 (1991)491. 10 G. Meyer, D.Woehrle, M. Mohl, G. Schulz-Ekloff, Zeolites, 4 (1984)30. 1 1 T. Bein, P. Enzel, F. Beuneu, and L. Zuppiroli, in M.K. Johnson et al. (Ed.) Electron Tranger in Biology and the Solid State, Inorganic Compounds with Unusual Properties (Advances in Chemistry Series 226) American Chemical Society, Washington D.C. 1990,p.433. 12 P. Enzel and T. Bein, J. Chem. SOC.,Chem. Comm., (1989) 1326. 13 P. Enzel and T. Bein, J. Phys. Chem., 93 (1989)1326. 14 Supramolecular Architecture. Synthetic Control in Thin Films and Solids., T. Bein (Ed), ( ACS Symposium Series 499), American Chemical Society, Washington, DC 1992. 15 S. Kowalak, M. Pawlowska, D. Szuba, M. Wejchan-Judek, J. Material Science Letters, 12 (1993)661. 16 S.S.Pollack, R.F. Sprecher, and E.A. Frommel, J. Molecular Catalysis, 66 (1991) 195. 17 S.Kowalak, J. Chem SOC.,Faraday Trans. 1, 84 (1988)2035. 18 S.Kowalak, M.Pawlowska, to be published. 19 S. Kowalak, Chemia Stosowana, 34 (1990)93. 20 J.P. Keene, E.J. Land and A.J. Swallow, J. Am. Chem. SOC.,87 (1965)5284. 21 T.Shida, Electronic Absorption Spectra of Radical Ions, Elsevier, New York, 1988. 22 B. Schrader, W. Meir, DMS RamadIR Atlas Organic Compounds, Verlag Chemie, Weinheim, 1974.

0s


Studies of Zeolite Single Crystals: Ethene Oligomerization in HZSM-5

Kenneth T. Jackson and Russell F. Howe* *Department of Physical Chemistry, University of New South Wales, Box 1, Kensington NSW, 2033, Australia.

ABSTRAm

Large (150 micron) crystals of HZSM-5 have been shown by electron microscopy, micro X P S and selective dissolution experiments to be highly zoned, with an aluminium rich exterior and an aluminium deficient interior. The oligomerization of ethene in the crystals has been studied by l3C, NMR, FI'IR microscopy and Raman microscopy. At low temperatures a largely linear oligomer is formed ; above 2000C chain branching occurs at channel intersections. Raman microspectroscopy can distinguish between oligomer in the interior and that near the outer surface of the crystals. INTRODUCTION The reactions of alkenes with HZSM-5 zeolites have been extensively studied spectroscopically, by FTIR [I-31, UV-VIS [3,4], 13C NMR [5-91 and thermal desorption spectroscopy [2,10]. Propene and higher alkenes oligomerize readily at room temperature, forming branched oligomer species within the zeolite consistent with a classical carbenium ion mechanism involving the Bransted acid sites. In the case of ethene, there are several reports that a linear oligomer is formed at low temperatures [4,5,8], although on heating chain branching also occurs. These previous studies have utilized polycrystalline zeolite samples and have provided no information about orientation or location of oligomeric species within the zeolite crystals. Recently, we and others have reported the feasibility of microspectroscopic experiments on single crystals of ZSM-5, using an FTIR microscope [ll-131. The microscope allows spectra to be. recorded from areas as small as ca. 20 microns, and the use of polarized radiation with an oriented single crystal gives information about the orientation of adsorbed molecules within the zeolite channels. Raman microscopy is capable of spatial resolution down to ca. 1 micron, but has not hitherto been applied to adsorbates in zeolite single crystals. In this paper, we present some preliminary data illustrating the power of a combination of the two forms of micro vibrational spectroscopy for studying ethene oligomerization in ZSM-5 single crystals.

I87

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K. T.Jackson and R. F. Howe

EXPERIMENTAL Zeolite single crystals were synthesised according to the process described by Komatowski [ 141, using tetrapropylammonium bromide as the template, fumed silica (Aerosil 200) and aluminium hydroxide. The as-synthesised crystals were calcined to remove the template at 600OC for 24 hours after initial heating at l W C for 8 hours. The H form of the samples was prepared by ammonium ion exchange followed by heating in air at 5OOOC for 5 days. Crystals were outgassed ( 10-5 torr, 400OC )for 24 hours and cooled to room temperature prior to admitting ethene, then heated to the desired temperatures in 100 torr of ethene for one hour before cooling in ethene, then exposing to atmosphere and analyzing. (Comparison of infrared spectra measured in-situ and ex-situ showed that the ex-situ analysis gave identical results to in-situ.) FTIR microscopy was carried out on a Spectra-Tech IR-Plan microscope coupled to a Bomem MB spectrometer, purged with dry nitrogen and fitted with a liquid nitrogen cooled, narrow band mercury-cadmium-telluride (HgCdTe) detector. A Spectra-Tech ZnSe wire grid polariser was placed in the IR beam before the sample. Double sided interferrograms were collected at 4 cm-l resolution. Typically 500 scans were co-added for both the reference and the sample. Raman spectra were measured with a Dilor Microprobe (CCD detector), using the 514.5 nm line of an argon ion laser at a power of 300mW. Scanning electron microscopy and EDX analysis were obtained on a Cambridge Scan 360. l3C CPMAS NMR spectra were obtained on a Bruker MSL-300 spectrometer with a magnetic field of 7.05T and a l 3 C frequency of 75.470 Mhz. A 7 mm magic angle spinning probe was used at a spinning rate of 2000 Hz.

RESULTS AND DISCUSSION The synthesis method used yielded in our hands uniform large crystals, ca. 150 x 40 x 40 microns, of pure ZSM-5, with no other phases detected by x-ray powder diffraction or visually in the scanning electron microscope. Figure 1 shows a cross-section of a cleaved crystal as measured in the electron microscope, together with aluminium and silicon profiles across the crystal determined by electron microprobe. The outer surfaces of the crystals gave an Si:Al ratio of ca. 12, whereas the aluminium content of the interior of the crystals was below detection limits (Si:Al > 200). This extreme zoning of the aluminium distribution within the crystals was confirmed by single crystal XPS depth profile experiments, described in detail elsewhere [ 12,151, and by selective dissolution experiments. As recently reported by Mobil workers [ 161, high silica ZSM-5 is soluble in sodium carbonate solution, whereas low silica materials are not. In cases of aluminium zoning, regions of low aluminium content can be selectively dissolved. Reaction of the crystals prepared here with sodium carbonate solution completely removed the interior of the crystals, leaving hollow shells of high aluminium ZSM-5 approximately 1 micron thick (confirmed by x-ray diffraction and 29Si NMR). From the viewpoint of reactivity towards ethene, these crystals thus represent microreactors containing a high density of Bronsted acid sites in an outer shell and a low density of Bronsted acid sites in the interior.

Ethene Oligomerization in HZSM-5 Single Crystals

Flgwe 1 Electron microprobe analysis across cleaved crystal

189

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K . T. Jackson and R. F. Howe

Ethene oligomerization occurred very slowly at room temperature in the zoned crystals. Figure 2 shows 13C NMR spectra of HZSM-5 after exposure to ethene at room temperature, IOOOC, 200OC and 3000C respectively ( 1 hour at each temperature). The room temperature spectrum (Figure 2 (a)) is dominated by the signal of physisorbed ethene, at a chemical shift of 121 ppm. A second weak signal at 32 ppm grows in intensity on heating to IOOOC (Figure 2 (b)), and is accompanied by several less intense features in the 0-40 ppm region. There is also a small but significant signal at ca. 59 ppm. The spectrum after heating to 200OC (Figure 2(c)) differs only slightly from that at IOOOC but at 3 W C relative intensities of the 0-40 ppm signals are dramatically altered (Figure 2 (d)). The NMR spectra are generally similar to those reported by Van den Berg a & for ethene oligomerization in microcrystalline ZSM-5 [ 51. The 32 ppm signal was assigned by these authors to CH2 groups in a linear oligomer species and a signal at 14 ppm to the terminal CH3 groups of the same species. The spectra at room temperature, lOOOC and 2OOOC in Figure 2 would, following Van den Berg & be attributed to a largely linear oligomer generated in HZSM-5. At 300OC on the other hand there is substantial development of signals attributed by Van den Berg to branched oligomer species (e.g., at 27,23 and 11 ppm). A recent study by 24imensional J-resolved NMR [ 9 ] has cast some doubt on the earlier assignments, but the spectrum reported in reference [ 9) for ethene oligomerized in ZSM-5 at I W C , and assigned to a mixture of branched and linear oligomers is quite similar to that obtained in the present work at 300°C (Figure 2 (d)). In view of these differences of opinion concerning NMR data, we undertook FTIR and Raman studies of ethene in HZSM-5, using microspectroscopy to examine single crystals of the zeolite. Figure 3 shows infrared spectra in the v(CH) region of ethene in ZSM-5 after exposure at room temperature, lOBC, 20BC, 300OC and 400OC, using unpolarized light on a single crystal. At low temperatures, the spectra are dominated by a pair of v(CH) bands at 2934 cm-l and 2860 cm-l. Such bands in the spectra of liquid alkanes are assigned to the asymmetric and symmetric stretching modes respectively of CH2 groups [ 171. The vibrational modes of alkyl oligomer chains in ZSM-5 may not be identical to those of liquid alkanes, but the group frequency concept is at least a first approach to analysis of the spectra measured here. At 30OOC, the CH2 bands are overtaken by those at 2960 and 2880 cm-1. which in liquid alkanes are due to asymmetric and symmetric stretching modes respectively of CH3 terminating alkyl chains. [ 171. The other feature present as a shoulder in all of the spectra in Figure 4 is a band at about 2900 cm-1 usually assigned to CH groups in liquid alkanes. Comparison of the NMR and infrmd data suggests that a largely linear oligomer is formed at low temperature, and that extensive chain branching occurs at elevated temperatures. These conclusions are supported by measurements using polarized infrared light. Figure 4 shows spectra recorded in the v(CH) region of a ZSM-5 crystal containing ethene after heating to 200OC, using polarized light. The spectra are recorded in transmission mode, through the (010) face of a crystal. Figure 4 (a) is the spectrum obtained with unpolarized light, in which all orientations of adsorbed oligomer are detected. In Figure 4 (b), the incident light is polarized parallel to the crystal a-axis. This polarization enhances particularly the asymmetric stretching mode of CH3 groups at

191

Figure 2 13C NMR Spectra after exposure to ethene at (a) morn temperature, (b) l W C , (c)200"C, (d) 300°C.

1.o

0.5

0

Figure 3 IR spectra after exposure to ethene at (a) room temperature, (b) l W C , (c)200"C, (d) 300°C.

192

K . T.Jackson and R. F. Howe

Figure 4 Polarised IR spectra after heating in ethene at 200°C. (a) Unpolarised beam, (b) polarised , (c) beam polarised .

F 1 Ramcvl Shift (an-1)

Figure 5 Raman spectra of cleaved crystal at (a) edge, (b) centre, (c) IR spectra of whole crystal

Ethene Oligornerization in HZSM-5 Single Crystals

193

2960 cm-1. Rotation of the plane of polarization through 900 (parallel to the crystal c-axis) gave the spectrum in Figure 4 (c), in which the CH3 modes are markedly diminished relative to the CH2 modes. Similar polarization effects were found in all crystals examined after exposure to ethene at room temperature, or after heating in ethene to lOOOC or 200OC. Crystals heated to higher temperatures however showed no variations in the v(CH) bands when the plane of polarization was rotated relative to the c-axis, and the spectra obtained with polarized light were identical to those measured with unpolarized light. (Figure 3). Crystals examined with polarized light incident on the (100) face showed the same effects; polarization parallel to the c-axis enhanced the CH2 bands relative to CH3 for samples heated in ethene up to 2oooC,but not above. The symmetric and asymmetric stretching modes of CH2 groups in linear polyethene oligomer chains will be observed most strongly when the plane of polarization is perpendicular to the linear axis of the oligomer. For terminal CH3 groups on such a linear oligomer, on the other hand, little difference would be expected between perpendicular and parallel polarization. The polarization effects observed for incident light along both the and c010> directions indicate that the linear oligomer chains are found along both the linear (cOlO>) and sinusoidal channels () of the ZSM-5 crystals.The formation of short chain highly branched oligomer at higher temperatures eliminates the polarization effects. The spectra of the ethene oligomers after heating to higher temperatures are similar to those of the tetrapropyl ammonium template cation in ZSM-5, which is also invariant to the direction of polarization [ 121; random orientation of highly branched oligomers at the channel intersections would account for these observations. The spatial resolution of the infrared microscope is insufficient to allow the distribution of oligorner between the high and low aluminium regions of the zoned crystals to be deterrnined.This problem can be overcome, in principle, by using Raman microscopy, since a visible laser can be focussed to a spot size of ca. 1 micron. We have recently succeeded in measuring Raman spectra from ethene oligomers in the HZSM-5 crystals used for the infrared and NMR studies, and some preliminary data are presented here [ 181. Zeolite crystals were heated in ethene to 200OC, cooled to room temperature, then cleaved across the c-axis and Raman spectra recorded from either the edge or the centre of the exposed crosssection. The samples fluoresced strongly, causing steeply sloping baselines in the Raman spectra above 500 cm-l, and poor signal to noise in the v(CH) region. Nevertheless, useful albeit noisy spectra could be measured, as shown in Figure 5. At low frequencies, HZSM-5 crystals show a set of characteristic Raman bands at 460 cm-l, 380 cm-l and 290 cm-l due to lattice modes. These bands were identical in all regions of the crystals and were unchanged by exposure to ethene. Figure 5 (a) was measured from the outer edge of a crystal after heating in ethene at 200OC. The same v(CH) bands detected in the infrared spectra (Figure 5 (c)) are also seen in the Raman spectrum, although with different relative intensities. Figure 5 (b) was measured from the centre of a cleaved crystal. The intensity of the oligomer v(CH) bands are in this case about twice those at the edge of the crystal (relative to the low frequency

194

K. T. Jackson and R. F. Howe

lattice bands), and the spectrum from the crystal centre shows a much greater contribution from the v(CH) bands due to CH2 groups at 2940 cm-1 and 2860 cm-l. Similar differences between the edges and centres of ethene loaded crystals were found for all crystals examined. Our interpretation of these initial results is that the oligomer in the centre of the crystals is less branched than that at the outer high aluminium regions of the crystal. We suppose that the degree of branching and average chain length depends on the density of Bransted acid sites in the crystal. At the outer edges of these zoned crystals there is a high density of Bransted acid sites, producing short chain highly branched oligomer, whereas in the interior of the crystals long chain linear oligomer is formed. Such a dependence of chain branching on acid site density may explain differences of opinion in the literature as to the extent of chain branching in ethene oligomers formed in HZSM-5. Further experiments are in progress to substantiate these tentative conclusions. It is clear however that microspectroscopic experiments with single crystals of zeolites can provide a great deal of new information about location and orientation of adsorbed reactants and reaction products, and many further studies of this type may be anticipated. We thank Drs. Leon Van Gorkom and Jim Hook for assistance with the *3C NMR experiments, and Dr. Dick Ashby at the University of Technology, Sydney, for access to the Raman Microprobe. This work was supported by research grants from the Australian Research Council. REFERENCES 1 A.K. Ghosh and R.A. Kydd, J.Catal., 100 (1986)185. 2 M.C. Grady and R.J. Gorte, J.Phys.Chem., 89 (1985)1305. 3 H. Forster and I. Kiricsi, J.Chem.Soc. Faraday Trans., I 84 (1988)491. 4 E.G. Derouane, J.P. Gilson and J.B. Nagy, J.Molec,Catal., 10 (1981)331. 5 J.P. Van den Berg, J.P. Wolthuizen, A.D.H. Clague, G.R. Hays, R. Huis and C. Van Hoof, J.Catal., 80 (1983)130. 6 E.A. Lombardo, J.M. Dereppe, G. Marcelin and W.K. Hall, J.Catal., 114 (1988)167. 7 J.F. Haw, B.R. Richardson, I.S. Oshiro, N.D. Lazo and J.A. Speed, J.Am.Chem.Soc., 111,

(1989)2052.

8

K.P. Datema. A.K. Nowak, J. van Braam Houckgeest and A.F.H. Wielers, CataLLett., 11

(1991)267. A.G. Stepanov, V.N. Zudin and K. Zamaraev, Solid State NMR, 2 (1993)89. T.J.G. Kofke and R.J. Gorte, J.Catal., 115, (1989)223. M. Nowotny, J. Lercher and H. Kessler, Zeolites, 11 (1991)454. K.Jackson and R.F. Howe, 9th International Zeolite Conf., Montreal, 1992,Abstract RP114 M. Schuth, J.Phys.Chem., 96 (1992)7493 J. Komatowski, Zeolites, 8 (1988)77. K.Jackson and R.F. Howe, to be published. R.M. Dessau, E.W.Valyocsik and N.H.Goeke, Zeolites, 12 (1992)776 L.J. Bellamy, "The Infrared Spectra of Complex Molecules" Chapman and Hall, London (2nd Edition) 1975. 18 A fuller account of this work will be forthcoming elsewhere.

9 10 11 12 13 14 15 16 17


Adsorption of Lower Hydrocarbons in Zeolite NaY and Theta-1. Comparison of Low and High Pressure Isotherm Data

J. A. Hampson and L. V. C. Rees Physical Chemistry Laboratories, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K.

ABSTRACT Low and high pressure adsorption isotherms over the range 0-20 atm have been measured for ethane, ethene, propane and propene in the zeolites NaY and Theta-1. Analysis of the isotherm data by the Langmuir-Freundlich and Toth equations are reported. INTRODUCTION The separation of simple gas mixtures e.g. CH4/Nz ; CH4/COz and mixtures of lower hydrocarbons is becoming of increasing commercial and environmental importance. The use of low energy methods such as pressure swing adsorption (P.S.A.) are being widely studied for the separation of the above mixtures. In order to improve the performance of such separation processes it is essential to use adsorbents with optimum characteristics for the specific mixture under consideration. Thus adsorbents have to have large separation factors for the mixture at, preferably, room temperature while still offering fast adsorptionldesorption kinetics. Zeolites have many advantages as adsorbents for the above mixtures. The optimum pore sue can be readily obtained fiom a reasonably wide selection of synthetic and natural zeolites. The separation factors can be modified by varying the SilAi ratio of the zeolite. High SiAl ratio zeolites are hydrophobic and thus if water as an impurity is present in the mixture a zeolite can be selected which has little selectivity towards the polar water molecule while still offering good selectivities towards hydrocarbons. In low SVAI ratio zeolites the cations present to balance the negative charge introduced into the fiamework by the Al atoms can be readily exchanged with other cations. Thus the size and the valency of the cations can be easily changed. Such changes allow controlled modificationof the electric field gradients which exist at the adsorption sites. Such changes alter the separation factors between polahon-polar gas mixtures and mixtures of saturatdunsaturated hydrocarbons. In the present paper an attempt will be made to show the importance of differences in pore size and WAl ratio on the adsorption of ethane, ethene, propane and propene adsorbates and indicate which type of I97

198

J . A. Hampson and L. V . C. Rees

adsorbent should be selected for separation of specific binary mixtures of these four adsorbates. EXPERlMENTAL The low pressure adsorption ( k 0.1 K. Sorption equilibrium parameters were measured by stepwise change of sorbent temperature along linear total isosteres. The isosteres became curved at high temperature and high pressure (qfbelow), because in this region isosteric condition was disturbed due to non-negligibly high desorption. The data corresponding to this phenomenon were rejected. Once equilibrium had been attained at a temperature chosen, the corresponding total pressure was measured and small gas samples, which would not distort the overall mass balance, were transferred into the ion source of the analyzer where a pressure of = 10 -6 Tom was maintained. Since the values of total and partial loading were already known due to the dosing procedure, only the values of total and partial pressures have to be determined and ascribed to the temperatures chosen. Thus, both the total and the corresponding partial isosteres were obtained. The slopes of the isosteres gave the corresponding sorption enthalpies of mixture components. To calculate the sorption entropy for all the components of the ternary mixture, the standard state was defined as the particular state of the gaseous mixture with a composition equal to that of the sorption phase at a total pressure of 760 Tom and at 25 "C [9]. This choice leads to the following expression for the partial sorption entropy, -@, of component i :

where, p , denotes the partial pressure of component i at temperature 7: and ---

and

-G are the

changes in enthalpy and entropy of component i due to the mixture sorption process, respectively, stands for the mole fraction ofthe component I in the gaseous (sorption) phase and H represents the universal gas constant

. ~ ~ , ~ , l ~ ~

-

The change in Gibbs' free sorption enthalpy, -AG,, was calculated using the hndamental relationship

By means ofthis quantity, the separation factor, a,,, defined for a binary mixture of components i I , 2 (where f denotes the preferentially sorbed component) as given by expression (3) was calculated for

Sorption Thermodynamic Functions for Gas Mixtures

21 I

each binary pair of sorbed ternary mixtures according to equation (4):

RESULTS AND DISCUSSION The svstem Ar-Kr-Xe/CaA zeolite. In Figure 1 are shown sorption isosteres of this ternary mixture

measured within the regions of temperature, (130 ._ . 190) K, and pressure, ( 5 x ... 80) Torr, for two different values of total zeolite loading, (0.222; 0.442) nimol g-1, respectively, but at approximately the same molar ratio of the sorption phase, 1 : I : I . The changes in the thermodynamic functions,

--w-@, and

-

for each mixture component are listed in Table 1 . lO?T,TinK 6.0 7.0 I I

TIK

5.0

-

8.0

I

1 - 9

-

\

I

.,

Fig. 1 . Sorption isosteres of the Ar-Kr-Xe/CaA system at x;" = 0.33 (sorbed amounts (mmol/g): 0,O...total: 0.222, 0.442; A,A...Ar: 0,0715 , 0.145; 0 ,.,Kr: 0.073.5, 0.149; V,V ..Xe: 0.0769, 0.149).

-

13

L &

-

12 .:

11

P 2

\

IQ

14 0-

-15

I1

10'11, T / K

-

:1

10

1

16 17

J 18 180

160

140

120

-TlK

Fig. 2 Dependence of Gibbs' free enthalpy change on temperature for Ar, Kr and Xe due to their siniultaiieous sorption on CaA-type zeolite at x/J = 0 . 3 3 .

The changes in Ciibbs' free sorption enthalpy, ---,

as dependent on 117' for the three mixture

components are given in Figure 2 at constancy of both total sorbed amount and sorption phase composition composition ( I : 1: 1). Separation factors, a,?.were calculated by both eqs. (3) and (4).

M . Biilow

212

--@/ kJ mol-1

System

n,,,,,, 1 mmol g-1 0 222 0 442

Ar-Kr-XelCaA Argon KIypton Xenon

135 179 283

I35 183 265

-

-qI

kJ mol-l grd-1

n,,,,., / mmol g-1 0222 0442 547 48 3 540 640 827 81 6

Table 2.Values a,, ofbinary mixtures Xe-Kr, Xe-Ar and Kr-Ar on CaA zeolite calculated from multicomponent data by eqs. (3) and (4)(total loading: 0.442mmollg, x,(" = 0 33) T/K

134.4 142.6 151.4 160.4 171.4 184.4

2.4 2.2

f

-

r

155.4 110.1 107.2 62.71

---

-

1.8

-

eq.(4) 155.6 110.4 107.4 62.73

,

1 I

---

Ia,,,

-

2.0

aXe-Ar

%':-KT

eq. (3)

A/

i l ~ mMY

bu

f/y'f ,

,*

101

-

-

aKr-Ar

er. (3)

eq.(4)

eq. (3) eq.(4)

3616 2082 1488 762.5 690.5 465.1

3683 2083 1488 762.1 689.8 465.4

23.25 18.88 13.86 12.11 9.95 7.68

I 1

23.67 18.81 13.86 12.15 9.59 7.68

3.9 3.7 3.5

3.3 4

Identical results were obtained for all binary pairs of* the two three-component nuxtures considered here, indicating the correctness of the approach expresd by eq.(4)together with the standard state cho-


213

cannot be due to non-equilibrium behaviour ofthe system, since the isosteres are linear. ( i i i ) In accordance with general thermodynamics, Gibbs' fiee sorption enthalpy depends on concentration. Except for the xenon system at high temperature, the temperature increment of

-a

is of comparable size for all systems considered. (iv) The values of the sorption enthalpy for xenon and krypton due to their simultaneous sorption together with argon are similar to the data obtained for the case of their mixture sorption by CaA zeolite previously described in literature [9], even with the following peculiarity: (v) Whereas for argon and krypton the sorption enthalpy and entropy decrease with increasing total loading, for xenon the opposite takes place over the concentration range considered. To explain the mutual interactions between three fluid components regarding their sorption behaviour, the experimental parameters should be varied over broader ranges, especially the total amount sorbed and the sorption phase composition. One has also to take into account that at low temperature strong interference etfects may occur even for noble gases, e . g between helium and xenon in CaA zeolite [13].

.. lozi : I

'

I

I

&8\

t1s l1o0o' P

4

at the same composition. For the three sets of experiments, at comparatively high values of temperature and pressure the isosteres decline fi-on1 linearity due to significant desorption. (The corresponding points belong to isosteres for lower sorbed amounts; these data were not taken into account for further consideration.) The changes of

\ > & thermodynamic functions, -qand -q-, for each

-

B

10.' c

mixture component are listed in Table 3. 'The changes in Gibbs' free sorption enthalpy,

-AC,, for the three-component mixture as depenI

1

,

4

I

6

\

I

-

-

8

dence on reciprocal temperature are given in Fibwre 5 at

214

M. Biilow

B

*

30

120

160

1

1

200

180

I

1

160 150

I

I

140

120

200

TJK

240

I

Fig. 5. Dependence of Gibbs' free enthalpy change on temperature for Ar, O2 and N, due to their siniultaneous sorption on NaX-type zeolite at x,(" = 0.33.

Fig. 7. Temperature dependence of gaseous phase composition equilibrated to sorption phase at total loading of 0.262 mmol g-I for the Ar-02-N2/NaXsystem at x$j= 0.33. The following main conclusions can be drawn: ( i ) Within the temperature region 200 ... 150 K, the following sequence of relative adsorbability holds: Nz > Ar > 0 2 . This result corresponds, quantitatively, to findings for the systems Ar-02-N2/CaA zeolite [ 141 and Ar-N2/NaX zeolite [IS]. ( i i ) At temperatures below 2 150 K, oxygen becomes sorbed preferentially over argon (Figure 7 supports this finding); the parameter a,+, approaches a value of = 2 at 100 K. (iii) Although nitrogen remains the most strongly sorbed component over the whole parameter range considered. at temperatures below I50 K. i.e. when -

~ i 6 , ~Dependence , lg a,, ,s I/T for the A,.-- oxygen starts to replace argon, the - A G N ~17.s I/T 02-N2/NaX systeiii calculated by eq44). relation deviates upward from linearity and assumes higher values (cf Figure 5). This behaviour is also reflected by the Ig a,l vs I l l ' dependence characterizing the competitive sorption of N2 with both O2 and Ar (Q Figure (6). Again this finding cannot be caused by non-equilibrium since the isosteres are straight lines.

-mNz


215

Table 3. Partial sorption enthalpies and entropies for the system Ar-02-N2/NaX at x,'" = 0.33

-=

System

-A,!$ / kJ mol-1 grd-I n,,,,,l / mmol g-*

ntClbl/ mmol g-1

Ar-07-N2/NaX 0.190 8.9 Argon 10.2 Oxygen 24.5 Nitroeen

(iv) N o clear dependence of

-

/ kJ mol-I

0.243 9.8

11.2 21.1

0.262 10.2 11.3 20.4

0.190 27.1 35.5 94.8

0.243 35.5 44.5 79.8

0.262 37.6 44.8 77.2

-qon the concentration of sorbed species could be observed because

of the small change in concentration of the same. (v) Whereas for argon and oxygen the absolute values of sorption enthalpy and entropy increase with increasing total zeolite loading, for nitrogen the opposite takes place in the concentration range considered With respect to both finding

(17)

and the

-3G

v.v I/T dependences for both ternary mixtures

considered, the components most strongly sorbed from the ternary mixtures behave in surprisingly similar manner I iowever, to explain the reason for this phcnomenon, more detailed experimental investigations are needed. CONCLUSIONS For sorption measuremcnts of two ternary gaseous mixtures on zeolites, isosteric equilibria and thermodynamic hnctions obtained therefrom demonstrate the usefulness of the isosteric method with minimum dead volume to investigate multi-coinponcnt sorption phenomena on microporous solids over wide ranges of temperature, pressure and concentration (composition) of the coexisting phases. The method is thought to be one of the most powerful experimental approaches to multi-component sorption equilibria REFERENCES I F.Meunier and h1.D. LeVan (Eds.),Adsorpt. Proc. Gas Separation, Lavoisier Tech. Docum., Paris 1991.

2 M Suzuki (Ed.), Sorptive Separation. University oflokyo Press, Tokyo 1991. 3 D.P. Valenzuela and A.L. Myers, Adsorption Equilibrium Data Handbook, Prentice Hall, N.J. 1989. 4 R.T. Yang, Gas Separation by Adsorption Processes, Huttenvorths, Boston 1987. 5 D.M. Kuthven, Principles of Adsorption and Adsorption Processes, Wiley. New York 1984. 6 M.M. Dubinin and V.V. Serpinsky (Eds.), Phys. Adsorpt. Multi-compon.. Phases (Russ) Nauka, Moscow 1972. 7 B.P. Bering, E.G. Zukovskaja, Ch.M. Rachmukov and V.V. Serpinsky, Z. Chem.. 9 (1969) 13. 8 W. Lutz, A. GroRniann and W. Schirmer, Chem. 'iechn. (Leipzig), 28 (1974) 739. 9 M. Bulow and P. Lorenz, Fundam. Adsorpt. I1 (Ed. A.I. Liapis) Eng. Found.. New York 1987, p. I 19. 10 P. Graham, A.D. Hughes and L.V.C. Rees, Gas Sep. Purif., 3 ( 1989) 56. 1 I L.V.C. Rees, P. Briickner and J. Hampson, Gas Sep. Purif., 5 ( 1991) 67. 12 S. Sircar, Ind. Eng.Chem. Research, 31 (1992) 1813. 13 Y. Yasuda and S. Shinbo, Bull. Cheni. SOC.Japan. 61 (1988) 194. 14 G.W. Miller, K. Knaebel and K.G. lkels, AlChE J., 33 (1987) 745. 1 S L. Vashclienko, V. Katalnikova and V.V. Serpinsky. Phys. Cond. State (Russ), Charkov, 34 (1974) 91.


Adsorption Characteristics of Hydrophobic Zeolites

Kazuo TSUTSUMI, Takae KAWAI and Takashi YANAGIHARA Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan

ABSTRACT Highly siliceous faujasites were obtained by Sic14 treatment as well as steaming and acid treatment. Adsorption of water depended significantly on the SifAl ratio of zeolites. As the ratio increased, the adsorption amount decreased, which can be explained by the decrease in the electrostatic field on the surface. On the other hand, the adsorption of chloroform decreased slightly with an increase in the ratio, suggesting that the field-dipole interaction became less significant. When the SVAl ratio exceeds about 10, the zeolite could be noted as hydrophobic and showed the adsorptive capability of organics such as surfactants or chloroform from their aqueous solution.

INTRODUCTION In zeolites of the alumino-silicate type, the presence of negatively charged aluminum atoms causes the formation of an electrostatic field, making zeolites capable of interacting strongly with polar molecules. On the other hand, nonpolar molecules can be adsorbed on zeolites by van der Waals interaction as well as pore filling in zeolite pores. However, the adsorption of nonpolar molecules is easily prevented in the presence of polar molecules. Limited adsorption of organic molecules from their aqueous solution is a typical example. The subject of this work is to study adsorption characteristics of hydrophobic zeolites and to analyze the effect of SiJAl ratio on them. EXPERIMENTAL Samples used were modified Na-Y faujasites with various SVAl ratios. The modification was carried out using Sic14 by a procedure similar to that reported by Beyer et al. [l]as well as by direct acidic dealumination in the presence of silicic acid [2,3]. High siliceous faujasites obtained by steaming and acid treatments were also used [4]. The SifAl ratio was determined by chemical analysis or infrared spectra [ S ] . The number in the sample name such as Na-Yq represents the SiOdAlz03 ratio, that is twice the SVAl ratio. Heats of immersion of water, nhexane and chloroform were measured by twin-conduction type calorimeter at 298 K. Adsorption of water, n-hexane and chloroform vapors were measured by gravimetric method at 303 K. Adsorption of various types of surfactants and chloroform was measured from their 217

218

K. Tsutsumi, T. Kawai and T. Yanagihara

aqueous solution at 298 K. RESULTS AND DISCUSSION Adsorption isotherms of water on Na-Y4.6, Na-Y47 and H-Y77o are shown in Fig. 1. The isotherm of Na-Y4.6 is of typical Langmuir type and exhibits a steep rise at the initial stage of adsorption due to the interaction between the electrostatic field of zeolite surface and water dipole. In the isotherm of Na-Yo, the saturated amount of adsorption greatly decreases compared to that in Na-Y4.6 and a little rise at the initial stage of adsorption due to the presence of sodium cations remaining in the framework of Na-Y47 was observed; these results indicate that Na-Yu becomes almost hydrophobic. An adsorption isotherm on H-Y77o is of Type 111, which exhibits very weak interaction of water with H-Yno surface. This sample was prepared by steaming and acid treatment, and observed to contain a number of "hydroxy nests". Such hydroxy nests proved to be inactive for approaching water because of closed hydrogen bonds among hydroxyls [4].

0

Equilibrium pressure / kPa 1 2

3

Adsorption of ti-hexane was not affected by the Si/AI ratio of zeolites, indicating that dispersive interaction was dominant in this system. This is consistent with the results of heats of immersion into n-hexane as shown later.

Hydrophobic Zeolites

219

Adsorption isotherms of chloroform from aqueous phase on several zeolites are shown in Fig.2. These isotherms are of typical Langmuir type irrespective of the SQAl ratio. The adsorption amount decreased slightly with an increase in the ratio. Since chloroform molecule can interact by its dipole with a zeolite surface field, a decrease in A1 atoms, which are the origin of the electrostatic field, should affect the adsorption behavior. The slight decrease in the adsorption amount indicates that the adsorption proceeds mainly by dispersive interaction.

Equilibrium pressure / kPa

0

0.2

0.4

0.6

0.8

1.o

P/Po Fig. 2. Adsorption isotherms of chloroform. Heats of immersion into water or n-hexane are shown in Fig. 3. Heats of immersion of ZSM-5 into water are also shown for the comparison. The heats of immersion of zeolites into water have three stages in their variation depending on the Al/(Si+Al) ratio or the Si/Al ratio. In the region where the Al/(Si+Al) ratio is higher than 0.08, heats of immersion of Na-Y series into water were constant with a value of about 500 mJ/m2. The high value in this stage is probably due to the adsorption by specific interaction of polar water molecules on cations on zeolite pore surface. Since the pore volume is limited, the number of water molecules which can interact specifically with cations is limited and vice versa. This may be the reason why the heat value was constant irrespective of the ratio in this stage.

220

K. Tsutsurni, T. Kawai and T. Yanagihara

In the region of the ratio between 0.03 and 0.08 heats of immersion of both Na-Y and ZSM5 series into water decreased linearly with the ratio. This suggests that the hydrophilicity of zeolites is dependent on the amount of Na+ or framework anion. The heats of immersion of ZSM-5 zeolites lie on the same relation, which indicating that the hydrophilicity is determined exclusively by the AV(Si+Al) or SQAl ratio and not by the zeolite structure. Below 0.03 in the ratio, the heats of immersion of ZSM-5 series into water were constant at about 120 mJ/m2. The low value shows that the immersion proceeds via weak physical interaction between water molecules and almost siliceous zeolite surface. Heats of immersion of Na-Y series into n-hexane were constant at about 100 d / m 2 for all ratios of Al/(Si+Al), suggesting that the interaction between n-hexane molecule and zeolite surface is only dispersive.

600

ylll I

I

I l l

-

0

I

I

I

I

I

0 (

0

0

Na-Y/H20 ZSM- 5/ H20

A

Na- Y/n- C6

-

- - - - - - - - - - - - - - - - - - - - - - ----------A’ A

I

I

I

I

1

Adsorption isotherms of sodium dodecylsulfate (SDoS) from its aqueous solution on various zeolites are shown in Fig. 4. Isotherms are of the Langmuir type, which reflects the adsorption on the pore structure of zeolites. No adsorption occurred on the low siliceous Na-Y4.6, while the siliceous faujasites became capable of adsorption and increases in amount with an increase in the Si/Al ratio. The Na-Y4.6 contains a great number of cations as well as framework anion sites,


221

inducing the adsorption of water molecules by dipole-electrostatic field interaction. In this case, adsorption of SDoS is inhibited by the presence of water. In the case of ZSM-5, the adsorption capacity is in the order: 25Na < lOOONa < 70 Na. The hydrophilicity is dependent on the SVAl ratio also in the case of ZSM-5 and in the order: lOOONa < 70Na < 25Na [6], which suggests the existence of the optimum adsorption condition with regard to the hydrophilic-hydrophobic balance of the adsorbent since the SDoS molecule is amphiphilic.

-

ZSM-5-70Na

A

0.2

V.l

0.2

0

0 Equilibrium concentration / mmol’l- 1 Fig. 4. Adsorption isotherms of SDoS.

The saturated adsorption amount calculated from a Langmuir plot of the isotherm is shown in Fig. 5 with results of sodium dodecylbenzenesulfonate (DBS). The amount is significantly dependent on the Al/(Si+Al) or SVAl ratio, and zeolites with their AV(Si+Al) ratio above ca. 0.1 or Si/Al ratio below cu. 10 have no adsorption capacity, which is consistent with the results of heats of immersion. In the system ZSM-SBDoS, amphiphilic character of SDoS was observed. Adsorption of a bulky molecule such as DBS was found to be difficult in the pore of ZSM-5 and occurred only on the outer surface of more hydrophobic ZSM-5. Adsorption behaviors of cationic surfactant, laurylpyridinium chloride, and nonionic surfactant, polyoxyethylene(n=l l)nonylphenylether, were almost similar. In the former, cation exchange adsorption occurred on the low siliceous zeolites. Adsorption of chloroform from its aqueous solution was found to occur only on the high siliceous zeolites, as shown in Fig. 6. In this case, zeolites modified by means of direct acidic dealumination in the presence of silicic acid followed by Sicktreatment were also used. Since the pore volumes of zeolites obtained by nitrogen adsorption at 77 K varied between 0.3 and 0.47

K . Tsutsumi, T. Kawai and T. Yanagihara

222

I

0.8

z

I

I

I l l

I

I

I

I

I

d

-

E \ c)

a

0.6

-

0

-

3

0

0

Na-Y/SDoS

1

-e

0

E cd c

0

0.4,~

-

.H Y

Ef.

s:

2 0 c1

2=r

0.2

-

ZSM-S/SDoS

0 . 0

n o

0

Na-Y/DBS

-

-

-

-

I

0

ZSM-5/DBS

0

Y

0.4 0.3 0.2

0.1 n

40 Equilibrium concentration / mmo1.g-' 20

Fig. 6. Adsorption isotherms of chloroform.

6 0"


223

The dependence of saturated amount of adsorption on the AV(SitA1) or Si/Al ratio shown in Fig. 7 was similar to that of surfactant adsorption and ca. 0.1 of Al/(SitAI) or ca. 10 of Si/Al was found to be the critical value.

A I/(Si+AI) Fig. 7. Dependence of saturated amount of chloroform on Al/(Si+Al) or SVAl ratio. This tendency was also observed in the results of heats of immersion of zeolites into chloroform, which is shown in Fig. 8. Heats into chloroform are almost constant on zeolites with relatively higher Al/(Si+Al) or lower Si/N ratio and decrease slightly as A1 content decreases. As observed in the results of adsorption isotherms in Fig. 2, chloroform adsorption from gaseous phase was governed mainly by dispersive interaction. However, the effect of dipole-field interaction was also evident, which is the reason for a slight decrease in heat values on samples with lower A1 content. The adsorption of chloroform in the presence of water became significant on samples whose values of heats of immersion into both water and chloroform were reversed. The adsorption of chloroform from the gaseous phase was favored on zeolites with higher Al content. However, its adsorption from aqueous solution was favored on more siliceous zeolites; this can be explained by the synergetic effect of the hydrophobic interaction of chloroform as well as of the repulsive interaction of water in both cases with hydrophobic zeolite surfaces.

K. Tsutsumi, T. Kawai and T. Yanagihara

224

S i/A 1 600

100

10

3

c.l I

E E

k 1

E

400

.& cn 0

h 0

E E

200

0 v)

U

0

Al/( S i+Al) Fig. 8. Dependence of heats of immersion of Na-Y into water and chloroform on AV(S1tAI) or SVAI ratio. REFERENCES 1 H. K. Beyer and I. Belenykaya, Stud. Surf. Sci. Catal., 5 (1980) 203. 2 K. Nakahara, T. Ida, Y.Arima and G. Sato, Ext. Abst. 9th IZC, RP175 (1992). 3 Japan Patent, 5-97427 4 T. Kawai, S. Ito and K. Tsutsumi, Netsusokutei, 19 (1992) 70. 5 T. Kawai and K. Tsutsumi, Colloid & Polymer Sci., 270 (1992) 711. 6 K. Tsutsumi and K. Mizoe, Colloids & Surfaces, 37 (1989) 29.

Measurements of Adsorption on Outer Surface of Zeolite and Their Influence on Evaluation of Intracrystalline Diffusiivity

T. Masuda and K. Hashimoto

Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Honmachi Yoshida, Sakyo-ku, Kyoto 606, Japan

ABSTRACT The influence of the amount of hydrocarbons, which were adsorbed on the outer surface of ZSM-5 zeolite crystallites, on the evaluated intracrystalline diffusivity was investigated. Six kinds of hydrocarbons; n -hexane, n-heptane, n -octane, benzene, toluene and p-xylene, were used as adsorbates. The uptake curves of the amounts of these hydrocarbons adsorbed on high siliceous ZSM-5 zeolites with different sizes and shapes were measured. The amount adsorbed on the outer surface of zeolite crystallites and the intracrystalline diffusivities were evaluated from a theoretical equation and the obtained uptake curves. The magnitude of the amount adsorbed on the outer surface was found to be about 10 to 50% of the total amount adsorbed (zeolite crystal size: 0.1 to 2 pm). The adsorption potential theory of Polanyi was found to well represent the adsorption isotherms considering only the outer surface of zeolite crystallite. The intrinsic uptake curve of the amount adsorbed within the zeolite crystallites was obtained by subtracting the amount adsorbed on the outer surface of the crystallite from the total amount adsorbed. Using the amount adsorbed within the crystallite, the intracrystalline diffusivities were reevaluated. These values were found to be about one-tenth of those evaluated from the uptake curves of the total amount adsorbed.

INTRODUCTION Zeolite catalysts such as ZSM-5 and Y-type zeolites are widely used in various chemical reactions, due to their high activity and selectivity. The high shape selectivity is closely related to the diffusion rate of hydrocarbon molecules [1,2], and could be predicted using the diffusivities of them [3]. Although the diffusivity data at temperatures lower than 373 K have been published for the adsorption processes, there are only a few available data of temperatures higher than 373 K [4-91. Hence, it is necessary to develop the efficient estimation methods to obtain accurate values at high temperature regions. Hashimoto e t d . [10,11] reported that the total amount adsorbed on the zeolite crystallites was the sum of the amount adsorbed on the outer surface of the crystallites and that within the crystallites. In this report, the curve calculated from the theoretical equation was adjusted to fit the uptake curve of the amount adsorbed by varying the diffusivity and the amount adsorbed on the outer surface of the crystallites, M,,and the intracrystalline diffusivity was evaluated. Thus, they measured the diffusivities of hydrocarbons within fresh and coked ZSM-5 zeolite crystallites in 225

226


Figure 1Scaling electron microscopy photographs of ZSM-5 zeolite crystallite: (a) cube, (b) hexagonal slab

the temperature range from 373 to 773 K, and developed methods to estimate them. However, the validity of the introduction of the M, value has not been examined. The main objective of this work is to measure the amount adsorbed on the outer surface of the zeolite crystallite, M,,and examine its reliability, and to investigate the influence of the Ms value on the magnitude of the evaluated diffusivity. The uptake curves of six kinds of hydrocarbons were measured using high siliceous ZSM-5 zeolites with different sizes and shapes by the constant volume method in the temperature range from 373 to 773 K. The diffusivity and the M s value were separately evaluated by adjusting the curve calculated from a theoretical equation to the experimental data. The dependencies of the Ms value on the zeolite crystal size and shape were investigated, and a method to estimate the M s value was developed. Furthermore, the influence of the Msvalue on the magnitude of the evaluated diffusivity was studied.

EXPERIMENTAL Adsorbents Ten kinds of high siliceous HZSM-5 zeolites with different crystal shapes and sizes were

Adsorption on Outer Surface of Zeolite

227

synthesized from silica gel and Na,,SiO, in the temperature range of 433 to 473 K. The geometric properties are listed in Table 1.

cube 2.7

Shape Crystal size, 2L [pm] Outer surface area, amx103[m2.kg-']

I1

2.6

I

0.6

0.7

hexagonal slab 1.1 1.2 1.3 1.4

4.2

3.1

2.2

1.9

1.8

1.7

1.5

1.6

1.9

1.5

1.3

1.1

Figures l(a) and l(b) show typical scanning electron microscopy photographs of the crystallites of cube shape and hexagonal slab shape, respectively. The zeolite crystallites had uniform size and was identified as pentasil zeolite by X-ray diffraction. Measurement of Uptake Curves on Zeolite Crystalhe Three paraffins: n -hexane, n -heptane, n -octane, and three aromatics: benzene, toluene, p xylene were used as adsorbates. Mesitylene was also used as an adsorbate. This hydrocarbon is only adsorbed on the outer surface of the zeolite crystallite, as it can not penetrate into the pores within the crystallite due to its larger molecular size than the pore size. The uptake curves of the amount adsorbed were measured by the constant volume method under the temperature range of 373 to 773 K and pressure range 0 to 1.33 kPa. The experimental procedure was the same as those of Hashimoto et.al. [3,10].

RESULTS AND DISCUSSION Evaluation of Diffusivity and Amount Adsorbed on the Outer Surface of the Crvstallite The uptake curve of the amount adsorbed within the crystallite is expressed by following theoretical equation [12,13]:

M 2a(l-a) '=1-2 M, n=l i t a t a2q:

04 exp[ - 3t ] L2

where M, and Me are the amount adsorbed within the zeolite crystallite at time 1 and that at the equilibrium state, respectively, and D represents the intracrystalline diffusivity. At the beginning of the adsorption process, the adsorption of hydrocarbon molecules on the outer surface of the crystallite occurs rapidly , and an equilibrium state is attained within a short period At, which is relatively short compared with the adsorption time t [14]. This amount is represented by M,. Therefore, the total amount adsorbed on the zeolite crystallite, (MJObS, at time t, which is measured experimentally, can be expressed by the sum of M, and Ms. The observed amount adsorbed at the equilibrium state, (Me)Obs,can be represented by the sum of Me and M,. Thus, the MJM, value can be expressed by

Both values M , and diffusivity D were simultaneously evaluated by adjusting the curve calcu-


228

square root

of

t i me-I aw

1.0 0. 8

0

Eqs. (1) and ( 2 )

0. 6

a2

2

. 0.4 1

VI

-:

ZI

~ =4 xii 0, - '

D =4 .

m2/ s

0XI 0

0.2 0 0

100

Figure 2 Uptake curve of amount adsorbed on crystallite of cubic shape and 2.7 pm in shape (adsorbatep -xylene, T=573 K,p,=92.5 Pa)

200

lated from Eqs.(l) and (2) to the uptake curve using a nonlinear least square method. Figure 2 shows a typical uptake curve of adsorbate, p-xylene. The solid curve was obtained from Eqs.(l) and (2) using the estimated parameters, Ms and D,and is in good agreement with the experimental data. The broken curve, which was obtained without considering the Ms value, does not represent the data. Thus, the amount adsorbed on the outer surface of the crystallite should be taken into account, when the diffusivity is evaluated from the uptake curve of the amount adsorbed. The straight chain line is the result of the square root of time-law, which also does not consider the M s value. The obtained D value is coincident with those reported by many researchers [7-91, and is about ten times larger than that obtained from the solid curve. In this against t f l , yieldmethod, the diffusivity is usually evaluated from the plots of the (Mt)obJ(Me)obs ing a large error in the evaluation, because the M s value is about 10 to 50% of the (MJobs. Adsorption Isotherms on the Zeolite Crystallite of aromatics on the crystallites. The Figure 3 shows typical adsorption isotherms [(MJObS] (Me)obsvalue was well proportional to the equilibrium pressure p,. Figure 4 shows isotherms of adsorption on the outer surface of the crystallite Ms €or aramatics. The M, value was evaluated from the uptake curve of the adsorption. The result for mesitylene, which was only adsorbed on the outer surface of the crystallite is also shown in the figure. The linear relationship between the Ms value and the equilibrium pressure p , was found to hold in the pressure range below 0.8 kPa, as well as the (Me)Obsvalue in Fig.3. The number of the adsorption layer was calculated using molecular size of the adsorbate, and the shape and the size of the crystallite, and is plotted in the figure. The adsorption of aromatics on the outer surface of the crystallite was found to form mono to tri-layer. The linear isotherms were usually observed


229

/

0 w

,”

a

0.06

Figure 3 Adsorption isotherms of aromatics on zeolite crystallite of hexagonal slab shape and 1.3 v m in size (T=573K)

z

0. 0 4 0.02

0 0

200

400

aoo

600

Figure 4 Adsorption isotherms of aromatics on outer surface of zeolite crystallite of hexagonal slab shape and 1.3 v m in size (T=573 K)

-4mulLt

0

200

400

600

800

as long as the number of the adsorption layer was within about three. This is well coincident with the result in this work. The same result as that for aromatics was obtained for paraffins. From Figs.3 and 4, the M, value was found to be about 10 to 50% of the (MJobsvalue, and could not be ignored, when the diffusivity was evaluated from the uptake curve of the adsorption.

-Effect of Crystal shave and Size on the MsValue The M , value per unit weight is dependent on the shape of the zeolite crystallite, which can be expressed by

Ms = (qS/P)(W

(3)

where q, is the amount adsorbed on the outer surface of the crystallite per unit surface are, p is

230

T. Masuda and K . Hashimoto I

I

-

0

I

I

1

0

E I

6

0

0 T

x -

Figure 5 Relationship between amount adsorbed on outer surface of crystallite and crystal size (T=573 K)

4

0)

I

-

2 0 0

2

4

0

6

(SIV) x10-6 [

10

m-'I

the density of the crystallite, and S and Vare the outer surface area and the volume of one single crystallite. When the qs value is only dependent on the equilibrium pressure and temperature, not on the shape and size of the crystallite, the Ms value would show a linear relation to the (SlV) value for each adsorbate. Figure 5 shows the linear relation of the Ms to the (SlV) value for aromatics including mesithylene. The linear relation was also obtained for paraffins. Thus, the M s value was confirmed to be valid, and should be taken into account to evaluate the diffusivity, because the Ms value was about 10 to 50% of the amount adsorbed at the equilibrium state, (MJObs. Estimation of MEValue by Adsorption Potential Theory The adsorption potential theory of Polanyi [15,16] could be applied to the description of the adsorption isotherm on the outer surface of the crystallite. On the basis of the theory, the adsorption potential A, which is the free-energy change from the gaseous to the adsorption state, can be represented by

A = RT.ln@s/pJ

(4)

where T is the absolute temperature, R is the gas constant andp, is the saturated liquid vapor pressure at T. p, was calculated using Antoine's equation. When the adsorption isotherm obeys the adsorption potential theory, the A value is a function of the volume of the adsorbed phase 9, and this function is independent on temperature. The curve, which represents the relation of A to q, is called the characteristic curve. Figure 6 shows the characteristic curve of paraffins, where the data were obtained using the zeolite crystallites with different shapes and sizes under the conditions of the temperature range of 373 to 773 K and the equilibrium pressure below 1.33 kPa. Here, T, is the critical tempera-


231

1o-8

-

1 o-'

N I

E

m

E I

3

10-'*

LA 30

40

50

A [ kJ*rnol

60

Figure 6 Characteristic curve of paraffins (crystal shape) cube:.A hexagonal slab: others

70

-'I

ture. At temperatures higher than T,, Antoine's equation is not available for the estimation of the p c value. However, since there are no items replacing p , above T,, Antoine's equation was conveniently employed above T, in this work. Furthermore, the volume of the adsorbed phase I) was usually calculated from the amount adsorbed by assuming that the molar density of the adsorbed phase was equal to that of the liquid. In super critical region, the molar density would be smaller than that of the liquid and be larger than that of the gas. Therefore, the figure shows two straight lines each corresponding to below T , and above T,, respectively. In the case of aromatics, a similar straight line was obtained. The characteristics curves were found to be formulated as follows: (aromatics) 2C, = 3.2~1O-~exp[ -0.21A ] (paraffins) 2C, = 1.7~1O-~exp[ -O.ISA ] = 7.7xlO-'exp[ -0.16A ]

; T > T, ; T < T,

The data were found to lie on straight lines. The characteristic curves were independent on the kind of the adsorbate, temperature, pressure, and the size and the shape of the zeolite crystalMe. The curves were dependent only on the kind of the adsorbent, namely high siliceous ZSM5 zeolite in this work. When the diffusivity is estimated from the uptake curve of the amount adsorbed, the following procedure should be performed; (a) calculate the adsorption potential A using temperature T and equilibrium pressure p , from Eq.(4), (b) estimate the volume of the adsorption phase 9 from Fig.5, (c) calculate the amount adsorbed on the outer surface of the zeolite crystallite Ms, and substitute it into Eq.(2), (d) calculate the diffusivity D using Eqs.(l) and (2), and the uptake

232

T.Masuda and K. Hashimoto

curve of the amount adsorbed.

CONCLUSION (1) The amounts adsorbed on the outer surface of the high siliceous ZSM-5 zeolite crystallites with different shapes and sizes were calculated from the uptake curves of six kinds of hydrocarbons: n -hexane, n-heptane, n -octane, benzene, toluene and p -xylene under conditions of the temperature range from 373 to 773 K and the pressure range below 1.33 kPa. (2) The linear relationship between the amount adsorbed on the outer surface of the crystallite Ms and the equilibrium pressure was found to hold. (3) The Ms value per unit weight was proportional to the ratio of the surface area to the volume of one single crystallite. This meant that the amount adsorbed per unit outer surface area of the crystallite is independent on the shape and the size of the crystallite. (4) The Msvalue was found to obey the adsorption potential theory of Polanyi. The characteristic curves were obtained for paraffins and aromatics. (5) The Ms value was about 10 to 50% of the total amount adsorbed at the equilibrium state, which was experimentally measured. When the Ms was not taken into account, the estimated diffusivity was found to be about ten times larger than that with the consideration of the M, value.

REFERENCES 1 W.O. Haag, R.M. Lago and P.B. Weisz, Furad.Dis.Chem.Soc., 72(1981)317. 2 N.Y. Chen and W.E. Ganvood, CataLRev.,Sci.Eng., 31(1989)385. 3 K. Hashimoto, T. Masuda and M. Kawase, Shrd.Sur-Sci.Cutuf.,46(1989)485. 4 D.M. Ruthven, M. Eic and E. Richard, Zeolites, 11(1991)647. 5 K. Beschmann, S.Fuchs and L. Riekert, Zeolites, 10(1990)798. 6 A. Zikanova, M. Buelow and H. Schlodder, Zeolites, 7(1987)115. 7 N.V.D. Begin, LV.C.Rees, J.Caro and M.Buelow, Zeolites, 9(1989)287. 8 D. Shen and LV.C.Rees, Zeolites, 11(1991)666. 9 N.V.D. Begin, L.V.C. Rees, J. Caro, M. Buelow, M.Hunger and J. Kaerger, J.Chem.Soc., Farad. Trans.I , 85( 1989)lSOl. 10 K. Hashimoto, T. Masuda and N. Murakami, Stud.Sut$Sci.Cutul., 69(1991)477. 11 T.Masuda, N. Murakami and K. Hashimoto, Chem.Eng.Sci.,47(1992)2775. 12 J. Crank (Ed.), The Muthemutics of D i m i o n , Clarendon Press, Oxford, 1975. 13 D.M. Ruthven (Ed.), Principles of Ahorption & Ahorption Process, John Wiley & Sons, New York, 1984. 14 I. Suzuki, S. Oki and S. Namba, J.Cutul., 100(1986)219. 15 W.K. Lewis, E.R. Gilliland, B. Chertow and W.P.Cadogan, IndEng.Chem., 42(1950)1326. 16 G. Halsey, J.Chem.Phys., 16(1948)931.

Interpretation of Xenon Adsorption Isotherms and Xe-l2!l NMR Chemical Shifts on Ion-exchanged NaY Zeolites

S.B. Liu,' C.S. Lee,' P.F. Shiu,' and B.M. Fung2

'Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10764, R.O.C. 2Department of Chemistry, University of Oklahoma, Norman, OK 73019-0370, U.S.A.

ABSTRACT A comprehensive study of the effect of cation substitution on the adsorption of xenon on NaY zeolites and on 129Xe chemical shifts has been made. The cations studied include proton, alkali, alkaline, and transition-metal, each with varied degree of exchange with Na'. The results of the adsorption isotherms were analyzed by Langmuir-type equation. The observed 129Xe chemical shifts of the adsorbed xenon were interpreted by virial expansion model which treat the adsorbed xenon as twodimensional gas, and successfully correlate the observed chemical shifts with xenon adsorption strength. INTRODUCTION 129Xe NMR of xenon adsorbed on zeolite has proven to be a sensitive probe of its local environment due to its chemical inertness and excellent sensitivity [l]. Numerous applications can be found in this area and the subject has been reviewed recently [2,3]. A systematic investigation of the effect of the cation on the chemical shift of 129Xe adsorbed on zeolites has been carried out by Fraissard and co-workers [2,4]. They suggested an explanation of the 129Xe chemical shift in terms of different contributions from Xe-Xe interactions and Xe-zeolite interactions. Cheung et al. [5] have offered a more quantitative interpretation by the use of a model in which the Xe atoms adsorbed on the wall of the superca.ge are in rapid exchange with those in the gaseous phase. However, neither group has treated the 129Xe chemical shift data quantitatively to obtain parameters related to the nature of the cations. We have carried out a detailed study of the effect of the substitution of Na' in zeolite NaY by other cations, the parameters obtained from regressional and non-linear least-square fittings of both the adsorption isotherm data and the 129Xe chemical shift data are discussed in terms of the effect of the cations on various interactions of the Xe atoms with the zeolite supercage. 129Xe NMR were done at 83.012 MHz [6]. 233

234

S. B. Liu, C. S. Lee, P. F. Shiu, and B. M. Fung

RESULTS AND DISCUSSION Xenon adsomtion isotherms Xenon adsorption isotherms on zeolite NaY and its relatives (MNaY), with Na+ partly replaced by various cations, are exemplified in Figs. 1-3. A quantitative interpretation of the Xe adsorption isotherms were done following Cheung et al. (71:

where N is the number of Xe atoms adsorbed in each zeolite supercage, V is the free volume of the cage, b is the adsorption strength, Ns is the total number of available sites per supercage, P is the equilibrium pressure. The value of r = (kT)-'= 3.2734 x atom nm-3 Torr-' for T = 295 I Fe B Zn as expressed by the kind of metal incorporated. However, the result of reaction, as described above, indicates much larger difference than that expected from the qualitative information of the NH3-TPD. In order to elucidate more precise difference in acidic property among those metallosilicates, we newly studied on the computer simulation of NH3 adsorption on the acidic sites of different kind of metallosilicates by applying Monte Car10 method and estimated the precise position of adsorption sites and amount of NH3 adsorbed, and the results were The positions of isomorphous substitution were visualized by computer graphics [24].

270

T. Inui

2.0

-

r

,

1

I .5

E

5 -

C

v

.!2 K

4 -

1.0

4 4

-a

3 -

2 -

0.5

I - 0

.' - '

J

-

d '

"=r'

0 0.2 0.4 0.6 0.8 1.0 1.2

C Axis (nm) (A) View of A-C face. (B) Adsorbed amounts of NH3 expressed on open circulars : postulated positions of Al. C axis. closed circulars : positions and amounts of NH3 adsorbed. Fig. 7. Simulated distribution of NH3 adsorbed on the unit cell of an Al-silicate (300K).

postulated as follows; among 96 silicon atoms in a unit cell of MFI, the most stable positions, which are obtained by quantum chemistry calculation, 8 silicon atoms of the T12 site were selected considering the Loewenstein rule as the object for calculation. Transition elements, Al, Ga, Fe, and Zn were chosen as the metal ions to be replaced. A molecule of NH3 was set at a certain position of the domain of the MFI unit cell according to the Monte Carlo method, and its potential energy was calculated. This calculation was repeated for other NH3 molecules, and up to lo6 NH3 molecules, which had lower potential energies were selected. The calculation was performed by graphic super computer TITAN 3000V with a software of Cerius for personal Iris 4D35. POLYGRAF was used for the calculation of electron change of metallosilicate framework and NH3 for applying to Monte Carlo method. As shown in Figs. 7 (A) and 7 (B), the Al-silicate strongly adsorbs NH3 on the very limited sites, typically on the site indicatedas a. A Ga-silicate showed fairly similar feature but different position from that of the Al-silicate. On the contrary, as shown in Figs. 8 (A) and 8 (B), in the Fe-silicate the adsorption sites for NH3 deviated more widely and the amounts of adsorbed NH3 on distinguished sites were much smaller than that on the strong adsorption site of the Al-silicate. This feature was more evident on Zn-silicate.

Novel Catalytic Functions of Metallosilicates

271

2.0

1.5 n

B E

v

-31.0

4

6 0.5

- 0

0 0.2 0.4 0.6 0.8 1.0 1.2

C Axis (nm) (A) View of A-C face. open circulars :postulated positions of Al. closed circulars : positions and amounts of NH3 adsorbed.

0

0.4

1.2

0.8

C Axis (nm) (B) Adsorbed amounts of NH3 exmessed on C axis. -

x

Fig. 8. Simulated distribution of NH3 adsorbed on the unit cell of an Fe-silicate (300K).

The results obtained by computer simulation and the figures depicted by computer graphics strongly support the results qualitatively obtained by NH3-TPD and the acid catalyzed reactions on those metallosilicate catalysts, and give a significant insight to the design of metallosilicate catalysts. CONCLUSIONS Isomorphous substitution of transition elements for Al of MFI type zeolite brought a wide variety of unique catalytic functions. The reasons of those novel catalytic functions are ascribed to the change in the location and strength of acid sites in the crystal domain and the intrinsic catalytic properties of the substituted elements. Stability of substituted elements were much more larger than that prepared by ion-exchanged method. The balance of acid strength and redox property of metallosilicate can be applied other unsolved but important problems such as deN& in the exhaust gas from diesel engine. It was found that H-Co-silicate is the best catalyst for this purpose [25], however extension to other silica-rich metallosilicates such as zeolite type would become important from the view points of both larger diffusivity and thermal stability [26].

272

T. I n u i

REFERENCES 1 S.L. Meisel, J.P. McCallough and C.H. Lechtzler, CHEMTECH, (1976) 86. 2 C.D. Chang, J.C.W. Kuo, W.H. Lang, S.M. Jacob, J.J. Wise and A.J. Silvestri, I. E. C. Proc. Dec. Dev., 17 (1978) 255. 3 D. Liedman, S.M. Jacob, S.E. Voltz and J.J. Wise, I. E. C. Proc. Dec. Dev., 17 (1978) 340. 4 C.D. Chang, W.H. Lang and A.J. Silvestri, J. Catal., 56 (1978) 268. 5 T. Inui, M. Takaki, T. Hagiwara and M. Yosikawa, Stud. Surf. Sci. Catal., 34 (1987) 639. 6 T. Inui, H. Nagata, H. Matsuda, J.-B. Kim and Y. Ishihara, Ind. Eng. Chem. Res., 31 (1992) 995. 7 F.X. Cormerais, G . Peret and M. Guisnet, Zeolites, 1 (1981) 141. 8 P. Dejaifue,A. Auroux, P.C. Gravelle and J.C. Vedrine, Appl. Catal., 70 (1981) 123. 9 T. Inui, J. Japan Petrol. Inst., 33 (1990) 198. 10 C.D. Chang, Catal. Rev. Sci. Eng. 25 (1983) 1. 11 E.G. Derouane, J.B. Nagy, P. Dejaifve, J.H.C. van Hooff, B.P. Spekman, J.C. Vedrine and C. Naccache, J. Catal., 53 (1978) 40. 12 T. Inui, Stud. Surf. Sci. Catal., 44 (1989) 189. 13 W.O. Haag, R.M. Lago and P.B. Weisz, Nature, 309 (1984) 589. 14 T. Inui, 0. Yamase, K. Fukuda, A. Ito, J. Tammoto, N. Morinaga, T. Hagiwara and Y. Takegami, Proc. 8th Intern. Congress on Catalysis, Berlin July 2-6, 1984, Verlag chemie, 1984, Vol. 111, p. 569. 15 T. Inui, ACS Symp. Series 398 (1989) 479. 16 T. Inui, H. Nagata, T. Takeguchi, S. Iwamoto, H. Matsuda and M. Inoue, J. Catal., 139 (1993) 482. 17 T. Inui, Y. Makino, F. Okazumi,S. Nagano and A. Miyamoto, Ind. Eng. Chem. Res., 26 (1987) 647. 18 T. Inui, Y. Makino, F. Okazumi and A. Miyamoto, Stud. Surf. Sci. Catal., 37 (1987) 487. 19 T. Inui, H. Matsuda, T. Takeguchi and M. Chaisupakitsin, Proc. 2nd Japan-Korea Symp. on Catal, Tokyo, March 17-18,1989, Tokyo Inst. Tech., 1989, p. 19. 20 T. Inui and F. Okazumi, J. Catal., 90 (1984) 366. 21 T. Inui, React. Kinet. Catal. Lett., 35 (1987) 227. 22 T. Inui, H. Matsuda, 0. Yamase, H. Nagata, K. Fukuda, T. Ukawa and A. Miyamoto, J. Catal., 98 (1986) 491. 23 T. Inui, T. Takeguchi, A. Kohama and K. Tanida, Energy Convers. Mgmt., 33 (1992) 513. 24 T. Hattori, N. Goto, Y. Nakazaki, M. Inoue andT. Inui, Preprints 65th Annual Meeting of Chem. SOC.Japan, I (1993) 435. 25 T. Inui, S. Iwamoto and S. Shimizu, Proc. 9th Intern. Zeolite Confer. Montreol, July 5-10, 1992, Butterworth-Heinemann, I1 (1993) 405. 26 K. Matsuba, Y. Tanaka, N. Goto, Y. Nakazaki and T. Inui, 65th Annual Meeting of Chem. SOC.Japan, I (1993) 436.

Selective Synthesis of Ethylenediamine from Ethanolamine and Ammonia over Zeolite Catalysts

K. Segawa*, S. Mizuno, Y. Fujimoto, and H. Yamamoto Department of Chemistry, Faculty of Science and Technology, Sophia University 7-1 Kioi-cho Chiyoda-ku, Tokyo 102, Japan

ABSTRACT The synthesis of ethylenediamine from ethanolamine with ammonia over acidic type of zeolite catalysts were investigated. Among the zeolites tested in this study, the protonic form of mordenite catalyst was the best catalyst: at 603 K, W/F=200 g h mol-1, and NH3/EA=50. The reaction proved to be highly selective for ethanolamine over H-mordenite, with small amounts of ethyleneimine and piperazine derivatives as the side products. The results suggest that the reaction for the formation of ethylenediamine from ethanolamine required the stronger acidic sites in the mordenite channels with higher yield and selectivity. INTRODUCTION Ethylenediamine (EDA) is made by ammonolysis of ethylene dichloride with ammonia [EDC process], or by reductive amination of ethanolamine (EA) with hydrogen and ammonia [MEA process] on a commercial basis. Disadvantages of the EDC process include difficulties in controlling higher selectivity of the EDA that contains polyamines as the side products, and corrosion associated with chlorine atoms [l]. On the other hand, the MEA process includes high pressure reactions (10-20 MPa) over transition metal catalysts, and shows lower selectivity for EDA [2]. As an alternative process for EDA synthesis, acid catalyzed amination of EA under atmospheric pressure has been studied. In order to suppress the formation of bulkier by-products such as piperazine derivatives, acidic forms of zeolite catalysts have been studied. METHOD Catalysts. Zeolites (JRC-Z) and silica-alumina (JRC-SAL-2, Si/AI=S.3) samples were supplied by the Catalysis Society of Japan (JRC: Japan Reference Catalysts). Three different types of zeolites were studied: Na-FAU (faujasite, JRC-Z-Y5.6, Si/AI=2.8), Na-MOR (mordenite: JRC-HMlO, Si/Al=S; JRC-M20, Si/AI=lO), and Na-MFI (ZSM5, JRC-ZS-25, Si/AI=12.5). KLTL (Linde type L, HSZ-SOOKOA, Si/AI=3.0) was supplied by TOSOH Co. Acid-type zeolites were prepared by ion-exchange of Na-form or K-form of zeolite with aqueous solution of

273

274

K. Segawa, S. Mizuno, Y . Fujimoto and H . Yamamoto

NH4NO3; ion-exchanged samples were dried at 373 K for 24 h and then calcined in the furnace at a

constant temperature increase (1 K min-1) from 373 K to 773 K and kept at 773 K for 5 h. Catalytic reactions. The reaction was camed out at 543-643 K by using a flow reaction system with a mixture of EA, N H 3 , and N 2 in the ratio of 1/50/25 under atmospheric pressure. The reaction products were analyzed by an on-line gas chromatograph (FLD) which was equipped with a 30-mcapillary column (TC1701). Adsorption measlrrenietits. Chemisorption of base molecules on acidic zeolites was confirmed by IR spectroscopy and high-temperature calorimetry. A vacuum-tight IR cell with KBr windows was designed to fit an infrared spectrometer (270-30, Hitachi) and to be attached to a vacuum system (10-4 Pa). The cell was arranged such that the zeolite wafer could be lowered into slots behveen the optical windows, and withdrawn upward by the action of a magnet into the heated portion for the pretreatment and adsorption of NH3 and EA. After evacuation at 773 K for lh, the zeolite sample was cooled to 373 K before adsorption of the base molecules to be studied. IR spectra were obtained at room temperature. High-temperature micro-calorimetry of N H 3 on zeolite catalyst was obtained by the calorimeter (HAC-450G, Tokyo Rikou). Each sample (1.5g) was charged into the calorimeter, and evacuated at 673 K for 4 h. N H 3 (15 mmol g-1)was admitted dose after dose at 473 K. RESULTS AND DISCUSSION Table 1 . Catalytic Activities and Selectivities of EDA synthesis* over various zeolite catalysts Pore Size Catalyst***

Si/A

/MI

Conversion P-%J

Selectivity** /% EDA

EI

PA

Others

7 69 76 32 7 4

49 13 9 21 9 10 31

43 14 9 21 3 8 31

i__-_______-________-______--.

Si02-AI203 H-CHA H-FAU H-LTL H-MOR H-MOR H-MFI

5.3 2.2 2.8 3.0 5.0 10.0 12.5

--

100

1

0.38 0.74 0.71 0.70 0.70 0.54

4 6 15 42 35 23

4 13 23 81 78 36

13

*Reaction Conditions: Tempenture=603 K, W/F=200 g h niol-l, NH3EA=50 ** EDA:ethylenedianiine, EI: ethyleneiniine, PA: pipemine dcnvatives *** CHA: chabazite, FAU: faujasite 0,LTL: Linde type L, MOR: mordenite, MFI: ZSMS

The catalytic activity and selectivity of EDA synthesis from EA and N H 3 over various zeolite catalysts are shown in Table 1 . Among the various solid acids, the protonic form of mordenite (H-MOR) catalyst showed the highest selectivity for EDA. The selectivity of EDA was about 80 % at 42 % of EA conversion in the presence of an excess amount of ammonia (NH3/EA=50). Small amounts of ethyleneimine (EI) and piperazine (PA) derivatives were formed as by-products. When the reaction was carried out over some other solid acid catalyst, such as

Selective Synthesis of Ethylenediamine

275

amorphous silica-alumina, 100 % of EA converted to PA and other oligomers (polyamines). On H-CHA (chabazite), H-FAU (faujasite-Y), and H-LTL &-type), only a small amount of EDA was formed. The major product was EI, and PA or other polyamine oligomers were formed. On HMFI (ZSMS) catalyst, the major products were PA and other higher polyamines. Figure I shows the reaction scheme for EDA synthesis and by-products

Main reaction H+

NH'\/\OH

+ NH3

* N H z m N H , + H20

EA

E DA

Side products

c

NH

HNANH U Nn J' N

ethylene imine

El

piperazine 1,4-diazabicyclooctane

U

NHZ4N-OH H

aminoethylpiperazine

aminoethylaminoethanol

Fig. 1. Reaction scheme and side products for EDA synthesis from EA and N H 3 160

-

0

8-

2

120

K

.-0

P 8

80 0 w

m Q

I 40 0.0

1 0.5

I 1 .o

IH-CH I A~ 1.5

2.0

2.5

NH3 adsorbed lmmol g-'

Fig. 2. High-temperature micro-calorimetry of N H 3 on various H-zeolites: N H 3 adsorbed at 7 K, Si/AI; H-CHA 2.2, H-FAU 2.8, H-MOR 5.0, H-MFI 12.5. The results (Table 1) suggest that the selective synthesis of EDA from EA and N H 3 requires stronger acidic sites in the limited pore channels in order to suppress the formation of bulkier PA derivatives and polyamines. Among the zeolite catalysts in this study, H-MOR showed much

276

K . Segawa, S. Mizuno, Y. Fujimoto and H . Yamamoto

higher acid strength (140 W mol-l) than those of other types of zeolites, as determined by hightemperature micro-calorimetry of NH3, the results are shown in Fig. 2. Deeba and coworkers [3] reported that the dealuminated H-MOR showed higher selectivity for EDA at lower conversion region: about 60 % selectivity at 30 % conversion (NH3/EA=l6). However, at higher conversions, selectivity for EDA was not high enough. In this study, if the reaction conditions included longer contact time (see Fig. 3: W/F=500 g h mol-I), conversion exceeds about 95 % of EA with 80 % selectivity of EDA. The time courses of EDA synthesis over H-MOR catalyst at 603 K (PEA=1.4 kPa, NH3/EA=50) are shown in Fig. 3. The initial product of reaction was EI, and the formation of EDA followed. However, the selectivity of EDA did not exceed 85 % at higher conversion region. The selectivity of PA derivatives increased with increasing contact time. The results suggest that the intramolecular condensation of EA occurred at the initial stage of reaction to produce EI, then EI was activated by the stronger protonic acid sites to produce EDA. The activation of EI over the stronger acidic sites is the rate-determining step for this reaction, the protonated EI may convert to EDA with presence of NH3. Intermolecular condensation of EA to form PA derivatives may occur at weaker acidic sites, which may locate on the external surfaces of mordenite crystals.

n " 0

100

200 300 400 500 600 WIF /h g m o i l

Fig. 3 . EDA synthesis on H-MOR (Si/AI=5.0) as a function of contact time: Reaction conditions; temperature=603 K, NH3/EA=50, Pm=1.4 kPa.

R spectroscopy (Fig. 4) suggested that The adsorption studies of EA or NH3 on H-MOR by I the reaction may proceed through ammonio-ion of EA over protonic acid sites to produce an EI intermediate. When H-MOR was exposed to 0.3 kPa of EA and evacuated at 473 K (Fig. 4A), NH3+ deformation bands were built up at 1597 cm-I and 1497 cm-l together with CH2-N+ deformation band at 1471 cm-I. At 1372 cm-I and 1324 cm-1 wave number regions (Fig. 4A), OH deformation bands were observed. The results suggest that EA is protonated and adsorbed as ammonio-ion of EA (NH3+CH2CH20H) on H-MOR, and not adsorbed as an oxonium-ion (NH2CHzCHzOH2+).

Selective Synthesis of Ethylenediamine

277

1497 1471

A I

1750

I

I

I

I

1500

1200

1750

Wave numbers /ern-'

1500

I

1200

Wave numbers /cm-'

Fig. 4. IR spectra of adsorbed EA and NH3 on H-MOR (Si/AI=S.O): (A) H-MOR exposed to 0.3 kPa of EA at 473 K and evacuated at 473 K, (B) evacuated at 523 K, (C) evacuated at 573 K, (D) evacuated at 623 K, (E) H-MOR exposed to 0.3 kPa of NH3 at 473 K and evacuated at 473 K, (F) after recording spectrum E, the sample was exposed to 0.3 kPa of EA at 473 K and evacuated at 473 K, (G) evacuated at 523 K, (H) evacuated at 623 K. When the ammonio-ion on H-MOR was evacuated at higher temperature (Fig. 4B-4D), the adsorption species were changed to secondary amines. The deformation bands of ammonio-ion are shifted to lower wave numbers: NH2+ deformation bands build up at 1600 cm-', together with CH2 deformation band at 1445 cm-I. The intensities of OH deformation bands (1370 cm-', 1324 cm-l) decreased with increasing evacuation temperature. The results suggest that the ammonio-ion transformed to protonated EL The intramolecular condensation of EA occurred at the initial stage of the reaction to produce EI; then EI was activated by the stronger protonic acid sites to produce EDA. The secondary protonated amines are strongly held at the surfaces of H-MOR. When H-MOR was exposed to 0.3 kPa of NH3 at 473 K and evacuated at 473 K (Fig. 4E), only protonated ammonia (NH4+)was observed: deformation band builds up at 1443 cm-1, together with a small amount of coordinated bond N H 3 (deformation) that attached to Lewis acid sites at 1624 cm-l. The major acidic sites on H-MOR are Bransted sites, as determined by pyridine adsorption studies: about 80 % of acidic sites are Bransted sites and the rest are Lewis acid sites [4, 51. After adsorption of NH3, 0.3 kPa of EA are admitted on H-MOR at 473 K (Fig. 4F): adsorbed NH3 is easily replaced by EA to produce deformation bands of NH3+(1597 cm-1, 1497 cm-I), CH2N+ (1471 cm-I), CH2 (1471 cm-I), and OH (1370 cm-1, 1324 cm-1). This spectrum is the same as the spectrum Fig. 4B. The results suggest that adsorption of EA is much stronger than that of NH3. When adsorbed EA is heated up to 623 K (Fig. 4G, Fig. 4H), the spectra are almost the same as the spectra in Fig. 4 8 and Fig. 4D.

m+

278

K . Segawa, S. Mizuno, Y. Fujimoto and H . Yamamoto

CONCLUSION The synthesis of EDA from EA with NH3 over an acidic type of zeolite catalyst was investigated. Among the zeolites tested in this study, H-MOR was the best catalyst. The reaction proved to be highly selective for EA over H-MOR with small amounts of EI and PA derivatives as the side products. The reaction obeyed first order kinetics with respect to the partial pressure of NH3. The initial product of reaction was EI, and the formation of EDA followed. The reaction pathways for the formation of EDA from EA are summarized in Fig. 5.

..

EA

PA

NH 2*NH2

EDA

Fig. 5. Reaction scheme of EA synthesis from EDA and NH3 over H-MOR. The results suggest that the reaction for the formation of EDA reqdired the stronger acidic sites in the mordenite channels with excess amounts of NH3. The mordenite channels may retard the formation of bulkier PA derivatives and other polyamines. The reactions of EA proceed through ammonio-ions by the addition of protons of H-MOR, then intramolecular condensation of EA occurred to produce EI intermediate. EI was protonated by the stronger acidic sites to produce EDA. REFERENCES 1 S . Kumoi, N. Kubota, and T. Hiroi, Kugaku Keizni, 1 1 (1985) 56. 2 J. R. Ninters, U.S. Pat., 4 404 405 (1983). 3 M. Deeba, M. E. Ford, T. A. Johnson, and J. E. Premecz, J. Mol. Catnl., 60 (1990) 1 1 4 K. Segawa, M. Sakaguchi, and Y .Kurusu, Stlid. Swf: Sci. Catnl., 36 (1988) 579. 5 K. Segawa, and H. Tachibana, J Cnlnl., 131 (1991) 482.

Para-Selectivity of Zeolites and Metallosilicates with MF'I Structure

S. Nambal, J.-H. Kim2 and T. Yashima3 1Department of Materials, The Nishi-Tokyo University Uenohara-machi, Kitatsuru-gun, Yamanashi 409-0 1, Japan 2National Institute of Materials and Chemical Research Higashi, Tsukuba, Ibaraki 305, Japan 3Department of Chemistry, Tokyo Institute of Technology Ookayama, Meguro-ku, Tokyo 152, Japan

ABSTRACT The reason for t h e generation of para-selectivity of zeolites and metallosilicates with MFI structure for alkylation of alkylbenzenes is distinct from that for disproportionation. In t h e alkylation, the weaker acid sites on the modified zeolites and metallosilicates provide the higher para-selectivity, because in the narrow pores of t h e MFl structure the primary product is only p-dialkylbenzene and t h e secondary reaction, isomerization, is suppressed to some extent through 'restricted transition-state selectivity' and requires strong acid sites t o t a k e place, compared with the alkylation. In t h e disproportionation, t h e isomerization takes place readily under such severe reaction conditions; hence 'product selectivity' is indispensable for high para-selectivity.

INTRODUCTION It is widely known that HZSM-5 zeolites modified with oxides exhibit a high paraKaeding selectivity for alkylations [ 1-41 or disproportionation [5-71 of alkylbenzenes. e t al. proposed that t h e high para-selectivity of modified HZSM-5 zeolites for the alkylation [Z] and t h e disproportionation [5,6] was due to 'product selectivity', namely t h e intracrystalline diffusivity of p-isomer was much higher than that of t h e other two isomers. Olson and Haag reported t h e evidence for diffusion control of paraselectivity solely in disproportionation of toluene on modified HZSM-5 zeolites [7]. Paparatto et al. reported that p-isomer formed selectively inside t h e ZSM-5 channels in t h e alkylation of toluene with ethanol, while t h e isomerization of p-isomer proceeded just on t h e external surfaces and that t h e improvement in para-selectivity by t h e modification was due to t h e inactivation of t h e acid sites on t h e external surfaces [B]. On t h e other hand, w e proposed that t h e primary product in t h e alkylation was only the p-isomer due to 'restricted transition-state selectivity' and t h a t t h e improvement in para-selectivity by the modification of HZSM-5 with oxides was due to the suppression of the secondary reaction, i.e., the isomerization of primarily produced pisomer [3,4]. This means that the modification results in a reduction in acid strength of t h e catalytic sites on which t h e isomerization is accelerated more preferentially 219

280

S. Namba. J.-H. Kim and T. Yashima

than the alkylation solely in t h e narrow pores. Recently, a proposition similar to ours has been reported by many researchers 191. In this study, we aim t o clarify t h e reason why the modified HZSM-5 zeolites and metallosilicates with MFI structure exhibit a high para-selectivity for the alkylation and t h e disproportionation.

EXPERIMENTAL Catalyst. HZSM-5 (Si/Al=96) [ 11, ferrisilicate (Fe-MFl, Si/Fe=56) [lo], borosilicate (B-MFI, Si/B=70) [ 111, chromosilicate (Cr-MFl, Si/Cr=260), gallosilicate (Ga-MFI, Si/Ga=64) 1121 were prepared hydrothermally by published procedures. Antimonosilicate (Sb-MFI, Si/Sb=l20) and arsenosilicate (As-MFl, Si/As=92) were prepared by t h e atomplanting method [ 13,141. T h e HZSM-5 and metallosilicates (Me-MFI) modified with Mg, P or B oxide (Mg, P or B(x)HZSM-5 and B(x)Me-MFI, where x was t h e amount of Mg, P or B added) were prepared by t h e impregnation method [3,4,151. HZSM-5 zeolites steamed a t 1073 or 1223 K for 1 h (Stm.(1073 or 1223)) and coked by treating with methanol a t 973 K for 1 or 20 h (Coked (1 or 20h)) were prepared according to a method described elsewhere [3,4,16]. Determination of pore tortuosity. Gravimetric measurements of 0- or p-xylene adsorption were performed on a highly sensitive thermal microbalance. From t h e results of 0- and p-xylenes adsorption experiments [4,14-16,20,21], we determined 'time t o reach 30 O/o of amount of o-xylene adsorbed a t infinite time', t0.3, as a parameter of pore tortuosity. W e also determined 'relative o-xylene adsorption velocities' as another parameter of pore tortuosity. The relative o-xylene adsorption velocity, VROA, is defined as follows;

VROA

= (Amount of o-xylene adsorbed a t 180 min)

/(Amount of p-xylene adsorbed at infinite time), where the amount of p-xylene adsorbed a t infinite time may correspond to the pore volume. Determination of acid strength. In general, t h e peak position in NH3-TPD corresponds intrinsically to the acid strength of catalysts, but sometimes shifts t o higher In temperatures due to the readsorption of ammonia and/or t h e diffusion limitation. this study, NH3-TPD measurements were performed using a very small amount (18 mg) of catalysts under vacuum to minimize t h e readsorption effect. Moreover, t h e unity of ammonia initial coverage was probably attained, because we have reported t h a t the absolute amount of ammonia desorbed from HZSM-5 zeolite coincides with the amount of the framework aluminum under such NH3-TPD operation conditions [17]. We used t h e peak position in NH3-TPD, Tmax, as a parameter of the acid strength of catalysts [4,14-16,20,21]. Para-selective reactions. The alkylations of ethylbenzene with ethanol and of toluene with methanol at 673 K and the disproportionation of toluene at 823 K were carried out with a continuous flow system under atmospheric pressure. The alkylation of ethylbenzene with ethanol was also carried out in t h e presence of 2,4-dimethylquinoline (2,4-DMQ), which selectively poisoned t h e acid sites on t h e external surfaces [18]. The cracking of 1,3,5-triisopropylbenzene (1,3,5-TIPB) was performed in t h e presence of 2,4-DMQ t o confirm t h e inactivation of the external surfaces [19]. In this paper, t h e para-selectivity is defined as a fraction of p-isomer in t h e dialk y lben zene produced.

Para-Selectivity o f Zeolites and Metallosilicates

281

RESULTS AND DISCUSSION Alkylation Para-selectivities of various catalysts. In order t o compare the para-selectivities of various catalysts, t h e para-selectivities a t a n almost constant yield of diethylbenzene (15 - 20 O/O) or xylene (19 - 22 %), t h a t is, at a n almost constant alkylation activity, were determined and a r e summarized in Table 1. Almost constant alkylation activity was achieved by adjusting W/F. In the alkylation of ethylbenzene with ethanol, t h e para-selectivities of Me-MFI catalysts were higher than those of HZSM-5. In particular, As-MFI exhibited a high para-selectivity of 94 %. The para-selectivities of HZSM-5 and Me-MFI were improved by modification. In particular, B( lO)HZSM-5, B(5)Ga-MFI and B( 1)Sb-MFI exhibited perfect para-selectivity, 100 %. The order of the para-selectivities of various Me-MFI catalysts for t h e alkylation of toluene with methanol was exactly t h e s a m e as t h a t for t h e alkylation of ethylbenzene with ethanol. Primary product in alkylation In order t o clarify t h e primary product in the alkylation of ethylbenzene with ethanol on HY (Tosoh Co., Lot Y-30), HZSM-5 and SbMFI zeolites, t h e change in the distribution of diethylbenzene isomers with decreasing WIF was determined. Table 1. Para-selectivities for ethylation of ethylbenzene, methylation of and disproportionation of toluene on various catalysts with MFI structure.

No

Catalyst

1 HZSM-5

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

B( 1)HZSM-5 B(3)HZSM-5 B(5)HZSM-5 B(9)HZSM-5 B(lO)HZSM-5 P( 1)HZSM-5 P(5)HZSM-5 Mg( 18)HZSM-5 Stm.(1073) Stm.( 1223) Coked ( I h ) Coked (20h) Ga-MFI Fe-MFI B-MFI Cr-MFI Sb-MFI AS-MFI B(O.5)Ga-MFI B( 1)Ga-MFI B(3)Ga-MFI B( 5)Ga-MFI B(0.5)Sb-MFI B( 1)Sb-MFI B(3)Sb-MFI

Ethylation DEB paraYield/% Selec./%

Me thy lation Xylene paraYield/% Selec./%

19.8 17.9 18.8 18.1 18.3 17.8 20.1 16.3 19.9 15.7 15.1 16.2

43.2 48.9 55.3 71.8 97.1 100.0 49.3 89.8 72.4 68.9 84.5 64.2

20.9

16.9 16.5 16.6

59.6 61.0 70.3

21.6

51.9

15.0 15.3 18.5 17.5 16.7 17.1 16.1 15.2 15.6

90.4 94.1 67.1 77.3 94.3 100.0 95.8 100.0 100.0

19.1 20.2 19.8 20.7

55.0 68.5 71.7 81.8

45.0

toluene,

Disproportiona tion Xylene paraYield/% Selec./% 4.7 4.9 4.9 4.3 4.6 4.2

24.2 24.2 25.8 27.9 32.2 51.0

4.8 4.3 4.7 4.9 4.6 5.0 4.6 4.4

24.3 53.4 24.0 24.6 26.0 27.8 24.0 26.1

S. Namba, J.-H. Kim and T. Yashirna

282

(a) HY 1OD

(b) HZSM-5

(c) Sb-MFI

h

w---*-------.-

\

para-

t

0.0

0.5

log(ltW/F)

0.0

0.0

log(1tw/!=)

- -

meta0.5

7 .O

log(1+WE)

Fig. 1.Change in fraction of each diethylbenzene isomer with W F in ethylation of ethylbenzene on HY at 548K(a), on HZSM-5 at 673K(b) and on Sb-MFI at 673K(c). The results on HY a t 548 K in t h e diethylbenzene yield range of 4 1 t o about 4 OO/ a r e shown in Fig. l(a). The fraction of m-isomer decreased with decreasing W/F. In the case of HY catalyst, t h e primary product is not clear, but may be p- and oisomers. HY is not thought t o exhibit shape selectivity for this alkylation and the para/ortho orientation for this alkylation generally predominates. The results on H E M - 5 in the diethylbenzene yield range of 26 t o about 3 OO/ are shown in Fig. l(b). Although t h e molecular dimension of o-isomer was almost t h e This same as that of m-isomer, little o-isomer was observed in these experiments. suggests t h a t t h e HZSM-5 catalyst may not exhibit product selectivity due to configurational diffusion effects. Instead i t exhibits transition-state selectivity due t o transition s t a t e restrictions. The formation of o-isomer through t h e alkylation of ethylbenzene with ethanol and through t h e isomerization of diethylbenzene produced is accordingly suppressed. The fraction of p-isomer increased t o 100 %, and t h e fraction of m-isomer decreased t o 0 % when W / F was decreased to 0. These results clearly indic a t e that t h e primary product of this alkylation on HZSM-5 is only p-diethylbenzene. The primary product on HZSM-5 is different from that of HY because of the transition-state selectivity of HZSM-5. From the results of t h e alkylation of toluene with methanol on HY and HZSM-5, we also found that p-xylene was t h e only primary product on H E M - 5 111. In t h e alkylation of ethylbenzene with ethanol on Sb-MFI, clearer results were obtained than those on HZSM-5 as shown in Fig. I(c), that is, these results clearly indicate t h a t t h e primary product in t h e alkylation on Sb-MFI is only p-diethylbenzene. Therefore, for t h e selective formation of p-isomer in alkylation, t h e secondary reaction, i.e., t h e isomerization of p-isomer produced as a primary product, must be suppressed. Selective poisoning of external surfaces In order t o clarify t h e effect on paraselectivity of t h e acid sites on t h e external surfaces, t h e alkylation of ethylbenzene with ethanol in t h e presence of 2,4-DMQ whose molecular dimension was too large t o e n t e r t h e pores [18] was carried out. The complete poisoning of t h e acid sites on t h e external surfaces was confirmed by t h e inactivation of the catalysts for t h e cracking of 1,3,5-TIPB, which was a suitable probe molecule for determining t h e activity of t h e external surfaces [ 191. The H E M - 5 and Me-MFI catalysts poisoned with 2,4-DMQ were completely inactive for 1,3,5-TIPB cracking. The para-selectivities of HZSM-5

Para-Selectivity of Zeolites and Metallosilicates

283

and Me-MFI catalysts were t o some extent improved by selective poisoning. However, on these catalysts, a very high para-selectivity was not achieved by selective poisoning of the external surfaces [4,20]. These results indicate t h a t a very high paraselectivity requires not only the inactivation of the external surfaces but also the suppression of t h e isomerization which proceeds even inside the narrow pores. Para-selectivity for alkylation and pore tortuosity. The relationship between the para-selectivity for t h e alkylation of ethylbenzene with ethanol and t h e pore tortuosity, t0.3 and VROA, is shown in Figs. 2(a) and 2(b), respectively. In the c a s e of HZSM-5 modified with oxides, a close relationship is observed as shown in Fig 3(b). However, the para-selectivities of Me-MFI zeolites as well as steamed and coked HZSM-5 zeolites a r e relatively high compared with t h e pore tortuosities. Thus, we could not find a close relationship between the para-selectivities and t h e pore tortuosities for all t h e catalysts examined here. From t h e results in Figs. 2(a) and 2(b), it is doubtful that the para-selectivity for t h e alkylation is directly caused by 'product selectivity'. The relationship between the para-selectivities of various Me-MFI catalysts for the alkylation of toluene with methanol and the pore tortuosities, t0.3, was also examined and results similar to those for the alkylation of ethylbenzene with ethanol were obtained [21]. These results also indicate that t h e para-selectivity of t h e Me-MFI catalysts for the alkylation of toluene with methanol is not caused by 'product selectivity'. Para-selectivity for alkylation and acid strength. The relationship between the para-selectivity for t h e alkylation of ethylbenzene with ethanol or for t h e alkylation of toluene with methanol and the acid strength, i.e., Tmax, is shown in Fig. 3(a) or 3(b). An extremely close relationship is observed through every zeolite in both alkylations. Namely, weaker acid strength of modified HZSM-5 and Me-MFI catalysts provides higher para-selectivity. This indicates that t h e para-selectivity for the alkylation on catalysts with MFI structure is related closely t o acid strength and not t o pore tortuosity.

. ae g $-

16

WlO

21

l1

&20 50-

0

9)

g

u)

0

14

0 2

0

..

3

0 0 7 2

7

1

0

1

n

0

a

10

to3 /mln

0.0

0.5

1 .o

VAOA

Fig. 2. Relationship between the para-selectivityfor the ethylation of ethylbenzene and (b). The numbers correspond to those of the the pore tortuosity, to.3 (a) and VROA catalysts in Table 1.

284

S. Namba, J.-H. Kim and T. Yashirna

(b) Methylation

lal Ethvlatlon

0 450

550

500 Tm*x

450

550

SO0

Tmax /K

Fig. 3. Relationship between the para-selectivity for the Ethylation of ethylbenzene (a), for the methylationof toluene (b) and the Tmaxin NH3-TPD profile. Generation of para-selectivity for alkylation The primary product in the alkylations on catalysts with MFI structure is only p-isomer, and the secondary reaction, t h e isomerization of p-isomer, therefore, must b e suppressed t o improve t h e para-selectivity. The weaker acid sites on t h e catalysts with MFI structure provide t h e higher para-selectivity, because in the narrow pores t h e isomerization of p-isomer is suppressed t o some extent through 'restricted transition-state selectivity' and requires Actually, catalysts strong acid sites to take place, compared with t h e alkylation. with higher para-selectivities exhibit lower isomerization activities [3,21]. Thus high para-selective catalysts must be as follows; catalysts (1) with pores a s narrow as those of HZSM-5 or a little narrower than those, (2) without strong acid s i t e s and (3) with weak acid sites on which alkylation proceeds but with little isomerization. Disproportionation Para-selectivities of various catalysts. In the disproportionation of toluene on t h e HZSM-5 zeolite at 823 K, t h e aromatic products were benzene and three xylene isomers. The molar ratio of benzene-to-xylene was about 1.0. A near equilibrium mixture of xylene isomers was obtained. To compare t h e para-selectivities of catalysts with MFI structure, t h e paraselectivities a t an almost constant yield of xylene (4.5 - 5.0 O h ) , t h a t is, a t an almost constant disproportionation activity, were determined and a r e summarized in Table 1. Almost constant disproportionation activity was achieved by adjusting W/F. Various Me-MFI catalysts as well as HZSM-5 were not para-selective, t h a t is, they provided an equilibrium mixture of xylene isomers. Primary product in disproportionation. In order t o clarify which xylene isomer was the primary product in t h e disproportionation of toluene on HZSM-5, t h e change in fraction of each isomer in t h e produced xylene with decreasing W / F was examined. The results on HZSM-5 are shown in Fig. 4. In t h e high W / F range, t h e xylene produced was an equilibrium mixture of 22 O h o-isomer, 54 %O m-isomer and 24 90' pisomer. The fraction of p-isomer was increased t o 39 %, t h e fraction of m-isomer was slightly decreased to 49 O/O and fraction of o-isomer was decreased to 12 O/O when

Para-Selectivity of Zeolites and Metallosilicates

"

285

n

1 .o

0.5

0.0

log(1+W/F)

Fig. 4. Change in fraction of each xylene isomer with W/F in disproportination of toluene on HZSM-5. W/F was decreased t o 0. From these results, i t is difficult t o conclude t h a t t h e primary product in the disproportionation is only p-xylene. On t h e other hand, the This primary product in t h e alkylation of toluene or ethylbenzene is only p-isomer. result for the disproportionation is distinct from that for t h e alkylations on HEM-5. Para-selectivity for disproportionation and acid strength. The relationship between t h e para-selectivity for the disproportionation of toluene and acid strength, i.e., the peak position in t h e NH3-TPD profile, for t h e Me-MFI catalysts as well as t h e H E M 5 zeolites modified with boron oxide and with coke is shown in Fig. 5. W e could not find a close relationship between t h e para-selectivity for disproportionation and acid strength, although the weaker acid strength of the catalysts with a MFI zeolite structure provides a higher para-selectivity for t h e alkylation. Para-selectivity for disproportionation and pore tortuosity. The relationship be100

loo

e.-i?

r------

13

>

-jj so

On 6

$

E

05

o.

0

l9

18

4 17~603~14 12 15 2

0 . 5

1

n 0

450

500

550

Tm,,

Fig. 5. Relationship between the paraselectivity for the disproportionation of toluene and Tmm.

1 0 0.00

0.10

0.20

log (1+VAOA)

Fig. 6. Relationshipbetween the paraselectivity for the disproportionationof toluene and the VROA.

286

S. Namba, J.-H. Kim and T. Yashima

tween the para-selectivity and t h e pore tortuosity, VROA, is shown Fig. 6. A close relationship is observed, These results clearly indicate that t h e enhancement of paraselectivity for t h e disproportionation of toluene on modified HZSM-5 zeolites is caused by 'product selectivity', as reported by Olson and Haag [7]. Thus i t is concluded t h a t t h e para-selectivity for t h e disproportionation of toluene is related t o pore tortuosity [ 161, and t h e reason for t h e enhancement in para-selectivity in t h e disproportionation of toluene is distinct from that for t h e alkylation of toluene or ethylbenzene.

CONCLUSION In t h e alkylation, t h e isomerization of t h e p-isomer, which is t h e primary product in the alkylation on t h e catalysts with MFI structure, must be suppressed for t h e enhancement of para-selectivity. The isomerization on catalysts with MFI s t r u c t u r e takes place under more severe reaction conditions or requires stronger acid sites than t h e alkylation. Therefore, t h e para-selectivity for t h e alkylation is remarkably affecte d by acid strength. On t h e other hand, t h e disproportionation takes place under more severe reaction conditions or requires stronger acid sites than t h e isomerization. Therefore, under disproportionation conditions t h e isomerization of xylene proceeds readily, hence t h e para-selectivity for t h e disproportionation is a f f e c t e d not by t h e acid strength but by the pore tortuosity. REFERENCES 1 T. Yashima, Y. Sakaguchi and S. Namba, Stud. Surf. Sci. Catal., 7 (1981) 739. 2 W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein and J. Butter, J. Catal., 67 (1981) 159. 3 J.-H. Kim, S . Namba and T. Yashima, Bull. Chem. SOC. Jpn., 61 (1988) 1051. 4 J.-H. Kim, S. Namba and T. Yashima, Stud. Surf. Sci. Catal., 46 (1989) 71. 5 W.W. Kaeding, C. Chu, L.B. Young and S.A. Butter, J. Catal., 69 (1981) 392. 6 W.W. Kaeding, J. Catal., 95 (1985) 512. Olson and W.O. Haag, in T.E. Whytes (ed.) Catalytic Materials, ACS Symp. 7 D.H. Ser. 248, ACS, Washington D.C., 1984, p.257. 8 G. Paparatto, E. Moretti, G. Leofanti and F. Gatti, J. Catal., 105 (1987) 227. 9 1 ) F. Lonyi, J. Engelhardt, and D. Kallo, Stud. Surf. Sci. Catal., 49 (1989) 1357. 2) F. Lonyi, J. Engelhardt and D. Kallo, Zeolites, 1 1 (1991) 169. 3) P.A. Parikh and N. Subrahmanyam, Catalysis Letters, 14 (1992) 107. 4) K.H. Chandevar, S.G. Hegde, S.B. Kulkarni, P. Ratnasamy, G. Chitlangia, A. Singh and A.V. Deo, in D. Olson and A. Bisio (eds.) Proc. 6th Int. Zeolite Conf., Butterworths, Guildford, UK, 1984 p.325. 10 D.M. Bibby, L.P. Aldridge and N.E. Mileston, J. Catal., 72 (1981) 373. 1 1 M. Taramasso, G. Manara, V. F a t t o r e and B. Notari, Ger. P a t e n t No. 2924915 (1989). 12 R.J. Argauer and G.R. Landolt, U S . P a t e n t No. 3702886 (1972). 13 K. Yamagishi, S. Namba and T. Yashima, Stud. Surf. Sci. Catal., 49 (1989) 459. 14 J.-H. Kim, Y. Yamagishi, S. Namba and T. Yashima, J. Chem. SOC., Chem. Commun., 1990, 1793. 15 J.-H. Kim, S. Namba and T. Yashima, Appl. Catal., 100 (1993) 27. 16 J.-H. Kim, S. Namba and T. Yashima, Appl. Catal., 83 (1992) 51. 17 T. Yashima, K. Yamagishi, S. Namba, S. Nakata and S. Asaoka, Stud. Surf. Sci. Catal., 37 (1988) 175. 18 S. Namba, S. Nakanishi and T. Yashima, J. Catal., 88 (1984) 505. 19 S. Namba, A. Inaka and T. Yashima, Zeolites, 6 (1986) 107. 20 J.-H. Kim, S. Namba and T. Yashima, Zeolites, 1 1 (1991) 59. 21 S. Namba, H. Ohta, J.-H. Kim and T. Yashima, Stud. Surf. Sci. Catal., 75 (1993) 1685.

Transition State and Diffusion Controlled Shape Selectivity in the Formation and Reaction of Xylenes

Gabriele Mirth, Jiri Cejka, Ernst Nusterer and Johannes A. Lercher Institute for Physical Chemistry and CD Laboratory for Heterogeneous Catalysis, Technical University of Vienna, A-1060 Vienna, Getreidemarkt 9, Austria.

ABSTRACT The influence of the nature and concentration of the adsorbed species on the reaction rates and selectivities in toluene methylation and xylene isomerization over the zeolite HZSM5 is reported. In the both reactions, the reaction rates were found to be of first order with respect to the surface concentration of the reactants, i.e. methanol in the toluene methylation and xylene in the isomerization. The selectivity in m-xylene isomerization is explained by restrictions to the transition state to form o-xylene. In the case of p- and o-xylene isomerization, the selectivity is controlled by restrictions of the transport of the primary product m-xylene out of the pores. In toluene methylation, all three isomers were found to be primary products but due to the different rates of transport, the bulkier isomers are accumulated in the pores. High selectivity to p-xylene was achieved at elevated temperatures (i.e. at 573 K and above), when the surface concentration of the bulkier xylene isomers was so high, that the rate of the isomerization of the formed xylenes was higher than the rate of alkylation. At low temperatures (i.e,, 473 K), low selectivity to p-xylene was found. The accumulated xylene molecules blocked the catalytically active sites and decreased the overall reaction rate. Thus, we conclude that for shape selective methylation of toluene, the rate of isomerization has to exceed the rate of alkylation. Transition statc selectivity does not play a major role for the product selectivites in the methylation. INTRODUCTION The shape selective syntheses of substituted aromatic molecules faced significant interest during the past years [e.g.1,2,3,4,5,6,7]. Although suitable methods have been developed to increase the selectivites to the desired products, the differentiation between chemical, restricted transition state and mass transport induced selectivity remains difficult. The availability of in situ vibrational spectroscopic methods in combination with rigorous kinetic analysis allows to tailor specifically the reaction conditions and the catalyst pretreatment to optimize the output of a specific product. As example, a study on toluene methylation and xylene isomerization over the zeolite HZSM5 is presented here. The reaction ratcs and surface coverages of the individual reaction steps can be

287

288

G. Mirth, J. Cejka, E. Nusterer and J . A . Lercher

linked to give a unified picture of the shape selective process in the zeolite pores. In addition, the conditions under which diffusion limitations will play a role, are discussed.

EXPERIMENTAL For all investigations, a zeolite HZSM5 with a Si/AI ratio of 35.5 was used, the diameter of the zeolite crystals was about 1 pm. The zeolite was provided in the ammonium exchanged form and converted to the protonic form by heating in He flow (20 ml/min) up to 773 K with a heating rate of 10 K /min. For the IR measurements, the zeolite powder was pressed into self supporting wafers which were analyzed in situ during all treatments by means of transmission absorption IR spectroscopy. A BRUKER IFS 88 FTIR spectrometer with a typical resolution 4 cm" was used for all investigations .The IR cell was constructed as continuously stirred tank reactor equipped with 1/16" gas in- and outlet tubings and CaF, windows. The partial pressures of the reactants were 16 mbar xylene in the xylene isomerization experiments and 42 (11.4) mbar toluene and 14 (3.8) mbar methanol in the alkylation experiments. In order to characterize the adsorbed species in the zeolite pores during the reaction, IR spectra of the catalyst were recorded after contacting the activated zeolite with a carrier gas stream containing the reactants (pressure transient response). Simultaneously, samples of the effluent gas stream were collected into the 16 loops of a VALCO multiport valve and subsequently analyzed by means of gas chromatography. This experimental setup allowed the synchronous analysis of the products inside the zeolite pores (IR spectroscopy) and in the gas phase (GC).

RESULTS Xvlene Isomerization For the xylene isomerization, the reaction rates for all three xylcne isomers were found to be equal at low reaction temperatures (473 K), at steady state the rate was about l.103mole~ules.[H']~~.sec-'. The adsorption constants were similar for all three isomers, but the rates of diffusion varied over 3 ordcrs of magnitudcs (diffusivities for p-: m-: o-xylene = 1OOO:lO:l [8]). This indicates that the surface reaction is the rate determining step in xylene isomerization under these experimental conditions. At higher reaction temperatures (573 K), the rates of isomerization were different for the three xylene isomers, indicating that the diffusion of the reactant isomer influenced the overall reaction rate. For p-xylene the highest TOF (8,8.10" mole~ules.[H*]~'.sec~~) was detcrmined, for o-xy-

[5]. The rate constants lene it was about 6.7 and for m-xylene 5,4.103 molecules.[H']~'.~ec~~

Transition State and Diffusion Controlled Shape Selectivity

289

(k=TOF/coverage), however, were again equal for the three isomers. Thus, we conclude that the difference in the diffusion rates caused the different surface coverages, which leads to different catalyst efficiencies for the three xylene isomers. After increasing the partial pressure of m-xylene from 0 to 16 mbar, an increase in the rate of m-xylene isomerization with time was found. This enhancement in rate was directly proportional to the increase in coverage of the catalytically active Si-OH-A1 groups of the zeolite with m-xylene (as determined from the IR spectra). This suggests that all acid sites which are capable of adsorbing m-xylene have the same catalytic activity. We further conclude that the reaction is of first order with respect to the surface concentration of the xylenes. The product ratio p- to o-xylene was 2 for all reaction temperatures under investigation (473, 523, 573 K). Thus, the energies of activation for the formation of

0-and

p-xylene are concluded to be identical. Because accumulation of products in the

pores was not observed, the differences in the rate of transport between p- and o-xylene cannot account for the high p-selectivily. Thus, steric restrictions to the transition state in the m- to o-xylene isomerization compared to the m- to p-xylene isomerization are concluded to be responsible for the observed selectivites. Molecular modelling indeed shows that the minimum kinetic diameter of the transition state complexes in the m- to

0- and

m- to p-xylene isomerization are 6.2 A and 6.7 A,

respectively (see Fig.1).

Fig.1. Modcl of thc transition state complcxcs in the m-xylcnc isomeriaition

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C . Mirth, J. Cejka, E. Nusterer and J. A. Lercher

Thus, the constraints for the formation of the transition state are larger for the m- to ointermediate than for the m- to p-intermediate. This suggests that the p-/o-product ratio of 2 in the m-xylene isomerization is a result of restrictions to the transition state [ 5,9]. The product distributions in the isomerization of p- and o-xylene indicate that the prevailing mechanism is the 1.2-methyl shift [lo]. The analysis of the adsorbed phase showed that m-xylene accumulated in the pores with time on stream. This accumulation was accompanied by a decrease in the selectivity to m-xylene in the gas phase resulting from an increase in the rate of secondary isomerization of the primary product m-xylene to p- and o-xylene.

ne M e t h y b For the methylation of toluene, two temperature regions with different catalytic reactivity and selectivity exist. At 473 K, enhanced selectivity to p-xylene was not observed. After a stepwise increase of the pressure of the reactants (pressure transient experiment,

pr' 0

+ 42 mbar, pM=0

+ 14 mbar) only toluene and methanol were found in the adsorbed phase. At this point all BrBnsted acid sites were covered with reactant molecules. With time on stream, accumulation of products, mainly m-xylene (and to a smaller extent o-xylene), in the zeolite pores was observed (for the i.r. spectra collected during the reaction and the assignment of the i.r. bands see ref. 9 and 11). Adsorption of the formed xylenes at the Briinsted acid sites lowered the concentration of methanol molecules bound to the catalytically active sites, the SiOHAl groups.

IR INTENSITY (2400 cm-I)

0

0,0005

0,oo I

0,oo 1 5

0,002

0,0025

TOF [molec./site.s] Fig.2 Correlation between the rate of methylation and the surface concentration of methanol (derived from the normalized intensity of the band at 2400 cm-') at 473 K.


291

It was found that the coverage of the methoxonium ions (characteristic band at 2400 cm-') decreased by more than 60 % within 1 hour on stream. The concentration of toluene hardly changed during that period. The main products in the effluent gas stream after short reaction times were p- and o-xylene. The initial rate of toluene methylation was about 2.103 molecules.[H+]~'.sec~'and decreased by approximately 60 % during 1 hour on stream. This change in the reaction rate was directly correlated to the surface concentration of adsorbed methanol (methoxonium ions) as shown in Fig.2. Thus, the decrease of the overall rate of toluene methylation as function of time on stream is attributed to the replacement of methanol at the Si-OH-A1 groups by xylenes. Note that toluene (although present in a threefold excess with respect to methanol and in more than tenfold excess to the xylenes) did not adsorb directly on the Si-OH-A1 sites. When the partial pressure of the reactants in the pressure transient experiment was lowered (pT=0 4 11.4 mbar, pM=0 + 3.8 mbar), the transient function of the products in the gas phase was different. At steady state, however, achieved after approximately 20 min., the reaction rate normalized to the surface concentration of methanol (methoxonium ions) was identical to the normalized rate at high partial pressure. Thus, we conclude that the reaction is of first order with respect to adsorbed methanol (see Fig.3).

TOF [molec./site.s]

0,003

T

0,002

0.00 I

A w -

0

10

20

30

40

so

60

TIME [min ]

Fig.3. Change of the rates of methylation with time on stream €or.+low and* high partial pressure of the reactants at 473 K

292

G . Mirth, J . Cejka, E. Nusterer and J . A. Lercher

At higher reaction temperatures (573 K, 673 K) the analysis of the gas phase revealed high selectivity to p-xylene. Steady state with respect to the surface concentrations of formed xylenes was reached within a few minutes on stream. The main product adsorbed at the acid sites was m-xylene. Lower concentrations of 0-xylene and trimethylbenzene isomers were also detected in the adsorbed phase. The concentration of p-xylene in the zeolite pores was below the detection limit. 1.2.4. trimethylbenzene was the only trimethylbenzene isomer in the gas phase while it was the least abundant isomer in the adsorbed phase indicating that the isomerization of the trimethylbenzenes was faster than the rate of diffusion of 1.2.4. isomer. The other isomers (1.2.3. tmb and 1.3.5. tmb) accumulate in the zeolite pores as they are too bulky to escape from the poresystem. The overall reaction rate of the methylation of toluene, 1.10.' mo1ecules.[H']~'.sec", was constant over one hour on stream.

1SOMERIZP;IION

OF m-XYLENE

TOF = 0 001 molec/site.s

-

@m-*ylene

TOLUENE METHyLAnON

TOF = 0 0054 rnolec /site. s

0 55

-+ k,,,=

TOF,,, = 0002 0

Tor,,,,

= 0 002 rnolec /site

Q m-xylanc

TOFIsa + k,,= 0

s

TOF

= 0 01 molec /site s

TOF,, = 0 001 molec /sire s

-

Orn-xy,ac 0 25

+ TOF,,,

(0OOOS) < TOF,,

-004

0 m-xylenc (0 001)

-

+ TOF,,(O

025

035) > TOF,,,(O

01)

Fig.4. TOFs and rate constants for the methylation of toluene and the isomerization of m-xylene over HZSMS. All coverages [O] refer to the IR spectra collected after 1 hour time on stream. DISCUSSION

For both reactions, methylation of toluene and isomerization of the xylenes, all results suggest that the catalytic activity of all strong BriSnsted acid sites (SiOHAl groups) is identical. Both reactions are concluded to be of first order with respect to the surface concentration of methanol (toluene methylation) and xylenes (isomerization). The differences in the activity (p- > 0- > m-xylene) observed in the xylene isomerization


over HZSMS at 573 K is explained - yb

.

,

. . .

293

of the bulkier isomers m- and

o-xylene. Although the rates were quite different for the 3 isomers, the rate constants k=TOF/O (and the true energies of activation) were found to be identical. The variations of the surface coverage were attributed to differences in the diffusivities. At low reaction temperatures (473 K), the rate of transport was faster than the rate of the surface reaction which was reflected in equal rates of isomerization for all three isomers. The selectivities, however, were determined by either steric (in the case of m-xylene isomerization) or diffusional constraints (in the 0-and p-xylene isomerization). The preferential formation of p-xylene in the m-xylene reaction was attributed to be due to restricted

..

selectivity because the transition state complex is larger for the m- to a- than for the m- to p- reaction. In the p- and o-xylene isomerization, the product distribution is mainly governed by the reaction mechanism (1.2. methyl shift) which yields m-xylene as the only primary product. But as the rate for the surface reaction (in the initial period) is higher than the rate for the transport of the m-xylene out of the pores, a considerable surface Concentration of the m-xylene builds up in the pores (-

.

.

. . .

diffusion 1-

) and undergoes secondary isomerization reactions.

Similar effects were found in the case of toluene alkylation, when the accumulation of mxylene and other bulky products was observed in the zeolite pores. The selectivites in the methylation over HZSMS were found to depend highly on the extent of the secondary isomerization in the pores. By determining the surface concentration of the xylenes in the toluene methylation and knowing the rate constants for their isomerization. the intrinsic rates for the secondary reaction, the isomerization of the xylenes were calculated. Under conditions (473 K). when the rate of isomerization is lower than the rate of alkylation, selectivity to p-xylene is not observed.

0-and

m-

Xylene accumulate in the pores until the rate of formation equals the rate of transport out of the pores. As these products reach quite high concentrations in the pores, they will replace methanol from the catalytically active sites and impede further methylation steps. Under conditions when the rate of isomerization exceeds the rate of methylation (573 K and higher), high p-selectivity was obtained. The accumulation of xylenes or trimethylbenzenes in the MFI pores did neither decrease the surface coverage of the reactants nor the overall rate of reaction. Also the selectivity remained constant with time on stream. Because m-xylene isomerizes to approximately 66 mol% p-xylene and

34 mol% o-xylene over HZSMS, the higher selectivity to p-xylene is well explained with the contribution of the secondary isomerization. This holds not only for m-xylene. but also for the other products which face diffusional constraints, i.e., o-xylene and the trimethylbenzenes. o-Xylene for example reacts to m-xylene (1,2 methyl shift mechanism) which in turn undergoes reactions as described above.

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G. Mirth, J. Cejka, E. Nusterer and J . A . Lercher

CONCLUSION The results show that shape selective alkylation of aromatic molecules is only possible under conditions in which fast isomerization is coupled with restricted transport of the bulkier molecules (diffusion control). For the isomerization reactions both, transition state and diffusion induced selectivity, could be identified. Chemical selectivity was concluded to play a minor role for the reactions studied. ACKNOWLEDGEMENTS The financial support of the Christian Doppler Laboratory for Heterogeneous Catalysis is acknowledged. We thank MOBIL OIL corporation for providing us with the zeolite samples. REFERENCES

1.

N.Y.Chen, W.W. Kaeding and T.Dwyer, J.Am.Chem.Soc.,lOl (1979) 6783.

2.

W.W.Kaeding, C.Chu, L.B.Young and S.A. Butter, J.Catal., 67 (1981) 159.

3.

L.B. Young, S.A. Butter and W.W: Kaeding, J.Catal., 67 (1982) 418.

4.

D.H. Olson and W.O. Hang, ACS.Symp.Ser. 248 (1984) 275.

5.

D. Fraenkel and M. Levy, J.Mol.Catal. I18 (1989) 10.

6.

B. Wichterlova and J.Cejka, Catal.Lett., 16 (1992) 421.

7.

M.B. Sayed and J.C. Vedrine, J.Catal., 101 (1986) 43.

8.

G. Mirth, J. Cejka and J.A. Lercher, J.Catal., 139 (1993) 24.

9.

G. Mirth and J.A.Lercher, J.Catal., submitted for publication.

10.

A.Corma and E.Sastre, J.Chem.Soc.Chem.Commun., (1991) 594.

11.

G. Mirth and J.A. Lercher, J.Catal., 132 (1991) 244.

Selective Synthesis of 4,4'-Diisopropylbiphenyl Using Mordenite Catalysts

T. Matsuda and E. Kikuchi* Department of Applied Chemistry, School of Science & Engineering Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169, JAPAN

ABSTRACT Alkylation of biphenyl with propene was carried out at 250°C using H-mordenite as a catalyst. Hmordenite was selective for 4,4'-diisopropylbiphenyl(4,4'-DIPB)production. The 4,4'-DIPB selectivity, however, decreased with increasing level of biphenyl conversion, due to isomerization of 4,4'-DIPB to thermodynamically more favorable isomers. The external acid sites were selectively poisoned by treatment with tributylphosphite(TBP) since the molecular dimension of TBP was large compared with the pore size of mordenite. The isomerization of 4,4'-DIPB was suppressed by treatment of H-mordenite with TBP, resulting in enhanced selectivity for 4,4'-DIPB even at high levels of conversion.

INTRODUCTION Polynuclear aromatic hydrocarbons are expected as raw materials for advanced polymers such as liquid crystals. Particularly, 2,6-dimethylnaphthalene and 4,4'-dimethylbiphenyl are valuable intermediates for preparation of monomers to make thermotropic liquid crystal polymers. These polynuclear aromatic hydrocarbons have been expected to be selectively synthesized by use of shape selective catalysts since 2,6-dimethylnaphthalene and 4,4'-dimethylbiphenyl have the smallest molecular dimensions aniong their isomers. It was shown in our previous papers [1,2] that 2methylnaphthalene was selectively disproportionated to 2,6- and 2,7-dimethylnaphthalene on H-ZSM-

5 catalyst, due to its shape selective property. Fraenkel et al. [3] and Weitkamp et al. [4] reported that alkylation of naphthalene or 2-methylnaphthalene with methanol on H-ZSM-5 catalyst gave slim isomers, namely 2,6- and 2,7-DMN. It was shown by several researchers [5-101 that H-mordenite catalyst exhibited high activity in liquid phase alkylation of biphenyl with propene to produce 4 4 diisopropylbiphenyl(4,4'-DIPB). Lee and co-workers [ 11-131 reported that dealuminated mordenite with a high proportion of mesopores behaved as a shape selective catalyst for the alkylation of biphenyl. Zeolites possess a variety of properties that make them attractive candidates as catalysts and/or catalyst supports for shape selective reactions. Although the external surface area of zeolite is only a very small percentage of the total surface area, product distribution is strongly affected by the presence of the catalytically active sites on the external surface. The shape selective property of zeolites can generally be improved by decreasing numbers of the unselective active sites by poisoning or eliminating them by various treatments. 29 5

296

T.Matsuda and E. Kikuchi

Modifications of zeolites with phosphorous compounds have recently been the subject of many investigations 114-181. It was reported [ 191 that treatment of RHO zeolite with trimethylphosphite (TMP) was effective to poison external acid sites. RHO treated with TMP exhibited high selectivity for the formation of dimethylamine from methanol and ammonia. A similar result was obtained with ZSM-5 [20]: the selectivity for p-xylene formation by alkylation of toluene with methanol was improved by treatment with TMP. The acid sites of mordenite are fully poisoned by TMP because the molecular dimension of TMP is larger than the pore size of mordenite. In this paper, we describe the treatment of mordenite with tributylphosphite(TBP) to improve the performance as a catalyst for the selective synthesis of 4,4'-DIPB by alkylation of biphenyl with propene.

EXPERIMENTAL Catalvst Sodium-type mordenite(Si02/A1203=20) supplied by Tosoh Corp. was exchanged 5 times with 0.1N NH4Cl solution at 70°C for 6 h. Thus obtained ammonium-type mordenite was calcined at 540°C for 4 h to form the proton-type. Treatment of H-mordenite with tributylphosphite(TBP) involved the following procedures. Under nitrogen atmosphere, 10 g of H-mordenite was added to 50 ml TBP solution and stirred for 2.5 h at 170°C. The product was filtered and washed with acetone to remove excess TBP, followed by calcination at 400°C for 2 h. H-mordenite and H-mordenite treated with TBP are abbreviated to HM and P-HM, respectively. Aciditv Measurement The acidity of catalysts was determined by means of ammonia temperature-programmed desorption (TPD). In each TPD experiment, a sample placed in a cell was evacuated at 540°C for 1 h, and ammonia was adsorbed at loO°C for 1 h followed by evacuation for 1 h. The sample was heated from 100 to 750°C at a rate of 10"Cfmin in a stream of helium (60cc/min, 100 Torr). A thermal conductivity detector was used to monitor the desorbed ammonia. Apparatus and Procedures The catalytic study was carried out at 250°C using a suspension of a catalyst in decalin as a liquid medium. The liquid phase reactor was a stainless steel autoclave, having an internal volume of 388m1, equipped with a stirrer. The reactor containing 1 g of catalyst, 50 mmol of biphenyl, and 40 ml of decalin was heated to 250°C in nitrogen atmosphere and then 50 mmol of propene was admitted. Conversion of 4,4'-DIPB was carried out at 250°C using a mixture consisting of 1 g of catalyst, 10 mmol of 4,4'-DIPB, and 40 ml of decalin. Liquid products were analyzed by means of FID gas chromatography using DB-1 glass capillary separation column with temperature programmed heating from 80 to 270°C. The activities for cracking of cumene and 1,3,5-triisopropylbenzene(1,3,5-TIPB) were determined at 450°C using a pulse technique.

Selective Synthesis of 4,4’-Diisopropylbiphenyl

297

RESULTS AND DISCUSSION In alkylation of biphenyl with propene, HM catalyst gave isopropylbiphenyl(1PB) and diisopropylbiphenyl (DIPB), and triisopropylbiphenyl(TIPB) was hardly produced. Figure 1 shows the variation in the level of biphenyl conversion, and the yields of IPB and DIPB on HM catalyst with reaction time. The level of conversion increased with reaction time, and leveled off at about 4 h of run. The yield of IPB,however, increased slightly even after 4 h of run. The yield of DIPB reached a maximum, and then decreased. Variation in the compositions of IPB and DIPB isomers produced on HM catalyst with reaction time S 60 is shown in Fig. 2. IPB and DIPB have three and E 50 twelve isomers. The equilibrium concentrations of 25 , 3-, and 4-IPB isomers are 2, 63, and 35%, i 40 0 respectively, and those of 44’-, 3,4’-, 3,3’-, and 3 30 other DIPB isomers are 9, 37%, 33%, and 21%, 0 20 respectively [6]. Since the isomers having isopropyl groups in the para-positions are of the smallest 10 molecular dimension among these isomers, 4-IPB n and 4,4’-DPB are expected to be selectively formed -0 2 4 6 8 Reaction time/h if shape selectivity of catalyst is operative. As apparently shown in Fig. 2, HM catalyst was Fig. 1. Variation in the level of biphenyl conversion selective for 4-IPB and 4,4‘-DIPB production in a and the yield of alkylated products on HM catalyst with reaction time. comparison with the thermodynamically attainable 0,conversion;A, yield of IPB; 0.yield of DIPB. levels. The selectivities for these slim isomers,

8

80 A l

0

2

4 6 Reaction timeh

8

n 0

2

4 6 Reaction timeh

8

Fig. 2. Variation in the composition of IPB and DIPB isomers on HM catalyst with reaction time. A: O,4-IPB; A , 3-IPB; O,2-IPB. B: O,4,4‘-DIPB; A, 3,4‘-DIPB; U,3,3’-DIPB.

298

T.Matsuda and E. Kikuchi

Table 1. Physical properties and catalytic activities of HM catalysts. Surface area P content Conversion (%) Catalyst Si02/A1203 (m2tz-1) (wt%) Cumene 1,3,5-TIPB 0 81 100 20 556 HM 73 3 19 545 0.63 P-HM

however, were reduced with reaction time, due to isomerization to thermodynamically more favorable 3-IPB and 3,4-DIPB isomers. It is well known that the shape selective property of a catalyst is strongly affected by the presence of external acid sites. If the external acid sites are responsible for isomerization of 4-IPB and 44'DIPB, the selectivities for these isomers are expected to be improved by eliminating or poisoning them. In order to poison the external acid sites, HM was treated with TBP. The catalytic activities of HM treated with TBP (P-HM) for cracking of cumene and 1,3,5-TIPB were compared with those of HM. Typical results are summarized in Table 1. A SiOZ/A1203 molar ratio and surface area of HM were not changed at all by this treatment. The content of P was 0.63% by weight. The level of 1,3,5-TIPB conversion decreased from 100% to 3% by the TBP treatment, while the activity for cracking of cumene changed little. Cracking of 1,3,5-TIPB seems to proceed only on the external acid sites because the molecular dimension of 1,3,5-TIPB is 8.519 and is large compared with the pore size of HM. As shown in Fig. 3, there was no appreciable difference in the ammonia-TPD spectra between HM and P-HM. We conclude from these results that TBP poisoned only the external acid sites of HM. Figwe 4 shows the catalytic activity of P-HM for the alkylation of biphenyl. The activity of HM catalyst was lowered by the TBP treatment,

Fig. 3. NH3-TPD Spectra Of HW-1 and P - H W -1.

Fig. 4. Variation in the level of biphenyl conversion and the yield of alkylated products on P-HM catalyst with reactiontime. 0,conversion; A , yield of IPB; 0,yield of DIPB.

Selective Synthesis of 4,4'-Diisopropylbiphenyi

n

r

i

n

-n

n

n

299

n

4 6 8 0 2 4 6 8 Reactiontimeh Reaction timeh Fig. 5. Variation in the composition of IPB and DIP6 isomers on P-HM catalyst with reaction time. A: 0,4-IPB; A , 3-IPB; O,2-IPB. B: O,4,4'-DIPB;A , 3,4'-DIPB;0,3,3'-DIPB. 0

2

0.03

c

'o)

\

% % Conversion of biphenyl

Fig. 6. Relationship between the level of biphenyl conversion and the selectivity for 4,4'-DIPB on HM(u) and P-HM(o) catalysts.

0.02

2E

. 3

3

3

0.01

0.00 0

100 200 300 Exposure time I min

400

Fig. 7. Amount of 4,4'-DIPB(o, 0)and 3,4'-DIPB (0,B)sorbed on HM(0, B)and P-HM(o.0) at 50°C

with exposure time.

300

T. Matsuda and E. Kikuchi

probably due to poisoning of external acid sites. The level of biphenyl conversion and the yields of alkylated products increased with reaction time, and 60% biphenyl was converted on P-HM after 7 h of run, while almost the same level of conversion was obtained on HM after 4 h of run. The yields of DIPB on P-HM and HM after 7 h of run were 20 and 14%, respectively, although there was no appreciable difference in the level of biphenyl conversion between them. The higher yield of DIPB on P-HM is not attributed to the suppression of TIPB formation because TIPB was hardly formed even on HM catalyst. The level of propene conversion on HM after 7 h of run was about 90%,and there was no difference in the level of propene conversion between HM and P-HM. The consumption of propene through alkylation, however, was affected by the TBP treatment. The conversion level of propene by alkylation on HM and P-HM were 66 and 80 %, respectively. In the case of HM, 24% propene was unrecovered, while unrecoverble propene on P-HM was 10%. It is obvious that undesirable reactions such as polymerization was suppressed by the TBP treatment. The selectivities for 4-IPB and 4,4'-DIPB were improved by the TBP treatment. As shown in Fig. 5(A), P-HM catalyst exhibited higher selectivity for 4-IPB formation than HM, and the 4-IPB selectivity became almost constant, irrespectively of reaction time. The selectivity for 4,4'-DIPB also increased from 78 to 82% by the TBP treatment. On P-HM catalyst, the high 4,4'-DIPB selectivity was retained even at high conversion levels, although that on HM was reduced. Figure 6 shows the relationship between the level of biphenyl conversion and the selectivity for 4A-DIPB. The selectivity for 4,4'-DIPB at high conversion levels was significantly improved by the TBP treatment. It is well known that shape selective property was affected by coke formation. The amount of deposited carbon on HM catalyst after 7 h of run was determined to be 5.3 C-mmol/g-cat by burning off the carbonaceous deposit on the used catalyst. There was no difference in the amount of deposited coke between HM and P-HM. Thus, the high selectivity of P-HM for 4,4'-DIPB production is considered to be due to the absence of external acid sites.

"

0

1 2 3 Reaction tirne/h

4

5

Fig. 8. Variation in the level of 4,4'-DIPB anversion on HM( o ) and P-HM( A ) catalysts with reaction time.

30 I

Selective Synthesis of 4,4'-Diisopropylbiphenyl

40

s

-

B

A

30

z

B

=I

3

20

Q

c

0

rr

s 5 10

'I

10

c

1

2 3 Reactiontimeh

4

5

0

0

1 1

2

3 4 5 Reactiontimeh

Fig. 9 . Variation in yield of products on HM(A) and P-HM(B) catalysts in the conversion of 4,4'-DIPB. O,4-IPB; .,3-IPB; A, 3,4'-DIPB; A, 3,3'-DIPB.

Adsorption experiments were carried out at 5OoCusing a mixture consisting of 2 g catalyst, 0.5 ml of 4,4'-DIPB or 34'-DIPB, and 15 ml of decalin. As shown in Fig. 7, adsorption of 3,4'-DIPB was markedly suppressed on HM, while 4,4'-DIPB was adsorbed. Thus, 3,4'-DIPB is considered to be formed mainly on the external acid sites. There was no difference in the adsorptive property between HM and P-HM, indicating that the TBP treatment hardly affected the pore size of HM. We conclude that isomerization of 4,4'-DIPB hardly occurred on P-HM due to the absence of catalytically active sites on the external surface, resulting in high selectivity for the slim alkylated products. Conversion of 4,4'-DIPB was carried out at 250°C to study the effect of external acid sites on the catalytic property of HM. Figure 8 shows the activities of HM and P-HM catalysts for this reaction. P-HM was less active than HM, indicating that 4,4'-DIPB was easily converted on the external acid sites. As shown in Fig.9(A), 4,4'-DIPB was converted to 4-IPB, 3-IPB, 3,4'-DIPB, and 3,3'-DIPB on HM catalyst. These results indicate that isomerization and dealkylation occurred on HM. As shown in Figs2 and 4, the yield of DIPB increased to reach a maximum at about 4 h of run and the 4,4'-DIPB selectivity decreased with reaction time. These results can be understood by taking dealkylation and isomerization of DIPB once produced into consideration. 4,4'-DIPB was converted to 4-IPB and 3,4'-DIPB on P-HM catalyst, and 3-IPB and 3,3'-DIPB were hardly formed. These results indicate that isomerization and dealkylation of 4,4'-DIPB proceeded even on this catalyst. P-HM catalyst, however, was less active for these reactions than HM catalyst. We conclude from these results that treatment of HM with TBP was an effective method to poison the external acid sites, which were responsible for isomerization and dealkylation of DIPB once produced, and P-HM catalyst exhibited high selectivity for 4,4'-DIPB production by alkylation of biphenyl with propene.

302

T. Matsuda and E. Kikuchi

CONCLUSION HM was a selective catalyst for alkylation of biphenyl with propene to produce 4,4'-DIPB. Isomerization and dealkylation of DIPB proceeded mainly on the external acid sites. These undesirable reactions reduced the selectivity of 4,4'-DIPB and the yield of DIPB. The catalytic activity of HM for cracking of 1,3,5-TIPB was remarkably lowered by treatment with TBP, although the acidity changed little. These results indicate that the treatment of HM with TBP is an effective method to poison the external acid sites since the molecular dimension of TBP is large compared with the pore size of mordenite. Isomerization and dealkylation of DIPB hardly occurred on P-HM due to the absence of external acid sites, resulting in high yield of DIPB and high selectivity for 4,4'-DIPB. REFERENCES 1 T. Matsuda, K. Yogo, T. Nagaura and E. Kikuchi, Sekiyu Gakkaishi, 33 (1990) 214. 2 T. Matsuda, K. Yogo, Y. Mogi, and E. Kikuchi, Chem. Lett., (1990) 1085. (1986) 273. 3 D. Fraenkel, M. Cherniavsky, B. Ittah, and M. Levy, J. Catal., 4 N. Neuber, H.G. Karge, and J. Weitkamp, Catal. Today, 3 (1988) 11. 5 T.Matsuzaki, Y. Sugi,T. Hanaoka, K, Takeuchi, H. Arakawa, T. Tokoro, and G. Takeuchi, Chem. Express, 4 (1989) 413. 6 G. Takeuchi, H. Okazaki, T. Kito, Y. Sugi, and T. Matsuzaki, Sekiyu Gakkaishi, 34 (199 1) 242. 7 Y.Sugi, T. Matsuzaki, T. Hanaoka, K, Takeuchi, T. Tokoro, and G. Takeuchi, in T. Inui, S. Namba, T. Tatsumi (Eds.), Stud. Surf. Sci. Catal., 60 (1991) 303. 8 N. Sakamoto, T. Takai, S. Taniguchi, and K. Takahata, Japan Kokai Tokkyo Koho 122636, 1988. 9 T. Nakamura, S. Hoshi, and K. Okada, Japan Kokai Tokkyo Koho, 227529 1988. 10 Y. Sugi, T. Matsuzaki, M. Morita, G. Takeuchi, Japan Kokai Tokkyo Koho 190639,1989. 11 G.S. Lee, J.J. Maj, S.C. Rocke, and J.M. Garces, Catal. Lett., 2 (1989) 243. 12 G.S. Lee, J.J. Maj, S.C. Rocke, and J.M. Garces, in S. Yostuda, N. Takezawa, Y. Ono, (Eds.), Catalytic Science and Technology, vol.1, Kodansha-VCH, 1991, p.385. 13 G.S. Lee and S.C. Rocke, Japan Kokai Tokkyo Koho 165531, 1989. 14 W.W. Kaeding and S.A. Butter, U.S. Patent 3,911,041, 1975. 15 J.C. Vedrine, A. Auroux, P. Dejaifve, V. Ducarme, H. Hoser, and S. Zhou, J. Catal., 73 (1982) 147. 16 K.H. Chandawar, S.B. Kulkami, and P. Ratnasamy, Appl. Catal., 4 (1982) 287. 17 J.A. Lercher and G. Rumplmayr, Appl. Catal.,25 (1986) 2 15. 18 A. Jentys, G. Rumplmayr, and J.A. krcher, Appl. Catal., 53 (1989) 299. 19 D.R. Corbin, M. Keane, Jr, L. Abrams, R.D. Farlee, P.E. Bierstedt, and T. Bein, J. Catal., 124 (1990) 268. 20 J. Nunan, J. Cronin, and J. Cunningham, J. Catal., 87 (1984)77.

Mechanism of the Activation of Butanes and Pentanes over ZSM-5 Zeolites

Yoshio Ono, Kazuaki Osako, Misa Yamawaki, and Katsumi Nakashiro Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, 152 Japan

ABSTRACT The mechanism of the cracking of butanes and pentanes over H-, Zn-, Ga-, and Ag-ZSM-5 was studied. Over H-ZSM-5, the C-C bond cleavage occurs more preferably than the C-H bond cleavage. The three cation-loaded zeolites showed 15 - 70 times higher activity for the hydride abstraction than H-ZSM-5. Zn-ZSM-5 and GaZSM-5 also show the enhanced activity for the C-C bond cleavage. On the other hand, Ag-ZSM-5 showed reduced activities for the C-C bond cleavage compared with H-ZSM-5. The rate of the C-H bond cleavage over Zn-, Ga-, and Ag-ZSM-5 among butanes and pentanes depends on the stability of the carbenium ions to be formed by the hydride abstraction. The rate of the C-C bond cleavage over Zn- and GaZSM-5 also depends on the order of the carbenium ion stability. INTRODUCTION Whilst the cracking of alkanes over proton forms of zeolites has been a subject of continuous interest for many years, the research on alkane cracking has been rather confined to larger molecules such as decane because of their practical importance. However, it is important to extend our basic knowledge on the cracking mechanism to the activation of smaller molecules such as butanes in order to find the routes for the transformation of smaller alkanes into more valuable chemicals as aromatics. It is known that the loading of zinc or gallium cations onto H-ZSM-5 zeolites greatly enhances the selectivity for aromatics in the transformation of lower alkanes[l]. Therefore, it is of importance to understand how the mechanism of the alkane activation is modified by the metal loading. We have recently found that Ag-ZSM-5 is also active for the aromatization of alkanes, alkenes, and methanol. In this work, the mode of the activation of butanes and pentanes over H-ZSM-5 is examined at first and then it is studied how the mechanism changes by loading Zn, Ga and Ag cations. The general features of alkane activation will be discussed. 303

304

Y . Ono, K. Osako, M.Yamawaki and K . Nakashiro

EXPERIMENTAL ZSM-5 prepared gallium

zeolites was first converted into NH4-form. by

Zn- and

impregnation method using aqueous solutions of

nitrate, respectively.

Ga-ZSM-5

zinc

Ag-ZSM-5 was prepared either with

were

acetate

and

impregnation

or ion exchange by using silver nitrate solutions. The reactions were carried out in a continuous flow reactor at pressure.

atmospheric

The catalyst grains (16 - 32 mesh) was then packed into a reactor

of

silica-tubing (10 mm i.d.) placed in a vertical furnace and then heated under an air

stream at 823 K for 90 min. By this treatment NH4-ZSM-5 was expected to

converted

into the proton form (H-ZSM-5). Butane, isobutane, and nitrogen

fed into the reactor through flow meters. Pentanes were fed by feeding through the saturator.

The partial pressure of alkanes was 8

kPa

be were

nitrogen to

avoid

bimolecular reactions. The products (hydrocarbons + hydrogen) were analyzed with gas chromatography. RESULTS Butane H-ZSM-5.

Fig.

1 shows the conversion of butane, the yields

of

methane,

ethane, and hydrogen as a function of the contact time over H-ZSM-5

(Si02/A1203 =

43.5) at 773 K. The hydrogen yield is almost equal to the yield

of

Similarly, pene

and

butenes.

the yields of methane and ethane were almost equal to those of ethylene, respectively.

A small amount of propane was

also

pro-

formed.

This shows the cracking of butane over H-ZSM-5 proceeds through pentacoordinated carbocations, as reported earlier[l-51.

The carbenium ions, C3H7+, C2H5+, and C4Hg+, may release protons to form corresponding alkenes. Under the present conditions no hydride transfer mechanism is involved. The ratio of the three reactions, (1) - (3), can be estimated from the product ratio.

The selectivities thus estimated indicates that the three

Mechanism of Activation of Butanes and Pentanes

WIF

305

I g h rnol-'

Fig. 1. The change in the conversion of butane, the yield of hydrogen and methane over H-ZSM-5. Reaction conditions: T = 113 K, C4H10 = 8 kPa.

The yields is defined as moles of the product(H2, CH4) from 100 moles of the reactant.

W/F

/ g h rno1-l

Fig. 2. The change in the conversion of butane, the yield of hydrogen and methane over Zn-ZSM-5. Reaction conditions: T = 113 K, C4H10 = 8 kPa.

306

Y . Ono, K. Osako, M. Yamawaki and K . Nakashiro

reactions occur with the ratio of 40 : 40 :20. The cleavage of the C-C bonds is more prevailing than that of the C-H bond cleavage. The ratio was in good agreement with that reported in our previous uork12,3]. The relative rates are also in reasonable agreement with those reported recently by Krannila et al., 30 : 36 : 34[4].

A similar argument has been made about the activation of butane

over H-ZSM-5, by Shigeishi et a1.[5] The absolute rates of the three reactions were estimated from the results of Fig. 1 and listed in Table 1. Zn-ZSM-5.

Fig. 2 shows the conversion of butane, the yields of

methane,

ethane, and hydrogen as a function of the contact time over Zn-ZSM-5 at 773 K. In this case, the yields of propene and ethylene were slightly higher than those of methane and ethane, respectively. On the other hand, the yield of butenes is

slightly lower than that of hydrogen. These results indicate that a part

of

butenes are converted into smaller alkenes under the reaction conditions. Nevertheless, the relative rates of the primary reactions can be estimated from the yield ratio of the primary products, methane, ethane and hydrogen.

The ratio of the rates of the three reactions was estimated as 11 : 5 : 84. Thus, the incorporation of Zn ions into H-ZSM-5 changes the main primary reaction to the dehydrogenation. The absolute rates of the three reactions are listed in Table 1. The dehydrogenation, eq,(6), over Zn-ZSM-5 is about 20 times faster than that over H-ZSM-5. The formation of methane is also faster over Zn-ZSM-5. This result indicate that the metal cations are directly involved in the C-C bond cleavage as well as the C-H bond cleavage. Aq-ZSM-5 The rates of the three reactions, (4) - (6) over Ag-ZSM-5 were estimated and listed in Table 1. As expected, the introduction of Ag+ ions to H-ZSM-5 enhanced the dehydrogenation activity about 3.7 times. In contrast to Zn-ZSM-5, the activity for the C-C bond cleavage was depressed by Ag-loading. This results in higher selectivity for the dehydrogenation compared with Zn-ZSM-5. Thus, the ratio of the rates of reactions(4)-(6) is 94 : 4 : 2. Isobutane H-ZSM-5. The cracking of isobutane over H-ZSM-5 plained by the following mechanism[2 - 4, 7, 81.

( SiO,/A1203

=

43.5) is ex-


307

Table 1 Rates of formation of primary p r o d u c t s in a l k a n e conversions over H-, Zn-, Ga-, and Ag-ZSM-5. React ants

Cata 1yst H- ZSM- 5 Zn-ZSM- 5 Ag- ZSM- 5

Butane

Isobutane

Pentane

Isopentane

1.3 7.0

0.3 5.8

4.8

4.5

CH4

C2H6

0.5

0.5 0.4

0.7 0.2

1.0 45

H- ZSM- 5 Zn- ZSM- 5 Ga-ZSM-5 Ag- ZSM- 5

1.5 11 17 8.4

14 7.8

0.3

0.6 1.0 0,7 0.5

H- ZSM- 5 Zn-ZSM- 5 Ga- ZSM- 5 Ag- ZSM- 5

2.2 23

1.0 19 38 20

0.9 4.2

0.3 0.2

0.5

53 20

0.5

36 27

27

8.1 0.2

0.3

0.4 8.9

~~

0.9 1.5

13 0

~

0.5 0.3 0.2

0 0 0 0

1.6 0

1 .o 28 0.4

28 0.7

mol h-'

Reaction conditions: 773 K, 8 kPa. (Rates: Table 2.

0.2

~~~~

1 .o

H-ZSM-5 Zn-ZSM- 5 Ag- ZSM- 5

C3H8

0.1

H-ZSM- 5 Zn-ZSM-5 Ag- ZSM- 5

~

Neopentane

H2

Total

9-l)

The distribution of primary products in pentane cracking.

Catalyst

CH4

C4H8

C2H6

C3H6

C3H8

C2H4

H2

H-ZSM-5 Zn-ZSM-5 Ga-ZSM-5 Ag-ZSM-5

19 8 9 4

17 6 9

38 9 4 6

61 63 54 15

17

34 64 52 17

26 79 81 93

0

4 2 2

C5H10 iso-cg 0 0

7 24 31 79

4 0

number of moles formed from LOO moles of pentane reacted at 773 K. Table 3.

The distribution of primary products in isopentane cracking.

Catalyst

CH4

C4Hg

C2H6

c3H6

H2

C5H10

c3H8

C2H4

H-ZSM-5 Zn-ZSM-5 Ga-ZSM-5 AS-ZSM- 5

39 18 24

33 18 23 3

18

50 25 26 29

43 81 73 100

16 57

0 0 0

32 24 39 28

1

1

3 0

51

69

0 ~~

-

number of moles formed from 100 moles of isopentane reacted at 713 K.

308

Y.Ono, K. Osako, M . Yamawaki and K. Nakashiro

The relative rates of the two reactions, (7) and (8), at 773 K was estimated as 50 : 50 from the rate of the yields of methane and hydrogen. The absolute rates of the two reactions were listed in Table 1. In our previous work, the ratio was 37: 63 over H-ZSM-5(Si02/A1203 = 56[2], while Shigeishi et al. reported the ratio of 31 : 67 at 823 K[4]. Zn-ZSM-5. The primary reactions of isobutane over Zn-ZSM-5 are explained by the following two reactions.

'

C3H6

1 H2 +

C4H8

/ iso-C4H10

CH4

From the initial rates of formation of methane and hydrogen, the ratio of the two reactions was estimated. By loading Zn on H-ZSM-5, the rate of dehydrogenation increased about 45 times. At the same time, the C-C bond cleavage increased about 16 times. As the result, the ratio of the two reactions is 18 : 82. The extents of the rate enhancement of the C-C and C-H bond cleavage by loading Zn into H-ZSM-5 is much greater in isobutane cracking than that in butane cracking. Aq-ZSM-5. The rates of the formation of methane and hydrogen were determined. Ag-ZSM-5 is very unique in the high selectivity for the dehydrogenation: The dehydrogenation over Ag-ZSM-5 was 54 times greater than H-ZSM-5, while the methane formation was greatly depressed by Ag loading. Consequently, the ratio of the reactions, ( 9 ) and (10) is 99 : 1. Pentane H-ZSM-5. Four primary reactions are possible in pentane cracking.

The distribution

of the primary products are determined by

extrapolating

the


309

yields of each product to zero conversion and listed in Table 2. The ratio of the products of CH4, C2H6, C3H8, H2 gives the ratio of the rates of the four reactions, (11) - (14). The smaller yield of pentenes compared with that of hydrogen indicates that pentenes(or the precursor carbenium ions) are decomposed to ethylene and propene, in conformity with their formation in excess of propane and ethane, respectively. The ratio of the rate of the C-C bond cleavage and that of the C-H bond cleavage is 73 ; 27. The rates of the four reactions are estimated and listed in Table 1. Zn-ZSM-5. The primary products of pentane cracking is also explained by the four reactions, (11) - (14). The distribution of the primary products are listed in Table 2. The most predominant product is hydrogen. Ethylene and propene formed significantly probably through pentene(or the precursor). The ratio of

the rates of the C-C cleavage and the C-H bond cleavage is 21

: 79.

The rates of the four reactions are listed in Table 1. The rate of the dehydrogenation over Zn-ZSM-5 is much higher than that over H-ZSM-5. At the same time, the rate of the C-C bond cleavage is enhanced more than two times. Though no listed, both the rates of the cleavage of the C-C bonds and C-H bonds are further increased by increasing the loading amount of Zn. Ga-ZSM-5. The distribution of the products over Ga-ZSM-5 is similar to that over Zn-ZSM-5 (Table 2). As shown in Table 1, both the rates of the cleavages of the C-C bonds and C-H bonds are enhanced. ACT-ZSM-5. The distribution of the primary products and the rates of the primary reactions are listed in Tables 2 and 1, respectively. Loading Ag’ cations on H-ZSM-5 enhances the dehydrogenation, but, in contrast to Zn or Ga loading, the rates of the C-C bond cleavage are rather depressed. Consequently, the selectivity for the dehydrogenation is higher over Ag-ZSM-5 than over Zn(or Ga)ZSM-5. IsoPentane . H-ZSM-5. The primary products in the cracking of isobutane over H-ZSM-5 are accounted for by the following three reactions.

‘3% ‘gH1O

The primary products in the reactions over H-ZSM-5 is shown in Table 3. The ratio of the yields of methane, ethane, hydrogen gives the relative rates of

310

Y . Ono, K . Osako, M. Yamawaki and K . Nakashiro

reaction (151, (16), and (17), namely 39 : 18 : 43. Thus, the rate ratio of the C-C bond cleavage and the C-H bond cleavage is 57 : 43. The high yields of propene and ethylene indicates the decompositions of C4Hg' and C5Hll' ions. The absolute rates of the primary reactions are listed in Table 1. Zn-ZSM-5. The three reactions, (15) - (17) explains the primary products over Zn-ZSM-5 as in the case of the reaction over H-ZSM-5. However, the distribution of the primary products and the absolute rates over Zn-ZSM-5 are very different from those over H-ZSM-5. The ratio of the three products, methane, ethane, and hydrogen, gives the relative rates of reactions, (15), (16), and (17); namely, 18 : 1 : 81. The rate of cracking over Zn-ZSM-5 is far greater than that over H-ZSM-5. The rates of the dehydrogenation (reaction 17) and the formation of methane (reaction 15) over Zn-ZSM-5 are 19 times and 4 times higher than those over H-ZSM-5, respectively. Ga-ZSM-5. The cracking over Ga-ZSM-5 is similar to that over Zn-ZSM-5, though the rate over the former is higher than that over the latter. The primary products and the rates of the primary reactions are listed in Table 3 and Table 1, respectively. Both the C-C and C-H bond cleavage are enhanced by loading Ga on H-ZSM-5. Aq-ZSM-5, The reaction of isopentane over Ag-ZSM-5 is unique. Over Ag-ZSM-5, the dehydrogenation exclusively proceeds, no C-C bond cleavage occurring. The dehydrogenation over Ag-ZSM-5 is about 20 times faster than that over H-ZSM-5. Neopentane H-ZSM-5. The cracking of neopentane over H-ZSM-5 gave only methane and isobutane as primary products in agreement with literature[9]. Zn-ZSM-5. The rate of isopentane cracking over Zn-ZSM-5 is 28 times higher than that over H-ZSM-5. Only the C-C cleavage occurred. These facts clearly show that Zn cations are directly involved in the C-C bond cleavage. Aq-ZSM-5. The cracking of neopentane is rather depressed by loading Ag' cations on H-ZSM-5. This shows the essential difference between Zn-ZSM-5 and AS-ZSM-5. MECHANISM OF CRACKING H-ZSM-5. The product distributions of the cracking of butanes and pentanes under the present conditions(8 kPa, at low conversion level, 773 K) are in accord with the monomolecular mechanism through pentacoordinated carbonium ions. The same conclusion has already been reported for cracking of butanes by Kanae and Ono[2,3], Shigeishi et a1.[4], and Krannila et a1.[5]. Kanae and On0 showed that the bimolecular activation is involved at higher conversion


31 I

level[2]. The C-C bond cleavage is faster than the C-H bond cleavage. The rate difference between n-alkanes and isoalkanes are not so large. Zn-ZSM-5. The cracking over Zn-ZSM-5 is also accounted for by monomolecular mechanism. Loading of Zn cations greatly enhances the rate of the dehydrogenation in the cracking of butanes and pentanes except neopentane. It also enhances the rate of methane formation from alkanes including neopentane. The order of the rate of the dehydrogenation among alkanes is as follows.

C

C

C-C-c

>

c-c-c-c

>

c-c-c-c-c

c-c-c-c

>

F c-c-c C

0

5.9

8.9

19

36

>

The numbers shown below each alkane are the rates in mol g-' h-I. The order follows the stability of the carbenium ions to be formed by hydride abstraction. Thus, isopentane and isobutane, which give tertiary carbenium ions by hydride abstraction, show the highest reactivity. Pentane and butane, which give secondary carbenium ions, show one-order smaller reactivity than isoalkanes. Neopentane, which would give primary carbenium ions by hydride abstraction, does not undergo the dehydrogenation. These results shows clearly that the reactivity of alkanes over Zn-ZSM-5 occurs through hydride abstraction from the reactant alkanes. The order of the rate of formation of methane is as follows.

F c-g-c

C

>

c-c-c

C

>

c-c-6-c

>

c-c-c-c-c

>

c-c-c-c

C

28

8.8

4.2

0.9

0.7

The rate order is again in agreement of the stability of the carbenium ions which are formed by the abstraction of methanide(CH3-) from the alkanes. Thus, neopentane, which gives the tertiary carbenium ion by methanide abstraction, shows the highest reactivity. Isopentane and isobutane, which give the secondary carbenium ions, show much less reactivity than neopentane. The reactivity of pentane and butane is very low. These results strongly indicate that the methane formation from alkanes over Zn-ZSM-5 involves methanide abstraction to form the carbenium ions. Ga-ZSM-5. The results for Ga-ZSM-5 are essentially same as those observed for Zn-ZSM-5. hq-ZSM-5. The rate of the cracking is increased by Ag-loading. In con-

312

Y . Ono, K . Osako, M. Yamawaki and K. Nakashiro

trast with the cracking over Zn-(or Ga-)ZSM-5, Ag-ZSM-5 is active only for the C-H bond cleavage. Thus, isobutane gives exclusively hydrogen and isobutene over Ag-ZSM-5, while it shows no activity for neopentane. The rate order for the dehydrogenation follows the stability of the carbenium ions, which would be formed by hydride abstraction from the alkanes. This indicates that the cracking occurs through the heterolytic cleavage of the C-H bonds. C

c-c-c

C

c-6-c-c

>

20

27

>

c-C-C-C-C 7.8

>

c-c-c-c 4.3

>

? c-c-c C

0

Alkane activation on metal cations. Based on the results discussed above, the mechanism for alkane activation on the metal cations is expressed as follows.

RH

+

Mn+ - .

[M-H]("-')+

+

R+

(M = Zn. Ga. Ag)

(18)

Metal cations, Mn+, in the zeolite channels abstract H- or CH3- ions from alkanes to form carbenium ions, (reaction (18), (20)), Mn+ ions are regenerated by reactions (19) and (21). The carbenium ions are decomposed into alkenes and H+. It is probable, however, that reactions (18) and (19) or reactions ( 2 0 ) and (21) occurs in concerted manner, instead of the consecutive reactions. '

REFERENCES 1. Y. Ono, Catal. Rev.- Sci. Eng., 34 (1992) 179. 2. K. Kanae, Y, Ono, J. Chem. SOC., Faraday Trans. 87 (1991) 663. 3. Y. Ono. K. Kanae, K. Osako, K. Nakashiro, Mat. Res. SOC. Symp. Proc., 233 (19911 3. 4. R. Shigeishi, A. Garforth, I. Harrris, and J. Dwyer, J Catal., 130 (1991) 423. 5. H. Krannila, W. 0. Haag, B. C. Gates, J. Catal., 135 (1992) 115. 6. K. Kanae, Y. Ono, J. Chem. SOC., Faraday Trans. 87 (1991) 669. 7. E. A. Lombardo, K. W. Hall, J. Catal., 112 (1988) 565. 8. W. K. Hall, E. A. Lombardo, J. Engelhardt., J. Catal., 115 (1989) 611. 9. E. A. Lombardo, R. Pierantozzi, K. W. Hall, J. Catal., 110 (1988) 171.

Conversion of Ethane into Aromatic Hydrocarbons on Zinc Containing ZSM-5 Zeolites Prepared by Solid State Ion Exchange

A. Hagen, F. Roessner

University of Leipzig, Department of Chemistry, Institute of Technical Chemistry, Linnistr. 3, D-04103 LEIPZIG, F.R.G.

ABSTRACT ZSM-5 zeolites modified with zinc by different methods were studied in the conversion of ethane into aromatic hydrocarbons. A solid state ion exchange proceeds at mechanical mixtures of ZnO+H-ZSM-5 at temperatures above about 720 K, leading to zinc ions located at cationic positions. These cations are proved to be the active species in the conversion of ethane. Moreover, Brgnsted acid sites seem to be not necessary for the formation of aromatic hydrocarbons starting from ethane. INTRODUCTION The requirements for higher processing of natural resources on one hand and for solution and prevention of ecological problems on the other have stimulated investigations in the field of conversion of lower paraffins into useful products. The title reactant is a constituent of natural oil and gas (depending on the source up to 20 %) as well as of the effluent of mineral oil refining processes like cracking and reforming and is normally combusted.

ZSM-5 zeolites modified with noble metals like Pt [l-31 or cations like Zn" [4-61 are proved to catalyze the conversion of lower alkanes into aromatic hydrocarbons. By using ethane as reactant, modifications are necessary to achieve relevant conversions. At temperatures of 773 and 823 K conversions of ethane of around 60 and 80 wt.%, respectively, could be observed on zinc containing H-ZSM-5 zeolites independently on the method of preparation (ion exchange with Zn(N03),-solution, impregnation with Zn(N0,Lsolution or mechanical admixing of ZnO) [5]. These results imply that, in all cases, the same kind of zinc species is responsible for the high aromatization activity. Zinc ions located at cationic positions of the ZSM-5 zeolite are discussed to be these species [5-91.

313

314

A. Hagen and F. Roessner

The aim of this paper is to get more insight in the formation and role of active zinc species and Bransted acid sites in the aromatization reaction starting from ethane.

METHOD Sample preparation and characterization ZSM-5 zeolites were synthesized with template (H-ZSM-S(t), B.V., The Netherlands) and without template (H-ZSM-5,

Si/Al=18, PQ Zeolites

Si/Al=15, Chemie AG Bitterfeld,

Germany). In both cases, crystallinities were around 100 % as proved by X-ray diffraction. Zinc containing ZSM-5 zeolites were prepared by (i) 3-fold ion exchange with zinc nitrate solution at 353 K (Zn-ZSM-5)

or (ii) mechanical admixing of ZnO (ZnOtzeolite). Zinc

contents were 2.4 wt.% (Zn-ZSM-5)

and 2.0 wt.% (ZnOtH-ZSM-5).

One sample of the

zeolite synthesized without template had been treated in a mill for 3 h before mechanical mixing with ZnO (ZnOtH-ZSM-S(t)'). A Na-ZSM-5

was prepared by 3-fold ion exchange with sodium nitrate solution at

353 K starting from H-ZSM-5

synthesized without template.

Crystal shapes and sizes of zeolite particles were determined by transmission electron microscopy ("EM). For i.r. spectroscopic investigations samples of 6 to 10 mg/cm2 were in-situ activated at 775 K for 2 h in ultra-high vacuum. Spectra were recorded in transmission in the range from 2000 to 4000 cm-' at 298 K. Pyridine was adsorbed at 475 K for 2 h with p,=0.7 kPa and desorbed at the same temperature for 1 h. X-ray absorption spectra were measured at the Zn K edge in transmission and excited using synchrotron radiation. Catalytic investigations An amount of 0.5 g of the catalyst as a grain fraction of 0.2-0.3 mm was diluted with

0.75 g SiO, and studied in a quartz plug flow microreactor at a GHSV of 600 v/vh (pure ethane) and normal pressure. Samples were in-situ pretreated at 723 K in air flow for 2 h. The reaction products were analyzed with on-line gaschromatograph on a PONA column applying a temperature programme from 243 to 493 K.

RESULTS AND DISCUSSION In Tab. 1 the results of "EM are summarized. The crystals of H-ZSM-S(t) than those of H-ZSM-5.

are smaller

Moreover, they form large agglomerates. After treating this sample

Ethane Aromatization on Zn-ZSM-5

315

Table 1. Sizes of zeolite crystals and agglomerates determined by TEM sample average size of diameter of agglomerates I*m zeolite crystals / pm 0.5*1.5 0.1*0.5 0.1*0.5

H-ZSM-5 H-ZSM-S(t) H-ZSM-S(t)'

no agglomerates 0.4 to 2.4 0.3 to 0.5

most of the agglomerates are broken into fragments. The zeolite

in a mill (H-ZSM-S(t)'),

crystals themselves, however, were not altered. The catalytic investigations based on the result that the method of introduction of zinc into H-ZSM-5

(ion exchange, impregnation or mechanical mixing) did not influence the

conversion of ethane at 773 and 823 K [S]. Furthermore, i.r. and t.p.d.

investigations

indicated that zinc ions located at cationic positions seeming to be active for this reaction can be formed via solid state ion exchange (SSIE) during thermal treatment at 773 K [S], [9] or 853 K

[lo]. This

process can be formally expressed by the following equation:

ZnO + H +zeoliteH +zeolite-

=

z

~ zeolite+ + H20

(1)

zeolite-

Looking at the course of the conversion of ethane with time-on-stream

at 773 K, an

induction period of about 1 to 2 h is observed for a mechanical mixture ZnO+H-ZSM-5 after which a steady conversion is maintained within about 15 h. Fig. 1 shows this steady state value in dependence on the reaction temperature on the mechanical mixture ZnO+H100

r 1

GI

9 5

+_

80 ....................................................................

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

60 _ .............................................................

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

c d

Ei L

:

40 _ ............................................

_

20 ...............

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

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

U 0

I

01 500

600 700 temperature I K

800 + 900

Fig. 1. Conversion of ethane vs temperature, GHSV=600 v/vh, 2 h time on stream, ZnZSM-5 (V), ZnOtH-ZSM-5 (0) ZSM-5 and, additionally, a zinc exchanged H-ZSM-5. as main products in both cases.

Aromatic hydrocarbons are formed

316


The conversion on Zn-ZSM-5

nearly corresponds to thermodynamic calculations which

were carried through on the basis of the following reactions:

C2H6

4CzH6 C2H6

\

-

Q

+ m

’

+

6H2

(2)

CH4 + 6 H 2

(3)

H

2

(4)

On the other hand, the mechanical mixture ZnOtH-ZSM-5

shows low conversions at

low temperatures but a sharp raise to the level of Zn-ZSM-5 at temperatures above 720 K. Obviously, this temperature is necessary to mobilize zinc ions or ZnO for SSIE. Salzer et al. [ll] concluded from results of in-situ diffuse reflectance Fourier transform i.r. (DRIFT) that the formation of water which is product of SSIE (reaction 1) takes places inside the zeolite channels and, consequently, ZnO is the migrating species and not the zinc ion. The short induction period in ethane aromatization on the mechanical mixture ZnO+HZSM-5 as well as comparative DRIFT investigations (111 reveal the fast procedure of the SSIE process on this zeolite which is finished within 1 to 2 h.

On the contrary, a mechanical mixture ZnO+H-ZSM-S(t) starting from the zeolite synthesized with template shows an induction period of about 3 to 4 h as depicted in Fig. 2. The slope of the curve is a measure for the rate of SSIE.

L.

0

1

2 3 4 time on stream I h

5

6-7

Fig. 2. Conversion of ethane related to conversion after 3 h time on stream vs time on stream, GHSV=600 v/vh, 773 K, Zn-ZSM-S(t) (0),ZnOtH-ZSM-S(t) (A), ZnO+H-ZSM5(t)’ (mill) (0) There are two possible explanations for the, obviously hindered, SSIE process: (i) inho-

Ethane Aromatization on Zn-ZSM-5

317

mogeneous distribution of aluminum in the zeolite lattice over the crystallites (enrichment within the crystal) or (ii) hindrance of the transport of zinc species from the outer surface to the protonic, exchangeable sites which could be caused by the crystallites forming large

agglomerates. The latter reason is apparent from TEM photos (see Tab. 1). The treatment of this H-ZSM-S(t)

in a mill leads to a break of the agglomerates thereby shortening the

ways of transport of zinc species for SSIE (see Tab. 1). Indeed, the induction period of the mechanical mixture ZnOtH-ZSM-S(t)'

decreases in comparison to the untreated sample

(see Fig. 2). Finally, as expected, a Zn-ZSM-S(t)

prepared by ion exchange in solution

shows no induction period (Fig. 2). The active zinc ions are located at cationic positions right from the start of the reaction. The dominating role of zinc species for the aromatization activity is apparent from Tab. 2.

Table 2. Conversion and yield of key products in the reaction of ethane at 773 K after 2 h time on stream (B: benzene, T toluene, G: xylenestethylbenzene, Cat: naphthalene, methylnaphthalene) sample conversion yield 1 I wt.% wt.%

ZnOISiO, Na-ZSM-5 H-ZSM-5 ZnOtNa-ZSM-5 ZnlNa-ZSM-5

0.44 0.49 2.11 5.36 5.71

methane

ethene

BTC,aromatics

C,t-aromatics

0.00 0.01 0.57 0.10 0.30

0.21 0.22 0.52 4.04 1.44

0.00 0.02 0.31

0.00 0.00 0.02 0.05 0.71

On ZnO supported at SO, (ZnOBiOJ and Na-ZSM-5

0.55 2.21

almost no conversion of ethane

can be achieved. Only ethene is formed on ZnOISiO,, traces of methane and lower aromatic

hydrocarbons could be detected as products on Na-ZSM-5. activity is observed on H-ZSM-5.

An increase of aromatization

Besides aromatic hydrocarbons and ethene remarkable

amounts of methane are formed. However, using a mechanical mixture of ZnOtNa-ZSM-5

the conversion increases

dramatically. Large amounts of ethene are formed and higher yields of aromatic hydrocarbons are obtained. If zinc is introduced by ion exchange in solution (denoted as m a ZSM-5) the conversion slightly increases. The highest amounts of aromatic hydrocarbons are formed.

318


Using temperature programmed desorption of ammonia no acid sites could be found at ZnO/SiO, [5]. Tab. 3 summarizes the concentrations of Bransted acid sites and zinc ions in Na-ZSM-5,

differently treated mechanical mixtures ZnOtNa-ZSM-5

and zinc exchanged

Na-ZSM-5 determined by i.r. spectroscopy and chemical analysis. Traces of Bransted acid sites are the reason for the activity in the conversion of ethane observed on Na-ZSM-5 (see also Tab. 2). Table 3. Concentrations of Bransted and Lewis acid sites caused by zinc ions calculated from intensities of i.r. bands at 3605 cm-', after adsorption of pyridine at 1455 and 1545 cm-' and from results of chemical analysis sample [Bronsted acid sites] [Zn] / mmol/g I mmol/g Na-ZSM-5 ZnOtNa-ZSM-5' ZnO+Na-ZSM-5* ZnAVa-ZSM-5

0.035 0.000 0.000 n.d."'

0.000 0.028 0.027 0.085

' after thermal treatment 5 h at 823 K in air '' after reaction with ethane 5 h at 773 K '" not determined Mechanical mixtures ZnOtNa-ZSM-5

do not contain Bransted acid sites after treatment

in air at 823 K or ethane at 773 K. The disappearance of these centers can be explained by SSIE between protons of the zeolite and zinc ions of ZnO which can be followed by i.r. spectroscopy. ZnNa-ZSM-5

incorporates the highest amount of zinc ions.

X-ray absorption spectroscopy (XANES) was used to support the assumption about SSIE in the system ZnO+Na-ZSM-5. The spectra depend on the geometrical arrangement of atoms in a local cluster around the absorbing atom [12]. Investigations showed that X A N E S analysis allows to distinguish between zinc ions chemically bound in ZnO and

those located at cationic positions in ZSM-5 zeolite. It could be estimated that less than about 0.06 mmol/g zinc are located at cationic positions in consequence of SSIE during 5 h reaction of ethane on ZnOtNa-ZSM-5

at 773 K. These results are within the precision of

the method in good agreement with those obtained using i.r. spectroscopy (see Tab. 3). Moreover, the intensity of the main absorption structure of the mechanical mixture ZnOtNa-ZSM-5

studied after the reaction with ethane indicates to ZnO finely dispersed in

the zeolite channels. This finding supports the results of Salzer et al. [ l l ] of ZnO being the migrating species in SSIE.

Ethane Aromatiration on Zn-ZSM-5

319

CONCLUSION Catalytic conversions of ethane on zinc containing H- and Na-ZSM-5

zeolites showed

that zinc ions located at cationic positions are necessary for high aromatization activities. These species can be formed via SSIE between protons of the zeolite and zinc ions. This process proceeds during the reaction with ethane at temperatures above about 720 K. The structure of the microcrystallites influences the rate of the SSIE process. Large crystallites provide a nonhindered transport of zinc species from the outer surface to Bransted acid sites within the zeolite channels and, consequently, a fast SSIE process. 1.r. spectroscopic investigations proved a SSIE in the system ZnOtNa-ZSM-5

whereas

traces of Brplnsted acid sites were left in Na-ZSM-5. These sites were completely exchanged by zinc ions thereby creating centers active for the conversion of ethane. The exchange of a small part of sodium ions can not be excluded but seems to be rather improbably. Thus recent investigations of e.g. Osako et al. [9] gave no evidence for a SSIE between sodium and zinc ions in a mechanical mixture of ZnO and Na-ZSM-5. At the same system it was shown that XANFS exhibits a useful method to follow SSIE. This technique is advantageously applicable to systems which can not be directly

investigated by other conventional methods like e.s.r. or i.r. spectroscopies. Furthermore, hydrated and coked zeolites can be treated without problems. The conversion of ethane into aromatic compounds is possible on ZSM-5 zeolites containing zinc ions located at cationic positions, even in the absence of Bransted acid sites. Zinc ions are effective centers for activation of ethane, possibly via hydride abstraction. Furthermore, the formation of aromatic hydrocarbons proceeds on zinc centers, probably via insertion of olefinic products into intermediate hydrocarbons strongly adsorbed at zinc centers. Due to their hydrogen abstraction properties zinc species catalyze the final dehydrogenation of cyclic intermediates to aromatic hydrocarbons. The formation of methane as undesired product is suppressed in comparison to the reaction on H-ZSM-5.

Because of

the decrease or even absence of Brensted acid sites, protolytic cracking leading to methane

is restricted. Other reactions accompanied by the formation of methane like dealkylation seem to be not catalyzed by zinc ions.

ACKNOWLEDGEMENT The authors thank Dr. S. Rock for providing zeolite samples H-ZSM-S(t). Moreover, we are grateful to Dr. H.G. Karge, Dr. K.-H. Hallmeier and Dr. H.-D. Neubauer for the

320


successful co-operation in the fields of i.r., XANES and electron microscopy. The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. REFERENCES 1 K.-H. Steinberg, U. Mroczek, F. Roessner, Appl. Catal., 66 (1990) 37. 2 E.S. Shpiro, R.W. Joyner, G.J. Tuleuova, A.V. Preobrashenski, O.P. Tkachenko, T.V. Vasina, O.V. Bragin, Kh.M. Minachev, Stud. Surf. Sci. Catal., 65 (1991) 357. 3 T. Inui, Y. Ishihara, K. Kamachi, H. Matsuda, Stud. Surf. Sci. Catal., 49 (1989) 1183. 4 M.S. Scurrell, Appl. Catal., 41 (1988) 89 5 F. Roessner, A. Hagen, U. Mroczek, H.G. Karge, K.-H. Steinberg, Stud. Surf. Sci. Catal., 75 (1993) 1707. 6 Y. Ono, H. Nakatani, H. Kitagawa, E. Suzuki, Stud. Surf. Sci. Catal., 44 (1988) 279. 7 T.V. Vasina, V.P. Sitnik, A.V. Preobrashenski, L.J. Lafer, V.J. Jakerson, O.V. Bragin, Izv. M a d . Nauk SSSR, 3 (1987) 528. 8 E. Iglesia, J.E. Baumgartner, G.D. Meitzner, Stud. Surf. Sci. Catal., 75 (1993) 2353. 9 K. Osako, K. Nakashiro, Y. Ono, Bull. Chem. SOC.Jpn, 66 (1993) 755. 10 Y. Yang, X. Guo, M. Deng, L. Wang, 2. Fu, Stud. Surf. Sci. Catal., 46 (1989) 849. 11 R. Salzer, U. Finster, F. Roessner, K.-H. Steinberg, Analyst, 117 (1992) 351. 12 A. Bianconi, X-ray Absorption, Principles , Applications. Techniques of EXAFS, SEXAFS and XANES, Wiley, New York, 1988, p. 573.

Platinum-Nickel/Lzeolite Bimetallic Catalysts Effect of Sulfur Exposure on Metal Particle Size and n-Hexane Aromatization Activity and Selectivity

Gustavo Larsen#ll Daniel E. ResascoZ*, Vincent A. Durante2, Jae Kim1 and Gary L.Hallerl 1Department of Chemical Engineering, Yale University, P. 0. BOX2159, New Haven, CT 06520, USA 2Sun Refining and Marketing Company, Research and Development, P. 0. Box 1135, Marcus Hook, PA 19061-0835,USA

ABSTRACT Ex erimental evidence that Ni can be used to stabilize Pt particles supported in L-zeolite against eactivation by sulfur is presented. A series of Pt-Nib-zeolite bimetallic catalysts prepared by co-impregnation were tested for n-hexane aromatization and characterized by Extended X-ray Absorption Fine Structure (EXAFS), hydrogen chemisorption and Transmission Electron Microscopy (TEM) before and after sulfiding. It was found that bimetallic particles are significantly less subject to sulfur-induced particle growth than their monometallic (Pt) counterpart. This mechanism of catalyst fouling is known to cause deterioration of Pt/L aromatization catalysts.

B

INTRODUCTION While conventional reforming catalysts are bi-functional (utilize support acidity as well as metal dehydrogenation/hydrogenation functionality), it has been demonstrated that Ptb-zeolite reforming catalysts [I] may accomplish aromatization using the Pt only functionality and that acidity can be detrimental to optimum performance [21. The low sulfur tolerance of Pt/L-zeolite catalyst has been well documented [31; recently it has been demonstrated that the effect of sulfur is not the result of simple poisoning “6 51. By mechanisms which are not fully understood, sulfur promotes Pt crystal growth and movement of Pt out of the zeolite channels. The sulfur intolerance of PtL-zeolite catalysts is recognized as one of the major inhibitions to the commercial use of these catalysts for the production of benzene or other aromatics or for petroleum reforming. Thus, it is of interest to find catalyst modifiers which might stabilize Pt against crystal growth. One approach is to use first row transition metal cations to anchor Pt particles, a strategy recently demonstrated for Pt in Y-zeolite using Fe2+ [a, 71. Another example of this approach in Y-zeolite is the use of C r 9 to anchor metallic Rh particles [8]. Current address: Department of Chemical Engineering,University of Nebraska, Lincoln,NE 66510 USA * Current address: Department of Chemical Eng.ineering, University of Oklahoma, Norman, OK 73019 USA 32 1

322

G. Larsen, D. E. Resasco, V . A. Durante, J. K i m and G. L. Haller

We have been interested in Pt-Ni combinations on non-zeolite supports for the purpose of influencing the selectivity between hydrogenation of C=C and C=O groups in the same molecule and as sulfided versions for dehydrogenation of alkanes [9]. Ni is also the most active hydrogenolysis catalyst among the group VIIIB first row transition metals (Fe, Co and Ni) [lo]. However, Ni is in the same periodic group as Pt, has a stable 2+ oxidation state (higher oxidation states tend to undergo hydrolysis and impart acidity in the zeolite) and will readily form alloys and bimetallic clusters with Pt [ll]. Thus, it was of interest to examine whether a Ni ion could affect the movement of Pt to the external surface of zeolite L in the presence of S. EXPERIMENTAL Preliminary work using co-ion exchanged and sequentially ion exchanged (Pt ion exchange and reduction followed by Ni ion exchange and reduction) metals was not successful as judged by the very poor dispersion, measured by H2 chemisorption, that resulted. Bimetallic catalysts prepared by co-impregnation of the KL-zeolite with appropriate amounts of NiN03 and Pt(NH3)4(NO3h (supplied by Alfa Products) aqueous solutions exhibited reasonable dispersions even when Pt loading exceeded 5 wt% [12]. The precursors were calcined at 723K in flowing 0 2 and subsequently reduced for 8 hr under H2 flow at 773K The pure Pt catalyst is labeled Pt/KL, while the bimetallic catalysts are referred to as Pt-Ni/KL with the Pt mole fraction indicated, e.g., 0.70 PtNi/KL designates the catalyst which is 70 at% Pt and 30 at% Ni. Portions of these catalysts were sulfided with dimethyl sulfoxide. The catalysts were rereduced at 773K for one hour in a 100cc/min flow of H2; then dimethyl sulfoxide was injected (200 cc per g of catalysts or a S/Pt exposure ratio of about lo), followed by a further 2 hrs of stripping in H2 flow at 773K The sulfided catalysts are identified with a further prefix, e.g., S,0.44 Pt-Ni/KL. All samples were analyzed for Ni, Pt and S by Galbraith Labs, Inc. The sulfided catalysts were examined by TEM (Phillips EM 410 electron microscope, bright field mode, 153,000 magnification). All four sulfided catalysts were examined in duplicate. X-Ray absorption experiments were performed at the National Synchrotron Light Source (NSLS), lines X18B and X23A2, and at the Comell High Energy Synchrotron Source (CHESS), line C-2. In-situ catalyst reduction was carried out and X-ray absorption spectra were collected at 77K for EXAFS analysis. The analysis programs have been described by McHugh [13]. All catalysts were tested for the n-hexanehydrogen reaction, which was carried out in a 6 mm U-tube Pyrex reactor at 753K and partial pressures of n-hexane and H2 of 4.2 and 33.8 kPa, respectively, with the balance He to maintain atmospheric pressure. The gases were pre-mixed/preheated in an inert a-Al2O3 bed and the amount of catalyst adjusted in order to effect the desired conversion level. Ultrahigh purity H2 and He (supplied by Matheson Research) were further purified with 0 2 traps capable of reducing the 02 concentration below 50 ppb. n-Hexane (W%t purity) was purchased from Alfa. The desired n-hexane partial pressure was achieved by passing a mass-flow controlled He stream through a saturator held at 273K.

PT-Ni/L-zeolite Bimetallic Catalysts

323

RESULTS AND DISCUSSION The unsulfided catalyst composition and chemisorption data are presented in Table 1. Hydrogen uptakes were roughly one half or less of those typically encountered in well-dispersed Pt/L catalysts and further decreased with added Ni. We propose that rather than reflecting a particle size effect of Pt clusters, these data on samples with high loading are a consequence of pore plugging of the unidimensional L-zeolite channels which restricts the accessibility of adsorbate molecules to only a fraction of the total metallic surface. This hypothesis is supported by EXAFS data. A quasi-linear correlation between EXAFS-derived coordination numbers and hydrogen uptake has been found in P t L when Pt loading was varied up to 3.5 wt% [14].However, when our current EXAFS data on 5.31 wt% Pt/KL (see Table 2) are compared to the data in ref. [14],one would predict a higher hydrogen uptake for the more highly loaded samples were the clusters freely accessible. Thus, we suspect pore blockage to exist based on an extrapolation of the correlation between coordination number and hydrogen uptake to the loading characteristic of the samples described here. Table 1. Catalyst composition. Catalyst Pt/KL 0.70 Pt-Ni/KL 0.53 Pt-Ni/KL 0.44 Pt-Ni/KL

H/M 0.58 0.50 0.45 0.39

wt% Pt 5.31 5.43 5.02 5.12

wt% Ni

0.71 1.33 1.93

XPt 1.00 0.70 0.53 0.44

While a careful statistical particle size distribution was not determined by TEM, most metallic particles were estimated to be in the range of 10-30nm on the S,Pt/KL catalysts and 1-3 nm on the three S,Pt-Ni/KL catalysts. The latter size range is consistent with the average particle size of the unsulfided catalysts [12]. The Pt only and the highest Ni loaded catalysts were studied by the EXAFS technique before and after sulfiding. Results of these runs are reported in Table 2 below. The coordination number Npt-pt = 7.1 is consistent with a particle size of about 1.2 nm for P t K L which grows to Npt-pt = 9.0 after sulfiding. The latter value is equivalent to a particle size of about 2.0 nm [8]. Thus, the EXAFS analysis confirms that particle size increased with sulfiding. Combining E M S and TEM measurements, a global picture of the particle size distribution may be formulated. Our results suggest a bimodal distribution of particles; those within the zeolite channels which are mostly not seen in the TEM and of order 1.2 nm and those particles outside the zeolite at about 25 nm. From a simple volume average mass balance (assuming that the large particles outside the zeolite have a Npt-pt = 12), one finds that about 40% of the Pt has migrated outside the zeolite. Although the particle size is larger following sulfiding (which usually implies that particles are more ordered), one should also note that the Debye-Waller term, DWpt-pt, has

324

G. Larsen, D. E. Resasco, V. A . Durante, J . Kim and G . L. Haller

increased indicating that sulfiding causes disorder as well as particle growth. Since the RPt-pt is identical to that of bulk Pt, Rpt-pt = 0.277 nm and there is no evidence for bulk PtS, this disorder is not short range. Table 2. EXAFS determined coordination number, Nx-y, interatomic distance, Rx-y, in nm, and Debye Waller term, DWx-y, in & (where X is the absorber and Y the scatterer).

EXAFS Parameters Npt-Pt NPt-Ni NNi-Ni NNl-Pt RPt-Pt RPt-Ni RNI-Ni RNI-Pt DWPt-Pt DWPt-Ni DWNI-Ni DWNI-Pt

Pt/KL

S,R/KL

7.1

9.0

0.277

0.277

0.0005

0.0010

0.44 R-Ni/KL

S,0.44 Pt-Ni/KL

3.8 1.6 3.9 1.4 0.272 0.263 0.254 0.263 0.0032 0.0017 0.0017 -0.0009

3.6 1.8 4.1 1.8 0.272 0.264 0.254

0.264 0.0028 0.0027 0.0020 0.0036

The bimetallic EXAFS results must be interpreted with some caution because the analysis is l complex requiring a total of 14 parameters (the 12 shown in Table 2 plus EO for both the Pt L ~ land Ni Kedges). Seven of these must be varied simultaneously for each edge spectrum to obtain a fit of the data to a model. One of variables can be eliminated by requiring that RPt-Ni = RNi-pt, but there may still be several local minima in the fit. Moreover, there is no simple relationship between the coordination numbers of the bimetallic particles and their size since this must depend on the structure and shape. One can say that the particles are bimetallic since the RNi-Ni = 0.263 nm is significantly different from that of bulk Ni where it is 0.249 nm. The RNI-Ni = 0.263 nm is the same as that for large Pt-Ni clusters on S i 0 2 which can be determined to be PtNi alloy by X-ray diffraction [15]. However, the bimetallic clusters on L-zeolite are not homogeneous PtNi alloys since this would require that 2Npt-pt = Npt-Ni and 2NNI-Ni = "I-pt for a 1:l PkNi ratio, which is clearly not the case [la]. The fact that neither (NPt-pt + Npt-Ni) nor (NNI-Ni + "I-pt) change much with sulfiding is consistent with the E M finding that the particle size does not grow with sulfiding. Note also that the Debye Waller terms for all the bimetallics (except one) are greater than that for Pt/KL which would be consistent with the existence of bimetallic clusters since such clusters are expected to be more disordered than a single phase. (The DWNi-pt = -0.0009 is probably in error since it implies that the disorder, from the perspective of the Ni absorber in the bimetallic moiety, is less than in the bulk Ni reference, a situation which is not physically reasonable.)


325

The reaction kinetics for conversion of n-hexane over Pt-Ni/L catalysts was investigated at below atmospheric pressure over a period of time on stream from 15 to 135 minutes at low and at high conversion. Measurement of differential rates would allow specific rates to be estimated at the low conversion. However, we observed that the selectivity increased monotonically with conversion so that from a practical point of view one would wish to compare the highest possible selectivity at high conversions. Because one observes integral rates at high conversion, it is not possible to extract a true kinetic rate ratio without obtaining detailed kinetics over the whole range of conversion. There is a further complication that the catalysts suffered significant deactivation with time on stream. The deactivation appeared to be mostly the result of coke deposition on the metal because reactivation by simple re-reduction produced about a 80% recovery of the initial rate. A further indication that the deactivation was primarily the result of coke deposition on the metal was made evident when the deactivation rates were compared on the sulfided and unsulfided catalysts (at both low and high conversion). The unsulfided catalysts lost about 50 60% of their activity in the first 135 min on stream, but the sulfided catalysts lost 4 0 % of their activity in the same period. A selection of data are presented in Table 3 at 75 minutes on stream, a period after which most of the deactivation had occurred. The sulfided catalysts show higher selectivities towards dehydrogenation (c6=formation) and lower selectivity towards methane formation than do non-sulfided catalysts. At low conversion the sulfided catalysts suffer a large lost of the desired selectivity to benzene because hexene, c6=, formation competes. However, at high conversion there is little decrease in the benzene selectivity. This is reassuring since low pressure reaction studies may be suspect because they may involve a component of gas phase cyclization (of hexatriene) which can be suppressed at high H2 pressure. The fact that sulfiding preferentially suppressed benzene formation relative to hexene formation at low conversion suggests that the gas phase contribution was small in these experiments if it can be assumed that dehydrogenations subsequent to hexene formation track hexane dehydrogenation. The turnover frequencies (for low conversion at the conditions given in Table 3) of the unsulfided catalyst differ by about a factor of two, from 0.14 s-1 for Pt/KL to 0.063 s-1 for 0.44PtNiKL. We have argued that a linear relation between EXAFS coordination number and H/Pt that exists for catalyst loadings between 0.98 and 3.5 wt% Pt breaks down for the 5.31 wt% Pt/KL of Table 1 above and implies pore blockage [9]. The incremental pore blockage that would then result by addition of more metal (Ni) in the bimetallic catalysts would then explain why H/M decreases with added Ni. Since the turnover frequencies given here are normalized to H/M,they should already account for pore blockage and the decrease in turnover frequency with increased Ni indicates that the bimetallic clusters are less active than pure Pt clusters for reactions of n-hexane. However, the relative activities per unit mass of catalyst (last column of Table 3) indicates that the improved sulfur tolerance of the bimetallics (measured at either low or high conversion) is improved by almost one order of magnitude in the 0.44Pt-NiKL catalyst relative to P t K L and that the sulfur tolerance increases monotonically with added Ni. Of course, this sulfur tolerance may be due to

-

326

G. Larsen, D. E. Resasco, V . A . Durante. J . Kim and G . L. Haller

either a resistance to sulfur induced sintering (clearly evidenced in both the "EM and the EXAFS) or resistance to sulfur poisoning or both. Table 3. Conversion and selectivity on n-hexane reaction at T = 753% P H =~ 33.8 @a, Pn-G = 4.2 kPa after 75 min on stream on fresh and sulfided catalysts. Catalyst

%Conv.

C1

C2-C5

Cg=

Bz

SBZ

4.4 6.2 2.9 6.9 2.8 10.2 2.4 10.7

5.3 1.2 4.7 1.2 3.1 1.5 2.6 3.1

0.51 0.16 0.58 0.15 0.49 0.13 0.48 0.22

Ra

Low Conversion Pt

s,pt 0.70Pt-Ni S,O.7OPt-Ni 0.53Pt-Ni S,O.53Pt-Ni 0.44Pt-Ni S,OAPt-Ni

10.3b 7.4

0.6

0.022

0.5 8.1 0.083 6.3b 0.4 0.12 11.7 5.4b 0.4 0.22 14.0 0.2 High Conversion Pt 39.6 4.8 34.8 0.88 s,pt 58.0 0.7 0.5 12.7 44.1 0.76 0.018 0.70Pt-Ni 39.8 5.3 0.3 0.1 34.1 0.86 S,O.7OPt-Ni 53.1 0.8 0.6 11.5 40.2 0.76 0.064 0.53Pt-Ni 54.4 7.5 0.4 46.5 0.85 S90.53Pt-Ni 55.1 1.1 0.9 12.2 40.9 0.74 0.077 0.44Pt-Ni 56.1 9.0 1.0 46.1 0.82 S,O.44Pt-Ni 61.4 4.5 3.4 9.2 44.3 0.72 0.16 YlXs is the relative activity of the sulfided catalyst based on conversion per unit mass of catalyst. wreating this as a differential conversion and using the H/M from Table 1 as a measure of site density, the turnover frequencies are estimated to be 0.14,0.14,0.11 and 0.063 s-1, respectively, at the conditions given. 8.lb

-

A new series of catalysts of lower Pt and Ni loading (scaled down by a factor of five) were prepared to improve metal efficiency (to avoid channel blocking). The same preparation and sulfiding procedure as described in the experimental section was used. However, in addition to the lower metal loading, we attempted to achieve a lower sulfur exposure, i.e., a S/Pt 1 was the goal. Only four of these catalyst have been characterized by chemisorption, n-hexane reaction and EXAFS.Unfortunately, the low Ni loading precluded Ni EXAFS. Compositional analysis is shown in Table 4. The labeling convention is as before, e.g., 0.42 Pt-Ni implies a 42 at% Pt. One must note that the low H/M observed for the 0.42 Pt-Ni is unlikely the result of metal pore blocking, as we suggested for the higher metal loaded catalysts. The increase in Npt-pt with sulfiding of the Pt only catalyst is consistent with particle growth and, within the rather large error,

-


327

the sum of Npt-pt + Npt-Ni is not changed by sulfiding. In this sense, the lower loaded catalyst confirm the results described for the higher loaded catalysts, i.e., Ni inhibits Pt agglomeration. Table 4. Chemical composition of low metal loading Pt-Ni catalysts. Catalyst Pt s,pt 0.42 Pt-Ni

wt% Pt

WM

NPt-Pt

1.0

1.6

3.9

1.0

0.27

5.6 4.8 4.1

0.71 0.43 S,0.42Pt-Ni 0.70 032 a Both S and Pt are from chemical analysis.

NPt-NI

spta

1.5 3.1

2.6

1.6

The activity (after 75 minutes on stream, high conversion) of the sulfided relative to the unsulfided 0.42 Pt-Ni catalysts is essentially the same as for the higher loading (0.12 compared to 0.16, see Table 3). However, the very small particles of Pt appear to be more resistant to sulfur effects since the relative conversion per unit mass of catalysts is 0.37 compared to 0.018 for the 0.44 Pt-Ni/KL which was 5.12 wt% Pt. Perhaps this can be understood based on the fact that even after sulfiding the increased Npt-pt (from 3.9 to 5.6) implies particles small enough to remain in the zeolite pores, but the higher loaded pure Pt catalyst has a Npt-pt of 9.0 (see Table 2) after sulfiding which indicates that much of the Pt is outside the zeolite pores. In any case, the advantage of Ni is less apparent for the catalyst which has both a lower Pt loading and a much lower sulfur exposure although in both cases the EXAFS indicated that the Ni inhibits sulfur catalyzed Pt aggomeration. Figure 1 shows the benzene yield of all the low loading catalysts (four sulfided and four unsulfided) at both low and high conversion levels (similar to the results presented in Table 3 for the higher loading). It appears that it is the number of sites active for benzene that is modified upon Ni addition and sulfiding rather than a mechanistic change in the reaction pathways since all data points fall onto the same curve. It should also be noted that these results are very similar to those of McVicker et al., see Figure 8 of ref. [5]. There were several experimental conditions different in the two sets of experiments, e.g., reaction temperature of 783K for McVicker et al. and 753K here. (The definition of selectivity and yield also differ slightly since ours are based on mole ratios and those of McVicker et al. on weight ratios.) However, the biggest difference is the total reactant pressure which was low in our case, 4.2 kPa n-hexane/33.8 kPa H2,and high in theirs, 118 kPa n-hexaneD07 kPa Ha.At any given benzene yield (conversion), the low pressure experiment produced a higher benzene selectivity The overall shape of the relationship in Figure 1can be understood in terms of a greater rate of conversion of n-hexane to cg isomers (methyl pentanes and methylcyclopentane) than to benzene [2], but as long as these are not lost to hydrogenolysis, they are ultimately converted to benzene as equilibrium is approached. While no kinetics studies of pressure effects have been published on the n-hexane to benzene reaction on Pt/KL catalyst, the qualitative kinetic behavior can be deduced from the kinetics of heptane over similar catalysts [17]. The rate of aromatiation has a somewhat lower hydrocarbon pressure dependence than isomerization (and a slightly slower H2 dependence).

328

G. Larsen, D. E. Resasco, V. A. Durante, J. K i m and G. L. Haller

Using the reaction orders given for heptane in Table 2 of ref. [17]for hexane, one can estimate a selectivity of about 25% and 50% for McVicker et al. and our conditions, respectively, which is about what is observed at low conversion or benzene yields for the two data sets.

1.0 I G

0.0

! 0.0

I

I

I

0.2

0.4

0.6

0.8

benzene yield Figure 1. Benzene selectivity versus benzene yield plot. The wt% Pt was 0.7 - 0.9 for all catalysts and the at% Pt relative to total metal is indicated by the prefix with sulfided catalysts identified by a further S prefix. The reaction conditions are the same as those given in Table 3. (+)Pt-Ni/L (fresh)y (open square) 0.64Pt-Nin (fresh), (open circle) 0.52Pt-NiL (fresh), (open triangle) 0.42Pt-NiL (fresh), (X)S, Pt-Nib, (filled square) S, 0.64Pt-Ni/Ly (filled circle) Sy0.S2Pt-Ni/L and (filled triangle) S,0.42Pt-Ni/L. SUMMARY The effect of sulfur on Pt agglomeration in L-zeolite [S] makes it clear that the first priority for a promoter must be stabilization of the small Pt particles in the L-zeolite pores. This stabilization against agglomerzation must be accomplished with little loss of aromatization activity (nor increase in hydrogenolysis activity) because selectivity to benzene depends on maintaining a high conversion. Both TEM and EXAFS indicate that particle size stabilization is accomplished by Ni and that hydrogenolysis activity is not changed. However, Ni does appear to make it more difficult to obtain a high dispersion of Pt (or Pt-Ni clusters) and sulfiding further reduces aromatization activity. ACKNOWLEDGMENTS This research was supported by DOE,Office of Basic Energy Sciences. Partial support from Sun Co. is also acknowledged. W e wish to thank the NSLS for beam time, Tosoh Corp., Japan, for samples of KL-zeolite and NSF for initiating work on PtL-zeolite catalysts.


329

REFERENCES 1 J. R. Bernard and P. J. Nury, US Patent No. 4,104,320 (1978). 2 J. R. Bernard, in L. V. C. Rees (Ed.), Proc. 5th Int. Conf. Zeolites, Heyden, London, 1980,

W. C. BUSS,P. W. Tamrn and R. L. Jacobson, in Y. Murakami, A. Iijima and J. 3 !?;6Hughes, W. Ward @is.), New Developments in Zeolite Science and Technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22,1986) Kodansha/Elsevier, Tokyo/Amsterdam, 1986, p.725. 4 M. Vaarkamp, J. T. Miller, F. S.Modica, G. S. Lane and D. C.Koningsberger, in L. Guczi, F. Solymosi and T6t6n i (Eds.), New Frontiers in Catalysis (Proc. 10th Int. Cong. Catal.,Budapest, July 19-24,1992) E sevier Sci. Pub., Amsterdam, 1993, p.809. 5 G. B. McVicker, J. L Kao, J. J. Ziemiak, W. E. Gates, J. L. Robbins, M.M.J. Tracy, S.B. Rice, T. H. Vanderspurt, V. R. Cross and A. K. Ghosh, J. Catal. 139 (1993) 48. 6 M. S. Tzou, J. H. Jiang and W. M. H. Sachtler, Appl. Catal. 20 (1986) 231. 7 V. R. Balse, W. M. H. Sachtler and J. A. Dumesic, Catal. Lett. 1 (1988) 275. 8 M. S. Tzou, B. K. Teo and W. M. H. Sachtler, Langmuir 2 (1986) 773. 9 C. G. Raab and J. A. Lercher, J. Mol. Catal. 75 (1992) 71. 10 J. H. Sinfelt, Advan. Catal. 23 (1973) 91. 11 P. Biloen, J. N. Helle, H. Verbeek, F. M. Dautzenberg and W. M. H. Sachtler, J. Catal. 63 (1980) 112. 12 G. Larsen and G. L. Haller, in R. V. Ballmoos, J. B. Higgins, M. M. J. Treacy, @Is.), Proc. 9th Intern. Zeolite Conf., Butterworth-Heinemann, Boston, 1993, v01.2, p.441. 13 B. J. McHugh, Ph.D., Yale University (1991). 14 G. Larsen and G. L. Haller, Catalysis Today 15 (1992) 431. 15 C. Raab, J. A. Lercher and J. J. G. Goodwin, J. Z. Shyu, J. Catal. 122 (1990) 406. 16 A. Jentys, G. L. Haller and 3. A. Lercher, J. Phys. Chem. 97 (1993) 484. 17 A. B. Kooh, W.-J. Han and R. F. Hicks, Catal. Lett. 18 (1993) 209.

r


Characterization and Catalytic Performance of the Platinum KL Zeolite Treated with Chlorotrifluoromethane

M. Sugimoto, T. Fukunaga and N. Ishikawa Central Research Laboratories of Idemitsu Kosan Co., Ltd. 1280 Kami -izumi, Sodegaura, Chiba, 299-02, Japan.

ABSTRACT In the aromatization of C 6 feedstock, the platinum catalyst supported on the CF3C1-treated KL zeolite (Pt/FKL) exhibits a high stability even a t a low H2/C6 mole ratio of 0.5 and a high aromatization activity, compared with the untreated catalyst (Pt/KL). The results of CO-FT-IR and 129Xe NMR show that the platinum particles on the Pt/FKL are rich in electrons, compared with those on Pt/KL and that most of the P t particles on both catalysts are located on the external surfaces. Therefore, it is thought that the electronic state of platinum particles affects more significantly the catalytic activity than the channel structure of the L-zeolites. INTRODUCTION Since the remarkably high selectivity of Pt/KL zeolite catalysts for the aromatization of hexane to benzene was discovered [ 1 1 , much attention has been directed to these catalyst systems [2-31. However, the intrinsic effect of the L zeolite on the unique activities of the platinum catalysts supported on it has been a matter of controversy for several years. We reported that the treatment of L zeolite with CFgCl markedly increased the catalytic activities for hexane aromatization [ 4 , 5 1 . Three items were inferred as the effects of the CF3C1 treatment. That is, halogen atoms supported by the replacement of terminal OH groups on the zeolite surface during the CF3C1 treatment, have important effects : ( 1 ) on the initial high dispersion of the platinum atoms, (2) on the maintenance of its dispersion, and (3) on the low accumulation rate of carbon deposits as a result of the low hydrogenolysis activity. In this paper, we report on the catalytic properties of the Pt/FKL and the state of platinum particles on it. 33 I

332

M . Sugimoto, T. Fukunaga and N . Ishikawa

EXPERIMENTAL The L zeolite used in this study was purchased from TOSOH Go., (TSZ-500). The CF3Cl treatment was carried out according to the methods described in our paper [51. Platinum was supported on the zeolite by incipient wetness impregnation or ion-exchange method, using an aqueous solution of Pt(NH3)4C12. The content of platinum metal was 1.0 wt%. The aromatization of hexane was carried out in a tubular reactor. A catalyst was activated in-situ in flowing hydrogen at 773 K for 1 h before introducing hydrogen and hexane feed. The aromatization reaction was carried out a t 773 K , 5 Kg/cm2G, a space velocity of 2.0 weight hourly space velocity (WHSV) and a hydrogen-tohydrocarbon mole ratio of 5.0. The aromatization of C 6 feedstock was carried out by adjusting the reaction temperature to obtain an aromatics yield of about 6 5 wt%, 5 Kg/cm2C, a space velocity of 2.0 WHSV and a hydrogen-to-hydrocarbon mole ratio of 0.5. The composition of C 6 feedstock was : 2,3-dimethylbutane : 0.7 wt% ; 2-methylpentane : 9.3 wt% ; 3-methylpentane : 15.3 wt% ; hexane : 59.7 wt% ; methylcyclopentane : 13.5 wt% ; 2,4-dimethylpentane : 1.0 wt% ; 3,3-dimethylpentane : 0.5 wt%. Its sulfur content had been decreased to less than 0.02 wt ppm by hydrotreating followed by sulfur sorption. The catalyst life time is defined a s a time when the catalyst average temperature reached 798 K after C6 feedstock introduction had been started. The infrared spectroscopy of adsorbed CO was carried out a s follows. A crushed sample was shaped into a wafer with a diameter of 20 mm under a pressure of 100 kg/cm2G. This was placed into a n infrared cell and pretreated for 1 h a t 813 K i n flowing hydrogen. Then the cell was evacuated a t 1.33 x 10-1 Pa at that temperature. The adsorption of CO was performed at a CO pressure of 4.0 x lo2 P a for 0.5 h and then the cell was evacuated at 4.0 x 10-1 Pa. The FTIR spectra were recorded a t room temperature using a JIR-100 spectrometer (JEOL). The NMR spectra of adsorbed l29Xe were measured a s follows. A powder sample was placed in a tube and evacuated a t 673 K and 1.33 x Pa for 1 h. Then the sample was reduced a t 673 K and 9.33 x 104 P a of hydrogen for 15 h, followed by evacuation a t 673 K for 10 h. Xenon was adsorbed at 298 K on each sample. The NMR signal was recorded a t 298 K using JEOL JNM CX270 operating a t 74.7 MHo.

Pt-KL Zeolite Treated with Chlorotrifluoromethane

333

RESULTS AND DISCUSSION Table 1 shows the results of hexane aromatieation over the platinum catalysts supported on the KL zeolite (Pt/KL) and the CFjCl-treated KL zeolite (Pt/FKL) by the incipient wetness method, along with the platinum catalyst supported on the KL zeolite by ion-exchange method. Table 1

Aromatieation of hexane over platinum catalysts supported on the L zeolites1)

Catalyst

Pt-KL

H/P t

Pt/FKL

1 .ll

0.93

94.2

99.7

99-8

23.3

12.2

6.8 1.9 1.7 89.6 89.5

1.01

Conversion (wt%) Selectivity (wtX) c1 - c '!+

4.6

30.h2)

c5

-

c6'

Aromatics Aromatics yield (wt%) ~

Pt/KL

~

~~~~

2.2

46.2

81 .O 80.8

43.5 ~

~~

1 ) Reaction conditions; 5 Kg/cm2G, 773K, WHSV 2 h-l, Hzlhexane 5 mole/mole. 2) Selectivity for C5 + c6

.

The Pt/FKL showed the highest selectivity for aromatics among the three catalysts. The three catalysts have almost the same hydrogen chemisorption uptake (H/Pt) measurement, indicating that the catalyst activities are independent of the platinum dispersion. In the case of Pt-KL, it is expected that a small quantity of acid sites is formed during platinum exchange of KL zeolite wjth Pt(NH3)4C12, followed by reduction, as Moretti and Sachtler reported [61. Thus, the lower selectivity of Pt-KL for aromatics is plausibly due to the presence of the acid sites, which promote the cracking of hexane and/or modify the electronic state of the Pt particles [7,8]. To evaluate the catalytic properties of the Pt/FKL and Pt/KL, exhibiting the higher selectivities for aromatics compared with PtKL, the measurements of turnover frequency (TOF) in hexane aromatization and the stability tests were carried out. The TOF values for the hydrocracked products (CT-C5) and aromatics were calculated on the basis of surface Pt atoms (H/Pt). The Pt/FKL (0.014 sec-l) exhibited 3.5 times higher TOF for C1-Cg than the Pt/KL (0.004 s-1). Furthermore, the Pt/FKL (0.825 s - l ) exhibited 9.1 times higher TOF for aromatics than the Pt/KL (0.091 sec-l). Thus the Pt/FKL exhibits the higher selectivity for aromatics and

334

M. Sugimoto, T. Fukunaga and N. lshikawa

the lower Cl-C5 selectivity than the Pt/KL. The stabilities of Pt/FKL and Pt/KL in the aromatization of Cg feedstock are illustrated in Fig. 1 . The stability of the Pt/FKL was extremely high under such a low hydrogen-to-hydrocarbon mole ratio of 0.5 and its catalyst life (3,650 h) was 21 times longer than that of the Pt/KL.

-

I

31 0

Fig. 1

I

I

I

1 4000

I

500 1000 1500 2000 2500 3000 3500 T i m e on S t r e a m / h r

Life tests of Pt/FKL and Pt/KL with C6 feedstock. ( 0 )Pt/FKL, ( 0 ) Pt/KL Reaction conditions; 5 Kg/cm2C, WHSV 2 h-l, H2/c6 0.5 mole/mole, Aromatics yield 65 wt%.

The conventional reforming catalysts, Pt/A1203 etc., deactivate very rapidly with decreasing hydrogen-to-hydrocarbon mole ratio due to increase in coke formation rate. It is of great significance that the aromatization of C6 feedstock, the Pt/FKL shows the extreme ly high stability even at such a low mole ratio. Amounts of carbon on the used Pt/FKL and Pt/KL are listed in Table 2, together with average accumulation rates of carbon. The average carbon accumulation rate of the Pt/KL was 14 times higher than that of the Pt/FKL. Since the sulfur content o f C6 feedstock has been decreased to less than 0.02 wt ppm in this study, it seems that the carbon formation rate largely affects the catalyst stability, rather than sulfur poisoning. Table 2

Catalyst Pt/KL Pt/FKL

Amount and average accumulation rate of carbon in used catalysts Process time (hr)

170

3,650

Amount (wt%)

Carbon Avg. accumulation rate (wt ppm/hr)

1.4

82.0

2.1

5.8

Reaction conditions; 5 Kg/cmZC, WHSV 2 h-l, H2/C6 0.5 mole/mole, Aromatics yield 65 wt%


335

To evaluate the electronic state of platinum particles on the KL and FKL, the CO-FT-IR measurements were carried out. Fig. 2 shows the IR absorption spectra of chemisorbed CO on the Pt/FKL and Pt/KL.

aa v e m a L

0

m

s

a --

a b

-r

2400

Fig.2

I

2200 2000 Wavenumber /cm.1

I

1800

IR absorption spectra of chemisorbed CO on : (a) Pt/FKL and (b) Pt/KL

Both the platinum catalysts have two strong bands i n the range of 2,000 - 2,070 cm-l , and a broad band around 1,785 cm-I The former bands are ascribed to CO linearly adsorbed on platinum sites and the latter to bridged CO adsorbed on platinum sites [91. Two strong bands of CO linearly adsorbed o n platinum sites of the Pt/KL were observed at 2,070 and 2,020 cm-l, and those of the Pt/FKL were observed at 2,045 and 2,005 cm-1. The remarkable shift of the bands suggests that the platinum particles on the FKL are rich in electrons relative to those on the KL. From this result, the higher activity for hexane arornatization of the Pt/FKL is attributed to the change in the electron density of the platinum particles. To evaluate the location of platinum particles on the Pt/FKL, Pt/KL and Pt-KL, 129Xe NMR spectra were measured. A single signal was observed for KL support, the Pt/KL and Pt-KL. I n contrast, for the FKL and Pt/FKL, two signals were the dominant feature of the spectra. This suggests that some change of the pore size of the K L zeolite occurred during the CF3C1-treatment. Fig. 3 shows the dependence of the 129Xe NMR chemical shift for the FKL, KL and K(H)L on xenon pressure. K(H)L was prepared by ionexchanging of KL with aqueous ammonium nitrate, followed by calcination a t 773 K. The degree of proton exchange was about 9 X . The

.

336

M. Sugimoto, T. Fukunaga and N. lshikawa

Fig. 3

Dependence of the 129Xe N M R chemical shift on xenon pressure : ( 0 ) FKL, ( 0 )KL, ( A ) K(H)L

xenon chemical shifts for the three samples linearly decreased with decreasing xenon pressure. At zero concentration of xenon, the FKL exhibits two chemical shifts, namely the slightly higher than and the same as that observed for the KL, indicating that the former has two types of the channels with smaller and the same sizes compared with the latter. Furthermore, at high concentration of xenon, the FKL exhibits the two higher chemical shifts than the KL. This means that the collisions between xenon atom and the internal wall and/or between xenon atoms increase in the channels ~f the FKL. As we have already reported [lo], the surface area decreased upon the CF3Cl-treatment, due to the replacement of OH groups on the KL zeolite with halogen atoms and the formation of a very small amount of AlX3 (X ; F , Cl), indicative o f the occurrence of degradation of the part of the crystal structure of KL zeolite. Therefore, these two higher chemical shifts are attributed to the existence of halogen atoms and the halogen compounds. Especially in case of the channel with higher chemical shift at zero concentration of xenon, the halogen compounds such as AlX3 seem to decrease the channel size. The K(H)L exhibited the same chemical shift at zero concentration of xenon as the KL and the higher chemical shifts at higher concentration of xenon relative to the KL. This suggests that the K(H)L has the channels with the same sizes as those o f the KL and that the protons interact with xenon atoms and the interaction increases the xenon chemical shifts. Fig. 4 shows the dependence of the 129Xe N M R chemical shift for the Pt/FKL, PtfKL and Pt-KL on xenon pressure. In the case of


337

Pxe / 10+a

Fig. 4

Dependence of the I29Xe NMR chemical shift on xenon Pt/KL, ( A )Pt-KL. pressure : ( 0 ) Pt/FKL, ( 0 )

the Pt-KL, the xenon chemical shift decreased with decreasing xenon pressure, and through the minimum, it increased with further decreasing xenon pressure. In contrast, the xenon chemical shifts for the PtfFKL and PtfKL linearly decreased with decreasing xenon pressure. In case of the metal supported zeolite, the chemical shift, which is attributed to the xenon-metal interaction, becomes much greater as xenon pressure decreases [ l l ] . The chemical shift, which is attributed to the interaction of xenon atom with the metal existing on the external surface, is negligible because the relaxation time of the polarization of xenon caused by the interaction is very short [ I l l . As shown in Fig. 3 , the xenon chemical shift for the K(H)L linearly decreased with decreasing xenon pressure in the presence o f the interaction between proton and xenon atoms. Therefore, the increase of the xenon chemical shift at low xenon pressure cannot be ascribed to the existence of proton in the Pt-KL, which are formed during platinum exchange of KL zeolite, followed by reduction. As the platinum content on the Pt-KL decreased from 1.0 to 0.5 and 0 . 3 wt%, the extent of the increase of the chemical shifts at low xenon pressure became smaller. The dependence of the xenon chemical shifts on platinum amount suggests that most of the platinum particles on the Pt-KL are located in the channels o f KL zeolite. As described above, the PtfPKL and PtfKL exhibited the

338

M.Sugirnoto, T. Fukunaga and N. Ishikawa

linear decrease of the chemical shift at low pressure, indicating that most of platinum atoms on the two catalysts are not located in the channels, but on the external surface of the zeolite supports. From these results, the effect of the channels of the Pt/FKL and Pt/KL prepared in this study on collimating the flux of hexane molecules proposed by Tauster and Steger [12] is not significant in our reaction system. In addition, much less coke formation in the channels of the Pt/FKL seems to be one of the reasons why the Pt/ FKL exhibits such a high stability in the aromatization of C6 feedstock even at a low hydrogen-to-hydrocarbon mole ratio. If the platinum particles on the Pt/FKL were located in the channels, coke formation there would affect significantly the diffusion of reactants and the Pt/FKL would not exhibit such a high stability. In cases of the Pt/FKL and Pt/KL used in this study, the former exhibited a higher selectivity for aromatics than the latter, irrespective of almost the same dispersion. Therefore, it is concluded that the modification of the electronic state o f platinum particles, through the interaction of them with the L zeolite support, is of primary importance in determining the selectivity for aromatics. REFERENCES 1 J.R.Bernard, in L.V.Rees (Ed.), Proc. 5th Int. Zeolite Conf., Heyden, London, 1980, p.686. 2 T.R.Hughes, W.C.Buss, P.W.Tamm and R.L.Jacobson, in Y.Murakami, A.Iijima and J.W.Ward (Eds.), New Developments in Zeolite Science and Technology (Proc. 7th Int. Zeolite Conf.,) KodanshaElsevier, Tokyo-Amsterdam, 1986, p.725. P.W.Tamm, D.H.Mohr and C.W.Wilson, in J.W.Ward (Ed.) Catalysis (Studies in Surface Science and Catalysis, vol. 3 8 ) , Elsevier, Amsterdam, 1987, p.335. 4 T.Fukunaga, H.Katsuno and M.Sugimoto, American Chemical Society Division of Petroleum Chemistry, No 4; August, 1991, p.723. M.Sugimoto, H.Katsuno and T.Murakawa, Appl. Catal., A: General, 95(1993)257. 6 C.Moretti and W.M.H.Sachtler, J. Catal., 113(1988)220. 7 G.Larsen and G.L.Haller, Catal. Lett., 3(1989)103. 8 G.Larsen and G.L.Haller, Catalytic Science and Technology, 1 (19911135. 9 C.besoukhanova, J.Guidot and D.Barthomeuf, J. Chem. SOC. Faraday Trans. I., 77(1981 )1595. 10 M.Sugimoto, H.Katsuno and T.Murakawa, Appl. Catal., A: General,

96(1993)201. 1 1 L.C.Menorva1, J.P.Fraissard and T.Ito, J. Chem. SOC., Faraday Trans. 1 , 78(1982)403. 12 S.J.Tauster and J.J.Steger, J. Catal., 125(1990)387.

Characterization and Catalytic Properties of ZeoliteSupported Platinum-Iridium Bimetallic Catalysts Prepared by Decoration of Iridium

I. C. Hwang and S. I. Woo Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea

ABSTRACT Highly dispersed bimetallic PI-Ir clusters supported in NaY zeolite were characterized using FTIR spectroscopy of adsorbed CO, 29Xe NMR spectroscopy and ethane hydrogenolysis reaction. The results of 129Xe NMR, FTIR and ethane hydrogenolysis reaction indicate that surface of the Pt clusters are decorated with Ir in supercage of NaY zeolite. Ir atoms on the surface of Pt-Ir bimetallic cluster were re-dispersed during the re-oxidation above 623 K. In n-decane reforming reaction, Pt-Ir bimetallic catalysts showed higher selectivities to aromatics than Pt and Ir monometallic catalysts. INTRODUCTION Bimetallic catalysts, comprising a combination of atoms of a group VIII metal have been widely used as the heterogeneous catalyst for reforming refinery naphtha [l-21. They show remarkable activity maintenance and selectivity compared to monometallic catalysts. This is due to the variation of surface structure and composition by metallic cluster formation. Structural studies on supported bimetallic catalysts are a prerequisite for rational catalyst design. Small metal cluster supported on faujasite zeolite such as NaY attracts much attention. Yang el nl. [3] demonstrated Pt-Ir bimetallic cluster are formed in supercage of Y zeolite after co-ion exchange of Pt and Ir, subsequently calcination in 0 2 and reduction in H2 at 573 K, and showed electronic interaction between Pt and Ir by characterization method such as 129Xe N M R and ethane hydrogenolysis probe reaction. It was suggested that the catalytic properties of Pt-Ir bimetallic catalysts are related to the interaction between Pt and Ir [4]. The chemical shifts of Xe adsorbed on Pt-Ir bimetallic catalysts were lower than those on physical mixture of corresponding monometallic catalysts. They suggested that this was due to the altered surface composition by formation of bimetallic cluster. But the chemical shifts may be highly sensitive to the number of clusters and the variation of cluster size as well as surface composition of bimetallic cluster. The variation of size and number of Pt-Ir bimetallic cluster, which could occur during the calcination of zeolite co-ion exchanged by Pt and Ir complex, can influence the chemical shift of Xe adsorbed on metal cluster. If small amount of Ir (111) ion exchanged to Pt/NaY containing Pt 339

340

I . C. Hwang and S. 1. Woo

clusters can form bimetallic clusters through adsorption on surface of Pt cluster followed by calcination and reduction, the number of Pt-Ir bimetallic clusters will not change. In this work, NaY supported small Pt-Ir bimetallic clusters prepared by Ir decoration were characterized by 129Xe NMR, hydrogen chemisorption, FTIR of adsorbed CO and ethane hydrogenolysis reaction. The change of metal phase of Pt-Ir bimetallic cluster during the reoxidation was studied. Through n-decane reforming reaction, the catalytic properties of small Pt-Ir clusters with various surface composition were also investigated.

EXPERIMENTAL Monometallic Pt4NaY and Ir4/NaY samples were prepared by ion exchange of Pt(NH3)42+ and Ir(NH3)5CI2+ into NaY zeolite, calcination in 0 2 and subsequently reduction in H2 at 573 K. The sample was placed into a Pyrex U-tube flow reactor which was joined with a N M R tube equipped with vertical ground-glass vacuum stopcocks. The bimetallic Pt-Ir/NaY samples with various IrPt ratios were prepared as followed. Pt4/NaY, which was prepared as described above, was ion-exchanged with [Ir(NH3)5C1]2+ in aqueous solution. The ion exchanged samples were calcined and reduced in the same way used for the preparation of Pt4/NaY. The bimetallic samples are designated as Pt4IrdNaY. The experimental details for the 129Xe NMR spectrum, hydrogen adsorption and ethane hydrogenolysis reaction were described earlier [3]. For FTIR studies, the wafers were prepared by pressing 0.015 g of samples in metallurgical die under 1 . 5 ~ 1 psi. 0 ~ The wafers of each sample were re-reduced under H2 at 573 K for lh in in sit14 IR cell equipped with KBr window, evacuated for 3h at 673 K under vacuum of 1x10-5 Torr and then cooled at room temperature. 40 Torr of CO was introduced into IR cell and equilibrated with sample for Ih, and then the sample was evacuated for sufficient time. FTIR spectra were obtained at room temperature using a Bomem(MI3-102) spectrometer with resolution of 4 cm-l. FTIR data were recorded by substrating the spectrum of CO-free background from that of CO adsorbed sample. n-Decane reforming reaction was carried out in a differential fixed bed reactor with 100 mg of catalyst at atmospheric pressure and 673 K. Catalysts re-reduced at 573 K with H2 for 2 h were used in the reaction. H2 and n-decane reaction mixture was generated by bubbling hydrogen through n-decane saturator kept at 364 K. The total flow rate (H2 + n-decane) was 42.7 mVmin. with a H2h-decane ratio 15. Products were analyzed by on-line HP 5890A gas chromatography equipped with a 50 m cross linked methyl silicon fused silica capillary column and a FID detector. RESULTS AND DISCUSSION The composition of catalyst and total number of adsorbed hydrogen atom per metal (Htotal/M) are listed in Table 1. Assuming an adsorption stoichiometry of HA4 = 1, Pt dispersion with Htota@i =1.2 indicates that small metal clusters of lnm are formed in Y zeolite. Htota$h4 was not changed by the decoration of small amount of Ir on Pt4/NaY, but decreased with increasing Ir loading above 1 wt%. These results suggest that size of metal cluster was not almost changed by

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decorating small amount of Ir on Pt4/NaY while the size of metal cluster increased when amount of Ir loading increased above 1 wt%.

Catalysts

Pt wt Yo

Ir wt %

IrPt ratio

4

0 0.2 0.4 1 2

0 1/20 1/10 114 112

Pt4/NaY Pt4Ir0.2/NaY Pt4Ir0.4/NaY Pt4Ir I/NaY Pt4Ir2/NaY

4

4 4 4

Htntsl/h4 (M = Pt+Ir) 1.2 1.2 1.1 0.9 0.9

Figure 1 shows the variation of chemical shift for Pt-Ir bimetallic catalysts with various Ir loading prepared by two methods, in which one is by decoration of Ir to Pt cluster and another is by co-ion exchange of Pt and Ir. The co-ion exchange method consisted of ion exchange of iridium and platinum with NaY maintaining the amount of Pt ion-exchange at 4 wt%, calcination in 0 2 and reduction in H2 at 573 K. In the case of Pt-Ir/NaY bimetallic catalysts prepared by Ir decoration,

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Figure 1. The l29Xe NMR chemical shifts of catalysts prepared by Ir decoration on Pt4/NaY ( 0 ) and by co-ion exchange of Ir and 4 wt% Pt ( ) as a fhction of Ir wt0/0,and ( V ) Ir4/NaY. the chemical shift decreased with increasing Ir loading on Pt4/NaY up to 1 wt%, but increased when Ir loading was more than 1 wt%. In the case of Pt-Ir bimetallic catalysts prepared by coion exchange method, chemical shift decreased with increasing Ir loading up to 2 w%, but

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increased above 2 wt%. The decrease in the chemical shift of Pt-IrMaY prepared by Ir decoration was much larger than that of Pt-IrMaY prepared by co-ion exchange of Pt and Ir. This can be explained by the preferential location of Ir atoms on the Pt clusters near the supercage window or by the fact that part of Ir atoms in Pt-Ir/NaY prepared by co-ion exchange method are present in the bulk phase. The 129Xe NMR chemical shifts of the Pt-Ir bimetallic catalysts containing the large amount of Ir started to increase. This is due to the increase of Xe-metal interaction by the formation of monometallic Ir cluster in addition to that of Pt-Ir bimetallic clusters. In Figure 2 are shown IR spectra of the various Pt-IrMaY's after exposure to CO followed by removal of gas phase CO at room temperature. AAer calcination and reduction at 573 K, the monometallic Pt4MaY catalyst showed a strong peak at 2086 cm-l. The peak at 2086 cm-1 is attributed to CO linearly adsorbed to h l l y reduced Pt monometallic cluster. IR spectra on the bimetallic Pt4IrO.2MaY shows two IR absorption peaks at 2099 and 2069 cm-1. When Ir loading on Pt4MaY increased from 0.4 wt% to 1 wt%, band intensity of the band at 2099 cm-1 decreased while that of the band at 2069 cm-1 hrther increased. Based on the CO stretching bands of PtMaY and Ir/NaY, the band at 2099 cm-l can be assigned to CO adsorbed on Pt and the band at 2060 2069 cm-1 to CO adsorbed on Ir. The decoration of Ir on Pt clusters will decrease the Pt sites on the surface of Pt cluster

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, 2059

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e

0

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-? 0 VI

13

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1900

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Figure 2. FTIR spectra of CO adsorbed on Pt, Ir and Pt-Ir bimetallic clusters prepared by decoration method. I. (a) Pt4MaY (b) Pt4Ir0.2/NaY (c) Pt4Ir0,4/NaY (d) Pt4Irl/NaY. 11. (a) IrlOMaY (b) Pt4Ir2MaY (c) Pt4Ir0.4/NaY (d) Pt4Ir0.2/NaY. This decreases the band intensity of CO linearly adsorbed to Pt. The enrichment of Ir atoms on Pt clusters would increase the band intensity of CO linearly adsorbed on Ir. This result indicates that

Zeolite-Supported Pt-lr Bimetallic Catalysts

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Pt monometallic clusters are decorated by Ir. Figure 3 shows the arrhenius plots for the Pt-Ir bimetallic catalysts in ethane hydrogenolysis reaction. At 558 K (l/T X 1000 = 1.79 ), the turnover frequency of catalysts increased from that of Pt4MaY by addition of Ir. When amount of Ir was 2 wt%, TOF approached to that of Ir4MaY. This result can be explained by the geometric effect of Pt-Ir bimetallic cluster. Ethane hydrogenolysis reaction requires the active sites comprising of adjacent metal atom ensemble. At reaction temperature where the activity of Pt clusters is negligible, the active sites of catalysts are Ir ensemble. TOF of Pt4IrO.2/NaY was a little bit higher than that of Pt4MaY and much lower than that of Ir monometallic catalyst. This is due to the small amount of Ir. When Ir atoms are dispersed on Pt clusters, Ir loading of 0.2 wt% is not enough to form Ir ensemble on which ethane can be adsorbed. When Ir loaded on Pt4MaY was increased more than 0.2 wt%, the number of Ir ensemble on the Pt clusters increased and therefore TOFs of the catalysts abruptly increased. In the catalysts containing large amount of Ir such as Pt4Ir2/NaY, the surface of metal cluster would be saturated by Ir and extra monometallic Ir cluster would be formed, therefore the activity approached to that of Ir4MaY.

0.1

-

0.01

I v)

W

LL

P 0.00 1

0.0001 1.6

1.8

2.0

1/T X 1000 ( K-' ) Figure 3. Arrhenius plots in ethane hydrogenolysis reaction on ( 0 ) Pt4/NaY, ( 0 Pt4Ir0.2MaY, ( V ) Pt4IrO,rl/NaY, ( ) Pt4Irl/NaY, ( 0 ) Pt4Ir2lNaY and ( Ir4MaY.

) )

The FTIR spectrum of CO adsorbed on Pt4IrlhIaY re-oxidized at 623 K followed by the reduction at 573 K is compared with that of Pt4IrllNaY without re-oxidation in Figure 4. FTIR spectrum of fresh Pt4IrlMaY shows a peak at 2060 cm-I and shoulders with slight intensities at

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about 2090 and 2002 cm-1. This indicates that the surface of Pt cluster is almost saturated by Ir atoms as shown in Figure 2. In Pt4Irl/NaY re-oxidized at 623 K, CO streching band intensity of the peak at about 2093 cm-1 significantly increased. This peak is attributed to CO adsorbed on Pt atoms, indicating that more Pt atoms were exposed on the surface of Pt-Ir bimetallic cluster after re-oxidation at 623 K. This was also confirmed by 129Xe NMR spectroscopy. Chemical shift of Xe adsorbed on fresh Pt4IrVNaY was 99 ppm. After re-oxidation, the chemical shift increased to 108 ppm. This is due to the increase of Xe-Pt interaction, which results from the increase of the number of Pt atom on the surface. From these results, it can be suggested that during the reoxidation at 623 K Ir atoms were segregated from the Pt-Ir bimetallic clusters and migrated on external surface or sintered on the surface of Pt-Ir bimetallic cluster.

0

0 c 0.2 0

e 0

v)

2 0.0 2200

2050

1900

Wave nurnber ( crn-' ) Figure 4. FTIR spectra of CO adsorbed on (a) fresh Pt4Irl/NaY, (b) PtrlIrllNaY re-oxidized at 623 K. Figure 5 shows the product distribution after 205 min. time on stream at 673 K in n-decane reforming reaction. Main product was light gas and higher aromatic compounds such as cumene, cymene and propylbenzene etc. The selectivities to C6 and C7 isomers and BTX, which are desired products for enhancement of octane value, were very low in all catalysts. The Pt-Ir bimetallic catalysts containing the small amount of Ir show higher selectivity to aromatic compound and lower selectivity to light gas than Pt and Ir monometallic catalysts.. As Ir content increased, the

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selectivity to light gas significantly increased and the selectivity to aromatics decreased. This is due to high cracking activity of Ir indicating that with increasing the Ir loading, the surface of Pt clusters were enriched by Ir atoms and Ir atoms on the surface of metal cluster mainly influence the product distribution.

Figure 5. Product distributions of Pt/NaY, Ir/NaY and Pt-Ir/NaY catalysts in n-decane reforming reaction.

CONCLUSIONS 129Xe NMR spectroscopy, complemented by FTIR spectroscopy of adsorbed CO and ethane hydrogenolysis probe reaction, shows that Pt clusters are decorated by Ir atoms after sequential ionexchange of Pt and Ir cations with Nay. Ir atoms on Pt-Ir bimetallic cluster are migrated from the surface after re-oxidation above 623 K. In n-decane reforming reaction, the cracking activity increased with the increase of Ir content. Ir atoms decorated to Pt cluster significantly influenced the distribution of product. REFERENCES 1. J. H. Sinfelt, Bimetallic Caialyst.s-Di.sco\teries,Coriceptsarid Applicatioti; Wiley: New York, 1983. 2. V. Ponec, In Advaiices it1 CalaIysis;Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: San Diego, 1987; Vol. 32, p149. 3. 0. B. Yang and S. I. Woo, R. Ryoo,J. Calal. 137 (1992) 357. 4. 0. B. Yang and S. I. Woo, in L. Guczi et a/.(Eds), New Frotitiers iri Catalysis (Proc. 10th Int. Cong. Catal., Budapest, Hungary, 19-24 July, 1992), Elsvier, Amsterdam, 1992, p67 1.


Infrared Spectroscopic Study of CO Adsorption on Pt-Co Bimetallic Particles Entrapped in Nay-Zeolite

Genmin Lul and Uszl6 Guczi* Department of Surface Chemistry and Catalysis, Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, Budapest, Hungary, H-1525

ABSTRACT IR swtra of the CO molecules adsorbed on Pt/NaY and Pt-ColNaY bimetallic catalysts -prepared by ion exchange (IE) and impregnation (IM) methods, have been measured at different temperatures. On different samples the absorption frequencies of the linearly bound CO molecules shift towards lower wavenumbers in the sequence of Pt/NaY(IE) > Pt-Co/NaY(IE) > Pt/NaY(IM). This red shift is attributed to the interaction between Pt and Co and to the influence of the particle size and locations of metals, e. g. inside the cages or on the external surface of the zeolite. Upon heating the samples in CO gas the absorption bands for the linearly bound CO on Pt/NaY(IE), PtCo/NaY(IE) and Pt/NaY(IM) shift to the lower wavenumbers due to re-dispersion of the adsorbed CO layer, which diminishes the dipole-dipole interaction between the adsorbed CO molecules. Upon adsorption and heating the Pt-Co/NaY(IM) in CO a new band for linear CO appears due to formation of the Co sub-carbonyl species. 1. INTRODUCTION Bimetallic catalysts usually exhibit catalytic properties which are very different from those observed on the single metals. While this phenomenon is still the subject of debates, the possible explanations include formation of bimetallic or alloy particles. Hence, surface properties of the metals including the electronic structure or the composition of surface active sites, are modified. Recently, several papers were published that deal with Pt-Co bimetallic systems. Bardi et al [1,2] found that the surface of Copt3 have a sandwich structure in which the outmost atomic layer consists of Pt and a second layer enriched in Co. The adsorption properties of Pt were modified by formation of intermetallic bonding between Co in the second layer and Pt in the top layer. For supported Pt-Co bimetallic catalysts, the situation is further complicated with regards to the reducibility and formation of different Co surface species. From IR spectra of the CO molecules adsorbed on Pt-Co bimetallic catalysts supported on

'Permanent address: Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, P. R. China 341

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aerosil, Dees et al [3] have concluded that the effect of Co addition on CO adsorption could be attributed to ensemble size effect by which the adsorbed CO layer is diluted. Recent works in our laboratory [4, 51 indicated that Pt and Pt-Co bimetallic particles as well as Co surface species, which are highly dispersed and hardly reducible, could be formed on Pt-Co/A12Og catalysts after different pretreatments. By encapsulating the bimetallic particles inside zeolite cages their properties are further modified by interaction with cations and protons in the zeolite [6, 71. It was previously found that Nay encaged Pt and Pd particles were electron deficient due to the effect of particle size or formation of metal-proton adults [8, 91. Our recent works [lo-121 on PtCo/NaY catalysts indicated that Pt-Co bimetallic particles could be formed inside zeolite cages with very high dispersion. The properties of Pt for H2,0 2 and CO adsorption as well as for CO hydrogenation were modified significantly with alteration of the Pt/Co ratios in the catalysts. However, the reason for the modification of Pt properties by addition of Co needs further clarification. In the present paper, FT-IR has been used to study the adsorption of CO on Pt-Co/NaY catalysts prepared by ion exchange method. The results are compared with those measured on impregnated samples using Nay as support to investigate the influences of particle size and zeolite cages on CO adsorption. 2. EXPERIMENTAL

The Pt and Pt-Co/NaY catalysts were prepared by ion exchange (IE) of NaY zeolite (Si/A1=2.5) with Pt(NH3)4(No3)2 and Co(NO3)2. Pt was introduced first when preparing the bimetallic catalysts. The detailed procedures were described elsewhere [lo]. For comparison the catalysts were also prepared by impregnation method (IM) with similar metal contents. Table 1 gives the catalyst designation, metal contents and metal dispersion measured by H2 and CO adsorption.

Table 1 Metal contents and dispersion for ion exchanged (IE) and impregnated (IM) samples Catalyst

Preparation

Pt,wt%

Pt/NaY(IE) PtCo/NaY(IE) Pt/NaY(IM) PtCo/NaY(IM)

ion exchange ion exchange impregnation impregnation

6.5 4.6 4.9 4.8

Co,wt%

2.6 3.4

H/Ptl

CO/Ptl

0.84 0.86 0.63 0.45

0.50 0.69 0.50 0.47

'Catalyst was calcined at 573 K for 2 h and reduced at 723 K for 1 h, respectively. Adsorption was measured at RT.

CO Adsorption on Pt-Co Entrapped in NaY

The IR spectra were recorded at room temperature (RT) on Digilab-275 FT-IR spectrometer with 4 cm-l resolution. Before IR measurements the sample was calcined at 573 K for 2 h. The calcined sample was then evacuated at 573 K for 1 h, which was followed by H2 reduction and evacuation at the Same temperature for 1 h each. Then gaseous CO was introduced into IR cell at RT to 100 mbar and the pellet was heated under CO atmosphere up to different temperature between RT and 523 K for 30 min. The IR bands between 2300-1600 cm-l for adsorbed CO and surface carbonaceous species were measured at each step. Background was subtracted by using the spectrum prior to CO admission on each sample.

3. RESULTS AND DISCUSSION 3.1 Ion Exchanged Samples In Fig. 1 the IR spectra of the adsorbed CO molecules on Pt/NaY(IE) as a function of adsorption temperatures are shown. In the region of linearly bound CO on Pt at RT, the stretching frequency appears at 2075 cm-l along with a shoulder at around 2010 cm-'. Both bands shift towards lower wavenumbers with increasing adsorption temperature. In the region of 1900-1700 cm-l two broad peaks appear at around 1840 and 1790 cm-l for bridged CO. With increasing adsorption temperature the intensity of 1840 cm-l peak diminishes and disappears at temperature higher than 470 K, while the 1790 cm-l peak remains unchanged, In the Figures the bands in the region of 1700 - 1600 cm-l, which is characteristic of the surface carbonaceous species, are not shown. The intensities of these bands increase with adsorption temperature. After re-reduction of the sample at 570 K with flowing H2 for 1 h the peak positions for linear and bridged CO are recovered ( Fig. If ). As shown in Fig. 2 the band for linearly adsorbed CO on Pt-Co/NaY(IE) catalyst appears at lower wavenumbers compared with Pt/NaY(IE) and further shifts towards lower wavenumbers with increasing adsorption temperatures. Similarly to Pt/NaY(IE) the bridged CO on Pt-Co/NaY(IE) at RT appears at around 1840 and 1790 cm-', but at temperature higher than 420 K, the 1840 cm-l band disappears leaving the band at 1790 cm-I unchanged. After repeated reduction in H2 the positions for the linearly bound and bridged CO are not completely recovered ( Fig. 2e ). It is known that the adsorption mode of the CO molecule and the corresponding IR frequencies depend upon the metal particle size and its chemical environment. Our previous results [lo, 111 indicated that the location of metal and its particle size in the ion exchanged PtlNaY and Pt-Co/NaY catalysts were controlled by calcination temperature. Under the conditions applied in the present study, calcination at 573 K will put most of Pt in the supercages of the zeolite for Pt/NaY(IE). Hydrogen reduction results in highly dispersed Pt particles inside zeolite cages as indicated by H2 and CO chemisorption in Table 1. For pure Co/NaY catalyst similar calcination temperature puts most of the Co ions to the small cages of the zeolite, where the Co ions are non-reducible at temperature

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lower than 923 K (for this reason the IR results for pure Co/NaY samples are not shown in this paper). For Pt-Co/NaY(IE) sample the calcination at 570 K results in the interaction of Co with Pt in the supercages. After H2 reduction most of Co are in reduced states and form bimetallic particles, which may distribute homogeneously in the zeolite cages as indicated by TPR and XRD results [101. Although it has been suggested that highly dispersed Pt particles encaged in zeolite was partially positively charged [8, 91, the present IR results for the CO molecules adsorbed on Pt/NaY(IE) with full coverage ( Fig. la ) show similar stretching frequencies for the linearly bound CO molecules as was observed on oxide supported pt catalysts [13]. The red shift with increasing adsorption temperature can be attributed to re-dispersion of the adsorbed CO molecules layer, by which the dipole-dipole interaction is diminished [14]. Heating the sample in CO also causes migration and coalescence of Pt particles inside zeolite cages. However, its effect on the IR band position maybe not appreciable, because the repeated reduction in HZ recovers the band positions as shown in Fig. If. The Pt particles is, therefore, confined by the zeolite cages. Concerning the multiple bands for the linear CO in Fig. 1, it can be assigned to CO adsorption on different Pt sites. Greenler et al [13] reported multiple bands for linear CO on Pt, e. g. 2085 cm-' for terrace sites and 2065 cm-l for cornededge sites on Pt crystal and 2081, 2070 and 2063 cm-l for terrace, comer and edge site, respectively, on Pt/SiO2. Similar assignment can be made for bridged CO. The decrease of band intensity at 1840 cm-I with increasing adsorption temperature indicates that it is related to the CO molecules adsorbed on relative smooth surface, where the site can be easily blocked by carbon deposit. It is worth noting that several authors [15, 161 suggested formation of Pt-carbonyl cluster inside Nay cages upon CO adsorption. We would not try to assign the above multiple absorption bands to Pt carbonyl species because similar spectrum is found on Pt/NaY(IM) sample, where bare Pt carbonyl cluster is unstable on the external surface of the zeolite. Addition of Co to Pt/NaY causes the frequency of the linearly bound CO molecules adsorbed at RT shifts towards lower wavenumbers, e. g. 2075 cm-' and 2068 cm-l for Pt/NaY(IE) and Pt-Co/NaY(IE), respectively. Two possible reasons must be considered for this red shift. First, formation of Pt-Co bimetallic particles would decrease the dipoledipole interaction among the adsorbed CO molecules by the dilution effect of Co. If this were true the band frequencies of linear CO on Pt-Co/NaY(IE) sample would not be further decreased with increasing adsorption temperature, or at least the red shift would be smaller than that observed on Pt/NaY(IE). As shown in Fig. 3 the absorption frequencies of linearly bound CO on different samples are plotted against the adsorption temperatures. The frequency of linear CO on Pt-Co/NaY(IE) shifts towards the lower wavenumbers by about 14 cm-l with increasing the adsorption temperature from RT to 523 K. This shift is

CO Adsorption on Pt-Co Entrapped in NaY

1845 1794 2200

2200

2000

1800 (cm-')

Fig. 1 IR spectra of CO adsorbed on Pt/NaY(IE) at different temperature. a. RT;b. 373 K, c. 423 K; d. 473 K;e. 523 K;f. RT after H re-reducbon at 573 K Fig.2 IR spectra of CO adsorbed on Pt-Co/NaY(IE) atdifferent temperature. a. RT;b. 373 K, c. 423 K;d. 473 K, e. RT after H2 re-duction at 573 K larger than that observed on Pt/NaY(IE) ( 11 cm-l ). The present result is in controversy with that observed on aerosil supported R-Co bimetallic catalysts by Dees et al [3]. The red shift caused by Co addition to Pt/NaY(IE) for linearly bound CO at RT can, therefore, be attributed to the electron interactions between Pt and Co. The suggested "sandwich" structure for Coptg intermetallic crystals, indeed, proves an electron interaction indicated by +0.5 eV shifts in Co 2p B. E. measured by XPS [l, 21. This model is further developed to the supported Pt bimetallic catalysts by Joyner et al [17]. The absence of Co sub-carbonyl species on Pt-Co/NaY(IE) sample is in agreement with this model and also consistent with our previous chemisorptionresults.

3.2 Impregnated Samples For CO adsorption on WNaY(IM) sample at RT the main band for linear CO appears at 2060 cm-l with a shoulder at around 2010 cm-l ( Fig. 4a ). The peak position shifts towards lower wavenumbers with increasing adsorption temperatures. The bridged CO appears as broad peaks at 1810 and 1790 cm-'. The band at 1810 cm-l disappears

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n

2100

F

B

Y

3

p

2050

E

3

C

Q)

>

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0

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2000 250

350

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Temperature (K) Fig.3 Influence of adsorption temperature on the band frequencies of linearly adsorbed CO on different samples. a. Pt/NaY(IE); b. Pt-Co/NaY(IE); c. Pt/NaY(IM); d. PtCo/NaY(IM)

--

2200

2000

1800 tcrn-9

2200

2000

1800 (cm-')

CO Adsorption on Pt-Co Entrapped in N a Y

at temperature higher than 573 K. As the Same for Pt/NaY(IE) sample, repeated reduction in H2 recovers the intensities and band positions of both linearly bound and bridged CO on Pt/NaY(IM) ( Fig. 4f ). The IR bands for the CO molecules adsorbed on Pt-Co/NaY(IM) sample are quite different from those measured on both ion exchanged samples and P t / N a Y o . A broad peak in the linear CO region and two weak broad peaks in the bridged CO region at 1840 and 1790 cm-l are found at RT ( Fig. 5a ). With increasing adsorption temperature the stretching frequencies of the linearly bound CO shifts towards lower wavenumbers. When the temperature is higher than 423 K additional peak at around 1990 cm-' is observed which also shifts towards lower wavenumbers with increasing adsorption temperatures. Similarly to Pt-Co/NaY@), the peak at 1840 cm-l for bridged CO on R-Co/NaY(IM) disappears when the adsorption temperature is higher than 423 K, while the second peak remains unchanged. After repeated reduction at 573 K the absorption bands for linearly bound and bridged CO can not completely be recovered ( Fig. 5f ). It can be expected that the oxidation states and the reducibilities of Pt and Co are very different for ion exchanged and impregnated samples. Hydrogen reduction would result in larger particles on the external surface of the zeolite for impregnated samples as shown in Table 1. This is further confirmed by our recent XRD results [MI. Although the difference in the interaction between Pt and zeolite support in Pt/NaY(IE) and Pt/NaY(IM) ( e. g. with cage well and external surface, respectively ) should not be excluded, the red shift for linearly bound CO on Pt/NaY(IM) may be considered as the change of Pt particle size. The distinct features for the CO molecules adsorbed on Pt-Co/NaY(IM) sample indicate the difference in the surface structure of the catalyst. Apparently, Co sub-carbonyl species has been formed on this sample upon CO adsorption. Our recent TPR and XRD results [18] show the complete reduction of both Pt and Co oxides in Pt-Co/NaY(IM) catalyst. Pt-Co bimetallic particles with -30 atom% Co has been found. In addition, there must be isolated cobalt metal because of the relatively higher Co content in this catalyst. This can explain why cobalt sub-carbonyl species is formed on Pt-Co/NaY(IM) but absent on PtCo/NaY(IE) catalyst. For Pt-Co/NaY(IE) catalyst, isolated Co cations, if present, are entrapped in small cages and remain unreduced. Finally, it has been found that the Pt-Co bimetallic particles may be decomposed either by mild oxygen reoxidation or by reaction with surface protons at higher temperatures in inert atmosphere [19]. In addition, different pretreatments, e. g. with prolonged H2 reduction, repeated CO adsorption and desorption as well as with CO hydrogenation reaction, may induce surface segregation of the bimetallic particles. The researches are still in progress in our laboratory. The influences of these pretreatments on the IR spectra of adsorbed CO molecules on various IE and IM catalysts should be investigated in the further studies.

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4. CONCLUSIONS

The present IR spectroscopic results for CO adsorption on different Pt-Co/NaY samples indicate that the surface properties of Pt particles are influenced by the particle size, its location and the presence of the second metal. The red shift of the IR frequencies for linearly bound CO on samples prepared by ion exchange and impregnation methods can be attributed to the influences of particle size and its location. The red shift for CO adsorption on the same sample with increasing adsorption temperatures can be considered as the decrease of the dipole-dipole interaction among the adsorbed CO molecules. While the red shift with addition of Co may be caused by the electron interaction of Pt with Co by forming bimetallic particles. Heating Pt-Co/NaY(IM) in CO atmosphere may result in Co sub-carbonyl species formation. The different features of CO adsorption on ion exchanged and impregnated samples indicates the influences of zeolite cages on constraining the formation and the stability of small Pt and Pt-Co bimetallic particles.

ACKNOWLEDGMENTS This work was supported by the Hungarian Scientific Research Fund (OTKA No. 1887). The authors are indebted to Professor J. Mink and his colleagues for the help of IR measurements and useful discussions. REFERENCES 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

U. Bardi, A. Atrei, P. Ross, E. Zanazzi, and G. Rovida, S q f Sci., 211/212, 441(1989) U. W d i , B. C.Beard, and P. N. Ross, J . Cutul., 124, 22(1990) M. J. Dees, T. Shido, Y. Iwasawa, and V. Ponec, J. Cutul., 124,530(1990) 2. Zsoldos, T. Hoffer, and L. Guczi, J. Phys. Chem., 95, 798(1991) 2. Zsoldos, and L. Guczi, J . Phys. Chem., 96, 9393(1992) T. Homeyer, and W. M. H. Sachtler, in "Zeolites:Facts, Figures, Future" ( a s . : P. A. Jacobs and R. van Santen), Elsevier, Amsterdam, 1989, p. 975 and references therein W. M. H. Sachtler, Z-C Zhang, A. Yu Stakheev, and J. S. Feeley, in L. Guczi, F. Solymosi and P. T6tknyi (Eds.), New Frontiers in Catalysis ( Proc. 10th Inter. C a d . Congress, Budapest, July 19-24, 1992 ), Elsevier/Amsterdam, 1993, Vol. 75, Part A, p 271 P. Gallezot, Cutul. Rev. Sci. Eng., 20, 121(1979) A. Yu Stakheev, and W. M. H. Sachtler, J. Chem. SOC. Furaduy Trans. I , 87, 3703(1991) G. Lu, T. Hoffer, and L. Guczi, Cutul. Lett., 14, 207(1992) G . Lu, T. Hoffer, and L. Guczi, Appl. Cutul., 93, 61(1992) L. Guczi, G. Lu, and Z. Zsoldos, Cutual. Today,in press R. G. Greenler, K. D. Burch, K. Kretzschmar, R. Klauser, A. M. Bradshaw, and B. E. Hayden, Surf. Sci., 152/153, 338(1985) M. Pnmet, J . Cutal., 88, 273(1984) A. De Mallmann, and D. Barthomeuf, Caul. Lett., 5 , 293(1990) M. Ichikawa, in L. Guczi, F. Solymosi and P. TBGnyi ( Eds.), New Frontiers in CatuZysis ( Proc. 10th Inter. Catal. Congress, Budapest, July 19-24, 1992 ), Elsevier/Amsterdam, 1993, Vol. 75, Part A, p 280 R. W. Joyner, and E. S . Shpiro, CutuZ. Left., 9, 239(1991) G . Lu and L. Guczi, to be published G. Lu, Z. Zsoldos, Z. Koppany and L. Guczi, Cutul. Lett., in press

Some Characteristics of Transition-metal Containing Y-Zeolite in CO Hydrogenation

Son-Ki Ihml, Dong-Keun Lee2 and Jin-Ho Lee3 Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusongdong, Yusonggu, Taejon, 305-701, Korea Department of Chemical Engineering, Gyeongsang National University, 900, Kajwadong, Chinju, 660-701, Korea Department of Chemical Engineering, Chungbuk National University, Gaesindong, Cheongju, 360-763, Korea

ABSTRACT The cobalt-containing Y-zeolite catalysts were prepared by excess water (EW), ionexchange (LE) and carbonyl impregnation (CI) methods. The activity and selectivity in CO hydrogenation is explained in terms of the acidity modification and metal distribution. Characterization by electron paramagnetic and ferromagnetic resonance spectroscopy and TPSR was in good agreement with the catalytic property. Temperature programmed surface reaction (TPSR) for methane formation is proposed as a tool to determine the location of metal species, especially in CI catalysts. INTRODUCTION The catalytic hydrogenation of carbon monoxide to hydrocarbons andlor other oxygenates has attracted much attention due to the need to develop alternatives to petroleum feedstocks. The recent interest in CO hydrogenation has been aimed mainly at improving product selectivity by circumventing the conventional Schulz-Flory distribution. For this purpose metalcontaining zeolites show great promise because they can be made to provide catalysts with highly dispersed metals to show molecular sieving selectivity, and to induce polfinctional activity [ 1-61, The preparation methods for highly dispersed metal in zeolite pores can be said to be the key to a better design of a selective catalyst. The characteristics of metal-containing zeolite catalysts can be changed by modieing the zeolite acidity and controlling the metal locations. For instance, metal(Fe, Co or Ru)N-zeolite catalysts prepared by thermally decomposing metal carbonyls within the zeolite cavities would give selective formation of hydrocarbons in the C1 - Cg range [I]. This paper discusses the characteristics of transition metal-containing Y-zeolites in CO hydrogenation [2-51. Special emphasis is placed on the cobalt-containing Y-zeolites prepared by three different methods. The effects of preparation methods and metal loading on the catalytic 35s

356

S.-K. Ihm, D.-K. Lee and J.-H. Lee

activity and selectivity for CO hydrogenation were investigated. Characterizations of catalyst samples were conducted through hydrogen chemisorption, transmission electron microscopy, electron paramagnetic and ferromagnetic resonance, and temperature programmed desorption. The temperature programmed surface reaction (TPSR) is proposed as a tool to determine the location of cobalt clusters in Y-zeolite. EXPERIMENTAL Three different preparation techniques for excess-water (EW), ion-exchange (IE) and carbonyl complex-impregnation (CI) were used to introduce cobalt on or into the NaY zeolite ( Strem Chem., Na,,.,[(A102)54.9(SiO~)~~,,11 ). Catalysts obtained by the excess water technique were prepared by filling the pores of the supports with a solution of cobalt salt. For the ion-exchanged catalysts cobalt nitrate solution (pH=4.5, 0.04N) was mixed with NaY zeolite by stirring for 48h at 85 OC. The carbonyl complex-impregnated catalysts were prepared by physically dispersing cobalt carbonyl (Co2(CO),) dissolved in n-pentane on NaY zeolite, followed by the decomposition of cobalt carbonyl to cobalt metal. All catalysts were reduced with hydrogen by heating to 500 OC for 18h. For the investigation of the location of cobalt metal, three different modes of temperature programmed surface reaction (TPSR) were carried out in a flow system. For the first mode, CO is preadsorbed at room temperature and TPSR were observed with hydrogen flow. For the second mode, CO is dissociated at 270 OC to deposit surface carbon and TPSR were made on the carbon deposit with H2. For the third mode, H, is adsorbed first and CO is adsorbed consecutively at room temperature. The intensities and locations of main peaks were correlated with catalytic properties. The activity and product distribution for CO hydrogenation were measured in a tubular microreactor under atmospheric pressure. The H+O ratio was 2 and the reaction temperature varied from 230 OC to 390 OC [3]. RESULTS and DISCUSSION Catalvst characterization The extent of reduction and dispersion of CON-zeolite catalysts obtained by the three preparation methods are listed in Table 1. The extent of reduction of the IE catalyst was very low (less than 10%) but the dispersion of reduced cobalt metals of the IE catalysts was much higher than that of the EW catalyst. Our investigation [3] by transmission electron microscopy (JEOL 200CX, 160 KeV) showed that large cobalt particles with diameter in the range of 20 - 50 nm were located on the outside of the zeolite crystals for the EW catalysts reduced at 500 OC for 18 h. When the IE and CI-10 catalysts were reduced at 500 *C for 18 h, no metal particles of detectable size were observed. The size of cobalt metal in the IE and CI-10 catalysts is thought to be less than that of faujasite supercage (- 1.3 nm). Figure 1 shows the TPD spectra of carbon monoxide. Sharp maxima appear at about 110 OC for the E and EW catalysts, while a very broad peak ranging from 30 OC to 300 OC appears for the CI-10 catalysts.

Transition-metal/Y-Zeolitein CO Hydrogenation

357

Table 1 . Extent of reduction and dispersion for the Con-zeolite catalysts reduced at 500 OC for 18 h. Catalysta

Extent of reduction(%)

Dispersion(%)d

EW-10 IE-6 IE-8 IE-9 c 1-10

88.9b 8.2' 7.2' 8.7' 10oe

0.23 50.0 60.0 60.0 10oe

~

the first term denotes the preparation method and the second term denotes cobalt metal loading in wt %. b calculated from 0 2 titration( 3C0° + 202 4 C03O4 ) [7]. C calculated from H2 consumption( Co2+ + H2 4 Coo t 2H+ ) [3]. d calculated from the irreversible uptakes of H2. e assumed that cobalt carbonyl decomposed completely to atomic cobalt metal. a

A

0

100

200

300

400

TEMPERATURE('C)

Fig. 1. TPD chromatogram of CO adsorbed on Co/Y-zeolite catalysts [3].

zoo0

H (

9

3000 4 00 Gauss

Fig. 2. EPR and FMR spectra of the IE and CI catalysts(detecti0n temperature = 100OC) [2].

The EPR and FMR spectra of the IE and CI catalysts are shown in Figure 2. EPR spectra of divalent cobalt ion in the unreduced IE-9catalyst appeared at a g-value of 1.99. In the CI catalysts broad and nearly symmetric peaks are shown at a g-value of 2.17 which was identified to be an FMR peak of cobalt metal in Y-zeolite [S]. In the IE catalyst, however, sharp peaks of divalent cobalt ion ( g = 1.99, AH = 10 Gauss ) appeared together with the broad FMR peak of cobalt metal. This implies that only part of the cobalt ion was reduced to cobalt metal ( or that the cobalt ion and metal coexist). Since the g-value and the peak-to-peak line width of Fh4R spectra in the IE and CI catalysts varied only slightly with detection temperature (20 OC - 300 OC) irrespective of cobalt loading, the size of cobalt metal is believed to be small enough not to show magnetoanisotropy [ 8 ] .

358


Since the line width of CI-10 catalyst decreased slightly with increase in the detection temperature, it is believed that the cobalt metal particles of CI-10 catalyst were somewhat larger than those of IE catalysts. Catalvtic activity and selectivity As expected from the aforementioned characterization by chemisorption, TEM, TPD and FMR study, the catalytic behaviors were greatly affected by preparation methods and metal loading, as shown in Table 2.

Table 2. Activity and selectivity of CON-zeolite catalysts in CO hydrogenation (270 OC, H2KO =2). Rate x 1O8

Catalyst (

mole CO ) g cobalt. sec _

EW-10 IE-6 IE-8 IE-9 c 1-10

_

1210. 0.283 13.1 188. 26.

Turnover frequency Product distribution(%) (sec-1) x 103 _

_

_

290. 3 . 3 2 ~lo4 1 . 2 7 ~10-2 1.85 x 10-1 1.53 x 10-2

_

~

__

~

11. 22. 16. 16. 14.

i -C4n-C4=

C2= C2

- -

C1 C2 C3 C4 C5

55. 49. 67. 63. 35.

Selectivity

~

18. 11. 5. 19. 10. 14. 3. 14. 7. 26. 18. 7.

~

0.11 0.50 0.18 0.04 0.20

0.6 3.3 6.8 7.1 0.1

The activity in terms of turnover frequency (TOF) of the EW-10 catalyst is about 1500 times higher than that of the IE-9 catalyst, and the TOF of the IE-9 catalyst is about 12 times higher than that of the CI-10 catalyst. The higher activity of the EW catalysts than that of the IE and CI catalysts could be ascribed to the structure-sensitive nature of CO hydrogenation [9]. The higher activity of the IE-9 catalyst than that of the CI-10 catalyst, however, would be due to the electrondeficient character of the metals from the electronic modification with acidic zeolite [lo]. Similarly, the reason for increase in the activity of the E catalyst with increasing metal loading and reduction temperature could be also attributable to the modification of zeolite acidity, while in the CI-10 catalyst the electronic modification could not be expected. For the product distribution with different preparation methods, the CI catalyst was the most selective to higher hydrocarbons of C3 and C4 and olefinic hydrocarbons. The decrease in the olefin fraction with the ion exchange in the IE catalysts involves the acidity of zeolite support, and the marked formation of iso-butylene in C4 fraction on the IE catalysts is attributed to their dual hnctional action between cobalt and Bronsted acid sites [ 111. Location of cobalt metal particles The location of metal particles in the reduced metal-zeolite catalysts are of great importance since very different catalytic properties have been reported by varying the location of metals in

Transition-metal/Y-Zeolitein CO Hydrogenation

359

zeolite. Characterization of metal location has been done conventionally either through a consecutive temperature programmed reductiodoxidation (TPWTPO) procedure or through magnetic characterization techniques [12]. As shown in Table 2, it is obvious that the differences in the catalytic properties among the EW, IE and CI catalysts were due to the differences in the modification of acidity as well as the location of the cobalt metal in zeolite. Therefore, it is necessary to distinguish the independent effects of the metal location on the characteristics of CO hydrogenation. Four types of CI catalysts having different locations of cobalt metals were prepared by heating fresh CI-10 catalyst in hydrogen flow. The treatment conditions are designated by the symbols in Table 3. The TEM photographs of the four types of CI catalysts are shown in Figure 3. TEM and FMR data indicate that the size and location of cobalt metals do vary with treatment conditions. As the treatment conditions become severe, finely dispersed cobalt clusters inside zeolite pores (CI-A) migrated out of zeolite pores and agglomerated outside the zeolite crystals showing bidisperse metal distribution (CI-B), and hrther migration resulted in the formation of large cobalt metal particles at the exterior surface of zeolite crystals (CI-C and CI-D).

Fig. 3. TEM photographs of CI-10 catalysts with treatment conditions [4].

360


Table 3 indicates that the CI-A catalyst showed two orders of magnitude lower activity than the other CI catalysts. This seems to be due to the suppression of hydrogen adsorption as proposed for dispersed platinum surface [13]. Even though the product distribution over CI-A favored C3 and C4 hydrocarbons, those over other CI catalysts followed the conventional Schulz-Flory distributions. This must be due to the effect of different particle sizes. Table 3. Treatment conditions, chemisorption data and catalytic properties of the CI-10 catalysts. Treatment H, uptakes Rate x 108 Product distribution(%) Catalyst condition (pmole/g cobalt) (mole CO/g cobaltsec) C1 C2 C3 C4 C5 CI-A CI-B CI-C CI-D

500-2- 18* 500-20-72 700-20-24 700-20-168

26. 1670. 2350. 3150.

0.9 92. 123. 76.

35. 50. 68. 70.

14. 20. 16. 14.

26. 18. 7. 18. 10. 2. 10. 5. 1 . 9. 6. 1 .

* indicates that the CI-A catalyst, for example, was heated to 500 OC at 2 OC/min and reduced at 500 OC for 18h. TPSR spectra of the CI catalyst for the first mode where preadsorbed CO reacted with hydrogen are shown in Figure 4 - (a). Two representative peaks of the lower temperature peak at around 170 OC and higher temperature peak at around 570 C ' were assigned to be Q and p peak respectively. With the treatment condition changing from A to D, the intensity of Q peak increased while that of j3 peak decreased. This change seems well correlated with the location of cobalt metal particles, i.e., a peak is due to large cobalt metal particles (CI-D) while j3 peak is due to small

-

a

-..01

A - : CI-A

. CI-A . CI-D

- ..- : CI-D

I

!! I :

i;

'.. 100 200 300 400 SO0 000 700

TEMPERATURE ('C)

. . !

I

'.

_...-.. -

100 200 300 400 SO0 000 700

TEMPERATURE ('C)

100 200 SO0 400 SO0 000 700

TEMPERATURE ("C)

Fig. 4. TPSR spectra of CI-A and CI-D catalyst (a); the 1st mode where CO is preadsorbed at room temperature, (b); the 2nd mode where CO is dissociated at 270 OC to deposit surface carbon, (c); the 3rd mode where Hz is preadsorbed first and CO subsequently.

Transition-metaI/Y -2eol ite in CO Hydrogenation

36 1

particles (CI-A). It is believed that this distinct difference in TPSR spectra with treatment conditions could be employed successfilly to distinguish the location of cobalt metals in Y-zeolite. It was found that when the mixture of H2 and CO (H2/CO upto 2) was preadsorbed at room temperature the TPSR spectra remained the same. This indicates that hydrogen is unstable to compete with CO for adsorption sites in the CI-A catalyst. The difference in temperature between a and p peak may be due to one or more of the following reasons; different dissociation rate of adsorbed CO or different rate of reaction between surface carbon and hydrogen. Figure 4 - (b) shows that the change of a and p peak from the second mode is similar to that from the first mode of TPSR, except for a new peak appearing at around 400 OC and both 01 and p peak being shifted to a lower temperature. The second mode of TPSR where predeposited surface carbon reacts with hydrogen can be considered to eliminate the effect of metal location on the formation of methane if the dissociation rate of adsorbed CO is the major factor for the difference in temperature between a and p peak. Accordingly the difference in temperature between a and p peak cannot be said to be totally due to the difference in CO dissociation rate. Figure 4 - (c) shows the results from the third mode of TPSR, where hydrogen was adsorbed first and CO consecutively. Since the adsorbed hydrogen is known to be very reactive the surface carbon can be removed easily as methane. The main peak of TPSR spectra is expected to shift to a lower temperature. Even though it is not conclusive, the shift seems to be drastic for the CI-A catalyst as seen in the figure. The difference in the relative rate of both CO dissociation and surface carbon hydrogenation must play important roles for the difference in the main peak of TPSR with the location of cobalt metal particles.

CONCLUSION Catalytic behaviors of cobalt on Y-zeolite were strongly affected by preparation method. The higher activity of the EW catalysts than that of the IE and CI catalysts is ascribed to the structuresensitive nature of CO hydrogenation. The CI catalyst was the most selective to higher hydrocarbons and olefinic hydrocarbons. The activity of the IE catalysts increased significantly by increasing cobalt loading and reduction temperature, while the activity of the CI catalysts was nearly constant. The increasing acidity with cobalt loading in the IE catalysts seemed to result in the decrease of the olefin fraction, as well as in the increase of branched hydrocarbons, but the CI catalysts showed little change. Schulz-Flory type product distributions were observed in the IE and EW catalysts, while the C3 and C4 hydrocarbons were favored in the CI catalysts. The difference in the product distribution between the IE and CI catalysts seemed to be due to the difference in chain propagating ability of the catalysts. The CI catalysts treated under various reduction conditions showed very different TPSR spectra of methane. It can be concluded at this time that a main peak appearing at around 570 OC is for the catalyst having finely dispersed cobalt clusters in the cages and a lower temperature peak

362


appearing at around 170 OC for the catalyst having bulky cobalt metal aggregates outside the zeolite. The different TPSR spectra due to metal location might be explained in terms of the difference in the relative rate of both CO dissociation and surface carbon hydrogenation. It is proposed that the TPSR technique could be used to characterize the cobalt metal locations in Y-zeolite, and hrther work in this direction is encouraged.

REFERENCES 1. D. B. Tkatchenko and I. Tkatchenko, J. Mol. Catal., II (1981) 1. 2. D.K. Lee and S.K. Ihm,J.Catal., 106 (1987) 386. 3. D.K.Lee and S.K. Ihm,Applied Catal., 2 (1987) 85. 4. C.H. Bartholomew and J.B. Butt(Eds.), Catalyst Deactivation 1991, Elsevier/Amsterdam, 1991, p.219. 5. R. Ryoo, S.J. Cho, C.H. Pak, J.G. Kim, S.K. Ihm and J.Y. Lee, J. Am. Chem. SOC., 114 (1992) 76. 6. F J . Wang and Y.W. Chen, Applied Catal., 22 (1991) 21, 7. C.H. Bartholomew and R.J. Farrauto, J. Catal., 45 (1982) 360. 8. L.E. Iton, R.B. Beal and P.J. Hamot, J. Mol. Catal., 27 (1984) 95. 9. L. Fu and C.H. Bartholomew, J. Catal., 92 (1985) 376. 10. J.G. Goodwin and C. Naccache, J. Mol. Catal., 14 (1982) 259. 11. Y.W. Chen, H.T. Wang and J.G. Goodwin, J. Catal., 82 (1 983) 41 5. 12. P.A. Jacobs, M. Tielen, J.P. Linart, H. Nijs and J.B. Uytterhoeven, J. Chem. SOC. Faraday I, 22. (1977) 1745. 13. T. Kubo, H. Arai, H. Tominaga and T. Kunugi, Bull. Chem., SOC. Japan, 45 (1972) 607.

Ni-Mo-Y Zeolites as Catalysts for the Water-Gas Shift Reaction

M. tanieckl Faculty of Chemistry, A. 60-780 Poznafi. Poland

Kickiewicz

University,

Grunwaldzka 6,

ABSTRACT Sulfided M o - Y and Ni-Mo-Y zeolites, as the catalysts for the water-gas shift (WGS) reaction, are described. Molybdenum loaded Y-zeolites were studied with TPR, sorption of NO and ammonia, ESR and FTIR spectroscopy. It was found that high activity of NiMo-Y zeolites is related not only to the high dispersion of molybdenum-sulfide like.species, synergy between Ni and Mo sulfides but is also influenced by surface OH groups. INTRODUCTION The water-gas shift (WGS) reaction is well known industrial catalytic process for hydrogen production. Two classes of catalysts are used almost exclusively in industry as shift catalysts: iron oxide and copper oxide based catalysts 111. Some years ago a new class of sulfur tolerant alumina supported Co-Mo sulfidesalso found the industrial application 121. The recent results IS51 indicate that sulfided Mo-Y and Ni-Mo-Y zeolites can be very efficient catalysts in the WGS process. It was found that catalysts prepared from molybdenum hexacarbonyl and medium pore zeolites in contrast to those obtained from ammonium heptamolybdate, show high dispersion of molybdenum sulfide-like species and very high activity in the WGS reaction. The present paper summarize certain earlier results and extends the investigation of Ni-Mo-Y sulfided zeolites as catalysts in the WGS reaction with TPR, IR and ESR spectroscopy.

-

EXPERIMENTAL NaY zeolite (Katalistiks: Si/A1=2.56) was used as a starting material f o r preparation of NH K, Cs and N i exchanged zeolites. 4'

363

364

M.taniecki

These zeolites were applied as supports for molybdenum. Zeolites after activation in H2 at 475 o r 675 K were saturated with molybdenum hexacarbonyl vapours. Sublimation of M o ( C O ) ~ was performed at room temperature f o r 12-15 hours in a stream of highly purified hydrogen. After partial decarbonylation at 425 K and exposition to air at room temperature, samples were next sulfided fer 2 hours at 675 K (for preparation pathways see also Fig.1). The catalytic experiments were performed at 625 K , and the concentration of H,S during the reaction was constant and equal 2 vo1.k. The TPR experiments were carried out according to the procedure described in ref.6. The details of the preparation, experimental conditions for ESR, IR and adsorption studies can be found elsewhere )3-5,7,81.

I I

steaming with loo\ H 2 0 vspour at 875 K

~ i " solution

Ni"

soluion

Activation (at 4 7 5 or 6 7 5 K(

I

1

Partial decarbonylation at 425 K

at 675 K

Fig.1. Scheme f o r the catalysts preparation.

RESULTS AND DISCUSSION

Data presented in Table 1 show that catalysts prepared by the incipient wetness method with ammonium heptamolybdate (AHIUI) are at least 2 to 4-fold less active than those obtained from Mo(CO)~. In both cases the Mo content was almost the same and equal I 0 wt.%. Assuming that NO adsorption can serve as the measure of Mo-sulfide

Ni-Mo-Y for Water-Gas Shift Reaction

365

Table I. Activity of the sulfided, molybdenum loaded Y-zeolites in the WCS reaction 131. Reaction temp. 625 K. Support

Impregnation AHM Conversion

l%l*

NaY NaH(28)Y+** NaH(61)Y+** NaH(78)Y***

8.7

8.5 10.8 2.3

Loading with Mo(C0l6

Amount of Conversion NO adsorb.** IfRI+ 0.085 0.040 0.034 0.019

Amount of NO adsorb.**

19.8 46.5 41.7

0.184 0.165 0.156

40.6

0.166

* - measured after 2 hours, cata4yst weight - 0.5 g +* - sorption capacity in mmol g*++- numbers in parantheses indicate the degree of exchange dispersion, the significant differences can be observed in both types of catalysts. A very poor dispersion and low catalytic activity indicate that application of ammonium heptamolybdate ( A H M ) as a source of Mo, results only in the external deposition of Mo-sulfided species on the zeolite surface. This proposal finds confirmation in the studies by Fierro et a1.191, who established that impregnation of zeolites with AHM leads to the formation of oxoanionic and neutral complexes that are not able to penetrate into the zeolite porous system. In contrast, preparation with WO(CO)~ due to the small particle dimensions, leads toward the zeolites where Mo atoms can penetrate zeolite cavities and in consequence the sulfided species can be also located inside the zeolite porous system. Moreover, it was found that activity for the NaHY supports is higher than for nonprotonated zeolite Nay, while the Mo content was the same in all cases. It is assumed that this effect as well as the deactivation of the zeolitic catalysts is related to the presence of surface acidic OH groups. Application of nickel-excyanged zeolites as the supports for Mosulfided species significantly enhances catalytic activity ( the Mo-free sulfided nickel zeolites were totaly inactive in the WCS). Data presented in Table 2 show the influence of the zeolite pretreatment temperature and Ni content on activity in th WCS reaction. In all cases under the same experimental conditions,

366

M. taniecki

Table 2. The WGS activity of sulfided Ni-Mo-Y zeolites 181. Ni content** Iwt.%l

Conversion I t e l *

NH upta$e+i* lmmol g I

Activation temperature IKI 4 75 675

0

0.65

1 2

0.82 0.92

31.2

3

1.39

63.3

53.4

55.6

48.3

3.6

15.6 18.5

1. I 9

9.9 8.6 17.6

* - measured at 625 K, after 2 hs, catalysts weight 0.25 g ** - Mo content const., 10.3 wt.k ***- after 2 hours sulfidation, support activated at 675 K pretreatment in hydrogen at lower temperatures gives much higher activity than for the samples activated at 675 K. Samples pretreated at temperature lower than 475 K showed lower activity due to the limited access of M0(COl6 inside the zeolite (lower Mo content). On the other side, migration of Ni particles with simultaneous formation of Ni clusters ( 4 0 to 50 nm crystallites) at temperatures higher than 675 K 1 7 0 1 significantly reduces catalytic activity (e.g.- activity drops 50% when NaNiY zeolite was pretreated at 825 K ) . In order to establish the influence of a pretreatment temperature on catalytic activity for NaNiY zeolites, a series of TPR

Activ. temp. [ K 415

-j.

I

n

____

675 - 675 ; Ni cont.- 5 wt. %

0

Y

C

0 .-+

a

Fig.2. Temperature-programmed reduction (TPR) of selected NaNiY zeolites. Heating rate 20 K minr’

E

rn 3 C

8 h

1

0

700

800

900

IMX)

Temperature [ K 1

1100

li


367

(temperature-programmed reduction) experiments was performed. Three peaks at 780, 900 and 1080 K appeared on TPR spectrum for NaNiY (3 wt.%) supports indicating the best catalytic activity. This corresponds to the reduction of at least three kinds o f Ni2+ ions with different reactivities for hydrogen toward zerovalent state. The reduction toward Ni' can be excluded because of the absence o f characteristic resonance line on ESR spectrum.According the appearance o f three to the findings by Suzuki et al.161 distinct peaks on TPR spectrum can be assigned to Ni2+ ions positioned at three kinds of exchanged sites SII (or/and SfI), S; and SI, respectively. As it can be seen from Fig.2. the reducibility of Ni2+ ions at SII position decrease with the increase of precalcination temperature. For the sample pretreated at 675 K at least part of the Ni ions is located at SII position migrate into SI (peak at 1080 K). The increase of Ni content (5 wt.k) increases the amount of Ni ions at SI position at the expense o f the SII location. Thus, it is belived that mainly Ni2+ ions located at SII undergo an easy transformation toward zerovalent nickel during the reduction and toward sulfur deficient nickel sulfides during sulfidation. TPS (temperature-programmed sulfidation) experiments confirm this assumption 1111. The TPR and TPS experiments of Ni-Mo-Y zeolites are under way and will be reported in due course. The WGS activity is therefore related to the dispersion o f Mosulfided species, location o f Ni2+ ions and i n consequence the ability o f nickel ions toward nickel sulfide species formation. The increased activity of nickel containing zeolites, however, is essentialy related t o the synergetic effect between Ni- and Mosulfided species. The synergy betwen these supported sulfides was confirmed in the IR experiments by the appearance of a band at 2082 cm-' after interaction o f sulfided species with CO at room temperature 112 I It was already mentioned that activity in the WCS can be also influenced by the support acidity. The results o f NH adsorption 3 over the sulfided Ni-Mo-Y catalysts ( Table 2 ) seems to confirm this assumption. The amount of adsorbed ammonia increases almost parallely with the Ni content (up to 3 wt.%). At higher Ni loadings up to 5 wt.% the decrease of ammonia sorption capacity was observed. The increase of NH adsorption is related to the 3 generation o f acidic OH groups during the Ni-exchange 1 4 1 .

.

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M.kaniecki

Series of additional experiments with decomposition of isopropyl and diacetone alcohols as well as measurments of ammonia, pyridine and BF3 adsorption 181 indicate that zeolite surface hydroxyl groups are probably involved in the WGS reaction. These groups can originate both from the support itself (e.g. NaHY or NaNiY) o r can be formed by a heterolytic splitting of H2S into H+ and OH- ions. The catalytic experiments in which WGS reaction was interrupted by the poisoning of catalysts at the reaction temperature with bases (pyridine, ammonia) and acids (BF HC1, CH3COOH) provided addi3’ tional arguments. The most active Ni-Mo-Y catalysts after the repeated injections of small aliquots of NH3 ( usually 0.09 mmol shots in helium at 625 K ) indicated almost no changes in activity at the initial stage. The drop in activity after poisoning with 0.45 mmol of NH usually was not higher than 3% (for 0.25 g of 3 catalyst). Next aliquots of NH, further reduced the activity which 2 finally was stabilized at level of 40% of the initial one. Similar effect was observed while poisoning with pyridine. As in the case of ammonia, pyridine (shots 5 or 15 ul in He) also reduced activity with final stabilization at 35% of the initial activity. A typical acidic support as NaH(78)Y, after poisoning with small amount of Py, showed the same behaviour as very active Ni-MoY catalyst: negligible decrease during first doses of a base, further decrease till the level of 508 of the initial activity and relatively stable activity during the next 20 hours of reaction. A prolonged exposition of pyridine or ammonia poisoned catalysts to the base-free feed resulted in all cases in partial recovery of the WGS activity. Presented results show that ammonia or pyridine reacts first with strong acid sites which probably are not engaged in the WGS, whereas weak or medium acid sites can be involved in the mechanism of U G S process. Similar studies with acid adsorption (mainly BF 1131 and acetic 3 acid ( 1 4 1 were applied ) showed that participation of basic sites in the WGS is rather doubtful under the reaction conditions but cannot be completely excluded. Poisoning of sulfided Mo-Nay, NoKY o r MO-CSY catalysts with BF3 at the reaction temperature caused only slight decrease in activity. However, for protonated supports (NaHY o r NaNiY) catalytic activity changed dramatically upon the BF adsorption. Usually after the interaction of 0.25 g of a 3


369

catalyst with 0.3 mmol of BF3 the complete deactivation of the catalysts was observed. After the reaction of surface hydroxyls or basic oxygen ions with BF formation of borate-like species 3 was expected 1131. However, presence of water (after the restart of WGS) leads to the formation of hydrated borium oxide inside the zeolite porous systemvia hydrolysis of the “borated” form. This was confirmed by the surface area measurments where a sharp decrease in surface (from 800 to 30 m2g”) was observed. Application of acetic acid as a probe for basic sites participatin g in the WGS, due to a rapid desorption at high temeperature, appeared to be not very informative. The use of HC1 resulted in noticable loss of zeolite crystallinity. Because BF reacted almost exclusively with hydroxyl groups, 3 causing the deactivation of catalysts with protonated surface,it is belived that OH groups participate in the WCS reaction under the sulfided feed. In addition, ammonia and pyridine adsorption strongly supports this idea and indicate that only moderate or weak acid sites can be engaged in the WGS reaction mechanism.

Fig.3.ESR spectra of ~lo/lo(~)6/Y-zeoliteFig.4.m spectra of Mo(CO) y-z-lite system decarbonylated at 425 K system after sulfida ion at and exposed to air. 675 K followed by air exposition. a- NaY b- NaH(28)Y a NaY after sulfidation c- NaH(61 )Y d- NaH(78)Y b as a/ after air exposition c NaH(78)Y after sulfidation d - as c/ after air exposition

e/

-

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M.taniecki

In order to establish the influence of a support acidity on catalytic activity, the ESR experiments with Ni-free zeolites were additionaly performed. The decarbonylated samples after exposition to air indicated ESR signal with g=1.93 ascribable to Ko5*.The intensity of this signal increases with support acidity (Fig.3.). Subsequent sulfidation at 675 K leads to the formation of sulfur and hydrosulfur radicals 151 and their amount is proportional to the support acidity. Sulfidation of decarbonylated Ko species at 675 K , without preexposition to air, gives simple spectrum for NaY support (Fig.4a) and more complicated for KaHY supports (Fig.4~). A comparison of these results with literature data (151 and with those obtained for molybdena-alumina, suggest that Mo+ and Mo' z + are formed upon sulfidation in oxygen-free atmosphere. Series of sulfur and hydrosulfur radicals are formed (Fig.4b,4d) after exposition of these samples to air and their amount is proportional to the support acidity. The detailed ESR study of sulfided EloY and Ni-Mo-Y zeolites will be published elsewhere. The ESR results as well as those from IR measurments (not shown here) indicate that molybdenum ions with lower than !to5+ oxidation state (Mo' o r No3+) and formate-like species can participate in the mechanism of the water-p,as shift reaction. Acknowledgments The author acknowledges the Stefan Batory Foundation for financial support: the Elsevier Sci. Publ. and Butterworth-kinemann Publ. for permission to use

published materials.

REFERENCES 1. L.Lloyd, D.E.Ridler and M.V.Wigg, Chapter 6,in "Catalyst Handbook", 2nd Ed. (M.V. 'hi&, Editor) Wolfe Publ. Ltd. 1989, England 2. D.S. Newsome, Catal.Rev.-Sci.Eng., 21 (1980) 275 3. M. taniecki and W. Zmierczak, Zeolites, 11 (1991) 18 4. M. taniecki and W. Zmierczak, Stud.Surf.Sci.Catal., 65 (1991) 337 5. M. taniecki and W. Zmierczak, Stud.Surf.Sci.Catal., 68 (1991) 799 6 . M. Suzuki, K. Tsutsumi, H. Takahashi and Y. Saito, Zeolites, 9 (I=) 98 7. M. taniecki, W. Zmierczak and G. Buntkowski, Proceed. 7th 1nt.Heterogeneous Catal., Sept.29-0ct.2, 1991, burgas, Bulgaria, (L.Petrov Ed.), Publ. House Bulg. Acad. Sci. , 1991, p. 283 8. M. kaniecki and W. Zmierczak, Stud.Surf.Sci.Cata1. , 75C (1993) 2569 9. J.L.G. Fierro, J.C. Coresa and L.A. @do, J. Catal., 108 (1987) 34 10. B. Coughlan and M.A. Keane, Zeolites, 11 (1931) 2 11. M. taniecki, in preparation 12. J. Leglise, A. Janin, J.C. Lavalley and D. Cornet, J. Catal.,ll4 (1988)3a8 13. H.G. Karge and I.G. Dalla Lana, J.F'hys.Chem., 88 (1984) 1538 14. W. Przystajko, R. Fiedorow and I.G. Dalla Lana, Zeolites, 7 (1987) 477 15. S. Abdo and R.F. Howe, J.Phys.Chem. , 87 (1983)1922

Synthesis, Characterization and Catalytic Performance of Nitro-substituted Fe-phthalocyanines on Zeolite Y

Rudy F. Parton, Cvetana P. Bezoukhanova, Jan Grobet, Piet J. Grobet and Pierre A. Jacobs Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Department Interface Chemistry, Kardinaal Mercierlaan92, B-3001 Heverlee, Belgium

ABSTRACT Nitro substituted iron- hthalocyanines on Y zeolite are synthesized via a ligand exchange reaction starting o!m a mixture of solids containing ferrocene, 4-nitro-1,2di anobenzene and d Y zeolite. The nature and loading of the catalyst is verified by VisN% and solid state DC-NMR. Purification by soxhlet extraction demonstrates that the nitro-substituted phthalocyanines are not located in the supercages but are adsorbed at the outer surface of Y zeolite crystals. On the contrary, unsubstituted hthalocyanines are encapsulated in the superca es. Nitro-substitution improves considerab y the activity of the complexes, as shown by t e catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanoneat 298 K and 0.1 MPa with tertiary butyl hydroperoxide as oxygen donor.

P

a

INTRODUCTION The rate and selectivity of biological reactions are regulated by enzymes, considered perfect catalysts. The function of the enzyme active site, in some cases a metal ion, is combined with tridimensional protein mantle, creating an environment that ensures highly specific sorption of the reagents. In the recent years zeolites are used as catalysts in the production of organic intermediates and fine chemicals [1-31. It has been suggested [4-71 zeolites to substitute the protein mantle of enzymes. Enzyme mimics have been prepared by in situ synthesis of the metallo-phthalocyanines into zeolite Y. Such systems are denoted as zeozymes [8]. Such heterogenized complexes have shown to be catalytical superior to their homogeneous dissolved analogues and are active in the oxidation of alkanes [6-91.A further increase in their catalytic activity eventually can be achieved by substitution of the phthalocyanine ligand. In homogeneous catalysis [10-11] it is claimed that an iron-oxo species constitutes the active site. Therefore, the active oxygen has electrophilic properties and will become more active by substitution of the phthalocyanine with electron withdrawing groups, Activity can also be enhanced by substitution of bulky groups, which protect the weak meso atoms of the macrocycle against oxidation [12]. Following the latter method Ichikawa et al. [13]claimed to be able to synthesize Fe-phthalocyanines substituted with 4 t-butyl groups in zeolite Y. 37 I

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R. F. Parton, C. P. Bezoukhanova, J . Grobet, P. J. Grobet and P. A. Jacobs

They explicitly claim that the iron r-butyl phthalocyanines are located inside the supercages of zeolite Y. In the present work, the synthesis of nitro-substituted Fe-phthalocyanines associated with zeolite Y is reported and its catalytic activity and selectivity is tested in the oxyfunctionalizationof cyclohexane with tertiary butyl hydroperoxide (tBHP). EXPERIMENTAL Commercial NaY with silicon to aluminum ratio of 2.7 is acquired from Zeocat. Cyclohexane (+99 %) and acetone (p.a) are purchased from Janssen Chimica; 1,2dicyanobenzene (DCB) (+ 98%), 4-nitro-l,2-dicyanobenzene (4NDCB) (+99%), dimethylformamide (99%) (DMF) and ferrocene (98%) from Aldrich. UV-vis-absorption, FT-IR-spectroscopy and solid state CP/MAS 13C-NMR are performed on a CARY 17, a Nicolet 730 and a Bruker 400 MSL spectrometer operating at 9.4 T, respectively. The solid state CP/MAS %NMR were run at 100.577 MHz, with a contact time of 2.5 ms, a pulse interval of 5 s, a spinning frequency of 13 kHz and an accumulation of 11,000 scans. Nitrogen adsorption isotherms were recorded using an Omnisorp-100analyzer from Coulter. The catalytic reactions are carried out in a batch reactor with continuous stirring in the liquid phase at room temperature (298 K), atmospheric pressure (0.1 MPa), with tBHP (100 mmol) as mono oxygen atom donor, acetone (30 ml) as solvent, with 0.5 g catalyst and 50 mmol substrate. Identification and quantification of products occurs by GC-analysis on a 50 m CP-Sil 88 capillary column purchased from Chrompack, using the appropriate sensitivity factors for a FID detector.

RESULTS AND DISCUSSION Synthesis of iron tetranitroDhthalocvanine-Y The procedure for the synthesis of iron-tetranitrophthalocyanine-Y (FeTNPcY) is a modified one of that described in an earlier publication [9]. NaY (10 g) is exchanged with sodium (1 1 of a 0.5 N NaCl solution) to remove cationic impurities and dried at 473 K for 24 h. FeTNPcY is prepared by mixing the dry zeolite under nitrogen atmosphere with 4.42 g 4NDCB and 0.84 g ferrocene. This corresponds to a loading of 32 4NDCB and 6 ferrocene molecules per unit cell. This mixture is autoclaved under nitrogen atmosphere at 453 K for 24 h. Half of this solid is soxhlet extracted with acetone, DMF and again acetone, while the other half is treated only with acetone. Acetone is used to remove low boiling products and reactants. The more drastic treatment with DMF removes all compounds including phthalocyanines at the outer surface of the zeolite crystals as well as products from the intracrystallinezeolite voids provided they are able to diffuse through the 12-MRwindows of this zeolite. The final extraction with acetone removes DMF from the zeolite. Finally, the catalyst is dried at 333 K.

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Iron-phthalocyanine-Yzeolite (FePcY) is synthesized via a ligand exchange reaction by mixing the dry zeolite u?ider nitrogen atmosphere with 3.15 g 1,2-dicyanobenzene(DCB) and 0.84 g ferrocene. This corresponds to a loading of 32 DCB and 6 ferrocene molecules for each unit cell. The synthesis and purification procedure is identical to that used for the nitrosubstituted form except for the short synthesis time used (4 h) and the extensive extraction procedure applied. The longer synthesis time used for the synthesis of FeTNPcY is required to obtain a maximal yield of supported complexes. After application of the extensive extraction procedure to FeTNPcY all phthalocyanines are removed from the zeolite, indicating that tetranitrophthalocyanines are not formed in the supercages of zeolite Y.Minor variations in the synthesis procedure always failed to encage nitro-substituted phthalocyanines. On the contrary, it is impossible to remove all phthalocyanines from FePcY by the same extraction procedure. Therefore, all phthalocyanines of the FePcY catalyst after completion of the extraction procedure are located in the supercages of zeolite Y. As the tetranitrophthalocyanines are at the outer surface, the extraction is stopped after the first acetone treatment when the samples are subjected to spectroscopic and catalytic characterization. Clearly, substitution of dicyanobenzene with bulky groups makes it impossible to synthesize such phthalocyanines in the supercages of zeolite Y. Consequently, it is doubtful that the t-butyl-substituted phthalocyanines made on a Y zeolite [I31 are really encaged in the supercages of zeolite Y. SDectroscoDic characterization of FeTNPcY Table 1: The loading of zeolite Y with phthalocyanines. material FePcY FeTNPcYb FeTNPcYC

Pc/super cagea 0.24

0 0

Pc concentration (mg/g)a 87.06 6.82 0

a, from the intensity of the 770 nm and 830 nm bands b, FeTNPcY which is extracted only with acetone C, FeTNPcY which is extracted with acetone, DMF and again acetone The number of Pc molecules associated with the zeolite is determined by dissolving 0.1 g of the zeozyme in 100 ml concentrated sulfuric acid and measuring the absorption by VisNIR-spectroscopy, after proper dilution and a contact time of 2 h with sulfuric acid. The concentration of phthalocyanines is calculated from a calibration curve. The loadings are shown in Table 1. Obviously, the loading with nitrophthalocyanines is much lower than that with unsubstituted phthalocyanines. Much higher loadings of unsubstituted phthalocyanines are formed as the interior surface of the zeolite is considerably higher than the outer surface. As the outer zeolitic surface measured is 40 m2 g-1, the nitrophthalocyanines (cross-section of 2.25 nm2) can be present as a monolayer on the external surface of the zeolite crystals. In

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R. F. Parton, C. P. Bezoukhanova, J. Grobet, P. J . Grobet and P. A. Jacobs

the case FeTNPcY which is extracted with the complete extraction procedure, no phthalocyanines are left on the zeolite. Phthalocyanines show bands in the visible (Q) (between 650 and 900 nm in sulfuric acid) and near-W spectrum (B or Soret) (between 400 and 500 nm in sulfuric acid), as porphyrines do. However, the Q bands of the former complexes are far more intense [14]. When the complex loses its metal ion, the Q band splits in a Qx and a Q band, resulting Y from a decrease of symmetry from D4h to D2h Both the Q, and Q,, band have an overtone vibration, denoted as Q,(l,O) and Qy(l,O), respectively. Principally the Q and B bands are (n,n*) transitions. The Soret band is broadened due to underlying (n,n*) transitions. The Vis-NIR spectra (Fig.1) show that FePcY as well as FeTNPcY exist predominantly as phthalocyanines devoid of iron, From calibration mixtures of iron chelated and metal-free phthalocyanines it is possible to estimate the concentration of iron phthalocyanines. Results indicate that 25% of the ligands contain iron. For FePcY this estimation is confirmed by FTIR-analysis as major bands at 1332 and 1287 cm-1 are splitted in the metalfree form. It was not possible to obtain this verification for the nitro-substituted phthalocyanines as the loading was too low to get acceptable resolution of the different bands. Another important feature of the spectra is the shift of the Q-bands to higher energy in case of nitro-substituted phthalocyanines. Normally the opposite is obtained [15]. However, as protonation causes a red shift and nitro-substitution by its electron withdrawing action produces a lower degree of protonation the occurence of a smaller red shift can be rationalized [16].

I

1

I

I

I

500

600

700

800

900

Wave length (nm) Fig. 1. The Vis and NIR absorption spectra in concentrated sulfuric acid of phthalocyanines (A) and nitrophthalocyanines(B), synthesized in presence of zeolite Y.

Substiruted Fe-Phthalocyanines on Zeolite Y

375

The 13C-NMR spectra of FeTNPcY and FePcY are shown in Fig. 2. Since the loading of FeTNPcY is lower (0.014 TNPc per supercage) than that of FePcY (0.24 Pc per supercage), the signal to noise ratio is at least 10 times lower on FeTNPcY than on FePcY. These results are consistent with the Vis-NIR-analysis, from which a twenty fold decrease in concentration can be derived for the nitro-substituted catalyst, and with IR spectroscopy, as the spectra do not show TNPc. Both spectra are similar and characteristic for metal-free phthalocyanines [17-181. The resonance lines at about 145 ppm and in the range between 134 and 110 ppm are representative of C1, C2, C3 and C4 of metal-free phthalocyanines. The line at 70 ppm for FeTNPcY can be attributed to residual ferrocene.

B

A

Fig. 2.

Solid state CP/MAS 13C-NMR spectrum of (A) FeTNPcY and (B) FePcY

Oynenation activitv of FeTNPcY Fig. 3 shows the catalytic activity of FeTNPcY and FePcY in the oxidation of cyclohexane as a function of time. The phthalocyanines are not soluble in acetone and

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R. F. Parton, C. P. Bezoukhanova, J. Grobet, P. J . Grobet and P. A. Jacobs

consequently any leaching of adsorbed TNPc is not possible. Although the absolute number of FePc in the reactor is smaller for samples with the substituted Pc ligand, the overall conversion is higher than for the unsubstituted. From the conversion curves it seems that FePcY suffers from deactivation which is absent or less pronounced in case of FeTNPcY. Recently we [19] suggested that deactivation is caused by sorption of polar compounds by the hydrophilic zeolite. As in case of FeTNPcY the active sites are located exclusively at the outer surface it is evident that such FeTNPc complexes will suffer less from adsorption of polar products in the interior void volume of the zeolite. Fig. 4 shows the turn-over number (TON) after 30 h reaction with FeTNPcY and FePcY based on the number of metallated phthalocyanines present in the catalysts. It is considered that two turn-overs are required to form one molecule of cyclohexanone out of cyclohexane. The selectivity for cyclohexanone is about 80 % after 30 h on both catalysts.

1

A

FePcY

FeTNPcY

1

25 20 15 10 5 0 0

500

1000

1500

2000

Time ( m i d Conversion of cyclohexane to cyclohexanone and cyclohexanol as a function of Fig. 3. time on FeTNPcY and FePcY. Fig. 4 undoubtedly shows that the TON obtained with FeTNPcY are about ten times higher than those obtained on a FePcY catalyst. Most likely this is attributed to the electron withdrawing effect of the nitro-substituent which enhances the electrophilic character of the active oxygen species and consequently its reactivity. The improvement in TON upon nitrosubstitution is comparable with that reported in literature for porphyrines where also a tenfold increase is found [12]. The activity enhancement for FeTNPcY can also be explained

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377

by the fact that all complexes are accessible because they are located on the outer surface. This would imply that in case of FePcY, intragranular diffusion determines the reaction rate.

TON in the oxidation of cyclohexane after 30 h reaction on FeTNPcY and Fi%cY, 4* based on the number of metallated phthalocyanines present in the catalysts and Fe taking into account that two turn-overs are required to obtain cyclohexanone. A

FoPoY

A

rloohol

0

FoPoY kotono

0

FoTNPoY

FoTNPoY

rloohol

kotono

1 0

10

20

30

Conversion (%) Product distribution in the oxidation of cyclohexane to cyclohexanone and cyclohexanol as a function of conversion on FeTNPcY and FePcY.

Fig. 5.

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R. F. Parton, C. P. Bezoukhanova, J . Grobet, P. J. Grobet and P. A. Jacobs

Fig. 5 shows the product distribution in the oxidation of cyclohexane as a function of conversion on FeTNPcY and FePcY. At low conversions FeTNPcY has a higher selectivity for cyclohexanol than FePcY. Indeed cyclohexanol once formed is strongly adsorbed in the interior volume of the zeolite due to its hydrophilic nature. In case of FePcY the active entities are also present in the interior part of the zeolite crystals. Therefore, consecutive oxidation to the ketone is facilitated. At higher conversions the selectivity is comparable.

CONCLUSION Irontetranitrophthalocyanines can be synthesized on Y zeolites. The purification procedure followed clearly shows that such nitro-substituted ligands are located exclusively at the outer surface of the zeolite, which is not the case for unsubstituted phthalocyanines. In agreement with the electrophilic nature of the active oxygen species, nitro-substitution enhances significantly the catalytic activity and stability of the catalyst in the catalytic oxidation of cyclohexane. Changes in selectivity are correlated with the location of the complexes.

ACKNOWLEDGEMENTS RP and PG acknowledge the Flemish N.F.W.O. for a research positions as Research Assistant and Senior Research Associate. CPB from the University of Sofia (Bulgaria) is grateful to KU.Leuven for a grant as Senior Research Fellow. The authors acknowledge sponsoring from the Belgian ministery of science in the frame of a UIAP-PAI project on Supramolecular Chemistry and Catalysis. The paper doesnot contain official viewpoints and its content is the scientific responsibility of the authors. REFERENCES 1. R.F. Parton, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs, Stud. Surf. Sci. Catal., 46 (1989) 163. 2. H. van Bekkum and H.W.Kouwenhoven,Recl. Trav. Chim. Pays-Bas, 108 (1989) 283. 3. W.F. Holderich, Stud. Surf. Sci. Catal., 49 (1989) 69. 4. N. Herron, G.D. Stuc and C.A. Tolman, J. Chem. SOC.Chem. Commun., (1986) 1521. 5. G. Meyer, D. Wohrle, . Mohl and G. Schulz-Ekloff,Zeolites, 4 (1984) 30. 6. B.V. Romanovsky, Proceed. 8th Int. Congr. Catal., Verlag Chemie, Weinheim, 4 (1984)

%

657.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19.

N. Herron, J. Coord. Chem., 19 (1988) 25.

R.F. Parton, D. De Vos and P.A. Jacobs, NATO AS1 Ser. (352, (1992) 531. R.F. Parton, L U tterhoeven and P.A. Jacobs, Stud. Surf. Sci. Catal., 59 (1991) 395. D. Mansuy, J.F. artoli, 0. Brigaud and P. Battioni, J. Chem. SOC.Chem. Commun., (1991) 440. T.K. Miyamoto, E. Takahashi and S. Tagaki, Chem. Letters, (1986) 1275. J.T. Groves and R. Quinn, J. Am. Chem. Soc.,107 (1985) 5790. T. Kimura, A. Fukuoka and M. Ichikawa, Shokubai, 31 (1989) 357. L. Edwards and M. Gouterman, J. Mol. Spectrosc., 32 (1970) 292. J. Metz, 0. Schneider and M. Hanack, Inorg. Chem., 23 (1984) 1065. V.M. Negrimovskii, V.M. Derkacheva, O.L. Kaliya and E.A. Luk’yanets, Z. Obshchei Khimii, 61 (1991 460. P.J. Grobet, H. eerts, R.Parton and P.A. Jacobs, in preparation. T. Enokida, R.Hirohashi and N. Morohashi, Bull. Chem. SOC.Japan, 64 1991) 279. R. Parton, C.P. Bezoukhanova, G. Peere and P.A. Jacobs, to be publishe .

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6

Zeolite Catalyzed Aromatic Acylation and Related Reactions

H. van Bekkum', A.J. Hoefnagel', M.A. van Koten', E.A. Gunnewegh', A.H.G. Vogt2 and H.W. Kouwenhoven2

' Delft

University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136, 2628 BL Delft, The Netherlands Technical Chemical Laboratory, ETH-Zentrum, UniversitatsstraRe 6, 8092 Zurich, Switzerland

ABSTRACT The development of catalytic procedures in aromatic acylation is a priority because the current industrial methods apply stoichiometric or excess amounts of metal chlorides or mineral acids as "catalysts". The paper reviews zeolite catalysis in this field and subsequently focusses on the acylation of phenols. Here, two reaction steps are involved: esterification and the so-called Fries rearrangement; both reactions are catalyzed by H-zeolites. The Fries rearrangement has been studied over various zeolites using phenyl acetate as a standard reactant. For the combined reaction especially the system resorcinol/benzoic acid has been examined with several catalysts. Zeolite H-Beta was found to be the best catalyst. When more bulky reactants are involved the new MCM-41 zeolitic material is a promising catalyst.

INTRODUCTION In many aromatic substitution reactions non-regenerable metal chlorides are used as the catalyst, sometimes in more than stoichiometric amounts. Zeolites which are tunable in many ways and regenerable would seem excellent candidates to take over the catalytic job (1). Aromatic acylation is of importance in various areas of the fine chemicals industry. For instance many synthetic fragrances of the musk type contain an acetyl group. Also the syntheses of several major pharmaceuticals (e.g. Ibuprofen, (S)-Naproxen) involve an aromatic acylation step. Present-day industrial practice generally involves the use of acid 319

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H. van Bekkum, A.J. Hoefnagel, M.A. van Koten, E.A. Gunnewegh. A.H.G. Vogt and H.W. Kouwenhoven

chlorides together with a stoichiometric amount of metal chloride (AlCl, FeC13, Tic&) which means a high amount of inorganic by-product ( 2 4 Cl per mol ketone product). The high catalyst requirement is related to the fact that the ketone coordinates more strongly to the catalyst than the acid chloride. Thus the association constant of FeCl, with acetophenone was found (2) to be 100 times larger than that of acetyl chloride with FeCl,. Some years ago our group succeeded in preparing a single crystal of the adduct of

4-t-butyl-2,6-dimethylacetophenoneand FeC13. The X-ray structure determination (3) showed the Fe to be tetrahedrally coordinated by 3 CI and the carbonyl oxygen. Other conventional methods include the use of anhydrides with

2 2

equivalents of

metal chloride and the use of benzotrichloride (towards benzophenones) with a catalytic amount of metal chloride. Both methods suffer from a substantial salt production. A salt-poor or salt-free synthesis would be most attractive. Approaches include the use of the (aromatic) acid chloride with solid super acids (4) or with silica-supported heteropoly acid ( 5 ) as the catalyst. Acid anhydrides have been applied in combination with polyphosphoric acid (2), fluorinated ion exchange resins, solid superacids, such as sulfated zirconia (6). and hydrogen fluoride (7). The latter system is applied industrially (Hoechst). Zeolite catalysts show promise in this respect (1) and options include: the use

of the acid chloride, the use of the anhydride, and the use of the free acid as the reactant. Application of the acid chloride in combination with a regenerable zeolitic catalyst means a substantial (75%) reduction of chloride waste. Reports include the acylation of anisole with phenylacetyl and phenylpropanoyl chloride over H-Y zeolite (8). the acylation of toluene with lower aliphatic acid chlorides over La-Y(9) and the acylation of thiophene over H-Y (10). Selectivity was highest in the latter case. Anhydrides, in particular acetic anhydride, have been successfully used by Holderich et al. (11) to acylate heteroaromatics. High selectivities were obtained over modified zeolites of the MFI-type. Several authors have reported (12) the conversion of benzene and phthalic anhydride towards anthraquinone using zeolites of the Faujasite-type. The recently reported mild homogeneous acetylation using lanthanide triflates (13) makes extrapolation towards Ln-zeolites attractive. Application of the free carboxylic acids as the acylating agent is the most attractive option. Geneste et a]. (14, 15) applied lanthanide-exchanged Y-zeolites in the acylation of toluene with a series of alkanoic acids. These authors (16) also showed cation-exchanged montmorillonites to catalyze the direct acylation. Recently zeolite H-Beta was found by French workers (17) as well as by our groups to catalyze the direct acylation of aromatics

Aromatic Acylation and Related Reactions

381

with benzoic acids. In the present contribution aromatic acylation will be discussed first in general terms.

Then the paper focusses on work carried out at Delft and Zurich. Especially zeolitecatalyzed acylation of phenols has been studied. Some general considerations Acylbenzenes may be approached not only by acylation of the aromatic nucleus but also by &-oxidation of hydroxyalkyl- and alkylbenzenes. The potential of redox molecular sieves has recently been demonstrated (18). It may be recalled however, that selective aromatic hydroxylation is not easily achieved (19). Also Friedel Crafts aromatic alkylation has its limitations because of isomerization of the intermediate carbenium ion. This leaves acylation as by far the most important route to acylaromatics. It may be noted that in the above reactions sometimes (activation of anhydrides or aldehydes) both Bronsted and Lewis catalysis can be applied whereas in other cases one type of catalysis seems to be preferred. Thus, a multivalent cation like a lanthanide ion will not - at moderate temperatures - activate a carboxylic acid towards an acylium ion but rather will form a carboxylate anion. The equilibria for a carboxylic acid in a H-zeolite lead to active species for acylation. An interesting point is whether the intermediate acylium ions are to be regarded as

(solvated) counterions or are parked on the zeolite walls by true bonding, on the analogy of alkyl groups (20). Side reactions for aliphatic acyl precursors involve the reversed Koch reaction, the formation of ketenes, ketonization and (cyclo) oligomerization (21). It may be noted that benzoic acids will not enter most of these reactions and consequently can stand higher reaction temperatures in acylation. When applying acid chlorides Lewis acid catalysis seems more appropriate than proton-catalysis (cf. (9)). In homogeneous medium not only the conventional metal chlorides but also metals equipped with oxygen-containing ligands (B, Al, Ga triflates) are applicable (22) under mild conditions. We are presently studying Zn-exchanged zeolites as catalysts in acylation with acid chlorides. Toluene can be p-acylated with several aliphatics over ZnY. With anisole, however, essentially quantitative ester formation was observed. The mild demethylation reaction, which does not take place in H-zeolites (8), shows resemblance to the ZnC1,catalyzed conversion of benzoyl chloride and tetrahydrofuran towards 4-chlorobutyl

382

H.van Bekkum, A.J. Hoefnagel, M.A. van Koten. E.A. Gunnewegh, A.H.G. Vogt and H.W. Kouwenhoven

benzoate. The Fries rearrangement of phenyl acetate An interesting type of reaction in this field is the acylation of phenols. Here, first the

carboxylic acid is converted into the phenyl ester followed by migration of the acyl group (the Fries-rearrangement). Results obtained in the Fries-rearrangement of phenyl acetate in batch reactions at

453 K are collected in Table 1. The product consisted of the expected components, the distribution depended strongly on the zeolite type, the p/o-HAP ratio decreased with increasing pore diameter. After a reaction time of 24 h, Z S MJ showed most conversion and relatively the lowest formation of phenol and by-products. NU-10 had a very low activity, the low p/o-HPA ratio indicates that the reaction occurred mostly on the outer surface of the NU-10 crystals. Table 1. Liquid phase Fries rearrangement of phenyl acetate with different types of zeolites in batch. Product composition (% w) after 24 h at 453 K Type

Si/AI

PhOAc

Phenol

o-HAP

p-HAP

p-AAP

p/o

SAPO-5 USY Beta ZSM-12 ZSM-5 NU-10

0.07 4.7 25 50 20 37

73.9 69.5 42.0 78.3 31.3 92.4

7.1 9.9 13.7 8.2 11.9 5.9

7.4 7.5 10.7 3.7 17.5 0.4

3.6 5.7 10.7 3.7 17.5 0.4

6.8 7.2 5.6 5.9 13.3 1.1

0.5 0.7 0.9 1.1 1.5 0.5

OH

0-HAP

0

OH

p-HAP

0

)-'

OH

p-AAP

The outer surface of the zeolitic catalysts can participate in the reaction and influence product composition. Another effect of outer surface activity with reactive feeds


383

is the gradual formation of polymeric species on the outer surface of a crystal, hindering access to its inner surface. The outer surface of ZSM-5, ZSM-12 and zeolite Beta was deactivated by reaction of triphenylchlorosilane followed by calcination. Table 2 shows the effects of this treatment on the reaction product and the zeolites. The N, adsorption data showed that for ZSM-5 and ZSM-12 deactivation had very little effect on the zeolite texture, an appreciable loss in BET surface area and pore volume occurred however upon deactivation of zeolite Beta. Activity, p/o-HAP ratio and selectivity of the surface deactivated ZSM-5 were somewhat higher than those of the base material. We conclude that the rearrangement of phenyl acetate into o-HAP is limited by the constrained environment in the MFI channels and that polymeric species formed during reaction gradually cover the outer surface of the non-modified ZSM-5 crystals. Also deactivated

ZSM-12 had an appreciably improved performance being more active and selective and showing a higher p/o-HAP ratio than the parent material. Passivated Beta showed an overall catalyst deactivation and no appreciable change in product distribution. We assume that partial pore mouth blocking occurs during passivation of zeolite Beta, in agreement with the loss in micropore volume. Table 2. Influence of the surface passivation in ZSM-5, ZSM-12 and Beta catalyzed reaction of phenyl acetate (T = 453 K, t = 24 h). Zeolite

ZSM-5 ZSM-5 ZSM-12 ZSM-12b Beta Betab a

Conv.

69.4 81.2 23.2 37.2 57.9 33.0 SBET= specific surface Passivated materials.

Sel.

P/O

(m /g)

Y

(cm /g)

398 1/5 2.0 398 1.1 387 1.6 378 0.9 665 0.7 545 (& I ) = specific micropore ' "P 90.1 91.1 70.2 94.4 49.5 50.9

0.121 0.111 0.122 0.113 0.204 0.169 volume.

Coke (wt

14.8 12.1 6.2 5.8 14.5 14.2

The effect of Si/Al ratio on the performance of MFI type catalysts was investigated using a series of samples with Si/AI ranging from 20 to 414. The number of acid sites had

a strong influence on catalyst activity, but not so much on the selectivity of the reaction. The rate of formation of o-HAP

+ p-HAP + p-AAP in moles/min per mol Al was 0.04

and more or less independent of the Si/AI ratio of the catalysts. The effect of solvent addition on the conversion of phenyl acetate over zeolite Beta is shown in Table 3. Conversion, p-selectivity and phenol formation increase with solvent

384

H. van Bekkum, A.J. Hoefnagel. M.A. van Koten, E.A. Gunnewegh, A.H.G. Vogt and H.W. Kouwenhoven

polarity, indicating ionic reactions to become favoured. In polar solvents enhanced formation of side products occurs due to unselective reactions of the acylium ion. Table 3. Effect of added solvent. Conditions: 453 K, 450 rpm, 1.5 g zeolite Beta, 7.5 g PhOAc, 10 ml solvent, 1 h. Solvent n-Decane Nitrobenzene Sulfolane PhOAc

€

Sel. wt %

k *lo3 mol/h.g

PI0

2.0 34.8 43.3 5.2

82.5 85.6 79.3 65.6

9.8 14.7 30.2 14.7

0.4 0.8 4.9 1.0

5, passivated ZSM-5 and zeolite Beta were tested in continuous iquid p lase experiments at 453 K and the former two also at 523 K at 5 bar. Results show that at 453 K the passivated Z S M J material had a very low activity, in fact much lower than expected on the basis of the batch experiments. ZSM-5 and zeolite Beta were initially active, stability was, however, low. At 523 K activity and stability of ZSM-5 were much better than at 473 K and the passivated material had a surprisingly good performance (see

Fig. 1). The results indicate that in the continuous liquid phase experiments most of the activity resides on the outer surface of the ZSM-5 crystals and that exchange between the liquid phase in the zeolite channels with the bulk liquid is a relatively slow process. Gas phase conversion of phenyl acetate at 693, 573 and 523 K confirmed data obtained by Perot et al. (23) at 673 K. The reaction was very unstable and the main product was phenol.

Continuous liquid phase reaction of PhOAc and 5

over

HZSM-5 (inert.) at 250 ' C

atm. WHSV = 0.86 h-'.

Figure 1.


385

The use of free carboxylic acids in the acylation of phenols (the direct Fries reaction) A "direct Fries reaction" in which the phenyl ester is made "in situ" would be attractive.

Starting compounds then are the phenol and the carboxylic acid. We have selected the reaction between resorcinol and benzoic acid as a model reaction for this approach. The industrial preparation of 2,4-dihydroxybenzophenone,which is an intermediate in the preparation of 4-0-octyl-2-hydroxybenzophenone- applied as UV absorbent

-

is

presently accomplished in good yield by the metal chloride catalyzed reaction of resorcinol with benzotrichloride with the co-production of three moles of hydrochloric acid and spent metal chloride, which forms together a highly corrosive mixture. Reactions were carried out batch-wise while monitoring the composition of the reaction mixture by GC analysis. Applied solvents were chlorobenzene, p-chlorotoluene, n-decane and n-butylbenzene, allowing a temperature range of 403-455 K at atm. pressure. The reaction of resorcinol and benzoic acid is formulated in the Scheme below.

Using chlorobenzene as the solvent various Brclnsted acid catalysts were tested with removal of the water formed in the first step of the reaction. The activity order found (24) is: sulfonic acid resins > zeolite H-Beta > polyphosphoric acid, heteropoly acid (H,SiW,,O,)

> zeolite H-ZSMJ, Filtrol 105 (clay).

Ion-exchange resins, especially Amberlyst-15, are the most active catalysts, however the formation of the resorcinol dibenzoate ( 5 ) and the formation of a coloured side product decrease the yield of the desired compound 4. The side product is assumed to be 3,6-

dioxy-9-phenyl-xanthydrolresulting from the reaction of two moles of resorcinol with one mole of benzoic acid. The side-reaction and the formation of resorcinol dibenzoate with zeolite H-Beta as the

386

H. van Ekkkum. A.J. Hoefnagel, M.A. van Koten, E.A. Gunnewegh, A.H.G. Vogt and H.W. Kouwenhoven

catalyst are absent or very low and this catalyst was chosen for a more detailed investigation. The side-product is too bulky to be formed in the pores of Beta. In order to shorten the reaction time 4-chlorotoluene (b.p. 435 K) was applied as the solvent. Starting from a 1:l molar mixture of resorcinol and benzoic acid mixture is obtained

(15 h) in which 70% of 2,4-dihydroxybenzophenone (4) is present together with 20% of resorcinol monobenzoate (3), 5% of benzoic acid, 2% of resorcinol and 3% of resorcinol dibenzoate. Upon cooling of the reaction mixture compound 4 crystallizes and can be collected. Because of the equilibrium nature of the reaction the filtrate can be recycled leading to a 88% yield of 4. Practically the same equilibrium mixture can be obtained by refluxing of a solution of 2,4-dihydroxybenzophenone in 4-chlorotoluene with zeolite H-Beta. This proves the equilibrium nature of the zeolite Beta-catalyzed Fries reaction. When subjecting various substituted phenols to the direct Fries reaction with benzoic acid the rate of benzophenone formation was as follows: 3-OH > 3-Me > 4-Me > 3-C1, H > 3-N02, 2-OH, 4-OH. Ester formation was always faster than Fries rearrangement.

As predicted by the Hammett relation a 3-OH substituent will slightly retard the esterification and will strongly accelerate the substitution at the ortho position (which is the para position with respect to the substituent); a 3-N02 group in the phenol will retard both the esterification and the rearrangement, moderately and strongly, respectively. As an example of the Fries reaction of a 2,3-disubstituted phenol, the reaction of 2-Me-

resorcinol with 2-Me-benzoic acid, catalyzed by zeolite H-Beta, was executed in the solvents p-C1-toluene and n-decane. Conversion into > 90% of the substituted benzophenone can be accomplished in both solvents within 2 h. The overall kinetic electronic effects for a series of substituted benzoic acids are well illustrated by comparing the percentages of substituted benzophenone formed over H-Beta after 3 hours of reaction with resorcinol at 435 K in 4-chlorotoluene: X = H, 36%; 4-Me, 40%; 4-OMe 55%; 4-C1 2%; 2-Me 91%. This sequence is in harmony with the expected stabilization or de-stabilization of the arylacylium ion. A methyl group in ortho position accelerates both the esterification and the rear-

rangement while leading to a high equilibrium conversion. A 2,6-dimethyl configuration also provides fast esterification but the subsequent rearrangement occurs just slowly on H-Beta.


387

In the case of 2,6-diMe-benzoic acid reacting with resorcinol the dimensions of 2,4-

dihydroxy-2’,6’-diMe-benzophenoneare 7.5 x 8.5 x 9 A whereas those of the mono-ester amount to 5 x 8.5 x 13 A. In view of the pore dimensions of zeolite Beta (25) the monoester might be formed in an intersection but desorption seems difficult if not impossible. Any benzophenone formed seems also too bulky to escape from the intersections. What remains then is the backward reaction towards resorcinol and 2,6-diMe-benzoic acid. The observed catalytic activity of zeolite Beta in the reaction between resorcinol and 2,6-diMe-benzoic acid is assumed to stem mainly from the relatively large outer surface of the zeolite. This was confirmed by performing an experiment with a H-Beta catalyst of which the outer surface was dealurninated. Here just ester formation was observed and the Fries rearrangement did not take place at all. The esterification might occur at lowacidic sites (silanol groups), also thermal conversion might contribute. MCM-41 as a catalyst in aromatic acvlation Recently a new family of superlarge pore zeolitic materials was discovered (26). We have prepared one of its members, MCM-41, and have tested this catalyst in the direct Fries reaction. MCM-41 was synthesized using cetyltrimethylammonium hydroxide as the template. Recently a mechanism of formation was advanced (27) in which first a layered surfactant-silicate structure is formed followed by transition towards a hexagonal mesophase. We were able to synthesize MCM-41 with Si/AI ratios from approximately 10 to

m.

When using synthesis mixtures with Si/AI below 5 no MCM-41 material was

detected. Charge compensation may play a role here. In this connection it was found that

a tetraalkylammonium compound (R

=

Me or Et) is an essential synthesis mixture

ingredient when Al is present. All-silica MCM-41 could be synthesized without problems in the absence of added R,N+. After calcination the MCM-41 material was examined by various techniques. Nitrogen adsorption and thermoporometry showed the material to have an average pore diameter

388

H. van Bekkum, A.J. Hoefnagel, M.A. van Koten, E.A. Gunnewegh, A.H.G. Vogt and H.W. Kouwenhoven

of approximately 40 A. A large BET surface area is found (950-1000 m2/g).

H-MCM-41, obtained by ion exchange of calcined material with 1 M aqueous NH,NO,, proved to be an interesting catalyst in the direct Fries acylation. When comparing

H-MCM-41 with H-Beta the latter catalyst is somewhat more active in the standard resorcinol/benzoic acid reaction. However, a major advantage of MCM-41 is its high accessibility which allows e.g. the conversion of resorcinol and 2,6-dimethylbenzoic acid to the corresponding benzophenone in > 80% yield. Zeolite Beta behaves poorly in this reaction. Some larger reactant (and product) systems were tested. 1-Naphthol can be converted selectively and in high yield to the 2-substituted naphthophenone when reacted with 2,6-dimethylbenzoic acid. Esterification is a fast first step here. The MCM-41 catalyst was recycled (up to four times) without significant loss of activity, though some loss of crystallinity was observed.

Acrvlic acid in the direct Fries reaction As an example of a bifunctional reactant acrylic acid was subjected to reaction with

resorcinol and 1-naphthol in the presence of zeolite catalysts and of Amberlyst-15. First esterification takes place, then a choice exists between o-alkylation and o-acylation (Fries). In the examples studied always o-alkylation took place leading to cyclic lactones, which may, inter alia, be converted by dehydrogenation into coumarins.


389

Acrylic acid as acylating agent. OH

OH

OH

CONCLUSIONS New zeolite-based technology in aromatic acylation is forthcoming:

-

reduced waste when applying acid chlorides or anhydrides, target reactants remain the carboxylic acids; still much work to be done, continuous liquid phase method beneficial in Fries rearrangement, two reaction steps successfully combined in the direct Fries reaction,

MCM-41 is a promising catalyst for conversion of bulky reactants.

References 1 W.F. Holderich and H. van Bekkurn, Stud. Surf. Sci. Catal. 58 (1991) 664. 2 J.J. Scheele, PhD thesis, Delft University of Technology, 1991. 3 H. van Koningsveld, J.J. Scheele and J.C. Jansen, Acta Cryst. C43 (1987) 294. 4 K. Tanabe, M. Misono and Y. Ono, “New Solid Acids and Basis; Their Catalytic Properties”, Kodansha, 1989. 5 Y. Izumi, N. Natsume, H. Takamine, I. Tarnaoki and K. Urabe, Bull. Chem. SOC. Jpn. 62 (1989) 2159. 6 S. Goto, M. Goto and Y. Kimura, React. Kin. Catal. Lett. 41 (1990) 27. 7 J.H. Simons, D.I. Randell and S. Archer, J. Am. Chem. Soc. 61 (1939) 1795. 8 A. Corma, M.J. Climent, H. Garcia and J. Primo, Appl. Catal. 49 (1989) 109. 9 D.E. Akporiage, K. Daasvatn, J. Solberg and M. Stocker, Preprints 3rd Int. Symp. on Heterogeneous Catalysis and Fine Chemicals, Poitiers, 1993, p. 179. 10 A. Finiels, A. Calmettes, P. Geneste and P. Moreau, Preprints 3rd Int. Symp. on Heterogeneous Catalysis and Fine Chemicals, Poitiers, 1993, p. 327. 11 W.F. Hblderich, H. Lermer and M. Schwarzrnann, DE 3.618.964, to BASF A.G. 12 E.g. Jap. Pat. 56.142.233 (1981) to Mitsui Toatsu Chemicals Inc. 13 A. Kawada, S. Mitamura and S. Kobayashi, J. Chern. Soc., Chem. Commun. 1993, 1157. 14 B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch, J. Org. Chem. 51 (1986) 2128. 15 C. Gauthier, B. Chiche, A. Finiels and P. Geneste, J. Mol. Catal. 50 (1989) 219. 16 B. Chiche, A. Finiels, C. Gauthier and P. Geneste, J. Mol. Catal. 42 (1987) 229.

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H. van Bekkum, A.J. Hoefnagel, M.A. van Koten, E.A. Gunnewegh, A.H.G. Vogt and H.W. Kouwenhoven

17 J-P. Bourgogne, C. Aspisi, K. Ou, P. Geneste, R. Durand and S. Mseddi, Fr. Pat. Appl. 90.1 1856 (1992), to PLASTO S.A. 18 R.A. Sheldon, J. Dakka, J.D. Chen and E. Neeleman, Proc. 2nd Conference on Microporous Solids, Nagoya, 1993. 19 M.H.W. Burgers and H. van Bekkum, Preprints 3rd Int. Symp. on Heterogeneous Catalysis and Fine Chemicals, Poitiers, 1993, p. 297. 20 V. Bosacek, J. Phys. Chem., in press. 21 E.g. Y. Servotte, J. Jacobs and P.A. Jacobs, Proc. Int. Symp. Zeol. Catal., Siofok, 1985, Acta Phys. Chem., Szeged, p. 609. 22 G.A. Olah, 0. Farooq, S.M.F. Farnia and J.A. Olah, J. Am. Chem. SOC. 110 (1988) 2560. 23 Y. Pouilloux, J.P. Bodibo, I. Neves, M. Gubelrnann, G. Perot and M. Guisnet, Stud. Surf. Sci. Catal. 59 (1991) p. 513. 24 A.J. Hoefnagel and H. van Bekkum, Appl. Catal. A97 (1993) 87. 25 J.M. Newsam, M.M.J. Treacy, W.T. Koetsier and C.B. de Gruyter, Proc. R. SOC., London, A240 (1988) 375. 26 J.S. Beck et al., J. Am. Chem. SOC. 114 (1992) 10834. 27 G.D. Stucky et a]., Science 261 (1993) 1299.

Diels-Alder Condensation of Methyl and (-)-Menthy1 Acrylates with Cyclopentadiene over Zeolites and Cation Exchanged Clays

F. Figueras*, C. Cathiela**, J.M. Fraile**, J.I. Garcia**, J.A. Mayoral**, L. C. de MCnorval* and E. Pires**

* Laboratoire de Chimie Organique Physique et CinCtique Chimique AppliquCes (URA 418 CNRS), E.N.S.C.M., 8 rue &ole Normale - 34053 Montpellier Cedex 1- France. **Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza- C.S. I.C., 5OOO9 Zaragoza, Espana. ABSTRACT Clays exchanged by different cations have been compared to HY and HBEA zeolites for Diels Alder condensation of methyl and (-)-menthy1 acrylates with cyclopentadiene. Two reactions are in competition: the Diels Alder reaction and diene polymerisation which deactivates the a!dysts and consumes the reactant. Polymerisation is catalysed either by protons or reducible cations. With methyl acrylate polymerisation is minimised, and activity is related to the number of acid sites. Selectivity is comparable for zeolites or clay catalysts with the two acrylates, then concentration effects or confinement in micropores play a negligible role. INTRODUCTION The Diels Alder reaction is a model reaction of organic chemistry, well known from the theoretical point of view, and a powerful tool for the creation of C-C bonds and total synthesis of natural products [I]. This reaction leads to cyclic products and the challenge is to control the regioselectivity and stereochemistry. Due to the fragility and molecular weight of the adducts the process is camed out in liquid phase, and catalyzed by Lewis acids, which increase both regioselectivity and stereoselectivity [2]. This reaction is very sensitive to electronic factors. It has been shown that the endo transition state is more compact, and the increased rate when water is used as solvent has been attributed to a confinement of the substrates at the interface [3]. Zeolites [4-71 have been reported as good catalysts for Diels-Alder synthesis and concentration or confinement effects, have been claimed [5-71 to explain their catalytic properties. Recent results indeed support this proposal and show that the heats of adsorption of hexane and benzene on aluminophophates [8] increase as the pore size decreases. Similar results have also been reported on faujasites with an increase of the adsorption coefficient with the SilAl ratio [9]. This reaction is therefore a good model to check the influence of confinement and shape selectivity effects in catalysis. Clays are also known to be good catalysts for this class of reactions 39 1

392

F. Figueras, C. Cativiela, J . M. Fraile, J . 1. Garcia, J . A . Mayoral, L. C. de Mcnorval and E. Pires

[lo]. The comparison of macroporous clay catalysts with microporous zeolites should then be informative, such as the comparison of heterogeneous and homogeneous catalysis. In the liquid phase, the control of the conformation of the enoate moiety of the dienophile by complexation with a Lewis acid permits to reach excellent diastereofacial selectivities in asymmetric Diels Alder reactions with c h i d dienophiles [ll-121, and it was then attractive to check the possibility of asymmetric synthesis using solid acids. Recent studies on the Diels-Alder cycloaddition of methyl acrylate (1) and cyclopentadiene (2) (Figure 1) catalyzed by clays have shown that the solvent [13], calcination of the solid [14] and exchanged cation [15] play a decisive role on the yield of the reaction. In the present work, non microporous clays, exchanged with diferent cations have been compared to HY and HBEA zeolites for Diels Alder condensation of methyl and (-)-menthy1 acrylates with cyclopentadiene.

1

COOCH,

2

3x

3n

Figure 1 EXPERIMENTAL METHODS 1) Preparation and characterisation of the catalvsts Zeolite beta (BEA) was synthesized using tetraethylammonium hydroxide (TEAOH) as organic template, following the procedure described by Nicolle [16]. The crystals appeared as spheroids with an average size of 0.6 pm. These samples were calcined at 773K for 8 h under dry air , then converted to the ammonium form by ion exchange in a 1M NH4N% solution at 373 K. The Y zeolite was Linde LZY62 (Si/Al = 2.5), in NH4 form. The sample was calcined in air for 5 hours at 773K or 823K to decompose the ammonium form into the H form. Table 1: Physico-chemical characteristics of the zeolites used in this work. Sample

SUAI

HY

2.5 15

H-BEA

mesoporous vol (m1.g-1) 0.04 0.04

micropore vol. (m1.g-1) 0.36 0.27

Acidity (meq.g-1) 1.5 0.85

The K10 original sample was purchased from Aldrich. This clay, prepared by Sud Chemie from montmorillonite, has suffered first a calcination at 873K, then an acid leaching, and has lost most of the original structure of the clay. I t contains A1 in tetrahedral coordination [17:, shows a BET surface area of 240 m2/g and a microporous volume of 0. lml/g. Cation exchange was performed by gmdually adding the clay to a stirred solution of the cation (Table 2) at room temperature and stirring the suspension for 24 h. After exchange, suspensions

Diels-Alder Condensation over Zeolites and Clays

393

were filtered and washed with deionised water. The resulting solids were dried on a thin bed at 3%K and ground. The solids were equilibrated over saturated salt solutions in order to give reproducible water contents. Calcination was carried out in air (25-30 mllmin) with the following temperature program: 20°C - 10"C/min - 120°C - 1"Clmin - 550°C (10 h) - 1"Clmin - 40°C. After calcination the exchanged clays are believed to contain the cations in their higher oxidation state. 2) Characterisation of the catalysts: these solids can be divided into mesoporous clays (mean pore sizes 1.5-2 nm) and microporous zeolites. BET surface areas of Zn, Fe, Ti, Zr and Ce clays were in the range 220-240 m2/g, with the exception of the Zr-clay (190 m2/g.). The number of acid sites was determined by stepwise thermal desorption of ammonia above 373 K, monitoring the amount of ammonia evolved from the solid by conductometry.

Table 2. Preparation of cation exchanged clays (for 10 g of clay). salt conc. volume starting clay Acidityf(mq/g) NaCl 1M 125 ml K10 K10 0.44 125 ml 1M HCl K10 125 ml 1M R FeCl3 K10 125 ml 1M Zn ZnCl2 K10 0.9 1M 125 ml cu CuC12 240ml K10 V VOSO4 0.25M 267ml K10 0.9 Ce(N03)3 0.25M Ce3+ 167 ml K10 Ce4+ ce(so4)2 0.25 Ma Ca CaCl2 1M 125 ml NaKlO zr ZrOC12 0.1 M 1.4 250 ml K10 in1 1 H20b c€ ccb 0.1 MC 1.25 I Kl@ 0.64 Ti 0.8 Me 126 ml K10 in 2.5 1 H2Ob 1.1 Tic4 aIn H2SO4 1 M. bSolution of the cation gradually added to the clay suspension. CNa2C03 (125 mmol) gradually added to the solution of Cr and solution then refluxed for 36 h. dSuspension refluxed for 1.5 h and then filtered. Tic14 added to HCI (4 ml, 6 M) under Ar atmosphere. The mixture was then diluted by slow addition of deionised water (122 ml). [Number of acid sites adsorbing ammonia at 373K for the solids calcined at 823K. Sample Na H

3) Reaction procedures The methods have been described in detail previously [13-15, 171. The reactions wcre carried out in Schlenk flasks, at 293K. A preweighed amount of catalyst was dried at 393K overnight or calcined. The flask was charged with the catalyst and methylene chloride (15 ml) under argon atmosphere at 293K. Methyl acrylate, or (-) menthyl acrylate prepared according to a procedure described in the literature [18] and freshly distilled cyclopentadiene were added via a syringe. Under these conditions a complete dissolution of the reagents is achieved. The reaction flask was shaken for 24 h and the reaction monitored by gas chromatography. Overall yields and endolexo ratios were determined from the solution obtained by filtration and washing of the catalyst with the reaction solvent. The absolute configurations of the adducts obtained in the case of (-)-menthy1 acrylate were assigned by comparison of the gas chromatograms obtained with those Fxviously reported by Oppolzer et al. [19] for Lewis acid catalysed reactions .

394

F. Figueras, C. Cativiela, J. M. Fraile, J . 1. Garcia, J . A. Mayoral, L. C. de Menorval and E. Pires

Table 3 Conversions and endolexo selectivities (3nl3x) obtained for the Diels-Alder reaction of methyl acrylate (1) and cyclopentadiene (Z), catalysed by H zeolites and cation exchanged K10 dried at 393K or calcined at 823K. 30 min Catalyst conversiona

2h 3d3xa

conversiona

24h 3n/3xa

conversiona

3d3xa

54

3.7

87 26

11.7 6.2

82 57 31b 78 83 87

9.1 9.8 12.2 12.3 9.9 7.8 8.3

99 77 91 74

11.3 6.5 9.4 5.2

In absence of catalyst 2

3.7

Zeolites (activated at 8238)

HY HBEA

11

10.3

65 22

13.7 6.7

Clay samples dried in air at 3938 Zn

R

cu V(IV)

cr

Na Ca H

zr

Ti Ce(111) Ce(IV)

35 26 15 23 28 8 11 21 23 16 45 10

9.4 9.7 12.2 12.3 13.0 8.3 9.8 10.1 11.2 10.2 9.6 7.6

63

44 23 47 46 21 33 3oc 65 31 67 19

9.4 10.0 12.5 12.1 12.2 8.3 10.0 10.6 11.4 8.5 9.6 6.5

44

Clay samples calcined at 823K 14.7 99 14.7 92 61 14.6 15.0 97 75 15.3 47 14.8 15.4 cu 90 15.6 99 59 15.9 89 99 14.5 14.0 52 14.5 V(W 0 99 78 14.7 14.7 42 14.5 9.1 65 Na 32 9.2 7 7.9 14.6 98 79 Ca 14.7 52 14.0 H 92 62 12.2 12.3 27 1!.7 zr 97d 14.3 14.4 81 Ti 95 14.6 14.6 76 Ce(II1) 99 87 14.5 14.4 67 14.1 CeiIVj 54 14.0 82 13.4 99 12.8 aDetermined by gas chromatography. bAt this time an additional 3 eq. of diene were added and the reaction reached, after another 24 h, 92 % of conversion with endolexo = 9.3. C% of conversion after 1.5 h. After this time further conversion was not observed. d% of conversion after 1.5h. Zn

Fi:


395

RESULTS AND DISCUSSION The results obtained for the reaction of methyl acrylate (1) with cyclopentadiene (2) (Figure 1) on zeolites and clays activated in different conditions are reported on Table 3. The non catalysed reaction is slow and non selective, then the endo selectivity must be ascribed to the catalytic reaction. Both HY and HBEA zeolites appear as active and initially selective to the endo adduct. However the selectivity decreases in function of time, mainly in the case of BEA. A polymer fraction, which precipitates in methanol, suggests a parallel polymerisation of the cliene. The competitive polymerisation of cyclopentadiene then eliminates this reagent from the solution, and the final yield is poor. Clays are also good catalysts: except for the Na-clay, the structure of which collapses upon calcination, calcination improves both catalytic activity and endo/exo selectivity. With Cu and H exchanged clays the reaction stops at low conversions. In fact, when an additional amount of diene is added, further progress of the reaction is observed, but the endofexo selectivity decreases. The decrease in selectivity indicates that the polymers formed in the side reaction partially poison the catalyst, so that the percentage of the less selective non-catalysed reaction increases. The poisoning of the clay also accounts for the decrease in endolexo selectivity with increasing conversions observed when Cr, Ti and Ce(1V)-exchanged clays are used as catalysts. Except for Ce(1V)-K10,this behaviour disappears after calcination and this sheds some light on the results obtained with zeolites: since calcination of clays eliminates most of the Bransted acidity [15],it can be concluded that protons greatly favour the polymerisation of the diene. The high activity of BEA for this side reaction simply reflects its higher acidity. In the case of the Ce(1V) clay an additional mechanism for diene polymerisation can be proposed because it has been reported [15] that the formation of radical cations accelerates this side reaction. In fact, EPR spectra of Ce(1V)-clays show, in the presence of cyclopentadiene, a narrow signal at g = 2 . U f 0 . 0 2 which could be characteristicof organic radicals.

cu

' Cr

m H Y Zeolites

.H HBEA

0

0,s 1 1,s Number of acid sites (meq/g)

Figure 2: Influence of acidity on the activity of zeolites and clays. All catalysts have been calcined at 823K under air.

396

F. Figueras, C. Cativiela, J . M. Fraile, J . I. Garcia, J . A. Mayoral, L. C. de Mtnorval and E. Pires

The initial rate estimated by the conversion after 30 min, is proportional to the total number of acid sites, and zeolites show a lower activity per acid site (Figure 2). Since on clays dehydroxylation induces a higher activity, it can be concluded that, like in homogeneous catalysis, Lewis acids are more effective for this reaction. Microporosity and steric factors play a negligible role compared to acidity. It is interesting to compare these results on the addition of methyl-acrylate on cyclopentadiene with those reported by Eklund et al. [7] for the addition of the same dienophile on isoprene which is less reactive . In that case the activity pattern obtained in close experimental conditions, on a series of zeolites with different SUAI ratios was: BEA (14.5) > FAU (2.8)> MOR (10) > FAU (15) > ZSM5 (175) The higher activity of BEA suggests that with less reactive substrates strong acidity is required to activate the dienophile. Small pore zeolites like ZSMS show a low activity, most probably because of diffusional limitations. With small dienophiles,like methyl acrylate good selectivitiesfor Diels-Alder addition can be reached with solid acids and the activity is then related to total acidity. The parallel reaction of polymerisation is promoted either by strong Bronsted sites or by reducible cations. The yield depends then on the relative reactivities of the diene towards the dienophile (Diels Alder addition) 3r itself (polymerisation).Bulky acrylates like (-)-menthy1acrylate are less reactive, but pennit to investigate the possibility of asymmetric synthesis using a chiral substrate.This reaction is described in Figure 3.

4

2

&COOR*& COOR*

6a Figure 3: Reaction of cyclopentadiene on (-)-menthy1acrylate.

6bR

As can be seen (Table 4) both clays and HY zeolite are good catalysts. With this less reactive dienophile (4) larger differences are noticed on the activities, but no correlation can be drawn between activity and acidity of the catalysts. Ti clay is the most active catalyst, leadin: to a high conversion with a 3: 1 diene:dienophile molar relationship, and HY compares well with clays, but shows a clear decrease of selectivity in function of time, characteristic of a fast deactivation attributed here also to the fact that the polymerisation of the diene has not been altered, but Diels Alder addition is now slower. The clays containing reducible cations such as Cu, Fe and Ce(IV) show a particular behaviour: with Fe or Ce(IV) clays, both selectivities decrease with increasing conversions, which is particularly noticeable with Ce(IV) clay. With Cu clay the reaction stops at


397

low conversions. This behaviour can again be attributed to the polymerisation of cyclopentadiene. Since calcination eliminates practically all Brgnsted acid sites, a cation radical mechanism must be invoked for the extensive diene polymerisation. Ce(IV), Fe and Cu are the most easily reducible cations of those used here and their EPR spectra in the presence of cyclopentadiene show the above-mentioned signal of organic radicals. The low reactivity of cyclopentadiene for Diels-Alder addition lets open the path for polymerisation. Table 4 Results obtained from the Diels-Alder reaction between (-)-menthy1acrylate (4) and cyclopentadiene (2).catalysed by HY and cation exchanged K10 calcined at 823K. Catalyst none

HY znc

l+

v Cr

zr Ti Ce(II1)g Ce(II1) Ce(I V)

cu Ca

2:4

3: 1 6: 1

3: 1 3: 1 5:l d 61 61 3: 1 6 le 61 3: 1 6 le 6: 1 3: 1 3: 1 6: If 3: 1 3:1 3: 1 3: 1 6: le 6: 1 3: 1 6 le 61 3: 1 6 le 6:1 3: 1 6 le 6: 1 3: 1 6: le 61

time (h) 51 0.5 2 24 1 2 24 2 24 1 2.5 24 1 5.5 24 1 2 24 1 2 24 1 2 24 1 5.5 24 1 2.5 24 1 3 24 1 3 24

% conversion*

a, 19

40 64 46 67 99 35 76 25 48 77 30 73 89

35 48 85 52 63 86 13 33 69 30

64 78 5 12 43 11 28 46 37 65 86

516a

3.8 9.6 8.3 5.8 11.2 11.0 11.0 8.0 6.4 11.8 12.0 8.8 13.1 11.8 10.8 11.2 11.0 10.7 10.3 10.1 10.1 13.6 12.8 12.3 12.6 11.6 10.4 7.4 4.7 4.7 12.2 11.6 10.9 13.4 12.4 11.7

% d.e.a*b

6 41 39 27 41 41 41 39 33

44 44 39 52 52 49 36 36 35 38 38 38 47 48 47 45 45 43 36 18 16 41 44 43

50 50 48

aDeterminedby gas chromatography. d.e defined as 100(5b-5a/5b+%).CRef.5. dAfter 2 h, 2 eq. of diene are added. eAfter 1.5 h, 3 eq. of diene are added. fAfter 2 h, 3 eq. of diene are added. gClay dried at 393k.

398

F. Figueras, C. Cathiela, J. M . Fraile, J . I . Garcia, J . A . Mayoral. L. C. de Menorval and E. Pires

The best asymmetric inductions are achieved with Cr and Ca clays calcined at 823K which suggests that enantioselectivity is controlled by the hardness of the acid site, which would also account for the intermediate enantioselectivity observed with protonic zeolites. This point clearly needs further experimentation, now in progress. In conclusion, the reaction scheme is complex, and the yield depends on the competition between Diels Alder and polymerisation of the diene. Polymerisation deactivates the catalyst with a loss of regioselectivity and is catalysed either by protons or reducible cations. Good regioselectivities can be obtained on non reducible Lewis acids. In that case asymmetric synthesis with chiral dienophiles is possible, and the selectivity is probably determined by the hardness of the acid sites. AKNOWLEDGEMENTS: The financial support of CICYT (project no 93 0224) and CNRS is warmly aklowledged. REFERENCES 1 (a) E. J. Corey, N. M. Weinshenker, T. K. Schaaf, W. Huber, J. Am. Chem. SOC.,91 (1%9) 5675. (b) E.J. Corey, H. E. Ensley, J. Am. Chem. Soc.,97 (1975) 6908. (c) R. V. Boeckman Jr., P. C. Naegely, S. D. Arthur, J. Org. Chem., 45 (1980) 754. (d) 0.Ceder, H. G. Nilsson, Acta Chem. Scand. B., 30 (1976) 908. (e) E.E.Smissman, J. T. Suh, M. Oxman, J. Daniels, J. Am. Chem. Soc.,84 (1%2) 1040. 2 P. Yates and P. Eaton, J. Amer.Chem. Soc. 82 (1%0) 4436. G.I. Fray and R. Robinson, J. Amer.Chem. Soc. 83 (1961) 249. 3 R. Breslow, Acc. Chem. Res 24 (1991) 159. 4 a J. Ipaktschi, Z. Naturfomch. 41b (1986) 4%. b. Y .V.S.Narayana Murthy and C.N. Pillai, Synthetic Commun. 21 (1991) 786. 5 R.M. Dessau, J. Chem. Soc. Chem. Commun. (1986) 1167. 6 D. Hochgraeber and H. Lechert, 9th Int. Zeolite Conf. , Montreal, 1992, (R. von Ballmoos, J. B. Higgins and M.M.J. Tracy Eds), Buttterwoth-Heinemann (Boston), 1993, vol2, p 483. 7 L. Eklund, A.K Axelsson, A. Nordahl and R. Carlson, Acta Chem. Scand 47 (1993) 581. 8 S.B. McCullen, P.T. Reischman and D.H. Olson, 9th Int. Zeolite Conf. ,Montreal, 1992, preprint

RP7. 9 A. Corma, F. Llopis and J.B. Monton, Roc. 10th Int. Cong. Catal., Budapest 1992, (L. Guczi, F. Solymosi and P. Tetenyi Eds) Elsevier, Amsterdam, 1993, vol.M, p 1145. 10 (a) P. Laszlo, J. Luchetti, Tetrahedron Lett., 25 (1984) 1567. (b) P. Laszlo, J. Luchetti, Tetrahedron Lett., 25 (1984) 2147. (c) P. Laszlo, J. Luchetti, Tetrahedron Lett., 25 (1984) 4387. (d) P. Laszlo, H. Moison, Chem. Lett. (1989) 1031. (e) J. Cabral, P. Laszlo, Tetrahedron Lett., 30 (1989) 7237. (f) C. Collet, P. Laszlo, Tetrahedron Lett., 32 (1991) 2905. 11 W. Oppolzer, Angew. Chem. Int. Ed. Eng. 23 (1984) 876. and Tetrahedron 48 (1987) 1969. 12 L.A. Paquette, in “Asymmetric Synthesis”, (J.D. Morrison , Ed.), Academic Press, New York, 3 (1984) 455. 13 (a) C. Cativiela, J. M. Fraile, J. I. Garcia, J. A. Mayoral, F. Figueras, J. Mol. Catal., 68 (1991) L31. 14 C. Cativiela, J. M. Fraile, J. I. Garcia, J. A. Mayoral, E. Pires, F. Figueras, L. C. de MCnorval, Tetrahedron, 48 (1992) 6467. 15 C. Cativiela, J. M. Fraile. J. I. Garcia, J. A. Mayoral, F. Figueras, L. C. de MCnorval, P. J. Alonso, J. Catal., 137 ( 1 m ) 394. 16 M.A. Nicolle, Ph. D. Thesis, University of Montpellier, France, 1991. 17 F. Figueras, C. Cativiela, J.M. Fraile, J.I. Garcia, J.A. Mayoral, L. C. de Mtnorval and E. Pires, Appl. Catal., 101 (1993) 253. 18 W. Oppolzer, M. Kurth, D. Reichlin, C. Chapuis, M. Mohnhaupt, F. Moffat, Helv. Chim. Acta 64 (1981) 2802. 19 W. Oppolzer, M. Kurth, D. Reichlin, F. Moffat, Tetrahedron Lett. 22 (1981) 2545.

Controlled Preparation of Neoalkanals and Novel Cyclic Dioxepenes, Depending upon the Use of Shape Selective and Non Shape Selective Catalysts

W.F. Hoelderich and M.E. Paczkowski Institute for Chemical Technology and Heterogeneous Catalysis, University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany

ABSTRACT Rearrangement of m-dioxanes to neoalkanals was researched in the temperature range between 250°C and 400°C with zeolitic molecular sieve catalysts. Generally, the conversions are about 40%-80%, while the selectivities often are higher than 90%. In case, that m-dioxanes with an additional ether-function in the side-chain were used, novel cyclic dioxepenes were obtained. Several investigations were done to improve the controlled preparation of the neoalkanals, among them the use of the CVD-method. INTRODUCTION In the last twelve years the range of applications has been enlarged by using the zeolites, particularly pentasil zeolites, in the organic synthesis of intermediates and fine chemicals [1,2]. The reasons for the increase and the wide utility of zeolitic catalysts in this new field of application have to be seen in their various chemical properties such as acidity, basicity, redox-feature and multifknctionality and shape selectivity. Also such catalysts contribute tremendously to the reduction of by-products (sometimes more than 50kg per lkg product) and help to develop processes which are environmentally more friendly. Oxygen containing compounds such as alcohols, ethers and aldehydes are particularly important in the chemistry of fine- and intermediate chemicals. Various reactions of such oxygenates in the presence of zeolites and other microporous materials as catalysts in particular concerning the cleavage of C-0 bonds have been published [3,4]. Presently, we are interested in the selective cleavage of the C-0 linkage of cyclic acetals in the presence of shape and non shape selective catalysts and in the reaction mechanism thereby. EXPERIMENTAL PART The borosilicate zeolite of the pentasil type is prepared, in a hydrothermal synthesis starting from 64,Og of highly disperse SiOz, 12,2g of H3BO3 and 800g of an aqueous 1,Qhexanediamine solution (mixture, 50% : 50% by weight) at 170°C under autogenous pressure in a stirred autoclave. The cristalline reaction product is filtered off and washed 399

400

W. F. Hoelderich and M . E. Paczkowski

thoroughly, after which it is dried at 100°C for 24 hours and calcined at 500°C for 24 hours. This borosilicate zeolite is composed of 94.2% by weight of Si02 and 2.3% by weight of B2°3.

Used catalvsts HZSM-5, HZSM-Sb USY boron pentasil zeolite aluminum phosphate cerium phosphate boron phosphate

Si02/A1203= 54 (a) Si02/A1203= 26.8 (b) SiO2/Al2O3 = 6 Na20 < 1% see preparation procedure above 22.12% Al,77.87% PO4 Ce: not measured, 42.0% PO4 10.22%B, 89.78% PO4

Uetikon AG Degussa AG Grace-GmbH BASF AG BASF AG BASF AG BASF AG

Before their use the zeolite powders were extruded without binders, sieved and calcinated at 550°C for 6h. The fractions between I.0mm and 1.6mm were used for catalysis and modification experiments. Modification of the zeolites with silicon alkoxides The modification of the external surface of the zeolites was done by the CVD-method [5,6]. As reagents for the modification silicon ethoxide and silicon methoxide were used. Calcination of the zeolite after modification will lead to a silica-coat on the outer surface. The modification also can reduce the pore diameter and thus can enhance the shape selective properties of the zeolites. The catalyst (usually 4g) was made in a round bottomed flask and heated at maximum 35OOC and at tom for up to four hours to remove the water from the sample. After that the catalyst was treated with gaseous silicon alkoxide between 3OOOC and 350OC and at 1 to 10 torr for up to three hours. The modification was stopped by removing the excess silicon tom. The alkoxide just by reducing the partial pressure of the modification reagent to modified zeolite then was calcinated at 550°C for 6h. BET-measurements were done to show that the pores of the modified catalyst were not blocked. Rearrangement of the cvclic acetals The reactions were camed out under isothermal conditions between 25OOC and 4OOOC with whsv = 2.5h-I 3.5h-I and normal pressure in a fixed-bed tube reactor (diameter: 0.6cm, length: 90cm in form of a coil) in the gas phase for eight hours. Nitrogen was used as carrier gas with a flow rate of 4.5Vh. In each experiment more than 96% of material was recovered. The quantitative determination of the reactants and the products was done by gas chromatography. The neoalkanals were identified by the NMR data of their neopentylglycole acetale-derivatives. These derivatives were obtained by conversion of the neoalkanals with

-

Preparation of Neoalkanals and Novel Cyclic Dioxepenes

401

neopentylglycole at 90°C - 100°C under acid catalysis.

RESULTS AND DISCUSSION The investigated reaction is a rearrangement of cyclic acetals, which can be easily synthesized from aldehydes or ketones and 1,3-diols by acid catalysis. When m-dioxanes were applied for the first time in heterogeneous catalysis with zeolites it was thought of an isomerization reaction similar to the aldehyde-ketone rearrangement [7].For this reason the migration of carbenium ion and formation of the ketal was expected. But this was not the case. Instead, the linear neoalkanals, which contain an ether oxygen in the P-position, were obtained [8,9] acccording to equation 1.

R2i$: -

R'

H

H

R2

R3 R5

I I I R'-C-0-C-C-CHO I I I H

H

(1 1

R4

R1,R2, R4, R5

= H, alkyl, alkenyl, aryl, arylalkyl, alkylaryl, arylalkenyl, alkenylaryl, heterocyclic residue, R3 = H, alkyl

This reaction is also very interesting from a mechanistic viewpoint. The mechanism of this isomerization reaction was proposed by Rondestvedt and Mantell who used silica and pumice as catalysts for that isomerization [lo-121. The rearrangement reaction is supposed to proceed via C - 0 bond cleavage and a 1,3-hydride-shift.This was the first evidence of hydride ions being formed in the synthesis of fine chemicals over a zeolite catalyst. This isomerization reaction has a wide range of application. Especially pentad zeolites employed at temperatures from 250°C to 400°C give good results in the rearrangement of cyclic acetals. The conversion is about 40% - 80% while the selectivity often is greater than 90%. Some examples are listed in table 1. Table 1, conversions and selectivities in the rearrangement of m-dioxanes with different aldehyde stems over a boron pentad zeolite

R1

TOS h

phenyl i-propyl n-propyl filryl

T "C 300 250 250 350

8 4 4 6

R2 = R3 = H, R4 = RS = CH

whsv h-1 3,9 3,3 3,l 2,8

conversion % 80 38 46 50

selectivity % 95 93 91 92

3

'H-NMR data of the neopentylglycole derivatives of the neoalkanals: = 0.69 (s, 3H, ring-CH,), 0.98 (s, 6H,

R1 = phenyl: lH-NMR (300 MHz, CDCI3): 6

402

W. F. Hoelderich and M. E. Paczkowski

C(CH,),CHOO), 1.14 (s, 3H, ring-CH,), 3.29 (s, 2H, PhCH20CH2), 3.35-3.42, 3.53-3.61 (m, 4H, ring-CHz), 4.32 (s, lH, CHOO), 4.49 (s, 2H, PhCH2), 7.22-7.34 (Ph-H) ppm. R1= i-propyl: IH-NMR (300 MHz, CDC13): 6 = 0.70 (s, 3H, ring-CH3), 0.88 (d, J = 6.8Hz, 6H, (CH&CH)), 0.95 (s, 6H, (CH3)2CHOO), 1.15 (s, 3H, ring-CH3), 1.78-1.92 (m, lH, (CH3)2CH), 3.13 (d, J = 6.5Hz, 2H, (CH3)2CHC&O), 3.20 (s, 2H, (CH3)2CHCH20CH2), 3.35-3.42, 3.56-3.62 (m, 4H, ring-CH2), 4.28 (s, lH, CHOO) ppm. R1= n-propyl: ‘H-NMR (300 MHz, CDC13): 6 = 0.70 (s, 3H, ring-CHj), 0.91 (tr, J = 7.5 Hz, 3H, CH2CH3), 0.94 (s, 6H, C(CH3)2CH00), 1.15 (s, 3H, ring-C&), 1.30-1.42, 1.48-1.58 (m, 4H, CH3CH_2Cf52CH2),3.21 (s, 2H, CH3CH2CH2CH20CH2),3.38 (tr, J = 6,s Hz, 2H, CH$H2CH2CH20), 3.35-3.42, 3.56-3.62 (m, 4H, ring-CH,), 4.26 (s, lH, CHOO) ppm. R1 = hryl: ‘H-NMR (300 MHz, CDCI,): 6 = 0.69 (s, 3H, ring-CH3), 0.91 (s, 6H, C(CH,),CHOO), 1.14 (s, 3H, ring-CH3), 3.28 (s, 2H, fkryl-CH20CH2), 3.34-3.41, 3.53-3.60 (m, 4H, ring-CHz), 4.26 (s, lH, CHOO), 4.41 (s, 2H, hryl-C&), m-hryl-H), 7.38-7.40 (m, IH, o-fkryl-Hi) ppm,

6.27-6.33 (m, 2H,

It must be pointed out that zeolites as acidic and shape selective catalysts are superior to conventional catalysts such as silicagel and alumina. Furthermore, it was supposed that the restricted transition state selectivity of the zeolite favours an increased yield of the linear neoalkanals which might fit in the zeolite framework. In case that cyclic acetals, containing an additional ether hnction in the side-chain, have been employed for that rearrangement, unexpected results have been obtained. Methoxyacetaldehyde neopentylglycole acetale I for example forms two compounds according to equation 2: the expected linear neoalkanal I1 (3-(2-methoxy-ethoxy)-2,2-dimethyIpropionaldehyde) and the unexpected, unknown cyclic dioxepene 111 (6,6-dimethyl-[ 1,4]dioxepene-(2,3)). The latter results from splitting off methanol and incorporation of CH2 in the ring system [13]. As by-products cleavage products of the starting material were detected, among them short-chain aldehydes like the one used for the preparation of the cyclic acetal itself

TCH H,C

CH,

+

NMR spectral data of the neoalkanal (11) lH-NMR (300 MHz, CDC13): 6 = 1.08 (s, 6H, C(CH3)2), 3.36 (s, 3H, OW3), 3.50 (s, 2H, OCflz), 3.49-3.60 (m, 4H, OCH2CH20),9.57 (s, lH, CHO) ppm.


13C-Nh4R (75 MHz, CDCI,): 6 = 18.979 (C(cH3)2), 47.147 (C(CH3)2), 59.032 (OcH3), 71.121, 71.885, 76.394 (cH2). 205.256 (CHO) ppm. NMR spectral data of the dioxepene (111) 'H-NMR (300 MHZ, CDC13): 6 = 0.96 (s, 6H, CH3), 3.68 (s, 4H, C&), 5.67 (s, 2H, CH) PPm. 13C-NMR (75 MHZ, CDC13): 6 = 22.058 (cH3), 37.015 (c(CH3)2), 81.1 14 (cH2), 131.584

(CH) PPm.

.-._._.c - .-.*

P.-.-.-

*

. *

0

S(dioxepene)

[%I

S(neoalkana1) [%I

I

0

2

6

4

8

10

TOS h

Fig. 1 dependence of the conversion of methoxyacetaldehyde neopentylglycole acetale I over HZSM-5b and of the selectivities of the neoalkanal I1 and the dioxepene I11 upon the time on stream The dependence of the conversion of I over HZSM-5, as well as of the selectivities of I1 and 111 upon the time on stream is shown in fig. 1. Under the chosen conditions (T = 300°C, whsv = 3.3h-l, 4 N h N2) it was found that the conversion decreases strongly i.e. the catalyst deactivates fast and the selectivity of I1 increases slightly whereas the selectivity of I11 decreases slightly. In fig. 2 it is demonstrated that the conversion is dependent upon the temperature resulting in a strong increase. However the selectivities of TI and 111 remain almost constant. That is true for different times on stream at least up to 10h. In an area of whsv = 2.5h-1 - 3.5h-1 it could be recognized that the conversion is increased at lower whsv and the selectivities remain nearly on the same level.

403

404


T

%

loo 00

.-8

f2

d; E 0

'g

-

;:: *O-

60

-

40

-

A

g 20 8

Slneoalkanal) [%I

30

10-

A 5

A

A

I

0 4 280

290

300

310

320

temperature

330 O

340

360

360

C

Fig. 2 dependence of the conversion of methoxyacetaldehyde neopentylglycole acetale I over HZSM-5b and of the selectivities of the neoalkanal I1 and the dioxepene 111 upon the temperature (whsv = 3.3 h-l, 4.5Vh Nz, TOS = 2h) The use of catalysts other than HZSM-5 (see table 2), like phosphates or USY, in the conversion of compound I shows, that phosphates such as boron- or cerium phosphates give a high selectivity to the dioxepene 111. Table 2. catalysts other than pentasil zeolites used in the conversion of methoxyacetaldehyde neopentylglycole acetale I (TOS = 2h) catalyst

T "C

BP04 CePO4

350 350 300 3 00

whsv h-1 conversion %

ALP04

USY

3.5 3.6 3.6 3.5

41 31 31 33

selectivity dioxepene % 79 51 43 45

selectivity neoalkanal % 4 4 30 28

BET-surface m2fg 10 139 54 593

One could assume that there are spatial constraints in a zeolite pore for the dioxepene I11 to be formed. Thus, it is supposed that the linear neoalkanal is build up preferably on the inner surface of the zeolites, whereas the probably sterically bulkier dioxepene on the outer surface. With respect to the finding that BPO, with low BET-surface yields high selectivity for dioxepene III it can be assumed that the reaction for its formation is much faster than for forming compound 11. However, these suggestions are not in agreement with our findings e.g. in the presence of AlP04, because we obtain compound I1 and I11 in almost similar selectivities. Chemical vapor deposition techniques (CVD) allow to deposit ultra thin iayers of silica on the external surface of zeolites by using e.g. silicon methoxide or silicon ethoxide. In the


presence of such modified zeolites in the conversion of methoxyacetaldehyde neopentylglycole acetale I the selectivity to the dioxepene I11 should obviously decrease whereas the selectivity to the neoalkanale I1 should increase. One of our first experiments using silicon ethoxide is shown in table 3. In fact the expected results were obtained. This could be a hint that our presumption about the places of formation of I1 or 111 is right. Table 3 , conversion and selectivities in the conversion of methoxyacetaldehyde neopentylglycole acetale I using non-modified and by CVD-method modified HZSM-5, (TOS = 2h) catalyst conversion % selectivity selectivity Am YO dioxepene YO neoalkanal YO HZSM-5, 72 62 10 unmodified 23 44 21 0.41 modified T = 300°C,whsv = 4,l h-1, Am% = weight-% between the modified and non-modified catalyst Nevertheless, the low selectivity to the neoalkanal on unmodified HZSM-5, when compared even to non-microporous catalysts would seem to indicate that the stronger acid sites on the surface of that zeolite would so accelerate the reaction - as suggested by the high conversion observed in this case - as to outweigh any influence of diffusion properties. Furthermore also it has to be mentioned that computer modelling on a silicon graphics using Biosym software did not support our presumption. It seems that methoxyacetaldehyde neopentylglycole acetale I and its reaction products just have rather small differences in spatial expansion. Most probably all these compounds fit in the canal of pentad zeolites. Also we do not have exact imagination of the transition states forming the linear or cyclic product. So the final judgement about the sites of reaction (outer or inner surface of the zeolite) can not be stated. Therefore, a lot of further attempts are necessary in order to get a clear idea about the reaction course on the inner or outer surface of the catalysts. In addition to the discussion about the influences of pore diameter, pore size distribution and acidic strength several other investigations have to be done to get more insight of the reaction mechanism. We thought of MAS-NMR, FT-IR. MAS-Nh4R- (in cooperation with Prof. E.G.Derouane and Dr. I. Ivanova in Namur/Belgium) and FT-IR-examinations will be done to solve this problem, but these examinations are not finished yet.

AKNOWLEDGEMENTS The authors express their sincere thanks to BASF AG, Degussa AG, Uetikon AG and GraceGmbH for the supply of zeolites and catalyst and to the state Nordrhein-Westfalen for the financial support.

405

406


REFERENCES 1 W.F. Hoelderich and H. van Bekkum, Stud. Surf Sci. Catal., 58 (1991)631. 2 W.F. Hoelderich, M.Hesse and F. Naumann, Angew. Chem., 100 (1988)232. 3 W.F. Hoelderich, in S. Yoshida et al. (Eds.) Catalytic Science and Technology, Vol. 1, Proceedings of TOCAT 1, Tokyo 1990,Kodansha Ltd. 1991 p. 3 1. 4 W.F. Hoelderich and N. Goetz, Proceedings of the 9th International Zeolite Conference, Montreal 1992,Butterworth-Heinemann 1993 p. 309. 5 M. Niwa and Y. Murakami, J. Phys. Chem. Solids, 50 (1989)487. 6 U. Dingerdissen, doctoral thesis, University of Technology Darmstadt, 1990. 7 W.F. Hoelderich, Pure & Appl. Chem. Vol. 58, 10 (1986) 1383. 8 W.F. Hoelderich, Stud. Surf. Sci. Catal., 49 (1989)69. 9 W.F. Hoelderich and F. Merger, Eur. Pat. 199210 (29.10.86)BASF AG. 10 C.S. Rondestvedt and G.J. Mantell, J. Am. Chem. SOC.,82 (1960)6419. 1 1 C.S. Rondestvedt and G.J. Mantell, J. Am. Chem. SOC.,84 (1962)3307. 12 C.S. Rondestvedt, J. Am. Chem. SOC.,84 (1962)3319. 13 F. Merger, W.F. Hoelderich, J. Frank, T. Dockner and M. Sauerwald, DE 3826303 (03.08.88)BASF AG.

Redox Molecular Sieves: Recyclable Catalysts for Liquid Phase Oxidations R.A. Sheldon, J.D. Chen, J. Dakka and E. Neeleman Delft University of Technology, Laboratory for Organic Chemistry and Catalysis, Julianalaan 136, 2628 BL Delft, The Netherlands

ABSTRACT Redox molecular sieves have been synthesized by isomorphous substitution in the framework of silicalite-1 and ALPO-5. CrAPO-5 and CrS-1 were shown to be effective catalysts for the decomposition of secondary alkyl hydroperoxides to the corresponding ketones. In the decomposition of cyclohexyl hydroperoxide the highest selectivity to cyclohexanone (86%) was observed with CrAPO-5. CrAPO-5 was also shown to be an effective catalyst for the oxidation of secondary alcohols to the corresponding ketones, alkylbenzenes to acetophenones and cyclohexane to cyclohexanone using tert-butyl hydroperoxide (TBHP) or 0, as the terminal oxidant. Evidence is presented in support of the reaction taking place inside the cavity of the molecular sieve.

INTRODUCTION Catalytic oxidation is widely used for the conversion of petroleum-derived hydrocarbons to commodity chemicals [I]. Moreover, in fine chemicals manufacture there

is increasing pressure to replace traditional stoichiometric oxidations with inorganic reagents such as dichromate and permanganate with cleaner, catalytic alternatives which do not generate excessive amounts of inorganic salts as byproducts. Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. However, solid catalysts offer several potential advantages over their homogeneous counterparts, such as ease of recovery and recycling and enhanced stability. Moreover, site-isolation of discreet redox metal centers in inorganic matrices can lead to oxidation catalysts with unique activities and selectivities. One approach to designing stable solid catalysts with unique activities is to incorporate redox metal ions, by isomorphous substitution, into the lattice framework of molecular sieves, such as silicalites, zeolites, aluminophosphates (ALF'Os) and 407

408

R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman

silicoaluminophosphates (SAPOs). Such redox molecular sieves [2-41 can be regarded as 'mineral enzymes'. Unlike conventional amorphous materials, such as silica and alumina, molecular sieves possess a regular microenvironment with homogeneous internal structures consisting of uniform, well-defined cavities and channels. Hence, redox molecular sieves can be tailor-made by fine-tuning of the size and hydrophobicity of the redox cavity to provide unique oxidation catalysts. Furthermore, incorporation of the redox metal ion into the stable lattice of a molecular sieve may provide enhanced stability towards leaching, a problem often encountered with conventional supported metal catalysts. A landmark in the development of redox molecular sieves was the titanium(1V)catalyst developed by Enichem workers [5]. TS-1 catalyzes a variety of silicalite (TS-1) industrially useful oxidations with 30% aqueous hydrogen peroxide, e.g. olefin epoxidation, phenol hydroxylation and cyclohexanone ammoximation. As part of an ongoing research program on redox molecular sieves we have synthesized and characterized a range of redox ALPOs, zeolites and silicalites. Chromium molecular sieves were of particular interest based on the widespread use of chromium(V1) compounds as stoichiometric oxidants in organic synthesis [6] and, more recently, the use of soluble chromium catalysts in combination with TBHP [7]. EXPERIMENTAL m l y s t synthesis CrAPO-5 was hydrothermally synthesized by essentially following a reported procedure [8], using the molar ratio: 0.05 Cr203:0.9 A1203:P205:Pr,N:S0 H,O. Crystallization was performed at 175 "C for 24 h. The template (Pr3N) was removed by subsequent calcination of the as-synthesized material at 500 "C for 10 h. Chromium silicalite (CrS-I) was hydrothermally synthesized by essentially following a reported procedure [9]. In order to obtain CrS-1 free of quartz it was necessary to rotate the autoclave, e.g. at 320 rpm, during the crystallization step. Other metal ALPOs and silicalites were prepared by similar procedures. m l v s t characterization CrAPO-5, CrS-1 and other metal ALPOs and silicalites were characterized by elemental analysis, X-ray diffraction (XRD), diffuse reflectance atomic absorption spectroscopy (DREAS) and scanning electron microscopy (SEM). XRD powder patterns

Redox Molecular Sieves

409

were recorded on a Philips PW 1877 automated powder diffractometer using CuKa radiation.

DREAS

spectra

were

measured

with

a

Hitachi

150-20 UV-VIS

spectrophotometer equipped with a diffuse reflectance unit. SEM spectra were obtained using a JEOL JSM-35 scanning microscope. The samples were coated with an Au evaporated film. Elemental analyses were obtained using inductively coupled plasmaatomic emission spectroscopy (ICP-AES) on a Perkin-Elmer Plasma I1 instrument. TvDical reaction Drocedu re5 Typically, oxidation reactions were carried out by stirring a suspension of the catalyst (ca. 1 mol %) with a solution of the substrate and TBHP in the solvent (e.g. chlorobenzene) at 85-110 "C for 5 hours. Reactions employing molecular oxygen were caried out at atmospheric pressure by bubbling oxygen through the reaction mixture or 5 bar 0, in an autoclave. Hydroperoxides were analyzed by iodometric titration and other substrates and products by gas liquid chromatography. RESULTS AND DISCUSSION A e l1 hydroperoxide decomposition

In the manufacture of cyclohexanone via cyclohexane autoxidation initially formed cyclohexyl hydroperoxide (CHHP) is decomposed, often in a separate step, to give a mixture of cyclohexanol and cyclohexanone. From the viewpoint of practical utility it is desirable to achieve a high ratio of cyclohexanone to cyclohexanol. The ideal situation corresponds to decomposition of CHHP according to the stoichiometry:

OOH

0 catalyst

Moreover, a stable recyclable catalyst for reaction 1 would be particularly attractive. Consequently, we have tested a variety of redox molecular sieves as catalysts for reaction 1 (see Table 1). Both CrAPO-5 and CrS-1 were active catalysts for CHHP decomposition.

The highest selectivity to cyclohexanone (86%) was observed with CrAPO-5. CrS-1 was even more active but gave a lower cyclohexanone/cyclohexanol ratio. Other metal APOs and silicalites gave both lower activities and selectivities.

410

R. A. Sheldon, J. D. Chen, J. Dakka and E. Neeleman

In one experiment with CrAPO-5 the catalyst was filtered, washed with cyclohexane, recalcined and reused with a fresh solution of CHHP. This was repeated five times without any noticeable loss of activity or selectivity. Recalcination is probably necessary in order to remove the water, formed in the reaction, from the pores of the catalyst. In practice this may be possible using other means, e.g. azeotropic distillation during reaction. Table 1. Catalytic decomposition of cyclohexyl hydroperoxide (CHHP) at 70 "C. Catalyst

Selectivity (%)

CHHP conversion (%)

CrAPO-5 Cr-silicalite VAPO- 11 CO-ZSM-5 VAPO-5 COAPO-5 MWO-5 V-silicalite TS-1 None

87 98

76 24 17 2 2 0

0 0

Cyclohexanone

Cyclohexanol

86 64 50 43 51 50 50 0 0

13 36 50 50 43 50 50

0

0 0 0

Conditions: CHHP (2.9 mmol) dissolved in cyclohexane (10 ml) stirred with the catalyst (0.029 mmol metal) at 70 "C for 5 hours. Similarly, other secondary hydroperoxides, e.g. ethylbenzene hydroperoxide and tetralin hydroperoxide, afforded high yields of the corresponding ketone with CrAPO-5. Tert-alkyl hydroperoxides were decomposed to the corresponding alcohol and dioxygen, together with small amounts of the ketone formed by 8-scission of intermediate alkoxy radicals (Table 2).


41 I

Table 2. CrAPO-5 catalyzed decomposition of alkyl hydroperoxidesa. ~~

R02H

Cyclohexyl tert-Butylb Cumene Triphenylmethyl

Solvent

C6H12 C,H,CI C,H,CI 1,2-C2H4CI,

~

~~~

~

Conversion (%)

87 49 24 1

Selectivity (%) Ketone

Alcohol

86 5 2

13 93 86

Conditions: R02H (2.9 mmol) in solvent (10 ml) stirred with CrAPO-5 (0.1 g containing 1.5% Cr = 0.029 mmol Cr) for 5 h at 70 "C. 50 "C.

a

Evidence for the reaction taking place inside the cavity of CrAPO-5 was provided by the observation that the bulky triphenylmethyl hydroperoxide, which cannot be accommodated in the cavity, was not decomposed. In contrast, homogeneous chromium(II1) acetylacetonate and the supported Cr02C12/silica-alumina were effective catalysts for the decomposition of this hydroperoxide giving 75% and 72% decomposition in 2 h, respectively, with equivalent amounts of catalyst (1% m) at 70 "C in 1,Zdichloroethane. Walyst structure and catalytic mechanism

In the case of both CrAPO-5 and CrS-1 the as-synthesized catalysts are green and contain chromium in the trivalent state. After calcination at 500 "C the catalysts were yellow and DREAS showed that most of the chromium is present as Cr(VI). ICP-AES analysis showed that chromium contents of up to 1% (CrS-1) to 1.5% (CrAPO-5) could be achieved. We tentatively assume that in the as-synthesized catalysts chromium(II1) is isomorphously substituted, in tetrahedral positions, for silicon (CrS-1) or aluminium (CrAPO-5). Subsequent oxidation during calcination is assumed to afford dioxochromium(V1) which is still attached to the internal framework in either tetrahedral or octahedral coordination. Decomposition of CHHP according to the stoichiometry of reaction 1 is consistent with a heterolytic mechanism. This can be envisaged as proceeding via /3-hydrogen elimination in an alkylperoxochromium(VI) complex (Figure 1). Such a mechanism is not possible with tert-alkyl hydroperoxides and we assume that a homolytic mechanism, involving tert-alkoxy radicals as intermediates, operates in this case. Acid-catalyzed heterolysis can be ruled out because it would lead to the formation of different products,

412

R. A. Sheldon, J . D. Chen, J . Dakka and E. Neeleman

e.g. phenol and acetone from cumene hydroperoxide.

>-,

VI

Cr=O

+ R,C=O + H,O

Fig. 1. Mechanism of decomposition of secondary alkyl hydroperoxide. CrAPO-5 catalyzed oxidation of secondary alcohols Based on the known use of homogeneous chromium catalysts for the oxidation of secondary alcohols with TBHP [7] we envisaged that CrAPO-5 should be an effective solid catalyst for this reaction. The results of CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 "C are shown in Tdhk 3. Good to excellent selectivities to the corresponding ketones were observed with respect to both substrate and TBHP in most cases. Carveol underwent chemoselective oxidation of its alcohol group to give carvone in

94% selectivity, without any attack at its double bonds. Moreover, one of the (cis/trans) isomers appeared to react much faster indicating that some shape selectivity is observed. l-Phenyl-1,2-ethanediol was selectively oxidized at the secondary alcohol group to give ahydroxy acetophenone (73% selectivity). In one experiment with a-methylbenzyl alcohol the CrAPO-5 catalyst was filtered, washed 3 times with chlorobenzene and recalcined before reuse. The catalyst was recycled 4 times without any noticeable loss of activity or selectivity. DREAS spectra showed that most of the chromium remained in the hexavalent state within the ALPO, framework after recycling. Hence, we conclude that CrAPO-5 is a stable, recyclable catalyst for the selective liquid phase oxidation of secondary alcohols, to the corresponding ketones, using TJ3HP as the terminal oxidant. Interestingly, when the oxidation of a-methylbenzyl alcohol with TBHP was carried out in air instead of N2 a yield of acetophenone on TBHP of 216% was observed, suggesting that 0, could also act as the terminal oxidant. This was confirmed in subsequent experiments (Table 4). The best results were obtained using a small amount

(10 mol %) of TBHP to initiate the reaction.


413

Table 3. CrAPO-5 catalyzed oxidations of secondary alcohols with TBHP at 85 "Ca. Substrate

a-Ethylbenzyl alcohol a-Methylbenzyl alcohol Cyclohexanol Carveol l-Phenyl-1,2ethanediol

Time (h)

Product

Conversion

Selectivity (%)

(%Ib Substrate

TBHP

7

propiophenone

77

100

91

16 12 16

acetophenone cyclohexanone carvone a-hydroxyacetophenone

77 72 62

96 85 94

89 73 66

54

73

40

16

a Conditions: substrate, 10 mrnol; TBHP, 5 mmol; CrAPO-5 (0.14 mmol), chlorobenzene (solvent), 10 rnl; stirred at 85 "C for 16 h under N,. Conversion of substrate based o n the amount of TBHP charged.

We tentatively propose that the oxidation of secondary alcohols with TBHP in the presence of CrAPO-5 proceeds via a heterolytic mechanism involving 8-hydrogen elimination from an oxochrornium(V1) alkoxide followed by reoxidation of the reduced chromium(1V) by TBHP (Figure 2). When 0, is the terminal oxidant the a-hydroxyhydroperoxide, formed by (chrorniurn-catalyzed) autoxidation of the alcohol, can reoxidize the chromium(1V). Table 4. CrAPO-5 catalyzed oxidations of secondary alcohols with OZa. Conversion (%)

Selectivity (%)

cyclohexanone

30

97

acetophenone

31

96

propiophenone a-tetralone l-indanone

38 26 78

90 73 72

Substrate

Product

Cyclohexanol a-Methylbenzyl alcohol a-Ethylbenzyl alcoho I a-Tetralolb l-Indanolb

Conditions: substrate, 250 rnmol; 0, pressure 5 atm or 20 atm air; TBHP 25 mrnol; CrAPO-5 (3.65 mmol Cr); chlorobenzene (solvent), 65 ml; 3 A molecular sieve (drying agent), h g; 110 "C, 5 h. Conditions: substrate, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol; chlorobenzene, 5 ml, 110 "C, stirring 1000 rprn, 19 h.

R. A. Sheldon, J . D. Chen, J . Dakka and E. Neeleman

414

c"j-0 o H

@ lv>

CR,

)

TBHP. R,CHOH -H,O.

+ R,CO

&-OH

I

TBA

Fig. 2. Mechanism of alcohol oxidation. CrAPO-5 catalyzed oxidations of hydrocarbons By analogy with the chemistry of soluble chromium catalysts [7] we reasoned that CrAPO-5 should also be an effective catalyst for benzylic oxidations with TBHP. Indeed, CrAPO-5 (1 mol % Cr) catalyzed the selective oxidation of ethylbenzene (reaction 3) and tetralin (reaction 4) with TBHP. 0

PhC1/7O0C/1 6 h

8 5 % selectivity

0

9 0 % selectivity

As was observed in alcohol oxidations (see above) when the oxidation of tetralin was carried out in air the selectivity to a-tetralone based on TBHP was greater than 100%. Subsquent experiments confirmed that CrAPO-5 is an effective catalyst for the autoxidation of benzylic hydrocarbons to the corresponding ketones. A small amount (10 mol %) of TBHP was added to initiate the reaction. For example, tetralin was oxidized with 0, at atmospheric pressure and 100 "C to give a mixture of a-tetralone (64%), a-tetralol (7%) and a-tetralin hydroperoxide (THP; 20%). Presumably, in practice a small


415

amount of the THP-containing product stream could be recycled to the oxidation reactor, thus obviating the need for TBHP as initiator. Recycling experiments showed that the CrAPO-5 could be recycled 5 times without loss of activity (Table 5). Recalcination of the catalyst prior to reuse was not necessary, presumably because the water formed was removed azeotropically during reaction. Table 5. Recycling of CrAPO-5 in the autoxidation of tetralin at 100 'Ca. Cycle nr.

Selectivity (%)

Conversion

(%I 1 2 3 4

44 58

57 53 57

a-tetralone

a-tetra101

64 65 60 61 65

7 6 5 5 6

THP~ 20 24 30 32 24

a Conditions: tetralin, 50 mmol; O,, 15 ml/min; TBHP, 5 mmol; CrAPO-5, 0.73 mmol Cr; 100 "C, stirring, 1000 rpm, 10 h. THP = cr-tetralin hydroperoxide. The CrAPO-5 was regenerated by calcination at 500 "C for 5 h.

CrAPO-5 was also found to catalyze the autoxidation of cyclohexane at 115 "C. At 3% cyclohexane conversion the major product was cyclohexanone (64%) together with cyclohexanol (10%) and CHHP (9%) and dicarboxylic acids (13%). In practice part of the CHHP-containing product stream could be recycled to the oxidation reactor to act as an initiator. 0

64%

OH

10%

OOH

9%

3% cyclohexane conversion

416

R. A. Sheldon, 1. D . Chen, J. Dakka and E. Neeleman

CONCLUDING REMARKS CrAPO-5, containing chromium(V1) in the A1P04-5 framework, is an active, recyclable catalyst for alkyl hydroperoxide decomposition and the selective liquid phase oxidation of secondary alcohols, alkylaromatics and cycloalkanes with TBHP or 0,as the terminal oxidant. The scope and mechanism of these and related liquid phase oxidations mediated by redox molecular sieves are under further investigation. REFERENCES 1 R.A. Sheldon and J.K. Kochi, ‘Metal-Catalyzed Oxidations of Organic Compounds’, Academic Press, New York, 1981. 2 R.A. Sheldon, CHEMTECH, (1991) 566. 3 R.A. Sheldon, Topics Curr. Chem., 164 (1993) 21. 4 J.D. Chen, J. Dakka, E. Neeleman and R.A. Sheldon, J. Chem. SOC.,Chem. Commun., in press. 5 U. Romano, A. Esposito, F. Maspero, C. Neri and M. Clerici, Chim. Ind. (Milan), 72 (1990) 610. 6 G. Cainelli and G. Cardillo, ‘Chromium Oxidations in Organic Chemistry’, SpringerVerlag, Berlin, 1984. 7 J. Muzart, Chem. Rev., 92 (1992) 113. 8 E.M. Flanigen, B.M.T. Lok, R.L. Patton and S.T. Wilson, US Patent, 4,759,919 (1988) to Union Carbide Corp. 9 M. Kawai and T. Kyoura, Japanese Patents, JP 0358,954 and JP 0356,439 (1991) to Mitsui Toatsu Chemicals; CA 115 (1991) 48863d and 48864e.

Titanium Silicalites as ShapeSelective Oxidation Catalysts

T. Tatstmi, I

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