Methane conversion: proceedings of a symposium on the production of fuels and chemicals from natural gas, Auckland, April 27-30, 1987

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Author: D. M. Bibby | C. D. Chang | R. F. Howe | S. Yurchak

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Studies in Surface Science and Catalysis

METHANE CONVERSION

36

This page intentionally left blank

Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates

Vol. 36

METHANE CONVERSION Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1 9 8 7

Editors

D.M. Bibby DSlR Chemistry Division, Private Bag, Petone, New Zealand C.D. Chang Mobil Research and Development Corporation, Central Research Laboratory, P. 0. Box 1025, Princeton, NJ 08540, U.S.A. R.F. Howe Chemistry Department, University of Auckland, Private Bag, Auckland, New Zealand and

S. Yurchak Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, NJ 08066, U.S.A.

ELSEVlER

Amsterdam - Oxford - New York - Tokyo 1 9 8 8

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 2 1 1, 1000 AE Amsterdam, The Netherlands Distributors for the United Stares and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, NY 10017, U S A .

LIBRARY OF CONGRESS

Library o f C o n g r e s s Cataloging-in-Publication

Data

Methane conversion p r o c e e d i n g s o f a s y m p o s i u m on t h e p r o d u c t i o n o f f u e l s and c h e m i c a l s f r o m n a t u r a i gas. A u c k l a n d , A p r i l 27-30. 1987 e d i t o r s . D.M. B i b b y [ e t ai.1. p. c m . -- ( S t u d i e s in s u r f a c e s c i e n c e and c a t a l y s i s 36) I n C l u d e S index. I S B N 0-444-42935-2 (U.S.) fl 3 0 0 . 0 0 c e s t . ) 1 . S y n t h e t i c fuels--Congresses. 2. G a s . N a t u r a l - - C o n g r e s s e s I. B i b b y . C. M. ( D a v i d M.) 11. I n t e r n a t i o n a l Symposiufr on M e t h a n e C o n v e r s i o n (1987 U n i v e p s i t y of A u c k l a n o ) 111. S e r i e s . T P 3 6 0 . M 4 7 1988 665.7 3--dC19 87-34762

...

.

CIP

ISBN 0-444-42935-2 (Vol. 36) ISBN 0-444-4 180 1-6 (Series)

0 Elsevier Science Publishers B.V., 1988 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Science &Technology Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulationsfor readers in the USA -This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Printed in The Netherlands

V

CONTENTS Foreword

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

O r g a n i z i n g and S c i e n t i f i c Committee F i n a n c i a l Support

XI

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

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

XI11 XITI

METHANE CONVERSION V I A METHANOL

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

The New Zealand g a s - t o - g a s o l i n e p r o j e c t (C.J. Maiden) F i f t y y e a r s o f r e s e a r c h i n c a t a l y s i s (S.L.

Meisel)

1 17

The steam r e f o r m i n g o f n a t u r a l gas: problems and some s o l u t i o n s (D.L.

Trimm)

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

39

The mechanism o f t h e d i s s o c i a t i o n of methane on a n i c k e l c a t a l y s t (S.T.

Ceyer, Q.Y.

Yang, M.B.

Lee, J.D.

B e c k e r l e and A.D.

Johnson)

.....

51

Methane r e f o r m i n g by carbon d i o x i d e and steam o v e r s u p p o r t e d Pd, P t , and Rh c a t a l y s t s (M. Masai, H. Kado, A. Miyake, S. Nishiyama and S. Tsuruya) Syngas f o r C1-chemistry. (J. Rostrup-Nielsen)

...

67

L i m i t s of t h e steam r e f o r m i n g process

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

73

The o p t i m a l r e a c t i o n assemblage i n t h e steam r e f o r m i n g o f methane t o

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

produce syngas ( Z . Renjun)

79

The i n f l u e n c e o f t h e s u p p o r t on t h e e f f e c t i v e o r d e r o f t h e steam r e f o r m i n g r e a c t i o n (A.S.

Al-Ubaid,

S.S.E.H.

E l n a s h a i e and H.E.E.

Abbashar)

.....

83

Steam r e f o r m i n g : k i n e t i c s , c a t a l y s t d e a c t i v a t i o n and r e a c t o r d e s i g n (S.S.E.H.

E l n a s h a i e , A.S.

Al-Ubaid and M.A.

Soliman)

...........

- e s t a b l i s h e d and f u t u r e .............................

89

C a t a l y t i c processes f o r methanol s y n t h e s i s (M.S. W a i n w r i g h t )

95

Mechanism o f methanol and h i g h e r oxygenate s y n t h e s i s (K. K l i e r , R.G.

Herman, J.C.. Nunan, K.J.

J.G. S a n t i e s t e b a n )

Smith, C.E.

Bogdan, C.-W.

Young and

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

Mechanism o f hydrocarbon f o r m a t i o n f r o m methanol (C.D. Chang)

109

. . . . . . . 127

VI

I s o t o p i c and mechanistic studies o f methanol conversion (T. Mole)

..

,

..

145

Methanol t o gasoline: spectroscopic studies o f chemistry and c a t a l y s t (R.F. Howe)

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

157

A re-examination o f evidence f o r carbene (CH2:) as an intermediate i n the

conversion o f methanol t o gasoline. The e f f e c t o f added propane (D.V. Dass, R.W.

Martin, A.L. Ode11 and G.W.

Quinn)

. . . . . . . . . . . . . . . . . . 177

Hydrocarbon formation from methanol using W03/A1203 and z e o l i t e ZSM-5 c a t a l y s t : a mechanistic study (G.J. Hutchings, R. Hunter, W. Pick1 and L. Jansen van Rensburg)

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

183

On the mechanism o f hydrocarbon formation from methanol over protonated z e o l i t e s (S. Kolboe)

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

Formation o f p-xylene from methanol over H - Z S M - 5 S. Kolboe)

;

. 189

( E . Unneberg and

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

195

Further studies on the process o f methanol conversion t o o l e f i n s (G. Chen, J. Liang, Q. Wang, G. Cai, S. Zhao,

H. L i e t a l . )

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

201

Reactions o f methanol and toluene over molybdenum z e o l i t e s (M.M. Huang and R.F. Howe)

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

207

The s e l e c t i v i t y o f c a t a l y s t s composed o f V 2 0 5 supported on Zr02-Y203 mixed oxides f o r methanol o x i d a t i o n (J.G. van Ommen, P.J. Gellings and 0.R.H.

ROSS)

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

213

Active species and mechanism f o r mixed alcohol synthesis over s i l i c a supported molybdenum c a t a l y s t s (T. Tatsumi H. Tominaga)

, A.

Muramatsu, K. Yokota and

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

Alcohols from methane (H. Dotsch, C.B.

B. Hijhlein)

219

von der Decken, H . Fedders and

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

229

Improvements t o Raney copper methanol synthesis c a t a l y s t s through z i n c impregnation. 111. A c t i v i t y t e s t i n g (H.E. Curry-Hyde, M.S. D.J. Young).

Wainwright and

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

Methanol carbonylation t o a c e t i c a c i d w i t h supported metal c a t a l y s t (K. Omata, K. Fujimoto, H. Yagita, H . Mazaki and H. Tominaga)

239

. . . . . . . 245

VII Development o f M o b i l ' s f i x e d - b e d m e t h a n o l - t o - g a s o l i n e (MTG) process ( S . Yurchak)

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

251

Conversion o f methanol t o l i q u i d f u e l s by t h e f l u i d bed M o b i l process ( a commercial concept) (H.R.

Grimmer, N. T h i a g a r a j a n and E. N i t s c h k e )

Topsfie i n t e g r a t e d g a s o l i n e s y n t h e s i s

-

.....

273

t h e TIGAS process

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 G a s o l i n e and d i s t i l l a t e f u e l s f r o m methanol (A.A. Avidan) . . . . . . . . . 307 ( J . Topp-Jdrgensen)

S y n t h e t i c g a s o l i n e components s u i t a b l e f o r chemical f e e d s t o c k s

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

(I.J. M i l l e r )

325

ALTERNATIVE ROUTES TO METHANE CONVERSION F e a s i b i l i t y o f e t h y l e n e s y n t h e s i s v i a o x i d a t i v e c o u p l i n g o f methane

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

(M.M. Bhasin)

343

D i r e c t c o n v e r s i o n o f methane t o methanol and h i g h e r hydrocarbons (J.H. L u n s f o r d )

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

359

O x i d a t i v e coup1 i n g o f methane o v e r promoted magnesium o x i d e c a t a l y s t s ; r e l a t i o n between a c t i v i t y and s p e c i f i c s u r f a c e area ( E . Iwamatsu, T. Moriyama, N. Takasaki and K. A i k a )

. . . . . . . . . . . . . . . . . . . 373

S y n t h e s i s o f C2H4 b y p a r t i a l o x i d a t i o n o f CH4 o v e r t r a n s i t i o n m e t a l o x i d e s w i t h a l k a l i - c h l o r i d e s (K. Otsuka, M. Hatano and T. Komatsu)

. . . . . . . . 383

C a t a l y t i c o x i d a t i o n o f methane o v e r A1P04-5 and metal-doped A1P04-5 (J.L.

G a r n e t t , E.M. Kennedy, M.A.

Long, C. Than and A.J. Watson)

. . . . . . 389

The p r o d u c t i o n o f l i q u i d f u e l s v i a t h e c a t a l y t i c o x i d a t i v e c o u p l i n g o f methane (J.H.

Edwards and R.J. T y l e r )

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

395

S e l e c t i v e o x i d a t i v e c o u p l i n g o f methane t o ethane and e t h y l e n e (K. Asami, S. Hashimoto, K. F u j i m o t o and

H. Tominaga)

. . . . . . . . . . . . . . . . . 403

O x i d a t i v e dehydrogenation o f methane t o f o r m h i g h e r hydrocarbons

.

(F.P.

L a r k i n s and M.R.

Nordin)

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

E f f e c t o f O3 versus O2 as o x i d a n t s f o r methane (G.J. M.S.

S c u r r e l l and J.R. Woodhouse)

409

Hutchinqs,

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

415

VIII The o l i g o m e r i z a t i o n o f o l e f i n s d e r i v e d from p a r t i a l methane o x i d a t i o n (V.W.L.

Chin, A.F.

Masters, M. Vender and R.J. T y l e r )

...........

421

The o x i d a t i v e c o u p l i n g o f methane: c a t a l y s t requirements and process c o n d i t i o n s (J.A. Roos, S.J. Korf, A.G.

J.G. van Ommen and J.R.H.

ROSS)

Bakker, N.A.

de B r u i j n ,

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

427

Coupling o f methane and e t h y l e n e over s u l p h a t e - t r e a t e d z i r c o n i a (M.S. S c u r r e l l and M. Cooks)

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

433

Engineering aspects o f a l t e r n a t i v e routes f o r t h e conversion o f n a t u r a l gas (P.J. Jackson and N . White)

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

The Sasol r o u t e t o chemicals and f u e l s (M.E.

439

. . . . . . . . . . . . . 447

Dry)

Mechanism o f the Fischer-Tropsch process (H. Schulz, K. Beck and E. E r i c h )

. 457

The S h e l l middle d i s t i l l a t e synthesis process (f1.J. van der Burgt, C.J.

van Leeuwen, J.J. del'Amico, S.T.

S i e and I . ?laxwell)

........

473

D i r e c t conversion o f methane t o 1i q u i d hydrocarbons through chlorocarbon intermediates (C.E. Taylor, R.P.

Noceti and R.R.

Schehl)

.........

483

Methane conversion v i a methyl c h l o r i d e : condensation o f methyl c h l o r i d e t o l i g h t hydrocarbons (K.-3.

Jens, S. Halvorsen and E. Baumann Ofstad)

. . . . 491

Synthesis gas t o motor f u e l v i a l i g h t alkenes (B.G. Baker and N.J. C l a r k ) Hydrogenation o f CO over molybdenum-zeolites (Y.-S.

Yong and R.F. Howe)

. 497

. . 503

Role o f supports f o r cobal t-based c a t a l y s t s used i n Fischer-Tropsch synthesis o f hydrocarbons (G.M. F.P. L a r k i n s )

Roe, M.J. Ridd, K.J. Cavell and

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

509

L i q u i d phase Fischer-Tropsch synthesis using u l t r a f i n e p a r t i c l e s o f i r o n as c a t a l y s t (E. Kikuchi and H. I t o h )

. . . . . . . . . . . . . . . . . . . 517

O l i g o m e r i z a t i o n o f lower o l e f i n s t o octane enhancers and d i s t i l l a t e range o l e f i n s by n i c k e l based homogeneous and supported c a t a l y s t s (K.J. C a v e l l )

. 523

L i g h t o l e f i n s from synthesis gas using ruthenium on r a r e e a r t h oxide c a t a l y s t s (L. Bruce, S. Hardin, M. Hoang and T. Turney)

. . . . . . . . . . 529

IX ZEOLITES AND OTHER CATALYSTS

. . . . . . . . . . . . . . . . . . . 537

Micropores i n c r y s t a l s (R.M. B a r r e r )

Aluminophosphates as p o s s i b l e a l t e r n a t i v e s t o z e o l i t e s (N.B. N.J. Tapp)

M i l e s t o n e and

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

553

The c o n v e r s i o n o f methanol t o hydrocarbons and t h e o x i d a t i o n o f methane on h e t e r o p o l y oxometalates (J.B. M o f f a t )

. . . . . . . . . . . . . . . . . . . 563

I n v e s t i g a t i o n o f a c i d i c p r o p e r t i e s o f H - z e o l i t e s as a f u n c t i o n o f Si/A1 r a t i o (K

. Segawa. M . Sakaguchi

and Y

S o r p t i o n o f a c e t i c a c i d on H'ZSM-5

. Kurusu) . . . . . . . . . . . . . . .

(L.M.

. . . . . . . . . . . . . 589

Parker)

Impedance and i n f r a r e d spectroscopy o f t h e z e o l i t e ZSM-5 (J.L. J.F.

C l a r e and R.G.

Buckley)

579

Tallon.

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

597

The c h e m i s t r y and c a t a l y t i c p r o p e r t i e s o f t r a n s i t i o n m e t a l oxyanions i n s o d a l i t e cages (L.M. Moroney. S

. Shanmugam and A.G.

Raman s p e c t r a o f o c c l u d e d c a t i o n s i n ZSM-5 (J.R. and D.M. Olefin

Langdon)

B a r t l e t t . R.P.

. . . . . . . 603 Cooney

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 609 r e a c t i o n s o v e r Mo-mordenite (J.R. Johns and R.F. Howe) . . . . . . . 615 Bibby)

. . . . . . . 621 C r a c k i n g o f some l o n g c h a i n hydrocarbons on HZSM-5 z e o l i t e s ( Z . Yulong. 0 . Guangyao and Z . Z h i j . . . . . . . . . . . . . . . . . . . . . . . . . . 627 Propene o l i g o m e r i z a t i o n o v e r H-ZSM-5 z e o l i t e (K.G.

Wilshier)

Regeneration o f coke d e a c t i v a t e d ZSM-5 by a i r / o x y g e n (G.D. McLellan. R.F. Howe and D.M.

Bibby)

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

633

Q u a n t i t a t i v e thermal desorption/mass s p e c t r o m e t r y o f A1 PO4-11 p r e c u r s o r s (N.J.

Tapp and N.B. M i l e s t o n e )

COMMERCIALISATION

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

639

OF THE GAS-TO-GASOLINE PROCESS

Advances i n methanol t e c h n o l o g y (J.D.

Korchnak)

From molecules t o c o n c r e t e and s t e e l (J.Z. Bem)

. . . . . . . . . . . . . . 647 . . . . . . . . . . . . . . 663

The f i r s t f i x e d - b e d m e t h a n o l - t o - g a s o l i n e (MTG) p l a n t : d e s i g n and scale-up c o n s i d e r a t i o n s (D.E.

Krohn and M.G.

Melconian)

. . . . . . . . . . . . . . 679

X

O p e r a t i o n o f t h e w o r l d ' s f i r s t g a s - t o - g a s o l i n e p l a n t (K.G. A.R.

Williams)

A l l u m and

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

691

The v a l u e o f computer s i m u l a t i o n t o t h e process i n d u s t r i e s (H.J. Weake and G.A. Robertson)

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

713

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

725

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

737

L i s t o f Participants Author Index

S t u d i e s i n S u r f a c e Science and C a t a l y s i s ( o t h e r volumes i n t h e s e r i e s )

. . 739

XI

F0REkD)RD

I t is with pleasure t h a t we present the Proceedings of the International

Symposium on Methane Conversion which t o o k place i n New Zealand from 27-30 April, 1987. The Symposium marked the successful inauguration i n October 1985 of the world's f i r s t comnercial plant f o r production of gasoline from natural gas, based on the Mobil methanol-to-gasoline process. The plant is operated by the New Zealand Synthetic Fuels Corporation, of which Mobil is a shareholder. The

symposium was held a t the University of Auckland and concluded w i t h a tour of the synfuel gas-to-gasoline plant a t Motunui, near New Plymouth. The objectives of the Symposium were t o present both fundamntal research and engineering aspects of the development and c o m r c i a l i z a t i o n of gas-toqasoline processes. These included steam reforming, methanol synthesis and methanol-to-gasoline. Possible a l t e r n a t i v e processes e.g. MOGD, Fischer Tropsch synthesis of hydrocarbons, and the d i r e c t conversion of methane to higher hydrocarbons were a l s o considered. More than 130 delegates from a wide range of d i s c i p l i n e s attended and the Symposium was successful i n p r m t i n g interchange of ideas between s c i e n t i s t s developing processes i n the laboratory and engineers responsible f o r c o m r c i a l i z a t i o n . The Symposium prcgrarnne consisted of a series of invited lectures from industrial, university and government speakers, contributed papers and two poster sessions. The call f o r contributed papers resulted i n submission of 85 abstracts, a large proportion of which came from i n d u s t r i a l laboratories. Follming the grouping of papers a t the Symposium, w e present them here i n t h e following broad categories:

-

Inethane conversion via methanol a l t e r n a t i v e routes t o methane conversion z e o l i t e s and other c a t a l y s t s c o m r c i a l i z a t i o n of the gas-tc-gasoline process

The work presented a t the poster sessions is included i n this volume as short papers. The Editors have f e l t obliged to carry out e d i t o r i a l changes i n papers where W e

obvious typing errors or lack of c l a r i t y affected the understanding.

apologise f o r not being able t o obtain an authorisation i n every case but this The a l t e r n a t i v e of not accepting these papers, or

was because of lack of time.

XI1

of delaying publication of these Proceedings further was not justified in view of the high quality and imnediacy of the work presented. The Editors would like to thank the Authors for the quality of their presentations, and for participating in this volume. The Editors also thank the Organizing Committee for willingly giving their time and expertise to the Symposium, the Chairman of Sessions and the referees for their numercus contributions. We thank George Dibley and his staff of the Auckland University Centre for Continuing Education for arranging the Symposium and associated extra-curricular activities, and Neil Milestone for assistance in the preparation of this volume. Finally, we thank Dame Catherine Tizard, Mayor of Auckland, for opening the Symposium and welcoming the delegates to New Zealand and the City of Auckland.

DAVID M BIBBY CLARENCE D CHANG RUSSELL F HOWE SEFGEI WRCHAK

XI11

ORGANIZING AND SCIENTIFIC COMMITTT K.G.

Allum

New Zealand S y n t h e t i c Fuels Corporation

D.M.

Bibby

DSIR Chemistry Division

C .D.

Chang

Mobil Research and Developnent Corporation

R.P. Cooney R.F. Howe N.B.

Milestone

R. Nicol C.G.

Pope

S. Yurchak

University of Auckland University of Auckland E I R Chemistry Division

DSIR I n d u s t r i a l Processing Division

University of Otago Mobil Research and Development Corporation

FINANCIAL SUF'poRT T h i s Symposium was w d e p o s s i b l e by f i n a n c i a l support sponsorship from t h e following o r g a n i z a t i o n s : American Chemical S o c i e t y American I n s t i t u t e of Chemical Engineers Mobil Research and Development Corporation Mobil O i l New Zealand Mobil South, Inc. New Zealand Department of S c i e n t i f i c and I n d u s t r i a l Research, Chemistry Division

New Zealand I n s t i t u t e of Chemistry New Zealand S y n t h e t i c Fuels Corporation, Ltd University of Auckland


METHANE CONVERSION VIA METHANOL


D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

1

THE NEW ZEALAND GAS-TO-GASOLINE PROJECT

C.J. MAIDEN Chairman, New Zealand Synthetic Fuels Corporation Ltd.,

New Zealand

ABSTRACT The Synfuel gas-to-gasoline complex i s s i t e d w i t h i n 180 hectares o f land a t Motunui, Taranaki. I t i s designed t o convert 52-55 PJ per annum of n a t u r a l gas i n t o 570,000 tonnes (14,450 b a r r e l s per stream day) o f h i g h octane gasoline. The conversion of gas t o gasoline (GTG) takes place i n two stages: f i r s t gas t o methanol (GTM) and second methanol t o gasoline (MTG). The two stages are integrated i n t o a single complex t o achieve optimum e f f i c i e n c y i n management and operation. The GTM p l a n t employs the I C I low-pressure Methanol Process sub-licensed from Davy McKee, and incorporates two methanol t r a i n s each capable o f producing 2200 tonnes per day o f methanol. The MTG p l a n t employs Mobills f i x e d bed c a t a l y t i c process using the Mobil p r o p r i e t a r y z e o l i t e c a t a l y s t , ZSM-5. The process i s very s i m i l a r t o conventional vapour phase c a t a l y t i c petroleum r e f i n e r y processes, such as hydro-treating and platinum reforming. The p l a n t i s owned and operated by New Zealand Synthetic Fuels Corporation Limited (Synfuel) with a shareholding o f Government 75 percent and Mobil 25 percent. Mobil provides technical and management services t o the company. The p l a n t was mechanically complete by June 30, 1985, ahead o f schedule and about 17 percent under the o r i g i n a l budget o f US$1475 m i l l i o n . Commissioning and startup proceeded s a t i s f a c t o r i l y w i t h the f i r s t gasoline produced on October 17, 1985. The gasoline i s w i t h i n s p e c i f i c a t i o n and 739,000 tonnes have been produced t o the end o f March 1987. INTRODUCTION

With more than a year's successful operation o f the world's f i r s t gas-togasoline (GTG) p l a n t i t i s an appropriate t i m e t o review the h i s t o r y and development o f the project.

The New Zealand Synthetic Fuels Corporation L t d

(NZSFC) was incorporated i n September 1980 with the o b j e c t i v e o f . designing, constructing and operating a p l a n t t o manufacture gasoline.

The Company was

t o convert n a t u r a l gas owned by the Crown i n t o gasoline, v i a methanol as an intermediate product, f o r a processing fee.

Gasoline produced from the p l a n t

was t o be owned by the Crown and subsequently sold t o the o i l marketing companies f o r d i s t r i b u t i o n t o the consumer.

The p r o j e c t has i t s o r i g i n s i n

recommendations from L i q u i d Fuels Trust Board t o Government i n 1979.

2

THE LIQUID FUELS TRUST BOARD I n 1973, p r i o r t o the f i r s t energy c r i s i s , New Zealand imported v i r t u a l l y a l l i t s l i q u i d f u e l requirements i n the form o f o i l or i t s products.

A t this

time such imports represented about 60 percent o f New Zealand's primary energy requirements and cost l e s s than 5 percent o f the country's export earnings. After the f i r s t o i l shock the cost of l i q u i d f u e l s jumped t o between 20 and 30 percent o f export earnings and thereby severely weakened the economy.

The second o i l shock i n 1979 f u r t h e r weakened the economy and a h i g h r e l i a n c e on I r a n i a n crudes

l e d t o the i n t r o d u c t i o n of

"carless days",

a

(temporary) attempt t o reduce the amount o f car usage. The L i q u i d Fuels Trust Board (LFTB) was formed j u s t p r i o r t o the I r a n i a n r e v o l u t i o n w i t h the aims o f assessing ways o f reducing the need f o r imported f u e l s f o r transport purposes i n New Zealand.

The second o i l shock created

immediate pressure on the Board t o determine how the products o f New Zealand's n a t u r a l gas f i e l d s could be used as or converted i n t o transport fuels. New Zealand's t w o l a r g e s t gas f i e l d s are the Kapuni and Maui f i e l d s .

The

Kapuni f i e l d has estimated recoverable gas reserves o f 419 petajoules (PJ) and

is

in

on-shore

Taranaki;

whereas

the

Maui

field

with

estimated

recoverable gas reserves o f 5193 PJ i s 33 km o f f the coast o f Taranaki. A number o f

alternatives

n a t u r a l gas (CNG),

were

examined by the LFTB i n c l u d i n g compressed

l i q u e f i e d petroleum gas (LPG),

methanol as a gasoline

extender, pure methanol as a gasoline or diesel s u b s t i t u t e and the production

o f synthetic fuels.

This work resulted i n the presentation o f two reports t o

Government, i n August and October i n i t i a l strategy Zealand.

1979, containing recommendations on an

for transport f u e l s supply and gas u t i l i s a t i o n i n New

Two major p r o j e c t s were recommended:

A synthetic gasoline p r o j e c t t o produce 500-600,000

tonnes per annum

o f gasoline from methanol produced from n a t u r a l gas using a process developed by the Mobil O i l Corporation; A "stand-alone"

methanol project,

using n a t u r a l gas as a feedstock,

with the m a j o r i t y o f product dedicated t o export as chemical grade methanol and the remainder f o r l o c a l use as a f u e l or as a chemical. I t was also recommended t h a t the approved expansion o f the Refinery a t

Marsden Point i n northern New Zealand take account o f f u t u r e production o f synthetic gasoline. The canbined use o f synthetic gasoline, indigenous condensate from Kapuni and Mad, and CNG and LPG i n vehicles, was predicted t o make New Zealand about

3

50 percent

self-sufficient

i n transport

i n the middle 19801s, and

fuels

thereby save several hundred m i l l i o n d o l l a r s annually i n overseas funds. The LFTB's recommendation t o use n a t u r a l gas t o produce gasoline using t h e Mobil process was made f o l l o w i n g a comprehensive analysis o f synthetic f u e l technologies.

alternative

To a s s i s t them i n t h e i r studies during 1979 the

LFTB contracted overseas consultants i n c l u d i n g Ralph M. Company, Davy McKee and Lurgi.

Parsons, the Badger

These consultants undertook i n v e s t i g a t i o n s

i n t o the s t a t e o f the a r t o f methanol and synthetic f u e l technology and c a r r i e d out comparative studies between the Mobil methanol-to-gasoline

(MTG)

process and the Fischer-Tropsch synthetic f u e l process as used by SASOL i n South Africa. The LFTB concluded, on the basis o f these studies, t h a t the Mobil process provided a more economic and e f f i c i e n t method o f producing synthetic f u e l than d i d Fischer-Tropsch synthesis.

The Board also concluded t h a t i t was not

possible f o r New Zealand t o reach 50 percent self-sufficiency f u e l s by the middle 1980's without synthetic fuel. and

LPG a

sufficiency

.

realistic

cmparable

goal

would

i n transport

With only condensate, CNG

be

about

25 percent

self-

THE MOBIL PROCESS The success o f the unique Mobil process l i e s i n the z e o l i t e c a t a l y s t ZSM-5. This c a t a l y s t is the most e f f i c i e n t known f o r the conversion o f methanol t o hydrocarbons i n the gasoline range. The process has i t s o r i g i n s during the 1960's. VrackingtV o r

means o f scientists

researched

aluminosilicates.

breaking a

down

range

This work resulted,

of

O r i g i n a l l y looking f o r a

components o f chemical

crude

oil

catalysts

Mobills

including

i n 1968, i n the synthesis o f the

z e o l i t e ZSM-5 which was found t o convert methanol i n t o water and a mixture o f hydrocarbons.

This hydrocarbon mixture i s e s s e n t i a l l y the same as gasoline

produced by the t r a d i t i o n a l r e f i n i n g o f crude o i l . Development studies on the MTG process have been conducted since the e a r l y 1970's.

Using a fixed-bed

process, c a t a l y s t ageing t e s t s o f over 200 days

were c a r r i e d out, optimum process conditions were defined, and the q u a l i t y o f the gasoline produced was determined.

I n response t o a request from the New Zealand Government, Mobil Research and Development Corporation b u i l t a four b a r r e l s per day fixed-bed p i l o t p l a n t t o demonstrate the f e a s i b i l i t y o f the gas-to-gasoline

process.

This was a

major scale-up o f the laboratory work and the p i l o t p l a n t performed p r e c i s e l y as

predicted.

Reports t o

the LFTB by L u r g i and Badger also

confidence t h a t the process would scale successfully from the

pilot

indicated plant t o

4

commercial size. The ZSM-5 c a t a l y s t does n o t have only one use. s i x processes using ZSM-5,

M o b i l has commercialised

f i v e o f which a r e l i c e n s e d f o r use i n 25 chemical

and r e f i n e r y p l a n t s around the world.

These v a r i e d a p p l i c a t i o n s have g i v e n

M o b i l wide experience i n d i r e c t l y s c a l i n g up l a b o r a t o r y - s i z e u n i t s t o l a r g e commercial

plants.

Commercial

experience

has

also

confirmed

laboratory

p r e d i c t i o n s o f h i g h s t a b i l i t y and l o n g l i f e f o r t h e c a t a l y s t . PROJECT CONCEPTS The p r o j e c t concepts were developed under t h e terms o f a 1980 Government/ M o b i l Memorandum o f

Understanding.

By

this

acjreement

a Joint

Executive

Committee (JEC) was t o prepare a r e p o r t which would i n c l u d e a p l a n f o r t h e design,

c o n s t r u c t i o n , and o p e r a t i o n o f a p l a n t t o manufacture gasoline, and

an assessment o f t h e v i a b i l i t y o f such a p r o j e c t . M o b i l assumed r e s p o n s i b i l i t y f o r o v e r a l l p r o j e c t management,

subject

to

t h e d i r e c t i o n o f t h e JEC, and Bechtel Petroleum I n c . was employed as P r o j e c t Services Contractor. The JEC Report, completed i n J u l y 1981, concluded t h a t the venture was c l e a r l y t e c h n o l o g i c a l l y f e a s i b l e and commercially a t t r a c t i v e .

The Government

and M o b i l e l e c t e d t o proceed and concluded t h e various c o n t r a c t s i n February, 1982.

I t was agreed t h a t t h e p l a n t would be owned and operated by t h e New Zealand S y n t h e t i c Fuels Corporation L t d (Synfuel) with a Government shareholding o f 75 percent

and a M o b i l shareholding o f

25 percent.

M o b i l contracted

to

provide t e c h n i c a l and management s e r v i c e s t o the company. The company was t o operate on a t o l l i n g b a s i s , with t h e Crown s u p p l y i n g t h e n a t u r a l gas t o t h e p l a n t f o r processing i n t o gasoline f o r a t o l l i n g fee. T i t l e t o t h e hydrocarbons was t o remain with t h e Crown, which would s e l l t h e gasoline t o t h e New Zealand p e t r o l e m marketing companies.

Synfuel g a s o l i n e

was t o be p r i c e d c o m p e t i t i v e l y with gasoline produced from t h e R e f i n e r y a t Marsden Point. Under t h e terms o f t h e Processing Agreement between NZSFC and t h e Crown, t h e t o l l i n g f e e t o be p a i d by t h e Crown, when t h e p l a n t operated a t design capacity,

was t o cover a l l o f NZSFC's c o s t s i n c l u d i n g t a x and debt service.

Also, a t design capacity,

t h e f e e was t o provide the shareholders with a t a x

p a i d discounted cash f l o w r a t e o f r e t u r n on q u a l i f y i n g c a p i t a l a t r i s k o f 16 percent adjusted f o r i n f l a t i o n .

DESIGN OF T M PLANT The Synfuel p l a n t i s s i t e d within 180 hectares o f l a n d a t Motunui, Taranaki

5

TARANAKI

I

WAITARA / MOTUNUI

Fig. 1.

Location o f Synfuel Complex

6

as shown i n Figures 1 and 2.

I t i s designed t o convert 52-55 PJ p e r annum o f

n a t u r a l gas i n t o 570,000 tonnes per annum (14,450 b a r r e l s per stream day) o f gasoline.

Bechtel,

as

Project

Services

Contractor,

Davy

McKee,

Foster

Wheeler and New Zealand engineering c o n s u l t a n t s c o n t r i b u t e d t o the design and engineering o f the p r o j e c t . The conversion o f gas t o gasoline takes place i n two stages, f i r s t gas t o methanol and second methanol t o gasoline.

Based on i n i t i a l s t u d i e s i t was

decided t o i n t e g r a t e t h e two stages i n t o a s i n g l e complex t o achieve optimum e f f i c i e n c y i n management and operation. The gas-to-methanol sub-licensed

p l a n t employs t h e ICI Low-pressure

from Davy McKee,

and i n c o r p o r a t e s two methanol t r a i n s each

capable o f producing 2200 tonnes per day. process

are

desulphurisation

Fig. 2.

Layout of Synfuel S i t e

Methanol Process,

The main steps i n t h e methanol

o f the n a t u r a l gas feed,

steam

reforming

to

r

I

synthesis gas, compression, and methanol synthesis. Although Maui gas i s very low i n sulphur, t h e incoming gas is desulphurised as a p r e c a u t i o n a g a i n s t poisoning c a t a l y s t s used i n t h e process. desulphurisation,

Following

water, i n the form o f medium pressure steam, i s added and

t h e m i x t u r e passed through reformer r e a c t o r tubes which c o n t a i n a n i c k e l catalyst.

The

tubes

are

located inside

the

reformer

furmce

where

the

process temperature i s r a i s e d t o 900°C and t h e r e a c t i o n t o form synthesis gas occurs.

The synthesis gas i s cooled t o 35OC, compressed t o 100 b a r , 0

reheated and reacted a t 250-300 C over a copper/zinc water-methanol m i x t u r e with about 17 percent water.

catalyst

t o form a

The methanol product i s

reduced i n pressure and passed t o t h e methanol-to-gasoline (MTG) p l a n t .

Fig. 3.

Methanol Vaporisation Assembly

8

As the MTG p l a n t operates on unrefined methanol, there i s no heat load required f o r product d i s t i l l a t i o n .

Instead, waste heat from the reforming

sections of the p l a n t i s turned i n t o high pressure steam (105 bar) and used t o d r i v e the compressors w i t h i n the methanol p l a n t s or exported t o MTG and u t i l i t y p l a n t s t o d r i v e l a r g e steam turbines. Methanol y i e l d i s a strong function o f the feed gas hydrogen t o carbon ratio.

Natural gas from the Maui f i e l d is lean i n carbon dioxide (7 percent)

therefore, t o give optimum y i e l d , feed gas t o the p l a n t also includes carbon dioxide r i c h (44 percent) n a t u r a l gas from the Kapuni f i e l d . Conversion o f methanol t o gasoline occurs i n two stages.

I n the f i r s t

stage, the crude methanol i s p a r t l y dehydrated t o an equilibrium mixture o f dimethyl

ether

temperatures reactor.

of

(DME),

methanol

3OO0C t o

and

42OoC over

water.

The

an alumina

reaction

catalyst

occurs

at

i n a fixed-bed

The DME e q u i l i b r i u n mixture i s then combined with recycle gas and

passed t o the gasoline conversion reactors where the second stage reactions t o form gasoline take place. The MTG process u t i l i s e d by Synfuel i s based on a fixed-bed r e a c t i o n system.

adiabatic

This reaction is h i g h l y exothermic and heat generated i s

removed by recycle gas which l i m i t s the temperature r i s e i n the MTG reactors t o 42OoC a t the reactor o u t l e t .

Hot reactor e f f l u e n t i s cooled with waste

heat being used t o preheat recycle gas and t o vaporise methanol feed t o the DME reactor. The methanol i s converted t o approximately 44 percent hydrocarbons 56 percent water.

are

also

formed.

and

Small amounts o f carbon monoxide, carbon dioxide and coke Coke accumulation

on

the

ZSM-5

necessitates

catalyst

To enable t h i s regeneration t o be

regeneration a t worst every fourteen days.

done M stream the f i v e conversion reactors are operated on a swing system. Liquid

hydrocarbons

are

separated

from

recycle

gas

and

water

and

fractionated i n three major d i s t i l l a t i o n columns t o produce a heavy gasoline stream, a l i g h t gasoline stream and a high vapour pressure gasoline (used f o r vapour pressure c o n t r o l ) .

Heavy gasoline i s treated f u r t h e r t o reduce the

h i g h melting p o i n t component durene which, i f present i n him concentrations, could adversely

affect

product

quality.

These intermediate products are

blended i n t o f i n i s h e d gasoline blendstock and piped t o storage near P o r t Taranaki. The o v e r a l l thermal e f f i c i e n c y o f the p l a n t as designed i s 53 percent. PROJECT FINANCING The Government / Mobil

Joint

Executive

Committee (JEC)

presented i n i t s

9

Report a 2 20 percent cost estimate f o r the p l a n t o f US$767 m i l l i o n i n mid1980 d o l l a r s .

The p r i n c i p l e aggregates i n t h i s estimate are shown i n Table 1.

For the purpose o f actual expenditure c o n t r o l , and t o determine the funding requirements o f the project,

allowance had t o be made f o r i n f l a t i o n during

the construction period, c a p i t a l i s e d pre-operating expenses and c a p i t a l i s e d i n t e r e s t during construction.

To t h i s end the JEC developed a " d o l l a r s o f

the day" estimate o f US$1475 m i l l i o n as shown i n the second p a r t o f Table 1.

I t was on t h i s basis, with shareholders equity budgeted a t US$275 m i l l i o n , t h a t i n July 1982 the New Zealand Synthetic Fuels Corporation entered i n t o a credit

agreement

with

Citicorp

I n t e r n a t i o n a l Group

and

a

syndicate

of

i n t e r n a t i o n a l banks f o r a term p r o j e c t financing f a c i l i t y f o r US$1700 m i l l i o n . This f a c i l i t y

provided f o r a term loan ( t o be repaid over 10 years) o f

US$1200 m i l l i o n , plus a standby f a c i l i t y o f US$500 m i l l i o n . TABLE 1

-

FROXCT BUDGET

us$ooo's

Direct Costs Methanol p l a n t s MTG p l a n t O f f s i t e s and u t i l i t i e s Total d i r e c t costs

169 100 91,300 122 000

I n d i r e c t Costs F i e l d distributables Contractor home o f f i c e Total i n d i r e c t costs

149 800 69,900

Other costs Capitalised engineering Capitalised spares Venture costs N.Z. develoment levy Wrap-up insurance Land Total other costs

, ,

35,000 3,000 24,500 3,300 10; 000 4,000

Sub t o t a l Contingency

382,400

219,700

80,830 682,400 84 600

TOTAL PLANT COST (JOINT EXECUTIVE COMMITTEE REPORT) Fees and start-up costs

767,000 119,000

TOTAL PROJECT COSTS I N 1980 DOLLARS I n f l a t i o n through July 1985 Estimated i n t e r e s t during construction ( c a p i t a l i s e d )

886 ,000 305,860 283 140

TOTAL COST I N DOLLARS OF THE DAY

1,475 000

10

CONSTRUCTION

On March 12, 1982 an appeal against planning consents obtained under the National Development Act was dismissed and on-site work commenced. F i e l d construction was

subcontracted by Bechtel i n a number o f

work

packages t a i l o r e d t o allow the greatest possible p a r t i c i p a t i o n by New Zealand contractors.

LEGEND:

SCHEDULED ACTUAL

Fig. 4.

----

-

BnsED ON ORIGINAL CONSTRUCTION PLAN, NOMMBER 1982

Construction Progress

11

Early i n the planning stage of the p r o j e c t i t had become evident t h a t i n s u f f i c i e n t s k i l l e d labour existed i n New Zealand t o complete the job w i t h i n

For t h i s reason the p l a n t was designed t o incorporate

the desired time scale.

l a r g e preassemblies t o be b u i l t off-shore.

However, even taking preassemblies

and other o f f - s i t e work i n t o account i t was estimated t h a t a t l e a s t seven m i l l i o n o f the nine m i l l i o n construction man-hours would be on-site labour. A l l 76 preassemblies,

b u i l t by Hitachi-Zosen L t d i n Ariake,

Japan, were

shipped t o Port Taranaki a t New Plymouth between August 1983 and March 1984. The

heaviest

of

these

preassemblies

measured 25 metres i n length,

weighed 588 tonnes

and the

largest

23 metres i n height and 12 metres i n width.

A l l preassemblies were transported the 25 kilometres from P o r t Taranaki t o

the

s i t e without

combinations.

incident

using h i g h l y

sophisticated t r u c k

and t r a i l e r

(Figure 3 ) .

Figure 4 shows t h a t a t June 30, 1985 on-site construction was e s s e n t i a l l y complete and ahead o f schedule. numbered about 1,800.

Peak on-site

sub-contractor

labour f o r c e

Performance on the s i t e was most s a t i s f a c t o r y and only

2.5 percent o f the budgeted t i m e was l o s t due t o i n d u s t r i a l disputes.

The

performance f a c t o r (PF) o f the workforce over the construction period was 0.83 compared t o a budgeted PF o f 1.

I t i s t o be noted t h a t the lower the PF

the more productive i s the workforce.

All

infrastructure

work

commissioning and startup.

was

completed

on

schedule

and

in

time

for

The managing contractor f o r t h i s work was the

M i n i s t r y o f Works and Development. Figures 5 and 6 show the completed Synfuel s i t e and MTG p l a n t . COMMISSIONING AND STARTUP

Although commissioning o f some u t i l i t y plants commenced i n l a t e 1984, most elements o f the p r o j e c t were commissioned during 1985 w i t h Methanol 2 being l e f t to last.

The objective was f i r s t t o s t a r t Methanol 1 and s h o r t l y a f t e r

t o produce gasoline from two o f the f i v e MTG reactors. Dryout o f the i n s u l a t i o n i n the reformer furnace o f Methanol 1 commenced

i n l a t e August 1985 and methanol was synthesised on October 12. 17th the f i r s t gasoline was produced from the MTG plant. problems were encountered

-

On October

Only minor s t a r t u p

a few valves operating u n s a t i s f a c t o r i l y ,

the odd

steam leak and i n i t i a l c o n t r o l system and heat balancing problems. The f i r s t 4000 tonnes o f blended gasoline was sent t o the M i n i s t r y o f Energy tank farm near Port Taranaki i n e a r l y November 1985. The r e s u l t s o f q u a l i t y t e s t s o f t h i s f i r s t batch o f gasoline were w i t h i n s p e c i f i c a t i o n and were as follows:

12

-

Reid Vapour Pressure, mbar

7OoC

% evaporation ~a

~OOOC

% evaporation

End p o i n t ,

0

the

37.9

-

55.8

-

Q 19OoC

C

Research octane No. Methanol 2 and

0.7420

-

Distillation % % evaporation 8

846

-

Density, kg/m3 Q 15OC

97.8 199.4 93.7

remaining MTG r e a c t o r s

became o p e r a t i o n a l

before

Christmas 1985. Curing commissioning and s t a r t u p a number o f M o b i l secondees augmented t h e operating

workforce.

With

the

project

fully

operational

a

permanent

workforce o f about 320 i s employed. OPERATING RESULTS On April 6, 1986 t h e s t a r t u p phase o f t h e p r o j e c t was completed with f u l l

For tne remainder o f the year t h e

commercial p r o d u c t i o n being achieved.

company received a processing fee from t h e Crown f o r c o n v e r t i n g gas i n t o gasoline. Some 448,000

tonnes o f gasoline were produced from commercial p r o d u c t i o n

on A p r i l 6 u n t i l 3 1 December 1986.

This p r o d u c t i o n was within two percent on

p l a n and a l l gasoline met r e q u i r e d s p e c i f i c a t i o n s .

The company achieved an

average y i e l d f o r t h e p e r i o d o f 10.97 tonnes o f gasoline per t e r a j o u l e of gas. Towards t h e end o f 1986 a h i g h e r y i e l d was achieved as t h e p l a n t reached i t s optimum o p e r a t i n g c o n d i t i o n .

The o v e r a l l thermal e f f i c i e n c y o f t h e p l a n t i s

54 percent compared t o the design e f f i c i e n c y o f 53 percent.

For 1986 as a whole 584,780 tonnes o f gasoline were produced which was equivalent consumption.

to

nearly

35 percent

of

New

Zealand's

premium

gasoline

The p l a n t continues t o run w e l l .

ECONOMICS The f i n a l US d o l l a r f o r e c a s t o f t h e p r o j e c t c o s t i n c l u d i n g c a p i t a l i s e d i n t e r e s t and working c a p i t a l i s US$1218 m i l l i o n , t h e o r i g i n a l budget o f US$1475 m i l l i o n .

about 17 percent l e s s than

Savings were made because o f lower

i n t e r e s t and i n f l a t i o n r a t e s and h i g h e r workforce performance than f o r e c a s t . Also t h e very c o m p e t i t i v e b i d d i n g environment f o r o f f s h o r e components o f t h e p l a n t c o n t r i b u t e d t o below budget costs. A p r o j e c t c o s t o f US$1218 m i l l i o n and US$1 = NZ$1.85 have been assumed i n

the c a l c u l a t i o n o f loan i n t e r e s t and repayments shown i n Table 2.

13 Table 2 a l s o presents some o f t h e o t h e r c o s t elements, p r o d u c t i o n o f Synfuel gasoline.

t o be met i n t h e

The c o s t s a r e i n 1987 cents p e r l i t r e o f

gasoline produced, f o r t h e years 1987, 1956 and 2000, on t h e assumption t h a t the p l a n t produces i t s nameplate c a p a c i t y o f 570,000 tonnes per annum a t Note t h a t 1987 w i l l be t h e f i r s t f u l l

design l e v e l s o f process e f f i c i e n c y .

year of commercial o p e r a t i o n of t h e p l a n t , with f u l l loan repayments, whereas t h e years 1996 and 2000 a r e r e p r e s e n t a t i v e of t h e s i t u a t i o n a f t e r 1995 when Also note t h a t the p r i c e o f Maui gas f a l l s i n r e a l

a l l loans a r e repaid.

terms over t h e l i f e o f t h e take-or-pay TABLE 2

-

contract.

SYNFUEL GASOLINE ECONOMICS

Loan InteresURepayments a t Nameplace Capacity 1987 NZ Cents per L i t r e

i

Construction Loan

ii

Working C a p i t a l , C a p i t a l Funding TOTAL

1987

1988

1989 1990

1991 1992

1993

37

30

35

34

27

25

31

24

18

1

1

1

1

1

2

1

2

1

38

31

36

35

28

27

32

26

19

Cost Elements a t Nameplate Capacity 1987 NZ Cents p e r L i t r e

Operating Expenses/Fees Loan Interest/Repayments Dividends: M o b i l : Crown Tax TOTAL

1987

1996

2000

16 38 3

16

12

10 1

10

11

68

47

42

57

22

17

13

9

8

70

31

25

2 4

15

1 4

14

Less: Crown Dividend Tax Net Cost t o Crown Cost o f Gas TOTAL COST

________--

1994 1995

14

I n 1981 t h e Government o f t h e day decided t h a t i n o r d e r t o e n s u r e marketing s t a b i l i t y Synfuel g a s o l i n e would be s o l d t o t h e o i l marketing companies a t t h e p r i c e of r e f i n e d g a s o l i n e from t h e expanded New Zealand Refinery a t Marsden Point. However, t h e p r e s e n t Government h a s changed t h i s p o l i c y and, t o d a t e , t h e Crown has s o l d Synfuel g a s o l i n e i n New Zealand a t import p a r i t y p r i c e s (which are s i g n i f i c a n t l y below t h e p r i c e of l o c a l l y r e f i n e d g a s o l i n e ) . With such a p o l i c y Synfuel g a s o l i n e is l i k e l y t o be s o l d a t a l o s s u n t i l t h e r e i s a s i g n i f i c a n t rise i n o i l p r i c e s or u n t i l a l l l o a n s are r e p a i d i n 1995. A f t e r 1995 Synfuel g a s o l i n e should be very c o m p e t i t i v e i n cost with g a s o l i n e r e f i n e d from crude o i l even a t p r e s e n t o i l p r i c e s (US$18 p e r b a r r e l , A p r i l 1987).

Fig. 5.

S y n f u e l Complex a t November 1985

15

The above conclusions derive from Table 2 where the net costs t o the Crown are equivalent t o about US$45, $18 and $14 per b a r r e l o f gasoline i n the years 1987, 1996 and 2000 respectively. Adding the cost o f gas raises the above f i g u r e s t o US$55, $25 and $20 per b a r r e l f o r the years i n question. should be observed t h a t under the take-or-pay

Here i t

contract, the Crown i s required

t o pay f o r the gas independent o f the operation o f the Synfuel plant.

Also i t

i s important t o note t h a t the above costs are f o r a b a r r e l o f gasoline and must be reduced by around US$7 per b a r r e l t o obtain equivalent crude o i l costs.

F i n a l l y these conclusions could be affected i f , i n the future, the

Government

follows

through

with

its

stated i n t e n t i o n t o

refinance the Synfuel debt.

Fig. 6.

MTG Plant with Methanol 1 i n the background

take

over

and

The Synfuel plant i s a longterm investment, i . e . 2003.

t i l l a t l e a s t the year

I t s economic benefits w i l l depend p r i m a r i l y upon what happens t o the

p r i c e of o i l o v e r the l i f e o f the p r o j e c t .

When the decision t o proceed w i t h

Synfuel was made i n February 1982 the p r i c e o f o i l was US$28 per b a r r e l .

In

November 1985 the p r i c e f e l l frm US$28 per b a r r e l t o reach a low i n 1986 of abwt

US$9 per

barrel.

US$18-20 per b a r r e l .

Since

then

the

price

has

recovered

to

around

Such r a p i d f l u c t u a t i o n s make i t impossible t o p r e d i c t Certainly i t

with any c e r t a i n t y the f u t u r e economic b e n e f i t s from Synfuel.

i s not possible t o make the p r o j e c t look good from an economic viewpoint a t the

present

significant

time.

However,

Table 2

shows

that

Synfuel

b e n e f i t s i n the medium and long term.

Also

should

produce

i n considering

b e n e f i t s i t must be noted t h a t condensate worth o v e r NZ$70 m i l l i o n per annum i s being obtained from the gas flow f o r the project. CONCLUSION Synfuel i s o f s t r a t e g i c importance t o New Zealand.

I n 1979 the view o f the

L i q u i d Fuels Trust Board was t h a t i t was desirable f o r New Zealand t o move t o about 50 percent s e l f - s u f f i c i e n c y i n transport fuels.

I t was considered t h a t

i f there were f u t u r e supply r e s t r i c t i o n s , or the p r i c e o f o i l continued t o r i s e , t h i s degree o f s e l f - s u f f i c i e n c y could be increased.

On the other hand

i f the p r i c e o f o i l were t o f a l l New Zealand would s t i l l gain s u b s t a n t i a l

b e n e f i t from the lower cost o f imports. With the operation o f the Synfuel p r o j e c t New Zealand i s about 50 percent self-sufficient

i n l i q u i d fuels.

I n 1987 t h i s s e l f - s u f f i c i e n c y

be made up approximately as follows:

figure w i l l

20 percent from condensate from the

Kapuni and Maui gas f i e l d s , 14 percent from Synfuel gasoline, 5 percent from t h e use o f

LPG and CNG i n vehicles and 11 percent from indigenous o i l

supplies. I t i s forecast t h a t New Zealand w i l l import 2,100,000

i n 1987.

tonnes o f crude o i l

This f i g u r e is l e s s than one h a l f o f the 4,257,000 tonnes o f crude

o i l and o i l products imported i n 1973/74. t h i s increased s e l f - s u f f i c i e n c y the medium t o long term.

The advantage t o the country o f

could be very s i g n i f i c a n t ,

particularly i n


FIFTY YEARS OF RESEARCH I N CATALYSIS

S. L . Meisel

Vice P r e s i d e n t , Research Department, Mobil Research and Development Corporation, P r i n c e t o n , New Jersey, USA

I t ' s not only a p l e a s u r e f o r me t o be p a r t i c i p a t i n g i n t h i s symposium, i t ' s a c t u a l l y a t h r i l l . Can you imagine being involved w i t h a p r o c e s s s h o r t l y a f t e r it was invented and s t u d i e d i n g l a s s equipment t h e s i z e o f a test t u b e , a l l t h e way t o a modern, first-of-a-kind p l a n t t h a t ' s working f l a w l e s s l y and making 14,000 barrels a day of high o c t a n e gasoline? I t ' s a b r e a t h t a k i n g experience.

T h i s morning I ' m going t o t e l l you about f i f t y y e a r s o f c a t a l y s i s r e s e a r c h a t Mooil t h a t started with t h e commercialization o f t h e first Houdry c a t a l y t i c c r a c k e r i n 1936 a t our Paulsboro, New J e r s e y r e f i n e r y , and culminated with t h e d e d i c a t i o n of t h e Motunui gas-to-gasoline p l a n t on February 27, 1986. I t ' s a s t o r y t h a t ' s r e p l e t e with research i n g e n u i t y and p e r s i s t e n c e . But b e f o r e t e l l i n g it t o you, I ' d l i k e t o s t a r t by t e l l i n g you a n o t h e r s t o r y -the i n s i d e s t o r y of how t h e methanol-to-gasoline (MTG) p r o c e s s was conceived, developea, and commercialized. As many of you know, t h e discovery o f t h e MTG r e a c t i o n was an a c c i d e n t .

Or maybe more p r e c i s e l y it r e s u l t e d from a couple o f a c c i d e n t s . One group a t Mobil was t r y i n g t o c o n v e r t methanol over our ZSM-5 z e o l i t e c a t a l y s t t o o t h e r oxygen-containing compounds. I n s t e a d , they g o t unwanted hydrocarbons. A l i t t l e later a n o t h e r Mobil group, working independently, was t r y i n g t o alkylate i s o b u t a n e with methanol over ZSM-5, and i d e n t i f i e d a mixture of p a r a f f i n s and a r o m a t i c s b o i l i n g i n t h e g a s o l i n e range -- a l l o f it coming from t h e methanol. They a l s o observed t h e formation o f a s o l i d material, l a t e r i d e n t i f i e d as durene.

17

18

P a r t i c i p a t i n g i n these s t u d i e s were Clarence Chang, one o f t h e i n v e n t o r s o f MTG, and a speaker and organizer o f t h i s Symposium; Tony S i l v e s t r i , whom some

o f you met d u r i n g h i s v i s i t t o New Zealand; and Heinz Heinemann, who r e t i r e d from M o b i l and i s p r e s e n t l y on t h e Chemical Engineering s t a f f o f t h e U n i v e r s i t y o f C a l i f o r n i a a t Berkeley, and who i s a t t e n d i n g t h i s meeting. We had discovered a new r e a c t i o n and recognized i t s p o t e n t i a l s i g n i f i c a n c e . But we faced a dilemma.

I t was a n t i c i p a t e d t h a t t h e development o f t h e

process would i n v o l v e a l e n g t h y and expensive program, and commercialization appeareo t o be many years down t h e road.

There was a n a t u r a l r e l u c t a n c e a t

M o b i l t o continue t h i s work s o l e l y with M o b i l funds. A t t h e time, t h e U n i t e d S t a t e s O f f i c e o f Coal Research ( l a t e r t o become p a r t o f t h e Department o f Energy) was very i n t e r e s t e d i n t h e conversion o f c o a l t o l i q u i d products. reserves.

As many o f you know, t h e U n i t e d S t a t e s has tremendous c o a l

Since methanol had oeen made commercially from b o t h n a t u r a l gas and

c o a l , i t was recognized t h a t t h e MTG r e a c t i o n had t h e p o t e n t i a l o f p r o v i d i n g t h e c r i t i c a l step i n t h e f i r s t new s y n f u e l s process s i n c e t h e p i o n e e r i n g work

i n Germany some 50 t o 60 years e a r l i e r . Heinz Heinemann was asked t o approach h i s f r i e n d Alex M i l l s , who was t h e n head o f e x p l o r a t o r y c o a l conversion a t t h e O f f i c e o f Coal Research.

From t h e very

beginning Alex was a s t r o n g supporter, and b e f o r e i t was over we had obtained two Government c o n t r a c t s over a t h r e e year p e r i o d and had developed t h e Drocess. D u r i n g t h i s period, t h e New Zealand Government was l o o k i n g for ways t o c o n v e r t some o f t h e i r huge n a t u r a l gas reserves a t Maui t o more s a l a b l e products. Discussions with t h e New Zealand Government i n v o l v e d many anxious moments. The Sasol process was i n c o m p e t i t i o n with MTG.

Although our process o f f e r e d

s i z a b l e economic advantages i n New Zealand over t h e Sasol process, ours was unproven commercially, w h i l e t h e Sasol technology had been commercialized.

We

were asking t h e New Zealand Government t o t a k e our word t h a t we c o u l d s c a l e up from f o u r b a r r e l s t o 14,000 b a r r e l s per Gay.

They d i d , and t h e commercial

process has performed as w e l l as predicted, and i n a number o f cases even oetter.

1'11 n o t say more about t h e program t h a t was c a r r i e d o u t t o g e t ready f o r commercialization, s i n c e i t w i l l be discussed i n o t h e r papers a t t h i s meeting.

19 Altnough the discovery o f MTG was accidental, i t occurred because o f our continuing and balanced e f f o r t i n c a t a l y s i s over many years. The story begins i n the e a r l y 1930s.

Our researchers were t r y i n g t o develop a

c a t a l y t i c cracking process, and they learned t h a t Eugene Houdry was doing s i m i l a r work i n France.

His work was f u r t h e r advanced and Houdry was

persuaded t o j o i n our l a b i n Paulsboro, New Jersey. Houdry was a wealthy, eccentric mechanical engineer whose hobby was racing cars.

He had the great i n s i g h t t o r e a l i z e t h a t h i s engines were hampered not

so much by mechanical design as by the low octane r a t i n g o f the a v a i l a b l e gasoline.

So Houdry had set out t o develop a c y c l i c c a t a l y t i c cracking

process t o boost the octane r a t i n g o f gasoline.

Up t o t h a t time, the common

commercial process was thermal cracking, but i t produced very low octane materiai.

Houdry's process showed such great promise t h a t Mobil, and l a t e r

Sun O i l , set up a j o i n t venture w i t h him, and i n 1936 the world's f i r s t successful commercial c a t cracker went on stream a t a Mobil r e f i n e r y i n New Jersey. The Houdry u n i t s g r e a t l y increased both the q u a l i t y and the y i e l d o f gasoline per b a r r e l o f crude, and during world War I1 high octane Houdry gasoline propelled the Royal A i r Force t o v i c t o r y i n the b a t t l e o f B r i t a i n . Even more e f f i c i e n t processes were introduced l a t e r i n the war.

Mobil

developed a moving bed process c a l l e d Thermofor C a t a l y t i c Cracking and a new synthetic amorphous cracking c a t a l y s t (Ref. l), while Exxon l e d a group o f companies i n developing f l u i d c a t cracking.

These processes provided t h e

Armed Forces w i t h p l e n t i f u l supplies o f 100 octane a v i a t i o n f u e l i n the l a t t e r stages o f world War I1 and they were a decisive element i n the f i n a l A l l i e d v i c t o r y i n Europe. A f t e r the war, i n d u s t r y made a number o f advances i n cracking catalysts, but a l l were high surface area, amorphous oxides, l i k e silica-alumina. mid-1950s the improvements became smaller and l e s s frequent.

By the

Many companies

c u t out or de-emphasized research i n t h i s area because i t was commonly believed t h a t c a t cracking had gone about as f a r as i t could go. But MoDil's work continued, and l e d t o a monumental discovery.

Our s c i e n t i s t s

showed t h a t c a t a l y t i c a c t i o n could take place w i t h i n the c r y s t a l l i n e spaces o f zeolites.

This discovery upset the o l d b e l i e f t h a t only amorphous structures

could provide the h i g h surface areas necessary f o r u s e f u l r a t e s o f conversion.

20

We launched an i n t e n s i v e e f f o r t t o develop these h i g h l y ordered c r y s t a l l i n e z e o l i t e s f o r commercial u s e , and by t h e early 1960s we commercialized t h e f i r s t s y n t h e t i c z e o l i t e c r a c k i n g catalyst (Refs. 2 , 3 ) . T h i s c a t a l y s t was

d e r i v e d from a s y n t h e t i c f a u j a s i t e , f i r s t made by Union Carbide. Converting t h e f a u j a s i t e i n t o a u s e f u l c r a c k i n g c a t a l y s t was a r a t h e r involved procedure, i n c i u o i n g t h e removal of sodium, t h e i n t r o d u c t i o n of r a r e e a r t h , and s e v e r e steam treatment.

IMPACT OF ZEOLITE CRACKING CATALYST Introduced 1962 Gasoline Yield from Cat Cracker

0 I

24

gal

16 gal

Amorphous Catalyst

Zeolite Catalyst

F i g . 1.

W e introduced t h e first z e o l i t e c r a c k i n g c a t a l y s t a t one o f our r e f i n e r i e s i n 1962 (Ref. 4 ) . I t was l i k e a s h o t heard around t h e world. F i g u r e 1 shows why.

catalyst, t h e c a t c r a c k e r was g e t t i n g about 16 g a l l o n s of g a s o l i n e from each b a r r e l o f feed. But with t h e new z e o l i t e c a t a l y s t , t h e same cracker squeezed o u t a n o t h e r 8 g a l l o n s of g a s o l i n e from t h e same barrel of feed. T h a t ' s a f i f t y p e r c e n t i n c r e a s e i n g a s o l i n e y i e l d -- an almost u n b e l i e v a b l e result. With t h e o l d amorphous

Over t h e y e a r s , the s a v i n g s i n petroleum r e s o u r c e s have been enormous. I n t h e U.S. a l o n e , t h e z e o l i t e c r a c k i n g c a t a l y s t has saved t h e U.S. petroleum i n d u s t r y t h e e q u i v a l e n t of some 3.5 b i l l i o n b a r r e l s (500 m i l l i o n tonnes) of crude o i l s i n c e 1962. I t lowered c r u d e imports even more and enabled i n d u s t r y t o produce higher g a s o l i n e volumes without a d d i t i o n a l investments i n r e f i n e r y expansion.

21

The success o f the z e o l i t e cracking c a t a l y s t l e d t o a more intense c a t a l y s t research e f f o r t i n Mobil

--

z e o l i t e s was very l i m i t e d .

and t o more milestones.

A t f i r s t our choice o f

Most z e o l i t e s were n a t u r a l l y occurring, and those

t h a t were synthetic, l i k e f a u j a s i t e , tended t o r e p l i c a t e n a t u r a l materials. But soon we learned t o synthesize z e o l i t e s t h a t had never been found i n nature. During the l a t e 1960s and 70s we invented about 40 new zeolites, and converted some o f them i n t o u s e f u l catalysts. the discovery o f ZSM-5 (Ref. 5). commercial-ready,

The major milestone i n t h i s e f f o r t was

So f a r , we have commercialized,

or made

eleven d i f f e r e n t processes t h a t use various modifications o f

t h i s very v e r s a t i l e c a t a l y s t (Ref. 6).

ZSM-5 PROCESSES Petroleum Refining

M-Forming Distillate Dewaxing Lube Dewaxing Cracking Catalyst

Chemical

Xylene lsomerization Toluene Disproportionation Ethyl benzene Para-ethyltoluene

Synthetic Fuels & Chemicals

Methanol t o Gasoline Methanol t o Olefins Olefins t o Distillate and Gasoline

r Fig. 2.

Tne l i s t i n Figure 2 includes four petroleum r e f i n i n g , four chemical, and three synthetic f u e l s and chemicals processes

--

a l l developed and brought t o

commercial-ready status i n the short span o f 15 years since we received the basic patent on ZSM-5.

Today, ZSM-5 i s being used i n more than 40 commercial

i n s t a l l a t i o n s on f i v e continents.

This i s a remarkable achievement.

1'11 be t a l k i n g l a t e r about the chemistry t h a t makes these processes possible.

But f i r s t , I ' d l i k e t o discuss the work t h a t determined the

s t r u c t u r e o f ZSM-5.

I t was a r e a l i n t e r d i s c i p l i n a r y approach.

A team o f crystallographers,

chemists, and chemical engineers tackled the s t r u c t u r a l determination.

22

Fig. 3.

Twinned ZSM-5 c r y s t a l .

The f i r s t major obstacle t h a t had t o be overcome was twinning:

the tendency

o f a c r y s t a l t o grow i d e n t i c a l twin components, symmetrically united, as you see i n the scanning e l e c t r o n micrograph i n Figure 3.

Twinning camouflages the

c r y s t a l ' s t r u e symmetry, and t h i s can lead t o i n c o r r e c t s t r u c t u r a l analysis. So we had t o devise procedures t o grow single, untwinned c r y s t a l s t h a t would

be l a r g e enough t o study.

Growing the s i n g l e c r y s t a l s required persistence,

patience, and a great deal of s k i l l a t c o n t r o l l i n g the growth environment. Figure 4 shows some s i n g l e ZSM-5 c r y s t a l s o f the q u a l i t y used i n our study. Now i t was possible t o look deeper i n t o t h i s unusual material.

We wanted t o

l e a r n i t s precise atomic structure, because t h a t would help us understand i t s s o r p t i v e and c a t a l y t i c behavior, and g i v e us clues about new ways t o use the catalyst. From s i n g l e c r y s t a l X-ray d i f f r a c t i o n patterns, we knew t h a t the basic b u i l d i n g block of ZSM-5 was a c e l l 20.1 by 19.9 by 13.4 Angstrom u n i t s , as

23

shown in Figure 5. alumina tetrahedra.

We also knew that the cell contained 96 silica and

Fig. 4. Single ZSM-5 crystal. ZSM-5 BUILDING BLOCKS

Fig. 5.

24

From hydrocarbon adsorption data, we suspected t h a t the tetrahedra formed ten-membered r i n g s , 5 t o 6 Angstrom u n i t s i n diameter.

But we d i d n ' t know the

precise arrangement o f these b u i l d i n g blocks i n the ZSM-5 structure. To solve t h i s problem, our a n a l y t i c a l team developed several hypothetical models, and computed an X-ray d i f f r a c t i o n p a t t e r n f o r each.

They then

compared these patterns with the X-ray spectrum f o r the experimental ZSM-5. None o f these early models matched ZSM-5 exactly, but we d i d f i n d some common s t r u c t u r a l arrangements.

X-RAY DIFFRACTION PATTERNS

Observed for ZSM-5

Diffraction Angle (2W)

Fig. 6.

From here on i t was a matter o f modifying these s t r u c t u r a l arrangements t o f i t the observed d i f f r a c t i o n data.

As Figure 6 shows, t h i s eventually produced a

very close match between the computed X-ray d i f f r a c t i o n p a t t e r n f o r the ZSM-5 model and the p a t t e r n observea f o r the ZSM-5 c r y s t a l . observed f o r ZSM-5 occur i n the computed pattern.

A l l o f the peaks

The differences i n peak

heights are due t o absorbed water. You can imagine our e x h i l a r a t i o n , when a f t e r more than three years o f exacting and o f t e n f r u s t r a t i n g work, the s t r u c t u r e o f ZSM-5 had been solved (Refs. 7, 8). Figure 7 shows the s t r u c t u r e as

our a n a l y t i c a l team unraveled it. You can see

t h a t the ten-membered r i n g s dominate the s t r u c t u r e and create t h e major

25

channels o f the crystal. place.

F i g . 7.

I t ' s inside these channels where the reactions take

ZSM-5 structure.

The structure also contains a l o t of five-membered rings, which we believe account f o r ZSM-5's high thermal stability. The white balls are aluminum atoms, which provide the acidic s i t e s that give ZSM-5 its catalytic properties. By varying the aluminum concentration, we can t a i l o r the catalytic activity over a very wide range (Ref. 9 ) . T h i s makes ZSM-5 special. But what makes ZSM-5 unique is the combination of its catalytic activity, its s t a b i l i t y , and its shape selectivity -- its ability t o l i m i t the size and shape (or

bulkiness) of the molecules that can be processed (Ref. 10). The secret is the size of ZSM-5's channels. openings o f three zeolites, shows why.

Figure 8, comparing the channel

ZSM-5 is the one i n the middle, the one w i t h an n-hexane molecule s i t t i n g

comfortably inside its ten-membered r i n g . A hexane w i t h a methyl branch would also f i t . The n-hexane, b u t not the isohexane, would f i t i n t o the pores of Erionite, the zeolite w i t h the eight-membered r i n g a t the l e f t . Both hexanes

26

would be swallowed up along w i t h a bunch o f l a r g e r molecules by Faujasite, the z e o l i t e cracKing c a t a l y s t w i t h the cavernous twelve-membered r i n g .

ZSM-5 COMPARED WITH ERlONlTE AND FAUJASITE

Erionite 8-Ring 3.6 x 5 . 2 i

ZSM-5 10-Ring 5.4 x 5 . 6 i

Faujasite 12-Ring 7.4 x 7 . 4 i

Fig. 8.

No other c a t a l y t i c a l l y active, stable z e o l i t e can match z s M - 5 ' ~o v e r a l l

s e l e c t i v i t y f o r the conversion o f intermediate s i z e molecules, although we have several z e o l i t e s i n the development stage t h a t match o r exceed i t s s e l e c t i v i t y f o r s p e c i f i c reactions. We c a n ' t see the organic molecules as they wiggle i n and out o f these channels.

But we can measure how quickly or slowly they d i f f u s e i n t o ZSM-5.

From d i f f u s i o n studies (Ref.

ll), we know t h a t only s t r a i g h t chain and

mono-methyl p a r a f f i n s and o l e f i n s , c e r t a i n one-ring aromatic and naphthenic molecules d i f f u s e a t u s e f u l r a t e s through ZSM-5. the f a s t e r the d i f f u s i o n r a t e .

The l e s s bulky the molecule,

Larger molecules e i t h e r d i f f u s e i n slowly, and

r e a c t a t a lower rate, or they are completely excluded.

We c a l l t h i s reactant

shape s e l e c t i v i t y . Actually, ZSM-5 e x h i b i t s

three types

t r a n s i t i o n state, and product.

reactant molecules i t w i l l admit... channels during the t r a n s i t i o n stage leave.

o f shape s e l e c t i v i t y :

reactant,

I t i s s e l e c t i v e i n the s i z e and shape o f o f molecules t h a t can form w i t h i n i t s

... and o f product molecules t h a t can

27

1'11 aescrioe each type of s e l e c t i v i t y , and show how they are used i n petroleum upgrading and chemical synthesis. Figure 9 shows an example of reactant s e l e c t i v i t y . On the l e f t is a view down a ZSM-5 channel, and on the r i g h t is a cross section of the same channel. Insicle the channel is a C12 paraffin. You can see i t u sa close f i t .

F i g . 9.

Reactant s e l e c t i v i t y .

Once admitted, the paraffins are catalytically cracked i n t o smaller gasoline-type hydrocarbons, which escape easily. T h i s gives u s the a b i l i t y t o crack unwanted paraffins out of our l i q u i d products, and, as a bonus, t o make a l i t t l e extra gasoline. We've developed four commercial processes based on the reactant shape s e l e c t i v i t y of ZSM-5: two for cracking out low octane paraffins i n gasoline, and two for removing waxy paraffins from d i s t i l l a t e fuels and lubricating o i l s (Refs. 12, 13). Now l e t % look a t t r a n s i t i o n s t a t e s e l e c t i v i t y , a theoretical mechanism t h a t ' s had a l o t of play i n the l i t e r a t u r e (Ref. 14). Proponents of t h i s mechanism say that the s t e r i c crowding o f the transition s t a t e can l i m i t the formation of certain reaction products.

28

F i g u r e 10, f o r example, shows how t h e channel s i z e of ZSM-5 retards an u n d e s i r a b l e r e a c t i o n -- t h e t r a n s a l k y l a t i o n o f xylene t o produce t h e bulky i n t e r m e d i a t e s which would y i e l d t r i m e t h y l benzene and t o l u e n e . The desirable r e a c t i o n is the i s o m e r i z a t i o n o f a xylene mixture t o para-xylene -- t h e raw material f o r p o l y e s t e r f i b e r s .

TRANSITION STATE SELECTIVITY

Fig. 10.

I t ' s a d i f f i c u l t mechanism t o prove and i t ' s questioned by some i n v e s t i g a t o r s . But t r a n s i t i o n state s e l e c t i v i t y does provide a n e a t e x p l a n a t i o n for some of t h e chemistry t h a t takes p l a c e i n s i d e t h e p o r e s o f ZSM-5.

a t p r o d u c t s e l e c t i v i t y , where t h e size o f t h e product molecule is restricted by t h e dimensions o f ZSM-5's channels.

Now l e t ' s looK

29

I

PRODUCT SELECTIVITY WITH MODIFIED ZSM-5

I

Para-Xylene

Benzene

F i g . 11. (After Ref. 11.)

An example of product s e l e c t i v i t y is t h e ZSM-5 p r o c e s s , s e l e c t i v e t o l u e n e d i s p r o p o r t i o n a t i o n (Ref. 151, which is t h e second g e n e r a t i o n o f our commercial t o l u e n e d i s p r o p o r t i o n a t i o n p r o c e s s t h a t produces x y l e n e s and benzene from toluene. Based on fundamental s t u d i e s t h a t r e v e a l e d t h e important v a r i a b l e s i n c o n t r o l l i n g t h e s e l e c t i v i t y , we have been able t o modify t h e ZSM-5 c a t a l y s t t o produce para-xylene from t o l u e n e with much g r e a t e r s e l e c t i v i t y . A c t u a l l y , what we've done is p a r t i a l l y plug t h e pore openings i n ZSM-5 t o make it more d i f f i c u l t f o r t h e b u l k i e r meta-xylene and ortho-xylene molecules t o come out. F i g u r e 11 is a h i g h l y s i m p l i f i e d diagram o f t h i s process. Toluene e n t e r s t h e ZSM-5 c r y s t a l and d i s p r o p o r t i o n a t e s a t t h e acid c a t a l y s t sites t o benzene and Because of t h e i r l a r g e r t h e t h r e e xylene isomers: para-, meta-, and ortho-. s i z e , meta- and ortho-xylenes d i f f u s e slower t h a n para-xylene. As you expect, t h e l o n g e r they stay i n s i d e t h e ZSM.5 c r y s t a l , t h e r i c h e r t h e product w i l l be i n para-xylene.

30

The r e l a t i v e d i f f u s i o n r a t e f o r para-xylene i n t h i s modified ZSM-5 c a t a l y s t i s a t l e a s t a thousand times f a s t e r than the d i f f u s i o n r a t e s o f the other isomers, and t h i s r e s u l t s i n a para-xylene concentration much higher than e q u i l i b r i u m (Ref. 11).

PARA-SELECTIVITY PERFORMANCE 100-

80 Para-Xylene 60 of Xylenes 40 -

-

I

O/O

20 =--------=-

Standard ZSM-5 I

Fig. 12.

I

I

I

Equilibrium

I

( A f t e r Ref. 15.)

Figure 12 compares the performance o f the modified ZSM-5 with the standard unmodified ZSM-5.

A thermodynamic e q u i l i b r i u m mixture o f xylenes contains

about 24% para-xylene.

I n laboratory t e s t s w i t h the modified c a t a l y s t , we

have achieved para-xylene s e l e c t i v i t i e s as high as 98% a t low conversions. Now i t ' s t i m e t o move along t o my f i n a l t o p i c

--

Figure 13 i s a photo o f the New Zealand complex.

Synthetic Fuels. Since i t began operation i n

l a t e 1985, i t ' s been converting n a t u r a l gas f r o m the huge Maui gas f i e l d i n t o high octane gasoline a t a r a t e o f about 14,000 b a r r e l s a day; o f New Zealand's needs. methanol-to-gasoline

about a t h i r d

A t the heart o f the f a c i l i t y i s Mobil's

technology.

31

F i g . 13.

New Zealand Synfuels plant.

MTG: THE MISSING LINK Coal

Natural Gas

Methanol

\

Dimethylether + Water

\

pG5-1 \

Gasoline Fig. 14.

1

As F i g u r e 14 shows, MTG p r o v i d e s t h e missing l i n k i n a c h a i n o f r e a c t i o n s t h a t begins with c o a l or n a t u r a l gas and ends with t h e g a s o l i n e we need t o keep our mechanized s o c i e t y moving.

N a t u r a l gas or c o a l i s converted t o synthesis gas,

a m i x t u r e o f C02 and H2, and then t o methanol.

The methanol i s then

converted t o an e q u i l i b r i u m m i x t u r e o f methanol, dimethylether and water, which i s f e d t o an MTG r e a c t o r and converted t o g a s o l i n e over a ZSM-5 c a t a l y s t .

F i g . 15.

Experimental 100 B/D f l u i d bed MTG p l a n t a t Wesseling, FRG.

We've developed MTG technology.

technology.

--

and demonstrated

--

f i x e d bed and f l u i d bed v e r s i o n s o f t h e

The New Zealand s y n f u e l s p l a n t uses f i x e d bed MTG

I t s performance has met or exceeded a l l expectations with r e s p e c t

t o product q u a n t i t y , product q u a l i t y , and c a t a l y s t behavior. A f l u i d bed v e r s i o n of t h e MTG process has been demonstrated s u c c e s s f u l l y i n an experimental 100 b a r r e l / d a y p l a n t a t Wesseling, F e d e r a l Republic o f Germany, and now awaits scale up t o commercial a p p l i c a t i o n (Ref. 16).

A

photograph of t h e 100 b a r r e l / d a y f l u i d p i l o t p l a n t i s shown i n F i g u r e 15. We're very proud t o see t h e New Zealand gas-to-gasoline complex running.

The

New Zealand and German experience w i l l p r o v i d e proven technology a g a i n s t t h e t i m e when s y n f u e l s a r e needed i n other p a r t s o f t h e world. Meanwnile, we've explored t h e MTG r e a c t i o n i n t e n s e l y s i n c e t h e i n i t i a l discovery, and we have learned t o make a l o t more than gasoline.

I

SIMPLIFIED MTG REACTION PATH 400" C

I F i g . 16.

-Residence Time

-

1

I

( A f t e r Ref. 17.)

The p o s s i b i l i t i e s i n t h e s i m p l i f i e d MTG r e a c t i o n p a t h (Ref. 17), a r e shown i n F i g u r e 16.

As t h e methanol f l o w s through t h e r e a c t o r , from l e f t t o r i g h t on t h e c h a r t , i t changes very r a p i d l y i n t o new chemicals and changes a g a i n b e f o r e i t f i n a l l y becomes t h e m i x t u r e o f p a r a f f i n s , aromatics and o l e f i n s t h a t we c a l l gasoline.

34

I f we allow the reaction t o proceed t o completion we obtain a mixture of aromatics and paraffins -- o r gasoline. B u t a t one point i n the MTG reaction the product mix is about 4 0 % C2-C5 olefins. If we were t o interrupt the reaction a t that p o i n t , we could harvest these l i g h t olefins. The MTG reaction r u n s a t a temperature around 4OO0C a t a methanol pressure o f several atmospheres, and uses a high a c t i v i t y catalyst. These a r e t h e optimal conditions for converting the olefins that form w i t h i n the ZSM-5 c r y s t a l s i n t o paraffins and aromatics. But i f we adjust the reaction conditions and modify the catalyst, we can

virtually double the olefin yield. T h i s discovery has led t o the development of another ZSM-5 process -- called methanol-bolefins, or MTD (Refs. 18, 1 9 ) .

SIMPLIFIED MTO REACTION PATH 500"c

Comp. Wt. Yo

I Fig. 17.

-Residence

Time

-

(After Ref. 18.)

For MTO, where the object is t o optimize the olefins, we r a i s e the temperature t o about 5OO0C, which favors olefin formation. We a l s o modify t h e catalyst

t o slow down the conversion of olefins t o aromatics and paraffins. These changes produce a dramatic change i n the reaction path. As Figure 17 shows, we have now decoupled the aromatics + paraffins plot from t h e olefins plot. I n e f f e c t , what MTO accomplishes is t o produce olefins f a s t e r than they can be converted t o paraffins or aromatics. T h i s r e s u l t s i n an olefin s e l e c t i v i t y that i n lab t e s t s has gone as high a s 80 percent, and even higher w i t h other

I

35

p r o p r i e t a r y c a t a l y s t s now under development.

MTO generates mostly propylene

and butylene with h i g h octane gasoline as a byproduct.

But we can modify the

ZSM-5 c r y s t a l t o be more s e l e c t i v e t o ethylene. So what we have here i s a chemical factory, t o be brought on stream as t h e

need arises.

And we can go a step f u r t h e r and convert these o l e f i n s t o an

e n t i r e spectrum o f products, through yet another ZSM-5 process: olefins-to-gasoline

+

Mobil

d i s t i l l a t e , or MOGD (Refs. 20, 21).

FOUR-STEP MOGD REACTION

C6=,Cg', C12=,etc. c6=, Cg',

C,2=,etc. 3=7 C4=,C,=,

C6=,C,=,etc.

C3=,C4=,.... C,c=,C,,', etc.

F i g . 18.

(Ref. 21.)

I n the MOGD reaction, ZSM-5 oligomerizes l i g h t o l e f i n s , from e i t h e r r e f i n e r y streams or MTO, i n t o higher molecular weight o l e f i n s t h a t f a l l i n t o the gasoline, d i s t i l l a t e , and l u b r i c a n t range. Actualiy, the chemistry o f MCGD i s a l i t t l e more complicated than simple oligomerization.

I n f a c t , i t involves a series o f reactions t h a t take you

forward a couple o f steps and then back a step, and then forward again. example, propylene (C;) CG, CT2 and so f o r t h .

oligomerizes i n the forward d i r e c t i o n t o C z , As i l l u s t r a t e d i n Figure 18, some o f t h i s

product cracks, isomerizes and disproportionates, and f i n a l l y polymerizes again t o form a broad spectrum o f materials.

For

36

The sequential nature o f the MCGD r e a c t i o n i s shown very c l e a r l y i n Figure 19.

I t shows a set o f gas chromatographs o f MCGD product sampled a t

d i f f e r e n t stages o f the reaction. oligomers

--

C6, C9 and C12

--

Note t h a t the e a r l y predisposition t o

indicated by the sharp peaks i n the lower

trace, decrease dramatically as the r e a c t i o n proceeds and more intermediate carbon nurrbers are produced by polymerization.

MOGD PRODUCT DISTRlBUTION Oligomeriration of Propylene

6 9 1215182124273033 Carbon No.

Fig. 19.

( A f t e r Ref. 21.)

A t equilibrium, the r e s u l t i s a wide d i s t r i b u t i o n o f product molecules,

ranging from Cj a l l the way t o C40 and higher. with p r a c t i c a l l y any o l e f i n feed.

We see s i m i l a r r e s u l t s

Because the r e a c t i o n takes place w i t h i n the

confines o f the narrow pores o f the ZSM-5 c r y s t a l , the end r e s u l t o f the M E 0 r e a c t i o n i s a product w i t h a w e l l defined structure:

a continuous carbon

number product with an occasional methyl group s i t t i n g on i t . While the dimensions o f the ZSM-5 channels c o n t r o l the cross s e c t i o n a l area o f the produced molecules, r e a c t i o n conditions.

c o n t r o l the length o f t h e molecule by a d j u s t i n g

By changing the temperature, pressure, and residence

time, we can make mostly gasoline or mostly a l l d i s t i l l a t e , between.

or any mix i n

37 MOGD and i t s companion process, MTO, are ready f o r commercial use.

MOGD has

been proved i n a commercial t e s t i n r e f i n e r y scale equipment, and the MTO process has been successfully demonstrated i n the same experimental 100 barrel/day p l a n t used t o prove the f l u i d bed MTG process i n Germany (Ref. 22). I n the beginning, our methanol-to-hydrocarbons technology was seen p r i m a r i l y as an e f f i c i e n t method for converting America's abundant reserves o f c o a l i n t o high octane gasoline.

But t h i s concept has long since been expanded, not j u s t

t o other fuels, b u t also t o chemicals and even lubricants.

I n f a c t , using our

new C - 1 technologies, we can make almost anything out o f coal or n a t u r a l gas t h a t can be made out o f crude o i l .

REFERENCES 1. R.W. Porter, Chemical and M e t a l l u r g i c a l Engineering, A p r i l 1946. 2. K.M. E l l i o t t and S.C. Eastwood, Proceedings o f the American Petroleum I n s t i t u t e , 1962, 43 (111) 272. 3. D.H. Stormont, O i r a n d Gas J., A p r i l 5, 1965. 4. S.C. Eastwood, R.D. Drew, F.D. H a r t z e l l , O i l and Gas J., Oct. 29, 1962, 152. 5. R.J. Argauer & G.R. Landolt, U.S. Patent 3,702,886 (1972). 6. N.Y. Chen & W.E. Garwood, Catal. Rev., Sci. Eng., 1986, 28, 185. 7. G.T. Kokotailo, S.L. Lawton, D.H. Olson & W.M. Meier, NaGre, 1978, 275, 119. 8. O.H. Olson, G.T. Kokotailo, S.L. Lawton, and W.M. Meier, J. Phys. Chem., 1981, 85, 2238. 9. P.B. Wssz, Ind. Eng. Chem. Fund., 1986, 25, 53. 64, 382. 10. P.B. Weisz & V.J. F r i l e t t e , J. Phys. ChemT; 1960, 11. P.B. Weisz, Pure & A p l . Chem., 1980, 52, 2091. 12. N.Y. Chen, J. Maziuky A.B. Schwartz & E B . Weisz, O i l & Gas. J., 1968, 66 (471, 154. 13. N.Y..Chen, R.L. Gorring, H.R. I r e l a n d & T.R. Stein, O i l & Gas J., 1977, 75 (23), 165. 14. T S . Csicsery, J. Catal., 1971, 23, 124. 1984. 15. D.H. Olson & W.O. Haag, American memica1 Society Symp. Ser., 16. K.-H. Keim, F. J. Krambeck, J. Maziuk, A. TEnnesmann, Erd61 Erdgas Kohle, 1987, 103, 82. 17. C.D. Chang & A.J. S i l v e s t r i , J. Catal., 1977, 47, 249. 18. C.D. Chang, C.T-W. Chu & R.F. Socha, J. Catal., 1984, 86, 289. 19. R.F. Socha, C.D. Chang, R.M. Gould, S.E. Kane & A.A. Azdan, American Chemical Society Symp. Ser. 328, 1987, 34. 20. W.E. Garwood, American C h e m i z Society Symp. Ser. 218, 1983, 383. 21. S.A. Tabak, F.J. Krambeck & W.E. Garwood, AIChE M t g y S a n Francisco, Nov. 1984, 22. D. Johnson, J. Soto, A.A. Avidan, H. G i e r f i c h & N. Thiagarajan, Report 1986, DOE/ET/14914-H2, NO. DE86015960.

248,

-


D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE STEAM REFORMING OF NATURAL GAS:

39

PROBLEMS AND SOME SOLUTIONS

D.L. TRIMM School o f Chemical E n g i n e e r i n g and I n d u s t r i a l Chemistry, U n i v e r s i t y o f New South Wales, PO Box 1, Kensington, NSW 2033, A u s t r a l i a ,3CTRACT A l t h o u g h t h e p r o d u c t i o n o f hydrogen by steam r e f o r m i n g i s a w e l l e s t a b l i s h e d process, some problems remain w i t h t h e i n d u s t r i a l o p e r a t i o n . Thermodynamics d i c t a t e s t h a t t h e r e a c t i o n be c a r r i e d o u t a t h i g h temperatures, where c o k i n g o r s i n t e r i n g o f t h e c a t a l y s t may occur. Coking i s d i c t a t e d by t h e b a l a n c e between coke f o r m a t i o n and removal. One i m p o r t a n t f a c t o r i s t h e number o f n i c k e l atoms i n an ensemble needed t o c a t a l y s e steam r e f o r m i n g o r c o k i n g , c o n t r o l o f which can r e s u l t i n a s m a l l l o s s o f steam r e f o r m i n g a c t i v i t y coupled t o a marked d i m i n u t i o n o f c o k i n g . D i f f e r e n t methods o f ensemble s i z e c o n t r o l a r e discussed. The k i n e t i c s o f n i c k e l and alumina s i n t e r i n g have a l s o been s t u d i e d and t h e e f f e c t o f d i f f e r e n t s t a b i l i s e r s e x p l o r e d . Some p o s s i b l e s i n t e r i n g processes can be r e j e c t e d , l e a d i n g t o a b e t t e r d e f i n i t i o n o f f a c t o r s i m p o r t a n t t o t h e process. INTRODUCTION The i n c r e a s e d i n d u s t r i a l demand f o r hydrogen o v e r t h e l a s t few y e a r s has r e s u l t e d l a r g e l y f r o m t h e use o f t h e gas i n t h e h y d r o p r o c e s s i n g o f heavy o i l s and i n t h e p r o d u c t i o n of ammonia and methanol.

The means o f p r o d u c i n g hydrogen

i s w e l l e s t a b l i s h e d , w i t h t h e v e r s a t i l e steam r e f o r m i n g r e a c t i o n b e i n g a p p l i e d w i d e l y on an i n d u s t r i a l s c a l e [1,2].

The r e a c t i o n can be used t o produce meth-

ane o r hydrogen f r o m l i g h t naphtha o r hydrogen f r o m methane

CnH2n+2

+ (T)H20 n-1

=

(-)CH4 3n+l

CH4

+ H20

=

co

co

+ H20

=

co2

+ (fl)C02 4

( f o r n > 1)

+ 3H2

+

H2

(4)

As a r e s u l t o f unwanted s i d e r e a c t i o n s ( s e e below), t h e p r o d u c t i o n o f hydrogen from naphtha i s u s u a l l y c a r r i e d o u t i n two stages, c o n d i t i o n s i n t h e f i r s t s t a g e b e i n g a d j u s t e d t o produce m a i n l y methane which i s c o n v e r t e d t o hydrogen i n a second r e a c t o r . Under i n d u s t r i a l c o n d i t i o n s , t h e r e a c t i o n proceeds t o n e a r thermodynamic e q u i l i b r i u m , and t h e c o m p o s i t i o n o f t h e p r o d u c t s t r e a m can be determined f r o m

40

knowledge of t h e e q u i l i b r i u m constants.

The dependence o f these on temperature

i s represented i n Figure 1. from which i t can be seen t h a t t h e p r o d u c t i o n o f methane i s favoured a t low temperature w h i l e t h e p r o d u c t i o n o f hydrogen i s favoured a t h i g h temperatures.

O v e r a l l , t h e r e a c t i o n i s endothermic, and i t i s

necessary t o supply heat t o t h e r e a c t o r .

12 114 116 1051 K1

1:O

Kp (5OOOC)

-AHo(5OOoC)kca~/mo~

1.

co

2.

CH4

+

+ H20

=

C02 + H2

H20

=

CO

+ 2H20

+

3H2

8.88

4.98

-53.07

9.89

-89.11

6.24

3.

C2H6

=

2CO + 5H2

4.

n-C4H10 + 4H20 =

4CO + 9H2

-165.7

1.21

5.

n-C7H16

7CO

-280.5

5.76

+

7H20 =

+

15H2

. .

5

10

2 atm 4 atm 8 atm

. 10l1 a d 4

E q u i l i b r i u m constants f o r reforming r e a c t i o n s .

Figure 1.

Depending on t h e product spectrum required, o p e r a t i n g c o n d i t i o n s a r e adjusted

-

t h e two important v a r i a b l e s being t h e r e a c t o r temperature and t h e

steam : carbon r a t i o .

Some i n d i c a t i o n o f t h e d i f f e r e n t c o n d i t i o n s r e q u i r e d t o

produce gas f o r d i f f e r e n t a p p l i c a t i o n s i s given i n Figure 2. Although c o n t r o l l e d by thermodynamics, t h e r e a c t i o n s a r e n o t n e c e s s a r i l y r a p i d and a c a t a l y s t i s used i n o r d e r t o a t t a i n e q u i l i b r i u m .

Metallic catalysts

promote t h e r e a c t i o n , t h e a c t i v i t y p a t t e r n being [l] Rh,Ru > Ni,Pd,Pt

> Re > Co

Cost e f f i c i e n c y d i c t a t e s t h e use o f supported n i c k e l i n i n d u s t r i a l s i t u a tions.

The nature o f t h e support i s important and t h e b a s i s o f s e l e c t i o n i s

d i scus sed be1ow.

41

d

600 700 TEMP. ('C)

000

900

Figure 2. Typical reforming conditions for various applications. The biggest problems with the system arise from the necessary presence of steam and the high temperatures needed to produce hydrogen. Steam accelerates many solid state reactions, including those involved in catalyst sintering and those producing interactions between catalyst and support. High temperatures favour these reactions and favour the production of coke, a major deactivating foulant produced by the reaction. These problems, and some of the solutions that have been developed, are dealt with individually. Coke formation and removal The coking of steam reforming catalysts is a well recognised problem which has received much attention [3,4,5,6]. At high temperatures coke deposition may result from reactions on the catalyst or in the gas phase [7], although the latter are less common in the steam reforming situation. The four most important catalytic reactions producing carbon may be represented by the equations

CO + H2

=

C + H20

(6)

2co

=

c + co2

(7)

CH4

=

C + 2H2

(8)

and the irreversible formation of coke from higher hydrocarbons CnHm

+

polymers

+.

coke + H2

(9)

The thermodynamic equilibria for the first three reactions are shown in Figure 3, from which it can be seen that carbon formation via catalysed or uncatalysed pyrolysis is the major problem at high temperatures.

42

TEMP. xlW(K1 F i g u r e 3.

E q u i l i b r i a f o r coking/de-coking reactions.

Carbon removal by t h e r e v e r s e o f r e a c t i o n s 6, 7 and 8 i s p o s s i b l e , and o p e r a t i n g c o n d i t i o n s a r e g e n e r a l l y a d j u s t e d t o ensure’ t h a t t h e f e e d and p r o d u c t gas c o m p o s i t i o n s a r e f a r f r o m v a l u e s t h a t , thermodynamically, would f a v o u r However, t h e approach t o e q u i l i carbon f o r m a t i o n ( c r i t i c a l carbon l i m i t ) [l]. b r i u m i s k i n e t i c a l l y c o n t r o l l e d and, depending on t h e f e e d and on l o c a l c o n d i t i o n s i n t h e r e a c t o r , coke f o r m a t i o n can and does o c c u r [1,3,4]. The n a t u r e o f t h e coke and t h e k i n e t i c s and mechanism o f coke f o r m a t i o n and Minimisation o f coking r e q u i r e s

removal have been s t u d i e d i n some d e t a i l [3-71.

m i n i m a l coke f o r m a t i o n and maximal coke removal

-

b y g a s i f i c a t i o n o f carbon o r

o f i n t e r m e d i a t e s which can l e a d t o carbon. Steam r e f o r m i n g o f methane proceeds t h r o u g h adsorbed s p e c i e s produced by dehydrogenation.

As seen i n F i g u r e 4, r e a c t i o n o f adsorbed s p e c i e s w i t h w a t e r

competes w i t h f u r t h e r dehydrogenation t o produce coke.

F i g u r e 4.

P o s t u l a t e d r e a c t i o n mechanism o f methane steam reforming.

Not a l l o f t h e processes a r e r e p r e s e n t e d i n F i g u r e 4.

Water i s b e l i e v e d t o

r e a c t i n t h e adsorbed s t a t e and carbon, produced by dehydrogenation, i s n o t a

43

I n f a c t , a t l e a s t f o u r forms o f carbon have been d e t e c t e d on

s i m p l e species.

The s p e c i e s o r i g i n a l l y produced has been d e s i g n a t e d a-carbon

t h e s u r f a c e [6,8].

and i s b e l i e v e d t o be a t o m i c carbon [9,10].

I t i s v e r y r e a c t i v e , and i s known

t o be an i m p o r t a n t i n t e r m e d i a t e i n r e a c t i o n s such as m e t h a n a t i o n o r F i s c h e r Tropsch [6,8]. reaction.

I t may w e l l be a m a j o r i n t e r m e d i a t e i n t h e steam r e f o r m i n g

I s o m e r i s a t i o n o f a- t o B-carbon (amorphous and l e s s r e a c t i v e ) can

o c c u r on t h e s u r f a c e i f t h e r e s i d e n c e t i m e i s s i g n i f i c a n t [8].

Isomerisation/

p o l y m e r i s a t i o n o f a- o r B-carbon t o c r y s t a l l i n e carbon can a l s o occur.

Finally,

d i f f e r e n t forms o f carbon may d i s s o l v e i n t h e metal t o f o r m metal c a r b i d e s [3,4]. B u i l d up o f carbon on t h e s u r f a c e can n e c e s s i t a t e replacement o f t h e catalyst.

C r y s t a l l i n e carbon encapsulates n i c k e l and d e a c t i v a t e s t h e c a t a l y s t .

However, carbon may a l s o d i s s o l v e i n n i c k e l and r e p r e c i p i t a t e a t a g r a i n boundary, r e s u l t i n g i n a n i c k e l p a r t i c l e b e i n g r a i s e d on a column o f carbon The c h a r a c t e r i s t i c w h i s k e r carbon produced i n t h i s way ( F i g u r e 5 ) [3,4]. b l o c k s t h e r e a c t o r and causes h i g h p r e s s u r e d r o p w i t h o u t , n e c e s s a r i l y , affecting catalytic activity.

This r e s u l t s from t h e f a c t t h a t t h e n i c k e l

p a r t i c l e on t h e t i p o f t h e carbon w h i s k e r remains a c c e s s i b l e t o t h e gas and c o n t i n u e s t o promote steam r e f o r m i n g . Coke g a s i f i c a t i o n by hydrogen, steam o r carbon d i o x i d e i s promoted by n i c k e l [ll] and by a l k a l i n e s a l t s [1,2,12].

As a r e s u l t , t h e c a t a l y s t s u p p o r t

i s u s u a l l y a l k a l i n e , w i t h d i f f e r e n t m a n u f a c t u r e r s r e l y i n g on potassium s a l t s o r on magnesia t o a c c e l e r a t e g a s i f i c a t i o n [1,2,12]. B-carbon, w h i s k e r carbon o r e n c a p s u l a t i n g carbon a r e much h a r d e r t o g a s i f y t h a n a-carbon o r CHx adsorbed i n t e r m e d i a t e s l e a d i n g t o a-carbon.

As a r e s u l t ,

a t t e n t i o n has been focussed on c o n t r o l o f c o k i n g b y c o n t r o l l i n g t h e dehydrogena-

tion/isomerisation/gasification o f adsorbed species. The b a s i s o f c o n t r o l can be e x p l a i n e d i n terms o f a s i m p l i f i e d p i c t u r e o f t h e processes o c c u r r i n g on t h e s u r f a c e .

I n s i m p l e terms, a d s o r p t i o n o f methane

t o produce a-carbon r e q u i r e s 8 a d j a c e n t s i t e s : CH4

+ 8x

=

C + xxxx

4H X

Again, i n s i m p l e f o r m a t , a d s o r p t i o n o f w a t e r r e q u i r e s 4 s i t e s : H20 +

4x

=

0 + xx

2H X

I n t h e s e s i m p l e terms, steam r e f o r m i n g t h e n r e q u i r e s 12 s i t e s a d j a c e n t t o each other:

C + 0 = xxxx x x 6H X

=

3H2

CO + 6x

+ 6x

44

a NySilica

- Sample A

N w l a s s - Sample A

-Sample

Nj/Glass - Sample B

X3000

b N-ilica

B

X3000

X990 Figure 5.

X5620

X2360

r

X5 8 5

Coked and uncoked nickel steam reforming c a t a l y s t s

45

The f o r m a t i o n o f forms o f carbon o t h e r t h a n t h e a f o r m r e q u i r e s e i t h e r a t l e a s t 16 a d j a c e n t s i t e s o r t h e a b i l i t y o f a-carbon t o m i g r a t e o v e r t h e s u r f a c e CH4

+ 8x

=

c

xxxx

2 ( o r more) C = xxxx multiple

+ 4H x

6 carbon

C + i n i g r a t i o n t o nucleus xxxx j.

i s o m e r i s a t i o n t o o t h e r forms o f coke Even f r o m t h i s s i m p l i f i e d p i c t u r e , i t i s o b v i o u s t h a t steam r e f o r m i n g r e q u i r e s fewer a d j a c e n t s i t e s t h a n coke f o r m a t i o n .

As a r e s u l t , i f t h e number

o f s i t e s i n an ensemble i s c o n t r o l l e d and i f m i g r a t i o n across t h e s u r f a c e i s l i m i t e d , t h e n steam r e f o r m i n g s h o u l d be promoted a t t h e expense o f c o k i n g . T h i s concept was f i r s t u t i l i s e d by R o s t r u p - N i e l s e n [13,14] number o f a c t i v e s i t e s i n an ensemble by t h e use o f s u l p h u r .

who l i m i t e d t h e L a r g e r amounts o f

s u l p h u r c o n t a i n i n g hydrocarbons i n t h e f e e d l e a d s t o t h e f o r m a t i o n o f b u l k metal s u l p h i d e s and t o c a t a l y s t p o i s o n i n g . n i c k e l as a r e g u l a r a r r a y [ 1 5 ] , d e l i n e a t i n g defined size.

Small amounts o f s u l p h u r adsorbs on ensembles o f c l e a n n i c k e l o f a w e l l

Steam r e f o r m i n g on such c a t a l y s t s would t h e n be expected t o

proceed w i t h m i n i m a l coking, p r o v i d e d t h a t t h e ensembles a r e b i g enough t o a l l o w r e f o r m i n g b u t s m a l l enough t o l i m i t c o k i n g . [13,14],

with

T h i s was f o u n d t o be t h e case

> 5 ppm H2S i n t h e f e e d b e i n g s u f f i c i e n t t o a l l o w steam r e f o r m -

i n g b u t t o m i n i m i s e coke f o r m a t i o n .

The o v e r a l l c o n v e r s i o n o f methane was

reduced ( f r o m 100% t o 64% a t ca 950OC) as a r e s u l t o f t h e f a c t t h a t some s u r f a c e

N i atoms were poisoned by s u l p h u r , b u t c o k i n g was reduced d r a m a t i c a l l y and, a f t e r much l o n g e r on l i n e , t h e s t r u c t u r e o f any coke t h a t was formed was d i f f e r e n t ( " o c t o p u s " carbon [14]).

A t h r e s h o l d coverage o f about 70% o f t h e n i c k e l

was found t o be s u f f i c i e n t t o s t o p c o k i n g [13,14]. I n a d d i t i o n t o l i m i t i n g ensemble s i z e , s u l p h u r may a l s o a c t t o r e s t r i c t m i g r a t i o n o f a-carbon across t h e s u r f a c e .

Surface d i f f u s i o n involves l o o s e l y

bound chemisorbed s p e c i e s [16] and a change f r o m n i c k e l t o n i c k e l s u l p h i d e a t t h e edge o f an ensemble may be s u f f i c i e n t t o l i m i t s u r f a c e m i g r a t i o n . A l t h o u g h s u l p h u r d o p i n g has many advantages, i t has t h e d i s a d v a n t a g e t h a t t h e amount o f adsorbed s u l p h u r depends on an e q u i l i b r i u m w i t h t h e gas phase NiS

+ 1i2

=

Ni

+ H2S

As a r e s u l t , s u f f i c i e n t s u l p h u r must be m a i n t a i n e d i n t h e f e e d t o ensure f o r m a t i o n o f t h e NiS a r r a y on t h e s u r f a c e .

The obvious n e x t s t e p i s t o t r y t o

a t t a i n t h e same r e s u l t w i t h a permanent dopant. T h i s p r e s e n t s some d i f f i c u l t y , i n t h a t a r e g u l a r a r r a y o f a second component i s h a r d t o achieve.

Assuming t h a t a second component moves t o t h e s u r f a c e t o

46

form a random network w i t h t h e n i c k e l , t h e n i t s h o u l d be p o s s i b l e t o add s u f f i c i e n t o f t h e second component t h a t t h e n i c k e l a r e a l e f t between c l u s t e r s o f t h e a d d i t i v e may be o f t h e r i g h t s i z e t o f a v o u r steam r e f o r m i n g and t o s t o p coking.

Experiments w i t h Ni-Cu m i x t u r e s show t h a t t h i s can be achieved [17],

b u t a t t h e expense o f a c t i v i t y .

Copper i s n o t a good steam r e f o r m i n g c a t a l y s t

and i t was found necessary t o add ca.80 atom % i n o r d e r t o produce octopus carbon [17].

T h i s r e s u l t s p r o b a b l y f r o m t h e f a c t t h a t a l t h o u g h Cu aggregates

on t h e s u r f a c e o f t h e a l l o y , i t i s necessary t o add a l o t o f t h e second metal t o c r e a t e , by random arrangement, ensembles o f n i c k e l o f t h e c o r r e c t s i z e . n e t r e s u l t i s t h a t c o k i n g i s m i n i m i s e d b u t t h a t steam r e f o r m i n g i s a l s o l o w as a r e s u l t o f t h e l o w s u r f a c e area o f exposed n i c k e l [17]. F u r t h e r e x t e n s i o n o f t h e concept seems p o s s i b l e , based on two i d e a s .

The

-

Firstly,

i t i s known t h a t rhodium and r u t h e n i u m a r e b e t t e r steam r e f o r m i n g c a t a l y s t s

t h a n n i c k e l [l].I t c o u l d be p o s s i b l e t o dope n i c k e l w i t h a p r e c i o u s metal t o t h e e x t e n t t h a t ensemble s i z e c o n t r o l i s achieved u s i n g an a d d i t i v e which i s a c t i v e f o r steam r e f o r m i n g i n i t s own r i g h t .

I n t h i s case, b o t h a c t i v i t y and

s e l e c t i v i t y s h o u l d be h i g h . Secondly, i t can be expected t h a t some coke w i l l be formed even on t h o s e c a t a l y s t s i n which t h e ensemble s i z e i s c o n t r o l l e d . e v e n t u a l l y , been observed on doped n i c k e l [17], e n t f r o m d e p o s i t s formed on n i c k e l .

Coke d e p o s i t s have,

albeit with a structure differ-

As a r e s u l t , i t c o u l d be argued t h a t

ensemble s i z e c o n t r o l s h o u l d b e s t be e f f e c t e d u s i n g a second component t h a t f a v o u r s g a s i f i c a t i o n o f coke o r coke f o r m i n g i n t e r m e d i a t e s .

M e t a l s such as P t

o r I r a r e known t o c a t a l y s e coke g a s i f i c a t i o n [18] and a r e o b v i o u s c a n d i d a t e s . Some r e l a t e d work has been c a r r i e d o u t w i t h N i - P t / S i 0 2 c a t a l y s t s used t o promote methanation.

C a r e f u l s t u d i e s o f t h e p r e p a r a t i o n and c h a r a c t e r i s a t i o n

o f t h e c a t a l y s t s were completed [19] and t h e c o m p o s i t i o n and s t r u c t u r e o f t h e Pure p l a t i n u m was found t o s u p p o r t e d a l l o y s were r e l a t e d t o performance [20]. be l e s s a c t i v e t h a n p u r e n i c k e l , w i t h t h e a c t i v i t i e s o f t h e a l l o y s f a l l i n g between t h e two extremes.

Carbon f o r m a t i o n was reduced by a l l o y i n g , b u t t h i s

appeared t o r e s u l t more f r o m p a r t i c l e s i z e c o n t r o l t h a n f r o m a c c e l e r a t e d gasi f i c a t i o n promoted by p l a t i n u m . c a r r i e d o u t a t ca.600-800

T h i s i s n o t t o o s u r p r i s i n g , as m e t h a n a t i o n i s

K, below t h e t e m p e r a t u r e a t which t h e p r e c i o u s m e t a l

can be expected t o have a s i g n i f i c a n t e f f e c t on coke removal [18].

Further

work on s i m i l a r systems used f o r steam r e f o r m i n g i s i n p r o g r e s s t o p r o v e o r d i s p r o v e t h e concept.

3 Heterogeneous c a t a l y s i s r e 1 i e s on f l u i d - s o l i d c o n t a c t , and c a t a l y s t s a r e prepared w i t h as h i g h as p o s s i b l e s u r f a c e a r e a i n o r d e r t o maximise such contact.

T h i s s t r u c t u r e i s thermodynamically u n s t a b l e and, g i v e n s u f f i c i e n t

a c t i v a t i o n energy, c a t a l y s t s w i l l r e o r g a n i s e towards s t r u c t u r e s o f minimal surface energy.

T h i s process, r e s u l t i n g i n l o s s o f s u r f a c e a r e a and p o r o s i t y ,

proceeds g e n e r a l l y t h r o u g h s u r f a c e d i f f u s i o n , volume d i f f u s i o n o r phase change c211. V a r i o u s s o l i d s t a t e r e a c t i o n s a r e i n v o l v e d i n t h e process.

I n addition t o t h o s e g o v e r n i n g s i n t e r i n g , c a t a l y s t - s u p p o r t i n t e r a c t i o n s may a l t e r t h e n a t u r e and c a t a l y t i c a c t i v i t y o f t h e s o l i d and may s t a b i l i s e o r d e s t a b i l i s e t h e s o l i d towards s i n t e r i n g . Thus, f o r example, t h e f o r m a t i o n o f n i c k e l a l u m i n a t e , NiA1204, i s w e l l e s t a b l i s h e d i n steam r e f o r m i n g c a t a l y s t s [21,22], and t h i s compound i s c a t a l y t i c a l l y i n a c t i v e .

However,

i t s presence may a f f e c t t h e

thermal s t a b i l i t y o f t h e s o l i d [23],

as i s t h e case i n cobalt-molybdenum and

nickel-molybdenum based c a t a l y s t s s u p p o r t e d on alumina and used f o r hydrot r e a t i n g [24]. The necessary presence o f steam i s disadvantageous, many s o l i d s t a t e r e a c t i o n s [21].

i n t h a t steam a c c e l e r a t e s

I n t h e presence o f steam, t h e g e n e r a l r u l e

t h a t s i n t e r i n g becomes s i g n i f i c a n t a t c a 1/3 t o 1/2 o f t h e m e l t i n g p o i n t o f t h e s o l i d [21] may i n d i c a t e o n l y an upper l i m i t f o r t h e appearance o f s i n t e r i n g .

25 TIME

0

F i g u r e 6.

so

75

Ihl

S u r f a c e a r e a changes as a f u n c t i o n o f steam a t 95OOC. Closed p o i n t s = y alumina, Open p o i n t s = Ni/A1203

*

o

p u r e oxygen

0 H2/5%H20

O r i g i n a l s u r f a c e areas : A1203 Ni/A1203

A

H2/10%H20

=

95 m2g-1

=

85 m2g-1

48

160

2 120 \

u

x

a 0

0

25

50 I5 PORE RADIUS i d ]

100

125

Figure 7. Pore size distribution in y-alumina. o reduced at 350°C for 24h a heated in 955 H2 : 5% H20 for 72h at 950°C * heated in H2 for 72h at 950°C

160

5" 120 v

-t

0

5 80

CL

'p

z 40 \

0

Figure 8.

25

50

PORE RADIUS

I5

(A)

100

125

Pore size distribution in steam reforming catalysts. o reduced at 350°C for 24h * heated in H2 for 72h at 95OOC a heated in 95% H2 : 5% H20 for 72h at 950°C

The sintering of steam reforming catalysts and possible preventive measures havebeen the subject of only limited attention in the literature, largely because the changes occur only over long periods of time. There is good

49

evidence that both nickel and steam accelerates sintering, the process leading to loss of surface area and porosity. This is seen clearly from Figures 6, 7 and 8. Surface area during the initial stages of heating decreases rapidly. Ni/A1203 sinters more rapidly than alumina even in hydrogen (Figure 6). In steam, the rate of sintering of both solids is higher, with nickel having a smaller relative effect on the stability of the solids (Figure 6). These changes were found to be accompanied by a collapse of pore structure (Figures 7 and 8) and by conversion of y-alumina to the 6 , 0 and, eventually, a-phase of a1 umi na. Once these initial stages of sintering are complete the catalyst becomes more stable. Several workers [22,23,25,26] find that metal surface area and total surface area stabilises after ca 60h on line, with the nickel crystallite size After 800h of remaining relatively constant after this time [22,23,25]. sintering, the total surface area was found to decrease substantially, but the nickel crystallite size remained the same [22]. Interestingly, Doesburg et a1 [22] find that although nickel aluminate is produced in the catalyst, the system reverts to alumina surrounded by nickel on reduction at high temperatures. Limited attention has been focussedon the possibility of stabilising alumina under steam reforming conditions. Alumina sintering is also a problem in car exhaust catalysts, which are designed to operate up to about 1300 K [27,28]. In this case, some stabilisation of support is achieved by the addition of small amounts of rare earth oxides (and, in particular, baria), compounds which have been suggested to migrate to vacancies in the alumina lattice and to eliminate defects which accelerate sintering. Following this argument, it was suggested that steam could adsorb to produce small amounts of oxygen which could increase the number of cation vacancies in the trivalent aluminium lattice. The increase in vacancy concentration would then lead to enhanced sintering. On this basis, the addition of small amounts of multivalent ions would then be expected to decrease vacancies and decrease sintering. In fact, the addition of traces of ions of different valency was found to have an accelerating effect on sintering [29] and other mechanisms have to be considered. Sintering of steam reforming catalysts remains a problem, but one that produces only slow changes in the catalyst. As a result, industrial application of the catalyst is possible, but with increased life being expected from better understanding and control of the sintering process. ACKNOWLEDGEMENTS Figures 1 and 2 are reprinted from reference 1 by kind permission of Haldor Topsoe A/S.

50

REFERENCES 1 J.R. Rostrup-Nielsen, "Steam Reforming Catalysts", Teknisk Forlag A/S, Copenhagen (1975). Catalyst Handbook, Wolfe Scientific Texts, London (1970). 2 3 J.R. Rostrup-Nielsen and D.L. Trimm, J. Catal., 24 (1977) 352. D.L. Trimm, Catal. Rev. Sci. Eng., 16 (1977) 155. 4 5 D.L. T r i m , Appl. Catal. 5 (1983) 263. 6 J.G. McCarty and H. Wise, J. Catal. 57 (1979) 406. 7 D.L. Trimm, Cnem. Eng. Process. 18 (9984) 137. 8 S.M. Davis, F. Zaera and G. Somorjai, J. Catal. 77 (1982) 439. 9 A.B. Anderson, 3. Amer. Chem. SOC. 99 (1977) 696. 10 B. Kneale and J.R.H. Ross, Farad. Trans. I, 79 (1983) 157. 11 C.A. Bernardo and D.L. Trimm, Carbon 17 (1979) 115. 12 J.R.H. ROSS, Spec. Rep. Roy. SOC. Chem., Surface and Defect Props. of Solids, 4 (1975) 34. 13 J.R. Rostrup-Nielsen and L.J. Christiansen, Proc. 6" Simposio IberoAmerican0 de Catalyse (Rio de Janiero 1978), pp 1615, Instituto Brasileiro de Petroleo (1981). 14 J.R. Rostrup-Nielsen, J. Catal. 85 (1984) 31. 15 J.J. McCarroll , Surface Sci . 53 (1975) 297. 16 D. Briggs, J. Dewing, A.G. Burden, R. Moyes and S.P.B. Wills, J. Catal. 65

(1980) 31. 17 C.A. Bernardo, I. Alstrup and J.R. Rostrup-Nielsen, J. Catal. 96 (1985) 517. 18 R.T. Rewick, P.R. Wentrcek and H. Wise, Fuel 53 (1974) 274. 19 A.F.H. Wielers, G.L. Zwolsman, G.U.G. Van der Grift and J.W. Geus, Appl. Catal. 19 (1985) 187. 20 P.C.M. Van Stiphout and J.W. Geus, Appl. Catal. 25 (1986) 19. 21 D.L. Trimm, Design of Industrial Catalysts, Elsevier, NV (1980). 22 E.B.M. Doesburg, P.H.M. de Korte, H. Schaper and L.L. van Reijen, Appl. Catal. 11 (1984) 155. 23 C.H. Bartholomew, and W.L. Sorensen, J. Catal. 81 (1983) 131. 24 A. Stanislaus, J. Catal. in press (1986). 25 A. Williams, G.A. Butler and J. Hamonds, J . Catal. 24 (1972) 352. 26 C.H. Ba,rtholomew, R.B. Pannell and R.W. Fowler, J. Catal. 79 (1983) 34. 27 M. Kotter, L. Riekert and F. Weyland, Verfahrenstechnik 17 (1983) 607. 28 B.R. Powell, Materials Research Society Annual Meeting, Boston, Nov. 16-21, 1980, paper H9. 29 P. Udaja, M.Sc Thesis, University of New South Wales, (1987).

D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

51

THE MECHANISM FOR THE DISSOCIATION OF METHANE ON A NICKEL CATALYST

S.T.

CEYER,

Q.Y.

YANG, M.B.

LEE, J.D.

BECKERLE and A.D.

JOHNSON

Department o f Chemistry, Massachusetts I n s t i t u t e o f Technology, Cambridge, Massachusetts 02139 USA

ABSTRACT The mechanism f o r t h e d i s s o c i a t i o n o f CH4 on N i ( l l 1 ) i s s t u d i e d by m o l e c u l a r beam t e c h n i q u e s coupled w i th h i g h r e s o l u t i o n e l e c t r o n energy l o s s spectroscopy. The p r o b a b i l i t y o f t h e d i s s o c i a t i v e c h e m i s o r p t i o n o f CH4 i n c r e a s e s e x p o n e n t i a l l y w i t h t h e normal component o f t h e i n c i d e n t m o l e c u l e ' s t r a n s l a t i o n a l energy and w i t h v i b r a t i o n a l e x c i t a t i o n . D i s s o c i a t i o n can a l s o be induced by t h e i m p a c t o f an A r atom i n c i d e n t on a monolayer of CH4 p h y s i s o r b e d on N i ( l l 1 ) . The nascent p r o d u c t s o f t h e d i s s o c i a t i o n a r e i d e n t i f i e d as an adsorbed methyl r a d i c a l and a hydrogen atom. The c h e m i s t r y and s t a b i l i t y o f t h e s e adsorbed methyl r a d i c a l s have a l s o been studied. These r e s u l t s , which have shown t h a t t h e r e i s a b a r r i e r t o t h e d i s s o c i a t i v e c h e m i s o r p t i o n , a r e i n t e r p r e t e d i n terms o f a d e f o r m a t i o n model f o r t h e r o l e o f t r a n s l a t i o n a l and v i b r a t i o n a l energy i n p r o m o t i n g d i s s o c i a t i v e chemisorption. The b a r r i e r a r i s e s l a r g e l y f r o m t h e energy r e q u i r e d t o deform t h e molecule s u f f i c i e n t l y t o a l l o w a s t r o n g a t t r a c t i v e i n t e r a c t i o n between t h e carbon and t h e N i surface. T u n n e l i n g i s suggested as t h e f i n a l process i n t h e C-H bond cleavage. The presence o f t h i s b a r r i e r t o d i s s o c i a t i v e c h e m i s o r p t i o n and c o l l i s i o n - i n d u c e d d i s s o c i a t i o n o f adsorbates p r e s e n t p l a u s i b l e e x p l a n a t i o n s f o r t h e p r e s s u r e gap i n heterogeneous c a t a l y s i s . INTRODUCTION The c h e m i s t r y o f methane i s p r e s e n t l y o f g r e a t i n t e r e s t .

This i n t e r e s t i s

f u e l e d by t h e d e s i r e t o make more e f f i c i e n t commercial use o f an abundant w o r l d w i d e r e s e r v e o f n a t u r a l gas, o f which methane i s t h e m a j o r c o n s t i t u e n t , f o r t h e p r o d u c t i o n o f more complex hydrocarbons.

P r e s e n t l y , h i g h e r hydro-

carbons a r e produced f r o m n a t u r a l gas i n a s e r i e s o f s e v e r a l steps. The f i r s t s t e p o f each s y n t h e t i c scheme i s t h e steam r e f o r m i n g o f n a t u r a l gas ( r e f . l), w h i c h i s t h e r e a c t i o n o f methane and w a t e r o v e r a N i metal supported c a t a l y s t t o f o r m carbon monoxide and hydrogen o r syn gas.

Since t h e goal i s t o produce

carbon-carbon bonded species, i t seems h i g h l y i n e f f i c i e n t t o f i r s t almost c o m p l e t e l y o x i d i z e t h e s t a r t i n g m a t e r i a l , methane, and t h e n reduce i t back t o C-H bonded species.

I d e a l l y , one d e s i r e s a c a t a l y s t t h a t cleaves one o r two of

t h e C-H bonds i n methane, l e a v i n g t h e r e s u l t i n g CH2 o r CH3 s p e c i e s t o recombine and desorb as h i g h e r hydrocarbons.

How t o e f f e c t t h i s k i n d o f c h e m i s t r y i s

c e r t a i n l y t h e goal o f many a r e s e a r c h p r o j e c t . One approach t o a r a t i o n a l d e s i g n o f such a c a t a l y s t i s t o seek i n f o r m a t i o n a b o u t t h e a d s o r p t i o n o f CH4 and t h e c h e m i s t r y o f i t s adsorbed fragments

52

p r o v i d e d by t h e p r o d i g i o u s f i e l d o f u l t r a h i g h vacuum s u r f a c e science.

However,

one q u i c k l y d i s c o v e r s t h a t l i t t l e i n f o r m a t i o n i s a v a i l a b l e from s u r f a c e s c i e n c e because no d i s s o c i a t i o n o f methane i s observed t o occur under t h e UHV s u r f a c e science conditions.

T h e r e i n l i e s an apparent c o n t r a d i c t i o n .

Methane appears

t o d i s s o c i a t i v e l y adsorb a t t h e h i g h pressures p r e s e n t under t h e c a t a l y t i c c o n d i t i o n s o f steam r e f o r m i n g b u t i s n o t observed t o d i s s o c i a t e under t h e low p r e s s u r e c o n d i t i o n s p r e s e n t i n an u l t r a h i g h vacuum s u r f a c e science experiment. How can we understand t h e d i s p a r i t y i n t h e r e a c t i v i t y o f methane

on a N i

c a t a l y s t surface? The answer t o t h i s q u e s t i o n can be e x t r a c t e d from an experiment performed by Bootsma and co-workers ( r e f . 2 ) .

They measured t h e amount o f methane exposure

necessary t o produce a monolayer o f carbon on s e v e r a l n i c k e l s i n g l e c r y s t a l c a t a l y s t s when t h e methane gas was p r e s e n t above t h e c a t a l y s t a t h i g h p r e s s u r e s

(>10-2 t o r r ) .

From t h a t measurement, t h e y c a l c u l a t e d a v a l u e f o r t h e p r o b a b i l -

i t y f o r d i s s o c i a t i v e c h e m i s o r p t i o n o f 10-9.

T h i s e x t r e m e l y small v a l u e f o r t h e

d i s s o c i a t i o n p r o b a b i l i t y immediately suggests why h i g h p r e s s u r e s o f methane a r e necessary f o r t h e o b s e r v a t i o n o f d i s s o c i a t i o n .

S i n c e t h e d i s s o c i a t i o n proba-

b i l i t y i s so low, t h e a b s o l u t e f l u x o f i n c i d e n t m o l e c u l e s must be l a r g e i n o r d e r f o r t h e d i s s o c i a t i o n r a t e t o be h i g h enough f o r carbon d e p o s i t i o n t o be o b s e r v a b l e i n a reasonable amount o f time.

The n e x t q u e s t i o n i s why i s t h e

d i s s o c i a t i o n p r o b a b i l i t y so low?

A p o s s i b l e e x p l a n a t i o n f o r t h e low d i s s o c i a t i o n p r o b a b i l i t y l i e s i n t h e presence o f a b a r r i e r a l o n g t h e d i s s o c i a t i v e r e a c t i o n c o o r d i n a t e o f methane t o f o r m an adsorbed methyl r a d i c a l and an adsorbed hydrogen atom.

The energy

r e q u i r e d t o surmount t h i s b a r r i e r must be s u p p l i e d as t r a n s l a t i o n a l o r v i b r a t i o n a l energy i n t h e i n c i d e n t methane molecules.

The molecules i n t h e methane

gas above t h e c a t a l y s t have a Maxwell-Boltzmann d i s t r i b u t i o n o f e n e r g i e s and o n l y t h o s e molecules i n c i d e n t on t h e s u r f a c e w i t h e n e r g i e s above t h e b a r r i e r

w i l l d i s s o c i a t e upon a d s o r p t i o n .

I f t h e b a r r i e r i s v e r y l a r g e , o n l y a small

f r a c t i o n o f molecules i n c i d e n t on t h e s u r f a c e w i l l d i s s o c i a t e .

Therefore, t h e

l o w d i s s o c i a t i o n p r o b a b i l i t y i s determined l a r g e l y by t h e f r a c t i o n o f molecules i n t h e gas w i t h e n e r g i e s above t h e energy o f t h e b a r r i e r .

A t h i g h pressure,

t h e f r a c t i o n o f t h e s e h i g h energy molecules remains t h e same b u t t h e a b s o l u t e number o f molecules w i t h s u f f i c i e n t energy i s g r e a t l y i n c r e a s e d and, t h e r e f o r e , t h e r a t e o f d i s s o c i a t i o n i s l a r g e enough f o r d i s s o c i a t i o n t o be observable. I f t h i s h y p o t h e s i s f o r t h e o r i g i n o f t h e low p r o b a b i l i t y f o r d i s s o c i a t i o n i s

c o r r e c t , t h e n t h e h i g h p r e s s u r e requirement f o r o b s e r v a t i o n o f d i s s o c i a t i o n can be bypassed by r a i s i n g t h e energy o f t h e methane gas i n c i d e n t on t h e surface. I n o r d e r t o v e r i f y t h e e x i s t e n c e o f t h i s b a r r i e r t o d i s s o c i a t i o n , we have used m o l e c u l a r beam t e c h n i q u e s coupled w i t h u l t r a h i g h vacuum s u r f a c e e l e c t r o n spect r o s c o p i e s t o m o n i t o r t h e e x t e n t o f d i s s o c i a t i o n as a f u n c t i o n o f t h e t r a n s l a -

53

t i o n a l energy of t h e CH4 i n c i d e n t on a N i ( l l 1 ) c r y s t a l s u r f a c e a t low pressures

torr).

Supersonic m o l e c u l a r beam t e c h n i q u e s a l l o w t h e t r a n s l a -

t i o n a l energy and t h e d i r e c t i o n o f t h e incoming adsorbate t o be v a r i e d over a wide range and t h e v i b r a t i o n a l energy t o be v a r i e d o v e r a l i m i t e d range. r e s o l u t i o n e l e c t r o n energy l o s s spectroscopy,

High

a surface v i b r a t i o n a l spectros-

copy, enables t h e chemical i d e n t i f i c a t i o n o f t h e p r o d u c t o f t h e d i s s o c i a t i v e c h e m i s o r p t i o n event.

I f a b a r r i e r t o d i s s o c i a t i v e chemisorption i s present,

t h e i n t e n s i t y o f a v i b r a t i o n a l mode b e l o n g i n g t o t h e p r o d u c t o f t h e d i s s o c i a t i v e l y chemisorbed species w i l l have a sharp onset when t h e energy o f t h e i n c i d e n t m o l e c u l e j u s t exceeds t h e b a r r i e r .

The i n c i d e n t energy a t which t h i s

t h r e s h o l d occurs corresponds t o t h e h e i g h t o f t h e b a r r i e r . The r e s u l t s o f t h e s e experiments form a p i c t u r e o f t h e dominant f e a t u r e s o f t h e methane-nickel s u r f a c e i n t e r a c t i o n p o t e n t i a l t h a t c o n t r o l t h e mechanism o f t h e d i s s o c i a t i o n o f methane,

We w i l l f i n d t h a t t h e r e i s indeed a b a r r i e r t o

t h e d i s s o c i a t i v e c h e m i s o r p t i o n o f methane and t h a t t r a n s l a t i o n a l and v i b r a t i o n a l energy o f t h e i n c i d e n t methane molecule a r e e f f e c t i v e i n overcoming it. The i d e n t i f i c a t i o n o f t h i s b a r r i e r a l o n g t h e d i s s o c i a t i v e r e a c t i o n c o o r d i n a t e a l l o w s t h e e s t a b l i s h m e n t o f a l i n k between l o w pressure, u l t r a h i g h vacuum s u r f a c e s c i e n c e and h i g h p r e s s u r e c a t a l y s i s ( r e f . 3). EXPERIMENTAL The experiments a r e c a r r i e d o u t i n an u l t r a h i g h vacuum apparatus designed s p e c i f i c a l l y f o r measurements o f t h e d i s s o c i a t i v e a d s o r p t i o n p r o b a b i l i t y as a f u n c t i o n o f t h e energy o f t h e i n c i d e n t m o l e c u l e and f o r s p e c t r o s c o p i c i d e n t i f i c a t i o n o f t h e p r o d u c t o f t h e d i s s o c i a t i v e c h e m i s o r p t i o n event. o f t h e apparatus i s shown i n Fig. 1. p r e v i o u s l y ( r e f s . 4-6),

A schematic

Since i t has been discussed i n d e t a i l

o n l y t h e m a j o r f e a t u r e s w i l l be d e s c r i b e d here. The

apparatus c o n s i s t s o f a beam source chamber ( l a b e l e d E i n Fig. l ) , two d i f f e r e n t i a l pumping chambers (F,G) main chamber

(N).

f o r t h e m o l e c u l a r beam and an u l t r a h i g h vacuum

The supersonic m o l e c u l a r beam n o z z l e source ( A ) i s a l i g n e d

a x i a l l y w i t h t h e quadrupole mass spectrometer ( K ) ,

passes p r e c i s e l y t h r o u g h

t h e c e n t e r o f a 127' c y l i n d r i c a l d e f l e c t o r - h i g h r e s o l u t i o n e l e c t r o n energy l o s s spectrometer (H) ( r e f . 7) and d i r e c t l y i n f r o n t o f a s i n g l e pass c y l i n d r i c a l m i r r o r e l e c t r o s t a t i c energy a n a l y z e r o r CMA ( J ) . The beam source ( A ) c o n s i s t s o f two c o n c e n t r i c i n c o n e l tubes j o i n e d a t t h e n o z z l e end.

The n o z z l e

end and about 3 cm o f t h e i n n e r t u b e can be heated r e s i s t i v e l y t o 1100 K.

The

m o l e c u l a r beam o f methane i s t y p i c a l l y produced by an a d i a b a t i c expan-sion of a m i x t u r e o f 1%CH4 i n He a t 100 p s i from a n o z z l e whose diameter i s 25.4 p . T h i s t y p e o f m o l e c u l a r beam p r o v i d e s a convenient source o f h i g h k i n e t i c energy (bythe Commonwealth Department of National Development REFERENCES 1. W.L. Marsden, M.S. Wainwright, and J.B. Friedrich, I. and E.C. Product Research and Development, 19,(1980) 551. 2. J.B. Friedrich, M.S. Wainwright and D.J. Young, J. Catal. 80 (1983) 1. 3. J.B. Friedrich, M.S. Wainwright and D.J. Young, J. Catal. 80 (1983) 14. 4 . A.J. Bridgewater, M.S. Wainwright and D.J. Young, Appl. Catal. 7 (1983) 369. 5 . H.E. Curry-Hyde,D.J. Young and M.S. Wainwright, Appl. Catal. 29 (1987) (1987) 31. 6. A.D. Tomsett, H.E. Curry-Hyde, M.S. Wainwright and D.J. Young, Appl. Catal. (in press, 1987). 7. H.E. Curry-Hyde,Ph.D. thesis, University of New South Wales (1987). 8. A . J . Bridgewater, M.S. Wainwright and D.J. Young, Appl. Catal. 28 (1986) 241. 9. French Patent 1,489,682 (Dec. 12, 1970); assigned to Imperial Chemical Ind., Ltd. 10. Collins B.M., German Patent 2,302, German Patent 2,302,658 (Aug. 2 , 1973) assigned to Imperial Chemical Ind. Ltd.


D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion

245

0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

METHANOL CARBONYLATION TO A C E T I C A C I D WITH SUPPORTED METAL CATALYST

K. OMATA. K.

FUJIMOTO, H. YAGITA, H. M A Z A K I and H. TOMINAGA

Department o f S y n t h e t i c Chemistry, Tokyo, Hongo 7-3-1.

Bunkyo-ku,

F a c u l t y o f Engineering, The U n i v e r s i t y o f

Tokyo 113 (Japan)

ABSTRACT

The vapor phase c a r b o n y l a t i o n o f methanol t o a c e t i c a c i d w i t h a n i c k e l on a c t i v e carbon c a t a l y s t s was g r e a t l y enhanced b y a d d i t i o n o f hydrogen was i n c o r p o r a t e d i n t h e products. a l t h o u g h 1 i t t l e hydrogen

INTRODUCTION L i q u i d phase c a r b o n y l a t i o n o f methanol t o a c e t i c a c i d w i t h a rhodium complex c a t a l y s t i s a w e l l known process ( r e f .

1).

The a u t h o r s have found

t h a t g r o u p 8 m e t a l s supported on carbonaceous m a t e r i a l s e x h i b i t e x c e l l e n t activity for

t h e vapor phase c a r b o n y l a t i o n o f methanol i n t h e presence o f

iodide promoter(ref.

5).

Especially,

a n i c k e l on a c t i v e carbon c a t a l y s t

gave a c e t i c a c i d and m e t h y l a c e t a t e w i t h t h e s e l e c t i v i t y o f 95% o r h i g h e r a t 100% methanol

c o n v e r s i o n under 10 atm and 250

I n the present

OC.

s t u d y i t has been found t h a t a s m a l l amount o f hydrogen which i s always contained i n the commercially

a v a i l a b l e CO and r e q u i r e s much c o s t f o r

b e i n g removed c o m p l e t e l y , a c c e l e r a t e s

g r e a t l y t h e carbonylation reaction.

EX PER IMENTAL The e x p e r i m e n t s were conducted i n a f i x e d bed f l o w t y p e r e a c t o r under p r e s s u r i z e d c o n d i t i o n s as have been r e p o r t e d i n d e t a i l elsewhere ( r e f . Methanol (MeOH) and m e t h y l i o d i d e (MeI) were mixed and f e d w i t h a highp r e s s u r e microfeeder.

C a t a l y s t s were prepared b y i m p r e g n a t i n g a Takeda S h i r a s a g i C,

c o m m e r c i a l l y a v a i l a b l e g r a n u l a r a c t i v e carbon (A.C., 20-40 mesh) w i t h n i c k e l a c e t a t e and d r y i n g a t 120 oven. The

f o r 12 h i n an a i r

They were used w i t h o u t any f u r t h e r p r e t r e a t m e n t . c a r b o n y l a t e d p r o d u c t s were a c e t i c a c i d (AcdH) and m e t h y l

a c e t a t e (AcOMe). (CH4).

OC

S m a l l amounts o f d i m e t h y l e t h e r (DME),

and carbon d i o x i d e (C02) were a l s o formed.

methane

2).

246 RESULTS AND D I S C U S S I O N Effect o f support The a c t i v i t i e s o f n i c k e l c a t a l y s t s s u p p o r t e d o n a v a r i e t y o f c a r r i e r m a t e r i a l s were i n v e s t i g a t e d (Fig.

W h i l e t h e c a t a l y t i c a c t i v i t y o f N i was

1).

q u i t e l o w when i t was s u p p o r t e d o n Si02, A1203, and TiO2, t h a t o f N i s u p p o r t e d on carbonaceous c a r r i e r such as c a r b o n b l a c k o r a c t i v e c a r b o n was e x c e l l e n t , its

a c t i v i t y was a p p a r e n t l y p r o p o r t i o n a l t o t h e s p e c i f i c s u r f a c e a r e a o f t h e

carrier. the

I t c a n be c o n c l u d e d t h a t c a r b o n i s e s s e n t i a l f o r t h e appearance o f

carbonylation a c t i v i t y o f n i c k e l catalyst.

The r o l e o f carbonaceous

c a r r i e r i s most probably a t t r i b u t e d t o i t s e l e c t r o n donor-acceptor c h a r a c t e r s as d e m o n s t r a t e d i n Fig. 2.

__ Promotion

e f f e c t s o f hydrogen

(i)E f f e c t o f p a r t i a l p r e s s u r e o f hydroqen.

The n i c k e l on a c t i v e c a r b o n

c a t a l y s t shows e x c e l l e n t a c t i v i t y f o r t h e m e t h a n o l c a r b o n y l a t i o n i r r e s p e c t i v e o f the precursor o f nickel,

many a d d i t i v e s , t h e p r e t r e a t m e n t such as

c a l c i n a t i o n i n a i r o r r e d u c t i o n i n hydrogen o r t h e p r e p a r a t i o n method ( r e f . However,

3).

t h e a d d i t i o n o f h y d r o g e n i n t h e f e e d gas caused a d r a s t i c

i n c r e a s e i n t h e a c t i v i t y . The e f f e c t o f t h e p a r t i a l p r e s s u r e o f hydrogen i s shown i n T a b l e 1.

B o t h m e t h a n o l c o n v e r s i o n and AcOH y i e l d i n c r e a s e d w i t h

i n c r e a s i n g t h e p a r t i a l p r e s s u r e o f hydrogen.

The y i e l d o f AcOH l e v e l s o f f a t

It i s n o t e w o r t h y t h a t t h e AcOH s e l e c t i v i t y

t h e hydrogen p r e s s u r e o f 0.9 atm.

i n t h e c a r b o n y l a t e d p r o d u c t s i s l a r g e l y increased.

Although t h e high Hp

p r e s s u r e i s r e s p o n s i b l e f o r t h e i n c r e a s e i n CH4 y i e l d ,

Carrier A.C. a

C.B.

a, Oxidative odd1t i o n

a

A1 203b

i t suppresses t h e

(NIO-

I

1

NIZt)

b . Formation o f

acetyl groups

I 0 Fig.

1

I

20

40

I

I

60 Yield

80

(2)

100

E f f e c t o f c a r r i e r on c a r b o n y l a t i o n

o f methanol.

11 atm, 250

OC.

a) W/F= 5 g - h/mol,

CO/MeOtl/MeI=100/19/1.

b ) w / F = l O g'h/mol.

CO/MeOH/Mel=20/19/1.

C.B.=carbon black

Reductive e l imination (NlZt+ NI')

c 9

*

CH31 CH

e-1

A,C,

333A.C,

CH30H

CH3COOCH3

h 57h

'

F i g . 2 R e a c t i o n model.

+HI

A,C.

247

E f f e c t o f H2 on Methanol C a r b o n y l a t i o n a )

Table 1 PH2

MeOH Conv. Y i e l d (Me-base X ) (X) AcOH AcOMe DME CH,

0 0.5 0.9 1.6

92.6 99.3 100 100

36.7 65.0 78.9 79.3

51.6 28.5 12.8 10.5

C02/CH4 (molar r a t i o )

3.7 1.6 0 5.8 0 8.3 0 10.2

1.9 0 0 0

- - - _ _ _ _ _ _ _ . ~ _ _

a) 250

OC;

P,-0=7.9,

PNeoH=1.5,

PMeI=0.08 atm; W/F=5 g'h/mol.

!

gen as a f u n c t i o n o f temperature.

1

0

The added hydrogen i n t o t h e f e e d

\

promotes t h e AcOH p r o d u c t i o n t o r e a c h the l e v e l o f a rhodium c a t a l y s t w i t h o u t hydrogen. (iii)R e a c t i o n p a t h p r o m o t e d b y

(i) co

m

B

1

1.01

Cf DME

Scheme 1.

,,..&.""'

..."...' ,.."

...'

,,fi.""Ni/AC

0.5.

I

-+

0

MeOH j AcOMe H20 AcOH

\

,A,' Ni/AC(HZ)

, ,

Ac20

THzO

L

.

248

Table 2 __

E f f e c t o f H2 on C a r b o n y l a t i o n a ) ~~

~~

Reaction

+

MeOH 2COb) -->AcOME + H20

+

CO +H2OC) AcOMe -->2AcOH

__

-~

_______.

No.

H2/C0

1 2

0.3

3 4

0.15

Conv.

(X)

AcOMe

0

13.6 38.6

10.3 36.0

0

58.7 85.9

Y i e l d(Me-base%) MeOH AcOH DME

-

-

0 0

1.5 0.3

55.1 80.0

CH4

3.3

0

0 0

2.1 5.6

1.8

0.8

a) 25OoC, 11 atm. b) C a t a l y s t , 0.1 wt%/A.C. : CO/MeOH/MeI=50/9/1. c ) C a t a l y s t , 2.5 wt%/A.C. : CO/AcOMe/MeI/H~O=lOO/9/l/lO.

which i s formed almost e x c l u s i v e l y a t low conversion l e v e l o f MeOH.

The

comparison o f experiments No. 1 and 2 i n Table 2 shows t h a t the hydrogen enhances t h e r e a c t i o n (i) i n Scheme 1 b y a f a c t o r o f about 3.

The r e s u l t s o f

No. 3 and 4 show t h a t the c o n v e r s i o n o f AcOMe t o AcOH m a i n l y v i a c a r b o n y l a t i o n

and h y d r o l y s i s i s markedly promoted by hydrogen. ( i v ) I n c o r p o r a t i o n o f hydroqen atom i n t h e product. i s t h e p r i m a r y p r o d u c t o f methanol c a r b o n y l a t i o n .

Methyl acetate, which

retards the carbonylation

o f methanol, d i m e t h y l e t h e r and methyl a c e t a t e i t s e l f ( r e f .

6).

Hydrogen

has been supposed t o decompose t h e methyl a c e t a t e adsorbed on c a t a l y s t f o l l o w i n g t h e r e a c t i o n (1)-(3) reaction.

hydrogen showed t h a t few AcOH,

r e s u l t i n g i n t h e a c c e l e r a t i o n o f the

However, t h e r e a c t i o n i n t h e presence o f

deuterium instead of

hydrogen atoms a r e i n c o r p o r a t e d i n

CH4. CH31 o r

suggesting t h a t t h e f o r m a t i o n o f CH4 should n o t be a t t r i b u t e d t o

r e a c t i o n ( 2 ) o r (4). CH31

CH3COOCH3 CH3COOCH3 (v) K i n e t i c s .

+

+ +

+

CHq

HI

-+

CH3COOH

2H

4

CH3COOH

2H

+

+ +

HI CH31 CH4

Fig. 4 shows t h a t t h e p a r t i a l pressure o f each r e a c t a n t a f f e c t s

the r e a c t i o n rate.

F o r each e x p e r i m e n t t h e r a t e i t s e l f i s b y about 2 t i m e s

h i g h e r i n t h e presence o f hydrogen.

The

rate equations are expressed as

equations ( 5 ) and ( 6 ) with and without H2 resp.

Part la1 Pressure(atm1 Fig. 4

Rate o f c a r b o n y l a t i o n o f methanol.

11atm. 250°C, Pco'3.5.

c a t a l ys t: 2 . 5 ~t % N i / a c t ive carbon,

P ~ l ~ o ~ = O . P~~1'0.07.P~2~0.8 6, atm

The c h a r a c t e r i s t i c features o f t h e r e a c t i o n order i s t h a t t h e o r d e r w i t h respect t o Me1 i s l o w e r and t h a t t o MeOH i s h i g h e r f o r hydrogen f r e e system.

It suggests t h a t t h e r e d u c t i v e e l i m i n a t i o n step (Fig.2 c) i s promoted by hydrogen. CONCLUSION (1) It i s found t h a t the present N i supported on a c t i v e carbon c a t a l y s t i s e f f e c t i v e f o r c a r b o n y l a t i o n o f methanol.

(2) Carbon i s e s s e n t i a l f o r the appearance o f the

carbonylation a c t i v i t y

o f n i c k e l c a t a l y s t because o f i t s e l e c t r o n donor-acceptor characters.

(3) The c a r b o n y l a t i o n r e a c t i o n i s g r e a t l y enhanced by co-existence o f hydrogen w i t h a s l i g h t increase o f methane formation.

REFERENCES F.E. P a u l i k and J.F. Roth, Chem. Comm.. (1968) 1578. K. Fujimoto. T. Shikada, K. Omata and H. Tominaga, Ind. Eng. Chem. Prod. Res. Dev.. 21. (1982) 429-432. 3 K. Omata, K. Fujimoto, M. Takagi and H. Tominaga, 5 8 t h Annual Meeting o f CATSJ. Nagoya. October 1986, p r e p r i n t s 4C14. 4 K. Omata. K. Fujimoto, T. Shikada. and H. Tominaga, Ind. Eng. Chem. Prod. Res. Dev., 24, (1985) 234-239. 5 K. Fujimoto, K. Omata, T. Sikada, and H. Tominaga. i n Yu.Yermakov and V.Likholobov (Eds. 1. Homoaeneous and Heteroaeneous Catalvsts. 1

2


D.M. Bibby,C.D. Chang,R.F. Howe and S. Yurchak (Editors),Methane Conversion 1988Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

251

DEVELOPMENT OF UOBIL ’S F lXED--l(bDUb~’fHA~lJl>-TOGASfl1,INE (MTG) PROCESS S . YCiRCHAK

Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, New Jersey 08066 (USA) ABSTRACT Mobil recently commercialized its fixed-bed methanol-togasoline (MTG) process in a 14,500 B/D (gasoline) plant, based on natural gas, hhich is located in New Zraland. Process development studies were carried out in a small pilot plant to define conditions f o r producing gasoline in good yield and with acceptable pioduct quality while also insuring satisfactory catalyst life. The process was successfully scaled-up by a factor of 100 to a drmnonstration unit size of 4 B/D. The scale-up factor from the Jci~onrtr-ationunit to the commercial plant was in excess of 3000. The characteristics of the fixed-bed MTG process, its development, scale up to demonstration unit, and assurance of acceptable product quality are discussed.

INTRUDliC‘r1 ON It has been a little over ten years since Mobil announced a process f o r converting methanol to high-octane gasoline from nonpetroleum sources (refs. 1-3).

In 1987, a commercial plant has

been in operation in New Zealand for more than one year, converting natural gas from the Maui and Kapuni fields into methanol and then into 14,500 B/D of gasoline via Mobil’s fixedbed MTG process.

The gasoline produced is fully compatible with

conventional gasoline. In the

MTG process, the conversion of methanol to hydrocarbons

and water is virtually complete and essentially stoichiometric. The reaction is quite exothermic with a heat of reaction of about

1.74 MJ/kg methanol (750 Btu/lb methanol); adiabatic temperature rise is about 60OoC. In the fixed-bed process, the reaction heat is managed by splitting the conversion in two parts.

In the first

part, methanol is converted to an equilibrium mixture of methanol, dimethylether

(DME), and water over

a non-zeolite catalyst.

About

15% of the heat of reaction is liberated in this step. In the second part, the equilibrium mixture is mixed with recycle gas and passed over ZSM-5 catalyst to form hydrocarbons and water. Most of the hydrocarbon product boils in the gasoline boiling range. A block diagram of the MTG process to produce specification gasoline is shown in Fig. 1. Methanol is heated and passed into

252

the

DME

reat t o r .

The effluent

is

mixed with recyclt. gas (not

shown) and converted to gasoline and water in the ZSM-5 reactor. After cooling, the effluent is separated into three phases: gas, liquid water, and liquid hydrocarbon. Most of the gas is recycled to the ZSM 5 reactor. The water phase contains about 0.1-0.2 wt% oxygenates (alcohols, ketones, and acids) and is treated

by

conventional biological means to give an acceptable effluent for discharge.

The hydrocarbon product is sent to distillation.

The

raw MTG gasoline contains substantial amounts of durene (1,2,4,5tetramethylbenzene), excessive concentrations of which can cause driveability problems.

The heavy gasoline fraction contains most

of the durene produced, which is removed in the heavy gasoline treating unit (HGT). The treated heavy gasoline is blended with other gasoline components to give specification finished gasoline. The

HGT process has no impact on gasoline yield

I , C2.

Superheat Vaporize Preheat

4

DME Reactor

ZSM-5 Reactors

4

HP

Sep.

or octane.

Light Gasoline

1

Distillation

HGT

Heavy Gasoline Water to

Finished Gasoline

Crude Methanol

Fig. 1 . Block diagram of gasoline

-

MTG process to produce specification

Development of the MTG process was carried out in a benchscale unit with a capacity of 4-8 l/day of methanol (refs. 4 - 6 ) . Scale-up of the process was confirmed by operating a 4 barrels per day (B/D) unit (refs. 6-9). Development of the HGT process was similarly conducted in a bench-scale unit.

Specification

finished gasoline was produced in a manner similar to that depicted in Fig. 1 from 4

B/D unit MTG gasoline.

In this paper, the development and laboratory scale-up of the

253

fixed bed UTG process are reviewed. The process chemistry, the characteristics of the process, the quality of the MTG gasoline product, and the process implic,ations of catalyst performance are discussed. MTG PROCESS CIIEMISTRY A.W YIELDS Catal y s t_s The key catalyst in the MTG process is zeolite ZSM-5, which catalyzes the conversion of methanol to hydrocarbons.

The

framework of ZSM-5has two types of intersecting channels: one nearly circular and the other elliptical (ref. 10).

The size of

the openings exerts a strong influence on product distribution.

ZSM-5’s high hydrothermal stability and low coke selectivity are critical for the MTG process to ensure satisfactory catalyst life. The low coke selectivity allows reasonable cycle lengths to be achieved without excessive catalyst requirements. Various catalysts can be used for converting methanol to and water (ref. 11).

DME

The fixed-bed MTG process uses a yalumina

catalyst which has high selectivity for methanol conversion to

DME

and water and low selectivity for methanol decomposition and coke. These properties are important as any loss of methanol to byproducts directly affects gasoline yield.

Commercially, it is

DME reactor per ZSM-5 train. Thus, high coke formation in the DME catalyst would necessitate the additional expense of multiple DME reactors.

preferred to have one

Reaction Path

The overall reaction path for converting methanol to hydrocarbons is shown in Fig. 2 . Detailed mechanisms are discussed elsewhere (ref. 12). The initial step is conversion of methanol to

DME and water. Methanol and DME react to give light

olefins which react further to heavier olefins.

The higher

olefins give rise to paraffins, naphthenes, and aromatics. Aromatics are formed from olefins via hydrogen transfer reactions as little molecular hydrogen is produced.

The conversion of

methanol is accelerated by the reaction products, probably olefins (ref. 1 3 ) .

As the catalyst ages, the product distribution will

shift towards higher olefin content.

An important reaction which

also occurs is aromatic alkylation by methanol/DME.

This reaction

254

2 CHIOH

+ CHsOCH3 + H20

-

m Light Olefins + H20

CHIOH, CH30CHI

Light Olefins

CS+Olefins

Ca+Olefins

Paraffins NaDhthenes Aimatics

Fig. 2. MTG reaction path

TABLE 1 Fixed-Bed MTG Yields (83/17 (w/w) Methanol/Water Charge) =mDeratures,.OC (OF) Dehydration Reactor Inlet 316 (600) Dehydration Reactor Outlet 405 (760) Conversion Reactor Inlet 360 (680) Conversion Reactor Outlet 415 (780) Pressure (kPa abs), psig 2170 (300) Recycle Ratio, mol/mol of Charge 9:l Conversion Reactor WHSV, kg Charge/kg Cat-Hr 2 Yield, wt% of Methanol Charged Methanol Plus Ether Hydrocarbons Water

co, co

Coke, 8xygenates Total Hydrocarbon Product, wt% Light Gas Propane Propylene Isobutane n-Butane Butenes C6+ Gasoline Total Gasoline (82.7 kPa

LPG

Fuel Gas

RVP, 93 R+O)

0.0 43.66 56.15 0.04 0.15 100.0 1.3 4.6 0.2 8.8 2.7

1.1 81.3

100.0 86.0 12.7 1.3

255

eventually leads to selective formation of 1 , 2 , 4 , 5 tetramethylbenzene (durene) over the ZSM-5 catalyst: CH,OH

-->

C,H,(CH,),

t

C,H, (CH3)3

t

H,O

Product Y i e l d s Typical process conditions, product yields, and hydrocarbon selectivity are given in Table 1 .

The conversion of methanol to

hydrocarbons and water is complete and essentially stoichiometric. The yield of by-products such as CO, CO,, very low.

coke, and oxygenates is

The hydrocarbon yields shown represent those expected averaged over the useful life of the ZSM-5 catalyst. gasoline contains C 4 ’ s for vapor pressure control.

Finished

For an 82.7

kPa (12 psi) R W (Reid Vapor Pressure) finished gasoline, the yield is 86 wt% of hydrocarbons and the clear Research octane number is 93. Additional gasoline could be made by alkylating the propene and butenes produced with isobutane.

A s the amount of

alkylate would be low, its manufacture would most likely be considered only for very large plants. are very low at 1.3 wt% of Light gas yields (C,-) hydrocarbons. This could be used to provide some of the energy The remaining C, and C, products are about 12.7 wt% and could be marketed as LPG. Depending on local

for plant operation.

conditions, this could be burned as fuel to back out, e.g., natural gas, but doing so will reduce the overall thermal efficiency of the plant. BENCH-SCALE DEVELOPMENT

Experimental(references 4-5) Several types of fixed-bed reactor systems were considered for the MTG process development (ref. 9). Perhaps the easiest to scale-up is the two-reactor configuration shown in Fig. 3; this was used for bench-scale studies.

Both reactors contained an

axial thermowell to monitor temperature profiles.

Special

precautions (e.g., adiabatic heaters, insulation) were taken to ensure proper accounting of heat effects, although in small reactors it is virtually impossible to be 100% adiabatic (ref.

14).

The bench-scale reactors were estimated to be 90-95%

adiabatic. A synthetic crude methanol blend containing 83 wt% methanol

256

(commercial, pure) and 17 wt% distilled water was used as the charge.

This water content is typical of crude methanol made

from natural gas. Impurities normally present in crude methanol (ref. 15) would not be expected to affect the performance of the

ZSM-5 catalyst (refs. 2, 4 ) .

This was confirmed by tests

conducted with crude methanol.

Methanol

Preheater 16 mm ID Reactor

32 mm ID Reacto

60 cc of Catalyst

Dehydration Reactor

Fig. 3. Schematic of fixed bed pilot plant The catalyst w a s regenerated by combusting the coke formed with air. Process Characteristics and Catalvst Aging In an adiabatic reactor the heat release gives rise to an ascending temperature profile down the reactor (Fig. 4 ) .

At

short time on stream, the maximum bed temperature is reached well before the reactor outlet, which, of course, implies an excess of catalyst. This is done, however, to assure a satisfactory cycle length between regenerations. These profiles are most interesting because they indicate that the catalyst band-ages. There is very little change in shape of the profile as the catalyst ages.

This suggests either that coke formation

downstream of the main reaction zone is low or that-its level is such that it does not appear to affect catalyst activity, as determined from temperature profiles.

Although profile

distortion due to heat conduction could mask true activity

257

changes (ref. 14) s i m i lar behavior was observed in the scale-up studies, as disclissed later. Any distortion would be more pronounced in the small srale reactor.

Eventually] the reaction

zone approaches the reactor outlet and significant quantities of methanol appear in the water product. An example of the high methanol conversion efficiency achieved is shown in Fig. 5. When the methanol conversion decreases sufficiently (e.g., to 99.9%), methanol breakthrough is deemed to have occurred, and the cycle

is terminated.

After regeneration] the catalyst is returned to

conversion service. W

v,

a W a

+ a

3

a W

n

+ L5

P W

! J

a

SECOND CYCLE ( ) DAYS ON STREAM

B

fz

s

1300 1000

I

-

500 -

-

i

UJO

223 w a sik ar n

-

10050-

0

0

-

0 0

10-

I

0

-

I

I

:

258

100

75

-&-----'

5 P

50

-

d

25

:

V

kr I-

2

5

m

0

C4- HYDROCARBONS

W

.-

I

C g f HYDROCARBONS

Fig. 6. Gasoline yields increase with cycle time The change in hydrocarbon type as the catalyst ages within a cycle is depicted in Fig. 7. Even though all data are for essentially complete methanol conversion, they support the overall reaction path given in Fig. 2 .

Normal paraffins decrease

and isoparaffins increase as aging progresses.

This helps to

compensate for the decline in aromatics and eventually results in a gasoline octane number which changes little with time.

The

ZSM-5 catalyst deactivates not only temporarily by coke deposition but also permanently.

Permanent deactivation is

caused by the presence of steam and is enhanced by increasing temperature.

Different segments of the catalyst bed are

subjected to varying degrees of water partial pressure and temperature for different times of exposure.

This variation

results in a permanent activity gradient in the catalyst bed. The loss in activity will be reflected by the manner in which product yields vary from cycle to cycle.

259

60

I

I

1

1

I

I = ISO-PARAFFINS P = N-PARAFFINS 0 = OLEFINS N ii NAPHTHENES A I AROMATICS 40

WEIGHT PERCENT OF HC

20

0

0

20 30 DAYS ON STREAM

10

50

40

Fig. 7. Variation in product P-0-N-A distribution over the first cycle 100

I

I

I

I

I

80 m c

e

"

L

0

I x

ae

I 40

0

1 0

50

100 150 Time-On-Stream, Days

200

250

Fig. 8 . Methanol-to-gasoline aging study One of the major tasks completed during the bench-scale studies was an eight-month aging test during which the ZSM-5 catalyst performed satisfactorily under realistic process conditions. Fig. 8 .

Gasoline yield data for this test are depicted in

The vertical lines represent the completion of a process

cycle, catalyst regeneration, and start of the next cycle. Start-of-cycle (SDC) gasoline yields increase f r o m cycle to cycle

260

End--of-cycle (EOC) gas01 ine yields are fairly constant as might be expected. Thus, as the as a consequence of permanent aging.

catalyst ages, the change in gasoline yield within a cycle decreases, and the cycle average gasoline yield increases.

TABLE 2 Cycle Lengths Durinv Aging Test -

CY&

Cycle Lenvth. Days

Cumulative Catalyst Age, Days

16 44 64 85 104 131 154 181 208

13 25 17 18 16 25 21 24 24

Even though the catalyst is being irreversibly deactivated, cycle lengths stabilized (Table 2).

Interestingly, cycle length

initially increases before undergoing a decline.

This behavior is

consistent with the interaction of permanent deactivation with coke deactivation, with the coke formation rate being dependent on the catalyst’s activity level (ref. 16). Evidence of permanent deactivation can also be obtained by comparing product composition as a function of catalyst age at constant temperature profile location (refs. 4, 5).

Additionally, since normal paraffins

correlate with aromatics, propane yield, or propane/propene ratio, could be used to track catalyst activity (Fig. 9 ) .

SOC

propane/propene yields show a sharp decline initially followed by a rather gradual decline, whereas the EOC values appear to approach a constant value.

Cycle lengths do not reflect the

decreasing differences between SOC and EOC propane/propene levels because the rate of decline of the propane/propene ratio within a cycle decreases with increasing catalyst age.

This is due to

reduced coking rate on deactivated catalysts (ref. 16). Eventually, of course, permanent deactivation will become sufficiently severe that cycle lengths will decrease.

261

200b

I

I

I

I

P

t

OPROPANElPROPENE AT SOC (1 DAY) OPROPANElPROPENE AT EOC

-

-

C

0

K

n

n I

-

50

OO

n

U I "

I

100

-

O

0

n

n

150

IP

200

250

Fig. 9 . Propane/propene as catalyst aging indicator Comparison of gasoline yield versus propane/propene ratio shows that inherent gasoline selectivity did not change throughout

MTG is somewhat unusual in that the

the aging test (Fig. 10).

selectivity of the ZSM-5 catalyst for the desired gasoline product actually increases as it ages.

y 100 8 + B

g

I

I

I

I

A

0 CYCLE 0 CYCLE CYCLE CYCLE x CYCLE t CYCLE

90-

v

>-

Y

-I

4

I

1 2 4 6 8 9

-

-

+ 00-

w

A '

E

$a 0

n

> U a n

0 0

0

0

70-

-

0 0

60

I

1

I

I

I

Fig. 10. Gasoline selectivity is unaffected by catalyst age

262

QME Catalyst .Performance The DME catalyst must Larry out equilibrium conversion of methanol to dimethylether and water with minimum by-product formation. 1,ess than equilibrium conversion will require more heat to be removed in the ZSM-5 circuit, which will result in higher reactor temperature rise.

This will increase catalyst

deactivation and decrease yield.

The higher temperature rise

could be reduced by increasing gas recycle,

but this will

Excessive decomposition of methanol will result not only in carbon loss,

increase operating costs. (e.g., to CO, CO,, H,)

thereby reducing gasoline yields, but will also affect the composition of recycle gas in the ZSM-5 circuit. For example, one percent methanol decomposition to CO and H, will increase the ZSM-

5 reactor temperature rise by 12%. The degree of conversion and extent of decomposition of methanol with good dehydration catalysts are shown in Table 3 . Essentially equilibrium conversion was attained with each catalyst, but catalyst B, which is a later version, gave reduced decomposition of methanol.

The temperature profiles in Fig. 1 1

indicate that equilibrium conversion should be attainable for a long time.

FRACTIONAL CATALYST BED LENGTH

Fig. 11. DME catalyst ages very slowly

263

TABLE 3

DME Catalyst Performance A

Catalyst

80 413

Time on Stream, Days Outlet Temperature,'C

Decamp,.. Prod. , % MeOH

H C8 CO,

MeOH Conv. to DME Experimental , % Equil., %

A 245 410

0.0081 0.0434 0.4647

+ H2Q

77.0 76.2

0.0018 0.0108 0.0848

77.4 76.4

B

78 404 0.0020 0.0098

0.0980

-

MTG SCALE-UP Having defined process conditions to obtain satisfactory gasoline yield, acceptable product quality, and adequate catalyst life in bench-scale tests, fixed-bed MTG development would normally be considered complete, and the process ready for commercialization.

However, to obtain large quantities of

gasoline for testing and to confirm scale-up, a demonstration unit with a capacity of 4 B/D of methaqol was built and operated. In vapor-phase, fixed-bed reactors, the only item that cannot be scaled is reactor length.

This is because as the reactor

length is increased, the linear velocity is also increased, as shown below: Plug Flow Reactor: where :

Ai f(pAi) G kj

L X

= =

= =

= = p = pB =

%

=

E kj pB

f(pAi)

concentration, mols i/kg of gas concentration dependence of reaction rate mass velocity, kg gas/m2-hour rate constant of jth reaction; first order: kj [=I m3 gas/kg catalyst-hour reactor length, m reactor length, dimensionless gas density, kg/m3 catalyst density, kg/m3 of reactor

Vapor-phase, fixed-bed process performance usually stays the same or improves as the reactor length is increased because any influence of external heat and mass transfer would diminish upon scale-up.

Diameter has no effect as long as the reactants are.

264

distributed uniformly across the reactor cross-section. Uniformity is particularly important for MTG because of its extremely high conversion level requirements (Fig. 5 ) . MTG scaleup was conducted with catalyst bed lengths approximately equal to that expected for a commercial size reactor (ref. 9). A schematic diagram of the 4 B/D demonstration unit is shown

in Fig. 12.

Catalyst bed dimensions were 50 mm (2 in.) dia by 3 m

(10 ft) for the DME, and 100 mm (4 in.) dia by 2 . 4 m (8 ft) for

the ZSM-5 reactors.

The linear velocities in these beds are about

10 times those in the bench-scale unit. The diameter of the ZSM-5 reactor was chosen to be 100 mm to reduce the influence of axial heat conduction along the reactor walls to a negligible value (ref. 14). Heat loss from the ZSM-5 reactor was estimated to be less than 1%. Except for size, the 4 B/D unit is very similar to the bench-scale unit.

Methanol Storage

Reactor

Reactor

Fig. 12. MTG fixed-bed demonstration plant The feed used for the scale-up study was a synthetic crude methanol blended from commercial methanol (DuPont) and distilled water.

A bench-scale unit and the 4 B/D unit were operated at the

same conditions with the same catalyst and methanol feed. Process conditions and product yields for the scale-up study are shown in Table 4 .

Methanol decomposition products (CO, CO,,

and H,) were somewhat lower in the 4

B/D unit, which suggests that

some decomposition may have occurred in the methanol preheater of

265

the bench scale unit, but catalyst effects dominate. Hydrocarbon selectivities are summarized in Table 5.

Light

hydrocarbon selectivity, and gasoline yield and octane are unchanged by scale--up. Heavy hydrocarbon selectivity was also unchanged, as exemplified by durene.

The excellent scale-up

results and ease of operation of the demonstration unit attest to the technical viability of the process.

TABLE 4 MTG Scale-Up: Averaae First Cycle Conditions and Product Yields

Charge Methanol/Water, w/w

DME Inlet Temp., "C("F) ZSM-5 Inlet Temp., 'C('F) ZSM-5 Outlet Temp., OC(OF) MeOH WHSV, kg/kg ZSM-5 Cat-Hr

Recycle Ratio, mols/mol Charge High Press. Sep. Temp., OC(OF) Pressure , kPa (psig) Product, wtX of Methanol Hydrocarbons Water CO, CO,, H MeOH + DMF!

Bench Unit

4 B/D Unit

83/17 315 (599) 358 (676) 404(759) 1.6 9 50(121) 2163 (299)

83/17 316(601) 360 (680) 407 (765) 1.6 9.2 52(125) 2 156 (298)

43.73 56.17 0.10 0.00 100.00

43.75 56.19 0.06

TABLE 5 MTG Hvdrocarbon Yields Scale-Up Hydrocarbon Products Methane Ethane Ethy1ene Propane Propene Isobutane n-Bu tane Butenes

%+

Averape First Cycle Yield, W t % Bench Unit 4 B/D Unit

1.33 0.82 0.02 8.54 0.15 8.45 4.06 0.71 75.92 100.00

Gasoline+Alkylate (62 kPa RVP)80.2 Clear Research Octane 95 Durene, wtX of HC

5.17

3.25 0.86 0.03 8.60 0.15 8.39 4.20 0.74 75.78 100.00 80.2 95 5.36

0.00

100.00

266

FRACTIONAL BED LENGTH

Fig. 13. Catalyst aging is slower in the 4 B/D unit Some positive effects of scale-up were also noted. For example, ZSM-5 cycle lengths are about 50% greater in the 4

B/D

unit than in the bench unit. The increase in cycle length is clearly related to the slower rate of movement of the catalyst bed temperature profile as shown in Fig. 13. Several factors were examined to determine if they could account for the difference: catalyst loading, wall effects, axial dispersion, and heat effects. Calculations indicated that none of these items were important, e.g., the flow in the bench-scale ZSM-5 reactor was at least 99.99%of true plug flow.

The key observations here are the

slower profile movement and the equivalent location of the temperature profile at methanol breakthrough. The slower profile movement is due to reduced rate of coke formation in the 4 B/D unit. The coke yield on methanol in the 4 B/D unit is two-thirds of that in the bench unit. This translates exactly to a 50% increase in cycle length.

The lower coke yield is attributed to

linear velocity effects, as this is the only significant difference in operation of the two units. Because of the difference in cycle length, gasoline yield at a given time on stream will not be the same for the two units. However, they are the same when compared at the same extent of reaction as measured by the propane/propene ratio (Fig. 14).

This

correspondence holds true not only for first but also for later

267

90

I

I

0 I

I

0 BENCH UNIT 0 4 BiD UNIT

8 2+ 80-

5

w

z-I

8

Q

70-

k

u

-

0

+

0

8

vo

0

0

0 60

I

'

0

0

I

I

Durene is particularly noteworthy as its concentration in MTG gasoline is the only significant difference between MTG and Although durene has a good octane number (110 blending clear Research octane) and boils within the gasoline boiling range (197OC, 386'F), its high melting point (79OC, 175'F) conventional gasoline.

can lead to driveability problems if its concentration in gasoline is too high (ref. 4 ) . For example, listed in Table 6 is the durene tolerance of several vehicles similar to the type sold in New Zealand (ref. 17).

Vehicle sensitivity to durene depends not

only upon manufacturer but also the particular model. From an overall product quality viewpoint, the concentration of durene in gasoline should be less than about 2 wt% (ref. 9).

This level

will ensure product acceptability not only for vehicle driveability but also for handling. As shown in Table 5, the MTG process will produce a gasoline product containing substantially more than 2 wt% durene.

Although

durene yield is affected by temperature, pressure, and particle size, use of these parameters for reducing durene could lead to difficulties.

For example, reduced catalyst particle size would

entail an operating cost penalty. by undercutting the gasoline.

The specification could be met

To avoid these unacceptable

solutions, an exploratory program was started to s e e if a gasoline

268

treating step could reduce durene

TABLE 6 Durene Tolerance of Selected Vehicles Durene in Gasol in.e,_W_t%(2) 6 5 4 3

("Repeated letter indicates different vehicle model. (2)Durene equal to or less than indicated concentration has no effect on vehicle performance.

HEAVY GASOLINE TREATING (HGT) Exploratory research showed that treating a heavy MTG gasoline fraction could reduce durene levels sufficiently to meet the specification.

Subsequent process development in isothermal and

adiabatic bench-scale units confirmed this.

TABLE 7 Heavv Gasoline Treating Yields (Wt%l

c*cC,,+ P+N c4 +

C, Aromatics

C, Aromatics C, Aromatics C, Aromatics 1,2,4,5- Me, Benzene 1,2,3,5-Me, Benzene 1,2,3,4-Me, Benzene Other CIB+ Aromatics

Product 0.3 1.96 4.52 0.03 0.52 5.74 29.41 13.62 16.06 3.60 24.24

Feed 0.65 0.00 0.00 0.74 23.04 43.69 8.13 2.80 20.95

~~

In the HGT process, (refs. 9, 18) a 177'C.- (350°F+) cut of MTG gasoline, comprised primarily of aromatics, is processed over a multifunctional metal-acid catalyst. composition is shown in Table 7.

A typical feed and product

On a finished gasoline basis,

Cs+ yield loss from the HGT process is insignificant as only 10 15% of the MTG gasoline is processed in the HGT unit (ref. 9). The following reactions occur:

-

isomerization, disproportionation,

transalkylation, ring saturation, and dealkylation/cracking.

The

low yields of methane and ethane indicate that methyl groups are removed and combined to form higher paraffins by what is known as

269

the ”paring” reaction (ref. 19). At n o r m a l conversions, hydrogen consumption is moderate. The HGT process fits into the MTG product recovery section well (ref. 9 ) . A typical product recovery section for a MTG unit including HGT is shown in Fig. 15. Liquid hydrocarbon from the product separator is passed into a de-ethanizer. The bottoms product is then sent to a stabilizer.

The stabilized gasoline is

split into light and heavy gasoline streams in the gasoline splitter. The heavy gasoline is processed i n the HGT unit to decrease its durene content. After stabilization, the treated heavy gasoline is blended with light gasoline and Cq’s to produce a finished gasoline containing less than 2 wt% durene.

MTG

Liq. HC

d

S

2

Stabilizer

Splitter

Treater

Stabilizer

Fig. 15. MTG heavy gasoline treater

PRODUCT QUALITY Tests conducted with gasoline from the bench-scale studies indicated that MTG gasoline was high quality (refs. 4 , 5). However, not enough gasoline was produced for fleet tests. demonstration unit resolved that problem.

The

Large quantities of

specification MTG gasoline for fleet testing were prepared by operating the MTG demonstration unit at the operating conditions for the New Zealand MTG unit. The demonstration unit gasoline was cut to produce a heavy gasoline stream. This was treated in a scaled-up

HGT unit to produce treated heavy gasoline which was MTG gasoline and components not recovered

then blended with light

270

during the distillation to give a rrpresentative finished gasoline suitable for testing. Typical composition and properties of a finished MTG gasoline are shown in Table 8. The hydrocarbon composition and distillation are typical of good quality gasolines. The gasoline is not corrosive and cont,ains negligible amounts of s u l f u r and nitrogen components. Using a conventional additive package, fixed-bed gasoline meets other quality standards such as storage stability, copper attack, multimetal corrosion, carburetor detergency, filterability, emulsion formation, and metals retention. Automotive exhaust emissions and fuel economy with MTG gasoline are essentially identical to those with conventional gasolines (ref. 17).

TABLE 8 Bpical-Finished Gasoline Properties Components, vol% C5+ Gasoline

95

5

C4'S

Octane Number Clear Research Clear Motor Distillation (D-86), "C("F) 10%

93 83 46 99 166 204 1 loo.

Because of the catalyst shape selectivity, most products are methyl-branched iso-olefins. Tn the C, to C,, range, branched iso-olefins have good octane rating. In the C,, to C,, range isoparaffins have good distillate fuel properties after hydrogenation. -

OLIGOMERIZATION

DISPROPORTIONATlON

/

I

CYCLO-OLEFIN

+ PARAFFIN

CYCLO-DIOLEFIN

+ PARAFFIN

ALKYL-AROMATIC

+ PARAFFIN

Fig. 8. Schematic of MOGD mechanism

A potential MOGD kinetic scheme is described in Fig. 8 (ref. 1 9 ) . Olefins react by oligomerization - for example C,= forms C6=, CB=, CI2=, etc. Olefins also undergo double-bond and skeletal isomerization, and disproportionation to produce intermediate olefins.

Cracking occurs at the same time, and

further reactions include cyclization and hydrogen transfer. Those latter reactions produce cyclo-olefins, alkyl-aromatics and paraffins.

MOGD olefin product distribution is determined by thermodynamic, kinetic, and shape-selective limitations.

The

equilibrium calculation was greatly simplified by assuming the isomers for a given carbon number to be at equilibrium (ref. 19). At low pressure and high temperature, olefin equilibrium is reached, while at higher pressure kinetic limits prevent equilibrations at commercially feasible space velocities. Isomerization reactions are fast at all carbon numbers, and isomer equilibrium is achieved for low carbon numbers.

Shape

selectivity determines isomer equilibrium for higher carbon

318

numbers.

Calculated MOGD equilibrium distributions are shown in

Fig. 9 .

30

25

1

15 10

20 15

5

10

0

5 0 1

CARBON NUllBER TEtlPERATURE

(C) 400

Fig. 9 A . Effect of temperature on olefin equilibrium distribution at 1 bar 30

CARBON

NUneER

P R E S S M E (BAR

Fig. 9B. Effect of pressure on olefin equilibrium distribution at 232°C The process scheme generally uses four fixed-bed reactors, three on line and one in regeneration. The three online reactors are connected in series with inter-reactor coolers and liquid recycle to control the heat of reaction.

The olefins

feed is mixed with a recycle stream and passes through the three

319

reactors.

Fractionation is used to generate a gasoline-rich

stream for recycle to the reactors. This recycle both helps control the heat of reaction and improve distillate selectivity. MOGD distillate and gasoline mode product yields are shown in Table 4 .

The charge stock was a C,-C,

main olefinic components of MTO product.

feed representing the Yields were 82%

distillate, 15% gasoline and 3% light gas.

A large-scale MOGD

test run was conducted in a Mobil refinery in 1981 (Fig. 10). The test run used commercially-produced zeolite catalyst, and the unit was a modified, commercial wax hydrofinisher.

Charge stock

was an LPG mixture of propane/propene/butanes/butenes

(62%

olefins) from an FCC unit. The test run lasted 70 days and product yields and selectivities were the same as in our smaller pilot plants.

TABLE

4

MOGD Process Yields With C,-C, Olefins Feed Max Distillate Mode

C,-C, c4 C -165°C Gasoline

Gasoline Mode

1

4

15

-

2

5

82

1h5"C+ Distillate C -200°C Gasoline 280"C+ Disti 11ate

84

7

MTO-MOGD PROCESS One possible combination of MTO and MOGD is shown schematically in Fig. 11 (ref. 2 0 ) .

High-octane MTO gasoline is

separated before the MDGD section and is later blended with MOGD gasoline.

Some MOGD gasoline is recycled to the MOGD reactors, as

described previously.

The distillate is hydrotreated and can be

fractionated into various products.

Typical distillate and

gasoline yields from the olefins yield demonstrated in the 100 B/D

MTO plant are 50/50 wt/wt.

The process is flexible, so that this

ratio can be varied considerably. For many applications, the desired distillate to gasoline ratio would be 1 or less.

A much

higher distillate to gasoline ratio can be obtained by increasing olefins yield in

MTO from the 60% level to the

70% level, by the

320

various options described previously (lower pressure, use of diluent, etc). RecycleCompressor

Fresh FWd

RecycleGasoline

Fig. 10. MOGD demonstration unit Typical MTO-MOGD combined product properties are shown in Tables 5 and 6. The gasoline is olefinic and aromatic, of better quality than FCC gasoline.

The motor octane number, in

particular] is higher than for a typical FCC gasoline.

MTO-MOGD

gasoline contains only small amounts of durene, which is not favored at the higher MTO temperatures. The distillate product is mostly iso-paraffinic and is an exceptionally good blending stock due to its high cetane index, low pour point and negligible sulfur content. Its physical properties] such as flash point, boiling range and viscosity are comparable with conventional distillate fuels. MOGD diesel fuel has somewhat lower density than typical conventional fuels (0.8 vs. 0.86). MOGD product makes excellent jet fuel, meeting or exceeding all commercial and military specifications. Its low aromatic level yields a highly stable fuel, as shown by high JFTOT (343'C vs a maximum of ZSO'C), with very little smoke emission during combustion.

321

TABLE 5 MTO-MOGD Typical Gasoline Proje-r-ti-es

0.738

Density, g/cc Octanes Research Clear Motor Clear Reid Vapor Pressure, kPa Sulfur, ppm Distillation, D86, " C 10% 30% 50% 70% 90%

EP

93.0 85.0 57.2

E 6

I!

\

TOTAL HCS C2’s

40

u) W

20 0

2

4

6

8

1

0

02 IN FEED GAS(VOI%I

F i g . 3. .Influence o f oxygen l e v e l i n feed gas on methane conversion, oxygef consumption and product s e l e c t i v i t i e s (770°C; pseudo-contact t i m e 1.5 g m l s ) .

400

expense of reauced hyarocarbon s e l e c t i v i t y and i n c r e a s e d f o r m a t i o n o f carbon o x i des. T o t a l methane c o n v e r s i o n r a t e s ( i n m i l l i r n o l e s o f CH4 p e r m i n u t e p e r gram of c a t a l y s t o r mmol min-lg-')

c a l c u l a t e d f o r t h e d a t a i n F i g s . 2 and 3 a r e shown

as a f u n c t i o n o f W/F i n F i g . 4 ( t h e range o f W/F v a l u e s corresponds t o space v e l o c i t i e s o f 2,000-19,000

h-'

on a volume b a s i s a t o p e r a t i n g c o n d i t i o n s ) .

As

expected, t h e methane c o n v e r s i o n r a t e i n c r e a s e d as t h e oxygen l e v e l i n t h e f e e d gas i n c r e a s e d .

However, a t each oxygen l e v e l t h e r e was a marked decrease i n

methane c o n v e r s i o n r a t e as W/F i n c r e d s e d ( i .e. r e a c t o r decreased).

as t h e gas v e l o c i t y t h r o u g h t h e

The p o s s i b i l i t y t h a t t h i s e f f e c t was caused by l i m i t a t i o n s

i n t h e mass t r a n s f e r o f r e a c t a n t s across t h e boundary l a y e r t o t h e e x t e r n a l s u r f a c e o f t h e c a t a l y s t was checkea u s i n g s t a n d a r d c a l c u l a t i o n procedures Lret.31.

I n a l l cases t h e r e a c t o r was found t o be o p e r a t i n g w e l l o u t s i d e (by

more t h a n an o r d e r o f rriagni t u u e ) t h e e x t e r n a l mass t r a n s f e r 1i m i t i n g reginie. Presumably t h e observed d e c l i n e i n methane c o n v e r s i o n r a t e w i t h i n c r e a s e d W/F i s t h e r e s u l t o f o p e r a t i o n o f t h e r e a c t o r i n an i n t e g r a l mode where t h e v a r i a t i u n i n N/F r e s u l t s i n d i f f e r e n t averaye r e d c t a n t and p r o d u c t c o n c e n t r a t i o n s ana hence d i f f e r e n t r e a c t i o n r a t e s .

The p o s s i b i l i t y o f r a t e

s u p p r e s s i o n by one o r more o f t h e p r o d u c t s r e d u c i n g c a t a l y s t a c t i v i t y must a l s o be c o n s i a e r e o . IhPLICATIURS FUR DtSIbEU UF LAKGE-SCALt REACTORS Since t h e OXCU r e a c t i o n s a r e b o t h r a p i d and h i g h l y exothermic, s e r i o u s problems a r e encountered i n t h e a e s i g n o t a f i x e d - b e d r e a c t o r f o r t h i s purpose.

The h i g h e x o t h e r m i c i t y can be i l l u s t r a t e d by c a l c u l a t i n g t h e

a o i a b a t i c teidperature r i s e f o r t h e two extremes i n oxygen l e v e l c o n s i d e r e d here

on t h e b a s i s o f e s t i m a t i n g t h e hydrogen and w a t e r i n t h e p r o d u c t gas by hyaroyen ana oxygen balances. fixeu

W/F v a l u e o f 1.5 were

The c a l c u l a t e d a d i a b a t i c temperature r i s e s a t a

46OL aria 347°C f o r ,the 1.1 and 9.4

v o l S oxygen

l e v e l s respectively. I n view o f these ternperature r i s e s , a f i x e a - b e d r e a c t o r would be f e a s i b l e o n l y i t t h e oxyyen c o u l a be addea i n stages i n a m u l t i - b e d r e a c t o r so t h a t t h e teiliperature r i s e across each be0 were 1 iiili t e a .

I n t e r s t a g e c o o l i n y c o u l a be

accoilipl i s h e u by i n j e c t i o n o f r e c y c l e inethane and i n d i r e c t h e a t exchange. exdniple,

For

i t t h e pertorniance i n eacn r e a c t o r beu i s as shown i n F i g . 2a, t h e

tei,lperature r i s e across each bed w i l l be 46OC. have shown

tlidt

L a b o r a t o r y experiments L r e f .4]

s e l e c t i v i t y i s n o t s i g n i f i c a n t l y a f f e c t e d by a ternperature r i s e

a t t h i s iliagni tuue. S t a s i n y o f t h e oxygen a o d i t i o n would a1 so ensure t h a t t h e r e a c t o r o p e r a t e d well outside the explosive l i m i t s . 1s

However, f o r t h i s concept t o be f e a s i b l e i t

necessdry t o denionstrdte t h a t n e i t h e r c a t a l y s t d e a c t i v a t i o n n o r f u r t h e r

401

r e a c t i o n o f t h e C2+ hydrocarbons w i t h oxygen o r steam o c c u r t o any a p p r e c i a b l e e x t e n t i n subsequent r e a c t o r beds.

Since methane c o n v e r s i o n woula be low, t h e

f i x e a be0 r e a c t o r would need t o be o p e r a t e d under p r e s s u r e (1-5 MPa) t o achieve reasonable p r o d u c t i o n c a p a c i t i e s .

The e f f e c t o f p r e s s u r e on t h e OXCO r e a c t i o n s

i s t h e r e f o r e Deing i n v e s t i g a t e d . If i t i s n o t p o s s i b l e t o use a f i x e d - b e d r e a c t o r f o r t h e OXCO process, some forrli o f f l u i d i sed-bea r e a c t o r w i 11 be necessary.

Heat removal and temperature

c o n t r o l a r e g r e a t l y f a c i l i t a t e a i n f l u i a i s e d - b e d s because o f t h e e x c e l l e n t backniixing o f t h e s o l i a phase.

Considerable o p p o r t u n i t y a l s o e x i s t s t o v a r y

t h e mode o f y a s / s o l i d s c o n t a c t by o p e r a t i o n i n e i t h e r t h e b u b b l i n g , t u r b u l e n t o r f a s t ( c i r c u l a t i n g ) f l u i d i s e a - b e d regimes. Uespi t e t h e o b v i o u s advantages o f f l u i d i s e d - b e d s f o r temperature c o n t r o l , t h e r e i s u n c e r t a i n t y as t o t h e i r s u i t a b i l i t y f o r c a r r y i n g o u t t h e OXCG r e a c t i o n s s i n c e t h e s e a r e a conibination o f gas phase r e a c t i o n s and r e a c t i o n s on the c a t a l y s t surface.

C S I R O has begun i n v e s t i g a t i n g t h e OXCO r e a c t i o n s i n a

s m a l l - s c a l e b u b b l i n g f l u i d i s e d - b e d r e a c t o r and i n i t i a l r e s u l t s have been encouraging i n t h a t t h e y a r e s i m i l a r t o t h o s e r e p o r t e d h e r e f o r a f i x e d - b e d reactor. CONCLUSIONS The p r o d u c t i o n o f h i g h e r hydrocarbons d i r e c t l y frois methane by c a t a l y t i c o x i d a t i v e c o u p l i n g i s a novel methane c o n v e r s i o n process which w a r r a n t s f u r t h e r study.

When combined w i t h an e t h y l e n e o l i g o r n e r i s a t i o n s t e p i t i s a p o t e n t i a l

a1 t e r n a t i v e t o c o n v e n t i o n a l processes, based on s y n t h e s i s gas, f o r p r o d u c i n g l i q u i u f u e l s froni methane.

However, f u r t h e r r e s e a r c h i s necessary t o p r o v i d e

t h e i n f o r i n a t i o n r e q u i r e d t o assess t h e comriiercial p r o s p e c t s f o r t h i s r o u t e . ACKI4UWLEUbCitkTS bir A. McCutcheon a s s i s t e u i n o p e r a t i n g t h e f i x e d - b e d r e a c t o r .

T h i s work

forms p a r t o f a l a r b e p r o j e c t i n v o l v i n g C S I K O and BHP, and s u p p o r t o f t h e program by t h e N a t i o n a l Energy Research, Development and Demonstration Council i s g r a t e f u l l y acknowledged. REFEREKES

1 2 3 4

T. I t o and J.H. L u n s f o r d , Nature, 314 (1985) 721-722. T. I t o , J-X. Wang, C-H. L i u and J.H. L u n s f o r d , J . Am. Cherri. SOC., (1985) 5062-5066. C.H. S a t t e r f i e l d , Class T r a n s f e r i n Heterogeneous C a t a l y s i s , M.1.T Cambridge, I4assachusetts, 1970. K.J. T y l e r and C.A. Lukey, hi. Cherii. SOC. D i v . Fuel Chem. P r e p r . press.

107 Press, 1987) i n


403

D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SELECTIVE OXIDATIVE COUPLING OF METHANE TO ETHANE AND ETHYLENE

KENJI ASAMI, SHIGERU HASHIMOTO, KAORU FUJIMOTO and HIRO-0 TOMINAGA Department o f S y n t h e t i c Chemistry, F a c u l t y o f Engineering, The U n i v e r s i t y o f Tokyo, Hongo 7-3-1,

Bunkyo-ku,

Tokyo 113

(Japan)

ABSTRACT

It has been found t h a t l e a d o x i d e c a t a l y s t s supported on b a s i c c a r r i e r s such as MgO o r @'-A1 O3 e x h i b i t e x c e l l e n t a c t i v i t y and s e l e c t i v i t y f o r t h e L a t t i c e oxygen i s proved t o be r e s p o n s i b l e f o r o x i d a t i v e c o u p l i n g o f methane. t h e f o r m a t i o n o f C2 hydrocarbon.

INTRODUCTION S y n t h e s i s o f ethane and e t h y l e n e b y t h e o x i d a t i v e c o u p l i n g o f methane i s a c u r r e n t s u b j e c t o f g r e a t s i g n i f i c a n c e t o a novel use o f n a t u r a l gas. c a t a l y s t s have been r e p o r t e d t o be a c t i v e f o r t h e r e a c t i o n ( r e f s .

Several

1-5).

The

p r e s e n t a u t h o r s have a l s o r e p o r t e d t h a t an MgO-supported PbO c a t a l y s t has h i g h a c t i v i t y and s e l e c t i v i t y f o r t h e f o r m a t i o n o f C2t hydrocarbons ( r e f .

6).

I n t h e p r e s e n t work, s e v e r a l supported PbO c a t a l y s t s have been t e s t e d f o r t h e i r r e a c t i v i t y and found t h a t b a s i c m a t e r i a l s such as MgO o r B"-A1203 a r e e x c e l l e n t c a r r i e r s f o r PbO which make C2 hydrocarbons f r o m methane.

The

d e t a i l s o f t h e c a t a l y t i c f e a t u r e s o f PbO/MgO and t h e r e a c t i o n mechanism have been s t u d i e d .

EX PER IMENTAL A l l c a t a l y s t s were prepared by i m p r e g n a t i n g t h e c a r r i e r m a t e r i a l s w h i c h had been c a l c i n e d a t 800

OC

f o r 2 h. w i t h l e a d n i t r a t e f r o m i t s aqueous s o l u t i o n ,

f o l l o w e d by d r y i n g i n a i r a t 120 O C f o r 12 h. wt%.

The standard PbO l o a d i n g was 20

They were a c t i v a t e d i n f l o w i n g a i r a t 750

OC

f o r 0.5-1

h.

Reactions were conducted w i t h a f l o w t y p e r e a c t i o n apparatus under a t m o s p h e r i c pressure. temperature:

750

OC.

The s t a n d a r d r e a c t i o n c o n d i t i o n s were as f o l l o w s : t i m e f a c t o r (W/F):

1.0 g'h/mol,

14%, 02 1.6%. N2 b a l a n c e , c a t a l y s t w e i g h t : 1.0 g.

feed gas c o m p o s i t i o n : CH4

404 RESULTS AND DISCUSSION Support e f f e c t on PbO c a t a l y s t

8

The r e l a t i o n s h i p between s e l e c t i v i t y and y i e l d o f C2 hydrocarbons o f a l l c a t a l y s t s t e s t e d ( F i g u r e 1) shows t h a t

6

t h e m a t e r i a l s which have b a s i c c h a r a c t e r . a r e t h e most e f f e c t i v e c a r r i e r s o f PbO f o r making

0 acidic A neutral

B"-Al203

.basic

U

Cz

hydrocarbons. The r e s u l t s a r e summarized as follows: (1) A c i d i c c a r r i e r s had h i g h a c t i v i t y and s e l e c t i v i t y t o carbon oxides.

0

(2) N e u t r a l and weakly a c i d i c o r b a s i c

20

40

60

C2 selectivity

c a r r i e r s showed l o w a c t i v i t y b u t

80

100

(C-mol%)

Fig. 1 E f f e c t o f c a r r i e r m a t e r i a l PbO l o a d i n g : 20 wt%. Temp.; 750 OC, W/F: 1.0 g'h/mol,

f a i r l y h i g h s e l e c t i v i t y t o C2 hydrocarbons.

(3) B a s i c c a r r i e r s had h i g h a c t i v i t y and s e l e c t i v i t y f o r C2 hydrocarbon formation. The reasons why a c i d i t y o r b a s i c i t y o f t h e supports s i g n i f i c a n t l y i n f l u e n c e d t h e r e a c t i o n w i l l be d i s c u s s e d i n a l a t e r paper. C h a r a c t e r i s t i c f e a t u r e s o f t h e PbO/MqO catalyst S i n c e an MgO-supported c a t a l y s t showed t h e h i g h e s t s e l e c t i v i t y f o r C2 hydrocarbon,

t h e d e t a i 1s o f i t s The e f f e c t s o f PbO

l o a d i n g a r e shown i n F i g u r e 2.

100

8

80

7 6

60

z 4

40

-

MgO,

3 0

hydrocarbon y i e l d ) .

Both the c a t a l y t i c

e

C

f r

E 0

C >

2

20

i t s e l f , showed some c a t a l y t i c a c t i v i t y

(0.8% o f C H 4 c o n v e r s i o n a n d 0.39% o f C2

-2 rl

e

0

c a t a l y t i c f e a t u r e s were studied. (i)PbO loading.

10

N

0

0

0 PbO loading

(wt%l

a c t i v i t y and s e l e c t i v i t y t o C2 hydrocarbon i n c r e a s e d w i t h i n c r e a s i n g PbO l o a d i n g r e a c h i n g a maximum a t 5 w t %

(72% C2 s e l e c t i v i t y ) and then decreased as t h e l o a d i n g increased.

Fig. 2

E f f e c t o f PbO l o a d i n g

~ ~ ~ p ) ' 0~~'=w~~:11iP69;h~~P:: ~57

405

800

OC

as demonstrated i n F i g u r e 3,

because t h e p r o m o t i v e e f f e c t o f

u 0

20

-

CH4

*-

-

I*

The i n i t i a l CH4 c o n v e r s i o n r a t e t o C2 hydrocarbons (4.0 mmol/g'h)

3 min.

-f:

. P

4 0

E

I 0)

d

e r(

Ll R

>C U

-n X

Fig. 4 T r a n s i e n t response o f methane c o n v e r s i o n on PbO/MgO c a t a l y s t PbO l o a d i n g : 20 wt%. Temp.: 750 OC, W/F: 4.3 g'h/mol.

,..

20 2

.

was

406

TABLE 1. Amount o f t h e consumed b u l k oxygen i n t h e c y c l i c r e a c t i o n . I n i t i a l amount o f PbO 0.896 (mmol) Temperature (OC)

Consumed oxygen (X) (mmol)

~

0.003

650

700

0.085 0.189 0.204

750

800

0.3

9.5 21.1 22.8

f a i r l y close t o t h a t i n the steady s t a t e r e a c t i o n (2.8 mmol/g'h).

whereas t h e

c o n v e r s i o n r a t e t o CO2 (0.2 mmol/g'h)

is

much l o w e r t h a n t h a t i n t h e s t e a d y s t a t e (1.4

mmol/g'h).

As gas phase oxygen

does n o t e x i s t i n t h i s r e a c t i o n system, l a t t i c e oxygen o f PbO must be responsible f o r t h e coupling reaction.

1

( C )

Table 1 shows t h e c a l c u l a t e d amounts

Pb

o f l a t t i c e oxygen o f PbO w h i c h were consumed d u r i n g t h e 12 m i n u t e s o f

I t s h o u l d be n o t e d t h a t t h e

reaction.

amount o f consumed l a t t i c e oxygen increased w i t h increased r e a c t i o n temperature,

w h i c h c o u l d be a t t r i b u t e d

t o t h e increased d i f f u s i o n r a t e o f t h e b u l k oxygen i n PbO. XRD p a t t e r n s o f a PbO/MgO c a t a l y s t

t

I

25

30 deg

under a v a r i e t y o f c o n d i t i o n s a t room

I

I

35

40

128

t e m p e r a t u r e show, as i l l u s t r a t e d i n F i g u r e 5,

t h a t both t h e fresh c a t a l y s t

and t h e used c a t a l y s t ,

w h i c h were

o x i d i z e d b y a i r a f t e r methane conversion,

gave s i m i l a r p a t t e r n s (a) o r

(b) i n w h i c h s t r o n g peaks o f PbO were

3

F'g.

5

a fresh

XRD p a t t e r n s o f PbO/MgO

b)after a i r oxidation i n the c y c l i c reaction ' ) a f t e r CHq c o n v e r s i o n i n t h e c y c l i c reaction.

407

observed.

P a t t e r n ( c ) was o b t a i n e d f r o m a c a t a l y s t a f t e r t h e c y c l i c methane

r e a c t i o n , showing weak peaks o f PbO and s t r o n g peaks o f m e t a l l i c Pb.

It

i n d i c a t e s t h a t PbO i s reduced t o Pb by r e a c t i n g w i t h methane and i s r e o x i d i z e d b y 02.

Thus t h e o x i d a t i v e c o u p l i n g o f methane i s i n f e r r e d t o proceed b y a

redox c y c l e between Pb(0) and Pb(I1).

as demonstrated i n e q u a t i o n s (1) and

(2). PbO

+

2

CHq-Pb

+

CH3CH3

t

H20

(1)

K i n e t i c s t u d y suggests s t r o n g l y t h a t r e a c t i o n (1) i s r a t e d e t e r m i n i n g .

The

s t u d y a l s o suggests t h e COX f o r m a t i o n i s m a i n l y c a t a l y z e d n o t b y l a t t i c e oxygen but by adsorbed oxygen on t h e PbO surface. REFERENCES 1

2

3 4 5 6

G.E.

K e l l e r , M.M. Bhasin, J. Catal., 73 (1982) 9-19. Hinsen, W. Bytyn, M. Boerns, Proceedings 8th I n t e r n a t i o n a l Congress on C a t a l y s i s , Vol. 3 (1984) 581-592. K. Otsuka, K. Jinno, A. Morikawa, Chem. Lett., (1985) 499-500. T. I t o , J.H. Lunsford. Nature, 314 (1985) 721-722. H. Imai, T. Tagawa, J. Chem. SOC., Chem. Commun., (1986) 52-53. K. Asami, S. H a s h i m o t o . T. S h i k a d a , K. F u j i m o t o , H. Tominaga, Chem. L e t t . , (1986) 1233-1236.

W.


409

D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors),Methane Conversion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

OXIDATIVE DEHYDROGENATION OF METHANE TO FORM HIGHEK HYDROCARBONS F.P. LARKINS and M.R. NORDIN Department of Chemistry, Wliversity of Tasmania, G.P.O. Tasmania 7001 (Australia)

Box 252C, Hobart,

ABSTRACT The oxidative dehydrogenation of methane over a range of materials has been investigated. Results obtained primarily at 800°C are reported here. It is shmn that the extent of CHI+ conversion and the selectivity t o C2 hydrocarbons depend on the nature of the catalyst. 'Ihe @O catalyst a c t i v i t y may be altered with L i loadings as low as 0.1 w%. Ibping with a transition metal can also have a significant effect on C2 selectivity. 'Ihe reaction is sensitive t o the presence of CO.

I~ O D U C T I O N Homlytic reactions

of

methane t o

form hydrocarbons

(HC's)

are not

thermodynamically feasible u n t i l 11200 K, while the oxidative routes t o HC's are favoured (ref. 1) a t lmer temperatures. Lhfortunately, the exhaustive oxidation of HC's t o carbon oxides are more thermodynamically favoured and are often kinetically preferred. In order t o reduce the gasaseous phase reaction the product should be immediately quenched; ideally the heating should be localised t o the catalyst bed. The reaction

kinetics

in

oxidative dehydrogenation

were

found

t o be

influenced by the presence of surfaces (refs. 2-5). The r e s u l t s of s t d i e s with a range of &,O

related catalysts for the oxidative dehydrogenation of methane

a r e reported i n t h i s paper. 'Ihe extent of gaseous phase oxidation i s also examined. EXPEKIMENTAL

Most @@based catalysts w r e prepared by impregnation using the slurry method of

Lunsford e t al.

(ref.

3).

For the three component system the

transition metal nitrate was used. b s t catalysts were precalcined i n air a t

900°C for 10 h r , crushed and sieved t o a p a r t i c l e size of less than 43 m. The microreactor used for evaluating catalyst activity wa s operated a t atmospheric pressure and consisted of Porter manual mass flow controllers, and a tubular reactor made from fused alumina of 11 m 0.d. measured by means of

'Ihe tenperature was

an i n e r t internal thermocouple and controlled by an

electronic temperature controller. Product gases were analysed by the g.c.

410

80 60

40

20 0

10'1

100

101

Log ( Wt.%(Li/(Li+Mg))) Fig. 1. Conversion and selectivity of J i / F Q O catalysts as a function of Li loading a t 800°C. technique. b l e s s otherwise stated the reactant gas mixture consisted of 100 T of Q , and 210 'I of CH,, in helium a t a t o t a l flaw r a t e of 25 ml/min. The c a t a l y s t (0.2 q ) was preheated i n a i r a t 800°C for 2 hr before the reactants were passed over it. 'Ihe conversion data a f t e r 2 hr on stream a t 800°C are used i n t h i s paper. For most of the run >95%carbon balance was achieved. Chemicals used i n t h i s w r k were obtained from BDH and Strem; C.P. Matheson's methane, 99.99% pure, helium and 0 2 gases from C.I.G. were used. RESULTS

1. Blank run The gaseous phase oxidation of methane a t different temperature w a s studied. Under the condition for catalytic a c t i v i t y determination ( 1988 Elqevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlands

M

433

S SCIJRRELL and M COGkS

Cataiysis Division: Nation& Counci

In;titutc?

for Chemica; Engineering Researcrl.

for Scientific and Industrial Research, P . 9 . B o x 395, Pretoria 0001,

Republic of South Africa

ABSTRACT Methane-ethylene mixtures react over sulphate-treated zirconia catalysts. to yield higher hydrocarbons. Initially, C products predominate but with increasing time on stream lighter produczs are preferentially formed. There is a concomitant decline in overall catalytic activity. With ethylene alone, a different pattern of behaviour is found and it appears that the methane in the methane-ethylene mixture exerts a decided effect on the course of the reaction. Sulphate-treated zirconia undergoes thermally induced recrystallization in a different manner from that displayed by sulphate-free zirconia. INTRODUCTION There is at present considerable interest in effecting the catalytic conversion of methane to higher hydrocarbons such as light alkenes or t o methanol [l-31.

Much of the work has been aimed at carrying out selective

oxidation [2,4], which would provide an alternative approach to the steam reforming of methane [ 5 ] that is conventionally employed as a means o f obtaining more desirable products via processes based on synthesis gas. Reactions involving the use of hydrocarbons alone have been less well studied, but examples of methane-ethylene coupling being catalysed by superacids such as TaF5-HF [6] or TaF5 on fluorided alumina [ 7 ] have been presented. Another type of catalyst claimed to exhibit superacidity [8] is exemplified by sulphate-treated zirconia.

This solid has now been investigated as a

catalyst for converting methane-ethylene mixtures to higher hydrocarbons.

434

EXPERIMEhTkL

Zr02 was obtained from ZrC14 via hydrolysis and subsequent precipitation with aqueous ammonia.

The precipitate was washed and dried at llOuC. Sulphate

ions were introduced (to a level of ca 3 wt%] by impregnation with a solution of ammonium sulphate (incipient wetness technique), followed by drying first at 45 " C , and then at 110 "C [ 8 ] .

The solids were screened and the 35-50 mesh

portion used for microreactor experiments, using a fixed-bed tubular reactor. The catalyst was pretreated in flowing nitrogen at ca. 500°C for at least 16 h and then cooled to the desired reaction temperature (usually 300 "C). The methane-ethylene (v. ethylene] feed was passed at a GHSV of 960, and a total pressure of 1.95 MPa. 2.0 and 3.0 was used. chromatography.

For mixed feeds, a mol ratio CH4: C2H 4 of between Products were analysed on line using gas-liquid

A l l lines downstream of the reactor were held at 200 'C in

order to prevent condensation of the products, Reaction conditions were chosen after consideration of the thermodynamics of the desired coupling reaction (Figure 1).

In most cases, direct evidence for methane consumption was

obtained by monitoring methane/argon fractions in the product obtained from feeds in which ca 3 mol % argon had been added as a tracer. Specific surface areas of ZrOZ and ZrOZ-SO$'

were determined by the BET

method, using nitrogen as the adsorbate. RESULTS AND DISCUSSION Based on the data depicted in Figure 1, the fraction of propane formed (Equation 1) at equi,librium at 600 K for 1:l CH4:C2H4 mixture was calculated to be 36, 63 and 79 mol % for total pressures of 0.1, 0.5 and 2.0 MPa respectively.

CH4

t

C2H4 - > C H 3 8

Therefore, under the reaction conditions employed, substantial conversion of the methane-ethylene mixture would be permitted on thermodynamic grounds. Figure 2 shows the product distribution obtained for reaction of methane with ethylene.

On fresh catalyst, the major products were C6 and C7

hydrocarbons, but with increasing time on stream lighter products appeared and eventually almost equal quantities of C3, C4 and C5 hydrocarbons were formed.

A large fraction ( > ca 70%) o f the hydrocarbons were saturated and,

in the C4+ products, iso-structures predominated at all times.

The product

spectrum for intermediate times on stream closely resembled that reported for the coupling of methane and ethylene over TaF5 on fluorided alumina [7]. change in the product spectrum with time on stream was accompanied by a

The

435

+ 80 t60

-2 t 4 0 E I-!

3

+20

\

OL (3

a

. -20 -40

FIGURE 1

Standard free energies o f reaction between methane and unsaturated

hydrocarbons.

60-

*

55 rnin

40.

I,

*

20-

QI

* I

3

0

4

.I 5

I

FIGURE 2

L 305 rnin

6

7

3

4

5

6

7

Cn PRODUCT

Product distributions obtained from reactions o f methane and ethylene

over Zr02-S02and, for comparison (from ref. 7), over TaF5-A1203(F). 4 stream are indicated for ZrO2-SO:-

data.

Times on

436

substantial deactivation of the catalyst.

Between 55 and 1380 min on stream,

the absolute activity fell by about 36 times (Table 1 ) . When ethylene alone was fed to a ZrO2-SO;again resulted (Figure 3).

catalyst, higher hydrocarbons

However, in contrast to the runs in which both

methane and ethylene were fed, the product spectrum consisted of relatively light hydrocarbons.

Further, with ethylene alone, the tendency is for h i g h e r

rather than l i g h t e r products to be formed at longer times on stream.

From

these observations, and from the direct evidence (argon tracer runs) for methane consumption on fresh catalyst, it is concluded that the presence of methane in the methane-ethylene mixture exerts a decided effect on the catalytic behaviour.

It would be of interest to investigate the extent of

incorporation of the methane carbon into the product hydrocarbons using, for example, 13CH4.

However, the rapid onset of deactivation, which is probably mainly due to the formation of carbonaceous residues on the surface, presents a major obstacle.

In the experiments described here

quantity of carbon converted is less than 0.05 g g-hat.

the total

Neve theless,

even deactivated catalysts possess activities for ethylene 01

gomerization

comparable to those displayed by HZSM-5 (Table 1 ) . TABLE 1

Rates of reaction of hydrocarbons.

~

Catalyst 2-

~~

Reactant(s)

Time on stream (min)

a

Rate g

CHq

t

C2H4

55

,a ZrG2-SOi-

CHO

t

C2H4

1380

2.2

HZSM-5b'C

2' H4

-

2.5

2 r0,-SO4

Experimenta I conditions:

aCH4:C2H4

HCS

-1

-1

gcat

ca 8.0 x

= 2.0-3.0 mol mol-l; pressure =

1.95 MFa; temperature = 300 "C; GhSV = 960

Opressure = 1.4 MPa: temperature = 288 " C Cfrom ref. 9 Finally, Figlire 4 deinonstrates that the recrystallization of ZrO,, i

reflected by a rrdhction in specific surface area on thermal threatment, i s retarded by the incorporation of su1y;hate ions.

XRO

studies

[lo]

reveal that,

dfter treatment at 500 " C (the temperature required to ach3eve conversion o f

437

methane-ethylene mixtures), sulphate-free zirconia exists in a monoclinic phase, whereas in sulphate-treated zirconia a cubic phase dominates.

It is

considered likely that the different recrystal lization pathway followed by sulphate-treated zirconia is connected with the superacidic properties exhibited by this solid.

This aspect is under further investigation at

present.

J

X 0 W

- N

0 0

26

g 3

C, PROWCT

FIGURE 3

Product distributions obtained from reaction o f ethylene over

ZrOq-SO~-. Times on stream are indicated.

I

200

ua

600

800

PRE-TREATMENT TEMPERATURE P C

FIGURE 4

Specific sLrface area as a function of pretreatment temperature for

( 0 ) Zr02 and

( 0 )Zr02-SOi-.

438

ACKNOWLEDGEMENTS This work was partly financed by the Foundqtion for Research Development of the CSIR. The authors express their thanks to M.G. tiowden, J. Vink and H.E.L.G. Schweigart for their assistance with this work. REFEREEvCES 1 C.A. Jones, J.J. Leonard and J.A. Sofranko, Energy and Fuels, 1 (1987) 12. 2 N.R. Foster, Appl. Catal., 19 (1985) 1. 3 M.S. Scurrell, Appl. Catal., in press. 4 G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, this meeting. 5 J.R. Rostrup-Nielsen in: Catalysis-Science and Technology, J.R. Anderson and M. Boudart (Ed.), Springer-Verlag, Berlin 1984, Vol. 5, p. 1. 6 M. Siskin and I . Mayer, U.S. Pat. 4 094 924 (1978). (Exxon Res. and Engng co. 1 7 G.A. Olah, Eur. Pat. Appl. 73 673 ( 1 9 8 3 ) . 8 K. Tanabe, T. Yamaguchi, K. Aikiyama, A. Mitoh, K. Iwabuchi an K. Isogai, Proc. 8th Int. Congr. Catal., West Berlin, 1984, Vol. V, p. 601. 9 W.E. Garwood, Amer. Chem. SOC.; Div. Petr. Chem, Las Vegqs Meeting, 1982. !O M.S. Scurrell, submitted for publication.

D.M. Bibby,C.D. Chang,R.F. Howe and S.Yurchak (Editors),Methane Conversion 0 1988 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

439

ENGINEERING ASPECTS OF ALTERNATIVE ROUTES FOR THE CONVERSION OF NATURAL GAS

P.J. JACKSON and N. WHITE

BHP Melbourne Research Laboratories, 245 Wellington Road, Mulgrave, Victoria, 3170. Australia

ABSTRACT An alternative route for the conversion of natural gas to liquid fuels, which employs acetylene rather than the more conventional synthesis gas intermediate, was studied to identify some of the engineering problems and cost-sensitive areas of processing. The limitations of this route as a model for other alternative processes are discussed. INTRODUCTION Interest in the development of alternative routes for the conversion of natural gas to "transportable" products has been stimulated by the projects in this area in New Zealand. The current technologies for gas conversion are nearly all based on the conversion of natural gas to synthesis gas as a first step.

In the so-called "alternative" technologies, other intermediates are

produced in the first step of the process.

These alternative processes fall

into three categories: 1.

Partial Oxidation/Pyrolysis.

This involves the reaction of natural

gas with or without a controlled amount of oxygen at elevated temperatures in the presence or absence of a catalyst to produce olefins/acetylene and/or oxygenates. The heat source may be a hot surface, plasma, laser or simultaneous exothermic reaction, e.g., from co-production of carbon monoxide, carbon dioxide and water in varying amounts.

2.

Protonation. The reaction of methane with superacids t o produce

protonated species which may then produce olefins and/or higher alkanes. 3.

Oxidation by Oxidants other than Oxygen. Methane can be reacted with

chlorine, for example, to produce chlorinated hydrocarbons which can be converted to olefins by subsequent reaction with oxygen. The potential advantages of these alternative processes are :

. . .

a higher thermal efficiency, a higher selectivity and/or conversion, and fewer processing steps

than synthesis-gas-based processes.

440

The goal of this sLudq. was t o select such a route, develop the ovcrall process concept and carry out

ail

engineering/economic feasibility study on it.

The process selected was based on the conversion of methane 2CHq

---->

C 11 2 2

+

to

acetylene,

3H2

and the subsequent hydro-oligomerisation of the acetylene to higher hydrocarbons C H

2 2

t H2

---->

2(-CH2-)

Our interest in selecting this route arose because of its superficial simplicity and the developments made i n our laborator!? in the hydro-oligomerisation of acetylene. be noted.

Nevertheless, a number of caveats need to

Firstly, compared to many of the other alternative processes, the

production of acetylene requires much higher temperatures, generally in the order of 1600°C. The reaction system must be quenched rapidly to prevent decomposition of the acetylene.

Secondly, acetylene possesses unique

detonation characteristics by comparison with other potential intermediates, and requires some special engineering considerations. Therefore, the problems associated with this acetylene-based route would not necessarily be encountered in other alternative routes. PRODUCTION OF ACETYLENE Acetylene is produced currently from natural gas but at conversions and selectivity that rule out consideration in the present study. A survey was made of the routes available which included commercial as well as proposed non-commercial processes (refs. 1-3).

These were:

Plasma Routes/Electric Arc These routes have some very attractive features, with hydrogen, acetylene and a little coke being almost the sole products.

The systems can be made

self-quenching which is a major advantage; however, the conversion per pass is at most 50% with unacceptable power consumption.

Indeed, we estimate that the

power generating facilities for such a route alone would cost about as much as a Fischer-Tropsch plant with the same net hydrocarbon production rate. To the best of o u r knowledge this route is not presently practised on a commercial scale. Hot Surfaces Two variations have been described.

In the cyclic route, a chequerbrick

surface is intermittently heated by combustion of natural gas with air followed by the introduction of natural gas alone for reaction, until the

441

temperature falls, where>Jponthe heating cycle is recommenced. The continuous route is typified by the tubular reactor which is heated electrically on the external surface and natural gas is then passed through the tube.

Processes

employing such routes tend to be difficult to control, and wasteful coke production is substantial. Only the cyclic route is practised commercially. Partial Oxidation and Pyrolysis A carefully controlled oxygen/natural gas flame can produce acetylene if

the flame is quenched after an appropriate interval. However, in all current processes, the final distribution of carbon is such that for every mole of carbon reporting to acetylene approximately two moles of carbon report to carbon monoxide, making this route primarily a synthesis gas producer. The vast amount of sensible heat in the exhaust gases needs to be recovered to maintain a reasonable thermal efficiency. By far, most of the acetylene produced presently employs this route. Carbide Route This is a variation of the coke-based route in which natural gas rather than coal or petroleum-derived coke is used to produce calcium carbide, which on treatment with water yields very pure acetylene, but with very poor overall thermal efficiency. The extent of usage of natural gas for the carbide route is not well documented. Clearly, none of these routes fulfilled our requirements, and a search commenced for a suitable alternative. A reactor system based on proprietary technology was eventually located which appeared to have the right characteristics, viz., good control of residence time, capable of maintaining isothermal conditions over a wide variety of residence times, and very compact. The principle of operation involved combustion in a hydrogen/oxygen flame; reactor exit gases emerged at 16OO0C, but provisions could be made for quenching facilities. Since this proprietary reactor system had not been tested under the conditions required for acetylene production, a model was used to simulate the conditions of acetylene production for the various process configurations that were investigated. The model had been applied under other conditions and the results verified experimentally. It incorporated 250 reactions and 40 species, with all the necessary thermodynamic data to achieve heat and mass balance. The model was run to determine product distributions for three different process parameters which had been identified as having the most impact on process economics. These parameters were the ratio of hydrogen plus oxygen to

442

methane feed, the overall methane feed and the oxygen consumption. Some general guidelines for ranking the preference for different products are:

HZ

required for reactor flame and hydro-oligomerisation

H20

the most desirable end-point for oxygen elimination

co

acceptable because it can be shifted to hydrogen the least desirable product, represents wasted carbon and

c02

unnecessary consumption of oxygen

CH4

unconverted feed, may need to be recovered for feed or fuel

C2H2

the desirable product acceptable by-product (can also be oligomerised)

C2H4 C(so1id)

undesirable product undesirable higher acetylenes, e.g., C4H2

c2+

RESULTS OF MODEL STUDIES Preliminary engineering studies revealed that a substantial capital saving would be made if the reactor stream could emerge at a pressure in vicinity of However, at pressures above 0.15 MF’a the model showed that a

about 0.5 MPa.

substantial l o s s in acetylene selectivity occurred, which is not unexpected in view of the extensive literature on the subject of acetylene formation. Other results to emerge were:

. . .

Oxygen consumption is 0.8-0.85 moles per mole methane Water make is 1.9 moles per mole methane Net hydrogen make (after allowing for flame hydrogen) and hydrogen from shifting of CO was 0.18 moles per mole methane; this is about 50% of the requirement for subsequent down-stream processing

,

The carbon distribution in the exit gases is: CZH2

‘ZH4

co

73.5%

5.6% 12.2%

co2

2.4%

C(S)

3.4%

CqH2, etc.

2.9%

. - The per

pass methane conversion is 58%

Consideration of these results led to the selection of the process configuration as shown in Fig. 1.

Three engineering problems need to be

considered: the recovery and utilisation of waste heat, the separation of the gas streams, and the overall process integration.

443

e l t Prebumer

LPG Sap.

I

CZt Gas

CH4

I c02 Quench

Waste Boiler

4

Gas Separation

L

Fig: 1:

H2

Conversion

C2+ Gas

I

METHANE TO LIQUID FUELS VIA ACETYLENE

ENGINEERING DESIGN SUMMARY Heat Recovery The major source of sensible heat is in the reactor product which emerges at 1500-1600OC.

The initial (and most cost-effective) quench step is carried

out using ethane which takes the temperature down to 1150°C. The ethane cracks to ethylene rapidly absorbing a considerable amount of heat. It is therefore useful to remove as much ethane from the feed gas as possible; by-product ethane is also available from the hydro-oligomerisation (see below). Two options were studied for quenching to a temperature at which acetylene no longer decomposes. The gas stream must be further cooled before entering the first phase of gas separation. The first and most obvious approach is to use a water quench. This does not permit any of the sensible heat to be recovered and results in a substantial "dirty" waste water stream which must be treated. The other approach, which was finally adopted, was to use a recycle gas quench (about 4 to 1) which cooled the reactor gas to 700OC. Following this, the total gas stream was passed through a series of Transfer Line (heat) Exchangers (TLE).

444

TLE's are capable of reducing the temperature of gases from about 700°C to 150°C in milliseconds, simultaneously generating high quality steam, and should have been ideal for the process under study. Enfortunately their use gave only an increase of 2.5% in thermal efficiency for a 6% increase in overall capital cost, making their incorporation into the process flowsheet a marginal proposition. Heat Utilisation

All power is generated through waste heat. One of the major power consumers was the Charge Gas Compressor (25% of total) which pressurises the cooled reactor gas to the pressure required for the first phase of gas separation. This is the penalty that must be paid f o r the low pressure operation of the first stage of conversion (cf. syngas generation which can be run at the relatively high pressures of about 2MPa).

The Oxygen Plant and

Feed Compressors also consume 25% of the power and attest to the need.to keep oxygen consumption as low as possible. The Cryogenic Separation (excluding Energy balance from waste heat

Oxygen Plant) consumes 20% of the waste heat.

was not possible and the overall natural gas feed had to be supplemented by 5% f o r fuel.

Gas Separation Since acetylene cannot be separated cryogenically (solid acetylene has detonating properties), it must be removed prior to Cryogenic Separation, which is the major gas separation step. The most cost effective method of acetylene recovery is gas absorption using dimethyl formamide (DMF) as a solvent. Prior to this step, however, it is necessary to remove the diacetylenes, which are sent to fuel, and C02, which is removed using a mono-ethanolamine ( M E A ) scrub. After removal of the diacetylenes, the C02 and C2H2, the remaining gas enters the Cryogenic Separator, where the following separations and disbursements are made: H2

to acetylene production and hydro-oligomerisation

CH4

recycled to acetylene production

co

shifted to hydrogen for hydrogen make-up

C2H4

to oligomerisation

PROCESS INTEGRATION When linking the hydfo-oligomerisation into the rest of the process, it was necessary to account for the hydrogen requirements of this step. A key issue was the fact that the major gaseous by-product of the hydro-oligomerisation is ethane, and this can be used to good effect in the reactor quench.

445

While a substantial amount of heat is generated in the hydro-oligomerisation step, no attempt was made to recover it because of its low quality. CAPITAL COST SUMMARY (ACETYLENE PRODUCTION ONLY) PercentaRe

Ma.jor Components

2

Natural Gas Treatment

9

Oxygen Plant and Compression Reactor System

11

Waste Heat Recovery and Quench

28

5

Gas Compression for Separation

12

Gas Separation Complex Water Treatment

5

Power, Steam, Other off-site

5 8

Utilities

15

Contingencies Total :

100%

It was estimated that this process route was approximately twice as costly as converting natural gas to liquid fuels via synthesis-gas-based routes such as Fischer-Tropsch. However, some novel developments in gas separation technology and high temperature heat recovery could bring the process back into contention.

CONCLUSIONS AND GUIDELINES FOR PROCESS DEVELOPMENT

.

Low pressure operation of the first stage of conversion leads to

.

Oxygen Consumption must be minimized because it is costly to make

.

Conversion and selectivity in the first stage should be high to

. .

penalties in capital cost and downstream processing. and needless consumption results in wasteful CO make.

2

minimise gas separation and recycle to feed. Acetylene routes have some peculiarities not shared by other alternative gas conversion routes. The ability to consume, in the upstream processing, a limited amount of the by-products of the oligomerisation has some benefits.

REFERENCES 1 S.A. Miller, Acetylene - Its Properties, Manufacture and Uses, Vol. 1, Ernest Benn, London, 1965. 2 R.E. Kirk and D.F. Othmer, Encyclopedia of Chemical Technology, Vol. 1, John..Wiley, New York, 3rd ed., 1978. 3 R. Muller and G. Kaske, Erdol und Kohle - Erdgas - Petrochemie, 37(1984) 149-155.


447

D.M. Bibby, C.D. Chang, R.F. Howe and S.Yurchak (Editors), Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE SASOL ROUTE TO CHEMICALS AND FUELS MARK E. DRY

P 0 Box 1, S a s o l b u r g 9570, S o u t h A f r i c a

Sasol R & D Department,

ABSTRACT The wide range o f p r o d u c t s formed i n t h e F i s c h e r - T r o p s c h process can be an economic advantage. The waxes have many i n d u s t r i a l a p p l i c a t i o n s . The C2 t o C4 o l e f i n s can be up t o 34% o f t h e t o t a l p r o d u c t . By a p p l y i n g b o t h o l e f i n o l i g o m e r i z a t i o n as w e l l as wax h y d r o c r a c k i n g t h e d i e s e l f u e l t o g a s o l i n e r a t i o can be v a r i e d from about 0.1 t o 3. The f u e l s meet a l l t h e r e q u i r e d s p e c i f i c a t i o n s . Sasol i s c o m m e r c i a l i z i n g i t s r e s e a r c h f i n d i n g s . A 4 5 b a r f.ixed bed r e a c t o r i s under c o n s t r u c t i o n . As an a l t e r n a t i v e t o t h e Synthol r e a c t o r a " f i x e d f l u i d i z e d bed" r e a c t o r has been s u c c e s s f u l l y developed. INTRODUCTION AND ECONOMICS The p r o d u c t i o n o f s y n t h e s i s gas i s an expensive process i r r e s p e c t i v e o f whether one s t a r t s w i t h methane o r c o a l . partial

o x i d a t i o n of

endothermic,

coal

or

The chemical reason i s t h a t t h e

methane w i t h

steam t o f o r m CO and H2 i s

and a h i g h energy i n p u t i s t h e r e f o r e needed.

from b u r n i n g a d d i t i o n a l CH4 o r c o a l .

T h i s energy comes

A p r e r e q u i s i t e f o r an economic process

i s t h e r e f o r e t h a t CH4 o r c o a l must be a v a i l a b l e a t low c o s t . The s y n t h e s i s gas can be c o n v e r t e d t o f u e l s and chemicals e i t h e r d i r e c t l y

( F T ) process o r i n d i r e c t l y ,

using t h e Fischer-Tropsch

by f i r s t producing

methanol and t h e n c o n v e r t i n g i t t o o l e f i n s and/or f u e l s . The c o s t o f p r o d u c i n g t h e s y n t h e s i s gas i s more t h a n double t h a t o f converting

it t o

oligomerization,

crude

fuels.

The

c o s t o f subsequent work-up

h y d r o f i n i n g , g a s o l i n e reforming,

t h e c o n v e r s i o n step.

(olefin

e t c ) i s about h a l f t h a t o f

The r e f i n i n g c o s t i s t h e r e f o r e o f l e s s e r importance

when d e c i d i n g on which r o u t e t o f o l l o w .

The c h o i c e should r e s t on t h e t y p e

o f chemicals and t h e spectrum o f f u e l s r e q u i r e d as w e l l as on t h e proven r e l i a b i l i t y o f t h e processes. The Sasol coal-based

FT processes have been o p e r a t i n g s i n c e 1955.

The

c u r r e n t c o a l consumption i n t h e t h r e e commercial p l a n t s i s 36 m i l l i o n t o n p e r year.

O r i g i n a l l y t h e South A f r i c a n Government h e l d t h e m a j o r i t y o f t h e Sasol

shares, b u t i n 1979 these shares were s o l d on t h e Johannesburg Stock Exchange and so Sasol became a p r i v a t e s e c t o r company.

I n 1986 S a s o l ' s p r e - t a x p r o f i t

was R1 189 m i l l i o n . I n South A f r i c a t h e p r i c e o f f u e l s i s based on t h e average p r i c e s a t t h e l a r g e r e f i n e r i e s e a s t of Suez. i m p o r t e d crude o i l o r from

T h i s a p p l i e s t o a l l f u e l s whether made from

l o c a l coal.

The Sasol f u e l p r i c e i s t h e r e f o r e

448

f u l l y m a r k e t r e l a t e d ( r e f . 1 ) . Table 1 shows t h e c o s t breakdown o f a l i t r e o f p e t r o l e u m - d e r i v e d g a s o l i n e i n 1986 i n Johannesburg which i s t h e main market area. TABLE 1 THE COST OF A LITRE OF GASOLINE I N RSA CENTS. Coastal market p r i c e

36.1

Rai 1age t o Johannesburg

7.4

Wholesaler m a r k e t i n g m a r g i n

5.0

5.2

R e t a i l e r marketing margin S t a t e t a x e s and l e v i e s

33.4 87.1

TOTAL

Methane gas f i e l d s have been d i s c o v e r e d o f f t h e southern coast o f South A f r i c a (Mosselbay) and r e c e n t l y t h e go ahead has been g i v e n f o r c o n v e r t i n g t h e CH4 t o f u e l s . Sasol w i l l n o t share i n t h i s new v e n t u r e . I n South A f r i c a t h e consumption r a t i o o f g a s o l i n e and d i e s e l f u e l i s about 50:50,

and so t h e

Mosselbay p l a n t w i l l have t o produce b o t h g a s o l i n e and d i e s e l f , u e l . A t t h e t i m e o f w r i t i n g t h e t e c h n o l o g y t o be used t o produce t h e l i q u i d f u e l s has n o t been announced.

Amongst t h e a l t e r n a t i v e s were t h e Sasol S y n t h o l r o u t e and a

t h r e e stage process i n which methanol i s f i r s t produced, t h e n c o n v e r t e d t o l i g h t o l e f i n s and these t h e n o l i g o m e r i z e d t o f u e l s .

The main problem w i t h

t h i s t y p e o f s t u d y i s t h a t t h e commercially-proven and t h e r e f o r e w e l l - d e f i n e d S y n t h o l r o u t e of which a c c u r a t e c a p i t a l c o s t s a r e a v a i l a b l e i s b e i n g compared w i t h t h e methanol r o u t e which c o n t a i n s key process s t e p s which have n o t as y e t been proven on commercial s c a l e and which cannot be c o s t e d a c c u r a t e l y . VERSATILITY OF THE SASOL FT PROCESSES The c h a i n growth mechanism o f t h e FT r e a c t i o n appears t o i n v o l v e s t e p w i s e addition o f

p r i m a r y carbon

atom

s u r f a c e complexes ( r e f .

2

- 3).

A wide

p r o d u c t spectrum i s hence i n e v i t a b l e . The p r o b a b i l i t y of c h a i n growth can, however,

be

manipulated

by

changing

the

catalyst

formulation,

or

the

temperature, o r t h e gas c o m p o s i t i o n ( r e f . 2 ) . Thus i t i s p o s s i b l e t o v a r y t h e CH4 s e l e c t i v i t y from 1 t o 100% o r t h a t o f t h e " h a r d " wax from zero t o o v e r

50%. (The IBP of "hard" wax i s 500 "C and i t c o n t a i n s molecules w i t h carbon numbers up t o a t l e a s t 250.)

As t h e p r o b a b i l i t y o f c h a i n growth increases,

t h e i n t e r m e d i a t e p r o d u c t s pass t h r o u g h maxima, about 50 % f o r g a s o l i n e and about 25 % f o r d i e s e l f u e l ( r e f . 2 ) . C u r r e n t l y Sasol uses two t y p e s o f commercial r e a c t o r s . The low t e m p e r a t u r e f i x e d - b e d process i s geared a t p r o d u c i n g waxes w h i l e t h e h i g h t e m p e r a t u r e

449

f l u i d i z e d - b e d u n i t s produce m a i n l y l i g h t o l e f i n s and l i q u i d f u e l s . T y p i c a l p r o d u c t s p e c t r a f o r t h e s e two o p e r a t i o n s a r e g i v e n i n Table 2. TABLE 2 S E L E C T I V I T Y (CARBON ATOM B A S I S ) OF SASOL PROCESSES

F i x e d bed CH4 C 2 t o C4 o l e f i n s C 2 t o C4 p a r a f f i n s

S y n t h o l ( f l u i d i s e d bed)

4

7

4

24

4

6

Gasol ine

18

36

Diesel f u e l

19

12

Heavy o i l s and waxes

48

9

3

6

l a t e r s o l u b l e oxygenates PRODUCTION OF CHEMICALS

Under normal o p e r a t i n g c o n d i t i o n s t h e FT s y n t h e s i s produces p r e d o m i n a n t l y s t r a i g h t chained molecules.

The o l e f i n s a r e almost e n t i r e l y a l p h a o l e f i n s

( r e f . 2 ) . From t h e f a c t t h a t a t h i g h space v e l o c i t i e s and l o w c o n v e r s i o n s t h e olefin,

alcohol

and

aldehyde

content

of

the

products

increase,

these

compounds a r e c o n s i d e r e d t o be p r i m a r y p r o d u c t s . The f o r m a t i o n o f branched hydrocarbons, a r o m a t i c s and ketones o c c u r s o n l y a t h i g h e r t e m p e r a t u r e s and so a r e c o n s i d e r e d t o r e s u l t f r o m secondary r e a c t i o n s ( r e f . 2). Using i r o n c a t a l y s t s , a t o t a l C 2 t o C4 o l e f i n s e l e c t i v i t y o f about 34% can be achieved. Sasol c u r r e n t l y produces more t h a n South A f r i c a ' s t o t a l need f o r e t h y l e n e and p r o p y l e n e . The excess o l e f i n s a r e c o n v e r t e d t o l i q u i d f u e l s . As t h e l o n g c h a i n o l e f i n s produced i n t h e low temperature FT s y n t h e s i s a r e

almost e x c l u s i v e l y l i n e a r a l p h a o l e f i n s , t h e y a r e i d e a l f o r p r o d u c i n g e a s i l y b i o d e g r a d a b l e d e t e r g e n t s . Cuts f r o m t h e Cg t o C15 p r o d u c t s a r e r e a c t e d w i t h benzene

to

detergents.

produce

alkylbenzenes

which

give

the

The p a r a f f i n s i n t h e f e e d s t o c k pass t h r o u g h t h e process,

are

c h l o r i n a t e d and used as p l a s t i c i s e r s .

are

sulphonated

to

L i n e a r o l e f i n s can a l s o be h y d r o f o r -

m u l a t e d t o y i e l d l i n e a r aldehydes and a l c o h o l s . The FT waxes produced i n t h e low t e m p e r a t u r e s y n t h e s i s a r e p r e d o m i n a n t l y s t r a i g h t c h a i n e d and a r e e n t i r e l y f r e e o f aromatics. The l a t t e r f e a t u r e i s i m p o r t a n t from t h e h e a l t h r e g u l a t i o n s a n g l e and alows them t o be used i n t h e p r o d u c t i o n o f , f o r i n s t a n c e , hand-creams and waxed paper and c a r t o n s used i n t h e food industry. The waxes a r e used i n t h e p r o d u c t i o n o f a wide v a r i e t y o f m a t e r i a l s , eg, petroleum j e l l i e s , m e l t adhesives

crayons,

l u b r i c a n t s and p l a s t i c i s e r s , p r i n t i n g i n k s , h o t

and even chewing gum.

The FT hard wax f r a c t i o n i s unique

amongst i n d u s t r i a l waxes i n t h a t i t has a h i g h c o n g e a l i n g p o i n t coupled w i t h

450

a low v i s c o s i t y .

The waxes a r e a l s o o x i d i z e d t o v a r i o u s degrees and a r e used

i n t h e manufacture o f p o l i s h e s ,

c a s t i n g waxes and a n t i - c o r r o s i o n c o a t i n g s ,

etc. I n t h e h i g h temperature S y n t h o l process as c u r r e n t l y operated, about 5% o f t h e r e a c t e d carbon ends up as l i g h t a l c o h o l s and ketones.

Extraction o f

t h e s e compounds from t h e r e a c t i o n water and t h e i r subsequent r e f i n i n g i s an expensive process b u t n e v e r t h e l e s s we1 1 w o r t h w h i l e .

.The ketones ( m a i n l y

acetone and MEK) f e t c h h i g h p r i c e s . Those a l c o h o l s which a r e n o t s o l d as such a r e added t o t h e g a s o l i n e pool t o boost i t s o c t a n e v a l u e and so lower t h e l e a d requirements. THE PRODUCTION OF FUELS Despite

the

high

profitability of

p r o d u c i n g chemicals,

S a s o l ' s main

p r o d u c t i s l i q u i d f u e l s produced v i a t h e Synthol process. The l o c a l market f o r h e a t i n g o i l s i s s m a l l . The consumption r a t i o o f d i e s e l f u e l and g a s o l i n e i n South A f r i c a i s about 50:50.

As can be seen i n Table 2 t h e p r o d u c t i o n

r a t i o i n t h e Synthol process i n o n l y 25:75.

To o b t a i n more d i e s e l t h e l a r g e

amount o f l i g h t o l e f i n s produced i n S y n t h o l a r e o l i g o m e r i z e d . plants

"Cat

t h e UOP

phosphoric acid. highly

branced.

values),

it

Poly"

process

is

used.

I n t h e present

The c a t a l y s t

is

supported

Because o f t h e carbonium i o n mechanism t h e p r o d u c t s a r e While

this

i s a negative

is

a plus

factor

for

f a c t o r f o r gasoline diesel

fuel

as

even

(high octane after

full

h y d r o g e n a t i o n t h e cetane number i s o n l y about 34 as a g a i n s t t h e r e q u i r e d 45. The FT process i t s e l f produces p r e d o m i n a n t l y s t r a i g h t chained p r o d u c t s and hence t h e q u a l i t y o f t h i s d i e s e l f u e l i s h i g h . (The cetane number o f S y n t h o l d i e s e l f u e l i s about 53.)

Because o f t h i s , a c o n s i d e r a b l e amount o f t h e poor

q u a l i t y "Cat P o l y " d i e s e l f u e l can be blended i n t o t h e f i n a l p o o l . The "Cat Poly"

product,

however

has

another

drawback.

Due t o i t s r e l a t i v e l y low

m o l e c u l a r mass, t h e v i s c o s i t y o f t h e p r o d u c t i s o n l y about 1.8 c S t a t 40 "C. (The degree o f o l i g o m e r i z a t i o n

i s h i n d e r e d by s t e r i c e f f e c t s due t o t h e

h i g h l y branched n a t u r e o f t h e o l i g o m e r s . )

These two f a c t o r s

(low cetane

number and low v i s c o s i t y ) l i m i t t h e o v e r a l l v e r s a t i l i t y o f t h e S y n t h o l r o u t e t o fuels,

and hence Sasol Research has i n v e s t i g a t e d a l t e r n a t i v e o l i g o m e r i -

zation catalysts. The t e s t s w i t h ZSM-5 z e o l i t e were v e r y s u c c e s s f u l . Due t o t h e r e s t r i c t e d pore size,

i t i s p h y s i c a l l y i m p o s s i b l e t o produce h i g h l y branched o l i g o m e r s

( o n l y m e t h y l branches can be formed), and because o f t h i s , t h e cetane number o f t h e d i e s e l f u e l i s about 50. As t h e c a t a l y s t i s v e r y a c t i v e , t h e e x t e n t o f o l i g o m e r i z a t i o n i s a l s o high, r e s u l t i n g i n a h i g h e r v i s c o s i t y p r o d u c t ( a b o u t 3.0 c S t a t 40 "C).

T h i s process has been s u c c e s s f u l l y t e s t e d on p i l o t p l a n t

s c a l e u s i n g Sasol

l i g h t olefins.

The c o s t o f t h e z e o l i t e c a t a l y s t i s ,

451

however, r e l a t i v e l y h i g h . Sasol has a l s o i n v e s t i g a t e d t h e use o f amorphous s i l i c a - a l u m i n a c a t a l y s t s . The d i e s e l f u e l produced w i t h FT o l e f i n s has a good v i s c o s i t y ( a b o u t 3 cSt a t b u t due t o t i l e h i g h degree o f branching, t h e cetane number i s s t i l l

40 " C ) ,

o n l y about 37. Ten y e a r s ago Sasol i n v e s t i g a t e d an a l t e r n a t i v e r o u t e t o h i g h d i e s e l f u e l yields.

I t was demonstrated on p i l o t - p l a n t

boiling

above

350 " C

produced

in

the

scale

fixed-bed

(ref. FT

4 ) t h a t t h e wax

reactors

could

hydrocracked t o e x t i n c t i o n y i e l d i n g 15% g a s o l i n e and 80% d i e s e l f u e l .

be The

l a t t e r had a cetane number o f about 60. The o v e r a l l d i e s e l f u e l t o g a s o l i n e ratio

can

be

as h i g h as 75:25.

The f i n a l

diesel

fuel

pool

has a h i g h

v i s c o s i t y and an e x c e l l e n t cetane number. The absence o f a r o m a t i c s makes such a f u e l premium grade w i t h r e s p e c t t o exhaust emissions. It i s o f i n t e r e s t t o n o t e t h a t t h e S h e l l M i d d l e D i s t i l l a t e Process ( r e f . 5 ) i s v e r y s i m i l a r i n concept. While t h e s t r a i g h t c h a i n n a t u r e o f FT p r o d u c t s r e s u l t s i n h i g h q u a l i t y d i e s e l f u e l , t h e same f e a t u r e r e s u l t s i n t h e FT g a s o l i n e h a v i n g a low octane number ( R O N ) .

A t Sasol t h e g a s o l i n e i s f i r s t c a t a l y t i c a l l y h y d r o f i n e d and

t h e n reformed

( p l a t i n u m c a t a l y s t ) . The f i n a l g a s o l i n e pool i s a b l e n d o f

i s o m e r i z e d FT C5/C6,

platformate,

"Cat P o l y " g a s o l i n e and about 10% e t h a n o l .

I n t h e Johannesburg area ( 1 700 metres above sea l e v e l ) t h e r e q u i r e d RON and MON o f premium g a s o l i n e i s 93 and 83,

respectively.

A t sea l e v e l t h e RON

needs t o be 98. T h i s can be produced by making s u i t a b l e adjustments i n t h e refinery.

From January, 1989, t h e maximum a l l o w a b l e Pb i n g a s o l i n e i n South and by 1994 i t w i l l be 200 mg/l.

A f r i c a w i l l be 400 mg/l, Both t h e

gasoline

and d i e s e l

f u e l s produced a t Sasol

are completely

c o m p a t i b l e w i t h , and meet a l l t h e s p e c i f i c a t i o n s l a i d down f o r , f u e l s d e r i v e d from crude o i l . As b o t h S and N a r e absent i n t h e Sasol p r o d u c t s , t h e exhaust emissions r a i s e no environmental problems. Sasol has n o t commercially produced j e t f u e l , b u t t e s t s have shown t h a t J e t A1 s p e c i f i c a t i o n can be met by a c u t from t h e hydrogenated "Cat P o l y " p r o d u c t (01 i g o m e r i zed C3/C4

o le f ins)

.

FT CATALYST DEVELOPMENTS To d a t e Sasol has used o n l y i r o n - b a s e d c a t a l y s t s . Not o n l y i s i r o n much cheaper t h a n t h e a l t e r n a t i v e metals, t h e fixed-bed prepared

b u t i t a l s o produces more o l e f i n s . F o r

r e a c t o r s t h e s i 1 i c a - s u p p o r t e d and a1 k a l i - p r o m o t e d c a t a l y s t i s

by p r e c i p i t a t i o n

techniques

(ref.

2).

A

recent

improvement

in

c a t a l y s t f o r m u l a t i o n has r e s u l t e d i n a more r e a c t i v e c a t a l y s t as w e l l as a lower

cost

reactors

is

per

r e a c t o r charge.

prepared

by f u s i n g

The

catalyst

suitable

used

i n the

i r o n oxides

fluidized-bed

together

with

the

452

r e q u i r e d promoters ( r e f . 2). One o f t h e key f a c t o r s c o n t r o l l i n g t h e o v e r a l l

product spectrum i s t h e

" b a s i c i t y " o f t h e c a t a l y s t . T h i s depends n o t o n l y on t h e amount and t y p e o f a l k a l i p r o m o t e r p r e s e n t b u t a l s o on i t s d i s p e r s i o n and how i t has i n t e r a c t e d w i t h o t h e r p r o m o t e r s and i m p u r i t i e s p r e s e n t fluidized-bed

Synthol

p r o c e s s t h e CH4

(ref.

2).

selectivity

I n t h e case o f t h e

has been p r o g r e s s i v e l y

l o w e r e d o v e r t h e y e a r s f r o m 15 t o t h e c u r r e n t 7%. A s t h e m a r k e t f o r f u e l gas t h e e x c e s s CH4 must be c a t a l y t i c a l l y r e f o r m e d

i s l i m i t e d i n South A f r i c a ,

back t o CO and H 2 ( r e f . 2 ) . N o t o n l y i s t h e p r o d u c t i o n o f CH4 w a s t e f u l i n t h a t i t consumes more H 2 and CO t h a n i s needed f o r t h e f o r m a t i o n o f o l e f i n s b u t t h e reforming process i t s e l f i s i n e f f i c i e n t . selectivity

C u t t i n g back on t h e CH4

has t h e r e f o r e g r e a t l y b e n e f i t e d t h e o v e r a l l

economics o f t h e

S y n t h o l FT p r o c e s s . There

are

two

areas

of

c o n c e r n when

using

iron

catalysts.

At

high

c o v e r s i o n s t h e p a r t i a l p r e s s u r e o f H20 i s i n e v i t a b l y h i g h and t h a t o f H 2 l o w at

the

reactor

magnetite,

and

exit.

so

The

the

iron

is

hence p a r t i a l l y o x i d i s e d t o i n a c t i v e

activity

of

the

CO

to

catalyses

the

decomposition

reaction)

and

the

of

presence of. a l k a l i

s u c c e s s f u l FT o p e r a t i o n ,

catalyst

is

elemental promoters,

lowered.

carbon which

(the

Iron

also

Boudouard

i s essential

for

u n f o r t u n a t e l y a l s o enhances t h e Boudouard r e a c t i o n .

I n S a s o l ' s experience carbon formation,

as such, does n o t m a r k e d l y d e a c t i v a t e

t h e ' c a t a l y s t , b u t i s does r e s u l t i n d i s i n t e g r a t i o n o f t h e c a t a l y s t p a r t i c l e s (ref.

2).

A f i x e d - b e d r e a c t o r w o u l d become i n o p e r a b l e as a r e s u l t o f f i n e s

p l u g g i n g t h e c a t a l y s t bed.

It i s f o r t h i s r e a s o n t h a t t h e f i x e d - b e d r e a c t o r s

a r e o p e r a t e d a t l o w e r t e m p e r a t u r e s where t h e r a t e o f c a r b o n d e p o s i t i o n i s l o w (ref.

2 ) . F o r f l u i d i z e d - b e d r e a c t o r s bed p l u g g i n g i s n o t a c o n c e r n , b u t t h e

fines

produced by c a t a l y s t d i s i n t e g r a t i o n r e s u l t s i n l o s s o f c a t a l y s t from

the reactors. M e t a l s o t h e r t h a n i r o n w h i c h a r e a c t i v e i n t h e FT p r o c e s s a r e N i , Co and Ru.

Under t h e normal

process c o n d i t i o n s these metals n e i t h e r o x i d i s e n o r

d e p o s i t c a r b o n and hence s h o u l d b e o f c o n s i d e r a b l e i n t e r e s t . T a b l e 3 g i v e s t h e costs o f these metals r e l a t i v e t o t h a t o f i r o n .

TABLE 3

Metal

Relative cost per unit mass

Fe

1 .o

Ni

50

co

300

Ru

35,000 It i s c l e a r t h a t

d e s p i t e i t s c h e m i c a l drawbacks i r o n has a huge c o s t

453

advantage. N i c k e l i s v e r y hydrogenating a t h i g h temperatures ( t h i s r e s u l t s i n h i g h CH4 and i n low o l e f i n y i e l d s ) , w h i l e a t low temperatures n i c k e l carbonyl i s formed and so N i i s c a r r i e d o u t o f t h e r e a c t o r . Research t o suppress t h e hydrogenation

activity

of

Ni

without

loss

i n FT a c t i v i t y

would

be

a

w o r t h w h i l e g o a l . The v e r y h i g h c o s t o f Ru, as w e l l as i t s l o w a v a i l a b i l i t y , makes l a r g e - s c a l e

commercial a p p l i c a t i o n o f t h i s metal v e r y u n l i k e l y . T h i s

appears t o l e a v e c o b a l t

as t h e o n l y p o s s i b l e a l t e r n a t i v e t o i r o n .

c o b a l t c a t a l y s t t o be c o m p e t i t i v e ,

For a

t h e Co-content w i l l have t o be low (say

l e s s t h a n 10% on a s u i t a b l e s u p p o r t ) , and t h e u s e f u l l i f e i n t h e r e a c t o r w i l l need t o be much l o n g e r t h a n t h a t o f i r o n .

FT REACTOR DEVELOPMENTS

I n i t s p r e s e n t commercial o p e r a t i o n s Sasol uses two d i f f e r e n t t y p e s o f FT reactors.

The m u l t i t u b u l a r f i x e d bed (see F i g u r e 1 ) produces waxes and t h e

c i r c u l a t i n g f l u i d i z e d bed (CFB, oils.

On a c r o s s - s e c t i o n a l

see F i g u r e 2 ) produces l i g h t o l e f i n s and

area b a s i s t h e gas t h r o u g h p u t as w e l l as t h e

amount c o n v e r t e d i s much h i g h e r f o r f l u i d i z e d - t h a n f o r f i x e d - b e d r e a c t o r s . F o r b o t h t y p e s o f r e a c t o r s p i l o t p l a n t s t u d i e s have shown t h a t i f t h e o p e r a t i n g p r e s s u r e i s doubled and t h e amount o f gas f e d t o t h e r e a c t o r i s a l s o doubled ( t h i s combination r e s u l t s i n no change i n t h e gas r e s i d e n c e t i m e ) t h e percentage c o n v e r s i o n remains t h e same. T h i s f i n d i n g means t h a t t h e p r o d u c t i o n p e r r e a c t o r volume is doubled. A l l t h i s i s i n keeping w i t h k i n e t i c model p r e d i c t i o n s . The b a s i c r a t e e q u a t i o n used i s t h a t a t any element i n s i d e a p l u g f l o w r e a c t o r t h e r a t e o f c o n v e r s i o n o f CO t o hydrocarbon p r o d u c t s equals m.PC0.PH2/

(Pco

+

a*PH20)

(ref. 2)

Because o f an i n c r e a s e d market demand f o r waxes,

a new f i x e d - b e d r e a c t o r

i s under c o n s t r u c t i o n and i s due t o come on l i n e l a t e i n 1987. T h i s r e a c t o r

w i l l o p e r a t e a t 45 b a r which r e p r e s e n t s a 65% i n c r e a s e i n pressure. I n the higher

case o f

pressures

high-temperature,

has an

additional

fluidized-bed

reactors operation a t

benefit

i n t h a t t h e r a t e o f carbon 2 d e p o s i t i o n on i r o n c a t a l y s t s i s p r o p o r t i o n a l t o P c o / P H2 ( r e f . 2 ) . As t h e p a r t i a l p r e s s u r e s increase, t h e carbon d e p o s i t i o n r a t e decreases. Because t h e d e p o s i t e d carbon has a h i g h area and a c t s l i k e a sponge f o r r e t a i n i n g wax, t h e r a t e o f wax accumulation on t h e c a t a l y s t a l s o decreases w i t h i n c r e a s i n g pressure ( r e f .

2). The b e n e f i t o f t h i s i s t h a t t h e c a t a l y s t p a r t i c l e s a r e

l e s s l i k e l y t o become " s t i c k y " and r e s u l t i n d e f l u i d i z a t i o n o f t h e bed. The CFB r e a c t o r s a t Sasol Two and Three o p e r a t e a t h i g h e r p r e s s u r e s t h a n t h e o l d e r u n i t s a t Sasol One, and t h e lower d e p o s i t i o n r a t e s o f carbon and wax on

454

z

!3

I L

I

1

I a I 3

0

2

I-

cn

Q

'i

5 0 X

455

t h e c a t a l y s t s have been confirmed. The p r o d u c t i o n c a p a c i t y o f each new CFB r e a c t o r i s t h r e e t i m e s h i g h e r t h a n t h a t o f t h e o l d e r r e a c t o r . T h i s i s due t o b o t h an i n c r e a s e i n diameter as well

as i n o p e r a t i n g pressure.

These new r e a c t o r s a r e v e r y l a r g e ,

f u r t h e r scale-up i n c a p a c i t y i s n o t c o n s i d e r e d p r a c t i c a l .

and a

Because o f t h i s

Sasol Research i n v e s t i g a t e d an a l t e r n a t i v e t y p e o f r e a c t o r , named t h e f i x e d fluidized-bed

(FFB).

It i s e s s e n t i a l l y an " e b u l l a t i n g bed" and because t h e

c a t a l y s t bed d e n s i t y i s much h i g h e r t h a n i n t h e CFB, smaller.

The

relative

sizes

t h e r e a c t o r i s much

o f t h e CFB and FFB r e a c t o r s w i t h t h e same

p r o d u c t i o n c a p a c i t y a r e i l l u s t r a t e d i n F i g u r e s 2 and 3. The FFB i s e s t i m a t e d t o c o s t 50% o f a CFB r e a c t o r .

The p r e s s u r e d i f f e r e n t i a l o v e r t h e r e a c t o r i s

a l s o lower, which r e s u l t s i n a lower o p e r a t i n g c o s t . I n 1984 a d e m o n s t r a t i o n FFB u n i t was commissioned and v a r i o u s d e s i g n aspects

were

investigated.

commercial-sized

FFB

at

A

the

few months Sasol

One

ago Sasol plant.

It

decided is

to

build a

scheduled

to

be

commissioned d u r i n g 1989. Sasol

R &

D has a l s o b e i n g i n v e s t i g a t i n g t h e v i a b i l i t y o f s l u r r y - p h a s e

r e a c t o r s which a r e e s s e n t i a l l y f i x e d - f l u i d i z e d

beds w i t h pdwdered c a t a l y s t

suspended i n a l i q u i d o f low v o l a t i l i t y ( i n t h e Sasol p i l o t p l a n t t e s t s FT wax i s used). The

structure

of

a

slurry reactor

i s much s i m p l e r

than

that

of

a

m u l t i t u b u l a r f i x e d - b e d r e a c t o r and so i t i s about 45% cheaper t o b u i l d . Since t h e s l u r r y phase i s an e x c e l l e n t h e a t exchange system, t h e r e i s no need, as i n t h e case o f

fixed-bed

reactors,

to

recycle a

large portion o f the

e f f f l u e n t gas i n o r d e r t o o b t a i n b o t h a h i g h c o n v e r s i o n ( f r e s h f e e d b a s i s ) and good temperature c o n t r o l . The o p e r a t i n g c o s t o f a s l u r r y r e a c t o r i s hence

I f t h e o b j e c t i v e i s t h e p r o d u c t i o n o f h i g h y i e l d s o f FT waxes, t h e n

lower.

t h e s l u r r y system appears t o be a b e t t e r p r o p o s i t i o n . The s l u r r y system, however,

would r e q u i r e an a d d i t i o n a l u n i t t o separate t h e f i n e c a t a l y s t f r o m

t h e wax p r o d u c t . At lower

h i g h temperatures Sasol t e s t s have shown t h a t t h e s l u r r y bed has a conversion

than

the

fixed

fluidized-bed

(FFB).

At

these

high

temperatures t h e wax i s hydrocracked, i e , t h e r e i s a n e g a t i v e wax p r o d u c t i o n . To a v o i d t h i s , means a f u r t h e r

t h e r e a c t o r temperature has t o be lowered which, drop i n conversion.

o f course,

It i s n o t p o s s i b l e t o l o a d as much

c a t a l y s t p e r u n i t r e a c t o r volume i n a s l u r r y phase r e a c t o r as i n a normal FFB r e a c t o r . T h i s g i v e s t h e l a t t e r system an i n t r i n s i c advantage. ( I n c r e a s i n g t h e c a t a l y s t c o n t e n t of a s l u r r y i n c r e a s e s i t s v i s c o s i t y , hence t h e bubble s i z e increases, r e s u l t i n g i n a s h o r t e r gas r e s i d e n c e t i n e . )

If t h e f e e d gas has a low H2/C0 r a t i o (e.g. 0.7) and t h e o b j e c t i v e i s t h e

456

p r o d u c t i o n of l i q u i d f u e l s u s i n g i r o n c a t a l y s t s , t h e n a s l u r r y bed o p e r a t i n g at

intermediate

fluidized-bed

temperatures

has an

advantage o v e r b o t h t h e f i x e d -

and

The h i g h r a t e o f carbon d e p o s i t i o n would make b o t h

reactors.

r e a c t o r s i n o p e r a b l e . To e f f i c i e n t l y u t u l i s e a low H2/C0 r a t i o gas d i r e c t l y i n t h e s l u r r y reactor, s h i f t reaction.

t h e c a t a l y s t w i l l have t o be a c t i v e f o r t h e watergas

Such a s l u r r y system needs t o be c o s t compared w i t h one i n

which t h e gas H2/C0 r a t i o i s a d j u s t e d i n a s e p a r a t e upstream watergas s h i f t r e a c t o r and t h e n f e d t o t h e h i g h e r c a p a c i t y FFB r e a c t o r .

REFERENCES

1

Dry, M.E.,

2

Dry, M.E.,

3

Anderson, R.B.,

4

Dry, M.E.,

5

Van

and Erasmus, H.B.

de W.,

Annual Energy Review, Energy Review

I n c . , P a l o A l t o , CA, USA, Vol 12, 1987. i n J.R.

Anderson and M Boudart ( E d i t o r s ) ,

C a t a l y s i s Science

and Technology, S p r i n g e r - V e r l a g , 1981, V o l . 1, Chapter 4. The F i s c h e r - T r o p s c h S y n t h e s i s , Academic Press, 1984.

and Hoogendoorn, J.C.,

C a t a l y s i s Reviews - S c i . Eng.,

23 ( 1 &

2 ) (1981) 265. der

Burgt,

M.J.,

Van

Klinken,

J.,

and Sie,

Worldwide Symposium, Washington, DC, Nov. 11, 1985.

S.T.,

5 t h Synfuels

D.M. Bibby, C.D. Chang, R.F.Howe and S. Yurchak (Editors), Methane Concersion 1988 Elsevier Science Publishers

R.V.,Amsterdam - Printed in The Netherlands

457

MECHANISV OF THE FISCHER TROPSCH PROCESS

Hans SCHULZ, Klaus BECK, Egon E R I C H Engler-Bunte-Institut,

U n i v e r s i t a t K a r l s r u h e , 7500 KARLSRUHE, FRG

ABSTRACT When comparing t h e competing processes f o r making hydrocarbons f r o m s y n t h e s i s gas - t h e F i s c h e r Tropsch CO h y d r o g e n a t i o n and t h e MTG c o n v e r s i o n t h e process f l o w sheets show as t h e main d i f f e r e n c e t h e a d d i t i o n a l s t e p o f methanol s y n t h e s i s f o r t h e MTG r o u t e . However, p r o d u c t s e l e c t i v i t y i s b a s i c a l l y d i f f e r e n t f o r b o t h t h e conversions. And f r o m t h i s p o i n t o f v i e w t h e one o r t h e o t h e r r o u t e can be t h e more f a v o u r a b l e o p t i o n as f i t t i n g b e s t t h e p a r t i c u l a r demand p a t t e r n . S e l e c t i v i t y d i f f e r e n c e s f u n d a m e n t a l l y r e s u l t f r o m t h e d i f f e r e n t k i n d s o f c h e m i s t r y which a r e i n v o l v e d : Hydrogenation on s p e c i a l metal t y p e c a t a l y s t s i n case o f t h e F i s c h e r Tropsch r e a c t i o n and a c o n v e r s i o n v i a c a r benium i o n i n t e r m e d i a t e s on a c i d i c s i t e s , which i s a d d i t i o n a l l y c o n s t r a i n e d by shape s e l e c t i v i t y i n case o f t h e MTG process. The k i n e t i c s o f t h e mechanism o f F i s c h e r Tropsch hydrocarbon f o r m a t i o n f r o m CO and H2 a r e n o d e l l e d i n t h i s paper as a "non t r i v i a l p o l y m e r i z a t i o n " which occurs on t h e s u r f a c e o f t h e s o l i d c a t a l y s t . The t e r m "non t r i v i a l " r e l a t e s t o t h e f a c t t h a t p r o d u c i n g a s t r a i g h t c h a i n a1 i p h a t i c "polymethylene" r e s p e c t i v e l y a "methylen oligomer", f r o m CO and H2 i n v o l v e s as t h e r e p e a t i n g p o l y m e r i z a t i o n s t e p o f p r o l o n g i n g t h e g r o w i n g c h a i n by one CH2 a s e t o f r e a c t i o n s , o f a c t i v a t i o n and t r a n s f e r o f hydrogen, a c t i v a t i o n o f CO and s p l i t t i n g o f t h e C/O bond and f o r m a t i o n o f a new C / C bond. A k i n e t i c model of t h i s s u r f a c e p o l y m e r i z a t i o n i s p r e s e n t e d i n t h e paper. I t s t a r t s with t h e i d e a l system and adopts s t e p w i s e a d d i t i o n a l assumptions which r e g a r d e s s e n t i a l f e a t u r e s o f observed t y p e s o f p r o d u c t d i s t r i b u t i o n s . The i d e a l system i s d e s c r i b e d w i t h o n l y one parameter, t h e q u o t i e n t o f t h e r a t e c o n s t a n t s o f c h a i n p r o p a g a t i o n and c h a i n t e r m i n a t i o n and i t i s r e l a t e d e a s i l y t o t h e m a c r o k i n e t i c s o f t h e CO consumption r a t e . A c t u a l r e a l cases o f p r o d u c t d i s t r i b u t i o n a r e d e s c r i b e d as e x t e n s i o n s o f t h e i d e a l model, t a k i n g i n t o account t h e f o l l o w i n g functions ( 1 ) Formation o f 3 k i n d s o f p r i m a r y p r o d u c t s ( o l e f i n s , p a r a f f i n s and a l c o h o l s p l u s aldehydes) ( 2 ) Methyl b r a n c h i n g d u r i n g c h a i n growth ( 3 ) Carbon number dependence o f c h a i n p r o l o n g a t i o n r a t e c o n s t a n t s ( 4 ) Carbon number dependence o f c h a i n b r a n c h i n g r a t e c o n s t a n t s . I n a d d i t i o n e x p e r i m e n t a l p r o d u c t d i s t r i b u t i o n s can s e r i o u s l y be a f f e c t e d by i n s t a t i o n a r i t y o f t h e system, e r r o r s d u r i n g sampling and a n a l y s i s and by-product f o r m a t i o n o r secondary r e a c t i o n s o f t h e compounds which a r e produced t h r o u g h t h e F i s c h e r Tropsch CO hydrogenation.

INTRODUCTION The thermodynamically most favoured p r o d u c t compound o f CO h y d r o g e n a t i o n w i t h i n a l l t h e range o f reasonable r e a c t i o n c o n d i t i o n s i s methane. Yethane i s e a s i l y o b t a i n e d v i a CO h y d r o g e n a t i o n on many metal c a t a l y s t s as Pd, P t , Pu and

N i . W i t h o x i d e h y d r o g e n a t i o n c a t a l y s t s as ZnO and CuO t h e carbon/oxygen bondof

458

t h e CO i s n o t s p l i t and t h e o b t a i n e d p r o d u c t i s methanol. A l i k e t o t h e methane s y n t h e s i s , F i s c h e r Tropsch CO h y d r o g e n a t i o n i s performed w i t h h y d r o g e n a t i o n metal c a t a l y s t s (Ru, Fe, N i , Co) and t h e carbon oxygen bond o f t h e CO i s a l s o broken. However, i n t h e F i s c h e r Tropsch system t h e C i s a r e n o t e a s i l y r e l e a s e d f r o m t h e s u r f a c e t o y i e l d t h e f a v o u r e d methane b u t undergo c h a i n p r o l o n g a t i o n r e a c t i o n s w i t h hydrocarbon s p e c i e s which a r e s i m i l a r l y s t r o n g l y chemisorbed on t h e s u r f a c e and which can undergo numerous c h a i n p r o l o n g a t i o n s t e p s u n t i l t h e y l e a v e t h e c a t a l y s t s u r f a c e . The f a v o u r e d p r o d u c t o f c h e m i - d e s o r p t i o n i s t h e a-olefin (refs. 1,2).

I t has been concluded f r o m t h e above mentioned f a c t s and

o t h e r o b s e r v a t i o n s ( r e f . 1) t h a t t h e most e s s e n t i a l f e a t u r e e o f t h e F i s c h e r Tropsch system i s a k i n e t i c one: The i n h i b i t i o n o f methane f o r m a t i o n . I t has a c c o r d i n g l y been shown t h a t c h e m i - d e s o r p t i o n f r o m F i s c h e r Tropsch

s i t e s as a p a r a f f i n i s a slow s t e p , as compared w i t h c h e m i - d e s o r p t i o n as an a - o l e f i n ( a p p r o x i m a t e l y f i v e t i m e s slower w i t h i r o n manganese c a t a l y s t s a t ca.

250 "C ( r e f . 2 ) ( F i g . 1 ) . Thus i t can be deduced a l s o t h a t c h a r a c t e r i z a t i o n o f the Fischer

CH3 -CH2- R

Tropsch system i s n o t so much a m a t t e r o f which b u i l d i n g b l o c k s f i n a l l y add t o the growing chain b u t t h e i n h i b i -

'

- 2HC&lC-&C

1

CH2= CH- R

much foster thon ( 2 )

t i o n o f chemi-desorption r e a c t i o n s i s e s s e n t i a l . Chain p r o l o n g a t i o n i s p o s s i b l e t h e n w i t h s e v e r a l s p e c i e s as

R

CO, CIH,

o r ethylen ( s i m i l a r l y an out-

s t a n d i n g monument o f a r c h i t e c t u r e c o u l d be b u i l t f r o m d i f f e r e n t t y p e s o f F i g . 1. K i n e t i c scheme o f p r o d u c t c h e m i - d e s o r p t i o n and c h a i n p r o longation. Desorption r e a c t i o n s a r e t h e s l o w s t e p s o f t h e mechanism.

s t o n e s o f even f r o m mixed ones. I t i s t h e c o n c e p t which c o u n t s ) . The e s s e n t i a l p r i n c i p l e o f t h e F i s c h e r Tropsch system t h u s i s a

chemical c o n s t r a i n t : t h e s l o w i n g down o f c h e m i - d e s o r p t i o n r e a c t i o n s and e s p e c i a l l y o f t h e a s s o c i a t i v e c h e m i - d e s o r p t i o n o f an a1 k y l - s p e c i e s t o g e t h e r w i t h a H-species t o f o r m a p a r a f f i n molecule. I n such a system t h e r e a c t i o n s between chemisorbed s p e c i e s become dominant and s u r f a c e p o l y m e r i z a t i o n i s p o s s i b l e. I n h i b i t i o n o f c h e m i - d e s o r p t i o n r e a c t i o n s i s p r o b a b l y m a i n l y caused by t h e

CO, which i s known t o be s t r o n g l y adsorbed on t h e F i s c h e r Tropsch c a t a l y s t m e t a l s and which i s a l s o known as a p o i s o n o f h y d r o g e n a t i o n r e a c t i o n s o r a

f i r m l y bound s - l i g a n d i n c o o r d i n a t i o n c h e m i s t r y , which r e a c t s v i a i n s e r t i o n . Thus i n t h i s paper a k i n e t i c c o n c e p t o f F i s c h e r Tropsch s u r f a c e p o l y m e r i z a t i o n i s developed, whereas t h e n a t u r e o f s u r f a c e s p e c i e s i s o n l y r e g a r d e d i n g e n e r a l .

459

COMPARISON OF FISCHER TROPSCH AND MTG SELECTIVITY

The b a s i c process f l o w sheets o f F i s c h e r Tropsch and t h e MTG c o n v e r s i o n ( F i g . 2) a r e r a t h e r s i m i l a r . S t a r t i n g f r o m n a t u r a l gas, t h i s has t o be p u r i f i e d and c o n v e r t e d t o syngas. Hydrocarbons a r e o b t a i n e d d i r e c t l y f r o m syngas through t h e F i s c h e r Tropsch r o u t e . The MTG hydrocarbon f o r m a t i o n uses a methanol f e e d and t h e methanol s y n t h e s i s has a d d i t i o n a l l y t o be performed. I n s p i t e o f b o t h processes b e i n g o r i g i n a l l y d e s t i n a t e d f o r g a s o l i n e p r o d u c t i o n , t h e p r o d u c t compositions show fundamental d i f f e r e n c e s which a r e due t o t h e t y p e o f c h e m i s t r y b e i n g i n v o l v e d . The F i s c h e r Tropsch r e a c t i o n produces m a i n l y nonbranched a l i p h a t i c hydrocarbons on metal c a t a l y s t s and t h e MTG r e a c t i o n t r a n s forms methanol on an a c i d i c c a t a l y s t (HZSM-5) m a i n l y v i a c a r b o c a t i o n i n t e r mediates which have a h i g h tendency f o r c h a i n b r a n c h i n g and t h e f o r m a t i o n o f aromatic rings.

NATURAL GAS

JATURAL GAS H

Z

F

Z

p

S

P U R I F I E O NATURAL GAS

02, H20 PRODUCTION

SYNGAS PRODUCTION

SYNTHESIS GAS

SYNTHESIS GAS METHANOL SYNTHESIS

ETHANOL

HEAT

OF

REACTION

CONVERSION

T A I L GAS (FUEL GAS, CH4, C2H6, H 2 ) C2

- C4

HYDROCARBONS PROPENE, WENES,

-(ETHENE,

PRODUCT RECOVERY AN0 SEPARATION

LPG)

GASOLINE (LINEAR OLEFINS) DIESEL FUEL -(LINEAR

OLEFINS) ,PARAFFINIC

I

HEAVY OIL

GASOLINE (AROMATICS UP TO OURENE)

b PARAFFIN WAXES ,ORGANIC

OXYGEN COMPOUNDS

(ETHANOL PLUS OTHERS)

F i g . 2. Schemes o f n a t u r a l gas c o n v e r s i o n t o g a s o l i n e v i a F i s c h e r Tropsch s y n t h e s i s ( l e f t ) and MTG process ( r i g h t ) .

I n Table 1 maximum v a l u e s o f s e l e c t i v i t y f o r t h e two modes o f hydrocarbon f o r m a t i o n a r e c o m p a r a t i v e l y shown. The c o n c l u s i o n can be drawn t h a t t h e t w o

460

routes are not only competing but also complementary in nature. Optimum application of MTG is for producing aromatic gasoline and olefins C2, C3. The unique nature of the Fischer Tropsch conversion allows selective formation of 1 inear a-olefins, paraffinic hard wax, polymethylene and high quality diesel fuel favourably in combination with ideal hydrocracking. TABLE 1 Maximum values of obtainable selectivity through Fischer-Tropsch-and MTG hydrocarbon synthesis. (The estimated figures of selectivity are given as % of carbon of the total product. The values in brackets include further processing of the primary product) Mobil Route Fischer-Tropsch MTG MT 0 low severity high severity normal severity high severity Gasol ine Diesel fuel C2 - Cq'olefins Aromatics linear paraffins linear olefins C4+ linear a-olefins Paraffinic wax Polymethylen (high molec. weight Maxim. C-number Degree of branching %

25 20

1) (a)

(D)1)

20

1000

-

-1

5

65 20

50

(85)3)

70 15

-

0 0

99.6-

-

-

-

98.4-

I

I

I

I

I

I

I

I

I

Fig. 2. Conversion of Chloromethane Over ZSM-5 as a Function of Time an Stream After 14 Oxidative Regenerations.

488

2-Chloropropane and 2-chlorobutane were also found in the chloromethane oligomerization products. These compounds possibly arise from gas-phase addition of hydrogen chloride, the by-product of chloromethane oligomerization, to propene and butene. It was hypothesized that these compounds could also be oligomerized over ZSM-5. To test this theory, several primary, secondary, and tertiary chlorides of propane, butane, and pentane were reacted over ZSM-5 under the conditions for chloromethane oligomerization. In each case the halocarbon was oligomerized to aromatic hydrocarbons and hydrogen chloride. To determine the tolerance of ZSM-5 to the presence of polychlorinated methanes, mixtures of mono- and dichloromethane were reacted over ZSM-5. Feed streams containing a molar ratio of 2.75:l mono- to dichloromethane reacted over ZSM-5, and coking of the catalyst was no greater than that experienced for either chloromethane alone or methanol. As expected, the product mixture contained a larger aromatic fraction than the product from chloromethane oligomerization. The presence of chlorinated aromatics was not detected for any of the 01 igomerization reactions conducted. CONCLUSION Methane has been converted to higher hydrocarbons boiling in the gasoline range by the two-stage process described above. Mono- and dichloromethanes are produced by the oxyhydrochlorination catalyst in ratios dependent upon feed stoichiometry, residence time, and temperature. The conversion of methyl chloride to gasoline-boiling-range hydrocarbons occurs under conditions similar to those for methanol-to-gasoline conversion. Mixtures of mono- and dichloromethane, in ratios greater than about 2.75:1, can also be converted without rapid coking of the zeolite. Work is currently in progress that will identify optimum operating conditions for both reactor stages. This information will be used for the design of a continuous, integrated, bench-scale system. DISCLAIMER Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply endorsement or favoring by the United States Department of Energy. REFERENCES 1 2

Chu, P., and Dwyer, F.G., U.S. Patent 4 513 092, 1985. Kroenke, W.J., and Nicholas, P.P., U.S. Patent 4 467 127, 1984.

489

3 4 5 6 7 8 9

Pitchai, R., and Klier, K., "Partial Oxidation o f Methane", Dept. o f Chemistry, Lehigh University, Bethlehem, Pa., 1985. Olah, G.A., Felberg, J.D., and Lammertsma, K., J. Am. Chem. SOC., 1983, 105, 6529.

Siskin, M., Abstracts o f Papers, Proceedings o f Workshop on Basic Research Opportunities in Methane Activation Chemistry, Houston Texas, Gas Research Institute, 1985. Chang, C.D., and Silvestri, A.L., J. Catal., 1977, 47, 249. Ione, K.G., Stepanov, V.G., Romannikov, V.N., and Shepelev, S.E., Khim. Tverd. Topl., 1982, 16(6), 35. Pieters, W.J.M., Carlson, E.J., Gates, E., and Conner, W.C. Jr., U.S. Patent 4 123 389, 1978. Pieters, W.J.M., Conner, W.C. Jr., and Carlson, E.J., J. Appl. Cat., 1984, 11, pp. 35-71.


D.M. Bibby,C.D.Chang,R.F. Howe and S.Yurchak (Editors),Methane Conoersion

01988Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

491

METHANE CONVERSION V I A METHYLCHLORI D E : CONDENSATION OF METHYLCHLORIDE TO LIGHT HYDROCARBONS

Klaus-Joachim Jens. Sir1 Halvorsen a n d Elizabeth Baumann O f s t a d Centre f o r Industrial Research, P.B. 3 5 0 Blindern, 0 3 1 4 OSLO 3 , Norway ABSTRACT Condensation of methylchloride catalyzed by acid catalysts (A120 ; Si02/A1 0 . %nA120 ; ZSM-5; H* -mordenite)leads predominantly to C2 hydrocarbons. In th; case o i zeolites C3 and some C4t hydrocarbons are also formed. ZSM-5 shows the hest stability. The product stream exhibits 4% selectivity towards chlorinated hydrocarbons. Metal exchanged mordenites (M=Zr,Cr,Sb,Zn)give initially medium conversion of methylchloride, but deactivate rapidly. A l l catalysts are prone to coking. INTRODUCTION The oxidative dimerization of methane in the presence of oxygen or air often gives substantial amounts of carbon oxides as byproducts [ I ] . Conversion of methane to methylchloride and further condensation to higher hydrocarbons and hydrogen chloride avoids this problem. The reoxidation of hydrogen chloride to chlorine, as well as the formation of methylchloride from methane is known technology. EXPERIMENTAL The catalysts were tested in a continous fixed bed micro test apparatus (500 mg catalyst) with automatic GC sampling (KC1 PLOT column; temp. prog. lOO(5)-5150(5)). The feed was 50% diluted with helium. All selectivities are based on gas phase product compositions. H* -mordenite was purchased from Ventron (Nr. 14-3841, ZSM-5 was prepared according to [2] (Si: A1 19:l) and used in the hydrogen form. Metal containing HI-mordenites were prepared by ion exchanging with ZrO(N03)2 , Cr(NO3I3 .9H20, SbC13 and Zn(S0,) .7H,O respectively. RESULTS Exploratory pulse reactor experiments showed that silica alumina, alumina and zinc aluminate spinel are active catalysts for the condensation of methylchloride to mainly ethylene and HC1. H* -mordenite gave additionally C3-5 hydrocarbons. H' -mordenite was subsequently tested for conversion of methylchloride at 4OO0C with a WHSV of 3 and 24. The deactivation curves and selectivites are shown in Figs. 1, 2 and 3. Initially 52% selectivity is observed for ethylene (WHSV=3). Ethane and propane become the main reaction products during deactivation. The amount of methane formed stays relatively constant. Testing of H* -mordenite at 200 and 25OoC (WHSV=11) gave conversions of methylchloride below 1%.

492

10ol

20

6\ / : ; SH W H' mordenite, 4OO0C,

m CH'

S (%I H*rnordenite. 400°C

40

60

50

,

80

WH&V= 24

Fig,2 . Hydrocarbon selectivities f o r 0 M'-mordenite (WHSV=24, T7400 C) .

Fig. 1 , Deactivation of Ht-mordenite.

s (%)

H'mordenite. 4OO0C, WHSV

10

20

30

40

50

60

I

3

70

80

time (min)

Fig. 3 . Hydrocarbon selectivities for Ht-mordenite (WHSV=3, T=400 C) . 0

0

The reaction was also run with ZSM-5 (WHSV=24) at 400 and 500 C (Fig. 4). De0 activation at 400°C is rather slow in contrast to reaction at 500 C. The 0 selectivities at 400 C remain nearly constant (Fig. 5). During the pronounced deactivation at 5OO0C, the product selectivity shifts towards methane (Fig. 6). In one case the product stream was analysed for chlorinated compounds (ZSM-5, 4OO0C, WHSV=24) as shown in Table 1. The total selectivity to chlorinated reaction products (chloroisobutane, 2-chloro-2-methylbutane) is 4% and stays

493

-. I

I

I

I

I

100

I

I

I

I

-.L.

200

ZSM-5ILOO'C

I

1

I

time [ m i n l

I

300

'

I

'

? H'wrnordeniteIL00'C

Fig. 4. Deactivation of ZSM-5 and H i - mordenite. A

80 30

f

S

P d60 -

LO -

0

LO

80

120 time[miij

Fig, 5. Hydrocarbon selectivities for ZSM-5 at 4OO0C (WHSV=24).

time [min] Fig. 6 . Hydrocarbon selectivities ZSM-5 at 5OO0C (WHSV=24).

All catalysts are prone to coking, especially at low space velocites. Ht-mordenite and ZSM-5 were analysed for coke formed after 6 hours on stream (Table 2). 4t

Metal exchanged mordenites show initially up t o 58% conversion (M=Zr.

)

but

494

TABLE 1 Chlorinated compounds in the product stream 0 (ZSM-5, WHSV=24, T=400 C). I

1 jcata-

I

I

ITime onlconverl Selectivities [%] I stream!-sion

j 56

1

336

j

28

21

j 58

I 55

jZZji2j

3

1241141 3

j

I

TABLE 2 Amount of coke formed on the catalyst after 6 hours on stream. llyst

1

1

1

i

i

jwH3y /T&mp. Coke I[h I I [ Cl M

I

I

ji~+-mord.i24 i 4001 3.oj1.1 i IIZSM-5 124 I 4001 4.810.8 I IZSM-5 124 I 500123.010.8 I I

I

I

I

1

I

have deactivated totally after 24 minutes on stream (Table 3). TABLE 3 0 Metal exchanged Hi -mordenites as catalysts (T=400 C, WHSV=24, time om stream: 1 minute).

OISCUSSION According to the stoichiometry of the condensation of methylchloride to hydrocarbons and HC1, unsaturated compounds should be the products. As expected ethylene and propene are observed mainly in the beginning of the reaction. A similar pattern is found for a high silica zeolite [4]. During deactivation, the selectivity towards saturated hydrocarbons increases, in the case of ZSM-5 at 5OO0C (WHSV=24) the selectivity becomes eventually nearly 100% in methane. It, can be assumed that these saturated compounds are produced either by hydrogen transfer from coke precursors to olefins or by thermal cracking of coke precursors. The latter may seem appropriate especially for methane formation. These cracking reactions take over significantly especially at 5OO0C with ZSM-5. Coke formation may proceed via high molecular precursors that do not desorb easily at the reaction temperature; raising the temperature from 400 to 5OO0C on a partially deactivated Ht -mordenite led to the desorption of a significant amount of higher hydrocarbons (as observed by GC) and a partial restoration of the catalytic activity. Coke formation decreases with higher gas velocities (Table 21, in the case of ZSM-5 at 5OO0C it may be assumed that hydrocracking is becoming a very fast secondary reaction, leading to a high selectivity towards methane and considerable amounts of coke. The nature of the maxima in the selectivities for ethylene and propylene for Ht-mordenite at WHSV=24 (Fig. 2) is at present not fully understood.

495

It is interesting that the selectivity to chlorinated hydrocarbons is rather low and constant over time, implying a highly selective side reaction towards chlorinated products. As observed for the MTG reaction [ 3 ] , metal exchanged the stability of the catalysts, in the case of methyl the deactivation was much more pronounced than in the initially enhanced activity may be because the metals sition of methylchloride to HC1 and carbene.

zeolites do not increase chloride condensation, metal free catalyst. The catalyze the decompo-

For the MTG reaction [3] carbenes have been suggested as initial species, leading to ethylene as the "first" olefin formed. This seems also to be a plausible mechanism in our case. Thus in the initial step methylchloride may form a surface carbene and HC1 catalyzed by acid and basic sites in the zeolite. Surface carbenes can subsequently react with each other to give ethylene. Another possibility may be that a carbene molecule reacts with methylchloride to ethylchloride, which leads to ethylene after HC1 elimination. Since no ethylchloride has been observed under our reaction conditions, the former reaction sequence may rather apply. Higher olefines may be formed by reaction of ethylene with methylchloride, followed by HC1 elimination, o r higher condensation of surface carbenes. The product HC1 may cause catalyst deterioration during long reaction times. CONCLUSION ZSM-5 is the most stable catalyst for condensation of methylchloride to light unsaturated hydrocarbons. The reaction shows 4% selectivity towards chlorinated hydrocarbons. The catalysts are prone to coking. ACKNOWLEDGEMENT The authors would like to thank Mrs. E. Myrvold and T. 0fsti for experimental help and Statoil 'A/S for permission to publish the results. REFERENCES 1. R. Pitchai and K. Klier, Catal. Rev. Sci. Eng., 28 (1986) 13-88. 2. C.D. Chang and A.J. Silvestri, J. Catal. 47 (1977) 249-259. 3. C.D. Chang, Catal. Rev. Sci. Eng.,a (1983) 1-118. 4. V. N. Romanikov and K.G. Ione, Kinet. Kataliz., 2, (1984) 92-98


D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

497

SYNTHESIS GAS TO MOTOR FUEL V I A LIGHT ALKENES

B.G.

BAKER and

N.J. CLARK

School o f P h y s i c a l Sciences, F l i n d e r s U n i v e r s i t y , South A u s t r a l i a , 5042

SUMMARY Two t y p e s o f c a t a l y s t have been developed: (i) Catalysts w i t h h i g h select i v i t y f o r a l k e n e p r o d u c t i o n f r o m s y n t h e s i s gas, c o n s i s t i n g o f h i g h l y d i s p e r s e d i r o n on an alumina s u p p o r t which has been impregnated w i t h a l a n t h a n i d e o x i d e . ( i i) C a t a l y s t s c o n t a i n i n g t u n g s t e n o x i d e , c o n d i t i o n e d and o p e r a t e d under s p e c i f i c c o n d i t i o n s , which have h i g h a c t i v i t y f o r t h e s k e l e t a l i s o m e r i z a t i o n o f Together t h e s e c a t a l y s t s p r o v i d e t h e b a s i s of a process t o c o n v e r t alkenes. s y n t h e s i s gas t o branched alkenes which a r e i m p o r t a n t i n t e r m e d i a t e s i n t h e p r o d u c t i o n o f high octane gasoline. The branched C s and C s alkenes a r e o f p a r t i c u l a r use f o r t h e s y n t h e s i s o f t h e e t h e r s , MTBE and TAME, which have except i o n a l v a l u e as o c t a n e enhancers f o r unleaded g a s o l i n e . D e t a i l s o f t h e c a t a l y s t s , t h e i r mode of o p e r a t i o n and an o u t i n e o f an o v e r a l l process a r e d e s c r i b e d . INTRODUCTION An o b j e c t i v e o f much r e c e n t work on s y n t h e t i c f u e l s has been t o produce h i g h v a l u e p r o d u c t s w h i c h meet s p e c i f i c needs.

This i s i n contrast w i t h resource

based s t r a t e g i e s w h i c h aim t o compete d i r e c t l y w i t h n a t u r a l p e t r o l e u m o v e r t h e whole f u e l range. The s y n t h e s i s o f l i g h t alkenes by m o d i f i c a t i o n o f t h e F i s c h e r - T r o p s c h p r o cess i s r e c o g n i s e d as an i m p o r t a n t r o u t e t o h i g h v a l u e f u e l components ( r e f s . 1,2).

The e s s e n t i a l r e q u i r e m e n t of t h e F-T c a t a l y s t i s h i g h s e l e c t i v i t y

towards alkenes, s u p p r e s s i o n o f methane and r e s i s t a n c e t o c o k i n g under cond i t i o n s w h i c h f a v o u r t h e f o r m a t i o n o f hydrogen d e f i c i e n t p r o d u c t s . L i g h t alkenes (C,-C,)

a r e i m p o r t a n t chemical feedstocks.

They may be con-

v e r t e d t o l i q u i d f u e l s by a number o f processes; a l k y l a t i o n , o l i g o m e r i z a t i o n and t h e s y n t h e s i s o f oxygenates ( r e f s . 3 , 4 ) .

Branched isomers o f t h e alkenes

a r e needed f o r t h e s e processes i f h i g h o c t a n e b l e n d s t o c k s a r e sought.

Since

t h e d i r e c t p r o d u c t o f t h e F-T s y n t h e s i s i s g e n e r a l l y s t r a i g h t c h a i n 1-alkenes, a f o l l o w i n g process i s r e q u i r e d t o c a t a l y s e t h e s k e l e t a l i s o m e r i z a t i o n w i t h o u t d e s t r o y i n g t h e d o u b l e bond. The p r e s e n t work d e s c r i b e s c a t a l y s t s f o r t h e s e l e c t i v e s y n t h e s i s o f a1 kenes and f o r t h e s k e l e t a l i s o m e r i z a t i o n . Processes based on t h e s e c a t a l y s t s c o u l d p r o v i d e i s o b u t e n e and i s o p e n t e n e f o r t h e s y n t h e s i s o f MTBE (methyl t e r t - b u t y l e t h e r ) and TAME ( t e r t - a m y l methyl

498

ether).

These oxygenates are i n high demand t o boost the octane content o f

unleaded gasoline ( r e f s . 3,4,5,6).

Since methanol from synthesis gas i s

reacted w i t h t h e iso-alkenes t o form the ethers an a l l s y n t h e t i c process could have advantages over c u r r e n t technology which depends on e i t h e r r e f i n e r y cracked product o r f i e l d butanes. SELECTIVE FISCHER-TROPSCH CATALYST Catalysts w i t h high s e l e c t i v i t y f o r alkene production by the Fischer-Tropsch synthesis have been developed.

They consist o f h i g h l y dispersed i r o n on an

alumina support which has been impregnated w i t h a lanthanide oxide.

The

e s s e n t i a l steps i n t h e c a t a l y s t preparation are (i) heat treatment o f y-alumina t o change the pore s t r u c t u r e , a c i d i t y and surface area.

( i i ) impregnation o f

t h e alumina w i t h a s o l u t i o n containing the lanthanide element i n s u f f i c i e n t q u a n t i t y t o coat a l l o f t h e support surface.

(iii)impregnation w i t h a f e r r i c

s o l u t i o n of c o n t r o l l e d pH and containing an amount o f i r o n comparable t o t h e amount o f lanthanide. D e t a i l s o f the preparation and performance o f the c a t a l y s t s have been desc r i b e d and patented ( r e f s . 7,8,9).

Best s e l e c t i v i t y towards l i g h t alkenes

exceeds 90% and methane production i s c o n t r o l l e d a t about 5%.

These f i g u r e s

are achieved w i t h CO/H2 = 2 a c o n d i t i o n under which very prolonged c a t a l y s t l i f e i s maintained w i t h no evidence o f coking. The e s s e n t i a l feature o f t h e c a t a l y s t i s t h e h i g h dispersion o f i r o n on 12.5

Total hydrocarbon 22.6

I

I

CO I H2= 2 Pressure 800 kPo

31.2

I

/

Propene

Ethene Butene Higher alkenes Pent ene Methane other alkanes Hexene

I

280

I 300 Temperature (‘C)

I

320

Figure 1. Products from F-T synthesis over the c a t a l y s t : Fe 2%, P r 2%, on heat t r e a t e d alumina support. A1 kene s e l e c t i v i t y 91-83%.

499

lor

a

15

a

1

2

3

Product

Figure 2.

1

P 5

b 6

7

2or

+

carbon number

1

1Alkones

b

2

3

Product

6

5

6

carbon number

F-T product d i s t r i b u t i o n s a t 26OOC ( a ) CO/H2 = 2; ( b ) CO/H2 = 0.5

alumina a c o n d i t i o n which favours t h e formation o f aluminates.

I t has been

shown t h a t alkene s e l e c t i v i t y i s markedly dependent on t h e Al/Fe and Na/Fe The e f f e c t o f t h e lanthanide a d d i t i o n i s t o improve alkene s e l e c t i v i t y and p a r t i c u l a r l y t o suppress t h e formation o f methane. Praseor a t i o s ( r e f . 10).

dymium i s most e f f e c t i v e and lanthanum and neodymium are useful. The s e l e c t i v i t y i s inherent and r e l a t i v e l y independent o f t h e extent o f conA t t h e upper Figure 1 shows product d i s t r i b u t i o n s a t 280-320OC. version. temperature 37.2% o f CO i s converted t o hydrocarbon (83% alkenes); major byproduct.

CO2

i s the

When hydrogen i s i n excess, water i s a major byproduct and

more methane i s formed, however, s e l e c t i v i t y (62%) t o alkenes i s s t i l l achieved at

CO/H2

= 0.5 ( f i g u r e 2).

The product d i s t r i b u t i o n s obtained from t h i s type o f c a t a l y s t show strong departure from t h e usual Schulz-Florey chain growth c h a r a c t e r i s t i c a t C 1 and These are favourable c h a r a c t e r i s t i c s if and small values o f ci (- 0.55).

C2

f u r t h e r s p e c i f i c upgrading o f t h e alkenes i s t o be undertaken. ALKENE ISOMERIZATION CATALYST A method f o r t h e preparation, c o n d i t i o n i n g and operation o f a c a t a l y s t conThe a c t i v e c a t a l y s t contains t a i n i n g tungsten oxide has been developed.

tungsten i n an intermediate valence s t a t e maintained by t h e alkene i n a c a r r i e r gas containing hydrogen and water vapour.

Results f o r t h e isomerization of

1-butene, 1-pentene and 1-hexene show t h a t t h e r a t i o o f branched t o s t r a i g h t chain alkenes i n t h e product approach t h e e q u i l i b r i u m r a t i o s , t h a t double bond s h i f t and s i n g l e chain branching adjacent t o the double bond are t h e major reactions and t h a t hydrogenation o f t h e a1 kene i s n e g l i g i b l e .

500

D e t a i l s o f t h e method have been d e s c r i b e d and p a t e n t e d ( r e f s . 8,9,11).

More

r e c e n t t e s t s o f t h e s e c a t a l y s t s have demonstrated t h a t i n c r e a s e d s p e c i f i c a c t i v i t y and c a t a l y s t s l i f e can be achieved ( t a b l e 1). TABLE 1 Performance o f a l k e n e i s o m e r i z a t i o n c a t a l y s t , 18 w t % WOs on alumina, p r e c o n d i t i o n e d a t 450°C i n H2/HZ0. Reactant

1-butene

1-pentene

r e a c t i o n temperature ("C) gas r a t i o H2/H20/alkene f l o w (hour-1) branched p r o d u c t ( X )

360 100/2.5/5 600 42

360 100/2.5/6 600 60

Under t h e t e s t c o n d i t i o n s o f t a b l e 1, c a t a l y s t l i f e f o r 1-pentene i s o m e r i z a t i o n exceeded 24 hours b u t i n t h e same p e r i o d c o n v e r s i o n of 1-butene decreased to

- 30%.

The c a t a l y s t was r e g e n e r a t e d a t 450°C by a i r (15 m i n ) t h e n

H2/H20 ( 1 h o u r ) .

Repeated r e g e n e r a t i o n s were s u c c e s s f u l .

SYNTHESIS OF ETHERS One o f s e v e r a l a p p l i c a t i o n s o f t h e above c a t a l y s t s i s t h e p r o d u c t i o n o f t h e h i g h o c t a n e e t h e r s , MTBE and TAME, which a r e i n c r e a s i n g l y i n demand as b l e n d s t o c k s for unleaded g a s o l i n e . f i g u r e 3.

S y n t h e s i s gas f r o m n a t u r a l methane would be c o n v e r t e d by an a l k e n e

selective catalyst.

STEAM

L T

'

s A (

The concept o f t h e process i s o u t l i n e d i n

++

k

The butene and pentene c o u l d t h e n be i s o m e r i z e d by t h e

METHANE

SYNTHESIS

F i g u r e 3. S y n t h e s i s o f MTBE and TAME

r

METHANOL

n

MTBE TAME

-

concept o f process. HEAVY PRODUCTS L I G H T ALKENES

++I

C a t a l y s t 1 : alkene s e l e c t i v e F-T c a t a l y s t . C a t a l y s t 2: a l k e n e s k e l e t a l isomerization catalyst.

!LO-ALKENES

501

tungsten c a t a l y s t described above.

The

Cb

and

C5

f r a c t i o n s would be separated

and reacted w i t h methanol, an e s t a b l i s h e d technology f o r t h e p r o d u c t i o n o f t h e ethers ( r e f . 12).

These l a t t e r r e a c t i o n s a r e s e l e c t i v e f o r t h e branched

isomers so t h a t unreacted butene and pentene c o u l d be r e c y c l e d through t h e i s o merization catalyst. Other processes f o r ethene, propene and heavy a1 kenes would o b v i o u s l y be needed t o achieve complete u t i l i z a t i o n . ACKNOWLEDGEMENTS This work was supported by t h e A u s t r a l i a n N a t i o n a l Energy Research Development and Demonstration Council. REFERENCES

1 R.A. Sheldon, Chemicals from Synthesis Gas, Reidel, Dordrecht, 1983, 71-73. 2 B. Bussemeir, C.D. Frohning and B. C o r n i l s , Hydrocarbon Process, 55(11) (1976) 105. 3 V.E. P i e r c e and A.K. Logwinuk, Hydrocarbon Processing, 64, Sept. (1985) 75-79. 4 R.M. Heck, R.G. McClung, M.P. W i t t and 0. Webb, i b i d , 59, A p r i l (1980) 185-191. 5 L.S. B i t a r , E.A. Hazbun and W.J. P i e l , i b i d , 63, Oct. (1984) 63-66. 6 E. Anderson, Chem. Eng. News, Oct. 13, 1986, 8. 7 B.G. Baker and N.J. C l a r k , I V I n t e r n a t i o n a l Symposium on t h e S c i e n t i f i c Bases f o r t h e P r e p a r a t i o n o f Heterogeneous C a t a l y s t s , E l s e v i e r (1986). 8 B.G. Baker, N.J. C l a r k , H . McArthur and E . S u m e r v i l l e , I n t e r n a t i o n a l Patent A p p l i c a t i o n PCT/AU83/00110. 9 B.G. Baker, N.J. C l a r k , H. McArthur and E. Summerville, U n i t e d States Patent 4610975, Sept. 9, 1986. 10 J. Abbot, N.J. C l a r k and B.G. Baker, A p p l i e d C a t a l y s i s 26 (1986) 141-153. 11 B.G. Baker and N.J. Clark, I n t e r n a t i o n a l Symposium on C a t a l y s i s and Automotive P o l l u t i o n C o n t r o l , Brussels 1986 ( E l s e v i e r ) . 12 J.D. Chase and B.B. Galvez, Hydrocarbon Processing 60, March (1981) 89-94.



503

HYDROGENATION OF CO OVER MOLYBDENUM-ZEOLITES YOU-SING YONG and R.F. HOWE Chemistry Department, University of Auckland, Private Bag, Auckland (New Zeal and) ABSTRACT The molybdenum-zeolites prepared by adsorption and decomposition of M o ( C O ) ~ have been shown to be active CO hydrogenation catalysts. The product distribution varies with the charge-compensating cation in the zeolite and Si/A1 ratio. An LPG selectivity of up to 57% has been observed. INTRODUCTION The scarcity of liquid fuel and the relative abundance of coal have refocussed on the possibility of synthesising hydrocarbons via the Fischer-Tropsch synthesis (FTS). In this process, the product distribution is usually broad due to the fact that the reaction is essentially a polymerization process (ref. 1). An improvement in the selectivity of the desired products is thus essential. The use of zeolite as support has been reported to improve the selectivity due to the "cage effect" (refs. 2-4). Jacobs gal. (ref. 4) have reported a sharp cut-off at around C9-Cl0 in the CO hydrogenation over ruthenium-zeolite catalyst. Further, the selectivity can be improved by the use of alkali metals, either as charge-compensating cations (refs. 5-6) or dopants (refs. 7-8) for non-zeolite supports. For example, Murchison (ref. 7) has reported a 70% selectivity to LPG (C2-C4) product for a potassium-promoted molybdenum catalyst supported on saran charcoal. Brenner et al. (ref. 8) have also reported a high selectivity to LPG product for a potassium-promoted molybdenum-alumina catalyst. In our previous papers (refs. 9-10), we described the activity and selectivity of some molybdenum-zeolite catalysts in the FTS reaction at sub-atmospheric and moderate temperature reaction conditions. In this paper, an extension of the earlier work will be described, paying particular attention to the influence of the charge-compensating cations, the pore size and aluminium content on the product distribution. EXPERIMENTAL The synthetic zeolites NaX and NaY were obtained from Strem Chemicals Inc. CsY, LaY and HY were prepared by repeated ion-exchange of NaY as described elsewhere (ref. 11). The unit cell composition of the zeolites was determined by flame spectroscopy and EDX. The preparation of the molybdenum-zeolites and the CD hydrogenation experimental procedures have been reported earlier (refs.

504

9-10,

12).

RESULTS AND D I S C U S S I O N I n t h e absence o f molybdenum, t h e b l a n k dehydrated z e o l i t e s showed no CO I n c o n t r a s t , measurable q u a n t i t i e s o f

h y d r o g e n a t i o n a c t i v i t y even up t o 400OC.

a l i p h a t i c hydrocarbons were d e t e c t e d o v e r t h e molybdenum-zeolite c a t a l y s t s a t 300°C and above.

F i g s . 1-2 show t h e t i m e dependence o f CO c o n v e r s i o n o v e r

HY and M o ~ ~CsY. ~a t 300OC.

The c o n v e r s i o n and p r o d u c t d i s t r i b u t i o n were

dependent on t h e r e a c t i o n c o n d i t i o n s , T a b l e 1.

a typical set o f results i s illustrated i n

The molybdenum-zeolites prepared b y a d s o r p t i o n and decomposition o f

M O ( C O ) ~resembled c l o s e l y t h e alumina-supported molybdenum c a t a l y s t s p r e p a r e d b y The r e s u l t s o b t a i n e d p r e s e n t l y c o u l d

decomposing M o ( C O ) ~on a l u m i n a ( r e f . 1 3 ) . n o t match t h e f i g u r e s r e p o r t e d b y Brenner

Ct. ( r e f .

8), b u t t h i s c o u l d be due

t o t h e s i g n i f i c a n t d i f f e r e n c e s i n t h e r e a c t i o n c o n d i t i o n s used b y t h e above authors.

However, a comparison w i t h t h e s i l i c a - m o l y b d e n a c a t a l y s t ( p r e p a r e d b y

i m p r e g n a t i o n o f ammonium molybdate) c l e a r l y i n d i c a t e s t h a t t h e molybdenumz e o l i t e s were more a c t i v e on p e r molybdenum b a s i s .

The improved a c t i v i t y i s due

t o t h e presence o f z e r o v a l e n t molybdenum ( f o r LaY and HY, r e s i d u a l z e r o v a l e n t molybdenum were r e s p o n s i b l e f o r t h e a c t i v i t y ) .

As shown i n T a b l e 1, t h e a l k a l i metal-exchanged z e o l i t e s were s i g n i f i c a n t l y more s e l e c t i v e t o b o t h LPG and alkenes t h a n LaY and HY. s t r o n g molybdenum-cation i n t e r a c t i o n i n t h e z e o l i t e s .

This c l e a r l y r e f l e c t s a I t has been e s t a b l i s h e d

t h a t t h e exchange o f sodium i o n s w i t h s m a l l e r , l a r g e r o r m u l t i v a l e n t i o n s and an i n c r e a s e i n t h e Si/A1 r a t i o a l t e r t h e a c i d s t r e n g t h o f t h e z e o l i t e s ( r e f . 14). Due t o t h e s t r o n g p e r t u r b a t i o n o f t h e z e o l i t e environment, t h e molybdenum i n t h e supercages becomes e l e c t r o n - d e f i c i e n t .

T h i s e l e c t r o n d e p l e t i o n depends on t h e

s u p p o r t and i n g e n e r a l , t h i s c h a r a c t e r i n c r e a s e s w i t h i n c r e a s i n g a c i d s t r e n g t h .

As a . r e s u l t ,

t h e metal-C0 bond weakens.

hydrogen c h e m i s o r p t i o n .

T h i s i s p a r a l l e l e d by an i n c r e a s e i n

I n f r a r e d evidence showed t h a t CO d i s p r o p o r t i o n a t e s

r e a d i l y o v e r t h e s e molybdenum-zeolites a t 200°C and above.

I n addition,

c h a r a c t e r i s t i c bending v i b r a t i o n s o f CHx ( x = 1-3) segments were observed between 1445 and 1325 cm-’.

These bands grew i n i n t e n s i t y w i t h time, s u g g e s t i n g

t h a t t h e c h a i n growth o c c u r r e d v i a t h e i n s e r t i o n o f CHx species.

However, an

i n c r e a s e i n adsorbed hydrogen causes an i n c r e a s e i n t h e r a t e o f h y d r o g e n a t i o n o f t h e s u r f a c e carbon s p e c i e s . and HY c a t a l y s t s . Jacobs

9

z. ( r e f .

Hence, l e s s LPG and alkenes were observed f o r LaY

S i m i l a r r e s u l t s have been r e p o r t e d b y L e i t h ( r e f s . 5-6) and

4) over ruthenium-faujasite c a t a l y s t s .

Under t h e l o w p r e s s u r e and moderate t e m p e r a t u r e r e a c t i o n c o n d i t i o n s , t h e l a c k o f h i g h e r hydrocarbons i n t h e p r o d u c t stream i s n o t s u r p r i s i n g s i n c e t h e s e

products are thermodynamically unfavourable.

A l t h o u g h t h e p o r e s t r u c t u r e s show

no d r a m a t i c s t e r i c i n f l u e n c e on t h e p r o d u c t d i s t r i b u t i o n s , t h e y a r e s u f f i c i e n t

505

L3

20 C

._ m c u

15

V 0 W

Is)

3 C

10

5t n

L

TIME Fig. 1:

(

MINUTE

)

Time-dependent CO conversion over Mo11.8 HY at 3OO0C, CO/H2 = 1, Ptotal = 0.28 atm., 23 hr reaction

8 C

0 ._

n al

2

u

6

V 0 0)

cn

3

4

0 W

a W

2

n

0

300

600

TIME

900 (

1200

1500

MINUTE 1

Fig. 2: Time-dependent CO conversion over M015.1 CsY at 3OO0C, CO/H2 = 1, Ptotal = 0.28 atm., 23 hr reaction.

TABLE 1:

Activity and Product Distribution of CO Hydrogenation over Molybdenum-Zeolite Catalysts.

I

I

I

Product Fraction ( % CO Conversion )

Catalyst

LPG

%

1

Mo11.8H46Na5Y

1

44.5

I

56.8

I

30.9

I- 1

57.4

12.0

l-. 9

43.2

M024. 3H46Na5Y b,

42.5

66.4

31.8

tr

M012. qLalZH U N a Z Y

15.8

52.0

31.8

tr

14.8

48.0

b, M O ~ 8La12H13Na2Y ~ .

41.4

59.1

32.7

tr

8.2

40.9

M015. 4Na71X

21.0

48.9

30.5

1.0

13.9

51.1

33.7

507 t o induce a non-Schulz-Flory p r o d u c t d i s t r i b u t i o n .

These r e s u l t s a r e con-

s i s t e n t w i t h t h e e a r l i e r r e s u l t s observed o v e r c o b a l t ( r e f . 2 ) , i r o n ( r e f . 3 ) and r u t h e n i u m ( r e f . 4 ) - z e o l i t e c a t a l y s t s .

I n a l l t h e c a t a l y t i c runs, no

oxygenates were d e t e c t e d . CONCLUSION The above s t u d i e s showed t h a t t h e molybdenum-zeolites were a c t i v e CO hydrog e n a t i o n c a t a l y s t s due t o t h e presence o f z e r o v a l e n t molybdenum.

Although, t h e

p o r e s i z e o f t h e z e o l i t e s d i d n o t i n f l u e n c e t h e p r o d u c t spectrum, t h e enhanced s e l e c t i v i t y t o LPG and alkenes was due t o t h e c l o s e molybdenum-zeolite association. ACKNOWLEDGEMENT We thank t h e Donors o f t h e Petroleum Research Fund o f t h e American Chemical S o c i e t y f o r t h e s u p p o r t o f t h i s work. REFERENCES 1 2 3

4 5 6 7 8 9 10 11 12 13 14

G. H e n r i c i - O l i v g and S. O l i v g , Agnew. Chem. I n t . Ed. Engl., 15 (1976), 136. B.G. Gates and D. Fraenkel. J. Amer. Chem. Soc., 102 (1980). 2478. L.F. Nazar, G.A. Ozin, F. Hugues, J. Godber and-D. Rancourt; Agnew. Chem. I n t . Ed. Engl., 22 (1983), 624. P.A. Jacobs, H.H. N i j s and J.B. Uytterhoeven, J. Chem. SOC. Chem. Comm., (1979), 1095. I.R. L e i t h , J. Chem. SOC. Chem. Commun., 93 (1983). I.R. L e i t h , J. Catal., 91 (1985), 283. C.B. Murchison i n " F o u r t h I n t e r n a t i o n a l Conference Uses and Chemistry o f H.F. B a r r y and P.C.H. M i t c h e l l Eds.", Climax Molybdenum Molybdenum Company, Ann Arbor, Michigan, (1982), 197. A. Brenner, S. Sivasanker, E.P. Yesodharan, C. Sudhakar and C.B. Murchison, J. Catal., 87 (1984), 514. Y.S. Yong and R.F. Howe, J. Molec. Catal., 38 (19861, 323. Y.S. Yong and R.F. Howe, Stud. S u r f . S c i . Catal., 28 (1986), 883. Y.S. Yong, Ph.D. Thesis, U n i v e r s i t y o f Auckland (1987). Y.S. Yong and R.F. Howe, J. Chem. SOC. Faraday Trans. 1, 82 (1986), 2887. A. Brenner and R.L. B u r w e l l , J. Catal., 52 (1978), 353. P.A. Jacobs i n 'Carboniogenic A c t i v i t y i n Z e o l i t e s " , E l s e v i e r , Amsterdam, (1977), 57.

-

-


D.M. Bihhy, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

509

ROLE OF SUPPORTS FOK COBALT-BASED CATALYSTS USED IN FISCHEK-TROPSCH SYNTHESIS OF HYDKOCARBONS

G.M.

ROE, M.J.

RIDD, K.J.

CAVELL and F.P. LARKINS

Department of Chemistry, University of Tasmania, G.P.O. Tasmania 7001 (Australia)

Box 252C, Hobart,

ABSTRACT A s u i t e of cobalt-based catalysts supported on high- and low-surface area s i l i c a s and on alumina, magnesia, zeolite A and t i t a n i a have been characterised f o r metal-support interaction effects using a range of techniques including temperature programmed reduction, X-ray photoelectron spectroscopy, gravimetric adsorption studies and diffuse reflectance spectroscopy. 'Ihe prepared catalysts were also screened for t h e i r Fischer-Tropsch (CO + H2) a c t i v i t y and tested by CO temperature programmed carbiding experiments. Correlations w e r e found t o e x i s t between the behavimr of the catalysts and the occurrence of various cobalt species. INTRODUCTION

It is w e l l established that iron, cobalt and nickel supported catalysts may be used t o convert CO and H2 t o hydrocarbons by a process hum a s the

Fischer-Tropsch

synthesis.

The problems of

elucidation of

mechanism and

identification of active metal species have usually been tackled separately. Despite extensive research, neither aspect is w e l l understood. In t h i s paper Fischer-Tropsch

a c t i v i t y is assessed i n term of the nature of the cobalt

species present a s part of the catalysts. EXPERIMENTAL

High purity

(99.9%) T i 0 2

(Strem chenicals,

NATL 93-2207),

99.8% pure

Si02 (Strem, NATL 93-1436), 99% pure NO (Strem, NATL 93-1243), high purity (99.9%) A1203 (Strem, NATL 93-1380), zeolite type 4A (BDH, W. 54004) and Si02 40A (Merck, No. 10181) were used as supports. The unsupported cobalt oxide used for calibration was CosO,, (Specpure, Johnson Matthey). Cobalt(I1) n i t r a t e hexahydrate (BDH 97.5% min. assay) w a s used for catalyst preparation. The cobalt supported zeolite catalyst was prepared by ion exchange. Powdered z e o l i t e A was slurried i n d i s t i l l e d water, and an aqueous solution of C O ( N O ~ ) ~ . ~was H ~ Oadded dropwise Over a period of five hours with s t i r r i n g and gentle heating. tanperature.

me

sample was

then

filtered

and air dried

at room

510

other catalysts were prepared by impregnation. The supporcs were slurried in d i s t i l l e d water, and aqueous solutions containing appropriate amounts of C O ( N @ ) ~ . ~ H were ~ O added. The mixture was then evaporated t o dryness with constant stirring. All

Each Catalyst was calcined a t 400°C i n a i r for 24 hours and passed through a BSS 100 mesh (.0063 inches) sieve.

The fraction 275"C on &/Ti02

and CoSi02

catalysts (Fig. 2) are related t o TFC evidence for carbide and coke formation. Methane is formed by hydrogenation of the coke i n competition with the FT polymerisation process. For Co/A1203 the yield of C2 and C3 hydrocarbons relative t o C, w a s greatest a t the highest temperature examined. h e Tpc study showed that t h i s catalyst had a very law coking r a t e , associated with the w e l l dispersed Coo derived from surface Co2+ ions. Here, the FT polymerisation synthesis does not have t o compete with coke formation, therefore a higher selectivity tmards higher hydrocarbons is achieved.

515

reduction of phase I (Coso,), which gave large metal particles (as on Co/SiO2 and Coll'iO,) and from the reduction of phase I1 Co2+ which led t o w e l l dispersed particles. Coking muld be expected t o become the aver-riding reaction on the large particles, by analogy with CoISiO2 and Co/Ti02, whereas the FTR muld predominate on the well dispersed clusters, by analogy with Co/A1203. The observed behaviour of the catalyst supports t h i s hypothesis. The increase i n CHI, yield with temperature, observed for the Co/Si02 and Co/Ti02 catalysts occurred. So too did the trend of increasing C2 and C3 yields observed f o r Co/A1203. The lack of a c t i v i t y of Co/MgO i s due t o the non availability of cobalt metal due t o the formation of a CoO/MgO solid solution during activation. In contrast, the relatively high dispersion of @/zeolite suggests that cobalt metal i s available on the catalyst surface. The s q l e is inactive possibly because the rnnnber of Coo atoms i n each cluster is less than a lmer l i m i t required for dissociative CO chemisorption. Chemisorption with retention of i n t e g r i t y , as is measured i n the dispersion experiment, is not subject t o such a constraint. CONCLUSION It has been shown that the dispersion of cobalt on various supports varies

with the surface area and nature of the support. In the as-prepared calcined s t a t e the cobalt exhibits three reduction phases. Characteristics of the catalysts have been used t o explain t h e i r Fischer-Tropsch activity. REFERENCES D.A. Castner and D.S. S a n t i l l i , i n T.E. Whyte, R.A. D a l l a Betta, E.A. Derwane and R.T.K. Baker (Editors), Catalytic Materials; Relationship between structure and reactivity, A.C.S. Syrnposim Series No. 248, Washington, U . S . A . , 1984, pp. 39-56. P. Arnoldy and J.A. Mouljin, J. Catal., 93 (1985) 38-54. E.E. lhrmth, L.H. Schwartz ana J.B. Butt, J. Catal., 63 (1980) 404-414. T. Paryjczak, J. Rynkwski and S. Karski, J. Chromatogr., 188 (1980) 254-256. H.F.J. van't Blik, D.C. Konningsberger and R. &ins, J. Catal., 97 (1986) 210-218. B.A. Sexton, A.E. Hughes and T.bL Tumey, J. Catal., 9 7 (1986) 390-406. P. Pascal, Noweau B a i t e de Chimie Minerale, Vol. 17, Masson e t cie, Paris, 2nd ed., 1963. G.M. Roe, B.Sc. Hons. Thesis, h i v e r s i t y of Tasmania, 1986. R.B. Anderson, 'Ihe Fischer Tropsch Synthesis, Academic Press, 1st ed., 1984.



517

LIQUID PHASE FISCHER-TROPSCH SYNTHESIS USING ULTRAFINE PARTICLES OF I R O N AS CATALYST

E. K I K U C H I and H. I T O H

Department o f Applied Chemistry, School of Science and Engineering, Waseda U n i v e r s i t y , 3-4-1 Okubo, Shinjuku-ku, Tokyo 160 (Japan)

ABSTRACT Liquid-phase Fischer-Tropsch synthesis has been i n v e s t i g a t e d using a s l u r r y bed reactor. The c a t a l y t i c a c t i v i t y o f u l t r a f i n e p a r t i c l e s (UFP) composed o f Fe was shown t o be greater than t h a t of a p r e c i p i t a t e d Fe c a t a l y s t . The d i f f e r e n c e was i n t e r p r e t e d as caused by the d i f f e r e n t nature o f surface s t r u c t u r e between these c a t a l y s t s , whether porous o r not. The obtained carbon number d i s t r i b u t i o n s over alkali-promoted Fe UFP c a t a l y s t s were simulated by a superposition o f two F l o r y type d i s t r i b u t i o n s . I t i s ascertained t h a t t h e surface o f alkali-promoted UFP c a t a l y s t s possesses promoted and unpromoted s i t e s e x h i b i t i n g d i f f e r e n t chain growth p r o b a b i l i t i e s . INTRODUCTION

Fischer-Tropsch (FT) synthesis i s accompanied by an extremely l a r g e heat e v o l u t i o n (exothermic). To improve t h e c h a r a c t e r i s t i c s o f heat t r a n s f e r , 1 i q u i d phase synthesis using a s l u r r y - t y p e r e a c t o r has been developed. Although l i q u i d phase synthesis has been operated using p u l v e r i z e d c a t a l y s t s ( r e f . I ) , i t i s i n t e r e s t i n g t o use a c a t a l y s t o f smaller p a r t i c l e s , so-called u l t r a f i n e p a r t i c l e (UFP), f o r t h e purpose o f enhancing the g a s - l i q u i d - s o l i d i n t e r f a c e contact. The o b j e c t i v e o f t h e present work i s t o determine the c a t a l y t i c a c t i v i t i e s and s e l e c t i v i t i e s o f alkali-promoted Fe UFP f o r the l i q u i d phase FT synthesis. EXPERIMENTAL Apparatus and procedures Hydrogenation o f carbon monoxide was c a r r i e d o u t i n a slurry-bed reactor, as p r e v i o u s l y described ( r e f . 2). Syngas submitted t o the s l u r r y r e a c t o r was l e t t o r e a c t on the c a t a l y s t suspended i n a l i q u i d c a r r i e r (hexadecane). The product gas was p a r t l y recycled, and c a t a l y s t p a r t i c l e s were a g i t a t e d i n t h e l i q u i d c a r r i e r by the recycled gas. An u l t r a s o n i c generator was used t o prepare the suspension o f UFP p r i o r t o r e a c t i o n ( r e f . 3). The procedure employed f o r a d d i t i o n o f a l k a l i metals ( L i , Na, K, Rb, Cs) t o

518

UFP was s i m i l a r t o t h a t d e s c r i b e d i n t h e p r e v i o u s paper ( r e f . 3 ) ; t h e c o l l o i d a l

s o l u t i o n o f a l k a l i metal prepared by u l t r a s o n i c i r r a d i a t i o n was added t o t h e suspension o f UFP c a t a l y s t . Catalysts An Fe UFP and a p r e c i p i t a t e d Fe c a t a l y s t s were used i n t h e p r e s e n t work. The Fe UFP d e s i g n a t e d as Fe UFP 200

was s u p p l i e d b y Vacuum M e t a l l u r g i c a l Co.,

Ltd.

(Chiba P r e f e c t u r e , Japan) and was prepared by t h e gas e v a p o r a t i o n method ( r e f s . 4,5). Transmission e l e c t r o n m i c r o s c o p i c (TEM) o b s e r v a t i o n showed t h a t t h e UFP was s i n t e r e d i n t h e course of p r e p a r a t i o n and was i n t h e f o r m o f " n e c k l a c e " as shown i n F i g . 1. T h i s UFP had a nonporous s t r u c t u r e and t h e BET s u r f a c e a r e a o f 0 2 36.8 m /g, i n accordance w i t h an average p a r t i c l e s i z e o f 210 A. The X-ray d i f f r a c t i o n a n a l y s i s o f t h e UFP gave d i f f r a c t i o n peaks o n l y due t o a-Fe.

F i g . 1. Transmission e l e c t r o n m i c r o g r a p h of Fe UFP c a t a l y s t . The p r e c i p i t a t e d Fe c a t a l y s t composed o f 100Fe:0.3Cu:0.6K2C03

was p r e p a r e d

a c c o r d i n g t o t h e method d e s c r i b e d by Kunugi e t a l . ( r e f . 6), u s i n g aqueous n i t r a t e s and p r e c i p i t a t e d a t pH=7.2. A f t e r c a l c i n a t i o n a t 32OoC, t h e c a t a l y s t was f i n e l y crushed t o powders s m a l l e r t h a n 74 urn (200 mesh), and a c t i v a t e d i n a

CO s t r e a m f o l l o w e d by a H2 s t r e a m ( r e f . 2 ) . The BET s u r f a c e area of t h i s K2 promoted p r e c i p i t a t i o n Fe c a t a l y s t was 42.2 m /g. RESULTS AND DISCUSSION The c a t a l y t i c a c t i v i t y o f Fe UFP

519

-

I

I I

L

.c

n

I

M -i 0

x

15

I

a, 0 V

2

0

None

L1

Na

K

Rb

Alkali promotion to UFP')

Cs

K-pro,moted prec i p i t a t ion

Fig. 2. Effect of alkali promotion on the average STY over Fe UFP catalysts. Reaction conditions: tempera ure, 22OoC; pressure, 30 atm; H2/CO, 1 mol/mol; W/F, 300 g-cat.min/CO-mol. a Alkali addition: 1 wt% of catalyst.

f

Liquid phase hydrogenation of carbon monoxide was carried out using unpromoted and alkali-promoted Fe UFP catalysts and the K-promoted Fe precipitation catalyst. The principal products formed on these catalysts were n-olefins, n-paraffins, alcohols, aldehydes, ketones, carbon dioxide, and water. Although addition of alkali metal reduced the catalytic activity of Fe UFP for FT synthesis, catalyst deactivation was suppressed by a1 kali promotion. Figure 2 shows the average STY'S of hydrocarbons, oxygenates, and C02 over the Fe UFP catalysts promoted by various kinds of alkali metals in a comparison with the precipitated catalyst. These data were taken for the products in the initial 6 hr of run. The activities of UFP catalysts were higher than that of the ordinary K-promoted Fe precipitation catalyst, in spite of comparable surface areas. This is interpreted as due to an effect of surface structure of catalyst. In the case of the precipitated catalyst having a rather porous structure compared with UFP, the reactant diffuses into the pores and reacts on the catalyst surface. If the reaction is faster than diffusion processes, the concentration of reactant falls along with the distance from the pore mouth. Thus, a limited portion of the surface of the precipitated catalyst can be used for reaction (ref. 7).

Selectivity of a1 kal i-promoted Fe UFP catalysts A further investigation into the selectivity of alkali-promoted Fe UFP catalysts was achieved. The Flory plot of hydrocarbons synthesized over these

520

promoted Fe UFP c a t a l y s t s c o u l d n o t be d e s c r i b e d by a s t r a i g h t l i n e , o r t h e p r o d u c t s formed on t h e s e a l k a l i - p r o m o t e d UFP c a t a l y s t s d i d n o t obey a normal F l o r y d i s t r i b u t i o n . A t y p i c a l F l o r y p l o t on t h e K-promoted c a t a l y s t i s shown i n F i g . 3. A s i m i l a r break i n t h e F l o r y d i s t r i b u t i o n has been r e p o r t e d f o r Kpromoted Fe c a t a l y s t s . And i t has been proposed t h a t carbon number d i s t r i b u t i o n s o f p r o d u c t s f r o m K-promoted Fe c a t a l y s t s can be d e s c r i b e d by a s u p e r p o s i t i o n of two F l o r y t y p e d i s t r i b u t i o n s ( r e f s . 8-10). The p r o d u c t d i s t r i b u t i o n s h o u l d be f i t t e d f o r t h e eqn. 1, i f two k i n d s o f s i t e s e x i s t , and g r o w i n g c h a i n s on each k i n d o f s i t e do n o t i n t e r a c t :

where mn i s t h e mole f r a c t i o n o f a p r o d u c t h a v i n g n carbon atoms, aA and a a r e B t h e c h a i n g r o w t h p r o b a b i l i t i e s f o r t h e two k i n d s o f s i t e s d e s i g n a t e d as s i t e A and s i t e B, r e s p e c t i v e l y . S i t e A and B a r e d e f i n e d as t h e s i t e e x h i b i t i n g a l o w e r and a h i g h e r p r o b a b i l i t y o f c h a i n growth, r e s p e c t i v e l y . CA and C B ( = l - C A ) a r e mole f r a c t i o n s o f o r g a n i c p r o d u c t s formed on s i t e s A and B y r e s p e c t i v e l y , i.e.,

t h e c o n t r i b u t i o n o f each k i n d o f s i t e s .

These parameters were e s t i m a t e d t o g i v e t h e b e s t f i t t o t h e d a t a f o r unpromoted and a l k a l i - p r o m o t e d UFP c a t a l y s t s by use o f a n o n l i n e a r l e a s t - s q u a r e s method.

The f i t n e s s between s i m u l a t e d and e x p e r i m e n t a l p r o d u c t d i s t r i b u t i o n was

I

I

5

I

I

I

10 15 20 Carbon number

I

25

F i g . 3. F l o r y p l o t of hydrocarbon p r o d u c t s o v e r potassium-promoted Fe UFP c a t a l y s t . R e a c t i o n c o n d i t i o n s : temperature, 22OoC; p r e s s u r e , 30 atm; Hz/CO, 1 mol/mol; W/F, 300 g-cat.min/CO-mol. Potassium a d d i t i o n : 1 w t % o f c a t a l y s t . S o l i d l i n e r e p r e s e n t s t h e s i m u l a t e d d i s t r i b u t i o n based on eqn. 1.

521

TABLE 1 E s t i m a t e d parameters f o r a l k a l i - p r o m o t e d Fe UFP c a t a l y s t s . A l k a l i metal

None

Li

Na

K

Rb

cs

0.83 0.13

0.83 0.17

0.83 0.37

0.84 0.55

0.85 0.33

0.88 0.35

R e a c t i o n c o n d i t i o n s : temperature, 22OoC; p r e s s u r e , 30 atm; H2/C0, W/F, 300 g-cat.min/CO-mol.

1 mol/mol;

s a t i s f a c t o r y , as t y p i c a l l y shown i n F i g . 3. The l o w e r growth p r o b a b i l i t i e s . f o r t h e s e a l k a l i - p r o m o t e d c a t a l y s t s were c l o s e l y equal t o t h a t f o r t h e unpromoted c a t a l y s t , and t h e y were. a p p r o x i m a t e l y 0.6.

The e s t i m a t e d parameters, aB

and CB a r e summarized i n Table 1. The h i g h e r c h a i n growth p r o b a b i l i t i e s (a,)

on

t h e s e a l k a l i - p r o m o t e d Fe UFP c a t a l y s t s i n c r e a s e d w i t h r e l a t i v e a b i l i t y o f t h e promoter t o donate e l e c t r o n s : L i < Na < K < Rb < Cs. Thus, i t i s a s c e r t a i n e d t h a t promoted s i t e s ( s i t e B ) and unpromoted s i t e s ( s i t e A) a r e p r e s e n t on t h e s u r f a c e o f a1 k a l i - p r o m o t e d UFP c a t a l y s t s . ACKNOWLEDGEMENT The p r e s e n t work was s u p p o r t e d by a G r a n t - i n - A i d f o r S c i e n t i f i c Research No.

61040012 f r o m t h e M i n i s t r y o f Education, Science and C u l t u r e , Japan. REFERENCES

1 H. Kb’lbel and M. Ralek, C a t a l . Rev.-Sci. Eng., 21 (1980) 225-274. 2 E. K i k u c h i , H. I t o h , M. M i y a z a k i and Y. M o r i t a , S e k i y u Gakkaishi, 29 (1986) 317-323. 3 H. I t o h , E. K i k u c h i and Y. M o r i t a , s u b m i t t e d f o r p u b l i c a t i o n . 4 K. Kimoto, Y. Kamiya, M. Nonoyama and R. Uyeda, Jpn. J. Appl. Phys., 2 (1963) 702-713. 5 S . Kashu, M. Nagase, C. Hayashi, R. Uyeda, N. Wada and A. Tasaki, Jpn, J. Appl. Phys. Suppl. 2, Pt, 1 (1974) 491-493. 6 T. Kunugi, T. Sakai, H. Ose and Y. Hamada, Kogyo Kagaku Zasshi, 69 (1966) 2244-2249. 7 R.B. Anderson, J.F. S h u l t z , L.J.E. H o f e r and H.H. S t o r c h , B u l l e t i n U.S. Bureau o f Mines, 580 (1959). 8 L. K 6 n i g and J. Gaube, Chem.-1ng.-Tech., 55 (1983) 14-22. 9 G.A. H u f f , Jr. and C.N. S a t t e r f i e l d , J. Catal., 85 (1984) 370-379. 10 B. S c h l i e b s and J. Gaube, Ber. Bunsenges. Phys. Chem., 89 (1985) 68-73.



523

OLIGOMERIZATION OF LOWER OLEFINS TO OCTANE ENHANCERS AND DISTILLATE RANGE OLEFINS BY NICKEL BASED HOMOGENEOUS AND SUPPORTED CATALYSTS

K.J.

CAVELL

Chemistry Department, U n i v e r s i t y o f Tasmania, G.P.O. 7001 ( A u s t r a l i a )

Box 252C, Hobart, Tasmania

ABSTRACT S e l e c t i v e c o n v e r s i o n o f l o w e r o l e f i n s , produced from methane, i n t o f u e l range o l i g o m e r s and p e t r o c h e m i c a l f e e d s t o c k may be c a r r i e d o u t homogeneously. A g r o u p o f h i g h l y a c t i v e , v e r s a t i l e , B - d i t h i o k e t o n a t e n i c k e l based homogeneous c a t a l y s t s have r e c e n t l y been developed. C a t a l y s t s t h a t a r e s e l e c t i v e f o r o l e f i n d i m e r s o r systems p r o d u c i n g s i g n i f i c a n t amounts o f h i g h e r o l i g o m e r s have been prepared. A v a r i e t y o f dimer p r o d u c t mixes may be produced s e l e c t i v e l y and c a t a l y s t s may be o p e r a t e d i n a d i v e r s i t y o f s o l v e n t s o r c h e m i c a l l y a t t a c h e d t o a p p r o p r i a t e s u p p o r t s and o p e r a t e d heterogeneously. However s e n s i t i v i t y t o oxygenated i m p u r i t i e s and l i m i t e d t h e r m a l s t a b i l i t y may r e s t r i c t t h e i r use i n f u e l s appl ic a t i o n s .

INTRODUCTION C u r r e n t t e c h n o l o g i e s f o r t h e c o n v e r s i o n o f methane i n t o g a s o l i n e , d i s t i llate

and

petrochemicals

require

initial

formation

of

middle

intermediate

f e e d s t o c k s such as s y n t h e s i s gas, methanol and l o w e r o l e f i n s which i n t u r n must b e c o n v e r t e d t o t h e d e s i r e d products.

W h i l s t adding an e x t r a s t a g e such an

approach may add f l e x i b i l i t y t o t h e o v e r a l l c o n v e r s i o n o f methane. F o r example, t h e p r o d u c t i o n o f l o w e r o l e f i n s such as e t h y l e n e and p r o p y l e n e f r o m methane has t h e p o t e n t i a l t o s a t i s f y needs i n t h e f u e l s , commodity and s p e c i a l i t y chemicals a reas. One o v e r a l l

approach

currently

receiving mch

c o u p l i n g o f methane t o y i e l d h i g h e r hydrocarbons.

attention

i s the direct

T h i s c o u p l i n g may be c a r r i e d

o u t c a t a l y t i c a l l y i n t h e presence o f oxygen o r o t h e r o x i d i z i n g agent o r by t h e pyrolysis

of

methane o v e r a s u i t a b l e c a t a l y s t .

Should such a r o u t e prove

s u c c e s s f u l t h e l i k e l y p r o d u c t s w i l l be r i c h i n l o w e r o l e f i n s . O l i g o m e r i z a t i o n o f l o w e r o l e f i n s t o b o t h h i g h octane motor s p i r i t and m i d d l e distillate

is

conceptually

appropriate s e l e c t i v i t y ,

attractive.

Dimerization of

p r o v i d e s a C6 p r o d u c t ,

propylene w i t h t h e

r i c h i n 2,3-dimethylbutenesY

which i s s u i t a b l e f o r b l e n d i n g w i t h motor s p i r i t t o y i e l d a q u a l i t y unleaded g a s o l i n e . The p r o d u c t i o n o f h i g h e r o l i g o m e r s w i t h t h e a p p r o p r i a t e c h a i n l e n g t h s and l i n e a r i t y may p r o v i d e m i d d l e d i s t i l l a t e range products.

524

S e l e c t i v i t y c o n t r o l i s again o f primary

importance i n t h e p r o d u c t i o n o f

p e t r o c h e m i c a l s , f o r example t h e p r o p y l e n e d i m e r 4-methyl-1-pentene for

the

production

l i n e a r a-olefins

of

the speciality

i s a monomer

polymer p o l y ( 4 - m e t h y l - 1 - p e n t e n e )

and

are required f o r t h e production o f biodegradable detergents

and s y n t h e t i c l u b r i c a n t s . Commercially v i a b l e o l i g o m e r i z a t i o n processes u t i l i z i n g homogeneous n i c k e l based c a t a l y s t s a r e o p e r a t i n g on a l a r g e scale.

Two o f p a r t i c u l a r i n t e r e s t i n

t h e p r e s e n t c o n t e x t a r e S h e l l ' s H i g h e r O l e f i n Process (SHOP) ( r e f s .

I F P ( I n s t i t u t F r a n c a i s du P g t r o l e ) Dimersol Process ( r e f s .

SHOP i s a t h r e e stage process i n v o l v i n g o l i g o m e r i z a t i o n ,

metathesis,

which

produces

l i n e a r a-olefins

1-3) and

4,5). i s o m e r i z a t i o n and

p r e d o m i n a n t l y as feedstock

for

d e t e r g e n t manufacture. O l i g o m e r i z a t i o n i s c a r r i e d o u t a t 80-120°C and 1000-2000 p s i y (6800-13800 kPa) u s i n g a t w o l i q u i d phase system. I n a fuel

c o n t e x t t h e S h e l l process i s unacceptable.

The r e l a t i v e l y low

a c t i v i t y o f t h e o l i g o m e r i z a t i o n stage and t h e h i g h c o s t of t h e o v e r a l l process means t h e SHOP i s s u i t a b l e o n l y f o r t h e manufacture o f h i g h v a l u e p r o d u c t s . The Dimersol Process was f i r s t operated c o m m e r c i a l l y i n 1977. There a r e now some 20 p l a n t s worldwide w i t h c a p a c i t i e s dimerizes

and

co-dimerizes

p r o d u c t i o n of h,igh

propylene

up t o

and

5

The process

5x10 t o n n e s / p r

butene

predominantly

for

the

octane g a s o l i ne b l e n d i ng components.

The main drawbacks o f t h e Dimersol process a r e t h e requirement f o r l i q u i d o l e f i n feed,

which makes t h e o l i g o m e r i z a t i o n o f e t h y l e n e f o r f u e l

purposes

uneconomic, and t h e apparent l i m i t e d l i f e s p a n o f t h e c a t a l y s t . As w i t h t h e SHOP t h e prospect

of

process

improvement o r

variation

by c a t a l y s t

modification

The Dimersol and SHOP processes demonstrate t h e commercial

v i a b i l i t y of

appears t o be l i m i t e d . homogeneous

oligomerization

c h e m i c a l s areas. such

as

catalysis

i n the fuels

r e l a t e d and s p e c i a l i t y

However t h e r e i s a c o n s t a n t demand f o r improvement i n a s p e c t s

activity,

selectivity

control,

process v e r s a t i l i t y ,

simplicity

and

innovativeness. C S I R O CATALYST

Research i n i t i a t e d a t t h e C S I R O D i v i s i o n o f M a t e r i a l s Science, Melbourne and n w b e i n g c o n t i n u e d a t t h e U n i v e r s i t y o f Tasmania, of

has l e d t o t h e development

a group o f h i g h l y a c t i v e and e x t r e m e l y v e r s a t i l e c a t a l y s t systems ( r e f s .

6-8).

The systems comprise n i c k e l d i t h i o - 6 - d i k e t o n a t e

phosphine complexes ( I )

and (11) a c t i v a t e d by a s u i t a b l e c o c a t a l y s t such as an a l k y l aluminium h a l i d e complex. The c a t a l y s t p r e c u r s o r s ( I ) and (11) a r e i n e r t t o oxygen and m o i s t u r e and are

readily

synthesized

i n high y i e l d

from

relatively

cheap

and abundant

reagents. A v a r i e t y o f s y n t h e t i c r o u t e s a r e a v a i l a b l e f o r t h e p r e p a r a t i o n of

525 t h e s e complexes. The e f f i c a c y o f each and hence t h e r o u t e o f c h o i c e i s dependa n t upon t h e n a t u r e of t h e s u b s t i t u e n t s .

Where R 1 = R3 = CH,,

R~ = H;

Scheme 1 i s a summary o f t h e s e routes.

R'C(S)C(R2)C(S)R3 = SacSac

PL1L2L3

1

Scheme 1

t

Nix,

526

R and L which may be r e a d i l y v a r i e d markedly a f f e c t

The s u b s t i t u e n t s catalyst

activity

substituents

and

also

product

allows

a

distributions.

variety

of

Selection

process

of

conditions

appropriate

to

be

chosen.

O l i g o m e r i z a t i o n may be c a r r i e d o u t w i t h n o s o l v e n t o t h e r t h a n l i q u i d s u b s t r a t e o r p r o d u c t hydrocarbons employed support

( T a b l e 1). materials to

1 ) o r a h i g h l y p o l a r o r g a n i c s o l v e n t may be

by

a

variety

of

approaches

and on

bound t o i n s o l u b l e activation

operated

Independent s e l e c t i o n o f R and L a l l o w s a v a r i e t y o f d e s i r e d

heterogeneously. properties

(Fig.

P r e c u r s o r s may a l s o be c h e m i c a l l y

be

built

into

a single

catalyst.

Thus

d i s t r i b u t i o n and a d e s i r e d s o l u b i l i t y may be designed, highly active catalyst.

I n general,

a

required product

independently,

into a

f l e x i b i l i t y i n d e s i g n o f t h i s scope i s n o t

a v a i l a b l e i n o t h e r systems.

,

Moles

/

C3H6

/

0.61

200

Fig.

1.

400 600 Time (min.)

Av. c a t a l y t i c a c t i v i t y = 20,500 mole CJmole N i / h r .

800

>

Diagram s h a v i n g t h e c a t a l y t i c performance o f t h e c a t a l y s t system + Et2A1C1, o p e r a t e d a t O°C a t V2 p s i g pressure.

Ni(SacSa~)[p(C1~H,,)~]Cl Variations

i n ligand substituents

also control the selectivity

of these

c a t a l y s t s tcwards dimers o r h i g h e r oligomers. A number o f t h e c a t a l y s t s produce d i m e n almost e x c l u s i v e l y ,

h m e v e r one example l i s t e d i n Table 2 produces a

p r o d u c t c o n t a i n i n g 80% dimers and 20% o f p r e d o m i n a n t l y t r i m e r s , pentamers.

t h e d i m e t h y l b u t e n e r i c h dimer c u t i s i d e a l l y s u i t e d , gasoline

t e t r a m e r s and

The advantage o f t h e p r o d u c t m i x f r o m t h i s c a t a l y s t system i s t h a t blend t o

r a i s e t h e octane number of

a f t e r hydrogenation,

as a

unleaded p e t r o l .

The h i g h e r

o l i g o m e r f r a c t i o n my be blended w i t h t h e m i d d l e d i s t i l l a t e pool.

Due t o t h e

sharp

cut

off

i n the

carbon

chain length o f

t h e p r o d u c t mix f r o m t h e s e

c a t a l y s t s v i r t u a l l y a l l p r o d u c t s f a l l w i t h i n t h e t r a n s p o r t f u e l range.

527

Tables 1 and 2 l i s t a s e l e c t i o n o f c a t a l y s t systems based on t h e complexes

(I).

Also i n c l u d e d i n t h e Tables i s an example of

a c a t a l y s t prepared from

c o q l e x e s o f t y p e (11) and an example of a heterogenized c a t a l y s t . Because o f t h e extremely high a c t i v i t y o f

some of

these c a t a l y s t systems t e s t i n g was

c a r r i e d o u t a t -15°C i n f l o w i n g s u b s t r a t e gas a t atmospheric pressure.

N i ( R2-R’SacR3Sac)PL’L2L3X

PL’L2L3

R’

R2

R3

Me Me Me Me Me CF3

H H H H H H

Me PEt3 Me PBu3 Me PPh3 Me PCy3 Me PPhZ(0Et) Me PBu3 ~ B u PBu3 CH3 PBu3

H

‘BU

CH3 a l l y l

Act iv i t y a X

Toluene

c1 c1 c1 c1 C1 c1 c1 c1

3,500 6,500 22,000

Chlorobenzene

-

18,500 22,000 30,000 20,000 3,500

-

10,000

-

15,000 21,300 2,300

CNi ( SacSac) (dppe)]BPh,

$:iriCH2CH2P(Ph)2Ni(SacSac)Cl 0’

-

2,500

a A c t i v i t i e s expressed as moles s u b s t r a t e consumed/mole Ni/hr.

,

TABLE 2

Ligand e f f e c t s on product d i s t r i b u t i o n s i n propylene o l i g o m e r i z a t i o n

’

N i ( R2 -R SacR3Sac) PL’ L2L3X

R’

R2

Me H H Me Me H Me H Me H CF3 H Me a l l y l tBU H

R3

PL’L2L3

X

Me

PEt3 PBu3 PPh3 PCy3 PPhp(0Et PBu3 PBu3 u PBu3

c1 c1 c1

M e M e

Me Me Me Me ~ B

C N i (SacSac) (dppe)]BPh,

c1 c1 c1 c1 c1

C 6 Products % DimethylMethylbutenes pentenes

% Higher 01 igoners Hexenes 1-2 4-5

27 29 10 77 8 26 25 27

67 67 73 22 72 69 69 69

’6 4 17 1 20 5 6 4

6

75

19

5

7

83

10

6-7

-

15-20

-

2- 3 1 1-2

528

Recent Deve 1opme n t s A p a r t f r o m t h e development of a l i p h a t i c hydrocarbon s o l u b l e c a t a l y s t s ( F i g . 1 ) and t h e i n v e s t i g a t i o n o f

new p r e p a r a t i v e r o u t e s t o c a t a l y s t p r e c u r s o r s

(Scheme J ) e x t e n s i v e m o d e l l i n g s t u d i e s a r e underway i n a n a t t e m p t t o e l u c i d a t e t h e a c t i v e s p e c i e s and i n t e r m e d i a t e s w i t h i n t h e c a t a l y t i c c y c l e . New approaches t o s u p p o r t e d c a t a l y s t s v i a unusual o r g a n i c s u p p o r t m a t e r i a l s a r e a l s o b e i n g It i s a n t i c i p a t e d t h a t t h e s e heterogeneous systems w i l l p r o v i d e i n

developed.

d e p t h i n f o r m a t i o n on t h e o p e r a t i o n o f t h e C S I R O c a t a l y s t s and t h e p o t e n t i a l f o r e x t e n s i v e c o n t r o l o v e r p r o d u c t d i s t r i b u t i ons. C u r r e n t l y , employing a c l o s e d flow psig,

in

chlorobenzene

solvent

and

system w i t h p r o p y l e n e d e l i v e r e d a t V2-1 at

0°C

the

catalyst

system

based on

N i (SacSac)PBu3C1 has an a c t i v i t y ~ 1 0 0 , 0 0 0 t u r n o v e r s p e r hour. The c a t a l y s t was operated f o r 8 hours w i t h o u t s i g n i f i c a n t deactivation. t h e a c t i v i t y i s considerably higher

-

approaching

With e t h y l e n e as feed

2 . 5 ~ 1 0 ~t u r n o v e r s p e r hour.

It i s l i k e l y t h a t t h e s e a c t i v i t y f i g u r e s a r e minimums and w i l l be c o n s i d e r a b l y

enhanced w i t h selection.

improved a g i t a t i o n ,

Should t h e s e

systems

i n c r e a s e d p r e s s u r e and c a r e f u l prove

stable

at

catalyst

h i g h e r temperatures

(say

40-100°C) t h e r e w i l l be a n a n t i c i p a t e d t e m p e r a t u r e e f f e c t on r a t e s also. CONCLUSION The m a j o r drawback

a s s o c i a t e d w i t h u s i n g t h e C S I R O t y p e c a t a l y s t systems

w i t h e t h y l e n e o r p r o p y l e n e s u b s t r a t e s d e r i v e d f r o m methane p a r t i a l o x i d a t i o n r o u t e s i s t h e s e n s i t i v i t y o f t h e s e Z e i g l e r t y p e systems t o 02,

H20 and CD.

Removal o f t h e s e contaminants f r o m t h e p r o d u c t stream o b t a i n e d f r o m p a r t i a l o x i d a t i o n would be necessary.

Further,

i t i s u n l i k e l y t h a t c a t a l y s t s based on

t r a n s i t i o n metal complexes w i l l be a b l e t o o p e r a t e a t t h e temperatures o f t h e p r o d u c t gases p a s s i n g from t h e p a r t i a l o x i d a t i o n stage. be required. feasible

A c o o l i n g stage would

A s c e n a r i o i n c o r p o r a t i n g a methane p y r o l y s i s r o u t e i s a more

p r o p o s i t i o n as such a p r o d u c t stream would be f r e e o f oxygenated

contaminants.

However a h e a t removal s t e p i s s t i l l a p r o b a b l e requirement.

The most l i k e l y r o l e o f homogeneous c a t a l y s t systems o f t h e t y p e d i s c u s s e d i n this

paper

would

be

i n the

production

of

higher

value

commodity

or

s p e c i a l i t y chemicals o r i n t h e p r o d u c t i o n o f o c t a n e b o o s t e r s f o r l e a d f r e e p e t r o l as p r a c t i s e d by I.F.P. REFERENCES

M. Sherwood, Chemistry and I n d u s t r y (Dec. 1982) 994-995. E.R. F r e i t a s and C.R. Gum, Chem. Eng. Progress (Jan. 1979) 73-76. E.F. LUtZ, J. Chm. Ed., 6 3 (1986) 202-203. W.J. Benedek and J-L. Mauleon, Hydrocarbon P r o c e s s i n g (May 1980) 143-149. J.W. Andrews and P.L. Bonnifay, Hydrocarbon P r o c e s s i n g (Apr. 1977) 161-164. K.J. C a v e l l and A.F. Masters, J. Chem. Research (S) (1983) 72-73. CSIRO, U.S. Pat. 4,533,651 (1985). K.J. C a v e l l and A.F. Masters, Aust. J. Chem., 39 (1986) 1129-34.

D.M. Hitiby. C.D. Chang, K.F. Howe and 5 . Yurchak (Editors),Methane ('oncersion C; 1988 Elsevier Science I'uhlishers R.V., Amsterdam - Printed in The Netherlands

LIGHT OLEFINS

FROM SYNTHESIS

GAS USING RUTHENIUM

ON-RARE EARTH OXIDE

529

CATALYSTS

L. RRUCE, S. HARDIN, M. HOANG and T. TURNEY D i v i s i o n o f M a t e r i a l s Science and Technology, C S I R O , Locked Rag 33, C l a y t o n Vic. 3168, A u s t r a l i a

APSTRACT The i n t e r a c t i o n of r u henium c a r b o n y l , Ru~(CO)I w i t h r a r e e a r t h o x i d e s o f h i g h s u r f a c e area, >50rn2g-', has been s t u d i e d . rRu3fu H ) ( C O ) ~ O ( ~ - O V S i) s] formed on holmia, b u t on l a n t h a n a o n l y [Ru(CO),], s p e c i e s a r e observed. R e d u c t i o n o f t h e carbonyl l i g a n d s t a k e s p l a c e a t 6573K t o g i v e c a t a l y s t s f o r t h e hydrog e n a t i o n o f carbon monoxide w i t h a c t i v i t y and s e l e c t i v i t y dependent on t h e p a r t i c u l a r r a r e e a r t h o x i d e and p r e t r e a t m e n t . Over c e r i a , t h e p r o d u c t i s up t o 55 w t X C2-5 o l e f i n s . A s i m i l a r s e l e c t i v i t y i s o b t a i n e d o v e r l a n t h a n a o n l y a f t e r redispersion through a reduction-oxidation-reduction cycle. INTRODUCTION N a t u r a l gas, by d i r e c t p a r t i a l o x i d a t i o n , can p r o v i d e o l e f i n s s u i t a b l e f o r o l i g o m e r i s a t i o n u s i n g t h e 'Wobil O l e f i n t o Gasoline and D i e s e l ' process. A l t e r n a t i v e l y , s y n t h e s i s gas r o u t e s t o o l e f i n s can be v i a methanol o r F i s c h e r Tropsch s y n t h e s i s .

I n t h e Fischer-Tropsch o p t i o n , t h e h y d r o g e n - r i c h n a t u r e of

t h e s y n t h e s i s gas r e q u i r e s t h a t t h e c a t a l y s t should have poor s h i f t a c t i v i t y and produce a narrow range o f l o w e r o l e f i n s . On many supports,

r u t h e n i u m c o n v e r t s s y n t h e s i s gas w i t h h i g h s e l e c t i v i t y i n t o

lower alkanes, n o t a b l y methane.

However, when ruthenium i s supported on

c e r t a i n b a s i c oxides, such as l a n t h a n a and c e r i a , h i g h l y s e l e c t i v e p r o d u c t i o n o f l o w e r o l e f i n s has been r e p o r t e d (1,Z).

One o f t h e aims o f t h i s r e s e a r c h was t o

improve t h e a c t i v i t y and s e l e c t i v i t y o f ruthenium supported on r a r e e a r t h o x i d e s (REO).

As c o m m e r c i a l l y a v a i l a b l e RE0 a r e o f low s u r f a c e area ( Ru(0) 0 He, l h r , 623K, c) Ru p a r t i c l e diam, d = 6/pA; p = Ru d e n s i t y (12.4 x 16$jm:i) and A, Ru s u r f a c e area, assumes H:Ru = 1:l and Ru s u r f a c e conc = 1.63 x 19 m (7).

r,

531 I R examination showed t h a t each o f t h e supported Ru c a t a l y s t s s t u d i e d here

s t i l l c a r r i e d t r a c e s o f s u r f a c e carbonate and h y d r o x y l species.

On t h e l a n t h a n a

c a t a l y s t t h e i n i t i a l carbonate species was r e a d i l y reduced by hydrogen t o form e x c l u s i v e l y methane a t about 575K.

I n c o n t r a s t , no such r e d u c t i o n t o o k p l a c e i n

t h e absence o f r u t h e n i u m as would be expected from t h e known s t a b i l i t y o f s u r f a c e carbonate on REO(9).

T h i s f a c i l e r e d u c t i o n o f s u r f a c e carbonate was

f u r t h e r demonstrated by TPR s t u d i e s i n which samples w i t h and w i t h o u t ruthenium, a f t e r r e d u c t i o n a t 623K, were exposed t o C02 a t 553K, cooled t o ambient i n N2 and t h e n examined by TPR. Only t h e sample c a r r y i n g ruthenium generated methane ( a t about 55310.

A s p i l l - o v e r mechanism of adsorbed atomic hydrogen from Ru t o

t h e support appears t o be o p e r a t i v e . The manner o f bonding o f t h e p r e c u r s o r carbonyl v a r i e s between REOs.

Thus,

on l a n t h a n a , bands a t 1955 and 2045 cm-l suggest t h e s p e c i e s i s p r o b a h l y a s u r f a c e d i c a r b o n y l , b u t on holmia, 2065, 2025 and 2000 cm-l bands i n d i c a t e s u r f a c e species r e l a t e d t o [Ru3( u-H)(CO)lO(

are present ( 1 0 , l l ) .

p-OM:)]

TPR p r o f i l e s demonstrate t h a t t h e s u r f a c e carbonyl l i g a n d s a r e reduced between 533K and 573K, w i t h methane f o r m a t i o n ; some p r e p a r a t i o n s a l s o show r e d u c t i o n w i t h o u t methane f o r m a t i o n a t l o w e r temperatures.

Preliminary studies

i n d i c a t e new c a r b o n y l species are produced on exposure t o CO o r s y n t h e s i s gas a t 553K under process c o n d i t i o n s . Thus t h e c a r b o n y l s p e c i e s formed on Ru/lanthana e x h i b i t s an I R band a t 1980 cm-l, a n i o n i c c l u s t e r C R U ~ cO)16-j2C(

a t t r i b u t a b l e t o t h e adsorbed

(10,12):

Reduced c a t a l y s t samples a r e s u s c e p t i b l e t o o x i d a t i o n e.g.

l h r i n 1% 02/He

a t 623K o x i d i s e s Ru(0) q u a n t i t a t i v e l y t o P u ( I V ) , as c o n f i r m e d by TPR. It i s a l s o found t h a t d i s p r o p o r t i o n a t i o n o f CO occurs on t h e reduced

c a t a l y s t s u r f a c e a t room temperature on l a n t h a n a , t o produce s e v e r a l species:a Ru suboxide species, s u r f a c e carbonate, and a t l e a s t two forms o f r e a c t i v e s u r f a c e carbon.

TPR s t u d i e s show t h e carbon s p e c i e s d i f f e r i n t h e i r r e a c t i v i t y

t o hydrogen, one r e d u c i n g t o hydrocarbon p r o d u c t s a t temperatures as l o w as 473K, whereas t h e second does n o t r e a c t u n t i l 673K.

U n l i k e C O Y CO2 m e r e l y

forms a s u r f a c e c a r b o n a t e w i t h o u t d i s p r o p o r t i o n a t i o n under t h e s e c o n d i t i o n s . ( i i ) Fischer-Tropsch A c t i v i t y and S e l e c t i v i t y .

I n i t i a l l y t h e o n l y hydrocarbon

produced i s methane, b u t a f t e r about 30 min t h e f i n a l p r o d u c t stream shown i n Table 2 i s established. disproportionation,

It appears t h a t a degree o f carbon b u i l d - u p ,

through a

i s necessary f o r t h e f o r m a t i o n o f h i g h e r hydrocarbons,

c h a i n growth o c c u r r i n g on a c t i v e carbon species so generated.

Typical

a c t i v i t i e s o f t h e c a t a l y s t s a f t e r l h r on stream a r e r e p o r t e d i n Table 3. The hydrocarbon a c t i v i t y decreases by about 30% over t h e f i r s t 5 hours and then remains almost c o n s t a n t over t h e next 15 h r , w i t h t h e s e l e c t i v i t y r e m a i n i n g unchanged.

Complete r e c o v e r y o f t h e o r i g i n a l a c t i v i t y i s e f f e c t e d by

h e a t i n g i n hydrogen f o r I h r a t r e a c t i o n temperature, d u r i n g which t i m e methane

532 i s generated,

i n d i c a t i n g t h a t t h e carbon species b l o c k i n g t h e r e a c t i o n i s s t i l l

r e a d i l y hydrogenated.

Carbon d i o x i d e p r o d u c t i o n i s s i g n i f i c a n t f o r a l l REO,

e s p e c i a l l y c e r i a f o r which t h e r a t e o f f o r m a t i o n i s about 60% o f t h a t f o r hydrocarbon p r o d u c t i o n .

Whether t h i s i s due t o w a t e r gas s h i f t o r t o t h e

Boudouard r e a c t i o n remains t o be determined. TABLE 2

SELECTIVITY OF RUTHENIIJC ON RARE FARTH nXIDE SUPPORTSa

Support

Ru wt%

Conv.

La203

1.8 4.5 1.8 5.0 5.0 1.0 1.8 4.5

24 43 6 15 44 52 23 33

Ce02 Yb2O3 Hop03

Hydrocarbon S e l e c t i v i ty/wt% C2 C3 C4 C5 C6+ C2-5ene

C1

%

42 38 17 23 20 100 65 56

14 17 16 11 11

17 19 22 23 25

11 11 14 15 16

6 6 9 10 11

10 9 22 18 17

15.5 6.5 55.3 51.1 49.0

32 12 91 88 78

7

11 14

8 8

5 5

4 6

8.3 19.1

27 50

-

11

-

-

-

a ) r e a c t i o n c o n d i t i o n s : 553K, 103kPa, H2/CO = 1.2, TABLE 3

Support La203 Ce02 Ho203 Yb203 a)

ACTIVITY

-

-

GHSV = 750mlg-1 h r - l

OF RUTHENIUM ON RARE EARTH O X I D E SUPPORTSa Rate/u moles CO s - l t y HC t y CO2 g- cat. g- c a t .

Ru/wt% 1.8 4.5 1.8 5.0 4.5 1.8 1.0

Xene C2-5

’

0.25 0.87 0.14 0.70 0.42 0.12 0.99

0.91 1.65 0.25 1.16 1.24 0.86 1.94

r e a c t i o n c o n d i t i o n s as Table 2; b)

Nb CO HC

ty

g - l Ru

S-

50.5 36.6 13.8 23.2 27.6 48 194

lo3

24

-

3 7 11 55

N = t u r n o v e r number per s u r f a c e Ru atom

The most a c t i v e c d ’ t a l y s t ( p e r g o f Ru) was 1%Ru/Yb203 and t h e l e a s t a c t i v e was 1.8% Ru/Ce02.

However, t h e s e l e c t i v i t i e s i n Tahle 2 i n d i c a t e t h a t , whereas

t h e y t t e r b i a support renders Ru a methanator, on Ce02 as much as 55 w t % o f t h e t o t a l hydrocarbon p r o d u c t i s o l e f i n s i n t h e range C2-5.

It i s notable t h a t t h e

hydrocarbons produced on c e r i a a r e 80% o l e f i n i c even a t h i g h conversion. a l s o shows t h e l o w e s t methane s e l e c t i v i t y o f t h e RE0 s t u d i e d .

Ceria

From Table 1 i t

can be seen t h a t c e r i a i s t h e s u p p o r t g i v i n g t h e h i g h e s t d i s p e r s i o n and s m a l l e s t p a r t i c l e s i z e f o r ruthenium. The low p r o d u c t i o n o f o l e f i n s on t h e l a n t h a n a supported c a t a l y s t c o n t r a s t s w i t h p u b l i s h e d r e s u l t s f o r Ru/La203 prepared from a RuC13 p r e c u r s o r ( 1 ) . Tables 1 and 4 show t h a t t h e Ru d i s p e r s i o n on t h e p r e s e n t l a n t h a n a c a t a l y s t s was s i g n i f i c a n t l y a l t e r e d by a r e d u c t i o n - o x i d a t i o n - r e d u c t i o n w i t h a c o n c u r r e n t f a l l i n a c t i v i t y f r o m 0.91 t o 0.25 p r o d u c t i o n o f hydrocarbons.

p

( r - 0 - r ) cycle,

mole CO g - l s - l f o r

The r e s u l t i n g s e l e c t i v i t y i s t h e n s i m i l a r t o t h a t

533 observed f o r t h e c e r i a supported and t h e c o n v e n t i o n a l l a n t h a n a c a t a l y s t s , w i t h 52% o f t h e t o t a l p r o d u c t b e i n g o l e f i n s i n t h e C2-5 range. TABLE 4

Support

E f f e c t of Reduction-Oxidation-Reduction

Ru wtl

Ybp63 Ho2O3

1.8 1.8 1.0 1.8

Cyclea

SelectivitylWt % Rate/u moles CO s - l t o (32 t f HCSl N CO 9 ggt o HCb C 1 C2-5 C6+ %ene cat. cat. RU s-1x1~3 ene C2-5 0.17 0.21 0.03 nmc

0.25 0.36 0.11 0.10

14 20 11 6

4 nmc 3 1

22 15 35 36

52.2 57.0 31.0 18.9

13.6 21.9 27.6 5.5

81 90 83 33

a ) H2, 623K, Zhr, 1%Op/He l h r , Hp, 623K 2hr; r e a c t . c o n d i t i o n s : a s Table 2 h ) as Table 3 c ) nm = not measured I n c o n t r a s t , c e r i a supported c a t a l y s t s show l i t t l e response t o t h e r-0-r procedure, b u t y t t e r b i a ceased t o he a methanator and showed t r e n d s s i m i l a r t o lanthana.

Holmia i s t h e o n l y RE0 which so f a r has n o t produced s i g n i f i c a n t

lower o l e f i n s e l e c t i v i t y , e i t h e r as prepared o r f o l l o w i n g t h e r-0-r procedure. Work i s i n p r o g r e s s t o determine t o what e x t e n t a Ru p a r t i c l e s i z e e f f e c t i s

c o n t r o l l i n g c a t a l y s t performance ( i e i s t h e r e a s t r u c t u r e s e n s i t i v i t y ) and t o what e x t e n t t h e n a t u r e o f t h e i n i t i a l Ru3(C0)12-oxide i n t e r a c t i o n and differences i n t h e surface crystallography o f t h e p a r t i c u l a r oxides a r e contributory factors.

M o r p h o l o g i c a l s t u d i e s by TEM and e l e c t r o n d i f f r a c t i o n as

w e l l as d e t a i l e d FTIR and ESCA s t u d i e s a r e c u r r e n t l y a d d r e s s i n g t h e s e problems. REFERENCES J.G. Goodwin Jr., Y.W. Chen and S.C. Chuang, Proc.Symp.Catal.Convers.Synth. Gas A l c o h o l s Chem., 1983, ed R.G. Herman, Plenum NY, 1984, pp 179-189. 4,508,846. 1985; Chem.Ind. 22 (1985)115-133. 2. R. P i e r a n t o z z i , U.S.Patent 3. L. Bruce, S. Hardin, M. Manh and T. Turney, i n p r e p a r a t i o n . Chem.Comm., 1984, 4. M. Audier, J. K l i n o w s k i and R.E. B e n f i e l d , J.Chem.Soc., 626-628. 5. K.J. Smith and R.C. Everson, J. C a t a l y s i s , 2 (1986) 349-357. 6. L. Bruce, H. McArthur and T. Turney, CHEMECA ‘84, Aust. Chem. Eng. Conf., 12th, (1984) 649-654. 7. K. Foger, i n C a t a l . Sci. Technol. 6, J.R. Anderson and M. Roudart ( E d i t o r s ) , S p r i n g e r , B e r l i n , 1984, pp.227-305. Searcy, J. Phys. Chem., (1985) 1695-1699. 8. D. Beruto, P.F. Rossi and A.W. 9. R.P. T u r c o t t e , J.O. Sawyer and L. E y r i n g , Inorg. Chem., 5 (1969) 238-246. 10. R. P i e r a n t o z z i , J. Mol. Cat., 2 (1983) 189-202. 11. J. Evans and G.S. McNulty, J. Chem. SOC., D a l t o n Trans., 1984, 1123-1131. 12. H.H. Lamb, T.R. Krause and B.C. Gates, J. Chem. SOC., Chem. Comm., 1986, 821-823.

1.

89


ZEOLITES AND OTHER CATALYSTS


D.M. Bibby. C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

537

MICROPORES IN CRYSTALS R.M. BARRER Chemistry kprtment, merial College, London sw7, England ABSTRACT

Crystals which can exhibit microporosity on the scale of molecules include layer silicates such as smectites and vermiculites; zeolites; porosils; aluminium phosphates (AlpO's); s m e Werner compounds and cyanmetallates; and clathrates. A short historical account of zeolites and some features of porosils and AlFO's have been given. As an example of a zeolitic Werner caipound B-(NiI1, &I1) - (4methylpyridine)4(SC~)2is cited, and as zeolitic cyanometallates three carplex cyanides are referred to. Dianin's ccmpound, a c h r m n , exemplifies a zeolitic, organic, clathrating host structure. The intracrystalline micropores in the 3-dimensional 4-connected nets of some zeolites, porosils and AlFQ's have been ccmpared in terms of the windows controlling access to the micropores; their volumes and internal dimensions; and the total intracrystalline pore volumes per cm3 or p r g of crystal. Examples are given of cluster sizes of guest molecules in cages of several kinds

.

IMlTODUCTION

The science of microporous materials has developed dramatically since intracrystalline porosity was first demonstrated in certain crystals. Categories of microporous crystals ncw include: Expanded clay minerals (smectites and vermiculites) Zeolites (Al-rich to Si-rich) Porosils (clathrasils and zeosils) Aluminium phosphates (AlW's, W's and MApo's) Some Werner ccanpounds and cyananetallates Clathrates Until well after 1945 sorbents and many catalysts were based on amorphous carhns, silica gels and other oxide and mixed oxide gels. Crystals were not considered. However, in the background there existed a remarkable class of microporous minerals, the zeolites. In the early 1940's quantitative single-step separations with zeolites as sorbents were abundantly demonstrated [ 1 , 2 , 3 , ] , separations based not on boiling or freezing point differences between

538

molecular s p e ci es , but on d i f f e r e n c e s i n shapes and s i z e s of m l e c u l e s . Molecules of t h e r i g h t shape and s i z e r e a d i l y permeated the outgassed c r y s t a l s and could be copiously sorbed; those of t h e wrong shape or s i z e were turned back a t the e x t e r n a l s u r f a c e s of t h e c r y s t a l s .

The molecule sie ving behaviour

o f a given c r y s t a l s i e v e could be f u r t h e r changed by ion-exchange.

The

p o t e n t i a l of such c r y s t a l l i n e sponges ev entua lly began to be r e a l i s e d elsewhere, and t h e p r es en t r i ch n es s of research and development involving microporous c r y s t a l s has r es u l t ed .

Even when completely f i l l e d with z e o l i t i c

water the c r y s t a l s remain d r y to touch. EXPANDED CLAY MINERALS Following demonstrations of molecule sie ving by z e o l i t e s it was next r e p o r t e d i n 1955[ 4,5] and subsequently t h a t sm e c tite s and ve rm ic ulite s could also be made permanently microporous and could then behave as molecular sie ve s. The way i n which permanent p o r o s i t y was achieved is shown schematically i n Fig.1 [ 51. The normally p r es en t i n t e r l a y e r c a tions, such as Na+ or &+, were exchanged f o r l a r g e r g l o b u l ar c a t i o n s such as (CH3)4fir CH3h3, H3fiCH$H2h3, [Co(en)3]3+ and t h e l i k e .

The W r t a n t d i s t a n c e s d l , and d3 can each be

a d j u st e d by varying t h e shape, s i z e and charge of the c a tion.

Also d3 can be

a d j u s t e d through changes i n the negative charge on the s i l i c e o u s l a y e r of the c l a y mineral. d l and d3 are i n the range of molecular dimensions. I n t e r e s t i n t h i s category of molecular s i e v e s is now increasing.

The

o r i g i n a l organic "props" have r ecen t l y been replaced by oxycations of A l l Zr, and T i , f o r example

[ 61

[Al1304(OH)24(H20)1217+

0;2E

~-~

(.)bm.ld* &.zz& I I I 1 1 1 I I I 1 1 I1 I I1 I I I I I I 1111111ll 1/1/11111111

Fi g 1. Schematic r ep r es en t at i o n of a l a y e r s t r u c t u r e rendered permanently

porous by exchange of Na+, Ca2+, etc. by l a r g e ions [ 51.

539

Cations based on substituted silsesquioxanes have also been intercalated 171. On heating the ion-exchanged clay minerals water is evolved, but oxycation or oxide pillars keep the siliceous layers apart. These materials have enhanced thermal stability carnpared with clay minerals expanded with organic cations. Expanded clay minerals cover at least as wide a range of accessibilities to the interlamellar micropore spaces as the zeolites, but the pore characteristics of clay minerals with inorganic pillars need mre detailed investigation. ZEOLITES

Three properties have mde the zeolites of major importance: their ion exchange and ion sieve behaviour; their selectivity as sorbents and mlecular sieves; and their catalytic and shape selective catalytic functions. Some dates of significance in their long history are given, with references, in [ 81. These dates are given below. 1756 First zeolite recorded (stilbite). 1852 Nature of ion exchange in soils clarified. 1858 Action of salt solutions on silicates investigated. 1862 First synthesis of a named zeolite (levynite) claimed. 1870-88 Various qualitative ion exchange studies on zeolites. 1875 A sedimentary zeolite deposit reported. 1898 Quantitative studies of water-zeolite equilibria. 1899 Ion exchange via vapour phase using NH4C1. 1902-5 Ion exchange via salt melts. 1910 Sorption of heavy vapours examined (12, Br2, Sr H g r HgS12r HgS)* 1932 Term "molecular sieve" introduced. 1941-4 Quantitative separations by mlecule sieving demonstrated, with 3 types of sieve. 1946 First synthesis of a zeolite with no natural counterpart[ 91. 1949 Hydrogen zeolites first made by heating NHq-zeolite. 1956 onward. Zeolites A, X, Y and L revealed. 1961 First direct syntheses of zeolites using only organic bases. 1962 Zeolite-based cracking catalysts introduced. 1964 Crystalline de-aluminated zeolitic Si02 made by acid extraction of clinoptilolite. 1967 Preparation of ultrastable faujasite (zeolite Y) reported. 1971 onward. Very siliceous synthetic zeolites described.

540

More recently still we have seen the development of bi-functional catalysts, involving metal atom clusters in and on zeolite catalysts; new procedures for de-alumination of zeolites, for example treatment with hot Sic14 vapour in a nitrogen carrier gas; continued syntheses of m r e and m r e silica-rich zeolites and other zeolites having no known natural counterparts; zeolite modification by chemisorption processes; isomrphous replacements of tetrahedrally co-ordinated framework elements, especially Al by Ca and Si by Ge; increasing numbers of structure determinations and refinemnts by X-ray and neutron diffraction; studies of Al and Si ordering; investigations of lattice defects by high resolution electron microscopy; numerous investigations of zeolite acidity in relation to catalysis; and many quantitative studies of molecule diffusion, sorption and ion exchange.

FOROSILS The porosils have appeared only recently upon the scene [ 101. They include the silica end-products of synthesis of zeolites richer and richer in silica, such as silicalites I and 11, and also other species which do not always have a zeolite counterpart. They can be sub-divided into clathrasils in which openings between intracrystalline cavities are too small for molecule migration; and zeosils in which these openings are adequate for molecule diffusion. Examples of each sub-division are: Clathrasils Silica sodalite[ 111 (4-&6-rings) Melanophlogite (5-&6-rings) Dodecasil 3 C (5-&6-rings) (4-,5-&6-rings) Jkdecasil 1 H Nonasil (4-,5-&6-rings)

Zeosils Silicalite I (mainly 5-&10-rings) Silicalite 11 (mainly 5-&10-rings) Deca-dodecasil 3R[ 121 (4-,5-,6-&8-rings)

The number and structural diversity of porosils should increase substantially as research into their synthesis continues. Interest could centre upon the zeosils as very stable, hydrophobic molecular sieves and catalyst carriers. Their synthesis requires at least partial occupation of channels or cavities by stabilising guest mlecules. ALWIINIUM PSIOSPHATES

A variety of porous 3-dimensional (3-D) 4-connected Alm4 nets have been synthesised, some of which have the same topologies as their zeolite counterparts while others have novel topologies [ 13, 141. As with porosils and silica-rich zeolites, basic organic species are involved in their synthesis, to occupy channels and cavities as space fillers and stabilisers. AlpO's have

541

m net framework charge. They f can be considered to be composed of alternating AlOz and PO2 units, so that they are mre polar than porosils. Ihe strict alternation of Al and P in AlP04 frameworks means that so far no AlW's with odd numbered rings have been made. Their synthesis requires media which have lower pH's than those from which zeolites normally form. After crystallisation the organic guest species can usually be remved by heating in air. Al/P = 1 and, like the porosils, ideally carry

WJERNER COMPOUNDS AND CYANOMETALJATES

Among other zeolitic phases l3-(Ni1I1 Co1I)-(4rnethylpyridine)4 (SCN)2 sorbs both non-polar and polar species (permanent and inert gases: alkyl halides; n-, iso- and cycloparaffins and aromatic hydrocarbons: alcohols and ethers). The host lattice can sometimes change at critical loadings of sorbate from one porous structure to another. This causes the steps seen in Fig. 2 for sorption isotherms of Xe at various temperatures [ 151.

I

I

1

20

10

I

30

I 40

I

50

p (crn.Hg)

Fig. 2. Isotherms of Xe in Co(4methylpyridine)4(SCN)2, showing step. Curves 1 to 6 are at temperatures -78, -62.5, -57.3, -51, -47.5 and -41.5OC, respectively [ 151. K2Zn3[ Fe(CN)6] 2.xH20 [ 161 , Zn3[&(CN)6] 2 of these compounds contains ellipsoidal "he first [ 171 and Zn[ Fe(CN)yO] [ 171 cavities which give a total free volume of -0.215 crn39-l. In each cavity there is room for -8.5H20 mlecules, and the windows linking the cavities have free dimensions of about 3.9 x 5.2. The compound was reported to be thermally stable to -493K and when outgassed readily sorbed Cot N2, C2H4 and C2 to C4 paraffins. The second of the above cyancmetallates has window dimensions Zwlitic cyanmetallates

.

542

estimated as 5.6 x 8.6 8. It readily sorbed n-hexane and 3methylpentane but excluded 2,2-dimethylpentane. The windows in Zn[ Fe(CN)$O] are smaller but admit molecules such as C02. It is noteworthy that cyanometallate sieves are based not on the tetrahedron but on the octahedron, T06. CLATHRATES Clathrates differ from zeolites in that the host structures are not usually stable in the absence of the guest. An exception is Dianin's compound (a chrcxnan) where the porous hydrogen-bonded framework persists with no guest molecules to stabilise it. This is also true of a , B- and y- cyclodextrins, where the individual host mlecules are like truncated sections of a hollow, gently tapering cone. If crystals of guest-free Dianin's compound (and various other clathrate formers) are shaken with small steel ball-bearings we discovered that they behaved like a zeolite and sorbed large amounts of permanent and rare gases, or hydrocarbons and other guest species of the right shape and size to occupy the hour-glass shaped cavities of the host lattice [ 181. Goldup and Smith [ 191 elegantly demonstrated the lock-and-key fit of host and guest as follms: Molecules which fit and clathrate

Molecules which do not fit and do not clathrate

n-Cg; n-C7 2-~e-c6;2 - ~ e - ~ 5 2,5-di-Me-Cg

n48 3* H-ZSM5 1. H-Y > La-Y. e x h i b i t e d d e a l u m i n a t i o n b e h a v i o r d u r i n g t h e p r o t o n a t i o n o f t h e ammonium forms o f synthetic zeolites. The r e s o n a n c e l i n e o f '7Al-MASNMR attributed t o o c t a h e d r a l aluminum ions(dea1uminated s p e c i e s from z e o l i t e framework) appeared f o r H-Y a f t e r c a l c i n a t i o n i n a i r a t 773 K, whereas t h e o c t a h e d r a l peaks i n La-Y, H-M and H-ZSM5 were a p p r e c i a b l y i n h i b i t e d . Changes o f microenvironments o f S i - and A l - t e t r a h e d r a d u r i n g t h e p r o t o n a t i o n o f H - z e o l i t e s h a v e been i n v e s t i g a t e d : t h e s e would affect t h e a c i d i c p r o p e r t i e s .

INTRODUCTION Currently,

much a t t e n t i o n

i s focused on t h e a c i d i c p r o p e r t i e s and shape

1 ) . Since t h e

s e l e c t i v i t i e s o f c a t a l y t i c reactions over acid-type z e o l i t e s ( r e f .

a c i d i t y as w e l l as t h e s t r u c t u r e s t r o n g l y a f f e c t s t h e c a t a l y t i c behavior,

it i s

o f extreme importance t o c h a r a c t e r i z e t h e z e o l i t e c a t a l y s t as a f u n c t i o n o f Si/Al

r a t i o from t h e viewpoint o f t h e a c i d i t y ( r e f .

during t h e preparation o f acid-type zeolites, acidic

some d e a l u m i n a t i o n

properties o f zeolites.

zeolites

has o c c u r r e d ( r e f . T h i s work w i l l

3):

1246.0).

H-M(Si02/A1203=9.

and a c i d p r o p e r t i e s .

On t h e o t h e r hand,

7-20.4)

o f Na-form

t h i s would

r e f l e c t the

H-Y(Si02/A1203=4.8-5.6),

and H-ZSMS(Si02/Al203

=24.6-

We s t u d i e d t h e micro-environments o f S i - and A l -

t e t r a h e d r a i n t h e z e o l i t e framework by h i g h r e s o l u t i o n s o l i d s t a t e "Si27Al-MASNMR(magic

of

e l u c i d a t e t h e dealumination

behavior d u r i n g t h e preparation o f acid-type zeolites:

La-Y(Si02/Al203=4.8-5.6),

2).

by ion-exchange

a n g l e s p i n n i n g NMR),

and

and we compared t h e a c i d p r o p e r t i e s o f

z e o l i t e s by TPD( temperature-programmed d e s o r p t i o n ) and high-temperature m i c r o c a l o r i m e t r y o f NH3

*

and by I R spectroscopy.

To whom a l l correspondence should be addressed.

580

EXPERIMENTAL Z e o l i t e samples Z e o l i t e s a m p l e s w e r e s u p p l i e d by t h e C a t a l y s i s S o c i e t y o f J a p a n ( J a p a n Reference C a t a l y s t s : zeolites

were

(Y-type,

from JRC-Z-Y),

H-ZSM5(24.6,

JRC-Z-)

studied:

80.0,

form:

H-Y(4.8, H-M(9.7,

5.6)(Y-type. 14.7,

1246)(ZSM5-type,

each code o f z e o l i t e ,

exchanged);

and H-M,

from JRC-Z-Y).

from JRC-Z5).

and

Numbers i n parentheses a f t e r

i s t h e Si02/A1203 mole r a t i o i n Nao f Na-form

L a ( N 0 3 ) 3 f o r La-Y

ion-exchanged

5.6)

La-Y(4.8, from JRC-Z-M)

by e m i s s i o n s p e c t r o c h e m i c a l a n a l y s i s .

z e o l i t e s were prepared by ion-exchange NH4N03 f o r H-Y

Four d i f f e r e n t a c i d - t y p e

20.4)(mordenite-type,

such as H-Y(4.8),

t h i s was determined

1).

i n Na-form(ref.

w i t h aqueous

Acid-type solution o f

a n d H C 1 f o r H-ZSM5(above 9 9 %

s a m p l e s w e r e d r i e d a t 373 K f o r 24 h and t h e n

c a l c i n e d a t 773 K f o r 5 h. MASNMR spectroscopy The MASNMR has been e m p l o y e d f o r z e o l i t e s a m p l e s a f t e r t h e y h a d been hydrated

i n a desicator

t e m p e r a t u r e f o r 24 h,

saturated

aq.

solution

o f NH4C1 a t room

l i n e w i d t h o f 27A1-MASNMR

The MASNMR s p e c t r a were o b t a i n e d a t 53.7 MHz f o r 2 9 S i and 70.4 MHz

spectra. f o r 27Al

with

i n order t o minimize t h e

on a F o u r i e r t r a n s f o r m p u l s e d NMR spectrometer(JE0L JNM-GX270)

was equipped w i t h a CP/MAS

u n i t (JEOL NM-GSH27MU).

w i t h magic a n g l e spinning(MAS) d u r i n g data acquisition.

which

A l l NMR s p e c t r a combined

s p e c t r a were measured w i t h p r o t o n d e c o u p l i n g

Cross p o l a r i z a t i o n ( C P )

p r o t o n s do n o t a t t a c h d i r e c t l y t o "Si

was n o t employed,

and 2 7 A l n u c l e i .

c o l l e c t e d w i t h 700 t o 1300 scans accumulated p e r spectrum. a r e c a l i b r a t e d by TMS and Al(H20)63+

f o r 29Si-

since the

8 K d a t a p o i n t s were The chemical s h i f t s

and 27A1-MASNMR measurements.

respectively. High-temperature m i c r o c a l o r i m e t r y M i c r o c a l o r i m e t r i c measurements o f NH3 on a c i d - t y p e w i t h a high-temperature

calorimeter(HAC-450G.

z e o l i t e s were performed

Tokyo R i k o ) a t 473 K.

c a l o r i m e t r i c experiments, t h e samples were evacuated o v e r n i g h t a t 673 a pressure o f

Before

K

down t o

Pa f o r t h e e l i m i n a t i o n o f w a t e r molecules.

A d s o r p t i o n measurements and TPD Both a d s o r p t i o n measurements and TPD o f NH3 were performed under vacuum conditions.

About 150 mg o f a sample was charged i n a q u a r t z b a s k e t a t t a c h e d

t o a s t a n d a r d vacuum s y s t e m ( l ~ l O - ~ Pa). hung down t o t h e sample basket.

A McBain-type q u a r t z s p i r a l s p r i n g was

The sample was evacuated f o r 5 h a t 773 K.

The w e i g h t change b e f o r e o r a f t e r NH3 a d s o r p t i o n a t 373 K was d e t e r m i n e d by t h e

581 change

i n t h e l e n g t h o f the spring.

meter(type-2U,

Shinko E l e c t r o n i c ) .

which was equipped w i t h a d i s p l a c e m e n t A f t e r a d s o r p t i o n o f NH3,

t h e sample was

e v a c u a t e d a t 373 K u n t i l t h e p r e s s u r e r e a c h e d I x I O - ~ Pa(3-4 determination o f a c i d i t y .

F o r TPD experiments,

for the

t h e p r e s s u r e change from t h e

e l i m i n a t i o n o f NH3 a t e l e v a t e d temperature(373-773 by an i o n i z a t i o n gauge(G1-K,

h).

K, 10 K m i n - I ) was m o n i t o r e d

ULVAC) which r e c o r d e d a u t o m a t i c a l l y .

I R spectroscopy

I R c e l l w i t h K B r windows was designed t o f i t an i n f r a r e d

A vacuum-tight spectrometer(270-30,

H i t a c h i ) and t o be a t t a c h e d t o a vacuum ~ y s t e m ( l x l O - Pa). ~

I R s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e w i t h t h e s p e c t r o m e t e r o p e r a t i n g i n t h e absorbance mode.

S e l f s u p p o r t i n g z e o l i t e wafers were pressed:

t h i c k n e s s was 10.2 mg could

be lowered

examination,

t h e sample

The c e l l was arranged such t h a t t h e z e o l i t e w a f e r

into slots

between t h e o p t i c a l

windows

for

spectroscopic

and withdrawn upward by t h e a c t i o n o f a magnet i n t o a heated

p o r t i o n f o r t h e p r e t r e a t m e n t and a d s o r p t i o n o f NH3, P y r i d i n e and C o l l i d i n e . RESULTS AND DISCUSSION

MASNMR spectroscopy I n framework a l u m i n o s i l i c a t e s , formula Si(nAl),

where n = 0.

t h e r e a r e f i v e p o s s i b i l i t i e s , d e s c r i b e d by t h e

1, 2, 3 o r 4.

These f i v e b a s i c u n i t s o f Si(nA1)

express t h e f a c t t h a t each silicon atom i s l i n k e d ,

A

v i a oxygens,

B

-90

-100

Ppm from TMS

-110

150

t o n aluminum

0

100

50

0

-50

ppm from A ~ ( H ~ o ) ~ ~ +

Fig. 1. "Si-MASNMR(A) and 27A1-MASNMR(B) s p e c t r a o f Na-Y(4.8). Open c i r c l e s denote A1 atoms, c l o s e d c i r c l e s S i atoms.

582 S

NH4-Y (4.8)

Si

H-Y (4.8) ( a i r )

H-Y(4.8) ( v a c )

La-Y (4.8) I

I

I

I

I

I

I

-60 -80 -100 -120

ppm from TMS

I

I

,

,

I

and 27A1-MASNMR s p e c t r a o f Y(4.8). Fig. 2. "Sia t 773 K; (vac), evacuated a t 773 K.

(air),

( S i /A1 ) nmr*

Si(4A1) Si(3A1) Si(EA1) S i ( l A 1 ) Si(OA1) ~~~

* *i6

0 0 0 0 0

11 11 4 8 10

44 43 14 38 37

c a l c u l a t e d by eqn. 1. determined by chemical a n a l y s i s .

36 37 38 42 39

~~

9 9 44 12 14

calcined i n a i r

with Si/A1 ratios.

peak i n t e n s i t i e s i n Y(4.8)

Normalized peak i n t e n s i t i e s

Na-Y (4.8) NH -Y(4.8) H-!( 4.8) ( a i r ) H-Y (4.8) (vac) La-Y (4.8)

I

2 7 -MASNMR ~ ~

29Si-MASNMR

TABLE 1 H i g h - r e s o l u t i o n "Si-MASNMR

I

0 -100 ppm from A1(H20)6 3+ 100

~~~

( S i /A1 )ca3b* ~~

2.5 2.6 5.1 2.8 2.8

2.4 2.4 2.4 2.4 2.4

583

H-Y (5.6)

La-Y (5.6)

H-M( 20.4)

H-ZSM5(80.0)

I

-60

I

I

I

I

,

I

l

I

-80 -100 -120

"Si-MASNMR "Si-

I

1

0

I

-100

2 7 ~-MASNMR 1

and E7A1-MASNMR s p e c t r a o f a c i d - t y p e z e o l i t e s .

The "Si-MASNMR

neighbors(ref.4).

d i f f e r e n t resonance peaks a t -89 kinds o f Si-tetrahedra Si(3A1),

I

ppm from A I ( H ~ O ) ~ ~ +

ppm from TMS

F i g . 3.

I

100

Si(ZAl),

s p e c t r a o f Na-Y(4.8) t o -105

i n Fig.

ppm from TMS,

1 showed f o u r

which r e p r e s e n t f o u r

a t t a c h e d t o d i f f e r e n t numbers o f a j a c e n t A l - t e t r a h e d r a ;

Si(lA1)

and Si(OA1).

The P7A1-MASNMR spectrum i n Fig.

1

g i v e s one sharp resonance peak a t 59 ppm f r o m A l ( H ~ 0 ) 6 ~ + ,which i s assigned t o t e t r a h e d r a l framework s i t e s occupied b y t h e aluminum(ref. shows d i r e c t l y s i x - c o o r d i n a t e d

aluminum(A1-octahedra)

o f ' t h e f o u r - c o o r d i n a t e d aluminum i n t h e by t h e dashed

l i n e i n t h e spectrum.

framework

4).

27A1-MASNMR a l s o

b u i l d up a t t h e expense

a t 0 ppm, as i s r e p r e s e n t e d

In case o f Na-Y(4.8).

no d e t e c t a b l e

584

o c t a h e d r a l aluminum has been observed.

Therefore,

a l l t h e aluminum atoms a r e

t e t r a h e d r a l l y c o o r d i n a t e d t o oxygens i n t h e framework. F i g u r e 2 g i v e s t h e "Si-

and 27A1-MASNMR

s p e c t r a o f NH4-Y(4.8),

c a l c i n e d i n a i r a t 773 K o r i n vacuum a t 773 K and La-Y(4.8). exchange f r o m Na-Y observed.

t o NH4-Y(see

When t h e NH4-Y

Fig.

1 and 2),

no s e r i o u s d e a l u m i n a t i o n was

was c a l c i n e d i n a i r a t 773 K ,

occurred.

B u t t h e o c t a h e d r a l aluminum peaks i n H-Y(4.8)

La-Y (4.8)

were a p p r e c i a b l y i n h i b i t e d ( s e e Fig. 2).

The d i s t r i b u t i o n s o f Si(nA1) i n "Si-MASNMR

H-Y(4.8) A f t e r ion-

some d e a l u m i n a t i o n

c a l c i n e d i n vacuum and

s p e c t r a o f Y(4.8)

were o b t a i n e d

by computer s i m u l a t i o n o f each spectrum, based on Gaussian peak shapes. and ( S i / A l )

1 summarizes t h e peak i n t e n s i t i e s o f Y(4.8) chemical

analysis,

t h a t denote ( S i / A l ) n m r

and ( S i / A l ) c a

respectively.

(Si/Al)nmr

r a t i o s a r e c a l c u l a t e d from t h e f o l l o w i n g e q u a t i o n ( r e f .

(si'A1)nmr

=

4 n=O

Y,

The

6):

4 . :Asi(nAl) n=O

i s t h e peak area o f each d e c o n v o l u t e d c u r v e o f "Si-MASNMR

Here As~(,,A~) spectra.

/

ASi(nAl)

Table

r a t i o s f r o m NMR and

When t h e d e a l u m i n a t i o n o c c u r r e d a f t e r h e a t t r e a t m e n t o f NH4-Y

o r La-

t h e aluminum atoms o f Si(3A1) and Si(2A1) a r e p r e f e r e n t i a l l y removed f r o m

t h e framework,

which may decrease t h e a c i d i t y .

(Si/Al)nmr from ( S i / A l ) c a calcined i n air,

And t h e i n c r e a s e d v a l u e s o f

i n d i c a t e t h e dealumination. E s p e c i a l l y f o r H-Y(4.8) s e r i o u s dealumination from t h e framework has been

observed. F i g u r e 3 and Table 2 show t h e NMR r e s u l t s o f H-Y(5.6). and H-ZSM5(80.0).

F o r H-M and H-ZSM5,

La-Y(5.6).

H-M(20.4)

t h e m a j o r A l - s i t e s would be S i ( l A 1 ) .

no s e r i o u s d e a l u m i n a t i o n o c c u r r e d d u r i n g t h e p r e p a r a t i o n . TABLE 2 H i g h - r e s o l u t i o n "Si-MASNMR with S i / A l ratios.

peak i n t e n s i t i e s i n a c i d - t y p e z e o l i t e s

Normalized peak i n t e n s i t i e s Si(4A1) Si(3A1) Si(2A1) S i ( l A 1 ) Si(OA1) ~

H-Y(5.6) La-Y (5.6) H-M( 9.7) H-M( 14.7) H-M( 20.4) H-ZSM5( 24.6) H-ZSM5( 80.0) H-ZSM5( 1246) 3tx

(Si/Al)nmr*

(Si/Al)ca+*

~

7 12 0 0 0 0 0 0

33 35 10 4 4 0 0 0

c a l c u l a t e d by eqn. 1. determined by chemical a n a l y s i s .

46 40 60 45 33 26 10 0

14 13 30 51 63 74 90 100

3.0 2.7 5.0 7.5 9.8 15.0 40.0 -

2.8 2.8 4.8 7.4 10.2 12.3 40.0 623

and

585

h

c,

.r

-0 .r

u

m

/j 0. 0

0.0

0.1

,

,

0 . O

0.2

0.3

0.4

1

0 : H-Y 0 : La-Y 0 : H-M

A: H-ZSM5

0.5

Al/Si ratio F i g . 4.

The a c i d i t y p e r S i atom as a f u n c t i o n o f A l / S i r a t i o .

2 f u n c t i o n o f Si/A1 r a t i o

Acidity

A c i d i t y p e r Si-atom o f a c i d - t y p e z e o l i t e s were measured by NH3 c h e m i s o r p t i o n a t 373 K as a f u n c t i o n o f A l / S i r a t i o .

We assumed t h a t one A l -

h e d r a l p a i r c o r r e s p o n d i n g t o one S i ( l A 1 ) g i v e s one a c i d s i t e . t h e observed a c i d i t y would be on t h e s o l i d l i n e i n Fig.

and S i - t e t r a I n t h i s case,

4.

The a c i d i t y o f

H-ZSM5 and H-M a r e c o n s i s t e n t w i t h t h e s o l i d l i n e , and t h e v a l u e s a r e i n c r e a s e d w i t h i n c r e a s i n g numbers o f Al-atoms.

B u t on H-Y and La-Y l o w e r v a l u e s t h a n t h e

e s t i m a t e d v a l u e a r e found.

This is because of the larger number of sites which do not result in a proportional increase in acid sites.

Si(d1)

High-temperature m i c r o c a l o r i m e t r y r e p o r t e d m i c r o c a l o r i m e t r y measurements o f NH3 a t 423 K t o

Vedrine e t al.,

c h a r a c t e r i z e t h e a c i d c e n t e r s i n H-ZSM5(ref.

7-8).

They found t h a t t h e a c i d

d i s t r i b u t i o n o f s t r o n g e r a c i d s i t e i s more' homogeneous i n H-ZSM5. the

temperature

at

473 K f o r

c a l o r i m e t r y measurements

information o f the stronger acid s i t e s .

We p r e f e r r e d

t o collect the

The r e s u l t s a r e summarized i n Fig. 5.

A l l m i c r o c a l o r i m e t r i c curves a r e decreased w i t h i n c r e a s i n g coverage o f NH3 on zeolites.

H-M(20.4)

showed a h i g h e r i n i t i a l

heat o f adsorption(higher

s t r e n g t h ) and a l a r g e r amount o f s t r o n g e r a c i d s i t e s t h a n o t h e r s . H-M(20.4)

and H-ZSM5(80.0)

acid

I n addition,

gave a d r a s t i c a l decrease o f t h e h e a t o f a d s o r p t i o n

a t p a r t i c u l a r narrow domain o f coverage o f NH3.

The r e s u l t s suggest t h a t H-M

and H-ZSM5

h a v e more homogeneous A1 d i s t r i b u t i o n a l o n g t h e c h a n n e l s o f

zeolites.

The c a l o r i m e t r i c c u r v e s o f H-Y(5.6)

indicate the wider acid

586

-

150

c 4)

7

0

E

. 3 Y

0 1

I

I

1 .o

0.0

2.0

NH3 adsorbed / mmol g - l Fig. 5.

High-temperature m i c r o c a l o r i m e t r y o f NH3 on a c i d type z e o l i t e s .

d i s t r i b u t i o n than H-M

and H-ZSM5.

From t h e m i c r o c a l o r i m e t r y and t h e TPD o f

NH3. we conclude t h e f o l l o w i n g sequence f o r t h e a c i d s t r e n g t h :

H-M > H-ZSM5

> H-Y > La-Y.

I R spectroscopy The OH s t r e t c h i n g v i b r a t i o n s o f acid-type z e o l i t e s a f t e r evacuation a t 773 K a r e shown i n F i g .

6.

Two o r t h r e e t y p e s o f OH g r o u p s a r e o b s e r v e d a t

wavenumbers from 3800 t o 3500 cm-l.

9-10)

zeolite(ref,

Terminal Si-OH groups a t o u t e r s u r f a c e o f

g i v e s a band a t 3738 o r 3735 cm-’.

Those t e r m i n a l Si-OH

groups showed very weak chemical i n t e r a c t i o n between p y r i d i n e and NH3.

The

a c i d i c OH bands whose i n t e n s i t i e s reduced o r disappeared a f t e r a d s o r p t i o n o f base molecules g i v e bands a t 3673 and 3648 cm-I La-Y(5.6).

3610 cm-l

f o r H-M(20.4)

expense o f those a c i d i c OH bands, region.

hydroxyls(ref. 3610 cm-l

NH bands

However, t h e band a t 3557 cm-I

base molecules,

f o r H-Y(5.6).

and 3613 cm-l

3676 cm-’

f o r H-ZSM5(80.0).

for

A t the

b u i l t up a t 3500 t o 3000 cm-’

i n La-Y(5.6)

does n o t i n t e r a c t w i t h

and t h e band can be assigned t o OH s t r e t c h i n g o f Lanthanum 11).

For H-M(20.4),

about 25-30 % o f t h e a c i d i c OH bands a t

s t i l l remained even a f t e r a d s o r p t i o n a t room temperature.

was i n t r o d u c e d t o H-M.

t h e a c i d i c band almost vanished.

When NH3

We conclude t h a t about

25-30 % o f OH groups i n H-M a r e l o c a t e d i n s m a l l e r channels, such as so-called s i d e pockets, and t h e r e s t a r e i n main channels o r on o u t e r surfaces. The r i n g v i b r a t i o n r e g i o n s o f I R s p e c t r a a f t e r ’ a d s o r p t i o n a t 473 K on acidt y p e z e o l i t e s are shown i n Fig. 7.

The major a c i d s i t e s a r e Bronsted sites(BPy

587

."..\

mmm

mm m, mm

mr.u

r.IDI0

mmm

1 ',

, 3000 4000 3000 4000 3000 4000 3000

4000

wave number

H-Y (5.6)

La-Y (5.6)

/ cm-l H-ZSM5 (80.0)

H-M(20.4)

F i g . 6. I R s p e c t r a o f OH s t r e t c h i n g o f a c i d - t y p e z e o l i t e s : ( a ) a f t e r e v a c u a t i o n a t 773 K, ( b ) p y r i d i n e adsorbed and evacuated a t 473 K. ( c ) NH3 adsorbed and evacuated a t room temperature.

h

1

0.1

cn

-

d

7

- m h m a M

-

h

b

----

1700

1400 1700

1400 1700

wave number

H-Y (5.6) Fig. 7.

La-Y (5.6)

1400 1700

/ cm-l H-M( 20.4)

A d s o r p t i o n o f p y r i d i n e on a c i d - t y p e z e o l i t e s .

1400

H-ZS?l5( 80.0)

588

at

1543 cm-’)

(5.6)(BPy:

for

65 %).

83 Z),

H-ZSM5(80.0)(BPy: However, f o r H-Y,

H-M(20.4)(BPy:

80 % ) and La-Y

o n l y 26 Z o f BPy was observed.

After the

d e a l u m i n a t i o n , t h e c o n c e n t r a t i o n o f BPy tended t o decrease. When more b u l k y base m o l e c u l e s ( c o 1 l i d i n e : i n t r o d u c e d on a c i d - t y p e z e o l i t e s ,

2,4,6-trimethyl-pyridine)

were

o n l y H-ZSM5 d i d n o t i n t e r a c t w i t h c o l l i d i n e .

The r e s u l t s suggest t h a t a l l t h e a c i d i c s i t e s o f H-ZSM5 a r e l o c a t e d i n t h e channels b u t n o t on t h e o u t e r surfaces. I n conclusion,

d e a l u m i n a t i o n o f a l u m i n o s i l i c a t e s has o c c u r r e d a t t h e

c a l c i n a t i o n s t e p from ammonium-form t o H-form. change t o (A10)’

The p a r t of A l - t e t r a h e d r a may

o r n o n - a c i d i c aluminum o x i d e species.

We found d e a l u m i a t i o n

o c c u r r e d on Si(3A1) o r Si(2A1) s i t e s w i t h w a t e r vapor which would a c t as an a c i d i c r e a g e n t a t h i g h e r temperatures.

Most o f t h e S i ( l A 1 ) s i t e s o f H-M and

H-ZSM5 a r e a c i d i c and 80 % o f a c i d i c s i t e s a r e s t r o n g e r Bronsted s i t e s .

REFERENCES 1

7

8 9 10 11

M. Niwa. M. Iwamoto and K. Seqawa, B u l l . Chem. SOC. Jpn., 59(1986) 37 3 5- 3 739. W.O. Haag. R.M. Lago and P.B. Weisz, Nature, 309(1984) 589-591. P.A. Jacobs and H.K. Beyer, J. Phys. Chem., 83(1979) 1174-1177. J.M. Thomas and J. K l i n o w s k i . Adv. Catal., 33(1985) 199-374. J. K l i n o w s k i , J.M. Thomas, C.A. F y f e and G.C. Gobbi, Nature, 296(1982) 533-537. C.A. Fyfe. J.M. Thomas, J. K l i n o w s k i and G.C. Gobbi, Angew. Chem. I n t . Ed. Engl., 22(1983) 259-267. J. Vedrine, A. Auroux, P. Dejave, V. Ducarme. H. Hoser and S Zhou, J. Catal., 73(1982) 147-160. A. Auroux, V. B o l i s . P. Wierzchowki, P.C. G r a v e l l e and J.C. Vedrine, J. Chem. SOC. Faraday Trans. 11, 75(1979) 2544-2555. P.A. Jacobs and R. von Ballmoos. J. Phys. Chem.. 86(1982) 3050-3052. M. B. Sayed, R.A. Kydd and R. P. Cooney, J. Catal., 88( 1984) 137-149. J. Scherzer and J.L. Bass, J. C a t a l . , 46(1977) 100-108.


589

SORPTICN OF ACETIC ACID CN H+ZSM-5

Linda M. Parker Chemistry Division, CGIR, Private Bag, Petone, New Zealand

ABSTRACT

Acetic acid sorbed on H+ZSM-5 held a t 15OOC w a s studied by tga, td/ms and I t was found that one m l e c u l e of a c e t i c acid was sorbed per z e o l i t e acid s i t e with only a partial transfer of the Bronsted proton to the acetic acid. FTIR.

INTRODUCTION

The sorption of weak bases such as m n i a on to acid c a t a l y s t s is a w e l l knwn technique f o r determining the number and strength of the acid sites [ l ] . In a similar manner, the sorption of weak acids, such as acetic acid, should give information on the basicity of the catalyst. The sorption of carboxylic acids on the acid form of z e o l i t e s has been studied

sites showed basic behaviour reacting with the acid to form acetate and water:

previously [2,3,4].

I t was claimed that the zeolite-

Ha+

RC=O

This scheme would r e s u l t in the removal of an oxygen from the z e o l i t e lattice

and an irreversible loss of acid sites. Since zeolites have a mch greater acid strength than that of carboxylic acids, it would appear to be thermodynamically mre favourable f o r the carboxylic acid to be protonated by the zeolite: R

590

The aim of this work was to determine the mode of acetic acid sorption (whether the zeolite acid sites showed amphiprotic behaviour as previously implied [2,3,4]), and also to determine whether any additional basic sites could be observed. EXPERIMENTAL

Thermogravimtric analysis (tga) and thermal desorption/mss spectrometry (td/ms) were used as described previously [5]. Sample sizes were typically 10 mg, with heating rates of 1O0C/min. In preparation for the tga and td/ms experiments, acetic acid (-20 Torr) was sorbed on to a previously evacuated H+ZSM-5 sample held at 15OOC. Any excess acetic acid was remved by trapping into a liquid nitrogen cold finger. In one case this was followed by sorption of m n i a (-100 Torr), with removal of the excess. Fourier Transform Infra Red (FTIR) spectra were obtained using a self supported H+ZSM-5 wafer in an in-situ, atmosphere and temperature controlled cell. The ETIR spectrometer used was a Bornem rrodel DA3. RESULTS AM) DISCUSSION Acetic acid, in the vapour phase, was strongly sorbed on H+ZSM-5 at 15OOC. The number of acetic acid molecules sorbed per zeolite unit cell was determined by tga for three,ZSM-5 samples with different aluminium contents (Table 1). TABLE 1 zeolite sample. no.

Al atoms/unit cell

1.3 2.5 3.1

f f

f

0.1

0.1 0.1

acetic acid mlecules/ unit cell 1.6 2.4 2.9

f f. f

0.1 0.1 0.1

Samples # 2 and #3 have been used in previous work [6]. The number of mlecules of acetic acid sorbed per zeolite unit cell was comparable to the number of aluminium a t m per unit cell (determined by Atomic Absorption analysis), and hence [7] the number of Bronsted sites per unit cell, suggesting that the strongly sorbed acetic acid is associated with the acid sites.

591

Td/ms showed the species that desorbed upon heating H+ZSM-5 a f t e r sorption of a c e t i c acid a t 15OOC. (Figure 1).

-._

-__ ----

........

I

200

I

300

-

-

-

-

-

-

-

400

I

-

-

Temperature I0C

acetic acid carbon dioxide acetone ethane methane water

I

-

-

-

-

-

500

-

-

i

600

FIGUFE 1. Td/m of acetic acid sorbed on H+ZSM-5 held a t 15OOC. I n i t i a l l y acetic acid desorbed unaltered.

However, as the temperature

increased, products of thermal decomposition and f u r t h e r reaction were observed.

a large a 2 peak, some ethene, methane, water and acetone. Integration of the desorption peaks, followed by scaling by their respective

A t 24OOC there w a s

s e n s i t i v i t y factors, gave the following mole percentages of products desorbed: C02 39, ethene 15.4, H 2 0 25.8, CH4 9.9 and acetone 9.9. A mass balance of the of 109:114:212, approximtely the This implies t h a t the products desorbed were

C, 0 and H atoms evolved gave a r a t i o of C:O:H

ratio f o r acetic acid (2:2:4). from the decomposition of acetic acid only. This supports the hypothesis t h a t acetic acid was sorbed i n t a c t (i.e. equation 2 r a t h e r than equation 1). If water had been eliminated u p n sorption on an acid site then the desorption products could not include acetic acid, as any water evolved would have rapidly desorbed f m the zeolite a t 15OOC. REACTION OF SORBED ACETIC ACID WITH AMMONIA OVER H+ZSM-5

I f the a c e t i c acid is in f a c t protonated by the zeolite, and behaves a s a weak base, it should be displaced from the acid sites by a stronger base, such a s amnia.

592

Acetic acid was sorbed onto H+ZSM-5 (sample #3) held a t 15OoC, with any The td/ms r e s u l t s are shown in

excess k i n g removed, then amnonia was sorbed. Figure 2.

- acetic acid

--........

-.-

2m

acetonitrile

500

400

300

ammonia water

ternperature/OC FIGURE 2.

Td/ms of acetic acid, then amnonia, sorbed on H+ZMS-5 held a t 150OC.

The amount of arnnonia t h a t desorbed (2.420.1 molecules per z e o l i t e u n i t cell) approached the number of z e o l i t e acid sites per u n i t cell (2.5tO.l). Very l i t t l e a c e t i c acid desorbed. This implies t h a t m n i a , being the stronger base, displaces a c e t i c acid from the Bronsted sites. However, mre than simple displacement of a c e t i c acid by m n i a may have Occurred, as acetic acid and a m n i a reacted to form the salt: CH3

OH

- CJ. + / r

3-zeolite

OH

+

2

~

+ ~

0

+ om4

~3 ~ 3 - c :

+-

~~40-zeolite

(3)

A t 18OOC the s a l t decomposed to give the a i d e and water with a sharp water

loss (observed i n Figure 2 ) :

B

CH3 - C G H 4

18OOC +

8

CH3X-NH2

+

H20

(4)

593

Further dehydration resulted in the formation of a c e t o n i t r i l e and water:

9

CH3-C-NH2

220-390°C + H3C-CZN + H 2 0

(5)

I t was thus not possible to show that m n i a displaced a c e t i c acid from the

This mst likely meant

acid sites because ammonia reacted with the a c e t i c acid.

t h a t m n i a displaced ammonium acetate from the acid sites rather than acetic acid. However, the observation of water loss a t 18OoC (from reaction 4 ) , provides f u r t h e r proof t h a t the acetic acid was i n i t i a l l y sorbed intact. FTIR OF ACETIC ACID CN H+ZSM-5

FTIR spectra of acetic acid sorbed on dry H+ZSM-5 ( a t 1 5 0 T under dry N 2 ) are shown in Figure 3. These are presented as difference spectra between a c e t i c acid on dry H+ZSM-5 and dry H+ZSM-5 only.

u

0.a

0 Z Q

m

E

2 m

0.4

Q

0.0

?-

8

-0.4-4000 FIGURE 3.

1 I

M 1 ,

,

1

,

1

1

300b

1 ,

1

I

1

t

1

*

1

200b

1

1

1

1

FTIR difference spectra between acetic acid sorbed on dry $ZSM-5

Spectrum A was recorded 30s a f t e r acetic acid injection, spectrum B a f t e r 12'min. and spectrum C,2 hours later.

and dry H+ZSM-5 (held a t 150°C under N2).

594

Spectrum A was recorded 30s a f t e r injection of acetic acid ( 2 ~ 1 1 ) . Two sharp peaks a t 1799 and 1772cm-1 resulted from the monomeric and dimeric gas phase and/or physisorbed acetic acid [8]. After 1 2 minutes, only the strongly sorbed acetic acid remained. The loss of the Bronsted 0-H s t r e t c h , shown by the negative -peak a t 3601 cm-1, is d i r e c t evidence t h a t the a c e t i c acid was associated with the acid sites. The Bronsted proton had been transferred to the a c e t i c acid with two broad bands centred a t -2870 and 2480 cm-1 observed.

Similar bands have been [9,10], but not f o r sorbed NH3. In the case observed f o r H20 sorbed a t 8OoC of m n i a sorption the proton is completely transfered giving NH4+ and no broad bands are observed.

The broad bands probably result from protons with a broad

range of energies shared between the zeolite and the acetic acid. The large doublet a t 1705 and 1678 cm-1 occurred a t lower wavenumbers than the C=O s t r e t c h of the gas phase or physisorbed acetic acid species.

Acetic acid protonated by fluorosulphonic acid in the c r y s t a l l i n e state a t 90K produces a symnetrical species with symstric and anti-syrranetric C-0 s t r e t c h e s a t 1615 and 1560 cm-1 [ l l l . The carbonyl stretches observed are closer to those observed in the gas phase or physisorbed species (species A i n figure 4 ) than they are to the species t h a t would be observed i f complete proton t r a n s f e r had occurred (species C in figure 4 ) . Therefore it is proposed that the protonated acetic acid is present as an intermediate species (species B i n figure 4 ) with only partial t r a n s f e r of the proton. 0

4

H3C-C,

6

+~+zeolite

0

H3C-C \----+H

OH

OH

A. No proton transfer

B.

--- &zeolite

P a r t i a l proton transfer.

OH

H3C42x+ S03FOH

C.

Figure 4.

Complete proton transfer.

Different degrees of proton t r a n s f e r f o r acetic acid sorbed on H+ZSM-5.

After two hcurs the acetic acid had almost ccmpletely desorbed, leaving an a h s t zero difference spectrum.

595

CONCLUSIONS

Acetic acid is strongly sorbed on to H+ZSM-5 held a t 150OC. Tga showed a one-to-one association with the z e o l i t e acid sites. The a c e t i c acid molecule remained i n t a c t with very l i t t l e or no f u r t h e r reacfion a t this temperature. ETIR spectra showed t h a t the Bronsted protons are only p a r t i a l l y transfered to

the acetic acid molecules and t h a t they occupy a wide range of energy s t a t e s . No behaviour w a s observed that could indicate the presence of basic sites.

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

B.M. Lok, B.K. Marcus and C.L. -ell, Zeolites, 6 (1986) 185. A. Bielanski, J. Datka, J. Catal., 32 (1974) 183.

T.M. IXlncan, R.W. Vaughan, J. Catal. 67 (1981) 49. T.M. IXlncan, R.W. Vaughan, J. Catal. 67, (1981) 469. L.M. Parker, D.M. Bibby and R.H. Meinhold; Zeolites, 5 (1985) 384. D.M. Bibby, N.B. Milestone, L.P. Aldridge and J.E. Patterson, J. Catal. 97, (1986) 493. 7. W.0. b a g r R.M. Lago and P.B. Weisz, Nature, 309 (1984) 589.


D.M. Bibby,C.D. Chang,R.F.Howe and S.Yurchak (Editors),Methane Conuersion 0 1988Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

597

IMPEDANCE AND INFRARED SPECTROSCOPY OF THE ZEOLITE ZSM-5

J.L. Tallon, J.F. Clare and R.G. Buckley Physics and Engineering Laboratory, DSIK, Private Bag, Lower Hutt, New Zealand ABSTRACT The zeolite catalyst ZSM-5 has been studied using impedance and infrared spectrocopies as part of an investigation of the loss of catalytic activity in the methanol to gasoline process. The complex impedance from 10 Hz to 10 MHz at a range of temperatures was measured for a range of values of aluminium content and for Li', Na+ and Cs' ion--exchangeto obtain activation energies for a relaxation process due to internal silanols. FTIR absorption spectra of a high-aluminium content, protonated ZSM--5at three temperatures around 750'C show the loss of Bronsted protons with annealing time. The kinetics of the process are second order and it would appear that there are two activation energies: 60.7 eV above and >3.4 eV below about 750'C. INTRODUCTION In an effort to probe catalytic sites and their stability, the zeolite catalyst, ZSM-5 was investigated by impedance and fourier transform infrared spectroscopies as a function of aluminium substitution and cation exchange. Samples were provided by Chemistry Division, DSIR, with (Si + Al)/Al ratios of m,

1000, 500, 200, 136 and 40.

Crystallite size and morphology varied somewhat

with aluminium content but typically the samples had crystal size distributions in the range 0 . 2

urn to

2

pm.

IMPEDANCE SPECTROSCOPY Impedance samples of 13 mm diameter and 0.3 mm thickness were pressed in a die, mounted between platinum electrodes in an oven and the complex impedance measured between 10 Hz and 10 MHz.

The spectra shown in Fig. 1 show a low

frequency inter-particle relaxation (of no further interest to

us)

and an

intra-particle relaxation associated with cation exchangeable sites. These relaxations have peak frequencies which exhibit the simple Ahrrenius behaviour shown in Fig. 2 , ie

The effective activation energies, heff, thus obtained are shown in Fig. 3 to be functions of both the exchanged ion and the degree of substitution, even at high silica/aluminiun ratios. The latter result is not expected'if the relaxation is associated with the exchanged Bronsted proton. Furthermore

598

M'

Id

I lo3

I

lo2

I

I

lo4

10'

I

1o6

lo7

Frequency (Hz) Fig. 1 A sequence of modulus spectra of ZSM-5 samples at temperatures of 322°C to 5 9 1 O C .

lo1

Lithium (vacuum) Sodium (vacuum) m Sodium (air) 0

09

-

106

lo4 1o3

Fie. 2 Ahrrenius behaviour of the peak frequencies.

Fig. 3 The effective activation energy as a function of exchanged ion and (Si + Al)/Al ratio.

599 (i) even without any substituted aluminium much the same relaxation peaks were

apparent ; (ii) the peaks remain stable until well above the temperature of Bronsted

dehydroxylation, and (iii) the height of the peaks is independent of the degree of substitution.

These observations suggest that the relaxation is due to silanols. External and internal silanols The relaxation was progressively lost on annealing at 950°C in vacuum at which temperature the IR silanol absorption is also lost (ref. 1).

Two types

Of

silanols may be considered: (i)

External terminating silanols. Charge transport may occur via the

terminating protons, the a.c. relaxation occurring because of the finite particle size. Proton mobility is confirmed by the fact that several minutes' exposure to D,O vapour at 1 mbar is sufficient to replace all terminating OH groups by OD (ref. 2).

For this surface effect the frequency prefactor is size

dependent and the data are consistent with particles 1 pm in diameter in agreement with the dimensions found by scanning electron microscopy.

(ii)

Internal silanols. Chester et a1 ( r e f . 3) and Dessau et a1 ( r e f . 4)

have shown that highlsilica ZSM-5 exhibits cation exchangeability which is independent, and in excess, of the framework aluminium content. They showed ( r e f . 4) that this arises from internal silanols which may be removed by

steaming. In our own IR measurements on the silanol absorption at 3740 cm-' of ZSM-5 with (Si + Al)/A1

-

4.0 we found that exchange of sodium ioos fur the

Bronsted proton resulted in a 20 to 25% reduction in the silanol peak in addition to the loss of the Bronsted peak. Moreover we note that defect sites in these samples are of sufficiently high density that Raman measurements are precluded by fluorescence of trapped electron-hole pairs. Two characteristics, the aluminium-substjtutionindependence of the impedance relaxation strength, and the cation-exchange dependence of the relaxation activation energy, suggest thal i t is the internal rather than the external silanols that we are observing. INFRARED SPECTROSCOPY Thermal dehydroxylation was examined in situ using a Bomem FTIR spectrophotometer. Samples with (Si self-supporting wafers of

A1)Al

= 40

were pressed into

- 20 mgm/cm2, dried in nitrogen at 4OO0C, then t

subjected to isothermal anneals, spectra being recorded, immediately after cooling, at 120°C. Fig. 4 shows spectra through a succession of anneals at 75OoC in air at ambient humidity.

600

w

U

z a

0

v)

m

a

FREQUENCY ( c m-'1 Fig. 4 Infrared spectra showing the hydroxyl absorption band for ZSM-5 [(Si + Al)/Si = 401 for a range of anneal times at 750°C. Bronsted dehydroxylation In contrast to the loss of the Bronsted proton at 3610 cm-', the silanol absorption at 3750 cm-' remains unchanged, though new absorptions appear at 3790 cm-' and 3670 cm-'.

The kinetics, as shown in Fig. 5, is second-order,

consistent with the requirement for pairwise dehydroxylation in order to generate H,O molecules. Such an interpretation requires mobility of the framework aluminiums. That the kinetics is second order (as opposed to first order with two time constants) will be checked by repeating these measurements at higher Si/A1 ratios. The time constant at a given temperature should vary as the square of the ratio. The Ahrrenius plot, figure 6 , which is preliminary, shows timr constants at three anneal temperatures. It suggests that, as with loss of crystallinity (ref. 5), there are two regimes of dehydroxylation with activation energies of $0.7

eV above and 33.4 eV below about 75OoC. The latter value is not

inconsistent with the activation energy of 4 . 3 eV for diffusion of framework aluminiums deduced from thermal decomposition studies (ref. 5 ) . measurements are in progress.

Further

601

Fraction a o f . Bronsted protons 7.0 -

remaining

3000

1

/ /

.I-

3001

/ / / /

/

c

/

-;I-* /

0

200

400

600 800 1000 12

Anneal Time at 75OoC (min)

Fig. 5 The fraction, a, of Bronsted protons remaining as a function of anneal time at 75OOC. Plotted as a-1 vs t the linearity implies second order kinetics. The same data are also plotted as In a vs t to illustrate that the kinetics is not first order.

Fig. 6 Preliminary results for the Ahrrenius behaviour of Bronsted dehydroxylation. The lines drawn are tentative.

REFERENCES S.E. Spiridinov et al., Kinet i Katal g? (1986) 201. (1987) 9-13. G.O. Brunner. Zeolites A.W. Chester, Y.F. Chu. R.M. Dessau. G.T. Kerr and C.T. Kresge, J. Chem. Soc Chem. Commun. 3 (1985) 289. 4 R.M. I)essau, K.D. Schmitt, G.T. Kerr. G.L. Woolery and L.B. Alemany, J. Catal. 100.4 (1987) 484-489. 5 J.L. Tallon and R.G. Ruckley, J . Phys. Chem. 91 (1987) 1469-1475. 1 2 3


D.M. Bibby,C.D.Chang,R.F. Howe and S. Yurchak (Editors),Methane Conversion 0 1988 Elsevier Science PublishersB.V.,Amsterdam - Printed in The Netherlands

603

THE CHEMISTRY AND CATALYTIC PROPERTIES OF TRANSITION METAL OXYANIONS IN SODALITE CAGES

L.M. MORONEY, S. SHANMUGAM and A.G. LANGDON Chemistry Department, University of Waikato, Hamilton, New Zealand ABSTRACT The loading of zeolite cages with catalytically active species provides a strategy for the modification of zeolite properties and the preparation of multifunctional catalysts. Hydroxy-sodalite has been used to study the loading of sodalite cages with chromate, molybdate and tungstate ions by a dry salt high temperature reaction. The resulting noseans were used as model systems for examining some of the properties of the occluded oxyanions. INTRODUCTION The widespread application of zeolites as catalysts has directed attention towards enhancing and extending catalytic activity through the introduction of catalytically active metal species principally by cation exchange with lattice cations. In zeolites such as the X, Y and A type zeolites, the sodalite cages provide possible sites for accommodating reactive metal species. It has long been known that sodalites with cages containing up to four water molecules, two NaOH molecules, one molecule of monovalent salts and one molecule of divalent salts shared between two cages, can be prepared by hydrothermal synthesis (ref, 1).

However except for a limited number of special

cases (refs. 2-3) it does not appear that salt occlusion by the sodalite cages of zeolites can be achieved during hydrothermal synthesis (ref. 4 ) .

Even

for the case of feldspathoid synthesis, the hydrothermal reaction involving the salts of transition metal oxyanions such as chromate, tungstate and molybdate yields different crystalline phases (sodalite and cancrinite) of low salt loading (refs. 5-6).

An alternative route to the occlusion of salts in zeolite systems is by means of high temperature reactions involving previously imbibed salts. This procedure has been successfully employed to fill the zeolite cages of X, Y and A zeolites with anions such as chloride, bromide, iodide, nitrate and chlorate (refs. 4,7).

It offered a possible means of occluding the oxyanions of trans-

ition metals such as Cr, Mo and W in the sodalite cages of hydroxy-sodalite and zeolites.

604

METHODS AND MATERIALS Hydroxy-sodalite was prepared from metakaolin in 4 m o l 1-' NaOH at 8 O o C . High temperature reactions were carried out in platinum crucibles, in contact with air and at atmospheric pressure.

Products were characterised by X-ray

diffraction (XRD) and the peak heights were used to obtain semi-quantitative data for the amounts of the crystalline phases present.

Oxidation/reduction

chemistry was studied using a vacuum line specially constructed to follow gas adsorption. RESULTS AND DISCUSSION Conversion of Hydoxy-Sodalite to Salt Loaded Noseans Salt loaded noseans were prepared by a high temperature reaction between excess salt and hydroxy-sodalite (ref. 8 ) : Na,Al6Si,O2,.4NaOH

+

Na,X

+.

Na6A1,Si,0,,.Na2X

+ Na,O +

2H20

where X = Cr0,2-.Mo0,2-,W0,2-. Nosean has a cage structure very similar to that of sodalite.

It can be

considered to be derived from sodalite by flattening the A1-0-Si bond angle to enlarge the sodalite cage.

Sodalite and nosean have common XRD peaks

corresponding to d = 6 . 2 4

and d = 3.71 A but because nosean does not exhibit

the systematic extinctions that occur in the XRD pattern of sodalite, it was possible to monitor both the total crystallinity of the sodalite/nosean mixture and the growth of nosean during the conversion reaction. Intermediate formation of nephiline, when it occurred, was monitored by the XRD line corresponding to d = 4.17

A.

Preliminary differential thermal analysis (DTA) experiments indicated that the melting points of the salts were depressed in the reaction mixture.

For

example, the melting point of Na2Cr0, in the sodalite/Na2Cr0, reaction mixture. was depressed by 22'C

to 77OoC. From studies at temperatures above and below

the effective melting points of the salts it was clear that whereas the reaction from sodalite to nosean at temperatures above the melting point was accompanied by significant initial structural collapse, the conversion at temperatures below the melting point was effected with little loss of crystallinity. The observation of increases in product nosean intensities without concomitant decreases of any other crystalline phase indicated the possibility that an amorphous phase was formed during the reaction.

However

these increases could have been due at least partly to the gradual improvement of the crystallinity ofthe already formed product phase.

From these and other

data (refs. 8-9) the following reaction scheme was devised for the high

temperature reaction: OH-Sodalite/Na,CrO, (1) J.

metastable expanded OH-Sodalite

Cr0,-Nosean At 900°C the rates of reactions 2 , 3 and 4 were very much increased. The rate of reaction 6 at 75OoC and 8OO0C were comparable but reaction 2 was much slower than reaction 6 at 75OoC. Direct conversion of sodalite to nosean is favoured by keeping the reaction temperature below the depressed melting point of the salt.

It would appear that when the loading of zeolite sodalite cages

is attempted, best results for high melting point salts can be expected if the reaction temperature is kept below the effective melting point of the salt in the zeolite system. Aspects of the Chemistry of Occluded Metal Oxyanions Although feldspathoid structures are generally thought to be insufficiently porous to find widespread catalytic applications, the reactions of the occluded oxyanions are of catalytic interest. Previous work with other systems has shown that occlusion affects the properties of both the encapsulated species and the lattice itself (refs. 1,6,10,11). (i) Thermal Stability. While chromate-nosean is stable in air and under vacuum at temperatures up to 750°C, molybdate- and tungstate-noseans appear to undergo partial decomposition under vacuum to produce pale blue-grey colours.

No measurable evolution of gas was observed however. (ii) Reduction with H,(g).

Reduction of chromate-nosean with H,(g) at

atmospheric pressure started above about 3OOOC and appeared to reach completion after about 1 hour at 48OoC. The H,(a)

consumed indicated that the green

colour of product formed was due to Cr,O,.

The nosean structure was found to

remain intact until virtually all of the chromate had been reduced whereupon a dispersion of Cr,03 in nephilene of low crystallinity was formed. For reactions in which the rate of H2(g) consumption was studied, it was found that the rate of reaction'was pressure dependent and after a temperature dependent induction period,varied initially with the square root of time as might be expected for a diffusion controlled reaction.

606

The molybdate- and tungstate-noseans were much more stable than chromatenosean under reducing conditions. This is consistent with the properties of the pure salts. Sodium chromate is relatively easily reduced to oxidation state I11 whereas molybdates and tungstates tend to form polymeric 'bronzes' with oxidation state between V and VI.

THe formation of this type of compound

is not possible when the oxyanions are separated in cages. The amount of H,(g) adsorbed by molybdate-nosean at 800°C was sufficient to account for less than 14% reduction of the oxyanion to the V oxidation state.

(iii) Reoxidation with O,(g). noseans with O,(g)

Quantitative reoxidation of reduced chromate-

was possible providing the reduction step had not resulted

in the destruction of the lattice.

If s o , reoxidation produced a mixture of

chromate-nosean, nephilene and free Na,CrO,. (iv) ESR Studies. Iron-free chromate-nosean necessary for these studies was prepared from iron-free Al(OH),,

H,SiO,, NaOH and Na,CrO,

(ref. 9 ) .

ESR

studies of the products formed during reduction and oxidation reactions gave two discrete signals with g values of 1.987

*

0.002 and 1.974

f

0.002

consistent with Cr(V) and Cr(II1) species. The line widths observed were as expected for the relatively dilute dispersion of Cr species in the aluminosilicate matrix.

Semi-quantitative data for the reduction and reoxidation

reactions were obtained by plotting A/GM (where A is the peak to peak first derivative signal amplitude, G is the gain and M is the amplitude modulation) versus time. These data indicated a sequential nature of the oxidation/ reduction reactions. The ESR experiment also provided a useful means of monitoring structural changes in the reduced noseans.

ESR line broadening and

reduced amplitude were associated with l o s s of X-ray crystallinity. (v) Exchange Reactions.

The sodium cations of the salt loaded noseans were

exchangeable with other simple cations.

In the case of NHb+ exchange, the

thermal stability of the encapsulated oxyanions was markedly reduced. (vi) Catalytic Activity.

Preliminary studies have shown that the nosean

systems with occluded transition metal oxyanions are active oxidation catalysts for reactions involving small molecules. CONCLUSIONS Studies of high temperature, dry salt reactions with hydroxy-sodalite have provided useful insights into the behaviour of zeolites when loaded with high melting point salts and reacted at elevated temperatures. Such reactions at temperatures above the effective melting point of the salt are likely to lead to the formation of both amorphous aluminosilicate phases and crystalline product phases such as nephilene and nosean. The salt loaded nosean systems and their cation exchanged forms provide convenient model systems for investigating the chemistry of metal oxyanions in

607

sodalite cages.

These systems also allow a means by which novel oxidation

states in a highly dispersed form may be prepared. REFERENCES R.M. Barrer and J.F. Cole, J. Chem. SOC. (A), (1970) 1516. G.H. Khul, Advan. Chem. Ser., 101 (1971) 75. R.M. Barrer and E.F. Freund, J. Chem. S O C . , Dalton Trans., (1974) 1049. J.A. Rabo, in J.A. Rabo (Editor), Zeolite Chemistry and Catalysis, A.C.S. Monograph No. 171, 1976, pp. 332-349. 5 R.M. Barrer and A.G. Langdon, personal communication. 6 R.M. Barrer, J.F. Cole and H. Villiger, J. Chem. SOC. (A), (1970) 1523. 7 J.A. Rabo, M.L. Poutsma and G.W. Skeels, in J.W. Hightower (Editor), Proc. Inter. Congr. Catal. 5th, Miami Beach, 1972, North Holland Publishing Co. Amsterdam, 1973, pp. 1353-1363. 8 L.M. Moroney, M.Sc. Thesis, University of Waikato, 1978. 9 S . Shanmugam, M.Sc. Thesis, University of Waikato, 1983. 10 R.M. Barrer and C. Marcilly, J. Chem. SOC. (A), (1970) 2735. 11 R.M. Barrer, E.A. Daniels and G.A. Madigan, J. Chem. SOC., Dalton Trans., 1 2 3 4

(1976) 1805.


D.M. Bibby, C.D. Chang, R.F. Howe and S.Yurchak (Editors), Methane Conversion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

609

RAMAN SPECTRA OF OCCLUDED CATIONS I N ZSM-5

J.R.

BARTLETTI, R.P.

COONEY2 and D.M.

BIBBY3

1Department o f Chemistry, U n i v e r s i t y o f Newcastle, 2308, N.S.W. (Australia) 2Department o f Chemistry, U n i v e r s i t y o f Auckland, P r i v a t e Bag, Auckland (New Zeal and) %hemistry D i v i s i o n , Department o f S c i e n t i f i c and I n d u s t r i a l Research, P r i v a t e Bag, Petone (New Zealand) ABSTRACT A s e r i e s o f ZSM-5 samples w i t h d i f f e r i n g framework aluminium contents ( c o n t a i n i n g tetrapropylammonium cations, [TPAltl have been characterised by Raman spectroscopy. Difference Raman spectra reveal evidence f o r two d i s t i n c t occluded species i n samples w i t h non-zero framework aluminium content. These species have been i d e n t i f i e d as [ T P A l t cations associated w i t h framework anionic s i t e s and non-framework anions such as B r - o r OH-, on the basis o f c o r r e l a t i o n s between the i n t e g r a t e d i n t e n s i t i e s o f d i f f e r e n c e spectra and z e o l i t e aluminium content. The r e l a t i v e abundance o f the two forms have been determined semiq u a n t i t a t i v e l y and empirical evidence f o r CTPAI' d i s o r d e r i n g i s reported. INTRODUCTION Z e o l i t e c a t a l y s t s l e n d themselves t o c h a r a c t e r i s a t i o n by a v a r i e t y o f d i r e c t spectroscopic method. ( a ) Fourier transform infrared spectroscopy IFtirl which has been i n t e n s i v e l y applied, can 'provide molecular and s t r u c t u r a l information on both the adsorbed phase and the z e o l i t e substrate.

I n recent studies by Cooney e t aZ., F t i r has

provided i n f o r m a t i o n on surface s i t e populations f o r ZSM-5 ( r e f . 11, conformations o f adsorbed aromatic bases on z e o l i t e s ( r e f . 2 ) and c a t i o n speciation i n r a r e earth-exchanged f a u j a s i t e s ( r e f . 3 ) . ( b ) Laser Ramun spectroscopy (LRsl i s an a l t e r n a t i v e and complementary source o f v i b r a t i o n a l s t r u c t u r a l data t o F t i r which i s a t t r a c t i n g sharply increasing i n t e r e s t i n the f i e l d s o f c a t a l y s i s and energy chemistry ( r e f s . 4,5). In its non-resonant form, LRs i s l e s s s e n s i t i v e than contemporary F t i r spectroscopy. Nevertheless, i t provides much wider z e o l i t e substrate windows than F t i r . This f a c i l i t a t e s d e t e c t i o n o f v i b r a t i o n s o f adsorbed species across t h e complete f i n g e r p r i n t region and o f t e n leads t o more secure s t r u c t u r a l conclusions. LRs has a l s o proved t o be very e f f e c t i v e i n recent studies o f z e o l i t e c a t i o n speciation i n actinide-exchanged f a u j a s i t e s ( r e f s . 6,7). I n i t s resonant form, LRs gains s u b s t a n t i a l l y i n s e n s i t i v i t y because o f e l e c t r o n i c and p o l a r i s a b i l i t y factors.

610

( c ) Cryogenic Lwninescence spectroscopy i s a s e n s i t i v e technique which i s based on l a s e r - e x c i t e d e l e c t r o n i c emission from selected cations. It has been employed i n recent studies o f c a t i o n s i t e s p e c i a t i o n i n r a r e earth-exchanged f a u j a s i t e s (ref.e 1. T h i s paper r e p o r t s t h e a p p l i c a t i o n o f l a s e r Raman spectroscopy t o t h e c h a r a c t e r i s a t i o n s o f a s e r i e s o f ZSM-5 m a t e r i a l s c o n t a i n i n g occluded organic cations. EXPERIMENTAL ZSM-5 samples i n v e s t i g a t e d i n t h i s study were provided by D S I R (Chemistry 1.3 and 2.4 aluminium atoms per u n i t c e l l (samples

D i v i s i o n ) , and contained 0.0,

are subsequently r e f e r r e d t o as ZSM-5(o.o) respectively).

, ZSM-5( 1.5)

and ZSM-5(2.4) Raman spectra were e x c i t e d using t h e 514.5 nm l i n e o f a Coherent

Radiation argon i o n laser, and were recorded on an Anaspec-modified Cary-81 spectrometer i n c o r p o r a t i n g an RCA 31034A p h o t o m u l t i p l i e r tube. Weighted d i f f e r e n c e spectra were constructed using a N i c o l e t 1074 hardwired instrument computer. Weighting f a c t o r s employed during the c a l c u l a t i o n o f these spectra were chosen t o minimise t h e i n t e n s i t y o f negative peaks. Integrated i n t e n s i t i e s were obtained by numerical i n t e g r a t i o n o f d i g i t i s e d spectra, and the r e l a t i v e l y l a r g e e r r o r s associated w i t h t h e values reported i n Table 2 r e f l e c t t h e small signal-to-noise (S/N) r a t i o s encountered i n t h e c a l c u l a t e d d i f f e r e n c e spectra. RESULTS AND DISCUSSION Ordering o f occluded tetrapropylamnonium c a t i o n s The spectral features associated w i t h [TPAI+ c a t i o n s i n t h e various z e o l i t i c and n o n - z e o l i t i c media i n v e s t i g a t e d i n t h i s study a r e i l l u s t r a t e d i n Figs. 1 A

and 2, and Table 1. Pelrker e t at. have p r e v i o u s l y reported Raman spectra ( w i t h i n t h e range < 1500 cm-1) o f as-prepared ZSM-5 samples i n c o r p o r a t i n g occluded [TPAl+ cations, and our spectra a r e comparable t o those observed d u r i n g t h i s e a r l i e r study ( r e f . 9). The C-H s t r e t c h i n g modes associated w i t h [TPAI' i o n s i n c r y s t a l l i n e [TPAI'Br-

and aqueous [TPAI+[OHl-

corresponding spectrum o f [TPAI'

are compared t o t h e

occluded i n s i l i c a l i t e (pure s i l i c a form,

ZSM-5(0.0))

i n Fig. 1. The spectrum o f the occluded phase e x h i b i t s a s e r i e s o f broad overlapping bands w i t h maxima a t 2745, 2881, 2933, and 2975 cm-1 (Fig. l ( A ) ) . This spectral p r o f i l e i s somewhat d i f f e r e n t t o t h a t o f c r y s t a l l i n e [TPAl+Br- which e x h i b i t s a complex s e r i e s o f sharp, w e l l defined bands (Fig.

1(C) and Table 11, suggesting t h a t the occluded ITPA]+ species are more I n support o f t h i s proposal, i t disordered than those o f c r y s t a l l i n e [TPAI+Br-. was found t h a t t h e spectrum o f an aqueous s o l u t i o n o f [TPAl+ (Fig. 1 ( B ) ) resembles t h a t o f the occluded species. Occlusion o r adsorption o f molecular species by z e o l i t e s and r e l a t e d

611

TABLE 1 Raman spectra (v/cm-l) o f ZSM-5 samples c o n t a i n i n g [TPAI' a d d i t i o n a l compounds containing [TPA]+. Model (TPA)+ Compoundsa 20 % i n H20 (OH- 1

Crystalline (Br-)

In MeOHb

758 sh 781 w (0.03)

761 w 788 m

755 sh 780 m

849 w

849 m

849 m

941 w

(0.75) (0.12)

939 973 1037 ms (0.36) 1033 1060

w 940 w w 973 vw ms --w

cations, and

ZSM-5 2.4

Assignments

A1 atoms per u n i t c e l l 1.3 0.0

753 772 825 848 868 920 935 980 1035

w w w w

w w

w

w ms

749 771 825 846 868 919 934 979 1035 1085

w w

w w w

w

w w ms

751 772 832 845 869 919 933 982 1036

w w vw w w w

w w ms

1105 mw (0.19) 1105 w 1104 m 1140 mw (0.07) 1141 m 1140 mw

1101 m 1140 w

1100 m 1138 w

1101 m 1140 w

vs (C-C)/ CH2 rock

1162 w

1168 w

1164 w

1169

w

CH3 rock

1318 sh 1339 m

1317 sh 1337 m 1349 sh

1317 sh 1338 m

1455 s

1454 s

1395 vw 1455 s

2748 w

2746 w

2745 w

2882 s

2883 s

2881s

2938 s

2935 s

2933 s

2975 vs

2978 vs

2975 vs

1322 m

(0.75) 1162 m 1160 w 1276 (0.59) 1321 ms 1320 mw 1354 w

1459 ms (0.64) 1456 2732 2751 mw 2760 2819 2887 s 2872 2905 2920 sh 2926 2947 vs 2986 s

1345 sh 1390 w ms 1457 ms w vw 2745 w w vs 2882 s s s

2949 vs 2940 2975 vs 2978 sh 2997 ms

aAnion associated w i t h [TPAl+ given i n parentheses. garentheses represent d e p o l a r i s a t i o n r a t i o . ---: obscured by solvent band. c r y s t a l l i n e oxide phases (e.9.

CH2 wag

HCH Def.

v(C-H)

MeOH, methanol.

Values i n

s o d a l i t e ) i s o f t e n accompanied by the formation

o f an ordered adsorbed s u p e r l a t t i c e w i t h i n t h e z e o l i t e framework ( r e f . 101, i n In c o n t r a s t t o the proposed nature o f occluded CTPAI+ cations i n ZSM-5. p a r t i c u l a r , the adsorption o f acetylene by z e o l i t e s r e s u l t s i n t h e formation o f a q u a s i - c r y s t a l l i n e phase as a consequence o f adsorbate-adsorbate, cationadsorbate and framework-adsorbate i n t e r a c t i o n s . However, the occluded CTPAI+ cations i n ZSM-5 a r e l o c a l i s e d a t tetrahedral channel j u n c t i o n s ( f o u r CTPAI' cations per u n i t c e l l , A1nSig6-nO192) w i t h each o f t h e associated propyl chains

612

Fig. 1. Raman spectra of CTPAIt occluded i n ( A ) s i l i c a l i t e , (B) 20 % aqueous s o l u t i o n o f tetrapropylammonium hydroxide and ( C ) c r y s t a l 1i n e tetrapropylammonium bromide. ( A l l traces, e x c i t i n g l i n e , 514 nm. Top trace: l a s e r power, 90 mW; bandpass, 10 cm-l; Middle trace: l a s e r power, 250 mW; bandpass, 10 cm-1; Bottom trace, l a s e r power, 60 mW, bandpass, 5 cm-1).

I

28'00

2600

i 30'00

c

31100

h

WAVENWBER I cm-?

D

Fig. 2. Raman spectrum of CTPAI' occluded i n s i l i c a l i t e ZSM-5(oao)). ( E x c i t i n g l i n e , 514 nm (90 mW); bandpass, 5 cm-1).

being d i r e c t e d down one o f t h e i n t e r s e c t i n g channels. As a r e s u l t o f t h i s r i g i d occlusion geometry, i n t e r a c t i o n s between adjacent CTPAIt cations a r e excluded. I n a d d i t i o n , t h e i n d i v i d u a l propyl chains (diameter ca. 0.4 nm) a r e o n l y l o o s e l y constrained by the l a t t i c e channel s t r u c t u r e (channel diameter ca. 0.6 nm) and would thus be expected t o e x h i b i t a degree o f conformational disorder, i n accord w i t h t h e spectra presented i n F i g . 1. B E f f e c t s o f framework aluminium content on t h e Raman spectra o f occluded t e t r a p r o p y l amnonium c a t i o n s A f e a t u r e of the data summarised i n Table 1 i s t h a t no systematic v a r i a t i o n s

i n t h e wavenumbers o r r e l a t i v e i n t e n s i t i e s o f dominant bands observed below 1500 cm-l a r e apparent as a f u n c t i o n o f l a t t i c e aluminium content f o r ZSM-5(0.0),

613

ZSM-5(1.5) and ZSM-5(2.4).

I n contrast, a small systematic v a r i a t i o n was

observed i n the p o s i t i o n o f one o f the C-H s t r e t c h i n g modes, which may be r e l a t e d t o framework aluminium content (2938*2 cm-I (ZSM-5(2.4)) V 6 2935+2 cm-1 (ZSM-5(1.3)) vs 2933*2 cm-1 (ZSM-5(o.o) 1. However, s i g n i f i c a n t spectral d i f f e r e k e s ( p a r t i c u l a r l y i n the region 2700-3000 cm-1) are revealed f o l l o w i n g weighted s u b t r a c t i o n o f the spectrum o f ZSM-5(oSo) from those o f ZSM-5(1.3) and ZSM-5(2.4). The i n t e g r a t e d i n t e n s i t i e s o f the d i f f e r e n c e p r o f i l e s observed i n t h e region 2700-3100 cm-1 (normalised w i t h respect t o t h e i n t e n s i t i e s o f the parent p r o f i l e s and subsequently r e f e r r e d t o as 1 ' ) are presented i n Table 2. A comparison of t h e values o f I' and nCA11 (where "A11

r e f e r s t o the number o f aluminium atoms per u n i t c e l l 1 reveals a semi-quantitative c o r r e l a t i o n between

the framework aluminium content and the i n t e g r a t e d i n t e n s i t i e s o f the d i f f e r e n c e p r o f i l e s (i.e. n [ A l I / I ' = 4k1). TABLE 2 Normalised i n t e g r a t e d i n t e n s i t i e s o f the spectral p r o f i l e s obtained f o l l o w i n g weighted s u b t r a c t i o n o f t h e spectrum o f ZSM-5(oSo) from the spectra o f ZSM-5(1.3) and ZSM-5(2.4) w i t h i n t h e region 2700-3100 cm-1.

1.3 2.4

0.32iO. 1

0.58k0.2

0.68kO. 1 0.42k0.2

4k1 4i1

0.47 1.5

0.5k0.2 1.4k0.4

% [ A l l r e f e r s t o the number o f aluminium atoms per u n i t c e l l . bI' r e f e r s t o t h e normalised i n t e g r a t e d i n t e n s i t y o f the Raman d i f f e r e n c e spectral p r o f i l e s .

These f i n d i n g s may be r a t i o n a l i s e d on t h e basis o f two d i s t i n c t types o f CTPAI+ species, one o f which i s comnon t o a l l ZSM-5 preparations i n v e s t i g a t e d i n t h i s study (regardless o f aluminium content) w h i l e the o t h e r i s o n l y observed i n ZSM-5 samples o f non-zero aluminium content (and i s thus absent from ZSM-5(0.0)). Since t h e z e o l i t e l a t t i c e i s n e u t r a l i n s i l i c a l i t e (nCAlI = 0.01, CTPAI' cations occluded i n t h i s l a t t e r preparation would be accompanied by nonframework anions such as Br',

o r C1-, and i t i s concluded t h a t a l l samples [TPAl+COHI-, o r CTPAI+Cl- (non-framework

[OH]-

i n v e s t i g a t e d c o n t a i n CTPAI+Br-, associated ITPA]', [TPAlnfa). The normalised i n t e g r a t e d i n t e n s i t i e s o f t h e spectral p r o f i l e s associated w i t h [TPAlnfa species are given by t h e expression (1-1'1, as i l l u s t r a t e d i n Table 2. Based on t h e c o r r e l a t i o n between nCAll and I ' derived above, i t i s a l s o proposed t h a t t h e a d d i t i o n a l CTPAI' species observed f o r ZSM-5 samples w i t h non-zero aluminium content a r e associated w i t h the negative CA1041' framework components o f these l a t t e r preparations ([TPAl'fa

614

c a t i o n s ) . I n p a r t i c u l a r , i t i s concluded t h a t each anionic framework s i t e i s associated w i t h a s i n g l e CTPAI'fa cation, based on the f o l l o w i n g aspects o f the data presented i n Table 2:-

( i 1 As-prepared ZSM-5 contains f o u r CTPAI+ cations per u n i t c e l l . Consequently,

if each anionic framework s i t e i n t e r a c t s w i t h a sJngle [TPAI'fa cation, then t h e cations i s given by t h e expression (4-nCA111. ( i i 1 The observation t h a t t h e numerical values o f n[Al1/(4-n[Al I ) are comparable t o the values o f I ' / ( l - I ' ) (Table 2) i s i n accord w i t h the postulated model associating one CTPAI'fa c a t i o n w i t h each anionic framework s i t e , assuming t h a t t h e i n t r i n s i c i n t e n s i t y o f spectral features associated w i t h [TPA]+nfa i s equivalent t o t h a t o f [TPAIfa cations. The m u l t i p l e [TPAI' species present i n ZSM-5(1.5) and ZSM-5(2.4) are a l s o r e f l e c t e d i n weighted d i f f e r e n c e spectra obtained w i t h i n t h e region from 90 t o 1500 cm-1. However, t h e lower signal-to-noise r a t i o s obtained f o r bands i n t h i s spectral region precluded a r e l i a b l e determination o f the r a t i o I ' / ( l - I ' ) i n t h i s l a t t e r case. number o f [TPAl'nfa

REFERENCES 1 2 3 4

5 6 7 8 9 10

M.B. Sayed, R.A. Kydd and R.P. Cooney, J. Catal., 88 (1984) 137. J.R. B a r t l e t t and R.P. Cooney, submitted t o Spectrochim. Acta. J.R. B a r t l e t t , R.P. Cooney and R.A. Kydd, submitted t o J. Catal. J.R. B a r t l e t t and R.P. Cooney, i n R.E. Hester and R.J.H. Clark (Editors), Spectroscopy o f Inorganic-Based Materials, John Wiley and Sons, London, 1987, Chapter 3. R.P. Cooney i n J.R. D u r i g ( E d i t o r ) , V i b r a t i o n a l Spectra and Structure, Vol. 15, Elsevier, Amsterdam, 1986, Chapter 3. J.R. B a r t l e t t , R.P. Cooney and R.A. Kydd, manuscript i n preparation. J.R. B a r t l e t t , R.P. Cooney and R.A. Kydd, manuscript i n preparation. J.R. B a r t l e t t , R.P. Cooney and R.A. Kydd, submitted t o J. Catal. C. Peuker, W. P i l z , B. Fahlke, E. L o f f l e r , J. Richter-Mendau and W. Schirmer, Z. Phys. Chem. (Liepzig), 266 (1985) 74. N.T. Tam, R.P. Cooney and G. Curthoys, J. Chem. S O C . , Faraday Trans. I,72 (1976) 2577.


615

OLEFIN REACTIONS OVER Mo-MORDENITE

J.R. JOHNS and R.F. HOWE Chemistry Department, University of Auckland, Private Bag, Auckland (New Zealand) ABSTRACT Mo-mordenite was prepared by vapour phase adsorption of MoC15 onto the hydrogen and sodium forms of the zeolite. The reaction of some simple olefins over these catalysts was studied and the results related to measurements of their relative acidity. INTRODUCTION Catalytic systems containing molybdenum continue to receive considerable attention in the literature. Molybdenum, however, is one of the most difficult elements to ion exchange into zeolites (refs. 1,2) although several other successful methods of preparation are reported in the literature (refs. 3-7). Perhaps due to the difficulty of preparation, there are only a few reports of catalytic reactions over Mo-zeolites in the literature. The objective of the present study was to investigate the reaction of some simple olefins over Momordenite prepared by vapour phase adsorption of MoC15 into the hydrogen and sodium forms of this zeolite. EXPERIMENTAL MoH-mordenite and MoNa-mordenite were prepared statically in a high vacuum cell by vapour phase adsorption of MoC15 onto dehydrated zeolites, as previously described (ref. 8). Some catalysts were also prepared in a flow system to be described elsewhere. Catalytic testing was carried out in an all glass fixed bed recirculating reactor attached to a vacuum line. Samples from the reactor loop were analyzed periodically by sampling into an evacuated sampling loop of a gas sampling valve. A Shimadzu CG-8A gas chromatograph (GC) with a 1 metre 80/100 mesh Durapak(TM) (n-octane on Porasil-C) column operating at 5OoC, produced separation of Cl’s to C5’s within 15 minutes, at a carrier gas flow rate of 20 mllmin. The relative acidities of the catalysts were determined by comparing the integrated intensities of the 1545 cm-’ infra-red band of adsorbed pyridine (BrBnsted acid sites) ratioed against the 1870 cm-’ zeolite lattice overtone band. These results are shown in Table 1.

616

RESULTS AND DISCUSSION M e t a t h e s i s o f Propene o v e r Mo-mordenites F i g u r e s 1 and 2 show t h e p r o d u c t d i s t r i b u t i o n as a f u n c t i o n o f t i m e o f t h e r e a c t i o n o f propene o v e r f u l l y o x i d i s e d (Mo6+ ) and f u l l y reduced M ~ ~ M+ ~ )H m o r d e n i t e (6.1 w t % Mo, 1.95 M o / u n i t c e l l ) r e s p e c t i v e l y .

The p r o d u c t s a r e n o t

t h o s e o f c l e a n m e t a t h e s i s , w i t h o n l y t r a c e s o f ethene produced, t h e predominant p r o d u c t s b e i n g c i s - 2 - b u t e n e Y pentenes and propane.

The p r o d u c t s p r o b a b l y a r i s e

from c r a c k i n g , o l i g o m e r i s a t i o n and s e l f - h y d r o g e n a t i o n r e a c t i o n s o f p r i m a r y metat h e s i s p r o d u c t s , c a t a l y s e d by r e s i d u a l a c i d s i t e s .

The p l a i n H-mordenite

c a t a l y s t a l s o e x h i b i t s t h e a b i l i t y t o o l i g o m e r i s e and hydrogenate propene a t t h e s e temperatures ( p r o d u c i n g c i s - 2 - b u t e n e and propane). F i g u r e 3 shows t h e same r e a c t i o n o v e r a MoNa-mordenite c a t a l y s t (1.7 w t %, 0.54 M o / u n i t c e l l ) .

The p r o d u c t s a r e t h o s e o f c l e a n m e t a t h e s i s , w i t h o n l y

t r a c e s o f propane formed.

On r e g e n e r a t i o n o f t h i s c a t a l y s t b y c a l c i n a t i o n i n

oxygen (400 t o r r , 773 K ) f o l l o w e d b y r e d u c t i o n i n hydrogen (400 t o r r , 77310 t h e above r e a c t i o n produces a v e r y d i f f e r e n t p r o d u c t d i s t r i b u t i o n ( F i g . 4).

The

o v e r a l l r a t e o f r e a c t i o n i s much l o w e r and t h e p r o d u c t s a r e more l i k e t h o s e produced o v e r reduced MoH-mordenite. F o r comparison, t h e m e t a t h e s i s o f propene o v e r an Mo03/A1203 (8 w t % Mo) c a t a l y s t under i d e n t i c a l c o n d i t i o n s i s shown i n F i g u r e 5. r e a c t i o n i s h i g h e r t h a n t h a t f o r t h e MoNa-mordenite,

The i n i t i a l r a t e o f

however u n l i k e t h e z e o l i t e ,

t h e conventional material r a p i d l y deactivates. Hydrogenation o f O l e f i n s o v e r MoH-mordenite F i g u r e s 6 and 7 show t h e h y d r o g e n a t i o n o f propene o v e r MoH-mordenite (6.1 w t % Mo, 1.95 M o / u n i t c e l l ) . The f u l l y reduced f o r m (Mo5+) e x h i b i t s s i g n i f i c a n t 6+ h y d r o g e n a t i o n a b i l i t y , however t h e o x i d i s e d f o r m (Mo ) e x h i b i t s almost p u r e m e t a t h e s i s . I t i s g e n e r a l l y accepted t h a t t h e l o w e r t h e o x i d a t i o n s t a t e o f Mo The r a t e must b e s i g n i f i t h e g r e a t e r t h e r a t e o f h y d r o g e n a t i o n ( r e f s . 9,IO). c a n t l y l o w o v e r - t h e f u l l y o x i d i s e d f o r m t h a t t h e m e t a t h e s i s r e a c t i o n predomin a t e s (some propane i s observed), and hydrogen has t h e a f f e c t o f i n h i b i t i n g a c i d - c a t a l y s e d secondary r e a c t i o n s . The h y d r o g e n a t i o n o f s e v e r a l C 4 - o l e f i n s was a l s o s t u d i e d . The c o n v e r s i o n o f I - b u t e n e was f o u n d t o be f a s t e r t h a n t h a t o f cis-2-butene, which was f a s t e r t h a n trans-2-butene. This correlates with the r e l a t i v e s t e r i c r e s t r a i n t s o f the m o l e c u l e s w i t h i n t h e z e o l i t e pores, however as t h e observed p r o d u c t s were due t o i s o m e r i s a t i o n o f t h e r e a c t a n t t o t h e e q u i l i b r i u m c o m p o s i t i o n f o r butenes ( r e f s . 11,12), no r e a l c o n c l u s i o n s can be drawn, as t h e r e l a t i v e r e a c t i v i t y ( s t a b i l i t y ) o f t h e isomers f o l l o w t h e same c o r r e l a t i o n . I s o - b u t e n e (2-methylpropene) r e a c t e d e x t r e m e l y r a p i d l y , p r o d u c i n g o n l y coke and no gas phase p r o d u c t s , and complete c o n v e r s i o n o c c u r r e d w i t h i n minutes.

617

REACTION OF PROPENE OVER MoH-MORDENITE N U Y OXlOlSED (UoO+). RE*CTK)N AT 2SM

0.28

I

E

7 C I S-2-EUTENE/

"99.4

0.12

-1

i

/ /

I

REACTION OF PROPENE OVER MoH-MORDENITE

,

N U Y RWUCED (NOS+). R W O N AT 2 S I K

3.54

REACTION

OF PROPENE OVER MoNa-MORDENITE

A S . MoNAT37W 1.7 1 -

TRANS-2-WTENEI

CIS-2-BUTENE 0.4 0.3

0.2 0.1 0

0

1Y)

250 nN€/yxI

460

14Y)

1470

618

REACTION OF PROPENE OVER MoNa-MORDENITE

n l Q A RCU7W4 AT 373 K 1.2 I - i -

FIG4

1.11-

h

g

\

f

0.7 0.6 -

0,s

0.1

-

0.5

O1 0.3 0.2 0.1 0

METATHESIS OF PROPENE OVER 8ZMo-ALUMINA NUI oumm (w). -R

2

AT 47w

1.S

1.1 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9

0.1 0.7 0.6

CIS-2-BUTENE

0.5 0.4

0.3 0.2 0.1 0

0

YI

10

PROPENE I .6 IA 1.3 1.2 1.1

-

-

I -

0.6 0.5 0.4 0.3 0.2 0.9

Od

0.7

0.1

-

FIG8

+

loo

60

nuE /

1%

uo

YIN

HYDROGEN OVER MoH-MORDENITE

FULY OXlDlSW (WE+). REKlKU AT W

1230

619 PROPENE

+

21 *

HYDROGEN OVER MoH-MORDENITE

N U Y RED

15

nuE

NIN

TABLE 1 Relative Acidity of Catalysts Catalyst

Relative Acidity

5.1 0.7 2.1 1 .I

H-mordenite Na-mordenite MoH-mordenite as prepared MoNa-mordenite as prepared

CONCLUSIONS MoH-mordenite prepared from H-mordenite was found to retain significant acidity causing typically acid-catalysed reactions to occur rather than clean metathesis as on a conventional Mo03/A1203 catalyst. The analogous material prepared from Na-mordenite exhibited only low acidity, and produced products of clean metathesis. On regeneration (calcination in oxygen at 473 K then reduction in hydrogen at 473 K ) of this material however, acidity appears to be introduced producing catalytic behaviour similar to the MoH form. Reduction can produce acidity by reaction with hydrogen as envisaged in the scheme (refs. 13,14): No6'

+

(I2-

+

1/2H2

=

Mo5+ + OH-

620

5+ Reaction of propene with hydrogen over fully reduced (Mo ) MoH-mordenite produced mainly the hydrogenated product propane, with the presence o f hydrogen appearing to suppress acid catalysed reactions. The fully oxidised (Mo6+) catalyst under the same conditions exhibited little hydrogenation ability, with products more like those of metathesis. It appears that to avoid acid catalysed reactions over a Mo-zeolite it is desirable t o prepare it from a non-acid form of the zeolite. However considerable acidity is introduced into such a zeolite on regeneration.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

E.L. Moorehead, U.S. Patent 4,297,243, Oct. 1981. M.M. Huang and R.F. Howe, in press. P. Gallezot, G. Coudurier, M. Prime? and B. Imelik, in "Fourth Int. Conf. Mol. Sieves", ed. J.R. Katzer, ACS Symp. Ser., 40 (1977), 144. P.S. Dai and J.H. Lunsford, J. Catal., 64 (1980), 173. M.B. Ward and J.H. Lunsford, IIProceedings o f the 6th International Zeolite Conference, 1983" (Eds. D.H. Olson and A. Bisio), Butterworths, 1984. Y.S. Yong and R.F. Howe., J. Chem. SOC. Faraday Trans. I, 82 (1986), 2887. S. Abdo and R.F. Howe, J. Phys. Chem., 87 (1983), 1722. J.R. Johns and R.F. Howe, Zeolites, 5 (1985), 251. E.A. Lombardo, M. LoJacono and W.K. Hall, J. Catal., 64 (1980), 150. E.A. Lombardo, M. Houlla and W.K. Hall, J. Catal., 51 (1978), 256. J. Goldwasser, J. Engelhardt and W.K. Hall, J. Catal., 70 (1981), 275. J. Goldwasser, J. Engelhardt and W.K. Hall, J. Catal., 71 (1981), 381. W.K. Hall and F.E. Massoth, J. Catal. , 34 (1974), 41. W.S. Millman, M. Crespin, A.C. Cirillo, S. Abdo and W.K. Hall, J. Catal., 60 (1979), 404.

D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors),Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

621

PROPENE O L I G O M E R I Z A T I O N OVER H-ZSM-5 ZEOLITE

K.G.

Wilshier

C S I R O D i v i s i o n o f M a t e r i a l s Science and Technology, Locked Bag 33, Clayton, V i c t o r i a , A u s t r a l i a 3168

ABSTRACT O l i g o m e r i z a t i o n o f propene over a f i x e d bed o f H-ZSM-5 z e o l i t e (1.22 wt.% A l ) a t 24 b a r and 462K g i v e s dimer ( c 6 ) , t r i m e r (Cg), t e t r a m e r (C12), etc. w i t h o u t shape s e l e c t i v i t y . R e a c t i o n p r o b a b l y occurs on t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e p a r t i c l e s , p r o d u c i n g branched o l i g o m e r s w i t h a d i s t r i b u t i o n n o t u n l i k e t h a t i n commercial polymer g a s o l i n e produced o v e r p h o s p h o r i c a c i d / k i e s e l g u h r c a t a l y s t s . A t h i g h e r t e m p e r a t u r e s a broad range o f o l e f i n s (cg, C5, c6, C7 etc.) i s formed by c r a c k i n g and i s o m e r i z a t i o n r e a c t i o n s . Poisoning t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e improves shape s e l e c t i v i t y , b u t a t t h e expense o f conversion. INTROOUCTION Conversion o f l i g h t o l e f i n s t o h e a v i e r hydrocarbons o v e r s t r o n g a c i d c a t a l y s t s has been known f o r many y e a r s and i s s t i l l o f importance i n p e t r o l e u m refining.

The U n i v e r s a l O i l Products Co. polymer g a s o l i n e process which uses a

phosphoric a c i d / k i e s e l g u h r c a t a l y s t e x e m p l i f i e s t h i s technology. a r e g e n e r a l l y h i g h l y branched o l e f i n s , w i t h some i s o - p a r a f f i n s ,

The p r o d u c t s naphthenes and

a r o m a t i c s ( r e f . 1-2). R e a c t i o n of (C2-C5) o l e f i n s o v e r H-ZSM-5 z e o l i t e produces a r o m a t i c g a s o l i n e range hydrocarbons up t o a p p r o x i m a t e l y C10 ( r e f .

3).

However, a t i n c r e a s e d

p r e s s u r e and moderate temperatures d i s t i l 1a t e range hydrocarbons a r e formed ( r e f . 4-7). O l i g o m e r i z a t i o n i s o f carbenium-ion c h a r a c t e r and so i s expected t o g i v e h i g h l y branched o l igomers.

Because o f t h e s h a p e - s e l e c t i v e n a t u r e o f ZSM-5

z e o l i t e ( c a t a l y t i c p o r e s i z e ca. 0.55

X 0.7nm.,

r e f . 8), a h i g h e r p r o p o r t i o n o f

s t r a i g h t c h a i n and s i n g l y branched o l i g o m e r s h o u l d be formed. at. (ref.

9-10),

Van den Berg e t

u s i n g h i g h r e s o l u t i o n s o l i d s t a t e 13C nmr spectroscopy,

observed o n l y l i n e a r o l i g o m e r s a t 300K, b u t some b r a n c h i n g o c c u r r e d a t 373K. A t h i g h e r temperatures t h e degree o f b r a n c h i n g i n c r e a s e d f u r t h e r , w i t h s i g n i f i c a n t c r a c k i n g above 400K.

R e s u l t s o f propene o l i g o m e r i z a t i o n a t h i g h

c o n v e r s i o n o v e r H-ZSM-5 z e o l i t e under p r a c t i c a l c o n d i t i o n s o f t e m p e r a t u r e and p r e s s u r e a r e d e s c r i b e d h e r e ( f o r f u r t h e r d e t a i l s see r e f .

11).

622

EXPERIMENTAL ZSM-5 z e o l i t e was prepared ( f o l l o w i n g Ruhin, P o s i n s k i and Plank, r e f . 12) u s i n g 0-Brand sodium s i l i c a t e s o l u t i o n and tetrapropylammonium bromide. samples were prepared w i t h d i f f e r e n t SiO2/Al203 r a t i o s .

The crude z e o l i t e s

were washed, d r i e d , c a l c i n e d a t 823K o v e r n i g h t , t w i c e r e f l u x e d w i t h 0.W

HC1

The samples, so o b t a i n e d i n t h e

s o l u t i o n , t h e n again water washed and d r i e d . p r o t o n form, c o n t a i n e d 1.27 wt.%

Two

A1 (Sample A) and 0.10 wt.%

A1 (Sample P).

Sample A c o n s i s t e d o f 5-10 pm diameter aggregates o f f i n e l a t h s , whereas Sample R comprised h i g h l y i n t e r g r o w n , e l l i p s o i d a l c r y s t a l s o f 2-3 l ~ mdiameter. A sample o f each z e o l i t e was p e l l e t i z e d , crushed and sieved t o 40-60 mesh p a r t i c l e s , t h e n d i l u t e d t o 20 wt.% w i t h 40-60 mesh a c i d washed q u a r t z sand f o r use i n t h e r e a c t o r .

01 i g o m e r i z a t i o n Experiments A schema%ic diagram o f t h e r e a c t o r i s shown i n F i g u r e l a .

L i q u i d propene

was pumped a t a r a t e o f 7cm3/h (approx. 0.3 WHSV) t h r o u g h a s t a i n l e s s s t e e l p r e h e a t i n g c o i l t o a f i n n e d r e a c t o r t u h e ( F i g u r e l b . ) c o n t a i n i n g 13.59 o f d i l u t e d catalyst.

Both p r e h e a t e r and r e a c t o r t u b e were supported w i t h i n a

pi

(a)

TC

8

T

= =

+= -

-

= =

Y

d

Q

a

Pressure Indicator Thermocouple Ball Valve Pressure relief valve shciwing relief pressure (kPa) Switching Valve Pressure Regulator Snubber

s Meter

Flowmeter Bubble -Liquid

S.S. Sheath

(b)

Catalyst Bed 120mm x 10.5mm wide

/

'Cajon' VCR Fikings with S.S. Gasket

F i g . 1 a) b)

Product

/

/

Silica-wool Plug

Cajon Fittings

/

Finned Reactor ~

i

l

i

S.S. Thermocouple

! ~ Preheated ~ - ~ Reactant

~

~

~

01 i g o m e r i z a t i o n r e a c t o r shown s c h e m a t i c a l l y . Reactor t u b e d e t a i l .

623

h i g h l y t u r b u l e n t a i r oven.

Product passed f r o m the r e a c t o r tube through a

back-pressure r e g u l a t o r (maintaining t h e system a t 24 bar) t o a l i q u i d s t r a p h e l d a t 273K.

Gaseous products passed t o a rotameter and t o t a l volume gas-

meter. Normal procedure was t o evacuate t h e e n t i r e r e a c t o r system p r i o r t o s t a r t up. The feed-pump was then s t a r t e d and t h e oven temperature s l o w l y r a i s e d u n t i l l i q u i d s began t o accumulate i n the t r a p . The c a t a l y s t bed temperature was monitored by a thermocouple s l i d i n g i n an a x i a l sheath. Analysis L i q u i d products were f i r s t hydrogenated over a Pd-on-charcoal c a t a l y s t t o s i m p l i f y the a n a l y t i c a l problem o f i d e n t i f y i n g t h e many isomeric o l e f i n s formed (no s k e l e t a l i s o m e r i z a t i o n occurred d u r i n g t h i s treatment).

Hydrogenated

product was then analysed by gas chromatography on a 50m SE-30 coated c a p i l l a r y column, and t h e i d e n t i t y o f i n d i v i d u a l isomers determined by gc/ms. RESULTS S u b s t a n t i a l recovery o f l i q u i d product from propene conversion over H-ZSM-5 z e o l i t e (Sample A,

1.22 wt.% A l ) was not achieved u n t i l t h e c a t a l y s t

temperature reached about 460K a t 24 bar pressure. were then obtained.

Cu. 90 wt.%

liquid yields

I H nmr spectroscopy showed t h e product t o be h i g h l y

branched o l e f i n i c hydrocarbons, f r e e o f aromatics.

Gc and gc/ms a n a l y s i s o f

hydrogenated product provided a d e t a i l e d a n a l y s i s o f t h e carbon d i s t r i b u t i o n and branching.

Table 1 shows t h e d i s t r i b u t i o n of C5-Cg hydrocarbons.

Detailed

i d e n t i f i c a t i o n o f t h e l a r g e number o f C10+ isomers was not possible, b u t r e t e n t i o n times p e r m i t t e d t h e i r o v e r a l l q u a n t i f i c a t i o n .

The product

d i s t r i b u t i o n over Sample A a l t e r e d d r a m a t i c a l l y a t higher temperatures (55710 i n d i c a t i n g s u b s t a n t i a l s k e l e t a l rearrangement ( i s o m e r i z a t i o n ) and c r a c k i n g reactions.

The p r o p o r t i o n of C5,

C7 and c 8 hydrocarbons increased and t h e r e

were more normal and s i n g l y branched alkanes present and fewer doubly branched hydrocarbons. 2,3 and 2,4 dimethyl-alkanes dominated i n t h e c5-c8 products w h i l e 3,4 and 3,5 dimethyl-heptanes made-up >40% o f t h e Cg product. With low-aluminium H-ZSM-5 (Sample B, 0.1 wt.% A l ) , much h i g h e r temperature was required t o achieve reasonable conversion t o l i q u i d ; l i q u i d y i e l d s were 36 wt.%

a t 571K, and 73 wt.% a t 632K.

Shape s e l e c t i v i t y (as measured by t h e

t o t a l percentage o f n-alkane and s i n g l y branched alkanes i n t h e products) increased g r e a t l y r e l a t i v e t o t h a t i n t h e l i q u i d product over Sample A a t lower temperature. The e f f e c t o f d e a c t i v a t i n g a c i d s i t e s a t t h e e x t e r n a l s u r f a c e . o f t h e z e o l i t e c r y s t a l s (Sample A) was examined i n two experiments.

I n one, propene feed was

624

doped w i t h 0.5 w t . % 4-methylquinoline

(4MQ), and i n t h e o t h e r t h e H-ZSM-5

z e o l i t e c a t a l y s t was t r e a t e d w i t h excess hexamethyldisilazane (HMOs) vapour a t 523K p r i o r t o charging t o t h e r e a c t o r .

Both treatments d r a m a t i c a l l y reduced

With 4MQ i n t h e feed conversion was 53 wt.%

t h e conversion t o l i q u i d product.

a t 577K, w h i l e w i t h t h e HMDS t r e a t e d z e o l i t e t h e conversion was o n l y 37 wt.% a t 624K.

The degree o f branching, as shown i n Table 1, was a l s o reduced i n each

experiment. TABLE 1 Product d i s t r i b u t i o n (C5-Cg hydrocarbons C%) from propene o l i g o m e r i z a t i o n . H-ZSM-5 Catalyst

Product O i s t r i b u t i on by s k e l e t o n

Sample A a t 460K

n-a1 kane S i n g l y Branched Doubly Branched T r i p l y Branched

Sample B a t 632K

--

-

2.0

4.8

58.8

29 71

13 87 -

7 90 3

-

-

-

-

5.8

7.8

18.5

10 77 13

6 73 22

3 58 39 -

2 27 64 7

2.2

20

7

7

31.1

63 n-a1 kane S i n g l y Branched 37 Doubly Branched T r i p l y Branched

23 67 11

24 47 26

17 50 33

4 29 67

5.2

12

12

13.2

21.3

33 67

21 67 12

13 60 8 16

10 48 24 6

8 31 39 2 5 9

-

--

n-a1 kanes S i n g l y Branched Doubly Branched T r i p l y Branched Aromatic Cycl oal kanes n-a1 kane S i n g l y Branched Doubly Branched T r i p l y Branched

Sample A a t 624K Treated w i t h HMDS

3.2 9 83 10 4.5

Sample A a t 577K Poi soned w i t b 4MQ

0.1 100

by carbon number c7 c8 c9

21 79


Sample B a t 571K

c6

0.9

Sample A a t 557K

c5


-

-

-

-

-

-

-

-

-

-

-

16

5.4

6.2

27.9

42 58

19 69 11

15 61 23

11 59 30

5 46 49

4.3

17.7

12.1

11.7

22.6

68 32 -

32 61 7

28 53 19

19 55 26

9 48 43

-

-

-

-

-

-

-

62.5

32.7

-

1.0

-

31.1

36.3

43.5

-

-

31.6

625

DISCUSSION

Product from o l i g o m e r i z a t i o n o f propene a t l o w t e m p e r a t u r e (460K) and 24 har pressure over H-ZSV-5 gasoline.

z e o l i t e (Sample A) c l o s e l y resembles commercial polymer

Roth a r e h i g h l y branched ( p a r t i c u l a r l y t h e dominant Cg components),

c o n s i s t e n t w i t h f o r m a t i o n by c a t i o n i c 01 i g o m e r i z a t i o n on a c a t a l y s t s u r f a c e l a c k i n g shape-selectivity. apparent.

Considerable s k e l e t a l i s o m e r i z a t i o n i s a l s o

It i s concluded t h a t r e a c t i o n proceeds w i t h o u t s h a p e - s e l e c t i v i t y on

t h e e x t e r n a l s u r f a c e o f t h e c a t a l y s t p a r t i c l e s i n b o t h cases.

This r e s u l t

i m p l i e s t h a t t h e r a t e o f d i f f u s i o n of p r o d u c t from t h e channels o f t h e z e o l i t e i s low a t 460K, r e l a t i v e t o f o r m a t i o n o f o l i g o m e r a t t h e z e o l i t e s u r f a c e .

The

channels may be considered t o he blocked by o l e f i n o l i g o m e r a t t h i s temperature, c o n s i s t e n t w i t h van den B e r g ' s o b s e r v a t i o n ( r e f . 9 ) t h a t a t 373K o l e f i n s o r p t i o n i s hindered, w h i l s t h i g h l y branched p r o d u c t s a r e desorbed a t 473-573K.

The r e s u l t s o b t a i n e d a f t e r p o i s o n i n g t h e e x t e r n a l s u r f a c e o f t h e

z e o l i t e c r y s t a l s ( T a b l e 1 ) p r o v i d e f u r t h e r evidence f o r r e a c t i o n a t t h e e x t e r n a l s u r f a c e o f t h e c a t a l y s t a t l o w temperatures. A t h i g h e r temperature (>560K), over poisoned o r unpoisoned c a t a l y s t ,

i n c r e a s e d r a t e s o f d i f f u s i o n o f hydrocarbon p r o d u c t s a l l o w t h e shapes e l e c t i v i t y o f t h e c a t a l y s t t o become apparent.

The i s o m e r i z a t i o n r e a c t i o n s

w i t h i n t h e channels which l e a d t o l i n e a r and s i n g l y branched hydrocarhons may a l s o occur i n r e v e r s e on t h e a c t i v e s i t e s o f t h e e x t e r n a l s u r f a c e (most a c t i v e s i t e s a t t h e z e o l i t e s u r f a c e occur a t t h e mouth o f t h e channels), which may t h u s mask some o f t h e s h a p e - s e l e c t i v i t y o f t h e z e o l i t e . REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12

R.E. Schaad, i n R.T. Brooks e t al., ( e d i t o r s ) , The Chemistry o f Petroleum Hydrocarbdns, Reinhold, New York (1955) 721-247. E.K. Jones, Advances i n Catal., 8 (1956) 219-238. E.I. Givens, C.J. Plank and E.J. R o s i n s k i ( t o Mobil O i l Corp.), 1I.S. P a t e n t 3,827,968 (1974). W.E. Garwood, Prepr. Div. Pet. Amer. Chem. SOC. 2 7 ( 2 ) (1982) 563-575; W.E. Garwood i n " I n t r a z e o l i t e Chemistry", A.C.S. Symposium Series, 218 (1983) 383-396. S.A. Tabak ( t o Mobil O i l Corp.), U.S. Patent 4,482,772 (1984). S.A. Tabak, R.S. Wright and H. Owen ( t o P o h i l O i l Corp.), 1J.S. Patent 4,504,693 (1985 1. 5.A. Tahak and F.J. Kramheck, Hydrocarbon Processing, Sept. (1985) 72-74. J.G. R e n d o r a i t i s , A.W. Chester, F.G. Dwyer and W.E. Garwood, Stud. Surf. S c i . Catal 78 (1986) (New nev. Z e o l i t e Sci. Technol.), 669-675. J.P. van den Rerg, J.P. Wolthuizen, A.D.H. Clague, G.R. Hays, R. t h i s and J.H.C. van H o o f f , J. C a t a l . 80 (1983) 130-138. J.P. van den Rerg, J.P. M o l t h u i z e n and J.H.C. van Hooff, J. C a t a l , 80 (1983) 139-144. K.G. W i l s h i e r , P. Smart, R. Western, T. P o l e and T. Pehrsing, Appl. Catal., " i n press". M.K. Rubin, E.J. R o s i n s k i and C.J. Plank ( t o Mobil O i l Corp.), 1J.S. Patent 4,151,189 (1979).

.,

This Page Intentionally Left Blank

D.M.Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

627

C R A C K I N G OF SOME LONG CHAIN HYDROCARBONS ON HZSM-5 Z E O L I T E S

ZHANG YULONG, OU GUANGYAO and ZHANG Z H I I n s t i t u t e o f Coal Chemistry, Academia Sinica, P.0.Box China

P.R.

165, Taiyuan, Shanxi,

ABSTRACT I n t h i s paper, t h e c r a c k i n g o f n-hexane, n-dodecane and n-hexadecane on ZSM-5 z e o l i t e s a t about atmosphere and temperatures o f 260-400°C were studied. The r e s u l t s showed t h a t both mono-molecular cracking and bimolecular r e a c t i o n ( d i s p r o p o r t i o n a t i o n ) f o r n-hexane cracking took place. A network f o r i n i t i a l r e a c t i o n s was proposed, and the apparent k i n e t i c parameters o f the r e a c t i o n s were estimated. An examination f o r the f a c t o r s a f f e c t i n g the product d e s t r i b u t i o n o f n-hexadecane i n d i c a t e d t h a t hydrogen t r a n s f e r on the surface o f HZSM-5 z e o l i t e s p l a y s an important role i n c r a c k i n g r e a c t i o n .

INTRODUCTION Recently, t h e s c i e n t i s t s and engineers i n s e v e r a l c o u n t r i e s are endeavouring t o develop a two-stage F-T process (MF-T) f o r synthesis o f h i g h octane gasoline from coal, i n which HZSM-5 z e o l i t e i s used i n the second stage t o upgrade t h e product o f F-T synthesis i n f i r s t stage. Cracking o f long chain hydrocarbons i s one o f important r e a c t i o n s . i n MF-T,

however, t h e l i t e r a t u r e on t h e c r a c k i n g o f

long chain hydrocarbons on HZSM-5 z e o l i t e s i s sparse. Therefore, i n the present paper the c r a c k i n g r e a c t i o n s o f n-hexane,

n-dodecane and n-hexadecane on HZSM-5

z e o l i t e s are studied. EXPERIMENTAL The ZSM-5 z e o l i t e s were synthesized w i t h n-butylamine as template ( r e f . 1 ) . Their p r o p e r t i e s were shown i n Table 1. The o r i g i n a l z e o l i t e s were transformed t o H f o r m w i t h H C 1 exchange. TABLE 1 ProDerties o f ZSM-5 z e o l i t e s . Zeolite

Si02/A1203

Na20 (W X )

S p e c i f i c area ( M2/g)

Adsorption capacity o f cyclohexane( W X )

z-1 2-2 2-3

53 54 59

0.09

545 468

8.7 8.2

0.10

0.25

-

9.3

The experiments were c a r r i e d o u t w i t h a continuous f l o w micro-reactor a t

constant pressure. Nitrogen carrier gas was bubbled into a saturator containing n-hexane at O°C, then passed through the reactor. In the case of n-dodecane and n-hexadecane a pump was used. The products were analyzed by GC using a flame ionization detector. RESULTS AND DISCUSSION Crackina o f n-hexane (i) Effects o f temperature and space velocity. The activity and selectivity of cracking reaction are not change during the experiments because of high stability o f HZSM-5 zeolites. The results are given in Fig.1. It can be seen from Fig.l(a) that the conversions increase and the selectivities change with increasing reaction temperature at the same space velocity. The selectivities to C2 and C j increase while those to C4 and C5 decrease. The results obtained at different space velocities and the same temperature, as shown in Fig.l(b), indicated that conversion has little effect on product selectivities at low conversion. Indeed, the secondary reactions could be restrained by using low reaction temperature, low hexane pressure and low conversion.

Temperature (a)

(Or)

Fig. 1. Selectivities as functions of temperature and conversion f o r n-hexane cracking. (ii) Initial reaction network for cracking of n-hexane. The initial selectivities were obtained by extrapolating the selectivity curve in Fig.l(b) to zero conversion and were listed in Table 2. It is interesting

to note the data in Table 2. C4/Cz ratio in primary products is much larger than 1. There is lots o f C5 but almost no C1 in product. The experiment results, i . e . almost no methane

detected, low C2 selectivity, and high ratio o f n-paraffidiso-paraffin, were

629 TABLE 2

Initial selectivities of n-hexane cracking on HZ-2 at 32OoC.* Products Selectivities (MX)

C1 0

C2 4.8

c3 35.4

c4 43.2

c5 10.1

i-Cg 6.5

*The hexane pressure was 45mmHg. consistent with those predicted by a carbenium mechanism. Unimolecular cracking o f n-hexane would yield equal amounts o f C2 to C4, and C1 to C5. According to

carbenium theory, C3 would predominate. The results obtained at higher temperatures showed that C3 was major product indeed. However, at lower temperatures selectivity is higher f o r C4 than f o r C3, and the lower the temperature, the higher the C4/C2 ratio and the selectivity o f C5. These results suggest that in addition to the unimolecular cracking, there would be bimolecular disproportionation reactions similar to those described by Corma et al (ref.2), in which a twelve carbon intermediate was formed and it quickly cracked to C J - C ~hydrocarbons. In summary the network o f initial reactions of n-hexane cracking on HZSM-5 zeolites could be described in Fig.2.

/

Disproportionation

n-Hexane

Cracking

Isomerization

c2

+

2c5

c2

+

c4

i-Cg

Fig. 2. Initial network o f n-hexane cracking reaction on HZSM-5 zeolites. (iii) Kinetic parameters for n-hexane cracking. Following Fig.2, the kinetic parameters of individual reactions we& estimated (Table 3), assuming that the reactions are first order. The results show that the activation energies o f n-hexane cracking to C2 + C4 is larger than to C3 + C3. Provided the rate-determining step was the formation of carbenium ion, there would be a quick equilibrium between carbenium ions in a hydrocarbon chain followed by a slow step o f scission of the carbon-carbon bond. The unimolecular cracking of n-hexane could

-

be described as follows.

C-C-C-C-C-C

+ c-c-c-c-c-c +

c-c-c-c-c-c

+ -HC-C-C-C-C-C +

or

c-c-c-c-c-c + c-c=c + c-c-c

+

C-C-C-C-C-C

Slowest

(1)

Fast Slow

(2)

(3)

630

TABLE 3 Kinetic parameters of initial reactions of n-hexane on HZSM-5 zeolites.

HZ-I Eaa kcal/mol 11.4

e

Disappearance o f n-hexane

Global cracking 22.1 Cracking t o 25.2 c2 c4 Cracking to 20.6 c3 c3

HZ-3

HZ-2 Ab Eaa ml/g. hr kcal/mol 1 . 5 ~ 1 0 ~ 11.0

ml/g.hr

Eaa kcal/mol 1 . 9 ~ 1 0 ~ 10.5

Ab ml/g. hr O6 9.0~1

3 . 7 ~ 1 0 ~21~.O

2 . 5 ~ 1 0 ~20.2 ~

1 .ox1010

1 . 6 ~ 1 0 ~ 24.1 ~

1 . 1 ~ 1 0 ~ 24.3 ~

8.9~10~~

7 . 5 ~ 1 0 ~ 18.5

2.2~109

1 .8x109

9 . 3 ~ 1 0 ~ 4.6

6 . 8 ~ 1 0 ~ 4.9

6.1~10~

Slow

(4)

~

Ab

+

18.6

+

Isomerization

5.7

aActivation energy. bpre-exponential factor.

+

c-c-c-c-c-c

- c-c-c=c + +c-c

The reaction (4), which led to C2 + C4 products, involved a formation o f less stable ethyl carbenium ion, therefore, a higher activation energy was required. This is consistent with the results. Cracking of n-dodecane and n-hexadecane HZSM-5 zeolite is a highly active catalyst for cracking o f n-dodecane and n-hexadecane, and more than 90% conversion can be obtained at 32OoC and WHSV o f

Carbon Number Fig. 3. Product distribution by carbon number for n-hexadecane cracking.

631 8-9 h r - l .

The conversion i s higher f o r n-hexadecane than f o r n-dodecane a t the

same operating c o n d i t i o n s , and t h e r e are a g r e a t deal o f aromatics i n t h e products f o r b o t h r e a c t i o n s . The major products f o r n-dodecane c r a c k i n g a r e C J - C ~ . The product d i s t r i b u t i o n f o r n-hexadecane c r a c k i n g i s shown i n Fig.3.

The change

o f d i s t r i b u t i o n w i t h r e a c t i o n temperature i s very l a r g e and w i t h apparent con-

t a c t time i s only a l i t t l e . For c r a c k i n g r e a c t i o n s catalyzed by acid, t h e f i r s t step i s t o form the carbenium i o n . I n general, t h e nearer the center o f t h e hydrocarbon chain, the lower t h e formation heat of t h e carbenium i o n , and t h e easier i t s formation. I f most o f t h e carbenium i o n s formed were on t h e t h i r d and f o u r t h carbon atoms, major products would be C4 and C5 hydrocarbons, which i s c o n s i s t e n t w i t h the experimental r e s u l t s . The d i s t r i b u t i o n r u l e p u t forward by Greensfelder e t a1 ( r e f . 3 ) or Van Hook e t a1 ( r e f . 4 )

c o u l d s a t i s f a c t o r i l y e x p l a i n t h e i r r e s u l t s f o r n-hexadecane crac-

king. However,,our

product d i s t r i b u t i o n , as shown i n Fig.3,

from t h e i r s ( c a l c u l a t e d curve i n Fig.3).

i s quite different

I t can be seen from Fig.3 t h a t t h e

experiment values are lower than c a l c u l a t e d values f o r C 3 and C4, b u t higher f o r C7-Clo.

S t a b l i z a t i o n o f l o n g chain hydrocarbons by i n t e r m o l e c u l a r hydrogen t r a n -

s f e r was n o t considered by Greensfelder e t a1 and Van Hook e t a 1 because the c a t a l y s t s they used were s i l i c a - a l u m i n a on which hydrogen t r a n s f e r was n o t signi f i c a n t . However, hydrogen t r a n s f e r i s f a c i l i t a t e d i n z e o l i t e s , which possess s u i t a b l e a c i d s i t e s and pore s t r u c t u r e s . Furthermore, higher concentrations of hydrocarbons are b e l i e v e d t o e x i s t i n z e o l i t e micropores than i n s i l i c a - a l u m i n a c a t a l y s t pores because o f l a r g e adsorption c a p a c i t y o f z e o l i t e c a t a l y s t s ( r e f . 5 ) . The secondary c r a c k i n g o f C:

hydrocarbons c o u l d be decreased by t h e hydrogen

t r a n s f e r r e a c t i o n s , which c o u l d increase t h e s e l e c t i v i t i e s t o C z hydrocarbons. S e l e c t i v i t y t o c6-Clo hydrocarbons c o u l d a l s o be increased because o f h i g h aromatization a c t i v i t y o f HZSM-5 z e o l i t e s . CONCLUSIONS

I t can be concluded t h a t t h e r e a r e bimolecular r e a c t i o n s i n n-hexane cracki n g , and t h e r e i s a q u i c k e q u i l i b r i u m between carbenium i o n s i n a hydrocarbon chain. The z e o l i t e c a t a l y s t s have s u i t a b l e micro-pore s t r u c t u r e s and a c i d s i t e s f o r i n t e r m o l e c u l a r hydrogen t r a n s f e r and a r e favorable t o gasoline production. REFERENCES

1 2 3 4 5

Z. Zhang, C.R. Fu, J o u r n a l o f F u e l Chemistry and Technology, 10 (1982) 53. A. Corma, A. Lopez Agudo, I.Nebot, and F. Tomas, J. Catal., 77 (1982) 159. B.S. Greensfelder, H.H. Voge, and G.M. Good, Ind. Eng. Chem., 41 (1949) 2573. W.A. Van Hook and P.H. Emmett, J. Am. Chem. SOC., 85 (1963) 697. P.B. Venuto, Chem. Technol. 1 (1971) 215.


D.M. Ribby, C.D. Chang, R.F. Howe and S. Yurchnk (Editors),Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

633

REGENERATION OF COKE DEACTIVATED ZSM-5 BY AIR/OXYGEN G.D.

McLELLAN,'

R.F.

HOWE' and D.M.

BIBBY

2

'Chemistry Department, U n i v e r s i t y o f Auckland, P r i v a t e Bag, Auckland (New Zeal and) 'Chemistry

D i v i s i o n , DSIR, P r i v a t e Bag, Petone (New Zealand)

ABSTRACT The r e g e n e r a t i o n o f z e o l i t e ZSM-5 c o n t a i n i n g v a r y i n g amounts o f coke b y e i t h e r h i g h t e m p e r a t u r e a i r o r oxygen t r e a t m e n t has been s t u d i e d b y i n - s i t u FTIR spectroscopy and temperature-programmed o x i d a t i o n (TPO) mass s p e c t r o m e t r y . These methods show t h a t c y c l i c a n h y d r i d e and o t h e r c a r b o n y l groups a r e formed on coke, t h e s e decompose t o t h e carbon o x i d e s hence f a c i l i t a t i n g t h e removal o f coke f r o m t h e z e o l i t e .

-

INTRODUCTION The r e g e n e r a t i o n o f z e o l i t e ZSM-5 c o n t a i n i n g v a r y i n g amounts o f coke by e i t h e r h i g h t e m p e r a t u r e a i r o r oxygen t r e a t m e n t has been s t u d i e d by i n - s i t u FTIR spectroscopy and temperature-programmed o x i d a t i o n (TPO)/mass s p e c t r o m e t r y .

To

d a t e t h e c h e m i s t r y i n v o l v e d i n t h e r e g e n e r a t i o n o f ZSM-5 has r e c e i v e d l i t t l e study.

Hutchings -e t a l . ( r e f s . 1 and 2 ) have examined t h e e f f e c t s o f coke

removal b y a i r combustion on c a t a l y s t e f f i c i e n c y and compared i t w i t h t h a t o f an a l t e r n a t i v e ozone r e g e n e r a t i o n method.

Employing I 3 C NMR spectroscopy ( r e f . 3 )

t h e y a l s o i d e n t i f i e d e t h e r and k e t o n e groups i n a l i g h t l y coked sample o f ZSM-5 exposed ex s i t u t o oxygen a t 410

OC.

Furthermore, t h e y d i s c o v e r e d t h a t oxygen

had removed b o t h a l i p h a t i c and a r o m a t i c compounds i n coke. An I R s t u d y made b y Demodov

eta. ( r e f .

4 ) observed c a r b o n y l s u r f a c e s p e c i e s

f o r coke d e p o s i t s i n z e o l i t e HNaY t r e a t e d w i t h oxygen o v e r 100°C t o 5DOOC.

At

t h e lower t e m p e r a t u r e s aldehydes, ketones and c a r b o x y l i c a c i d groups were seen, w h i l e a t h i g h e r t e m p e r a t u r e s c a r b o x y l a t e and a n h y d r i d e groups were i n s t e a d observed.

They a l s o f o l l o w e d changes t o coke on o x i d a t i o n .

This study i d e n t i f i e s ( i ) removal o f coke and ( i i )

t h e o x i d a t i o n s p e c i e s t h a t occur i n t h e o x i d a t i v e

t h e p a r t s o f coke t h a t a r e l o s t t h r o u g h o x i d a t i o n .

The e f f e c t o f r e g e n e r a t i o n on t h e a c i d i t y o f ZSM-5 i s d i s c u s s e d elsewhere ( r e f .

5). EXPERIMENTAL The t h r e e ZSM-5 samples used i n t h i s s t u d y have been p r e v i o u s l y d e s c r i b e d ( r e f . 6) ( d e s i g n a t e d "ZSM-5 no. 2 " , "ZSM-5 no. 3" and "Mobil ZSM-,5").

The

samples c o n t a i n e d ca. 1.3,

Samples

2.5 and 3.2 A1 ( u n i t c e l l ) - '

respectively.

634

were u n i f o r m l y coked i n a t u m b l i n g bed r e a c t o r a t 37OOC w i t h methanol i n t r o duced i n a stream o f n i t r o g e n . V o l a t i l e p r o d u c t s were s u b s e q u e n t l y desorbed i n n i t r o g e n a l o n e f r o m coked samples a t c o n v e r s i o n temperature. Coke c o n t e n t s were determined i n a thermobalance ( r e f . 6 ) . I n - s i t u o x i d a t i o n experiments were conducted i n a h e a t a b l e I R f l o w c e l l o f t h e t y p e d e s c r i b e d by K a t z e r ( r e f . 7 ) . Z e o l i t e samples were pressed i n t o s e l f mounted i n t h e c e l l and a c t i v a t e d b y h e a t i n g s u p p o r t i n g wafers (7-10 mg 3 i n f l o w i n g n i t r o g e n (ca. 120 cm m i n - I ) t o 400OC. O x i d a t i o n o f t h e coke was 3 c a r r i e d o u t i n a stream o f d r y a i r (100 cm m i n - I ) a t 4OOOC t o 580 O C . I n f r a r e d s p e c t r a were measured on a N i c o l e t 5-DX FTIR spectrometer. T y p i c a l l y 1000 ( I s ) i n t e r f e r o g r a m s were averaged. The 160-180 m i x t u r e was s u p p l i e d b y Cambridge Isotope Laboratories. Temperature-programmed o x i d a t i o n (TPO) o f coke on ZSM-5 was c a r r i e d o u t i n a 1.0% oxygen-in-argon

mixture.

Coked samples were dehydrated a t 3OOOC f o r 30

m i n u t e s i n f l o w i n g argon b e f o r e TPO commenced. a t 10°C min-’

f r o m 3OOOC t o 750OC.

Samples (ca. 10 mg) were heated

P r o d u c t s e v o l v e d f r o m t h e z e o l i t e were

analysed i n a c o n t i n o u s l y scanning E x t r a n u c l e a r SpectrEL mass s p e c t r o m e t e r (model no. 275-50). F l o w o f t h e gas m i x t u r e was c o n t r o l l e d by a f i x e d c a p i l l a r y 3 1 l e a k ( 1 0 cm min- ) . RESULTS AND DISCUSSION F o r a l l t h r e e samples o f ZSM-5 a band appears a t ca. 1780 cm-’

when coke i n

ZSM-5 i s exposed t o a i r a t c o n v e r s i o n temperatures and above (see F i g . 1 ( a - c ) ) . T h i s i s accompanied by t h e l o s s o f some I R bands due t o coke between 1700 and 1300 cm-’,

e s p e c i a l l y a t temperatures above 500OC.

T h i s band i s s t a b l e up t o

45OOC and a t c o n v e r s i o n temperatures t a k e s a few h o u r s t o r e a c h maximum i n t e n s i ty. H e a t i n g above 55OOC produces a c l e a n spectrum o f t h e z e o l i t e which o n l y c o n t a i n s a p a i r o f bands a t 3740 and 3610 cm-’ due t o t h e h y d r o x y l groups o f t h e

zeolite.

The r a t e o f growth o f t h e 1780 cm-’

band i s independent o f z e o l i t e

aluminium c o n t e n t f o r s i m i l a r coke c o n t e n t s . Curve ( d ) i n F i g . 1 shows t h e r e s u l t o f t h e spectrum o f dehydrated 9.14 w t % coked ZSM-5 no. 2 s u b t r a c t e d f r o m t h e spectrum o f t h e sample a f t e r 170 m i n u t e s o f r e g e n e r a t i o n a t 420°C.

P o s i t i v e bands a t above 1700 cm-’ correspond t o

s p e c i e s formed d u r i n g t h e i n i t i a l s t a g e o f r e g e n e r a t i o n w h i l e t h e n e g a t i v e peaks f r o m 1700 t o 1300 cm-I correspond t o s p e c i e s l o s t d u r i n g t h i s stage.

The

p o s i t i v e bands o c c u r i n a r e g i o n o f t h e i n f r a r e d spectrum where c a r b o n y l s t r e t c h i n g modes a r i s e ( r e f . 8 ) . 1785 cm-’

We a s s i g n t h e s e bands a t ca. 1850 cm-’

and

t o c y c l i c a n h y d r i d e groups and t h e o t h e r bands t o k e t o n e groups, b o t h

formed i n coke.

T h i s i s c o n s i s t e n t w i t h t h e I R assignments o f Meldrum , e t f l .

( r e f . 9), f o r c a r b o n y l s p e c i e s observed f o r a c t i v a t e d carbon t r e a t e d i n oxygen and t h o s e o f Demodov e t a l . ( r e f . 4 ) .

635 Curve ( e ) i n F i g . 1 i s a s u b t r a c t e d spectrum of 7.20 w t % coked ZSM-5 no. 2 a f t e r -ex s i t u t r e a t m e n t w i t h 50% I 8 0 - e n r i c h e d oxygen a t 35OOC f o r 69 minutes. The a d d i t i o n a l bands i n t h e upper c u r v e a r e t h e r e s u l t o f I 8 O s u b s t i t u t i o n i n t h e c a r b o n y l groups formed i n coke.

Coincidences i n t h e two curves o c c u r a t

1856, 1850: 1791, 1785: 1733, 1720 and 1690, ca. 1685 cm-’. These a r e t h e 18 v(C 0) modes o f u n s u b s t i t u t e d c a r b o n y l groups i n coke o f b o t h samples. ‘*Oinduced c a r b o n y l bands i n c u r v e ( e ) , hence, appear a t ca. 1827, 1763 and 1661 16 A s h o u l d e r between 1856 and 1791 cm-’, i n d i c a t e t h a t t h e observed C 0cm-’. 18 C 0 spectrum i s more complex t h a n would be expected ( r e f . 10) f o r an i s o t o p i c m i x t u r e of uncoupled c a r b o n y l groups.

I n s t e a d t h i s spectrum i s c o n f i r m a t i o n

t h a t a s p e c i e s c o n t a i n i n g v i b r a t i o n a l l y c o u p l e d c a r b o n y l groups i s formed i n coke on ZSM-5, t h o s e o f say a c y c l i c anhydride. A coalesced d o u b l e t o f t r i p l e t s 16 occurs, where t h e symmetric and asymmetric v ( C 0) modes o f such a s p e c i e s a r e each s p l i t i n t o a 1:2:1 t r i p l e t because s t a t i s t i c a l l y t h e a n h y d r i d e would e x i s t as 25% [(C160)O],

25% [(C180)O]

and 50% [(C160)(C180)O].

The appearance o f a band a t ca. 1660 cm-’ on i s o t o p i c o x i d a t i o n ( c u r v e ( e ) ) i s c o n s i s t e n t w i t h bands between 1720 and 1680 cm-’ b e i n g due t o k e t o n i c c a r b o n y l groups ( r e f . 10). I r r e s p e c J i v e o f coke c o n t e n t t h e same o x i d a t i o n s p e c i e s a r e seen which suggests t h a t s p e c i f i c p a r t s o f t h e coke a r e o x i d i s e d a t 300-450°C t o g i v e a n h y d r i d e and k e t o n i c species, t h e n l o s t a t h i g h e r temperatures. Since bands below 1650 cm-’

a r e l o s t as coke i s o x i d i s e d , t h e y must b e

a s s o c i a t e d with t h o s e p a r t s o f coke must s u s c e p t i b l e t o o x i d a t i o n . between 1590-1470 cm-’, 1365 cm-’.

e s p e c i a l l y a t 1590 and 1530 an-’,

Losses occur

ca. 1430 cm-’

and ca.

The 1530 and 1365 cm-’ bands each s u f f e r a s i g n i f i c a n t l o s s i n

i n t e n s i t y ’ b e l o w 5OO0C, t h e s e correspond t o t h e l o s s o f a r o m a t i c r i n g s and methyl A s s o c i a t e d w i t h t h i s l o s s i s a l o s s o f v(CH)

groups i n coke, r e s p e c t i v e l y .

bands between 3100 and 2850 cm-’.

However, t h e p r i n c i p a l coke band a t 1590 cm-

1

i s t h e l a s t t o be l o s t and t h e r e f o r e i s r e l a t e d t o s t r u c t u r e s i n coke t h a t a r e t h e most r e s i s t a n t t o o x i d a t i o n o f ZSM-5 ( r e f . 11).

-

namely g r a p h i t i c p a r t t o coke on t h e e x t e r i o r

Over t h e e n t i r e range o f coke c o n t e n t s (3.58-15.4

w t %, I R

e x p e r i m e n t s ) t h e same bands a r e always p r e f e r e n t i a l l y l o s t , a g a i n t h i s suggests t h a t s p e c i f i c p a r t s o f coke a r e o x i d i s e d d u r i n g r e g e n e r a t i o n . cm-’

The r i s e a t 1470

i n c u r v e ( d ) o f F i g . 1 c o u l d be due t o i s o l a t e d a r o m a t i c r i n g s formed as

t h e d e g r a d a t i o n o f coke t a k e s p l a c e . t h o s e o f Demodov

gal. ( r e f .

These o b s e r v a t i o n s a r e i n agreement w i t h

4 ) f o r o x i d i s e d coke i n NaHY z e o l i t e .

The temperature-programmed o x i d a t i o n (TPO) o f ZSM-5 no. 2 c o n t a i n i n g v a r y i n g amounts o f coke r e s u l t e d i n t h e d e s o r p t i o n o f two species: m/z = 28 (CO’.)

m/z = 44 (C02+*) and

due t o d e s o r b i n g C02 and C O Y r e s p e c t i v e l y ( F i g . 2

).

s i n g l e d e s o r p t i o n e v e n t i s observed i n TPO thermograms o f coked ZSM-5. a l l t h e c o m b u s t i b l e m a t e r i a l i n coke i s o x i d i s e d between 350 and 650OC.

Only a Hence, A trend

636

e x i s t s between t h e amount o f C02 desorbed and i n i t i a l coke c o n t e n t .

The tempera-

t u r e o f maximum C02 d e s o r p t i o n i n c r e a s e s f r o m about 4OOOC f o r low l e v e l s o f coke t o 6OOOC a t ca. 4 w t % coke, presumably t h i s r e f l e c t s t h e more r e f r a c t o r y n a t u r e o f coke towards h i g h e r coke c o n t e n t s . The FTIR d a t a i n d i c a t e t h a t two t y p e s o f o x i d i s e d hydrocarbon s p e c i e s i n coke on ZSM-5 exposed t o a i r a t h i g h temperatures.

This correlates with a loss o f

m e t h y l groups and some a r o m a t i c r i n g s i n coke. t o b e a c o m p e t i t i v e process i n coke removal.

A l s o p y r o l y s i s o f coke i s l i k e l y TPO demonstrates t h a t

decarbonylation-decarboxylation i s t h e f i n a l s t e p i n r e g e n e r a t i o n .

2

4000

1365

1300

-4

2000

1763

1300

wavenumber

F i g . 1. FTIR s p e c t r a o f 9.14 w t % coked ZSM-5 no. 2: ( a ) B e f o r e r e g e n e r a t i o n . ( b ) F o l l o w i n g i n s i t u r e g e n e r a t i o n o f t h i s sample i n a i r f o r 170 mins. a t 4 2 O O C . ( c ) 1 7 5 m i n s a t 540°C. The l e f t h a n d a x i s r e f e r s t o t h e l o w e s t spectrum. S p e c t r a ( a ) and ( c ) were r e c o r d e d a t 350°C. ( d ) S u b t r a c t e d spectrum o f 9.14 w t % coked ZSM-5 no. 2 a f t e r i n t r e a t m e n t . 170 mins a t 420°C. ( e ) S u b t r a c t e d spectrum o f 7.20 w t 7 coked ZSM-5 no. 2 a f t e r s i t u t r e a t m e n t w i t h 50% I 8 0 - e n r i c h e d oxygen. 69 mins a t 350°C.

-

ex

637

m/z 44 m/z 28 m/z 2 L

C

9 .... ......................... .. ...:*............. I.

300

400

500

600

700

800

Temperature ("C )

Fig. 2. Mass s p e c t r o m e t r i c thermogram o f t h e TPO o f 2.25 w t % coked ZSM-5 no. 2, o x i d a n t 1.0% oxygen-in-argon, h e a t i n g r a t e = 10°C min-I. REFERENCES R.G. Copperthwaite, G.J. Hutchings, P. Johnston and S.W. Orchard, J. Chem. SOC. , Chem. Commun. , (1985), 644-645. R.G. Copperthwaite, G.J. Hutchings, P. Johnston and S.W. Orchard, 3. Chem. SOC., Faraday Trans. I, 82 (1986), 1007-1017. L. Carlton, R.G. Copperthwaite, G.J. Hutchings and E.C. Reynhardt, J. Chem. SOC., Chem. Commun., (1986), 1008-1009. A.V. Demodov, A.A. Davidov and L.N. Kurina, Z. P r i k l a d o i Spek., 43 (1985), 845-848. D.M. Bibby, G.D. McLellan and R.F. Howe, submitted f o r p u b l i c a t i o n , Studies i n Surface Science and C a t a l y s i s , E l s e v i e r , Amsterdam. ( a ) D.M. Bibby, N.B. Milestone, J.E. P a t t e r s o n and L.P. Aldridge, J. Catal., 97 (1986), 493-502; ( b ) T.R. Forester, S-T. Wong and R.F. Howe, J. Chem. SOC., Chem. Commun. , (1986), 1611-1613. S.H. Moon, H. Windawi and J.R. Katzer, Ind. Eng. Chem. Fundam., 20 (1981), 7 396-399. L.J. Bellamy, "The I n f r a r e d Spectra o f Complex Molecules", Chapman and H a l l , 8 London, 3 r d ed., 1975. B.J. Meldrum, J.C. Orr and C.H. Rochester, J. Chem. SOC., Chem. Commun., 9 (1985), 1176-1177. 10 S. Pinchas, " I n f r a r e d Spectra o f L a b e l l e d Compounds', Academic, London, 1971. 11 B.A. Sexton, A.E. Hughes and D.M. Bibby, t o be published.


D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 1988Elsevier Science Publishers B.V., Amsterdam - Printed in 'l'he Netherlands

639

QUANTITATIVE THERMAL DESORPTICN~SSPEcrwlMETRY OF AlPO4-11 PRECURSORS N.J.

TAPP and N.B.

MILESlONE

Chemistry Division, CGIR, Private Bag, Petone (New Zealand) ABSTRACT

Crystalline products with the s t r u c t u r e of AlW4-11 have been produced using f o u r d i f f e r e n t m i n e s as structure-directing species. A method f o r quantifying t h e amunt of m i n e i n the precursors has been developed. This involves the use of thermal desorption/mass s p e c t m t r y i n combination with t h e m gravimetry. This method also provides information as to the d i s t r i b u t i o n of t h e water within the structure. INTRO~CTION

Microporous aluminophosphates ( A l P O 4 ' s ) are synthesised by hydrothermal treatment of a reactive g e l containing aluminium, phosphorus and an organic structure-directing species. A series of these AlP04's has been produced (ref.1) by using a wide variety of organic m i n e s and quaternary Qrmonium cations. Each c r y s t a l l i n e product as synthesised contains the structured i r e c t i n g species trapped within the s t r u c t u r a l voids, but these species can be removed by heating i n a i r a t 500-600°C and generally without any s t r u c t u r a l collapse of the AlPO4. By changing the reaction conditions, several d i f f e r e n t s t r u c t u r e s can be produced from one structure-directing species (ref.2). Furthermore, a p a r t i c u l a r s t r u c t u r e can be produced from more than one structure-directing species (ref.2). I t has been reported (ref.3) that AlP04-ll* could be produced from several secondary d n e s and a relationship w a s found between t h e success

or f a i l u r e of the production of AlP04-11 and t h e s i z e of the amine. Determination of the amount of structure-directing species which remains i n t h e channels of these precursors may' help shed l i g h t on the role of these organic conpcunds. W e have developed a mthod f o r quantifying the amount of amine i n Alp04 precursors. This involves t h e use of thermal desorption/mass spectrometry (td/ins) i n combination w i t h thermogravimetry (tg). It provides an a l t e r n a t i v e t o conventional chemical analysis and gives information on the temperature of &sorption of the amine and on the location of the water trapped i n the s t r u c t u r e during synthesis.

* The series is denoted AlPO4-n, where the acronymAlP04 is derived from (AlxPy)02.

The s u f f i x "n" demtes a s p e c i f i c s t r u c t u r e type

640

EXPERIMENTAL Four samples of AlP04-ll were prepared as i n reference 3 using di-ethylamine (Et2NH), di-n-propylamine (n-Pr2NH), di-iso-propylamine (i-Pr2NH) and n-butylethylamine (n-BuEtNH) as s t r u c t u r e - d i r e c t i n g species. K a o f i n i t e N o 5, from Lamar P i t , South Carolina w a s shown by t g t o c onta in 1 3 . 3 0 . 2 mass % water and was used as an i n t e r n a l sta nda rd i n the td/m experiment. Thermogravimetric a n a l y s i s was performed on a Stanton Redcroft TGA Model 770.

q were heated (from 15 t o 700OC) a t 5OC min-l i n an Sample masses of 15-20 r argon flow of 50 m l f i n - l . The td/ms experiment was performed on approximately 40 mg of AlpO4-ll p r ecu r s o r , mixed with approximately 15 mg of k a o l i n i t e . This mixture was heated a t 5°C min-1 i n a low-pressure mini-furnace i n t e r f a c e d d i r e c t l y to an Extranuclear SpectrEl cpadrupole mass spectrometer. Argon w a s passed through a f i x e d c a p i l l a r y leak (24 ml f i n - l a t STP) and over the sample. The desorbed prcducts were pumped p a s t the mass spectrometer i n l e t by a r o t a r y vacuum pump and t h e g as stream was sampled through a leak valve. The mass spectrum w a s continuously scanned and t h e data s t o r e d on an HP9835 computer as

a f u n c t i o n of s q l e t e q e r a t u r e .

More d e t a i l e d information on the software

and equipment is given elsewhere ( r ef . 4 ) . RESULTS AND DISCUSSION The s t r u c t u r e of AlFO4-11 ( r e f . 5 ) i l l u s t r a t e d i n f i g u r e 1, is a m p r i s e d of a l t e r n a t i n g A104 and Po4 t e t r a h e d r a joined by the sha ring of t h e oxygen atoms (white s p h e r es ) . There are three sets of channels which are bounded by 4, 6 and 10 membered r i n g s r es p ect i v el y . The u n i - d i m n s i o n a l channel system bounded by a lomembered r i n g is of mst i n t e r e s t .

The u n i t - c e l l c-dimension looking

d m these l a r g e channels is 8.44 A ( r ef . 3) and we have defined a p o r t i o n of t h e channel of this length as a c h a n n e l u n i t .

Fig. 1. Model of AlFO4-ll shown w i t h the t e t r a h e d r a l c e n t r e s joined and the oxygen at(white s p h er es ) p r e s e n t i n only h a l f of the s t r u c t u r e

641

The size of the m i n e s (ref.3) used t o produce AlP04-11 is such t h a t they must reside in the large channels. A finding of one amine per channel-unit would suggest t h a t the channel formed around the amine i.e. t h a t the m i n e acted as a template f o r t h a t structure. Thermogravimetric ( t g ) and d i f f e r e n t i a l thermogravimetric (dtg) results from the four AlFO4-ll precursors are given i n Figures 2a-b. A l l of the samples gave similar weight loss p r o f i l e s consisting of a low temperature (

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