Studies in Surface Science and Catalysis 83 ZEOLITES AND MICROPOROUS CRYSTALS Proceedings of the International Symposiu...
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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
viii
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
List of Contributors
ix
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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List of Contributors
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
List of Contributors
xi
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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List of Contributors
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
xiii
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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List of Contributors
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
List of Contributors
xv
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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List of Contributors
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
List of Contributors
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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
xviii
List of Contributors
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
List of Contributors
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.
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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
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Contents
Organization
v
List of Contributors
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
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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
Zirconophosphate Molecularly Engineered Layered Structures
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
Zirconophosphate Molecularly Engineered Layered Structures
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
Pyridine Intercalates of Bismuth Molybdic Acid
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
Pyridine Intercalates of Bismuth Molybdic Acid
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
S.-J. Jong, J.-T. Lin and S. Cheng
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,
n i l 2 of
(a) to 20 mL of
spectrum of TiClJethanol solution. The one at 16 ppm is assigned to carbon at the methyl group, and that at 68 ppm is the carbon of methylene group in ethanol. Comparing with free ethanol, the latter peak shifts downfield (from ca. 56 to 68 ppm), indicating the possible formation of
Titanium Pillared C[ay Using Organic Medium
39
titanium ethoxide. After the solution was mixed with glyceridwater solution, four peaks appear in the spectrum (Fig. 5@)). The two strong peaks at 63.1 and 72.5 ppm are consistent with those of free glycerin. The other two weak peaks appeared at 18.5 and 56.1 ppm are attributed to free ethanol. These results demonstrate that titanium ethoxide is hydrolyzed and ethoxy groups are released and form free ethanol. After the mixture was diluted with a 100 mL of water, the NMR spectrum showed only two peaks corresponding to free glycerin (Fig. 5(c)). The latter experiment was to imitate the pillaring condition where clays were suspended in the same amount of water. Accordingly, glycerin does not seem to form bonding with titanium. Instead, it probably dilutes water concentration, increases the hydrophobic character of the solution, and slows down the hydrolysis reaction of titanium ethoxide species so that titanium polynuclear clusters of narrow distributed size are obtained. Aciditv and shape-selectivitv The acidity of Ti-PILC was compared with that of AI-PILC and Zr-PILC by carried out temperature programmed desorption of ammonia. Fig. 6 shows that the profiles can be roughly divided into three regions: <473 K, 473-573 K and 573-673 K. The desorption bands appeared above 673 K are attributed to condensation of hydroxyl groups of clays. The total acid amount of Ti-PILC is less than that of either Al-PILC or Zr-PILC. However, the amount of strong acid
sites that corresponding to NH, desorption at 573-673 K region decreases in the order of A1-PILC
> Ti-PILC > 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
S.-J. Jong, J.-T. Lin and S. Cheng
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.
Oxygenated Stabilizing Agents in Zeolite Synthesis
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.
Oxygenated Stabilizing Agents in Zeolite Synthesis
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.
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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
Discrete Colloidal Zeolite Particles
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
Discrete Colloidal Zeolite Particles
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.
Isomorphous Substitution by Tetravalent Elements
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
Isomorphous Substitution by Tetravalent Elements
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
70
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
74
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
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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.
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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.
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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.
Fine Structures of Zeolites: HREM Study
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).
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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.
Fine Structures of Zeolites: HREM Study
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).
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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
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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
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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.
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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.
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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.
108
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.
Topography of Vanadium in Silicalite-2
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
Topography of Vanadium in Silicalite-2
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.
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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
Molecular Dynamics and Computer Graphics
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
Molecular Dynamics and Computer Graphics
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)
Re Impregnated Zeolite Y and ZSM-5
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
Re Impregnated Zeolite Y and ZSM-5
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
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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
Solid-state Reactions of Zeolites
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.
Solid-state Reactions of Zeolites
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
Solid-state Reactions of Zeolites
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
Solid-state Reactions of Zeolites
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-
Solid-state Reactions of Zeolites
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.
152
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.
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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
HZSM-5 Modified by CVD
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
HZSM-5 Modified by CVD
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
Pore Size Regulated MFI Zeolites
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
Pore Size Regulated MFI Zeolites
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
Intercalation Compounds as Photofunctional Materials
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
Intercalation Compounds as Photofunctional Materials
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.
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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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S. Kowalak, M. Pawbwska. A. B. Wieckowski and J . Goslar
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 .
Polymerization Inside Molecular Sieves
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.
184
S. Kowalak, M. Pawlowska, A. B. Wieckowski and J . Goslar
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-!
Polymerization Inside Molecular Sieves
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.
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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 <100>, (c) beam polarised <001>.
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.
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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 (
Adsorption of Lower Hydrocarbons in N a Y and Theta-1
199
Fig 1. Low (0) and high (0)pressure adsorption isotherms for (a) ethane, (b) ethene, (c) propane, (d) propene at (i) 273K , (ii) 298 K, (iii) 323K on zeolite Nay. The initial slopes of the ethane and propane isotherms are steeper in Theta-1 than in Nay due to the increased interaction energy brought about by shorter adsorbatdadsorbent dispersion force interaction distances in the channels of Theta-1 than in the much larger supercages of Nay. In contrast the initial slopes of the ethene isotherms are much greater in NaY than in Theta-1 because of the large additional electrostatic interactions of the x electrons of the double bond with the electric field gradient at the adsorption sites in Nay.
200
J. A. Hampson and L. V. C . Rees
1 0
00 0
0 00
-9 g
0 00
0
l
-
0
0 0
0 0 0 0
0 0 00
0
0 0
0 0
0.1:
2 : - (i)(ii)(iii) 0 0 0
0.01
10-210-' loo 10' lo2 1(
I
'
0
0
. . . . . . . . . . 10'' loo 10' lo2 lo3 10-l loo 10' lo2 lo3
PressurekPa Fig 2. Low (0)and high (0) pressure adsorption isotherms for (a) ethane, (b) propane, (c) ethene at (i) 273K, (ii) 298 K, (ui) 323K on zeolite Theta-1. The subtle differences in the adsorption behaviour of these adsorbates in NaY and Theta-1 arising from differences in the total free volumes, pore geometries and electric field gradients at the sorption sites can be more clearly seen when various parameters are compared in Table I . The limiting adsorption capacities n, in mmolgl of the various adsorbates as the equilibrium pressure approaches P,,are some 3.4-3.5 times larger for the C, hydrocarbons and some 3.2 times larger for the C, hydrocarbons in NaY compared with Theta-]. Since the ratio of the crystallographic free volumes is -2.5 these experimental ratios indicate the more efficient packing of these hydrocarbons molecules in the larger interconnected cavities of NaY compared with the smaller one dimensional channels of Theta-1. The Henry'sLaw constants KH in Table 1 obtained fiom the initial slopes of the isotherms at 298K show the stronger interaction of ethane and propane molecules with the adsorption sites in Theta-1 due to enhanced dispersion force interactions of the non-polar molecule with the fimework oxygens of Theta-1 compared with those in the Nay cavities. This enhanced interaction arises fiom the shorter distance between the adsorbate molecule and the fimework oxygens associated with the adsorption site. Polarisation of the saturated hydrocarbon molecules by the cations present in NaY introduce a catiodinduced dipole
201
Adsorption of Lower Hydrocarbons in NaY and Theta-I
contribution to the adsorption energy , but this contribution is smaller than the enhanced dispersion force contribution from the smaller channels of Theta-1 . In comparison the Henry's Law constant for the adsorption of ethene in Nay in Table 1 is over an order of magnitude larger than that of ethene adsorbed in Theta-1 and the Henry's Law constant for propene adsorbed in NaY is over twelve times largw than the corresponding value for ethene adsorbed in Nay. These very large values arise from the large electrostatic interaction of the x electrons of the double bond with the large electric field gradients which exist in Nay. The initial heats of adsorption, -AHK calculated from the variation of the Henryk Law constant with temperature using the Van't HOEisochore, mirror the differences in the Henry's Law constants for these adsorbates in these two zeolites. Table 1 shows that the initial heats of adsorption of ethane and propane are larger in Theta-1 than in NaY while the corresponding heats of adsorption of ethene show the enhanced interaction energy from the double bond in NaY compared with Theta-1. These initial heats of adsorption, however are not as sensitive as the Henry'sLaw constants to the differencesin the pore geometries and electric fields of these two zeolites. It is interesting to note that -AHKfor ethene in Theta-1 is smaller than that for ethane. The difference of some 2 Wmol-I arises from the loss of the two hydrogen atoms in ethene which results in a smaller net dispersion force interaction between the atoms of the adsorbate molecule and the framework oxygens. The difference in the initial heats of ethane and propane is some 12 kJmol-Lin Theta-1 and 11 kJmol-l in Nay. Thus the contribution to the heat of adsorption for an additional CH, group is very slightly larger in the smaller pores of Theta-1 than in the larger cages of Nay. The initial heat of adsorption of ethene in Nay is larger than that of propane demonstratingthat the specific interaction energy of the double bond i.e. 16 Hmol-1 is somewhat larger than the contribution of 11 Mmol-1 for an additional CH, group. Table 1 also lists -AH, for ethane and propane adsorbed in the channel network of silicalite-1, another pure silica zeolite [2]. Silicalitel has channels of similar dimensions to those in Theta-1 but sicalite-1 has intersecting channels which give rise to small cavities at the intersections. The -AHKvalues for ethane in the two zeolites W e r only by lklmol-1 but the heat of adsorption of propane in Theta-1 is some 4kJm01-~larger than for silicalital. The latter adsorbate molecule is larger than the channel segments between intersections in silicalite-1 and the lower heat of adsorption probably reflects the increased separation distances between the ends of the propane molecule and the nearest framework oxygens. Finally in Table 1 there is a comparison between the ratios of to -AHfi,,, the heat of liquefaction of these adsorbate molecules. The ratio increases from 2 to 2.04 to 2.22 on going from ethane to ethene to propane in Theta-1. The corresponding values for the saturated hydrocarbons in Nay are much smaller at 1.69 and 1.90 for ethane and propane respectively demonstrating once again the lower heats of adsorption of these adsorbates in the more open
-
202
J . A. Hampson and L. V. C. Rees
channels of Nay. However, the enhanced electrostatic interactions of the double bond of ethene in NaY results in the large value of 3.02 for this ratio which is very much larger than the correspondingvalue of 2.04 in Theta-1. Table 1. Henry's Law constants, K, at 298K , Initial Heats of Adsorption -AHK, Initial Isosteric Heats %, and limiting adsorption capacity % for the hydrocarbonsin Nay and Theta1 Zeolite Sorbate K" qst -m,ia /mmolglkPa-l fldmol-1 AcJmol-l klmol-1 /mmolgl %I
NaY
Ethane Ethene Propane Propene
0.055 0.651 0.569 7.948
24.97 40.94 35.82
23.6 36.5 29.6 -45
14.72 13.54 18.77 18.40
1.69 3.02 1.90
4.5 4.9 3.8 3.6
Theta-1
Ethane Ethene Propane
0.091 0.060 0.700
29.50 27.64 41.73
31.9 33.0 42.0
14.72 13.54 18.77
2.00 2.04 2.22
1.33 1.41 1.19
Silicalite-1
Ethane Propane
30.54 37.79
In Table 2 the Henry's Law constants at 273,298 and 323 K for the four adsorbates for NaY and for the three adsorbatesin Theta-1 may be compared. Table 2 : Henry's Law constants KHin mmolgl for hydrocarbonsin NaY and Theta-I 298K 323K Zeolite Adsorbate 273K NaY
Ethane Ethene Propane Propene
0.123 2.717 1.581 32.50
0.055 0.650 0.569 7.948
0.027 0.225 0.210 2.628
Theta-1
Ethane Ethene Propane
0.270 0.167 3.274
0.091 0.060 0.700
0.036 0.025 0.190
Adsorption ol' Lower Hydrocarbons in NaY and Theta-I
203
These KHvalues at 273 and 323K show similiar differences as discussed above for the 298K values, but there are subtle differences e.g. KH for propane is smaller in Theta-1 than in NaY as the temperature is raised to 323K. It has been shown previously [2], that the ratio of the Henry'sLaw constants for any two adsorbates in a specific zeolite gives a v g r good estimate of the experimentally obtained separationfactor a,defined by eq. 1.
where X,Y represent the mole fractions of components 1 and 2 of the binary mixture in the adsorbed and gas phase respectively. The ability of NaY and Theta-1 zeolites to separate binary mixtures of the adsorbates listed in Table 2 can be assessed by calculations of the appropriate ratio of the Henry's Law constants for the chosen binary mixture [2]. For example, predicted separation factors for propandethane mixtures at 298K in Nay and Theta-1 are 0.569/0.055 = 10.34 and 0.700/0.091= 7.69 respectively. The isosteric heats of adsorption , &t (&t =AH) for ethane, ethene, propane and propene in NaY as a finction of coverage are given in Figure 3(a) and for ethane, ethene and propane in Theta-I in figure 3@). Ethane adsorbed on NaY shows a small drop in &t &om 24.6 to 22.5 kJmol-1at low coverage demonstrating that this small adsorbate molecule can pick out small degrees of heterogeneity in the adsorption sites in the supercages of zeolite Nay. The heterogeneity is most probably associated with the Na+ ions sited in the SIII sites of the Nay framework where large electric field gradients exist. The isosteric heat then steadily increases by about 1.5kJmok1as the coverage increases due to adsorbateadsorbateinteractions. The large propane molecule shows only a steady but much larger increase in Q with coverage because of adsorbate-adsorbate interactions and there is no sign of the heterogeneity observed with ethane. Larger adsorbate molecules are usually found to be incapable of picking out heterogeneity which arises fiom cation sites. The heat of adsorption of propane is larger than that of ethane because of the additional CH, contribution to the dispersion-repulsion interactions. Both ethene and propene show much larger e values at low coverage compared to the heats for the saturated hydrocarbons, because of the additional electrostaticinteractions of the x electrons of the double bond with the electric field gmhents in the supercages of Nay. The trend in q, as a finction of coverage for propene is quite similar to that for ethane, i.e. an initial indication of site heterogeneity followed by a small, steady increase in 4, due to adsorbatsadsorbate interactions. The trend in Q for ethene is quite different. A small discontinuity in &t at -0.8 mmolgl i.e. 12 molecules per unit cell can be seen, which must be
204
J . A. Hampson and L. V. C. Rees
associated with SIII Na+ cations [3]. The trend continues with an almost constant value of Q of -36 klmol-1 followed by a sigdcant decrease in q, at higher loadings. The small ethene molecule seems to pick out heterogeneity in the supercages of Nay which saturated hydrocarbonscannot see.
45 40
B
35
3
30 25
20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 'B/mlgl
0.5
1 .o
I
iig. 3. Isosteric heats of adsorption as a finction of coverage for (0) ethane, (A] Shene, (0)propane, (V) propene on (a) NaY and (b) Theta-1 at 298K. decreases as shown in figure 3(b) for both ethane and ethene in Theta-1 in a similar manner which also resembles the initial behaviour of ethane in Nay. This decrease indicates that some heterogeneityexists wen in the pure silica channels of Theta-1. This heterogeneity is difficult to explain but presumably is associated with hydroxyl nests and other defect sites in the channels. This heterogeneityis smoothed out by the larger propane molecule. The heats of adsorption of the saturated hydrocarbons are significantly larger in the smaller channeSs of Theta-1 compared with the correspondtng heats in the large supercages of NaY as discussed previously. Ethene on the other hand has smaller isosteric heats in Theta-1 compared to NaY because of the lack of electric fields in the pure silica channels of Theta-1. The isosteric heats of adsorption obtained fiom the slopes of the isosteres at low loadings are in reasonable agreement with the AHKheats of adsorption as can be seen in Table 1 as would be expected. Experimental values of Q for ethane, ethene and propane in NaY measured by the isosteric technique are in excellent agreement with values quoted in the literature [4,5,6]. The isosteric system used in this study was designed to cope with binary mixtures. Because of the design when a dose of a binary mixture is admitted to the adsorbent the Q
Adsorption of Lower Hydrocarbons i n NaY and Theta-l
205
adsorbed phase composition remains sensibly constant across the temperature range of the isostere. Only the gas phase composition, which is analysed with an on-line mass spectrometer, and the equilibrium pressure, changes with temperature. From the resulting set of isosteres accurate thermodynamic data can be determined not only for the mixture but for each component of the binary mixture. Such analyses have been reported for the first time [2,7]. However, the determination of these binary mixture data is very time consuming and the ultimate goal is to develop a model for binaq mixture adsorption which would provide the extensive mixture data over the wide equilibrium pressure range which is necessary in the design of a P.S.A. separation unit. Such models are available e.g. Ideal Adsorbed Solution Theory, Vacancy Solution theory and Extended Langmuir models, and have been tested for their predictive accuracy [8]. However, in the use of such models it is necessary to have isotherm models which fit the two single component adsorption isotherms accurately over the wide equilibrium pressure range to be used in the P.S.A. process. We have therefore attempted to fit the accurate isotherm data shown in figures 1 and 2 with two isotherm models a) Langmuir-Freundlich, b) the Toth model [9]. The Langmuir-Freundlich equation is given by
n=
n,(K*P)f
and the Toth equation by n =
I+(K*P)
~
nm
where n is the amount adsorbed at the
(b+Pt)”
equilibrium pressure P and % is the saturation capacity of the adsorbent at the temperature of the isotherm and K’, b and t are adjustable constants. These isotherm equations were fitted to the experimental data using non-linear simplex routines developed by either Powell or Nelder-Mead [lo]. The goodness of fit was determined by measuring the Root Mean Square ofRelative Residuals (R.M.S.R.R.)given in eq.2.
R.S.M.R.R.(n,) = R,=
Jm] -
where M is the number of data points and P is the number of adjustable constants. AU of the isotherms in figure 1 and 2 have been fitted to the above models but only the parameters obtained from the fits of the ethane isotherms at 298K are presented in table 3. These data demonstrate the general features of the models. The Lanpuir-Freundlich equation gives excellent fits to both the Nay and Theta-1 low pressure isotherms and the predicted saturation coverages, n, are in reasonable agreement with experiment. Although the Langmuir-Freundlich equation still gives excellent fits to the
206
J . A . Hampson and L. V. C. Rees
high equilibrium pressure region and the complete isotherm for Theta-1 the corresponding fits for NaY are poor. The Toth equation gives a good fit to the high pressure region and the complete isotherm for Theta-1, the fit for the complete isotherm for NaY is again poor as well as the fit of the low pressure region for Nay. Table 3 : Application of Langmuir-Fremdlich and Toth equations to the Low, High and (Low+High) equilibrium pressure data for the adsorption of ethane at 298K in NaY and Theta1 Zeolites. Equation Low Equilibrium High Equlibrium Low + High Experimental Pressure Pressure platm) Equilibrium (
n, K* t (Qx100
5.105 0.017 1.173 0.71
1.229 0.083 0.850 0.02
4.507 0.017 0.976 15
1.390 0.086 0.724 0.07
4.338 0.02 1.118 39
1.175 0.576 0.693 1.4
4.5
1.33
-
1.405 2.974
4.367 95.38
1.195 0.697
4.5
1.33
0.659 0.05
1.116 45
0.585 0.8
Toth
n, b t (R,,dxlOO
9.781 81.80 1 0.885 2.95
The quality of these fits are more clearly seen in Figure 4 where the ethane isotherms for Theta-1 predicted by the Langmuir-Freundlich equation are compared with experiment; (a) low pressure region only, (b) total equilibrium pressure range. Isotherms predicted fiom the three sets of constants listed in Table 3 are included in Figures 4(a) and 4(b) The correspondingplots for the adsorption of ethane in NaY are given in Figures 5(a) and 5(b) respectively.
Adsorption of Lower Hydrocarbons in NaY and Theta-I
0
20
40
60
80 100 200
400
600
207
800
PressurekPa Fig 4. Predicted (Langmuir-Freundlich)and expermental isotherm data for ethane adsorbed on Theta-1 at 29813. (a) Low Pressure region, (b) High Pressure region. Constants fiom Table 3 fiom low pressure (--); High pressure (-); and Low + High pressure (-) isotherms.
0
20 40 60 80
0
1000
2000
3000
PressurekPa Fig. 5. Predicted (Langmuir-Freundlich)and expermental isotherm data for ethane adsorbed on NaY at 298K. Symbols same as fig. 4.
208
J. A. Hampson and L. V. C. Rees
CONCLUSIONS Subtle differences in the adsorption behaviour of saturated hydrocarbons in two zeolites with different pore geometries have been demonstrated. Large differences in Si/AI ratios of these two zeolites have only minor significance in the adsorption behaviour of these saturated hydrocarbons. However , when unsaturated hydrocarbons are the adsorbates the strong interaction of the x electrons of the double bond with the large electric field gradients which exist in the zeolite with a low SdAI ratio has been shown to introduce a large contribution to the adsorption potential. REFERENCES
1 L.V.C. Rees, P. Brueckner and J.A. Hampson, Gas Sepn. Purif., 5 (1991) 67. 2 J.A. Hampson and L.V.C. Rees, J. Chem SOC.Faraday Trans., 16 (1993) 0000. 3 J A . Hampson and L.V.C. Rees, in ( Proc. 4th Conference on the Fundamentals of Adsorption,, Kyoto, Japan, 1992), to be published. 4 A.G. Bezus, A.V. Kiselev and Pham Quang Du, J. Coll. Inter. Sci., 40 (1972) 223. 5 T.A. Egerton and F.S. Stone, J.Coll. Inter. Sci., 38 (1972) 195. 6 O.M. Dzhigit, A.V. Kiselev and T.A. Rachmanova, Zeolites, 4 (1984) 389. 7 M. Bulow and P. Lorenz in A.I. Liapis (Ed.), (Fundamentals of Adsorption II), Eng. Foundation, New York, 1987, p 119. 8 J.A. Hampson and L.V.C. Rees in E.F. Vansant (Ed.), (Proc. 3rd Symp. on Sep. Tech., Antwerp, Belgium, August 22-27, 1993), Elsevier, Amsterdam. 9 J.Toth, Acad.Sci.Hung.(Budapest)., 69 (1971) 31 1 . 10 W.H. Press, B.P. Flannery, S.A. Taikolsky and W.T. Vetterling, Numerical Recipes in Pascal., Univ. Press. Cambridge, 1989, p.326.
Determination of Sorption Thermodynamic Functions for Multi-component Gas Mixtures Sorbed by Molecular Sieves
M. Bulow The BOC Group Technical Center, 100 Mountain Avenue, Murray tiill, N.J. 07974-2064, U.S.A
ABSTRACT The isosteric technique with minimal dead volume of sorption cell allows experimental detennination of sorption equilibria of multi-component mixtures on microporous sorbents with high accuracy whereby calculation of the changes of thermodynamic fiinctions due to sorption is facilitated. The application is illustrated by measurements of sorption isosteres fcx ternary mixtures of argon, krypton and xenon, as well as of argon, oxygen and nitrogen on zeolites of types CaA and NaX, respectively. The corresponding sorption enthalpies, entropies and Gibbs’ free enthalpies were also determined for each system. These quantities were correlated with the factor of sorptive separation The success of the isosteric technique for mixtures with more than three components depends niairlly 011 the accuracies with which the sorption phase is prepared and the gaseous phase analysis is perfornied INTRODUCTION New processes of gas separation, c . ~ those . involving the PSA principle, require a knowledge of both rhe equilibrium and non-equilibrium sorption properlies of multi-component mixtures on poroiis solids [ 1-61, To investigate the sorption equilibria of mixtures, the isosieric method based upon the schemes proposed by Bering t’l cri. [7], as well as GroOniann rl a/. 181 was described in detail in [9]. Despite its potential, this method has only been used for binary mixtures [9-I I]. A thermodynamic analysis of the method has been given in [12]. The goal of the actual paper is to illustrate that the method can be utilized to determine sorption thermodynamic data for at least ternary mixtures, which were hitherto unexplored. METHOD The underlying principle, advantages and limits of utilization of the isosteric technique for adsorption of mixtures were described elsewhere [8- 121 The particular experimental setup included a sector field mass spectrometer (CH6, Varian) which had the following properties mass resolution up to 2000 a u , ( u ) strong reproducibility of spectra,
(I)
(111)
(IV) (1.9)
a 70 eV electron energy ion source, a reference inlet system, a differential pumping system and a secondary electron multiplier, high resolution at maximum of electron emission current (qualitative analysis), 209
210
M. Biilow
( v i ) linearity with respect to pressure and signal intensity ensured over six orders of magnitude for quantitative analysis, (vii) high oxygen concentrations are measurable without destroying the cathode.
For all the measurements, the results of which are shown below, the ratio of the sorption-vessel dead volume to the mass of zeolite crystals inside amounted to = 6.0 cm3/g. The zeolites used were laboratory-synthesized samples of CaA- and NaX-types having crystal diameters of 5 2 pm. For case of the CaA sieve, the cation exchange of Na' to Ca2+ was = 90 %. A predetermined mass of mixture with desired composition was dosed on the zeolite after calibration of the individual gas components in the analyzer, heating out the inlet system and activating the zeolite sample (during 8 .., 10 hours at 5 400 "C it7 I Y I L ~ N O ) . The dosing step is best done at low temperature, e.g that of liquid nitrogen. The temperature of loaded sorbent was controlled with an accuracy > 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-
Sorption Thermodynamic Functions for Gas Mixtures
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
Sorption Thermodynamic Functions for Gas Mixtures
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.
This Page Intentionally Left Blank
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,
Hydrophobic Zeolites
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"
Hydrophobic Zeolites
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
T. Masuda and K. Hashimoto
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-
T. Masuda and K. Hashimoto
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
Adsorption on Outer Surface of Zeolite
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-
Adsorption on Outer Surface of Zeolite
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<. A t low coverage (brP << l ) , the value of initial bN,) can be obtained. slope m = r(V For isotherms with appreciable curvatures, the parameters b and Ns can be obtained by least-squares fitting of the experimental data into Eq. 1 while keeping T and V as constants. Small variation in V for different MNaY has been proven to be negligible in the above treatment. Experiincntal data are given in Table 1 and fitted by solid curves in Fig. 1. The values of Ns = 15.6 0.5 for parent NaY was obtained from a second-order empirical fitting by using the Ns for MNaY zeolites (M = Rb, Cs, Sr, and Ba) which was plotted against the percentage of Na+ exchanged with a common intercept and appropriate weighting factors. For isothermal change which do not show appreciable curvatures, i t is straightforward to measure m, but impossible to obtain Ns from the experimental data, the values of Ns are assumed to be the same as that for parent Nay. For comparison, the monolayer capacity for Xe (diameter 0.44 nm) packing in the inner surface of a spherical supercage (diameter 1.3 nm) is about 15.3. Moreover, a rough estimate of the monolayer capacity through the relationship with the specific surface [7] obtained a value of Ns = 14.9. Therefore, for parent Nay, the adsorption of Xe is non-specific and probably unrelated to the presence of the Na+ ions: since there are only 4 Na+ in each supercage, it is unlikely for one Na+ to attract 4 Xe atoms due to the difference in their sizes. For ions having greater values of b than Na+ (Table l ) , e.g. Rb', CS+, Sr*+, and Ba2+, their ionic diameter [6] exceeds the aperture of the smaller cages (0.22 nm) and hence they can only be located in the supercages. Moreover, because the adsorption of Xe on zeolites is mainly carried by the van der Waals attraction, the increased polarizability of the electron-rich ions located in the supercages would favor the adsorption of xenon atoms and increase of b. Therefore, we suggest that the strong adsorption of xenon on MNaY zeolites ion-exchanged with the large ions may be site-specific, with a 1-1 correspondence to the number of cations replacing Na+. With less than complete substitution, the observed values of Ns and b would be a weighted
+
T a b l e 1 : Data obtained fran the Analysis of Adsorption Isotherms and lgSXeEMR Chemical S h i f t s for Xenon Adsorbed on Z a l i t e Nay w i t h Na+ Partly Replaced by D i f f e r e n t Ions.a
~a
H
Li
K
Rb
cs
MS
ca
Fi/A1=2.49 15.6(0.5)1 Si/A1=2.57 15.6(0.5) 27 44 60 77
c
14 32 59
C
c
C
c
3.88( 0.02) 3.94(0.02)
7.05I0.03) 7.65( 0.03)
56.9 58.6
14.78 15.55
3.50 ( 0.02 ) 3.22 (0.04) 2.99( 0.04) 2.76(0.03)
6.60(0.03) 6.25(0.06) 5.80(0.07) 5.36(0.05)
59.5
14.78 14.79 15.28 14.88
3.81(0.01) 3.77( 0.01 ) 3.15(0.03)
7.41( 0.04) 7.34 (0.05) 6.12( 0.06)
61.3
61.6 63.3 58.5 57.8
56.1
14.41 14.45 15.52
27 48 65 97
[14.3( 1.0) ] [ 13.9( 0 . 6 ) ] [ 13.5( 1.0) ] 12.9( 0.6)
5.4(0.2) 6.5(0.21 7.7(0.2) 9.7(0.2)
11.q2.1) 14.2( 1.6) 18.4(2.2) 28.0(4.5)
64.6 68.9 71.0 76.9
13.75 12.85 12.19 10.57
17 40 69
[14.6(1.0)]" 12.4( 1.1) g.l(l.4)
5.4(0.2) a.O(O.2) 13.9(0.2)
11.5(1.7) 23.4(2.9) 64.1 ( 7.8 )
66.8 76.9 87.4
14.07 12.61 11.25
23 41 56 66 68
[ 13.1( 0.6) Id
6.5(0.2) 8.0(0.2) 12.1 (0.2i 18.1( 0.2) 19.3(2.0)
15.6(0.9) 24.q2.4) 58.6(3.2) 83.4(6.7) 103.2(9.0)
76.5 88.9
14-05 13.27 12.82 10.91 11.73
44 56 61 67
38 61 65 71
11.8(0.8) 8.2(0.2) 8.3(0.3) 7.8(0.3) e c
[ 12.1( 1.0)1" [ 11.8( 1 .O) ] 1'
8.8( 1.0) 6.8(1.3)
2.85(0.02) 2.82(0.03) 3.27(0.03) 2.69(0.03) 3.4(0.1) 3.5(0.1) 3.3(0.1) 4.4(0.2)
5.53( 0.03) 5.47( 0.03) 6.35(0.06] 5.21 10.06)
9.0( 1 . 0 ) 10.0(1.0) 12.1( 3.3) 20.7(5.4)
105.2
121.4 125.8
63.1 11.30 62.6 12.34 69.1 3.01 71.8 -2.28 58.1 70.8 90.1 114.2
--
----
Sr
ea
41 63 75
10.7( 1 . S ) 6 . 7 ( 1.3) 5.4(0.8)
41 51
7.8(0.3) 6.9( 0.3) 6.4(0.2) 5.9(0.1) 6.0(0.3)
--
66
--
68 76
------
co
Ni
--
-
---3.04 3.05 4.19 8.53
13.40 0.63 -0.63 4.98 -25.77 15.23 -32.42 12-52
cu
Zn
3.7(0.1) S.E(O.5)
6;3(0.3) 6.1I0.2) 8.7(0.5)
lO.B(O.5) 11.5(0.7) 13.2(0.2)
11.2(2.4) 23.3(6.1) 39.1(8.7)
67.7 98.3 113.4
28.0(1.7) 43.8(3.2) 61.5(3.3) 75.2(3.3)
112.2 118.3 123.1 124.6 123.9
88.9(6.4)
4.36
3.89
-8.92 -7.87
5.07 3.72
5.11
-
6.55 7.29 7.24
-
3.88(0.04) 3.83(0.02) 3.72(0.03) 3.69(0.03) 3.67(0.01)
7.54(0.03) 674.8 -414.89 7.45(0.03) '1199.5 -784.65 7.23(0.05) 1055.9 -706.37 7.17(0.04) 968.0 -640.88 7.13(0.06) 1156.8 -724.05
3.79(0.02) 3.81(0.03) 3.68(0.02) 3.49 ( 0.03 ) 3.64(0.03] 3.58(0.02)
7.36(0.02) 7.41(0.03) 7.15(0.03) 6.79(0.05) 7.08(0.03) 6.%(0.02)
6 13 28 35 49 57
3.74(0.03) 3.66(0.03) 3.51(0.02) 3.35(0.03) 3.21(0.03) 2.97(0.03)
7.28[0.03) 7.11(0.04) 6.82(0.04) 6.50I0.03) 4.23110.02) 5.75(0.06)
59.6 56.6 51.7 51.1 47.6 45.9
14.35 15.04 15.86 16.39 17.15 18.60
6 11 29 36
3.75(0.02) 3.67 (0.04 ) 3.48(0.01) 3.40(0.01) 3.46(0.02) 3.60( 0.04 )
7.29[0.02) 62.7 7.13(0.03) 61.8 6.76(0.03) 73.1 6.60[0.02) 76.2 6.71(0.05) 87.5 6.99(0.12) 108.7
12.02 12.60 4.48 2.79 -5.10 -16.49
6 11 26 43 57
6 11 27 34 42 51
c c c
c
c
41 56
c
--
5.68
115.4 -16.07 136.6 -25.15 145.6 -32.01 137.5 -23.90 169.4 -51.30 380.1 -176.80
80.90
152.10
142.73 130.64 142.71
5.63 7.05 8-%
7.78 15.27 37.05
--
-I
0.45 0.36 2.16 2.53 4.13 5.34
value of Ns,b,and m are obtained by n o n - f i ~ leastsquare f i t t i n g of 4.1. The standard errors in the parentheses folloklirmg by regrssional f i t t i n g of wch data were obtained from the least-sqwe calculations. The values of g o , C, and Cz are the qrimental r e s u l t s t o m.5 frm a second4qree polynomial. bFrm the extrapolated data; see text. cAssumd to be 15.6 atun cage-'. dInterpdated values.
S. B. Liu, C. S. Lee, P. F. Shiu, and B. M. Fung
236
M Or6
?5
2
1
./5
4
B
4
f 3 v)
-0
m2
e,
CI 11
E
4 0
- .
0
200
%51
(b)
400
600
Pressure, torr
800
1000
0
200
400
600
Pressure, torr
800
1300
Fig. 1. Room temperature adsorption isotherms of xenon adsorbed on NaY zeolites with Na' partly replaced by various monovalent cations (a), group IIA divalent cations (b), and transition-metal cations (c). The number indicates the percentage of Na' exchanged. The solid curves were calculated by using Eq. 1; the dashed lilies are empirical straight lines. -. 0
200
400
600
Pressure, torr
800
1000
averages for the strong and weak adsorptions. The overall trend' for the adsorption strength b is: Ba > Cs 2 Rb > K 2 Sr > Ca > Na ? Co 2 Ni 2 Li 2 Zn > Cu N H > Mg. In fitting our data with the Langmuir model, no attempt has been made to distinguish between weak (nonspecific) and strong (site-specific) adsorptions. Experimentally, it is difficult to separate these two types of contributions. Nevertheless, the present study of the adsorption isotherms based on this simple model bears important implications in the interpretation of the 129Xe chemical shift data.
chemical shifts It is known that, for NaY and its homologs with Na' being substituted by various monovalent cations, the 1*9Xe chemical shifts vary linearly with xenon loading [ 2 4 ] and that the slopes of chemical shift depend on the type and the extent of cation substitution (Fig. 2a). For divalent cations, the dependence of the 129Xe chemical shift on xenon loadi?g is not linear, except for Ba (Fig. 2b) and Cu (Fig. 2c). Ito and Fraissard [ 2 4 ] used the following expression to explain the dependence of 129Xe chemical shifts on the replacement of Na' in NaY by various ions: I2%e
Xenon Adsorption Isotherms and Xe-129 NMR
237
where 60 is the reference, 15, corresponds to the shift at zero Xe loading, Cgpg is the Xe-Xe interaction term in the gaseous phase, 6, is due to the "electric effect" causes by the cations, and 6, is due to the "magnetic effect" of the paramagnetic ions. The equation can only give qualitative features about the complex dependence of the 129Xe chemical shift on the type of cation and the level of Na+ exchanged. More recently, Cheung et al. (51 suggested that the observed 129Xe chemical shift is actually a weighted average between those of two species in rapid exchange, one adsorbed on the wall of the supercage, and another staying in the middle of the supercage as gaseous Xe:
3 a
O
0 120-
L
1
2
3
2
3
4
5
0
1
0
1
O
2
3
c 2
3
L
4
0
'i 0
1
2
3
4
5
Xenon loading. atom/cage
6
6
100
80
60. P 120
0
1
O
L 4 4
I 4
Xenon loading, atom/cage
LOO 80
-460
P --a 60
0
1
2
3
Xenon loading, atom/cage
1
Fig. 2. '"Xe chemical shifts of xenon adsorbed on NaY zeolites with Na+ partly re laced by various monovalent cations (a!, group IIA divalent cations (b), and transition-metal cations (c). The number indicates the percentage of Na+ exchanged. The solid curves were calculated using, Eq. 5; the dashed lines are empirical straight lines.
238
S.
B. Liu, C. S. Lee, P. F. Shiu, and B. M.
Fung
where os is the chemical shift of the adsorbed Xe atoms. By considering that the chemical shift of gaseous Xe, og , changes linearly with its density (at low xenon loading) [9], i.e. ug = Cgpgr they were able to derive an equation by which they explained why some of the chemical shift plots are linear, but some others are curved (Fig. 2). However, no quantitative treatment of the experimental data was given. Besides the lack of quantitative data, there is discrepancy between the above mentioned models and experimental results. Both groups of authors suggested that systems with weak adsorption strengths lead to a linear dependence whereas those with strong adsorption strengths yield parabolic plots. However, four of the five systems with the largest value of b, I<+, Rb+, Cs+ and Ba2+, give essentially linear plots (Fig. 2). On the other hand, most systems showing upward concave curves (Ca2+ in Fig. 2b and Co2+, Ni2+ and Zn2+ in Fig. 2c) have very small values of b (Table 1). Obviously, a different approach must be sought so that the data can be interpret and be consistent with results of the adsorption isotherm. Theoretically, the experimental data of u vs N can be fitted to Eq. 3 by the use of non-linear least-squares calculations with two variable parameters ( os and ug) while the values of and Ns were held constant. For parent N a y , Cg = 20.7 ppm nm3 atom-' at 295 K [9]. The remaining parameter Ns and its correlation with b can be obtained from the adsorption isotherms (Table 1). However, nearly all of the experimental data failed to converge when we tried to fit them by Eq. 1. Detailed examination showed that the failure arised from the imbalance i n the relative contributions of ug and us: in these systems the term involves ug contributes only 5 1% of the observed chemical shift (a), while the dominant contribution of the 129Xe chemical shift in the system is due to as.
ss
The virial exnansion model In order to obtain a more quantitative interpretation of the observed *29Xe chemical shift ( ~ 7 ) we ~ developed a new model which treats the adsorbed xenon as a twodimensional gas. If the major force responsible for the adsorption of Xe in zeolite is due to van der Waals interaction, the adsorbed Xe can change their locations by moving around the surface of the supercage or by exchange with other gaseous Xe. Then, there exist at least two types of adsorbed Xe: one in direct contact with the cage wall of the zeolite and the other not in such direct contact (cf: Fig. 3). If the number of gaseous Xe atoms and the two types of adsorbed Xe atoms are denoted by Ng, Nd, and Nil respectively, the total number of adsorbed Xe in each zeolite supercage is N = Nd + Ni Ng, and the observed chemical shift can be written as o = fdod fioi hog N (1 - fi)cTd )oil where fd, fi, and fg refer to the fraction of the directly adsorbed, indirectly adsorbed, and gaseous Xe, respectively. For simplicity, the term hog will be neglected because its contribution to the observed chemical shift ( 5 1%) is negligibly small within the numerical uncertainty of the treatment. The fraction of indirectly adsorbed Xe, fi, is expanded a.s a Taylor series of the surface coverage:
+
+
+
+
Xenon Adsorption Isotherms and Xe-I29 NMR
h
=
Pd-)
N
N
+ P2(-)2
Ns
NS
+ ....,
239
(4)
where the p’s are the coefficients of the expansion. Also, we would express the 129Xe chemical shifts for two types of adsorbed Xe as virial expansion similar to that used by ..., and Ci = Jameson et ai. 191 for gaseous Xe as follows, fJd = uo + ( t N + (zt\lL ( I N (z@ + ..., where go, as before, denotes the effect of the zeolite surface on the chemical shift of the directly adsorbed Xe in absence of Xe-Xe interactions, and (1 and (2 are constants. Hence, we have
+
+
u =
00
+
C1N
+
C2M
+ ..”,
(5)
Since the existence of more than two adsorbed layers of Xe is unlikely due to the limited size of the MNaY supercages, we shall not consider the higher order terms which involve mutual interactions of more ttian three xenon atoms. In this context, we shall focus on the two virial coefficients C1 and C2, which represent coefficients of binary and three-particle Xe-Xe interactions, respectively. For a quantitative analysis, regressional fitting of the experimental values of u to Eq. 5 by a second-order polynomial was performed. The results for no, CI and C2 are listed in Table 1 for each MNaY system; they are also shown in Fig. 2 as solid curves. Because the plots for monovalent ions Ba and Cu are essentially linear, their C2 values were taken as zero. Since we treat the adsorbed Xe as a twowdiiiensional gas, CI and CZ are expected to be related to b. However, because the directly and indirectly adsorbed Xe atoms interact with the surfxe of the supercage differently and there is no direct experimental method to distinguish between them, the value of b obtained from each adsorption isotherm fitting must be considered as a weighted average of the two types of &orbed xenon atoms. For a Schematic diagrams showing Pig. 3. the two types of adsorbed Xe atoms, quantitative analysis of Cl and CZ,we namely the directly adsorbed (open circle introduce two empirical relations for the and the indirectly adsorbed (full circle coefficients P in Eq. 6, such that Xe.
S. B. Liu, C. S. Lee, P. F. Shiu, and B. M. Fung
240
where 71, 72, p, and v are constants. By substituting Eq. 7 into Eq. 6, we have
Then, the values of CI and C2 were fitted independently (Eq. 8) by the use of nonlinear leastsquares calculations with three variable parameters each; 51, 71, and p for the former, and (2, 72, and Y for the latter. The remaining parameters in Eq. 8, uo, b, and Ns were held constant for each MNaY sample. Since only 2 to 6 different exchange levels are available for each MNaY sample, a non-linear least-square fitting of each MNaY is not practical. Therefore we proceeded the fitting by grouping different exchangeable cations listed as group A-I in Table 2. It is interesting to note that the values (1, 71, and p obtained for M2+ ions is in general greater than that for M+ ions. When all the C1 data are taken together (group F), we obtained the values of (1 = 58.4 * 2.0, 71 = 36.2 * 2.4, and /-I = 0.6. Similarly for group I in which all of the MNaY samples having lion-zero C2 values are pooled, we obtained values for (2 = -9.3 f 1.0, 72 = 4 3 8 . 7 f 82.9, and Y = 1.3 f 0.1. In view of the fact that the types of systems studied were very comprehensive, the range of the chemical shifts covered was very wide, and a single equation with only three variable parameters was used for all systems, the quality of the fitting can be regarded as rather satisfactory.
Table 2 :Data Obtained from Non-Linear Least-squares Calculations of !3q. 8 for Groups of MNaY Sarrples Having V a r i e d cams3n Propertiesa. Groupb
5 1
U
7 1
ppn cage atan-'
f z
7
A
15.2(2.3)
0.2(0.7)
-0.10(0.82)
--
-
B
16.7(2.7)
0.6(1.3)
0.07(0.41)
-
C
51.3(6.6)
34.8(5.6)
0.66(0.03)
-
D
62.9(4.6)
5.1(4.2)
-0.39(0.41)
--
E
63.8(3.2)
37.9(3.4)
0.62(0.05)
--
58.4(2.0)
36.2(2.4)
0.60(0.03)
-
___ ---
___ __
-
F G
H I
-___ --
S.D.R.C
U
2
ppm cage
__ __
1.47
-
5.75
__ -__
-11.8( 3.3) -743.3( 192.7)
1.30
14.58 12.87 11.53
1.43( 0.06)
2.30
-
-9.5(1.3)
-48.3(53.5)
O.lg(0.56)
3.88
--
-9.3(1.0) -438.7(82.9)
1.30(0.10)
3.71
remaining parameters g o , b , and Ns can be &bindfrm Table 1. bThe assignments of each group are: Qoup A:(Na, H, L i , K, Rb, Cs); Group B:(Group A + Cu); Qoup C:(Mq, Ca, Sr, Ba); Group D:(Cn, Ni, Zn); Group E:(Qoup C t Group D); Group F:(Qoup A t Qoup C Qoup G:(bQ, Ca, Sr); Group H:(Co, Ni, Z n ) ; Group I: (Group G + Group H) CStandard deviatim of residuals obtained.
%e
.
t Qoup
D
+ Cu');
Xenon Adsorption Isotherms and Xe-129 NMR -30
o Na v
K
A
Rb
A
-10
10
241
30
Sr 0 CU 10 Zn
OBaO
- 3
u
i0 I
-301 -601 -901
00-600-300
0
t
Fig. 4. Plots of the C;; vs Cl lor ~ ~ O U J FN (left) and that of vs C2 for groups I (right). The solid line scross the diagonal axis i n each plot represents a perfect match between the fitted and calculated vslues.
In order to further judge the quality of the fitting, the fitting results listed in Table 2 were substituted into Eq. 8 to obtained the calculated values of Ci and Ci for each MNaY samples together with each value of go, b, and Ns (Table 1). The plots of CI vs C; and CZ vs for various groups (Table 2) of MNaY having similar chemical shift and/or adsorptive properties are depicted in Fig. 4. Although the data in Fig. 4 exhibit certain fluctuations, the fact that most of them fall near the diagonal line indicates a good correlation between the fitted and calculated values. Although there are limitations in the virial expansion model, it gives a meaningful quantitative description of a large number of systems and sheds new light on the nature of 129Xe chemical shifts of adsorbed xenon in zeolitic systems.
4
REFEItENCES 1 J. Reisse, Nouv. J. Chim., 10 (1986) 665.
2 3
4 5 6
7 8 9
J. Fraissard and T. Ito, Zeolites, 8 (1988) 350; and J. Chem. Ph s. 76 (1982) 5225. P.J. Barrie and J. Klinowski, Prog. NMR Spectro., 24 (19927 91; and reference therein. Q.J. Chen and J. Fraissard, J. Phys. Chem., 96 (1992) 1809; and references therein. T.T.P. Cheung, C.M. Fu and S. Wharry, J. Phys. Chem. 92 (1988) 5170. S.B. Liu, L.J. Ma, M.W. Lin, J.F. Wu and T.L. Chen, J. Phys. Chem., 96 (1992) 8120; and references therein. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1967, chapter 2. R.D. Shamron and C.T. Prewitt, Acta Crystallogr., B25 (1969) 925. C.J. Jameson, J. Chem. Phys., 63 (1975) 5296.
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Adsorption of c8 Aromatic Isomers on Faujasite Zeolite
K. Iwayama and M. Suzuki
Chemicals Research Laboratories, Toray Industries,Inc., 9-110e-cho, Minato-ku, Nagoya 455, Japan ABSTRACT The adsorption of para-xylene in C8 aromatic hydrocarbons was studied on cation exchanged faujasite zeolite. The effect of the basicity of aromatic hydrocarbons, ionic potential of exchanged cations, and the packing of C8 aromatics in the cavity of zeolite was examined. Paraxylene was adsorbed more selectively with decreasing ionic potential of cation,while para-xylene was strongly subject to the restriction of packing in the cavity of zeolite compared with other C8 aromatics. The adsorption selectivity was determined from the balance of the basicity of adsorbed components and the restriction for the packing of component molecules in the cavity of zeolite. INTRODUCTION Para-xylene is an important raw material for terephthalic acid used asa monomer of polyester. Each CS aromatic hydrocarbon has a similar boiling point. In particular, the difference between the boiling points of para-xylene and meta-xylene is less than one degree centigrade. At first, the crystallization process was indusmally developed to separate Cs aromatic components taking advantage of the difference in their melting points. Recently, an indusmal adsorptive separation technique using zeolite adsorbents has been industrially developed [ 1-31. Para-xylene is economically recovered from Ca aromatics through selective adsorption on zeolites and the recovery of para-xylene can reach 100%. The selectivity in the adsorption of para-xylene is dependent on the characteristics of adsorbent. Faujasite typz zeolite is used industrially as an adsorbent because of its large capacities of adsorption and ion exchange. A variety of researches have been published on xylene adsorption [4,5]. In this research, the selective adsorption of para-xylene in Cs aromatics on faujasite zeolite was studied to clarify the effect of exchanged cations, i.e.,their ionic potential, size and concentration. EXPERIMENTAL Sample preDarah‘on Adsorbents, 3.2NaY(sodium form faujasite type zeolite with SiOZ/A1203 molar ratio 3.2), 4.8NaY and 5.5NaY, were molded with A1203 sol binder. The adsorbents were dried at 120°C overnight, then calcined at 500°C in air for 1 h. The calcined NaY moldings were ion-exchanged with an aqueous solution of metal nitrate(5wt% as metal) at 80°C. KY with various K+ content(5.5K-H-Y) was prepared by ion-exchangeof 5.5KY with an aqueous solution of ammonium chloride(5wt% as NH4Cl) and calcined in air at 500°C for 1 h to convert NH4+ into H+.The contents of metal ions 243
244
K. Iwayama and M. Suzuki
in 4.8Y and 5.5K-NH4-Y are shown in Tables 1 and 2, respectively. In Table 1, the decrease of BET surface area from Li ion-exchanged Y to Cs ion-exchanged Y is correspondent with the increse of adsorbent's weight induced by atomic weight of alkali metal. Table 1. Cation contents in 4.8Y with 10wt% of A1203 sol binder.
a) Determined by atomic absorption spectrophotometry. The numbers in parentheses are values re-calculated from Na+ and metal ion per unit cell. b) unit cell : (Na,M)s6. [(A102)56 (Sio2)136].
-
Table 2. Cation contents in 5.5K-Na-NH4-Y as a precursor of 5.5K-H-Y adsorbents, and number of cations per unit cell.
a)Determined by atomic absorption spectrophotometry(K+ and Na+) and Kjeldahl method(NH,+). b)Determined by Si-MAS-NMR.
Definitions of a d s m h'on selectivitv and adsomtion c
im
Adsorption selectivity and adsorption capacity are representative characteristics of adsorbent. Adsorption selectivity for component A to component B is defined by the following equation.
Adsorption of C, Aromatic Isomers on Faujasite Zeolite
245
In this equation, (A/B)s is the weight ratio of A to B in adsorbed phase and (A/B)L is the weight ratio of A to B in non-adsorbed phase under the adsorption equilibrium condition. As a prerequisite for measuring the adsorption selectivity, it should be noted that the adsorption of nnonane, used as an internal standard, is negligible in the presence of aromatic hydrocarbons. According to this prerequisite, the adsorption capacity (C) is calculated from the following equation under the adsorption equilibrium.
F C=$l-
2)
where F, W and R are as defined in the nomenclature at the end of this paper. The adsorption capacity of each component is calculated using a similar procedure. The adsorption selectivity for A to B is calculated from the following equation.
When am is more than 1, A is adsorbed more selectively than B. When a A B is 1, A and B are equally adsorbed, that is, the adsorbent does not have any selectivity between A and B. Adsorption measurement Adsorption measurement was conducted by the following method. At fust, exactly weighed feedstock and adsorbent were put into a micro autoclave with a capacity of 5ml. The sealed autoclave was kept in an oil bath at 60°C. It was confirmed that the adsorption equilibrium was reached when the adsorption time was more than half an hour at 60°C. Therefore, the raffinate was sampled from the autoclave after 1 h. Feedstock and raffinate were analyzed by gas chromatography. N -nonane was used as an internal standard material in feedstock. To check the validity of this evaluation, the adsorption capacity of 4.8 NaY without A1203 binder was measured using a mixture of n-nonane 30wt% and para-xylene 70wt% at 60°C as feedstock. The adsorption capacity of 4.8NaY was 24.3wt%. On the other hand, the adsorption capacity was calculated by the following procedure. The volume within the supercage of 2.5NaX is 822A3[6]. The lattice constants of 2.5NaX and 4.8NnY are 24.932 and 24.681A. respectively[7]. From these data, the calculated void volume of 4.8NaY is 0.301 cclg. As the density of para-xylene at 60°C is 0.822 g/cc, the calculated adsorption capacity is 24.7wt% and the value is coincident with the experimental value. The effect of nnonane concentration in feedstock on the adsorption capacity was examined in the range from 30 to 50wt%. The adsorption capacity was hardly affected by the n-nonane concentration. The effect of A1203 binder in the adsorbent was also studied. It was confirmed that only 0.5wt% of the total adsorption capacity was due to the adsorption on the A1203 binder, so the influence of the A1203 binder has been omitted from the following discussion. RESULTS AND DISCUSSION .. Effect ion on -tion selectivu The effect of alkali metal ion in 4.8Y on the adsorption selectivity is summarized in Table 3.
246
K. lwayama and M. Suzuki
Table 3. Effect of metal ion in 4.8Y zeolite on adsorption selectivitya) for para-xylene Adsorbent 4.8LiY 4.8NaY 4.8KY 4.8RbY 4.8CsY
Cb)(wt%) 23.3 22.6 20.8 16.9 15.6
aPX/EB 1.52 2.02 1.75 0.88 0.58
aPX/OX 0.68 0.82 3.70 1.68 1.oo
aPX/MX 0.39 0.34 3.66 2.33 1.47
The adsorption selectivity for para-xylene to ortho-xyleneand me&-xylene was enhanced from Lit to K+. Ionic potentials(ion valence /ionic radius) of alkali metal ions are as follow.
Ionic potential(A-1)
Li+ 1.67
Na+ 1.05
K+ 0.75
Rb+ 0.68
Cs+ 0.59
The basicities of aromatic hydrocarbons are as follow [4].
Relative basicity
PX 1.OO
EB OX 1.06 1.13
MX 1.26
The ionic potential of alkali metal ion decreases from Li+ to Cs+. AlOd-, which constitute the zeolite structure, are affected by alkali metal ions. The interaction between A104-and alkali metal ion increses the electron density of adsorption site from Li+ to Cs+. As a result, para-xylene, which is less basic, is adsorbed more selectively than the other Cs aromatic isomers on 4.8KY. However, the adsorption selectivity of para-xylene decreased inversely from K+ to Rb+,Cs+,which suggests that there is another factor controlling the adsorption selectivity of para-xylene in the case Of 4.8RbY and 4.8CsY. From the adsorption measurements. the number of adsorbed molecules per supercage was calculated and plotted against the ionic radius, as shown in Fig. 1. The amount of para-xylene adsorbed per supercage decreased from KY to CsY. It is assumed that the packing of para-xylene in the supercage may be hindered sterically by Rb+and Cs+,which have larger ionic radii than K+. Effect of SiOdAlz03 ratio in fauiasite zeolite on adsorption selectivity The adsorption selectivities of KY having different SiWAlz03 ratios were investigated. The results are shown in Table 4. The number of alkali metal ions per unit cell increases with decreasing SiOdA1203 ratio. From Table 4, 3.2KY showed a smaller amount of para-xylene adsorbed per supercage than 4.8KY and 5.5KY. Using 3.2Y and 4.8Y adsorbents having different alkali metal ions, the adsorption results of each isomer of Cs aromatics are shown in Fig 2.
Adsorption of C, Aromatic Isomers on Faujasite Zeolite
247
Fig. 1 Effect of metal ion in 4.8Y on the amount of adsorbed molecules Adsorbent 4.8Y(A1203 lOwt%), feedstock n-C9 : EB : PX :OX : MX = 4 : 1 : 1 : 1 :1, adsorption temp. 60't3, S.C. : supercage. (0) : EB, (e): PX, (0) : OX, (A) : MX, (.) : Total
Table4. Effect of SiOz/A1203 molar ratio in K form zeolite on the amount of adsorbed molecules Adsorbent 3.2KYa 4.8KYb 5.5KYC
Total 3.34 3.49 3.67
Adsorbed molecules I S.C. EB PX ox 1.02 1.03 0.72 1.07 1.48 0.46 1.11 1.64 0.43
MX 0.57 0.48 0.49
Figure 2 shows that in the case of 3.2Y, the amount of para-xylene adsorbed is low on all kinds of metal ion-exchanged Y compared with ethylbenzene and ortho-xylene. On the other hand, in the case of 4.8Y, the amount of para-xylene adsorbed is higher than other c8 aromatic isomers, but decreases suddenly on RbY and CsY. From these results, the packing of para-xylene in supercage is hindered more by alkali metal ions, especially Rb+, Cs+, than that of other cs aromatic isomers. Therefore, it is suggested that there be a proper amount of K+ in faujasite for para-xylene adsorption. -+-
.. -Na-y o n o n =lectlvltY 4.8K-Na-Y adsorbents with different amounts of K+ were prepared by ion-exchange method.
248
K . lwayarna and M. Suzuki
In Fig. 3, the amounts of Cs aromatic isomers adsorbed changed with the amount of K+per unit cell. Sodium ions, which exist on Se sites in supercage accessible by molecules adsorbed, may be exchanged with K+ at first. As a result, it is considered that the amounts of para-xylene and ethylbenzene adsorbed are increased with K+ion-exchange. The amount of para-xylene adsorbed with increasing the amount of K+from 0 to about 40. 4.5
4.0
3.5
-
PX 14.8Y
-
3.0 Na 2.5 I 0.5
I
' 0.7
0.9
K .
,
.
1.1
,
1.3
Rb .
t
cs .
1.5
,
1.7
Ionic radius (A) Fig. 2. Effect of metal ion in 4.8Y and 3.2Y on the amount of each C8 isomer adsorbed. Adsorbent 4.8Y(Alz03 lOwt%), 3.2Y(A1202 13wt%),feedstock n-C9 : each C8 isomer = 1 :1, adsorption temp. 60 C. @I, (0): PX, a), 0 :EB. W, 0:OX.
c 0
10 20 Amount of
30 40 50 (number / unit cell)
60
' in 4.8K-Na-Y zeolite Fig. 3. Effect of the amount of K on the amount of adsorbed molecule Adsorbent 4.8K-Na-Y(A1203 10wt%), feedstock n-C9 : EB :
PX : OX : MX = 4 : 1 : 1 : 1 : 1, adsorption temp. 6OoC,
-
unit cell :(K,Na)s [(A102)56.(Si02)136]
Adsorption of C, Aromatic Isomers on Faujasite Zeolite
249
Effect of the amount of K+ in 5.5K-H-Y on adsorptlo * n se1eca'vi tv Adsorbent 5.5KY was ion-exchanged with ammonium chloride aqueous solution to decrease the amount of K+ ion, then calcined in air at 500OC for 1 h to convert NH4+ into H+. When the amount of K+ ion was decreased from 5.5KY(K+/u.c.=49), the adsorption selectivity for paraxylene increased slightly to reach maximum at K+/u.c.=47. Further decrease in the amount of K+ resulted in a decrease in the adsorption selectivity to approach unity. H+, having larger ionic potential, may interact with A104- strongly, and the electron density of the adsorption site may be reduced. Therefore, the adsorption selectivity decreases when K+ is replaced by H+.
8
r#;3
.. .. 0
..............................................
10 20 30 40 Amount of K+(number / unit cell)
Fig.4. Effect of the amount of K' zeolite on adsorption selectivity
50
in 5.5K-H-Y
Adsorbent 5.5K-H-Y(AIz03 low%), feedstock n-C9 : EB :
PX :OX :MX = 4 : 1 : 1 : 1 : 1, adsorption temp. 6OOC. unit cell :(K,Na)5i
[(AIOz)5i.(Si02)141]
CONCLUSIONS The adsorption selectivity for Cs aromatic isomer on faujasite zeolite depends on the electron density of adsorption site, which is affected by the interaction between metal ion and A104-. The adsorption selectivity for para-xylene, which is less basic, increases with decresing ionic potential of exchanged cations in faujasite, that is, the selectivity increases in the order LiY, NaY and KY. Change in the packing of para-xylene in zeolite pores affects the adsorption selectivity. As Rb+ or Cs+ has a large ionic radius, so the packing of para-xylene in supercage is restricted by the ion, which decreases the adsorption selectivity for para-xylene. The adsorption selectivity for para-xylene on KY depends on its SiOZ/A1203 ratio. As the SiOZ/A1203ratio increases from 3.2 to 5.5, the amount of metal ion in the unit cell decreases and the packing of para-xylene in the supercage becomes easier, enhancing the selectivity for paraxylene.
250
K. lwayama and M. Suzuki
NOMENCLATURE F: weight of feedstock (g) w weight of adsorbent (g) F9 concentration of n-nonane in feedstock (wt%) concentration of n-nonane in raffinate (wt%) R9 FA,FB: concentration of component A or B in feedstock (wt%) RA,RB: concentration of component A or B in raffinate (wt%) ACKNOWLEDGMENT We are grateful to Professor T. Yashima, Department of Chemistry,Tokyo Institute of Tecnology, for helpful advice. REFERENCES 1. USP3,558,730(1971), assigned to UOP. 2. USP3,761,533(1973), assigned to Toray. 3. USP4,069,172(1978), assigned to Toray. 4. D.Barthomeuf, A.de Mallmann, Ind. Eng. Chern. Res.,29 (1990) 1435. 5. A.de Mallrnann, D.Barthorneuf, Proceedings of the 7th International Zeolite Conference, Kodansha, Elsevier. (1986) 609. 6. M.M.Dubinin et.al. Izv.Akad.Nauk.SSR,Ser.Khim.. (1962) 760. 7. D.W.Breck, Zeolite Molecular Sieves, John Wiley & Sons, (1974) 94.
Study on a New Humidity Controlling Material Using Zeolite for Building
Akio Sagael, Hiroki Wami', Yoshinobu h a i l , Hiroshi Kasai', Tetuo Sate* and Hiroshi Matumoto2
* Kajima Technical Research Institute, Tobitakyu, Chofu City, Tokyo 182, Japan
* Shin Tohoku Chemical Industry Itd., Kamisugi, Aoba Ward, Sendai City, Miyagi 980 Japan ABSTRACT The authors have been examining a method of using natural mordenite zeolite as a solid absorbent for the building industry. Zeolite can be used as (1)a humidity controlling material for building, (2)a heat accumulating material and (3)a passive cooling system. Development of (1)is now under way. Mortar is mixed with natural zeolite as a substitute for sand and made into molded mortar panels or plastered walls. Exhibiting physical properties equal to those of wood, zeolite-mixed mortar has a larger moisture absorbingldischarging capacity than wood materials. Such panels can be used not only for storage warehouses and art museums but for homes and production facilities. These panels will be incorporated in a complex system as part of the technology for controlling indoor temperature and humidity environment. INTRODUCTION Excluding glass, metal plates, plastic plates, etc., all construction materials exhibit moisture absorbing/discharging property to some extent. Of various types of construction materials, those with especially great humidity controlling capacity are shown in Table 1. According to the grouping of building interior finishes in terms of moisture absorbingldischarging properties, materials with a higher rate of water vapor diffusion and large absorbing quantity are excellent in controlling humidity. Both wood and wood materials exhibit such tendencies. Excellent characteristics of wood are highly evaluated. However, wood is subject to deterioration with age due to its moisture absorbingldischarging property and lacks fire resistance and dimensional stability. Fine-quality wood is difficult to obtain, and is often at expensive. The above are the reasons why non-wooden humidity controlling materials are being developed. It is reported that resinoozing from Japanese cypress has an adverse effect on works of art. Therefore, a substitute for wood material is desired for warehouses where works of art are stored. Humidity controlling performance means the ability to maintain a proper fluctuation range of the desired relative humidity in the interior space, irrespective of humidity fluctuation in the outside environment. There are various types of construction spaces requiring humidity control, which is 25 I
252
A . Sagae, H Warni, Y . Arai, H. Kasai, T. Sato and H. Maturnoto
Table 1. Humidity contorolling materials 'Material Aqueous solution of various salts Silicagel Zeolite Nickapellet Wood: Poulownia, cedar, Used as construction Japanese cypress, spruce material Cloth: Rayon Ceramic-type porous material Calcium silicate material ALC Gypsum board Rock wool Soft fiber board Cemented excelsior board Paper Natural zeolite Zeolite-mixed mortar panel especially needed for the space for storing works of art or industrial products which are susceptible to Group Used as chemicals
I
temperaturehumidity. The moisture absorbing/discharging performance is dependent on the temperature, so control of temperature alone may influence relative humidity. When room temperature drops, rise in relative humidity is controlled to prevent dew condensation. When dew forms, outdoing the dew-condensation prevention performance, waterholding performance will prevent water droplets from falling. Two-stage dew-condensation prevention performance is required in such a method. In Japan, natural zeolite is distributed widely in the Tohoku, Hokuriku and San-in districts. Synthetic zeolite is avaliable as well for industrial purposes. However, this paper discusses natural zeolite alone in view of cost performance. The major features of zeolite used as a desicating agent are as follows: 1. Absorbs water better than other materials. 2. Has greater absorbing capacity than silicagel or activated aluminum in a place with low water
vapor pressure. 3.The moisture absorbing and discharging performance is highly temperature dependent, discharging moisture as temperature rises and absorbing moisture as temperature drops. Feature 3 is especially desirable with regard to controlling humidity and preventing dew condensation in buildings exposed to substantial changes in temperature and fluctuation of relative humidity. FUNDAMENTAL EXPERIMENTS The authors prepared a zeolite (natural mordenite)-mixed mortar panel (mold-form and pressform) to be used as a humidity-controlling material. The following are advantages of the zeolite-mixed
New Humidity Controlling Material
253
mortar panel, having the features mentioned above: moldability, fire and corrosion resistance, strength, low cost, temperature dependency of moisture absorbing and discharging properties, dimensional stability, and durability. 1. Mold-form mortar panel
The dynamic characteristics of zeolite mortar panel, shown in Figs. 1-6, are obtained through experiments. The size of the test piece is 40*40+16O(mm). The natural mordenite zeolite mixture ratio stands for the ratio of zeolite substituted for sand during mortar preparation. The bending and compressive strength of zeolite mortar decreases as the zeolite mixture ratio increases. After a period of 28 days, the bending strength of the mortar with the zeolite mixture ratio of 100% will decrese by about 50% and the compressive strength will decrease by about 45% as compared to those of zeolite free mortar. However, these values will not pose any meaningful problem when mortar is used as secondary components for building interior finish. In view of manufacturing, transportation and installation of panels with high zeolite mixture ratio, vinylon fiber may be mixed as a reinforcement, at a ratio of about 1% of the volume of mortar. To understand the moisture absorbingldischarging property, 400*400*20(mm) size test pieces of zeolite panels were put in and out of a conditioned chamber, and the weight variation was measured. Wood (spruce), calcium silicate board and ordinary mortar board of the same size were also tested for
100
Mixture ratio
isM
Mixture ratio
Fig. I to 6 Relationship between mixture ratio and bending strengthkompressive strength (kgfkm’)
254
A. Sagae, H. Wami, Y . Ardi, H . Kasai. T. Sato and H . Matumoto
comparison. The results are shown in Fig.7. The moisture absorbingldischarging quantity of zeolite panels was equal to that of wood and about three times larger than that of ordinary mortar board. At about 5 weeks after installation, the discharge of excess water decreased, indicating almost the same cycle. When zeolite panels are made on an industrial basis, kiln-drying will permit them to exhibit specified moisture absorbinddischarging performance at an early stage. The humidity controlling effect due to moisture absorbingldischarging was verified with the help of an experiment box. A steel box was prepared under conditions where the space was 1 m3, the thickness of the roof board and baseplate was 3.2 mm and the thickness of the side panel mounting plate was 4.5 mm. The total size of zeolite panels on the north, south, east and west sides was 2.56 m2, accounting for 42%of the entire surface area. Box A with 16 zeolite panels 400*400*20(mm) volume and Box B with aluminum panels of 0.5 mm in thickness used for comparison purposes were installed outdoors. The dry-bulb temperature and the dew-point temperature were measured to compare the temperaturehumidity fluctuation in boxes A and B, and the following facts were revealed
as a result. The data on March 24 indicates that the dew-point temperature rose as the daytime temperature rose in Box A as shown in Fig.8; and zeolite panels discharged moisture to stop any sudden drops in relative humidity. Fig.9 shows the combination of dry-bulb temperature and relative humidity for every measurement. In Box B with aluminum panels, the relationship between dry-bulb temperature and relative humidity was almost the same as that in the outside air, while in Box A with zeolite panels, the humidity controlling effect was observed. When the situation is compared using the standard deviation u (%) of daily fluctuation of relative humidity, u =15.19 in the outside air, u =8.28 in the zeolite box, and a=19.16 in the aluminum box. Fig.10 shows the differences in
temperature and absolute humidity inside and outside of the boxes. In the sealed boxes, ventilation is considered to be zero. Therefore, the quantity of absorbed/discharged moisture observed in the experiment is considered to be that of the zeolite panels alone. After the above experiment, the aluminum panels were replaced by ordinary mortar panels without zeolite and subjected to a similar experiment. As a result, the moisture absorption by the ordinary mortar panels was about 1/3 that by the zeolite panels; thus the result was almost identical to that obtained through the measurement of weight change in single test pieces. The behavior of zeolite panels against dew condensation in attic space was calculated by forming a space with zeolite panels. The extent of the effect of zeolite panels on preventing vapor condensation in attic space was checked with the help of vapor-condensation prevention performance judgment prograrn(1) on the basis of the result shown in Fig.7. The quantity of moisture that one panel 400+400*20(mm)absorbs/discharges was assumed to be 54 glday, which was averaged per day to be used as the quantity of absorbed and discharged moisture. Fig. 1 1 shows the specification
New Humidity Controlling Material
255
to model building. Fig.12 shows the calculation results of dew-condensation in the attic space without zeolite panels, indicating dew formation on the outer walls of the attic space. Fig. 13 shows the comparison of temperaturehumidity between a case where 80 zeolite panels were installed in the attic space and a case where no panels were used. It is estimated that the humidity when zeolite panels were installed is 20 to 30% lower than that when no zeolite panels were used. With zeolite panels, there was no vapor condensation in the attic space.
0
2
1
3
5
4
6
Passage of time (Hour) +
Fig.7 4 materials weight change values
%
I00
-
80
).
2
=
4B
60
40
oL~i li2i ii
4 ii ti ii zi
j
B
ti
1;
How
Fig.8 Difference in dry bulb temperature and absolute humidity inside and outside the box
20
O
10 Dry-bulb lempenturc
1s
Fig9 Measurements of temperature and humidity in experimental box
20
c
256
A. Sagae, H . Wami, Y . Arai, H . Kasai, T. Sato and H. Matumoto
Moisture discharge + *
U ~
Moisture absorption
Fig. 10 Relationship between dry bulb temperature and absolute humidity inside and outside of the boxes
400x400
120-1
X80
:
I
Attic space external wall
IA
180
rh\
External wall
RC 180 Roof slab
Insulation 25 Plaster board 12
RC
800 2500
-_
1
0
Float glass 4
Ceiling P
2000(
Glass wool 25 Plaster board 9
, \
Floor
7 2oo
Fig. 11 Specifications of calculation model building
New Humidity Controlling Material
257
50
-
.-A
40
2
0-
3 30
8
-3g
0
20
a
0
Y
0
3
1
6
9
12
15
18
21
Wall on the south 24 ~i~~
Fig. 12 Calculation results of accumulated dew condensation of attic space in Tokyo winter season
"I 10
I # > . ,. . . . . . . . . . . . . . . . . . . . . . 3 6 9 12 15 18 21 24 Time
A: Relalive humidity (zeolite panels were no1 provided) B: Relative humidity (zeolite panels were provided) C: Dry-bulb temperature (zeolite pancls wcn: not provided) D: Dry bulb tempcralure (zeolile panels were provided)
Fig.13 Calculation results of temperature and humidity in attic space (Tokyo in February; TAC 50%)
258
A. Sagae, H. Wami, Y. Arai, H. Kasai, T. Sato and H. Matumoto
2. Press-form mortar panel The authors are presently developing these materials and adapting the press-form machine for building finishing surfaces. To understand the moisture absorbing and discharging properties, tests were conducted as follows. Test panels, 200*200*20(mm) in size (ratio of zeolite substitute for sand
loo%), were placed in an artificial environmental test room, the conditions in which were altered as shown in Fig.14. The moisture absorption and desorption were measured in terms of the weight variation. The humidity controlling effect due to moisture absorbing/discharging was verified with a calculation method: The analysis was conducted using the simultaneous heat and moisture transfer model. The numerical method was a fully implicit differential scheme with non-uniform space differentials. The measured values were used for the physical properties required in the calculations. For comparison with measurement values, the predominant parameter for the purpose of the calculations under the test conditions was the equilibrium water content gradient b. Calculations were carried out using several different values for this water content gradient. The results closest to the measurement values were obtained when a value in the medium humidity range of b=0.039 was used (Fig. 15). 201----+
I I
,
I
,
,
,
,
8 Ib Elapsed Days
6
4
Fig.14 Boundary conditions for calculation in artificial chamber
;:r
-
7-
(a) Weight variation [ r,',,f~
(b) Water content [ m ] <'
-.-I
.
Cal. (bd.059)
U
4-
38
'
>
Gal. (bS.012) '
"
'
'
'
I
18
Elapsed Days
Fig. 15 comparison between measured and calculated weight variation data
New Humidity Controlling Material
259
APPLICATION OF THE PANELS IN AN ART MUSEUM The new Meguro-Gajyoen. Tokyo, Japan, was completed in October I99 1. It is a large, modern complex facility containing a hotel, office, a banquet tower, and museum. The natural mordenite zeolite mortar panels(press-form ratio of zeolite substituted for sand 100 %) used in its art museum show high performance in humidity control for art storage space, where the pressed-form zeolite panels are installed on the wall and ceiling surfaces. Photo 1 shows the art storage space and Fig. 16 shows a schematic diagram of the storage space. The room's temperature and humidity and other characteristics are being measured, and Fig. 17 shows measurements which attest to its successful humidity condition.
7
Ventilation
#I-
Zeolite Mortar Panel
Fig. 16 Schematic diagram of storage space
Fig. 17 Measurements of temperature and humidity in storage space
260
A. Sagae, H. Wami, Y . Arai, H . Kasai, T. Sato and H. Matumoto
Photo 1 The new Meguro-Gajyoen, Tokyo, Japan, art storage space CONCLUSION Humidity controlling devices are highly needed by art museums, storage warehouses and other types of buildings which are susceptible to excessive air conditioning. It is considered that the dew condensation problem results from an imbalance between heat insulating performance and humidificatiodair conditioning in buildings. To solve such a problem, building construction methods and materials must be reviewed. When due attention is paid to interior finishes and stored objects, then external walls and roofs themselves installed by the heat insulation construction method may be economically used to prevent dew condensation. Zeolite panels are being examined for applicability in the storage of paintings and other works of art in some buildings, and experiments for that purpose are under way. The number of humidity controlling materials to be developed will increase in the future. ACKNOWLEDGMENTS The authors are grateful for guidance and cooperation to Dr. Taku Yamazaki, Dr. Yasuhiro Kameda, Dr. Muneshige Nagatomo and Dr. Shumpei Ohara. References
1 Shumpei Ohara, Akio Sagae, Hiroo Izumiyama; Estimation of Dew Condensation, June 1984, Annual Report of Kajima Institute of Construction Technology 2 Garry Thomson: The Museum Environment 2nd 1986. 3 Akio Sagae, Hiroki Wami, Yoshinobu Arai, Hiroshi Kasai; “Study on a New Humidity Controlling Material” , CIB W 67 SEP.1990
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Novel Catalytic Functions of Metallosilicates Exerted by Isomorphous Substitution
Tomoyuki Inui Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
ABSTRACT In order to imurove and extend the catalytic function of H-ZSM-5, isomorphous substitution of transition elements for Al in the framework of ZSMJ maintaining the favorable feature of its pore structure was achieved by adopting the rapid crystallization method. The synthesized crystals, called metallosilicates, exhibited a variety of unique catalytic function different from H-ZSMJ, in reactions related to hydrocarbon conversion. The large difference among them cannot be explained by mere global acidic properties measured by NH3-TPD. A microscopic description of both the location of acid sites and their strength was tried through computer simulation by applying Monte Car10 method for NH3 adsorption, and a clear difference of each metallosilicate,which can elucidate the catalyticperformance, was found. PROMINENT PROPERTIES OF ZEOLITE MFI AND ITS LIMITATION IN CATALYTIC FUNCTION A silica-rich zeolite MFI (ZSM-5) gave a breakthrough in the catalysis for synthesis and conversion of hydrocarbons [ 13. The protonated form of ZSM-5, designated as H-ZSM-5, has strong acid sites originated from the unbalance of valency between tetra-valent silicon and trivalent aluminum. On the typical H-ZSM-5 having a SVAl atomic ratio around 20-40, for example, exhibits an evident aromatization function in the conversion of methanol [2,3], some other oxygen containing organic compounds [4,5], and olefins [6]. Compared with other conventional zeolitic catalysts such as H-Y and H-M, H-ZSM-5 exerts a remarkable longer catalyst life due to its high resistance against the coke formation [7-91. The main reason for this property is ascribed to its proper size of pore diameter and its unique pore structure which do not allow the growth of fused-ring aromatics as the precursor of coke. In spite of these excellent properties, H-ZSM-5 cannot be got away from the intrinsic fatal property of zeolite or aluminosilicatethat, as shown in the following sequential reaction scheme [ 10,111, the hydrogen evolved during the course of aromatization shifts toward intermediate olefins and hydrogenates them into corresponding paraffins, resulting the decrease in the yield of more valuable gasoline range fractions or aromatics [ 121. It has been emphasizedthat the activity of n-hexane cracking 263
264
T. Inui
(a) + CH30H (b) + C =2-5
2-5
-H2
-
-
H2°
--
Higher molecular weight olefins, c z6+
\
C =6+ ____t Aromatics (4 Fused-ring aromatics
coke)
(3)
+“2
C =2-5
(4)
Aliphatics
Reaction scheme of the methanol conversion on a typical H-ZSM-5.
is strictly proportional to the content of Al in the ZSM-5‘s framework even in the range of ppm order [13]. Reflecting this nature, the quality of acidic property of H-ZSM-5 cannot be changed by mere decrease in Al content. ISOMORPHOUS SUBSTITUTION BY RAPID CRYSTALLIZATION METHOD In order to overcome the disadvantage in H-ZSM-5 and to investigate a wider extension as the microporous crystalline catalysts, we have conducted isomorphous substitution of transition elements for Al in the framework of ZSMJ maintaining the favorable feature of its pore structure [ 141. In general, conventional zeolitic materials consume a considerably long time for nucleation and successive crystallizationat a definitetemperature like as shown in Fig. 1 [ 151.
100 *I.--
Crystallldty,
@ /,
220
o^
”0
CrystalUzation time (day)
Fig. 1. Conceptionalillustration for the change of crystallization rate and crystallinity with time on hydrothermal treatment at a constant temperature in a conventional slow-crystallization method for the synthesis of typical ZSM-5.
Novel Catalytic Functions of Metallosilicates
265
However, in this kind of slow-crystallization process, ununiformity in crystal size and distribution of active species in the crystal particles are involved, and furthermore, elements to be incorporated are apt to be expelled from the framework except Al and some other small number of elements. On the other hand, the rapid Crystallization method [15] increases the possibility of occlusion of the hetero compounds in the early stage of crystallization, and then they are incorporated into the framework more easily resulting that the formation of small and uniform crystals. The feature of the rapid crystallizationmethod is as follows; (i) the gel mixture was prepared keeping the concentration of the mother liquid at a constant levels as much as possible, and the supernatant solution for the crystallization was separately synthesized to adjust the crystallization. (ii) The precipitated gel was milled before crystallizationin the hydrothermal condition, which was essential for obtaining uniform and fine crystals and for providing rapid crystallization. (iii) The temperature was programmed under the hydrothermal treatment to minimize the time necessary for crystallizationas shown in Fig. 2 [ 151.
6
c
200
E fl I c
180 c
.R
H
160
140
4 Ti c;
Crystallization time ( h )
Fig. 2. Conceptional illustration for the change of crystallization rate and crystallinity with time on hydrothermal treatment in the rapid-crystallization method with a programmed temperature rise for the syntheses of typical metallosilicates such as Fe-silicates, Ga-silicates, and Znsilicates. The incorporated elements exist much more stably in the framework than those prepared by slow crystallizationmethod [16]. It was confirmed that the three control conditions mentioned above were very effective in the rapid synthesis of crystals and increasing the catalytic activity as compared in Fig. 3 [15]. This indicates that the Al is more highly dispersed in the crystals and forms of acid sites more effectively.
266
T. Inui
Fig. 3. Comparison of catalyst performance for MeOH conversion on H-ZSM-5s prepared by conventional slow crystallizationmethod and the rapid crystallizationmethod. S U N atomic ratio = 40, 30 % MeOH - 70 % N2, GHSV 1100 h-l. Conversion of MeOH was 100 %, Reaction temperature, (a) 4OO0C, (b) 300°C. CATALYTIC FUNCTION OF METALLOSILICATES Among the various kinds of metallosilicates,Ga-silicate [ 171 and Zn-silicate [18] exhibited a remarkable activity and selectivity for aromatization of paraffinic hydrocarbons [17,181 and methanol [ 191 with evolving hydrogen into gas phase from the catalyst surface. Modification with a small amount of Pt not only enhanced the activity markedly but also moderated the coke deposit. In Table 1, catalytic performance of H-ZSM-5, H-Ga-silicate, and H-Zn-silicate for propane conversion was summarized. The effect of Pt-modification [ 17-20] was also compared in this table. Fe-silicate exerted an extraordinary high conversion rate and selectivity in olefin oligomerizationinto gasoline range fraction [21], and showed a very high selectivity to ethylene Table 1. Propane conversion on Pt-loaded and non-loaded metallosilicatecatalysts. Catalysts
H-ZSM-5
Si/Me ratio
Si/AI=40
Reaction Temp. (“C) Propane Conv. (96) Selectivity (C-wt%)
c1
c2 c2= c3=
C4+ aliphatics Aromatics
Pt/HZSM-5 SUAk40
H-GaPt/H-Ga- H-ZnPt/H-Znsilicate silicate silicate silicate Si/Ga=21 Si/Ga=21 Si/Zn=40 Si/Zn=40
600 63.3
500 92.0
600 83.6
600 92.3
600 64.2
600 81.6
27.2 7.1 30.2 17.8 6.6 11.1
13.8 56.2 1.8 3.1 0.6 24.5
10.4 7.5 7.6 6.4 0.7 67.4
4.8 27.7 1.7 4.9 0.2 60.7
10.6 11.8 9.6 21.3 3.2 43.5
7.1 21.9 3.6 18.7 2.3 46.4
Reactant, 20% propane and 80%N2, SV 2000 h-l.
Novel Catalytic Functions of Metallosilicates
267
Fig. 4. Comparison of catalytic performance of typical H-ZSMJ, H-Ga-silicate, and H-Fesilicate catalysts in methanol conversion. 0 : paraffins, I: olefins, A aromatics. and propylene in methanol conversion at lower temperature range [22]. In Fig. 4, a marked difference in the selectivity to products of methanol conversion on H-Al-silicate (i.e. H-ZSMJ), H-Ga-silicate, and H-Fe-silicate is shown. Since the reaction advancing on Fe-silicate is not affected by hydrogen, it can be used as the second-reactor catalyst for hydrogenation of carbon oxides to synthesize high octane number gasoline with high selectivity avoiding hydrogenrelated unfavorable reactions for the products [23]. The results are summarized in Fig. 5. When a mixed catalyst, which consisted of methanol synthesis catalyst prepared by conventionalprecipitation method (MSCp) and H-ZSM5, as the catalyst in a single reactor for syngas conversion, gasoline fraction in the products was only 24% and main products were light paraffins. This low performance is ascribed to the nature of H-ZSM-5; i.e., the rapid hydrogen shift on H-ZSM-5 to hydrogenate the intermediate olefins, and unfitness of the optimum reaction condition of methanol synthesis catalyst (MSCp) and H-ZSM-5. On the other hand, when improved methanol synthesis catalyst prepared by our intrinsic uniform gelation method (MSCg) was combined with metallosilicate catalysts, results were markedly improved. In case of R/H-Ga-silicatewas packed in the 2nd reactor, an aromatic-rich high octane-number gasoline was obtained from syngas with 50% in selectivity, however, the other products were almost occupied by C2-C4 paraffinic hydrocarbons, although methane produced was negligible. In contrast with this case, when H-Fe-silicate was packed in the 2nd reactor, gasoline fraction was obtained with a selectivity of 57%. Although the selectivity to aromatics decreased, aliphatic hydrocarbons involved in gasoline range were mainly iso-mono-internal
268
T. h i
Fig. 5 . Hydrocarbon synthesis from syngas and COZ-H2 mixture using two-stage series reactors in which different kinds of catalysts were packed. olefins indicatinghigh octane value. It is noteworthy that the produced hydrocarbons other than gasoline range still remained in valuable intermediate olefmic hydrocarbons, and unfavorable light paraffins especially methane were produced very little. Furthermore, very important things for COZ hydrogenation are that, by using improved MSCg with La modification and somewhat higher-pressure condition than in case of syngas conversion, almost the same feature of hydrocarbon distribution was obtained. This means that the process for gasoline synthesis from COZ can be designed as same as the process for syngas transformation. MICROSTRUCTURE OF METALLOSILICATES ANALYZED BY COMPUTER SIMULATION The size of pentasil pore opening of ZSM-5 was delicately modified by the isomorphous substitution of other kinds of elements for Al and the amount of incorporated elements. These factors would affect on the diffusivity of reactants and shape selectivity of the reaction products.
Novel Catalytic Functions of Metallosilicates
0
100
200
300
400
500
269
600
Temperature (“C) Fig. 6. NH3-TPD profiles for the four kinds of metallosilicates. However, more evident effects of isomorphous substitution are that the change in acidic property and the exertion of intrinsic metallic and/or metal oxide catalysis. The change in effective diffusivity and different kind of catalysis caused by the metal incorporation can be observed by experiment. The change in acidic property is usually evaluated by NH3-TPD method, and actually the profiles of different kind of metallosilicates can be differentiated both in the integral area of profiles and in the shift of peak temperature as shown in Fig. 6. The former corresponds to the amounts of acid sites and the latter indicates the strength of acid sites. Through the measurementsof NH3-TPD, one can recognize in general, the order of acid strength among the metallosilicates is in the order of Al 2 Ga > 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
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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+
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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
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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.
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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
290
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.
Transition State and Diffusion Controlled Shape Selectivity
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
Transition State and Diffusion Controlled Shape Selectivity
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.
294
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-
Mechanism of Activation of Butanes and Pentanes
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
Mechanism of Activation of Butanes and Pentanes
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
Mechanism of Activation of Butanes and Pentanes
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
A. Hagen and F. Roessner
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
A. Hagen and F. Roessner
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
A. Hagen and F. Roessner
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.)
PT-Ni/L-zeolite Bimetallic Catalysts
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,
-
PT-Ni/L-zeolite Bimetallic Catalysts
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.
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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.
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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%
Pt-KL Zeolite Treated with Chlorotrifluoromethane
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
Pt-KL Zeolite Treated with Chlorotrifluoromethane
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
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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
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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
Zeolite-Supported Pt-lr Bimetallic Catalysts
341
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,
E
a a
80
0
1
2
3
4
Ir Loading
5
6
/ wt
%
7
8
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
1. C. Hwang and S. 1. Woo
342
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
-
I
I1
, 2059
n
5 4
U
e
0
c 0
-? 0 VI
13
6
2200
2050 Wavenurnber (crn-')
1900
2200
2050 Wavenurnber (cm-')
1900
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
343
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
Zeolite-Supported Pt-Ir Bimetallic Catalysts
345
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.
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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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G.Lu and L. Guczi
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
351
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G. L u and L. Guczi
n
2100
F
B
Y
3
p
2050
E
3
C
Q)
>
C
0
3
2000 250
350
850
480
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 ] .
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S.-K. Ihm, D.-K. Lee and J.-H. Lee
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].
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S.-K. Ihm, D.-K. Lee and J.-H. Lee
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
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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
Ni-Mo-Y for Water-Gas Shift Reaction
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 .
.
368
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
Ni-Mo-Y for Water-Gas Shift Reaction
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/
-
370
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
372
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.
Substituted Fe-Phthalocyanines on Zeolite Y
373
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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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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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
Substituted Fe-Phthalocyanines on Zeolite Y
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 .
i
\--
-I
2
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
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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
Aromatic Acylation and Related Reactions
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
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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.
Aromatic Acylation and Related Reactions
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.
Aromatic Acylation and Related Reactions
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
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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.
Aromatic Acylation and Related Reactions
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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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:
Diels-Alder Condensation over Zeolites and Clays
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
Diels-Alder Condensation over Zeolites and Clays
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.
Preparation of Neoalkanals and Novel Cyclic Dioxepenes
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
W. F. Hoelderich and M. E. Paczkowski
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
Preparation of Neoalkanals and Novel Cyclic Dioxepenes
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
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W. F. Hoelderich and M. E. Paczkowski
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
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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).
Redox Molecular Sieves
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.
Redox Molecular Sieves
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
Redox Molecular Sieves
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<. Yr\n:\gisilw:\, K. Asano, M. Nakamura and H. Toniinaga Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo, 113, Japan
ABSTRACT Titanium silicalites, TS- I iInd TS-2, effectively catolyze oxidation of a variety of organic compounds with aqueous €-1202. There is a marked difference i n the reactivity between linear i\lki\nes/iilkenesiind cyclic alkanes/alkenes, resulting from reactant shape selectivity. In the competitive oxidation shape selectivity is enhanced. The influence of cosolvent, product and oxidant on the oxidation reaction is discussed. Chemospecificity in the intramolecular competitive oxidation reveals the iliiporti\lice of coordinntion of the group to be oxidized to the active site present in the sterically constrnined environment in the zeolites. INTRODUCTlON Titanium silicalite TS-1 is i1 derivative of high silica ZSM-5, in which titanium is thought to be incorporated i\t the tetriiliedrd site of the framework. This new zeolite has been found to be effective in the oxidation of :I variety of organic compounds with aqueous hydrogen peroxide as oxidant under mild conditions: hydroxylation of aromatic compounds 111, epoxidation of alkenes [2], oxygenation of alkanes [3,4], and oxidation of alcohols [ 5 ] . Incorporating catalytic sites within zeolite channels and cages of molecular dimensions provides the basis of molecular shape selective catalysts. We have already reported the remarkable shape selectivity in the epoxidation of alkenes [6-81 and oxidation of alkanes [31. Titanium silicalite TS-2 which has an MEL structure has been prepared and found to exhibit similar catalytic behavior [91. Here we have investigated the factors determining the rates of oxidation of various substrates which have different steric and electronic characters on titanium silicalites TS-1 and TS-2. Effects of cosolvent, oxidant and product have been investigated. To clarify these factors the intra- and intermolecular selectivity in the epoxidation of C=C double bonds, oxidation of alkyl groups, hydroxylation of arenes, and oxidative dehydrogenation of alcoholic groups has been also examined. EXPERIMENTAL TS-1 [lo] and TS-2 11 1 1 were prepared from tertaethyl orthotitanate or tetrabutyl orthotitannte, tetraethyl orthosilicate nnd rilkylrminonium hydroxide as a template. A typical oxidation run used 50 mg of ;I catalyst i n 2.5 cm3 of substrates or a mixture of substrates in a round-bottom flask, 417
418
T. Tatsumi, K. Yanagisawa, K. Asano, M. Nakamura and H. Tominaga
to which was added 2.5 cm3 of 30% iiqueous solution of H202. The resulting mixture was stirred at 323 K for 3 h. Adsorption experiments were carried out at 303 K using 1,3,5-triisopropylbenzene, whose dimensions was too large to enter the pores of titanium silicalites, as a solvent [12]. Gravimetric merisurenients of o-xylenes were conducted on a highly sensitive microbalance. After zeolite samples (0.25 g) was evacuated iit 473 K, the measurements were performed at 393 K and an oxylene pressure of 0.39 kPa to estimate the time taken to reach 30% of the amount of o-xylene adsorbed ;it i nl’in i re ti me. RESULTS AND IIISCUSSION Shaoe selectivitv i n oxidation of ;ilkanes and alkenes Linear hvdrocarbons vs. cvclic hvdrocarbons. We have reported [3, 6-81 that cyclic alkanes and illkenes lire much more difficult to oxidize than linear ones on TS-1. Similar shape selectivity is demonstrated on TS-2. The results we summarized in Table 1. The oxidation turnover of liexane is 40-80 times iis high ;is that of cyclohexnne on both titanium silicalites. Analogously to alkanes, lineilr nlkenes is epoxidized much faster than cyclic alkenes. Such subswate selectivity presumably arises from the IiIolecdiir sieving iiction of titanium silicalites. The molecular dimensions of cyclohexiine tire 0.47 x 0.62 x 0.69 nni and those of liexane are 0.39 x 0.43 x 0.91 nm. The two smaller parameters define the critical dimensions. Since the pore dimensions of the MFI structure are 0.53 x 0.56 nm for the straight chonnel and 0.51 x 0.55 nm for the sinusoidal channel and those of the MEL structure ;ire 0.53 x 0.54 n m 1131, cyclohexane seems to have some difficulty in diffusing in TS-1 or TS-2. Table 1. Oxidation of dkanes and alkenes on TS-1 and TS-2a Catalyst Alkanes TS-1
Substrate
- hexane TS-2
Alkenes TS-1
heptane octane cyclohexone Ilexi\ne heptane octane cyclohexnne
7
‘I‘S-2
2- hexene 3-hexene cycloliexene 2-hexene 3-hexene cyclohcxnne
Yield/mol/mol-Ti Distribution/% 2-01 2-one 3-01 3-one 4-01 4-one 9.2 16 26 42 17 12.4 9 7 56 20 8 trace 7 29 6 4.0 14 7 37 0.12 15.2 13 28 39 20 14.1 7 9 4431 8 1 6.1 15 13 29 13 23 7 0.34 translcis epoxide 65.0 59/41 76.1 53/47 3.2 58.0 56/44 48/52 63.S 1.7
“CiIIiilySt(TS-1: Sini = 70, TS-2: SiRi = 85) 50 mg, H20,(30 wt% aqueous solution) 2.5 cm3,substrate 2.5 cm3,323 K, 2 Ii.
Titanium Silicalites for Shape-Selective Oxidation
50 -
0.
....
419
b.3
40'
30 TI
5fn
z
0
20
'
10
'
Fig. 1. The ratio of oxidation rilte for hexane iind cyclohexane on different preparations of TS-2. Catalyst (TS-2"J: Si/Ti = 85, TS-21SJ:S i p = 140, TS-2[R]: Si/Ti = 109) 50 mg,H,0,(30 wt% aqueous solution) 2.5 cni3, substrote 2.5 cm ,323 K, 2 h.
Fig. 2. (u)Oxidation and (b)iidsorption of C h drocarbons on TS-2(Si/ri = 85). (a) Catalyst 50 nig, H,0,(30 wt% nqueous solution) 2.5 cni3, substriite(totn1) 6 Y 2.5 cni', 323 K, 2 h. (b) Adsorbent 100 nig, solvent: 1,3,5-triisopropylberizene 3.0 g, aclsorbote(each) I wt%,303 K, 3 h.
420
T. Tatsumi, K. Yanagisawa, K. Asano, M. Nakamura and H. Tominaga
Figure 1 shows dependence of the ratio of oxidittion rate for hexane and cyclohexane on different TS-2 preparations, i n which the temperature progriim in hydrothermal synthesis is varied. The ratio pitriillels well the 30% sorption time for o-xylene (to.3),which was proposed by Olson and Hiiag ;is ;I direct measure of the critical niass transfer property 1141. These data cleorly establish the major role of diffusion i n the shape selective oxidittion of nlkanes. Competitive ox id,ltioq . Figure 2 (a) conipiires the results of co-oxidation of C, hydrocarbons with those of single oxidntion of each component on a TS-2 catalyst. In the presence of 2-hexene, the oxidiition of hexane is stopped. In the presence of hexane, the hydroxylation of benzene is retarded. On the other hand, tlie epoxidittion of 2-hexene and the oxidation of hexane are virtually uninfluenced by the presence of hexane and benzene, respectively. Figure 2 (b) illustrates the adsorption of those C, hydrocnrbons on the TS-2 sample. The adsorption amount of benzene is sharply decreased by the presence of hexane, while that of hexane is hardly affected by the presence of benzene. We could not determine the precise rate of adsorption. However, judging from initial uptakes, there is u clear correlution between the rate and amount of adsorption of these hydrocarbons, iilthotigh the difference in the former is greater than that in the latter. Because of the molecular dimensions of benzene (0.34 x 0.62 x 0.69 nm), its diffusion into the 10-membered ring pore openings i s made unfrivored in the presence of hexane with smciller dimensions. In contrast, there is no difference between the itdsorption iiniount of hexane and 2-hexene; hexane enters into the pores and hence there should be iiliother reason for the inhibition of hexane oxidation by 2-hexene (vide infra)
.
Effect of cosolvent a n d oxidant Triinsfer of substriitcs to the ;iuiieous phose. The oxidation of organic substrates on titanium silicalites uses ;iqaeoiis 1-I2O, iis the oxitlant. Hence tlie ciitiilytic reaction system is rather complicated. For tiiost substretcs the liquid phase is separated into ;in orgnnic phase iind an aqueous phase. Since [lie solid ciititlyst exists ;ilniost exclusively in the ii(1ueotls phase, the substrates must trnnsfer to the ric.yieous phase, where the catalyst itnd H,O, iire present. Figure 3 shows the effect of iiddition of nlcoliol in the oxidation of hexane on a TS-I ciitiilyst. Addition of a small amount of methanol results in the etihiit1Celiictit of oxidiitioti of hexitne [ 31. Hexane is pritctically insoluble in the iiqueoils phase. In tlie presence of methanol, however, its solubility in the aqueous phase is remarkably increased. ‘This iiccoutits for the promoting effect of organic cosolvents in the oxidation of hexnne. Methanol tiiiiy be effective 111 promoting desorption of oxidized products and this may be the reason for the enhonced iictivity. However, this hypothesis suffers difficulty in explaining the effect of excess methanol (vide irlfrrr). The effect of methanol iind ethanol in the epoxidiition of 2-hexene ir. not so remarkable as in the oxidntion of hexane. Furthermore, for the substrates with hydrophilic groups cosolvents are unnecessary. For example, Thangamj et al. I151 have found thiit the oximation of cyclohexnnonc on TS-1 proceeds fiister in the absence of solvents. The effect of co-iidsorptioq. As shown in Fig. 3, addition of the excess amount of methanol results in a decrease in the turnover. In the competitive adsorption of hexane and methanol on TS-I
Titanium Silicalites for Shape-Selective Oxidation
421
-e Methanol
+ Ethanol
0
5
10
Alcohol added / rnl Fig. 3. Effect of oddition of olcohols in oxidation of hexane. Catalyst (TS-1: SiFi = 60) 50 mg, liexane 10 cm3, H20, (30 wt% aqueous solution) 10 cm3, 323 K, 3 11.
i n the absence of WiiIer, tlie ;imouiit of hexane adsorbed decreases ~iionotonouslywith increasing iimount of 1i1ethiin01. 'l'lii~sit is assiimed that although niethnnol in a small amount is useful in increasing the solubility of hexcine in tlie aqueous phase, excess methanol leads to suppression of tlie adsorption of hexane. While ethanol is only slightly effective in promoting the oxidation of hexane, 1-propiinolproduces an adverse effect (Fig. 3). The ineffectiveness of these alcohols may be due to their preferential oxidation; the formation of the corresponding aldehydes is observed. Benzene is oxidized to phenol on TS-1 1161 and TS-2[ I 11. We have found that the rate of benzene hydroxylation is seriously retarded by the addition of cosolvents such as methanol, acetone ~ l . a very small amount of cosolvent results in the decrease in the formation and t-butyl i i l ~ ~ l i Only rate of phenol. The lidsorption of benzene is hindered by the presence of these cosolvents. Presumably the diffusion of benzene wliicli has similiir critical dimensions to the zeolites is strongly affected by tlie co-existing niolecules. As described above, only a smnll amount of benzene is sorbed in the presence of hexane. It is dso noteworthy that in the reaction system water is present and may reduce the effective pore dimensions. In contrast, the hydroxylntion of phenol takes place in the presence of ii large amount of cosolvents such as methanol and acetone. This may be due to the large polarity of phenol compared to that of benzene. Organic substrates with a polar group such as alcohols, phenols and ketones and with suitable dimensions are more adsorbable on TS-1 or TS-2 than hydrocarbons. These findings are supportive of our claim that the titanium silicalites are not so hydrophobic as silicalites [17]. Similar adsorption phenomenit may be important in the oxidiition of hydrocarbons. It has been found that the yield of oxidation products from hexane levels off within several hours 131. This is not due to catalyst destruction but due to pore blocking by the products, since the catalytic activity
422
T. Tatsumi, K. Yanagisawa, K. Asano, M. Nakamura and H. Tominaga
Oxidant
H202
TBHP
H202
H202
TBHP
Substrate CH3CH=CH(CH2)2CH3 CH3CH4HCH(OH)CH3 CH3(CH2)3CH(OH)CH3
Fig. 4. Effect of oxidniit in the oxidiition on TS-1 nnd amorphous Ti0,-SiO,. Catalyst (TS-1: SiDi = 52, Ti0 -SiO : SiDi = 26) 50 mg, substrate 10 cm3, H,O (30 wt% aqueous solution) 10 cm3 or rBuOOH (+BHh (80% aqueous solution) 10 c1n3, 323 K, 3 h. is completely recovered by calcination at 823 K. Adsorption of hexane on TS-1 is seriously retarded by the presence of 2-hexnno1, which is preferentially adsorbed. This suggests that the oxidation products from hexane are rather strongly adsorbed on TS-1, slowing the diffusion of the reactant into the pores. Hydrogen peroxide vs. t-butvl hvdrooeroxide. Figure 4 shows the effect of oxidant in theoxidation on TS-1 and amorphous 730,-SiO,. Both H,O, and t-BuOOH (80% aqueous solution) are effective in the oxidation of alkenes and alcohols on TS-1, although the former is superior to the latter. On the other hand, ;tlcohols are oxidized faster with t-BuOOH on Ti0,-SiO,, where no epoxidation of simple ctlkenes trike plnce with H,O, [18]. However, H,O, can be utilized for the epoxidation of unsaturated alcohols on Ti0,-SiO,. These differences can be interpreted in terms of competitive adsorption of substrate and oxidant. Excess water should prevent non-polar substrates such as simple iilkenes froin adsorbing on Ti0,-SiO,, which is much more hydrophilic than TS-1. Unsaturated alcohols, being more polar than alkenes, can be adsorbed on Ti0,-SiO,. Chemospecificitv in lhe in1r;imolecular comoetitive oxidation Coortlinntion to the itctive sites. I n the competitive oxidiition of 2-hexene and hexane, no oxidation of hexitne occurs although hexane enters into the pores (Fig. 2). It is suspected that this is it result of ;I preferential coordination of C=C double bonds to the iictive sites, preventing hexane
Titanium Silicalites for Shape-Selective Oxidation
423
Table 2. Oxidation of unsaturated alcohols on TS-1" Entry
Substrate
Products yield/moVmol-Ti Ketondaldehvde
Eooxide
0
19
0
16
3
0
30
4
31
95
5
37
4
6
7
27
7
43
65
44
141
98
94
18
10
75
17
1 2
8 9
&OH %OH
&OH
-OH
10 11
P
O
H
?OH "Catalyst (TS-I: Si/Ti = 52) 10 mg, substrate 2.5 cm3,H202 (30 wt% aqueous solution) 2.5 cm3, 323 K, 3 ti. molecules from interacting with active oxygen species. A more concrete example is demonstrated by the intramolecular competitive oxidation of unsaturdted alcohols [19]. Table 2 shows the chemospecificity for oxidation of the alcoholic group and epoxidation of the C=C double bond. In this intramolecular competitive oxidation the influence on the chemospecificity between terminal and internal double bonds is found quite different. The terminal CH,=CH- double bond is oxidized preferentially, while oxidation of the alcoholic group is inhibited or significantly retarded. With unsaturated alcohols having an internal -CH=CH- double bond, epoxidi\tion of the double bond and the oxidation of the alcoholic group occurs competitively. The terininal double bond, being less sterically demanding, appears to be able to interact strongly with the Ti active site, preventing the alcoholic group from approaching the active site. Similar effect of the alkene structure has been observed for the intermolecular competitive oxidation of alkenes and alcohols. Factors governing the reactivitv inside the channels. It is obvious that access to the active site is not the sufficient condition for the reaction. As can be seen in Table 2, in the oxidation of unsaturated alcohols reactivity toward epoxidation is opposite to the coordination ability; higher turnover
424
T. Tatsumi, K. Yanagisawa, K. Asano, M. Nakamura and H. Tominaga
of epoxidation of the internal double bond thiln thnt of the terminal double bond would result from ;in elecaophilic nature of the attack of nctive oxygen species against C=C double bonds. However, the trisubstituted double bonds (entry 11 in Table 2) and asymmetrically disubstituted double bonds (entries 5 and 10) are only slowly epoxidized on TS-1. These double bonds should have difficulty in gaining access to the active site present in the sterically constrained environment in the zeolites. Such steric regulation is not encountered for ainorphous Ti0,-SiO,; epoxidation is promoted by the electron-relensing methyl su bstituent. CONCLUSIONS It has been revealecl that iliere are several prerequisite conditions to the completion of the H,O, oxidation of organic substrates on the titriniuni silicnlites. First, the substrates must transfer to the aqueous plinse, where the cntrilyst and H,O, we present. The diffusion into the zeolite pores is difficult for cyclic hydrocarbons whose dimensions are similnr to those of pore openings. Cosolvent and oxicl;iiit iilso ilffect the adsorption of substrates on the catalysts. Further, the group to be oxidized must nppronch the i\CtiVC site in the sterically constrained environment i n the zeolites. The renclivity reflects the eleclrophilic nature of the nttnck of nctive oxygen species. Finally, desorption of the products must Ii\kc place siiioothly and otherwise pore blocking by the products results in the leveling-off of the reaction. REFERENCES 1 e.g., 13. Noturi, Stud. Surf. Sci. C:iI:il., 37 (1987) 413. 2 Eur. Pat. 100 119 (1984). 3 1:Tntsumi, M. Nt~k~iitiri~, S . Negishi and 1-1, Tominaga, J. Chem. SOC.Chem. Commun., (1990) 476. 4 D.R.C Huybrechts, L. Re Bruycker and P.A. Jacobs, Naturc, 345 (1990) 240. 5 P. R. Huri Pritsi\tln RiIo, A, l’h;tngariij nnd A.V. Ramaswamy, J. Chem. SOC.Cheiii. Commun., (1991) 1130. 6 T. Tmumi, M. Nitki111i~ri1,K. Yiiitsa iind H. Tominagii, Cheiil. Lett., (1990) 297. 7 T. T;itsumi, M. N ; ~ k ~ i i ~nnd r i i 1.1. Toniinagu, in S. Yoshidil, N. Tiikezawii and T. Ono (eds.), Catitlytic Science nnd Tcchnology, Vol. 1, Kodnnsha-VCH, Tokyo, 1991, p. 213. I , YUiISa and H. Toniinaga, Catal. Lett., 10 (1991) 259. 8 T. Tatsumi, M. N ~ U I I I W ~K. 9 J.S. Reddy, R. Kuinar iind P. Ratnxiiiny, Appl. Catal., 58 (1990) L1. 10 U.K. Put. GB 2071071 (1981). 11 J.S. Reclcly illid R. Kumar, J. Catitl., 130 (1991) 440. 12 S. N ~ I I I IY. ~ ~Knnni, I , H. Shoji mid T. Yashim~i,Zeolites, 4 (1984) 77. 13 W.M. Meier itnd D.H. Olson, Atlas of Zeolite Structure Types, Butterworth-Heinemaiiii, Stoneham, 1902. 14 D.H. Olson nnd W.O. Hniig, in T.E. Whyte, Jr., R.A. Dalla Bettit, E. Derouane and R.T.K. Baker (Eds.), Catalytic M;\terii\ls(ACS Symposium Series 248), Am. Chem. SOC.,Washington D,C., 1984, p.275. 15 A. Thangaraj, S. Sivasanker and P. Rntnasitiny, J. Cml., 131 (1991) 394. 16 A. Tliangciraj, R. Kuiniir and P. Ratnasamy, Appl. Catal., 57 (1990) L1. 17 T. Tatsumi, M. Nakamurct i ~ i dH. Tominaga, Shokubai (Catalyst), 33 (1991) 444. 18 R.A. Sheldon, J. Mol. CiItol., 7, (1930) 107. 19 T. Tiitsti~iii,M. Y ~ I ~M. o ,Ni~kit~iit~r:~, Y. Yuhara and H. Toniiniiga, J. Mol. Cdtal., 78 (1993) L41.
Carbon Supported TS-1 Catalysts
P. Birke', P. Kraak', R. Pester', R. Schodell and F. Vogtz 1 Geschaflsbereich Katalysatoren, Leuna-Werke AG, D-06236 Leuna, Germany 2 Fachbereich Chemie, Martin-Luther-Universitat Halle-Wittenberg, D-062 17 Merseburg, Germany
ABSTRACT It is well known that titanium silicalite catalyses a number of oxidation reactions. Within the scope of our studies we have made an attempt at in situ synthesising of TS-I on different carriers. In dependence on the manufacturing method we can observe an intensive interaction between the titanium silicalite and the carrier. TS-1 fixed on active carbon will be the focal point of this contribution. Catalysts prepared in this manner were investigated in the ammoximation of cyclohexanone to cyclohexanone oxime. INTRODUCTION In recent years a great number of scientific papers has been published on the use and the characterization of titanium silicalite catalysts, e.g. [ 1 - 51. Reasons for this great interest are on the one hand the mild reaction conditions at which these catalysts work and on the other hand the great variety of reactions being catalyzed by titanium silicalites, for instance oxidation of alcohols to aldehydes or ketones, oxidation of hydrocarbons, epoxidation of olefinic compounds, hydroxylation of aromatics, synthesis of glycol monomethylethers, ammoximation of carbonyl compounds. Noteworthy are high activity and selectivity values for the ammoximation of cyclohexanone. This reaction has been investigated very thoroughly by members of MONTEDIPE S.P.A. and ENICHEM ANIC S.P.A. [GI, resp. as well as of RATNASAMY and co-workers [7]. As we see it two problems are linked with the use of the powdery titanium silicalite catalysts: the handling of the fine powdery titanium silicalite and the high manufacturing costs for these catalysts. BELUSSI and co-workers [8] improved the handling of the catalyst by incorporating titanium silicalite into a SiOz sol and by the subsequent spraying of the heterogeneous mixture. The catalyst particles obtained by this method have, however, the drawback that the active zeolite phase i s to a considerable extent enveloped by SiOz. Based on much experience in the development and manufacture of active component carrier catalysts we asked ourselves the question whether it would be possible to synthesize titanium silicalites on carriers in order to improve thereby the handling and to reduce the manufacturing costs 425
426
P. Birke, P. Kraak, R. Pester, R. Schodel and F. Vogt
considerably, Doing so we did not aim at the manufacture of the already well known Shell-type catalysts. Our objective was rather to synthesize titanium silicalite with MFI structure supported in situ. In this connection we used A l 2 0 3 , Si02, Ti02, ZrOz, amorphous aluminosilicates, and active carbon. We ask you to understand that due to patent rights we can not yet present any information on the manufacture of the catalysts. METHODS The titanium content was determined by means of the X-ray fluorescence analysis. The X-ray diffractometry was used to measure the crystallinity and the volume of the unit cells. The IR-spectroscopic characterization of the Si-0-Ti vibration was conducted by means of the KBr-press method. By means of a CHNS-analyser from FISONS Instruments the carbon portion of the titanium silicalites being supported on active carbon was measured. Statements on the size of the titanium silicalites on the carrier have been made on the basis of photographs obtained by a scanning electron microscope. For the characterization of the catalytic properties of the supported titanium silicalites in the ammoximation of cyclohexanone the "batch" mode of operation was selected. The catalyst, t-butanol and ammonia were charged into the reaction vessel. While strongly agitating cyclohexanone and 30 % H202 were subsequently continuously charged at a temperature of 80 "C within five hours. The tests were conducted at catalyst loads of 1.4 to 13 g of C,H,oO per g . h. The molar ratio of cyclohexanone : H 2 0 2 : NH, was maintained constant at 1 : 1 : 2.2. For processing the reaction mixture was extracted with cyclohexane after the catalyst had been separated. Cyclohexanone oxime, unreacted cyclohexanone and by-products were analyzed by means of capillary gas chromatograp hy. A long-term investigation at selected catalysts was conducted in accordance with a continuously working suspension method.
RESULTS Phvsico-chemical characterization The properties of the titanium silicalites fixed on active carbon will be the focal point of our expositions. It is well known that the substitution of larger titanium ions for silicon ions in the silicalite lattice will result in an expansion of the lattice [2, 91. Simultaneously, a transformation of the monoclinic structure into the orthorhombic structure is connected with the incorporation of titanium ions into the silicalite lattice. Figure 1 illustrates the dependence of the volume of the unit cells on the Ti/Ti+Si atomic ratio for unsupported titanium silicalites with MFI structure. The curve clearly demonstrates the expansion of the lattice with increasing titanium content. This dependence makes it possible to determine the portion of titanium being incorporated into the zeolite lattice. Using this curve for the characterization of the titanium silicalites with MFI structure being supported on the outer surface and also in the pores of active carbon we, however, found out that
Carbon Supported TS-I Catalysts
427
the connection between the volume of the unit cells and the titanium content, which has been established at titanium silicalites without any carrier, does not apply. UNIT CELL VOLUME in nrn' 594
5,32
0,Ol
0
0,02 x = nTi/(nTi + nSi)
0,04
0,03
Fig. I . Dependence of the unit cell volume of unsupported TS-I on the Ti content In Table 1 the volumes of the unit cells for titanium silicalite-active carbon-carrier catalyst, prepared by various methods, are listed. It is surprising that the values found are below the expected volumes of the unit cells. If the active carbon is carehlly burnt off within 24 to 30 hours and subsequently the volumes of the unit cells of the remaining unsupported titanium silicalite are measured, a significant increase of the volume of the unit cells can be proven. The extent of the lattice expansion, which can be identified on the basis of the differences in the volumes of the unit cells, is first of all determined by the manufacturing method. We were in a position to verify the shrinkage of the unit cell described here also at titanium silicalites being supported on oxides Table 1 .
Unit cell volume and position of the Si-0-Ti vibration for supported and unsupported TS- 1 -catalysts
CATALYST
CONTENT OF ACTIVE CARBON IN Ye
nm + 51
TS-1-A1 TS-1-A2 TS-1-A3 TS-l-A4
23.9 24,5 58,6 75.6
0,0294 0,0259 0,0303 0.0244
0
TS-1 TS-1-A1 TS-1-A2 TS-l-A3 TS-l-A4
C burnt off C bum off C burnt off C burnt off
TS-1 + active carbon 60 % TS-1 (mechan. mixture) 40 % active carbon
UNIT CELL VOLUME IN nm'
POSITION OF THE si-0-n VIBRATION IN c m '
5,3372 5,3390 5,3522 5.3574
949 947 950 961
0,0381
5,3721
966
0,0294 0,0259 0,0244
5.3720 5,3744 5,3695 5,3632
966 965 942 955
0,0381
5,3752
966
0,0303
P. Birke, P. Kraak, R. Pester, R. Schodel and F. Vogt
428
These experimental findings suggest a structure effect of the carrier on the zeolite component. Also the locations of the Si-0-Ti vibration bands within the IR-spectrum point out an interaction between carrier and titanium silicalite (Table 1): in the most cases in the supported titanium silicalites they are located at lower wave numbers than in the case of unsupported ones. At the moment we are not yet in a position to present a scientific explanation on the lattice shrinkage of the titanium silicalites caused by the influence of the carrier. We hope that we will be able to clear up this phenomenon by investigations on the crystallization mechanism. What influence is exercised on the catalysts by the verified lattice disturbances? Catalytical results Table 2 lists the conversion and the selectivity in the ammoximation of cyclohexanone obtained with various titanium silicalite-active carbon catalysts and an unsupported titanium silicalite. We can recognize that the supported catalysts with a zeolite portion of 40 to 76 weight per cent show similar high activities as the unsupported titanium silicalite. If the catalyst amount is decreased and if the measurements are conducted in the range of partial conversions of 30 to 60 % and if the velocity of the oxime formation referred to the zeolite portion is determined we recognize that the supported titanium silicalite phases show higher ammoximation activities than the unsupported titanium silicalite. This effect is particularly distinct in the case of catalysts with high carrier portions. The measured activity data point out that the lattice defects obviously exercise a positive effect on the ammoximation activity. In addition, however, also the positive effect of the better access of the supported titanium silicalites regarding the formation of cyclohexanone should be taken into consideration. Table 2.
Activity and selectivity of active carbon supported TS-1 catalysts at a high degree of conversion
Catalyst
1 ~
~
Content of active carbon in %
Conversion in %
Selectivity in %
TS-1Al
23.9
90.8
99,7
TS-1-A2
24,5
91,5
99,8
TS-1-A3
58.6
88,l
99,8
TS-l-A4
75.6
84.2
99,8
TS-1A5
39,4
92,4
99,6
0
93,l
99,7
TS-1
The catalytic properties of the supported titanium silicalites are strongly determined by the purity of the used active carbon. In Table 3 the activity and selectivity data for a supported titanium silicalite with acid washed active carbon and for a catalyst with non-pretreated active carbon have been compiled. The comparison of the measuring data shows that the active carbon, which has not
Carbon Supported TS-I Catalysts
429
been washed with acid, has a distinctly lower ammoximation activity than the pure active carbon. We attribute this decrease on the metal ion content (mainly Ca2+ ions) of the unwashed active carbon resulting in a Hz02 decomposition and, therefore, reduces the H,02/NH3 ratio. Table 3 .
Influence of the purity of the active carbon on the activity and selectivity of the TS-Iactive carbon catalysts
Catalyst
Content of active carbon in weight %
Pretreatment
Oxime yield in %
Selectivity in %
TS-1 -A5
39,4
acid washed
92,4
99,6
TS-l-A7
40,O
untreated
22,7
94.6
The ammoximation activity of the titanium silicalites supported on active carbon furthermore depends on the titanium content. In Figure 2 the oxime yields are plotted against the atomic proportion Ti/Ti+Si for titanium silicalite-active carbon carrier catalysts with zeolite portions of 40 to 42 weight per cent. The behaviour of the curve shows that the ammoximation activity increases with a growing titanium content. The increase of the activity is explained by the concentration of the catalytically active Si-0-Ti-OH species growing with an increasing incorporation of titanium. 94
OXIME YIELD in %
92 90 88 86
84
82 80 0,02
I
0,03 x = nTi/(nTi+ nSi)
I
0,04
Fig. 2 . Oxime yield dependence on the Ti content of active carbon supported TS-1 catalysts with zeolite portions of 40 to 42 weight percent The investigations regarding the influence exercised by the particles of the supported titanium silicalites on the catalytic properties revealed that catalysts with comparable compositions but with different particle sizes in the range of 0.1 to 0.8 pm do not show any significantly different activities and selectivities in the ammoximation reaction.
430
P. Birke, P. Kraak, R. Pester, R. Schodel and F. Vogt
The tests to optimize the reaction conditions ended in the following results. Figure 3 illustrates the correlation between conversion and selectivity with the example of two titanium silicalites supported on active carbon and one unsupported titanium silicalite.
Fig. 3 . Influence of the oxime yield on the selectivity of the oxime formation by changing the space velocity In the case of low conversions the selectivity for the oxime formation will dramatically decrease. Obviously, under those conditions the unreacted cyclohexanone increasingly enters into side reactions. Thereby first of all the following undesirable products develop: the azine of the cyclohexanone and the peroxy-dicyclohexylamine; additionally, 2-cyclohexylidene cyclohexanone and 2-( 1 -cyclohexen-1 -yl) cyclohexanone will develop by condensation reaction. 100
SELECTIVITY in
CONVERSION in %
100 99,a
90
99,6 */--
ao __---
__--__--___---
99,4
+ - - - - - -
*- - - t ~ ~ r i e e i ~ o n - ( r S - l A l O-t) Selectivity (TS-1A10) !+Conversion ITS-11 70
t Selectivitv (TS-11
1
99,2
I 99
Fig. 4. Influence of the mole ratio H202/Cyclohexanone on the conversion and the selectivity of the ammoximation; solvent: in the case of TS-1 t-butanol, in the case of TS-1 A10 toluene; (sample TS-IAlO: 38.3 % C, TilTi + Si = 0.0226)
Carbon Supported TS-I Catalysts
431
The same circumstances result in the tests regarding the dependence of the activity and selectivity on the molar ratio H202/cyclohexanone as is shown by Figure 4 for active carbon supported TS-1 and non supported TS-1. High conversions correspond to high selectivities with reference to each of these two catalysts. Consequently, the ammoximation must be conducted under conditions of high conversions in order to minimize the formation of by-products. As shown by Figure 5 , the conversiodselectivity are also strongly influenced by the reaction temperature. At the test conditions selected by us a reaction temperature of 80 "C proved to be most suitable. 100
SELECTIVITY in '
CONVERSION in %
ao
100 80
Selectivity (TS-IAIO) *Conversion (TS-1) 13
60
60
40
40
20
20
0
20
I
I
30
40
I
I
I
I
50 60 70 80 TEMPERATURE in "C
I
I
90
I00
0
D
Fig. 5 Influence of the temperature on the conversion and the selectivity of the ammoximation; solvent: in the case of TS-I t-butanol, in the case of TS-IAlO toluene; (sample TS-IAIO: 3 8 . 3 % C, Ti/Ti + Si = 0.0226) Based on the obtained results optimum test conditions for a continuous mode of operation were stipulated and a long-time test with a titanium silicalite supported on active carbon was conducted over a period of 3 5 0 hours without any activity decrease at a conversion of 99.9 % and a selectivity of 99.9 YO. CONCLUSIONS 1. Titanium silicalites with MFI structure supported on active carbon show lattice shrinkages. 2. Referred to the zeolite portion, supported titanium silicalites have a higher ammoximation activity than unsupported titanium silicalites. 3 . The ammoximation activity increases in the range of the Ti/Ti+Si ratio of 0.02 to 0.042 with increasing titanium content. 4. In order to minimize the formation of by-products the oxime formation must be conducted under the conditions of high conversions.
432
P. Birke, P. Kraak, R. Pester, R. Schodel and F. Vogt
REFERENCES 1 A. Zecchina, G. Spoto, S. Bordiga, A, Ferrero, G. Petrini, G. Leofanti and M. Padovan, in P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova (Ed.), Zeolite Chemistry and Catalysis, Amsterdam, 1991, p. 251. 2 A. Thangaraj, R. Kumar, S.P. Mirajkar and P. Ratnasamy, J. Catal., 130, (1991) 1. 3 A. Thangaraj, R. Kumar and P. Ratnasamy, J. Catal., 131, (1991) 294. 4 D.R.C. Huybrechts, P.L. Buskens and P.A. Jacobs, J. Mol. Catal., 7 1 (1992) 129. 5 G. Belussi, A. Carati, M.G. Clerici, G. Madinelli and R. Millini, J. Catal., 133 (1992) 220. 6 P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti and R. Vagarnolo, Chim. Industr., 72 (1 990) 598. 7 A. Thangaraj, S. Sivasanker and P. Ratnasamy, J. Catal., 131 (1991) 394. 8 G. Belussi, M. Clerici, F. Buonomo, U. Romano, A. Esposito and B. Notari, EP 200 260. 9 R. Millini, E. Previde Massara, G. Perego and G. Bellusi, J. Catal., 137 (1992) 497. ACKNOWLEDGEMENT The authors gratefblly acknowledge the financial support of the Ministerium fiir Wirtschafi, Technologie und Verkehr des Landes Sachsen-Anhalt and the Treuhandanstalt Berlin
Alteration of Alumina Pillared Clays for Enhanced Catalytic Activity
A. Clearfieldl, H. M. Aly, R. A. Cahill, G. P. D. Serrette, W.-L. Shea and T.-Y. Tsai Department of Chemistry, Texas A&M University, College Station, Texas 77843, U.S.A.
ABSTRACT Saponite and Montmorillonite A113 pillared clays were subjected to heat treatments up to 75OOC. The pillars were found to rehydrate and exhibit enhanced cation and anion exchange capacities. Treatment of the pillared clays with dilute HCl solubilized increased amounts of aluminum accompanied by an increase in surface area and pore size. Treatment of the pillared clays with sulfate or phosphate ion increased the number of Bronsted and Lewis acid sites. 27Al solid-state NMR spectra indicated formation of a ?A1203 type structure for the pillars. INTRODUCTION The field of pillared layered materials is a rapidly expanding one. Reviews cover the literature to 1990 [l-41 and furnish a good overview of the research in progress. The attractive feature of this research is the conversion of two-dimensional layered compounds into porous threedimensional materials. By control of the synthetic process it was hoped that the size of the pores could be controlled within certain size ranges. The original expectation was that pores larger than those available in zeolites could be prepared for use in cracking of the resid portions of petroleum. This hope has not been fulfilled, principally because the pillared clays used in such reactions collapse during the decoking process. Never-the-less interesting porous materials have been produced. Through the choice of layered compound and pillaring agents a wide variety of pillared types of products can be synthesized. We have arbitrarily divided these material types into pillared clays and pillared non-clay layered products. The clays usually chosen for pillaring are the smectites. These clay minerals have a low layer charge ranging from 0.37 e- to 1.1 e- per unit cell, corresponding to 0.5 to 1.5 meq/g of exchange capacity. Because of the low charge, members of this family of clay minerals spontaneously swell in water so that a polyoxymetal cation can be inserted between the layers to prop them open. Heating this assembly to above 5OOoC fixes the pillars permanently. However, it was shown that this heating step largely destroys the Bronsted acid sites and the resultant pillared clays are essentially Lewis acid catalysts [5,6]. Thus, there is a need to increase the Bronsted acidity for reactions to be carried out at temperatures lower than required for cracking. A general procedure which is just now emerging is to add sulfate or phosphate ion to the pillared clay [7,8]. It has been shown that sulfate ion added to hydrous oxides, 433
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A. Clearfield, H . M. Aly, R. A . Cahill, G . P D. Serrette, W.-L. Shea and T.-Y. Tsai
particularly hydrous zirconia, converts the oxide to a superacid [9]. The effect of such treatments on pillared clays is worthy of further study. The problem with pillaring of layered compounds other than clays is their high layer charge. This charge prevents the layered materials from swelling and as a result no exchange with the large pillaring cation is obtained. The problem was solved by first intercalating an amine between the layers to enlarge the interlayer spacing [lo]. The pillaring cation can then exchange for the alkyl ammonium ions formed by the intercalation reaction. This reaction is quite general and has been carried out for a variety of phosphates and oxides [ l l , 121. The difficulty is to achieve microporosity in the face of the higher layer charges of these compounds. Methods of achieving high porosity in layered phosphates and oxides will be presented in a subsequent paper. In this paper we shall confine the discussion to certain aspects of the chemistry of pillared clays relevant to their use as catalysts. EXPERIMENTAL on of Pillared Clay (PILm Two clay minerals were used for this study, a saponite and a montmorillonite. Two versions of the same montmorillonite were utilized. Both were the well-known STx-1. However, one was obtained from the clay repository and purified by us by wet sedimentation. The other was obtained in purified form from the Southern Clay Products, Inc. The aluminum Keggin ion was prepared by dissolving 15 g of AlCly6H20 (0.062 mol) in 250 ml of water. To this solution was added 350 ml of 0.043M NaOH (0.150 mol, OWAl = 2.43). An NMR spectrum of 27Al showed a single peak at 63.3 ppm indicating the presence of the All3 Keggin ion. Several procedures were used to pillar the clays. Both dilute (7.95 x lO-3M) and moderately concentrated solutions (0.0347M) of the A113 Keggin ion were used at room temperature and 60°C. The ratio of Keggin ion to clay varied from 5 to 1 to 1.5 to 1. Contact was maintained from 2 to 10 h. The pillared clays were collected and washed by centrifugation and then heated to different temperatures up to 750°C for 3-4 h. .. cidified In order to improve the acidity of the pillared clays, portions were treated with NH4H2PO4 or NHqHSO4. Approximately 1 g of the pillared montmorillonites (Southern Clay Products, Inc.) were slurried in 80 ml of the 0.1M ammonium phosphate or sulfate solution for 10 h, filtered and dried at 65°C without washing and then heated at 300OC for 15 h. Prior to this treatment the clays were heat treated at 200°C or 344°C for 15 h. Other samples were treated similarly for 36 h.
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Titration's were carried out by the static method. 0.25 g samples of the pillared clay was added to 25 ml of solutions 0.1 M in (NaCl t NaOH) or (NaC1 + HCl) and equilibrated for 24 h. The pH was then recorded and the amount of H+ or OH- generated by the reaction determined as a difference between the measured pH and that of a blank to which no clay had been added. At the high and low pH ends of the titration the filtrates were recovered and analyzed for aluminum. Acid/Base Stability One gram samples of the Al-PILCs, heat treated to temperatures ranging from 200 to 7OO0C, were added to 100 ml of HC1 or NaOH ranging in concentration from 0.005 to 0.1 N. The
Alteration of Alumina Pillared Clays
435
suspensions were shaken for 2 days at room temperature and separated by filtration. The solid was washed free of acid or base and the filtrate and washings analyzed for Al3+. The unpillared clays were also treated in this fashion as a control. The solids were dried at 60°C and their XRD patterns and surface areas obtained. Insmmental Metho& Infrared patterns were obtained with a Digilab FTS-40 spectrometer. Pellets for pyridine adsorption were prepared by pressing 25 mg of sample into a thin wafer and then placed in a special vacuum cell fitted with CaF2 windows. The samples were heated at 300°C for 10 h under reduced pressure. Adsorption of pyridine was carried out at 5OoC by exposure of the wafer for 10 h at a pressure of -75 mm. Desorption of physisorbed pyridine was carried out at l00OC under vacuum for 10 h. Surface areas were determined on a Quantachrome Autosorb-6 automated gas adsorption system. Approximately 0.5 g samples were degassed at 200°C for 24 h in a vacuum oven and subsequently at 25OOC under high vacuum in the Autosorb unit. Nitrogen sorption/desorption isotherms were obtained at -196OC and analyzed both by the BET and Langmuir models. The micropore volume was determined by the t-method of DeBoer. NMR spectra for 27A1 were obtained on a Bruker MSL-300 solid state NMR spectrometer at 78.2 MHz. The aluminum chemical shifts were referenced to external A1@€20?: as derived from a 0.1M aqueous AICl3 solution. The rotor was spun under dry N2 at a rate of 3.5 KHz and the pulse angle was set at 3OOC. Elemental analysis was carried out by electron microprobe at the Texas A&M Electron Microscopy Center. RESULTS AND DISCUSSION Pillared Clavs X-ray diffraction patterns of the pillared saponite, unheated and calcined at 500°C are shown in Figure 1. It is seen that the pillared clays show a high level of regularity with 4 orders of (00l)
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A. Clearfield, H. M. Aly, R. A. Cahill, G . P. D. Serrette, W.-L. Shea and T.-Y. Tsai
reflections present. The montmorillonitecontained about 5 7 % of quartz as shown by reflections at 4.276, 3.354 and 2.463A. The saponite also contained a small amount of quartz and an impurity with d-spacing of 4.595A, that has not been identified. A typical nitrogen sorption/desorption isotherm is presented in Figure 2. Surface areas were determined by the BET N2 sorption method and found to depend upon the method of pillaring and the subsequent heat treatment. For example, for two of the pillared montmorillonites the surface area increased from -300 m2/g when dried at 200°C to maximum values of 414 and 392m2/g at 400°C and then decreased by about 12-14% when kept at 500OC for 3 h. In each case the percentage of the surface area in the micropore region was -85%. Pore volumes were of the order
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,
.
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Fig. 2. B.E.T. N2 isotherm for aluminum pillared montmorillonite STx-1 preheated to 400°C for 3 hours (S.A.=414 m2/g). of 0.25 cc/g. Formulas for the clay samples are presented in Table 1. They are based on microprobe analysis without standard and therefore are of an approximate nature. However, the pillared clay obtained from Laporte Absorbents was analyzed by them. This sample had a surface area of 250 m2/g and had been calcined at 550°C. The saponite pillared clay was more thermally stable than the montmorillonite. On heating to 700OC it still exhibited a dml reflection at 17.6A and retained its microporosity. Further heating at 75OOC resulted in collapse of the layers. The pillared montmorillonite was stable to 650°C at which temperature the basal spacing was reduced to 17.5A. On heating at 700°C this sample collapsed with loss of the micropore volume. Dyer and Gallardo, in an important paper [13], showed that both an A113 and a zirconia pillared montmorillonite exhibited much higher cation and anion exchange capacities than the original clay. This increased capacity could only arise from contributions by the pillars. However, it has been assumed that on heating to 500-600"C, the pillars have essentially been converted to oxide. Therefore, the pillars must rehydrate in order to participate in ion exchange reactions. We
Alteration of Alumina Pillared Clays
437
Table 1. Formulas of clay minerals before and after pillaring with aluminum Keggin ion Name
Formula
1.13 from formula 1.10 with impurity 1.02plus 1.62H2O 1.05 from formula 0.94 with impurities 0.88 plus 1.5H20
STx-lb Nao.31~Ko.o78[Mgo.38gA11.61lS~01o(OH):! Montmorillonite Pillaredb STX-1
0.841 from formula 0.729 with 1.5H20 impurities
[A11304(0H)27.11(H20)8.8910.10 [All .61MgO.3891 Si401o(OH)2
0.826 calcined Pillaredc NaO.1 b.O16CW.O22[A1130 19.5lO.i02H0.263[Ali.577 0.98 DV Laporte Mg0.4231SUO10(OH)2 0.95 with impurities aContains 0.2 mole Si02 as quartz. b Contains 0.086 mole Fe2O3, -0.5 mole quartz and 0.012 mole Ti02 as impurities. CContains 0.092 mole Fe2O3 and 0.012 mole Ti02 as impurities. have carried out a study earlier [14] on the stability of an A113 pillared montmorillonite, from Laporte Industries, calcined at 55OOC. Titration data yielded a Na+ ion capacity of only 0.2 meq/g @H=l1.1) and an anion exchange capacity of 1.75 meq/g (pH=2.7). At higher and lower pH values aluminum was removed from the clay with decrease in capacity. Figure 3 shows titration data for our STx-1 pillared montmorillonite. A similar curve was obtained for the pillared saponite. Each of these had been heated to 500°C prior to exchange. For the pillared montmorillonite the Na+
0
2
4
6
8
10
12
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PH Fig. 3. Effect of pH on the cation and anion exchange capacity of NaCl on the aluminum Keggin ion pillared montmorillonite STx-1calcined at 500OC for 3 hours. capacity was 0.63 meq/g at pH=11.2. At higher pH increasing amounts of aluminum were solubilized accompanied by a sharp drop in sodium ion uptake. In acid solution C1- ion uptake increase started at pH=4.9 and reached a maximum of 1.55 meq/g at a pH of 2.3. At higher levels
A. Clearfield, H . M . Aly, R . A. Cahill, G. P. D. Serrette, W.-L. Shea and T.-Y. Tsai
438
of acidity increasing amounts of aluminum were solubilized accompanied by decreased C1- uptake. The comparable values for the pillared saponite were 1.23 meq Na+ uptake per gram of exchanger at pH=12.36 and 1.76 meq/g at pH 2.7. We note that the anion exchange capacity is fairly constant but the cation exchange capacity was quite different for the three samples. Saponite derives its exchange capacity from substitution of Al3+ for Si4+ in the tetrahedral sites. Thus, none of the protons which neutralize the layer charge are lost by diffusion into the layer as may occur for montmorillonite. This is undoubtedly the case for the Laporte sample. Much less loss of protons was observed in our montmorillonite, probably because the calcination temperature was 50°C lower than used for the Laporte sample. In Table 1, we have listed the theoretical cation exchange capacity of the saponite (unpillared) as 1.10 meq/g. The estimated cation exchange capacity for the pillared sample (with no contribution from the pillars) is 0.909 meq/g. This value is considerably less than the 1.23 meq/g observed. Clays have very low anion exchange capacities so we may assume that almost all of the C1- exchange capacity results from exchange with the pillars. A simple calculation, taking into account the presence of impurities and water content, shows that the observed exchange capacities for C1- amount to about 9 C1- per pillar. This number of chloride ions indicates that a considerable amount of rehydration must occur when the pillared clays are contacted with water. Evidence for the amount of rehydration was obtained by observation of the water content of the calcined pillared clays. Typical thermogravimemc weight loss curves are shown in Figure 4. Reference to Figure 4A shows that the air dried pillared saponite lost 16.28% H 2 0 to 695OC. This water includes that held in the pores and the water coordinated to the alumina pillars together with
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Fig. 4. Thermogravimemc weight loss curves of (A) air dried pillared saponite and (B) the same sample heated at 55OoC for 3 hours. water formed by the split-out of hydroxyl groups. Heating to 550OC for 3 hours followed by rehydration for 24 h yielded the TGA curve shown in Figure 4B. We note that the rehydrated sample lost almost as much water (15.42%) as the original sample. Of greater importance is the fact that the rehydrated sample lost 3.3% water between 200OC and 700OC. The original pillared saponite lost 5.34% water in this same temperature interval. Thus, more than 50% rehydration
Alteration of Alumina Pillared Clays
439
occurred. Even when the sample was kept at 65OOC for 3 h about 2.2% water was lost from the rehydrated sample in the hydroxyl split out temperature interval. The results reported here form the basis for understanding the instability of pillared clays to the decoking process. The steam generated rehydrates the pillars which are then open to attack by acids and bases or thermal collapse. Reference to Figure 3 shows that solubilization of the pillars takes place if the titrating solution is too acid or too basic. To determine how much aluminum is solubilized we subjected 0.5 g samples of both the pillared and unpillared clay minerals to 50 ml portions of solutions of HCl and NaOH of varying concentrations from 0.005 to 1.OM. The suspensions were shaken intermittently while being held at room temperature for times up to 5 days. A typical dissolution rate curve is shown in Figure 5 . It is seen that the solubility increases over a 2 day period to a maximum of 1.44 mmole of A1 in 0.1M HC1. The comparable value for
P -a El E 0
0
50
100
150
TIME, h.
Fig. 5. Rate of dissolution of aluminum in pillared montmorillonite that had been calcined at 500°C for 3 hours. the unpillared montmorillonite was 0.065 mmoles/g. Thus, it is clear that the bulk of the aluminum dissolved arises from the pillars and amounts to slightly over 40% of the pillar material. Somewhat less aluminum was removed in 0.1M NaOH, the value for STx-1 being 1.08 meq/g. Surprisingly this treatment did not always destroy the porosity (vida infra). The solubility decreased by about 20% in 0.05M acid or base and by 85% in 0.01M solutions. These solubility values also depended strongly on the temperature and time of calcination, decreasing rapidly at temperatures above 5OOOC. This decrease is apparently tied to the amount of rehydration of the pillars. The higher the temperature and t i e of calcination, the lower is the degree of rehydration and solubilization in acid or base. In addition, there was less chance that the pillars would collapse with the samples calcined at higher temperatures. In the case of the Laporte clay, it was found 1141 that not only was the interlayer spacing maintained on acid treatment, but the surface area and pore volume increased by about 30% for the 0.1M HCl treatment. Treatment of the clay samples with 1M HC1 or NaOH resulted in increased Al3+ removal with consequent collapse of the pillared structures. This instability of the pillars, the change in ion exchange capacity, pore volume and surface area indicates that heat treatment of the pillared clays does not lead to a simple formation of aluminum oxide.
440
A. Clearfield, H . M . Aly, R . A. Cahill, G . P. D. Serrette, W.-L. Shea and T.-Y. Tsai
It is well known that when aluminum hydroxide is heated to above 45OOC it forms y-AlzO3. This phase of alumina has the spinel structure with 2$ A1 atoms distributed among the 16 octahedral and 8 tetrahedral sites [15]. Therefore, on heating the All3 pillars, it would be expected that the 27Al resonance peak for tetrahedral aluminum might increase relative to the resonance peak for octahedral aluminum. We attempted to obtain evidence for this effect from NMR studies. However our montmorillonite samples contained 5 6 % iron which broadened the signals and prevented any clear detection of tetrahedral 27Al. In the case of the saponite all the aluminum in the unpillared clay is in the tetrahedral silicon sites. This fact made it difficult to assess whether additional tetrahedral aluminum was generated by the dehydration process. However, there appeared to be an increase in the resonance peak at 63.1 ppm at the expense of the resonance peak at 1-2 ppm on heating. The presence of overlapping side bands further clouded the issue. We therefore examined the All3 Keggin ion pillared perovskite HCa2Nb3010 [12,16]. The spectrum is shown in Figure 6. The unheated pillared calcium niobate, yielded a spectrum with one peak at approximately 0 ppm. Upon heating at 35OOC for 15 h a second peak of equal intensity was obtained positioned at -64 ppm. This peak decreased in size with heating to higher temperatures until at 80O0C the peak for octahedral aluminum was twice the size of the one at 64 ppm. Clearly dehydration of the Keggin ion pillars appears to yield a form of y-alumina. In this state the alumina
+--+
i--"~"""'"~''' 0 -200 -400 -600
600 400 200
PPM
Fig. 6. Aluminum-27 solid state magic angle spinning NMR spectrum for aluminum pillared HCa2Nb3010 heated to different temperatures. Asterisks denote spinning side bands. can readily rehydrate yielding a reactive hydrous oxide with both cation and anion exchange capability. The pillars can thus be altered to impart added catalytic activity by coating them with metals or oxides as is done with ordinary y-alumina. Furthermore, calcination at higher temperatures shifts the tetrahedral alumina back to octahedral coordination, indicative that the structure of the alumina is shifting towards that of corundum (a-Al2O3).
Alteration of Alumina Pillared Clays
441
and Phospbted Pillared Clays Based on the results of our acid treatment of the clays, we chose to use 0.1M (NH4)HSOd and 0.1M (NH4)2HP04 as reagents to treat our clays. The pillared montmorillonite prepared in this study, unless heat treated to 344°C (or higher temperatures), completely collapsed into nonporous structures upon treatment with the diammonium hydrogen phosphate solution. Even the sample heated to 344OC showed a major decrease in surface area from 254 m2/g to 145 m2/g. The ammonium bisulfate treatment proved to be much less destructive, as only a 7% reduction in surface area was noted for the sample preheated to 344OC. By way of comparison the Laporte sample, treated similarly, maintained its surface area and microporosity. The change in acid character of the treated samples is shown in Figure 7. The untreated STx-1 montmorillonite heated to 344°C contained a high level of Lewis acid sites as shown by the lower curves in Figure 7.
2600
1500
1400
1300
1600
1500
1400
1300
(B)
(A)
Wavenumbers Fig. 7. Infrared spectrum in the pyridine sorption region of the spectrum. (A) bottom curve is unsulfated aluminum pillared montmorillonite, top curve sample treated with ( N b ) H S 0 4 , (B) same as A but treated with (NH&HPO4. Both treated samples exhibited large increases in both Bronsted and Lewis acid sites as shown by the upper curves in Figure 7. The band at 1483 cm-I represents pyridine sorbed on both types of acid sites. The stability of these acid sites under catalytic conditions is under investigation.
CONCLUSIONS We have shown that it is possible to alter the properties of alumina pillared clays by post pillaring treatment. These alterations include increasing the surface area and pore size, incorporating a greater number of Bronsted and Lewis acid sites, and incorporation of active species
442
A. Clearfield, H. M. Aly, R. A . Cahill, G . P. D. Serrette. W.-L. Shea and T.-Y. Tsai
through ion exchange. Thus, the catalytic chemist or engineer has many options available to build in specificity for catalysis of desired reactions. ACKNOWLEDGMENT This research was supported by the National Science Foundation under grant number DMR9107715 for which grateful acknowledgment is made. One of us (G.P.D. Serrette) wishes to thank the State of Texas for support with a minority fellowship under the State's Advanced Technology Program. We wish to thank Laporte Absorbents, Chisire, U.K. for supplying a sample of pillared montmorillonite. REFERENCES
1 R. Burch (Ed.), Pillared Clays. Catalysis Today, Elsevier, New York, 1988 Vol2. 2 I. V. Mitchell (Ed.) Pillared Layered Structures: Current Trends and Applications; Elsevier, London, 1990. 3 A. Clearfield and M. Kuchenmeister in Pillared Layered Materials, Supramolecular Architecture, T. Bein (Ed.), ACS Sym. Series No. 499, Am. Chem. Soc.,Wash. D.C., 1992. 4 T. J. Pinnavaia, Science, 220 (1983) 365. 5 (a) M. L. Occelli, Catalysis Today, 2 (1988) 339. (b) M. L. Occelli and R. M. Tindwa, Clays Clay Miner., 31 (1983) 22. 6 M.-Y. He, Z. Liu and E. Min, Catalysis Today, 2 (1988) 321. 7 Y. F. Shen, A. N. KO and P. Grange, Appl. Catal., 67 (1990) 93. 8 E. M. Farfan-Torres and P. Grange, Catalytic Science and Technology, S . Yoshida, N. Takezawa, T. Ono (Eds.), KodanshaElsevier, Tokyo, 1 (1991) 103. 9 K. Arata and M. Hino, Mater. Chem. Phys., 26 (1990) 213. 10 A. Clearfield and B. D. Roberts, Inorg. Chem., 27 (1988) 3237. 1 1 A. Clearfield in Multifunctional Mesoporous Inorganic Solids, A. C. Sequeira, M. J. Hudson, (Eds.), Kluwer Acad. Publ., Amsterdam 1993, pp.159-178. 12 S. Hardin, D. Hay, M. Millikan, J. V. Sanders and T. W. Turney, Chem. Mater., 3 (1991) 977. 13 A. Dyer and T. Gallardo in Recent Developments in Ion Exchange, P.A. Williams and M. J. Hudson, (Eds.), Elsevier Sci. Publ. 199 ,pp.75-84. 14 A. Molinard, A. Clearfield, H.Y. Zhu and E. F. Vansant, Microporous Solids, submitted. 15 A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, U.K. 5th Edition, 1984, pp.55 1-2. 16 R. A. Mohan Ram and A. Clearfield, J. Solid State Chem., submitted.
Development of Pillared clays for Industrial Catalysis
Enze Min Research Institute of Petroleum Processing, China PetrochemicalCorporation, P.O.Box 914, Beijing 100083, China
ABSTRACT With a purpose of industrial application of pillared clays, Directed Basic Research has been conducted on their acidity, thermal stability, and catalytic characteristics. A hydrocracking catalyst was developed through combining Al-pillared montmorillonite and intercalated palladium metal for use at mild temperatures. The stability of the layer structure and the interaction between layer and pillar are essential for the thermal stability of the pillared clay. A pillared rectorite based catalytic cracking catalyst was developed. Pillared clays have also been proved to be suitable for aromatintion and benzene alkylation with larger hydrocarbon molecules.
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Pillared clays are relatively new catalytic materials. They are characterized in their pillaredinterlayered structure which provides two dimensional large open pores [ I]. The open pore structure is obviously attractive for catalytic reactions of large size molecules such as catalytic cracking of petroleum residue [2]. The great diversity [3-41 of clay and pillar combinations offer a new, technologically unexplored fiontier. It is well known that unceasing efforts have been made to pursue the industrial applications of pillared clays since last decade. This paper focuses on our Directed Basic Research program for development of industral pillared clay catalysts with emphasis on: nature of acid sites and the reaction environmental requirement for its maintenance; structural factors and hydrothermal stability; catalytic characteristicsof open pore structure. ACIDITY CHARACTERISTICS AND APPLICATION IN MILD TEMPERATURE HYDROCRACKING Nature of acidic properties of Al-pillared montmorillonite (Al-PMJ Lewis acidity. Ammonia and pyridine adsorption combined with IR and differential thermal analysis was conducted to investigate the dependence of Lewis acidity of Al-PM on pretreatmen! temperature [S]. Al-PM pretreated at 500 OC displays mainly Lewis acidity. Lewis acidity of Al-PM increases with the increase of pillar density almost proportionally.
443
444
E. Min
Bronsted aciditv r5l. IR and ammonia or pyridine adsorption were again used to examine the dependence of Bronsted acidity of Al-PM on pretreatment temperature. The results show that Bransted acidity declines rapidly when the pretreatment temperature is higher than 350 "C. However, Bronsted acid sites are abundant on AI-PM samples pretreated at a temperature lower than 350 "C. Thus it is suitable for catalyzing hydrocarbon conversion reactions that occur at mild temperatures. BrOnsted aciditv maintenance Bronstcd acidity of A-PM mainly originates from hydroxyls associated with four coordinated aluminum and six coordinated aluminum in layer-pillar structure. An important new finding [6] is that the water associated with alumina pillars in the interlayer space of the pillared clay can dissociate to provide protons. Moreover, proton migration occurs between layer structure and interlayer space and hence influences BrOnsted acidity significantly at mild temperatures. Based on the understandingof water dissociation and proton migration, IR and ammonia adsorption experiments were performed to examine the dehydroxylation of the layer structure of AI-PM with temperature increase and the reverse hydroxylation upon water vapor uptake, and also, the corresponding diminishing and restoring of Bronsted acidity [7]. The results indicate that the existence of water vapor in the reaction environment of AI-PM in certain temperature range plays an important role in retaining BrOnsted acidity of AI-PM. Application in mild temperature hvdrocraeking Based on the results of Directed Basic Research conducted, the following guidelines could be established for industrial application of AI-PM : 0 since Al-PM displays intensive Bransted acidity only at a mild temperature below 350 "C, the preparation and reaction condition for Al-PM should be in a temperature range below this temperature; 0 water vapor can be used as an agent for reaction environmental control to maintain Bronsted acidity. Due to the temperature limitation, the exploratory work aimed at a hydrocrackingcatalyst used at mild temperatures was performed. Palladium was selected owing to its high hydrogenation activity at mild temperatures. A catalyst, designated as Pd-Al-PM, was thus prepared by pillaring montmodonite with a complexed pillaring agent which was prepared from polyhydroxyaluminium cations and palladium chloride. The compositionand properties of Pd-Al-PM are listed in Ta6le 1. T h e prope&x of a gas oil feed are listed in Table 2. Since Y type zeolites are usually used as the active components in hydrocracking catalysts, the hydrocracking activity of Pd-Al-PM is compared with that of Pd-HMgY and Pd-REUSY catalysts at 250 "C in a micro-reactor with a catalyst loading of 2 g under a reaction condition of: 3.0 MPa, LHSV 1.5 and a moil volume ratio of 800. The results listed in Table 3
Pillared Clays for Industrial Catalysis
445
show that the activity of Pd-AI-PM is sigruficantly higher than those catalysts based on Y type zeolites at mild temperatures such as 250 "C. Table 1. Composition and physical properties of Pd-AI-PM. Composition, wt% A L a Fez03 CaO
M@
LO
N&O SiOz Pd
19.22 1.76 0.14 2.21 0.20 0.60 68.33 0.59
Basal spacing, nm
1.86
Surface area, m*/g
386
Pore volume, mYg
0.24
Table 2. Properties of feed for hydrocracking. Density (20 "C), g/d Aniline point,"C Gum, mg/lOoml s, PPm N: PPm Distfflation range, "C Initial point 10 % 30 Yo 50 Yo 70 Yo 90 Yo
End point BMCI
0.8004 94.8 15.2 2.0 4.02 233 260 287 305 3 19 338 353 9.12
Table 3 . A comparison of hydrocracking activity at 250 "C. Catalyst Pd-AI-PM Pd-HMgY Pd-REUSY
Conversionlevel, wt% 74 38 33
446
E. Min
Hydrocracking of the feed was conducted in a 100 ml reactor under a reaction condition of 6.0 MPa, 260 "C and a Woil volume ratio of 1000. For maintaining Bronsted acidity, water vapor content in the reaction environment was carefully controlled. The results listed in Table 4 show thai high conversion level and liquid yield are attained at a rather mild temperature of 260 "C using PdAl-PM catalyst. Hydrocracking product was cut into several distillates for evaluation. The saturate content in the whole product was more than 98 wtYo according to the results of adsorptionseparation using silica gel column. Olefins and aromatics in the product were undetectable. Table 5 lists the properties of different product cuts, which seem good enough to meet the requirements of high quality kerosene and solvents. For further checking the aromatic content of the products, both gas chromatography and W spectroscopy were used for detection. The results indicated that their aromatic contents were undetectable by these methods. The stability of Pd-Al-PM catalyst was tested in the 100 ml reactor at 6.0 MPa for 2000 hrs. At the end of the run the conversion still remained at a level of 80 wt% when the reactor temperature was only 260 "C. It is evident that PdAI-PM catalyst has good stability. Pilot plant hydrocracking of the feed over Pd-.N-PM catalyst was performed in a 10 liter reactor. The conversion could reach a level between 60-80 wt% at 230 "C. The liquid yield attained 100 wt%. The properties of the product were similar to that of the product from the 100 ml reactor. Therefore, a mild temperature hydrocracking process was developed for producing aromatics-freekerosene and light solvents from light gas oils. Table 4. Product distributionof light gas oil hydrocrackingon Pd-AI-PM catalyst. Liquid Hourly Space Velocity. hr 1.5 2.5 Conversion, wt% Liquid yield, wt% Product distribution,wt% -120°C 120-154 "C 154-260 "C gas Product, < 260 "C, wt% Product, > 260 "C, wt%
83.0 92.0
67.9 92.0
33.1 11.1 34.5 7.4 89.5 10.2
27.0 11.8 33.5 7.4 73.8 24.1
STRUCTURAL STABILITY AND APPLICATION IN CATALYTIC CRACKING Thermal and hydrothermal stability is a critical property for a catalyst used under the condition of fluid catalytic cracking (FCC). However, the thermal and hydrothermal stability of Al-PM do not meet the requirements of catalytic cracking catalyst [8]. Although the thermal stability of Al-PM at 800°C has been improved recently [9], its hydrothermal stability remains to be a problem. Hence, the collapse process of Al-PM under thermal treatment was studied in order to find the way of' improveing its thermal and hydrothermal stability.
Pillared Clays for Industrial Catalysis
447
Table 5. Properties of hydrocracking product cuts. 150- 175°C
Density, 20 "C,g/cm3 Bromine number, g Br/ml s, PPm. Dynarmc viscosity, 40 "C,
0.7468 0.03 0.25
175-200°C
200-235°C
0.7604 0.03 0.21 I .48
0.7753 0.00 0.26 2.11
< -40
< -40
161 I73 181 I86 202 218
186 I 99 208 214 233 25 I
mm2Is
Pour point, "C Distillation range, "C Initial point 10 %
50 % 70 Yo 95 % End point
134 I46 156 161 177 202
Collapse Process and Structural factors affecting the stability of pillared clays The results of a differential thermal analysis (DTA) study show that the endothermic and exothermic profiles and also the phase transformation temperature (ca. 940 "C)are similar for both AI-PM and its parent montmorillonite. To follow up the thermal collapse process of montmorillonite, XRD patterns of 001 was observed for montmorillonite samples pretreated a1 different temperatures for 2 hrs. The diffuse nature of the (001) peak of the sample pretreated at 55C "C shows that destruction of interlayer zone has started. The 001 peak of the sample pretreated a1 750 "C appears again indicating a basal spacing of 0.96 nm, which is exactly equal to the spacing 01' the 2:1 clay layer. This means that the interlayer zone has completely collapsed while the laya structure of the clay is retained. The characteristic 001 peak disappears completely for the samplc pretreated at 800 "C showing the complete collapse of the clay structure. The montmorillonite studied was a Na-type montmorillonite, which means Na cations were located in the interlayer zone for balancing the positive charge of the layer structure of' montmorillonite. Upon heating, sodium oxide is readily formed from the sodium cation. However, the stability of sodium oxide should not be responsible for the collapse of the monrmorillonite sample pretreated at 550 "C, since sodium oxide is stable at least up to a temperature of 800 "C.Therefore, the observed destruction of the interlayer zone at 550 "C is attributed to the destruction of the possible connection between cations and the 2: 1 layer structure. It is evident that the interaction between pillars or cations and layer structure is also an important factor for the stability of pillared clay. For montmorillonite, the negative charge of the layer structure usually originates from the isomorphous substitution of Mg for Al in the octahedral sheet. The interaction between interlaya cations and the octahedral sheet is weak because the octahedral sheet is sandwiched by twc tetrahedral sheets. The isomorphous substitution of Al for Si in the tetrahedral sheet will more effectively strengthen the interaction between interlayer pillars or cations and the layer structure.
448
E. Min
Two montmorillonite samples from different sources with different extents of tetrahedral substitution, designated as H and L respectively, were selected to check the influence of the tetrahedral substitution on the stability of their pillared counterparts, Al-PM-H and Al-PM-L. The comparison shows that because of the higher content of aluminum in the chemical composition of Al-PM-L, it exhibits higher cation exchange capacity (CEC) and also higher thermal stability, as indicated by the higher surface area retention aRer heat treatment. The results are listed in Table 6.
Table 6. Chemical composition and thermal stability of Al-PM-H and Al-PM-L.
Al-PM-H
Al-PM-L
14.05 I .95 73.39 0.75
22.52 I .89 61.05 I .34
Chemical composition, wt% .&03
Fez03 Si02 CEC, meq./g
d001, nm Calcination temp., "C 300 400 500
1.88 1.58-1.77 1.57
Surface area,m2/g d001, nm 236 198 191
1.84 1.84 1.73
Surface area,mz/g 384 373 345
The selection of parent clav material for preuaring hvdrothemallv stable pillared clavs Based on the above discussion, the guideline for selecting parent clay material for preparing hydrothemally stable pillared clays could be summarized as follows: the layer structure should be highly stable and, of course, expandable; the tetrahedral aluminum in tetrahedral sheet of the layer structure should be relatively abundant. An extensive work for searching and evaluating various clays leads to the discovery of
rectorite, which belongs to a regularly interstratified mineral clay family and possesses a regular alternation of mica-like layers and expandable layers having smectite compositions. An aluminum pillared rectorite, Al-PR, was prepared by the same method used for the preparation of Al-PM. The chemical compositions and physico-chemical properties of A-PR are listed in Table 7. For examining the hydrothermal stability of Al-PK, the surface area and pore volume were also measured after a pretreatment o f : 800 "C, 100 % steam, 17 hrs. In the case of Al-PM, the structure collapses completely after pretreating at 800 "C for 1 hr in the presence of steam. The results in Table 7 filly proves the extraordrnary t h d and h y d r o t h d stability of Al-PR.
Pillared Clays for Industrial Catalysis
449
Table 7. Chemical and physical properties of AI-PR
Chemical composition, wt% Mz03
si02
NaO Fa03 unknown Surface area, m' I g
fresh steaming at 800 "C for 17 hrs Pore volume, ml I g fresh steaming at 800 "C for 17 hrs
45.50 47.60 2.00 0.29 4.60
1 74 129
0.17 0.20
AI-PR cracking catalvst r9.101 The preparation of AI-PR cracking catalyst is characterized in that a mixture of rectorite, kaolin and a binder is spray dried to form microspherical catalyst particles followed by pillaring with aluminum chlorohydroxide and calcination. The distinctiveXKD 001 peak can be observed even for an AI-PR catalyst sample steamed at 800 "C for 17 hrs indicating the superior hydrothermal stability of the catalyst. The properties of the AI-PR catalyst is compared with those of a commercial REY catalyst in Table 8. The reaction condition of the microactivity (MAT) test for light gas oil (235-337 "C) cracking was : 500 OC, WHSV 16 h-l, and cadoil ratio 4. Evaluation in a pilot plant riser cracker A pilot plant riser cracker was used for evaluating AI-PR catalyst using a mixed feedstock of' vacuum gas oil and vacuum resid with a density of 0.8845 g/cm3. a Conradson carbon residue of' 3.6, a Ni content of 3.1 ppm, and a V content of 0.1 ppm. The catalyst was aged at 760 "C for 6 hrs in the presence of 100 % steam under atmospheric pressure. The reaction condition for the pilot plant riser cracking test was : 490 "C, and catloil ratio 5.4-5.7. The results in lable 9 show that the gasoline selectivity and coke selectivity for both AI-PR and REY catalysts are quite similar. It is conceivable that AI-PR can be used as an active component for FCC catalyst. 'Ihe extraordinary hydrothermal stability and the distinctive large pore structure would further make AI-PR an attractive component for designing various FCC catalysts, however, its coke selectivity needs further improvement. OPEN PORE STRUCTURE AND CATALYTIC CHARACTERISTICS The open pore structure of pillared clay is an important aspect of extensive research interest. The fiee interlayer space of AI-PM has a distance of ca. 0.9 nm as shown before. The reaction characteristicsof pillar clays in petroluem r e W g such as hydrocrackingand catalytic cracking were
Table 8. A comparison between Al-PR catalyst and a commercial REY catalyst. Al-PR catalyst Surface area, mzlg fresh steamed at 800 "C for 17 h surface retention, % Pore volume, d g fresh steamed at 800 "C for 17 h pore volume retention, % Microactivity,% fresh steamed at 800 "C for 17 h activity retention, %
Commercial REY catalyst
I26 70 55
315 93 29
0.13 0.11 84
0.59 0.33 55
58 42 72
88 61 69
Table 9. Pilot riser unit cracker evaluation.
Catalyst Product yield, wt% Cracking gas dry gas liquefied gas Gasoline LCO HCO Coke Loss Conversion Selectivity,% Cracking gas Dry gas Gasoline Coke
Al-PR catalyst
REY catalyst
16.63 2.17 14.46 36.49 20.10 16.83 8.29 1 .oo 62.41
19.37 2.57 16.80 38.66 23.55 9.10 8.59 0.73 67.55
26.7 3.5 58.9 13.3
28.8 3.8 57.4 12.8
studied. In order to explore the reaction characteristics of such open pore structure material in the reactions of petrochemical processes, aromatization of paraffin and alkylation of benzene with long chain a-olefins were investigated. Aromatization of n-Cbto n-Cs A series of bfinctional catalysts were prepared by impregnating Al-PM, KL zeolite, and alumina with platinum, respectively and designated correspondingly as WAl-PM, WKL, and Walumina. The aromatization of n-C6 to n-Cm was performed in a pulse reactor at 500 "C. The
Pillared Clays for Industrial Catalysis
451
results listed in Table 10 show that while FWalumina does not exhibit any particular selectivity in aromatization due to the wide pore size distribution, WKL selectively catalyzes the aromatization 01' n-Cs and, interestingly, WAI-PM displays high selectivity for the aromatization of CS and C9. The different behavior of Pt/KL and FWAI-PM in selectivity may be ascribed to the 0.7 I nm pore sue of' L zeolite and 0.9 nm interlayer distance of AI-PM respectively. When n-Cs was used as feed, the main reaction products on AI-PM were CSaromatics, whereas light hydrocarbons, benzene, and toluene were the predominant products on F't/KL. Tablt: 10. The aromatization of n-Cs to n-Clo hydrocarbons
Feed n-Cs n-C. n-Cs n-C9 n-Clo
Yield of aromatics of correst?ondmacarbon number. wt% WAI-PM pt/KL Walumina 11.7 43.5 64.2 60.9 47.5
76.0 38.8 16.6 26.8 26.1
31.0 51.1 53.9 55.8 51.0
Table 1 1. Alkylation of benzene with ole6ns. Conversion. wt% I-octene 1-dodecene AI-PM HY zeolite H-mordenite p zeolite HZSM-5
56.3 100.0 52.8 28.1 0.0
99.2 99.3 20.0 38.0 0.0
Alkylation of benzene with olefins The alkylation of benzene with 1-octene and I-dodecene was conducted in a batch reactor at 80 "C and atmospheric pressure for 0.5 hr using catalysts with different pore structure. The results in Table I I show that different orders of activity for Al-PM among all tested catalysts in I-octene and 1-dodecene alkylation seem to be correlated with their pore sizes.
CONCLUSION 1. AI-PM displays intensive Bronsted acidity only at a mild temperature below 35OOC and water vapor can be used as an agent for reaction environmental control to maintain the Bronsted acidity. Thereby a mild tempereture hydromaking process was developed to produce aromatic-free solvents from light gas oil.
452
E. Min
2. A microspherical cracking catalyst prepared with Al-PR as cracking component has superioi hydrothermal stabiity, it has performance similar to a commercial ReY catalyst as evaluated in a pilot plant riser cracker. 3. AI-PM with open pore structure has potential to be developed into catalysts for aromatization of C g -Cg paraffin and alkylation of benzene with a-olefins.
REIFERENCES 1 ’VaughanD. E. W., et al., U. S.Patent 4,176,090 (1979) 2 Shabtai J., ct al., Prac 7th Inter. Congress on Catalysis, Tokyo, (1980) Kodansha, Ltd., Tokyo, (1981) p323 3 ‘VaughanD. E. W., ACS Symp. Vol368, Perspect Mol. Sieve Science, p 308-23, Ed by William IN. Flankand, et al., 195th National Meeting of A. C S., Toronto, Canada (1988) 4 ‘ThomasJ. Pinnavaia, Zeolite Microporous Solids: Synthesis, Structure and Reactivity, p 9 1- I 0 4 ]Ed by Derouane E. G., Kluwer Academic Publishers, Netherland (1992) 5 IMing-Yuan He, Zhonghui Liu and Erne Min, Catalysis Today, 2 (1 988) 32 I. 6 Guanhua Liu, Ming-Yuan He, Xuanwan Li and Enze Min, in Xianglin Hou (Ed), Proc. Intern. Cod.Petrol. Refin. & Petrochem. Proc., Beijing, Sept. 1 1-15, 1991, Intern. Acad. Publ., IBeijing, (1991) p.696. 7 Enze Min, Zhiqun Wang, Xia Zheng and Ming-Yuan He, Roc. 18th Seminar on Petroleum Chemistry, Nigata Conference of Japan Petroleum Society, Nigata, October 26-27, 1988, p.141. 8 C3ccelli, M. L., I & EC Prod Res. and Dev., p553 (1993) 9 Espinosa J. et al., lhirteenth North American Meeting of The Catalysis Society (1993), Final ]Program and Abstracts of Presentations D 17 10 .rigjie Guan, Enze Min and Zhiqing Yu, in M. J. Philips and M. Ternan (Eds), Proc. 9th Intern. Catal. Congr., Vol. 1, June 26-July 1, 1988, Calgary, Canada, p. 104. 11 iligjie Guan, Enze Min and Zhiqing Yu,U.S.Patent 4,757,040 (1988). 12 .rigiie Guan, Qlnglin Liu and Zhiqing Yu, in Xianglin Hou (Ed), Proc.Intern. Cod.Petrol. IRefin. & Petrochem. Proc., Beijing, Sept. 11-15, 1991, Intern. Acad. Publ.,Beijing,(1991) 13.1255. 13 :Geqing Wan& Huisheng Liao, Yanxiou Hu,Zhijian Da and Erne Min, in Xianglin Hou (Ed), IProc. 1ntern.Cod.Petrol. Refin. & Petrochem. Proc., Beijing, Sept. 11-15, 1991, Intern. .bad. h b l . , Seijing, (1991) p.709.
Ni-Exchanged Sepiolite as a Fibrous Clay Catalyst for Selective Dehydration of n-Butyl Alcohol to Dibutyl Ether
Kazuo Ur&, Sei-ichiro Iida and Yusuke Izumi Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
ABSTRACT The cation exchange capacity (CEC) of sepiolite is greatly enlarged by the ion exchange in an aqeous solution at a high temperature of 95°C as well as the precalcination of original sepiolite at temperatures over 500°C. The Ni ions incorporated for Mg ions in sepiolite crystal exist in the divalent state and are octahedrally-coordinated. An original sepiolite without Ni is completely inert as a catalyst for the dehydration reaction of n-butyl alcohol. However, the fixation of Ni confers much higher catalytic activity on it. INTRODUCTION Sepiolite is a fibrous magnesia-silicate clay mineral which contains zeolitic water (depicted as @ in Fig.1) in its one-dimensional intracrystalline channel, as illustrated in Fig.1. The magnesium ion in sepiolite crystal is known to be exchangeable with other ions [l]. However, the applicability of sepiolite as a catalyst has been restricted [2] due to its small cation exchange
c 11
o on 0 4% 0 4or..,
Fig. 1. Schematic structure of sepiolite [21.
453
454
K. Urabe, S. Iida and Y. Izumi
capacity (CEC) of about 40 meq/100g [3,4]. The present work reports a successful attempt to enlarge CEC of sepiolite and apply it to catalytic dehydration of n-butyl alcohol. EXPERIMENTAL An original sepiolite (Aid-plus G, Takeda Chem. Ind.) was calcined for 4 h at various temperatures prior to cation exchange manipulation. Then 1.6 g of the calcined sepiolite was added to lOOml of 0.1 N nickel acetate aqueous solution and the dispersed solution was stirred at 95’C for 24 h for cation exchange. The product was filtered, washed repeatedly with deionized water and air-dried overnight at 60’C. For comparison, Ni-exchanged sample was also prepared at room temperature. The catalytic reactions were carried out at 200°C under hydrogen flow using a conventional flow reaction apparatus with a fixed bed of catalyst. The catalyst was reduced at 400°C for 1 h under hydrogen flow prior to dehydration reaction. The reaction products were analyzed by gas chromatography. RESULTS AND DISCUSSION Enlareernent of the cation exchange caDacitv (CEO of seDiolite The cation exchange experiments on sepiolite using nickel acetate solution were carried out under various conditions. The contents of Ni and Mg ions in the Ni-sepiolites were determined analytically and shown in Table 1. These amounts are expressed as mol% in the table. The amount of fixed Ni is also expressed for convenience as meq/lOOg-clay. It is estimated to be 49 meq/l OOg for the sample exchanged at room temperature of uncalcined sepiolite(conventional Table 1. Contents of Ni and Mg ions in the Ni-sepiolites exchanged under various conditions. Ni-sepiolite Precalcination Ni-exchange temperaturef’c temperaturef’c 750 700 600 500 400 300 60(dried) 700 600 500 60(dried)
95 95 95 95 95 95 95 r.t.d r.t. r.t. r.t.
Contents/mol% CEC/meq(1OOg)-’ Surface of Ni’ of Mga for fixed Nib areac/ m2g-I 0.264 0.254 0.135 0.135 0.101 0.089 0.094 0.083 0.053 0.032 0.027
0.272 0.263 0.412 0.580 0.486 0.519 0.502 0.469 0.580 0.638 0.601
548 525 276 276 209 183 195 173 109 66 49
182 190 132 132 150 120 110
a) Determined by the ICP spectroscopic method. b) See the text. c) B.E.T. type N, adsorption isotherm. d) Room temperature.
Ni-Exchanged Sepiolite
455
exchange method). This value agrees with the cation exchange capacity (CEC) of about 40 meq/100g reported commonly in the literature [3,4]. On the other hand, the exchange at a high temperature of 95°C causes fourfold increase of fixed Ni (195 meq/100g) compared to that at room temperature. Furthermore, marked increase of fixed Ni was brought about by the calcination of original sepiolite at higher temperatures. Fig. 2 shows the amount of fixed Ni as a function of the precalcination temperature. Up to a precalcination temperature of 4OO0C,the amount of fixed Ni in the Ni-sepiolite, whether exchanged at 95'C or at room temperature, hardly increases. However, it increases dramatically at temperatures over 500'C. For example, 525 meq/100g of Ni was fixed for the sample exchanged at 95°C of sepiolite calcined at 700'C. This value is one order of magnitude larger than that by the conventional exchange method. It means that 165% of constituent Mg ions (see Fig. 1) exposed to the channel space is exchanged for Ni ions. On the other hand, 173 meq/100g was fixed for the Ni-sepiolite exchanged at room temperature of sepiolite calcined at 700°C.
600
0.3-
E
-z Y
c
0.2-
400
C
Y
C
6
-
exchanged at 95'C
A
8
0.1 b
-
at room temperature
'
'
9'
$____-__----a
0
260
'
460 ' 660
Shoo
Precalcination temperature/'C Fig. 2. Amount of fixed Ni as a function of the precalcination temperature of original sepiolite. It can also be seen from Table 1 that the decreased amount of Mg is comparable to the fixed amount of Ni in almost all Ni-sepiolites. This means that the cation-exchange between the Mg ion in sepiolite crystal and the Ni ion in solution is a key mechanism. Thus, it seems that the great enlargement of CEC of sepiolite is brought about by both the ion exchange in an aqueous solution at a high temperature of 95°C and the precalcination of original sepiolite at temperatures over 500°C. The value of the enlarged CEC in sepiolite extends to several times that of montmorillonite, or a typical smectite clay.
456
K . Urabe, S. Iida and Y . Izurni
Structure of Ni-sepiolite exchanged at high temperatures By the calcination of original sepiolite, its specific surface area gradually decreased from about 200 m2/g at 200°C to 70 m2/g at 800°C. The extensive decrease (200 m2/g at 200°C to 130 m2/g at 300'C) that occurs by heating at about 300°C is related to the formation of well-known folded structures [3], resulting from the loss of bound water molecules (depicted asOin Fig.l) in the channels. However, it seems that the subsequent Ni exchange at a high temperature of 95°C brings about recovery of the surface area. For example, a high surface area of 182 m2/g is observed with the Ni-sepiolite sample exchanged at 95'C of sepiolite precalcinated at 750'C. Any Ni-sepiolite exchanged at 95'C holds a high value of 130-180 m2/g.
Uncalcined sepiolite
without
t
sepiolite precalcinated at 650'C
4
Ni exchanging at 95'C I
3
1
I
10
I
I
20 28
I
30
(deg)
Fig. 3. XRD spectra of sepiolites treated under various conditions. Fig. 3 shows XRD spectra of sepiolite samples treated under various conditions. Uncalcined sepiolite, with or without Ni exchanging at 9 5 T , reveals an intense typical (1 10) reflection peak of about 12.2 A. On the other hand, an original sepiolite precalcinated at 650'C shows almost loss of (1 10) reflection, indicating the formation of folded sepiolite at this temperature but not the destruction of the fundamental structure in sepiolite crystal [3]. The Ni exchange at 95°C does not seem to bring about the re-appearance of the (1 10) reflection peak. In order to examine the chemical state of fixed Ni, the diffuse reflectance UV-Vis spectra of powdered Ni-sepiolites were measured and shown in Fig. 4. The spectrum of Ni-sepiolite exchanged at 9 9 2 , with or without precalcination at 650"C, has a strong resemblance to that of well-known [Ni(H20)J2' aqueous ions [5], indicating that the Ni in the Ni-sepiolite is divalent and octahedrally-coordinated.The valency of fixed Ni was also confirmed by means of XPS spectroscopy. Characteristic quadruple peaks were observed in the range of 860 to 900 eV of
Ni-Exchanged Sepiolite
457
binding energy unit with the Ni-sepiolite sample exchanged at 95°C of sepiolite precalcinated at 650°C. This spectrum pattern has been assigned to divalent Ni.
1 Uncalcined
1'
?recalcinated at 650°C Ni-sepiolite exchanged at 95'C
Ni-sepiolite
w
original sepiolite
'
I
,
-1% 0-%?o
300 ' ' ' 500 I ' I 750' Wavelenglh (nm)
Wavelength (nrn)
Fig. 4. Diffuse reflectance UV-Vis spectra of powdered Ni-sepiolites. Fig. 5 shows the 29Si MASNMR spectra of sepiolites treated under various conditions. Uncalcined original sepiolite gives a symmetric triple signal [ 6 ] ,indicating three types of Si nucleus, as shown in Fig. 1. The calcination at 650°C causes the breaking of symmetry in its spectrum, evidencing the change in the chemical environment of Si nucleus byfolding, as
Ni-sepiolite
original sepiolite
h
precalcinated
r'l exchanged at 95°C
precalcinated at
,,.C-/
if,,,temperature
PRI
4 -
-M
- 0
Fig. 5. 29SiMASNMR spectra of sepiolites treated under various conditions.
458
K. Urabe, S . lida and Y . lzumi
described above. The subsequent Ni-exchange at room temperature almost never influences the "Si NMR spectrum of sepiolite. In contrast, the exchange at a high temperature of 95°C causes a substantial change in spectrum. The "Si NMR spectrum of Ni-sepiolite exchanged at 95°C is nearly identical to that of uncalcined original sepiolite, demonstrating a cancellation offolded structure, or a structural recovery of Si-0 linkage in sepiolite crystal by the ion exchange at the high temperature of 95°C. This is in harmony with the result of recovery of surface area by the ion exchange at 95°C described above. From comparative DTA-TG experiments of both samples exchanged at 95°C and room temperature, it is confirmed that the Ni-exchange at 95°C brings about a recovery of bound water molecules (depicted a s O i n Fig.1) in the sepiolite channels. From these results, it seems that the fixation of Ni does not destroy the sepiolite structure in substance and the divalent Ni ions constitute a part of the structure for Mg ions. In addition, pore distribution analyses by N, gas adsorption for the sepiolite samples before and after the ion exchange at 95°C are also consistent with the result described above. It seems that the precalcination of original sepiolite at temperatures over 500°C causes a lattice deformation round the Mg ions exposed to the channel space (folded structure) to enhance a cation-exchange of the Mg ion in the lattice with the Ni ion in solution at the high temperature of 95'C. Catalytic efficiencv of Ni-exchaneed seoiolite for the dehvdration of n-butvl alcohol An original sepiolite without Ni is completely inert as a catalyst for the dehydration reaction of n-butyl alcohol. Table 2 illustrates the catalytic efficiencies for the same reaction of Niexchanged sepiolites prepared under various exchange conditions. The Ni-exchanged sepiolite (Ni content = 49 meq/lOOg) at room temperature (conventional exchange method) manages only a 5% yield of dibutyl ether at 200°C and WHSV = 1 h-'. In contrast, the sample exchanged at Table 2. Catalytic efficiencies of the Ni-sepiolites prepared under various conditions for the dehydration of n-butyl alcohola. Ni-sepiolite Precalcination Ni-exchange temperature/'C temperatureK 700 95 650 95 600 95 400 95 60(dried) 95 60(dried) rat? Original sepiolite without Ni
Contents of Nib/mol% 0.254 (525)d 0.147 (292) 0.135 (276) 0.101 (209) 0.094 (195) 0.027 (49) 0
Catalytic efficiencies Yield of Selectivity for ethef/% ether/% 28 30 26 21 28 5.3 trace
47 47 46 69 66 71
a) Reaction temp. = 200"C, WHSV= 1.0 h-', flow rate of H carrier gas=1000 ml/h, averaged initial efficiency for 1 h after feeding of reactant. b3 Determined by the ICP spectroscopic method. c) Dibutyl ether. d) Figure in parentheses denotes the content of Ni expressed as CEC equivalent (see Table 1 and the text). e) Room temperature.
Ni-Exchanged Sepiolite
459
95°C of sepiolite calcined at 600°C (Ni content = 276 meq/100g) displays a high catalytic activity, with a dibutyl ether yield of 26%, five times as much as the sample exchanged at room temperature. The Ni content of 276 meq/100g corresponds to an exchange rate of 85% in constituent Mg ions exposed to the channel space. Fig. 6 shows the values of dibutyl ether yield given by various Ni-exchanged sepiolites as a function of Ni content of each catalyst. The yield of dibutyl ether as a catalytic efficiency increases linearly with a rise in Ni content, coming to a saturated value at a Ni content of more than 0.1 mol%. Interestingly, the Ni content causing this saturation of catalytic efficiency is nearly equal to the exchange rate of 100% in exposed Mg ions, suggesting that excess Ni ions incorporated into bulk do not work as effective catalytic sites for the conversion of n-butyl alcohol. It is concluded that the Ni incorporated into sepiolite crystal plays not only a crucial role as a catalyst but also that the number of Ni ions situated on proper sites regulates the catalytic efficiency of dehydration reaction. This work is an example of 'soft chemistry routes [7]' to catalytic materials. In fact, this method is applicable to other cations, such as Zn2+,Mn2+,Cii2+.
3
Ni content/ mol% Fig. 6. Dibutyl ether yield of Ni-sepiolite as a function of Ni content. ACKNOWLEDGMENT We thank the Izumi Science and Technology Foundation and the Mikiya Science and Technology Foundation for financial supports of this work, JGC Co., Ltd. for permitting us to use the ICP spectrometer and obtaining the MASNMR spectra, Dr.T. Wada (Mizusawa Chemhd.) for supplying the sepiolite samples, and Dr.K. Shimosaka (GIRI, Nagoya) for his helpful discussions.
460
K. Urabe, S. Iida and Y . lzumi
REFERENCES 1 Y. Oguchi, Japan Kokai Tokkyo, 1978, 1978-7592. 2 J-B. d’Espinose de la Caillerie and J.J. Fripiat, Catalysis Today, 14, (1992) 125. 3 B.F. Jones and E. Galan, in S.W. Bailey (Ed.), Hydrous Phyllosilicates (Reviews in Mineralogy, Vol. 19), Mineralogical Society of America, Chelsea, 1988, p.631. 4 L.W. Zelazny and F.G. Calhoun, in J.B. Dixon and S.B. Weed (Eds.), Minerals in Soil Environments, Soil Science Society of America, Madison, 1977, p.435. 5 F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th edn., John Wiley & Sons, New York, 1988, p.744. 6 P.F. Barron and R.L. Frost, Amer. Miner., 70, (1985) 758. 7 12th Intern. Symp. on Reactivity of Solids, Topic C; Intercalation and Soft Chemistry, Madrid (Spain), Sept. 24-30 (1992); Intern. Symp. on Soft Chemistry Routes to New Materials, Nantes (France), Sept. 6-10 (1993).
Role of the Zeolite Catalysts in the New Refining Strategies
Avelino Coma Instituto de Tecnologfa Quimica, UPV-CSIC, Universidad Polittcnica de Valencia, Camino de Vera sfn, 46071 Valencia, Spain.
INTRODUCTION The demands for new investment in refining come from: compliance of tougher operating standards for health, safety and the environment, changes in the product structure and product qualities required by the market, and changes in the quality of crude slate. Today, the citizens of many countries are conscious of the necessity that economical growth should not be achieved at any price, and have forced the governments to do this with a higher respect to our environment. The 1970 Clean Air Act (CAA) and 1977 amendments failed to control two major pollutants: CO and ozone. For this reason the 1990 amendments to the CAA have required new pollution control measures in order to reduce emissions from stationary sources such as refineries, service station, chemical plants and power plants, as well as from mobile sources as for instance motor vehicles, trains, and airplanes. Such requirements affect the petroleum refining operations with respect to air toxic emissions and water discharge limitations, land disposal of hazardous waste, oil spill prevention, underground storage tanks, etc. In addition to implications for production costs, the new environmental requirements will have a significant impact on fuels, and specially on the demand for cleaner mator vehicle fuels. In this way, the EPA has asked the refiners to refomulat gasoline (RFG), for the period 1995-1997, by reducing RVP down to 9.0 psig, benzene content below 1% vol, add oxygenates up to a minimum of 2.7 and 2.0 wt% in carbon monoxide and ozone nonattainment areas respectively. Meanwhile no NO, increase has to occur, and the sulfur, T,, and olefins should be kept at the refiner's 1990 average. The sulfur in diesel is also expected to come down to 0.05 wt%. After march 1997, emissions from gasoline, and therefore its composition, will be controlled by EPA's complex model which will add sulfur T, and olefins reduction. Thus, solutions to the problems derived from the production of new reformulated gasolines will have 46I
462
A. Corma
to consider, as objective, not only to meet product specifications but also maximum economic benefits. To do this integrated solutions involving new processes together with revamping existing ones, catalyst changes and revision of the operating conditions will be necessary.
In this paper we describe how the use of new zeolitic catalyst can help to design an integrated solution to deal with reformulated gasoline.
THE GASOLINE POOL When the composition of the different gasoline streams are considered (Table 1) it may be seen that the reformate introduces too much benzene and aromatics; the FCC gasoline is relatively low in octane, and introduces too much olefins and sulfur into the gasoline pool. The LSR naphtha, while being low in aromatics and olefins has a slightly high RVP and the octane number is too low. It is also clear that from the point of view of reformulated gasolines the alkylate is the most suitable. Table 1. Typical blending component properties of the main gasoline streams.
Blendstock
RONC
MONC
RVP (psig)
Aromatics
92.1 92.6 91.9
80.7 80.8 91.3
7.1 9.9 2.6
29.2 13.5 60.3
29.1 39.8 16.5
97.7 82.8 104.9
87.4 77.5 94.5
5.3 7.9 1.3
62.6 31.1 87.2
0.7 1.o
0.5
93.2 93.9 84.0
91.1 91.2 81.5
7.9 6.4
0.4 0.6
0.5 0.3
78.6 74.1 53.7
76.0 72.1 54.2
10.0 13.3 2.7
Pol%)
Olefins
Pol%)
FCC gasoline Full range Light Heavy Reformat Full range Light Heavy Alkylate Full range Light Heavy Naphtha Full range Light Heavy
9.8 2.8 10.7
2.2 0.8 0.2
Zeolite i n New Refining Strategies
463
Concerning the introduction of oxygenates, in Table 2 the blending characteristics of different potential additives are given. It appears that C,-C, alcohols while having good blending octane numbers have a too high RVP. Thus the most adequate oxygenate additives are methyl-text-butyl ether (MTBE), ethyl-tert-butyl ether (ETBE), and tert-amyl ether (TAME). Following this, and in order to look for an integrated solution, one has to look not only to optimize the different gasoline streams, but also to produce enough isobutylene and isoamylenes needed for MTBE and TAME production, as well as more isobutane and n-butenes required for alkylation if, as expected, a new solid catalyst will soon come on stream, and the alkylation capacity will increase. Table 2. Blending characteristics of oxygenates. Additive
RVP @sig)
Octane (R+M/2)
Methanol
60
120
Ethanol
€9
115
Isopropanol
€4
108
MTBE
€08
ETBE
111
TAME
106
on the f Taking into account that up to 75% of the benzene in the gasoline pool comes from the reformate, it appears that the most effective strategies in reducing benzene should be focused on this process. There are two approaches to this problem. The first one, and probably the most logical, is to remove the benzene precursors before they go into the unit. In this way, the Cc could be removed from the reformate feed, and probably be sent to an isomerization unit, in which a zeolite catalyst can isomerize the n-paraffins and increase their value as gasoline components. There is a second option to reduce benzene in the reformate gasoline without loosing gasoline and which involves zeolite catalysts. It is possible by using a medium pore ZSM-5 zeolite as catalyst to reduce reformate benzene by alkylation with G-C, olefins from FCC off gas, cocker fuel gas etc. [€I. However the economics of the benzene reduction process is strongly dependent on the price of short chain olefins.
464
A. C o m a
FCC With the introduction of the complex model which will limit olefins and sulfur in gasoline, some people thought that the importance of this unit in the refinery will strongly decrease. However this does not appear to be the case owing to the following reasons: a surplus in the supply of the heavy fractions requires conversion units, and here FCC plays an important role. New FCC catalysts are developed to produce higher bottoms conversion and higher gasoline octane. The sulfur in the FCC gasoline is mainly concentrated in the heaviest part, and therefore it can be strongly reduced by lowering the distillation end point of the gasoline. And last but not least, the FCC unit is an important producer of today highly demanded isobutene, isoamylenes, n-butenes, isobutene and propylene. Then, as the ave Fenix, each time that some people have predicted its death, the FCC has again emerged from the ashes. As it is well known, Y zeolite is the main zeolite component of actual FCC catalysts. Since both conversion and quality products strongly depend on the zeolite preparation, an extensive work has been carried out to optimize the activation procedure of zeolite Y. In this way by studying the influence of the framework composition, it has been shown that: by increasing the framework Si/Al ratio of HY zeolites, the total number of Bronsted acid sites decreases while the acid strength of the remaining sites increases [2,3]. These two factors combine to give a maximum in activity for gasoil cracking on HY zeolites with a framework Si/Al ratio of 5-8, or what is equivalent, zeolite Y with a unit cell size of 24.36-24.40
A
[3-51. Indeed, a
correlation exists between the framework Si/Al ratio of zeolite Y, and the unit cell size (UCS) [6,71.
From the point of view of product quality, it has been seen that by decreasing the UCS of the Y zeolite, the selectivity to dry gas first decreases up to 24.32 A and then increases. Inversely, gasoline selectivity first increases up to
- 24.32 A and then decreases upon further
UCS reduction. Coke decreases upon dealumination up to a UCS of
- 24.28 A and then remains
practically constant. Furthermore the olefinicity of the C, and C, gases increases slowly until the UCS is reduced at
- 24.35 A, and then strongly increases upon further reduction in UCS.The
changes in selectivity owing to UCS changes in the zeolite, also have a strong effect on gasoline octane [3,4]. In general, it can be said, that if high conversions and high middle distillate yields are prioritary, then a zeolite which stabilizes at 24.36-24.40 A UCS should be used. However, if one seeks for gasoline octane and olefins UCS as low as 24.25 A are preferred. On the other hand,
Zeolite in New Refining Strategies
465
if maximum octaneharrel is the objective, 24.28 8, stabilized Y zeolite should be in the FCC catalyst. Besides the framework composition,the presence of extraframework species in the zeolite also has a direct impact on gasoil cracking activity and selectivity. In this way, it has been found that Na+ in the dealuminated Y zeolite decreases activity, and inhibits the formation of high octane gasoline [4,8]. The different types of extraframework Al (EFAL), i.e., tetra, penta, and octahedrally coordinated, with their associated Lewis acidity [5,9-121 also play a role in gasoil cracking. The relative proportion of those depends on the dealumination procedure, i.e., steam, SiCl,, (NH,)F,Si etc. [11,13]. The role of the EFAL in equilibrated (steamed) zeolite cracking catalyst is shown in Figure 1 [14]. Yield (%)
15’
30
Convtrslon (%)
Converrfon (%)
Yield (%)
50
40
80
70
Conversion (%)
YIdd (%)
2s,
1
4,
I
to
I
Diesel
14
I
10‘
30
40
50
60
Converrlon (%)
70
80
QO
50
60
70
do
Converrlon (%)
Fig.1. Selectivity to gases, C,tC;, gasoline, diesel and coke for samples USY-3 (0)and UIF-25S (0)in gasoil cracking Today, it is believed that if most of the EFAL generated during the activation of the zeolite to produce the fresh catalyst is removed, then the equilibrated catalyst gives more gasoline of a higher octane, lower coke, and more olefinic gases [14-181. Finally, there is a third important variable in Y zeolite preparation for FCC catalyst, and which considers the accessibility of the bulky reagents into the zeolite pores. This accessibility
I
80
466
A. Corma
could be increased by creating mesopores in the zeolite crystals [19-241, and/or by decreasing zeolite crystallite size [25,26]. However, in this last case better results are obtained with Y zeolite crystallized in small crystals and with a higher framework SVAI ratio (3.1) [26], i.e., conversion and gasoline octane increase, coke decreases, and the selectivity to olefinic gases increases.
USE OF ZEOLITE ADDITIVES However, at the end, Y zeolite is still limited to give the desired catalyst flexibility towards the products required for reformulated gasoline. Thus, the concept of multizeolite FCC catalyst was developed, and zeolites other than Y were also introduced as FCC catalyst additives. In this way, the introduction of 1-3 wt% zeolite ZSM-5 increases the RON of the gasoline
produced, while increasingthe selectivity to G-C, olefins, specially to propylene [27-311. Other medium pore size zeotype additives such as SAPO-11 produces an increase in the olefidparaffin ratio in the C, and C, fraction, without penalty in gasoline [32]. In order to obtain higher yields of the desired isobutylene and isoamylenes, a twelve member ring tridirectional zeolite such as the Beta zeolite has been studied for gasoil cracking. This zeolite can be synthesized with high SifAl ratios and it is formed by the intergrowing of polymorphs A and B with two system of channels with 5.5-6.5, and 6.3-7.3 %, [33]. This zeolite presents acid sites which are stronger than those of the HY zeolite, and with a higher turnover than HY zeolites for cracking short chain alkanes [34]. When used as the zeolitic component for gasoil cracking, it is hydrothermally stable, active and selective, but it produces less gasoline and more gases than HY samples. However its hydrogen transfer activity is lower, giving more olefinic gases and gasoline (Fig. 2 and Table 3) [34-361. Its final properties depend on the synthesis procedure [56], and when used as an FCC additive gives higher amounts of isobutylene and isoamylenes than ZSM-5 additives [37-391.
Zeolite in New Refining Strategies
467
Table 3. Ratio of i-alkanes to i-alkenes, and n-alkanes to n-alkenes ratio in the gasoline fraction, during cracking of gasoil at 755K and 50% conversion level. Catalyst
iC&'
iCdiC;
iC,/iq
USY-2
1.70
1.13
0.84
8-0.4
St
0.26
0.16
0.10
Catalyst
nCJnC;
nCdnC,'
nGhV
USY-2
0.10
0.14
0.20
8-0.4 st
0.08
0.12
0.15
Fig.2. Comparison of Beta and USY zeolites for producing C, and C, gases during gasoil cracking in a MAT unit, at 482*C.
SYNTHESIS OF MTBE AND TAME The reformulated gasolines include oxygenated compounds such as MTBE and TAME
as additives to boost octane. MTBE is commercially produced by the etherification of isobutene with methanol, catalyzed by sulphonated ion-exchange resins [40]:
CH3
I
Acid
CH,-C=CH2 t CH30H
CH,
I
CH3-C-CH3 Cat
I
r11
OCH, The sulphonated ion exchange resins catalysts are able to work at relatively low temperature and therefore can minimize the formation of by products, while shifting the thermodynamic equilibrium versus MTBE, while close to 95% conversion of isobutene are achieved in a single stage operation. These catalysts work with a near stoichiometric methanol to isobutene ratio, and catalyst life times of up to two years can be achieved. There is not doubt that the sulphonated ion exchange resins appear as a well optimized catalyst for olefins etherification. However, there is still room for improving: the acid effluents produced, to lower the sensitivity to methanoVisobutene ratio, and to achieve higher thermal stability. All these improvements could be achieved by using zeolites. Indeed, the fact that in zeolites one can modify, besides the acidity, the pore size and the polarity, can make the concentration of reactants in the pores and in the reaction media to be quite different. For instance, in medium pore zeolites with MFI and MEL structures, methanol diffuses more rapidly than isobutene and, consequently, the ratio of methanol/isobutene is much higher inside the channels than in the reaction media. Thus, when used in liquid phase, ZSM-5 and ZSM-11 zeolites give superior selectivity than Amberlyst 15, in a wider range of methanol to isobutene ratios [41,42]. It has been reported that increasing the Si/Al ratio of the ZSM-5,the selectivity to MTBE increases while coke formation decreases. When large pore zeolites are used, the relative concentration of the two reactants can be modified, by changing the polarity of the zeolite. In this way, highly polar zeolites such as a
REY, preferentially adsorb methanol versus isobutene (Table 5 ) [41]. Isopentene can also efficiently be etherified with methanol on large pore zeolites [43].
Zeolite in New Refining Strategies
469
Table 4. Formation of MTBE over zeolite catalysts in vapor phase at 3.4 h-' total weight of methanol and isobutene per gram of catalyst. Zeolite SIOJAI,O,
Mordenite 26
REHY 5.3
Beta 22
REAlY 5.1
ZSM-5 70
ZSM-11 25
Btactinn MlIB Molar
1.07
1.14
1.46
1.53
1.20
1.118
1.06
1.04
1.04
1.05
0.98
0.98
82
93
82
93
82
93
82
93
ratio PC, Reaction Temperature
8 2 9 3
8 2 9 3
u to MTBE %
8.4
7.1
13.9
9.1
11.3
10.6
25.3
23.4
30.5
25.3
25.1
21.0
to C, olefins %
6.0
14.1
23.0
27.3
0.47
1.9
0.3
1.3
0
0.1
0.1
0.2
MTBE
58.3
33.5
37.7
25.0
96.0
85.0
98.8
94.9
100
99.6
99.4
99.0
selectivity (%)
An integrated process can be visualized using zeolites and metallosilicate catalyst, in
which the isomerization of the n-olefins contained in the feed stock is combined with the etherification [44-461. Zeolite catalysts are also used in a combined methanol to olefid etherification process [47].
ISOMERIZATION OF n-OLEFINS
A way of increasing the economic benefit is to introduce a process which isomerizes the unreacted n-olefins to the corresponding isoolefins. While acidic zeolites are able to carry out this isomerization, it should be taken into account that the thermodynamics of the process is favored at low temperatures. However, at lower temperatures olefin dimerization is an important competing reaction. Then it appears that the most suitable zeolite catalyst should be of mild acidity and work at relatively high temperatures. In this way, when zeolites such as Y, Beta,
ZK5, ZSM-5,ZSM-12, ZSM-22, and ZSM-23 are used, higher selectivities to isoolefins, and longer catalyst lifes are obtained if the acidity of the zeolite is reduced by exchanging protonic sites by alkaline earth cations [48]. SAP0 materials such as SAPO-11 and SAPO-31 with a milder acidity than zeolites, are active and selective for n-butene isomerization when operated at reaction temperatures higher than 300T [48-501. The incorporation of transition elements into the crystal structure can greatly enhance the skeletal isomerization. In this way, SAPO-11 (FeApO-ll), MnAPO-11, and FAPO-31, MnAPO-31, give better results than the corresponding
SAPO-11 and SAPO-31 [51]. Acid ferrierite has been shown to be very selective for n-butene
3 L
-4
FCC UNIT r- -- - --
-
7
DEHY DHO-
- --- ---- 3-----
I
ISOMERIZATIONJ
L---
-- 1-- - -
I
Zeolite in New Refining Strategies
471
a n-pentene isomerization [52]. Thus, by combining the processes here described, a fully integrated refinery scheme which makes a complete use of FCC products, while producing suitable streams for reformulated gasoline can be summarized in the following scheme. ACKNOWLEDGEMENTS Financial support by the Comisih Interministerial de Ciencia y Tecnologia of Spain (MAT 91-1152) is gratefully acknowledged. REFERENCES 1 S.D. Evitt, G. Gong, A.R. Goelzer, M.N. Harandi, H. Owen and N.A. Collins, Fuel Reformulation, December (1992) 60. 2 D. Barthomeuf and R. Beaumont, J. Catal., 30 (1973) 288. 3 L.A. Pine, P.J. Maher and W.A. Watcher, J. Catal., 85 (1984) 466. 4 R.E. Ritter, J.E. Creighton, T.G. Roberie, D.S.Chin and C.C. Wear, NPRA Annual Meeting, 1986. 5 A. Coma, V. FornCs, A. Martinez and A.V. OrchillCs, ACS Symp. Ser., 368 (1988) 542. 6 H. Fichtner-Schmittler, U. Lohse, G. Engelhardt and V. Patzelova, Cryst. Research and Technol., 19 (1984) K-1. 7 J.R. Shon, S.J. De Canio, J.H. Lunsford and D.J. ODonnell, Zeolites, 6 (1986) 225. 8 S.M.Brown, W.J. Reagan and G.M. Wolterman, US Patent, 4,325,813 (1982). 9 J. Klinowski, C.A. Fyfe and G.C. Gobi, J. Chem. SOC.Faraday Trans. I, 81 (1985) 3003. 10 J.P. Gilson, G.C. Edwards, A.W. Peters, K.Rajalopagan, R.F. Wormsbecher, T.G. Roberie and M.P. Shatlock, J. Chem. SOC.,Chem. Commun. (1987) 91. 11 J. Sam, V. FomCs and A. Corma, J. Chem. SOC.Faraday Trans. I, 84 (1988) 3113. 12 A. Corma, E. Herrero, A. Martinez and J. Prieto, Prepr. ACS Pet. Div., 32 (1987) 639. 13 J. Dwyer, F.R. Fitch, G. Quim and J. Vickeman, J. Phys. Chem., 86 (1982) 4574. 14 A. Corma and J. Nieman, AKZO FCC Symposium, 1991, p. 217. 15 D.A. Keyworth, R. Gilman and J.R. Pearce, NPRA Annual Meeting, 1989. 16 A. Corma, V. FornCs and F. Rey, Appl. Catal., 59 (1990) 267. 17 A. Hurnphries, J.J. Yanik, P.H. Desai, L.A. Gerritsen and P. O'Connor, AKZO FCC Symposium, 1991, p.125. 18 W. Letzch, D. Michaelis and J.D. Pollock, NPRA Annual Meeting, 1987. 19 M. Ogata, T. Masuda, Y.Nishimura, G. Satoh and S. Egashira, Sekiyu Gakkaishi, 29 (1985) 105. 20 H. Stach, U. Lohse, M. Thamm and W. Schirmer, Zeolites, 6 (1988) 74. 21 V. Patzelova and N.I. Jaeger, Zeolites, 7 (1987) 240. 22 J. Lynch, F. Raatz and P. Dufkesne, Zeolites, 7 (1987) 333. 23 J. Lynch, F. Raatz and Ch. Oclalande, in K.K.Unger et al. (Ed.), Characterization of Porous Solids, Elsevier, 1988, p.547. 24 A. Yoshida, H. Nakamoto, K. Okanishi, T. Tsurn and H. Takahashi, Bull. Chem. SOC. Japan, 55 (1982) 581. 25 K. Rajalopagan, A.W. Peters and G.C. Edwards, Appl. Catal., 26 (1986) 69.
472
26
27 28 29 30 31 32 33 34
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
A. Corma
M.A. Camblor, A. Coma, A. Martinez, F. Mocholf and J. Ptrez-Pariente, Appl. Catal., 55 (1989) 65; M.A. Camblor, A. Coma, F. Mocholf, E.Iglesias and M.Ptrez, in H.J. Lovink and L.A. Pine (Eds.), The Hydrocarbon Formation of FCC Naphtha Formation (Technip), 1990, p.25. D.M. Nace and H. Owen, US Patent, 3,894,931 (1975). A.W. Chester, W.E. Cormier and W.A. Stover, US Patent, 4,368,114 (1983). C.D. Anderson, F.G.Dwyer, G. Koch and P. Niiranen, Roc. 9th Iberoamerican Symp. on catal., 1984. S.Yanik, R.J. Campagna, E.J. Demmel and A.P. Humpries, NPRA Annual Meeting, 1985. G.W. Young, W. Suarez, T.G.Roberie and W. Cheng, NPRA Annual Meeting, 1991. RJ. Pellet, P.K. Coughlin, M.T. Staninlis, G.N. Long and J.A. Rabo, US Patent, 4,791,083 (1988). J.M. Newsam, M.M.T. Treacy, W.T.Koetsier and C.B. Gxuyter, Proc. R. SOC.London A., 420 (1988) 375. A. Corma, V.Fornts, F.V. Melo and J. Ptrez-Pariente, ACS Syrnp. Ser., 375 (1988) 49.; A. Corma, V. Fornts, J.B. Mont6n and A.V. OrchillCs, J. Catal., 107 (1987) 288. G.C. Edwards and A.W. Peters, US Patent, 5,102,530 (1992). E.G. Campbell and PA. Winthrop, EP 243629 (1987). L. Bonetto, M.A. Camblor, A. Corma and J. Ptrez-Pariente, Appl. Catal., 82 (1992) 37.; L. Bonetto, A. Corma and E. Herrero, loth Int. Zeolite C o d , Montreal, 1992. W.Ch. Cheng, N. Suarez and G.W. Young, AIChE Annual Meeting, Nov. 17 (1991). N.Y. Chen, A.B. Ketkar, P.M. Nace, A.Y. Kam, C.R. Kennedy and R.A. Wase, EP 186446B1 (1991). G.J. Hutchings, C.P. Nicolaides and M.S. Surrell, Catalysis Today, 15 (1992) 23. P. Chu and G.H. Kuhl, Ind. Eng. Chem. Res., 26 (1987) 365. LM. Tau and B.H. Davis, Appl. Catal., 53 (1989) 263. W.K. Bell and W.O. Haag, Eur. Pat. Appl. 309177A1 (1989). M.N. Harandi and H. Owen, US Patent, 4,814,519 (1989). W.O. Haag and H. Owen, US Patent, 5,132467 (1992). W.K.Bell, W.O.Haag, M.N. Harandi and H. Owen, US Patent, 5,013,329 (1991). M.N. Harandi and H. Owen, US Patent, 4,826,507 (1989); M.N. Harandi and H. Owen, US Patent, 4,831,195 (1989). M. Janssen, W.J. Mortier and C.W. van Oorschot, Appl. WO 9118851 (1991). S. Barri, I. Ali and D.A. Kidd, Eur. Pat. Appl. 485145 (1992). A.M. Gaffney and CA. Jones, US Patent, 5,107,050 (1992). P.R. Pujad6, J.A. Rabo, GJ. Antos and S.A. Gambichi, Catalysis Today, 13 (1992) 113. H.H. Mooiweer, K.P.de Jong, B. Kraushaar-Czametzki, W.H.J. Stork and P. Granvallet, Paper presented at the Europacat I - First European Congress on Catalysis, Montpellier, 1993.
Liquid-Phase Hydration of Cyclohexene with Highly Silicious Zeolites
H. Ishida. Y. Fukuoka, 0. Mitsui and The late M. K6no Chemical Development Laboratory, The Asahi Chemical Industry Co. , LTD, Shionasu, Koj irna, Kurasiki-shi, Okayama 711. Japan
ABSTRACT Liquid-phase hydration of cyclohexene t o form cyclohexanol was carried out i n the presense of zeolites as a catalyst. Highly silicious zeolites w i t h SiOz/AI2O3molar ratio 0 2 0 ) gave high activity. Especially, ZSM-5 and ZSM-11 gave high activity and good selectivity. I n the case of ZSM-5, the activity depended on the crystal size and the acidity. The reaction system consists of three phases, an oil phase, an aqueous phase and a catalyst phase. The reaction rate and the equilibrium composition of the oil phase depended on the d i s tribution ratio between the o i l phase and the aqueous phase.
INTRODUCTION Homogeous acid catalysts such as sulfuric acid and heteropoly acid have been known to give high yield for the hydration of cyclohexene. [ I , 21 However, these catalysts cannot be industrialized because of the difficulty of the product separation and the corrosion of the equipment. Solid acids such as ion-exchange resign [31 and Y type zeolite C41 have been reported as catalysts, though they also could not be indutrialized because of their low activity and low thermal stability. Here, we report the highly silicious zeolites as the catalyst having strong acid strength and high thermal stability.
METHOD Catalyst The zeolites except for the Y type zeolite and mordenite were synthesized by the methods using organic templates, which were 1,3-dimethylurea for ZSM-5, 1.8-diarninooctane for 2 3 - 1 1 , methyl-tri-ethylammonium chloride for ZSY-12 and 1.4-diaminobutane for 23-35. The obtained materials were calcined a t 400 C and ion-exchanged w i t h aqueous 1 N HNOs solution. Y type zeolite and mordenites were obtained from the Toso Co.,Ltd. Reaction apparatus and procedures The reactions were carried out batchwise i n a 300 m l glass autoclave. The reaction mixture was stirred vigorously and the oil phase was withdrawn periodically and analyzed by gas chroma tograghy. 413
474
H. Ishida, Y. Fukuoka, 0. Mitsui and The late M. K6nO
Measurement of acidity The acidity of zeolites was measured by the titration of the filtrate samples after the zeolites were ion-exchanged with aqueous NaCl solution. [51
DISCUSSION Comparison of activity and selectivity of various zeolites Table 1 shows the activity and selectivity of various zeolites. [61 Highly silicious zeolites with SiOz/A120smolar ratio (:20) gave high activity. Namba et al. [71 obtained similar results in the hydrolysis reaction of ethyl acetate and concluded that the hydrophobic property of highly silicious zeolites was necessary for the approach of the organic molecules to the active sites in the presence of water. The same explanation would account for the high activity observed in the present study since the hydration of cyclohexene occurs in the aqueous phase. ZSM-5 and ZSM-11 having channels formed by 10-membered rings gave good selectivity for cy clohexanol( >99%). On the other hand, mordenites and ZSM-12 of which channels are formed by 12 - membered rings gave low selectivity due to high yield of dicyclohexylether. These results could be due to the shape selectivity of zeolites: dimension of channels formed by 10-membered rings was most suitable for the cyclohexanol formation. RESULTS
AND
Table 1. Activity and selectivity of various zeolites for hydration of cyclohexene Catalyst Channels SiO, Crystal Time Composition of oil phase(wt%) Selectivity for system /Al2O3 size Cyclo- Methvlcvclo- Dicvclo- Cvclohexanol (fim) (hr) hexanol pentenes hexylether - ( % )
ZSM-5
u***
ZSM-I1
ZSM-35 Mordenite
25 30
0.6 1.0
2 2
12.8 9.0
0.084 0.070
0.008 trace
99.3 99. 3
@K**
25
1.0
2
11.5
0.045
0.018
99. 0
a*
15
10
2.0 0.7
72 4
0.3 0.3
trace trace
0. 01
10 33 45 94
0.5 0.5 0.5
4 4
0.5
trace trace trace trace
trace 2.72 1.67 4.25
100
4 4
0.2 8.2 4.1 3.4
@ 8%
trace
100 96
74 70 43
EM-12
E*
35
1.0
16
6.7
0.024
2. 16
74
Y
12***
5
0.5
72
0.3
trace
0.05
98
Underlined : The number of oxygen atoms that constitute the rings of pore Number of asterisks : The number of dimension of channel Conditions : Cyclohexene / Water / Cat weight ratio = 2.4 / 2.7 / 1.0 Temp 120 “C
Liquid-Phase Hydration of Cyclohexene
475
Apparent reaction rate Figure 1 shows a typical time course of cyclohexanol concentration in an oil phase using 2.9-5 as a catalyst. The oil phase is separated from the aqueous phase and the catalyst always exists in the aqueous phase. Therefore, the time course o f cyclohexanol concentration in the oil phase shows an apparent reaction rate. 20
30 40 50 60 Time ( hr 1 Fig. 1. Hydration of cyclohexene catalyzed by 2 9 - 5 under batch conditions. Initial charge : Cyclohexene/Water/Cat weight ratio=O.39/2.7/1.0 Pressure:608 kPa under an atmosphere of N2 0 llO°C 0 12OoC 0 13OoC
0
10
20
The rate equation is derived as follows. Water is assumed to exist in large excess and reaction is to proceed reversiblly with first order with respect to both cyclohexene and cyclohexanol.
Each symbols indicates as follows. CHEo : Initial concentration of cyclohexene [ mol/l 3 CHE : Concentration of cyclohexene at time t [ mol/l 1 CHL : Concentration of cyclohexanol at time t [ mol/l 1 XHE : Conversion of cyclohexene N H E o : Initial amount of cyclohexene [ 1 1 V : Amount of catalyst [ kg 1
KC
: Ki/Kz
476
H. Ishida, Y . Fukuoka, 0. Mitsui and The late M. Kano
The r a t e equation (2) is transformed into equation (3) which i s a basic equation for the batch reaction.
from which a linear relation Figure 2 shows the relation between t and -ln{l-(l+l/Kc) X,,), s h i p between the two factors is obtained. The activation energy calculated from the Arrhenius plot was 87.4 and 119.2 kJ m o l - ' for the forward and reverse reactions, respectively.
20 30 40 50 time ( min ) 0 :llO°C , A:120°C Fig.2. Time vs - l n ( l - ( l t l / K c ) . X ~ ~profiles. ) 0
10
.O : 1 3 O o C
Influence of the crystal size Figure 3 shows the dependence of the reaction rate on the crystal size of ZSM--5 zeolite. 0
x,
0. 1 1.0 10 Crystal size ( ,urn) Fig.3. Dependence of the reaction rate on the crystal size for 2%-5 (SiOz/A1208=28) k4
0.01
Liquid-Phase Hydration of Cyclohexene
477
The reaction rate increased as the crystal size decreased below 0.5 p m. On the other hand, w i t h the crystal size less than 0.5 pm, the influence of the crystal size on the reaction rate was very small. These results suggest that a l l H' sites are available for the reaction if the crystal size i s small.
Influence of the acidity There was no correlation between the rate constant K, and the theoretical acidity calculated from Si02/A120smolar ratio of ZSM-5 zeolite with the crystal size less than 0. 1 ,urn. Therefore, we measured actual acidity by the titration of f i l t r a t e samples after the zeolites were ion-exchanged by aqueous NaCl solution. L61 Figure 4 shows the titration curves under various ion-exchange conditions. The titration curves for the f i l t r a t e samples were the ordinary type, while for the slurry sample the curve was different from the ordinary one i n that the slope was very gentle.
I
0. 5 1.0 NaOH (mmol/g-cat) with crystal size less than 0.1 pm. Fig.4. Titration curves for B Y - 5 zeolite (SiO2/Al2OS=28) Ion-exchange conditions: Temp ?C ) Time ( min 1 Samples A 2 10 Pi 1trate 0 50 10 n (> 80 10 N Q 80 360 // 0 50 360 Slurry 0
Figure 5 shows the dependence of the titration endpoints on the time of ion exchange a t vari ous temperature. These results suggest that at every temperature the ion-exchange occurs very
fast almost to an equilibrium within 10 min. This fast ion-exchange may be due to the absence of deep sites such as S, sites in faujasite and to the smallness of the crystal. The titration endpoints varied slightly w i t h temperature and time. Furthermore, a t 80'C the point of inflection a t pH 5 was observed on the titration curve. (Fig.4) Skeels e t al.obtained similar results and concluded t h a t these phenomena were due to the buffer effects of non-lattice aluminium i n zeolites. [51 Furthermore, Breck et al. mentioned that the non-lattice alu minium i n Y type zeolite varied as the following equation during the ion-exchange by the aque-
478
H. Ishida, Y . Fukuoka, 0. Mitsui and The late M. Kano
ous NaCl solution. [ E l The equilibrium for the hydroxide ion of the non-lattice aluminium i s established according t o equations (5)-(7). Upon the ion-exchange by aqueous NaCl solution, the equilibrium shifts to the left to generate H' which did not exist before the ion-exchange. The change of the titration endpoints with the temperature and time can be explained similar ly. Therefore, the acidjty measured under a mild condition of 2 " C and 10 min represents the nearly actual value. L A l ( O H ) '+ + H' A 1'' + 1120 A l ( O H ) *+ + H 2 0 A l ( O H ) 2 + + H' + H ' e A 1 ( O H ) 2' -t H z O A l (OH) 3
-
10
100
Time
(
1000
min 1
Fig. 5. Dependence of the titration endpoints on the temperature and the time of ion-exchange, 0 Z ' C , CD 25'C. @ 50°C. 80'C Figure 6 shows the correlation of reaction rate constant K , w i t h acidity t h u s obtained. The reaction rate was proportional to the acidity. This suggests that i n the case of 2 3 - 5 zeolite w i t h small crystal size, the reaction proceeds a t most of acidic sites. 1.5
1.0
0.5
0
0.2 0.4 0. 6 0.8 Acidity ( mmol/g-cat 1
Fig.6. Correlation of K , at 120'C w i t h acidity.
Liquid-Phase Hydration of Cyclohexene
479
Reaction model for the three-phase system The reaction system consists of three phases: the oil phase, the aqueous phase and the catal y s t phase, the catalyst staying always i n the aqueous phase. Figure 7 shows a model for this system. The hydration reaction occurs as follows. Cyclohexene i n the o i l phase dissolves i n the aqueous phase and adsorbs on the catalyst, where the hydration reaction occurs to form cyclohexanol. The cyclohexanol dissolves i n the aqueous phase and is extracted by the o i l phase. The chemical equilibrium is established on the catalyst. On the other hand, the distribution equilibrium is established between the aqueous phase and o i l phase. Therefore, the reaction rate and the equilibrium concentration of cyclohexanol i n the o i l phase are affected by the distribution ratio.
O i l phase
Aqueous phase
Catalyst phase Pig. 7. Reaction model for the liquid-phase hydration of cyclohexene catalyzed by zeolites. Figure 8 shows the effect of the addition of various solvents on the cyclohexanol concentration i n the o i l phase.
Pig.8. Effect of addition of solvent on the reaction rate. Conditions :Temp; 120 ' C. Initial charge;Cyclohexene/SoIvent/Water/Catweight ratio =I. 2/1.2i 2.7/1.0, In the case of non solvent: Cyclohexenez2.4, non. 0 Phenol, @ Benzyl alcohol. @ Metyl ethyl ketone, A Methanol, A Di isobutyl ether
480
H. Ishida, Y . Fukuoka, 0. Mitsui and The late M. KOno
Table 2 shows the influence of phenol on the distribution ratio. Phenol distributes between both phases and gives the advantage of the distribution of cyclohexene to the aqueous phase. Table 2. The distribution ratio between the oil phase and the aqueous phase a t 120 "C Presence or o i l phase (mol%) aqueous phase (mol%) Distribution ratio absence of Cyclo- Cyclo- Phenol Water Cyclo- Cyclo- Phenol Water Cyclo- CycloPhenol hexene hexanol hexene hexanol hexene hexanol Presence Absence
16.0 84.0
24.9 13.0
36.7 22.0 3.0 ~
0. 024 0.022
0.32 0.22
1.31 98.3 99.8 ~
680 3818
78 59
Cyclohexene or Cyclohexanol in the o i l phase Distribution ratio
=
Cyclohexene or Cyclohexanol i n the aqueous phase CONCLUSIONS 1. Highly s i l i c i o u s zeolites ( SiOz/Alz03 molar r a t i o :20 give high c a t a l y t i c activity for the liquid-phase hydration of cyclohexene. Hydrophobic property of these zeolites accounts
for the high activity. 2. ZSM-5 and ZSM-11 having channels formed by 10-membered rings give high s e l e c t i v i t y for cyclohexanol because of the fitness of dimensions of the cyclohexanol and the channel.
3. The reaction rate increases w i t h decreasing the crystal size of ZSM-5 with more than 0.5 pm
of the crystal size.
4. The reaction r a t e is proportional to the acidity of ZSM-5 w i t h less than 0. 1
p m
of the
crystal size.
5. The reaction system consists of three phases. The reaction rate and the equilibrium composi tion of the o i l phase depends on the distribution ratio between the o i l phase and the aqueous phase. REFERENCES
I 2 3 4 5 6 7 8
T6y6 RFyon. Jpn. Kokai Tokkyo Koho. 48 447 (1969). Ube K6san. Jpn. Kokoku Tokkyo Koho. 58 1089 (1983). T6y8 REyon, Jpn. Kokoku Tokkyo Koho. 44 26656 (1969). Toyo Reyon, Jpn. Kokoku Tokkyo Koho, 47 45323 (1972).
G. W. Skeels, W. H. Flank, Intrazeolite Chemistry, American Chemical Society., (1983) 369. M. KEno, Y. Fukuoka. 0. Mitsui. H. Ishida, Nihon Kagaku Kaisi., 3 (1989) 521. S. Namba, N. Hosonuma. T. Yashima, J. Catal .72 (1981) 16. D. W. Breck. Zeolite Molecular Sieves, A Wiley Interscicnce Publication, (1974) 514.
The Synthesis of Methyl Isobutyl Ketone over Palladium Supported Zeolites
P. Y. Chen, S. J. Chu, W. C. f i n , K. C. Wu,and C. L. Yang Union Chemical Laboratories, Industrial Technology Research Institute, 321 Kuang Fu Road, Section 2, Hsinchu, Taiwan.
ABSTRACT The one-step liquid phase synthesis of methyl isobutyl ketone (MIBK) on supported Pd/ZSM5 from acetone was extensively studied. Effects of supports, treatment reagents, and reaction variables on the activity and selectivity of MIBK formation were investigated. The catalyst life was also examined. It was found that organic sacid pretreatment could enhance MIBK selectivity and catalyst life. It is suggested that this enhancement is resulted from an expansion of secondary pores and an increased acidity. INTRODUCTION Methyl isobutyl ketone is an important industrial solvent. The available commercial production procedures involve three steps that include: Aldol condensation, dehydration, and In addition to being a complicated process, this 3-step approach also involves hydrogenation(*-2). corrosion problems, contamination by acids and bases, and the employment of low temperature equipment. On the contrary, a process utilizes Pd/resin catalyst to form MIBK from acetone and hydrogen in just one step". This process is not only simpler than the 3-step process but also has less corrosion and contamination problems. For this reason, the 1-step process is believed to be '~. the current 1-step process the major process for MIBK formation in the f ~ t u r e ' ~ However, catalysts are not stable at high temperature and are time-consuming upon regeneration. This paper presents our results using improved Pd/ZSM-5 zeolites as bifunctional catalysts to convert acetone and hydrogen to MIBK in one step.
48 1
482
P. Y . Chen, S. J. Chu, W. C . Lin, K . C. Wu. and C . L. Yang
EXPERTMENTAL 1. Catalvst meparation The ZSM-5 zeolites were prepared following the Mobil patents@).Before loading Pd, the HZSM-5 Zeolites were pretreated (with organic or inorganic acids) under different concentrations, temperatures, and time. The pretreated zeolites were then washed, and dried. The zeolites were then ion exchanged with 0.01M Pd(NH,),Cl, solutions until the maximum exchanged capacity was reached. They were followed by filtering, washing, drying , and pellet formation.
2. Catalvst characterization The chemical compositions of the zeolites were analyzed according to the procedures of Hillebrand et a1.O. The acidity was determined by NH, temperature programmed desorption (TPD)css. The pore size distribution was measured by Hg porosimetry, and the surface area was measured by N2physical adsorption.
3. Catalyst activitv evaluation Activities were tested in a fixed-bed reactor at 135-160 "C under 200-420 psig. The 318" cylindrical reactor contained 3g of 12-20 mesh catalyst. The reactor effluent was cooled and collected in a cold trap. The collected liquid product was analyzed by a HP-5890 gas chromatograph with a HP-FFAP column, RESULTS AND DISCUSSION
- supports 1. Influence of various catalvst The conversion of acetone and the selectivity of each component over various catalysts are shown in Table 1. As indicated by the data, the alkaline-free supports are superior. For example, KL zeolite contains alkaline metal ions has the lowest activity; ZSM-5 and AAB have the
highest MIBK selectivity (around 60%). In terms of diisobutyl ketone (DIBK) by-product, AAB is superior to ZSM-5. According to the mechanism of DIBK synthesis, the pore size of support also influences the formation of DIBK. In general, the pore size of AAB is larger than ZSM-5.
Synthesis of Methyl lsobutyl Ketone
483
Table 1. Influence of various catalyst supports catalyst
conversion selectivity mole% acetone mole% MIBK IPA DIBK other
0.3 wt% PdIA120, 0.4 wt% PdIKL 0.3 wt% PdIAAB 0.5 wt% PdIZSM-5
51.78 31.12 42.26 52.24
9.20 14.43 60.13 61.12
79.08 35.47 5.69 12.83
0.36 11.15 0.23 49.87 24.37 9.81 1.88 24.17
Reaction condition: T=160 "C, LHSV=1,6 hf', H21acetone=l, P=420 psig time on stream=6-8 hr *AAB:mixed oxide (A1203and B,OJ 2. Influence of treatment reagents In order to obtain better activity and MIBK selectivity, ZSM-5 zeolite was treated with various reagents, and then loaded with Pd, The results are shown in Table 2. As indicated in the table, steam treatment (at 7WC, 2hrs) increase IPA selectivity as well as acetone conversion. This result suggest that steam pretreatment might lower the surface acidity, thus to increase hydrogenation activity. Strong inorganic acid (such as hydrogen chIoride, nitric acid etc) pretreated ZSM-5 enhanced MIBK selectivity nearly 20%. However, the activity was lower by more than 29% compared to that of untreated ZSM-5. This may be due to the partial elimination
of active sites by the dealumination. Pretreatment with mild organic acids (such as citric acid, benzoic acid, malic acid, tartaric acid etc) had the highest MIBK selectivity and insignificant effect toward catalyst activity. The contents of IPA and light molecules are also decreased. This result may indicate that organic acids can only react with free A1203,but not the aluminum in the framework structure. In terms of selectivity enhancement, the pore size change has a significant impact. Table 2 The effect of reagent treatment on activity of PdIZSM-5 treatment reagent untreated steam inorganic acid organic acid
conversion acetone mole % 52.24 69.16 23.31 41.24
selectivity mole % MIBK IPA DIBK other 61.12 5.23 81.01 90.98
12.83 72.68 0.28 0.94
1.88 24.17 0.09 22.00 1.56 17.15 2.09 5.99
Reaction condition: T=160 "C,LHSV=1.6 hf', H,/acetone=l, P=420 psig time on stream=6-8 hr
484
P. Y . Chen, S. J . Chu, W. C. Lin, K. C. Wu, and C. L. Yang
3. Catalytic properties of ZSMJ treated with or-
.
.
The analytical data of treated and untreated ZSM-5 zeolites and the reactivity of zeolites after loading Pd are shown in Table 3. From TPD data, it is discovered that the acid amount between 200 to 400 "C increased significantly; acid amount at 400 to 600 "c also increased slightly. Pore
size increase is most apparent at less than 1000 A. These property changes acetone conversion only slightly, but the selectivity toward MIBK was enhanced by 30%. From this, it is concluded that acetone conversion and MIBK selectivity depend on the acidity between 200-400 "C. The increase of lo00 A pores may imply that, in this reaction, the shape selectivity of ZSM-5 is not a deciding factor.
Table 3 The effect of organic acid on catalytic properties of ZSM-5 untreated
properties Of ZSMd
BET (m2/g) acidity (mmol/g) < 200 "C 200-400 "C 400-600 "C pore volume (ml/g) <0.1 pm 0.1-10 pm > 10 pm conversion acetone mole% selectivity mole% MIBK IPA DIBK other I
activity of Pd/ZSM-5
treated
378
386
0.54 0.49 0.21
0.54 0.75 0.32
0.003 0.07 0.13
0.42 0.03 0.24
52.24
41.24
61.12 12.83 1.88 24.17
90.98 0.94 2.09 5.99
Reaction condition: T=160 "C, LHSV=1.6 hr-', H,/acetone=l, P=420 psig time on stream=6-8 hr 4. Influence of reaction condition
(1) Effect of temuerature The acetone conversion and MIBK selectivity at 135-160 "c are described in Fig. 1. Both the conversion and selectivity increase with increasing temperature. However, the selectivity is at the highest at 145 OC, and then selectivity decreases by further increasing the temperature, but still remains above 87%. IPA by-product content is not dependent on the temperature variation;
Synthesis of Methyl lsobutyl Ketone
485
DIBK by-product increases slightly with reaction temperature. Both by-product concentrations
are less than in the case of zirconium phosphate catalyst?@. I00
00 -
100 A
n
s 60 z
U
o_
2
W
40-
%.-
A A /
w
A
MlBK
fir-
-80 n i60-, s
- + -5 3 -
-4
I-
Fig. 1. Effect of temperature. Reaction condition: P=420 psig, LHSV=1.6 hf', H2/acetone=l, Time on stream=6-8 hr. 12) Effect of pressure
Fig. 2 shows the effect of total pressure on the conversion and selectivities to each component in the range of 200-420 psig. Acetone conversion slightly increases with increasing pressure. I
LLI
> z
0 0
Fig. 2. Effect of pressure. Reaction condition: T= 150 "C, LHSV= 1.6 hr', H,/acetone= 1.6, Time on stream=6-8 hr.
486
P. Y. Chen, S . 1. Chu, W. C . Lin, K .
C . Wu, and C. L. Yang
MIBK selectivity is around 90%and is not dependent on pressure changes. The IPA by-product and MIBK selectivities are also independent of pressure. However, DIBK selectivity decreases with increasing pressure.
13, Effect of mole ratio of hvdroPen/acetong Fig. 3 illustrates the effect of hydrogedacetone mole ratio in the range 1-25.No appreciable change in either acetone conversion and MIBK selectivity was observed when the mole ratio was changed. From HJDMK 1.0 to 2.0,IPA selectivity decreases but DIBK increases. Beyond this range, the H,/DMK has little imDact on the selectivities. I00
n
400 A
80.
s u60z 0 -
UI
7"
.
-
MIBK
o DMK r
-80
. s
-
n
-
CJ
-Lo>- t -5 3
Fig. 3. Effect of mole ratio of H,/acetone. Reaction condition: T=150 T,P=420 psig, LHSV=1.6 hf', Time on stream=6-8 hr. [4) Effect of acetone mace velocity
The effect of acetone space velocity (SV) on both conversion and MIBK selectivity at 150 "C were shown in Fig. 4, Although the acetone conversion gradually decreased with increasing
SV, the MIBK selectivity still remained above 92% in the SV range 0.4-3.9. The major by-
products, IPA and DIBK, decreased with increasing SV. No mesityl oxide was detected under these reaction conditions.
Synthesis of Methyl lsobutyl Ketone
487
Fig. 4. Effect of acetone space velocity. Reaction condition: T=150 "C, P=420 psig, H2/acetone=1.6, Time on stream=6-8 hr.
5. Catalyst life The catalyst life data was shown in Fig. 5. Both conversion and selectivity of untreated catalysts decreased rapidly during the first 24 hours of operation at 160 "C, whereas the activity of catalysts treated with organic acid decreased slowly from 52% to 39% and MIBK selectivity decreased from 90% to 80% during the first 180 hours.
I00
100 v
n
u $
00A*.
n
A
untreated
treated
-
$ c)
30 2_ I- -
> -
-40+ 0 0
0 .ld LJ
-x)cn
20-
0
80 120 ' Id0 mo TIME ON STREAM C hr 1 Fig. 5. Effect of treatment on catalyst life. T= 150T,P=420 psig, LHSV= 1.6 hi', H2/acetone=1. 0
40
488
P. Y. Chen,
S. J. Chu, W. C. Lin, K. C. Wu, and C. L. Yang
CONCLUSION The following remarks could be made on the basis of the above discussion: (1) The Pd/ZSM-5 are potentially useful catalysts for one-step, liquid phase MIBK preparation.
(2) The Pd/ZSMd selectivity and catalyst life can be enhanced by organic acid pretreatment. (3) Both acidity and secondary pore size of modified ZSMd are responsible for enhancing the activity and catalyst life. ACKNOWLEDGEMENTS We acknowledge the financial support of the Lee Chang Yung Chemical Industry Co. LTD. REFERENCES 1 S. Kudo, J. Chem. SOC.Jpn Ind. Chem. Sect.,(1955)785. 2 D. Showa, Jp 2009(1971). 3 K. Takay et al., German 1,936,203(1970). 4 P. Y. Chen et al., Stud. Surf. Sci. Catal., 46(1989)231. 5 P. Y. Chen et al., US 5,059,724(1991). 6 R. J. Argauer, and G. R. Landolt, US 3,702,886(1972) 7 W. F. Hillebrand et al., Applied Inorganic Analysis, John Wiley and Sons, New York,1953. 8 K. J. Chao, B. H. Chiou, C. C. Chu, and S. Y. Jeng, Zeolite, 4(1984)2. 9 J. C. Post, J. H. C. Van Hooff, Zeolite,4(1984)9. 10 Y.Watanabe, Y. Matsumura, Y. Izumi, and Y. Mizutani, Bull. Chern. SOC. Japan, 47(1974)2922.
Influence of Zeolite Secondary Porosity on Performance of Resid Hydrocracking Catalysts
P.E. Dai',
D. E. Sherwood Jr., and B. R.Martin
Texaco Inc. Research and Development P. 0. Box 1608 Port Arthur, Texas 77641 USA
ABSTRACT Conventional USY-containing hydrocracking catalysts, though exhibiting conversion improvements over alumina based catalysts, were not suitable for hydroprocessing heavy oils in the mild hydrocracking mode because of high sediment formation. The secondary porosity of zeolites and macroporosity of finished catalysts played a decisive role in the activity and reactor stability of novel zeolite/alumina catalysts when hydrocracking vacuum resid containing feedstocks. INTRODUCTION In recent years, the decreasing demand for heavy fuel oils requires that refiners find ways for converting heavy hydrocarbon feedstocks to higher value mid-distillate products. To increase mid-distillate production, the refiner can choose from several processing options such as mild hydrocracking (e.g. Texaco's T-STAR Pmcess), fluid catalytic cracking, hydrocracking (e.g. HRI/Texaco's H-OILm Process) and coking with mild hydrocracking (h4HC) being the least capital-intensive option. Using a conventional hydrotreating catalyst, an MHC process typically converts up to 30 vol% of a vacuum gas oil feedstock boiling above 627 K (627 K+) to middle distillates boiling at or below 627 K (627 K-). For a residuum-containing feedstock, MHC usually gives less than 10 vol% conversion of the 627 K+ fraction. With alumina-based hydrotreating catalysts, the
conversion of resid components boiling above 8 10 K (8 10 K+) into products boiling at or below 810 K (810 K-) is achieved primarily by thermal cracking reactions. A difficulty which arises
in resid hydroprocessing units employing currently known catalysts is the formation of insoluble carbonaceous substances (also called sediment) when the conversion is high (above 50 ~01%). The higher the conversion level for a given feedstock, the greater the amount of sediment formed. HY and USY zeolites have been widely used in catalysts for vacuum gas oil hydrocracking 489
490
P. E. Dai, D. E. Sherwood Jr., and B. R. Martin
processes for the production of gasoline and middle distillates [l-41.The commercial catalysts which contain USY zeolites also give rise to the formation of high sediment when employed in resid hydrocracking processes. This problem is more acute at low hydrogen partial pressures and high reaction temperatures. The general objective of this work is to identify a zeolite catalyst that will give a higher conversion level for heavy hydrocarbon feedstocks containing significant amounts of vacuum resid while maintaining low sediment formation. A more specific objective is to investigate the effects of secondary porosity of zeolite components and macroporosity of finished catalysts on reactor stability in resid hydrocracking services.
EXPERIMENTAL PreDaration of dealuminated Y zeolites The preparation of steamed and steamedacid leached Y-zeolites are reported in the references
[3,4,6].Texaco's DAY Zeolite was prepared by PQ Corp. according to the reference [4].Three kinds of Y-zeolite, PQ's CP300-56USY, CP300-35SUSY and Texaco's DAY were selected for use in the preparations of zeolite/alumina catalysts. Characterization of dealuminated Y zeolites The secondary porosity, defined as pore volume of pores having diameters in the range of
50-600k was measured by nitrogen adsorption using a Digisorb 2500 instrument.
X-ray
diffraction using a Scintag PAD-V instrument was carried out to determine the unit cell size and crystallinity. The bulk SilAl ratio was calculated using the equation: Si/AI=[ 1.6704/(A0-24.19)]-1. The surface Si/AI ratio was determined by XPS using a VG surface analyzer. The macroporosity of the formed materials is defined as the pore volume of pores with diameters greater than 250
A (denoted as PV > 250 A, cc/g) measured by using an Autopore 9220 instrument. Catalvst DreDarations The catalysts were prepared from commercially available porous supports composed of
1 alumina and Y-zeolite/alumina. All of the supports were extrudates with diameters of 0.035-0.04 inches made by Cytec Industries. Each support was impregnated with the requisite amounts of
Mo and Ni metal oxides and phosphorus oxide to yield a finished catalyst containing a NiO in amount of 3-3.5 wt%, MOO,in amount of 14.5-16.5Wto? and P,O, in amount of 0-1.5wt??. Stabilizers such as phosphoric acid, hydrogen peroxide and citric acid monohydrate, were employed to vary the surface distributions of active metals. The impregnated supports were then oven-dried and calcined at 8 10 K-894K for 20 minutes to 2 hours in flowing air. Mild hydrocracking catalyst evaluation
Secondary Porosity i n Resid Hydrocracking
491
A Berty reactor was used to determine mild hydrocracking activities of the candidate catalysts in a diffusion controlled regime at a low rate of deactivation. After being loaded in the reactor, the catalyst was presulfided and then the reaction was carried out at a single space velocity for 38 hours. Sample cuts were taken every 4 hours and tested for boiling point distribution and sediment content. The feedstock was a blend of 60 vol% desulfurized vacuum gas oil (VGO) and 40 vol% Arabian Mediummeavy vacuum resid and contained 89.2 vol% of
616 K+ and 33.5 vol% of 810 K+. The operating conditions for the reactor evaluations are as follows: temperature 722 K, pressure 6.9ma, hydrogen feed rate 300 sccm, liquid feed rate 82.5 cchr, and catalyst loading 36.9 grams. The mild hydrocracking activity was determined by comparing the percentages of products in the 616 K- fraction and 810 K- fraction when various catalysts were evaluated under constant mild hydrocracking conditions with the same feedstock. The conversions of the 616 K+ and 810
K+ fractions were calculated using the equation: Conversion={[Y(F)-Y(P)]N(F)}XlOO% where Y(F) =volume percentage of the 616 K+ or 810 K+ fraction in the feedstock, and Y(P)
= volume
percentage of the 616 K+ or 810 K+ fraction in the product. Robinson reactor evaluation of catalysts The catalytic performances of catalysts in vacuum resid hydrocracking were evaluated using a Robinson Reactor. A Robinson Reactor is a continuous stirred tank reactor which evaluates catalyst deactivation rate at conditions simulating the first stage of a two-stage BOIL ebullated bed unit. The feedstock used was an Arabian Mediummeavy vacuum resid, and the evaluations were typically carried out for 3-4 weeks. This feedstock contained 5.0 wt% of sulfur, 49 ppm of Ni and 134 ppm of V and 88.5 wt% of the feedstock had a boiling point greater than 810 K The Robinson Reactor run conditions were as follows: temperature 683 K,pressure 15.5 MPa, space velocity 0.56hf’, catalyst weight 30 g, normal conversion of resid fraction boiling above
810 K to distillate fraction boiling below 810 K 41 wt%.
RESULTS AND DISCUSSION Catalvst DroDerties
SN-6556,a bimodal NiMo catalyst supported on alumina, is characterized by having a pore mode of about 100 hi and a macroporosity of 0.20 cc/g. SN-6448was made with a support that contained 20 wt% of USY and 80 wt% of precipitated alumina which contained no silica. Upon final calcination in the hydrothermal environment, a fraction of starting USY zeolite was destroyed. The actual zeolite contents of finished catalysts are given in Table 1. SN-6571 is a NiMoP catalyst supported on a carrier that consisted of 20 wt% of USY zeolite of unit cell size
492
P. E. Dai, D. E. Sherwood Jr., and B. R. Martin
about 24.41 A and 80 wt% of a precipitated alumina. Compared to SN-6448, SN-6571 has a larger pore mode (98 vs. 82 A) and a smaller macroporosity (0.13 vs. 0.24 cc/g). SN-6571 also has a higher percentage of pore volume in pores of diameter between 100-160 A than SN-6448. SN-6572 was prepared by using a support which comprised 20 wt% of an SUSY zeolite with
unit cell size of 24.29 A and silica-alumina molar ratio of about 32 and 80 wt?hof a precipitated alumina. The charge SUSY zeolite is featured by having a lower acid site density relative to the USY zeolite and a higher pore volume in pores with diameter greater than 50 A as well as having
A. The pore modes of SN-6572 and SN-6571 are about equivalent, but the pore volume in the 100-160 A range is greater for SN-6572 (52% vs. 42% of
a secondary pore mode of about 85
TPV). SN-6603 was prepared using the support which comprised 20 wt% of acid-treated,
dealuminated Y zeolite (DAY) and 80% of alumina that contained 2 wt% silica. Phosphorus promoter was employed in the impregnating solution. SN-6603 showed uniform distribution of Molybdenum since the Mo gradient was 1 . 1 . SN-6603 has a higher macroporsity than SN-6571 and SN-6572. The properties of two additional DAY-containing catalysts, SN-6726 and SN-6785 are shown in Table 2. Bern reactor mild hvdrocracking activities In our previous work [5], we reported that the 616 K+ conversion activity decreased and sediment decreased with increasing macroporosity of both NiMo and NiMoP catalysts on alumina supports. The presence of phosphorus did not affect the 616 K+ conversion activity. SN-6556 showed 9 vol% advantage in 616 K+ conversion and 6 vol% advantage in 810 K+ conversion compared to a standard H-Oil catalyst (Catalyst A in [51).
Because SN-6556 has a pore
structure more similar to the zeolite/alumina catalysts than a standard H-Oil catalyst, it was chosen as the reference catalyst for the comparison with the mild hydrocracking (MHC) activities of zeolitelalumina catalysts. The results of MHC activities are summarized in Table 1. The best catalyst SN-6603 gave 1 1 vol% improvement in 616 K+ conversion and 6 vol% advantage in 810 K+ conversion relative to the SN-6556 catalyst. Most importantly, there is little increase in the sediment-make accompanying with the increased hydrocarbon conversion. Compared to the SN-6556, the SN-6572 and SN-6571 exhibited about 5-7 vol% improvements in 616 K+ and 810 K+ conversion activities. The NiMoP zeolite/alumina catalysts in general exhibited improvements in both 616 K+ and 810 K+ conversion activities than SN-6556. In our earlier work, both SN-6446 and SN-6447 catalysts, which have bimodal pore size distributions with macropore modes (2000-4000 A) similar to those of standard H-Oil catalysts,
Secondary Porosity in Resid Hydrocracking
493
were evaluated for their MHC activities. The supports of SN-6446 and SN-6447 comprise respective 20 and 35 wt% of the USY zeolite having a unit cell size of 24.42 A and a silicaalumina molar ratio of 12. The alumina used in the two catalysts contains about 2 wt?hof silica
as a thermal stabilizer. Both SN-6446 and SN-6447 have pore modes between 69-80 A and high macroporosities between 0.21-0.30 cc/g.
SN-6446 and SN-6447 did not give significant
improvement over the standard catalyst (2-3 vol% 616 K+ conversion) despite containing 1 1 and 23 wt% of USY zeolite, respectively. It is believed that the lower than expected conversion activities resulted from high macroporosities and high acid site density of USY zeolite, leading to a rapid deactivation of zeolite component. Also noted is that SN-6447 generated less sediment than any other catalysts presumably due to its highest macroporosity. SN-6448 which had a pore structure similar to SN-6446 not only gave the same vol% improvement in the 810 K+ conversion compared to SN-6556, but also gave only slightly higher sediment-make.
SN-6448 and SN-6446 differ only in the composition of alumina matrix.
Precipitated alumina was used in the preparation of SN-6448, whereas, 2 wt?hsilica-stabilized alumina was used in SN-6446. The results suggest that catalyst made of the precipitated alumina appears to give a reduced sediment-make. SN-6571 catalyst showed high activities for 616 K+ and 810 K+ conversion. Unfortunately, it caused minor plugging problems probably because its macroporosity was reduced and the micropore mode was enlarged relative to SN-6448.
Idemitsu-Kosan's R-HYC4 catalyst is a
proprietary zeolite-containing catalyst for hydroprocessing of atmospheric resids. It showed 616 K+ and 810 K+ conversion activities very comparable to SN-6571, however, R-HYC4 also caused an early reactor unit shutdown due to plugging by sediment. These results indicate that zeolite
catalysts which comprise acidic Y-zeolites with unit cell sizes between 24.40 and 24.60 A and matrix compositions similar to those taught in U. S. Pat. 4,600,498 are not particularly suited for use in the MHC of heavy oils containing high fractions of vacuum resids. To improve the unit operability, Valfor CP300-56 USY zeolite in SN-6571 was replaced with a dealuminated Y zeolite (PQ's CP300-35 SUSY) in the preparation of SN-6572. SN-6572 and SN-6571 have almost identical pore structures and metal compositions. It is seen in Table 2 that SN-6572 gives about 5 vol% advantage in 616 K+ conversion and about 3 vol% improvement in 810 K+ conversion with small incremental sediment relative to SN-6556. There is no unit plugging problem for SN-6572. The results demonstrate that the combined modification of acidity and secondary pore structure of Y-zeolite provides a solution to the unit operability problem occurred on zeolite/alumina based catalysts. TO further improve conversion and reactor operability, SN-6603, SN-6726 and SN-6785
494
P. E. Dai, D. E. Sherwood Jr., a n d B. R. Martin
catalysts that contain DAY zeolites were investigated. As shown in Table 1, SN-6603exhibited higher 616 K+ and 810 K+ conversions than SN-6572,whereas, the IP sediment was maintained at similar or lower level. SN-6603 did not have reactor plugging problem during the whole period of evaluation. This improvement may be attributed to the nature of DAY zeolite and the higher macroporosity of the SN-6603 catalyst compared to SN-6572. Three catalysts, SN-6448,SN-6572,and SN-6603 not only provided advantages over a standard H-Oil catalyst, in both 650 F+ and 1000 F+ conversion activities but also maintained the sediment-make at a level similar to the conventional bimodal alumina based catalysts. Furthermore, the zeolite/alumina catalysts exhibited significant improvements over the newgeneration bimodal alumina supported catalyst- SN-6556. Robinson reactor results Table 2 presents the results of Robinson reactor evaluations of zeolite catalysts along with
SN-6556 reference catalyst. It is seen that the zeolite-containing catalysts showed 1000 F+ conversion advantages between 1.7and 7.4vol% at constant reaction temperature relative to SN-
6556. However, the sediment levels increased rapidly with increasing 810 K+ conversion levels. As a result, there were reactor plugging problems for the SN-6571,SN-6572and particularly,
SN-6726catalysts. SN-6785having a macroporosity at the same level as SN-6556did not have any problem with reactor plugging. In the present work, the secondary porosity of a SUSY zeolite was generated by hydrothermal treatment of USY zeolite, and the secondary porosity of a DAY zeolite was created by combined hydrothermal and acid treatments of a USY zeolite. Pore modes increase in the order of 23A (USY) < 85A (SUSY) < 135A (DAY). The secondary porosities of zeolites, defined
as pore volumes of pores with pore diameters in the range of 50-600A, are 0.05,0.17,0.22cc/g, for USY, SUSY, and DAY zeolites, respectively. By contrast, the total acidities measured by
NH, TPD decrease in the order of 1.4 (USY) > 0.31 (SUSY) > 0.13 (DAY) mmole/g. It was postulated that the addition of acidic zeolite will enhance the catalytic conversion of heavy oils to lighter products. It was further suggested that dealuminated Y zeolites that have a substantial amount of secondary pore volume and a pore mode of the secondary pores within 50-325 A can facilitate the transport of large oil molecules into the cracking sites in the zeolite, thereby leading to enhanced conversion. It was speculated that the dealuminated Y zeolite will make less coke or sediment precursor because of its reduced acidity relative to ultrastable Y zeolite (USY). The gas oil cracking activity for the steamed, the acid-leached, and the steamedacid-leached samples along with the untreated CP300-56USY zeolite are reported in the reference
[6]. The activity maximum may depend on the zeolite preparation procedure. For the steamed
Secondary Porosity in Resid Hydrocracking
495
and steamed acid-leached samples, the maximum is in the range of 24.33-24.35A which is in good agreement with the literature. There is no correlation between the gas oil cracking activity and the total acidity measured by TPD of ammonia for all of these samples. The secondary porosity of zeolites is beneficial for enhancing the cracking of heavy resid feedstocks. The results of hydrocracking catalyst performance in the Berly reactor indicate that the catalyst containing DAY zeolites gave improvements in both the 616 K+ and 810 K+ hydrocarbon conversions, and most importantly this catalyst improved reactor operability. It is implied that the a zeolite-containing resid hydrocracking catalyst requires a balance among several key parameters;
pore mode of secondary porosity, total acidity of the zeolite component, and
macroporosity of the finished catalysts. The results of Robinson reactor evaluations also suggest that secondary pores can facilitate the transport of resid molecules into the cracking sites in the zeolite, and that the macroporosity of the finished catalyst plays an important role in determining reactor stability. CONCLUSIONS The conventional USY-containing hydrocracking catalysts were not suitable for processing of heavy oils in the mild hydrocracking mode because of high sediment formation. SUSY and DAY zeolite-containing catalysts provide conversion improvement with small incremental sediment compared to alumina based catalysts.
The secondary porosity of zeolites and
macroporosity of finished catalysts play a decisive role in the activity and reactor stability of zeolite/alumina catalysts when hydrocracking the vacuum resid containing feedstocks. REFERENCES 1. J. W. Ward, U S . Patents 4, 600,498, 4,686,030, and 4,826,587. 2. K. 2. Steigleder, U.S. Patent 4,894,142. 3. P. E. Dai, D. E. Sherwood, Jr, and B. R. Martin, U.S. Patent 5,087,348. 4. P. E. Dai, D. E. Sherwood, Jr, U.S.Patents 5,112,473 and 5,143,878. 5. P. E Dai and C. N. Campbell, Symposium on Hydroprocessing of Petroleum and Distillates, AIChE 1993 Spring National Meeting, March 28-April 1, 1993, Houston, Texas. 6. P. E. Dai, L. D. Neff, and J. C. Edwards, Symposium on Advances in Fluid Catalytic Cracking, ACS National Meeting, August 23-25, 1993, Chicago
496
P. E. Dai, D. E. Sherwood Jr., and B. R. Martin
Table 1. BERTY reactor performance of zeolite hydrocracking catalysts Catalyst
SN-6556
SN-6448
SN-6571
SN-6572
SN-6603
~
Zeolite Type
NONE
USY
USY
SUSY
DAY
Zeol. Content, wt%'
0
11
15
17
16
ucs, A SiO,/Al,O,
NA NA
24.56 6
24.56 6
24.35 18
24.28 36
PV > 2 5 0 q cc/g
0.20
0.24
0.13
0.1 1
0.17
Reactor Plugging
NO
NO
YES
NO
NO
650 F+ Conv. Vol%
38
42
46
43
49
1000 F+ Conv. Vol%
84
84
91
87
90
TLP, IP Sediment, % Existent
0.4
0.6
1.1
0.7
0.6
Table 2. Robinson reactor performance of zeolite catalysts SN-6556
SN-6571
SN-6572
SN-6726
SN-6785
Zeolite Type
NONE
USY
SUSY
DAY
DAY
Zeol. Content, wt%b
0
15
17
16
26
ucs, A S i O,/Al ,03
NA NA
24.56 6
24.35 18
24.28 36
24.28 36
PV > 2 5 0 q cc/g
0.20
0.13
0.11
0.12
0.20
Reactor Plugging
NO
YES
YES
YES
NO
1000 F+ Conv. Advantage, vol%
BASE
+3.1
+3.4
+7.4
+1.7
Catalyst
TLP, IP Sediment, % 0.001 0.63 0.78 1.55 0.001 Accelerated 'Zeolite Contents shown in Table 1 are actual values determined by XRD for finished catalysts. The supports have a nominal zeolite content of 20 wt?hprior to impregnation and final calcination. bZeolite Contents shown in Table 2 are actual values determined by XRD for finished catalysts. the nominal zeolite contents for the supports of SN-6726 and SN-6785 are 20 wt% and 35 wt%, respectiviely.
Regeneration Behaviors of Hydroisomerbation CatalystS
Li Shi, Fan Liu, Lin-Li Liu, Cheng-Lie Li and Lu-Ting Li Petroleum Processing Research Center East China University of Chemical Technology 200237 Shanghai China
ABSTRACT Deactivation of a Pd / HM hydroisomcrization catalyst is described. Catalyst deterioration during elimination of coke by combustion was examined. The main cause was sintering of loaded palladium particles on Pd / HM catalysts. It was noted that not only the amount but also properties and place of coke deposits can have a dramatic effect on restoration of catalytic performance. Optimum tempcraturcs were in thc range of 530-550 C for ex-situ regeneration by oxidative burnoff of coke. INTRODUCTION As lead is phased out of gasolinc for cnvironmcntal rcasons, it has become increasingly necessary to rearrange the structure of thc hydrocarbons i n gasoline fractions in order to achieve high octane ratings. C, / C, hydroisomerization is one af economical processes for this upgrading. Noble metal loaded hydrogen form mordenite catalysts (Pt / H M or Pd / HM) have been effectively applied for this proccss.Under smooth normal conditions, the life of this kind of long lived catalysts is limited by very slow rates of coking. After a certain period of operation,called a cycle.the increasing reaction temperature becomcs so high that the unit must be shut down for elimination of coke by combustion. It is proved that burnoff conditions, which would give a dramatic impact to catalytic behaviors of regenerated catalysts, is also related to thc isomerization process itself. Thcy are, therefore, closely related with both isomerization and coke burnoff conditions. These points are discusscd i n this paper.
ESPERIMENTAL Catalysis -__The synthetic sodium mordenite was produced at the Nanjin Refinery, whrre a pilot plant has been installcd. It was waslicd with an acid, and therefore consists of 1 2.22%AI20,, 80.52% S O , and 8.023% Na20.The surl'iicc area is 115 ni2 / g. In our work, it was ion cxchanged with HCI and NH4CI aqueous solutions separately several times to remove sodium 497
498
L. Shi, F. Liu, L.-L. Liu, C.-L. Li and L.-T. Li
and enhance the silica -alumina ratio. H-form mordcnite ( H M ) was formed after calcination. The palladium mctal component was incorporated into mordenite from solutions of the amine complex [Pd(KH,),]C12. Our Pd / HM catalysts contained 0.5wt% palladium on zeolite base, They were mixed with alumina gel, then kneaded and calcincd. The calcination temperature was increased stcpwise and kcpt at 823K for two hours [1,2]. It was proved that crystallinity remained unchangcd in our ion cxchange procedures by X R D data [4]. Reactors Elcctrolytic hydrogen of 99% purity was purified by passing Microreactor system through a Deoxo purifier and a molecular sieve 5A trap. Passing through a mass flow meter, it was admixed with the feed before entering the reactor The catalyst applied in the microreactor was crushed and screcncd to 40- 60 mesh. 0.5g of catalyst was loaded in tlie center of the reactor tubc. Tlic fluidizcd sand bath and tlie control system maintained tlie temperature within? 0.5 K. Before rcaction, catalysts werc rcduced in situ at 673K under hydrogen pressure of 1.96MPa with a flow rate of 50 ml / min. The effluent was on-line sampled and analyzed by G.C. with a 56m OV-101 capillary column a t 273K, a hydrogen flame ionization detector (FID) and a sample splitter. Good resolution for all five isomers, pentanes and gaseous products (C,-C, o r CJ was obtaincd [3]. The laboratory reactor systcm ___ and pilot plant. Thc laboratory reactor was made of a 0 3 9 x 1350 stainless tube with a thermocouplc tubc inside. 30g of catalyst was loaded in the bed with a height of I85mm. n-Hcxane and ti-pentane of 99.0% purity wcre also used as feedstocks. In the pilot plant, a 0250 stainless tube was used as the reactor. T o simulate the industrial reactors, the height of tlie bed with about 100 kg catalysts was near 4,500 mm. The bulk density was 520 kg / m3. Elcctrolytic liydrogcn or 90%pure hydrogen from platforming was applied in the isomerization rcaction. but only the elcctrolytic hydrogen or that from a hydrogen-producing unit could be itscd for reduction, which was pcrformed at 673K. In the pilot plant, hydrogen was scparated from the effluent and recycled to the isomerization zone. The feedstocks included n-pentanc (n C5) a n d / o r n-hexane or light naphtha, etc. Catalyst particle size both in the laboratory and pilot reactors was in the range of 1.5-2.0 x 2-5 mni, much larger than those in the microreactor. The operating conditions were determined in microreactors. It was proved they werc able to bc applied in these two reactors for alkane isomerization, althougli the pilot rcactor system was operated i n an adiabatic state. Unless otherwise noted, hydroisomerization rcaction in these three reactors ivas performed at a tenipcraturc of 533K, a WHSV of 1-211-' and a hydrogen / alkanc molar ratio of 4, while maintaining a pressure of 1.96 MPa. Catalyst activity is noted as wt% of n-hexane convertcd. Due to the markedly higher octane numbcr of dinithylbutancs (DMB) in hcxane isomers and morc notable changes of 2,2 dimethyl butane (2.2DMB ) conccntratioii in the effluent products, liexanc selectivity is shown by 2,2 DMB / ZC, while yiclds of cracking gaseous products are indicated by C;. Catalvst characterization Infrared spectra \vcre rccorded on a Kicolet 20 SX FTIR spectrometer for nicosurment ~
Regeneration Behaviors of Hydroisomerization Catalysts
499
of acidic propertics. Pd / HM catalyst samples were pressed into a wafer with a density of 8-10mg/cm2. The cell was made of quartz and thewindow consisted of CaF,. The wafer, held in a quartz holder, could be raiscd to thc upper part of the cell where it was treated. The vacuum system reached up to Torr and pyridinc was uscd as a probe base. Dispersion state analysis of the palladium metal on thc catalyst was performed in a convcntional hydrogen-oxygen titration apparatus. For some samplcs, observations were made by transmission electron microscopy (TEM). TEM of JEM-200 CX,was made by JEOL Ltd and operated at 1250KV with final magnification of 29,OOOX . Electron micrographs were obtained from inspection of over 100 particles on one sample. XRD results were obtained on Rigaku D / max-rB.
RESULTS AND DISCUSSIOK Coked catalyst samples Alkane isomcrization reactions were performed in our pilot plant. In the first stage it was operated under normal conditions. After 3000 hours on stream, activities and selectivities remained almost at the same level. I n this period, pressures were in the range of 2-22 MPa with a hydrogen / hydrocarbon ratio of 4-8. Temperatures were all lower than 533K. To elucidate the effect of coke formation. an accelerated test was then performed stepwise as indicated in Table 1. Table 1 Stee
Temperature(C I
se accelerr d test
310
315
H, / oil (mot)
3.5
2.3
S.V. (hr-')
1.2
1.06
Pressure (Mpa)
Axial Distance (m.) Fig. 1 Carbon contcnt of coked catalysts along the reactor As the axial gradients are much larger than the radial ones, the discharge of coked catalysts was carried out in such a way as to obtain the coked catalyst samples n t various dcpths along the reactor. The carbon content of discharged catalysts is given in Fig. 1. It was shown cokc deposit was only 3 wt% in the upper end (inlct section) of the reactor, while there was a dramatic increase. in the catalyst bed near the outlet tubc. Regeneration by elimination of coke
L. Shi, F. Liu, L.-L. Liu, C.-L. Li and L.-T. Li
500
Deactivated catalysts wcre ex-situ regenerated by coke burning in an attempt to restore catalytic performance. Activi tics and sclcctivities presented i n Fig. 2 and Fig. 3 demonstrated possibilities to thoroughly rcstorc initial levels for dcactivatcd catalysts discharged from the upper section of the pilot reactor with lowcr coke contcnt. 100 n
u“
w \
-li *
Cbll
rLyenrrated
0
200
240
2HIl
320
Temperaturc (c Fig. 2 Activities of the trcsh and regcnerated catalyst in thc uppcr section
Temperature (“c ) Fig. 3 Selectivites of thc fresh and regenerated catalyst in the upper section
During a high degrce o f coke formation, superhcating of catalysts in the combustion stage are involved on the surhce and in the channels of mordenitc for effective elimination of coke deposits. Restoration profile for catalysts in the lower section of that reactor. wliere coke content on the catalyst reached up to 6%, arc givcn in Fig. 4 and Fig. 5. It was seen that deterioration occurred in LIIC rcgcncration stage under elevated temperatures and catalytic properties were, thereforc. oiily partly restorcd. This aging was more serious under prolongcd coke burning. as shown is Fig. 6 and Fig. 7.
200
240
30
320
Temperature (C ) Fig. 4 Activitics of thc fresh and rcgencrated catalysts a: frcsli catalyst b: catalyst regenerated at X23K c: catalyst regenerated at X43K d: catalyst rcgcncrated at 873K
200
1-40
?SO
3 20
Temperature (“c ) Fig. 5 Sccletivities of the fresh and rcgenerated catalysts a: fresh catalyst b: catalyst rcgenernted at S23K c: catalyst regcncratcd at 833K d: catalyst regenerated at S73K
Regeneration Behaviors of Hydroisomerization Catalysts
501
s h + 240
280
Temperature ('c Fig. 6 Activities of the fresh and regenerated catalysts
'* a n
?on
13h LO
Temperature (C) Fig. 7 Selectivites of the fresh and regenerated catalysts
In industrial reactors Rrolongation of a cycle is very profitable, but it can cause not only an increase in amount but also a change in location and properties of coke deposits that make it more difficult to be burned off. These phenomena may be explained by results of accelerated coking tests in the microreactor and related temperature programmed oxidation (TPO) profiles of the deactivated catalysts. A typical thermogram of TPO of coke is recorded and shown in Fig.8 where a fairly good resolution of two peaks is exhibited for coked isomerization Pd / HM catalysts. Those for Pd / cr-A1203and H-form mordcnite are also given. Because acidic function arose from H-form mordenite and a Pd / t2-A1203 was considered to be a nonacidic metallic function. it was therefore noted that the first zone of the Pd / HM catalyst corresponded to the burning of the coke deposited on the metallic function, while the second zonc in the higher temperature range corresponded to the coke depositcd on the acidic function. It was observed in our test that higher temperature peaks. which were suggested to be attributable to the coke deposited on HM acid sitestwere enhanced much faster than those for lower temperature peaks during increasing isomerization temperature and time on stream. At this time, more severe reganeration conditions should be applied for coke burnoff. Similar patterns were obtained for decreasing pressures and hydrogen-hcxane ratio. These were in line with results reported in the literature for reforming reactions [5]. It was concluded, therefore, that coking conditions in hydroisomerization reaction would have a strong impact on coke burnoff behavior and it was suggested that coke content on the catalyst must be limited to a certain level and a reasonable timc on stream should be determined as a cycle before shutdown for elimination of coke by combustion to thoroughly restore activity and selcctivity. _of catalyst deterioration in the regeneration process Cause Pd / HM catalysts were considered to have both metallic and acidic active centcrs. In our previous work, it was been reported that acidity is one of thc most important propcrtics in
502
L. Shi, F. Liu, L.-L. Liu, C.-L. Li and L.-T. Li
alkane hydroisomerization. Strong Bronsted acid sites in the main channels of mordcnite are catalytically relevant. Loaded palladium as a hydrogenation center is able not only to accelerate isomerization reaction but also postpone coke formation. In addition, it enhances strength of acidic sites and B- to L-acid sites ratio by interaction between loaded palladium and H-form mordenite. Therefore, changes in acidic properties were examined first. Strength and concentration of acid sites on fresh and regenerated catalysts under various temperatures, shown in Fig.4 and Fig.5, are listed in Table 2
518
718
618
818
K
Temperature Fig. 8 TPO spectra of coked samples
Temperature
Catalyst regenerated at
Fresh catalyst
570 C
550C
250C
350C 450C 550C
cB
cL
6.26 3.57 1.42 0.58
0.83 7.09 0.61 4.18 0.19 1.61 0.11 0.69
c
II 5.43 CB
1
I
600C
I
I
cL
0.73 4.20 0.60 2.87 0.16 0.73 0.08
c
CB 3.57 3.08 0.95 0.70
6.18 4.80 3.03 0.81
CL c CB cL 0.53 4.10 3.61 3.61 0.29 3.37 2.42 2.42 0.31 1.26 1.06 1.06 0.11 0.87 0.68 0.68
c 0.51
0.45 0.35 0.09
i
It was noted that concentration and strength of Bronsted and Lewis acidic sites under regeneration temperatures of 850K and higher were considerably weakened. At 823K, however. changes in those acidic properties were not important enough to explain the aging effect of temperature. The same conclusion was drawn from XRD results obtained from measurement of crystallinity and crystal pore constants. It was indicated catalyst deterioration, which was observed in regenerated catalysts, was not mainly attributable to changes in acid properties and crystallinity of H-form mordenite. Sintering of metal zeolite has bcen reviewed rccently [6]. Cliemisorption is by far the most commonly used tcchnique for dispersion dctcrminations. Hydrogen is the prefered adsorbate for thc characterization of supported metal catalysts. Because of the well known solubility of hydrogen in palladium, it can be uscd for palladium surface area determinations only under
Regeneration Behaviors of H ydroisomerization Catalysts
503
carefully controlled conditions, given in the literature [7] . It is reported that absolute determination of dispersion by chemisorption is sometimes difficult, relative changes, however, give reliable measure of the extent of sintcring [8] . In our work, palladium dispersion stale analysis was performed in a conventional hydrogen-oxygen titration apparatus to determine the average particle size of the palladium metal in thc various regenerated catalysts. Results are presented in Table 3. From these results, relative decrease in dispersion of loadcd palladium particles was observed under elevated regeneration temperatures and it was more dramatic undcr prolonged coke burning. They are in line with data in Fig. 4 to Fig. 7 for catalysts, discharged from lower section of the reactor, therefore it is clear that the loss in activity and selectivity may be explained by palladium sintering. Table 3 Relative changes in dispersion of loaded pnlladiiim during regeneration
I I
Regeneration conditions temperature (K)
I
time (h.)
Change in
I
metal area
diameter
a32
5
1 .oo
1.00
a32
13
0.507
1,97
a43
5
0.796
I .26
a73
5
0.788
1.36
Optimum regeneration temperature. __ To simulate the coke content in thc upper section of the pilot reactor, accelerated coking tests were carried in the lab scale reactor system. Conditions were controlled to obtain a discarded coked catalyst with a carbon lcvel of 2.63 wt%. Catalytic properties of coked and regenerated catalysts were measured i n the microrcactor. It is shown in Tablc 4 that n-hcxane conversion and selcctivity for 2,2 DMB werc enhanced with increasing regcneration temperature until 823K. Recovery to its original fresh catalysic bchavior was observed at burnoff temperature around 823K. Inspection of clcctron micrographs of the metal particlcs on H M were summarized. An appreciation of the growth characteristics of palladium can be obtained from detailed inspection of the crystallitc size distribution under various regenerating tcmperalures. It is clear that very fine palladium particles appeared a t a temperature of 823K.Thesc rcsults were in line with the catalytic performance of rcgcncratcd catalysts shown in Table 4. Table 4 Effect of regeneration temperature Regen. t e m n (K)I 673
I
723
1
773
I
803
I
823
1
833
I
843
I
873
I Fresh cat.
iiC6 Conv. (w%) 86.0
87.5
87.4
83.0
84.1
87.8
89.3
90.1
83.4
12.8
12.0
11.6
13.5
15.3
12.8
11.1
6.33
16.0
1.2DMB (W%)
504
L. Shi, F. Liu, L.-L. Liu, C.-L. Li and L.-T. Li
CONCLUSION Under smooth normal conditions a Pd / HM hydroisomerization catalyst is deactivated by very slow rates of coking. The amount of coke deposit should be limited to a ccrtain level in a cycle before shutting the reactor down for elimi nation of cokc by combustion. Propertics and location of coke have a strong impact on regeneration stages. Both optimum temperature and prolonged time on coke burnoff must be chosen to refresh the catalyst to an acceptable level. Catalyst deterioration during combustion of coke were mainly attributable to sintering of loaded palladium particles on Pd / HM catalysts. In view of these results, a stepwise coke burning procedure with increasing tempcrature was successfully developed to prevent from superhcating on the surface of catalyst in the ex- situ regeneration process and it is possible to almost restore the activity and selectivity to their initial level for the second cycle.
REFERENCES China Patent CN86,106388A(1986) I . C.L. Li and G.X. Huang, App. Catal., 71(1991)283. 2. M. Guisnet and V. Fouch. 3. C.L. Chai et al. J. of East china Instutute of chem Tech.(1981)2,19. 4. Z.M. Chai et al. A.C.S. Symp. Scr., 36(1991)864. 5. N.S. Figoli et al. App. Catal., 26(1986)39. 6. S. Bhatia et al. Catal. Rev. Sci. E n g . 3 I ( 19891.13 1, J. Chromatogr.. 11 I ( 1975)443. 7. T. Paryjczak et al. 8. S.E. Wanke in J. L. Figueiredd Ed. ) Sintering of Commercial Supported Platinuni Group Metal Catalysts (Proc. of the NaTo Adv. Study Inst on Catal. Dcact. Algarve. Portugal. May 18-29(1981)). Martinus Niyhoff Hague / Boston / Lonton, 1982, P315.
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, Universityof Pittsburgh, Pittsburgh, PA, U S A .
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Preparation of Catalysts IScientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-1 7,1975 edited by B. Delrnon, P.A. Jacobsand G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processesin Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delrnon Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September4-7,1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-1 1,1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine,G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis,Tokyo, June3&July4,1980. Parts Aand B edited by T. Seiyarna and K. Tanabe Catalysis by Supported Complexes by Yu.1. Yerrnakov, B.N. Kuznetsov and V.A. Zakharov Physicsof Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. LBzniEka Adsorption at the Gas-Solid and LiquidSolid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Irnelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24,1982 edited by P.A. Jacobs, N.I. Jaeger, P. JiN and G. Schulr-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September I-4,1982 edited by C.R. Brundleand H. Morawitz,
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Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13,1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jil,V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3,1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acidsand Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 2527,1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June28-29,1984 edited by M. Che and G.C. Bond Unsteady Processesin Catalytic Reactors byYu.Sh. Metros Physicsof Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoroi-Portorose, September 3-8,1984 edited by B. Driaj, S.HoEevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo,July 4-6,1985 edited by T. Keii and K. Soga Vibrationsat Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-windermere, September 1519,1985 edited by D.A. King, N.V. Richardson and S.Holloway Catalytic Hydrogenation edited by L. Cervenq New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22,1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knotinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1,1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September I-4,1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P. Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1,1987 edited by B. Delmon and G.F. Froment
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Keynotes in Energy-RelatedCatalysis edited by S.Kaliaguine Methaneconversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S.Yurchak Innovation in Zeolite MaterialsScience. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPACSymposium (COPSI),Bad Soden a.Ts., April 2649,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1,1987 edited by J. Koukal HeterogeneousCatalysis and Fine Chemicals. Proceedings of an International Symposium, Puitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P6rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pail Catalytic Processesunder Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16,1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference,Arnsterdam, July 10-14,1989. PartsA and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G. Anthony New Solid Acidsand Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori RecentAdvancesin ZeoliteScience. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19,1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8,1989 edited by D.L. Trimm, S.Akashah, M. Absi-Halabi and A. Bishara
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Future Opportunities in Catalytic and Separation Technology edited by M.Misono, Y. Moro-okaand S. Kimura New Developments in Selective Oxidation. Proceedings of an International Volume 55 Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and F. Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium Volume 56 on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Introduction to Zeolite Science and Practice Volume 58 edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals, Tokyo, June 2649,1990 edited by T. Inui. S. Namba and T. Tatsumi Volume 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, 0slo.August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPSIll, Alicante, May 6-9,1990 edited by F. Rodriguez-Reinoso,J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of CatalystsV. Proceedings of the Fifth international Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, September 3-6,1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon Volume 64 New Trends in CO Activation edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT90, Leipzig, August 20-23,1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Volume 66 Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonfured, September 10-14,1990 edited by L.I. Simandi Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in HeterogeneousCatalysis. Boston, MA, April 22-27,1990 edited by R.K. Grasselli and A.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Evanston, IL, June24-26,1991 edited byC.H. Bartholomew and J.B. Butt Volume 69 Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia,September 8-13,1991 edited by P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova
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Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive Pollution Control 11. Proceedings of the 2nd International Symposium (CAPoC2), Brussels, Belgium, September 10-13.1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by P. Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 2528,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congresson Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P. TBtenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of theThird International Conferenceon Spillover, Kyoto, Japan,August 17-20,1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals 111, Proceedings of the 3rd International Symposium, Poiters,April5 -8,1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. PBrot and C. Montassier Catalysis: An IntegratedApproach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the 4th International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion 11. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, Australia, July 4-9, 1993 edited by H.E. Curry-Hyde and R.F. H o w e N e w Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmhdena, Spain, September 20-24, 1993 edited by V. Cort6s Corberhn and S. V i c Bell6n Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and 1.Yashima
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